The role of deformation in the formation of metamorphic gradients: Ridge subduction beneath the Oman Ophiolite

Abstract

Two tectonic scenarios have been proposed for genesis and emplacement of the Oman ophiolite. One suggests that the ophiolite was generated at a spreading center, the other suggests generation within an intraoceanic arc. An integrated thermal and kinematic model of the temperature, stress, rock type, and displacement fields during early stages of the emplacement of the Oman ophiolite was developed to test these two possibilities. The thermal evolution was calculated by a finite difference algorithm for heat conduction, considering heats of metamorphic reactions, deformational heating, heat advection by flowing rock, mantle heat flow, and radioactive heating. The stress and displacement fields were calculated by an analytical model using a velocity boundary condition, power law constitutive relations, and a brittle frictional sliding relationship. Field observations in Oman can be satisfied with the spreading center model but not with the arc model. Moreover, simulations indicate that the ophiolite was probably <2 m.y. old at the time of intraoceanic thrusting and that shear stresses of ∼100 MPa were attained during thrusting. In addition to satisfying currently available field constraints, these simulations also indicate specific further work that will help resolve the controversy over thevolution of this unique orogen. Future work should be directed toward (1) using thermobarometry and thermochronology to constrain the spatial variation in PTt paths of the metamorphic sole; (2) dating the igneous genesis of the ophiolite; and (3) using quartzite and dunite recrystallized grain size paleopiezometers to infer stresses during thrusting. These simulations point out the important effect synmetamorphic deformation can have on the formation of metamorphic field gradients. In contractional fault zones, metamorphic field gradients may be normal or inverted and contracted or extended relative to the initial thermobarometric gradient, and adjacent fault zone rocks may display crossing PT paths. Unusually contracted or extended metamorphic field gradients may also develop in the footwalls of normal fault zones where portions of the heating hangingwall accrete to the uplifting, cooling footwall.