Uplift and subsidence from oblique slip: the Ganos-Marmara bend of the North Anatolian Transform, Western Turkey

Abstract

Bends that locally violate plate-motion-parallel geometry are common structural elements of continental transform faults. We relate the vertical component of crustal motion in the western Marmara Sea region to the NNW-pointing similar to18degrees bend on the northern branch of the North Anatolian Fault (NAF-N) between the Ganos segment, which ruptured in 1912, and the central Marmara segment, a seismic gap. Crustal shortening and uplift on the transpressive west side of the bend results in the Ganos Mountain; crustal extension and subsidence on the transtensional east side produce the Tekirdag Basin. We propose that this vertical component of deformation is controlled by oblique slip on the non-vertical north-dipping Ganos and Tekirdag segments of the North Anatolian Fault. We compare Holocene with Quaternary structure across the bend using new and recently published data and conclude the following. First, bend-related vertical motion is occurring primarily north of the NAF-N. This suggests that this bend is fixed to the Anatolian side of the fault. Second, current deformation is consistent with an antisymmetric pattern centered at the bend, up on the west and down on the east. Accumulated deformation is shifted to the east along the right-lateral NAF-N, however, leading to locally opposite vertical components of long- and short-term motion. Uplift has started as far west as the landward extension of the Saros trough. Current subsidence is most intense close to the bend and to the Ganos Mountain, while the basin deepens gradually from the bend eastward for 28 km along the fault. The pattern of deformation is time-transgressive if referenced to the material, but is stable if referenced to the bend. The lag between motion and structure implies a 1.1-1.4 Ma age for the basin at current dextral slip rate (2.0-2.5 cm/year). Third, the Tekirdag is an asymmetric basin progressively tilted down toward the NAF-N, which serves as the border fault. progressive tilt suggests that the steep northward dip of the fault decreases with depth in a listric geometry at the scale of the upper crust and is consistent with reactivation of Paleogene suture-related thrust faults. Fourth, similar thrust-fault geometry west of the bend can account for the Ganos Mountain anticline/monocline as hanging-wall-block folding and back tilting. Oblique slip on a non-vertical master fault may accommodate transtension and transpression associated with other bends along the NAF and other continental transforms. (C) 2004 Elsevier B.V. All rights reserved.