GRAVITY MODELLING DIRECTED AT STRUCTURAL-TECTONIC EVALUATION OF THE DIKILI DEPRESSION (IZMIR-TURKEY)

The Dikili Depression (near Dikili, Izmir Province, Turkey) is located in the north-western part of the Aegean extensional regime, an area that has been influenced by the Kozak Pluton and Bergama Graben. The general geology of the area is characterized by plutonic rocks and sedimentary units. Local geology, under a cover of Quaternary deposits (sand, gravel and clay), includes the Soma Formation (andesite/pyroclastics), Yuntdağ Formation (hornblende andesite and limestone) and Pre-Tertiary basement. In order to examine the structure and tectonic evolution of the Dikili area, gravity data were collected at 245 sites using a Scintrex CG-5 Autogravity meter. Standard corrections applied to the gravity data include those for instrumental drift, Earth tides and latitude, and the free–air and Bouguer corrections. Density was determined using the Nettleton approach. The corrected gravity data were imported into a Geosoft database to create a grid and map of the Bouguer gravity anomaly (grid cell size of 130 m).

The gravity map displays a distinct gravity low aligned along the Dikili Depression, a graben structure oriented NW-SE along the coast that links with the Bergama graben at its south-eastern extremity. The low is bounded to the north-west and south-east by steep gradients that seemingly, and somewhat surprisingly, trend at high angles across the trend of the depression, suggesting the presence of a structure having a NE-SW orientation. Possibly the Dikili Depression extends laterally to the south-west and north-east. Uncertainty related to this conjecture arises from a general lack of gravity measurements outside the depression. The geological significance of the gravity low was investigated by quantitatively modelling a NW-SE gravity profile extracted from the gravity grid and defined by values at 200 m intervals. The profile is 20,000 m long and extends south-eastward beyond the area of gravity coverage. A simple layered model (overburden, Soma Formation, Yuntdağ Formation, basement) indicates the presence of a fault-bounded basin coincident with the Dikili Depression, that possibly extends laterally beyond the limits of the depression. The basin has a maximum thickness of about 2,800 m and a width of about 13,800 m at surface between bounding faults.