Darby, "Mesozoic intraplate deformation in the east Asian tectonic collage: The enigmatic northwest Ordos region, China," 2003

Brian Joseph Darby, Ph.D.

Abstract (Summary)
The stable Ordos block is surrounded by Mesozoic intraplate fold-thrust belts, which include the north-trending Western Ordos fold-thrust belt (WOFTB) along its western margin, and the ?east-trending Lang Shan fold-thrust belt (LSFTB) along its northwestern margin. Strength contrasts between Ordos and the surrounding crust have localized intraplate strain along the margins of the block.

The WOFTB involves rock units from Archean to Late Jurassic(?) in age. A paleocurrent reversal within a Lower-Middle Jurassic section marks the onset of contraction and mountain building. Synchronous with contraction, and cutting across the WOFTB, is a major ?E-W striking right-lateral strike-slip fault. It is interpreted as an accommodation or tear structure within the thrust belt.

Following E-W contraction, the WOFTB was dismembered by a ?N15°W-trending sinistral strike-slip fault. This fault, here named the Western Ordos sinistral fault system, has displaced the frontal, Zhuozi Shan portion of the thrust belt ?60 km northward relative to the more internal Helan Shan. The Western Ordos fault system is exposed ?100 km north in the western portion of the Lang Shan where it only has ?33 km of displacement. This discrepancy in displacement along the left-lateral system can be accounted for by greater Early Cretaceous contraction in the eastern Lang Shan than areas to the west of the Western Ordos system. The LSFTB, involves basement and Mesozoic strata including Early Cretaceous synorogenic units.

Synchronous development of the WOFTB and east-west dextral strike-slip faulting implies relative westward movement of the Ordos with respect to the North China craton. Younger fold-thrust belt development along its northern margin and north-trending sinistral faulting along its western margin, indicate younger relative northward movement of Ordos with respect to the North China craton. Although the intraplate location of the Ordos block and the complexities of Mesozoic Asian plate interactions make it difficult to relate Ordos-bounding deformation to specific plate boundaries, the N-S and E-W orientation of the fold-thrust belts suggest a kinematic linkage with similarly oriented subduction zones along the margins of evolving eastern Asia.

Advisor: Davis, Gregory A.