Research ArticleGEOLOGY

Hydrous oceanic crust hosts megathrust creep at low shear stresses

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Science Advances  27 May 2020:
Vol. 6, no. 22, eaba1529
DOI: 10.1126/sciadv.aba1529
  • Fig. 1 Geological background and geotectonic setting.

    (A) Current tectonic setting and location of Kyushu island. (B) Simplified map of accretionary terranes and high P-T metamorphic belts on Kyushu island after (43). (C) Geological map of Coastal Makimine mélange (15). (D) Geological map of a coastal exposure of the NMR. (E) Cartoon cross section of a generic convergent margin showing approximate settings of exhumed shear zones.

  • Fig. 2 Outcrop and microscale appearance of metabasalts in coastal Makimine mélange.

    (A) Metabasalt exposed as foliated layers intercalated with red mudstone. (B) Photomicrograph of anastomosing solution selvages, which define a poorly developed foliation. (C) EDS element map for Si highlights reduction in Si concentration within solution seams, caused by dissolution of albite. (D) Backscattered electron image of solution selvages, which consist of very fine (mostly <10 μm) grains of chlorite, prehnite, and magnetite. (E) Photomicrograph of asymmetric stress shadows around titanite, indicating noncoaxial shear within solution selvages. Photo credit (A): Å. Fagereng, Cardiff University.

  • Fig. 3 Outcrop and microscale appearance of metabasalts in inland Makimine mélange.

    (A) Metabasalt layers with low amplitude, long wavelength pinch, and swell geometry interlayered with metasediment. (B) EDS element map of inland Makimine metabasalt, where actinolite and chlorite form in stress shadows adjacent to clinopyroxene grains. (C) Backscattered electron image of actinolite in an asymmetric stress shadow about a clinopyroxene grain. (D) Albite and clinopyroxene grains have an SPO with long axes parallel to foliation, more pronounced in larger aspect ratio grains. Photo credit (A): C. Tulley, Cardiff University.

  • Fig. 4 Outcrop and microscale appearance of amphibolite schist (metabasalt) in the NMR.

    (A) Amphibolite schist and pelitic schist separated by a thin (<1 m) layer of chlorite-actinolite schist. (B) In thin section, prolate grains of albite are enveloped by chlorite, muscovite, and actinolite, with an asymmetry showing a dextral sense of shear in the provided photomicrograph. Inset shows oriented inclusions of actinolite within albite and quartz. (C) EDS element map of amphibolite. Albite shows no chemical zonation, but actinolite has pronounced zoning with less aluminous composition in the rims. (D) Albite and dispersed quartz grains have a strong SPO with long axes parallel to lineation. Grains with larger aspect ratios are generally more closely aligned to the foliation. Photo credit (A): C. Tulley, Cardiff University.

  • Fig. 5 Pole figures (equal area, lower hemisphere) for grain orientations in metabasalt from the NMR.

    Contours in units of multiples of a uniform distribution (MUD); regions with MUD > 10 are shown in dark red. Quartz (boudinaged veins) shows two clusters each inclined between the Y and Z kinematic directions. Albite shows no clear preferred orientation. Muscovite and chlorite have strong preferred orientations with (001) planes parallel to foliation. Muscovite [100] forms a continuous girdle parallel to foliation, whereas in chlorite [100] is weakly clustered parallel and perpendicular to the shear direction. Actinolite [001] axes are aligned with the shear direction, and (100) and (101) planes lie parallel and orthogonal to foliation, respectively.

  • Fig. 6 Constraints on the shear strength and deformation temperature of exhumed hydrous oceanic crust and comparison to experimentally determined rheologies for subducting lithologies.

    (A) Piezometer-derived shear stress estimates and RSCM-derived temperature estimates for quartz veins deformed in inland Makimine mélange and the NMR. The experimentally determined shear strength of quartz undergoing dislocation creep (20) gives stresses close to the estimated values for the exhumed shear zones at a strain rate of 1012 s1. (B) The piezometer and geothermometer estimates define a strength-temperature curve for hydrous oceanic crust, which is far weaker than dry diabase (9). Coulomb strengths with effective friction μeff = 0.12 and μeff = 0.04 represent the cohesion-less strength of unaltered oceanic crust (9) and chlorite (13), respectively (assuming a pore fluid factor λ = 0.8). The sketched strength curve suggests a frictional-viscous transition near ~400°C. The mica schist flow law (35) represents the viscous strength of phyllosilicate-rich subducted sediment. (C) Cartoon cross section shows the broad plate interface shear zone extending into hydrated oceanic crust, as deformation by solution-transfer creep becomes effective with increasing temperature. As represented here, the frictional-viscous transition in hydrated oceanic crust would occur up-dip of the mantle wedge corner.

Supplementary Materials

  • Supplementary Materials

    Hydrous oceanic crust hosts megathrust creep at low shear stresses

    Christopher J. Tulley, Åke Fagereng, Kohtaro Ujiie

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