Mesozoic contractional deformation in the middle of the Asian tectonic collage: the intraplate Western Ordos fold-thrust belt, China

摘要

Intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of stranconcentrations into weak zones. These weak zones may be pre-existing crustal heterogeneities such as faults, thermally weakened zones due to magmatism and/or the effects of a thick sedimentary cover, or mechanical contrasts between adjacent crustal blocks. Intraplate deformation is nt exculsively related to continent-continent collision; it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries. Asia has many examples of active intraplate deformation that are thought to be related to the Cenozoic Indo-Asian collision. Although much attention is placed on active examples, discerning controls on pre-Cenozoic Asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of Asia. Situated on either side of the Yellow River along the western margin of the Ordos Plateau, the intraplate Western Ordos fold-thrust belt involves units as old as Archean basement and as young as Late Jurassic(?). The fold-thrust belt is spectacularly exposed in two ranges, the Helan Shan and the Zhuozi Shan. Crosssections drawn through the north-trending fold-thrust belt suggest 30% minimum shortening. A paleocurrent reversal within the Lower-Middle Jurassic section is interpreted to mark the onset of contraction and mountain building. A Late Jurassic (?) synorogenic boulder conglomerate records the final deformation in the fold-thrust belt. Synchronous with contraction, and cutting across the fold-thrust belt, is an ～E-W-striking strike-slip fault that displays rightlateral drag and sub-horizontal striae. We interpret this fault as an accommodation structure in the thrust belt. Following ～E-W contraction, the thrust belt was dismembered by an ～N15°W-trending left-lateral strike-slip fault. The left-lateral fault displaces the forntal portion of the thrust belt (the Zhuozi Shan) 62 km to the north, relative to amore internal portion (the Helan Shan). We propose that the anomalous orientation and intraplate location of the Western Ordos fold-thrust beltis a function of mechanical contrasts betwen a previously deformed area and a stable crustal block (Ordos) to the east. Pre-existing crustal weakness or anisotropy along the western margin of the Ordos block is most likely related to either a Late Proterozoic-Early Paleozoic aulacogen, a Triassic fault system, or to both. Although its intraplate location and the complexities of Mesozoic Asian plate interactions make it difficult to relate deformation in the Western Ordos fold-thrust belt to a specific contemporaneous plate boundary, its N-S trend may suggest a link with paleo-Pacific subduction along the eastern margin of Asia.