sgr ms thesis - University of Maine

moment of sudden fracture tip failure and sudden reduction of
(Dutrow and
Norton, 1995). As hydraulic brecciation is a low stress intensity mechanism, the
rate of propagation is generally slow and fractures will tend to develop along
inherent planes of weakness in the rock. Hydraulic breccias tend to have a
shallow slope for size distribution due to an inability to produce new fractures
(D s =1-2; Jebrak, 1997; Clark and James, 2003; Barnett, 2004; Clark et al., 2006;
Farris and Paterson, 2007).
Abrasive breccias form during shear failure in a rock (Goodman, 1980;
Jebrak, 1997). Shear along ruptured surfaces can lead to abrasive fragmentation
along fracture walls. Like hydraulic breccias, abrasive breccias can also form
incrementally, but simple shear kinematics produce more complex fragmentation
patterns (Sammis et al., 1987; Blenkinsop, 1991). Continued grinding, plucking,
and reduction of clast size results in the development of fault gouge. Rotation
and flow of elongate clasts cause a preferred alignment with respect to flow
(Jebrak, 1997). The characteristics of abrasive breccias are more difficult to
constrain due to the complexities that arise in shear kinematics. D s values can
range widely based on strain rate, the amount of stress normal to the fracture
plane, and the number and duration of abrasion events (Sammis et al., 1986;
Sammis, 1987; Blenkinsop, 1991). For a collapse breccia, a single fragmentation
event linked to caldera subsidence is assumed. This breccia would produce D s
values on the order of 2-2.7 (Sammis, 1987; Blenkinsop, 1991). Fabric produced
by sense of shear and transport of clasts would also be visible, and clasts would
show evidence of imbrications and preferred orientation.
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