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Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California

Tobisch, Othmar T. and Fiske, Richard S. and Saleeby, Jason B. and Holt, Elizabeth and Sorensen, Sorena S. (2000) Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California. Geological Society of America Bulletin, 112 (7). pp. 1043-1058. ISSN 0016-7606. doi:10.1130/0016-7606(2000)112<1043:STOMRB>2.3.CO;2.

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For ∼200 km along the eastern Sierra Nevada continental magmatic arc, Mesozoic metavolcanogenic rocks dip steeply to the southwest (∼80°), a feature that must reflect fundamental processes in magmatic-arc construction. Although tight folds can account for such steep bedding tilts, folds in the metavolcanogenic sections are sparse and small scale. We propose that the high bedding tilts were produced by a combination of thrusting, downward displacement, and ductile deformation of the beds. The last two processes accompanied emplacement of the Sierra Nevada batholith. The Ritter Range pendant lies within this ∼200 km belt and provides a relatively large and well exposed Mesozoic volcanic section ranging in age from Late Triassic to mid–Cretaceous. Detailed mapping and ages from U-Pb zircon dates and fossils within the volcanic section reveal five structural blocks (I–V) that are separated by bedding-parallel thrusts, some of which are cryptic. To explain the present difference in bedding orientations between blocks III and IV, we suggest that the thrusting may have had a duplex geometry, which produced a maximum bedding dip of ∼45° in some blocks. Downward displacement of wall rock and ductile strain account for the remaining ∼35° of the observed average bedding dip (∼80°SW). The exact time of thrusting and duplex formation of Late Triassic to Early Jurassic rocks in blocks I–IV is uncertain, but these structures developed either (1) between 105 and 164 Ma, well before the other rotational processes were active, or (2) mostly around 105 Ma, and closer to the time when other rotational processes were active. Much of the subsequent (ca. 91–76 Ma) bedding tilting is related to downward displacement of beds associated with the emplacement of voluminous Late Cretaceous plutons, and to regional ductile deformation of the wall rocks during that period: the majority of the tilting probably took place between ca. 91 and 86 Ma. Bedding tilts of early to mid-Cretaceous rocks in blocks IV and V is bracketed between ca. 98 and ca. 90 Ma. Comparisons with metavolcanic sections to the northwest near Tioga Pass and to the southeast in the Mount Morrison, Mount Goddard and Oak Creek pendants, suggest that bedding rotation by thrusting(?), downward displacement and ductile strain of wall rock may explain the steep dips along this entire ∼200 km segment of the continental arc. Similar mechanisms may operate at midcrustal levels in other continental arcs.

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Additional Information:© 2000 by the Geological Society of America. Manuscript accepted July 30, 1999. Manuscript received by the Society October 2, 1998. Revised manuscript received 20, May 1999. We are grateful for support from Sprague and Becker funds of the Smithsonian Institution (RSF, SSS), the National Science Foundation grant EAR9316105 (JBS), the U.S. Geological Survey, and the Department of Earth Sciences, University of California, Santa Cruz. P. S. Simony generously contributed to discussion and references on the maximum bedding tilts found in thrust belts in the Canadian Cordillera, for which we are grateful. We thank R. Brooks Hanson for performing zircon separations, and John Bartley, Sven Morgan and associate editor Doug Walker for reviews that greatly improved the manuscript.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Smithsonian Institution Sprague and Becker fundsUNSPECIFIED
University of California, Santa Cruz, Department of Earth SciencesUNSPECIFIED
Subject Keywords:batholith, structure, volcanogenic wall rocks.
Issue or Number:7
Record Number:CaltechAUTHORS:20140826-145557977
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Official Citation:Othmar T. Tobisch, Richard S. Fiske, Jason B. Saleeby, Elizabeth Holt, and Sorena S. Sorensen Steep tilting of metavolcanic rocks by multiple mechanisms, central Sierra Nevada, California Geological Society of America Bulletin, July, 2000, v. 112, p. 1043-1058, doi:10.1130/0016-7606(2000)112<1043:STOMRB>2.0.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48944
Deposited By: Ruth Sustaita
Deposited On:26 Aug 2014 22:39
Last Modified:10 Nov 2021 18:38

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