Geology Reference
In-Depth Information
11.7.3
Sensitivity of Prediction
For both vertical and oblique simple shear, the relationship between fault shape and
the hangingwall rollover geometry is sensitive to (1) the shear angle, (2) fault dip be-
tween the hangingwall and footwall cutoffs of the reference horizon, (3) the stratigraphic
correlation across the fault, and (4) the geometry of the reference bed near the fault
(Withjack and Peterson 1993). Shape predictions are less sensitive to the exact loca-
tion of the fault and to the rollover shape far from the fault.
Predictions are quite sensitive to small changes in the exact positions of the fault
cutoffs. This is because small differences in the initial fault dip lead to errors that
accumulate down the fault. From the opposite perspective, the agreement between
observed and predicted fault trajectories is a sensitive indicator of the correct fault
cutoff locations. Predictions are somewhat less sensitive to small changes in the cor-
relation of the reference bed across the fault (Rowan and Kligfield 1989). Incor-
rect correlations also result in a mismatch between the predicted and observed
fault trajectories. Additional discussions of simple-shear methods can be found in
Wheeler 1987; Dula 1991; Withjack and Peterson 1993; Withjack et al. 1995; Hague
and Gray 1996; Buddin et al. 1997; Shaw et al. 1997; Spang 1997; Hardy and McClay
1999; Novoa et al. 2000.
The major uncertainty in the simple-shear technique is in the choice of the shear
angle. The next section describes the relationship between layer-parallel strain and the
shear angle, followed by a discussion of how to choose the best shear angle.
11.7.4
Layer-Parallel Strain in Hangingwall
Simple shear oblique to bedding produces bed length and bed thickness changes
(Sect. 11.3, Eqs. 11.14, 11.15) in beds above a fault. The relationships among the vari-
ables in Eq. 11.14 are illustrated by Fig. 11.51. For a normal fault, if the angle of simple
shear is constant, the layer-parallel strain increases with the dip of the median
surface of bedding, i.e., as a plane bed is rotated to steeper dips. Vertical simple
shear produces much less strain for a given amount of bedding dip change than does
oblique simple shear. Substantial dip changes without much strain imply vertical
simple shear or that some other model is required. Synthetic simple shear (shear
direction parallel to master fault) at angles lower than about 80° will cause layer-
parallel contraction. Antithetic simple shear (shear direction dips opposite to master
fault) produces layer-parallel extension. Small dip changes accompanied by readily
visible second-order normal faults imply antithetic oblique simple shear, probably at
angles of 60° or less.
The mechanism for the layer-parallel strain produced by oblique simple shear
(Fig. 11.51) is usually small-scale faulting. The small-scale faults are probably not par-
allel to the shear direction, as logical as that may seem. The shear direction describes
the geometry, not the mechanics. The faults produced by layer-parallel extension and
the correlative thinning will be in response to the stress generated in the layer by the
