SALEEBY, JASON B., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, and CATHY J. BUSBY, Department of Geological Sciences, University of California, Santa Barbara, CA
ABSTRACT: Paleogeographic and Tectonic Setting of Axial and Western Metamorphic Framework Rocks of the Southern Sierra Nevada, California
In 1978, we postulated that much of the southern Sierra prebatholithic metamorphic framework consisted of lower Mesozoic siliciclastic, carbonate, and pelitic strata with variable arc volcanic admixtures (Kings sequence). Recent syntheses, however, have attempted to minimize the importance of early Mesozoic strata in the region and to extend large length scale coherent Paleozoic terranes into the framework as the predominant protoliths. Neither lithologic correlations nor structural analysis can substantiate such a view, however, and the proposed configuration of the Paleozoic terranes is in conflict with the petrochemical zonation pattern of the Cretaceous batholith.
Stratigraphic relations for the relatively well-preserved lower Mesozoic stratified rocks of the southern Sierra generally support our 1978 synthesis. As pointed out by more recent syntheses, however, we now recognize the likelihood of Paleozoic strata occurring in some or many of the Kings sequence pendants. Such rocks are more likely to be disparate fragments of a highly dismembered polygenetic basement composed of Shoo Fly, miogeoclinal, and possibly Antler-belt rocks rather than coherent terranes or crustal blocks. The lower stratal levels of the lower Mesozoic Kings sequence may have formed part of a regional post-Sonoman (Triassic) marine overlap sequence above this basement complex Dismemberment and accretion of the basement complex involved transform truncation tectonics and F othills ophiolite belt emplacement prior to and coincident with Sonoman thrust tectonics. Following the establishment of a Carnian-Norian carbonate platform, as part of the overlap sequence, the region subsided and became part of a regional Early Jurassic forearc basin with the deposition of Kings sequence turbidites and olistostromes. The forearc basin was destroyed by the late Early to Middle and Late Jurassic thrusting, which may have been diachronous with a northward migration pattern.
The assertion that much of the Kings sequence is Paleozoic is based on the discovery of probable Eocambrian-Cambrian miogeoclinal strata in the Snow Lake pendant of the east-central Sierra Nevada. The discovers offer a reconstruction of the displacement of these strata as part of a large crustal block from the western Mojave region through the axial Sierra Nevada along a now cryptic fault. The bounds of the hypothetical crustal block, however, are at odds with batholithic petrochemical patterns. A more conservative offset history for the Snow Lake pendant rocks preserves the petrochemical patterns and considers a broader uncertainty in the bounds of the possible source area for the rocks.
AAPG Search and
Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach,
California, May 5-7, 1993.