Earthquake seismology requires an active, shallow, low-angle (∼20°) north-dipping normal fault beneath the western Corinth-Patras rift. However, extensional faults that crop out south of the gulf, in northern Peloponnesus, are steeper (40°–50°), and this difference of dip has remained unexplained. In this area, the geometry of successive synrift deposits that were controlled by the steeper faults indicates a chronology showing that these faults and the related depocenter of the gulf have shifted to the north with time. The southernmost and earliest fault is different: it is a low-angle detachment fault, more than 70 km long, that cuts obliquely to the north through the nappe pile of the Hellenides. This major detachment fault was active in the early rifting stage. Then, steeper faults formed successively northward, as southern parts of the detachment became inactive and stranded the shallow parts of its hanging wall. This may have resulted from uplift and backtilting of the southern part of the detachment. This northern Peloponnesus detachment probably connects north to the one that is still active under the gulf. It explains the successive steep faults that branch from it, and the northward migration of the gulf and of its depocenter.

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