Late Middle Pleistocene Tectonic Inversion in the Mazarrón Graben (Betics, SE Iberia): Seismogenic Implications

Abstract

The Carrascoy (CAF) and Palomares (PF) faults are two major active faults of the Eastern Betic Shear Zone (SE Iberia), both controlling conspicuous mountain fronts. However, the area in between both faults, corresponding to the northern border of the Mazarrón Graben (MG), is a nearly flat plain bounded by a relief of smooth hills. The MG trends N–S and developed during the late Miocene through the activity of conjugate normal fault sets that controlled a marine basin, which subsequently evolved into a continental alluvial environment with pedogenic calcrete formation throughout the Pleistocene. Based on morphotectonic analysis, geophysical survey, and paleoseismological trenching, we provide evidence for a positive tectonic inversion along the northern border of the MG since the late Middle Pleistocene, indicating a local reorganization of the stress tensor. This inversion is expressed by folds nucleated at high-angle reverse faults striking N020°–040°, without a well-defined deformation front. These faults, herein named as the Paretón–Saladillo Fault System (PSFS), correspond to the favorably oriented subset of the former conjugates normal faults. Our observations reveal at least five reverse-to-oblique deformation events since the onset of the inversion, implying recurrence intervals of several tens of thousands of years and single-event displacements on the order of meters. Estimated vertical slip rates for the system range from 0.1 to 0.2 mm/yr. These fault parameters indicate that earthquakes exceeding Mw 7 could occur, implying the propagation of multi-fault ruptures from the CAF and/or PF across the PSFS. These findings provide relevant insights for regional seismic hazard assessments.