FAST SUBDUCTION, SLOW EXHUMATION: DATING CONTINENTAL SUBDUCTION BENEATH THE OMAN OPHIOLITE

Continent subduction beneath continental upper plates has occurred at a broad range of rates that correlate with the size of the subducted fragment: small slivers subduct and exhume fast and cold while larger fragments undergo much slower and hotter orogenesis (Kylander-Clark et al., 2012). The few extant examples of continental fragments subducted beneath oceanic crust record cold geothermal gradients (<10 °C/km), but rates associated with this process are uncertain. To better constrain these rates, we dated garnet, zircon, and rutile from the (U)HP As Sifah unit in the Saih Hatat of Oman. Existing geochronology from As Sifah suggests that either i) continental subduction occurred at ~110 Ma (Gray et al., 2004), predating ophiolite formation by ≥15 My, or ii) occurred at ~79 Ma (Warren et al., 2005), 15 My after initial emplacement of the ophiolite nappe. New dates from multiple lithologies in the As Sifah unit show that 1) previously published Sm-Nd dates are likely inaccurate and 2) multi-grain U-Pb zircon TIMS dates may be averages of long-term metamorphism. Our data suggest that garnet growth started by ~85 Ma – ~11 Myr after formation of the ophiolite crust – and lasted until at least ~74 Ma. Combined with other thermobarometric and structural constraints, these data indicate a subduction rate of 7±2 mm/yr and an exhumation rate of 3±2 mm/yr for the Arabian continental margin. The duration of HP metamorphism is similar to other continental fragments subducted beneath ophiolites (e.g., New Caledonia, Corsica). Our data show that As Sifah behaved as a small continental sliver during subduction yet like a larger continental body during exhumation – consistent with subduction of extended continental crust followed by thickening.