Seismic Consequences of Warm Versus Cool Subduction Metamorphism: Examples from Southwest and Northeast Japan

Abstract

Warm and cool subduction zones exhibit differences in seismicity, seismic structure, and arc magmatism, which reflect differences in metamorphic reactions occurring in subducting oceanic crust. In southwest Japan, arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth; in northeast Japan, arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth. Thermal-petrologic models predict that oceanic crust subducting beneath southwest Japan is 300 degrees to 500 degrees C warmer than beneath northeast Japan, resulting in shallower eclogite transformation and slab dehydration reactions, and possible slab melting.

Source and path of magma for volcanoes in the subduction zone of northeastern Japan

“…The general idea that dehydration in the descending slab plays an important role in generating the magma supply for subduction zone volcanism has been proposed long ago [Anderson et al, 1980;Peacock, 1993]. It is now argued on mineralogical grounds that dehydration at 140 to 150 km depth should occur [Abers, 2000;lwarnori, 1998;lwarnori and Zhao, 2000;Peacock and Wang, 1999]. Thus, our model of an inclined path for the magma (Fig.…”

Thermal structure due to solid-state flow in the mantle wedge beneath arcs

“…Based on heat flow in the forearc [Peacock and Wang, 1999], van Keken, et al [2002] adopt a shear heating rate of about 30 mW/m2 along this fault for NE Japan. A simple estimate of the temperature increase along the fault can be obtained from the solution for transient heat conduction into a halfspace with a prescribed heat flux at the halfspace surface [Carslaw and Jaeger, 1959, p. 75, equation (8) Figure 7B.…”

“…Thermal models with symbols: Open squares and circles, NE Japan and Cascadia, nonlinear rheology [van Keken et al, 2002]; open diamonds and closed circles, 40 km and 70 km decoupling models, NE Japan [Furukawa, 1993a]; closed squares, fast subduction of thin plate, assuming mantle potential temperature of 1400°C [Kincaid and Sacks, 1997], heavy solid line with no symbols, isoviscous corner flow, NE Japan [van Keken et al, 2002]. Other thermal models: NE and SW Japan [Peacock and Wang, 1999]; Izu-Bonin ; Aleutians [Peacock and Hyndman, 1999]; 100 km decoupling model, NE Japan [Furukawa, 1993a]; slow subduction of thin plate, old plate and young plate, plus fast subduction of old plate and young plate, all assuming potential temperature of 1400°C [Kincaid and Sacks, 1997]; Alaska Range hot and cold models [Ponko and Peacock, 1995]; fast and slow subduction, with and without shear heating [Peacock, 1996]; general models [Peacock, 1990a]. Triangular grey field encloses geotherms inferred from heat flow data in the Oregon Cascades arc [Blackwell et al, 1982]; although interpretation of heat flow data is often controversial, the inferred geotherm is broadly consistent with metamorphic PT estimates for arc crust.…”

“…These calculations are essentially of three types: (1) analytical approximations including various assumptions about coupling between the subducted crust and the overlying mantle and about convection in the mantle wedge [e.g., Davies, 1999;Molnar and England, 1995;Molnar and England, 1990], (2) purely plate-driven models with uniform viscosity, in which the thermal regime is calculated numerically using analytical expressions for corner flow in the mantle wedge, with model results depending on various input parameters including the thickness of the arc "lithosphere" and the depth of coupling between subducting crust and overlying mantle [e.g., Peacock, 2002;Peacock and Hyndman, 1999;Peacock and Wang, 1999;Iwamori, 1997;Peacock, 1996;Ponko and Peacock, 1995;Peacock et al, 1994;Pearce et al, 1992;Peacock, 1991;Peacock, 1990a;Peacock, 1990b], and (3) dynamic models in which the mantle flow field as well as the thermal regime are calculated numerically, with model results depending on parameters such as thermal buoyancy, chemical buoyancy and mantle viscosity [van Keken et al, 2002;Furukawa and Tatsumi, 1999;Kincaid and Sacks, 1997;Furukawa, 1993a;Furukawa, 1993b;Davies and Stevenson, 1992]. These models differ in many respects, but most agree that subduction of oceanic crust that is more than 20 million years old at down-dip rates greater than 20 km/Myr will not produce temperatures at the top of the subducting plate that are high enough to allow fluid-saturated melting of sediment or basalt.…”

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

“…Fig. 3 shows the pressure-temperature (P-T) of the subducted oceanic crust for the six subduction zones, from Syracuse et al (2010), superimposed on metamorphic dehydration reactions of the subducted oceanic crust from Peacock and Wang (1999). We selected the P-T paths calculated for the case where the mechanical decoupling between the slab and the overriding plate occurs where slab temperature reaches 550°C (see also Wada et al, 2008).…”