EARTHQUAKES AND EXHUMATION OF UHP ROCKS: OCCURRENCE, COMPOSITION, AND UV-LASER ABLATION 40AR/39AR GEOCHRONOLOGY OF PSEUDOTACHYLYTE FROM THE DORA MAIRA MASSIF, ITALY

In the Dora Maira Massif, Italy, pseudotachylytes occur within a ~50 m interval near the base of a 300-400 m section of moderately west dipping protomylonitic gneiss overlying the ~34 Ma coesite-bearing ultra high pressure (UHP) unit. All kinematic critera observed in the gneiss are consistent with top-to-the west extensional shear. Fresh pseudotachylyte veins up to 1 cm thick root in composite (ultramylonite/cataclasite/pseudotachylyte) bands roughly parallel to the mylonitic foliation. Strongly overprinted pseudotachylyte veins are also present. XRF analyses show minimal compositional differences between fresh pseudotachylyte and the surrounding gneiss. At a finer scale, UV-laser ablation ICP-MS analyses indicate that homogenization of REE elements within the pseudotachylyte occurs by ultracataclasis of REE carriers within the gneiss (epidote, apatite, and zircon) immediately prior to melting. In situ UV-laser ablation ⁴⁰Ar/³⁹Ar dating of different parts of a ~1 cm-thick fresh pseudotachylyte yields analytically indistinguishable (2s ) ages with a weighted mean of 20.1 ± 0.5 Ma, and are temporally consistent with zircon fission track ages from the UHP unit (29.9 ± 1.4 Ma, Gebauer et al., 1997). These data place the UHP rocks in a near-surface position at this time. The pseudotachylyte investigated reflects deformation and melting at relatively shallow (~10 km) crustal depths during a seismic event that occurred after the rapid exhumation of UHP rock (estimated at 2.0 to 2.4 cm/a; Gebauer et al., 1997). However, the occurrence of numerous other pseudotachylytes, some strongly overprinted, together with layers of phengite-rich quartzites, compositionally identical to the UHP whiteschists, suggests that the entire sequence of protomylonitic rock was once at UHP depths and has witnessed numerous seismic events separated in time. Rapid exhumation of the UHP rocks probably occurred during large magnitude displacements associated with pseudotachylyte producing seismic events. Such a model explains the unusually fast exhumation rates inferred from the geochronological data and the preservation of coesite in these UHP rocks.

Reference: Gebauer et al. (1997) Lithos, v. 41, p. 5-24.