Type‐B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Regions
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Abstract The crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b ‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.Keywords:
Forearc
Abstract Tectonic and seismogenic variations in subduction forearcs can be linked through various processes associated with subduction. Along the Cascadia forearc, significant variations between different geologic expressions of subduction appear to correlate, such as episodic tremor-and-slip (ETS) recurrence interval, intraslab seismicity, slab dip, uplift and exhumation rates, and topography, which allows for the systematic study of the plausible controlling mechanisms behind these variations. Even though the southern Cascadia forearc has the broadest topographic expression and shortest ETS recurrence intervals along the margin, it has been relatively underinstrumented with modern seismic equipment. Therefore, better seismic images are needed before robust comparisons with other portions of the forearc can be made. In March 2020, we deployed the Southern Cascadia Earthquake and Tectonics Array throughout the southern Cascadia forearc. This array consisted of 60 continuously recording three-component nodal seismometers with an average station spacing of ∼15 km, and stations recorded ∼38 days of data on average. We will analyze this newly collected nodal dataset to better image the structural characteristics and constrain the seismogenic behavior of the southern Cascadia forearc. The main goals of this project are to (1) constrain the precise location of the plate interface through seismic imaging and the analysis of seismicity, (2) characterize the lower crustal architecture of the overriding forearc crust to understand the role that this plays in enabling the high nonvolcanic tremor density and short episodic slow-slip recurrence intervals in the region, and (3) attempt to decouple the contributions of subduction versus San Andreas–related deformation to uplift along this particularly elevated portion of the Cascadia forearc. The results of this project will shed light on the controlling mechanisms behind heterogeneous ETS behavior and variable forearc surficial responses to subduction in Cascadia, with implications for other analogous subduction margins.
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This chapter examines the basin-filling stratigraphy and major unconformity events of the Cenozoic forearc basins in the NE Japan, SW Japan, Ryukyu and Izu-Bonin forearc territories along the northwestern Pacific margins to obtain information on the background tectonic scenarios along the plate subduction zones. The forearc basin type and tectonic history are characteristic for each forearc territory, reflecting the differences in plate tectonic processes. Several major unconformity events seem to be synchronous for a forearc territory or whole forearc territories around Japan, suggesting that these events originated from more or less wider scale plate tectonic events. In the NE Japan forearc territory, the Oligocene unconformity can be the largest events, which transformed the forearc basin styles from the trench slope break-uplifted, fluvial system-dominated type to the tensional, deeper marine sloped type. In the SW Japan and Ryukyu forearc territories, the latest Oligocene to Middle Miocene gap was the transformation phase from the Palaeogene Shimanto-type forearc and accretionary complex, to the Neogene compressive, sloped to ridged forearc basins, developments of which have been interrupted by several unconformity events possibly related to changes in plate tectonic condition. These transformations of the forearc basin styles may reflect the changes in plate tectonic conditions in the northwestern Pacific region.
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Efficient recycling of subducted sedimentary nitrogen (N) back to the atmosphere through arc volcanism has been advocated for the Central America margin while at other locations mass balance considerations and N contents of high pressure metamorphic rocks imply massive addition of subducted N to the mantle and past the zones of arc magma generation. Here, we report new results of N isotope compositions with gas chemistry and noble gas compositions of forearc and arc front springs in Costa Rica to show that the structure of the incoming plate has a profound effect on the extent of N subduction into the mantle. N isotope compositions of emitted arc gases (9-11 N°) imply less subducted pelagic sediment contribution compared to farther north. The N isotope compositions (δ15N = -4.4 to 1.6‰) of forearc springs at 9-11 N° are consistent with previously reported values in volcanic centers (δ15N = -3.0 to 1.9‰). We advocate that subduction erosion enhanced by abundant seamount subduction at 9-11 N° introduces overlying forearc crustal materials into the Costa Rican subduction zone, releasing fluids with lighter N isotope signatures. This process supports the recycling of heavier N into the deep mantle in this section of the Central America margin.
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Based on structural deformation analysis in the oblique Sumatra subduction system, we review uplift mechanisms of the forearc high and formation of the forearc basin. The development of the forearc high has been attributed to the flexural uplift, basin inversion, uplift of older accretion wedge, and backthrust in the landward margin of the accretion wedge. Observation of recently acquired seismic reflection data shows that the interplay between trenchward-vergent thrusts and arcward-vergent backthrusts has played a major role in the uplift of forearc high. The uplifted sediments on the forearc high were previously formed in a forearc basin environment. The present-day morphology of the forearc high and forearc basin is related to the uplift of the accretionary wedge and the overlying forearc basin sediments during Pliocene. Regardless of obliquity in the subduction system, the Sumatran forearc region is dominated by compression that plays an important role in forming Neogene basin depocenters that elongated parallel to the trench.
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Accretionary wedge
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Subduction of the Nazca plate under the peruvian and chilean segments of the Andes is associated with variable characteristic structures of the corresponding forearc. The forearc constitutes this specifie zone recognised to deform slowly (of the order of 1mm/yr or less according to different sources), squeezed inbetween the oceanic plate subducting at around 8 cm/yr, and the growing Andes that thicken the border of the South American continent. Depending on its latitud inal pos ition , as weil as in east-west directions, this forearc has developped numerous features of either extensional or inverse signature, sometimes co-eval: the coastal escarpment (coastal cordillera), the Central Depression, or the Atacama fault zone, are only some of the most proeminant yet still enigmatic structures that compose the chilean forearc.
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Seismotectonics
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In nature subducting slabs and overriding plate segments bordering subduction zones are generally embedded within larger plates. Such large plates can impose far-field boundary conditions that impact the style of subduction and overriding plate deformation. Here we present 3D dynamic analogue models of subduction, in which the far-field boundary conditions at the trailing edges of the subducting plate (SP) and overriding plate (OP) are varied. Four configurations are presented: Free (both plates free), SP-Fixed, OP-Fixed and SP-OP-Fixed. We investigate their impact on the kinematics and dynamics of subduction, with a special focus on overriding plate deformation. Our models indicate that in natural (narrow) subduction zones, assuming a homogeneous overriding plate, the formation of backarc basins (e.g., Tyrrhenian Sea, Aegean Sea, Scotia Sea) is generally expected to occur at a comparable location (300-500 km from the trench), irrespective of the boundary condition. Furthermore, our models indicate that the style of forearc deformation (shortening or extension) is determined by the mobility of the overriding plate through controlling the force normal to the subduction zone interface (trench suction). Our geodynamic model that uses the SP-OP-Fixed set-up is comparable to the Calabria subduction zone with respect to subduction kinematics, slab geometry, trench curvature and accretionary wedge configuration. Furthermore, it provides explanation for the natural observations of both backarc extension in the Tyrrhenian Sea and forearc extension in the Calabria region, which have been active since the Miocene. We explain the observations as a consequence of subduction of the narrow Calabrian slab and the immobility of the subducting African plate and overriding Eurasian plate. This setting forced subduction to be accommodated almost entirely by slab rollback (not trenchward overriding plate motion), while trench retreat was accommodated almost entirely by backarc and forearc extension (not trenchward overriding plate motion), similarly to our SP-OP-Fixed model. This tectonic setting induced strong trench suction, which caused the forearc extension in Calabria.
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geodynamics
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