SCEC Award Number 12164 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Stress transfer and the structure of lithospheric fault zones
Investigator(s)
Name Organization
John Platt University of Southern California Thorsten Becker University of Southern California
Other Participants Adam Holt
SCEC Priorities 1b, 2d, 3d SCEC Groups SDOT, FARM
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 The long-term aim of this project is to develop a generic model of the structure of lithosphere-scale continental fault zones below the seismogenic layer, incorporating the initiation of a ductile shear zone, the rheology of the rocks making up the shear zone, and the flow stress, strain-rate, width and geometry of the shear zone as a function of slip-rate, depth, temperature, and crust/lithosphere composition. During project year 2012 we worked on mechanisms of microstructural weakening that lead to strain localization and the initiation of ductile shear zones, and specifically on the formation and rheology of fine-grained two-phase mixtures in ultramylonites, for which we have developed a new theoretical flow law. We have also worked on the contribution of dissipative heating to strain localization in shear zones with a width pre-defined by microstructural weakening, and on the feedback between thermal weakening and grain-growth in grain-size sensitive creep. Our results improve our understanding of the strength and structure of lithospheric shear zones beneath the seismogenic layer, and their interaction with the seismogenic faults that lie above them.
Intellectual Merit The long-term aim of this project is to develop a generic model of the structure of lithosphere-scale continental fault zones below the seismogenic layer, incorporating the initiation of a ductile shear zone, the rheology of the rocks making up the shear zone, and the flow stress, strain-rate, width and geometry of the shear zone as a function of slip-rate, depth, temperature, and crust/lithosphere composition. During project year 2012 we worked on mechanisms of microstructural weakening that lead to strain localization and the initiation of ductile shear zones, and specifically on the formation and rheology of fine-grained two-phase mixtures in ultramylonites, for which we have developed a new theoretical flow law. We have also worked on the contribution of dissipative heating to strain localization in shear zones with a width pre-defined by microstructural weakening, and on the feedback between thermal weakening and grain-growth in grain-size sensitive creep. Our results improve our understanding of the strength and structure of lithospheric shear zones beneath the seismogenic layer, and their interaction with the seismogenic faults that lie above them.
Broader Impacts The project has supported a female postgraduate student.
Exemplary Figure Figure 2: Deformation mechanism map (stress vs grain-size) for granite/two-phase mylonite. Dynamic recrystallization and mixing at 100 MPa stress will reduce the grain-size from 1 mm (A) to 2.5µm on the feldspar piezometric line (B). This would result in an increase of strain-rate by a factor of 1000.