Paleogeographic and tectonic setting of axial and western metamorphic framework rocks of the southern Sierra Nevada, California

Abstract

This paper represents an update of our 1978 S.E.P.M. Mesozoic Paleogeography synthesis for the southern Sierra Nevada. We originally postulated that much of the southern Sierra Nevada pre-batholithic 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 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. We present stratigraphic relations for the relatively well-preserved lower Mesozoic stratified rocks of the southern Sierra which in general supports our 1978 synthesis. As pointed out by more recent syntheses, however, we now recognize the likelihood of Paleozoic basement rocks occurring in some or many of the Kings sequence pendants. Such rocks are disparate fragments of a highly dismembered polygenetic basement composed of Paleozoic ophiolitic, 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 appears to 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 of the southwest Cordillera and Foothills 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 to intra-arc extensional basin system with the deposition of Kings sequence turbidites and olistostromes. The basin system was destroyed by Middle and Late Jurassic thrusting. The assertion that much of the Kings sequence is Paleozoic in age is based on the discovery of probable Eocambrian-Cambrian miogeoclinal strata in the Snow Lake pendant of the east-central Sierra Nevada (Lahren and others, 1991). These authors 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. We propose a more conservative offset history for the Snow Lake pendant rocks which considers a broader uncertainty in the bounds of the possible source area for the rocks, and satisfies offsets of both batholithic petrochemical patterns and igneous-metamorphic assemblages of the Sierran batholithic complex.

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