Structural Analysis in the Coast Range Ophiolite near Paskenta, California: Implications for Tectonic Uplift Processes

Since ~155 Ma, the Franciscan complex of Central California accumulated in front of, and beneath the Coast Range Ophiolite. Blueschist facies rocks which formed at ~15 km depth are now juxtaposed against upturned ophiolitic rocks and overlying sediment that are <10 km thick along the Coast Range Fault. This fault boundary has been interpreted as 1) a rotated, subduction thrust fault, 2) a high-angle west-dipping reverse fault, and 3) a high-angle normal fault. Notably, kinematic data concerning Coast Range Fault movement and tectonic models for ophiolite uplift are sparse.

To constrain the geometry and kinematics of the Coast Range Fault and ophiolite deformation, three geologic transects were made across the Coast Range Ophiolite near Paskenta. Lithologies were mapped along ~30 km of roadcut exposures. The attitudes of 1426 fault planes were measured; 46% of which had lineations and 32% record full kinematics. Most of the faults were minor, but 266 had trace length ≥ 1 m.

Mapping revealed that while most parts of the ophiolite suite are present, the pieces are well shuffled. Additionally, faults and serpentinite shear zones are heterogeneously distributed. An ophiolitic mélange is present near Paskenta.

The bordering Coast Range Fault strikes roughly north-south and has a near vertical dip. Most faults within the ophiolite are subparallel: striking north-south and steeply dipping. Normal offsets (n=226) dominate over those with reverse offset (n=97).

Uplift of the Franciscan complex and Coast Range Ophiolite near Paskenta was accomplished not just by near-vertical, normal movement along the Coast Range Fault, but also by slip along many faults and shear zones within the ophiolite belt. As observed faulting within the ophiolite does not correspond with mapped lithologic heterogeneity, ophiolitic mélange formed prior to uplift, probably in an oceanic transform setting.