scholarly journals Multiple veining in a paleo–accretionary wedge: The metamorphic rock record of prograde dehydration and transient high pore-fluid pressures along the subduction interface (Western Series, central Chile)

Geosphere ◽  
2020 ◽  
Vol 16 (3) ◽  
pp. 765-786 ◽  
Author(s):  
Jesús Muñoz-Montecinos ◽  
Samuel Angiboust ◽  
Aitor Cambeses ◽  
Antonio García-Casco
Keyword(s):  
Bulk Composition ◽  
Matrix Material ◽  
Geothermal Gradient ◽  
Seismogenic Zone ◽  
Accretionary Wedge ◽  
Central Chile ◽  
Internal Fluid ◽  
Extensive Field ◽  
Dehydration Reactions ◽  
Fluid Production

Abstract High pressure–low temperature metamorphic rocks from the late Paleozoic accretionary wedge exposed in central Chile (Pichilemu region) are characterized by a greenschist-blueschist lithological association with interbedded metasediments that reached peak burial conditions of ∼400 °C and 0.8 GPa during late Carboniferous times. We herein combine new extensive field observations, structural measurements, and geochemical and petrological data on vein and matrix material from Pichilemu transitional greenschist-blueschist facies rocks. The studied veins were first filled by albite, followed by quartz and calcite as well as glaucophane and winchite. Field, structural, and microscopic zoning patterns show that these rocks underwent a protracted sequence of prograde vein-opening events, which have been largely transposed to the main foliation before and during underplating in the basal accretion site near 25–30 km depth. While some of the earliest albite-filled vein sets may have formed after prograde breakdown of sub–greenschist facies minerals (<250 °C), our thermodynamic modeling shows that relatively minor amounts of fluid are produced in the subducted pile by dehydration reactions between 250 and 400 °C along the estimated geothermal gradient. It also confirms that the formation of interlayered blueschist and greenschist layers in Pichilemu metavolcanics is a consequence of local bulk composition variations, and that greenschists are generally not formed due to selective exhumation-related retrogression of blueschists. The early vein sets are a consequence of prograde internal fluid production followed by sets of hydrofractures formed at near-peak burial that are interpreted as a record of external fluid influx. We postulate that such a fractured sequence represents a close analogue to the high-Vp/Vs regions documented by seismological studies within the base of the seismogenic zone in active subduction settings.

The effect of temperature-dependent thermal parameters in thermal models of subduction zones

2021 ◽  
Author(s):  
Iris van Zelst ◽  
Timothy J. Craig ◽  
Cedric Thieulot
Keyword(s):  
Subduction Zones ◽  
Thermal Parameters ◽  
Dislocation Creep ◽  
Seismogenic Zone ◽  
Effect Of Temperature ◽  
Temperature Dependent ◽  
Thermal Models ◽  
Intermediate Depth ◽  
Dehydration Reactions ◽  
Model Setup

<p>The thermal structure of subduction zones plays an important role in the seismicity that occurs there with e.g., the downdip limit of the seismogenic zone associated with particular isotherms (350 °C - 450 °C) and intermediate-depth seismicity linked to dehydration reactions that occur at specific temperatures and pressures. Therefore, accurate thermal models of subduction zones that include the complexities found in laboratory studies are necessary. One of the often-ignored effects in models is the temperature-dependence of the thermal parameters such as the thermal conductivity, heat capacity, and density.<span> </span></p><p>Here, we build upon the model setup presented by Van Keken et al., 2008 by including temperature-dependent thermal parameters to an otherwise clearly constrained, simple model setup of a subducting plate. We consider a fixed kinematic slab dipping at 45° and a stationary overriding plate with a dynamic mantle wedge. Such a simple setup allows us to isolate the effect of temperature-dependent thermal parameters. We add a more complex plate cooling model for the oceanic plate for consistency with the thermal parameters.<span> </span></p><p>We test the effect of temperature-dependent thermal parameters on models with different rheologies, such as an isoviscous wedge, diffusion and dislocation creep. We find that slab temperatures can change by up to 65 °C which affects the location of isotherm depths. The downdip limit of the seismogenic zone defined by e.g., the 350 °C isotherm shifts by approximately 4 km, thereby increasing the maximum possible rupture area of the seismogenic zone. Similarly, the 600 °C isotherm is shifted approximately 30 km deeper, affecting the depth at which dehydration reactions and hence intermediate-depth seismicity occurs. Our results therefore show that temperature-dependent thermal parameters in thermal models of subduction zones cannot be ignored when studying subduction-related seismicity.<span> </span></p>

The uplift of high-pressure-low-temperature metamorphic rocks

1987 ◽  
Vol 321 (1557) ◽  
pp. 87-103 ◽  
Keyword(s):  
Tectonic Setting ◽  
Geothermal Gradient ◽  
Dominant Mode ◽  
Metamorphic Rocks ◽  
Return Flow ◽  
Accretionary Wedge ◽  
Moderate Amount ◽  
Low Viscosity ◽  
Buoyancy Forces ◽  
Flowing Material

The uplift of high- P -low- T metamorphic rocks has been attributed to buoyancy, diapirism, or hydrodynamically driven return flow. Buoyancy forces can return material subducted into the mantle only if subduction slows or ceases, reducing the downward traction. The buoyancy forces will be reversed within the crust, because of the increased density of high- P assemblages, and therefore can not cause the subducted material to rise beyond the base of the crust. Diapirism and hydrodynamic flow processes require a low-density, low-viscosity matrix, and can only explain the emplacement of relatively small bodies of high- P rock entrained in the flowing material. The tectonic setting of coherent regional high- P —low- T terrains can be explained in terms of the mechanical behaviour of an accretionary wedge with negligible yield strength, where underplating is the dominant mode of accretion. Underplating thickens the wedge from beneath and increases its surface slope. This causes the upper part of the wedge to extend horizontally, even though convergence is continuing. Continued underplating beneath and extension above can allow the oldest high- P rocks to rise to within reach of a moderate amount of erosion on a time scale of the order of 10 Ma. As long as subduction continues beneath the wedge, the geothermal gradient will not relax to a normal value. This process explains (a) the evidence that high- P -low- T rocks are commonly uplifted while convergence is continuing; (b) the absence in many cases of significant overprinting by higher- T assemblages; (c) the position of the oldest and highest pressure rocks in the upper rear of orogenic wedges; (d) the lack of adequate tectonic thicknesses of overlying rock to explain the metamorphism; and (e) the common occurrence of post-metamorphic faults that excise parts of the metamorphic zonation.

Underplating altered oceanic crust: Insights from numerical modelling

2021 ◽  
Author(s):  
Zoe Braden ◽  
Jonas B. Ruh ◽  
Whitney M. Behr
Keyword(s):  
Numerical Modelling ◽  
Oceanic Crust ◽  
Fluid Pressure ◽  
Geothermal Gradient ◽  
Pore Fluid ◽  
Surface Erosion ◽  
Accretionary Wedge ◽  
Pore Fluid Pressure ◽  
Mafic Rocks

<p>Observations of several active shallow subduction megathrusts suggest that they are localized as décollements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.  Exhumed accretionary complexes from a range of subduction depths, however, preserve underplated mafic slivers, which indicate that megathrust faults can occasionally develop within the mafic oceanic crustal column. The incorporation of mafic rocks into the subduction interface shear zone has the potential to influence both long-term subduction dynamics and short-term seismic and transient slip behaviour, but the processes and conditions that favour localisation of the megathrust into deeper oceanic crustal levels are poorly understood.</p><p>In this work, we use visco-elasto-plastic numerical modelling to explore the long-term (million year) factors influencing the incorporation of mafic volcanic rocks into the subduction interface and accretionary wedge through underplating. We focus on the potential importance of oceanic seafloor alteration in facilitating oceanic crustal weakening, which is implemented through a temperature-dependent pore-fluid pressure ratio (lambda = 0.90-0.99 between 160 and 300oC). We then examine the underplating response to changes in sediment thickness, geothermal gradient, sediment fluid pressure, and surface erosion rates. Our results indicate that a thinner incoming sediment package and a lower geothermal gradient cause oceanic crustal underplating to initiate deeper beneath the backstop (overriding plate) compared to thicker incoming sediment and a higher geothermal gradient. Relative pore fluid pressure differences between sediments and altered oceanic crust control the amount of altered oceanic crust that is underplated, as well as the location of underplating beneath the backstop or accretionary wedge. When sediments on top of the altered oceanic crust have the same fluid pressure as the altered oceanic crust, no oceanic crustal underplating occurs. Modelling results are also compared to exhumed subduction complexes to examine the amount and distribution of underplated mafic rocks.</p>

Warm thermal structures in subduction zones lead to ample dehydration at the depths of deep slow slip and tremor and resultant transformations in viscous rheology 

2021 ◽  
Author(s):  
Cailey Condit ◽  
Victor Guevara ◽  
Melodie French ◽  
Adam Holt ◽  
Jonathan Delph
Keyword(s):  
Subduction Zones ◽  
Thermal Structure ◽  
Fluid Pressure ◽  
Plate Boundary ◽  
Pore Fluid ◽  
Seismogenic Zone ◽  
Plate Interface ◽  
Dehydration Reactions ◽  
Episodic Tremor And Slip ◽  
Increased Strength

<p>Feedbacks amongst petrologic and mechanical processes along the subduction plate boundary play a central role influencing slip behaviors and deformation styles. Metamorphic reactions, resultant fluid production, deformation mechanisms, and strength are strongly temperature dependent, making the thermal structure of these zones a key control on slip behaviors.</p><p> </p><p>Firstly, we investigate the role of metamorphic devolatilization reactions in the production of Episodic Tremor and Slip (ETS) in warm subduction zones. Geophysical and geologic observations of ETS hosting subduction zones suggest the plate interface is fluid-rich and critically stressed, which together, suggests that this area is a zone of near lithostatic pore fluid pressure.  Fluids and high pore fluid pressures have been invoked in many models for ETS. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or via up-dip transport from greater depths remains an open question. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface along predicted pressure–temperature (P-T) paths for the plate boundary along Cascadia, Nankai, and Mexico which all exhibit ETS at depths between 25-65 km. Our models suggest that 1-2 wt% H<sub>2</sub>O is released at the depths of ETS along these subduction segments due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across this narrow P-T range to cause high pore fluid pressures. Punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction margins, and up-dip migration of fluids from deeper in the subduction zone is not required to produce ETS-facilitating high pore fluid pressures. These dehydration reactions not only produce metamorphic fluids at these depths, but also result in an increased strength of viscous deformation through the breakdown of weak hydrous phases (e.g., chlorite, glaucophane) and the growth of stronger minerals (e.g., garnet, omphacite, Ca-amphibole). Lastly, we present preliminary data on viscosity along warm subduction paths showing the locations of these dehydration pulses correlate with viscosity increases in mafic lithologies along the shallow forarc.</p>

Composition of fluids released along a subduction interface with increasing depth: insights from fluid inclusions analysis on the Schistes Lustrés - Monviso transect (Western Alps)

2020 ◽  
Author(s):  
Clément Herviou ◽  
Anne Verlaguet ◽  
Philippe Agard ◽  
Hugues Raimbourg ◽  
Michele Locatelli ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Fluid Inclusions ◽  
Subduction Zones ◽  
Shear Zones ◽  
Free Fluid ◽  
Accretionary Wedge ◽  
Continuous Increase ◽  
Dehydration Reactions ◽  
Eclogite Facies ◽  
Fluid Sources

<p>Important amounts of fluids are released in subduction zones by successive dehydration reactions occurring both in the previously hydrated oceanic crust (and mantle) and overlying sedimentary cover. The release and circulation of such fluids in rocks have major consequences on both their mechanical and chemical behavior. Indeed, the presence of a free fluid phase strongly modifies the rock rheology, fracturing properties, and could be implicated in both intermediate-depth earthquake and slow slip events nucleation. Moreover, the scale of mass transfer, associated chemical changes in infiltrated rocks and element recycling in subduction zones are controlled by both the rock permeability and the amount and composition of such fluids. Thus, there is a crucial need to identify the major fluid sources, amounts and pathways to better constrain their impact on subduction dynamics.</p><p>Metamorphic veins, as well as mineralized fractures and shear zones in exhumed fossil subduction zones are the best witnesses of fluid-rock interactions and fluid circulation pathways. However, their interpretation in terms of fluid sources, residence time, scale of circulation requires a good knowledge of the composition of potential fluid sources. In order to determine the composition of the fluid released by both oceanic crust and sediments at various depth along their subduction, we analyzed the composition of fluid inclusions contained in vein minerals formed at peak P-T conditions, in rock units buried at various depths in the Alpine subduction zone.</p><p>The Schistes Lustrés complex is a slice-stack representing the deep, underplated part of the former Alpine accretionary wedge. These Alpine Tethys rocks are mainly composed of oceanic calcschists with fewer mafic and ultramafic rocks, buried to various depths before exhumation. From West to East, the juxtaposed Schistes Lustrés units show increasing peak P-T conditions from blueschist (300-350°C - 1.2-1.3 GPa) to eclogite facies (580°C - 2.8 GPa). This study focuses on the Schistes Lustrés - Monviso transect, which shows an almost continuous increase in metamorphic grade.</p><p>In the Schistes Lustrés blueschist-facies sediments, fluid inclusions were analyzed in quartz from high-pressure veins, i.e. quartz that co-crystallized with prograde to peak metamorphic minerals such as lawsonite and Fe-Mg carpholite. In the metamorphosed mafic rocks, we analyzed fluid inclusions from the peak metamorphic assemblages, i.e. glaucophane +/- omphacite in blueschist facies rocks, omphacite in eclogite-facies slices. Raman spectroscopy data on these fluid inclusions suggest that fluids released during dehydration of calcschists in blueschist-facies conditions are aqueous fluids with low-salinity and small amounts of CO<sub>2</sub> and CH<sub>4</sub>. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N<sub>2 </sub>content. These results, which will be associated with LA-ICP-MS analysis of fluid inclusions in metasedimentary quartz veins, will contribute to better constrain the evolution of composition of the fluids liberated by dehydration reactions with depth and protolith composition along the subduction interface.</p>

Thermal maturity of the accretionary wedge

2020 ◽  
Author(s):  
Utsav Mannu ◽  
David Fernández-Blanco ◽  
Ayumu Miyakawa ◽  
Taras Gerya ◽  
Masataka Kinoshita
Keyword(s):  
Thermal Conductivity ◽  
Phase Transitions ◽  
Vitrinite Reflectance ◽  
Geothermal Gradient ◽  
Accretionary Wedge ◽  
Thermal Maturity ◽  
Thermomechanical Model ◽  
Borehole Data ◽  
Different Temperatures ◽  
The Impact

<p>Records of thermal maturities in boreholes have led to a better understanding of the formation of geological structures, especially the duration of thrusting during the evolution of accretionary wedges. The temporal extent of thrusting is controlled by a host of factors such as the nature of sedimentation, the topography of the incoming plate and so on. As a result, estimating the peak heating through the thermal maturity of organic material can help elucidate which mechanism has played a prominent role in wedge evolution. However, the thermal maturity value expressed as the distribution of vitrinite reflectance is the combined effect of two factors: the geothermal gradient and the time the sediments were exposed to different temperatures. Thus, the distribution of vitrinite reflectance in accretionary wedges does not necessarily reveal the deformational pathway of individual thrusts. Moreover, since the conductivity of the sediments close to the surface (<10 km) is most accessible in borehole data and predominantly controlled by porosity, models of accretionary wedge simulating thermal maturity ought to incorporate the impact of porosity on thermal conductivity. Additionally, phase transitions of the sediments in the wedge, such as smectite-illite transition and the formation of zeolite facies, that lead to increased thermal conductivity and internal angle of friction for sediments at structurally deeper locations within the wedge, must be accounted for in modeling studies. Therefore, we use a 2D thermomechanical model of subduction with empirical porosity values form the Nankai subduction margin and incorporate the effect of phase transitions to simulate the formation of the accretionary wedge under several sedimentary conditions and track the evolution of the vitrinite reflectance. As a result, we gain a holistic picture of deformation in accretionary wedges exploring different scenarios using geodynamic modeling alongside field data.</p>

scholarly journals Seismogenic Zone in the Nankai Accretionary Wedge: General Summary of Japan-U. S. Collaborative 3-D Seismic Investigation

2000 ◽  
Vol 109 (4) ◽  
pp. 531-539 ◽  
Author(s):  
Shin'ichi KURAMOTO ◽  
Asahiko TAIRA ◽  
Nathan L. BANGS ◽  
Thomas H. SHIPLEY ◽  
Gregory F. MOORE ◽  
...  
Keyword(s):  
Seismogenic Zone ◽  
Accretionary Wedge ◽  
Seismic Investigation

scholarly journals Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

Geosphere ◽  
2021 ◽  
Author(s):  
Donald M. Fisher ◽  
John N. Hooker ◽  
Andrew J. Smye ◽  
Tsai-Wei Chen
Keyword(s):  
Fluid Flow ◽  
Fluid Pressure ◽  
Seismogenic Zone ◽  
Chemical Components ◽  
Low Grade ◽  
Coseismic Slip ◽  
Seismic Slip ◽  
Earthquake Simulator ◽  
Interseismic Deformation ◽  
Fluid Production

Subduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward.

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