Paleomagnetic Evidence for 25–15 Ma Crust Fragmentation of North Indochina (23–26°N): Consequence of Collision With Greater India NE Corner?

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 kmwide 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.