HYPERSOLIDUS DISPLACEMENT PATTERNS IN THE LOWER CRUST OF THE JOSEPHINE OPHIOLITE

Foliation patterns within the lower crustal, plutonic sections of ophiolites are commonly interpreted to reflect the ridge-perpendicular extensional displacement field within the lower crust. In well-studied ophiolites such as Oman and Bay of Islands, steep magmatic foliation in the upper gabbros shallow down section and become subparallel with the crust-mantle transition. In many numerical models and schematic diagrams, this transition in fabric orientation is interpreted to reflect ridge-perpendicular flow coupled with brittle extension via dike intrusion in the sheeted dikes of the upper crust. New mapping and structural observations in the lower crust of the Josephine ophiolite, Northern California provide insights into the style of crustal accretion and the geometry and kinematics of flow beneath a supra-subduction zone spreading center. The lower crust, defined here as the sequence of rocks overlying serpentinized 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. Igneous textures and hypersolidus fabrics (i.e., formation in the presence of melt) are the dominant microstructures observed. No pervasive crystal-plastic deformation is observed. Restoration of the hypersolidus foliations and igneous layers to their on-axis orientation indicates that they have strikes that are perpendicular to the strike of the paleo-spreading center as defined by the orientation of sheeted dikes and on-axis, oceanic faults. Hypersolidus lineations define a 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 local ridge-parallel flow in the partially molten lower crust, partitioning of deformation between the upper and lower oceanic crust, and highlight the need for caution when evaluating the geometry and kinematics of hypersolidus flow beneath contemporary oceanic spreading centers and ophiolites where it is difficult to restore igneous structures to their on-axis orientations.