Timing of the switchover from thrusting to normal faulting in the Cretan nappe pile, Greece

Abstract

Contractional faults and shear zones are often reactivated by normal faulting and the timing of this kinematic switchover is critical for better understanding orogeny, especially the formation and exhumation of high-pressure rocks. We report two fault gouge ages of ∼30 and ∼25 Ma from the contact zone between the high-pressure Phyllite-Quartzite Unit and the overlying, weakly metamorphosed Tripolitza Nappe in central Crete, southern Aegean Sea, Greece. This contact, the Damasta shear zone, is commonly regarded as a segment of the Cretan Detachment, the age of which is not well known. The dated gouge dominantly shows early top-to-the-S kinematic indicators, with some indication of a top-to-the-N reactivation. Illite/muscovite grain-size fractions of 0.5–0.2 μm and 0.2–0.1 μm yielded, within error, similar KAr ages of ∼30 Ma. These internally consistent ages can be interpreted as the timing of a first faulting event, which we interpret to be associated with the dominant set of top-to-the-S kinematic indicators. Three KAr ages of ∼25 Ma were obtained from two separate <0.1 μm and a single <0.2 μm grain-size fraction. This robust age of the finest grain-size fractions reflects the final faulting increment, considered to date top-to-the N normal shearing. Because the ∼25 Ma age overlaps with high-pressure metamorphism and subsequent rapid exhumation of the Phyllite-Quartzite Unit, we regard the age to be related to the Cretan Detachment in central Crete. Published data show that the upper parts of the Phyllite-Quartzite Unit started to be underthrust to the north between 36 and 29 Ma. Therefore, we relate the fault gouge ages of ∼30 Ma to this underthrusting event. We conclude that the switchover from contractional to normal faulting on the Cretan Detachment occurred at about 25 Ma.