ANNALS OF GEOPHYSICS, 56, 6, 2013, S0679; doi:10.4401/ag-6208 S0679 Morphotectonics of the Padul-Nigüelas Fault Zone, southern Spain Jochen Hürtgen, Andreas Rudersdorf * , Christoph Grützner, Klaus Reicherter RWTH Aachen University, Neotectonics and Natural Hazards, Aachen, Germany ABSTRACT The Padul-Nigüelas Fault Zone (PNFZ) is situated at the south-western mountain front of the Sierra Nevada (southern Spain) in the Internal Zone of the Betic Cordilleras and belongs to a NW-SE trending system of normal faults dipping SW. The PNFZ constitutes a major tectonic and lithological boundary in the Betics, and separates the metamorphic units of the Alpujárride Complex from Upper Tertiary to Quaternary deposits. Due to recent seismicity and several morphological and geological indi- cators, such as preserved fault scarps, triangular facets, deeply incised val- leys and faults in the colluvial wedges, the PNFZ is suspected to be a tectonically active feature of the south-eastern Granada Basin. We per- formed morphotectonic GIS analyses based on digital elevation models (DEM, cell size: 10 m) to obtain tectonic activity classes for each out- cropping segment of the PNFZ. We have determined the following geo- morphic indices: mountain front sinuosity, stream-length gradient index, concavity index and valley floor width to height ratio. The results show a differentiation in the states of activity along the fault zone strike. The western and eastern segments of the PNFZ indicate a higher tectonic ac- tivity compared to the center of the fault zone. We discuss and critically examine the comparability and reproducibility of geomorphic analyses, concluding that careful interpretation is necessary, if no ground-truthing can be performed. 1. Introduction The Padul-Nigüelas Fault Zone (PNFZ) is located at the southwestern border of the Sierra Nevada in southern Spain (Figure 1) and comprises a strongly seg- mented northwest-southeast trending system of nor- mal faults that dip to the southwest. Several authors have already studied the tectonic and seismic activity of the PNFZ as a separated part of the Granada Basin [Riley and Moore 1993, Keller et al. 1996, Calvache et al. 1997, Alfaro et al. 2001, Ruiz et al. 2003, 2007, El Hamdouni et al. 2008]. It is suspected to be a tectoni- cally active region due to its recent low to moderate seismicity and several morphological and geological in- dicators, such as preserved fault scarps both in bedrock and in alluvial fans, triangular facets, deeply incised channels and hanging valleys (Figure 2). Nevertheless, only a few paleoseismological records of a paleo-earth- quake or any historical earthquake reports have been found up to now [Alfaro et al. 2001]. Peláez Montilla et al. [2001] and Sanz de Galdeano et al. [2003] have al- ready classified the seismic potential of active faults in the Granada Basin, our aim is to semi-quantitatively as- sess the activity of the PNFZ. Analyses of geomorphic indices offer an appropriate method to obtain quanti- tative data concerning the seismogenic potential of a fault zone in a specific area. Some studies have analyzed morphotectonics for areas of 300 km 2 to 2500 km 2 in southern Spain [Silva et al. 2003, El Hamdouni et al. 2008, Pérez-Peña et al. 2010, Azañón et al. 2012, Giaco- nia et al. 2012]. We selected the morphotectonic ap- proach in a smaller scale by choosing a relatively narrow area around the main faults of the PNFZ, which corre- sponds to an area of approximately 200 km 2 . Since the PNFZ does not consist of one particular fault, but com- prises a fault zone consisting of three segments, we are interested in the tectonic activity of each segment and if there is a differentiation between each segment. This information is crucial for determining the seismic po- tential of the fault zone. 2. Geology and tectonic setting 2.1. Betic Cordilleras The WSW-ENE striking Betics make up the west- ernmost part of the Alpine chain, and are a result of the collision of the Eurasian and African plates in N- S/NW-SE direction [Sanz de Galdeano 1990, Morales et al. 1999]. The Betic Cordilleras are made up of two main domains, the Internal and the External zones (Fig- ure 1B). Non-metamorphic Mesozoic and Paleogene units of the External Zone form the northern and Article history Received September 27, 2012; accepted November 18, 2013. Subject classification: Padul-Nigüelas Fault Zone, Fault activity, Morphotectonics, Geomorphic index, Granada Basin. Special Issue: Earthquake geology