|
Jack Tung,
2006,
Examining the Style and Uplift Along the Claremont Strand of the
San Jacinto Fault Zone; Committee
Members: Dr.Nate Onderdonk (Chair), Dr. Kim Bishop, Dr.
Pedro Ramirez
Potrero Valley is a small basin in the northwestern-most San Jacinto Mountains
and is currently undergoing uplift as a result of slip along the
San Jacinto fault zone. The valley is characterized by
Mio-Pliocene to late Pleistocene sedimentary rocks and a
sequence of alluvial terraces while the surrounding ridges are
composed of Triassic metasedimentary rock and Cretaceous
plutonic rock. Potrero Creek flows through the length of Potrero
Valley and is currently in an incisional stage as it attempts to
reach base level. Potrero Creek passes through Massacre Canyon
at the southwestern end of Potrero Valley and feeds into the San
Jacinto River in San Jacinto Valley.
The San Jacinto fault is a right-lateral fault, yet has
apparently produced a significant amount of topographic
development. The mode of uplift and topographic development of
Potrero Valley and the adjacent San Jacinto Mountains are not
completely clear. Relatively little research has been conducted
in Potrero Valley due to the lack of access into the area in the
past. The presence of preserved fluvial terraces within the
valley provides a record of fluvial deposition and uplift. From
the preserved terraces, the rate of deposition, incision and
uplift can be inferred. In order to understand the topographic
development of Potrero Valley and the San Jacinto Mountains,
research consisted of geologic field mapping, aerial photo
interpretation, structural data collection, and radiometric
dating of detrital charcoal found within terrace units.
A series of preserved alluvial fill and strath terraces,
representing at least four distinct periods of aggradation,
deposited in the valley record a history of deposition, uplift,
and incision. Using radiometric dating of recovered detrital
charcoal from two terraces and the height above the active
channel where the samples were collected, the data suggest
deposition rates of 2.0 to >5.4 mm/year during the Holocene, an
incision rate of 2.9 mm/year since late Pleistocene, and uplift
rates of 1.1 to 1.2 mm/year since late Pleistocene. Uplift is
accomplished by right-lateral reverse oblique slip along
segments of the Claremont fault and the Soboba-Campbell Ranch
fault. |
