Chilled margins were recovered from the sheeted dike complex (SDC) of superfast (>200 mm/y)-spreading East Pacific Rise-spread crust during drilling of Integrated Ocean Drilling Program Hole 1256D on the Cocos Plate.The chilled margins contain stretched spherules, oriented plagioclase laths, grain-size segregation, and color banding.These rheomorphs locally crosscut veins but are elsewhere crosscut by veins.Electron microprobe investigations found that the chilled margins comprise dispersed micrometerscale minerals and veins including chlorite, actinolite, quartz, anhydrite, sphene, calcite, sphalerite, K-feldspar (adularia and/or orthoclase), magnetite, pyrite, and chalcopyrite.Though many of these phases are present throughout the SDC, anhydrite and calcite have not been previously recognized >100 m below the SDClava transition zone, and, with one exception, K-feldspar has not been previously identified in Hole 1256D core.Microstructures include quartz clasts surrounded by anhydrite, K-feldspar veins and clasts cut or surrounded by chilled margin material, and lenses of ductily deformed sphene.Some of the crosscutting relationships and distribution of mineral phases could be explained by hydrothermal alteration that occurred roughly simultaneously with dike intrusion.

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Integrated ocean drilling program expedition 309 preliminary report: Superfast Spreading Rate Crust 2 A complete in situ section of upper oceanic crust formed at a superfast spreading rate / JC Alt, DAH Teagle, S. Umino, S. Miyashita, NR Banerjee, DS Wilson, F. Einaudi, A. Belghoul, C. Cordier, L. Crispini, L. Galli, Y. Gao, J. Geldmacher, LA Gilbert, E. Herreo-Bervera, SA Holter, C. Laverne, HL Lledo Vasques, S. Rodriguez Durand, T. Sakuyama, T. Sano, CE Smith-Duque, S. Tominaga, P. Tartarotti, EA Veloso Espinosa, M. Reichow, R. Anma, J. Carlut, DM Christie, R. Coggon, NW Hayman, N. Hirano, S. Ingle, J. Koepke, J. MacLennan, S. Morgan, N. Neo, SH Park, B. Scheibner, SA Swift, AA Tikku, T. Yamazaki, S. Yamazaki. - College Station, TX … Integrated ocean drilling program expedition 309 preliminary report: Superfast Spreading Rate Crust 2 A complete in situ section of …

Oligocene to Mid-Miocene Somuncura basaltic plateau widely covers an area of over 20,000 km2, northern Patagonia, which is one of the largest Cenozoic basaltic field in extra-back arc province (e.g., Ardolino et al., 1999). Previous studies suggested three contrasting models for the magamatism in extra-back arc region from 34 to 52 S, so-called extra-Andean domain (Ramos et al., 1982); 1) upwelling of either small-scale hot spot (Kay et al., 1992; 1993), 2) asthenospheric upwelling resulted by the slab roll-back of the Farallon plate (Ignacio et al., 2001) or by opening of slab window beneath the region, that followed ridge subduction (e.g., Ramos and Kay, 1992; Gorring et al., 1997) and 3) partial melting of continental lithosphere (and/or asthenosphere) due to thermal and mechanical events of the mantle related to subducted slab beneath the western margin of the continent (Stern et al., 1990). With much discordant models, however, there is still repetition of controversy and further discussion supported by concrete evidence is required to specify magmatism of the Patagonian basalts. In this study, we focused on the Somuncura province, and preliminarily determined both major & trace element compositions and K-Ar

In the high-grade Cretaceous Sanbagawa high-pressure (HP) metamorphic belt, our new 1:5000 scale mapping of eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite-facies schists has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boudnary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is subparallel to a lithological boundary. Protoliths of the upper unit are gabbro, basalt, minor quartz rock, and pelite, and those of the lower unit are pyroxentie, gabbro, basalt, chert, and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient toward the Higashi-Akaishi body. Eclogite-facies metapsammite is intercalated between the Iratsu and Highashi-Akaishi bodies. Our mapping has revealed the following: (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin; (2) reconstructed ocenaic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert, and pelite, suggesting that different part of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite; and (3) a change in the convergent motion of the oceanic plate from NW to Ne during the accretion of the large oceanic island or plateau.

The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid-oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Twenty-nine K-Ar ages for lavas and juvenile ejecta obtained from Hudson volcano in the southern end of the Southern Volcanic Zone and Lautaro volcano in the northern end of the Austral Volcanic Zone, which are separated by a 350 km-long volcanic gap near the Chile ridge subduction zone, were determined using unspiked method that has significant sensitivity for dating young rocks (<0.1 Ma). It is newly revealed that Hudson is a significantly long-lived volcano; its activity started at ca. 1.0 Ma and continues to the Recent. The Hudson volcano has a well-preserved summit caldera complex of approximately 10 km in diameter, previously thought to be formed by a single event during the Holocene, perhaps at 6700 years BP. Our results for the K-Ar dating, however, indicate that the northeastern and southeastern flanks of the volcano formed at different times; formation of NE flank preceded that of SE flank. Aero-photographic observations indicate the presence of two or even three caldera. rims. These data suggest that the Hudson volcano had a complex evolution, superimposing or partially nesting calderas rather than a simple caldera. The activity of the Lautaro volcano, began at ca. 0.17 Ma and has continued to the Recent, as it is indicated by our K-Ar first results. Though Lautaro volcano is a relatively large stratovolcano for Chilean Patagonia, the chemical and radiometric results indicate a narrow range in its variability when compared with those of the Hudson volcano. These narrow compositional and geochronological ranges suggest that the Lautaro volcano developed from a homogeneous magma chamber produced by slab melting during the late Quaternary, assuming that the sampled part of this heavily ice-mantled volcano, spans its full lifetime.

Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure-high-pressure (UHP-HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP-HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.

The central part of the Kokchetav Massif is exposed in the Chaglinka-Kulet area, northern Kazakhstan. The ultrahigh-pressure-high-pressure (UHP-HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I-IV) metamorphosed under different pressure-temperature (P-T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP-HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW-SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II.

The Kokchetav Massif of Kazakhstan includes high to ultrahigh-pressure (HP-UHP) metamorphic rocks (some of which were recrystallized at depths in excess of 150 km), juxtaposed against much lower pressure metamorphic components. We investigated the relationship between the HP-UHP metamorphic unit and the low pressure (LP) unit (Daulet Suite) in the Sulu-Tjube area, where the metamorphic rocks have previously been interpreted as constituting a megamelange with subvertical structural attitudes. Analyses of fold structures suggest that the HP-UHP metamorphic unit overlies the LP unit across a west-dipping subhorizontal boundary. In addition, kinematic indicators display top-to-the-north senses of shear along the tectonic contact between the two units, indicating that the HP-UHP unit has been extruded northward onto the LP unit. Following the juxtaposition of the two units, upright folds developed in both units, and these are associated with the previously reported steeply dipping metamorphic foliations. These data have important implications for the mode of exhumation of the UHP rocks from upper mantle to shallow crustal depths.

Field data suggest that the epizonal Yakushima granite rose obliquely into the Ryukyu are of Japan (Anma, this volume). Analogue models test the hypothesis that the oblique rise of this buoyant diapir was due to drag by the stiff oceanic plate sinking obliquely beneath it. A simplified model of a solid plate sinking obliquely along its length into a ductile medium induces wedge flow in the overlying asthenosphere. Buoyant inclusions in the model lithospheric/asthenospheric wedge trace possible tracks of plutons above subduction zones. The inclusions rise obliquely toward the trench axis in the nearby wedge where rotational shear is greatest, leaving an inclined diapir tail. By contrast, near the surface away from the trench axis, where are-normal horizontal extension prevails, buoyant inclusions become tabular in shape and are carried toward the trench. Overall, the observations account for the geometries of the Yakushima and other Miocene granites in Southwest Japan.

Field observations and structural analyses of the orientations of ductile flow fabrics and brittle fractures are used to infer diapiric rise of the Yakushima pluton into epizonal Paleogene sediments of an accretionary prism in Miocene times. Inside the pluton, preferred orientations of orthoclase megacrysts define a weak primary flow fabric. Shape and intensity of fabric ellipsoids derived from patterns in the alignment of the megacrysts reveal a circulation cell within the domal structure of the Yakushima granite. The asymmetry of this circulation pattern suggests that the pluton rose obliquely upward to the southeast, toward the Ryukyu trench. Fold axes in the ductile strain aureole of the pluton are distorted into conformity with the pluton's shape in the direction of magma ascent. Late aplite sheets, whose orientations record the local paleostress pattern, were emplaced into a concentric fracture system in and around the pluton. The aplites indicate the lifting, and tangential spreading with concentrically disposed intermediate stress axes, of a brittle roof above a buoyant ellipsoidal body of residual magma. Comparable with strain patterns in theoretical models of a viscous sphere rising in ductile surroundings, the deformation pattern in and around the pluton is attributed to the final upsurge of oblique diapiric emplacement.