Changes in dip and frictional properties of the basal detachment controlling orogenic wedge propagation and frontal collapse: the External central Betics case

Thin-skinned fold-and-thrust belts (FTBs) have been extensively studied through both field examples and modelling. The overall dynamics of FTBs are, therefore, well understood. One less understood aspect is the combined influence of across-strike changes in the detachment properties and the basement topography on the behaviour of an orogenic wedge. In this paper, we use field data together with reflection seismic interpretation from the External Zones of the Central Betics FTB, S Spain, to identify a significant increase in the wedge basal dip (a basement "threshold") coinciding with the pinch-out of a weak substrate. This induced both changes to the wedge geometry and to the basal friction, which in turn influenced the wedge dynamics. The changing dynamics led to a transient “stagnation” of the FTB propagation, topographic build-up and subsequent collapse of the FTB front. This in turn fed an important Langhian depocenter made up of mass transport deposits. Coevally with the FTB propagation, extension took place both parallel and perpendicular to the orogenic trend. This case study illustrates how across-strike changes in wedge basal properties can control the detailed behaviour of a developing FTB front, but questions remain regarding the time-space interaction and relative importance of the basal parameters.