| dc.description.abstract | A ~8000 km wide ocean called the Neo-Tethys Ocean disappeared underneath the Eurasian plate during long time plate convergence between India and Eurasia since 130 Ma ago (and likely even earlier). The only rock records that are left of the large Tethyan ocean are ophiolitic belts that are nowadays found on top of the continents. The Semail ophiolite in Oman is one of them, and has formed while the Indian plate moved in a NE direction to collide with the Eurasian plate. Nowadays the Semail ophiolite forms a 500 km long belt composed a complete ophiolitic sequence of mantle rocks, gabbros, sheeted dykes, lava flows, and deep marine sediments, making it the most intact and most thoroughly studied ophiolite in the past 30 years.
The aim of this study is (i) to find the amount of rotation around a vertical axis experienced by the ophiolite, and (ii) to find the paleospreading direction of the oceanic ridge during the formation of the ophiolite, as well as along what kind of plate boundary the ophiolite formed. Paleomagnetic analyses of the upper crustal sequence (sheeted dykes and pillow lavas) of the Oman ophiolite was the adopted technique to reconstruct the initial orientations of the spreading ridges, determining their paleodirection and rotation, as well as the paleolatitude at which the ophiolite formed. In this work more than 400 cores from 12 localities were paleomagnetically analysed through both alternating field and thermal demagnetization treatments. Remanence components were isolated using standard paleomagnetic techniques and the mean values were calculated via Fisherian statistics of the characteristic remanence magnetisations. We used a Net Tectonic Rotation approach to calculate the rotation pattern and establish the initial dyke orientation necessary to infer on the paleospreading directions, for dykes are formed parallel to the spreading ridge. The results of this analysis evidenced the presence of two sectors rotated in opposite directions: clockwise in the northern and middle part of the ophiolite, and counterclockwise in the eastern part. Using NTR analysis we also documented the spreading directions, which are perpendicular to the N-S strike found for the magmatic spreading centres obtained from the initial orientation of the sheeted dykes. From the pillow lavas a paleolatitude of 8.6° N was found for the ophiolites at the moment of formation. From the found pattern, with both CW and CCW rotation, a N-S strike of the spreading ridge, and the paleolatitude, we argue that the ophiolite was not formed along a Neo-Tehyan ocean spreading ridge, but rather along a N-S transform segment of the western limit of the Indian plate, above a newly initiated subduction zone. | |
