Magnetic fabric constraints on the Cenozoic compressional strain changes in the northern Qaidam marginal thrust belt and their tectonic implications

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 (ATF). 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
are perpendicular to the rotation-corrected magnetic lineations
and exhibit a regular trend from nearly N-S before ~33 Ma to NE-SW
at ~10 Ma. Integrated with other lines of evidence from
the northern Qaidam Basin, we attribute the ~45° successive clockwise
rotation of the
compressional strain directions to the
propagation of significant strike-slip faulting along
the ATF into the northern Qaidam marginal thrust
belt that resulted in significant clockwise
rotations in the region.