Article
Cajon Pass Redux: Analysis of the Cajon Pass drill hole core, and insights into fault processes and fault geometry of the San Andreas fault system
Geological Society of America Bulletin
(2017)
Abstract
We examined 109.3 m of core in deformed
crystalline rocks, acquired at 50 spots over
3013 vertical meters in the Cajon Pass deep
drill hole, along with samples from nearby
outcrops in southern California, to determine
the nature of deformation and alteration
near faults of the San Andreas fault
system. The upper 3–3.5 km section of the
crustal column is deformed by narrow brittle
fractures and faults. The lower 1.5 km section
of the core is cut by at least 25 steeply
dipping faults, 11 of which are newly identified
here. Altered damage zones up to 20 m
thick along faults in the lower 1.1 km of the
hole are composed of indurated foliated laumontite
+ chlorite ± epidote cataclasites and
exhibit semibrittle to plastic deformation
mechanisms, diffusive mass transfer, and
solution transfer. Where the fault-related
alteration produced lower-density rocks,
well-defined borehole geophysical signatures
and in situ stress measurements deviate
from background levels. Some of the faults
encountered in the shallower part of the
borehole are probably related to the steeply
dipping, slightly listric left-reverse Cleghorn
fault, whereas indurate, sheared, and hydrothermally
altered fault-related rocks cored
near the base of the borehole, which sampled
a damage zone of hydrothermally altered
rock, are most likely to be related to subsidiary
traces on the North American side of an
~50° NE-dipping San Andreas fault zone,
which has several traces nearby. Although
the bulk of the fault rocks might be related
to a steep downdip continuation of a planar
Cleghorn fault zone, this interpretation is less
likely because the Cleghorn fault is a reactivated
reverse fault that is likely to be listric at
depth. The Cajon Pass borehole intersected a
complex suite of faults with pervasive faultrelated
damage and synkinematic hydrothermal
alteration to at least 5 km depth. Fluids
impacted all phases of deformation.
Disciplines
Publication Date
2017
DOI
doi: https://doi.org/10.1130/B31681.1
Citation Information
Forand, D., Evans, J.P., Janecke, S.U., and Jacobs, J., 2017, Cajon Pass Redux: Analysis of the Cajon Pass drill hole core, and insights into fault processes and fault geometry of the San Andreas fault system: Geological Society of America Bulletin doi: https://doi.org/10.1130/B31681.1