资源环境科技发展态势分析平台

The Tibetan Plateau accommodated major upper crustal shortening during Indian Plate oceanic and continental lithosphere subduction. Deciphering whether shortening was continuous or episodic, and how it correlates to major geodynamic changes is challenging. Here we apply anisotropy of magnetic susceptibility (AMS), a sensitive synsedimentary strain indicator, to a similar to 3 km thick magnetostratigraphically dated sedimentary section (69-41.5 Ma) in eastern Tibet. AMS shows "earliest deformation" fabrics from 69-52 Ma, followed by a sudden change to "pencil structure" fabrics with increasing anisotropy degree at similar to 52 Ma, dating a sudden increased synsedimentary shortening strain. This change coincides with enhanced sedimentation rates and synsedimentary vertical-axis rotations of the Gonjo Basin, suggesting a causal link to a marked India-Asia convergence rate deceleration. We show that AMS analysis provides a strong tool to distinguish between climatic and tectonic causes of sedimentological change and is an asset in identifying discrete tectonic pulses in intensely deformed terrane.

Plain Language Summary How the Tibetan Plateau evolved during India-Asia convergence and collision is notoriously challenging to decipher. Use of sedimentary records to date periods of tectonic activity is a popular approach, yet distinguishing between tectonic versus climate signals in sediment records can be challenging. The anisotropy of magnetic susceptibility (AMS) is an effective and sensitive technique that reveals tectonic stress fields during sedimentation and changes therein, even in weakly deformed clastic sedimentary rocks. We report a detailed record of AMS data from a similar to 3 km thick section of redbeds from the Gonjo Basin in eastern Tibet. Previous magnetostratigraphy dated deposition from 69 to 41.5 Ma; during this time period marked sedimentation rate increases and simultaneous vertical-axis rotations were interpreted to reflect shortening pulses. Our new AMS data indicate an increased shortening strain at similar to 52 Ma, demonstrating that the sedimentation rate changes are tectonic rather than climatic in origin, showing that a pulse in crustal shortening of Tibet occurred simultaneous with a marked similar to 52 Ma onset of deceleration of India-Asia convergence. We show that applying a suite of paleomagnetic and rock magnetic techniques, including magnetostratigraphy and sedimentation rate calculation, vertical-axis rotation analysis, and AMS analysis, provides a powerful tool to differentiate tectonic versus climatic influence on a sediment archive, allowing the precise dating of discrete deformation phases in intensely deformed regions that evolved over long periods of time.