ʹForebergsʹ, flower structures, and the development of large intra-continental strike-slip faults: the Gurvan Bogd fault system in Mongolia

The Gurvan Bogd mountains of the Gobi–Altay, Mongolia contain a system of strike-slip faults with a reverse component, part of which moved in a large earthquake (Mw∼8.0) in 1957. Adjacent and sub-parallel to the main ranges are numerous thrust-related folds, thrust faults, and elongated low ridges (`forebergsʹ), all of which result from the shortening component on the fault zone. The appearance of these thrust-related structures is varied, depending on their stage of development, preservation, and exposure. Evidence from geomorphology and surface ruptures suggests that they may all serve a common function, which is to broaden the deforming zone by creating new structures that are able to accommodate both the strike-slip and the shortening components of motion. The geomorphology further suggests that these new structures then evolve by lateral propagation and increase in amplitude, to eventually merge and form through-going new faults subparallel to the old. In their early stages the new faults and related structures appear to be influenced by the underlying sediments adjacent to the main range, which may include weak layers such as lake beds that can ultimately cause the collapse of foreberg ridges in landslides. The migration of faulting away from the main range is likely to be driven by stresses associated with topography, which in turn is a consequence of the shortening component. The evolution described here is thus peculiar to strike-slip faults with a reverse component, and can form many of the features of the `flower structuresʹ that are often described in such oblique-shortening zones. Although the shortening component is often localized in restraining bends, its origin may ultimately be related to rotations about vertical axes, which are common in deforming continental regions.