Long-standing debates exist over the timing and mechanism of uplift of the Tibetan Plateau and, more specifically, over the connection between lithospheric evolution and surface expressions of plateau uplift and volcanism. Here we show a T-shaped high wave speed structure in our new tomographic model beneath South-Central Tibet, interpreted as an upper-mantle remnant from earlier lithospheric foundering. Its spatial correlation with ultrapotassic and adakitic magmatism supports the hypothesis of convective removal of thickened Tibetan lithosphere causing major uplift of Southern Tibet during the Oligocene. Lithospheric foundering induces an asthenospheric drag force, which drives continued underthrusting of the Indian continental lithosphere and shortening and thickening of the Northern Tibetan lithosphere. Surface uplift of Northern Tibet is subject to more recent asthenospheric upwelling and thermal erosion of thickened lithosphere, which is spatially consistent with recent potassic volcanism and an imaged narrow low wave speed zone in the uppermost mantle.

Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures and vent communities occur in different tectonic settings. The discovery of the Lost City hydrothermal field in the Mid-Atlantic Ridge has stimulated interest in the role of serpentinization of peridotite in generating H 2 -and CH 4 -rich fluids and associated carbonate chimneys, as well as in the biological communities supported in highly reduced, alkaline environments. Abundant vesicomyid clam communities associated with a serpentinite-hosted hydrothermal vent system in the southern Mariana forearc were discovered during a DSV Shinkai 6500 dive in September 2010. We named this system the "Shinkai Seep Field (SSF)." The SSF appears to be a serpentinitehosted ecosystem within a forearc (convergent margin) setting that is supported by fault-controlled fluid pathways connected to the decollement of the subducting slab. The discovery of the SSF supports the prediction that serpentinite-hosted vents may be widespread on the ocean floor. The discovery further indicates that these serpentinite-hosted low-temperature fluid vents can sustain high-biomass communities and has implications for the chemical budget of the oceans and the distribution of abyssal chemosynthetic life.Challenger Deep | convergent margin | hydrothermal vent | Shinkai Seep Field | vesicomyid clam H ydrothermal activity plays an important role in Earth evolution by modifying the composition of oceanic crust, affecting ocean chemistry, forming metal-rich deposits, and providing energy and nutrient sources for chemosynthetic biological communities. Several varieties of seafloor hydrothermal vents with widely varying fluid compositions and temperatures occur in different tectonic settings. Along divergent plate margins, three basic vent types have been identified. The first type is a basalt-hosted, hightemperature hydrothermal system with fluid temperatures up to approximately 400°C and low H 2 and CH 4 concentrations, but high metal concentrations (e.g., TAG hydrothermal field, 26°10′

[1] Subduction zone magmas are produced by melting depleted mantle metasomatized by fluids released from the subducted slab. In most subduction zones, formation of backarc basin (BAB) and arc magmas depletes the mantle source toward the trench, resulting in more depleted mantle beneath the forearc. Slabderived fluids are aqueous beneath the forearc where the slab dehydrates, and the deeper subduction component is increasingly dominated by sediment melt at !100 km depth. In this study, we present new data for the Southeast Mariana forearc rift (SEMFR), an unusual region of forearc igneous activity, where 2.7-3.7 Ma lavas were recovered by Shinkai 6500 diving and dredged during the TN273 Thomas Thompson cruise. SEMFR is divided into SE (near the trench) and NW (near the arc) sectors. NW SEMFR lavas and glassy rinds are more depleted in melt-mobile elements (e.g., Nb and Yb) and more enriched in fluid-mobile elements (e.g., Cs, Rb, and Ba). SEMFR lavas were produced by partial melting of a BAB-like mantle source, metasomatized by sediment melt and aqueous fluids released from dehydrating the subducted oceanic crust, and the forearc serpentinized peridotites. Evidence of sediment melt, even in SE SEMFR lavas, could be explained by inheritance of BAB-like Th/Nb in the SEMFR mantle source. Geochemical mapping demonstrates that the subduction components and mantle depletion increase towards the arc, suggesting (i) input of a less-depleted mantle beneath SE SEMFR that flowed toward the arc and (ii) aqueous slab-derived fluids become increasingly important at $50-100 km depth, reflecting that phengite and barite from the downgoing plate and forearc serpentinite broke down beneath the arc volcanoes.This article was corrected on 25 MAY 2015. See the end of the text for full details.

26The southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough 27 backarc basin in late Neogene time, erupting basalts now exposed in the SE Mariana Forearc Rift 28 (SEMFR) 3.7 -2.7 Ma ago. Today, SEMFR is a broad zone of extension that formed on 29 hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth ≤ 30 -50 km). 30It comprises NW-SE trending subparallel deeps, 3 -16 km wide, that can be traced ≥ ~ 30 km 31 from the trench almost to the backarc spreading center, the Malaguana-Gadao Ridge (MGR). 32While forearcs are usually underlain by serpentinized harzburgites too cold to melt, SEMFR crust 33 is mostly composed of Pliocene, low-K basaltic to basaltic andesite lavas that are compositionally 34 similar to arc lavas and backarc basin (BAB) lavas, and thus defines a forearc region that recently 35 witnessed abundant igneous activity in the form of seafloor spreading. SEMFR igneous rocks 36 have low Na 8 , Ti 8 , and Fe 8 , consistent with extensive melting, at ~ 23 ± 6.6 km depth and 1239 ± 37

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