Preliminary Geologic Map of the San Bernardino 30' x 60' quadrangle, California

The San Bernardino 30'x60' quadrangle, southern California, is diagonally
bisected by the San Andreas Fault Zone, separating the San Gabriel and San
Bernardino Mountains, major elements of California's east-oriented Transverse
Ranges Province. Included in the southern part of the quadrangle is the northern
part of the Peninsular Ranges Province and the northeastern part of the
oil-producing Los Angeles basin. The northern part of the quadrangle includes
the southern part of the Mojave Desert Province. Pre-Quaternary rocks within the
San Bernardino quadrangle consist of three extensive, well-defined basement rock
assemblages, the San Gabriel Mountains, San Bernardino Mountains, and the
Peninsular Ranges assemblages, and a fourth assemblage restricted to a narrow
block bounded by the active San Andreas Fault and the Mill Creek Fault. Each of
these basement rock assemblages is characterized by a relatively unique suite of
rocks that was amalgamated by the end of the Cretaceous and (or) early Cenozoic.
Some Tertiary sedimentary and volcanic rocks are unique to specific assemblages,
and some overlap adjacent assemblages. A few Miocene and Pliocene units cross
the boundaries of adjacent assemblages, but are dominant in only one. Tectonic
events directly and indirectly related to the San Andreas Fault system have
partly dismembered the basement rocks during the Neogene, forming the modern-day
physiographic provinces.
Rocks of the four basement rock assemblages are divisible into an older suite of
Late Cretaceous and older rocks and a younger suite of post-Late Cretaceous rocks.
The age span of the older suite varies considerably from assemblage to assemblage,
and the point in time that separates the two suites varies slightly. In the
Peninsular Ranges, the older rocks were formed from the Paleozoic to the end of
Late Cretaceous plutonism, and in the Transverse Ranges over a longer period of
time extending from the Proterozoic to metamorphism at the end of the Cretaceous.
Within the Peninsular Ranges a profound diachronous unconformity marks the
pre-Late Cretaceous-post-Late Cretaceous subdivision, but within the Transverse
Ranges the division appears to be slightly younger, perhaps coinciding with the
end of the Cretaceous or extending into the early Cenozoic. Initial docking of
Peninsular Ranges rocks with Transverse Ranges rocks appears to have occurred at
the terminus of plutonism within the Peninsular Ranges. During the Paleogene
there was apparently discontinuous but widespread deposition on the basement rocks
and little tectonic disruption of the amalgamated older rocks. Dismemberment of
these Paleogene and older rocks by strike-slip, thrust, and reverse faulting began
in the Neogene and is ongoing. The Peninsular Ranges basement rock assemblage is
made up of the Peninsular Ranges batholith and a variety of metasedimentary rocks.
Most of the plutonic rocks of the batholith are granodiorite and tonalite in
composition; primary foliation is common, mainly in the eastern part. Tertiary
sedimentary rocks of the Los Angeles Basin crop out in the Puente and San Jose
Hills along with the spatially associated Glendora Volcanics; both units span the
boundary between the Peninsular Ranges and San Gabriel Mountains basement rock
assemblages.
The San Gabriel Mountains basement rock assemblage includes two discrete areas,
the high standing San Gabriel Mountains and the relatively low San Bernardino
basin east of the San Jacinto Fault. The basement rock assemblage is
characterized by a unique suite of rocks that include anorthosite, Proterozoic
and Paleozoic gneiss and schist, the Triassic Mount Lowe intrusive suite,
extensive deformed and undeformed Cretaceous granitic rocks, the Pelona Schist,
and Oligocene granitic rocks. Internal structure of the assemblage includes the
Vincent Thrust Fault, at least two old, abandonded segments of the San Andreas
Fault system, and extensive areas of well-developed to pervasively mylonitized
rocks.
The main body of the San Gabriel Mountains is bounded on the north by the San Andreas
Fault and on the south by the Sierra Madre-Cucamonga Fault Zone. East of the San
Jacinto Fault, the San Bernardino basin is an asymmetric pull-apart basin bounded by
the San Andreas Fault on the east, and underlain by many of the same rock units that
characterize the San Gabriel Mountains. Cretaceous and older rocks of the San Gabriel
Mountains basement rock assemblage are divided into two structurally and lithologically
distinct groups by the Vincent Thrust Fault, a regional, low-angle thrust fault that
predates intrusion of Oligocene granitic rocks. The Vincent Thrust separates the
Mesozoic Pelona Schist in its lower plate from highly deformed gneiss, schist, and
granitic rocks in the upper plate. The fault, along with its far-offset, dismembered
analogs in the Orocopia and Chocolate Mountains east of the Salton Sea, may underlie
much of southern California.
Crystalline rocks between the Mill Creek Fault and the main trace of the San
Andreas Fault Zone range from highly deformed gneiss of unknown age to relatively
undeformed Mesozoic biotite-hornblende diorite. They are overlain by Miocene
sedimentary rocks and cut by the Wilson Creek Fault, that is considered to be an
older segment of the San Andreas Fault system. Crystalline rocks of this basement
assemblage are similar to rocks in the Little San Bernardino Mountains to the
southeast, and appear to have been displaced about 50 km by the Wilson Creek and
Mill Creek Faults.
About 80 to 85 percent of the San Bernardino Mountains bedrock assemblage in the
quadrangle is Mesozoic granitic rocks, and the rest, highly metamorphosed and deformed
Late Proterozoic and Paleozoic metasedimentary rocks. There is a pronounced gradient
from east to west, and to a slightly lesser degree from south to north, in the
magnitude of both deformation and metamorphism of the Late Proterozoic and Paleozoic
metasedimentary rocks. In addition to the east to west gradient of increasing
metamorphism and deformation, east of the quadrangle there appears to be a sharp break
between highly deformed and relatively undeformed Late Proterozoic and Paleozoic rocks.
Late Proterozoic and Paleozoic units comprise a thick sequence of metasedimentary
rocks generally consisting of a lower quartzitic sequence and an upper carbonate
rock sequence. The entire lower quartzitic part is Late Proterozoic and Early
Cambrian, and includes the Stirling Quartzite, Wood Canyon Formation and Zabriskie
Quartzite; the upper carbonate rock sequence includes the Cambrian Carrara and
Bonanza King Formations, the Devonian Sultan Limestone, the Mississippian Monte
Cristo Limestone, and the Pennsylvanian Bird Spring Formation.
Mesozoic intrusive rocks in the San Bernardino Mountains and southern Mojave
Desert include numerous Triassic and Jurassic plutons. The Triassic rocks are
relatively alkalic and quartz deficient, and contrast with the voluminous,
quartz-rich, calc-alkalic Cretaceous granitic rocks, which make up the largest
part of the San Bernardino Mountains assemblage. The voluminous tonalitic rocks
in the San Gabriel Mountains and Peninsular Ranges assemblages are essentially
absent in the western San Bernardino Mountains. Many areas of dominantly
Cretaceous granitic rocks are mapped as Mesozoic mixed-rock units, because they
are extremely heterogeneous, and include large components of older rocks.
The relatively young, active San Andreas Fault system is by far the dominant
structure in the San Bernardino quadrangle. Based on offsets of many of the rock
units found in the San Bernardino quadrangle, different amounts of lateral
displacement have been proposed for the San Andreas Fault system within and south
of the Transverse Ranges. The Neogene evolution of the Transverse Ranges
Province, and its relationship to the San Andreas Fault system in particular, are
complicated by several abandonded segments and the shifting locus of the fault
during the late Cenozoic. Most recent structural interpretations require
relatively large rotations within the Transverse Ranges Province.
Other active faults in the quadrangle include the San Jacinto Fault and the
reverse faults bounding and within the Transverse Ranges. Older faults considered
to be abandoned segments of the San Andreas Fault system include the San Gabriel
Fault, Punchbowl Fault, Mission Creek Fault, and Wilson Creek Fault. The Vincent
Thrust and Squaw Peak Fault are both older faults, the Vincent probably of late
Mesozoic to early Tertiary age.