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Unraveling the rifting history along the southeastern margin of the South China Block has important implications for establishing the origin and evolution of the South China Sea (SCS). However, the onset time of rifting associated with the opening of the SCS remains unclear. The Littoral Fault Zone is the largest boundary fault between the South China Block and the SCS, and its fault activity provides information about the evolutionary history of the SCS and crucial constraints on the opening of the SCS. Here we present new apatite (U Th)/He (AHe) ages obtained from the Sunset Peak and Tai Mo Shan, Hong Kong, along the footwall of the extensional Littoral Fault Zone, to constrain the timing of fault activity. The AHe age-elevation relationships and thermal modeling results document an accelerated exhumation during ∼60–55 Ma, which we interpret to be associated with extensional movement along the Littoral Fault Zone. The activity of this extensional fault at ∼60–55 Ma is considered to be coeval with the initial deposition of conglomerate at the bottom of the Pearl River Mouth Basin in the hanging wall of the Littoral Fault Zone. Integration of thermochronological data, magmatic characteristics, and seismic and drilling data support that the Paleocene extension occurred widely in the southeastern margin of the South China Block, representing a prelude to the seafloor spreading in the SCS. • New apatite (U Th)/He data in Sunset Peak and Tai Mo Shan, Hong Kong. • Accelerated exhumation at ∼60–55 Ma in response to extension of the Littoral Fault. • Paleocene regional extension is the preclude to the opening of the South China Sea. [ABSTRACT FROM AUTHOR]

Abstract In this work we present data on fractures and mesoscale folds exposed in the Triassic to Miocene sedimentary succession of the Lurestan region (Zagros Belt). Data have been collected in tens of field sites, in a 104 km2 area extending across the High Zagros Zone and the Folded Belt. Fractures and mesoscale folds have been characterised in terms of orientation, cross-cutting and abutting relationships with the other structures, and relative timing with respect to sedimentation. Outcrop-scale folds, stylolites, and reverse faults are NW-SE oriented and developed during NE-SW directed shortening. Extensional fractures, including joints, veins, and normal faults, are arranged into two assemblages including: (i) NE-SW and NW-SE striking fractures and (ii) NNW-SSE and WSW-ENE striking fractures. Syn-sedimentary fractures of the two extensional assemblages occur in pre- and syn-rift Jurassic rocks, but also in post-rift Cretaceous rocks and Cenozoic syn-orogenic rocks. We infer that the development of these extensional fractures in the Lurestan region was controlled by: (i) NE-SW-directed extension during both Early Jurassic rifting and early orogenic foreland extension; and (ii) WSW-ENE-directed stretching caused by differential compaction and related subsidence above NNW-SSE elongated basement structures, during tectonically quiescent periods. Highlights • We present fracture data from the Lurestan region of the Zagros. • Fractures developed during Jurassic rifting and Cretaceous to Cenozoic shortening. • Extensional fractures developed also during tectonically quiescent periods. • Differential compaction on top of inherited structures explains these fractures. [ABSTRACT FROM AUTHOR]

What are the conditions under which lithospheric extension drives exhumation of the deep orogenic crust during the formation of gneiss domes? The mechanical link between extension of shallow crust and flow of deep crust is investigated using two-dimensional numerical experiments of lithospheric extension in which the crust is 60 km thick and the deep-crust viscosity and density parameter space is explored. Results indicate that the style of extension of the shallow crust and the path, magnitude, and rate of flow of deep crust are dynamically linked through the deep-crust viscosity, with density playing an important role in experiments with a high-viscosity deep crust. Three main groups of domes are defined based on their mechanisms of exhumation across the viscosity-density parameter space. In the first group (low-viscosity, low-density deep crust), domes develop by lateral and upward flow of the deep crust at km m.y −1 velocity rates (i.e. rate of experiment boundary extension). In this case, extension in the shallow crust is localized on a single interface, and the deep crust traverses the entire thickness of the crust to the Earth's near-surface in 5 m.y. This high exhuming power relies on the dynamic feedback between the flow of deep crust and the localization of extension in the shallow crust. The second group (intermediate-viscosity, low-density deep crust) has less exhuming power because the stronger deep crust flows less readily and instead accommodates more uniform extension, which imparts distributed extension to the shallow crust. The third group represents the upper limits of viscosity and density for the deep crust; in this case the low buoyancy of the deep crust results in localized thinning of the crust with large upward motion of the Moho and lithosphere-asthenosphere boundary. These numerical experiments test the exhuming power of the deep crust in the formation of extensional gneiss domes. [ABSTRACT FROM AUTHOR]

Lithospheric stretching is the key process in forming extensional sedimentary basins at passive rifted margins. This study explores the stretching factors, resulting extension, and structural evolution of the Møre segment on the Mid-Norwegian continental margin. Based on the interpretation of new and reprocessed high-quality seismic, we present updated structural maps of the Møre margin that show very thick post-rift sediments in the central Møre Basin and extensive sill intrusion into the Cretaceous sediments. A major shift in subsidence and deposition occurred during mid-Cretaceous. One transect across the Møre continental margin from the Slørebotn Subbasin to the continent-ocean boundary is reconstructed using the basin modelling software TecMod. We test different initial crustal configurations and rifting events and compare our structural reconstruction results to stretching factors derived both from crustal thinning and the classical backstripping/decompaction approach. Seismic interpretation in combination with structural reconstruction modelling does not support the lower crustal bodies as exhumed and serpentinised mantle. Our extension estimate along this transect is ~ 188 ± 28 km for initial crustal thickness varying between 30 and 40 km. [ABSTRACT FROM AUTHOR]

The Mediterranean geodynamic system is an exceptional natural laboratory to study the complexities of back-arc extension during subduction rollback. Corsica is a key locality that recorded the Alpine subduction-collision history followed by 30 Ma of the Apennines slab rollback, responsible for the successive opening, from West to East, of the Liguro-Provençal basin and the Tyrrhenian Sea. The overall strain pattern of the Alpine metamorphic units of Alpine Corsica is studied through a synthesis of i) available geological maps, ii) geochronological data and iii) newly acquired structural data relative to successive events of ductile and brittle extension. Three main stages of extension are recorded: 1/ Syn-metamorphic Oligocene extension accommodated by N140° trending ductile shear zones, that correspond to the reactivation of a major Alpine thrust; 2/ Burdigalian ramp-flat extension with N60°– 70 ° trending fault zones that controlled the deposition of onshore piggy-back basins (e.g. St Florent basin), and 3/ Serravalian N–S trending fault zones. From a geodynamical point of view, these successive extensional deformations in Alpine Corsica can be correlated to the history of the Apennines slab rollback. The two first stages are related to a southeastward rollback of the Appennines slab that successively triggered: 1/ Continental rifting in Oligocene leading to the opening of the Liguro-Provençal basin and 2/ Burdigalian extension with N60°–70° trending fault zone, oblique to the Alpine thrust that, after a counterclockwise rotation of the Corso-Sardinian block, became unable to accommodate extension. Finally, the third stage that gave NS-trending fault zones developed in Serravalian, at the onset of Tyrrhenian rifting, related to an eastward rollback of the Apennines slab, as a response of slab tearing in Sicily. [ABSTRACT FROM AUTHOR]

The timing, kinematics, and processes responsible for the rapid transition from subduction to oblique rifting and the localization of the Pacific-North America plate boundary in the Gulf of California are not well understood. Well exposed volcanic rocks deposited between ~ 15 and 10 Ma in the Sierra Bacha (coastal Sonora, México) preserve a record of late Miocene deformation on the eastern rifted margin of the Gulf of California and offer new insights into the timing and kinematic evolution of oblique rifting. Detailed geologic mapping, fault kinematic analysis, U–Pb and 40Ar/39Ar geochronology, and paleomagnetic data reveal that the > 2 km-thick composite volcanic section is cut by a series of southwest-dipping, domino-style normal faults and uniformly tilted down-to-the-northeast. Palinspastic cross-section restoration suggests that the region experienced ca. 55–60% northeast-southwest-directed extension between ~ 11.7 and ~ 10–9 Ma. Fault kinematic data reflect relatively minor dextral transtension either following or during the later stages of extension. Paleomagnetic results indicating modest clockwise vertical-axis block rotation suggest that dextral shear was concentrated in the southwest of the study area near the modern coastline. These results support an emerging model in which dextral strain was not ubiquitous across Sonora and did not initiate immediately following the ~ 12.5 Ma transition from subduction to oblique rifting. Instead, strain east of the Baja California microplate at this latitude evolved from extension-dominated transtension prior to ~ 8 Ma to dextral shear-dominated transtension by ~ 7–6 Ma. The onset of dextral shear in coastal Sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at ~ 8 Ma. The increase in rift obliquity and resultant onset of significant strike-slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern Gulf of California by ~ 6 Ma and continental rupture at ~ 2–1 Ma. [ABSTRACT FROM AUTHOR]

Varying amounts of gold mineralisation is occurring in all young and active collisional mountain belts. Concurrently, these syn-orogenic hydrothermal deposits are being eroded and transported to form placer deposits. Local extension occurs in convergent orogens, especially oblique orogens, and facilitates emplacement of syn-orogenic gold-bearing deposits with or without associated magmatism. Numerical modelling has shown that extension results from directional variations in movement rates along the rock transport trajectory during convergence, and is most pronounced for highly oblique convergence with strong crustal rheology. On-going uplift during orogenesis exposes gold deposits to erosion, transport, and localised placer concentration. Drainage patterns in variably oblique convergent orogenic belts typically have an orogen-parallel or sub-parallel component; the details of which varies with convergence obliquity and the vagaries of underlying geological controls. This leads to lateral transport of eroded syn-orogenic gold on a range of scales, up to > 100 km. The presence of inherited crustal blocks with contrasting rheology in oblique orogenic collision zones can cause perturbations in drainage patterns, but numerical modelling suggests that orogen-parallel drainage is still a persistent and robust feature. The presence of an inherited block of weak crust enhances the orogen-parallel drainage by imposition of localised subsidence zones elongated along a plate boundary. Evolution and reorientation of orogen-parallel drainage can sever links between gold placer deposits and their syn-orogenic sources. Many of these modelled features of syn-orogenic gold emplacement and varying amounts of orogen-parallel detrital gold transport can be recognised in the Miocene to Recent New Zealand oblique convergent orogen. These processes contribute little gold to major placer goldfields, which require more long-term recycling and placer gold concentration. Most eroded syn-orogenic gold becomes diluted by abundant lithic debris in rivers and sedimentary basins except where localised concentration occurs, especially on beaches. [ABSTRACT FROM AUTHOR]

The Isparta Angle is an important area of SW Anatolia where extensions in all directions (N-S, NE-SW, NW-SE and E-W) meet. These extensions were determined by normal faulting structures as well as by shallow earthquakes. All extensions, except the E-W one, were attributed to the deviatoric stresses in relation to slab forces and/or extrusion of Anatolia. The moment tensor inversion of 40 shallow earthquakes which occurred in the inner part of the Isparta Angle give focal mechanisms mostly indicating normal faulting. Inversion of all focal mechanisms of the earthquakes obtained from the moment tensor inversion yields normal faulting characterized by an approximately E-W (N268°E) σ 3 axis. The calculated stress ratio R is 0.6944 indicating a triaxial stress state. Commonly accepted geodynamic models for the eastern Mediterranean region do not include plate boundary forces acting in the east or west direction. Our hypothesis is that the cause of the E-W extension is the combined forces of Gravitational Potential Energy and the hot asthenosphere upwelling through a tear fault in the subducted African plate between the Hellenic and Cyprus arcs beneath the Isparta Angle. [ABSTRACT FROM AUTHOR]

We compile field data collected along the eastern part of the North Pyrenean Zone (NPZ) to point to a tectonic evolution under peculiar thermal conditions applying to the basin sediments in relation with the opening of the Cretaceous Pyrenean rift. Based on this compilation, we show that when thinning of the continental crust increased, isotherms moved closer to the surface with the result that the brittle-ductile transition propagated upward and reached sediments deposited at the early stage of the basin opening. During the continental breakup, the pre-rift Mesozoic cover was efficiently decoupled from the Paleozoic basement along the Triassic evaporite level and underwent drastic ductile thinning and boudinage. We suggest that the upper Albian and upper Cretaceous flysches acted as a blanket allowing temperature increase in the mobile pre-rift cover. Finally, we show that continuous spreading of the basin floor triggered the exhumation of the metamorphic, ductily sheared pre-rift cover, thus contributing to the progressive thinning of the sedimentary pile. In a second step, we investigate the detailed geological records of such a hot regime evolution along a reference-section of the eastern NPZ. We propose a balanced restoration from the Mouthoumet basement massif (north) to the Boucheville Albian basin (south). This section shows a north to south increase in the HT Pyrenean imprint from almost no metamorphic recrystallization to more than 600 °C in the pre- and syn-rift sediments. From this reconstruction, we propose a scenario of tectonic thinning involving the exhumation of the pre-rift cover by the activation of various detachment surfaces at different levels in the sedimentary pile. In a third step, examination of the architecture of current distal passive margin domains provides confident comparison between the Pyrenean case and modern analogs. Finally, we propose a general evolutionary model for the pre-rift sequence of the Northeastern Pyrenean rifted margin. [ABSTRACT FROM AUTHOR]

The exhumed post-Caledonian Eidsfjord and Fiskfjord extensional shear zones of northern Norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte. Geothermobarometric calculations on garnet-bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low-angle ductile extension. Average pressures from Eidsfjord (570 ± 115 MPa at ca. 650 °C) and Fiskfjord (1120 ± 220 MPa at ca. 650 °C) correspond to faulting depths of 21 ± 4 km and 41 ± 9 km, respectively. The Fiskfjord results agree with previous thermobarometric calculations on mylonitic Cpx + Grt-bearing pseudotachylyte at Fiskfjord. The calculated depths are 5–25 km below the depth of the standard seismogenic zone. These results demonstrate that low angle normal faults may cut through a large portion of continental crust. This occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and/or unusually high shear stresses to account for coseismic rupture at such depths, implying a direct connection with a seismogenic normal fault in the upper crust. [ABSTRACT FROM AUTHOR]

The constraints of surface heat flow and lithospheric structure were used to investigate the dynamics of the Tengchong volcanic region based on thermal–mechanical modeling. Three different boundary conditions representing three types of tectonic movements were modeled to discuss the thermal state and lithospheric deformation. The compressive model shows that the subduction of the Indian plate causes the upwelling of asthenosphere beneath the Tengchong block. In this process, upwelling of the asthenosphere causes thinning of the lithosphere beneath the Tengchong block. However, the surface heat flow of this model is not consistent with the observations. The regional extension model shows that, under the extension, the asthenosphere upwells along the preexisting weak suture zone in the southeastern Tibetan plateau. The thickness of lithosphere is the thinnest along the suture zone, and the surface heat flow is the highest. However, the extension time was found to generally need 10–15 Myr to conform to the surface heat flow, however, this is not consistent with accepted geochronological data. When a boundary condition from compression to extension is imposed, both the surface heat flow and the lithospheric structure fit well with the observations. The results show that the subduction of the Indian plate leads to the upwelling of the asthenosphere which causes a thinned lithosphere beneath the Tengchong block. Since the late Miocene (8–5 Ma), the extension or extrusion has been predominant. The thermal erosion of the lithosphere caused by upwelling of the asthenosphere renders the lithosphere even thinner along the Bangong–Nujiang suture zone. The upwelling of the asthenosphere provides an upward driving force and high-temperature environment for the Tengchong volcanic region. These findings suggest that the origin of the Tengchong volcanic region is caused by both the subduction of the Indian plate and regional extension across the Bangong–Nujiang suture zone. [ABSTRACT FROM AUTHOR]

In the Chalkidiki Peninsula of northern Greece a thrust complex made of a basement (Vertiskos Unit), a cover (Circum–Rhodope belt) and arc/back-arc units (Chortiatis Magmatic Suite and eastern Vardar Ophiolites) is exposed in the Chalkidiki Peninsula of northern Greece. The complex forms the western part of the Rhodope Metamorphic Province and lies on the hanging-wall of the Kerdylion Detachment, the structure responsible for the exhumation of the Southern Rhodope Core Complex and the most prominent and visible ductile structure related to the Tertiary Aegean extension. The Chalkidiki thrust complex arguably preserves a complete record of Cretaceous deformation and related fabrics. In this contribution we describe the geometry of foliation, stretching lineation and shear sense(s) on a regional scale. The regional foliation strikes NW–SE and displays different patterns in the three studied units: (i) dominantly dipping at low angle in the Vertiskos Unit, (ii) affected by upright folding in the Circum–Rhodope belt and (iii) systematically steeply dipping to the NE in the Chortiatis Magmatic Suite. Stretching lineation trend dominantly SW–NE in the three mentioned units. On the basis of new mapping, neglecting local perturbations and deformation related to Tertiary extension, we infer the regional kinematics of Cretaceous syn-metamorphic thrusting and subsequent exhumation of the metamorphic units. Thrusting took place toward the SW (in present-day coordination) and the related fabrics are recorded throughout the metamorphic pile. On the contrary, exhumation-related fabrics are related to shear toward the NE and are preferentially recorded in the uppermost part of the metamorphic pile suggesting that extension was more localised and of less finite intensity compared to thrusting. [ABSTRACT FROM AUTHOR]

Exhumation of the Eastern Alps from the early Tertiary to the late Miocene was localized mainly in the Tauern Window, a thermal and structural dome located in front of the Dolomites indenter. Stress inversions based on new structural investigations over the entire Tauern Window indicate a regional zoning of the paleostress field with a predominance of strike-slip states of stress in the core of the investigated area, and dominant extensional regimes in the eastern and western borders of the dome. Few inverse fault structures have been highlighted. We propose a two-stage deformation history in order to explain the different types of structures that characterise the ductile and the brittle domains. During the first stage of exhumation of the Tauern Window, corresponding to the folding event, the brittle crust was probably dominated by N-S shortening and compression. The second stage of exhumation was marked by normal faulting at the borders of the dome and strike-slip faulting in the core. During the second stage, the brittle part of the crust that was previously affected by compressive structures belonging to the first stage was eroded. Normal faulting associated to E-W extension along the eastern and western borders of the Tauern Window was accommodated by strike-slip faulting located in the core of the Tauern Window, yielding E-W extension and N-S shortening. The orientation of the extensional axes and the nature of the stress tensors are similar to the ones inferred for the late-stage, brittle overprint of the internal basement massifs of the Central Alps pointing to a possible common, large-scale, state of stress acting well beyond the area of the Eastern Alps. [ABSTRACT FROM AUTHOR]

This work analyzes through field, geophysical, geochemical and geochronological data, a particular setting of the Southern Andes in which an extensional system is flanking the mountain front. This setting is represented by the Loncopué Trough, whose origin is discussed for this part of the Andes. This work reconstructs 3-dimensionally the structure of this basin and its evolution through time, since Jurassic times as a series of isolated depocenters, to Late Cretaceous-Eocene times with the construction of a positive relief that subsequently was relaxed through two extensional stages during the late Oligocene-early Miocene and the Pliocene-Quaternary respectively. The last episode of extension is characterized by an initial stage with a series of caldera-collapses in the latest Pliocene-early Quaternary. This passed to a stage of development of a broad basaltic plateau in pre- and inter-glacial times, circumscribed with new radiometric data to the Pleistocene. Quaternary products in northern and central Loncopué Trough have chemical relations that are intermediate between the arc front and the southern Loncopué Trough volcanic rocks studied in previous works. Thus, retroarc eruptions would be part of an extended arc configuration that goes from typical arc series in the northern part of the trough to within-plate series in the south. Low elastic thicknesses computed from gravity data in this work correlate with the area of retroarc volcanic activity. Magnetic data have allowed determining the Curie isotherm, showing two areas of relatively abnormal heat flow, one along the Loncopué Trough itself and the other in the foreland zone. This scenario is discussed through three main hypotheses: an occurrence linked to a slab-steepening after a shallow subduction in the area; co-seismic crustal stretching linked to giant earthquakes in the subduction zone; and, finally slab-tearing associated with asthenospheric upwelling. [ABSTRACT FROM AUTHOR]

Abstract: Deep seismic reflection profiling has imaged different patterns of the Moho across the Indian shield with a variety of tectonic environments from Archean to Recent. The character of the Moho varies from a discrete strong event, the base of strong coherent lower crustal sub-horizontal reflections, the base of dipping lower crustal reflections into mantle, to no clear reflection boundary. The seismic reflection data suggest a laminated lower crust in several places and offsets in the Moho at others. Kinematic and dynamic modeling of wide-angle reflection data across the Mesoproterozoic South Delhi Fold Belt and the Central Indian Tectonic zone suggests a laminated lower crust for these regions. In general, the lower crust of the Indian shield is heterogeneous. A clear Moho is identified in some of the Precambrian orogenic belts and sedimentary basins along with reflective lower-crust, whereas the cratonic areas exhibit a diffused Moho. Post-collisional extensional process, such as orogenic collapse, delamination, magmatic intrusions, low-viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger Moho. The termination of lower-crustal reflectivity at the Moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle. [Copyright &y& Elsevier]

Abstract: The marine magnetic anomaly record and plate kinematics derived from that data provide evidence of numerous cases of significant changes in plate motions and plate interactions. However, in most cases the temporal resolution provided by the marine record does not discriminate between punctuated (abrupt) or smooth variations in plate motions. During a 10million year period (~30Ma to 20Ma), the Euler poles (stage poles) describing Australia–Pacific relative plate motion migrated more than 20° in latitude. The New Zealand segment of the plate boundary is close to the location of the Euler poles and their migration making the tectonics of its plate boundary structures particularly sensitive to the history of plate kinematics. Based on the rapid southward migration of the Euler (stage) poles from north of New Zealand to south of the landmass, there is an expected rapid, migrating transition from an extensional (stage pole north of the site) to transpressional (stage pole south of the site) plate boundary system. The geologic signature of this rapid change in plate boundary kinematics is preserved in the stratigraphic record of a suite of basin deposits that span the latitudinal sweep of the migrating Euler poles. Analyses of these deposits indicate that the timing of the transition from extensional to transpressional tectonics shows a continuous and systematic southward sweep, indicating that the changes in Australia–Pacific plate motions during the 30Ma–20Ma interval were smoothly continuous and not punctuated. [Copyright &y& Elsevier]

Abstract: Several tectonic processes have been proposed to be important in the Alpine to the recent evolution of the Polish Outer Carpathians, i.e., Paleogene–Neogene thrusting, post-thrusting extension, Quaternary reactivation of compression. This work tests the effect of these processes on exhumation of the wedge, in order to verify their timing and relative role in shaping the Polish Outer Carpathians. AFT, AHe and ZHe analyses were performed on samples collected along the study region. This allowed the reconstruction of their post-depositional minimum heating, depth of burial and timing of cooling and exhumation. After deposition samples from the innermost units were heated up to temperatures of ca. 60–120°C (ca. 4–9km), whereas samples from the outer units were heated to temperatures lower than 60°C (ca. 4km). Cooling and exhumation occurred progressively later from west to east (between ca. 25 and 10Ma in the western sector, between ca. 15 and 5Ma in the eastern sector). These results have been put in relationship with single structural features to understand the relative role of thrusts and normal faults. In the eastern sector of the study region samples at the footwall of significant normal faults show a higher degree of reset and younger ages than samples located at the hangingwall. This implies that, in this sector, extensional tectonics played an important role in exhumation. However, proper tectonic exhumation only had a minor role, the major exhumation mechanism being erosion, which is enhanced in the footwalls and reduced in the hangingwalls. In the western sector of the Polish Outer Carpathians only in few cases evidence of enhanced erosion to the footwall of major normal faults could be observed based on the present data. In this region the wedge exhumed mainly as a response to thrusting and normal faulting only played a minor role, locally modifying the pattern of erosion. Finally no major exhumation events occurred after Miocene, pointing to a minor relevance of recent compression. Unreset ZHe dates were used to estimate exhumation of source rocks through the ZHe PRZ and to outline the provenance of sediments forming the Polish Outer Carpathians. Exhumation occurred both in response to the Variscan (Late Devonian to Triassic) and Alpine (Late Jurassic to Early Paleocene) orogeneses, suggesting sedimentary provenance both from the southern margin and from the basement heights within the Outer Carpathian basin. [Copyright &y& Elsevier]

Abstract: This study defines the Plio-Quaternary to present day stress regime in the Burdur Basin, located at the northeastern end of the Fethiye–Burdur Fault Zone in SW Turkey. This fault length, which is considered the landward continuation of the Pliny-Strabo trench, is an important feature in SW Turkey. The inversion slip vectors measured on fault planes indicate a consistent normal faulting stress regime during Plio-Quaternary time, continuing into recent times as indicated by earthquake focal mechanism inversions. Both states have consistent NW–SE trending horizontal minimum stress axes (σ 3). The orientation of fault sets is predominantly around the NE–SW direction in the major Fethiye–Burdur Fault Zone, making the extension NW–SE. The mean stress ratio is 0.74 indicating a triaxial stress state, which is clearly different from radial extension. The NW–SE extension is probably responsible for the formation of the Burdur Basin during Plio-Quaternary time. This extension, which is probably caused by slab-pull force due to the subduction process along the Cyprus arc, produces a dominant normal motion along the FBFZ. [Copyright &y& Elsevier]

Abstract: Our study of the Late Cretaceous–Cenozoic extension in Eastern China is a kinematic analysis of faults (≈1500 striated fault data, 175 stress solutions at 90 sites) that has permitted us to separate stress fields belonging to six tectonic events. A WNW–ESE extension (1) has affected Eastern China from the Campanian to the Late Palaeocene–Early Ypresian. Under this extension, the Qinling (QLFZ) and Southern Tan-Lu (STLFZ) Fault Zones have been dextral transtensional fault zones. A first transpressional event (2) (NNE–SSW shortening) has occurred during the Late Ypresian–Lower Lutetian; the QLFZ and the STLFZ have been dextral transpressional fault zones. A NE–SW extension (3) has affected the Weihe graben during the Palaeogene; after a second transpressional event (4), the NE–SW extension (5) has been rejuvenated during the Late Miocene and the QLFZ has been a normal fault zone again and the STLFZ a sinistral transtensional fault zone; synrift subsidence has occurred in the Weihe graben. The second transpressional event (4) (WNW–ESE shortening) has occurred during the Late Oligocene–Early Miocene, during a period of uplifting and exhumation. At that time, the STLFZ and the QLFZ have become sinistral transpressional fault zones. A period of extension (6) has taken place during the Late Pliocene–Quaternary; the extensional direction has trended ≈NNW–SSE; during this event, the QLFZ has been a sinistral transtensional fault zone and the STLFZ a dextral transtensional fault zone. These results are compared with those previously published in order to evaluate the regional significance of these tectonic events. [Copyright &y& Elsevier]

Abstract: We characterize and map the stress fields acting during plate breakup along the West Greenland volcanic margin. The determination of interpolated stress fields is based on an inversion of fault-slip data sets and magma-driven fractures, crosscutting mainly an exposed inner seaward-dipping basaltic wedge (i.e., SDRi: inner Seaward Dipping Reflectors). This SDRi is segmented along-strike, with differently oriented segments. Relative chronology of stress fields is inferred from published age results on oriented dykes. We identify two distinct tectonic episodes (P1 and P2) with a P1–P2 change over at ~54Ma, i.e. during magnetic chron C24R. P1 is syn-magmatic and purely extensional. It is associated with the major crustal stretching event affecting the margin. P1 probably acted as early as the Late Palaeocene. This stress field was first homogeneous with the minimum principal stress σ3 trending ~N060E, defining a P1A stage. During development of the SDRi, σ3 locally reoriented to become orthogonal to each margin segment and, thus, to the continentward-dipping detachment faults bounding the SDRi (P1B). P1 is coeval with lithosphere breakup and is associated with an extension orthogonal to the Labrador–Baffin axis, which is inherited from the Mesozoic. A regional and radical change of σ3 to a ~NS trend takes place during P2, which follows on immediately from P1. P2 is also syn-magmatic. It is associated with only minor extension. σ3 runs parallel to the North American (NAM)/Greenland (GR) kinematic vector from C24R to C13. We establish therefore that the minimum horizontal stress σ3 for P1 and P2 is parallel to the relative displacement of Greenland related to NAM but not to its absolute displacement during the Tertiary. Taking into account those results as well as variations in magma chemistry from P1 to P2, we suggest that tectonic stresses at a volcanic margin could arise from the local dynamics of the melting mantle. [Copyright &y& Elsevier]

Abstract: The heterogeneous Sundaland region was assembled by closure of Tethyan oceans and addition of continental fragments. Its Mesozoic and Cenozoic history is illustrated by a new plate tectonic reconstruction. A continental block (Luconia–Dangerous Grounds) rifted from east Asia was added to eastern Sundaland north of Borneo in the Cretaceous. Continental blocks that originated in western Australia from the Late Jurassic are now in Borneo, Java and Sulawesi. West Burma was not rifted from western Australia in the Jurassic. The Banda (SW Borneo) and Argo (East Java–West Sulawesi) blocks separated from western Australia and collided with the SE Asian margin between 110 and 90Ma, and at 90Ma the Woyla intra-oceanic arc collided with the Sumatra margin. Subduction beneath Sundaland terminated at this time. A marked change in deep mantle structure at about 110°E reflects different subduction histories north of India and Australia since 90Ma. India and Australia were separated by a transform boundary that was leaky from 90 to 75Ma and slightly convergent from 75 to 55Ma. From 80Ma, India moved rapidly north with north-directed subduction within Tethys and at the Asian margin. It collided with an intra-oceanic arc at about 55Ma, west of Sumatra, and continued north to collide with Asia in the Eocene. Between 90 and 45Ma Australia remained close to Antarctica and there was no significant subduction beneath Sumatra and Java. During this interval Sundaland was largely surrounded by inactive margins with some strike-slip deformation and extension, except for subduction beneath Sumba–West Sulawesi between 63 and 50Ma. At 45Ma Australia began to move north; subduction resumed beneath Indonesia and has continued to the present. There was never an active or recently active ridge subducted in the Late Cretaceous or Cenozoic beneath Sumatra and Java. The slab subducted between Sumatra and east Indonesia in the Cenozoic was Cretaceous or older, except at the very western end of the Sunda Arc where Cenozoic lithosphere has been subducted in the last 20million years. Cenozoic deformation of the region was influenced by the deep structure of Australian fragments added to the Sundaland core, the shape of the Australian margin formed during Jurassic rifting, and the age of now-subducted ocean lithosphere within the Australian margin. [Copyright &y& Elsevier]

Abstract: The Yuanma Basin, situated in central South China with more than 800km far away from the active plate boundaries, is a key region in deciphering the Late Mesozoic–Early Cenozoic tectonic evolution of South China. Based on a field analysis of fault–slip data collected from different rock units, we document polyphase tectonic stress fields and address the changes in the orientation of the principal stresses, and finally establish a five-phase stress evolution of the basin during the Late Mesozoic–Early Cenozoic. The earliest one with E–W compression and N–S extension occurred in the Mid–Late Jurassic and resulted in regional folding and west-directed thrusting along the eastern margin of the basin. This deformational event constrains the timing of the significant intracontinental orogeny in South China to be Mid–Late Jurassic. The subsequent NW–SE extensional regime was responsible for the initial opening and sedimentary infill of the basin during the earliest Cretaceous, which occurred coevally with the extensive Early Cretaceous magmatism, volcanism and extensional doming in South China. The tectonic regime then changed, in the late Early Cretaceous, to a compressional one with NW–SE compression and NE–SW extension, causing the inversion of this extensional basin. This compression affected the whole South China, leading to an apparent magmatic quiescence at ~108–117Ma and sinistral strike–slip faulting on the NNE-trending faults. The N–S extension prevailed in the Late Cretaceous, and activated the east-trending normal faults that controlled the Late Cretaceous subsidence. This extensional phase seems to have lasted through the Early Paleogene and ended in the Late Paleogene by a compressional regime with NE–SW compression and NW–SE extension, as attested by the stratigraphic unconformity at the base of the Neogene in the Jianghan Basin. The temporary evolution of these tectonic stress fields documented in the Yuanma Basin provides a new insight into the influences of different plate tectonics exerting on the South China Block over the Late Mesozoic to Early Cenozoic time. We infer that the Yuanma Basin was influenced, in the Middle Jurassic–Early Cretaceous, by the process of westward subduction of paleo-Pacific plate; and in the Late Cretaceous–Early Paleogene by a combined effect of the subduction along the west pacific margin and in the Tethyan domain; and finally in the Late Paleogene by the far-field effects of India–Asia collision. [Copyright &y& Elsevier]

Abstract: To advance our understanding of the Mesozoic to Eocene tectonics and kinematics of basement units exposed in the south-western Central Iran plateau, this paper presents new structural and thermochronological data from the Chapedony metamorphic core complex and hangingwall units, particularly from the Posht-e-Badam complex. The overall Paleogene structural characteristics of the area are related to an oblique convergent zone. The Saghand area represents part of a deformation zone between the Arabian and Eurasian plates, and can be interpreted to result from the Central Iran intracontinental deformation acting as a weak zone during Mesozoic to Paleogene times. Field and microstructural evidence reveal that the metamorphic and igneous rocks suffered a ductile shear deformation including mylonitization at the hangingwall boundary of the Eocene Chapedony metamorphic core complex. Comparison of deformation features in the mylonites and other structural features within the footwall unit leads to the conclusion that the mylonites were formed in a subhorizontal shear zone by NE–SW stretching during Middle to Late Eocene extensional tectonics. The Chapedony metamorphic core complex is characterized by amphibolite-facies metamorphism and development of S and S-L tectonic fabrics. The Posht-e-Badam complex was deformed by two stages during Cimmerian tectonic processes forming the Paleo-Tethyan suture. [Copyright &y& Elsevier]

Abstract: The continental crust on Earth cannot be extracted directly from the mantle, and the primary crust extracted directly from an early magma ocean is not preserved on Earth. We review geophysical and geochemical aspects of global crust–mantle material exchange processes and examine the processes which, on one side, form and transform the continental crust and, on the other side, chemically modify the mantle residue from which the continental crust has been extracted. Major mechanisms that provide crust–mantle material exchange are oceanic and continental subduction, lithosphere delamination, and mafic magmatism. While both subduction and delamination recycle crustal material into the mantle, mafic magmatism transports mantle material upward and participates in growth of new oceanic and continental crusts and significant structural and chemical modification of the latter. We discuss the role of basalt/gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, together with basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal (lithosphere) recycling. The processes of crust–mantle interaction have created very dissimilar crustal styles in Europe, as seen by its seismic structure, crustal thickness, and average seismic velocities in the basement. Our special focus is on processes responsible for the formation of the thin crust of central and western Europe, which was largely formed during the Variscan (430–280Ma) orogeny but has the present structure of an “extended” crust, similar to that of the Basin and Range province in western USA. Major geophysical characteristics of the Variscan lithosphere are discussed within the frame of possible sequences of crust–mantle material exchange mechanisms during and after main orogenic events in the European Variscides. [Copyright &y& Elsevier]

Abstract: Quartz-monzodioritic dykes have intruded the Hercynian continental lower crust of Calabria at 323±5Ma. Pseudosection and average PT calculations have been performed using THERMOCALC. The obtained P (average PT) and T (pseudosection and average PT) values indicate that the quartz-monzodioritic dykes have intruded the 750°C hot metapelitic–migmatitic country-rocks at a depth of about 32km, with a temperature of ca. 850°C. Microstructural analysis suggests that deformation of the dykes started at hypersolidus conditions during emplacement and continued after cooling at sub-solidus granulite facies conditions together with the country rocks. During this deformation event, which started not later than 323±5Ma ago, the main Sn+1 schistosity evolved. Vorticity analysis of the quartz-metadioritic dykes and of the metapelitic country rocks shows that the Sn+1 schistosity evolved during a general shear deformation characterized by 60 to 70% of pure shear. This high percentage of pure shear suggests that the main Sn+1 schistosity evolved in the hot lower crust undergoing extension during vertical shortening. A horizontal attitude of this extensional schistosity is confirmed by retro-deformation of the today obliquely SE-ward dipping Sn+1 schistosity. Extension continued with time and propagated upwards within the crust, affecting the middle crust in a time interval of 306±1 to ca. 300Ma when huge masses of granitoid rocks intruded it. The deepest batches of these granitoid bodies show a gneissic schistosity which developed under ductile conditions, while granitoids at higher levels became deformed in a brittle fashion. Our whole data set is consistent with a late Hercynian evolution in Calabria characterized by a progressively crust overlying a mantle that gradually rose during lithospheric extension. This extension was asynchronous, starting at 323±5Ma in the lower crust and affecting higher crustal levels at progressively later times. [Copyright &y& Elsevier]

Abstract: A continental rift is conventionally described as a thinning process of the lithosphere ultimately leading to the rupture of the continent and the formation of a mid-oceanic ridge. Rifting is the initial and fundamental process by which the separation of two continents into two tectonic plates takes place. Previous classifications, particularly the one into “active” and “passive” rifting, are briefly presented, together with their limitations. The new classification presented here links continental rifts to the major plate tectonics structures which are at the origin of their formation. Thus, four types of rift can be defined: the subduction-related rift, the plume-related rift, the mountain-related rift and the transform-related rift. A number of examples representative of these four types of rift are then presented. This classification is shown to lie at the heart of our understanding of the major plate tectonic processes at work on Earth. [Copyright &y& Elsevier]

Abstract: A history of deformation has been determined for the Cameli Basin located in the western part of the major Fethiye–Burdur Fault Zone, interpreted as the on-land continuation of Pliny–Strabo fault system (e.g., eastern boundary of the Hellenic Arc). Inversion of fault slip vectors affecting Mio-Pliocene to Quaternary formations in the Cameli Basin, in the southwestern segment of the transtensional Fethiye–Burdur Fault Zone, yields two different normal faulting stress regimes characterized by a roughly orthogonal set of extensional axes; a NW–SE (N129±19°E) σ 3 axis and a NE–SW (N50±16°E) σ 3 axis. The orientation of fault sets is predominantly around the NE–SW direction in the major Fethiye–Burdur Fault Zone, making the extension NW–SE. The mean R values are 0.74 and 0.69 for both extensions indicating a triaxial stress state, which is clearly different from radial extension and from transitional to strike-slip stress state. The NW–SE extension is probably responsible for the formation of the Cameli Basin during Mio-Pliocene time. In contrast, the inversion of focal mechanism solutions of shallow earthquakes occurring within the Cameli basin identifies a present-day, predominantly extensional stress regime, characterized by an approximately N–S (N184°E) σ 3 axis, which has an R value of 0.637 indicating a triaxial stress state. The nearly orthogonal extensional stress regimes seem to have been acting contemporaneously with each other at different intensities from the Mio-Pliocene onwards in SW Turkey. This may be attributed to the geodynamic effects related to the subduction of the African plate beneath Anatolia, diffusing along Hellenic and Cyprus arcs and in the west-southwestward extrusion of Anatolia. Our hypothesis is that the cause of the early NW–SE extension is the slab-pull force due to the subduction process along the Cyprus arc, considered to be dominant up to Plio-Quaternary. The later NE–SW to present-day ∼N–S extension, dominant since the Plio-Quaternary, is related to the combined forces of the Anatolian extrusion and the subduction process along the Hellenic arc. [Copyright &y& Elsevier]

Abstract: The two major crustal blocks of the Pannonian basin, Alcapa (Alpine–Carpathian–Pannonian) and Tisza, underwent a complex process of rotation and extension of variable magnitude during the Tertiary. The northward push of the Adriatic Block initiated the eastward displacement and rotation of both the Alcapa and Tisza blocks. We have constructed geodynamical models of the rotation and extension of the two Pannonian blocks. We analyse this process in terms of the competing influences of a NE push by the Adriatic block, a NE pull from a retreating subduction zone on the eastern Carpathians, and the internal buoyancy forces arising from crustal thickness variations. Deformation of the blocks included substantial strike-slip movements, together with shortening and possible extension across the Mid-Hungarian Line, which now separates the two domains. We use an idealised thin viscous sheet model of the continental lithosphere in which deformation is described by a nonlinear viscous constitutive relationship. Our approach is based on the finite element method, and we consider several distinct models of initial geometry, boundary conditions, and constitutive parameters. Rotation and deformation vary across both blocks, with anticlockwise rotation occurring in the Alcapa plate, and clockwise rotation in the Tisza block. The opposite rotations of the two plates led to NW–SE convergence in the space between the two Intra-Carpathian terranes. Subsequently both domains underwent extension dominantly in the NE–SW direction. Extension and internal rotation can be represented using a thin sheet model in which deformation is driven by an east- or northeast-directed outward traction of order 1 to 3MPa. Internal buoyancy forces produced by thick crust from a prior phase of Alpine-style convergence appear to be a necessary element in the force balance that drives the extension process. [Copyright &y& Elsevier]

Abstract: This paper deals with the structural evolution along a transect across the inner Northern Apennines (Monte Amiata volcano-geothermal area–Monte Cetona), mainly focussing on the post-orogenic Middle Miocene–Quaternary structures. The tectonic setting of the study area derives from the superimposition of three main faulting events: the first and second events developed during the Middle–Late Miocene and Early–Middle Pliocene, respectively, and took place in response to extensional tectonics. The third faulting event developed in a transcurrent/transtensional setting during the Quaternary. Quaternary faults mainly strike NE–SW and show left-lateral kinematics with a minor dip–slip normal component. They caused the Monte Amiata volcanic eruptions (300–190 ka) and controlled the widespread Pleistocene and present hydrothermal fluids circulation. Quaternary strike-slip faults have been documented in the Monte Amiata–Monte Cetona area for the first time in this paper. Correlation with the adjoining areas (e.g. Vulsini region) identifies the Quaternary strike-slip faulting recognised in the study area as a feasible source for the seismic activity in the Monte Amiata region, where the origin of diffuse low-magnitude seismicity is still under debate and poorly understood. [Copyright &y& Elsevier]

Abstract: Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6±4.7 Ma and 87.3±4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9±3.6 and 37.3±2.3 Ma with mean track lengths between 13.26±0.16 µm and 13.95±0.19 µm whilst AHe ages are marginally younger 47.5±1.9–15.3±0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin. [Copyright &y& Elsevier]

Abstract: Tertiary I- and S-type plutonism is widespread in the Aegean region and frequently associated with extensional tectonic structures. This investigation presents the first evidence of protracted magmatism on the island of Serifos in the western Cyclades and at least two discrete generations of plutonism and coeval dyke generations can be distinguished: (1) An older granitoid with S-type characteristics intruded the Attic–Cycladic crystalline rocks. The granitoid and its host rocks were mylonitized by an earlier phase of deformation at temperature conditions >~450 °C and WSW directed shear. This crustal-scale ductile shear zone has been reactivated during the initial stage of a metamorphic core complex formation recording top-SSW sense of shear. Rb–Sr muscovite data from the orthogneiss yield cooling, respectively deformation ages of 15–11.3 Ma. (2) An I-type hornblende–biotite–granodiorite pluton with dacitic to andesitic dyke generations intruded syn- to post-kinematically during ongoing extensional deformation at shallow crustal levels. Single grain zircon TIMS U–Pb ages of the main granodiorite body and its associated dykes are 11.6–9.5 Ma. Cooling ages of these samples scatter between 8.5 and 7.7 Ma (Rb–Sr on biotite). [Copyright &y& Elsevier]

Abstract: In this paper we explore the possibility of reconstructing the initial geometry of inverted basins using combined geophysical and geological methods: seismic reflection profiles, paleomagnetism, analogue modelling and 3D geometrical analysis. These techniques allow for some unknowns in the geometry and evolution of extensional sedimentary basins (mainly the normal drag–reverse drag geometrical problem) to be solved. The example chosen for checking these techniques is the Cameros Basin (northern Iberian Chain), formed during the Late Jurassic–Early Cretaceous, and inverted during the Tertiary compressional stage. According to the reconstruction proposed, the Cameros Basin can be characterised as a lens-shaped, syncline basin with maximum thickness of deposits of about 8000 m in its depocenter, gradually diminishing toward the North and South. The extensional geometry of the Cameros Basin is defined by: 1) a large-scale growth syncline over the major basement fault, with southward-dipping beds in the northern basin margin, forming a southward-opening sedimentary wedge, with upward-decreasing dip values (large-scale normal drag geometry); 2) roll-over anticlines at the contact between the pre-rift series against the normal fault limiting the basin (smaller-scale reverse drag geometry). The combined syncline–anticline syn-extensional structures were characterised by means of paleomagnetic analysis of a remagnetization acquired at the time of extension. Analogue models indicate that the development of syncline basin geometries in extensional settings, as the one interpreted for the Cameros Basin, is closely related to the presence of a detachment level in the pre-rift sequence above the main basement faults. [Copyright &y& Elsevier]

Abstract: Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0–7.2 km·s−1) in the lower crust of the shelf edge. The thin crust of the continent–ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (>30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (<5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to Newfoundland–Iberia rift. [Copyright &y& Elsevier]

Abstract: We present numerical models of extensional visco-elasto-plastic 3D continental lithosphere containing weaker areas within its mantle section. We aim at understanding the 3D crustal structure of volcanic passive margins that is characterized by both across-strike and along-strike finite strain gradients, with maxima around central igneous complex or their feeding magma reservoirs. It is suggested that localized hot melting zones within the lithosphere act as mechanical soft points and result in the local focusing of extension. To test this hypothesis 3D thermo-mechanical models of extensional continental lithosphere containing thermally induced soft points are implemented. Results show that crustal extension initiates and is focused over soft points in the mantle, reproducing the tectonic segmentation and zig-zag pattern of VPMs (volcanic passive margins). [Copyright &y& Elsevier]

Abstract: Magma-poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting, the serpentinization of the mantle beneath crust thinned to less than 8±2 km, and the unroofing of a broad zone of mantle within the continent–ocean transition, accompanied by the development of detachment and other large-offset faults. These observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes. Although the paucity of magmatism may be explained by depth-dependent extension of the lithosphere, and by pre-depleted sub-crustal lithosphere, rifting above initially cool sub-lithospheric mantle also may explain the subsidence deficit observed at some margins: synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere. As it extends, thins and cools, ductile creeping layers in the mid- and deep crust become progressively more brittle, resulting in increased coupling between the upper and lower crust, and eventually the embrittlement of the entire crust, faults cutting from the surface across the Moho, bringing water into the mantle and causing its serpentinization. Increased coupling and the development of serpentine detachments predict the development of late-stage asymmetry once the entire crust is brittle. Such detachments are imaged on some margins and inferred on others; analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric. Similar analyses show that depth-dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning. Instead this “extension discrepancy” may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting, related to the changes in the rheological character and strength of the lithosphere as it is thinned. Complex polyphase faulting continues after complete crustal separation, resulting in the exhumation of broad expanses of peridotitic basement, the top of which is everywhere marked by an exhumed slip surface, similar to the corrugated surface observed at mid-ocean ridges. The similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the COT and true oceanic crust difficult and possibly moot. [Copyright &y& Elsevier]

Abstract: The dynamics and evolution of rifts and continental rifted margins have been the subject of intense study and debate for many years and still remain the focus of active investigation. The 2006 AGU Fall Meeting session “Extensional Processes Leading to the Formation of Basins and Rifted Margins, From Volcanic to Magma-Limited” included several contributions that illustrated recent advances in our understanding of rifting processes, from the early stages of extension to breakup and incipient seafloor spreading. Following this session, we aimed to assemble a multi-disciplinary collection of papers focussing on the architecture, formation and evolution of continental rift zones and rifted margins. This Tectonophysics Special Issue “Role of magmatism in continental lithosphere extension” comprises 14 papers that present some of the recent insights on rift and rifted margins dynamics, emphasising the role of magmatism in extensional processes. The purpose of this contribution is to introduce these papers. [Copyright &y& Elsevier]

The offshore area of northern Taiwan is currently in an extensional regime, known for a post-collision area of the Taiwan mountain belt that has formed due to the oblique collision of the Philippine Sea Plate against the Eurasian continental margin since the late Miocene. The active collision has undergone a progressive migration of the Taiwan mountain belt from the northeast to the southwest direction. After the relaxation of the compressional stress due to the westward subduction of the Philippine Sea Plate, the northern Taiwan orogen has collapsed together with the opening of the southern Okinawa Trough. Here, we show the transitional process from compressional to extensional regime in the area off northern Taiwan and reconstruct the tectono-sedimentary evolution by using multi-channel reflection seismic data. We process and interpret multi-channel seismic profiles and use borehole data to determine the ages of sedimentary units and stratal surfaces/unconformities. Based on the seismic facies, we have recognized two distinctive domains separated by the Offshore Shanjiao Fault (OSF): a shelf basin to the northwest and a collapsed zone to the southeast. The tectono-sedimentary evolution off northern Taiwan since the late Miocene can be described in 4 stages. (1) The compressional structures off northern Taiwan were formed during the collision period starting around 6 Ma, a series of folds-and-thrusts aligned in the NE-SW direction. (2) Due to the oblique collision, some NW-SE-trending strike-slip faults were developed across the collision belt to accommodate the differential displacements between two adjacent segments of the folded belt. (3) During the transition from compressional to extensional regime at about 2.7 Ma, a post-collisional erosion and subsidence occurred in the collapsed zone to form the basal unconformity. This post-collisional collapse is associated with the opening of the southern Okinawa Trough, a back-arc basin, due to the westward migration of the Philippine Sea Plate subduction beneath the southern Ryukyu arc. (4) The offshore area of northern Taiwan has subsided continuously with sediment accumulation since the early Pleistocene (~2.0 Ma), accompanying the rifting of the southern Okinawa Trough. [ABSTRACT FROM AUTHOR]

Abstract: We analyse the present-day deformation in two key areas of the Taiwan collision belt, the Ilan Plain to the NW and the Pingtung Plain to the SW. Our approach is mainly based on consideration of horizontal displacement revealed by recent geodetic (GPS) surveys, derived strain rate tensors indicating horizontal deformation and three-dimensional seismotectonic stress regimes issued from inversion of focal mechanisms of earthquakes. We reconstruct a consistent, albeit complex, tectonic pattern involving non-rigid rotations (clockwise in NE Taiwan, anticlockwise in SW Taiwan), transitions from pure compression near the mountains to transtension near the sea and simple shear affecting the deforming domain (NW–SE left-lateral in NE Taiwan, NNE–SSW right-lateral in SW Taiwan). These tectonic patterns reflect lateral extrusion processes towards mechanically weak domains with respect to collision zone, i.e., adjacent subduction zones (Ryukyu to the NE, Manila to the SW). The extrusion of the Ilan Plain area occurs towards the SE, whereas that of the Pingtung Plain area occurs towards the SW. The extrusion in NE Taiwan is facilitated by the opening of the Okinawa Trough, so that velocity and deformation rates are higher than in SW Taiwan despite less active collision in this northern area. The symmetry of the extrusion patterns is altered by differences in relative positions and orientations of collision belt and adjacent subduction zones, which depend on the plate tectonic configuration and tectonic history of the Taiwan region. [Copyright &y& Elsevier]

Abstract: In November 2003, 15 ocean bottom seismometers were deployed in the southwestern Okinawa Trough. More than 3300 microearthquakes were located during the 10-days passive seismic experiment. The earthquake activity is characterized by the ceaseless occurrence of small earthquakes in the vicinity of all the instruments. The seismicity is essentially restricted to the central part of the Southwestern Okinawa Trough, except for one cluster of events situated in the southern part of it (cluster 2). The seismic activity terminates abruptly against the NE–SW trending prolongation of the Lishan fault. Most of the microearthquakes are aligned along the E–W trending normal faults, showing where the present-day active normal faulting occurs and how it accounts for the N–S extension in the Okinawa Trough. According to the P-wave velocity spectra estimated from some deep earthquakes located beneath the cross backarc volcanic trail area, the existence of a lower crustal/upper mantle magma chamber is confirmed by the presence of low frequency earthquakes in the 3–10 Hz bandwidth. [Copyright &y& Elsevier]

Available data suggest that the breakup of the Neoarchean Kenorland supercontinent at 2.5–2.4 Ga was likely triggered by a large mantle plume upwelling that caused significant magmatism. Here, we present 2D high-resolution magmatic-thermomechanical numerical models of extension of the continental crust underplated by a hot mantle plume material. Using this model, it is demonstrated that mantle plume underplating generates a large amount of mafic melt by decompression melting. This melt penetrates into the extending continental crust along normal faults thereby forming multiple generations of mafic dyke-like intrusions along normal faults. In case of extension velocity of 0.2–1 cm/yr, lower crustal heating and hot mafic melt emplacement may cause partial melting of the continental crust that can generate significant volume of felsic melts. This in turn triggers emplacement of felsic intrusions that temporarily and spatially associate with the mafic dyke-like intrusions. The modeling results agree well with geological data from the Karelian Craton and provide possible explanation for the observed association of Paleoproterozoic mafic dykes and felsic intrusions which formed in a relatively short time interval (up to 20 Myrs) in the early stages of the supercontinent breakup. • Melting of the plume produces multiple generations of mafic dyke-like intrusions in the extending continental crust. • Thermal impact of mantle plume with slow extension causes generation of felsic intrusions coeval with the mafic intrusions. • Results provide an explanation for the association of Paleoproterozoic mafic and felsic dykes within the Karelian Craton. [ABSTRACT FROM AUTHOR]

Abstract: We explore the sensitivity of extensional systems to the thermal structure of the lithosphere using numerical simulations that fully couple the energy, momentum and continuity equations. The rheology of the lithosphere is controlled by weakening processes, such as shear heating, that localises strain into shear zones and faults. Numerical models show that during extension of an initially unpatterned lithosphere, structures develop spontaneously out of basic thermodynamic energy fluxes, and without the imposition of ad hoc rules on strain localisation. This contrasts with the classical Mohr–Coulomb theory for brittle localisation, which prescribes the angles of faults by a mathematical rule. Our results show that the mode of extension is sensitive to subtle changes in rheology, heat flux and geometry of the system. This sensitivity lies at the core of the variety and complexity observed in extensional systems. Localisation processes make the lithosphere weaker than previously estimated from the Brace–Goetze quasi-static approach. Consequently, typical estimates for plate tectonic forces are capable of splitting the lithosphere under extension, even without the role of active magmatism. [Copyright &y& Elsevier]

Abstract: Lithospheric extension is a fundamental plate tectonic process controlling the collapse of mountain belts, the break-up of continents and the formation of new oceanic basins. This review summarises major advances on understanding: a) the nature of the ocean–continent transition, b) the origin and evolution of detachment faults, c) the dynamic strength of continental lithosphere during extension, and d) the role of magmatism during extension. Major steps have been made in mapping out the complexities of the ocean–continent transition zone, particularly in magma-poor rifted margins, where extensional structures are not masked by voluminous magmatism. The role of detachment faulting during rifting seems to be crucial and explains observations on depth-dependent stretching of the lithosphere. Despite new insights into the dynamic behaviour of the lithosphere, its rheological response remains a major uncertainty in modelling the evolution of extensional systems. Likewise, the multiple roles that basaltic magmatism may have in modifying extension remain a key question. Future insights into the global behaviour of extensional systems will be gained by constraining lithospheric rheology through integrating structural evolution and magmatism in the field, geodetic measurements of active extension, thermochronology, seismic surveys and drilling programs. [Copyright &y& Elsevier]

Abstract: Rollover anticlines are syn-sedimentary extensional structures, common in salt tectonics, that are classically considered to result from layer bending during hangingwall displacement above a listric normal fault. However, theoretical considerations and seismic images of natural examples put in question the validity of the listric fault model from which rollover growth is steady state, by nature, and controlled by the relative displacement of two blocks. We used laboratory experiments of gravity driven deformation of brittle–ductile models simulating salt tectonics processes to study in which circumstances rollovers are obtained both in time and space. It is argued that rollover growth is a nonsteady process that results from a mechanical instability whose critical lifetime can be divided in three characteristic stages: birth, growth, and decay. In kinematic terms, the development of a synthetic or antithetic rollover requires a three-block system: an upslope block and a downslope block that move downslope and that separate from each other on top of an extending décollement layer lying above a basement block. Synthetic and antithetic rollovers developed with upward concave shapes whatever the sense of shear in the salt layer and can pass laterally to each other without transform fault. This points out that the concave shape of a rollover base is not an argument in favour of listric faulting. The concave shape simply results from the connection between a steeply dipping normal fault and a flat lying or gently dipping décollement. [Copyright &y& Elsevier]

Abstract: Numerical modelling, incorporating coupling between surface processes and induced flow in the lower continental crust, is used to address the Quaternary evolution of the Gulf of Corinth region in central Greece. The post-Early Pleistocene marine depocentre beneath this Gulf overlies the northern margin of an older (Early Pleistocene and earlier) lacustrine basin, the Proto Gulf of Corinth Basin or PGCB. In the late Early Pleistocene, relief in this region was minimal but, subsequently, dramatic relief has developed, involving the creation of ∼900 m of bathymetry within the Gulf and the uplift by many hundreds of metres of the part of the PGCB, south of the modern Gulf, which forms the Gulf''s main sediment supply. It is assumed that, as a result of climate change around 0.9 Ma, erosion of this sediment source region and re-deposition of this material within the Gulf began, both processes occurring at spatial average rates of ∼0.2 mm a−1. Modelling of the resulting isostatic response indicates that the local effective viscosity of the lower crust is ∼4×1019 Pa s, indicating a Moho temperature of ∼560 °C. It predicts that the ∼10 mm a−1 of extension across this ∼70 km wide model region, at an extensional strain rate of ∼0.15 Ma−1, is partitioned with ∼3 mm a−1 across the sediment source, ∼2 mm a−1 across the depocentre, and ∼5 mm a−1 across the ‘hinge zone’ in between, the latter value being an estimate of the extension rate on normal faults forming the major topographic escarpment at the southern margin of the Gulf. This modelling confirms the view, suggested previously, that coupling between this depocentre and sediment source by lower-crustal flow can explain the dramatic development in local relief since the late Early Pleistocene. The effective viscosity of the lower crust in this region is not particularly low; the strong coupling interpreted between the sediment source and depocentre results instead from their close proximity. In detail, the effective viscosity of the lower crust is expected to decrease northward across this model region, due to the northward increase in exposure of the base of the continental lithosphere to the asthenosphere; in the south the two are separated by the subducting Hellenic slab. The isostatic consequences of such a lateral variation in viscosity provide a natural explanation for why, since ∼0.9 Ma, the modern Gulf has developed asymmetrically over the northern part of the PGCB, leaving the rest of the PGCB to act as its sediment source. [Copyright &y& Elsevier]

Abstract: Since the early 1990s, a prolific literature has developed on the interpretation within western Turkey of apparent Mid to Late Cenozoic low-angle normal faults. Extension on these structures has been thought responsible for the exhumation of this region''s principal metamorphic massif, the Menderes Massif, which has thus been interpreted as a metamorphic core complex. Nonetheless, no convincing supporting structural evidence has emerged: some reported instances of low-angle normal faulting affecting the Menderes Massif can be shown from field relationships to have formed as steep normal faults and to have since become back-tilted; others seem to be misinterpretations of structures with no demonstrable relationship to extension at all. The main evidence for low-angle normal faulting in this region has instead emerged through thermochronology, which indicates highly non-uniform cooling histories that have seemed to lack any other explanation. However, this evidence can alternatively be explained by a combination of the cooling effects caused by flat subduction and by erosion. There is thus no evidence for Cenozoic low-angle normal faulting affecting the Menderes Massif. Extension started at ∼11 Ma in parts of western Turkey; but most of this region''s Late Cenozoic extension—on normal faults with steep initial dips—has occurred since ∼7 Ma, synchronous with slip on the North Anatolian Fault Zone. [Copyright &y& Elsevier]

Abstract: Analysis of structural rift architecture shows that the graben formation is commonly controlled by the contemporaneous activity of two fault trends with an angular obliquity of approximately 40°. Inspection of the crustal basement and geophysical data reveals that these faults are parallel to inherited oblique crustal and lithospheric discrete fabrics, which are reactivated during the extension event. We conducted experiments at crustal scale to determine the role of the coeval reactivation of such oblique inherited fabrics in the graben and depocentre development. Experimentally the oblique inherited lithospheric faults were simulated by a basal discrete velocity discontinuity (VD) characterised by two different angles (α) with respect to the extension direction at the intersection of the VDs. Our models show that besides the extension direction which induces the formation of linear or independent en-echelon grabens, the intersection of the two oblique VD segments controls the location of the depocentre and concentrates subsidence. For different stretching direction values, the depocentre geometry varies from strongly asymmetric to symmetric when the stretching direction corresponds to the bisecting line of the two VD segments. Applied to the Upper Rhine graben (central segment of the West European rift), our models allow interpretation of the development of the Late Eocene–Oligocene depocentres at the intersection of two main oblique inherited structures, as the result of a constant NW–SE extension direction. Concerning the southern East African rift, the graben geometries and the evolution of the subsidence during the Mio-Pliocene period may be explained by the reactivation of two main Pan-African inherited shear zones with an E–W direction of extension. [Copyright &y& Elsevier]

Abstract: We determine seismic strain rate of tectonic earthquakes along the Central America Volcanic Arc. We then compare this result to those obtained from earthquakes related to the convergence of the Cocos and Caribbean plates and to earthquakes in the back-arc region of northern Central America. The seismic strain-rate tensor for shallow-focus earthquakes along the Central America volcanic arc since 1700, has a compressive eigenvector with a magnitude of 0.7×10−8 year−1, and oriented in a 357° azimuth. The extensive eigenvector is oriented in a 86° azimuth, with a magnitude of 0.82×10−8 year−1. When only Centroid Moment-tensor solutions (CMT) are considered, the respective eigenvectors are 1.2×10−8 year−1 and 1.0×10−8 year−1. The compressive eigenvector from the seismic strain-rate tensor for earthquakes along the Cocos-Caribbean convergent margin is 2.0×10−8 year−1, plunging at 25°, and oriented in a 29° azimuth. Its magnitude and direction are similar to those of the compressive eigenvector for earthquakes along the volcanic arc. The extensive eigenvector along the convergent margin, on the other hand, has a large vertical component. The compressive and extensive eigevenvectors are 4.9×10−8 year−1 and 4.6×10−8 year−1, using only CMTs as the database. Earthquakes along the grabens of northern Central America yield a seismic strain-rate tensor whose extensive eigenvector has a magnitude of 2.4×10−8 year−1, oriented in a 109° azimuth. Magnitude and direction are similar to those of the extensive eigenvector for earthquakes along the volcanic arc. The compressive eigenvector along the grabens is practically vertical. Similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along the Central America volcanic arc is the result of compression along the convergent Cocos-Caribbean margin, and extension in the back-arc region, along the grabens of northern Central America. This field is resolved as strike-slip faulting along the arc. [Copyright &y& Elsevier]

Abstract: The geometry of extensional structures is described for the first time in the active setting of the Venezuelan Andes using remote sensing imagery. We favored the use of a mosaic of Synthetic Aperture Radar (SAR) scenes of the Japanese Earth Resources Satellite-1 (JERS-1) assisted by complementary remote sensing devices (Landsat TM, digital elevation models) and field observations to make a structural analysis at regional scale. Radar images are particularly efficient in the Venezuelan Andes where dense vegetation and frequent cloud covering earlier lent difficulties to remote sensing studies. We focused our analysis in the Valera–Rio Momboy and Bocono faults corner and in the Mucujun area. We show that, in an area where ongoing compression and strike–slip deformations occur, brittle extension can be detected independently from previous knowledge. Extensional structures correspond to elongated tilted blocks with dimension less than 10 km in width. Blocks are bounded by curved faults in plan view, the concavity being turned towards the axial part of the belt. The geometry and kinematics of such structures suggest that syn-orogenic extension started together with initiation of right-lateral strike–slip motion along the Bocono Fault in the Plio-Quaternary. [Copyright &y& Elsevier]