Paper No. 1
Presentation Time: 1:40 PM
TECTONIC AND MAGMATIC TEMPOS IN THE MESOZOIC CENTRAL SIERRA NEVADA ARC (Invited Presentation)
The evolution of continental margin orogens and magmatic arcs involve non–steady-state processes of subduction, orogeny, magmatism, erosion and exhumation. Recent studies have begun to examine the tempo of these processes and particularly the cyclic volumetric addition rates to arcs of plutonic and volcanic materials both at the scale of arcs and single magmatic systems (e.g., DeCelles et al. 2009). A full evaluation of these arc tempos requires the development and syntheses of large databases with high-precision temporal control. One of the legacies of Jason Saleeby’s studies in the Sierra Nevada is his remarkable ability to collect and synthesize a wide range of different datasets. In this spirit I present new datasets collected by my research group and colleagues in the central Sierra Nevada including 1:10,000 scale mapping, structural syntheses, CA-ID-TIMS and LA-ICPMS zircon geochronology of plutonic, volcanic, and sedimentary units (>6000 detrital zircon ages), >600 strain measurements, and an integration of new and published geochemistry including Sri, ЄNd, δ18O, and Pb isotopic studies of plutonic, metavolcanic, and metasedimentary units. We integrate these data sets in an attempt to develop a more robust understanding of the links between the magmatic and tectonic evolution of this segment of the arc. Our main conclusions are that (1) both the plutonic and volcanic components of this ~250 to 85 Ma arc are spatially, temporally, and geochemically linked and thus parts of the same vertically connected magmatic systems; (2) magmas in this arc are derived from mixing of mantle plus crustal melts and show clear evidence of fractionation and remixing during ascent; (3) that the fluctuations in arc magmatism (i.e., lulls and flare-ups and changing chemistries) are not directly related to plate motions or overall tectonic style, but may be loosely linked to the rates and magnitudes of the tectonic deformation that resulted in the >100% vertical thickening and >50% horizontal shortening of the arc; and (4) that both these tectonic and magmatic processes require the growth of large crustal roots beneath the arc consisting of downward displaced host rocks and large magmatic cumulate piles, which in turn led to modification of the underlying mantle lithosphere and temporal evolution of the isotopic signals in the arc.