A TECTONIC ORIGIN FOR LISTWAENITE IN THE SEMAIL OPHIOLITE AT WADI MANSAH, NORTHERN OMAN

Al Hajar Mountains in northern Oman are underlain by a series of allochthonous deep sea sediments and oceanic to mantle rocks of the Semail ophiolite that were thrust onto shallow carbonate rock on the Arabian Plate during Tethys subduction in the Late Cretaceous. Listwaenite, a distinctive carbonate-quartz rich rock interpreted to have formed by carbonation of serpentinzed ultramafic rocks, occurs near or at the base of the ophiolite. Previous workers infer that listwaenite formed in the mantle wedge during obduction, while others note that listwaenite may have formed during post-obduction extensional brittle faulting. In this study, we further investigate the origin of listwaenite near Wadi Mansah using a combination of structural and petrographic analyses. We mapped 25 km 2 through a combination of field work and satellite image interpretation.

The listwaenite near Wadi Mansah formed at least partially during postobductional extensional faulting. The rock occurs along multiple generations of NW-to-SE striking oblique normal faults, as well as another set of older shallowly-dipping normal faults. In one instance, a klippe of listwaenite overlies metamorphic sole, amphibolite facies rocks formed at the base of the ophiolite as it obducted. Previous studies have identified these features as Late Cretaceous to Early Eocene in age, which suggests the listwaenite formed after obduction. In hand sample and thin section many of the samples exhibit brittle deformation features - such as brecciation – that could only have formed after the ophiolite had cooled after obduction. These clastic listwaenites also appear to have undergone further listwaenization after brittle deformation occurred. Other microstructures include undulose extinction, recrystallized quartz, spherulites, and deformation lamellae.