Structural analysis in the Coast Range Ophiolite near Paskenta, California : implications for tectonic uplift processes




Dewhurst, Andrew David

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California has been a site of subduction since about 155 Ma. The Franciscan complex accumulated in front of, and beneath, the crystalline leading edge of the North American Plate: a belt of oceanic crust and mantle known as the Coast Range Ophiolite. Near Paskenta, California, the blueschist facies Franciscan rocks (South Fork Mountain Schist) are juxtaposed against the ophiolitic rocks along the Coast Range Fault. These Franciscan rocks have risen ~15 km and the ophiolitic rocks at least 3 km and perhaps as much as 10 km. The intervening Coast Range Fault has a near vertical dip. It has previously been interpreted as a rotated east-dipping thrust fault that formed by subduction underthrusting, an east-dipping décollement to a series of out-of-sequence imbricated thrusts, and a high-angle west-dipping reverse fault that formed by collisional overthrusting. To explain the unroofing of the Franciscan rocks, it has been interpreted as a high-angle normal fault. Notably, fault-related kinematic data in support of tectonic models for Coast Range Fault movement and ophiolite emplacement are sparse. To constrain the geometry and kinematics of the Coast Range Fault and ophiolite emplacement, three geologic transects were made along sinuous roads near Paskenta. Approximately 30 km of roadcut is present along the ~60 km length of road. Exposures are typically less than two meters high, but locally as tall as 20 m, along roadcuts that extend for tens to a few hundred meters. Lithologies were mapped in the roadcut exposures and the attitudes of fault planes and direction and sense of slip was measured. Data were obtained on 275 faults traceable for ≥1 meter. Within serpentinite shear zones, the attitudes of 1159 minor faults were measured. Mapping revealed that although all parts of the ophiolite suite are present near Paskenta, the pieces are well shuffled. For example, slabs of layered radiolarian chert are found in the western part of the ophiolite near the Franciscan rocks and masses of serpentinite are present in the eastern part near the steeply eastwards dipping strata of the depositionally overlying Great Valley Group (Knoxville Formation). In addition, faults and serpentinite shear zones are heterogeneously distributed. In the Paskenta area, the ophiolite is not simply an upturned slab of oceanic crust. It is well described as an ophiolitic mélange. Stereographic analysis reveals the pattern of faulting along each transect is similar with most faults striking roughly north-south and steeply dipping - near-parallel to the Coast Range Fault. For major faults with full kinematic observations, normal offsets (n = 226) dominate over those with reverse offset (n = 97). Movement patterns recorded in the minor faults found in the serpentinite shear zones are less defined, but overall patterns appear similar. Uplift of the Franciscan complex and Coast Range Ophiolite near Paskenta was accomplished not just by normal, near-vertical movement along the Coast Range Fault, but also by slip along the numerous faults and shear zones within the ophiolite belt. However, faulting associated with the ascent of Franciscan rock does not appear to be sufficient to account for the thorough disruption of the ophiolite terrane in this area. The ophiolitic mélange was probably generated prior to ophiolite emplacement in an oceanic transform setting


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