Geology Reference
In-Depth Information
and thereby greatly increases the amount of strain (to 58%) given by Eq. 11.37 using
horizontal as the reference line. Again, using the median line as the reference, the
strain is 18.3%.
Seismically active faults in the Basin and Range province of the western U.S. pro-
vide an example of domino-block deformation. The faults are planar and steeply dip-
ping to depths of at least 10 km, breaking the crust into a series of dominoes (Fig. 11.26).
How much crustal extension is implied by the geometry of the dominoes? The exten-
sion can be found from Eq. 11.38 and the measurements shown in Fig. 11.26b. Because
the cutoff angles are different for each block, so are the extension strains. The horizon-
tal extension is, from the left domino to the right domino, 12.9%, 24.9%, 12.5%, and
19.9%. Rigid dominoes should rotate the same amount and so the different amounts
of rotation and the different strain magnitudes could be viewed as representing depar-
tures from perfect rigid-block deformation.
11.5.3
Circular-Fault Predictive Model
Circular-arc faults permit rigid rotation of the hangingwall block. Rarely are faults
complete circular arcs, but the upper part of a listric thrust may be approximately cir-
cular. Quantitative kinematic models for block rotation on listric thrusts have been
developed for and successfully applied to normal faults (Moretti et al. 1988) and base-
ment-involved thrust faults (Erslev 1986, 1993; Seeber and Sorlien 2000). The model
below is for a listric thrust fault. It allows the prediction of the location of the lower
detachment and the displacement from the fault dip and the width of the structure.
The kinematic model (Fig. 11.27) is based on a fault that is a segment of a circular
arc with radius
R
from a center C. The circular portion of the fault meets the planar
Fig. 11.27.
Kinematic model of a rotated block above a circular-arc thrust. The hangingwall is displaced
D
along the lower detachment.
Light shading
is rigid;
:
rotation angle;
R:
radius of curvature;
C: center
of curvature; R*: radius on a line ort
hogonal to the regional;
θ
0
is the dip at the fault at regional;
d:
displacement of a marker at a distance
r
from the center of curvature;
W:
width of the structure at
regional;
H:
depth to detachment from the regional. The
dashed pin line
is the original trailing edge. The
solid-head pin
is the trailing edge of a constant-bed-length structure;
δ
d
is the length imbalance with
respect to a vertical trailing edge
∆
