Early Cretaceous supradetachment basins in the Hohhot metamorphic core complex, Inner Mongolia, China

摘要

Lower Cretaceous basins of the Hohhot metamorphic core complex in the Daqing Shan of Inner Mongolia, China, formed coevally with top to the SE faulting on the Hohhot detachment, as indicated by cross-cutting relationships between the basin and the detachment, sediment-source relationships between the exhumed footwall of the detachment and the basin, and concordant ages between cooling of the footwall and sedimentation in the basin at about 122 Ma. The basins grade through two phases of deposition, marked by distinct sedimentologic characteristics. Early, debris flow, rock avalanche and streamflow deposits in alluvial fans filled multiple structurally segmented basins. Later, a widespread, integrated basin formed and was filled by conglomeratic fluvial and sheetflood deposits supplied by drainage systems flowing to the SE, subparallel to the extension direction. Throughout deposition of the Lower Cretaceous strata, sediment was derived dominantly from the lower plate of the detachment; with progressive unroofing of the lower plate, higher grade lithologies including gneiss and mylonite provided clasts to the Lower Cretaceous conglomerate. The basin fill is thin throughout the basin (< 1200 m), but is much thicker on the SE side of the core complex, where detachment faulting was longer lived. These sedimentary rocks record the progressive exhumation of the lower plate of the Hohhot detachment, folding of the Hohhot detachment, and growth of the Hohhot metamorphic core complex, a classic metamorphic core complex that formed in response to fast, large-magnitude extension. Accordingly, the basin associated with the detachment is consistent, in terms of structural setting and sedimentary style, with the end-member model of supradetachment basin sedimentation, and supports the applicability of the supradetachment basin concept to sedimentation in highly extended terranes. These results also illustrate the dominance of lower plate relative uplift in localizing accommodation space in the overlying basins and providing sediment supply to the syn-extensional basins.