DEFORMATION WITHIN THE SOLITE QUARRY, DANVILLE
BASIN: SMALL NORMAL FAULTS, FRACTURE
PARTITIONING AND THE SPATIAL-TEMPORAL EVOLUTION
OF NORMAL FAULT SYSTEMS
ACKERMANN, Rolf V.
SCHLISCHE, Roy W.
PATIÑO, Lina C.
JOHNSON, Lois A.
YOUNG, Scott S.1
All at: Department of Geological Sciences, Rutgers
University, Busch Campus, Piscataway, NJ
The Cow Branch Formation within the Solite Quarry has been
deformed both continuously and via all three brittle failure modes, and
exhibits fracture partitioning such that lithologies comprised of less
than 10% weak minerals failed in tension, those with 20-30% weak
minerals formed hybrid fractures, and rocks with more than 38%
weak minerals failed in shear (small normal faults). All structures are
neo-formed. Stress orientation analysis based on Andersonian theory
suggests that all structures formed in response to the same remote
applied tectonic stress (Triassic rifting). Extension estimates for rocks
that failed in tension vs. those that failed in shear are comparable.
There do not appear to be detachment horizons between them,
implying that the units failed coevally or semi-coevally, placing
specific constraints on Mohr-Coulomb failure models for these rocks.
The small normal faults are synthetic to the border fault
system of the basin, and are of particular interest since they are the
smallest normal faults studied in detail to date. These very small faults
(L = <0.5 cm - 130 cm) dip at 70š to bedding and are in all ways like
their larger cousins. They occur both as isolated features and as
segments of relay systems, and exhibit slickensided, mineralized fault
surfaces, footwall uplift, hanging-wall subsidence, relay ramps,
elliptical fault surfaces, and displacement that is at a maximum at the
center of the fault and tapers to zero at the tips. These small faults
extend the global length-displacement data set from 7 to 9 orders of
magnitude of fault length, and demonstrate that maximum
displacement scales linearly with length over that scale range,
suggesting that fault growth is fractal (Schlische et al., 1996).
These small faults can be broken down into two subsets of
length based on their spatial (plan-form) distribution within the rock
volume. Larger master normal faults (L ‰ 50 - 300 cm) accommodate
the majority of the strain and are infrequent. At least one master fault
was reactivated as a reverse fault during basin inversion. The other
subset of faults (L ‰ <0.5 cm - 20 cm) is ubiquitous within the rock
volume, but exhibits anti-clustering with respect to the larger
structures, forming "shadow zones" around the master faults. The
shadow zones are elliptical in shape, approximating the deformation
fields (areas of footwall uplift and hanging-wall subsidence) of the
master faults. There appears to be a complete absence of brittle failure
within these zones, suggesting they are akin to Mode I crack
shields/stress reduction shadows. Anti-clustering patterns vary
between isolated master faults and linked systems consisting of
multiple segments, and depend on the stage of linkage of those
systems and breaching of relay ramps. Smaller faults commonly
change from left stepping to right stepping on either side of a given
This spatial distribution of faults complicates strain estimates,
with implications for the temporal evolution of this neo-formed
system. The master faults likely formed first and the smaller faults
later, but not within the deformation fields (stress reduction shadows)
of the larger structures. The smaller faults perhaps formed when some
strain threshold was exceeded outside the master fault deformation
field. This sequence of fault formation places geometric constraints on
the spatial distribution of faults consistent with field observations.
Schlische, R.W., Young, S.S., Ackermann, R.V., and Gupta, A., 1996, Geometry
and scaling relations of a population of very small rift-related normal faults,
Geology, in press.
1Now at: The Rock Fracture Project, Dept. of Geological and Environmental
Sciences, Stanford University
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