Exhumation of a Cretaceous rift complex within a Late Cenozoic restraining bend, southern Mongolia: implications for the crustal evolution of the Gobi Altai region

摘要

The Gobi Altai region of southern Mongolia contains intramontane basins dominated by Jurassic–Cretaceous clastic and volcanic deposits. The origin of the basins is poorly documented because Late Cenozoic transpressional mountain building and associated alluvial sedimentation have overprinted and obscured the older Mesozoic history. In this study, we report the discovery of Cretaceous high-angle normal faults and a major low-angle extensional detachment fault bordering the Altan Uul range in southern Mongolia, which indicates that NW–SE crustal stretching was responsible for creating the accommodation space for thick Jurassic–Cretaceous sedimentary accumulations in southern Mongolia. The detachment fault separates chlorite-grade metasedimentary rocks in the footwall from intensely sheared and flattened unmetamorphosed conglomerates and breccias in the hanging wall. The significant metamorphic break across the detachment fault, associated extensional structures in the hanging wall, and domal form of the footwall block indicate that NW Altan Uul has some structural and topographic characteristics typically associated with metamorphic core complexes. However, NW Altan Uul lacks ductile extensional fabrics along the detachment fault and in the adjacent footwall, and limited stretching in the upper plate suggests less total extensional slip than is reported from typical metamorphic core complexes. Unconformable relations in hanging-wall strata and palynological data indicate that extensional detachment faulting at NW Altan Uul occurred in the Aptian (120-112 Ma) similar in time to metamorphic core complex development previously reported in Mongolia–China border areas and the Daqing Shan of northern China. Post-detachment fault Cretaceous sedimentation buried NW Altan Uul, which is now exposed only as a result of erosional denudation associated with uplift of the recent Nemegt-Altan Uul restraining bend. The discovery of significant Aptian crustal extension in the southern Gobi Altai suggests that Early Cretaceous diffuse rifting encompassed an even wider region than was previously recognized, including areas of the eastern Altai, central, southern and eastern Mongolia, and adjacent areas of northern and northeastern China and Transbaikalia, constituting one of Earth's largest continental interior extensional provinces. Workers investigating the neotectonic development of the Gobi Altai should consider the extent to which the pre-existing rift basin architecture may have influenced the recent range development and network of seismically active faults in the region.