Synmagmatic normal faulting in the lower oceanic crust: Evidence from the Oman ophiolite

Structural and petrological study of the Oman ophiolite shows that normal faults at oceanic spreading centers can root in incompletely crystallized cumulates and form at the same time as melt is being supplied to the magma chamber. In the vicinity (<= 200 m) of a ridge-facing normal fault located a couple of kilometers off a former axial mantle diapir, blocks of layered cumulates were tilted at high angles relative to the Moho. Block rotation was accommodated by a swarm of anastomosed fault planes connected to the main fault. These planes are underlain by screens of gabbroic micropegmatites (former melt layers) and by ptygmatic folds, pointing to viscous deformation of a compacting crystal mush. Flat-lying, undeformed cumulate layers from the same parent melts as the deformed ones settled directly over the tilted blocks. Water introduction into the melt is evidenced by the crystallization of anomalously high anorthite (An 90%-95%) cumulus plagioclase, regardless of the degree of evolution of the magma. Shearing continued at subsolidus temperatures along some, but not all, fault planes, attested to by the local development of plastic deformation structures. The subsolidus development of amphibole-spinel coronas around olivine grains points to pervasive percolation of high-temperature hydrothermal fluids through the cumulates. As the crust cooled further, water penetration progressively focused within the main fault zone, where the crust was fractured and eventually altered in greenschist facies conditions.