ABSTRACT: Extensional Disruption of the Coast Range Ophiolite at Mt. Diablo
JONES, DAVID L., University of California at Berkeley, Berkeley, CA
Mafic and ultramafic rocks of the Coast Range Ophiolite are disrupted by extensional faults on the north side of Mt. Diablo. Four major rock units are present: (1) a lower unit of unsheared harzburgite and pyroxenite overlain by (2) sheared serpentinite, (3) a middle unit of sheeted sills and minor pillow basalt, and (4) an upper unit of pillow basalt and basalt breccia. Upper Jurassic and Lower Cretaceous strata of the lower part of the Great Valley sequence are present on the flanks of the mountain. All contacts between these units are extensional faults. Unit 1 of the Coast Range Ophiolite overlies
Franciscan central belt melange containing large blocks of Marin headlands terrane; rodingite is developed locally along the malange-harzburgite fault contact. The structurally lowest extensional faults trend northwesterly and drop upper parts of the ophiolite down onto sheared serpentinite. Joining this structure at nearly right angles are a series of northerly trending detachment faults that separate the sheeted complex from pillow basalt and also drop strata of the Great Valley sequence down into contact with all preserved members of the ophiolite. The estimated minimum thickness of excised ophiolitic elements is 5-6 km. Detachment faulting in the Mt. Diablo area appears to have commenced in the early Cenozoic and may coincide with a major episode of crustal thinning and basin form tion that occurred throughout the Coast Ranges in the early Eocene.