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[1] The Sea of Marmara is presently the site of pure dextral strike-slip faulting that connects the Izmit Fault in the east to the Ganos Fault in the west along a linear strike-slip fault that is part of the northwestern branch of the North Anatolian Fault. This active Main Marmara Fault (MMF) cuts across a succession of depocenters and highs. Analysis of a large data set that includes multichannel seismic lines, sparker and deep towed high definition seismics, shows that most of the N120[degrees]E trending normal faults framing these depocenters are now inactive as they are sealed by an average thickness of 300 m of sediments. The highs correspond to a series of NE-SW trending anticlines and reverse faults blanketed by a thin cover of undeformed sediments. We interpret these structures as inverted former transfer faults of the now inactive Sea of Marmara pull-apart system. Depocenters and highs are the relicts of a former pull-apart basins and transfer faults system that accommodated the North Anatolian Fault motion within the Sea of Marmara prior to the formation of the MMF. We propose that a relatively short (a few hundreds of thousands of years) period of basin inversion accompanied the localization of strain along the present Main Marmara Fault, until it was fully formed, 200 [+ or -] 100 Kyr ago. The preexisting pull-apart basin system accounts for about 30 km of motion along this fault system and thus probably appeared sometime in late Pliocene or early Pleistocene as a reactivated portion of the old Thrace basin. During the Plio-Quaternary the Sea of Marmara has been the site of progressive localization of strain. INDEX TERMS. 8107 Tectonophysics: Continental neotectonics; 8010 Structural Geology: Fractures and faults; 8122 Tectonophysics: Dynamics, gravity and tectonics; 8109 Tectonophysics: Continental tectonics--extensional (0905); KEYWORDS: Sea of Marmara, tectonics, geodynamics, strain localization, strike-slip fault, basin inversion.

We use a detailed 6500 km commercial seismic reflection survey over the lower northern margin of the Bay of Biscay to discuss the structure of the thinned lower margin crust. Our study demonstrates that the 'S' reflecting horizon, at the base of the tilted block layer, progressively cuts into higher levels of the upper continental crust oceanward. This S horizon is best interpreted as a low-angle normal fault within the upper crust with an original dip toward the continent. We use previously acquired seismic refraction work to demonstrate that the S horizon is the top of a highly thinned high-velocity lower continental crust layer. The crustal layers above and below S both thicken toward the continent. We consequently interpret the lower crust as a ductile zone of decoupling between the upper brittle crust and the upper presumably brittle lithospheric mantle. We present a model which incorporates those features. [ABSTRACT FROM AUTHOR]