Research ArticleGEOPHYSICS

Dynamic earthquake rupture in the lower crust

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Science Advances  31 Jul 2019:
Vol. 5, no. 7, eaaw0913
DOI: 10.1126/sciadv.aaw0913
  • Fig. 1 Structure of the lower crustal fault in the Bergen Arcs.

    (A) The damage zone is present on the northern side of the fault and is locally intruded by injection veins branching from the pseudotachylyte (Pst). (B) The interior of the fault consists of a zoned pseudotachylyte (inner and outer pseudotachylyte) bounded by cataclasites. The wall rock contains garnet (Grt), plagioclase (Plg), and diopside (Di). Clasts of cataclasite have been entrained in the outer pseudotachylyte (white arrows). (C) The damage zone contains fragmented wall rock minerals with little to no shear strain. Along this portion of the fault, cataclasites are absent. North is up in (B) and (C). (D) X-ray computed microtomography image of the fault shown in (B). The outer pseudotachylyte and wall rock plagioclase are transparent for better three-dimensional rendering. The orientation of the reference frame is as follows: x, west; y, perpendicular to the topographic surface; and z, north. Photo credit: (A, B, and C) Arianne Petley-Ragan, University of Oslo and (D) Francois Renard, University of Oslo.

  • Fig. 2 Microstructures in the cataclasites and damage zone.

    (A and B) Three-dimensional x-ray computed microtomography images looking southwest showing extracted garnet clasts (Grt) in the cataclasite (dark red). The entire volume of the cataclasite is shown in (A), and the volume adjacent to the outer pseudotachylyte is shown in (B). The x-y-z coordinate system is the same as in Fig. 1D. (C and D) Backscatter electron (BSE) images of the (C) southern and (D) northern cataclasite. The boundary with the outer pseudotachylyte is marked by elongated aggregates of garnet fragments with growth rims. Insets show garnet clasts with growth rims and the grain size distribution [probability density function (PDF)] of garnet clasts with associated power law exponents (see Methods). (E and F) BSE images of the (E) fragmented garnet and (F) fragmented diopside (Di) in the damage zone. Fragmented garnet has a corona of diopside, K-feldspar (Kfs), and kyanite (Ky). Fragmented diopside has a corona of hornblende (Hbl), K-feldspar, and kyanite. Note the fragmented plagioclase (Plg) with clinozoisite (Czo), K-feldspar, kyanite, and quartz (Qtz) growth (see also fig. S7). Insets show the grain size distribution of garnet and diopside clasts with associated power law exponents. Scale bars, 0.25 mm. North is up in (C) to (F). Photo credit: (A and B) Benoit Cordonnier, University of Oslo; (C) through (F), Arianne Petley-Ragan.

  • Fig. 3 Microstructures in the cataclasites and damage zone.

    EBSD results for (A) an elongated aggregate of garnet fragments in the cataclasite (Fig. 2C), (B) garnet in the damage zone (Fig. 2E), and (C) diopside in the damage zone (Fig. 2F). Grain boundaries (≥10°) are black, and low-angle boundaries (1° to 9°) are gray. Orientation maps use inverse pole figure coloring in relation to the x-vector (horizontal in image). Insets display the distribution of misorientations for neighboring (gray bars) and non-neighboring (blue line) grains compared with an expected uniform distribution (orange line).

  • Fig. 4 Temporal evolution of deformation processes in the fault.

    The fault propagates from right to left, and the time since the rupture tip passed increases to the right. The illustration is based on a rupture zone with clear damage and melting products observed in the field (Fig. 1A). (i) Fragmentation occurs near the rupture tip and into one wall rock. (ii) Dextral shear motion concentrates within the rupture zone behind the tip, and the damaged rock undergoes cataclasis. (iii) Accumulation of shear motion within the cataclasites leads to frictional heating and melting. Melt products are injected into the damage zone, and cataclasites are completely melted where the volume of the damaged wall rock is greatest. (iv) Metamorphism (green) creates secondary phases in the pseudotachylyte, growth rims around the clasts in the cataclasites, and coronae around fragmented grains in the damage zone. Differential cooling of the pseudotachylyte to create zoned veins is not included in the illustration.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaaw0913/DC1

    Fig. S1. Field structures of the fault in the Bergen Arcs.

    Fig. S2. Images of damage in the southern wall rock.

    Fig. S3. Microstructures of the zoned pseudotachylyte.

    Fig. S4. Microstructures of the recrystallized pseudotachylyte.

    Fig. S5. BSEs of the growth on garnet clasts in the cataclasites.

    Fig. S6. Ternary plots of garnet and diopside compositions associated with the fault.

    Fig. S7. BSE of fragmented plagioclase in the damage zone.

    Fig. S8. Pole figures of the microstructures analyzed with EBSD (Fig. 3).

    Fig. S9. Grain size distribution of the garnet fragments with growth rims in the cataclasites (Figs. 2C and 3A).

    Fig. S10. Asymmetric damage and shear along a neighboring fault.

    Table S1. Average mineral compositions for garnet and diopside clasts and associated growth rims within the cataclasites.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Field structures of the fault in the Bergen Arcs.
    • Fig. S2. Images of damage in the southern wall rock.
    • Fig. S3. Microstructures of the zoned pseudotachylyte.
    • Fig. S4. Microstructures of the recrystallized pseudotachylyte.
    • Fig. S5. BSEs of the growth on garnet clasts in the cataclasites.
    • Fig. S6. Ternary plots of garnet and diopside compositions associated with the fault.
    • Fig. S7. BSE of fragmented plagioclase in the damage zone.
    • Fig. S8. Pole figures of the microstructures analyzed with EBSD (Fig. 3).
    • Fig. S9. Grain size distribution of the garnet fragments with growth rims in the cataclasites (Figs. 2C and 3A).
    • Fig. S10. Asymmetric damage and shear along a neighboring fault.
    • Table S1. Average mineral compositions for garnet and diopside clasts and associated growth rims within the cataclasites.

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