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1. Under-thrusting of continental lithospheric mantle controls the architecture of crustal deformation: numerical modelling of the northern margin of the Tibetan Plateau

   https://doi.org/10.1144/jgs2024-183  

   Chen, Qizhi; Hu, Caibo; Zuza, Andrew_V; Zhang, Kai-jun; Zhang, Huai; Shi, Yaolin (February 2025, Journal of the Geological Society)

   Lithospheric shortening can be described by one of two end-member modes: indentation of the lithosphere and subduction of the lithospheric mantle. Deciphering the difference between these modes is crucial in the interpretation of past and present orogens and in predicting their structural architecture at depth. It is therefore important to establish how observable upper crustal proxies reflect deep lithospheric kinematics and dynamics. Over the last few decades, geological and geophysical data have provided valuable constraints on the northern margin of the Tibetan Plateau. This margin is defined by the Qilian Shan thrust belt, which developed in response to the far-field convergence between the Indian and Eurasian plates. The primary mechanism for this development is the southward subduction of the Asian lithospheric mantle beneath the Tibetan Plateau. We conducted numerical modelling to simulate the kinematics and response of the upper crust to the southward subduction of the lithospheric mantle. Our results show that subduction of the lithospheric mantle can result in upper crustal deformation that matches the records in the Qilian Shan, where pure shear shortening alone does not generate similar upper crust proxies, including the broad width and architecture of the bivergent orogenic wedge, the timing of fault initiation and evolution, seismicity and fault activity, the topography and geomorphology. The geometry of the subducting lithosphere impacts the width and asymmetry of the bivergent orogenic wedge. Our results demonstrate how records of crustal strain can be used to better interpret the deep structural architecture of past and present orogenic wedges. 

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2. Whole-lithosphere shear during oblique rifting

   https://doi.org/10.1130/G49603.1  

   Lutz, Brandon M.; Axen, Gary J.; van Wijk, Jolante W.; Phillips, Fred M. (December 2021, Geology)

   Abstract Processes controlling the formation of continental whole-lithosphere shear zones are debated, but their existence requires that the lithosphere is mechanically coupled from base to top. We document the formation of a dextral, whole-lithosphere shear zone in the Death Valley region (DVR), southwest United States. Dextral deflections of depth gradients in the lithosphere-asthenosphere boundary and Moho are stacked vertically, defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of shear. These deflections underlie an upper-crustal fault zone that accrued ~60 km of dextral slip since ca. 8–7 Ma, when we infer that whole-lithosphere shear began. This dextral offset is less than net dextral offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca. 8–7 Ma, weak middle crust decoupled upper-crustal deformation from deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma, detachment slip thinned, uplifted, cooled, and thus strengthened the middle crust, which is exposed in metamorphic core complexes collocated with the whole-lithosphere shear zone. Midcrustal strengthening coupled the layered lithosphere vertically and therefore enabled whole-lithosphere dextral shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal strengthening is probably a necessary condition for whole-lithosphere shear. 

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3. Relict Back‐Arc Basin Crustal Structure in the Western Greater Caucasus, Georgia

   https://doi.org/10.1029/2024GC012036  

   Vasey, Dylan A; Cowgill, Eric; VanTongeren, Jill A; Anderson, Catherine O (May 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Back‐arc basins frequently form within subduction zones, creating sources of lithospheric weakness that can accommodate subsequent compressional deformation. The crustal structure of these basins, including whether they contain extended preexisting crust and/or new crust formed by seafloor spreading, can thus exert a major influence on strain partitioning in orogenic belts. Here, we present field observations, petrographic analyses, and major/trace element geochemical data from the Caucasus Basin, a back‐arc basin that initiated in continental lithosphere in the Jurassic and subsequently localized deformation in the present‐day Greater Caucasus during the latter stages of Cenozoic Arabia‐Eurasia continent‐continent collision. Our results reveal distinct lithologic and geochemical domains separated by south‐vergent thrust faults within the North Georgia fault system (NGFS) in the Republic of Georgia. Along the Enguri River, shallow intrusive and volcanic rocks are thrust over dominantly volcaniclastic cover, whereas along the Tskhenistskali River, intrusions into metasedimentary rocks are juxtaposed against volcanic flows. The presence of a minor depleted mantle geochemical signature in intrusive rocks from the Tskhenistskali traverse supports an episode of Jurassic seafloor spreading in the Caucasus Basin, with the resulting lithosphere facilitating Cenozoic basin closure by north‐dipping subduction during Arabia‐Eurasia collision. The Khaishi fault along the Enguri River and the Lentekhi fault along the Tskhenistskali river mark major juxtapositions in back‐arc crustal structure and may be components of the terminal suture indicating Caucasus Basin closure. Our results highlight how magmatic rocks in relict basin rocks can yield key insights into basin structure and orogenesis, even when no ophiolite is present. 

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4. Mode of intracontinental mountain building controlled by lower crustal composition and mantle lithosphere depletion

   https://doi.org/10.1038/s41467-025-63468-1  

   Xu, Xi; Zuza, Andrew V; Gerya, Taras; Chen, Lin; Kuang, Xingtao; Chen, Hanlin; Wang, Baodi; Liu, Jingao; Shi, Xuhua; Sun, Yanyun; et al (December 2025, Nature Communications)

   Tectonic plate convergence is accommodated across the continental lithosphere via discrete lithospheric subduction or distributed shortening and thickening. These end-member deformation modes control intra-plate mountain building, but their selection mechanism remains unclear. The variable composition of the continental crust and lithospheric mantle, which impacts its density and rheology, can be inferred by the distribution of magnetic-indicated crustal iron. Here we demonstrate that vertically coherent pure-shear shortening dominated the active Tian Shan orogen, central Asia, based on high-resolution aeromagnetic imaging and geophysical-geodetic observations. Integrating these findings with thermomechanical collisional models reveals that the mode of intracontinental deformation depends on contrasts in lower crust composition and mantle lithosphere depletion between the converging continents and central orogenic region. Distributed shortening prevails when the converging continents have a more iron-enriched mafic crust and iron-depleted mantle lithosphere when compared to the intervening orogenic region. Conversely, continental subduction occurs without such lithospheric contrasts. This result explains how the Tian Shan orogen formed via distributed lithospheric thickening without continental subduction or underthrusting. Our interpretations imply that iron distribution in the crust correlates with lithospheric compositional, density, and rheological structure, which impacts the preservation and destruction of Earth’s continents, including long-lived cratons, during intracontinental orogeny. 

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5. Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith

   Klepeis, K; Schwartz, J; Miranda, E; Baskin, J; Castro_Mendez, A; Mora_Klepeis, G (April 2025, Geological Society of America bulletin)

   Structural analyses combined with U‐Pb zircon petrochronology show the influence of arc magmatism on the evolution of two transpressional shear zones in the deep root of the Late Cretaceous Southern California batholith. The mid-crustal Black Belt and lower-crustal Cucamonga shear zones (eastern San Gabriel Mountains) formed at ~84 Ma shortly after a large mass of tonalite and granodiorite intruded the lower crust. Both shear zones were active until at least ~74 Ma and probably until 72-70 Ma. In the mid-crustal shear zone, rheological contrasts between mingling magmas localized deformation at dike margins. The deformation began as hypersolidus flow in partially crystallized dikes and then transitioned to deformation below the solidus when alternations between viscous creep and brittle faulting produced interlayered pseudotachylyte, cataclasite, and mylonite. As the dikes solidified, strain hardening drove shear zone growth and created thin (10-30 m) high-strain zones and faults that are widely spaced across ~1 km. In contrast, the lower-crustal Cucamonga shear zone was magma-starved, lacks the variety of shear zone fabrics exhibited by its mid-crustal counterpart, and formed by the reactivation of a pre-existing fabric that records pure reverse displacements at 124-93 Ma. The two shear zones created a partitioned style of intra-arc transpression where sinistral-reverse (mostly arc-parallel with some arc-oblique) displacements were accommodated on moderately dipping faults and shear zones and arc-normal shortening was accommodated by coeval folds. This study shows how a magmatic surge influenced the architecture and style of Late Cretaceous transpression in the Southern California batholith, including the evolution of high-strain zones that record alternating episodes of brittle, ductile, and hypersolidus deformation. The results illustrate how magmatism localizes strain on deep-crustal faults during orogenesis and oblique convergence. 

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