Landscape Evolution at an Active Plate Margin - Biological Science ...

Figure 1-15A. Light colored stratigraphic layer in Paleozoic Keeler
Canyon Formation exposed along Tinemaha Road on the western
edge of the Poverty Hills. The bed, shown outlined, was distorted by
cataclastic breccia flow. Compare the distorted shape of the layer to
that of a known rock avalanche deposit shown in Figure 1-15b. [K.
M. Bishop photo]
2002). A recently advocated second model suggests
the hills are a landslide mass derived from the Inyo
Mountains (Bishop, 1999; Bishop and Clements,
2006), although the landslide model was first suggested
in the 1960s by Pakiser et al. (1964) based on
a gravity survey of the valley. If the hills are indeed a
landslide, the long-run out character classifies it as a
rock avalanche. Correct interpretation of the origin
of the hills is important because it has implications
for understanding the kinematics of the Owens Valley
fault. The landslide model will be advocated here. The
presence of widespread brecciation across the hills
suggest a landslide deposit (Bishop and Clements
2006). At this stop we will hike down Tinemaha Road
to view breccia exposures in road cuts.
Please watch for cars.
Of particular interest in the road cuts is the exposure
of a brecciated light-colored stratigraphic unit within
dark colored units near where we park. The color
contrast of the units allows one to clearly delineate
the results of cataclastic flow deformation of the breccia.
The light-colored unit thickens and thins across
the outcrop and has a contorted shape (Fig. 1-15A).
It is proposed that the heterogeneous stress field required
to create this pattern is the result of fluctuating
internal stresses created by vibration and/or variable
traction as the landslide mass moved rapidly across
an irregular ground surface. The deformed geometry
of the light-colored unit is reminiscent of the geometry
of deformed stratigraphic units present in the
Blackhawk landslide, a well-known rock avalanche
deposit in the southern Mojave Desert (Fig. 1-15B).
D. R. Jessey and R. E. Reynolds
Figure 1-15B. Light colored (outlined) stratigraphic layer in the
Paleozoic Furnace Formation exposed in the Blackhawk rock
avalanche in the southern Mojave Desert. [K. M. Bishop photo]
Advocates of the transpressional model for the Poverty
Hills suggest the road cuts expose fault breccia
(Tayor, 2002). In this hypothesis, a thrust fault accommodating
uplift is present just below the road level.
A counterargument to this idea is that the stress field
from tectonic thrust faulting could be expected to
be relatively homogeneous across the width of the
outcrop. Thus, fault activity would not be expected to
cause the contorted shape of the light-colored unit.
If the hills are a landslide mass, the landsliding occurred
at least several hundred thousand years ago
based on the degree of erosional dissection and the
age of basalt flows that must post-date emplacement.
The source area of the landslide is likely the Inyo
Mountains where bedrock units of the same age and
lithologies are exposed in the Santa Rita Flat pluton
area.
Big Pine Volcanic Field. Basalt flows and scoria cones
of the Big Pine volcanic field can be seen along both
sides of U.S. 395 from the base of the Sierra Nevada
Mountains to the west, to the Inyo Mountains to the
east. The field encompasses an area of approximately
150 square miles from one mile south of the town of
Big Pine (Stewart Lane, mp 217.5) to 8 miles north of
Independence (Sawmill Road, mp 196.9). Volcanism
dates from 1.2 Ma to as recently as 25 ka (Manley,
et al., 2000; Moore and Dodge, 1980). Basaltic rocks
dominate, but a small outcrop area of rhyolite is present
to the west of the Poverty Hills.
Numerous active faults have been mapped within
the Big Pine field. The Independence fault lies to the
west of the basalt field and along the Sierra Nevada
front. Le et al. (2007) reported that movement along
24 2009 Desert Symposium