Rates of scarp retreat: A means of dating Neotectonic activity

Abstract

Tectonically induced scraps have attracted much geomorphic and geologic interest in recent years. The distance of a cuesta scarp from a fault line or a domal structure is an age indicator of tectonic movements, if information on the rate of scarp retreat can be obtained. In Southern Morocco rates of retreat were determined by using the information given (1) by dated volcanic or sedimentary material in the foreland of scarps, (2) by gravels that were deposited on the cuesta backslope before the formation of the scarp, and (3) especially by talus relics or sequences of talus relics in the foreland of scarps. The methods were applied to a number of different cuesta scarps in Mio-Pliocene conglomerates, in Paleogene and Cretaceous limestones. The rates of retreat avarage 1,3 km/Mio. years for the thin conglomeratic caprocks, and 0,5 km/Mio. years for the more resistant and thicker limestone caprocks. Sequences of talus relics proved to be most valuable for determining rates of retreat, because the talus and pediment flatirons represent former scarp slope positions. As caprock lithology does not differ very much in the area under investigation the recession rates calculated so far yield a reliable basis for estimating the time of activation of scarps in geodynamically relevant positions. The transition from depositional environments to subaerial erosion in the Pre-Saharan depression happened much earlier between Goulmima and Errachidia (Middle Tertiary) than in the western part (Late Miocene to Early Pliocene).