Chen, Siyu

During the collision of India and Eurasia, regional-scale strike-slip shear zones played a key role in accommodating lateral extru- sion of blocks, block rotation, and vertical ex- humation of metamorphic rocks as presented by deformation on the Ailao Shan-Red River shear zone (ARSZ) in the Eastern Himala- yan Syntaxis region and western Yunnan, China. We report structural, mica Ar/Ar, apatite fission-track (AFT), and apatite (U- Th)/He (AHe) data from the Diancangshan massif in the middle segment of the ARSZ. These structural data reveal that the massif forms a region-scale antiform, bordered by two branches of the ARSZ along its eastern and western margins. Structural evidence for partial melting in the horizontal mylonites in the gneiss core document that the gneiss experienced a horizontal shear deformation in the middle crust. Muscovite Ar/Ar ages of 36–29 Ma from the core represent cooling ages. Muscovite Ar/Ar ages of 25 and 17 Ma from greenschist-facies mylonites along the western and southern shear zones, respec- tively, are interpreted as recording deforma- tion in the ARSZ. The AFT ages, ranging from 15 to 5 Ma, represent a quiescent gap with a slow cooling/exhumation in the mas- sif. AHe results suggest that a rapid cooling and final exhumation episode of the massif could have started before 3.2 Ma, or likely ca. 5 Ma, and continue to the present. The high-temperature horizontal shearing layers of the core were first formed across the In- dochina Block, locally antiformed along the tectonic boundaries, and then cooled through the mica Ar-Ar closure temperature during Eocene or early Oligocene, subsequently re- worked and further exhumed by sinistral strike-slip movement along the ARSZ during the early Oligocene (ca. 29 Ma), lasting until ca. 17 Ma, then final exhumation of the mas- sif occurred by dextral normal faulting on the Weixi-Qiaohou and Red River faults along the limbs of the ARSZ since ca. 5 Ma. The formation of the antiform could indicate local crustal thickening in an early transpressional setting corresponding to India-Asia conver- gence. Large-scale sinistral ductile shear along the ARSZ in the shallow crust accom- modated lateral extrusion of the Indochina Block, and further contributed to the vertical exhumation of the metamorphic massif from the late Oligocene to the middle Miocene. Furthermore, the change of kinematic re- versal and associated cooling episodes along the ARSZ since the middle Miocene or early Pliocene imply a tectonic transfer from strain localization along the major tectonic bound- aries to continuous deformation correspond- ing to plateau growth and expansion.

Existing models describing continental crust deformation require the coexistence of strike‐slip faults and crustal blocks rotating between them, although the dimension and shape of the blocks and the location and offset of the faults are mostly unconstrained. Here we report on the paleomagnetism of middle Cenozoic (<45 Ma) continental red beds exposed along the 40 km long and 2–8 km wide NW‐trending Mula basin (East Tibet), unconformably lying above Triassic marine strata and plutons and mildly deformed by two subparallel thrust faults. A tectonic magnetic fabric and magnetic lineations subhorizontal and parallel to the compressive fronts show that thrust tectonics guided basin formation and continued soon after sediment deposition. Characteristic and high‐temperature components isolated at 17 sites support a positive fold test and suggest primary detrital magnetization acquisition. The comparison with East Asia paleopoles defines several 2–5 km wide crust fragments yielding variable rotations from ~30° counterclockwise to ~90° clockwise without clear rotation trend. No strike‐slip fault with offset exceeding 1 km occurs among blocks, and no regional‐scale strike‐slip fault is documented at basin vicinity, implying that the East Tibet rotation pattern is different from all existing block rotation models. A regional high thermal flow and vigorous geothermal activity are consistent with the occurrence of a ductile crust layer identified by seismological data at 13–30 km depths. We suggest that midlower crust, flowing SE‐ward toward Indochina, drag upper crust fragments that were randomly rotated depending on the local torque exerted on lower block boundaries by a ductile crust flow.

The Cenozoic deformation of SE Asia is classically related to India‐Asia collision and Tibet Plateau rise, supposedly resulting in the southeastward drift of lithospheric blocks bounded by strike‐slip faults with displacements in the order of 1,000 km. Here we report on the paleomagnetism of 44 Triassic‐Cretaceous red bed sites from the northern Simao, Chuandian, and Lanping “blocks,” along both sides of the Ailao Shan‐Red River shear zone (north Indochina). In the Simao domain, remagnetization predates folding and subsequent 48–70° clockwise rotation of three 2–5 km wide subblocks separated by two unrotated blocks. A primary magnetization component from the Lanping domain center suggests variably clockwise rotated (up to 95° ± 24°) sites, interrupted by a 2–6 km wide block that is rotated counterclockwise by 27° ± 6°. Thus, the Lanping and Simao “blocks” are far from being rigid, being made of a mosaic of independently deforming subblocks, whose kinematics and association with documented tectonics are speculative. It is unclear whether both folding and widespread remagnetization were synchronous or diachronous across north Indochina, but (considering previously published results) strike‐slip activity along major shear zones, remagnetization, rotations, and crustal shortening overlapped within the 32–15 Ma time window, thus were likely genetically related. As opposed to previous models, we suggest that in early to mid‐Cenozoic times, north Indochina was under the influence of oblique Neo‐Tethys subduction. Collision between the NE corner of Greater India and Indochina at ~30 Ma yielded ENE‐WSW shortening and strike‐slip reactivation of preexisting faults, in turn fragmenting the crust into small, independently rotating, blocks.