Progress in Tectonic and Petrogenetic Studies in an Exposed Cross-Section of Young (~100 Ma) Continental Crust, Southern Sierra Nevada, California

Abstract

The southern Sierra Nevada offers an oblique section through young Cordilleran-type batholith generated crust spanning surface (volcanic) to deep (granulitic) levels. Regional mapping and Pb/U zircon geochronology reveal structural continuity through this crustal section for volcanic, plutonic and metamorphic assemblages developed at ~100 Ma, making it one of the youngest sections in the world. Construction of a synthetic cross-section is well-constrained by the oblique section map pattern, abundant age data, and a published crustal structure section that was based on geophysical and lower crustal xenolith data from across the shallow levels of the batholith. The synthetic cross-section depicts the state of the sialic crustal section during its ~100 Ma petrogenesis. Critical features of the section are as follows: (1) Much of the pre-Cretaceous crust was completely reconstituted by batholith generation; (2) Major influxes of mantle-derived tonalitic to gabbroic magmas drove crustal-level melting and magma mixing; (3) Pre-existing sialic components within the batholithic magmas were contributed primarily from partial to complete melts of craton derived metasedimentary material; (4) Silicic magmas emplaced at shallow crustal levels represent moderately to well-mixed and fractionated systems, while those frozen at deeper levels appear to be more heterogeneous both lithologically and geochemically; (5) Granulite facies metamorphic assemblages in the deep crustal rocks developed primarily in a retrograde regime descending from gabbro and tonalite solidus conditions; (6) Upward rise of silicic magmas was accompanied by downward return flow of metamorphic host rocks, and locally ignimbrite sections of only slightly older age than enclosing plutons were transported to considerable depth as well; (7) Much of the upper crust responded to the intrusion of silicic magmas by large-scale extension which also promoted major ignimbrite eruptions; and (8) Within the deeper levels of the composite batholith and off its flanks, at moderate to shallow crustal levels, low-angle detachments may have developed as a primary structural feature. Each of these points as well as a number of other interesting problems form the basis for a broad spectrum of current and future research efforts.