Active faulting and folding along the Jumilla Fault Zone, northeastern Betics, Spain

Abstract

The Jumilla Fault Zone (JFZ) is an ENE-WSW topographic lineament in the external part of the eastern Betic Cordillera. It represents an active fault. Three small basins are aligned along the JFZ, the La Celia-, Alqueria- and Jumilla basins. The tectonic geomorphology of the La Celia- and Alqueria basins consists of folds, a set of normal fault scarps, strike-slip lineaments, fault-springs, tectonically-modified drainage lines and elevated gypsum/anhydrite diapirs. Two of the scarps are normal faults generated by the extensional collapse of one of the folds. The other scarps are secondary normal faults generated by transtensional left-lateral motions along the JFZ. Normal fault scarps that developed on conglomerates are considerably steeper (~ 30°) than those affecting softer marly materials (<10°) as a consequence of their different erodibilities. Two natural springs reveal the sealing nature of the normal faults, which has resulted in a segmented groundwater system. These springs are the sources of the two largest ramblas (gullies) in the area. The onset of fault activity is constrained by the late Tortonian-Messinian age of the sedimentary rocks in the basins. Interbedded lavaflows and lamproitic dykes in the lacustrine sediments have been dated at 7.6-7.2 Ma. However, the morphology of fault scarps and relief of the diapirs suggest a younger age of the deformation. Results from modelling of fault scarp evolution indicate Middle Pleistocene and younger ages of faulting and scarp formation. Furthermore, in at least one of the fault scarps, Quaternary alluvial-pediment deposits are involved in the deformation. The drainage pattern anomalies and moderate earthquake activity (Mb 4.2) in the JFZ also indicate slow ongoing tectonic activity. In addition, the ENE prolongation of the JFZ (outside the study area) shows geodetic displacement rates of approximately 2 mm/year. We thus conclude that the JFZ is still active, despite the general lack of post-Messinian deposits in the studied basins. The discrepancy between the age of the tectonic landforms and the late Neogene age of the basin infill can be explained by an endo- to exhorheic change in the drainage system, due to the capture of the ancient basin lake system by tributaries of the nearby Segura river. The cessation of sedimentation in the basins resulted in the preservation of tectonic landforms.