Geomorphic evidence of active tectonics in the Sierra Alhamilla (eastern Betics, SE Spain)

摘要

Active tectonic deformation influencing the streams and hillslopes of the Sierra Alhamilla ridge (anticlinor-ium) in southeast Spain is related to the dextral-reverse Polopos Fault Zone to the north and east of the ridge and to normal faulting in the southern side of the ridge. A fault segmentation and fluvial terrace mapping is presented in this paper. The following geomorphic indices were calculated in the area: the mountain-front sinuosity, valley floor width-to-height ratio, drainage basin asymmetry factor, basin hypsometric curve and integral, longitudinal river profile and stream-length gradient index normalized by the graded river gradient (SLk index). The SLk map shows two sets of high anomalies: a) an E-W-striking anomaly, due to the dextral-reverse Polopos Fault Zone located in the northern limb of the anticlinorium; b) a group of NNW-SSE-striking anomalies, associated with Pliocene-Quaternary NW-SE- to NNW-SSE-striking high-angle normal faults that cut the southern limb. Late Pleistocene-Holocene activity occurred along the North Alhamilla Reverse Fault and the dextral transpressive South Gafarillos Fault, two segments in the Polopos Fault Zone. The former fault produces rejuvenation of the streams at the base of the northern slope. In this area, mountain-front sinuosity and valley floor width-to-height ratio suggest Quaternary uplift rates between 0.05 and 0.5 m ky~(-1). Holocene to present-day activity of the South Gafarillos Fault is demonstrated by faulted late Pleistocene terraces and Holocene colluvial deposits and suggested by deflected streams that have complex or convex hypsometric curves. Pliocene-Quaternary high-angle normal faults control the relief in the southern limb and the western end of the Sierra Alhamilla. These faults produce low mountain-front sinuosity and valley floor width-to-height ratio indices typical of uplift rates quite higher than field-based rates from some other studies.