Structural and strain analysis of the metasediments in Pacheco Pass, California
Structural mapping at 1:200 is applied to 8 kilometers of near continuous but isolated outcrop in Pacheco Pass, California, to understand outcrop-scale structures created during subduction.
Pacheco Pass, ∼177 km SSE of San Francisco, is part of the deeply subducted Eastern belt of the Franciscan accretionary-complex. Here, road and reservoir-shoreline outcrops provide exceptional exposures of distal turbidites, which are well known for their mid-Cretaceous blueschist-facies minerals.
Using geometric and domainal analyses, I determined four progressively overprinted phases of deformation recorded in interbedded Franciscan metagraywackes and argillites at 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 S2 discrete-crenulation cleavage, which is locally developed in clay-rich hinge zones. These structures are overprinted by D3 structures that consist of open to tight F3 folds, and S3 zonal-crenulation cleavage. Finally, the structures of D4 consist of F4 kink bands and late high-angle normal faults.
The consistency in orientations and discrete phases of deformation demonstrates a coherent and systematic deformation of the rock within these deeply-subducted terranes. I interpret these deformations as the result of initial subduction (D1), accretion (D2, D3?), and late strike-slip faulting (D4).
Further, I describe the taphonomy of Equisetum-like stems in siltstone layers at Pacheco Pass. These long, slender, and straight stem segments look like flattened hollow tubes. Textures and overprinting relationships indicate they settled out onto the sea floor between turbidity flows, were shallowly buried, collapsed from decay, and then silicified.
I also measured 40% extensional strain recorded by three sets of fibrous veins that cut and separate siltstone layers associated with the plant stems. These boudinaged-siltstone layers lie between coarse graywacke beds that are unbroken by the stretching, an inverse behavior for deep crustal deformation. Furthermore, S1 cleavage and D1 veins overprint the fibrous veins, indicating extension was pre- to early subduction.