THE ENIGMATIC WESTERN ORDOS FOLD-THRUST BELT, CHINA: INTRAPLATE FORMATION AND SUBSEQUENT DISMEMBERMENT

The intraplate Western Ordos fold-thrust belt (WOFTB) lies along the western margin of Ordos Plateau on both sides of the Yellow River. It is spectacularly exposed in two ranges, the Helan Shan and Zhouzi Shan. Its north-south trend is anomalous vis-à-vis the E-W structural grain found in most of China and eastern Asia.

The WOFTB deforms units from Archean to Late Jurassic(?) in age and is composed of dominantly east-directed, basement-involved thrust faults, reverse faults, and folds. An E-W trending, right lateral strike-slip fault that cuts, or is synchronous with contractional structures, crosses the northern portion of the belt. In the Zhouzi Shan, this strike-slip fault cuts the base of a Late Jurassic(?) synorogenic conglomerate section that is involved with thrust belt deformation. The upper portion of the Late Jurassic(?)conglomerate section is not cut by the right-lateral strike-slip fault, nor is it folded by structures related to the thrust belt. Foreland basin analysis studies have constrained the onset of contraction in the WOFTB to be Middle Jurassic in age on the basis of a major paleo-current reversal. Our cross-sections suggest ~30% minimum shortening across the belt. Following Middle thru Late Jurassic contraction, the WOFTB was dismembered by a ~N15°W striking left-lateral fault system now hidden beneath Yellow River alluvium. This left-lateral fault system displaced the Zhouzi Shan ~62 km northward with respect to the Helan Shan

The location and orientation of the WOFTB may be explained by tectonic inheritance or reactivation of structures related to Precambrian deformation, an early Paleozoic aulacogen, and/or a Triassic fault system. Although the WOFTB was far from contemporaneous subduction/collisional zones that bordered Mesozoic Asia, its N-S orientation might indicate far-field plate interaction with a paleo-Pacific subduction zone along Asia's eastern margin.