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The Molucca Sea region is an active arc-arc collision zone whose structure and mantle dynamics are essential for reconstructing the tectonic evolution of Southeast Asia. The key to understanding the tectonic processes of the region is to clarify the morphology of the subducted slabs. However, geometric details of the subducted Molucca Sea slab and other slab segments under this region remain controversial due to the lack of high-resolution mantle tomography. In this study, we jointly invert a large number of local and teleseismic travel-time data recorded at 99 local seismic stations to determine a high-resolution model of P-wave mantle tomography beneath the Molucca Sea region. Our tomography reveals two aseismic slab segments in the deep upper mantle and the mantle transition zone, which may reflect the earlier subducted Molucca Sea slab. The total length of the Molucca Sea slab is estimated to be ∼1800 to 1900 km, being ∼300 km longer than previous estimates. • The Molucca Sea slab has experienced break-offs. • Aseismic slab fragments broken off from the early subducted Molucca Sea slab are revealed in the mantle transition zone. • The maximum length of the Molucca Sea slab is ∼1800 to 1900 km. [ABSTRACT FROM AUTHOR]

The modern volcanoes of Indonesia, represent the cycling of material and heat along the Southeast (SE) Asian subduction zone. This volcanic activity is well exemplified in the middle section of Java Island, which is found on the Sunda arc portion of the SE Asian convergent system. Central Java's volcanoes inherit their geochemistry from silicate melts that formed due to hydrous melting at a range of depths above their subducting slab: ∼136 to ∼272 km. It has been shown that across-arc variations in magmatic trace elements like boron (B) can shed light on the quantity and type of slab-derived fluid released in a forearc to back-arc sequence. In this study, we present major, trace and radiogenic chemistry data for five volcanoes arranged in a roughly N-S lineament across Central Java. We show that the observed chemical changes across the section of the Sunda arc can be explained by variations in different kinds of source from fluid mobile material being devolatilized across the arc. Our mixing models suggest that the magmas underlying these volcanoes see a decreasing ratio of sediment to altered oceanic crust (AOC) contributions from the volcanic front (Merapi volcano) toward the rear-arc (Muria). We estimate that a maximum ∼1% contribution of sediment-derived fluid is sufficient to explain the chemical systematics of this arc segment. Shallow slab dehydration, mostly from the sediment cover, at low temperature (at <60 km of slab depth) transfers the fluid high in B/Nb, Ba/Nb, 87Sr/86Sr and 207Pb/204Pb to the mantle beneath this region's arc front, inducing flux melting. Farther from the trench, the signature of the subducting oceanic plate increases and peaks at around ∼160–170 km of slab depth, carrying substantial slab liquids with moderate to low B/Nb and Ba/Nb with lower 87Sr/86Sr and 207Pb/204Pb to the mantle beneath middle- and rear-arc region. Beyond 200 km, in the rear-arc, evidence for slab liquid is low, but not non-existent. Unexplained Ba-enrichment in magmas from across the arc may account for the role of subduction serpentinite dehydration in the subduction channel or local crustal assimilation. This study provides a conceptual model for material transfer from the subducting slab in SE Asia. Our findings have important implications for the understanding of geochemical cycling in this region, as well as the impact of these cycles on present and past volcanism. • Slab liquid with different proportion of SED/AOC is evident across Central Java, high at arc front and low at the rear-arc • Shallow slab liquid is high in FME, 87Sr/86Sr, 207Pb/204Pb; deep slab liquid has moderate/low levels of those components • Elevated Ba/Nb across the arc magmas may hint to the importance of subducting serpentinite in subduction channel [ABSTRACT FROM AUTHOR]

Subducted slabs provide the primary driving force for mantle convection and plate tectonics. The strength of the slab directly controls the transfer of forces between it and the lithospheric plates at the surface. However, slab viscosity constrained by surface topography and gravity data is significantly lower than that suggested by mineral physics laboratory experiments. It is unclear whether the free-slip top boundary condition used by these studies affects the inverted slab viscosity. In this study, we constrain the subducting slab viscosity structure by comparing the computed topography and gravity in slab subduction models with a free surface with observations. The free-surface model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region, consistent with previous studies with a free-slip top boundary. The viscosity of the slab below the bending region barely affects the surface topography and gravity field, and both strong and weak slabs fit the observed topography and gravity field, suggesting that extra independent observations are needed to constrain the deep slab viscosity. We investigate the comprehensive relations between subduction interface viscosity, surface topography and gravity anomaly, and trench motion. Models with trench advance have significantly low topography and gravity above the volcanic arc, contradicting subduction zone observations. Together with present trench motion observations and previous studies, we support the idea that the trench retreats under normal single-slab subduction conditions. • We constrain slab viscosity with topography and gravity anomalies in slab subduction numerical models with a free surface. • Model results support relatively weak slabs (20–120 times more viscous than the upper mantle) at the bending region. • Trench advance requires strong subduction interfaces, which result in significant arc depressions that contradict the observation. [ABSTRACT FROM AUTHOR]

The complex tectonic setting of south-central Alaska is characterized by a transition from normally-dipping subduction of the Pacific plate in the west to near-flat slab subduction of the overthickened Yakutat microplate in the east. Many previous studies have characterized both the isotropic and anisotropic subsurface structure of this region, but only a few studies have characterized anisotropy using receiver functions. Here, we present anisotropy-aware receiver function analysis for two transects of permanent seismic stations in south-central Alaska, one covering a normally-dipping segment of the subduction zone, and one covering the adjacent flat slab segment. Beneath the normally-dipping segment, there is evidence for shearing and possibly serpentinization at the top of the slab in the shallow forearc, and for variation in mantle flow geometry with depth, possibly a result of oblique subduction and/or the adjacent flat slab segment, or an arc magmatism-related process. Additionally, there appears to be significant crustal deformation associated with the volcanic arc. We also identify significant crustal deformation and anisotropy along the flat slab segment, likely a result of the subducting Yakutat microplate, with crustal deformation geometry appearing to vary along the transect. There also appears to be evidence for water-rich conditions at the top of the flat slab, shedding light on the distribution of volatiles in a flat slab setting that lacks an active volcanic arc. • Anisotropy-aware receiver functions for normally-dipping and flat slab segments. • Complex deformation in the mantle wedge above the normally-dipping Pacific plate. • Complex crustal deformation and possibly water-rich mantle above the flat Yakutat slab. [ABSTRACT FROM AUTHOR]

We used numerical modeling to investigate the geodynamic effects of subducted seamounts at the Manila Trench. A series of numerical modeling experiments were conducted with variable parameters, including the activation volume (V act) and cohesion (C), which influence lithospheric rheology, the plate convergence velocity, and the age of subducting slab. Modeling results indicate that varying the V act and C within an appropriate range have limited effects on the geodynamic process of subduction. A lower V act allows the slab to sink more easily and results in a steeper dip angle. A slab break-off is more likely to occur under subduction at depths of 100–300 km, while the existence of a seamount further promotes the break-off process. The convergence rate is a key parameter affecting the break-off timing and depth. In contrast, under subduction where subducted oceanic plate move faster upper plate, the model results exhibit non-break-off, steady subduction. Slab age is another factor controlling break-off, where break-off time extends with slab age. A subduction without seamount will cause a ~2 Myr delay in break-off timing. We suggest that the low-velocity zone under the Manila Trench at 17o N is the result of a break-off event due to subduction of the Zhenbei-Huangyan Seamount Chain. Further to the north, such as the location at 19o N, the absence of seamount and an older oceanic crust would favor a delay in break-off timing during subduction. • Slab break-off is more likely to occur under continental overthrusting, while a seamount will promotes the break-off process. • The convergence rate is a key parameter affecting break-off timing and depth. • A single-sided continental overthrusting without seamount will cause a ~ 2 Myr delay in break-off timing. • The low-velocity zone under the Manila Trench at 17o N is the result of a break-off event due to subduction of the Huangyan Seamount. [ABSTRACT FROM AUTHOR]

The origin of the high velocity anomalies in cold subduction slabs revealed by seismological tomography remains unknown. Garnet is a major component of the pyrolitic mantle and subduction slabs and crucially affects their physical properties, such as densities and velocities. Based on previous studies, the predicted velocities of garnets with various compositions are much different from the measured values. Here, we measured the velocities of four hot-pressed natural garnet samples, namely, Alm 65.1 Grs 17.5 Prp 8.5 Sps 2.0 And 6.9 , Alm 60.9 Grs 25.9 Prp 8.2 Sps 2.0 And 3.0 , Alm 49.6 Grs 5.3 Prp 37.8 Sps 1.7 And 5.6 , and Grs 89.0 Prp 0.4 Sps 6.3 And 4.3 , at pressures up to 11 GPa using the ultrasonic interference method in conjunction with a Walker-type multi anvil, to further constrain the composition dependence on sound velocities. Our results show that the velocities and the bulk and shear moduli monotonically increase with pressure but display a complicated dependence on composition. Accordingly, we have determined the dependences of P - and S -wave velocities of garnets on pressure and composition and reconstructed the velocity profiles of pyrolite, eclogite, harzburgite, and a garnet layer at depths of 90–300 km in a cold subducted slab. Our model proposes that the presence of a garnet layer with a thickness of 5.9–15.9 km atop the slab could cause velocity contrasts of 4.0–6.0% for V P and 4.1–7.7% for V S between the slab and the overlying pyrolite. The presence of a layer with a high proportion of garnet could contribute to observed high-velocity anomalies in cold subduction zones. • The velocities of four hot-pressed garnet samples up to 11 GPa were obtained. • The relationship between garnet wave velocity and composition was determined. • A garnet layer could cause high velocity anomalies in cold subduction zones. [ABSTRACT FROM AUTHOR]

The subduction of seismic oceanic ridges often results in the formation of slab windows, which can affect not only the heat flow and retroarc volcanism, but also the exhumation and topographic evolution of the upper plate. An active and world-class example of a slab window is southern Patagonia, in southernmost South America, which is related to the subduction of the seismic oceanic South Chile Ridge between the middle–late Miocene and the Present day. How the subduction of the ridge and formation of the slab window have influenced the evolution of the Patagonian landscape, exhumation and topography is still under debate. Some works have proposed orogenic deformation mostly affecting the Pacific margin and hinterland areas, or an inherited early Miocene tectonic relief generated before the slab window formation. Others have preferred epeirogenesis hypotheses, such as dynamic uplift or isostatic rebound as a result of lithospheric thinning associated with asthenospheric or lithospheric mantle changes. In this work, we analyze the landscape evolution at medium (orogen-scale) and long wavelengths (embracing the whole of southern Patagonia, from coast to coast) using FastScape a landscape numerical model. This program was coupled with an optimization scheme (the Neighborhood Algorithm) suitable for nonlinear inverse problems. The "goodness" (fit to data) of our landscape evolution models was evaluated using: i) cooling ages, and ii) maximum elevations, in order to provide constraints on the uplift rates, erosion efficiency and effective elastic thickness. We then used the best values to compare two forward models representing medium- versus long-wavelength processes. Our results indicate that long-wavelength uplift geometry (including dynamic uplift and/or lithospheric rebound from thinning) involving areas from the Andes to the Atlantic coast was required from 12 Myr to the Present day in order to reproduce not only the youngest cooling ages but also Present-day topography. • Southern Patagonia records rock uplift and plateau formation from Miocene-Present. • Long-wavelength uplift geometry is required for reproducing Present-day topography. • Long-wavelength uplift geometry is useful for reproducing Present-day cooling ages. • A slab window is responsible for long-wavelength uplift in southern Patagonia. [ABSTRACT FROM AUTHOR]

In the complicated force equilibrium of subduction dynamics, the slab pull resulting from the negative buoyancy of the lithosphere is considered the primary driving force of plate tectonics. In contrast, the isostatic restoring force due to lighter materials covering the oceanic crust (e.g., sediment) has been recognized as a relatively minor contributor. However, as the restoring force counteracts the gravitational pull, the resultant interaction has the potential to control the force balance governing the subduction system. Here, we conducted a series of three-dimensional viscoelastic-free subduction simulations with varying sediment distributions, thicknesses, and densities. We found that a higher restoring force suppressed the trench retreat and decreased the trench velocity. The trench curvatures increased with a high differential restoring force along the trench strike over time. Our results consistently showed a negative correlation between the effective slab pull and the isostatic restoring forces. Moreover, the magnitude and distribution of the isostatic restoring force along the trench strike determined the deviatoric stress level and location of the stress concentration within the subducting slab. We also found that locally abundant sediment margins may lead to a high deviatoric stress in the trench, which is associated with the occurrence of large earthquakes. We argue that sediment buoyancy can significantly affect the subduction kinematics and dynamics to a greater degree than previously suggested. Our findings provide fresh insights into the development of a global predicted velocity model, that reflects the effect of the isostatic restoring force, and provides a better understanding of subduction earthquakes. • We developed a systematic 3D subduction model to analyze sediment buoyancy. • Degree and distribution of isostatic restoring force control subduction kinematics. • Sediment buoyancy strongly affects stress state and slab pull in subducting slab. [ABSTRACT FROM AUTHOR]

Suruga Bay lies along the Tokai segment of the Nankai-trough, which last ruptured in the 1854 M8.4 Ansei-Tokai Earthquake, making it a potential locus for the next major earthquake on the Nankai-trough. Moderate size earthquakes have occurred in the Suruga Trough axial region (M6.5 in 2009 and M6.2 in 2011) that resulted in a prominent excitation of seismicity. Unfortunately, our ability to accurately locate these events is hampered by a sparse routine seismic observation network around the bay. In order to improve location accuracy, a total of 2 sets of 7 OBSs were deployed inside Suruga Bay, 6 from the spring of 2017, and an additional OBS from spring 2019. By combining the data from our OBS stations with that of existing land-based stations surrounding Suruga Bay, we were able to accurately relocate the observed seismic activity inside the Philippine Sea plate (PHS). Our combined processing revealed a 25° northwest-dipping 2–5 km wide zone on which the regional seismic activity occurs. Overall, seismic activity is observed to be 5–10 km shallower than previously determined from routine network data alone. A striking feature of the analyzed activity is the presence of a prominent strike-slip component in the focal mechanisms, which exhibit P-axes almost exclusively oriented towards the north. Furthermore, taking into consideration the apparent difference in the direction of the P-axis of the stress field to the calculated sense of movement of Izu peninsula by GNSS data, and the ongoing internal deformation observed by geodetic and geological data, we propose the existence of a shear zone inside the subducting PHS starting from the southern tip of Izu Peninsula and extending across the bay under Shizuoka city. This zone of strain partitioning acts as a buffer between the northward-colliding Izu Peninsula and the northwest-directed subduction of the PHS observed by GNSS. • First OBS deployment to monitor the Tokai Megathrust inside Suruga bay in 30 years, • Observed seismicity is 10 km shallower that reported in JMA catalogue, • Focal Mechanisms inside the PHS slab are mostly strike slip/oblique events, • Stress field on the megathrust boundary not favorably oriented for thrust faulting, • Slip partitioning sear zone inside the PHS slab impeding plate boundary rapture.. [ABSTRACT FROM AUTHOR]

Located in the eastern Himalaya and surrounded by Yalaxiangbo dome, the Cona Woka rift may provide valuable information for the extensional mechanism of crust and distribution of the Tethyan Himalayan polymetallic deposits. Here, for the first time we obtain the three-dimensional electrical structure reaching a depth of approximately 45 km across the Cona Woka rift. Underneath the Cona Woka rift, the electrical model reveals the intersection between the deep electrically conductive body (effective conductivity value of 0.29 s/m, 10–45 km depth) and the shallow conductive layer (conductivity value of 0.3–1 s/m, <10 km depth). We propose that the tearing of Indian slab should have contributed to the partial melting of the crust. The upper and middle crust has a melt fraction of approximately 17–20 vol%, which may deform locally to form the Cona Woka rift. The Yalaxiangbo dome is also influenced by the local deformation of the crust. Moreover, the local deformation may drive the upward migration of the fluids along the rift zone to mix with the shallow meteoric water, especially in the intersection of the Cona Woka rift and the east-west trending faults, affecting the distribution of Tethyan Himalayan polymetallic deposits. • The 3-D electrical model imaged the partial melting system of the Cona Woka rift. • The crust under the rift may deform locally and formed the Cona Woka rift. • The dome and polymetallic deposits were influenced by the east-west extension. [ABSTRACT FROM AUTHOR]

Seismic nests (or clusters) are typically defined as regions of high seismic activity at intermediate depths that do not move over time and are not associated with volcanic activity at the surface (e.g. Zarifi and Havskov, 2003; Zarifi et al., 2007; Prieto et al., 2012). In this study, we focus on a seismic nest located in central Peru beneath the Amazon basin city of Pucallpa. The location of this nest is just beyond the eastern-most extent of the Peruvian flat slab, similar to the settings of two other South American clusters, the Bucaramanga and Pipanaco nests. While the Pucallpa nest is visible in the figures of many earlier papers on intermediate depth South American seismicity, it has not to our knowledge been described as a seismic nest before. We present the first detailed description of the Pucallpa nest, compare it to other established nests, and discuss its implications for our understanding of the Peruvian flat slab. Our results indicate that the Pucallpa nest demarcates the northern margin of a sag in the horizontally subducting Nazca plate in central Peru. The position of the nest along the projected location of the downgoing Mendaña fracture zone is consistent with local variations in b-values and could help to explain both the nest's genesis and the co-located change in slab geometry. The nest is also spatially well correlated with the northern margin of the thick-skinned Shira Mountains and with the high heat flow associated with the Agua Caliente dome (Hermoza et al., 2006; Navarro Comet, 2018). Further study is needed to understand the effects of the complex Peruvian slab geometry on the formation of thick skinned deformation and heat flow anomalies on the overriding South American continent. • We present information on the previously undescribed Pucallpa nest in central Peru. • The location of the Pucallpa nest is at the eastern edge of the Peruvian flat slab. • The Pucallpa Nest comprises two legs with different source characteristics. • The nest is likely affected by both slab pull and the Mendaña fracture zone. • The nest aids our understanding of the Peru flat slab geometry and genesis. [ABSTRACT FROM AUTHOR]

Paleogeographic reconstructions place the Chortis block adjacent to southern Mexico in the Late Cretaceous and earlier time, forming the tectonically disturbed southwestern tip of the North American plate. This study assesses the relative motion between Chortis and North America during the Laramide orogeny, later development of the North America-Caribbean plate boundaries. We carried out paleomagnetic analysis of rotations and latitudinal displacement using a total of 90 sites from the Átima Limestone and the Valle de Ángeles Formation redbeds sampled at seven localities in Honduras. Two secondary but ancient magnetizations post-date Late Cretaceous folding at most localities. The oldest magnetization is a ChRM of high laboratory unblocking temperature in the redbeds (> 650 °C), and a moderate temperature in the limestones (500–600 °C). This component is WNW directed at five localities, varying somewhat in declination, having a consistent moderate positive inclination (mean I = 39.7°; α95 = 6.8°, n = 53 sites). The age of the ChRM is bracketed between 85 and 63 Ma, based on youngest zircon age and the age of a cross-cutting dike. An intermediate unblocking temperature component present in both units (Dec = 146.2°, Inc = − 28.5°; k = 13.1, α95 = 6.8°, n = 37 sites), is interpreted as an overprint. The ChRM indicates an average counterclockwise rotation of 55.4 ± 5.7°, with respect to the expected direction. The younger overprint was acquired between ~ 50 and 15 Ma; its mean is also discordant with the expected North America direction, implying some Neogene rotation of Chortis. Abundant late Albian-Turonian zircons in Valle de Ángeles redbeds explicitly require extending the Laramide arc along western Mexico into Chortis. A reconstruction of Chortis south of Mexico that takes into account 55 degrees of rotation shows a gap between both continental blocks at about 70 Ma. We propose a model where this gap formed by back-arc extension creating the Late Cretaceous Chontal basin at the southern termination of the Laramide flat-slab. [ABSTRACT FROM AUTHOR]

Abstract Stress-dependent nonlinear upper mantle rheology has a firm base in rock mechanical tests, where this nonlinearity results from dislocation creep of minerals. In the last few decades there has been some attention to nonlinear, power-law, materials for application in scaled analogue experiments for tectonic processes. However, studies describing the rheology of analogue materials with the same nonlinear dependency on stress as observed for lithospheric mantle materials at relevant stress levels, are still lacking. In this study we have developed and rheologically tested materials based on combinations of silicone polymers and plasticine, with the aim of obtaining a material that can serve as a laboratory analogue to the power-law rheology of olivine aggregates at lithospheric mantle conditions. From our steady-state creep tests we find that it is possible to obtain such a power-law material, with effective viscosities over relevant model stress ranges [5–4000 Pa] that allow for nonlinear deformation at laboratory time scales. We apply the developed material to a process where localized deformation of the lithosphere can be expected: slab break-off. We study this process using analogue models, where we apply the new nonlinear material to the lithospheric mantle domains, while we use Newtonian glucose to represent the low viscous asthenosphere. Now that we properly manage power-law behavior in our analogue lithosphere materials, we are able to model localized lithospheric tearing. Highlights • New materials to represent the lithospheric mantle in analogue models. • find power-law behavior comparable to olivine rheology. • We observe localized deformation in an analogue slab break-off model. [ABSTRACT FROM AUTHOR]

In the standard paradigm, continental crust is formed mainly by arc magmatism, but because the compositions of magma rising from the mantle are basaltic and continental crust is estimated to contain about 60% SiO 2 and much less MgO than basalt, the two do not match. To resolve this paradox, most researchers argue that large amounts of magmatic fractionation produce residual cumulates at the base of the crust, which because arcs are inferred to have magmatically thickened crust, form eclogites that ultimately founder and sink into the mantle. Not only are there problems with the contrasting bulk compositions, but the standard model also fails because prior to collision most modern arcs do not have thick crust, as documented by their eruption close to sea level, and in cases of ancient arc sequences, their intercalation with marine sedimentary rocks. Our study of Cretaceous batholiths in the North American Cordillera resolves the crustal composition paradox because we find that most are not arc-derived as commonly believed; but instead formed during the waning stages of collision and consequent slab failure. Because the batholiths typically have silica contents >60% and are derived directly from the mantle, we argue that they are the missing link in the formation of continental crust. Slab failure magmas worldwide are compositionally similar to tonalite-trondhjemite-granodiorite suites as old as 3.8 Ga, which points to their collective formation by slab failure and long-lived plate tectonics. Our model also provides (1) an alternative solution to interpret compiled detrital zircon arrays, because episodic peaks that coincide with periods of supercontinent amalgamation are easily interpreted to represent collisions with formation of new crust by slab failure; and (2) that models of early whole-earth differentiation are more reasonable than those invoking progressive growth of continental crust. [ABSTRACT FROM AUTHOR]

Marginal parts of a plate and subducting slab can play important roles in geodynamics. This is because in areas where a plate interacts with other plates or with the mantle thermal, geochemical, and mechanical interactions are expected. The Philippine Sea (PHS) slab that subducts beneath the Japan arcs has such an edge. To examine the relationship between arc magmatism and the slab edge in the transition zone from Northeast Japan to Central Japan, we investigated isotopic systematics of the regional volcanic rocks, incorporating data from literature and new data for five isotopic ratios of Sr, Nd, and Pb. The new data included major element compositions of 22 samples from the back-arc area, and 5 isotopic ratios for 6 samples selected from Pleistocene to early Quaternary epochs. Consequently, several findings were determined based on the spatial variation of the isotopic ratios and the estimated amount of slab-derived fluid: (1) the amount of fluid derived from the two subducting slabs (i.e., the Pacific slab and the PHS slab) decreases northward from a significantly high value (~ 5 wt% fluid added to the source mantle), away from the seismically determined edge of the PHS slab; (2) the proportion of the PHS component in the total slab-derived fluid also decays northward; and (3) the PHS component spreads to the north beyond the seismically determined edge of the PHS slab. These observations strongly suggest that the existence of an aseismic PHS slab beneath southernmost parts of Northeast Japan delivers the PHS component to the arc magmatism. As was indicated by previous geodynamical studies, subduction of the PHS and PAC slabs may generate suction force towards the corner of mantle wedge, which might account for the large amount of fluid near the seismically determined slab edge as described in (1) above. [ABSTRACT FROM AUTHOR]

Water transported by deep subduction to the mantle transition zone (MTZ) that is eventually released and migrates upwards is invoked as a likely cause for hydroweakening and cratonic lithosphere destruction. The destruction of the North China Craton (NCC) during the Mesozoic has been proposed to be related to hydroweakening. However, the source of water related to large-scale craton destruction in the NCC is poorly constrained. Some suggest that the water was mainly released from a flat-lying (or stagnating) slab in the MTZ, whereas others posit that most water was released from a previously existing strongly hydrous MTZ then perturbed by the stagnating subduction in the MTZ layer. In this study, we use numerical modeling to evaluate the water carrying ability of flat-lying slabs in the MTZ with different slab ages and water contents to simulate its maximum value and discuss its potential role on large-scale hydroweakening and craton destruction. Our results reveal that a single flat-lying slab in the MTZ cannot provide enough water for large-scale cratonic lithosphere hydroweakening and thinning. Water estimates invoked for craton destruction as experienced by the NCC can only be the result of long-term piling of multiple slabs in the MTZ or penetrating deeper into the lower mantle. [ABSTRACT FROM AUTHOR]

It has been established that almost all known mantle plumes since the Mesozoic formed above the two lower mantle large low shear velocity provinces (LLSVPs). The Hainan plume is one of the rare exceptions in that instead of rising above the LLSVPs, it is located within the broad global mantle downwelling zone, therefore classified as a “lone plume”. Here, we use the Hainan plume example to investigate the feasibility of such lone plumes being generated by subducting slabs in the mantle downwelling zone using 3D geodynamic modelling. Our geodynamic model has a high-resolution regional domain embedded in a relatively low resolution global domain, which is set up in an adaptive-mesh-refined, 3D mantle convection code ASPECT (Advanced Solver for Problems in Earth's ConvecTion). We use a recently published plate motion model to define the top mechanical boundary condition. Our modelling results suggest that cold slabs under the present-day Eurasia, formed from the Mesozoic subduction and closure of the Tethys oceans, have prevented deep mantle hot materials from moving to the South China Sea from regions north or west of the South China Sea. From the east side, the Western Pacific subduction systems started to promote the formation of a lower-mantle thermal-chemical pile in the vicinity of the future South China Sea region since 70 Ma ago. As the top of this lower-mantle thermal-chemical pile rises, it first moved to the west, and finally rested beneath the South China Sea. The presence of a thermochemical layer (possible the D″ layer) in the model helps stabilizing the plume root. Our modelling is the first implementation of multi-scale mesh in the regional model. It has been proved to be an effective way of modelling regional dynamics within a global plate motion and mantle dynamics background. [ABSTRACT FROM AUTHOR]

Across the entire mantle we interpret 94 positive seismic wave-speed anomalies as subducted lithosphere and associate these slabs with their geological record. We document this as the Atlas of the Underworld , also accessible online at www.atlas-of-the-underworld.org , a compilation comprising subduction systems active in the past ~ 300 Myr. Deeper slabs are correlated to older geological records, assuming no relative horizontal motions between adjacent slabs following break-off, using knowledge of global plate circuits, but without assuming a mantle reference frame. The longest actively subducting slabs identified reach the depth of ~ 2500 km and some slabs have impinged on Large Low Shear Velocity Provinces in the deepest mantle. Anomously fast sinking of some slabs occurs in regions affected by long-term plume rising. We conclude that slab remnants eventually sink from the upper mantle to the core-mantle boundary. The range in subduction-age versus – depth in the lower mantle is largely inherited from the upper mantle history of subduction. We find a significant depth variation in average sinking speed of slabs. At the top of the lower mantle average slab sinking speeds are between 10 and 40 mm/yr, followed by a deceleration to 10–15 mm/yr down to depths around 1600–1700 km. In this interval, in situ time-stationary sinking rates suggest deceleration from 20 to 30 mm/yr to 4–8 mm/yr, increasing to 12–15 mm/yr below 2000 km. This corroborates the existence of a slab deceleration zone but we do not observe long-term (> 60 My) slab stagnation, excluding long-term stagnation due to compositional effects. Conversion of slab sinking profiles to viscosity profiles shows the general trend that mantle viscosity increases in the slab deceleration zone below which viscosity slowly decreases in the deep mantle. This is at variance with most published viscosity profiles that are derived from different observations, but agrees qualitatively with recent viscosity profiles suggested from material experiments. [ABSTRACT FROM AUTHOR]

A study of teleseismic P-wave tomography was carried out along the Central Orogenic Belt of China (COBC) consisting of the Qinling, Tongbai, Hong'an, Dabie and Sulu orogens that were created by northward subduction of the South China Block beneath the North China Block to subarc depths in the Paleozoic to Mesozoic. The results show the presence of high velocity anomaly underneath the COBC and its adjacent cratonic regions but the absence of high velocity anomaly in the lower lithosphere beneath the COBC. The high velocity anomaly is deep underneath the COBC, possibly representing the relic of the subducted Paleo-Tethyan oceanic slab predating the continental deep subduction of the South China Block. The high velocity anomaly is also deep underneath the southeastern part of the North China Block, possibly being a relic of the subducted Paleo-Pacific slab. However, no relic of the deeply subducted continental slab is seismically detected in the lower lithosphere beneath the Dabie and Sulu orogens, though the occurrence of ultrahigh-pressure metamorphic rocks in the two orogens has clearly demonstrated continental deep subduction to subarc depths in the Triassic. Instead there is only the relatively flat and uniform Moho boundary beneath the COBC region, indicating that the subduction-thickened continental lithosphere along the COBC has lost its root. It is inferred that the orogenic root beneath the COBC region has been foundered into the asthenospheric mantle at the postcollisional stage. The foundering would have taken place in the Early Cretaceous as indicated by the extensive occurrence of magmatic and migmatitic rocks along the COBC. As a consequence, the seismic tomography cannot detect the deeply subducted continental slab remnant from the Triassic period. It is possible that the tomographic visibility of continental subduction in the lower lithosphere is limited to the past 100 Myr. [ABSTRACT FROM AUTHOR]

The Pacific and Okhotsk plates are converging at a rate of about 9 cm/a, thereby forming the Japan Trench subduction zone. Two large earthquakes occurred in the offshore region of Fukushima on 13 February 2021 (M w 7.1) and 16 March 2022 (M w 7.3). The occurrences of the two earthquakes are close in space and time, thus constituting the Fukushima-Oki earthquake doublet. The joint inversions of the onshore and offshore strong-motion data and the teleseismic P waves reveal that both earthquakes belong to intraslab events, and the unilateral ruptures of the 2021 and 2022 events propagate southwestward and northeastward, respectively. Both events are dominated by thrust motions, and major fault slips are mainly localized away from the rupture initiating area. The geometries and locations of the two seismogenic faults suggest that the Fukushima-Oki doublet arises from the reactivations of two preexisting bending-related faults inside the subducting Pacific plate, which is driven by the downdip compressional stress regime caused by plate unbending. Furthermore, the regional tectonic settings indicate a complicated seismogenic environment for the Fukushima-Oki doublet, in which the dehydration embrittlement and seamount subduction are likely to play significant roles. Additionally, the distinctive rupture patterns of both events are also associated with the variation of fluid content and the small-scale structure of a slab slice enclosed by the two seismogenic faults within the subducting plate, which may control the heterogenous distributions of fault slips and aftershocks. The analysis of Coulomb failure stress changes suggests that the 2021 event and the 2022 foreshock collectively increase the stress loading, which may facilitate the occurrence of the 2022 mainshock, and the seismicity of the seismic belt consisting of the three intraslab sequences following the great 2011 Tohoku-Oki earthquake is also likely to remain active in the future. • Source rupture processes of two Fukushima-Oki earthquakes in northeastern Japan. • Joint inversions of onshore and offshore strong-motion and teleseismic waveforms. • Two unilateral ruptures separately propagating southwestward and northeastward. • Both intraslab events caused by reactivations of preexisting outer-trench faults. • Seismogenesis attributed to heterogenous fluid content and regional tectonics. [ABSTRACT FROM AUTHOR]

An unprecedented number of broadband seismic data from 165 stations were utilized to constrain the topographic variations in seismic discontinuities at the top and bottom of the mantle transition zone (MTZ) in the Java subduction zone. After moveout corrections under a nonplane wavefront assumption and time-to-depth conversions based on the IASP91 Earth model, the depth undulations of two discontinuities that border the MTZ (d410 and d660) were obtained by stacking P receiver functions that were grouped into circular bins with a radius of 1°. The apparent depths exhibit systematically spatial variations that range from 385 to 427 km for the d410 and 608 to 700 km for the d660, which result in a MTZ thickness of 217–309 km. The mean values are 406 ± 12, 665 ± 19, and 258 ± 18 km, suggesting thicker than normal MTZ by an average of 8 km. The major factor that accounts for the systematic depth variations across the study area originates from the subducted slab that perturbs the entirety of the MTZ, which induces a negative thermal anomaly, high velocity zone, and abnormal water presence concentrated in the lower MTZ of western Java. In Lombok and Sumbawa Islands, after the thermal and velocity heterogeneities from the subducted slab were considered, the remnant d660 uplift is found due to a significant thermal anomaly with a magnitude of up to +200 K near the d660. In eastern Java and adjacent regions, advective thermal upwelling associated with toroidal mantle flow that traversed a slab window in central Java induces a low velocity anomaly with a magnitude that is greater than the high velocity anomaly caused by the subducted slab, which apparently depressed the d410 and d660 by the same amount. • MTZ discontinuities beneath Java and adjacent areas are imaged using receiver functions with an unprecedented resolution. • The abnormally thin MTZ underneath Lombok and Sumbawa Islandssuggests the presence of significant positive thermal anomaly. • The discontinuity depth undulations in eastern Java are associated with toroidal mantle flow traversing a slab window. [ABSTRACT FROM AUTHOR]

We report a first estimate of the extension rate of the onshore Kamchatka island arc, it central wider part. This average rate is 17 ± 3 mm/yr over mid-late Quaternary time. The extension is absorbed by slip on major normal active faults of Central Kamchatka, and graben-producing faulting in its volcanic belt. Probable extension of the underwater portion of the arc, its rate remaining unknown, may add up to the total value. The onshore extension rate, established by remote fault scarp measurements on DEMs resembles the numerical modelling estimate of Schellart et al. (2007), suggesting that the primary driving force responsible for the extension at Kamchatka is slab and trench retreat. [ABSTRACT FROM AUTHOR]

A large volume of continental lithosphere should have been emplaced beneath the Tibetan Plateau during the convergence between the Indian and Asian plates. However, geophysical studies have imaged an obviously thickened crust and a normal to slightly thickened mantle lithosphere beneath the plateau. Most of the additional mantle lithosphere is proposed to have detached and descended into the deep mantle to the mantle transition zone (MTZ). Recent studies suggest that the Indian lithospheric slab has torn into sections with different subduction angles. However, the locations of these slab sections, especially the high-angle one, require further structural constraints. Here, we used newly collected data from the broadband seismic network SANDWICH in central Tibet to study the thickness of the MTZ using the receiver function method. Our results show that there are two MTZ regions that are approximately 15 km thicker than the average one. One region is beneath the Lhasa terrane centered at 90°E, 31°N, and the other one is beneath the Qiangtang terrane centered at 88°E, 33°N. Because the upper and lower boundaries of the MTZ are both rock phase transition discontinuities that are sensitive to temperature. We interpret the thickened MTZ beneath the plateau as evidence of the presence of cold Indian slab and mantle lithosphere. The high-angle Indian lithospheric slab between the Yadong-Gulu Rift and the Pumqu-Xianza Rift is subducting deep into the MTZ, producing a decrease in temperature and thickening the MTZ beneath Lhasa terrane. In contrast, the Qiangtang anomaly corresponds to a piece of delaminated thickened Tibetan lithosphere. Based on the extensive distributions of Eocene to Oligocene potassic rocks and Mid-Miocene to Quaternary potassic rocks, we suggest that the thickened mantle lithosphere beneath the northern Qiangtang terrane and Songpan-Ganzi terrane detached in a piecewise manner, resulting in subsequent plateau uplift and recent potassic volcanism. [ABSTRACT FROM AUTHOR]

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21 ± 0.12 s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34 ± 0.47 s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy — trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates. [ABSTRACT FROM AUTHOR]

Seismograms of about 12,000 aftershocks of the M w 6.7 Coquimbo-La Serena earthquake of 20 January 2019 that were recorded during six months by an array of 33 short-period seismic stations are used to generate wavespeed images of the subduction wedge beneath the Chilean forearc between 28.0°-30.6°S. This part of the margin is near the northern terminus of the 2015 M w 8.4 Illapel earthquake and the southern terminus of the 1922 M w 8.5 Copiapo earthquake. The distribution of hypocenters and wavespeeds at the slab interface and within the subduction wedge, are like those determined by Comte et al. (2019) in the Illapel aftershock region, and we infer that the same processes of subduction erosion and re-accretion inferred by them are active here. Moreover, anomalous wavespeeds are found in the immediate vicinity of a dense cluster of aftershocks located in a well-defined double seismic zone, within the subducting Nazca plate. Vp/Vs ratios in the upper seismic zone are uniformly high (∼1.9), those in most of the lower seismic zone are average to low, and those in the intraslab region of the aftershock cluster are exceptionally low (∼1.6). These values suggest dehydration of the slab interior and hydration of the mantle near the slab interface. While the origin of this feature is enigmatic, its limited spatial extent suggests that it most likely is due to an inherited structure, such a seamount or petit-spot volcano, in the Nazca plate, rather than a feature that developed in situ. The high rate of activity coupled with the low wavespeeds suggests a region of local weakness that could eventually evolve into a tear or hole in the slab, and its location at the bounds of two megathrust earthquakes suggests that it plays a significant role in delimiting the rupture of such events. • Most Coquimbo-La Serena earthquake aftershocks are confined to a dense cluster. • Wavespeeds show dehydration of the slab interior and hydration of the overlying mantle. • Remnants of a seamount or petit-spot volcano are found in the subducted Nazca plate. • The aftershock region may arrest the rupture of Andean margin megathrusts. • The region is subject to the same subduction erosion processes as Illapel to the south. [ABSTRACT FROM AUTHOR]

Forward and inverse P- wave velocity models along an E -W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent. • Thinned continental crust (TCC) west of the Manila transcurrent fault and Manila slab. • Two adjacent stripes separated by an intra-continental velocity difference boundary composed the TTC. • The left-lateral motion guides the driving force controlling the collision of Luzon arc and Eurasian TCC. [ABSTRACT FROM AUTHOR]

In the middle of the Mediterranean, the partly still active Apennines subduction system has been usually defined using tomographic images and available shear wave splitting measurements. In this paper we describe the new seismic anisotropy dataset for Central Italy, the region where the transition between Northern and Southern Apennines occurs. The new measurements show NW-SE fast polarisation directions beneath the belt, due to the retreat of the slab, NN E -SSW orientations from proper Adriatic mantle sources, and E-W directed anisotropy, attributed to mantle convection flow at the Thyrranian side. Additionally, the new data suggest the presence of a toroidal mantle flow through a tear in the Apenninic slab, from the Adria to the Tyrrhenian side. However, mantle circulation and flows, identified by the pattern of shear wave splitting results, seem different from what was proposed in previous geodynamic models. Indeed, our results support the presence of a vertical slab tear with limited dimension. In the geodynamic model we propose, the tear acts to accommodate a differential slab retreat. The slab partitioning results in a different pattern and strength of seismic anisotropy traced from the Central Apennines with respect to the adjacent Northern and Southern Apennines. • New SKS data consistent with previously defined mantle deformation patterns • Geodynamic model for Central Mediterranean from new seismic anisotropy data • NW-SE Apenninic mantle anisotropy interrupted by a narrow well developed mantle flow • Seismic anisotropy consistent with shallow horizontal slab tear beneath central Italy [ABSTRACT FROM AUTHOR]

Late Mesozoic magmatism in eastern China was extensive and closely related to the subduction of the Paleo-Pacific plate, but genetic relationship between magmatism and subduction remains unclear. Cretaceous magmatic rocks distributed along the eastern coast of China can provide an opportunity to evaluate the influence of subduction on the evolution of the magmatism. Here, a combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages was carried out for the Cretaceous granitoids in Liyang volcanic basin, eastern China. Zircon U-Pb dating results indicate that the Liyang granitoids were emplaced in the Early Cretaceous (ca. 130–125 Ma). These samples (SiO 2 = 70.8–75.3 wt%) are mainly composed of high-K calc-alkaline granite and granite porphyry. Their geochemical characteristics are similar to I-type granites, with enrichment in LREEs, Rb, Th and U, and negative Eu (δEu = 0.04–0.51), Ba, Sr, P and Ti anomalies. All samples show negative whole-rock ε Nd (t) (−9.8 to −5.7) and zircon ε Hf (t) (−15.7 to −3.60) values (corresponding crustal Hf model ages of 1.40–2.17 Ga), and high radiogenic Pb isotopic compositions. The integrated interpretation of geochemical data reveal that rollback of the Paleo-Pacific subducted slab resulted in lithospheric extension and thinning, and the upwelling of hot asthenosphere in eastern China, which induced more intensive underplating of the mantle-derived mafic magma. The ascent of heat and mafic magma, intruded in Meso-Neoproterozoic crust, and triggered partial melting of the Meso-Neoproterozoic crust. The Liyang I-type granitoids were the result of the Meso-Neoproterozoic crustal components mixing with a small number of mantle-derived magmas in the extensional tectonic setting, which was response to slab rollback of the Paleo-Pacific plate. • The Liyang I-type granitoids consisting of granite porphyry and granite were formed in Early Cretaceous; • The Liyang granitoids were derived from Meso-Neoproterozoic crustal components mixing with mantle-derived magmas; • The Early Cretaceous (130–125 Ma) magmatism might be response to the Paleo-Pacific slab rollback. [ABSTRACT FROM AUTHOR]

We investigate the 3-D P and S wave velocity structure of the East Japan forearc using a large number of high-quality arrival-time data of local earthquakes recorded by a permanent seafloor seismic network (S-net) and the land-based Kiban seismic network. Geometries of the subducting slab boundary and the Conrad and Moho discontinuities under the forearc are refined by referring to results of previous studies including active-source seismic soundings and seismicity located by the S-net. The obtained P and S wave velocity images are quite similar to each other, suggesting that they reflect robust features of the 3-D seismic structure. The subducting Pacific slab exhibits high velocities, whereas significant low-velocity (low-V) anomalies are revealed in the overlying Okhotsk plate beneath the forearc, which may reflect accretionary materials containing abundant fluids. The subducting oceanic crust atop the Pacific slab is revealed clearly as a thin low-V layer extending from the trench axis down to ~100 km depth under the volcanic front. Our results of earthquake relocation using the 3-D velocity model indicate that the double seismic zone in the subducting Pacific slab occurs from the trench axis down to ~180 km depth. The occurrence and focal mechanisms of the double seismic zone are caused by slab deformation during its subduction, such as slab bending or unbending, as well as other processes including hydration, dehydration, compositional variations, and phase changes in the subducting slab. [Display omitted] • Seafloor S-net and Kiban network data are used to study the 3-D forearc structure. • The Conrad, Moho and Pacific slab geometries in the East Japan forearc are refined. • P and S wave velocity images are quite similar, reflecting robust structural features. • The subducting oceanic crust is revealed down to ~100 km depth under the volcanic front. • The intraslab double seismic zone occurs from the trench axis down to ~180 km depth. [ABSTRACT FROM AUTHOR]

The Andean orogenic belt, located in the Central Andes “Pampean flat-slab” segment in northern Chile (27–29°S), is composed of two major tectonic regions: the Coastal Cordillera and the Frontal Cordillera. To understand their internal tectonic styles, history of growth and the shortening absorbed by the upper crustal structure of this segment, we combined regional geological mapping data, new ages obtained from radiometric U–Pb dating, and a semibalanced and restored cross-section 225.18 km in length. The results as shown in the previous Mesozoic extensional fault systems, established in northern Chile by the Gondwana breakup, have played a fundamental role in the orogenic buildup. The central structure is characterized by an asymmetric basin (Upper Cretaceous–Paleocene) confined by a doubly vergent fault system composed of inverted faults related to the edges of the Mesozoic Chañarcillo and Lautaro Basins. The U–Pb geochronological data obtained from synorogenic volcano-sedimentary deposits and the angular unconformities recorded between the Cenozoic geological units have revealed that the compressive deformation in this segment started at around ~ 80 Ma by tectonic inversion in the eastern Coastal Cordillera and western Frontal Cordillera, however, the presence of Paleocene and Miocene synorogenic successions at the footwall of the basement reverse faults of the Frontal Cordillera suggests a migration of Andean deformation from the west to the east during the Paleocene–Miocene by propagation of ramps involving inherited basement highs. The pre-compression restoration makes it possible to estimate 40.94 km of minimum shortening, concentrated by inversion anticlines and fault-controlled basement highs across the Frontal Cordillera. [ABSTRACT FROM AUTHOR]

The Laramide orogeny (~ 80–50 Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000 km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (> 100 Myr) and denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. With a continental velocity of 4 cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15 Ma after plateau subduction. The flat slab underthrusts the continent at ~ 200 km depth, eventually extending > 1500 km inboard of the trench. [ABSTRACT FROM AUTHOR]

Zones of fluid-bearing anomalies occur in volcanic regions, on a plate interface, or in the mantle wedge of a subduction zone. The various causes include slab dehydration, serpentinization or metamorphic reactions in different thermal regimes. To reveal the impact factors that might have contributed to slab melting and arc magmatism behind the Japan Trench, we determined high-resolution three-dimensional (3-D) structures of seismic velocities (Vp, Vs), Vp/Vs ratio and temperatures. The temperatures (T) were calculated from the inverted seismic structures, and two petrological models of the upper mantle: a peridotite assemblage and a pyrolite assemblage, which differ from the results of the thermal simulation. The multi-parameter structures obtained for Vp, Vs, Vp/Vs and T show good consistency in the upper mantle behind the Japan Trench. The temperatures derived from the peridotite assemblage were found to match the mean geotherm and simulated temperature of the upper mantle more closely than those from the pyrolite model. A layer approximately 10 km thick with low-Vp and Vs, high-Vp/Vs, and slightly high-T anomalies was observed on the upper boundary of the subducting Pacific slab. This distinctive anomalous layer is interpreted as partial melting of the oceanic crust due to deep-seated metamorphic reactions whose characteristics are dependent on the source of fluids, mineral composition, and the thermal regime. Such a process enriches the peridotite content of the basalt underneath the island arc in the mantle wedge. Localized zones of significantly low-Vp and low-Vs, high-Vp/Vs and high-T perturbations were imaged in the mantle wedge under the active volcanoes, suggesting partial melting of peridotite-rich mantle material to produce tholeiitic magma. The present study demonstrates that fluids released from slab dehydration, mineral composition and the thermal regime play crucial roles in both arc magmatism, and slab melting in the subduction zone. [Display omitted] • High resolution seismic and thermal structures are imaged in the NE Japan subduction zone. • Slab melting is controlled by the source of fluids and thermal regime behind the Japan Trench. • Fluids and mineral composition play a key role in slab melting and arc magmatism. [ABSTRACT FROM AUTHOR]

We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low-velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk. • Aseismic portions of the subducted slabs beneath SE Asia are revealed. • Hot upwellings in the big mantle wedge caused intraplate volcanism in Borneo. • The Hainan plume has a complex geometry due to mantle wind. [ABSTRACT FROM AUTHOR]

The geodynamic evolution of the Western Mediterranean related to the closure of the Ligurian-Tethys ocean is not yet fully resolved. We present a new 3D numerical model of double subduction with opposite polarities fostered by the inherited segmentation of the Ligurian-Tethys margins and rifting system between Iberia and NW Africa. The model is constrained by plate kinematic reconstructions and assumes that both Alboran-Tethys and Algerian-Tethys plate segments are separated by a NW-SE transform zone enabling that subduction polarity changes from SE-dipping in the Alboran-Tethys segment to NW-dipping in the Algerian-Tethys segment. The model starts about late Eocene times at 36.5 Ma and the temporal evolution of the simulation is tied to the geological evolution by comparing the rates of convergence and trench retreat, and the onset and end of opening in the Alboran Basin. Curvature of the Alboran-Tethys slab is imposed by the pinning of its western edge when reaching the end of the transform zone in the adjacent west-Africa continental block. The progressive curvature of the trench explains the observed regional stress reorientation changing from N-S to NW-SE and to E -W in the central and western regions of the Alboran Basin. The increase of the retreat rates from the Alboran-Tethys to the Algerian-Tethys slabs is compatible with the west-to-east transition from continental-to-magmatic-to-oceanic crustal nature and with the massive and partially synchronous calc-alkaline and alkaline magmatism. • We propose a 3D numerical model of opposing subduction simulating the tectonic setting of the Western Mediterranean. • The retreat rates of the slabs are consistent with the duration of opening of the Alboran and Algerian basins. • The progressive curvature of the Alboran-Tethys slab is consistent with the regional stress evolution of the Alboran Basin. [ABSTRACT FROM AUTHOR]

The motivation for this work was to model the crustal thickness, and the geometry of the Mohorovičić (Moho) discontinuity, the crust-upper mantle boundary around the Japanese archipelago. The western Pacific Ocean is one of the most eminent subduction zones in the world. The subduction zone complexity is due to alteration within the geometry of the stagnant slab. We compiled a three-dimensional density distribution model for subduction plates around the Japan Islands, and estimated the depth to Moho using the constrained non-linear inversion of gravity data derived from the latest Gravity field and Ocean Circulation Explorer, or GOCE, model known as GOCO06s. The study had three steps, starting with the data corrections followed by the inversion, and ending with the modeling and comparing of the estimated results with previous models and studies. In this study, we used available receiver function results, seismic tomography models, global crustal thickness, and geological and tectonic information as constraints. The model estimated the crustal thickness, surface geometry of the Moho, and geometry of the stagnant slab in the Kuril Trench, Japan Trench, Nankai Trough, and Izu-Bonin Trough. In gravity inversion, we used 34 km and 600 kg m−3 as the Moho depth reference and density contrast, respectively. Results showed that the crustal thickness ranged from 14 to 43 km with a − 0.78 mean misfit for Moho depths derived from seismic P-wave analysis. The calculated gravity data had a 0.16 mGal misfit with the observed data. The normalized analytical signal helped to sharply delineate the different plates and their edges. Three cross-sections were conducted to model the stagnant slab along the subduction zone of the Eastern Eurasian plate, western Pacific plate, Okhotsk plate, and Philippine Sea plate. The eastern area of the entire region (Pacific plate) had a low crust thickness and high density compared to the western area (Eurasian plate), which had a high crust thickness and low density. • 3D gravity inversion of the Northwestern Pacific Plate at the Japan Islands. • Estimating the Moho depths around the Japan Islands. • 3D density modeling of the Stagnant Slabs and Subduction plates. [ABSTRACT FROM AUTHOR]

How deep the two opposite dipping subduction zones in the vicinity of Taiwan, northwestward-dipping Philippine Sea Plate (PSP) and eastward-dipping South China Sea Slab (SCS), penetrated down into mantle, are hotly on debate and insufficiently imaged by seismic tomographic inversions, either due to inadequate quantities of seismic data or limited vertical resolution of the techniques used by previous investigations. Here we employ an unprecedented number of 55,847 high-quality P-to-S radial receiver functions recorded by 304 broadband seismic stations located in SE China and adjacent areas to image the topography of the mantle transition zone (MTZ) discontinuities (d410 and d660) to delineate the configurations of the subducted slab segments and analyze the impacts of thermal and water content anomalies on the MTZ. The close-to-normal wavespeed-corrected d410 and a ~ 10 km depression of the corrected d660 to the east of northern Taiwan are revealed, indicating that the PSP breaks off above the d410 and the delaminated PSP drops into the lower MTZ along with slab dehydration. To the east of southern Taiwan, where the SCS begins to subduct eastward, the uplift of the apparent d410 is nearly twice as much as that of the d660. The positive wavespeed anomaly above the d410 and low temperature in the vicinity of the d410 are enough to contribute to the uplift of MTZ discontinuities, suggesting that the SCS just reaches the d410, rather than penetrating the entire MTZ. • The MTZ discontinuities in Taiwan and its adjacent regions are imaged using receiver functions of unprecedented resolution • The geometry of subducted slab segments in the vicinity of the MTZ is constrained • The break-off Philippine Sea Plate drops into the lower mantle transition zone along with dehydration • The eastward-dipping South China Sea Slab penetrates the d410 [ABSTRACT FROM AUTHOR]

Convergent plate dehydration is of significance in determining the coupling strength and seismicity activity at variable subduction contacts. Characterized by decadal M > 7.5 devastating earthquakes offshore and a lack of a magmatic arc in north-central Peru, the Peruvian megathrust remains enigmatic in terms of the along-strike heterogeneity of the thermal regime, slab dehydration, and seismotectonics. Hydrothermally controlled petrological metamorphism of multiple facies within the subducted oceanic lithosphere and subsequent fluid channeling and migration lead to fault instability and seismic and aseismic slips on the megathrusts. Based on 3D thermal modeling, we estimate the features of the Peruvian subduction hydrothermal regime. Our results show that (1) the shallow seismogenic zone (M > 7.5 earthquakes) along coastal Peru exhibits high slab dehydration rates and gradients and is associated with fault segmentation and the updip migration of fluids controlling the occurrence of great megathrust earthquakes; (2) the low slab dehydration rate (<0.01 wt%/km) in north Peru contributes to the absent volcanism in north Peru, while the high slab dehydration rate (>0.02 wt%/km) beneath south Peru facilitate the recurrence of magmatic volcanism; (3) the subduction-related slip behaviors at a certain depth resulting from heterogeneous variations in the coupled plate interface are determined by slab dehydration and petrological metamorphism in the subducted Nazca plate. The variation in slab dehydration contributes to the seismotectonic segmentation and slip variation associated with changes in the pore fluid pressure on the subducted megathrust. • Dehydration embrittlement resulting from the eclogitization of the MORB layer and harzburgitization of the ultramafic layer influence the occurrence of subduction earthquakes at various depths beneath Peru. • The along-strike fluid content variation contributes to the forearc slab segmentation and seismotectonics which are influenced by the plate topographic features entrained into subduction channels. • Migration of subducted oceanic ridge with a thick oceanic crust leads to the MORB-dominant flattened slab and multi-layered complicated hydrothermal regimes. [ABSTRACT FROM AUTHOR]

Geological observations have revealed a rapid evolution and obvious east–west differential evolution in the North Sulawesi subduction zone. The Celebes Sea plate has inserted itself under the north arm of Sulawesi Islands, which has simultaneously rotated clockwise. The rotation of the north arm of the Sulawesi Islands might be critical in facilitating tectonic processes, such as the slab subduction and rollback of the Celebes Sea plate. For the east–west differences along the North Sulawesi subduction zone, a numerical model with the convergence rate of the plates as the basic variable is established to quantitatively describe the evolution process of the North Sulawesi subduction zone. Our results reproduced the east–west differences of the subducting Celebes Sea plate, showing a shallow–deep–shallow subduction style. We propose that the variable velocity ratio of the overriding plate to the subducting slab could be the principal reason for the differential subduction along the strike of the North Sulawesi subduction zone. We conclude that the residual slab and the rate of the eastern continental plate limit the downward movement of the subducted slabs of the eastern Sulawesi. Furthermore, the reason for the shorter subducted slab at the extreme western Sulawesi is the fact that subduction occurs outside the rotation radius. Moreover, the widespread extension at the western Sulawesi has a limited correlation with the clockwise rotation. • We combined 2D numerical simulation with seismicity to obtain an evolution model of the North Sulawesi subduction zone. • Variable velocity ratio of the overriding plate to the subducting slab induced the East-West subduction difference. • Connection between the subduction of Celebes Sea and the rotation of the north arm of the Sulawesi Islands is discussed. [ABSTRACT FROM AUTHOR]

The Indian Ocean geoid low (IOGL) is one of the lowest geoid anomalies on Earth. Several theories have been proposed to explain this geoid low, including past subduction (Nerlich et al., 2016; Rao and Kumar, 2014), subduction coupled with low velocity anomalies in the upper mantle (Spasojevic et al., 2010) and hot, low density anomaly in the upper to mid mantle depths (Reiss et al., 2017; Ghosh et al., 2017). It was also argued by Ghosh et al. (2017) that subducted slabs in the lower mantle have minimal role to play in contributing to this geoid signal. We further investigate that claim here by looking at the geoid contributed from various processes (density vs dynamic topography) as well as by inspecting the contribution from different spherical harmonic degrees. Our current findings substantiate the earlier claim that lower mantle slabs indeed play a minimal role in creating this anomalous geoid low. • Individual contribution from density anomaly and surface dynamic topography show that low density anomaly causes the Indian Oceean geoid low. • Lower mantle slabs have negligible effect in creating this anomaly. • Amongst the recent tomography models, GyPSuM-P and S yield the highest fit to the IOGL. [ABSTRACT FROM AUTHOR]

• Effects of anisotropic thermal conductivity on the thermal structure are investigated. • Two regions with a high degree of anisotropy are observed in the mantle wedge. • Slab temperature changes only slightly due to the anisotropic thermal conductivity. The effects of anisotropic thermal conductivity due to the crystallographic-preferred orientation (CPO) of olivine in the mantle wedge on the steady-state thermal structure of the Tohoku subduction zone, Northeast Japan, are investigated using geodynamic modeling. Four different olivine crystal fabrics are modeled based on theoretical CPO development in the mantle wedge: random (i.e., isotropic thermal conductivity), A-type, C-type, and E-type fabrics. The thermal conductivity in the mantle wedge is highly anisotropic in the regions just above the descending slab and near the base of the overriding plate. The obtained temperature differences between the anisotropic and isotropic cases are only up to 30 °C within the slab. Therefore, the impact of anisotropic thermal conductivity on the slab temperature may be limited when anisotropy is only considered in the mantle wedge. [ABSTRACT FROM AUTHOR]

Although it is well understood that mantle flow in a subduction setting can develop seismic anisotropy, many tomographic studies still assume an isotropic mantle. In this study, we explore how unaccounted-for seismic anisotropy contributes to P-wave anomalies, focusing on apparent sub-slab low velocities. We use data from the westernmost Mediterranean, where the Alboran slab has subducted beneath the Gibraltar Arc. Our isotropic P-wave velocity model shows a low-velocity anomaly at depths of 75–225 km to the west of the high-velocity-anomaly that represents the Alboran slab. We explore whether this low-velocity anomaly could be an anisotropy-induced artifact as a significant shear-wave-splitting (SWS) along the arc is observed. We consider five different hypothetical anisotropy models representing different mantle configurations. The anisotropy models are based on 1) the SWS observations, 2) the depth distribution of the low-velocity anomaly and 3) geodynamic modeling of the development of entrained and toroidal mantle flow in generic subduction zones. Given the underdetermined nature of an anisotropic inversion, we include the anisotropy models as a priori constraints on the tomography. The tomographic results including anisotropy models have smaller travel time residuals and isotropic anomalies in the region of interest while fitting the SWS observation better than the isotropic model. We propose that unaccounted-for seismic anisotropy can produce artifacts in isotropic tomography of real datasets, and this is at least partially the case for the low-velocity anomaly below the Alboran slab. Further, we show that not taking this into account can lead to erroneous conclusions about mantle temperature and the presence of melt. From the insights that our anisotropy models provide, we also suggest that toroidal mantle flow is currently dominant below and around the Alboran slab. • Seismic tomography considering seismic anisotropy in the westernmost Mediterranean. • Anisotropic models improve fit to travel time and SWS observations. • Best-fit model includes only anisotropy from toroidal flow. • Results show isotropic models overestimate the magnitude of sub-slab low-velocities. • Inclusion of anisotropy affects interpretations of temperature and melt content. [ABSTRACT FROM AUTHOR]

Abstract: Global compilations have revealed a wide range of trench margin motions, from advancing to retreating, a complexity for which there is as yet no single dynamic explanation. While the buoyancy force of the sinking slabs in the mantle can solely account for retreating trench motions, additional forces are required to explain trench advance. Here, the role of gravitational sliding of oceanic lithosphere from the ridge rise to the trench, which is in opposition to the hinge retreat, is considered. Because lithospheric ageing and thickening have a first-order control on the driving forces of slab buoyancy and ridge push, controls on trench migrations should be age-dependent. A force balance model is used to show that gravitational sliding remains smaller than hinge retreat for a young (<30–40M.years) trench; with predicted trench motions in slow retreat due to the low buoyancy of young lithosphere. Only when older lithosphere is subducted does gravitational sliding become larger than hinge retreat, opposing the natural tendency of the hinge to rollback, and trench advance occurs. This model can explain the range of observed trench motions, from advance to retreat, and their anti-correlation with trench age along most of the convergent margins. [Copyright &y& Elsevier]

Abstract: Transformation kinetics associated with the presence of a metastable olivine wedge in old and fast subducting slabs has been the subject of many studies in the last years. Even with improvements in kinetics models, many of the parameters are still not well constrained. In particular, there is no consensus on the blocking temperature that could inhibit the transformation from olivine to spinel. Recently, based on anomalous later phases in the P wave coda and differential P wave slowness, a wedge of metastable olivine was detected in the Marianas subduction zone, that is approximately 25km wide, at a depth of 590km and is truncated at 630km. In this work, a thermomechanical model was used to mimic the subduction in the Marianas and try different blocking temperatures for the olivine/spinel transformation. The model includes, among other features, non-linear elasto-visco-plastic rheology based on laboratory data, phase transformations, latent heat, proper coupling between stress and thermal state of the slab and force balance of the system. The results show a positive correlation between the blocking temperature, depth of the wedge and its distance from the trench (or subduction angle). We compare these results to the situation in the Marianas and suggest that a blocking temperature for the olivine/spinel transformation of approximately 725°C would be the most likely. The volume of the wedge presents some oscillations that we relate to a runaway effect of the transformation kinetics in the mantle transition zone. Namely, the interaction between latent heat release and the advection of the isotherms due to the subduction velocity. The inclusion of shear heating in the model was fundamental to modeling such a subduction zone. Without shear heating, the slab shows a higher level of internal stress and the necessary bending to mimic the Marianas subduction zone cannot be reached. [Copyright &y& Elsevier]

Abstract: We determine a 3-D P-wave velocity structure of the mantle down to 700 km depth under the Kamchatka peninsula using 678 P-wave arrival times collected from digital seismograms of 75 teleseismic events recorded by 15 portable seismic stations and 1 permanent station in Kamchatka. The subducting Pacific slab is imaged clearly that is visible in the upper mantle and extends below the 660-km discontinuity under southern Kamchatka, while it shortens toward the north and terminates near the Aleutian–Kamchatka junction. Low-velocity anomalies are visible beneath northern Kamchatka and under the junction, which are interpreted as asthenospheric flow. A gap model without remnant slab fragment is proposed to interpret the main feature of high-V anomalies. Combining our tomographic results with other geological and geophysical evidences, we consider that the slab loss may be induced by the friction with surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then it was enlarged by the slab-edge pinch-off by the asthenospheric flow and the presence of Meiji seamounts. As a result, the slab loss and the subducted Meiji seamounts have jointly caused the Pacific plate to subduct under Kamchatka with a lower dip angle near the junction, which made the Sheveluch and Klyuchevskoy volcanoes shift westward. [Copyright &y& Elsevier]

Abstract: The Princeton Group is an assemblage of terrestrial volcanic and clastic sedimentary rocks in south-central British Columbia, and is part of the Challis–Kamloops belt that stretches from central British Columbia to the northwestern United States. The volcanic rocks were largely deposited as cinder cones and composite volcanoes, and are composed of basaltic andesite (olivine+clinopyroxene), andesite and dacite (hornblende+plagioclase+clinopyroxene), and rhyolite (biotite+quartz+K-feldspar), with calc–alkaline affinity. New 40Ar/39Ar dates on hornblende and groundmass separates, and whole rock indicate that magmatism took place during the Early to Middle Eocene, from 53–47 Ma. New neodymium isotopic measurements, in conjunction with previously published results, indicate that the Princeton Group has an ɛNd50 =1.2–6.4 and therefore represents primarily juvenile additions to the continental crust. The major and trace element abundances of Princeton Group rocks resemble those of many modern continental arcs. The compositions are notable, however, because they have an “adakitic” signature that extends throughout their entire compositional range, including high-Mg# basaltic andesite. Trace element modelling indicates that this signature was not derived from anatexis of normal oceanic crust, but from an “arc-like” source enriched in large-ion lithophile elements. This source may have been basaltic dykes that were emplaced into the lithospheric mantle during Mesozoic arc magmatism and subsequently partially melted during an event of lithospheric heating in the Eocene. The heating may have been caused by upwelling asthenosphere related to a slab window or slab tear. [Copyright &y& Elsevier]

Abstract: Carbonatites and kimberlites (~48 to ≤46 Ma) occur along a 700-km-long, N40°W linear trend of alkalic magmatism (~55 to ≤46 Ma) that extends from the Black Hills of South Dakota and Wyoming, across eastern Montana to southern Alberta. Isotopic and trace-element concentrations point to a genetic linkage among alkalic igneous rocks, carbonatites, and kimberlites along the trend (group 1), with Sr–Nd isotopic ratios flanking bulk silicate earth. In contrast, alkalic rocks southwest of the N40°W trend (groups 2 and 3) have significantly lower initial epsilon-Nd values and different trace-element contents (e.g. higher Ba, but lower REE, U, and Th). A tectonomagmatic model is proposed here to account for placement of the genetically related group 1 magmas along the N40°W linear zone; the model suggests that the edge of the Kula plate was lodged in the mantle transition zone directly below the N40°W trend by ~50 Ma. Mantle material rising through the Kula–Farallon slab window may have been concentrated at the southwest edge of the Kula slab, focusing melts upward along the trend. Geochemical attributes of group 1 magmas along the trend can be explained by an extremely small partial melt of carbonated peridotitic mantle (0–1%). Initiation of extension at ~53–49 Ma is important because it may have stimulated mantle upwelling and decompressional melting, as well as facilitated intrusion of asthenospheric melts into the Wyoming Archean craton. Although groups 2 and 3 are thought to be largely lithospheric melts, their Sr–Nd isotopic compositions reflect a greater proportion of asthenospheric component after ~50 Ma, following onset of extension in the west. [Copyright &y& Elsevier]

Abstract: The Tonga Wadati–Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60–300 km) and the deep part (400–700 km). To address this problem, we investigated the Wadati–Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722–743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852.]. The stress tensors were computed using the Gephart''s program [Gephart, J.W., Forsyth, D., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305–9320.]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14° and 19°S, the stresses at intermediate depth (60–300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment. [Copyright &y& Elsevier]

Abstract: We present a series of three-dimensional numerical models investigating possible thermal and mechanical effects of ridge subduction and slab window migration. Ridge subduction is a fundamental process of plate tectonics, and the geologic manifestations in the overriding lithosphere have been investigated in many convergent margin settings. Many of the geologic effects of slab window migration (e.g., anomalous high-T metamorphism in the forearc, non-arc-like magmatism in the volcanic arc) are explained primarily by the introduction of a region of hot, upwelling asthenospheric mantle in the subduction zone environment. Using end-member boundary conditions and idealized geometries, our models address the thermal and corresponding mechanical manifestations of slab window migration, and our results provide a general agreement with observations from interpreted regions of slab window migration. Our models show that protracted heating in the forearc region should result in protracted high-temperature metamorphism, associated rheologic weakening and strain partitioning and changes in the topographic uplift pattern in regions affected by slab window migration. Although these models are idealized, they do provide valuable insight into the geodynamics of slab window environments and provide a valuable starting point for further exploration. [Copyright &y& Elsevier]

Abstract: The detachment of subducted lithosphere (slabs) has been proposed in a number of locations as the cause for observed volcanism and regional tectonic evolution. For example, along-strike progression of uplift in the Adriatic Sea and west-to-east progression of volcanism in the Trans-Mexican Volcanic Belt may be caused by propagating detachment of subducted slabs. An understanding of the dynamics of slab detachment is necessary to relate the timing of detachment to surface observations. We demonstrate using two-dimensional (2D) numerical models that the timing, occurrence, and surface effects resulting from the detachment of subducted lithosphere are strongly dependent upon the stiffness (i.e. stress supported viscously) of the lithosphere and rheology of the upper mantle. In particular, in models with a Newtonian upper mantle where plastic yielding is the only weakening process within the slab, detachment does not occur. Models accounting for non-Newtonian upper mantle deformation demonstrate two modes of detachment: (1) slow and deep detachment of strong slabs (500 MPa or greater maximum yield strength) controlled by heating of the slab, and (2) fast and shallow detachment of weaker slabs (300 MPa) controlled by yielding within the slab interior. For slabs with a maximum yield strength of 500 MPa, the time to detachment increases with slab age (for ages ranging from 40 to 120 My), indicating that detachment time is controlled by the integrated stiffness of the slab, which limits the sinking rate. However, the detachment time for weaker slabs (300 MPa) is independent of age because weaker slabs sink rapidly, and the time to detachment is limited by the rate of flow of the surrounding mantle. [Copyright &y& Elsevier]

Abstract: Many subduction margins that rim the Pacific Ocean contain complex records of Cenozoic slab-window volcanism combined with tectonic disruption of the continental margin. The series of slab windows that opened beneath California and Mexico starting about 28.5 Ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime. The timing and areal extent of the resultant slab-window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin. The initial Pioneer slab window thermally weakened the overlying western Transverse Ranges and California Borderlands region starting about 28.5 Ma. A second thermal pulse occurred in the same region starting about 19 Ma during growth of the Monterey slab window. This additional heating, combined with the capture of a partially subducted Monterey plate fragment by the Cocos plate, initiated the pulling apart and rotation of the adjacent continental margin. Similarly, the capture of Guadalupe and Magdalena plate fragments by the Pacific plate and initiation of the Guadalupe–Magdalena slab window about 12.5 Ma are coeval with Baja California pulling away from the Mexico continental margin, with the break along the Comondú arc, in crust already thermally weakened by about 10 My of volcanism. In coastal California, distributed crustal extension and subsidence accompanied the new transform plate boundary, and continued until the slab windows cooled and plate motion coalesced along a through-going system of strike-slip faults. The transform boundary continues to evolve, and forward modeling predicts an instability with the current configuration as a result of convergence between the Sierra Nevada and Peninsular Ranges batholiths, starting about 2 My in the future. The instability may be resolved by a shift in the locus of transform motion from the San Andreas fault to the eastern California shear zone, or by breaking off another fragment of the Mojave or southern Sierra Nevada crustal blocks and translating it northward. [Copyright &y& Elsevier]