Earth Planets Space, Vol. 56 (No. 12), pp. 1103-1109, 2004
Susan Ellis1 and Bernhard Stöckhert2
1Geological and Nuclear Sciences, 69 Gracefield Rd, Lower Hutt, New Zealand
2Institute of Geology, Mineralogy, and Geophysics, Ruhr University, Bochum, Germany
(Received April 30, 2004; Revised November 19, 2004; Accepted December 3, 2004)
We show using numerical model experiments that upper crustal faults can impose ductile localization in the mid and lower crust over the seismic cycle, with strain-rates and integrated creep strain enhanced by a factor of 10, or a factor of 100 if lower crust is also thermally weakened. Imposed ductile localization is caused by the transfer in stress from the lower tip of the frictional fault to the mid-crust. Within the weak ductile mid-lower crust, this stress transfer also promotes significantly enhanced creep rates in a lobe that extends down-dip from the lower end of the fault. Comparison of model results with the Alpine Fault of New Zealand, shows how the interaction of faulting with other localization mechanisms can account for key aspects of the geodetic strain accumulating across the Alpine Fault. Localization of ductile strain in the lower crust imposed by faulting in the upper crust could explain the extension of major faults into the lower crust observed in seismic imaging.
Key words: Strain, localization, model, crust, rheology, fault, Alpine Fault, stress.