Accretionary wedge and oceanic crust thickness effect on surface deformation: Study case the Mentawai segment

Observations of several active shallow subduction megathrusts suggest that they are localized as d&#233;collements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.&#160; 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.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.

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

10.5194/egusphere-egu2020-16975 ◽

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

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.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.The Schistes Lustr&#233;s 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 Lustr&#233;s units show increasing peak P-T conditions from blueschist (300-350&#176;C - 1.2-1.3 GPa) to eclogite facies (580&#176;C - 2.8 GPa). This study focuses on the Schistes Lustr&#233;s - Monviso transect, which shows an almost continuous increase in metamorphic grade.In the Schistes Lustr&#233;s 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 CO2 and CH4. In contrast, eclogitic fluids released from metagabbros are highly saline brines with low N2 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.

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The role of the remarkable Seram-Kumawa strike-slip fault in the tectonic process of the northern Banda Arc system

10.5194/egusphere-egu2020-7300 ◽

2020 ◽

Author(s):

Xiaodong Yang ◽

Satish C. Singh ◽

Ian Deighton

Keyword(s):

Subduction Zones ◽

Surface Deformation ◽

Active Tectonics ◽

Strike Slip ◽

Strike Slip Fault ◽

Benioff Zone ◽

Accretionary Wedge ◽

Geophysical Research ◽

Fault Plane Solutions ◽

Banda Arc

The Banda Arc system is sited in a junction of convergence between the Eurasian, Indo-Australian, Philippine and Pacific plates. It has a remarkable 180&#176; curve in the Benioff zone. Two fundamental ideas have been invoked to explain this significant subduction-arc orientation change: (1) bent subduction zone around the Banda Sea (Hamilton, 1979; Spakman and Hall, 2010; Hall, 2012), or (2) oppositely dipping subduction zones (Cardwell and Isacks, 1978; McCaffrey, 1989), but no general agreement exists as to the cause of this curvature. However, a WNW-trending strike-slip fault, i.e. Seram-Kumawa fault, is observed at the north-eastern end of the Arc, cutting through the Seram accretionary wedge, prism and trench and seems to continue on the subducting plate (Hall et al., 2017). This fault is either inactive or locked temporarily at the present day, because there are very few strike-slip events along its trend while there are many thrust earthquakes on its north and northwest side. A few essential questions remain unanswered about this fault in relation to the evolution of the Banda Arc. For instance, what is the origin of this fault, what role does it play in the tectonic processes and large earthquakes along the Banda Arc. Could this fault eventually break-up the Banda Arc? What is its tectonic implication on the evolution of other highly curved subduction-arc systems? To address these questions, we will carry out a comprehensive investigation into active tectonics and seismicity occurrence along the northeast Banda Arc using high-resolution bathymetry, 2D marine seismic profiles and earthquake data. Reference:Cardwell, R.K. and Isacks, B.L., 1978. Geometry of the subducted lithosphere beneath the Banda Sea in eastern Indonesia from seismicity and fault plane solutions. Journal of Geophysical Research: Solid Earth, 83(B6): 2825-2838.Hall, R., 2012. Late Jurassic&#8211;Cenozoic reconstructions of the Indonesian region and the Indian Ocean. Tectonophysics, 570: 1-41.Hall, R., Patria, A., Adhitama, R., Pownall, J.M. and White, L.T., 2017. Seram, the Seram Trough, the Aru Trough, the Tanimbar Trough and the Weber Deep: A new look at major structures in the eastern Banda Arc.Hamilton, W.B., 1979. Tectonics of the Indonesian region. US Govt. Print. Off.McCaffrey, R., 1989. Seismological constraints and speculations on Banda Arc tectonics. Netherlands Journal of Sea Research, 24(2-3): 141-152.Spakman, W. and Hall, R., 2010. Surface deformation and slab&#8211;mantle interaction during Banda arc subduction rollback. Nature Geoscience, 3(8): 562.&#160;

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Metamorphic Conditions of Neotethyan Meliatic Accretionary Wedge Estimated by Thermodynamic Modelling and Geothermobarometry (Inner Western Carpathians)

Minerals ◽

10.3390/min10121094 ◽

2020 ◽

Vol 10 (12) ◽

pp. 1094

Author(s):

Ondrej Nemec ◽

Marián Putiš ◽

Peter Bačík ◽

Peter Ružička ◽

Zoltán Németh

Keyword(s):

High Pressure ◽

Oceanic Crust ◽

Early Cretaceous ◽

Continental Margin ◽

Late Jurassic ◽

Hanging Wall ◽

Western Carpathians ◽

Accretionary Wedge ◽

Metamorphic Evolution ◽

Lower Temperature

Metamorphic evolution of an accretionary wedge can be constrained by a reconstructed P–T conditions of the oceanic and continental margin fragments. This paper deals with the metamorphic overprinting of the Inner Western Carpathians (IWC) Meliatic Triassic–Jurassic paleotectonic units after the closure of the Neotethyan Meliata Basin. Medium to high-pressure and lower temperature conditions were estimated by Perple_X pseudosection modelling, combined with garnet–phengite, calcite–dolomite and chlorite thermometers and chlorite–phengite and phengite barometers. The Late Jurassic subductional burial to a maximum 50 km depth was estimated from the Bôrka Unit continental margin fragments at 520 °C and 1.55 GPa. This is compatible with the metamorphic peak garnet–glaucophane–phengite assemblage of blueschist facies in metabasites. The Jaklovce Unit oceanic fragments were subducted to maximum 35–40 km at 390–420 °C and 1.1–1.3 GPa. Metabasalts and metadolerites contain winchite, riebeckite, actinolite, chlorite, albite, epidote and phengite. A glaucophane-bearing metabasalt recorded an intra-oceanic subduction in blueschist-facies conditions. Rare amphibolite-facies metabasalts of this unit indicate the base of an inferred oceanic crust sliver obducted onto the continental margin wedge. The Meliata Unit oceanic/continental margin flysch calciclastic and siliciclastic metasediments suggest the burial to approximately 15–20 km at 250–350 °C and 0.4–0.6 GPa. This is indicated by a newly formed albite, K-feldspar, illite–phengite and chlorite associated with quartz and/or calcite and dolomite in these rocks. Magnesio-hastingsite to magnesio-hornblende bearing metagabbro with newly formed metamorphic magnesio-riebeckite and actinolite is an inferred detached Meliatic block tectonically emplaced in a Permian salinar mélange in the Silica Nappe hanging wall. Reconstructed P–T paths indicate variable metamorphic conditions from the medium-pressure to high-pressure subduction of the Bôrka and Jaklovce units to the Meliata Unit shallow burial in an accretionary wedge during Late Jurassic to Early Cretaceous Meliaticum evolution. Mélange blocks of Meliaticum incorporate different juxtaposed Meliatic paleotectonic units exposed in nappe outliers overlying the IWC Gemeric and Veporic superunits.

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Covering of cometary nucleus by refractory crust and its evolution into asteroid-like body

International Astronomical Union Colloquium ◽

10.1017/s0252921100031675 ◽

1999 ◽

Vol 173 ◽

pp. 365-370

Author(s):

Kh.I. Ibadinov

Keyword(s):

Laboratory Experiments ◽

Cometary Nucleus ◽

Sublimation Rate ◽

Cometary Nuclei ◽

Short Period Comet ◽

Short Period ◽

Brightness Decrease ◽

Crust Thickness ◽

Ice Sublimation ◽

Period Comet

AbstractFrom the established dependence of the brightness decrease of a short-period comet dependence on the perihelion distance of its orbit it follows that part of the surface of these cometary nuclei gradually covers by a refractory crust. The results of cometary nucleus simulation show that at constant insolation energy the crust thickness is proportional to the square root of the insolation time and the ice sublimation rate is inversely proportional to the crust thickness. From laboratory experiments resulted the thermal regime, the gas productivity of the nucleus, covering of the nucleus by the crust, and the tempo of evolution of a short-period comet into the asteroid-like body studied.

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