Basins beyond Bends along Continental Transforms in NW Turkey and Southern California: Rapid, Asymmetric, and Time-Transgressive Growth

Basins along transcurrent faults have long been of interest as petroleum reservoirs and as markers of transform tectonics. We investigate a class of basins that develop on the transtensional side of fault bends, based on examples along two well-known continental transforms. The Tekirdag (western) and the Cinarcik (eastern) basins in the Marmara Sea are similar active structures on the ‘downstream’ side of prominent bends along the North Anatolian fault (NAF). Much has been learned about these structures from submarine exploration in the Marmara Sea following the disastrous 1999 earthquakes on the NAF. The Ridge Basin along the San Andreas fault system in southern California exhibits similar features, despite being an exhumed Miocene structure viewed from a different perspective. The comparison points to signature characteristics of a type basin associated with transform bends and helps in developing hypotheses about the 3D development of these basins. The basins are asymmetric in two ways. They are half grabens bordered by the transform and are tilting progressively toward it. This fault is dipping toward the basin and is slipping obliquely, accommodating both transcurrence and extension. In addition, these basins are tilting toward the bend on the fault, which is ‘upstream’ from the basin, considering the motion of the side of the fault where the basin is forming relative to the other side. As a result, the most rapid subsidence is near the fault and near the bend. But, the fastest subsidence and the deepest part are at opposite ends of these basins and keep getting further apart, because their growths are time-transgressive. This key feature stems from the asymmetry, which is not just geometric, but also kinematic. The basin and the deformation that accommodates the fault bend are confined to one side of the fault. This implies that the bend is fixed on the other side. The basin-side of the fault ‘flows’ over the bend and, like water in a stream, subsides after the bend. This subsidence is fixed to the bend and moves upstream relative to the material (i.e., is time transgressive) at transform velocity, forming a typical onlap. Thus the basin subsides fastest where it is youngest and shallowest close to the bend, but is deepest where subsidence has progressed the longest, far from the bend. While the Marmara basins are young and still in transient growth, the Ridge Basin reached a steady state where the oldest parts of the basin have translated beyond the effect of the bend and basin growth is strictly horizontal. The Ganos and the Tuzla bends, responsible for the Tekirdag and Cinarcik basins, respectively, are fixed to opposite sides of the NAF and are thus expected to move away from each other at transform speed. At this stage in its evolution, the Marmara Trough is primarily growing longer not deeper. This notion is the basis for an evolutionary model of the Marmara Trough that accounts for the Marmara basins with ~30km of dextral motion on the transform.