Sedimentary and structural records of late Mesozoic high-strain extension and strain partitioning, East Gobi basin, southern Mongolia

Contrasting styles of late Mesozoic sedimentation in the East Gobi basin of southern Mongolia in part reflect varying rates and structural modes of local extension. High-strain extension was associated with formation of an Early Cretaceous metamorphic core complex in the Onch Hayrhan area. Recent mapping illustrates newly recognized structural features of this core complex, including ductile structures that indicate subhorizontal south-southeast-directed extension ca. 126 Ma. Aregionally extensive, east-west-trending detachment fault is locally domed, as defined by the rollover of foliation planes from north to south dipping. This detachment fault terminates to the east against a north-south-trending dextral strike slip transfer zone. Lower Cretaceous strata associated with this transfer zone are generally synextensional breccias and alluvial-fan deposits consisting of unsorted boulder conglomerate, interbedded debris-flow units, and rock-fall deposits that represent rapidly deposited components of proximal, syntectonic depositional systems. In contrast, several hundred kilometers to the northeast of this core complex late Mesozoic extension was characterized by more typical synrift structural geometries (e.g., high-angle normal faults bounding half-graben subbasins). Comparatively mature depositional systems represented by dominantly fluvial to lacustrine environments characterized the Early Cretaceous record in this part of the basin. The combined structural and stratigraphic studies we present in this chapter demonstrate that the East Gobi basin was segmented into regions of high- and low-strain extension, and that transfer zones accommodated these variations. In southern Mongolia and other parts of central Asia, late Mesozoic metamorphic core complex formation is typically associated with a rapid transition from compressional to extensional tectonics, particularly along orogenic belts associated with convergent terrane and plate margins. This implies that gravitational collapse along tectonic boundaries may have played an important role in the localization of high-strain extension throughout the region.