Sequential Folding During Subduction of the Franciscan Metasediments in Pacheco Pass, California
SAJA, David B., and PHIPPS, S. P., Department of Geology, University of Pennsylvania, Philadelphia, PA 19104-6316.
Geometric and domainal analyses are applied to 8 kilometers of near continuous but isolated outcrop in Pacheco Pass, California, to elucidate the heterogeneity of outcrop-scale structures created during subduction.
Pacheco Pass, ~ 177 kilometers SSE of San Francisco, is part of the deeply subducted Eastern Franciscan belt. Here, road and reservoir-shoreline outcrops provide exceptional exposures of accretionary-prism rocks. These rocks are famous for their mid-Cretaceous blueschist-facies minerals, and have been previously mapped as semi-coherent terranes.
Our mapping has determined four progressively overprinted phases of deformation as recorded in the interbedded Franciscan metagraywackes and argillites (distal to mid-fan turbidites) that outcrop in Pacheco Pass. The structures of deformation D1 consist of pre-cleavage veins, close to isoclinal interstratal F1 folds, and a regionally dominant, weak axial-planar S1 slaty cleavage. The structures of deformation D2, which deform D1 structures, are pinch-and-swell structures, tight to isoclinal F2 folds, and an S2 crenulation cleavage that is locally developed in clay-rich hinge zones. These structures are overprinted by D3 structures which consist of open to tight F3 folds, and an S3 crenulation cleavage. Finally, the structures of D4 consist of F4 kink bands and late high-angle normal faults. A range of deformation styles exist. In outcrop, the bulk deformation ranges from bedding-parallel shear bands in coherent planar beds to highly deformed almost chaotic folds to stratal disruption close to that of a melange. Even within each of the above phases of deformation there are contrasting contractional (fold) and extensional (vein) structures. However, our synoptic stereo nets of geometrically homogeneous domains show consistent orientations across a range of domain sizes.
In contrast to the generally accepted view that deep accretionary-prism structures are intractable, the above consistency in orientations and the discrete phases of deformation demonstrate a coherent and systematic deformation of the rock within these deeply subducted terranes. We interpret this deformation to be the result of initial subduction (D1), accretion (D2, D3?), and late strike-slip faulting (D4).
Saja, D. B., and Phipps, S. P., 1997, Sequential folding during subduction of the Franciscan metasediments in Pacheco Pass, Californina: Geological Society of America Abstracts with Programs, Vol. 29, No. 6, p. A-222.
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