Paleomagnetism of the Middle Cenozoic Mula Basin (East Tibet): Evidence for km‐Scale Crustal Blocks Rotated by Midlower Crust Drag

Abstract

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.