NW Pacific slab rheology, the seismicity cutoff, and the olivine to spinel phase change

Abstract

Along the Kamchatka-Kuril-Japan-Izu-Bonin-Mariana subduction zones, the old age of the subducting Pacific Plate and the rapid subduction rate together suggest that earthquakes should occur to the bottom of the transition zone. However, the seismicity cutoff varies in depth between 350 km and 650 km. Along these subduction zones, the largest deep-focus earthquakes invariably occur near the depth of the local seismicity cutoff regardless of its depth. The events near the seismicity cutoffs also have systematically different focal mechanisms than shallower events. Furthermore, data from S 660 P arrivals, residual sphere analysis, and tomographic studies all show that the slab dip consistently steepens to a near-vertical orientation at the seismicity cutoff. This change in slab dip indicates a strength loss in the slab. We hypothesize the following causal connection among all these observations: The cold temperatures in the slab kinetically hinder the olivine to spinel phase change and allow the olivine to persist metastably to depths well below its equilibrium pressure. When the phase transition occurs, it nucleates very finegrained spinel which acts as a lubricant, allowing the initiation of earthquake faulting at high confining pressures which further nucleates additional fine-grained spinel. The cold anomaly of the slab severely inhibits the growth of the nucleated spinel crystals. The presence of the fine-grained spinel crystals reduces the strength of the coldest part of the slab by several orders of magnitude, allowing high slab deformation rates. Additionally, the phase change, by increasing the density, provides a negative buoyancy force. Combined, these processes reduce the slab membrane strength and allow the slab to descend at a steeper dip.

Can Earth's rotation and tidal despinning drive plate tectonics?

“…;Engdahl et al (1998);Gudmundsson and Sambridge (1998);Castle and Creager (1998);Gutscher et al (1999);Chen et al (2001);Karato et al (2001);Engdahl and Villaseñor (2002);Rivera et al (2002);Pardo et al (2002);Billen et al (2003);Hirth and Kohlstedt (2003);Das (2004);Lallemand et al (2005);Milsom (2005);Syracuse and Abers (2006);Frepoli et al (1996);Reyners et al (2006);Billen and Hirth (2007); Vinnik et al (2007); Chiarabba et al (2008); Espurt et al (2008); Pérez-Campos et al (2008); and Scalera (2008).presently not following the standard kinematics of W-directed subduction zones where the subduction hinge migrates away from the upper plate; the hinge moves toward Eurasia and in fact the Japan Sea is shrinking. On the other hand, New Hebrides and Central America subduction zones have a relevant transpressional component (oblique subduction), which could contribute to the steepness of the slab.…”