Structural and strain analysis of the metasediments in Pacheco Pass, California

1999 1999

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Abstract (summary)

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.

Indexing (details)

0372: Geology
0373: Geophysics
0345: Paleobotany
Identifier / keyword
Earth sciences; California; Deformation; Equisetum; Metasediments; Pacheco Pass; Strain
Structural and strain analysis of the metasediments in Pacheco Pass, California
Saja, David Bryan
Number of pages
Publication year
Degree date
School code
DAI-B 60/04, Dissertation Abstracts International
Place of publication
Ann Arbor
Country of publication
United States
059925954X, 9780599259546
Phipps, Stephen Paul
University of Pennsylvania
University location
United States -- Pennsylvania
Source type
Dissertations & Theses
Document type
Dissertation/thesis number
ProQuest document ID
Database copyright ProQuest LLC; ProQuest does not claim copyright in the individual underlying works.
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