Strain partitioning and relief segmentation in arcuate fold-and-thrust belts: a case study from the western Betics

Abstract

Within arcuate orogenic belts, strain is commonly partitioned between arc-parallel stretching and arc-perpendicular shortening. Arc-parallel stretching can be accommodated by arc-oblique strike-slip faults and arc-perpendicular normal faults, whose localization in fault systems may provoke significant along-strike structural relief drops. In this work, we have studied the Ubrique area, located in a frontal segment of the external western Betics (northern branch of the Gibraltar Arc), where one of the most significant relief discontinuities along the orogenic grain is defined. We have found that this discontinuity is determined by two main types of structures: (1) arc-parallel, kilometric-scale folds and reverse faults that control the conformable relief of the fold-and-thrust belt; (2) the tectonic lineation related to the SW segmentation of this relief, which is composed of the Colmenar fault and the Ubrique Normal Fault Zone (UNFZ). This tectonic lineation seems to have localized arc-parallel extension. Qualitative and quantitative geomorphological analyses together with the age of the deformed rocks indicate that these structures have been active from the Tortonian to Holocene. Our results suggest that post-Serravallian outward radial thrusting and arc-parallel stretching accommodate a strain partitioning typical of progressive arcs, thus suggesting that the Gibraltar Arc is still protruding.