STRUCTURAL CONTROLS ON SEDIMENTATION IN SUPRADETACHMENT BASINS: AN EXAMPLE FROM THE EARLY CRETACEOUS HOHHOT DETACHMENT, INNER MONGOLIA, CHINA

Previous workers have argued that the geometry and sedimentology of supradetachment basins are distinct from half-graben because distinct structural geometry on the basin-bounding fault asserts dominant control on basin formation. Parameters such as low fault dip and high rate of footwall uplift relative to hangingwall subsidence promote transverse transport of sediment across the hangingwall to a distal depocenter. These factors prevent formation of a deeply subsided sediment ‘trap’ adjacent to the fault, as is characteristic of half-graben.

We have examined a series of supradetachment basins above the Hohhot detachment in north-central China to evaluate the Friedmann and Burbank (1995) supradetachment end-member extensional basin model. The Hohhot detachment is a low-angle normal fault related to the Hohhot metamorphic core complex, which formed on the North China craton during Early Cretaceous extension. A series of supradetachment basins formed as a result of this faulting, and they preserve distinct records of sedimentation in a variety of structural settings relative to the metamorphic core of the Hohhot core complex. These syn-extensional basins contain up to 1200-1400 m of syn-extensional strata, mostly consisting of coarse conglomeratic facies, primarily deposited in alluvial fan and fan delta settings. Depositional processes were streamflow and sheetflood, with significant additional contribution from debris flow and rock avalanche deposition. These basins have a decrease in clast size and megabreccia frequency away from the breakaway of the detachment. Paleocurrent indicators show strong southward flow, transverse to the detachment, indicating footwall-derivation of sediment, also supported by clast composition, which consists of recognizable footwall lithologies.

These characteristics of supradetachment basins along the Hohhot detachment match the predictions of the supradetachment basin model, supporting its applicability. We further conclude that distinct structural settings within the detachment-core complex system exert subtle, but recognizable, controls on basin formation that can be discerned in the ancient record.