HR: 0830h
AN: V21D-0557    [PDF]
TI: Structure, metamorphism and timing of an exhumed Cretaceous subduction zone beneath the Oman Ophiolite
AU: Searle, M P
EM: Mike.Searle@earth.ox.ac.uk
AF: Dept. Earth Sciences Oxford University, Parks Rd., Oxford, OX1 3PR
AU: Warren, C J
AF: Dept. Earth Sciences Oxford University, Parks Rd., Oxford, OX1 3PR
AU: * Waters, D J
EM: davew@earth.ox.ac.uk
AF: Dept. Earth Sciences Oxford University, Parks Rd., Oxford, OX1 3PR
AU: Parrish, R R
EM: r.parrish@nerc.ac.uk
AF: NIGL, British Geological Survey, Keyworth, Nottinghman, NC12 5AA
AB: The Semail ophiolite in Oman was emplaced from NE to SW at least 200 km over the Arabian passive margin, probably over 450 km in total, during the late Cretaceous (95-70 Ma). The first phase of obduction involved NE-directed subduction of Triassic-Jurassic basalt at least 45-50 km beneath the ophiolite, whilst the crustal sequence was forming (U-Pb zircons from plagiogranites, ca. 95 Ma). Amphibolites accreted beneath the mantle sequence peridotites have P-T conditions of 840-870§C and 10-12 kbar with 40Ar/39Ar hornblende cooling ages of 95-92 Ma. During the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite-bearing rocks (6-8 kbar), blueschist (12-15 kbar) and eclogite (ca. 20 kbar) facies metamorphism formed in a ductile deforming NE-dipping subduction zone. Five concordant U-Pb ages from the As Sifah eclogites constrain the HP metamorphic peak at 79.1ñ0.3 Ma. Detailed structural mapping and restoration of the continental margin, combined with P-T and U-Pb geochronology confirms the model of one protracted phase of ophiolite obduction along a NE-dipping subduction zone, at convergence rates of ca. 17 mm/a-1. NE-directed extensional crenulation schistosity and NNE oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with SW-directed exhumation of footwall HP rocks. NE facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds, with shortening in the upper plate balanced by the subduction of the lower plate, consistent with a NE-directed subduction of the continental margin rocks beneath the SW-obducting ophiolite, Haybi and Hawasina thrust sheets. Recent suggestions of a nascent SW-directed subduction beneath the Oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates, the geological structure of Saih Hatat, or the U-Pb geochronology of the ophiolite and eclogites.
DE: 1020 Composition of the crust
DE: 1065 Trace elements (3670)
DE: 3035 Midocean ridge processes
DE: 5120 Plasticity, diffusion, and creep
DE: 8005 Folds and folding
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting