Recent work shows that localization at the brittle ductile transition (BDT) arises naturally as a consequence of brittle failure in polymineralic rocks where both brittle and viscous behavior (semibrittle) occur in at least two different mineral phases (i.e., quartz and feldspar between 300°C and 450°C) [e.g., Mancktelow and Pennacchioni, 2005; Pennacchioni, 2005; Pennacchioni and Manktelow, 2007; Fusseis and Handy, 2008; Brander et al., 2011]. Assuming that seismogenic fault zones may be loaded by localized ductile shear at the BDT [e.g., Gilbert et al., 1994; Lister and Davis, 1989; Scholz, 2002], variations in creep rates may be expressed as strain rate variations in the seismogenic fault rooted in the ductile shear zone. Such strain rate variations may partly explain the increasing body of evidence that slip rates vary in space and time [e.g., Wallace, 1987; Friedrich et al., 2003]. Here we present a model of nonsteady strain accumulation in the form of creep events within localized shear zones at the base of the seismogenic lithosphere. Practically, we model the initiation of a network of shear fractures as a stress perturbation at the transition from brittle to ductile behavior. We study the different creep behaviors predicted by the model for physical properties suitable for the Earth's crust. We model large variations in creep rate at the BDT on both short time scales (seconds to years) and long (years to thousands of years) time scales. Finally, we apply this model to well-documented strain rate variations occurring over long time scales along the Wasatch fault and adjacent fault zones which appear to have experienced rapid increases in Holocene slip rate relative to late Pleistocene rates [Friedrich et al., 2003].
1.1. Observational Background
 We seek to model regions of the crust where temperatures and pressures are such that mineral assemblages undergo a transition between brittle and viscous behavior. For example, for quartzofeldspathic crust, we would expect the viscous behavior to be exhibited by quartz, which behaves ductilely at ∼200°C, whereas feldspar should remain brittle until ∼450°C. The resulting media in this transition zone has the attributes of a brittle material forming the load bearing framework and a viscous material allowing for creep [e.g., Handy, 1990; Handy et al., 1999; Handy et al., 2007].
 Field observations have shown that the formation of fractures in the load-bearing framework of such media is a likely precursor to the development of localized ductile shear and flow [Simpson, 1985, 1986; Segall and Simpson, 1986; Mancktelow and Pennacchioni, 2005; Pennacchioni, 2005; Wightman et al., 2006; Pennacchioni and Manktelow, 2007; Fusseis and Handy, 2008; Brander et al., 2011]. The main assumption is that the presence of compositional heterogeneities and brittle discontinuities (i.e., fractures filled with veins) leads to localization in ductile shear. Often, the evolution of the ductile shear zone is accompanied by the formation of a network of (dilatant) fluid-filled fractures at near-lithostatic fluid pressure (Figure 1a). Fractures and fluid infiltration in fractures creates veins filled with relatively weak ductile minerals by chemical alteration and other fluid assisted mechanisms. These processes decrease the stiffness of the shear zone and increase the amount of viscous material over time, decreasing the ability of the strong phase to bear the load [e.g., Segall and Simpson, 1986; Stel, 1986; Tourigny and Tremblay, 1997; Christiansen and Pollard, 1997; Fusseis and Handy, 2008; Brander et al., 2011]. Under strain a subset of fractures links through new fractures or microcracks (Figure 1b). Eventually, this generates a reorientation of the stresses so that shear stresses are resolved on the fractures and veins. As a consequence the strain localizes as creep within the weak material filling the fractures. The progressive increase in surface area generated by new fractures intensifies the efficiency of fluid rock interaction and facilitates mass transfer and viscous flow in the ductile shear zone (Figure 1c). Multiple fracture events lead to a reduction of grain size [Rutter and Brodie, 1988; Stewart et al., 2000] (Figure 1d). One can envision a shear zone that grows through time and becomes progressively more ductile as fractures increase the ratio of surface area to volume and the ratio of viscous to brittle material (Figure 1). In the long term and at large spatial scales, multiple discrete viscous shear zones are thought to organize into a system of ductile shear bands that in turn forms a larger zone of localized shear [e.g., Segall and Simpson, 1986; Lister and Davis, 1989; Christiansen and Pollard, 1997].
 Field examples of fractures or veins that are occupied by weaker minerals and eventually form an interconnected weak viscous phase such as calcite, quartzite, serpentinized peridotite are observed [e.g., Vissers et al., 1991; Herwegh and Kunze, 2002; Nüchter and Stöckhert, 2008; Fusseis and Handy, 2008]. For example, in the greenschist metamorphic facies typical of the middle crust, the alteration of feldspar to mica-rich aggregates along shear fractures can provide the softening necessary for the initiation of ductile shearing [Tourigny and Tremblay, 1997; Fusseis and Handy, 2008]. The formation of quartz-filled veins along fractures [Wightman et al., 2006; Nüchter and Stöckhert, 2008] may also provide weakening necessary for a shear zone to creep at the BDT. Dehydration reaction in a subducting slab also provide the fluids necessary to form fractures filled with weak serpentine mineral that eventually form mylonites anastomosing around strong olivine peridotite [e.g., Vissers et al, 1991; Evans, 2004].
 Herein, we hypothesize that strain at the BDT accumulates over multiple creep events over a fracture or network of fractures that are oriented in the direction of the shear stress in the shear zone. We do not model the period preceding creep when the fractures and shear stresses are not aligned. However, we model single creep events in the shear zone that occur when the fractures are optimally oriented and can accumulate slip. We present an analytic formulation that allows us to explore the effect of the physical properties of the crust, the shear zone and the BDT in between the crust and ductile shear zone, on creep at the BDT.