SCEC Award Number 15117 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Ductile fracture in feldspar aggregates - Insights into the role of pore-fluid pressure on fault behavior in the lower crust
Investigator(s)
Name Organization
Greg Hirth Brown University
Other Participants
SCEC Priorities 1b, 3a, 5c SCEC Groups FARM, SDOT
Report Due Date 03/15/2016 Date Report Submitted 11/10/2016
Project Abstract
 To understand the physics of crustal faulting near the brittle-ductile transition, we conducted a microstructural study on experimentally deformed quartz sandstones deformed at high pressure and temperature. Using quantitative microstructural data, we address: (1) how the relative contribution of grain-scale deformation mechanisms vary from brittle faulting to semibrittle faulting (ductile shear fracture) with changing deformation conditions; and (2) whether interactions of tensile or shear microfractures govern shear localization within these regimes. Our analysis of energy partitioning suggests that the deformation (plastic energy) prior to shear localization in both brittle faulting and semibrittle faulting regimes is accommodated primarily by grain-scale brittle mechanisms. Our analysis also indicates that the relative importance of tensile and shear microfracture and grain crushing remains similar over a wide range of the PT conditions. On the basis of characterizing the ratios of fracture spacing to length, we conclude that the interaction of mm-scale intergranular shear fractures is the primary mechanism of macroscopic fault formation in both brittle and semibrittle faulting within our granular rock samples.
Intellectual Merit To investigate processes responsible for the evolution of fault strength, permeability and modes of fault slip, we explore mechanisms of ductile fracture at conditions scaled to be appropriate for the brittle-ductile transition. The experiments focus on high-temperature processes responsible for grain-scale crack/pore growth, and how these processes respond to variations in stress, temperature and strain rate. These properties are critical for several of the science objectives of SCEC, including: (1) Stress transfer from plate motion to crustal faults: The long term strength of faults depends on pore-fluid pressure, thus investigating where the long-term strength and stress state of faults is actually controlled by friction rather than ductile creep – and how fault strength evolves during the seismic cycle - is key (priority 1b). (5) Causes and effects of transient deformations: slow slip events and tectonic tremor. The presence of fluids (promoting low effective stress) and frictional behavior near the slip stability transition are invoked in most models for the generation of tectonic tremor. However, the interactions among crystal plastic processes and pore-fluid pressure are not well constrained at these conditions (priority 5c and 5d). (3) Evolution of fault resistance during seismic slip: Key initial conditions here are the stress state and scale of strain localization at the base of the seismogenic zone during interseismic periods.
Broader Impacts The results of this study have broad applications in the field of geomechanics relevant to understanding earthquake processes, as well as the exploration, and mitigation of environmental impacts of, energy exploration. The results are presented to a wide range of audiences within our University as well as professional meetings.
Exemplary Figure Select Figure 2 in the report