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The left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene. Plain Language Summary The Altyn Tagh Fault (ATF), serving as the northern boundary of the Tibetan Plateau, demarcates the Tarim Basin from the Qaidam Basin. Understanding how intracontinental deformation across the boundary region would better inform the uplift and expansion of the plateau. This study reveals the fine crustal structure by analyzing seismic data from a ∼NW–SE oriented linear dense array across the central ATF. Combined with fault slip rates, we propose that the Tarim lower crust is underthrusting beneath the Tibetan Plateau, leading to the extrusion of upper crustal materials and the rapid uplift of the Altyn Tagh Range since the late Miocene, which provides insight into the lateral growth of the plateau. Key Points Detailed crustal structure beneath the central Altyn Tagh Fault was imaged by receiver functions of a dense 2‐D seismic array The Tarim lower crust is underthrusting to ∼75 km depth beneath the Tibetan Plateau The Altyn Tagh range was uplifted rapidly since late Miocene through the thickening of the upper crust

The Cenozoic Qilian Shan thrust belt is the northern margin of the Tibetan Plateau, which developed in part due to progressive India-Asia convergence during Himalayan-Tibetan orogeny. Available geologic observations suggest that this thrust belt started deforming shortly after initial India-Asia collision at 60-55 Ma, and thus its kinematic development is intrinsically related to the construction and evolution of the Tibetan Plateau. Here, we present new field observations from a geologic traverse across the Qilian Shan to elucidate the style of deformation across the active thrust belt. In particular, we infer protracted out-of-sequence deformation here that is consistent with this thrust system remaining a stationary northern boundary to the Tibetan Plateau since the early Cenozoic. We present a lithosphere-scale model for this region that highlights the following: (1) coupled distributed crustal shortening and underthrusting of the North China craton beneath Tibet, which explains the spatial and temporal distribution of observed crustal shortening and thickness, (2) this underthrusting exploited the south-dipping early Paleozoic Qilian suture paleo-subduction melange channel, and (3) development of a lower-crustal duplex at the lithospheric underthrusting ramp. This last inference can explain the relatively high elevation, low relief, and thickened crust of the central Qilian Shan, as well as the comparative aseismicity of the region, which experiences fewer earthquakes due to less upper-crustal faulting. Both the northern and southern margins of the Himalayan-Tibetan orogen appear to have developed similarly, with continental underthrusting and crustal-scale imbrication and duplexing, despite vastly different climatic and plate-velocity boundary conditions, which suggests that the orogen-scale architecture of the thrust belt is controlled by neither of these forcing mechanisms. Instead, strength anisotropies of the crust probably control the kinematics and style of deformation, including the development of northern Tibet, where thrust systems are concentrated along pre-Cenozoic suture zones.

Crustal shortening and thickening to c. 70-85 km in the Tibetan Plateau occurred both before and mainly after the c. 50 Ma India-Asia collision. Potassic-ultrapotassic shoshonitic and adakitic lavas erupted across the Qiangtang (c. 50-29 Ma) and Lhasa blocks (c. 30-10 Ma) indicate a hot mantle, thick crust and eclogitic root during that period. The progressive northward underthrusting of cold, Indian mantle lithosphere since collision shut off the source in the Lhasa block at c. 10 Ma. Late Miocene-Pleistocene shoshonitic volcanic rocks in northern Tibet require hot mantle. We review the major tectonic processes proposed for Tibet including \"rigid-block', continuum and crustal flow as well as the geological history of the major strike-slip faults. We examine controversies concerning the cumulative geological offsets and the discrepancies between geological, Quaternary and geodetic slip rates. Low present-day slip rates measured from global positioning system and InSAR along the Karakoram and Altyn Tagh Faults in addition to slow long-term geological rates can only account for limited eastward extrusion of Tibet since Mid-Miocene time. We conclude that despite being prominent geomorphological features sometimes with wide mylonite zones, the faults cut earlier formed metamorphic and igneous rocks and show limited offsets. Concentrated strain at the surface is dissipated deeper into wide ductile shear zones.

Successive indentations of Eurasia by India have led to the Tibet-Himalaya E–W orthogonal collision belt and the SE Tibetan Plateau N–S oblique collision belt along the frontal and eastern edges of the indenter, respectively. The belts exhibit distinctive lithospheric structures and tectonic evolutions. A comprehensive compilation of available geological and geophysical data reveals two sudden tectonic transitions in the early Eocene and the earliest Miocene, respectively, of the tectonic evolution of the orthogonal belt. Synthesizing geological and geochronological data helps us to suggest a NEE–SWW trending, ~450 km-long, ~250 km-wide magmatic zone in SE Tibet, which separates the oblique collision belt (eastern and SE Tibet) into three segments of distinctive seismic structures including the mantle and crust anisotropies. The newly identified Yongping basin is located in the central part of the magmatic zone. Geochronological and thermochronological data demonstrate that (1) this basin and the magmatic zone started to form at ~48 Ma likely due to NNW–SSE lithosphere stretching according to the spatial coincidence of the concentrated mantle-sourced igneous rocks on the surface with the seismic anomalies at depth; and (2) its fills was shortened in the E–W direction since ~23 Ma. These two dates correspond to the onset of the first and second tectonic transitions of the orthogonal collision belt. As such, both the orthogonal and oblique belts share a single time framework of their tectonic evolution. By synthesizing geological and geophysical data of both collision belts, the indenting process can be divided into three stages separated by two tectonic transitions. Continent–continent collision as a piston took place exclusively during the second stage. During the other two stages, the India lithosphere underthrust beneath Eurasia.

The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ∼9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.

At subduction zones, downgoing topographic features exert first-order structural and hydrologic effect on the plate boundary and the upper plate. Such process has been rarely documented by clear observations, especially at great depths, and it remains elusive how the altered structural and physical characteristics of the upper plate control seismogenic behavior and tectonic evolution of margins. Here, we present a reprocessed multichannel seismic (MCS) profile together with bathymetry and earthquake data in the central Lesser Antilles. A reflector imaged at 15-18 km depth ahead of the Tiburon ridge delimits the base of inner forearc crust with pervasive reflective anomalies. It is interpreted to represent a shallow fluid-rich decollement warped over the rough topography, where the underlying materials consist largely of oceanic sediments identical to those accreted at the Barbados prism and basement fragments from basal erosion. Our results suggest that fluids are expelled upward from the band of subducted sediments, leading to a NW-SE elongated zone of hydrofractured and weakened crust above a serpentinized mantle corner coinciding with a prominent aseismic corridor. The high interplate seismic activity offshore Martinique at ~30–65 km depths may correspond to deeply subducted indurated sediments that act as a strong asperity on the plate interface. Deep subducted sediments along the Lesser Antilles subduction zone act as a strong asperity on the plate interface and release fluids that drive hydrofracturing and interplate seismicity, according to analysis of a reprocessed multichannel seismic profile.

The Swakane Gneiss, interpreted to represent sedimentary strata metamorphosed at 8-12 kbar, is the deepest exposed crustal levels within the exhumed North Cascades continental magmatic arc, yet the nature and age of its protolith and the mechanism by which it was transported to deep-crustal levels remains unclear. Zircons from 11 paragneiss and schist samples were analyzed for U-Pb age and Hf-isotope composition in order to investigate the tectonic history of the Swakane Gneiss from protolith deposition to metamorphism within the North Cascades arc. Zircons interpreted to have crystallized in situ during metamorphism and/or melt-crystallization within the Swakane Gneiss at depth have ca. 74-66 Ma ages. Detrital-zircon age and Hf-isotope characteristics demonstrate provenance shifts that correlate with maximum depositional ages of ca. 93-81 Ma. Samples deposited between ca. 93 and 88 Ma have dominantly Mesozoic age peaks with initial εHf values between depleted mantle and chondritic uniform reservoir (CHUR), whereas ca. 86-81 Ma sample show the addition of distinct Proterozoic populations (ca. 1380 and 1800-1600 Ma) and Late Cretaceous zircons with unradiogenic Hf-isotope compositions. Similar detrital-zircon age and Hf-isotope patterns are observed in several Upper Cretaceous forearc and accretionary wedge units between southern California and Alaska along the North American continental margin. The connection between the Swakane Gneiss and these potential protoliths located outboard of Cordilleran arc systems indicate burial by either underplating of accretionary-wedge sediments or underthrusting of forearc sediments. Therefore, the protolith and incorporation history for the Swakane Gneiss is likely similar to those of deep crustal metasedimentary units elsewhere in the North Cascades (i.e., the Skagit Gneiss Complex) and to the south along the continental margin (i.e., the Pelona-Orocopia-Rand schists and Schist of Sierra de Salinas). These observations suggest that burial of sediment outboard of continental magmatic arc systems may be a major mechanism for the transfer of sediment to the deep levels of continental arcs.

AbstractThe morphostructures and paleoseismicity of the southern slope of the Kungei Ala-Too Range (Northern Tien Shan) have been studied. It has been found that the main seismogenic structures in the Late Pleistocene and Holocene were adyr (foothill) faults, that is, thrusts, strike-slips, and normal dips. With a general thrusting of block folds of the basement southwards (toward the Issyk-Kul Basin), there are also northward-moving underthrusts. The past seismic activity along the adyr faults is expressed by single seismogenic scarps, as well as by stair-like series of alluvial-proluvial and fluvioglacial surfaces, upthrown in the hanging walls of faults. In addition to the easily identifiable vertical component of slips along the faults, we have also identified significant horizontal slips, which often greatly exceed the vertical ones. In addition, a unique structure, that is, a neotectonic ramp graben bounded from the south and north by oppositely directed thrusts, was studied on the slope of the Kungei Range. The mentioned faults were also studied by trenching. It was found that in the middle–second half of the Holocene three strong morphogenetic seismic events occurred on the South Tegerek adyr fault and three strong earthquakes occurred on the North Tegerek adyr fault. The ages of four strong earthquakes were determined by the radiocarbon method. The maximum ages of the first three events are 3970–3755 BC, 220–405 AD, and 1380–1450 AD, while the minimum age of the last event is 1720–1815 AD. The magnitudes of historical and paleoearthquakes that occurred on the studied adyr faults range from M = 6.6–6.9 ± 0.5 (South Tegerek fault) to M = 6.8–7.4 ± 0.5 (Kultor adyr fault). Seismic events of similar magnitudes lead to deformations of the earth’s surface and in building constructions, which can be estimated on a macroseismic scale at I ≥ IX. Our data obtained in this study can be used for a comparative analysis of similar post-platform orogens in Russia (e.g., Altay and Sayan mountains), as well as in compiling the new general seismic zoning map for the Kyrgyz Republic.

The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M> 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro‐oki and 2003 Tokachi‐oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of ∼3 km/s, has been ruptured by the 1952 Tokachi‐oki earthquake. The seismic gap has also slipped incidental to neighboring asperities. The difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface, such as a reflectivity of the plate interface. We estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun–ocean bottom seismometer experiment on the along‐trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi‐oki source area including the seismic gap, rather than in the 1973 Nemuro‐oki source area. The strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi‐oki and 1973 Nemuro‐oki earthquakes. Our results suggest that the 1952 Tokachi‐oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake. Key Points Reflectivity survey in the Kuril Trench subduction zone, SE off Hokkaido, Japan Strong variation of the reflectivity along the Kuril Trench Two features of the 1952 Tokachi‐oki earthquake: tsunami and typical underthrust