This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle ...xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.
Determination of paleocurrent directions is a robust approach to establish the mechanism of sediment transport and of deposition. However, in tectonically controlled (or affected) basins where ...rotations are suspected, it is of critical importance to correct previously calculated paleocurrent directions to establish the original (pre-tectonic) transport and deposition patterns. In order to illustrate this, we determined the orientations of the anisotropy of magnetic susceptibility (AMS) fabrics from 14 sites distributed along the sedimentary deposits interbedded within the Late Miocene Main Volcanic Unit (MVU) of the Taitao ophiolite, Southern Chile. Data were used to estimate characteristics (directions and relative velocities) of the sediment transporting paleocurrents and orientations of the depositional slope. Sedimentary deposits consist of a rhythmic sequence of conglomerate (base), sandstone and mudstone (top) beds that present a systematic decrease in grain size up-sequence as a whole, and within individual beds. Sedimentary deposits of the MVU, as well as overlaying effusive volcanic products, are disrupted by small and discrete dextral/normal brittle faults. The distribution of fault planes and their kinematic characteristics suggest the development of a dextral negative flower-like fault zone in the area.
Previously reported NW-ward paleocurrent directions obtained from the orientations of sedimentary structures roughly agree with our calculations based on the orientations of AMS fabrics. Calculated paleocurrent directions are sub-parallel to the NW-SE trend of Estero Cono channel, the main lineament in the area. The spatial distribution of AMS axes between and within sites suggest that coarse grain deposits were transported by relatively high velocity flows whereas fine grained deposits were by low to nearly null velocity flows. Previously reported paleomagnetic analyses indicate that counterclockwise rotations affected the whole Taitao ophiolite, including the MVU. By applying the necessary corrections we obtained paleocurrent directions yielding mostly NNW-ward directions. Restoration of the orientations of beddings indicates that the depositional surface dipped steeply at its easternmost edge and shallowly at the westernmost end. We propose here that sediments were transported by turbidite flows generated by slope instabilities in the innermost part of the Estero Cono. These turbidites followed the Estero Cono lineament while they filled the channel and transported sedimentary material (eroded from the upper portions of the ophiolite and nearby units) towards the Pacific Ocean. Sediments were deposited on a NW- to W-facing slope and sorted in decreasing grain size along the channel. The instability of the slope was most probably triggered by the activity of a transtensional fault zone developed along the mid-axis of the Estero Cono channel.
We describe in detail the deformation structures and textures of a large-scale landslide body that developed in the Betto-dani Valley in northern central Japan. We studied the shape-preferred ...orientation of clasts and clay flakes and the development of internal shear planes within the slip zone of the landslide. The slip has an average rate of 5-10 cm/year under the overburden pressure of approximately 1.6 MPa; these values are similar to those of the proto-decollement zone of the Nankai accretionary prism in SW Japan. The anisotropy of magnetic susceptibility of samples obtained from the slip zone reveals that the long axes of clay flakes define an imbricate structure. The slip was due to a long-term periodical creep, which occurs during the thaw seasons with an average slip rate of 0.16-0.32 μm/min. During the creep, the long axes of grains including clay flakes in the slip zone are developed from parallel to perpendicular to the slip direction. The observed textures provide a clue to elucidate the deformation textures and process in the decollement zone of the Nankai prism.PUBLICATION ABSTRACT
The Chile Triple Junction (CTJ), an RTT-type triple junction located at 46°13′ S, 75°48′ W off the western coast of Chile, is characterized by the subducting Chile Ridge, which is the constructive ...plate boundary that generates both the Nazca Plate and the Antarctic Plate. The ridge subduction mechanism and the regional tectonics around the CTJ were investigated primarily using marine geophysical data (topography, gravity, geomagnetic field and single-channel seismics) collected during the SORA2009 cruise (Cruise ID = MR08-06) by R/V MIRAI together with other cruise data from the National Geophysical Data Center. The segment of the ridge axis just before the subduction around the CTJ is associated with an axial deep covered with thick sediment unlike that seen in typical ridge crests. The profiles of both topography and the free air anomaly around the CTJ show quite different patterns from those of ordinary subduction zones. However, topographic features typical of a slow-spreading type ridge, including a median valley and both flanks, remain in the seaward side of the trench. Even after the subduction of the eastern flank, the topographic features of the western flank remain. A slight Outer Rise and an Outer Gravity High, which are common in the western Pacific area, were observed in an area far away from the CTJ on both Nazca and Antarctic plate sides. The geomagnetic anomaly pattern around the Chile Ridge near the CTJ shows that the estimated spreading rate decreases gradually towards the ridge crest. This suggests that volcanic activity diminishes gradually towards the subducting ridge axis. The lithosphere under the Chile Ridge might have amalgamated with the surrounding oceanic lithosphere due to heat loss after the cessation of volcanic activity. The oceanic lithosphere towards the trench also thickens rapidly due to heat loss. Consequently, shallow-angle subduction of the youngest and most immature oceanic plate occurs smoothly via slab-pull force without any resistance along the interface between the South American continental plates.
The repeated north and southward migration of the Chile Triple junction, offshore the Península de Taitao, is expected to have imposed contrasting stress fields in the forearc for the last 6 Ma ...because of changes in convergence direction and rate of subducting plates. NNW-SSE to E-W and minor NE-SW striking brittle faults developed in the plutonic units of the Mio-Pliocene Taitao Ophiolite, whereas NNE-SSW and minor NW-SE trending faults developed in its eastern border (Bahía Barrientes fault-zone). These brittle faults are studied to elucídate the style of ophiolite emplacement and the tectonic effects resulting from the alternated migration of the Chile Triple junction in the área. Analyses of heterogeneous fault-slip data on both áreas suggest that faults were activated by different stress fields. Two different compressional stress fields were identified in the plutomc units (A and B), whereas three different stress fields, ranging from compressional to strike-slip, were identified in the BahíaBarrientos fault-zone (C, D and E). Calculated directions of Oj axes for A, C, D and E solutions are mostly E-W trending, roughly similar to the convergence direction of subducting plates, whereas that for B solution is counterclockwise rotated ca. 60° with respect to the previous E-W trend. Brittle structures related to solution B were attributed to an early deformation of the ophiolite, most probably developed shortly after its emplacement {ca. 6 Ma). These structures were further counterclockwise rotated, while new structures (related to solution A) developed in the plutomc units in order to absorb the continuous deformation. In the eastern margin of the ophiolite, the stress field divided inte compressional and strike-slip components. During periods of relatively strong compression (fast subduction of the Nazca píate), the fault-zone experienced well defined compressional and strike-slip movements (solutions C and D). In contrast, during periods of relatively weak compression (slow subduction of the Antarctic píate), the fault-zone experienced a complex mixture of thrust and strike-slip movements (solution E). Thus, the wide range of calculated stress ratios for all solutions could be attributed to the alternated change in convergent velocity of the subducting plates beneath the Taitao área.Es esperable que la repetida migración norte-sur del Punto Triple de Chile cercana a la costa de la Península de Taitao haya impuesto campos de esfuerzos distintos en el antearco por los últimos 6 Ma debido a cambios en la dirección de convergencia y tasa de subducción. Fallas con rumbos NNO-SSE y E-O y otras menores con rumbos NE-SO se desarrollaron en las unidades plutónicas de la Oflolita Miocena-Pliocena de Taitao, mientras que fallas con rumbos NNE-SSO y otras menores con rumbos NO-SE se desarrollaron en el borde Este de esta (Zona de falla de Bahía Barrientes). Estas estructuras frágiles son estudiadas para dilucidar el estilo de emplazamiento de la oflolita y los efectos tectónicos resultantes de la migración alternada del Punto Triple de Chile en al área. Análisis de datos heterogéneos de fallas en ambas áreas sugieren que fueron activadas por campos de esfuerzos diferentes. Dos campos de stress compresionales distintos fueron identificados en las unidades plutónicas (A y B), mientras que 3 campos de stress diferentes, desde compresionales a transcurrentes, lo fueron en la zona de falla de Bahía Barrientes (C, D y E). Las direcciones de los ejes a1 calculadas para las soluciones A, C, D y E son principalmente este-oeste, similares a la dirección de convergencia de las placas subductantes, mientras que aquella para la solución B está rotada antihorario ca. 60° con respecto del patrón anterior este-oeste. Las estructuras frágiles relacionadas con la solución B fueron atribuidas a una deformación temprana de la ofiolita, muy probablemente desarrolladas después de su emplazamiento (ca. 6 Ma). Estas estructuras fueron posteriormente rotadas antihorario, mientras nuevas estructuras (relacionadas con la solución A) se desarrollaron en las unidades plutónicas para absorber la deformación continua. En el margen este de la ofiolita, el campo de esfuerzo se dividió en componentes compresionales y transcurrentes. Durante períodos de compresión relativamente alta (subducción rápida de la placa de Nazca) la zona de falla experimentó movimientos compresionales y transcurrentes bien definidos (soluciones C y D). En contraste, durante períodos de relativa baja compresión (lenta subducción de la placa Antartica) la zona de falla experimentó una compleja mezcla de movimientos inversos y transcurrentes (solución E). Así, el amplio rango de radios de stress calculados para todas las soluciones puede ser atribuido al cambio alternado en la velocidad de convergencia de las placas subductantes bajo el área de Taitao.
To investigate the melting history of the Patagonian Ice Sheet (PIS) during the last deglaciation, we reconstructed Al2O3 flux in sediments from the Magellanes fjord (53°S) and southern Patagonia ...(55°S: Drake Passage) over the past 12 and 31 kiloyears (kyr), respectively. We confirm that the melting history of the PIS is recorded as changes in sedimentological signals and find that the melting ceased between ~10 and 8 Calendar kyr before present (cal kyr BP). The timing of major melting events of the PIS was similar to that of the Northern Hemisphere ice sheets.
PDF
