Cenozoic normal faulting and regional doming in the southern Hangay region, Central Mongolia: implications for the origin of the Baikal rift province

Abstract

The Hangay Dome in central Mongolia is a mountainous region covering more than 200,000km2 with numerous flat-topped peaks over 3000m that preserve a Late Cretaceous–Paleogene erosion surface. Doming began in the middle Oligocene producing more than 2000m of regional topographic uplift. The range represents an important kinematic link between the Baikal rift province to the north and Altai transpressional ranges to the south and west. Structural field investigations of major faults visible on satellite imagery indicate that the southern Hangay Dome region is dominated by Late Cenozoic normal faults that bound small half-graben. Most faults are NE-striking and faults at the highest elevations are the most recently active. Most graben appear to be isolated systems that constitute small sediment sinks perched on the flanks and crests of the dome. The first-order feature is the dome itself and most sediment eroded off of the dome is deposited in the Mongolian Valley of Lakes or is carried northwards by the Selenga river and its extensive tributaries.The basement of the dome is a Precambrian craton although the shape and dimensions of the craton are poorly constrained to the north and east. Late Cenozoic uplift of the southern dome region appears to be confined to the area underlain by cratonic basement whereas the Altai region to the south and west is underlain by mechanically weaker Palaeozoic arc and accretionary belts. With respect to the regional northeast directed SHmax, the Hangay craton appears to have acted as a rigid passive indentor focusing Late Cenozoic transpressional deformation around its west and southern margins. Models invoking a Late Cenozoic plume as the driving force for doming and widespread alkaline volcanism on the dome are not strongly supported by geochemical and isotopic data on Neogene–Recent volcanics and the spatial correlation between areas that are domed and older cratonic crust appears too coincidental to be ignored. Convective removal of an overthickened lithospheric root leading to adiabatically decompressed asthenosphere could explain regional doming and volcanism, however major crustal thickening last occurred in the Permian in the southern Hangay region and the time lag between thickening and postulated root removal and plateau uplift (>200Myr) appears too long. An alternative model is explored based on speculated lithospheric mantle flow patterns driven by India's continued northeastward indentation. It is suggested that lithospheric mantle flow diversion around the overthickened Hangay craton crustal keel could cause lithospheric thinning beneath the craton and passive asthenospheric upwarp leading to regional topographic uplift and decompression melting/alkali volcanism. In general, the angular relationship between Precambrian craton boundaries and the prevailing northeasterly SHmax appears to control the kinematics of late Cenozoic deformation throughout the Hangay, Altai, Sayan and Baikal regions.