Connecting shallow and deep slow-slip-events with geologic observations and an elastic-viscoplastic solution
Abstract
Deep subduction-zone slow-slip events (SSEs) occur in a thermomechanical environment that likely favors viscoplastic deformation mechanisms, and analogous geologic exposures exhibit mylonites, pressure-solution fabrics, and syn-tectonic quartz veins. Such observations are contradictory with seismological evidence for deep tremor from sliding on frictional interfaces. One class of models may reconcile this discrepancy by envisioning elastic strain accumulation across a <1-km thick zone filled with mixtures of strong and weak material. These materials fail plastically, but then the slip pulse is damped viscously. Shallow SSEs, on the other hand, occur at the up-dip limit of the seismogenic zone where conditionally stable friction and variable pore-fluid pressure are thought to govern slip dynamics. Geologic exposures of exhumed shallow subduction-zone sections, such as Paleozoic thrusts in northeastern North America, contain lenses of well-lithified sandstone suspended within finer-grained material, with veins cutting across the tapered ends of the lenses. The veins have irregular boundaries and comprise carbonate material that is compositionally (and crystallographically) equivalent to pore-filling carbonate in surrounding material. Near the veins the sediment underwent pervasive dissolution and removal of primary components. Shear bands are cm-scale and filled with <1-mm grains that exhibit little evidence of transgranular fracture or grain-boundary abrasion. In some places deformation was accommodated by pressure-solution creep, but the material is sub-metamorphic, overall, approaching peak temperatures of 200°C only in the highest-grade parts of the thrust belt. As in deeper subduction-zone systems, strain can be highly localized in narrower slip surfaces within the >10-m-scale shear zones (at the map scale). Altogether, pressure-solution-like kinetics, poroelasticity, and granular friction are likely prominent rheological controls in the shallow SSE environment. Yet, these can be incorporated into a geologically-scaled viscoelastoplastic model that should apply both above and below the seismogenic zone, thereby providing insight into some intriguing differences between the two and how they relate to the earthquake cycle.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T42D..04H
- Keywords:
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- 5112 Microstructure;
- PHYSICAL PROPERTIES OF ROCKSDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS