Coeval volcanism and rotation of Neotethyan oceanic crust in the Oman ophiolite – fact or fiction?

The upper crustal volcanic section of the Oman suprasubduction zone ophiolite is divided into an older V1 sequence, overlain by slightly younger V2 lavas and (in places) a final V3 sequence. Paleomagnetic data from the V1 and V2 sequences of the northern massifs of the ophiolite have been used previously to infer that clockwise rotation of the Oman lithosphere began while the upper crust was actively accreting, with V1 lavas apparently more rotated than the overlying V2 units. This inference has been largely accepted by the geological community and has influenced models for the spreading history and geodynamic evolution of the Oman ophiolite.

Here we present new paleomagnetic data from well-exposed and structurally well-constrained volcanic sequences in the Salahi and Fizh massifs of the ophiolite that discredit this interpretation. In contrast to previous studies that employed standard structural tilt corrections, we use a net tectonic rotation approach to determine rotation parameters, taking confidence limits on input variables into account using Monte Carlo modelling. Importantly, we correct the magnetization direction and structural orientation of the older V1 lavas for the effects of the net tectonic rotation of the younger V2 lavas prior to calculating rotation parameters for the older units. Results demonstrate that both massifs rotated ~120° clockwise around steeply-plunging rotation axes after eruption of the V2 lavas. This rotation occurred during roll-back of the Neotethyan subduction zone in response to impingement of the Arabian margin with the trench. Early rotation of the Salahi V1 lavas around shallowly-plunging, broadly ridge-parallel axes indicates only simple tilting between eruption of the V1 and V2 sequences, and no early rotation of the Fizh V1 lavas is required at all. These new constraints on the evolution of the ophiolite therefore provide no evidence of vertical axis rotation during accretion of the Oman volcanic sequences.