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Structure and tectonics of an Early Mesozoic oceanic basement in the northern Sierra Nevada Metamorphic Belt, California: Evidence for transform faulting and ensimatic arc evolution
Author(s) -
Dilek Yildirim
Publication year - 1989
Publication title -
tectonics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.465
H-Index - 134
eISSN - 1944-9194
pISSN - 0278-7407
DOI - 10.1029/tc008i005p00999
Subject(s) - geology , ophiolite , tectonite , geochemistry , greenschist , pillow lava , metamorphism , shear zone , metamorphic facies , oceanic crust , island arc , mylonite , petrology , volcanic rock , subduction , seismology , tectonics , geomorphology , facies , volcano , structural basin
The N‐S trending structural grain of the Sierra Nevada metamorphic belt bends sharply to an E‐W orientation along the Big Bend fault zone at the northern end of the Smartville complex. Multiply deformed and metamorphosed, early Mesozoic and older (?) rocks of the Jarbo Gap ophiolite crop out within and north of the Big Bend fault zone. The Jarbo Gap ophiolite includes serpentinized harzburgite tectonite, dunite, metagabbro, amphibolite, and metavolcanic‐metasedimentary rocks which are commonly separated from each other by steeply dipping and along‐strike discontinuous fault contacts. Locally, serpentinite occurs as diapiric intrusions and/or fault‐bounded blocks in metavolcanic rocks or is stratigraphically overlain by metavolcanic and metasedimentary rocks. Metasedimentary rocks include hemipelagic sandstones and mafic talus breccias composed of ophiolitic detritus. Amphibolite and metagabbro display multiple deformation features associated with mineral assemblages, indicating retrograde metamorphism from amphibolite to greenschist facies. Raser gabbro within the metagabbro unit shows mylonitic bands and shear zones and is intruded by a leucodiorite, which has a U/Pb zircon age of 204±2 Ma. The internal structure and rock types of the Jarbo Gap ophiolite characterize a heterogeneous oceanic crust that is consistent with an oceanic fracture zone setting. The Jarbo Gap ophiolite is intruded by ∼200 Ma dikes that have a geochemical composition similar to high‐magnesium andesites or boninitic lavas of arc volcanoes. These intrusions are coeval with calc‐alkaline plutonic and hypabyssal rocks of the Smartville and Slate Creek complexes and are interpreted as feeders for an incipient early Jurassic arc. Both complexes include ∼200 Ma plutonic and volcanic rocks that are intruded by ∼160 Ma dikes and plutons. These relations suggest that the Jarbo Gap ophiolite may be part of the oceanic basement of a Jurassic island arc terrane, which includes the Smartville and Slate Creek complexes. The E‐W orientation of the Big Bend fault zone is interpreted to be a manifestation of a regional tectonic discontinuity separating the northern Sierra Nevada and the Klamath Mountains provinces. Regional reconstructions and correlations and the paleomagnetic data from both regions suggest that this discontinuity is either a sinistral transform boundary between the two provinces or the northern termination of the Sierra Nevada province, which was probably translated northward for nearly 500 km in and/or prior to Early Cretaceous time.