Fault interaction and strain partitioning deduced from deformed fluvial
terraces of the Eastern North Qilian foreland, NE Tibetan Plateau
Abstract
Faulting and folding of basement rocks together accommodate convergence
within continental orogens, forming complex zones of intraplate
deformation shaped by the fault interaction. Here we use the river
terraces along the Dongda river to examine the tectonic deformation
patterns of the hinterland and the foreland of the eastern North Qilian
Shan, a zone of crustal shortening located at the northeast margin of
the Tibetan Plateau. Five Late Pleistocene-Holocene terraces of Dongda
river are displaced by three major reverse faults: Minle-Damaying fault,
Huangcheng-Ta’erzhuang fault, and Fengle fault, from south to north.
Based on displaced terrace treads, we estimated vertical slip rates
along the Minle-Damaying fault as 0.7–1.2 mm/a, and along Fengle fault
as 0.5–0.7 mm/a. Deformed terraces suggest additional uplift of
~ 0.2 mm/a through folding of the Dahuang Shan
anticline. Inhomogeneous uplift of the intermontane basins between the
Minle-Damaying fault and the Dahuang Shan anticline indicates a 0.9 ±
0.2 mm/a uplift rate along the Huangcheng-Ta’erzhuang fault. Kinematic
modeling of this thrust system shows that deformation propagated
northward toward the foreland along a south-dipping 10° décollement
rooted into Haiyuan fault at the depth of 20–25 km. This system
accommodates 2.7–3.8 mm/a total crustal shortening rate. We suggest
this broad thrust belt and the relatively high rate of shortening within
this part of the eastern Qilian Shan is as a result of the oblique
convergence along a restraining bend of Haiyuan fault system. The
elevated shortening rate within this area indicates high potential
seismic hazard.