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Theses Doctoral

Stratigraphy, structural geology, and tectonic implications of the Shoo Fly Complex and the Calaveras-Shoo Fly thrust, Central Sierra Nevada, California

Merguerian, Charles

Mylonitic rocks of the Shoo Fly Complex form a region of epidote-amphibolite grade quartzose and granitoid gneiss, subordinate schist and calcareous rocks, and rare amphibolite in the foothills of the Sierra Nevada range in central California. The Shoo Fly has endured a complicated Phanerozoic structural development involving seven superposed deformations at variable crustal depths. The first four of these (D1-D4) involved tight to isoclinal folding and shearing under medium grade metamorphic conditions. The last three (D5-D7) are marked by open folding and retrograde metamorphism of older fabric elements.

The Shoo Fly is in ductile fault contact with east-dipping argillite, chert, and marble of the Calaveras Complex. The Calaveras-Shoo Fly thrust formed during D3 and is a 1-2 km wide syn-metamorphic ductile shear zone. Recognition of D3 overprinting of older Dl+D2 fabrics along the thrust zone indicates that upper plate Shoo Fly rocks record an earlier and more complex structural history than the lower plate Calaveras rocks.

Paleozoic gneissic granitoids, an important lithologic component of the Shoo Fly, were intruded as a series of plutons ranging from calc-alkaline gabbro to granitoid (predominate) to syenite. They truncated the early S1 foliation in the Shoo Fly and were folded during regional D2 and D3 events when they were penetratively deformed into augen gneiss, blastomylonite, and ultramylonite.

The Sonora dike swarm occurs as an areally extensive (> 1500 km2) subvertical consanguineous suite of andesite, lamprophyre, and basalt dikes that trend east-west across the Calaveras and Shoo Fly Complexes. The metamorphic complexes form the basement to a middle Jurassic calc-alkaline plutonic arc (Jawbone granitoid sequence). A middle Jurassic K-Ar age on the dikes (157-159 m.y.) together with the data of this report indicate that they are petrogenetically related to the Jawbone granitoid sequence and that the dikes probably formed during subduction beneath a continental arc.

The dikes provide an important structural marker in the Shoo Fly and Calaveras Complexes. Intrusion of the dike swarm was sensitive to a structural anisotropy in the basement complexes. Since they intruded east-west along a spaced regional schistosity developed during folding of the Calaveras-Shoo Fly thrust, thrusting and subsequent folding were clearly pre-middle Jurassic events.

Available geochronologic data sets middle Ordovician to late Devonian intrusive ages for the gneissic granitoids, establishing a pre-late Devonian depositional age for the Shoo Fly. D1 and intrusion of the orthogneiss protoliths may have been precursors of the Late Devonian to Early Mississippian Antler orogeny or, alternatively, may have occurred significantly earlier than the Antler orogeny. Based on cross-cutting relations, D2 formed during the Antler orogeny, D3 and possibly D4 during the Sonoma orogeny, and D5 and D6 during the Nevadan orogeny.

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More About This Work

Thesis Advisors
Schweickert, Richard A.
Degree
Ph.D., Columbia University
Published Here
January 6, 2020