Geomorphic evidence for tilting at a continental transform: The Karamursel Basin along the North Anatolian Fault, Turkey

Continental transform boundaries are characterised by regional transcurrent faults that locally slip obliquely and spawn rapidly subsiding and tilting basins. A type example is the North Anatolian Fault (NAF) that accounts for the westward motion of the Anatolian “platelet” relative to Asia at about 25 mm/yr. Many of the basins along the NAF are asymmetric half grabens that border a strand of the NAF on the extensional side of a fault bend and tilt obliquely toward the fault. A prominent example is the 17 km-long Karamursel half graben south of the NAF in Izmit Gulf, one of the starved basins flooded by the Marmara Sea. A tilt rate of 3°/10 kyr has been proposed for the submerged part of the basin within 2 km south of the NAF, on the basis of a tilted early Holocene paleo-shoreline. Very rapid tilt and subsidence have been reported for similar basins along the NAF, but the locus of tilt shifts relative to the basin and is short lived at any one place. We find evidence of recent tilting from surface flow patterns on the steep flank of the basin above the southern coastline, up to 6-8 km from the NAF. Drainage divides between 13 rivers mark a northward 8-10° tilted sub-planar surface that extrapolates down into the basement below the progressively tilted sediments of the basin. River profiles are only slightly concave below this surface. We interpret this as the bevelled surface that preceded tilting, and the immature comb-like drainage to be symptomatic of recent tilt. On the north flank of the basin, the drainage is equally steep but markedly different. It suggests a backward eroding fault scarp. Northward drainage into the Black Sea suggests a subtle but regional northward tilt of the Kocaeli Plateau, possibly a flexural response to unloading along the fault. Along the western NAF, rapid progressive tilting of sediments is typical of submarine transform basins. They provide ground truth for investigating the geomorphic effects of rapid tilting on land.