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

• Published in: Journal of geophysical research. Solid earth Vol. 122; no. 12; pp. 10,643 - 10,665
• Main Authors: Stewart, Craig A., Miranda, Elena A.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-12-2017
• Subjects: Backscatter; core complex; Crystallization; Crystallography; Deformation; Dislocation; Dislocations; Ductile-brittle transition; Dynamic recrystallization; earthquake cycle; Earthquake prediction; Earthquakes; Electron backscatter diffraction; Evolution; Fracture mechanics; Grain boundary sliding; Grain size; Mountains; Particle size; Piezometers; Precursors; Preferred orientation; pseudotachylyte; Quartz; Rheological properties; Rheology; Rock; Rocks; Seismic activity; Seismicity; Shear zone; Size reduction; Slumping; Solifluction; South Mountains; Stresses; Studies; South Mountains; United States > US; Arizona
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB014680

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.
Key Points
We investigated coeval mylonites and pseudotachylytes in the footwall of the South Mountains core complex, Arizona
The onset of grain boundary sliding in quartz within host mylonites is a ductile precursor to coeval pseudotachylyte development
Rock microstructures record interseismic and coseismic parts of the earthquake cycle during cyclic seismic events