Tectonic geomorphology of the Acıgöl Graben, western Turkey

The Acıgöl Graben is located in an extensional province of western Turkey, related to combining effects of the southwestward retreat of the Hellenic trench and the westward displacement of the Anatolia Plate. This is a 50 km long and 10 km wide neotectonic depression with a NW-SE orientation that veers into an E-W trend in its southern part. The basin has a late Miocene to Quaternary fill, and according to borehole data Quaternary sediments exceed 600 m. The Acıgöl and Maymundağı Quaternary faults, respectively, control the SE and NW margins of this internally drained basin, which hosts a playa-lake. The higher elevation of the SE mountain front and the proximity of the lake to this margin suggests some asymmetry in the graben structure. This study analyses the graben-bounding faults by means of detailed geomorphological mapping, and demonstrates their late Quaternary activity by geomorphic evince and Optically Stimulated Luminiscence (OSL) dating. The Acıgöl Fault on the SW margin juxtaposes Mesozoic carbonate rocks against Quaternary deposits along the linear mountain front-piedmont junction. The uplifted range, with a local relief of around 1 km, shows geomorphic evidence of deep-seated gravitational slope deformation (DSGSD), such as ridge-top depressions, uphill-facing scarps, internally drained troughs and bulged toes. The SW margin on its southern sector displays E-W intra-basin faults that offset and tilt recent basin-fill deposits, producing backtilting on geomorphic surfaces. These deformations control the anomalous orientation of some drainages that flow towards the basin margin. OSL ages provide a maximum age of 24 ka for the youngest displacement event on these faults. On the NW margin of the graben, the Maymundağı Fault, with a NW-SE to E-W orientation, is expressed as a linear mountain front with triangular facets up to 700 m high. This fault juxtaposes Oligocene conglomerates against Quaternary alluvial fan deposits. In the southern section where the basin attains an E-W trend, the fault displays two parallel fault strands. The northern fault strand is the master basin bounding fault located at the toe of the mountain front, whereas the southern one is an intra-basin fault expressed as a scarp that offsest alluvial fan surfaces and deposits. Here we differentiate two generations of alluvial fans: (1) oversteepened and dissected old fan surfaces restricted to the footwall block; and (2) active fans in the downthrown block with their apex at intersections points associated with the fault scarp. Numerical ages indicate fault activity younger than 3 ka.