[1] The northward motion of the Pamir indenter with respect to Eurasia has resulted in coeval thrusting, strike-slip and normal faulting. The eastern Pamir is currently deformed by east-west oriented extension, accompanied by uplift and exhumation of the Kongur Shan (7719 m) and Muztagh Ata (7546 m) gneiss domes. Both domes are an integral part of the footwall of the Kongur Shan Extensional System (KES), a 250-km-long, north-south oriented graben. Why active normal faulting within the Pamir is primarily localized along the KES and not distributed more widely throughout the orogen, has remained unclear. In addition, relatively little is known about how deformation has evolved throughout the Cenozoic, despite refined estimates on present-day crustal deformation rates and microseismicity, which indicate where crustal deformation is presently being accommodated. To better constrain the spatiotemporal evolution of faulting along the KES, we present 39 new apatite fission-track, zircon U-Th-Sm/He, and ⁴⁰Ar/³⁹Ar cooling ages from a series of footwall transects along the KES graben shoulder. Combining this data with present-day topographic relief, 1D thermo-kinematic and exhumational modeling documents successive stages, rather than synchronous deformation and gneiss dome exhumation. While the exhumation of the Kongur-Shan-commenced during the late Miocene, extensional processes in the Muztagh Ata massif began earlier and has slowed down since the late Miocene. We present a new model of synorogenic extension, suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting Alai slab localizes extensional upper-plate deformation along the KES and decouples crustal motion between the Central/Western Pamir and Eastern Pamir/Tarim basin.

[1] At the NW corner of the Pacific region, just south of the Kamchatsky Peninsula, the northern tip of Pacific plate subduction and associated volcanic arc interacts with the western end of the Aleutian-Komandorsky dextral transform plate boundary and associated arc. Study of both Holocene and Pleistocene sequences of uplifted marine terraces and also of fluvial drainage patterns on the Kamchatsky Peninsula allows us to highlight active tectonics produced by complex plate interaction. Our results show that the central - eastern coast of the peninsula is currently divided into four different zones consisting in uplifted blocks associated with various uplift rates in front of a fold and thrust zone to the west. Our main tectonic benchmark--the altitude of the shoreline correlated to last interglacial maximum (MIS 5e)--yields Late Pleistocene uplift rates ranging from 0.2 to 2.74 mm/yr. One of the main active faults bounding the coastal blocks is dextral and is interpreted as a prolongation of an offshore fault of the Aleutian-Komandorsky dextral transform plate boundary. We suggest that structures on the Kamchatsky Peninsula accommodate a part of the transform motion, but that mainly the arc-continent collision of the Aleutian arc against Kamchatka produces a “bulldozer” effect on the Kamchatsky Peninsula.

[1] We analyze the tectono-sedimentary and thermochronometric constraints of the Tertiary Piedmont Basin (TPB) and its adjoining orogen, the Ligurian Alps, providing new insights on the basin evolution in response to a changing geodynamic setting. The geometry of the post-metamorphic faults of the Ligurian belt as well as the fault network that controlled the Oligo-Miocene TPB deposition have been characterised through a detailed structural analysis. Three main faulting stages have been distinguished and dated thanks to the relationships among faults and basin stratigraphy and thermochronometric data. The first stage (F1, Rupelian-Early Chattian) is related to the development of extensional NNW-directed faults, which controlled the exhumation of the orogen and the deposition of nearshore clastics. During the Late Chattian the basin drowning is marked by mudstones and turbidites, which deposition was influenced by the second faulting stage (F2). This phase was mainly characterised by NE- to ENE-striking faults developed within a transtensional zone. Since the Miocene the whole area was dominated by transpressive tectonics. The sedimentation was represented by a condensed succession followed by a very thick, turbiditic complex. At the regional scale, this succession of events reflects the major geodynamic reorganization in the central Mediterranean region during the Oligo-Miocene times, induced by the late-collisional processes of the Alps, by the eastward migration of the Apennines subduction and by the opening of extensional basins (i. e. the Liguro-Provençal Ocean).

[1] Structural studies of synorogenic basins in Timor using field and remote sensing techniques provide new structural and geomorphic evidence for syn-collisional extension in the converging plate boundary zone between the Australian Plate and Banda Arc. Fault mapping and kinematic analysis at scales ranging from outcrop (<1 m²) to the dimensions of the active orogen in East Timor (~100 km²) identify a predominance of NW-SE oriented dextral-normal faults and NE-SW oriented sinistral-normal faults that collectively bound large (5-20 km²) bedrock massifs throughout the island. These fault systems intersect at non-Andersonian conjugate angles of approximately 120° and accommodate an estimated 20 km of NE-directed extension across the Timor orogen based on reconstructions of fault-dismembered massifs. Major orogen-parallel ENE-oriented faults on the northern and southern sides of Timor exhibit normal-sinistral and normal-dextral kinematics, respectively. The overall pattern of deformation is one of lateral crustal extrusion sub-parallel to the Banda Arc. Stratigraphic relationships suggest that extrusion began prior to 5.5 Ma, before pronounced rapid uplift of the orogen. We link this to progressive coupling of the arc to an underthrust plateau on the Australian Plate, and subduction of its ocean crust. Our results enable us to track the structural evolution of the upper crust during dramatic plate-boundary reorganizations accompanying the transition from subduction to collision. The deformation structures that we document suggest that both upper and lower plate deformation during incipient island arc-continent collision was largely controlled by the geometry and topography of the lower plate.

[1] Apatite (AHe) and Zircon (ZHe) (U-Th)/He thermochronometric data from the southern Wassuk Range (WR) coupled with ⁴⁰Ar/³⁹Ar age data from the overlying tilted Tertiary section are used to constrain the thermal evolution of an extensional accommodation zone and tilt-domain boundary. AHe and ZHe data record two episodes of rapid cooling related to the tectonic exhumation of the WR fault block beginning at ~15 Ma and ~4 Ma. Extension was accommodated through fault-block rotation and variably tilted the southern WR to the west from ~60° -70° in the central WR to ~15° - 35° in the southernmost WR and Pine Grove Hills (PGH), and minimal tilting in the Anchorite Hills and along the Mina Deflection to the south. Middle Miocene geothermal gradient estimates record heating immediately prior to large-magnitude extension that was likely coeval with the extrusion of the Lincoln Flat andesite at ~14.8 Ma. Geothermal gradients increase from ~19° ± 4 ° C/km to ≥ 65° ± 20 ° C/km towards the Mina Deflection, suggesting that it was the focus of Middle Miocene arc magmatism in the upper crust. The decreasing thickness of tilt blocks towards the south resulted from a shallowing brittle/ductile transition zone. Post-magmatic Middle Miocene extension and fault-block advection were focused in the northern and central WR and coincidentally moderated the large lateral thermal gradient within the uppermost crust.

[1] The volcanic basement of the Oregon and Washington Coast Ranges has been proposed to represent a pair of tracks of the Yellowstone hotspot formed at a mid-ocean ridge during the early Cenozoic. This interpretation has been questioned on many grounds, especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and North America. Updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised. Forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55-49 Ma, and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments. This kinematic model indicates that there is no firm reason to reject the near-ridge hotspot hypothesis on the basis of plate motions.

[2] A novel element of the model is the Resurrection plate, previously proposed to exist between the Farallon and Kula plates. By including the defunct Resurrection plate in our reconstruction, we are able to model the Farallon hotspot track as docking against the Oregon subduction margin starting about 53 Ma, followed by docking of the Resurrection track to the north starting about 48 Ma. Accretion of the Farallon plate fragment and partial subduction of the Resurrection fragment complicates the three-dimensional structure of the modern Cascadia forearc. We interpret the so-called ‘E’ layer beneath Vancouver Island to be part of the Resurrection fragment. Our new kinematic model of mobile terranes within the Paleogene North American plate boundary allows re-interpretation of the three-dimensional structure of the Cascadia forearc and its relationship to ongoing seismotectonic processes.

[1] A series of high-resolution seismic reflection surveys was carried out in 2008, 2010, and 2011, providing a total of 5 new seismic profiles constraining the location and character of the Meeman-Shelby Fault (MSF), about 9 km west of Memphis, Tennessee in the Central U.S. The MSF is the best documented fault closest to Memphis yet discovered, and shows a recurrent fault history. The fault, as imaged by the reflection profiles, is ~45 km-long, strikes N25° E, and dips west-northwest ~83°, exhibiting an up-to-the-west sense of motion with a possible right-lateral strike-slip component. The data show that on average the MSF offsets the Paleozoic unit ~77 m and folds the top of the Cretaceous unit and the Paleocene-Eocene Wilcox Group ~44 and ~25 m, respectively. One seismic profile acquired along the Mississippi River images the bottom of the Quaternary alluvium was warped up ~ 28 m, indicating recent activity of the MSF. Calculated vertical slip rates of the MSF during the deposition of the Upper Cretaceous, Paleocene, Eocene, and Quaternary sediments are 0.0022, 0.0010, 0.0004, and 0.2154 mm/yr, respectively, suggesting an increase in fault activity during the Quaternary. Consistent with the present stress field and the deformation of the New Madrid seismic zone fault system, we interpret the MSF as a P-shear fault in the context of a left-stepping, right-lateral constraining strike-slip fault system under a nearly east-west oriented compressional stress field. Source scaling estimates indicate that the MSF is capable of generating a M6.9 earthquake if rupturing in one event.

[1] This paper deals with the Tertiary tectonic and sedimentary evolution of the western Tarim basin based on an integrated stratigraphic, sedimentary, structural and tectonic analysis. Basin evolution is divided into three stages: Paleogene, Miocene and Pliocene. The western Tarim basin was the easternmost part of the Tethyan realm from Late Cretaceous to Paleogene, and marine sedimentation continued into the Early Miocene. Miocene development of the western Tarim basin was chiefly governed by West Kunlun right-slip faulting and the simultaneous northward thrusting of the Pamir salient and Tianshuihai terrane. Yecheng subbasin developed as a pull-apart basin owing to synchronous activity of the West Kunlun and the Shache-Yangdaman right-slip faults. Hotan foreland basin formed in response to northward displacement of the Tianshuihai terrane, and another might have developed in front of the advancing Pamir salient in the Miocene. Basinward thrusting became predominant in the orogenic belts adjacent to the western Tarim basin in the Pliocene. North-directed displacement and uplift of the Tiklik thrust terrane fragmented the pre-existing Hotan foreland basin, and collision of the Pamir with the southern Tian Shan deformational fronts caused complete destruction of the Miocene Pamir foreland basin.

[2] Eastward displacement of the Qimugen fold-thrust system led to flexural subsidence of the Yecheng subbasin in the Pliocene. Kashi subbasin developed as part of the southern Tian Shan foreland basin, and was controlled by the eastern Pamir as well. A tectonic scenario is proposed to illustrate complicated interplay of the western Tarim basin with its peripheral orogens in the Tertiary.

[1] Contrasting models of upper crustal shortening versus lower crustal flow have been proposed to explain the formation of thickened crust in the Longmen Shan (LMS), eastern Tibetan Plateau (TP) margin. These models require different structural kinematics along the LMS, whose structural geometry is defined by three parallel NW-dipping fault zones. From foreland (southeast) to hinterland (northwest), they are the Guanxian-Anxian Fault, Yingxiu-Beichuan Fault (YBF) and Wenchuan-Maowen Fault (WMF). Newly derived and previously published low-temperature thermochronology data from the LMS were synthesized to constrain the spatial exhumation and test previous models. The results show that: (1) exhumation increases abruptly across the range-bounding YBF, suggesting the fault being the main thrust boundary between the LMS and the Sichuan Basin to the east; (2) Younger Late Cenozoic cooling ages are found on the hinterland WMF, where a dichotomy of ages on the hanging wall versus footwall suggests Late Cenozoic thrust activity. (3) Towards the hinterland to the west, exhumation rates decrease twofold over a distance of ~30–40 km. This exhumation pattern indicates a westward decrease of tectonic uplift, providing the regional topography approached a steady-state, whereby exhumation is in balance with tectonic uplift. The observed exhumation estimates support an upper crustal configuration where thrusts in the LMS merge gradually into a gentle detachment seated at a depth of ~20–30 km. Results of this study support a revised upper crustal thrusting model.

[1] Results of a detailed field study of the southern onshore portion of New Zealand's Alpine Fault reveal that for 75 km along strike, dextral-normal slip on this long-lived structure is highly-localized in phyllosilicate-rich fault core gouges and along their contact with more competent rocks. At three localities (Martyr River, McKenzie Creek, Hokuri Creek), we document complete cross sections through the fault. New ⁴⁰Ar/³⁹Ar dates on mylonites, combined with microstructural and mechanical data on phyllosilicate-rich fault core gouges show that modern slip is localized onto a single, steeply-dipping 1 to 12 m-thick fault core composed of impermeable (k = 10^-20 to 10^-22 m²), frictionally weak (μ_s = 0.12 – 0.37), velocity-strengthening, illite-chlorite and saponite-chlorite-lizardite fault gouges. Fault core materials are (1) comparable to those of other major weak-cored faults (e.g., San Andreas Fault), and (2) most compatible with fault creep, despite paleoseismic evidence of quasi-periodic large magnitude earthquakes (M_w > 7) on this portion of the Alpine Fault. We conclude that frictional properties of gouges at the surface do not characterize the overall seismogenic behavior of the southern Alpine Fault.

[1] Basement-cored ranges formed by reverse faulting within intra-continental mountain belts are often composed of poly-deformed lithologies. Geological data capable of constraining the timing, magnitude and distribution of the most recent deformational phase is usually missing in such ranges. In this paper, we present new low temperature thermochronological and geological data from a transect through the basement-cored Terskey Range, located in the Kyrgyz Tien Shan. Using this data, we are able to investigate the range's Late Cenozoic deformation for the first time. Displacements on reactivated faults are constrained and deformation of thermochronologically-derived structural markers is assessed. These structural markers post-date the earlier deformational phases, providing the only record of Cenozoic deformation and of the reactivation of structures within the Terskey Range. Overall, these structural markers have a southern inclination, interpreted to reflect the decreasing inclination of the reverse fault bounding the Terskey Range. Our thermochronological data is also used to investigate spatial and temporal variations in the exhumation of the Terskey Range, identifying a three stage Cenozoic exhumation history: (1) virtually no exhumation in the Paleogene, (2) increase to slightly higher exhumation rates at ~26–20 Ma, and (3) significant increase in exhumation starting at ~10 Ma.

[1] Svalbard is an anomalous, subaerial part of the Barents Shelf, Northeast Atlantic Ocean. In this study we performed both, one and two-dimensional subsidence analyses based on basin structure, water depth and thermochronology, to quantify and date the phases of uplift affecting Svalbard during the Cenozoic. Svalbard has experienced two phases of uplift, from > 36to ~10 Ma, and since ~10 Ma, similar in timing to uplift phases identified in Greenland, Scandinavia and the Barents Shelf. Total uplift across much of the Central Tertiary Basin of Svalbard is > 1.5 km and exceeds 2.5 km in parts of the West Spitsbergen Foldbelt (WSFB). Uplift from > 36 to ~10 Ma accounts for the greatest part of the vertical motion and like the younger phase reduces in magnitude towards the east. Flexural rigidity of the lithosphere is estimated to be low (Te ≈ 5 km), so that post -36 Ma erosion of the WSFB contributes little to the uplift, whose permanent nature and proximity to the synchronous Yermak Plateau favors a link to regional magmatic underplating. Plume dynamic support and flexural unloading along the western transform plate margin can be ruled out as influences on vertical motions. Since ~10 Ma renewed uplift, generating the modern topography may be linked to thermal erosion of the mantle lithosphere under Svalbard. We suggest that a likely cause of much of the surface uplift is the northward propagation of the Knipovich Ridge to establish continuous seafloor spreading through the Fram Strait after ~10 Ma.

[1] The Greater Caucasus are the northern most extent of the Arabia-Eurasia collision and are thought to represent the main locus of shortening within the central portion of the collision zone between 40º and 48ºE. Recent work suggests that in detail, since the Plio-Pleistocene, much of the shortening in the eastern portion of the Caucasus system has been focused within the Kura Fold-Thrust belt along the southeastern margin of the Greater Caucasus. Here we present new field mapping and stratigraphic investigations of the eastern termination of the Kura fold-thrust belt in Azerbaijan to better constrain the structural geometries, magnitude of shortening, and initiation age for this portion of the fold-thrust belt. Our work suggests that this area of the fold-thrust belt exhibits significant along-strike variations in structural style and evolution, and can effectively be divided into two distinct domains at ~48ºE. The western domain is characterized by a subcritical median surface slope and isolated folds and thrusts propagating out of sequence, whereas the eastern domain is dominated by a single duplex structure and a history of in-sequence development in a critically tapered wedge. We hypothesize that these variations result from changes in relative rates of syn-tectonic sedimentation, erosion, and convergence velocity along-strike. We find that within the western domain, the fold-thrust belt has accommodated ~12 km of total shortening. An unconformity within the western domain brackets the initiation age of this portion of the fold-thrust belt to between 1.8 and 0.88 Ma yielding permissible average shortening rates of between 6.7 and 13.6 mm/yr. Comparison of these average shortening rates to the geodetically measured shortening rate of 8 mm/yr indicate that since initiation, the fold-thrust belt has accommodated 83–100% of convergence between the Greater and Lesser Caucasus at this longitude.

[1] In the Aegean back-arc domain, some 30–35 Ma ago, an increase of the rate of slab retreat led to the initiation of post-orogenic extension, largely accommodated by large-scale structures such as the North Cycladic Detachment System (NCDS). Although this extension is still active nowadays, an E-W compressional regime developed in the Late Miocene with the propagation of the North Anatolian Fault. On Mykonos island (Cyclades), the NE-SW back-arc extension is particularly well expressed with the Livada and Mykonos detachments that belong to the NCDS and that are associated with NW-SE barite veins emplaced during the synkinematic cooling of the Mykonos intrusion. This study shows that the formation of the mineralization occurred when the pluton crossed the ductile-to-brittle transition during its exhumation below the NCDS at ~11-10 Ma. In addition, the kinematics of mineralized structures evolved with time: (1) most of the displacement was accommodated by the top-to-the-NE Livada and Mykonos detachments accompanied by the formation of mineralized normal faults that were (2) reworked in a strike-slip regime with an E-W direction of shortening and a persistent NE-SW stretching and (3) a latepost-mineralization E-W compressional stage with a minor reworking of shallow-dipping faults (locally including the detachments themselves). We interpret this increase of the E-W shortening component recorded during the mineraldeposition as a consequence of the initiation of the westward motion of Anatolia from 10 Ma, thus 4 Ma before the propagation of the North Anatolian Fault in the Dardanelles Strait and the localization of the strain on the Aegean Sea margins.

[1] Morphotectonic analysis and fault numeric modeling of uplifted marine terraces along the Ionian Sea coast of the Southern Apennines allowed us to place quantitative constraints on Middle Pleistocene-Holocene deformation. Ten terrace orders uplifted to as much as +660 m were mapped along ~80 km of the Taranto Gulf coastline. The shorelines document both a regional and a local, fault-induced contribution to uplift. The intermingling between the two deformation sources is attested by three 10 km-scale undulations superimposed on a 100-km scale northeastward tilt. The undulations spatially coincide with the trace of NW-SE striking transpressional faults that affected the coastal range during the Early Pleistocene. To test whether fault activity continued to the present, we modeled the differential uplift of marine terraces as progressive elastic displacement above blind oblique-thrust ramps seated beneath the coast. Through an iterative and mathematically based procedure we defined the best geometric and kinematic fault parameters as well as the number and position of fault segments. Fault numerical models predict two fault-propagation folds cored by blind thrusts with slip rates ranging from 0.5 to 0.7 mm/a and capable of generating an earthquake with a maximum moment magnitude of 5.9-6.3. Notably, we find that the locus of predominant activity has repeatedly shifted between the two fault systems during time, and that slip rates on each fault have temporally changed. It is not clear if the active deformation is seismogenic or dominated by aseismic creep; however, the modeled faults are embedded in an offshore transpressional belt that may have sourced historical earthquakes.

[1] A crustal geotraverse through the Iberian Variscides is presented by integrating the available geological and seismic profiling data. Different modes of orogenic shortening are identified, with varying degrees of coupling between upper and lower crust. In northern and southern regions of the geotraverse, a decoupling in the middle crust permits the lower crust to subduct / underthrust, thus compensating for strong upper crustal shortening. This behaviour does not result in great crustal thickening, except in sectors of crustal underthrusting. In southern Central Iberia, moderate upper crustal shortening is due to a mechanically strong lower crust impeded to subduct/underthrust. In this region, shortening is partitioned between upper and lower crust, deformation being distributed in the upper crust while localized at major fault zones in the lower crust. Finally, the central region of the geotraverse (northern Central Iberia) shows a coupled crustal deformation, having given way to the largest orogenic thickening in the Iberian Variscides. The thermal maturity of this much thickened crust originated voluminous crustal melting, and concomitant normal-fault detachments developed while shortening dominated in other regions. Theoretical models suggest that compressive stresses may prevail in the lower crust beneath the extending upper crust, thus explaining the efficient syn-collisional exhumation in this part of the orogen. A particular feature of the Iberian Variscan geotraverse is the great importance of out-of-section mass movements, mainly left-lateral shear zones concentrated in two suture boundaries, which displaced to the NW (present coordinates) central and northern Iberia with respect to southern Iberia.

[1] The Sierra de Aconquija and Cumbres Calchaquíes in the thick-skinned northern Sierras Pampeanas, NW Argentina present an ideal setting to investigate the tectonically and erosionally controlled exhumation and uplift history of mountain ranges using thermochronological methods. Although these ranges are located along strike of one another, their spatio-temporal evolution varies significantly. Integrating modeled cooling histories constrained by K-Ar ages of muscovite and biotite, apatite fission track data as well as (U-Th)/He measurement of zircon and apatite reveals the structural evolution of these ranges beginning in the late stage of the Paleozoic Famatinian Orogeny. Following localized rift-related exhumation in the central part of the study area and slow erosion elsewhere, growth of the modern topography commenced in the Cenozoic during Andean deformation. The main activity occurred during the Late Miocene, with varying magnitudes of rock uplift, surface uplift, and exhumation in the two mountain ranges. The Cumbres Calchaquíes is characterized by a total of 5–7 km of vertical rock uplift, around 3 km of crestal surface uplift, and a maximum exhumation of 2–4 km since that time. The Sierra de Aconquija experienced 10–13 km of vertical rock uplift, ~4–5 km of peak surface uplift, and 6–8 km of exhumation since around 9 Ma. Much of this exhumation occurred along a previously poorly recognized fault. Miocene reactivation of Cretaceous rift structures may explain along strike variations within these ranges. Dating of sedimentary samples from adjacent basins support the evolutionary model developed for the mountain ranges.

[1] The Qinling orogen is an important orogenic belt formed by the collision between the North and South China blocks along the Mianlue suture during the Triassic. The orogen is customarily divided into the western and eastern Qinling terranes. However, the boundary has long been a matter of debate. There are many Triassic granitoid plutons in the orogen, especially in South Qinling, along the southern part of the Shangdan suture. Systematic analysis of U-Pb ages and Hf isotopes in zircons from six granitoid plutons, including Guangtoushan, Gaoqiaopu, Xiba, Laocheng, Dongjiangkou, and Zhashui from west to east, allows us to trace their formation ages and source regions. All plutons yield ages that vary only from 218 to 211 Ma. However, zircon Hf-isotope data for these Triassic plutons cluster in two distinct groups. Granitoids from the western segment (i.e. Guangtoushan, Gaoqiaopu and Xiba) of the South Qinling belt have negative εHf(t) values (-20.9 to -5.2) and relatively old two-stage Hf model ages (1.58 to 2.57 Ga). In contrast, those from the eastern segment (i.e. Laocheng, Dongjiangkou and Zhashui) show higher εHf(t) values (-5.4 to +6.8) and younger two-stage Hf model ages (0.82 to 1.60 Ga). Integrating these analyses with published Sr-Nd-Pb-Hf isotopic data, we suggest that the division between the western and eastern South Qinling segments is located near the Taibai-Chenggu line, between the Baoji-Chengdu railway and 108°E longitude. The integrated data suggest that the western South Qinling segment separated from the North China block during the Paleoproterozoic and early Mesoproterozoic, then switched into continental convergence with the eastern South Qinling segment (northern margin of the Yangtze block) during the late Meso- to early Neoproterozoic; finally, the two segments amalgamated during the late Neoproterozoic.

[1] The Nankai accretionary complex is the most recent addition to the accretionary complexes of southwest Japan and has preserved a record of sediment flux to the trench during its construction. In this study we use U-Pb zircon and fission track analysis of both zircons and apatites from sediments taken from the forearc and trench of the Nankai Trough, as well as rivers from southwest Japan to examine the exhumation history of the margin since the Middle Miocene. Modern rivers show a flux dominated by erosion of the Mesozoic-Eocene Shimanto and Sanbagawa accretionary complexes. Only the Fuji River, draining the collision zone between the Izu and Honshu arcs, is unique in showing much faster exhumation. Sediment from the Izu-Honshu collision is not found 350–500 km along the margin offshore Kyushu indicating limited along strike sediment transport. Sediment deposited since 2 Ma on the mid-trench slope offshore the Muroto Peninsula of Shikoku (ODP Site 1176) and on the lower slope trenchward of the Kumano Basin (IODP Sites C0006E and C00007E) shares the dominant source in the Shimanto and Sanbagawa Complexes seen in the modern rivers. Prior to 5 Ma additional sediment was being sourced from further north in more slowly exhumed terrains, ~350 km from the trench axis. Around 9.4 Ma U-Pb zircon ages of ~1800 Ma indicate enhanced erosion from the North China Craton, exposed in northern Honshu. In the middle Miocene, at ~15.4 Ma, the sediment was being derived from a much wider area including the Yangtze Craton (U-Pb ages ~800 Ma). We suggest that this enhanced catchment may have reflected the influence of the Yangtze River in supplying into the Shikoku Basin prior to rifting of the Okinawa Trough at 10 Ma and migration of the Palau-Kyushu Ridge to form a barrier to transport.

[2] The restriction of Nankai Trough provenance to Mesozoic source partly reflects continued uplift of the Shimanto and Sanbagawa Complexes since the Middle Miocene.

[1] The deformation and exhumation history of an orogen reflects the interactions between tectonic and surface processes. We investigate orogenic wedge deformation, erosion, and sedimentation in the Pyrenees by (a) quantifying the spatiotemporal patterns of exhumation across the southern fold-thrust belt with bedrock apatite fission-track (AFT) thermochronology and (b) comparing the results with existing deformation, exhumation, and sedimentation chronologies. Eighteen new samples record exhumation during and after orogenesis between 90 to 10 Ma. Rocks from the range core (Axial Zone) record rapid exhumation that progresses east to west and north to south consistent with patterns of tectonically-driven uplift. Synorogenic sediments shed into piggyback basins on the southern fold-thrust belt during mountain building retain a detrital exhumation signal from the Axial Zone. In contrast, samples from other structural positions record exhumation of the thin-skinned Pyrenean thrust sheets, suggesting sedimentary burial and heating of sufficient magnitudes to reset the AFT system (> ~3 km). In some locations, exhumation of these fold-thrust structures is likely an erosional response to thrust-driven rock uplift. We identify an exhumation phase ~25-20 Ma that occurs along the entire central and eastern portions of the Spanish Pyrenees at the boundary between thick- and thin-skinned portions of the wedge. We suggest that this distributed exhumation event records (a) a taper response in the southern orogenic wedge to sediment loading and/or (b) a shift to wetter, stormier climate conditions following convergence-driven uplift and full topographic development. A final exhumation phase between ~20-10 Ma may record the excavation of the southern fold-thrust system following base level lowering in the Ebro Basin.

[1] K-Ar dating of fault rocks coupled with hydrogen isotope analysis allows constraining the timing of brittle faulting and the influx of meteoric fluids into such fault systems. Here we apply this approach to resolve the spatio-temporal activity of three detachment-fault systems in western Turkey and to evaluate how deep meteoric fluids infiltrated these fault systems. K-Ar ages of cataclasites and gouges from two detachment fault systems that accomplished the bivergent extension of the central Menderes Massif suggest diachronous brittle deformation. The Büyük Menderes detachment in the south was already active at ~22 Ma, whereas the earliest brittle deformation recorded at the Gediz fault system in the north occurred at ~9 Ma. K-Ar ages of secondary and splay faults indicate that both fault systems continued to be active until 4–3 Ma – consistent with rapid Pliocene cooling inferred from published thermochronological data. In the northern Menderes Massif, the boundary fault of the Simav graben became active at 17–16 Ma, after the end of faulting on the Simav detachment. Hydrogen isotope (δD) values of –109 to –87 ‰ for fault gouge, cataclasite, and mylonites document that meteoric fluids infiltrated the upper crustal normal faults and penetrated into the detachments and the uppermost levels of their mylonitic footwalls. This explains the ubiquitous retrogression of biotite to chlorite in extensional shear zones and the growth of chlorite in detachment-related cataclasites. Our results document that brittle normal faults were active over ~20 Ma of the extensional history and provided effective pathways for meteoric fluids.

[1] The Qinling–Dabie orogenic collage, central China, constitutes the geographic, geologic, and cultural heart of China; it plays a key role in understanding the amalgamation and break-up of the Rodinia supercontinent and the subduction and exhumation of continental crust under ultrahigh-pressure conditions. Herein, we investigate the Proterozoic evolution of the Qinling–Dabie orogenic collage and surrounding segments of the bounding South China craton (SCC) and North China craton (NCC), employing published and new U/Th–Pb geochronology. The Kongling, Hong'an–Dabie, and Douling–Foping complexes constitute the nucleus of the Yangtze block, recording a common ~2.0 Ga orogenic event that integrated the Yangtze block into the supercontinent Columbia. The ~1.10–0.95 Ga Miaowan ‘ophiolite’–Shennongjia arc association of the Huangling dome–Shennongjia massif seems to have split and reassembled that nucleus. It formed earlier than or contemporaneously with the Sibao orogeny along the southeastern margin of the Yangtze block. The ~0.95–0.80 Ga Mian–Lue complex comprises an oceanic accretionary wedge that formed outboard of an associated fore-arc–arc system represented by the Bikou–Hannan–Micangshan massifs along the north(western) margin of the Yangtze block. The Qinling complex, currently sandwiched between the SCC and NCC, lacks pre-Mesoproterozoic cratonal basement and its igneous rocks intruded a ~1.7–1.0 Ga old clastic wedge that incorporates meta-basites; it might have been part of the extended passive margin of East Antarctica and/or Australia. Neoproterozoic Qinling-complex magmatism spanned ~260 Myr and evolved from partial melting of the thick clastic sequence over an arc to a rift setting; most Qinling-complex paragneisses are erosional products of these igneous rocks. The ~1.0–0.85 Ga Qinling-complex magmatism formed independently from that along the north(west)ern Yangtze-block margin, but its ~0.8–0.7 Ga magmatism, peaking at ~750 Ma, is widespread throughout the Yangtze block; this suggest post- ~ 825 Ma accretion of the Qinling complex to the Yangtze block. The Daba and Wudang Shan, Douling, and Hong'an–Dabie areas of the northern Yangtze block are dominated by ~0.8–0.6 Ga bimodal continental-rift igneous rocks; in accordance with similar ages in the Qinling complex and the entire SCC, continental rifting appears to have been most active at ~750 Ma. Our Rodinia scenario suggests that the Qinling–Dabie orogenic collage records the final stages of the assemblage of the core of Rodinia and this was completed not earlier than ~825 Ma, and its break-up, which was most active at ~750 Ma.

[1] Granitoid magmatic sheets emplaced syntectonically during growth of the Orlica–Śnieżnik mantled gneiss dome (Central Sudetes, European Variscan belt) were examined by means of structural geology, quartz and anisotropy of magnetic susceptibility (AMS) fabric studies. Magmatic emplacement was localized along the eastern transpressive margin of the high metamorphic core and below the low–grade detached mantle rimming the southern margin of the dome. In the first area, the magmatic sheets were emplaced parallel to the dilated subhorizontal foliation of an anisotropic pre–orogenic block. The resulting melt–host rock multilayer localized the transpressive zone along which the bulk of magma was emplaced. The AMS study shows an across–strike fabric zonation underlying a strongly transpressive solid–state deformation in the hanging–wall, magmatic subhorizontal fabric in the foot–wall and a transitional fabric in the centre of the intrusion. In contrast, along the southern dome margin, magmatic sheets intruded along steep foliations of weakly metamorphic mantle rocks, and are affected by recumbent folding and subhorizontal shearing. The bulk of the sheets present a shallow–dipping magmatic foliation and an along–strike magmatic lineation. The variations of the quartz and magnetic fabrics are attributed to superimposition of pure shear–dominated ductile thinning followed by simple shear–dominated detachment onto the original steep fabric. AMS modeling confirms the role of variation of orientation of pre–intrusive anisotropy during progressive deformation on the resulting fabric pattern and helps explaining observed variations in fabric orientations and symmetries. This study highlights contrasting mechanical behavior of syntectonically emplaced magmas in different parts of a growing crustal–scale mantled gneiss dome.

[1] Quantitative paleoelevation histories can help to explain both why and how widespread Cenozoic extension occurred in the Basin and Range Province of western North America. We present new estimates of pre-extensional paleoelevations for the northern and central Basin and Range using clumped isotope (Δ₄₇) thermometryof lacustrine carbonates collectedfromeach region. Comparison of carbonate Δ₄₇-derived mean annual air temperature (MAAT) estimates (~ 16–20°C) for the late Cretaceous-Eocene Sheep Pass basin of east-central Nevada with published MAAT estimates for the Eocene, coastal northern Sierra Nevada (~ 20–25°C) suggests that the early Paleogene Sheep Pass basin hada paleoelevation of ≤ 2 km. Such a modest paleoelevation suggests that either (1) the proto-northern Basin and Range did not attain maximum paleoelevations of 3-4 km until the late Eocene-early Oligocene or (2) the Sheep Pass basin was a local, high relief (> 1 km) setting contained within a > 3 km orogenic highland(‘Nevadaplano’). Similarity of Δ₄₇-derived MAAT estimates (~ 17–24°C) for carbonates from thecentral Basin and Rangeand the near-sea-level southern Sierra Nevada Bena basin indicate that Middle Miocene paleoelevations in the Death Valley region were≤ 1.5 km.These fairly low paleoelevationsare incompatible with pre-extensional crustal thicknesses > 52 km and indicate that mean elevation change was minor (≤ 500 m) andlithospheric mass was not conserved during > 100% Neogene extension of the central Basin and Range,but was instead likely compensated by synextensional magmatic additions to the crust.

[1] Large topographic belts along convergent margins have been recognized with the ability to slowdown the kinematics of subduction over geologically short time-periods (i.e. few Myr), because their associated gravitational spreading provides significant resistive forces within the framework of plate tectonics. The record of past and present-day plate kinematics provides important constraints on the dynamics of the lithosphere, as plate-motion changes must reflect temporal changes in the balance of driving and resisting forces. Here we focus on the convergence between the Arabian and Eurasian plates, across the Zagros mountain belt. Relative motion across this plate boundary is reconstructed since 13 Ma from published paleomagnetic and geodetic data, and features a slowdown of ~30% from ~5 Ma to present-day. We employ global dynamic models of the mantle/lithosphere system to test whether the most recent uplift across the Arabia-Eurasia collision zone, including the Zagros orogeny, may induce the observed slowdown since 5 Ma. Specifically, we use constraints from the geologic record to infer past topography and quantify its influence on the convergence rate between Arabia and Eurasia. We test the sensitivity of our models to assumptions made in estimating the paleo-elevation by perturbing the orogeny parameterisation within reasonable ranges. Finally, we speculate on the potential effects of Tethys slab break-off, changes of the deformation style within the collision zone, and the Afar plume on the dynamics of convergence. Our results indicate that orogenic uplift across the Arabia-Eurasia collision zoneplays a key role in slowing down convergence since ~5 Ma.

[1] Since the Middle Eocene, the northwest Arabian Platform has been emerging from the water and rising above sea level, whereas the adjacent Levant Basin has been subsiding and accumulating a thick sedimentary section. This study investigates these opposing vertical motions and the relative roles of tectonic-driven vs. isostatic adjustment forces involved. Such a distinction is strongly dependent on a reliable estimation of the paleo-bathymetry, which in pelagic environments may vary substantially. We use two different methods to estimate the paleo-water-depth in the deep Levant Basin in the Tertiary time. Isostatic balancing calculations with the adjacent inland region are employed to estimate the paleo-water-depth in the deep Levant Basin in the Middle Eocene, just before the commencement of the Late Tertiary tectonic phase. For later periods, we use morpho-structural elements such as abrasion surfaces and incised canyons to build laterally changing topo-bathymetry profiles. Our results indicate that in the Mid-Eocene water depth in the deep Levant Basin was 2-3.5 km, which gradually decreased to 1.5 km today. Based on these results our analysis shows that the enhanced subsidence of the Levant Basin reflects an isostatic response to sedimentary filling of a pre-existing deep-water basin with no involvement of a downward tectonic force. On the contrary, we suggest that the regional tectonic force was upward, counteracting a sedimentary loading effect. Further regional implication of this understanding is that the cause for uplifting and exposure of the NW Arabian Platform in the Late Tertiary extended far westward beyond the inland region.

[1] The southern Tibetan Plateau margin between ~ 83°E - 86.5°E is defined by an abrupt change from the low-relief Tibetan Plateau to the rugged topography and deep gorges of the Himalaya. This physiographic transition lies well to the north of active thrusting and thus, the mechanism responsible for the distinct topographic break remains the focus of much debate. While numerous studies have utilized thermochronology to examine the exhumation history of the Himalaya, few have done so with respect to variations across the Himalaya-Tibetan Plateau transition. In this work, we examine the nature of the transition where it is accessible and well-defined in the Nyalam valley of south-central Tibet. We employ several new and previously published thermochronologic datasets (with a closure temperature range of ~ 70°C - 300°C) in conjunction with river incision patterns inferred by the longitudinal profile of the Bhote Kosi River. The results reveal a sharp change in cooling rate at ~ 3.5 Ma at a location corresponding to a pronounced river knickpoint representing a sharp increase in river gradient, and presumably incision rate (a proxy for rock uplift). Margin retreat models for the physiographic transition are inconsistent with the cooling pattern revealed by low-temperature thermochronologic data. Models invoking passive uplift of the upper crust over a mid-crustal ramp and associated duplex to account for the physiographic transition do not explain the observed break in cooling rate there, although they may explain a suggesting in the thermochronologic data of progressively increasing exhumation rates south of the transition. The simplest model consistent with all observations is that passive uplift is augmented by contemporaneous differential uplift across a young (Pliocene-Quaternary) normal fault at the physiographic transition. Drawing on observations elsewhere, we hypothesize that similar structural relationships may be characteristic of the Tibetan Plateau-Himalaya transition from ~83°E – 86.5°E.

[1] New constraints on pressures and temperatures experienced by rocks of the Himachal Himalaya are presented in order to test models for the emplacement of the Himalayan crystalline core here. A variety of methods were employed: petrographic analysis referenced to a petrogenetic grid, exchange and net-transfer thermobarometry, Ti-in-biotite thermometry, and analysis of quartz recrystallization textures. Rocks along three transects (the northern Beas, Pabbar, and southern Beas transects) were investigated.Results reveal spatially coherent metamorphic field gradients across amphibolite-grade and migmatitic metamorphic rocks. Along the northern Beas transect, rocks record peak temperatures of ~650-780 °C at low elevations to the north of ~32°10’ N; rocks in other structural positions along this transect record peak temperatures of <640 °C that decrease with increasing structural elevation. Rocks of the Pabbar transect dominantly record ~650-700 °C peak temperatures to the east of ~77°55’ E, and ~450-620 °C peak temperatures farther west. Peak temperatures of ~450-600 °C along the southern Beas transect record a right-way-up metamorphic field gradient.Results are integrated with literature data to determine a metamorphic isograd map of the Himachal Himalaya. This map is compared to metamorphic isograd map pattern predictions of different models for Himalayan crystalline core emplacement. This analysis excludes models involving large magnitude (>20-30 km) extrusion and permits (1) models involving small magnitude (<20-30 km) extrusion that is discontinuous along the orogen and (2) tectonic wedging models, in which the crystalline core was emplaced at depth between a sole thrust and a back thrust in the Early-Middle Miocene.

[1] In the Hengchun Peninsula, southern Taiwan, the Miocene Loshui Sandstone of deep-sea fan facies provides an opportunity and challenge to investigate the brittle deformation history of an emerging accretionary prism. A survey of brittle fractures was carried out along the coastline. The brittle fractures include joints of various sets, a few faults and veins. The intricacy of joints lies in the large number of joint sets that often vary not only from one site to another site but in bed thickness at a given site as well and in the complicated abutting relationships between the joint sets. In order to investigate the joint sequence, we define a full spectrum of possible systematic joint sets and develop an algorithm to determine the shortest joint sequence from the observed abutting relationships. Little difference between the shortest joint sequences for the southern and northern parts of the study transect attests to the evolution of overall uniform stress throughout the area that is responsible for these various systematic newly formed or reactivated joint sets. The fracture sequence derived reveals a complicated stress evolution during the transition from the passive to active continental margin.

[1] Recent normal and strike-slip faulting on the Puna Plateau of NW Argentina has been linked to lithospheric foundering, gravitational spreading, plate boundary forces and a decrease in crustal shortening from north to south. However, the timing, kinematics and rate of extension remain poorly constrained. We focus on the Pasto Ventura region (NW Argentina) located on the southern Puna Plateau and recent deformation (<1 Ma). Field mapping and kinematic analysis across offset volcanic cinder cones show that the overall extension direction is subhorizontal, is oriented NE-SW to NNE-SSW, and occurs at a slow, time-integrated rate of 0.02 to 0.08 mm/yr since at least 0.8–0.5 Ma. A regional compilation from this study and existing data shows that recent extension across the Puna Plateau is subhorizontal but varies in azimuthal orientation dramatically. Data from the Pasto Ventura region are consistent with a number of models to explain normal and strike-slip faulting on the Puna Plateau, all of which likely influence the region. Some role for lower lithospheric foundering through dripping appears to be seen based on the regional extension directions and ages of mafic volcanism in the southern Puna Plateau.

[1] The spatial distribution of monogenetic vents and the geochemistry of their erupted products can be used to probe heterogeneity in lithospheric strain across a rift. We show that Quaternary volcanic belts in the central Main Ethiopian Rift (MER) exhibit differences in vent fractal clustering with an exponent indicative of more clustering and a shallower magma reservoir for the Wonji Fault Belt (WFB), in comparison to the Silti-Debre Zeyit Fault Zone (SDFZ). The range of lengths that exhibit vent fractal clustering is bounded by (1) a lower cutoff of few hundreds of meters that correlates with the depth of emplacement of intrusive material and is likely linked to evolving silicic magma systems and (2) an upper cutoff which we interpret to scale with the depth from which dikes originate just prior to eruption: ~10 km for WFB and ~7 km for SDFZ. We attribute this difference to strain partitioning within the MER, which favors dike formation at greater depths beneath the more highly strained eastern margin of the MER (below the WFB), in comparison to the western rift margin (below the SDFZ). Statistical analysis of monogenetic fields in the MER show, when reviewed in light of a priori geophysical and geodetic data, that the plumbing system of monogenetic volcanism style is strongly controlled by crustal strain state. Such statistical techniques may have application in probing the magma systems of other environments where less geophysical or geochemical controls exist.

[1] The Fish Lake Valley–northern Death Valley–Furnace Creek fault zone, a ~250 km long, predominantly right-lateral structure in California and Nevada, is a key element of tectonic reconstructions of the Death Valley area, Eastern California Shear Zone and Walker Lane, and central Basin and Range Province. Total displacement on the fault zone is contested, however, with estimates ranging from ~30 to ~63 km or more. Here we present a new synthesis of available constraints. Preextensional thrust faults, folds, and igneous rocks indicate that offset reaches a maximum of ~50 km. Neogene rocks constrain its partitioning over time. Most offset is interpreted as ≤ ~13–10 Ma, accruing at ~3–5 mm/yr in the middle of the fault zone and more slowly toward the tips. The offset markers imply ~68 ± 14 km of translation between the Cottonwood Mountains and Resting Spring–Nopah Range (~60 ± 14 km since ~15 Ma) through a combination of strike-slip and crustal extension. This suggests that a previous interpretation of ~104 ± 7 km, based on the middle Miocene Eagle Mountain Formation, is an overestimate by ~50%. Our results also help to mitigate a discrepancy in the ~12–0 Ma strain budget for the Eastern California Shear Zone. Displacement has previously been estimated at ~100 ± 10 km and ~67 ± 6 km for the Basin and Range and Mojave portions of the shear zone, respectively. Our new estimate of ~74 ± 17 km for the Basin and Range is within the uncertainty of the Mojave estimate.

[1] Based on field investigations, aerial-photo morphological analysis, topographic profiling, and optically stimulated luminescence (OSL) dating of alluvial surfaces, we estimate vertical components of the slip rate along the South Heli Shan thrust fault, which lies on the northern margin of the Hexi Corridor and the northeastern edge of the Tibetan Plateau. The fault consists of three segments with scarp heights ranging from less than 1 m to more than 16 m. OSL dating indicates that most of the alluvial fans cut by fault scarps formed during the transition from the last glacial stage to the present interglacial stage from ~19 to ~9 ka along southern Heli Shan and from ~27 ka to ~22 ka along its northern margin. In addition, remnants of older alluvial fan have been abandoned after ~67 ka. Scarp heights increase from west to east and reach a maximum of more than 16 m near the eastern end. Using three approaches, we calculate late Quaternary slip rates for each of the three fault segments along the southern margin and the fault on the northern flank. These approaches yield maximum vertical slip rates from 0.18 to 0.2 mm/a for the western segment, 0.3 to 0.43 mm/a for the central segment, 0.36 to 0.53 mm/a for the eastern segment, and 0.21 mm/a for the Wutongjing Fault, which lies on the north side of the Heli Shan. For a range of likely fault dips, these correspond to 0.1–0.2 mm/a of average horizontal shortening for the western segment, and increase to 0.4–0.5 mm/a across the eastern segment of the southern Heli Shan Fault. Combining the height of the eastern parts of the Heli Shan (Daqing Peak) above the Hei He (a major river that incised the western end of the range) and the vertical component of the slip rate of the eastern segment, we suggest that the Heli Shan was uplifted by motion on the South Heli Shan Fault beginning sometime between 1 and 4 Ma, most likely since ~2 Ma. This age suggests that the Tibetan Plateau continues to grow northeastward across the Hexi Corridor.

[1] The eastern Himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the India-Asia collision. Spatially, most of these regions are proximal to the South Tibetan detachment system, an orogen-parallel normal-sense detachment system that operated during the Miocene, suggesting that it played a role in their exhumation. Here we use geo- and thermochronological methods to study the deformation and cooling history of footwall rocks of the South Tibetan detachment system in northern Sikkim, India. These data demonstrate that the South Tibetan detachment system was active in Sikkim between 23.6 and ~13 Ma, and that footwall rocks cooled rapidly from ~700 to ~120 °C between ~15-13 Ma. While active, the South Tibetan detachment system exhumed rocks from mid-crustal depths, but an additional heat source such as strain heating, advected melt and/or crustal thinning is required to explain the observed isothermal decompression. Cessation of movement on the South Tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed. A regional comparison of temperature-time data for the eastern South Tibetan detachment system indicates a lack of synchronicity between the Sa'er-Sikkim-Yadong section and the NW Bhutan section. To accommodate this requires either strike-slip tear faulting or local out-of-sequence thrusting in the younger segment of the orogen.

[1] Although much work has been done on active tectonics of eastern Tibet, little is known about the Longriba fault zone and its role in strain partitioning. Whether its two sub-parallel strands (Longriqu and Maoergai faults) can rupture simultaneously in a large earthquake remains unknown. We conducted trenching combined with the interpretation of satellite imagery, field investigations, topographic surveys, and radiocarbon and Optically Stimulated Luminescence (OSL) dating to reconstruct paleoseismic history, and we used displaced terrace risers to estimate geological slip rates. Our results demonstrate that the Longriba fault zone is predominantly right-lateral with a small southeast-verging thrust component. Four surface-rupturing events occurred on the Longriqu fault at 5080 ± 90, 11,100 ± 380, 13,000 ± 260, and 17,830 ± 530 cal yr B.P. Together with our previous trenches on the Maoergai fault, we found that the last event probably ruptured both the Longriqu and Maoergai faults. Prior to the last event, the two strands of the Longriba fault zone experienced alternating earthquakes. The fault zone has a high potential for an earthquake larger than Mw 7. The slip rate of the Longriba fault zone decreases from ~7.5 mm/yr in latest Pleistocene to ~2.1 mm/yr in the Holocene, probably related to a slowing down of the eastern motion of the Tibetan Plateau. The comparison with slip rates at the Longmen Shan fault zone suggests that the Longriba fault zone has an equally important role in strain partitioning in eastern Tibet. This study is helpful to seismic hazard assessment and an understanding of deformation mechanism in eastern Tibet.

[1] An intense tectonic activity in eastern Sicily and southern Calabria (Italy) is well documented by the differential uplift of Late Quaternary coastlines and by the record of the strong historical earthquakes. The extensional belt that crosses this area is dominated by a well-established WNW-ESE–oriented stretching direction. However, this area is largely lacking of any structural analysis for defining the tectonics at a more local scale. The analysis of systematic extension joint sets affecting Pleistocene deposits presented in this paper allows to infer the causative tectonic stress tensor by means of a quantitative inversion technique. Local perturbations of the first-order regional stress field are consequently recognized. Such perturbations are interpreted as due to interferences between large active faults and their particular geometrical setting. These results contribute to the understanding of the Quaternary tectonic evolution and the present-day stress regime.

[1] Predominant stretching structures in the Greater Himalayan Crystalline Complex (GHC) trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the GHC between the South Tibet Detachment (STD) and the Main Central Thrust. However, our field investigations in southern Tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen-parallel stretching lineation, which separates the Tethyan Himalayan Sequence and the underlying GHC. The shear zones are well preserved in the upper part of the GHC, south to and structurally lower than the STD. Field criteria, microstructures, and quartz fabrics indicate top-to-the-east shearing in the Yadong shear zone (eastern GHC), coexistence of top-to-the-east and top-to-the-west shearing in the Nyalam shear zone (central GHC), but top-to-the-west shearing in the Pulan shear zone (western GHC). Characteristic microstructures and slip systems of quartz in the high-grade GHC rocks resulted from the lateral flow under upper amphibolite (up to 650–700 °C) to greenschist facies conditions. U-Pb ages of metamorphic zircon rims by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses yield 28–26 Ma for the initiation of the Yadong and Nyalam shear zones and 22–15 Ma for the activation of the Pulan shear zone. In addition, ⁴⁰Ar/³⁹Ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13–11 Ma on the Yadong shear zone, which is coeval with the activation of the STD. Combined with previous studies, we propose that initiation of orogen-parallel extension marks the transition from burial/crustal thickening to exhumation of the GHC. Due to lateral crustal thickness gradients in a thickened crust, orogen-parallel gravitational collapse occurred within the convergent Himalayan orogen in the late Oligocene-Miocene. This tectonic denudation triggered and enhanced partial melting and ductile extrusion of the GHC in the Miocene.

[1] The evolution of the southeastern Gondwana margin in the Papua New Guinea segment is manifested by ophiolites signifying plate collision, volcanic arcs marking subduction, and metamorphic core complexes that have developed during extensional events and exhumation associated with sea floor spreading in the Woodlark Basin to the east. In detail the Papuan Ultramafic Belt marks a well-documented collision between continental crust and a subduction system. However, to the southeast, an extensive sequence of basaltic rocks known as the Milne Terrain is more problematic. Geochemical data indicate that these upper Cretaceous and Eocene rocks have MORB-type affinities, and their most likely tectonic association is with the opening of the Coral Sea Basin. Milne Terrain rocks represent the lower plate in the obduction system along which the Papuan Ultramafic Belt was emplaced, and thus they are the structural equivalent of the continental crust which was separated from the Australian continental block by the opening of the Coral Sea. Spectacular uplift (>4 km) of the oceanic basaltic crust of the Milne Terrain may be due to the underlying presence of underplated material associated with a Late Miocene-Pliocene episode of subduction immediately prior to the encroachment of the Woodlark spreading center into the Papuan area.

[1] In the Basque-Cantabrian Basin (Spain), normal faulting and associated folding occurred during Late Jurassic to Early Cretaceous rifting. Cenozoic Pyrenean thick-skinned transpressive inversion in the western parts of the basin preserved the first-order extensional architecture. Integration of geological maps and seismic profiles has permitted to fully constrain the style of extensional deformation and subsequent inversion in the western portion of the Basque-Cantabrian Basin. Extensional faults offset the Paleozoic basement up to Lower Triassic rocks. The presence of an efficient décollement level represented by Triassic evaporites produced the decoupling between basement rocks and the Upper Triassic to Middle Jurassic prerift cover sequence. Extensional forced folding occurred in the cover, driven by basement faulting and the migration of evaporites toward the hanging wall of the extensional faults, with salt welds developing away from them. Upper Jurassic to Lower Cretaceous syn-rift sediments deposited synchronously with forced folding, which led to the development of extensional growth geometries associated with both master faults and nearly-transverse faults. Syn-rift growth sequences are characterized by downlap and onlap relationships with the underlying prerift units, interpreted as the result of along-strike variations of master fault extensional displacement rate. Cenozoic Pyrenean contraction generated the right-lateral transpressive inversion of basement master faults and the almost dip-slip reactivation of transverse extensional faults.

[1] This paper shows how the Turkish-Iranian Plateau grows laterally by incrementally incorporating adjacent parts of the Zagros fold-and-thrust belt. The limit of significant, seismogenic, thrusting in the Zagros (M_w > 5) occurs close to the regional 1250 m elevation contour. The seismicity cutoff is not a significant bedrock geology boundary. Elevations increase northward, toward regional plateau elevations of ~2 km, implying that another process produced the extra elevation. Between the seismogenic limit of thrusting and the suture, this process is a plausibly ductile thickening of the basement, suggesting depth-dependent strain during compression. Similar depth-dependant crustal strain may explain why the Tibetan plateau has regional elevations ~1500 m greater than the elevation limit of seismogenic thrusting at its margins. We estimate ~68 km shortening across the Zagros Simply Folded Belt in the Fars region, and ~120 km total shortening of the Arabian plate. The Dezful Embayment is a low strain zone in the western Zagros. Deformation is more intense to its northeast, in the Bakhtyari Culmination. The orogenic taper (across strike topographic gradient) across the Dezful Embayment is 0.0004, and across the Bakhtyari Culmination, 0.022. Lateral plateau growth is more pronounced farther east (Fars), where a more uniform structure has a taper of ~0.010 up to elevations of ~1750 m. A >100 km wide region of the Zagros further northeast has a taper of 0.002 and is effectively part of the Turkish-Iranian Plateau. Internal drainage enhances plateau development but is not a pre-requisite. Aspects of the seismicity, structure, and geomorphology of the Zagros do not support critical taper models for fold-and-thrust belts.

[1] The Upper Paleozoic geodynamic evolution is discussed at the scale of a wide part of Gondwana from North Africa to Arabia. With the aim of giving an integrated tectonic scenario for the study domain, we revisit six key areas, namely, the Anti-Atlas Belt (Morocco), the Bechar Basin (west Algeria), the Hassi R'Mel High (central Algeria), the Talemezane Arch (south Tunisia), the Western Desert (Egypt), and, finally, the High Zagros Belt (Iran). Below the so-called “Hercynian unconformity,” which is in reality a highly composite discontinuity, surface and subsurface data display a well-known arch-and-basin geometry, with basement highs and intervening Paleozoic basins. We show that this major feature results mainly from a Late Devonian event and can no longer be interpreted as a far effect of the Variscan Orogeny. This event is characterized by a more or less diffuse extensional deformation and accompanied either by subsidence, in the western part of the system, or by an important uplift of probable thermal origin followed by erosion and peneplanation. By the end of the Devonian, the whole region suffered a general subsidence governed by the progressive cooling of the lithosphere. Such a primary configuration is preserved in Arabia with typical sag geometry of the Carboniferous and Permian deposits but strongly disturbed elsewhere by the conjugated effects of the Variscan Orogeny during the Carboniferous and/or by subsequent uplifts linked to the central Atlantic and Neo-Tethys rifting episodes. In conclusion, we try to integrate this new understanding in the geodynamics of the Late Devonian, which at world scale is characterized by the onset of the Variscan Orogeny on the one hand and by magmatism, rifting, and basement uplift on the other hand.

[1] The Pyrenees, north of the North Pyrenean fault, display a complex structure involving a succession of peridotite massifs, basement massifs, and mid-Cretaceous to Late Cretaceous basins located in a narrow domain, which was affected by a mid-Cretaceous, preorogenic, high-temperature, low-pressure metamorphism. The Late Cretaceous basins were interpreted either as pull-apart basins formed during transcurrent motion of Iberia relative to Eurasia or as remnants of a larger extensional basin. Recent models support that peridotite massifs result from the exhumation of the mantle during this preorogenic event. The northern boundary of the Agly basement massif shows evidence of ductile deformation of the basal formations of the Agly sedimentary cover. Macroscopic and microscopic kinematics indicators consistent with asymmetry of crystallographic fabrics suggest normal sense of shear and thus suggest detachment, at least partial, of the Mesozoic cover from its basement. Triassic to Early Cretaceous limestones are mylonitic and consistently shows a foliation, a NS- to NE-trending lineation, shear criteria suggesting top-to-the-north shearing and locally boudinage. At the microscopic scale, mylonites are characterized by a very fine grain size, frequently <10 µm. They contain larger, partially recrystallized calcite parent grains and undeformed quartz grains with calcite fringes crystallized in pressure shadows. In these mylonites, calcite systematically shows a weak but well-defined crystallographic-preferred orientations, suggesting HT dislocation creep combined to diffusion creep and possibly grain boundary sliding in the finest fraction of the mylonites. Paleotemperatures determined using Raman spectrometry suggest synkinematic temperature conditions in the range 340–390°C, in good agreement with observed microstructures and calcite CPO. The mylonitic fabric in Mesozoic limestones is folded by EW-trending Pyrenean folds north of the Agly basement massif, attesting that this fabric formed before the Pyrenean orogeny. These data consistently support preorogenic extension under medium-temperature conditions of the northern Agly massif and likely of the massif itself. Since simultaneously (~100 Ma) a mid-Cretaceous basin opened south of the basement massif, we suggest a model of preorogenic exhumation of the Agly massif in response to the regional extension associated to the rotation of Iberia. This model may explain the exhumation of the North Pyrenean massifs during a single preorogenic event that allowed the opening of extensional basins and the exhumation of the lithospheric mantle. All these structures being subsequently reworked during the Pyrenean orogeny.

[1] The spatial distribution of monogenetic vents and the geochemistry of their erupted products can be used to probe heterogeneity in lithospheric strain across a rift. We show that Quaternary volcanic belts in the central Main Ethiopian Rift (MER) exhibit differences in vent fractal clustering with an exponent indicative of more clustering and a shallower magma reservoir for the Wonji Fault Belt (WFB), in comparison to the Silti-Debre Zeyit Fault Zone (SDFZ). The range of lengths that exhibit vent fractal clustering is bounded by (1) a lower cutoff of few hundreds of meters that correlates with the depth of emplacement of intrusive material and is likely linked to evolving silicic magma systems and (2) an upper cutoff which we interpret to scale with the depth from which dikes originate just prior to eruption: ~10 km for WFB and ~7 km for SDFZ. We attribute this difference to strain partitioning within the MER, which favors dike formation at greater depths beneath the more highly strained eastern margin of the MER (below the WFB), in comparison to the western rift margin (below the SDFZ). Statistical analysis of monogenetic fields in the MER show, when reviewed in light of a priori geophysical and geodetic data, that the plumbing system of monogenetic volcanism style is strongly controlled by crustal strain state. Such statistical techniques may have application in probing the magma systems of other environments where less geophysical or geochemical controls exist.