EXHUMATION OF HIGH-PRESSURE ROCKS BY PERVASIVE BRITTLE THINNING OF THE UPPER CRUST, CRETE, GREECE

Normal faulting is an important exhumation process. In some settings, such as the Basin and Range province, large slip on low-angle detachment faults is thought to be the dominant mechanism for exhumation. A similar scenario has been proposed for the exhumation of high-pressure/low-temperature (HP/LT) rocks in Crete, Greece. There, HP/LT rocks are exposed in the footwall of the low-angle Cretan detachment fault. Previous work has shown that erosion was not a major exhumation process, so models have emphasized the role of the Cretan detachment fault in exhuming the HP/LT rocks. Here, we review aspects of the geology of Crete and conclude instead that exhumation is accomplished primarily through pervasive brittle extension of the upper crust. Peak metamorphic temperatures for several basement units have been measured using Raman spectroscopy of carbonaceous materials. In central Crete, temperatures of ~345-325°C in the footwall and ~265°C in the hangingwall give a break of 60-80°C across the detachment. Thermobarometry suggests a paleothermal gradient of about 10°C/km, indicating that the detachment excised only about 6-8 km of crust. Existing thermochronometric data constrain the exhumation of the footwall units to the interval 19 to 9 Ma. If the Cretan detachment was primarily responsible for exhumation of the footwall units, slip must have occurred at a rate of 8-12 km/m.y. Modeling studies show that slip at these rates will heat the hangingwall by ~40-80°C as it is juxtaposed against the relatively hot footwall. However, apatite fission-track ages from the hangingwall in Crete have ages of about 17.5 Ma, indicating cooling rather than heating during early exhumation. Together, the temperature and thermochronometric data are inconsistent with significant exhumation on a fast-slipping Cretan detachment fault.

As an alternative, we draw attention to the numerous high angle normal faults that dissect Crete. These structures affect all of the basement units, including both the hangingwall and footwall of the detachment. We argue that these observations are consistent with a scenario in which sustained underplating beneath Crete drives surface uplift and pervasive brittle extension of the upper crust. Slip on these higher angle structures is primarily responsible for the exhumation of the HP/LT rocks.