1. Abstract
The Black Hills uplift and Powder River Basin are separated by the northwest-
trending Black Hills (BHM) and northerly-trending Fanny Peak (FPM) monoclines that
intersect about seven miles east of Newcastle, Wyoming. The oblique trends of the axial
traces of these structures relative to the approximately N70°E strike of regional Laramide
horizontal maximum stress (σ1) proposed by Gries (1983) suggests possible high-angle,
oblique movement (a combination of dip- and strike-slip movement) for basement faults
that form the cores of the folds. Based upon map-scale features associated with these
monoclines, e.g., restraining bends, in-line anticlines, oblique faults and structural lenses
such as the Newcastle Terrace, Shurr and others (1988) suggested that the larger features
formed with such a component of strike-slip. This concept was not tested at the scale of
individual outcrops, however.
This study uses the kinematics of mesoscopic fault and joint data collected at 25
locations along the monoclines and analyzed using fault kinematic software (FaultKin 7)
to test the concept of a component of oblique-slip within the two larger folds. Fossen and
others’ (1994) geometric model of transpressional and transtentional deformation was
used in determining a singular maximum regional compressive stress capable of creating
all of the brittle strain observed in the field area.
The results of these techniques show two significant aspects. First, the fact that
the local computed σ1 directions are not perpendicular to either of the major fold trends
necessitates that there be a strike-slip component related to the genesis of these folds,
left-oblique for the Black Hills monocline and right-oblique for the Fanny Peak
monocline. Second, the orientations of the local σ1 stresses vary along the trends of the
larger folds (from east to west, N60°E to N40°E along the Newcastle Terrace within the
BHM, and from north to south, N88°E to N54°E along the southern portion of the FPM)
and are not aligned with the proposed N70°E regional Laramide stress. This illustrates
the presence of strain partitioning at two, or possibly more, scales. The Fossen model
calculates a maximum regional σ1 direction of N50°E from field measurements, which
does not equal the principal stress of N70°E proposed by Gries (1983). The variance
along the trends of the individual monoclines indicates partitioning at the local scales; the
variation of σ1 orientations across the intersection of the two large folds results from a
larger component of partitioning. In such a case the regional geometric models are not
sensitive enough to recognize local scale variation in principal stress directions due to
strain partitioning. Areas of local partitioning within complex deformation zones must be
modeled independently prior to conducting regional tectonic studies.
The intersection of monoclines of this magnitude (3,000 feet of structural relief) is
believed to be unique within the Rocky Mountains. As such, it is an ideal location for the
examination of the partitioning of regional strain into variously trending, large, local
structures.