The same rationale proposed for subduction zones ( Hyndman et al., 1997), for cases in which the locked fault zone does not extend deeper than the fore-arc Moho, seems to hold for intracontinental megathrust faults as well. This is also in keeping with the observation that the down-dip extent of the seismogenic zone generally coincides with the 350 ☌ isotherm, if this temperature is reached above the Moho ( Oleskevich et al., 1999). Indeed, according to laboratory experiments and field observations, this thermally activated transition occurs at a temperature of 300–350 ☌ for quartzo-feldspathic rocks ( Blanpied et al., 1991 1995).
This temperature range suggests that the onset of creeping dislocation could correspond to the transition from slip-weakening friction to aseismic stable sliding. The location of this point is relatively well constrained and, when compared to the thermal structure of Figure 12, falls in a temperature range of 250–350 ☌ (though this large range ignores uncertainties in the thermal structure itself). However, the exact location of the transition cannot be constrained tightly from the GPS campaign data, and there are a variety of possible slip-rate-distribution solutions with a tapering zone of finite width, including the one produced from the mechanical model, that would fit the data equally well.Īssuming that the transition is abrupt, it is possible to invert the geodetic data for the location of the up-dip end of the creeping dislocation in the 2-D model of Figures 36 and 38. Close inspection of the velocity field reveals that a portion of the fault near the base of the prescribed fault geometry undergoes stable sliding.
The mechanical model of interseismic strain shown in Figure 38 produces a smooth transition of the fault zone from fully locked to ductilely flowing along its northward continuation, where temperatures exceed 450 ☌. Geodetic and seismic data provide some information on the position of the down-dip end of the locked portion of the MHT. Avouac, in Treatise on Geophysics, 2007 6.09.6.4 What Controls the Down-Dip end of the Locked Portion of the MHT?
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