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1. Crust and Mantle Flow From Central Tibetan Plateau to the Indo‐Burma Subduction Zone

   https://doi.org/10.1029/2023JB027540  

   Timsina, Prakash; Hearn, Thomas M; Ni, James F (October 2024, Journal of Geophysical Research: Solid Earth)

   Bostock, Michael (Ed.)

   Abstract The extremely oblique Indo‐Burma subduction zone exhibits dextral strike‐slip faulting along the Sagaing, Kabaw, and Churachandpur‐Mao Faults as well as east‐west shortening between the Sagaing Fault and Bengal Basin. Through regional stress analysis, considering areas from central Tibet, around the eastern Himalaya Syntaxis, to Burma, it has been determined that the principal compressive stress directions align with the principal strain rates. The northeast‐southwest oriented compressive stress direction from the western Shan Plateau continues into Burma. Notably, P axes align with the topographic gradients, and T axes are sub‐parallel to the topographic contours in the Shan Plateau region south of 27°N. These stress patterns are consistent with a gravitational potential energy induced crustal and mantle flow. The alignment of the fast shear wave with the maximum strain rate and the colinear NW‐SE to E‐W fast direction of the SKS wave and T axis determined from focal mechanisms in the Shan Plateau suggest that the mantle lithosphere deforms in concert with the crust. We suggest crust and mantle flow south of the Red River Fault has resulted in widening of the lithosphere in the Shan Plateau in an east‐west direction. Therefore, the Sagaing Fault has bowed approximately 50–100 km westward if we assume that the Sagaing Fault was originally straight. Our results of regional stress inversion are consistent with late Miocene to present E‐W shortening in the Indo‐Burma subduction zone resulting from the release of gravitational potential energy from the central Tibetan Plateau. 

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2. KINEMATICS OF COMPLEX MULTI-FAULT EARTHQUAKE RUPTURE RECORDED BY PSEUDOTACHYLYTES FROM THE IKERTÔQ SHEAR ZONE, GREENLAND: AN NSF REU STUDY

   https://doi.org/10.1130/abs/2022AM-380367  

   Lubin, Ella; Burns, Brooke; Calderon, Anna; Allen, Joseph L; Shaw, Colin (January 2022, Geological Society of America)

   Flow connectivity between master and relay faults in the Ikertôq shear zone demonstrates that multiple ruptures during ancient earthquakes occurred during a single seismic event. The Ikertôq shear zone (ISZ) is part of the Paleoproterozoic Nagssuqtoqidian orogeny continental collision in West Greenland that includes a > 50 km pseudotachylyte system. As part of an NSF REU, this team mapped various faults throughout a 2 km transect on high-resolution UAV images of exhumed pseudotachylyte vein systems on the western end of Sarfannguit island to investigate the kinematics of multi-fault ruptures during individual seismic events. Pseudotachylyte veins exhibit a complex rupture geometry with linked kinematics between oblique reverse master faults striking approximately 240 and steep east-west relay faults dominated by strike-slip movement. Near complete exposure of veins provide a unique opportunity to document fault linkages and the partitioning of slip, including the interconnectivity of flow patterns of melt in pseudotachylyte veins, as well as angular ladders of melt. We measured the thickness of pseudotachylyte fault veins and injection veins along transects to examine slip partitioning between multiple reverse faults and strike-slip relay faults. Melt thickness is used as a proxy for earthquake slip since the pseudotachylyte melt occurred on faults that exhibit preexisting brittle displacement. The results of preliminary calculations from energy balance equations show that typical slip on some oblique reverse master faults was on the order of a meter or less, while typical slip on some east-west relay faults was cm scale. Our data clarify that most slip occurred on oblique reverse master faults with subsidiary slip on east-west relay faults. 

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3. A Reappraisal of Nappe Structures and Strain Patterns in the Core of the Variscan Orogen (French Massif Central)

   https://doi.org/10.1029/2024TC008366  

   Vanardois, Jonas; Trap, Pierre; Teyssier, Christian; Whitney, Donna L (October 2024, Tectonics)

   Abstract Major influences on the architecture of orogens include the overall thermal conditions of orogeny (hot vs. cold) and the angle of collision (orthogonal vs. oblique). In the French Massif Central of the Variscan orogen, a cold‐orogen style crustal nappe architecture was interpreted in the Haut‐Allier, located in the core of the orogen. Based on this model, the Variscan orogenic crust is described as the superposition of three main allochthonous tectonic units juxtaposed along major thrust faults. However, based on a detailed structural analysis, we have found instead that the region is dominated by a network of anastomosing conjugate and coeval dextral and sinistral shear zones striking WNW‐ESE and ENE‐WSW, respectively. The dextral WNW‐trending shear zones are predominant, whereas the sinistral NE shear zones are mainly located in the eastern part of the massif. Between these sub‐vertical shear zones, a sub‐horizontal foliation is observed. Our results indicate that all planar fabrics were partially synchronous during suprasolidus low‐pressure‐high‐temperature conditions. Strain partitioning occurred from high‐temperature suprasolidus conditions to subsolidus retrogression and may represent orogen‐parallel flow, suggesting hot‐orogen style. These results call into question the validity of the crustal nappe model in the Haut‐Allier. Based on new structural data and related observations, we propose a new model in which metamorphic gaps between tectonic units are explained by the juxtaposition of different structural domains by displacement along strike‐slip shear zones. 

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4. Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

   https://doi.org/10.1130/GES02409.1  

   Say, Michael C.; Zuza, Andrew V. (September 2021, Geosphere)

   Abstract The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, magnitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the westernmost structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (∼15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (∼150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ∼150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable. 

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5. Bhumichula plateau: A remnant high-elevation low-relief surface in the Himalayan thrust belt of western Nepal

   https://doi.org/10.1130/B36481.1  

   Sherpa, Tshering Z.L.; DeCelles, Peter G.; Carrapa, Barbara; Schoenbohm, Lindsay M.; Wolpert, Joshua (December 2022, GSA Bulletin)

   The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya. 

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