Creep burst coincident with faulting in marble observed in 4D
synchrotron X-ray imaging triaxial compression experiments
Abstract
Faults in carbonate rocks show both seismic and aseismic deformation
processes, leading to a wide range of slip velocities. We deformed two
centimeter-scale cores of Carrara marble at 25°C, under in-situ
conditions of stress of 2-3 km depth, and imaged the nucleation and
growth of creeping faults using dynamic synchrotron X-ray
microtomography with micrometer spatial resolution. The first sample was
under a constant confinement of 30 MPa and no pore fluid. The second
sample was under a confinement in the range 35-23 MPa, with 10 MPa pore
fluid pressure. We increased the axial stress by steps until creep
deformation occurred and imaged deformation in 4D during creep. The
samples deformed with a steady-state strain rate when the differential
stress was constant, a process called creep. However, for both samples,
we also observed transient events that include the acceleration of
creep, i.e., creep bursts, phenomena similar to slow slip events that
occur in continental active faults. During these transient creep events,
strain rates increase and correlate in time with strain localization and
the development of system-spanning fault networks. In both samples, the
acceleration of opening and shearing of microfractures accommodated
creep bursts. Using high-resolution time-lapse X-ray micro-tomography
imaging, and digital image correlation, during triaxial deformation
allowed quantifying creep in laboratory faults at sub-grain spatial
resolution, and demonstrates that transient creep events (creep bursts)
correlate with the nucleation and growth of faults.