Hypersolidus deformation in the lower crust of the Josephine ophiolite: evidence for kinematic decoupling between the upper and lower oceanic crust

Abstract

New mapping and structural observations in the lower crust of the Josephine ophiolite provide insights into the geometry and kinematics of hypersolidus flow beneath an oceanic spreading center. The lower crust, defined here as the sequence of rocks overlying mantle peridotite and beneath exposure of 100% sheeted dikes, can be divided into lower wehrlite–dunite and upper gabbroic sections. The contact between the two is mutually intrusive where exposed. Hypersolidus fabrics are the dominant structures observed. No pervasive crystal-plastic deformation is observed. Restoration of the hypersolidus foliations and igneous modal layers to their on-axis orientation indicates that they strike approximately perpendicular to the strike of the inferred paleo-spreading center as defined by the orientation of sheeted dikes and on-axis, oceanic faults. Hypersolidus lineations define a dispersed 3-D flow pattern in the lower crust, whereas extension directions in the upper crust (i.e. poles to sheeted dikes, oceanic normal faults) are unidirectional and perpendicular to the paleo-ridge axis. Collectively, these observations are consistent with partitioning of deformation between the upper and lower crust, and local ridge-parallel extension in the partially-molten lower crust due to possible subsidence of the thickened, axial upper crust. However, some component of kinematic coupling between the lower crust and mantle peridotite driven by asthenospheric flow cannot be ruled out.