The western extent of the Sierra Nevada batholith in the Great Valley basement and its significance in underlying mantle dynamics

An accurate understanding of the extent to which the Sierra Nevada batholith (SNB) lies beneath the Great Valley (GV) is essential in properly constraining dynamic models for underlying mantle lithosphere removal. Example: the southern Sierra Nevada mantle drip is commonly misrepresented as being strongly offset in map view to the west of the SNB, beneath the GV "forearc". A synthesis of petrographic data on over 200 GV basement cores, complimented by select single crystal U/Pb zircon ages and Nd-Sr isotopic data, shows clearly that the SNB extends westwards to at least the axis of the GV, beneath Upper Cretaceous forearc basin strata. This westernmost zone of the SNB yields zircon ages of 130 to 140 Ma, has depleted mantle Nd-Sr isotopic signatures, and consists of abundant hornblende rich mafic cumulates, as well as diorites and tonalites. Its metamorphic host rocks appear typical of the western Sierra Nevada Foothills, particularly the Middle Jurassic Smartville intra-arc igneous rift complex, Jurassic epiclastic-volcaniclastic slates and schists, and Late Jurassic deformed "Nevadan" plutons. Lower Cretaceous strata of the western GV lying depositionally above the Coast Range ophiolite (CRO) to the west are the remnants of the forearc for this westernmost zone of the SNB. This Early Cretaceous mafic to intermediate composition batholithic belt was exhumed to modest crustal depths in the mid-Cretaceous and then buried nonconformably by Upper Cretaceous forearc basin strata sourced from the felsic axial to eastern SNB. High density and strongly magnetic SNB gabbroids beneath the GV yield regional gravity-magnetic anomalies with amplitudes as high as 50 mgal and 1000 gamma. These coupled anomalies have been mistaken by numerous investigators as the mark of the eastern edge of the CRO having been "obducted" eastwards over "Sierran basement". The complete absence of depleted mantle peridotite core samples from the GV alone argues strongly against such a regional CRO obduction geometry, and in the light of mafic SNB rock types dominating core samples taken along the anomalies the CRO obduction model is rendered obsolete. An extensive westernmost mafic zone of the SNB beneath the GV has important implications for the dynamics of underlying mantle lithosphere removal. At total batholith scale, such a west to east primary compositional zonation pattern would impose a strong primary horizontal density gradient that would favor nucleation of convective instability of the lower crust and upper mantle along the west side of the SNB, as observed. Compounded on this effect is that western zone SNB mafic cumulates, as exposed at 1 GPa levels in the southwesternmost SNB, show preferential copious garnet production during cooling through solidus to hot sub- solidus conditions. Hornblende breakdown is the key to the observed reaction, and solidus to sub-solidus garnet modes tend to mimic protolith hornblende modes. This reaction, as observed in the southwesternmost SNB, could represent a window into incipient eclogitization of lower SNB crust preferentially developed in hornblende rich cumulates. Such a primary compositional control on lower crustal eclogitization would amplify the regional transverse horizontal density gradient across the SNB, and further assist in the preferential nucleation of convective instability to the west. Curiously, the southern Sierra Nevada mantle drip is centered over the largest known concentration of hornblende rich SNB mafic and ultramafic cumulates.