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 
			08855-1179

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 
master fault. 
	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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