Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′-48°30′S)
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Abstract Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The ɛ Nd (T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.Keywords:
Accretionary wedge
Continental Margin
Passive margin
Wedge (geometry)
Margin (machine learning)
Isotope Geochemistry
The oblique convergence of Eurasia and Iberia since the Early Cretaceous, caused the formation of the Pyrenean intracontinental collisional orogen in the east, and progressed to continent‐ocean collision with subduction of the Bay of Biscay oceanic crust beneath the North Iberian Margin in the west. Two deep multichannel seismic profiles (IAM‐12 and ESCIN‐4), integrated with gravity modeling and other geological and geophysical data, provide the crustal‐scale architecture of this margin and its tectonic evolution during the convergence. The North Iberian Margin is modeled with a South or south‐southeast dipping oceanic crust beneath the outer part of the continental shelf. Mesozoic basins on the shelf were inverted during the Tertiary, and compressional activity continued until recent times in the ESCIN‐4 section, while a shallower, probably Neogene age basin is subjected to active recent erosion in the IAM‐12 section. In the oceanic areas, a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement. The accretionary prism increases in size from west to east (18–56 km), and its internal structure and morphology varies along strike. The prism is buried by postcorivergence sediments in both sections and in the IAM‐12 section appears to have been active at least during Lutetian to Burdigalian times. The crustal‐scale structure of the North Iberian Margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely.
Continental Margin
Accretionary wedge
Passive margin
Trough (economics)
Basement
Oceanic basin
Neogene
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The provenances of Ordos Basin and its surrounding regions, which all comes from the upper crust, are dominated by felsic rocks, which consist of ancient metamorphic rocks, such as metamorphic volcanic rocks and sedimentary rock of Archaeozoic and Proterozoic, and certain amount of granite and alkali basalt. But compositions of source and structural setting of provenances in the north and south are different. Compositions of major and rare earth elements suggest that sedimentary rocks in both north and south show some differences in area and stratum, and the changes of major element, REE and Eu anomaly are in accordance with the variation trend from oceanic island arc, continental island arc, and active continental margin to passive continental margin. Analysis of major elements indicates that the north provenance derived mainly from plate subduction zones and were related to active continental margin and passive continental margin, with minor related to the island arc of passive continental margin, and were related to tectonic setting of active continental margin and passive continental margin until middle-late Paleozoic. REE contrast analysis shows that the source for the northern basin has affinities to Archeozoic and Proterozoic metamorphotic rocks, such as granitic gneiss, diorite gneiss, adamellite, metamorphotic litharenite, phyllite, etc; that for the southern basin was deeply affected by passive continental margin source, characterized by high SiO2 , low Na2O features, and K2O/Na2O1. All these features are consistent to those (high SiO2 and K2O/Na2O1) of rocks of Archean-Proterozoic Taihua Group, Qinling Group and Kuanping Group. Until the end of Late Paleozoic, the provenance was not affected gradually by active continental margin. Beiqinling intermontane basins characterized by rapid accumulation m langes belong to the outer margin of Ordos Basin and have continuous transitional relationship with the basin. Therefore, both show some inherited relation. The Gd content and (Gd/Yb)N ratios of the Late Paleozoic sedimentary rocks in the Ordos basin vary with time changing, and the analysis shows that the northern source area in Taiyuan period was in rapid tectonic activity period, while the southern source area in Shanxi period was fast active period. This is consistent with the regional tectonic evolution, i.e. the uplift of the northern source area was earlier than that in the south.
Continental Margin
Passive margin
Felsic
Continental arc
Island arc
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Abstract Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The ɛ Nd (T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.
Accretionary wedge
Continental Margin
Passive margin
Wedge (geometry)
Margin (machine learning)
Isotope Geochemistry
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Abstract Phase equilibria modeling of sodic-calcic amphibole-epidote assemblages in greenstones in the northern Appalachians, USA, is compatible with relatively shallow subduction of the early Paleozoic Laurentian margin along the Laurentia-Gondwana suture zone during closure of a portion of the Iapetus Ocean basin. Pseudosection and isopleth calculations demonstrate that peak metamorphic conditions ranged between 0.65 GPa, 480 °C and 0.85 GPa, 495 °C down-dip along the subducted Laurentian continental margin between ∼20 km and ∼30 km depth. Quantitative petrological data are explained in the context of an Early Ordovician geodynamic model involving shallow subduction of relatively young, warm, and buoyant Laurentian margin continental-oceanic lithosphere and Iapetus Ocean crust beneath a relatively warm and wet peri-Gondwanan continental arc. A relatively warm subduction zone setting may have contributed to the formation of a thin, ductile metasedimentary rock-rich channel between the down-going Laurentian slab and the overriding continental arc. This accretionary channel accommodated metamorphism and tectonization of continental margin sediments and mafic volcanic rocks (greenstones) of the Laurentian margin and provided a pathway for exhumation of serpentinite slivers and rare eclogite blocks. Restricted asthenospheric flow in the forearc mantle wedge provides one explanation for the lack of ophiolites and absence of a well-preserved ultra-high-pressure terrane in central and northern Vermont. Exhumation of the subducted portion of the Laurentian margin may have been temperature triggered due to increased asthenospheric flow following a slab tear at relatively shallow depths.
Forearc
Continental Margin
Laurentia
Passive margin
Volcanic arc
Accretionary wedge
Continental arc
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Forearc
Accretionary wedge
Continental Margin
Back-arc basin
continental collision
Collision zone
Passive margin
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Continental Margin
Convergent boundary
Accretionary wedge
Seafloor Spreading
Passive margin
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Accretionary wedge
Underplating
Continental Margin
Convergent boundary
Volcanic arc
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Summary form only given. The area offshore southwestern Taiwan is the a place where the Luzon subduction complex first encroaches on the passive Chinese continental margin. Distinctive fold-and-thrust structures of the convergent zone and horst-and-graben structures of the passive margin are separated by a deformation front that extends NNW-ward from the eastern edge of Manila Trench and Penghu Canyon to the foot of the continental slope. Due to the outgrowth of the accretionary wedge as the influx of orogenic sediments increases toward Taiwan, the trend of the frontal folds and thrusts of the submarine accretionary wedge changes from a NNE-SSW direction near Luzon Island to a NW-SE direction north of 20 N. The NE-SW trending Chinese continental margin blocks the westward advance of the growing accretionary wedge, forces the NW-SE trending ramp anticlines gradually turning into a NNE-SSW trending direction across the continental slope and the Kaoping Shelf, then connects to the frontal thrusts of the mountain belt on land Taiwan. The Kaoping shelf and slope are frontal portions of the submarine incipient collision zone that are separated from the South China Sea shelf and slope by the Penghu submarine canyon. Presence of the complex Penghu submarine canyon system made exact location of the deformation front and nature of many morphotectonic features offshore SW Taiwan unclear.
Accretionary wedge
Submarine canyon
Continental Margin
Passive margin
Collision zone
Eurasian Plate
Thrust fault
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