Rotational deformation of the southeastern margin of Tibet: A paleomagnetic study of the Yanyuan basin, Sichuan Province

Abstract: Investigating rotational deformation of the southeastern margin of the Tibet is the key to understanding how and why the Tibet material extruded to the southeast. Until now there are two sets of strata used to study the rotational deformation on the southeastern margin of the Tibet: the Jurassic-Eocene and the Miocene-Quaternary strata. Paleomagnetic studies of the Jurassic-Eocene strata indicate widespread clockwise rotational deformation, similar studies of the Miocene-Quaternary strata suggest multiple deformation patterns, including clockwise rotation, counter-clockwise rotation and also no rotation. However, it is still not well constrained as to the timing and degree of rotational deformation on the southeastern margin of Tibet. Two sets of strata (the Paleocene-Eocene and Pliocene-Quaternary sediments) are well exposed in the Yanyuan basin, Sichuan Province. Magnetostratigraphic studies indicate an age of >3.6-0.6 Ma for the Pliocene-Quaternary strata. Declination data suggest that the Pliocene-Quaternary sediments experienced counter-clockwise rotation (-14.4°), and the Paleocene-Eocene deposits underwent notable clockwise rotation (10°-21.5°). As the age of the youngest strata that experienced clockwise rotational deformation on the southeastern margin of Tibet is the Eocene, the clockwise rotational deformation likely occurred during a time interval between the Eocene and the Miocene. The counter-clockwise rotational deformation recorded in the Yanyuan basin happened before 3.6 Ma with an average rotational rate of 4°/Ma. Since anisotropy of magnetic susceptibility (AMS) data indicate that the Pliocene-Quaternary sediments underwent no compressional deformation at all, the counter-clockwise rotational deformation was initiated most likely by strike-slip faults around the Yanyuan basin.