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Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis

Tobisch, Othmar T. and Paterson, Scott R. and Saleeby, Jason B. and Geary, Edward E. (1989) Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis. Geological Society of America Bulletin, 101 (3). pp. 401-413. ISSN 0016-7606. doi:10.1130/0016-7606(1989)101<0401:NATODI>2.3.CO;2.

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Detailed mapping and structural analysis combined with new age dates, using U-Pb and ^(40)Ar/^(39)Ar techniques, have allowed us to constrain the timing of pre-ductile and ductile deformation in the Foothills terrane of the central Sierra Nevada. By using strain and other data, it can be shown that rigid rotation of beds (folding/faulting) predated the onset of ductile deformation and probably occurred between 160 and 151 Ma. Ductile structures, consisting of continuous and secondary cleavages and associated folds and lineations, started forming ca. 151 Ma in the Bear Mountains fault zone and then ca. 145 Ma, began moving away from the fault zone, forming diachronously over an ∼30-m.y. period. The last documented ductile structure formed ca. 123 Ma, although some secondary structures may be younger. Metamorphism of these rocks is generally upper greenschist facies, although higher-grade belts (one bearing staurolite, andalusite, and sillimanite) are present. Strain was preferentially partitioned into one of these belts of higher metamorphic grade (and sporadically elsewhere). The structural history here is much more complex, and at least one and locally two complete transpositions of the original cleavage have occurred. In these zones of complex deformation, it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases, all lying mutually parallel. Timing constraints indicate that the pre-ductile structures may correspond to a very late stage of the Nevadan orogeny (that is, 155 ± 3 Ma), but the ductile structures postdate that orogeny (as defined) by as much as about 25-30 m.y. Models which relate the ductile structures in the central Foothills terrane to Nevadan plate-tectonic events are untenable. In addition, recent work indicates that Late Cretaceous ductile deformation in the central and southern Sierra Nevada may be relatively widespread, indicating that tectonic models for the Sierra Nevada need to be reassessed.

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Additional Information:© 1989 Geological Society of America. Manuscript received by the Society October 5, 1987; Revised manuscript received July 20, 1988; Manuscript accepted August 18, 1988. This research was supported by National Science Foundation Grants EAR-8318212 and EAR-8607017 (to OTT) and EAR-8407788 (to JBS), which we gratefully acknowledge. We thank Elizabeth Miller, John Oldow, and Tim Little for very helpful comments on an earlier version of the manuscript. Ken Morrison, Forrest Fuller, and many other ranchers and landowners generously allowed us access to their land during the study. Mike Wolf assisted in zircon sampling and mineral separation work on the Guadalupe Igneous Complex, and Cherilyn Saleeby is acknowledged for patience and expertise in hand sorting of zircon populations.
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Official Citation:OTHMAR T. TOBISCH, SCOTT R. PATERSON, JASON B. SALEEBY, and EDWARD E. GEARY Nature and timing of deformation in the Foothills terrane, central Sierra Nevada, California: Its bearing on orogenesis Geological Society of America Bulletin, March, 1989, v. 101, p. 401-413, doi:10.1130/0016-7606(1989)101<0401:NATODI>2.3.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:48853
Deposited By: Tony Diaz
Deposited On:25 Aug 2014 21:22
Last Modified:10 Nov 2021 18:36

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