Three-quarters of the ocean crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the seafloor.Despite the significance of these rocks, sampling them in situ is extremely challenging due to the overlying dikes and lavas. This means that our models for

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Highlights ► The youngest known (2 Ma) volcanically-active subduction system. ► Exceptionally diverse range of magma compositions coeval and spatially juxtaposed. ► Mixing of an upwelling asthenospheric mantle melt and a slab melt. ► Modern example of an immature subduction system building its proto forearc. ► Modern analog of the environment where SSZ ophiolites lithosphere forms.

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...