Magnetic Fabric Constraints on the Cenozoic Compressional Strain Changes in the Northern Qaidam Marginal Thrust Belt and Their Tectonic Implications

摘要

Reconstructing the compressional strain history of the northern Qaidam marginal thrust belt has significant implications for understanding the mountain building processes of the northeastern Tibetan Plateau and the tectonic evolution of the Altyn Tagh Fault. However, the variation of compressional strain in this region has rarely been studied. Here, we present Cenozoic anisotropy of the low-field magnetic susceptibility records from the fluvial-lacustrine Hongliugou profile within the middle part of the northern Qaidam marginal thrust belt. The magnetic fabrics of the Shizigou Formation are dominated by a primary sedimentary magnetic fabric, whereas those of the lower Lulehe Formation are undetermined, most likely due to the conglomerate lithology; in contrast, the magnetic fabrics of the middle Lulehe to Shangyoushashan Formations, all of which are dominated by antiferromagnetic hematite and paramagnetic components with crystallographic preferred orientations, are mostly embryonic tectonic fabrics induced by prefolding layer-parallel shortening. Compared with previously reported paleomagnetic rotation results within the Hongliugou locality, the rotation-corrected compressional strain directions that are perpendicular to the rotation-corrected magnetic lineations exhibit a regular trend from nearly N-S before similar to 33 Ma to NE-SW at similar to 10 Ma. Integrated with other lines of evidence from the northern Qaidam Basin, we attribute the similar to 45 degrees successive clockwise rotations of the compressional strain directions to the propagation of significant strike-slip faulting along the Altyn Tagh Fault into the northern Qaidam marginal thrust belt that resulted in significant vertical axis clockwise rotations in the region.