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Two fundamental questions of interest with regard to the Andean Plateau are the mass balance of material needed to create and sustain a 3-4-km-high plateau and the relationship between the plateau and the bending of the Bolivian orocline. The link between these two questions is the distribution of shortening through the Central Andes. Is crustal shortening sufficient to support an isostatically compensated 60-70-km-thick crust? Is differential shortening between the hinge of the orocline and its limbs sufficient to account for both physiographic curvature and measured rotations? Three-dimensional (3-D) models of deformation within curved orogens require, in addition to paleomagnetic data, viable two-dimensional estimates of displacements. To this end, we present new map data and use those data to derive estimates of shortening across the northern margin of the Andean Plateau. The cross-section extent, from the eastern edge of the volcanic arc to the foreland basin, is approximately one half of the physiographic width of the Andean Plateau in Peru. Cross-sectional shortening estimates in southern Peru (12-14[degrees]S) provide a preferred estimate of 123 km or 40% shortening, with a minimum estimate of 58 km or 25% shortening and maximum estimate of 333 km or 65% shortening. The largest controlling factor on the magnitude of shortening is where basement becomes involved in thrust belt deformation. Using our preferred shortening estimate, the differential shortening between our cross section on the northern margin of the Bolivian orocline (12-14[degrees]S) and cross sections across the axis of the Bolivian orocline (17-18[degrees]S) is 177 km, corresponding to 23[degrees] of rotation. The differential shortening for the young (younger than 15 Ma) deformation of the Altiplano and Subandean region between 12-14[degrees]S and 17-18[degrees]S is 88 km, correlating to 11.5[degrees] of rotation. While differential shortening can account for young (younger than 15 Ma) vertical-axis rotations, it cannot completely account for early 45-35 Ma rotations (37[degrees] [+ or -] 15[degrees]). This suggests that forearc rotations reflect both bending due to differential shortening, ~12[degrees], as well as block rotations (~25[degrees]-30[degrees]) which facilitated deformation in a tightening core and transported material toward the center of the orocline. The preferred estimate of shortening is well short of the required 240-300 km of shortening needed to account for a 60-70-km-thick crust under the entire plateau. This suggests that for an isostatically equilibrated crust, either (1) there is a significant amount of shortening (~100-150 km) in the western half of the plateau that is hidden by the volcanic arc; or (2) crustal material is being added to the Peruvian section of the Andean Plateau, most likely through lower-crustal flow. 10.1130/B26477.1

Using global correlations of effusive eruption style with convergence rates and surface heat flux at volcanic arcs, this study identifies several arcs with excess surface heat flux, related to the presence of unusually hot and [H.sub.2]O-poor magmas. Geophysical evidence suggests that these melts are linked to discontinuities in the subducting slab, which would facilitate upwelling of hot sub-slab asthenospheric mantle. It is demonstrated that excess heat flux arcs are sites where Sr-rich adakitic compositions ate generated, and that the arc proportion erupting Sr-rich adakitic magmas is directly proportional to the degree of observed heat flux excess. A model is presented in which the formation of Sr-rich adakitic compositions, which require garnet as a residual phase, is facilitated at excess heat flux arcs as a result of a decrease in [H.sub.2]O content through contributions from sub-slab asthenospheric mantle, leading to an expansion of the garnet stability field. Future geochemical, geophysical and experimental work required to test this model is addressed.

The Tiger Gabbro layered intrusion is one of the few mafic intrusive bodies found along the ancient Antarctic Gondwana margin. Major and trace element data and Sr--Nd isotope compositions for gabbronorites indicate an island arc signature for the Tiger Gabbro parental magma. This is the first evidence for an islam arc from plutonic rocks in northern Victoria Land. The interpretation of the Tiger Gabbro as the roots of an Early Cambrian islam arc (535 [+ or -] 21 Ma, Sm--Nd age), integrated with geochemical and geochronological data from the literature, matches the occurrence of the Glasgow volcanic rocks in the southern Bowers terrane, which possibly represents its effusive counterpart. A scenario for the early Palaeozoic Antarctic Gondwana margin is hence proposed in which the Tiger volcanic arc developed on the Robertson Bay microplate in response to subduction of the palaeo-Pacific plate. The Tiger arc igneous activity was coeval to the Wilson continental arc (represented by the Granite Harbour intrusive rocks), with the two subduction zones merging southwards into one. The migration of the Wilson arc towards the forearc region in turn generated the Middle Cambrian Bowers arc. Supplementary material: Major element, trace element, mineral and [sup.40]Ar--[sup.39]Ar data are available at http:// www.geolsoc.org.uk/SUP 18351.

Explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans. The link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence. A compilation of 2814 K-Ar and [sup.40]Ar/[sup.39]Ar age determinations from four circum-Pacific arcs indicates that periods of high volcanic output (i.e., flare-ups) have been episodic and, in some cases, synchronous. Peak periods and subsequent lulls in arc magmatism over the past 50 m.y. have occurred coeval with major fluctuations in global climate, including the Eocene-Oligocene transition, one of the most prominent global climate reorganizations in the Cenozoic. Hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum-Pacific volcanism beginning in the late Eocene, which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the Pacific Ocean. We provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted Antarctic ice sheet growth ~34 m.y. ago.

The Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet-clinopyroxene-rutile residues. Garnet growth and melting began around 850[degrees]C at 10 kbar and the tonalitic melts were most probably generated around 1050[degrees]C at P [greater than or equal to] 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P-T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust. Keywords: Gourma, dehydration-melting, island arc, Neoproterozoic, granulite.

The Penon Blanco arc of the Jurassic-Triassic arc belt in central California is composed of the Jasper Point Formation, Penon Blanco Formation, and coeval Don Pedro intrusive suite, all exposed in the core of the Cotton Creek anticline. The Jasper Point Formation consists of ~900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts. It passes upward into the Penon Blanco Formation, which is made up of ~700 m of crystal-lithic basaltic tuff, 1-3.5 km of augite-rich volcaniclastic rocks, and up to 3.5 km of massive to brecciated flows of augite-phyric basalt. The Penon Blanco Formation is paraconformably overlain by the Oxfordian-Kimmeridgian Mariposa Formation, which provides a minimum age of juxtaposition for the Penon Blanco arc against the inboard Calaveras Complex. New geochemical data from the Penon Blanco arc show that the two volcanic suites are geochemically distinct. Jasper Point basalts are tholeiitic and are characterized by high large ion lithophile element (LILE) abundances, moderate high field strength element (HFSE) and heavy rare earth element (HREE) abundances, and low Ti/V ratios. Penon Blanco basalts are calc-alkaline, have higher LILE abundances, lower HFSE and HREE abundances, and lower Ti/V ratios. Geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel-bearing mantle source at 30-60 km depth by low to moderate amounts of partial melting (~3%-5% for Jasper Point and 5%-7.5% for Penon Blanco). The geochemical modeling and field data suggest that the Jasper Point basalts are similar to normal mid-ocean-ridge basalt (N-MORB) and were associated with forearc rifting, while the Penon Blanco basalts represent the transition to arc volcanism and head-on subduction. This model is consistent with interpretations for a single ensimatic arc on the Jurassic margin of North America. Keywords: California, volcanic arcs, Cordilleran margin, Jurassic, tectonics, geochemistry.

Recent work suggests that hydrated partially molten thermal-chemical plumes that originate from subducted slab as a consequence of Rayleigh-Taylor instability are responsible for the heterogeneous composition of the mantle wedge. We use a two-dimensional ultrahigh-resolution numerical simulation involving 10 x [10.sup.9] active markers to anticipate the detailed evolution of the internal structure of natural plumes beneath volcanic arcs in intraoceanic subduction settings. The plumes consist of partially molten hydrated peridotite, dry solid mantle, and subducted oceanic crust, which may compose as much as 12% of the plume. As plumes grow and mature these materials mix chaotically, resulting in attenuation and duplication of the original layering on scales of 1-1000 m. Comparison of numerical results with geological observations from the Horoman ultramafic complex in Japan suggests that mixing and differentiation processes related to development of partially molten plumes above slabs may be responsible for the strongly layered lithologically mixed (marble cake) structure of asthenospheric mantle wedges. Keywords: numerical modeling, subduction, volcanic arcs, mantle wedge structure, subducted crust melting.

East-central Alaska is a tectonically significant region between parautochthonous North America and the Arctic Alaska terrane to the north. The tectonic evolution of the area is recorded in Late Jurassic to early Tertiary sedimentary rocks of the Kandik River terrane. Fine-grained clastic rocks deposited in the area from Late Jurassic to Valanginian time represent a continuation of the passive margin sequence that persisted since the late Precambrian on the nearby North American margin. A pulse of quartz arenites (sandstones in the uppermost Glenn Shale and the Keenan Quartzite) reflects Valanginian rift-shoulder uplift, marking initiation of crustal upwarping and thinning that would later lead to the opening of the Canada basin. Fine-grained turbidites interpreted as distal submarine fan deposits (Biederman Argillite), Valanginian to possibly as young as Aptian, were next deposited along the passive margin. These quartz wackes contain framework grains with the same continental source as the quartz arenites, but geochemical evidence points to arc detritus in the fine-grained (pelagic) intervals. Compressional tectonics began in the Aptian, and are recorded by arc-derived volcaniclastic turbidites (Kathul Graywacke) that were quickly overthrust, buried to depths of 5-8 km, then uplifted and eroded in the Late Cretaceous. These events record the accretion of a previously unreported island arc to the Porcupine and Kandik River terranes, followed by accretion of this complex to North America. A second orogenic event followed in the latest Cretaceous to early Tertiary, when extension, possibly associated with emplacement of Yukon-Tanana terrane along the Tintina fault to the south, produced a half graben (Nation River basin) and a pull-apart basin that were filled with litharenites derived from local sources (unit TKs). These deposits were broadly uplifted and warped in the middle Tertiary, probably by regional Laramide-age events. These results are significant in that they provide evidence for a previously unreported Aptian-Albian volcanic arc in east-central Alaska. In addition, comparison of sedimentation events in east-central Alaska with those recorded in the North Slope sequence of northern Alaska provides constraints on tectonic models for the evolution of northern Alaska. Keywords: Alaska, geochemistry, paleogeography, paleotectonics, petrography, sandstone, volcanic arc.

Early Ordovician volcanic rocks exposed in the South Mayo region of western Ireland document the history of a volcanic arc complex, produced following the initiation of south-dipping subduction within the Iapetus Ocean in the Late Cambrian or Early Ordovician. Lavas of the Lough Nafooey Group (Tremadoc - Arenig) show an eruption history marked by an initial period of tholeiitic basaltic volcanism, followed by a shift to increasing silica compositions of calc-alkaline affinity. Subsequent volcanism of the Tourmakeady Group (Arenig) shows a mixture of rhyolitic and andesitic compositions, which are in turn succeeded by tuffs of the Rosroe and Mweelrea Formations of the Murrisk Group (Llanvirn). These are also rhyolitic and andesitic but are generally more siliceous. The overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening. Trace element studies of the basaltic members of the Lough Nafooey Group show a strong subduction zone influence. While absolute levels of immobile trace and rare earth elements decrease upsection, relative depletion of the most incompatible elements decreases over the same time period. These trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection. The basaltic Bohaun Volcanic Formation (Tremadoc - Arenig ?) shows greater depletion in its incompatible trace elements than the Lough Nafooey Group, so indicating a more depleted mantle source. Comparison with modern arc systems suggests that the Bohaun Volcanic Formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the Lough Nafooey Group (i.e. in the forearc). This confirms a south-dipping polarity to the subduction zone, and suggests an age of initiation during the Late Cambrian or earliest Ordovician.

Global systematics in the location of volcanic arcs above subduction zones (1,2) are widely considered to be a clue to the melting processes that occur at depth, and the locations [...]

Mantle-derived spinel harzburgites from the Kamchatka arc have fractionated Pd-group element patterns, and Pt is clearly enriched relative to Pd and Rh. This fractionation is also consistent with chondritic Ir-group-platinum-group element distribution and super-chondritic Pt/Pd, chondrite-normalized [(Pt/OS).sub.N] and [(Pt/Ir).sub.N] found in harzburgite xenoliths from the Tubaf seamount in the Lihir island group of the Tabar-Lihir-Tanga-Feni island arc in Papua New Guinea. The nonchondritic Pt/Pd ratios and Pt enrichment in the island-arc mantle are best explained by extraction of melt in a back-arc volcanic-arc setting, which left refractory Pt-Fe alloys in the residual mantle. Pt-Fe alloys selectively concentrate Pt relative to Pd and, overall, increase the Pt/Pd in a subarc mantle wedge. Partial melting of this Pt-enriched mantle wedge above the subduction zone, due to hydrous flux from the subducting lithosphere, is potentially capable of producing Pt-rich primitive melts parental to the Alaskan-type uitramafic-mafic complexes. Keywords: island-arc mantle, peridotite xenoliths, platinum group elements.

Global Positioning System geodesy and structural analysis of parts of the Aleutian island chain support the interpretation of forearc migration westward along right-lateral transcurrent faults. Measured displacements are essentially arc parallel in the eastern (Unalaska Island) and western (Attu Island) parts of the arc and vary from 3.1 mm/yr to 31.4 mm/yr, respectively. At the center of the arc (Adak) subduction of the Amlia fracture zone disrupts the pattern of arc-parallel displacement and records an upper plate velocity of 9.6 mm/yr parallel to plate motion. Where active, displacement partitioning accommodates 30%-50% of the strike-slip component of plate convergence. In the area of impeded subduction partitioning ceases, suggesting that the process is sensitive to the degree of coupling across the plate-boundary megathrust. The differential arc-parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc-parallel extension, which may play an important role in exhumation of high-pressure-low-temperature metamorphic rocks in ancient arc systems. Keywords: arc-parallel extension, displacement-field partitioning, GPS geodesy, structure.