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Lower Plate (lower + plate)
Selected AbstractsThe processes of underthrusting and underplating in the geologic record: structural diversity between the Franciscan Complex (California), the Kodiak Complex (Alaska) and the Internal Ligurian Units (Italy)GEOLOGICAL JOURNAL, Issue 2 2009F. Meneghini Abstract Existing studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid-related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska. Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment-starved oceanic sections results in diffuse deformation and mélange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak. The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during underthrusting and accretion. Transient hydrofracturing occurs through the development of a system of dilatant fractures grossly parallel to the décollement zone. Copyright © 2009 John Wiley & Sons, Ltd. [source] Debris flow and slide deposits at the top of the Internal Liguride ophiolitic sequence, Northern Apennines, Italy: A record of frontal tectonic erosion in a fossil accretionary wedgeISLAND ARC, Issue 1 2001Michele Marroni Abstract In the Northern Apennines, the Internal Liguride units are characterized by an ophiolite sequence that represents the stratigraphic base of a late Jurassic,early Paleocene sedimentary cover. The Bocco Shale represents the youngest deposit recognized in the sedimentary cover of the ophiolite and can be subdivided into two different groups of deep sea sediments. The first group is represented by slide, debris flow and high density turbidity current-derived deposits, whereas the second group consists of thin-bedded turbidites. Facies analysis and provenance studies indicate, for the former group, small and scarcely evoluted flows that rework an oceanic lithosphere and its sedimentary cover. We interpret the Bocco Shale as an ancient example of a deposit related to the frontal tectonic erosion of the accretionary wedge slope. The frontal tectonic erosion resulted in a large removal of materials, from the accretionary wedge front, that was reworked as debris flows and slide deposits sedimented on the lower plate above the trench deposits. The frontal tectonic erosion was probably connected with subduction of oceanic crust characterized by positive topographic relief. This interpretation can be also applied for the origin of analogous deposits of Western Alps and Corsica. [source] Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New ZealandJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2009A. H. ALLIBONE Abstract In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high- P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ,200,1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10,14 kbar. Similar P,T conditions are recorded in high- P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ,200,1000 m from the Malaspina Pluton retain medium -P mid-amphibolite facies assemblages, despite having been subjected to pressures of 10,14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional-scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3,4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10,14 kbar. Similar high- P metamorphic events may therefore be more common than presently recognized. [source] High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P,T paths and geotectonic significanceJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2002J. H. Guo Abstract High-pressure basic granulites are widely distributed as enclaves and sheet-like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P,T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low-Ca core of growth-zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750,870 °C and 11,14.5 kbar (M2) is defined by high-Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770,830 °C and 8.5,10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500,650 °C and 5.5,8 kbar (M4). These results help define a sequential P,T path containing prograde, near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P,T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high-pressures (6,14.5 kbar) of the four metamorphic stages and the geometry of the P,T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper-plate might be the tectonically overlying Khondalite series, which was subjected to medium- to low-pressure (MP/LP: 7,4 kbar) granulite facies metamorphism with a clockwise P,T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90,1.85 Ga in a collisional environment created by the assembly process of the North China craton. [source] Regional variation in exhumation and strain rate of the high-pressure Sambagawa metamorphic rocks in central Shikoku, south-west JapanJOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2002K. Yagi Abstract Regional variation in the P,T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P,T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P,T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P,T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low-grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio , 4.0) than in the northern part (aspect ratio< 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate. [source] | ||||||||||