DEVELOPMENT OF FOLIATION IN UPPERMOST GABBROS IN THE OMAN OPHIOLITE AND IMPLICATIONS FOR CRUSTAL ACCRETION ALONG A PALEORIDGE AXIS

Foliated gabbros in middle to upper oceanic crust are commonly thought to develop by directed magma flow either steeply upward to replenish the axial melt lens or steeply downward due to subsidence away from the axis. This hypothesis is supported by the fact that many outcrops of upper to middle oceanic crust in the Oman ophiolite show steeply-dipping magmatic foliations defined by aligned plagioclase and pyroxene. However, mesoscopic lineations that would indicate flow direction are rare or covered by desert varnish, so other methods are necessary to measure lineations.

We studied a small-scale area in the Semail Massif near Mahram to test the hypothesis for the generation of foliated gabbros and document magma flow directions using anisotropy of magnetic susceptibility (AMS). AMS provides a measure of the magnetic fabric of core samples as a proxy for the silicate fabric (mineral foliations and lineations) that can then be used to constrain magma emplacement and flow.

We find predominantly subhorizontal- to moderately-plunging magnetic lineations (Kmax) that are consistent in both foliated and isotropic gabbros across several hundreds of meters perpendicular to the spreading direction. A comparison of shape-preferred orientation of phases using image analysis of orthogonal thin sections shows generally good agreement with the AMS fabrics within the few sites examined, supporting the link between magnetic and silicate fabrics.

The consistent Kmax directions suggest a subhorizontal to oblique magma flow direction during the development of foliation in gabbros in this area. Although subhorizontal Kmax directions have been documented in the Troodos ophiolite near the Arakapas Transform Fault, shallow-plunging Kmax directions have not yet been documented in Oman, where lineations have instead been found to be predominantly steeply plunging or more variable within comparable sections of adjacent massifs. Correcting the AMS data using paleomagnetic directions does not significantly change the Kmax inclinations.

The significant component of lateral flow suggests that the Maqsad diapir located to the NW could be the magmatic source for these foliated gabbros, and implies that future refinements to accretionary models for the middle oceanic crust should also consider along-axis components of flow.