Lower Crustal Flow Research Articles

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

Extensional structures and Cenozoic magmatism in the northwestern South China Sea

This study investigates the crustal structure and Cenozoic magmatism in the northwestern South China Sea (SCS), based on two long-cable multi-channel seismic reflection profiles, together with gravity and magnetic data, and adjacent wide-angle refraction profiles. Basins/sags are bounded by large listric-normal faults (fault throws ≥ 0.5 km) and massifs are cut off by normal faults with small offsets (fault throws < 0.5 km) in the northwestern SCS. These structures are penetrated by magmatic edifices showing positive gravity and magnetic anomalies. syn-Rift magmatic intrusions/extrusions were intense in the basins/sags and continent-ocean transition zone while post-rift magmatism was widespread from basins/sags to massifs with the most intense stage occurring from 5.5 to 2.6 Ma. Based on previous geophysical and geochemical results, we suggest that syn-rift mantle upwelling from partial melting initiated seafloor spreading magmatic activities, whereas plume-related mantle upwelling contributed to the magmatism during and after seafloor spreading in the northwestern SCS. Stretching factors show that the upper and lower crusts have experienced differential extension from basins/sags to massifs. The non-uniform crustal extension resulted from upper crustal faulting and lower crustal flow. Particularly, the lower crustal flow was probably linked with the combined action of magmatic heating, mantle flow shearing stresses, and sediment loading, resulting in crustal boudinage and reestablishment of an equilibrium state over long distances.

Magnetostratigraphic Evidence From the Gengma Basin for the Fast Response of the Crustal Rotation of the Northern Part of the Shan‐Thai Block to the India‐Eurasia Convergence

AbstractThe Cenozoic tectonic evolution of the southeastern edge of the Tibetan Plateau is interpreted in terms of two alterative dynamic mechanisms: lateral extrusion of coherent lithospheric blocks and viscous lower crustal flow. To contribute to this debate, we conducted a magnetostratigraphic study of the Early Miocene sedimentary strata of the Gengma Basin, in the northern part of the Shan‐Thai Block (STB). The results show that variations in the deposition rate were synchronous with the rate of clockwise rotation during ∼18.05–15.93 Ma, indicating that the crustal clockwise rotation of the northern STB was the driver of the tectonic transformation of the NE‐SW trending strike‐slip system within the STB by at least ∼18.05 Ma. The rapid decrease in the clockwise rotational rate of the Gengma Basin was also synchronous with the sharp decrease in the convergence rate between the Indian Plate (IDP) and Eurasia at ∼17.23 Ma, which is similar to that occurred in the interior of the Tibetan Plateau during the Paleocene and Eocene, demonstrating the fast response of the southeastern edge of the Tibetan Plateau to the convergence of the IDP and Eurasia, in the form of the continuous upper crustal ductile deformation of the Tibetan Plateau and its periphery. The lateral flow of the viscous lower crust beneath the southeastern edge of the Tibetan Plateau may be the predominant driver of the tectonic evolution of the southeastern edge of the Tibetan Plateau since the Early Miocene.

P-Wave Velocity Structure of the Lower Crust and Uppermost Mantle beneath the Sichuan–Yunnan (China) Region

Abstract We conduct a tomographic inversion for the 3D P-wave velocity structure in the lower crust and uppermost mantle of the Sichuan–Yunnan region in the southeastern margin of the Tibetan Plateau. A total of 43,450 reliable arrival times of P waves are picked from over 300,000 regional seismic records using an automatic algorithm based on deep learning. A two-stage iterative inversion process in which events are relocated, leading to a significant reduction in travel-time residuals, is adopted. A statistical resolution matrix analysis suggests that our model has an optimal spatial resolution length of ∼0.4° in the lower crust and ∼0.2° in the uppermost mantle. Our 3D model shows that both the lower crust and uppermost mantle in the region are characterized by strong lateral heterogeneities. The unusually low velocities in the lower crust may indicate the existence of lower crustal flow, whereas the high-velocity anomalies in the uppermost mantle in and around the Sichuan–Yunnan Rhombic block (SYRB) may be an important factor in preventing the ductile materials in the lower crust from moving eastward. Our model also indicates a coupling between the surface deformation and the material flow in the lower crust. Finally, the lower crustal flow may influence the materials in the shallow part of the uppermost mantle beneath the SYRB, and the crust–mantle transition zone beneath the Songpan–Ganzi block may be influenced by weak materials from both the uppermost mantle and the lower crust.

Instant far-field effects of continental collision: An example study in the Qinling Orogen, northeast of the Tibetan Plateau

The ongoing Cenozoic Indo-Asia continental collision not only formed the highly elevated Tibetan Plateau but also reactivated the crustal deformation in adjacent Paleozoic-Mesozoic orogens. This includes the renewed phase of deformation in the Qinling Orogen, northeast of the Tibetan Plateau, which was formed initially by Paleozoic closure of the Paleo-Tethys and subsequent early Triassic continental collision between the North and South China blocks. Debates continue as to when and how the reactivation initiated. To investigate this question, this study reports a vertical profile of apatite and zircon (UTh)/He thermochronometric ages from the Guangtoushan Granite to estimate the exhumation history of the central Qinling Orogen, where previous models for the growth of the Tibetan Plateau inferred late Cenozoic northeastward growth of the Tibetan Plateau by outward flow of the lower crust from the plateau interior. Age-elevation relationship and thermal history modelling suggest two phases of relatively rapid exhumation during the early Cretaceous (~130–110 Ma) and the latest Cretaceous – early Paleocene (~70–50 Ma), respectively. The early Cretaceous exhumation coincided with the time of the Lhasa-Qiangtang collision, whose far field effects have been widely reported in regions of the central-northern Tibetan Plateau. In the southern and central Qinling, our finding of an early Paleocene (~70–50 Ma) increase in exhumation rate followed by minimal exhumation during the late Cenozoic suggests negligible impact of the lower crust flow from the expanding Tibetan Plateau on the rock exhumation of the Qinling. Instead, our study suggests instant strain migrations from the Lhasa-Qiangtang and Indo-Asia collision zones to the northeast margin of the Tibetan Plateau during the early Cretaceous and the early Paleocene, respectively, supporting the models highlighting the importance of rigid blocks in facilitating intracontinental strain migration.

Aseismic Slip and Cascade Triggering Process of Foreshocks Leading to the 2021 Mw 6.1 Yangbi Earthquake

AbstractUnderstanding the nature of foreshock evolution is important for earthquake nucleation and hazard evaluation. Aseismic slip and cascade triggering processes are considered to be two end-member precursors in earthquake nucleation processes. However, to perceive the physical mechanisms of these precursors leading to the occurrence of large events is challenging. In this study, the relocated 2021 Yangbi earthquake sequences are observed to be aligned along the northwest–southeast direction and exhibit spatial migration fronts toward the hypocenters of large events including the mainshock. An apparent static Coulomb stress increase on the mainshock hypocenter was detected, owing to the precursors. This suggests that the foreshocks are manifestations of aseismic transients that promote the cascade triggering of both the foreshocks and the eventual mainshock. By jointly inverting both Interferometric Synthetic Aperture Radar and Global Navigation Satellite Systems data, we observe that the mainshock ruptured a blind vertical fault with a peak slip of 0.8 m. Our results demonstrate that the lateral crustal extrusion and lower crustal flow are probably the major driving mechanisms of mainshock. In addition, the potential seismic hazards on the Weixi–Weishan and Red River faults deserve further attention.

Crustal flow and fluids affected the 2021 M7.4 Maduo earthquake in Northeast Tibet

• Fluids exist in the Maduo source zone and affected the rupture nucleation. • Depth-varying seismic anisotropy occurs and reflects lower-crustal flow. • The ductile flow in the mid-lower crust affected the seismotectonics. We present detailed 3-D tomographic images of P and S wave velocity (Vp, Vs), Poisson's ratio (ν) and Vp azimuthal anisotropy of the crust and uppermost mantle beneath the source area of the 21 May 2021 Maduo earthquake (M 7.4) in the NE Tibetan plateau. The images are obtained by inverting a large number of P and S wave arrival-time data of 11,235 local earthquakes and Maduo aftershocks recorded at 67 seismic stations. Our results show that the 2021 Maduo mainshock occurred in a low-Vs and high-ν anomaly, probably reflecting crustal fluids that affected the rupture nucleation. Our Vp anisotropy results show that at 40 km depth under the southern part of the study region, the fast-velocity direction (FVD) is NW-SE, which is mainly controlled by the India-Eurasia collision. At 60 km depth under the study region and at 40 km depth under the northern part of the region, the FVDs are NE-SW to N-S, reflecting lower crustal flow. The FVDs are roughly E-W at 60 km depth beneath the Qilian mountain range, which reflect the lower crustal flow that is blocked by a rigid terrane in the north. The lower crustal flow may lead to intra-crustal and crust-mantle decoupling and affect seismotectonics in NE Tibet.

Oligocene onset of uplift and inversion of the Cascadia forearc basin, southern Oregon Coast Range, USA

Abstract An extensive detrital zircon U-Pb data set (n = 6324 dates) from Eocene to Miocene sandstones and modern river sands establishes the onset of arc magmatism and forearc uplift along the Cascadia convergent margin in southwestern Oregon (United States). Middle to late Eocene marine strata in the Coos Bay area were primarily sourced from the Klamath Mountains and coeval Clarno-Challis volcanoes in central Oregon and/or Idaho. Ancestral Cascades arc magmatism initiated at 40 Ma and supplied sediment to a broad forearc basin in western Oregon during late Eocene time. Major reduction of Ancestral Cascades arc (40–12 Ma) and Clarno-Challis (52–40 Ma) zircon in the Tunnel Point Sandstone (ca. 33–30 Ma) records the isolation of the Coos Bay area from the Ancestral Cascades arc due to Oligocene onset of forearc uplift, basin inversion, and emergence of the southern Oregon Coast Range. The Tarheel formation (ca. 18–15 Ma) is characterized by disappearance of Ancestral Cascades arc zircon and a substantial increase in Clarno-Challis zircon recycled from underlying forearc strata. The ~15–20 m.y. delay between subduction initiation (ca. 49–46 Ma) and the onset of forearc uplift (ca. 33–30 Ma) supports insights from thermomechanical models that identify tectonic underplating and thermally activated lower-crustal flow as major drivers of deformation and uplift in active forearc regions.

3D electrical structure and crustal deformation of the Lajishan Tectonic Belt, Northeastern margin of the Tibetan Plateau

• A steep high-conductivity layer exists in the middle–lower crust to the south of the belt. • The high-resistivity body on the northern side may have blocked the high-conductivity layer, which formed the positive flower structure of the Lajishan tectonic belt and resulted in intense orogenic and seismic activities. • Crustal shortening may dominate the tectonic deformation of the NE Tibetan Plateau. The Lajishan tectonic belt is the most recently active orogenic belt in the northeastern Tibetan Plateau and represents a topographical boundary between the high Tibetan Plateau and the low-lying Longzhong basin to the east. Imaging the crustal structure beneath the Lajishan tectonic belt can improve our understanding about the regional tectonic deformation pattern, mechanisms of the outward growth of the Tibetan Plateau, and seismogenic environment in this area. In this study, we conducted two magnetotelluric (MT) profile measurements across the Lajishan tectonic belt in each section. Based on our newly-collected MT data and previous dataset in this area, a three-dimensional (3D) image of the deep electrical structure of the Lajishan tectonic belt is obtained by using 3D MT inversion and imaging technologies. Furthermore, by including 3D crustal deformation field information, a tectonic deformation model of the Lajishan tectonic belt and its adjacent area was constructed. Our present results show that a steep high-conductivity layer developed beneath the Lajishan tectonic belt and the adjacent Jianzha–Xunhua Basin and West Qinling Block, which then connected with a horizontal high-conductive layer underneath the Tibetan Plateau. Findings suggest that the Lajishan tectonic belt acts as a physical property boundary zone separating a high-resistivity body to the north from a deep high-conductivity layer in the middle-lower crust to the south. The high-conductivity layer, with a shallower depth from southwest to northeast, may be related to the NE-trending compression of the northeastern Tibetan Plateau, which was blocked by the high-resistivity body underneath the Xining basin. This led to the formation of a positive flower structure in the Lajishan tectonic belt, resulting in intense orogenic and seismic activities. The south-dipping Northern Lajishan Fault is the main boundary fault of the Lajishan tectonic belt and plays an important role in the mountain formation process, as suggested by asymmetry surface topography. The absence of a large-scale NE-SW-trending continuous high-conductivity layer in middle-lower crust indicates that the tectonic deformation of the northeastern Tibetan Plateau is dominated by a continuous upper-crustal compressing and shortening, instead of the lower-crustal flow.

Formation of the Hengshanli granitic gneiss dome in the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton

A group of granitic gneiss domes that may provide constraints on the tectonic evolution of early Precambrian orogenic belts are well preserved in the LiaoJi segment (or LJOB-Liaoji orogenic belt) of the Jiao-Liao-Ji Orogenic Belt (JLJOB), eastern North China Craton. To deeply understand the genesis of the domes and how the crustal mass flowed in collisional orogenic belt, we carried out structural analysis of the Hengshanli dome in the western part of the LJOB. Detailed analyses of deformation structures, microstructures and fabrics, and geochronology of syntectonic granitic dykes demonstrate that three episodes of structural deformation contributed to the formation of the dome structure. During D1, a pure shear-dominated deformation, penetrative foliations (S1), symmetrical lenses or boudinages, intrafolial folds (F1) and mineral lineations (L1) developed in the interlayered rocks of viscosity contrasts. The D2, the WNW-ESE orientated sub-horizontal simple shear-dominated deformation, is characterized by asymmetrical A-type folds (F2), stretching lineations (L2) , mylonitic foliations (S2), localized ductile shear zones, axial planar foliations, rotated porphyroblasts, L>S tectonites and S-C fabrics that indicate high-strain simple shear deformation. A D3 brittle deformation is evidenced by E-W directed open folds (F3) by flexural folding mechanism. LA-ICP-MS U-Pb dating of zircons from the syntectonic granitic dykes reveals that the main deformation event occurred from ca. 1946 to 1844 Ma. The dating also reveals that the Liaoji granites at the core were emplaced at ca. 2147 Ma and experienced anatexis at ca. 1864 Ma and 1846 Ma. It is suggested that the mechanism of doming is a combination of middle and lower crustal sub-horizontal flow, diapirism and folding superposition. Convergent flow of remobilized magma at depth and sub-horizontal simple shearing of metasedimentary rocks triggered middle and lower crustal flow in association with diapirism during the doming. The granitic gneiss domes are the products of dominant lateral and subsidiary vertical crustal mass flow under E-W directed compressional regime in the LJOB. We suggest that the orogenic belt experienced peak deformation at ca. 1946-1844 Ma, accompanied with syntectonic metamorphism and magmatism at nearly the same time.

Stratigraphic record of continental breakup, offshore NW Australia

AbstractContinental breakup involves a transition from rapid, fault‐controlled syn‐rift subsidence to relatively slow, post‐breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn‐breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth‐dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift‐associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup‐related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non‐erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra‐Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra‐Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth‐dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition.

Electrical resistivity structure across the Tethyan tectonic belt in western Yunnan, SW China

The Tethyan tectonic belt in western Yunnan, SW China, has witnessed a complex, multistage Tethyan evolution since the Paleozoic, including a late stage of continental collision between the Indian and Eurasian plates in the Cenozoic. Consequently, this belt has become a collage of Gondwana-derived microcontinental blocks and arc terranes wedged between two continents, including the Tengchong block, Baoshan block, Lincang block, and Simao block from west to east. To better understand the structure of these ancient continental blocks and the dynamic mechanism of the Indian-Eurasian continental collision, we investigate the electrical resistivity structure of the crust and uppermost mantle in the region. Magnetotelluric data were collected along a NW-SE striking transect crossing multiple tectonic terranes and were then modeled with a 3-D approach. The final resistivity model shows conductors beneath the Tengchong volcanic area, which may represent magma chambers that feed the active volcanism in the area. The electrical structure between the Tengchong area and the Wanding fault consists of layer-like lower crustal conductors along the transect, which may indicate mechanically weak layers where the tectonic escape of the Tibetan Plateau may occur via lower crustal flow. The Lincang block shows higher resistivity characteristics than the surrounding crust and demonstrates a trend of eastwardly napping resistors onto the Simao block, making up a crustal boundary of high-resistivity and low-resistivity bodies. This observation suggests that the Changning-Menglian suture between the Baoshan block and the Lincang block may be the boundary between Gondwana and Cathaysia.

Crustal shear wave velocity and radial anisotropy beneath Southern Africa from ambient noise tomography

Southern Africa is an amalgamation of a distinct Archean core, several Proterozoic mobile belts, and large igneous provinces, making it an ideal natural laboratory for studying Precambrian crustal evolution and deformation. Here we present a new radially anisotropic shear-velocity model of the crust and uppermost mantle beneath Southern Africa using Rayleigh- and Love-wave ambient noise tomography from multiple seismic networks. In the upper and middle crust, the distribution of the isotropic shear velocity correlates well with known surface geology. However, in the lower crust there is no systematic lateral variation of shear velocity between the Archean and Proterozoic terranes, which is likely due to the intensive modifications of Precambrian crust by several later thermo-tectonic events through processes such as lower crustal flow. The strong positive radial anisotropy (VSH > VSV) in the lower crust, when combined with weak azimuthal anisotropy imaged previously, suggests a complex flow pattern, the directions of which likely varied on local-scale due to inhomogeneous shortening or extension during the multiple post-cratonization thermo-tectonic events.

Timing and kinematics of the Variscan orogenic cycle at the Moldanubian periphery of the central Bohemian Massif

The high-grade metamorphic complexes along the northern Moldanubian periphery of the central Bohemian Massif provide an outstanding structural record of all episodes of the collisional evolution of the Variscan Orogeny. The kinematics and timing of the orogenic processes were examined by structural and microstructural studies of middle and lower crustal rocks combined with xenotime and monazite geochronology. Four distinct tectonic events were identified. A first relict sub-horizontal fabric S1 associated with high-pressure–high-temperature metamorphism was only developed in the lower crustal rocks and was related to back-arc extension or lower crustal flow in a supra-subduction domain. This fabric was completely reworked to the subvertical foliation S2 at c. 340 Ma by major collisional thickening, leading to juxtaposition of the lower and middle crust. Thereafter, extensional collapse of the thickened orogenic system caused strong refolding to the high-temperature sub-horizontal fabric at c. 325 Ma. The region was subsequently affected by NNE–SSW-oriented horizontal shortening related to dextral shearing and the clockwise rotation of crustal blocks adjacent to the large-scale dextral shear zone (the Elbe Zone). This led to fragmentation and reorientation of the Moldanubian margin to its current position. Supplementary material: A summary of the main microstructural features related to the defined deformation phases, the results of the U–Pb laser ablation inductively coupled plasma mass spectrometry analyses of xenotime and monazite, back-scattered electron images and rare earth element compositional maps are available at https://doi.org/10.6084/m9.figshare.c.5708800.v1

Laramide structure of southeastern Arizona: Role of basement-cored uplifts in shallow-angle subduction

AbstractLaramide reverse faults in southeastern Arizona commonly are obscured by mid- to late Cenozoic extension and subsequent cover, resulting in debate about their configuration and origin. A new mid-Cenozoic paleogeologic map depicts the structural configuration before extension, and new structural reconstructions characterize Laramide shortening in terms of structural style, magnitude, evolution, and timing.Reverse faults restore to moderate to high angles, are associated with fault-propagation folds, and involve significant basement and thus constitute thick-skinned deformation. The paleogeologic map suggests several major basement-cored block uplifts, many of which are newly identified. The largest uplifts may measure 150 km along strike, similar to those in the classic Laramide province of Wyoming and Colorado. Estimated shortening across the central study area is 14% or 23 km, whereas it is only 5% (9 km) to the north and 11% (12 km) to the south. Shortening by this mechanism is inadequate to explain previous estimates of crustal thickening in the region (∼50–60 km). Therefore, magmatic underplating, lower-crustal flow, or underplating of trench sediments and lithospheric material also may have contributed to thickening. Shortening largely occurred from 86 Ma to 64 Ma and possibly as late as 53 Ma, with initiation being younger to the northeast or north. Integration with data from southwestern New Mexico implies complex geometry for the subducting flat slab. Finally, reverse faults generally do not appear to have reactivated older faults, as previously suggested, primarily because reverse faults have associated fault-propagation folds in rocks that predate supposed reactivated structures.

Layered crustal azimuthal anisotropy beneath the northeastern Tibetan Plateau revealed by Rayleigh-wave Eikonal tomography

The uplift of the Tibetan Plateau in the Cenozoic has greatly affected the tectonics in Asia. But the evolution of the plateau is still unclear. The northeastern Tibetan Plateau is under ongoing expansion and is therefore essential for understanding the tectonic evolution of the plateau. Using data from 675 stations deployed by the ChinArray project, we obtained an anisotropic shear-wave velocity model by Rayleigh-wave phase-velocity Eikonal tomography and further investigated layered anisotropy and deformation patterns in the northeastern Tibetan Plateau. In the upper crust (<20 km depth), the fast directions are parallel with major strike-slip faults. In the lower crust, the fast directions are mostly margin-parallel along the plateau margin (at depths of ∼20 km to Moho), but they are perpendicular to the plateau margin while parallel to the topographic gradient under the high plateau (at depths of ∼40 km to Moho). Our result indicates that there is lower crustal flow under the high plateau due to the differential lateral gravity potential. However, the deep crustal flow is blocked by the surrounding blocks, which results in pure shear along the plateau margin in the northeastern Tibetan Plateau.

Oligocene-Early Miocene exhumation and shortening along the Anninghe fault in the southeastern Tibetan Plateau: insights from zircon and apatite (U-Th)/He thermochronology

ABSTRACT The growth mechanisms of the southeastern Tibetan Plateau are debated between models of Oligocene shortening and lateral extrusion of coherent crustal blocks versus Late Miocene flow of low-viscosity lower crust. To test these models, we examine brittle deformation of the north-striking Anninghe fault, which has been acting as a left-lateral strike-slip fault since Late Miocene time, using structural and bedrock low-temperature thermochronologic methods. Different from previous interpretations of Late Miocene strike-slip motion, our structural observations indicate westward contractional dip-slip along the southern segment of the fault. New zircon and apatite (U-Th)/He thermochronologic data along a vertical profile of Precambrian volcanoclastic cover both sides of the Anninghe fault. The age-elevation relationships and inverse thermal modelling suggest a phase of Oligocene-Early Miocene (~24-18 Ma) differential rock exhumation and cooling across the fault, with a mean erosion rate of ~0.30 ± 0.12 km/m.y. and ~0.007 ± 0.007 km/m.y. in the hanging wall and footwall, respectively. Consistent with the structural observations, the higher rate of rock exhumation in the hanging wall indicates an Oligocene-Early Miocene phase of west-verging reverse faulting. Combined with previously published data from adjacent sites, we suggest that the Anninghe fault has experienced a tectonic transition from Oligocene-Early Miocene east-west contraction to Late Miocene strike-slip motion. The early to Mid-Cenozoic crustal shortening contributed to the uplift of the southeastern Tibetan Plateau, supporting the crustal shortening and extrusion model.

Crustal-scale wedge tectonics at the narrow boundary between the Tibetan Plateau and Ordos block

The Tibetan plateau is bounded in part by cratonic blocks, and in part by tectonothermally younger and weaker crust. How the Tibetan plateau interacts with bordering non-cratonic crust is a continuing debate, whether the plateau is currently enlarging its area, and if so, whether by thrusting or by lower-crustal flow, or both. The gently sloping northeastern margin of Tibet has been inferred to incorporate a lower-crustal channel flow outward towards the Ordos block that is a part of the North China Craton. We calculated receiver functions using teleseismic waveforms recorded by a dense nodal seismic array crossing from the Longxi basin of northeastern Tibet across a transition zone marked by the Liupan Shan (mountains) into the Ordos block. Our image shows the Longxi upper crust overthrusts onto the Ordos block building the Liupan Shan, and the Longxi lower crust underthrusts beneath the Ordos forming a 10-km thick crustal root. The Ordos crust forms a crustal-scale wedge inserted into the Longxi crust, and deformation is limited to a narrow zone of <50 km either side of the Liupan Shan. Convergence between the Longxi Basin and Ordos block is producing a narrow mountain range rather than broadening the plateau. The larger-scale gentle topography is likely an expression of negligible crustal deformation of the Longxi basin. Even if the Longxi basin of NE Tibet, between the Qaidam and Sichuan basins, is underlain by a weak lower crust, tunneling channel flow does not extend beyond the Liupan Shan, and the Tibetan Plateau has not grown outward beneath the Ordos block.

Crustal structure underneath central China across the Tibetan Plateau, the North China Craton, the South China Block and the Qinling-Dabie Orogen constrained by multifrequency receiver function and surface wave data

Abstract Investigating the crustal structure at the juncture region of the Tibetan Plateau, the North China Craton, the South China Block and the Qinling-Dabie Orogen is important to understand processes of uplift of the Tibetan Plateau and the craton destruction and preservation. In this study, we dropped the H-k stacking of receiver functions (RFs), RF nonlinear inversion, and joint inversion of RF and surface wave dispersion with multiple Gaussian factors successively to invert the crustal thickness (H), average Vp/Vs ratio (k) and S-wave velocity in the juncture region. The imaging results and the synthetic tests of multifrequency H-k stacking were further used to study the crustal structure and related tectonic evolution. Our results show that the H and k values generally increase and decrease, respectively, as the Gaussian factor increases in the study region, which is consistent with synthetic tests of H-k stacking. The cores of the Ordos Block and the Sichuan Basin exhibit a typical cratonic crust, whereas the Zhongtiao Mountains and adjacent Qinling-Dabie Orogen to the south may have undergone delamination of the lower crust. Local low-velocity anomalies in the middle-lower crust were observed in the Western Qinling Orogen and Qilian Orogen, whereas high S-wave velocities in the middle-lower crust were observed at the juncture region between the Western Qinling Orogen and the Qinling-Dabie Orogen. Our results suggest that the lower crustal flow is probably restricted to the central area in the Western Qinling Orogen and does not flow into the Qilian Orogen or the Qinling-Dabie Orogen.

The Cenozoic Evolution of Crustal Shortening and Left‐Lateral Shear in the Central East Kunlun Shan: Implications for the Uplift History of the Tibetan Plateau

AbstractThe timing of crustal shortening and strike‐slip faulting along the East Kunlun Shan provides insight into the history of surface uplift and may constrain the time at which the Tibetan Plateau reached high elevations. We investigate a series of extensional basins and restraining bends along the Xidatan strand of the Kunlun strike‐slip fault, which provide an ideal setting to unravel the tectonic history of the northern plateau margin. We present new apatite (U‐Th)/He, apatite fission track, and zircon (U‐Th)/He ages and QTQt thermal modeling, 40Ar/39Ar fault gouge dating, and structural mapping from the central East Kunlun Shan. Our data suggest that the East Kunlun Shan experienced slow to negligible exhumation until late Cretaceous time, followed by an increase in rate by 65–50 Ma. Along with a ~47 Ma fault gouge age, we posit that the Paleocene–early Eocene was a time of crustal shortening along the northern plateau. Rapid exhumation along transpressional portions of the Xidatan fault initiated by 23–20 Ma, which we interpret as the local onset of strike‐slip faulting. An early Miocene transition from north‐south crustal shortening to left‐lateral shear along the East Kunlun Shan, the onset of normal and strike‐slip faulting in central and southern Tibet by 18 Ma, and lower crustal flow in eastern Tibet by 13 Ma suggest the establishment of orogen‐wide east‐west oriented extension and extrusion by the middle Miocene. The plateau‐wide shift in stress accommodation implies that high gravitational potential energy, and likely high elevation, was attained by the middle Miocene.