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A blanket of sedimentary and regolith material covers approximately three-quarters of the Australian continent. This poses a significant exploration challenge, with future mineral and energy resour...

Earth’s surface topography is a direct physical expression of our planet’s dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a substantial proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth’s deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth’s oceanic residual topography power spectrum. This indicates water-loaded power of 0.5 ± 0.35 km2 and peak amplitudes of up to ~0.8 ± 0.1 km at long wavelengths (~104 km), decreasing by roughly one order of magnitude at shorter wavelengths (~103 km). We show that geodynamical simulations can be reconciled with observational constraints only if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-wavelength) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth’s lithosphere. The structure of the lithosphere and its impact on mantle flow significantly influence the impact of Earth’s interior dynamics on surface topography, suggest statistical analyses of Earth’s topography.

The broadband seismic component of the iMUSH project was supported by National Science Foundation grants EAR‐1144568, EAR‐1144351, EAR‐1460291, and EAR‐1444275. CME acknowledges support from the Australian Research Council (DE190100062). We thank the 2017 IRIS undergraduate summer intern program for providing support to A. W. to work with E. A. W. at the University of Washington. The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR‐1261681.

Our crew braved rough Southern Ocean seas, endured pandemic precautions, and adapted plans on the fly for the chance to observe a possible subduction zone in the making below the Macquarie Ridge.

Australia is an old stable continent with a rich geological history. Limitations in sub-surface imaging below the Moho, however, mean that is unclear to what extent, and to what depth, this rich geological history is expressed in the mantle. Scattering of surface waves at ~150km depth by lateral gradients or boundaries in seismic anisotropy, termed Quasi-Love waves, offer potential new insights. The first such analysis for Australia and Zealandia shown here detects over 300 new scatterers that display striking geographical patterns. Around two-thirds of the scatterers are coincident with either the continental margins, or major crustal boundaries within Australia, suggesting deep mantle roots to such features. Within the continental interior such lateral anisotropic gradients imply pervasive fossilized lithospheric anisotropy, on a scale that mirrors the crustal geology at the surface, and a strong lithosphere that preserves this signal over billions of years. Along the continental margins, lateral anisotropic gradients may indicate either the edge of the thick continental lithosphere, or small-scale dynamic processes in the asthenosphere, such as edge-drive convection, tied to the transition from oceanic to continental crust/lithosphere.

S. A. and C. M. E. are currently supported by Australian Research Council Grant DE190100062.

Numerous studies have provided insights into one of the key problems of the Earth Sciences: subduction zone initiation (SZI). The insights into SZI are both numerous and diverse with evidence from multiple disciplines in Earth Sciences. SZI studies exploit the geological record, reconstruct regional or global plate motion back in time, interpret seismic tomography to identify the tip depth of sunken plate portions, and diagnose theoretical and numerical models of rock and plate deformation based on known physics.Getting and keeping an overview over the many discipline-specific advances is challenging for many reasons: one being the dispersed sources of information, another being the missing communication across the individual disciplines. The latter shortcoming also arises from the multiple incompatible scientific jargons currently in use.The SZI database now unifies the scientific jargon, and brings together old and new insights relating to SZI into a common, community-wide platform online (www.SZIdatabase.org). The SZI database builds bridges between individual communities, opening a community-wide discussion by making SZI data readily available and understandable. This keeps data and knowledge up-to-date, and can therefore provide the most complete picture of our current understanding of SZI.In this presentation, we outline where to find, how to use, and why to contribute to the SZI database. This community-wide project has already yielded interesting results regarding the fascinating question about how and where SZI occurs on present-day Earth and back to around 100 Ma. Work thus far suggests ‘subduction breeds subduction’, highlighting the beginning of crucial insights from these ongoing cross-disciplinary efforts.

From 2008 to 2011 a broadband seismic array (BILBY) spanned the Australian continent from North to South, crossing the suture zone between the North and South Australian cratons, and traversing the Central Australian orogenic belt. Past tectonic events, including orogenies during the Proterozoic and Paleozoic, have left a long-lasting impression on the crustal and Moho structure of this inter-cratonic region. However, the impact of past tectonic activity on the lithosphere has been less clear. Here we present the first shear-wave splitting results for the BILBY array using a combination of SKS and PKS teleseismic phases to investigate patterns of deformation and seismic anisotropy within the upper mantle beneath central Australia. Null *KS observations are found to be abundant compared to observations of splitting, as has been widely reported by previous studies, but this appears to be largely due to a coincidental alignment of the inferred anisotropic fast direction with the back-azimuthal range at which most available events occur (140°-160°). Across the central Australian belt the station averaged fast directions tend to orientate ENE-WSW parallel with topographic, gravity, and magnetic trends. Northwards however, the fast directions switch orientation, instead following the NW-SE elongated geometry of the Tennant Creek Inlier, thus delineating a sharp lateral change in the underlying seismic anisotropy. Overall, evidence suggests that the splitting pattern likely reflects anisotropy inherent within the lithosphere generated by past deformational events over 300 million plus years ago, as opposed to the present-day mantle flow in the asthenosphere. While two distinct layers of anisotropy, present in both the asthenosphere and lithosphere, is supported by other evidence, it is not necessarily required by our current dataset. Instead, we can sufficiently model our results with only a single layer of anisotropy, consistent with the expected geometry of azimuthal anisotropy in the lithosphere.

We performed shear wave splitting analyses to investigate seismic anisotropy across the northern extent of the Peruvian flat-slab subduction region. We used core-mantle refracted SKS, SKKS and PKS phases from teleseismic events (88° > Δ

Crustal receiver functions have been calculated from 128 events for two three-component broadband seismomenters located on the south coast (FOMA) and in the central High Plateaux (ABPO) of Madagascar. For each station, crustal thickness and Vp/Vs ratio were estimated from H- κ plots. Self-consistent receiver functions from a smaller back-azimuthal range were then selected, stacked and inverted to determine shear wave velocity structure as a function of depth. These results were corroborated by guided forward modeling and by Monte Carlo error analysis. The crust is found to be thinner (39 ± 0.7 km) beneath the highland center of Madagascar compared to the coast (44 ± 1.6 km), which is the opposite of what would be expected for crustal isostasy, suggesting that present-day long wavelength topography is maintained, at least in part, dynamically. This inference of dynamic support is corroborated by shear wave splitting analyses at the same stations, which produce an overwhelming majority of null results (>96 %), as expected for vertical mantle flow or asthenospheric upwelling beneath the island. These findings suggest a sub-plate origin for dynamic support.

The Central Andean Plateau, the second-highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source-side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench-parallel to trench-oblique, contradicting predictions from the simplest two-dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two-dimensional mantle flow regime.

Subducting oceanic plates are often considered as cold, rigid slabs. Analysis of seismic anisotropy in the subducted Nazca Plate beneath Peru suggests that the plate has deformed internally during subduction. Within oceanic lithosphere a fossilized fabric is often preserved originating from the time of plate formation. Such fabric is thought to form at the mid-ocean ridge when olivine crystals align with the direction of plate spreading1,2. It is unclear, however, whether this fossil fabric is preserved within slabs during subduction or overprinted by subduction-induced deformation. The alignment of olivine crystals, such as within fossil fabrics, can generate anisotropy that is sensed by passing seismic waves. Seismic anisotropy is therefore a useful tool for investigating the dynamics of subduction zones, but it has so far proved difficult to observe the anisotropic properties of the subducted slab itself. Here we analyse seismic anisotropy in the subducted Nazca slab beneath Peru and find that the fast direction of seismic wave propagation aligns with the contours of the slab. We use numerical modelling to simulate the olivine fabric created at the mid-ocean ridge, but find it is inconsistent with our observations of seismic anisotropy in the subducted Nazca slab. Instead we find that an orientation of the olivine crystal fast axes aligned parallel to the strike of the slab provides the best fit, consistent with along-strike extension induced by flattening of the slab during subduction (A. Kumar et al., manuscript in preparation). We conclude that the fossil fabric has been overprinted during subduction and that the Nazca slab must therefore be sufficiently weak to undergo internal deformation.

a r t i c l e i n f o a b s t r a c t The Peruvian flat slab is by far the largest region of flat subduction in the world today, but aspects of its structure and dynamics remain poorly understood. In particular, questions remain over whether the relatively narrow Nazca Ridge subducting beneath southern Peru provides dynamic support for the flat slab or it is just a passive feature. We investigate the dynamics and interaction of the Nazca Ridge and the flat slab system by studying upper mantle seismic anisotropy across southern Peru. We analyze shear wave splitting of SKS, sSKS, and PKS phases at 49 stations distributed across the area, primarily from the PerU Lithosphere and Slab Experiment (PULSE). We observe distinct spatial variations in anisotropic structure along strike, most notably a sharp transition from coherent splitting in the north to pervasive null (non-split) arrivals in the south, with the transition coinciding with the northern limit of the Nazca Ridge. For both anisotropic domains there is evidence for complex and multi-layered anisotropy. To the north of the ridge our ∗ KS splitting measurements likely reflect trench-normal mantle flow beneath the flat slab. This signal is then modified by shallower anisotropic layers, most likely in the supra-slab mantle, but also potentially from within the slab. To the south the sub-slab mantle is similarly anisotropic, with a trench-oblique fast direction, but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab. Overall the regional anisotropic structure, and thus the pattern of deformation, appears to be closely tied to the location of the Nazca Ridge, which further suggests that the ridge plays a key role in the mantle dynamics of the Peruvian flat slab system.

Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse, however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this, we utilize the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterization of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localize strain to the plate boundary.

Convection in the Earth?s mantle is mainly driven by cold, dense subducting slabs, but little is known about how 3D variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response (i.e. dynamic topography). We investigate this problem by studying the dynamics of an active region of flat-slab subduction located in Peru in South America. Here the slab geometry is well known, based on the regional seismicity, and we have observations from the local geological record to validate our models. Of particular interest is the widespread subsidence and deposition of the Solimões Formation across western Amazonia that coincided with the development of the Peruvian flat-slab during the Mid-Late Miocene. This formation covers an extensive area from the foredeep to the Purus Arch located ~2000 km away from the trench. Close to the Andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure. Based on an estimate of the Andean loading we predict 2.8 to 3.6 km of accommodation space that spans 100 km. The spatial and temporal history of the Solimões Formation however, particularly the thick distal foreland accumulations up to 1.2 km deep, can only be matched with the addition of a longer-wavelength dynamic source of topography. Following the transition from normal to flat subduction, we predict over 1 km of dynamic subsidence (~1500 km wide) that propagates over 1000 km away from the trench, tracking the subduction leading edge. This is followed by a pulse of dynamic uplift over the flat segment behind it. We therefore propose that a combination of uplift, flexure and dynamic topography during slab flattening in Peru is responsible for the sedimentation history and landscape evolution of western Amazonia that eventually led to the configuration of the Amazon Drainage Basin we know today. Fil: Eakin, Caroline M.. University of Yale; Estados Unidos Fil: Lithgow Bertelloni, Carolina. University College London; Estados Unidos Fil: Davila, Federico Miguel. University College London; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina

Flat or shallow subduction is a relatively widespread global occurrence, but the dynamics remain poorly understood. In particular, the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail. Here we present measurements of seismic anisotropy to investigate mantle flow beneath the Peruvian flat-slab segment, the largest present-day region of flat subduction. We conduct a detailed shear wave splitting analysis at a long-running seismic station (NNA) located near Lima, Peru. We present measurements of apparent splitting parameters (fast direction ? and delay time ?t) for SKS, ScS, and local S phases from 80 events. We observe well-defined frequency dependence and backazimuthal variability, indicating the likely presence of complex anisotropy. Forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench-normal fast direction underlying a layer with a more trench-oblique (to trench-subparallel) fast direction. In order to further constrain the anisotropic geometry, we analyzed the source-side splitting from events originating within the slab measured at distant stations. Beneath the flat-slab segment, we found trench-normal fast splitting directions in the subslab mantle, while within the dipping portion of the slab further to the east, likely trench-subparallel anisotropy within the slab itself. This subslab pattern contradicts observations from elsewhere in South America for “normal” (i.e., more steeply dipping) slab conditions. It is similar, however, to inferences from other shallowly dipping subduction zones around the world. While there is an apparent link between slab dip and the surrounding mantle flow, at least beneath Peru, the precise nature of the relationship remains to be clarified.

[1] The thermal and compositional structure of oceanic lithosphere, which exerts an important control on plate behavior, is still debated. Our set of 60,000 PP-P and SS-S traveltime differences with oceanic PP and SS bounce points provides a good constraint on both compressional- and shear-wave velocity. By calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach, we test whether lithospheric cooling can explain PP-P and SS-S traveltime variations with plate age. The PP-P and SS-S traveltimes have substantial scatter but, on average, decrease by 0.2 and 0.7 s/Myr½, respectively, when the PP and SS waves reflect off progressively older oceanic crust. Both a half-space and a plate cooling model with a mid-ocean ridge basalt-source mantle potential temperature (1315° ± 50°C) explain the average values of the PP-P and SS-S anomalies and their decrease with plate age. Residual PP-P and SS-S anomalies relative to a cooling model reveal large-scale patterns. Along a few paths (e.g., Tonga–Fiji to western North America), seismic heterogeneity in the deep mantle is responsible for a significant fraction of the PP-P and SS-S traveltime variation. Most anomalies probably correspond to broad temperature variations in the upper mantle, such as a very slow central–northern Pacific (which may require a 100°C excess temperature) and high- and low-velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.

A series of fluid-carved channels in the Sulci Gordii region of Mars were investigated. Numerous channel networks exist in Sulci Gordii, part of the Olympus Mons aureole, and this area comprises some of the youngest volcanic terrain on Mars. The channels ranged in length from 43 to 155 km, with widths of 128–288 m. The morphology of the channels was analysed assuming both lava and water as possible agents. For three of the four channels studied, water appears to be the likely agent, while one channel is probably lava-formed. For the water-formed channels, discharge rates were estimated at 8000–36 000 m3 s?1. The lava channel was probably formed from short-lived episodic activity by a low-viscosity lava. The age of the channels and surrounding area was estimated using crater counting to be 100 Ma. Water has appeared to have flowed for almost 150 km under the climatic conditions at this time. There is some evidence for later tectonic activity, possibly as recent as 10 Ma, but crater-dating accuracy was limited by the lack of high-resolution images of some areas. Sulci Gordii is therefore a dynamic site with evidence of hydrological and volcanic activity extending into the recent geological past.

Mantle flow associated with the Cascadia subduction zone and the Mendocino Triple Junction is poorly characterized due to a lack of shear wave splitting studies compared to other subduction zones. To fill this gap data was obtained from the Mendocino and FACES seismic networks that cover the region with dense station spacing. Over a period of 11–18 months, 50 suitable events were identified from which shear wave splitting parameters were calculated. Here we present stacked splitting results at 63 of the stations. The splitting pattern is uniform trench normal (N67°E) throughout Cascadia with an average delay time of 1.25 s. This is consistent with subduction and our preferred interpretation is entrained mantle flow beneath the slab. The observed pattern and interpretation have implications for mantle dynamics that are unique to Cascadia compared to other subduction zones worldwide. The uniform splitting pattern seen throughout Cascadia ends at the triple junction where the fast directions rotate almost 90°. Immediately south of the triple junction the fast direction rotates from NW–SE near the coast to NE–SW in northeastern California. This rotation beneath northern California is consistent with flow around the southern edge of the subducting Gorda slab.

The dynamics of flat subduction, particularly the interaction between a flat slab and the overriding plate, are poorly understood. Here we study the (seismically) anisotropic properties and deformational regime of the mantle directly above the Peruvian flat slab. We analyze shear wave splitting from 370 local S events at 49 stations across southern Peru. We find that the mantle above the flat slab appears to be anisotropic, with modest average delay times (~0.28 s) that are consistent with ~4% anisotropy in a ~30 km thick mantle layer. The most likely mechanism is the lattice‐preferred orientation of olivine, which suggests that the observed splitting pattern preserves information about the mantle deformation. We observe a pronounced change in anisotropy along strike, with predominately trench‐parallel fast directions in the north and more variable orientations in the south, which we attribute to the ongoing migration of the Nazca Ridge through the flat slab system. Por pares

International audience; Haiti, on the island of Hispaniola, is situated across the North American-Caribbean plate boundary at the transition point between oblique subduction in the east and a transform plate boundary in the west. Here we use shear wave splitting measurements from S waves of local (0-50 km) and intermediate depth (50-150 km) earthquakes as well as SK(K)S phases from teleseismic earthquakes to ascertain good spatial and vertical resolution of the azimuthal anisotropic structure. This allows us to place new constraints on the pattern of deformation in the crust and mantle beneath this transitional region. SK(K)S results are dominated by plate boundary parallel (E-W) fast directions with~1.9 s delay times, indicating subslab trench parallel mantle flow is continuing westward along the plate boundary. Intermediate depth earthquakes originating within the subducting North American plate show a mean fast polarization direction of 065°and delay time of 0.46 s, subparallel to the relative plate motion between the Caribbean and North American plates (070°). We suggest a basal shear zone within the lower ductile crust and upper lithospheric mantle as being a potential major source of anisotropy above the subducting slab. Upper crustal anisotropy is isolated using shear wave splitting measurements on local seismicity, which show consistent delay times on the order of 0.2 s. The fast polarization directions indicate that the crustal anisotropy is controlled by the fault networks in close proximity to the major strike-slip faults, which bisect the north and south of Haiti, and by the regional stress field where faulting is less pervasive.