The Rheological Evolution of Brittle‐Ductile Transition Rocks During the Earthquake Cycle: Evidence for a Ductile Precursor to Pseudotachylyte in an Extensional Fault System, South Mountains, Arizona

Abstract

We investigate how the rheological evolution of shear zone rocks from beneath the brittle‐ductile transition (BDT) is affected by coeval ductile shear and pseudotachylyte development associated with seismicity during the earthquake cycle. We focus our study on footwall rocks of the South Mountains core complex, and we use electron backscatter diffraction (EBSD) analyses to examine how strain is localized in granodiorite mylonites both prior to and during pseudotachylyte development beneath the BDT. In mylonites that are host to pseudotachylytes, deformation is partitioned into quartz, where quartz exhibits crystallographic‐preferred orientation patterns and microstructures indicative of dynamic recrystallization during dislocation creep. Grain size reduction during dynamic recrystallization led to the onset of grain boundary sliding (GBS) accommodated by fluid‐assisted grain size‐sensitive (GSS) creep, localizing strain in quartz‐rich layers prior to pseudotachylyte development. The foliation‐parallel zones of GBS in the host mylonites, and the presence of GBS traits in polycrystalline quartz survivor clasts indicate that GBS zones were the ductile precursors to in situ pseudotachylyte generation. During pseudotachylyte development, strain was partitioned into the melt phase, and GSS deformation in the survivor clasts continued until crystallization of melt impeded flow, inducing pseudotachylyte development in other GBS zones. We interpret the coeval pseudotachylytes with ductile precursors as evidence of seismic events near the BDT. Grain size piezometry yields high differential stresses in both host mylonites (~160 MPa) and pseudotachylyte survivor clasts (> ~200 MPa), consistent with high stresses during interseismic and coseismic phases of the earthquake cycle, respectively.

The ultimate fate of a synmagmatic shear zone. Interplay between rupturing and ductile flow in a cooling granite pluton

“…In contrast, the Moyagee fault represents a case where cataclasites formed in an otherwise ductile regime, were spatially and temporally coeval with ductile strain concentrations and were subsequently mylonitized. This outcome suggests that a transient rheological behaviour, and not only the geometry of precursor structures, played an important role in the generation of cataclasites along the Moyagee fault (White, 1996(White, , 2003(White, , 2004(White, , 2005(White, , 2012Kelemen and Hirth, 2007;Stewart and Miranda, 2017).…”

High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites

“…Ductile instabilities potentially develop from the positive feedback between shear heating and strain rate in a shear zone, a process known as thermal runaway (Kelemen and Hirth, 2007;Braeck and Podladchikov, 2007;John et al, 2009;Thielmann et al, 2015). This process has been suggested to explain the association of coeval pseudotachylytes and mylonites in the deep crust (Hobbs et al, 1986;White, 1996White, , 2012Stewart and Miranda, 2017).…”

“…Kelemen and Hirth, 2007;Braeck and Podladchikov, 2007;Thielmann et al, 2015), seismological studies (Prieto et al, 2013), and experiments (Ohuchi et al, 2017) have shown that thermal runaway is possible within the lithospheric mantle and the lower crust (John et al, 2009). Few exhumed pseudotachylyte/mylonite associations have been interpreted to have resulted from ductile instabilities (in the middle to lower crust: Hobbs et al, 1986;White, 1996White, , 2012Stewart and Miranda, 2017). Nevertheless, unambiguous microstructural evidence for the process of thermal runaway is missing.…”

The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

“…This may occur at realistic background flow stresses in a shear zone [38,41], provided that grain size reduction occurs prior to thermal runaway. However, although some cases of middle and lower crustal pseudotachylyte/mylonite associations have been interpreted as being due to plastic instabilities [30,37,[45][46][47][48], unambiguous microstructural evidence for this process is still missing [41].…”