Untitled

Abstract

Volcanic rock and mantle xenolith compositions in the Sierra Nevada (western United States) contradict a commonly held view that continental crust directly overlies asthenosphere beneath the Sierran range front, and that ancient continental mantle lithosphere (CML) was entirely removed in the Pliocene. Instead, space-time trends show that the Walker Lane is the principle region of mantle upwelling and lithosphere removal in eastern California, that lithosphere loss follows the migration of the Mendo cino Triple Junction (MTJ), and that the processes of lithosphere removal are not yet complete beneath the Sierra Nevada and its range front. Key evidence is provided by volcanic rock compositions. The 87 Sr/ 86 Sr ratios for mafi c volcanics of the Sierra (MgO > 8%) are mostly > 0.705, and 143 Nd/ 144 Nd < 0.5127, much unlike eastern sub-Pacifi c asthenosphere (where 87 Sr/ 86 Sr < 0.7027 and 143 Nd/ 144 Nd > 0.5129), but very much like CML. Similarly, Sierran volcanics carry CML-like trace element ratios, with La/Nb > 3 and Th/Nb > 1, values that are signifi cantly higher than asthenosphere-derived melts (La/Nb < 1.5, Th/Nb < 0.08). Spinel-bearing mantle xenoliths contained in Pleistocene-Holocene volcanics from the Sierran range front also have a CML composition, with 87 Sr/ 86 Sr and ε Nd ratios that range to 0.7065 and-3.6, respectively. New estimates of melt extraction depths using Si activity and mineralogy-sensitive trace element ratios (Sm/Yb, Lu/Hf) show that CML extends from the base of the crust (40 km) to depths of 75 km beneath the range front, and to 110 km for Pliocene volcanics of the southern Sierra. This means that garnet-bearing lithologies could not have been dislodged from beneath the southern Sierra until after the Pliocene. Only in the Walker Lane do young (0.18 Ma) volcanic rocks, from the Coso volcanic fi eld, approach asthenosphere-like compositions, which occurs only 20 Ma after MTJ arrival. Temporal trends show that MTJ arrival at any given latitude south of 37°N signals lithosphere heating, probably due to asthenosphere that upwells to replace the sinking Farallon plate. Partial melts of the asthenosphere, and perhaps the asthenosphere itself, intrude into and cause heating and partial melting of overlying CML; this culminates after 10 Ma. After 20 Ma, CML becomes highly degraded and asthenosphere-derived melts are dominant. North of 37°N, volcanic rocks approach asthenosphere-derived compositions to the west, not the east, and 87 Sr/ 86 Sr ratios increase from 18 to 0 Ma, indicating that this region has entered a phase of lithosphere heating, but not yet a phase of lithosphere removal. We propose a new model of lithosphere degradation, where asthenosphere or its partial melts pervasively invade CML beneath the Walker Lane. This process is now nearly complete beneath Coso, and is migrating west, so that it is only partly complete at the southern Sierra range front, or within the Sierra Nevada, at any latitude. This model of intermixed asthenosphere and lithosphere better explains the comp...

Epeirogenic transients related to mantle lithosphere removal in the southern Sierra Nevada region, California, part I: Implications of thermomechanical modeling

“…Each age group shows evidence for hybridization of mantle lithosphere and depleted mantle source components, with local increase of depleted mantle components eastward with time (Van Kooten, 1981;Ducea and Saleeby, 1998a;Farmer et al, 2002;DePaolo and Daley, 2000;Blondes et al, 2008;Putirka et al, 2012). The depth of melt extraction decreases with time from ~100-125 km to ~40-75 km, and fi nally into the lower felsic crust for silicic members (Van Kooten, 1980;Ducea andSaleeby, 1996, 1998a;Putirka et al, 2012). Figure 1 shows the areas over which the ca.…”

“…3A, 3B), and geochemical data indicating that basaltic rocks erupted during and following lithosphere removal contain substantial continental lithosphere components (cf. Putirka et al, 2012). Our preferred model predicts complex regional interlayering of ascended asthenosphere, mobilized mantle lithosphere, delaminated root, and entrained lower felsic crust, as well as compressed isotherms that migrate across the layering as it develops.…”

Basin and Range volcanism as a passive response to extensional tectonics

“…In Putirka et al (2012), it was shown that K 2 O contents are a key discriminator between Cascade and Basin and Range magmas; the latter group tends to have higher K 2 O contents at any given SiO 2 content. Furthermore, in Putirka and Busby (2007) it was suggested that high K 2 O contents may signal high crustal extension rates.…”

“…Whether mantle processes play an active role in promoting volcanism (e.g., a mantle plume; Wang et al, 2002) or magmatism is a passive response to tectonism and crustal extension (McQuarrie and Oskin, 2010) is a fundamental issue of Cordilleran evolution. To test whether plate tectonic actions control volcanism, we add a geochemical perspective to structural and palinspastic reconstructions of Snow and Wernicke (2000), McQuarrie and Wernicke (2005), and McQuarrie and Oskin (2010), and test the models in Putirka et al (2012) using a broader context. We do this by comparing temporal distributions and chemical compositions of volcanic rocks to time-space patterns of extensional deformation (Snow and Wernicke, 2000; McQuarrie and Wernicke, 2005) and an evolving plate boundary (Atwater and Stock, 1998).…”

“…Furthermore, in Putirka and Busby (2007) it was suggested that high K 2 O contents may signal high crustal extension rates. To test these ideas, we applied the equation (Putirka et al, 2012) that defi nes the boundary between Cascade and Basin and Range compositions, the K-index (Fig. 3):…”