Progressive weakening within the overriding plate during dual inward dipping subduction

Dual inward dipping subduction often produces complex deformation patterns in the overriding plate. However, the geodynamic process of how dual inward dipping subduction relates to this deformation is still poorly understood. Here we apply a composite viscosity, dependent on multiple parameters, e.g., temperature, pressure, strain rate etc., in 2-D thermo-mechanical numerical modelling to investigate how dual inward dipping subduction modifies the rheological structure of the overriding plate. Three variables are investigated to understand what controls the maximum degree of weakening. We find that the initial length and thickness of the overriding plate are negatively correlated with the magnitude of viscosity reduction. While the initial thickness of the subducting plate positively relates to the magnitude of viscosity reduction. The progressive weakening can result in a variety of stretching states ranging from 1) little or no lithosphere thinning and extension, to 2) limited thermal lithosphere thinning, and 3) localised rifting followed by spreading extension. Compared with single sided subduction, dual inward dipping subduction further reduces the magnitude of viscosity of the overriding plate. It does this by creating a dynamic fixed boundary condition for the overriding plate and forming a stronger upwelling mantle flow underlying the overriding plate. Three types of feedback weakening cycles are recognised, among which the strain rate weakening mechanism plays the dominant role in lowering the viscosity of the overriding plate throughout the simulation. Strain rate weakening is also a precondition for initiating thermal weakening, strain localisation and lithosphere thinning.

The late Paleozoic paired metamorphic belt of southern Central Chile: Consequence of a near-trench thermal anomaly?

The hypothesis of a subduction-related Miyashiro-type paired metamorphic belt for the origin of the late Paleozoic igneous and metamorphic complex in the Andean Coastal Cordillera has remained unquestioned since its proposal in the early seventies. A synthesis of the advances in the study of these metamorphic rocks between 33°S and 42°S, revising field relations among geological units, and geochemical and geochronological data from the contemporaneous granitoids of the Coastal Batholith, highlights inconsistencies in this model. The record of short-lived forearc magmatism in the late Paleozoic intruding the partially synchronous accretionary prism, and geochemical and isotopic data from the igneous rocks indicating sources from the accretionary prism sediments and the back-top lithosphere, suggest a departure from typical subduction settings. I conclude that the anomalous configuration of the paired metamorphic belt and the associated Coastal Batholith resulted from a complex geodynamic process involving a near-trench thermal anomaly caused by the subduction of a trench parallel mid-ocean ridge.

Geodynamic processes preceding to deep Okhotomorsky earthquake May 24, 2013 with magnitude MW = 8,3

The set of parameters of geophysical fields and geodynamic events before the strongest Okhotsk Sea earthquake May 24, 2013 with MW = 8,3 and a focal depth of 630 km in this paper analyzed. In the temporary vicinity of the earthquake (May 19-20), there was a temporary synchronization of geodynamic processes in areas of the Kamchatka Peninsula, separated by hundreds of kilometers. The simultaneous occurrence of anomalies in the behavior of tilts, in the dynamics of flow of subsoil radon, conditional deformations of remote foreshock activation in Avacha Bay, speaks of a single geodynamic process on a regional scale, preceding the earthquake. A possible reason for the observed in-phase geodynamic phenomena may be the geodeformation wave that arose at the last stage of the preparation of the Okhotsk Sea earthquake.