Geologic Map of the Devore 7.5' quadrangle, San Bernardino County, California, USGS, OFR 00-173

The data set for the Devore 7.5' quadrangle was prepared under the U.S.
Geological Survey Southern California Areal Mapping Project (SCAMP) as part of
an ongoing effort to develop a regional geologic framework of southern
California, and to utilize a Geographical Information System (GIS) format to
create regional digital geologic databases. These regional databases are being
developed as contributions to the National Geologic Map Database of the
National Cooperative Geologic Mapping Program of the USGS.

The digital geologic map database for the Devore 7.5' quadrangle has been
created as a general-purpose data set that is applicable to other land-related
investigations in the earth and biological sciences. For example, it can be
used for groundwater studies in the San Bernardino basin, and for mineral
resource evaluation studies, animal and plant habitat studies, and soil studies
in the San Bernardino National Forest. The database is not suitable for
site-specific geologic evaluations.

This data set maps and describes the geology of the Devore 7.5' quadrangle, San
Bernardino County, California. Created using Environmental Systems Research
Institute's ARC/INFO software, the data base consists of the following items:
(1) a map coverage containing geologic contacts and units, (2) attribute tables
for geologic units (polygons), contacts (arcs), and site-specific data
(points). In addition, the data set includes the following graphic and text
products: (1) A PostScript graphic plot-file containing the geologic map,
topography, cultural data, a Correlation of Map Units (CMU) diagram, a
Description of Map Units (DMU), an index map, a regional geologic and structure
map, and a key for point and line symbols; (2) PDF files of this Readme
(including the metadata file as an appendix), Description of Map Units (DMU),
and the graphic produced by the PostScript plot file.

The Devore quadrangle straddles part of the boundary between two major
physiographic provinces of California, the Transverse Ranges Province to the
north and the Peninsular Ranges Province to the south. The north half of the
quadrangle includes the eastern San Gabriel Mountains and a small part of the
western San Bernardino Mountains, both within the east-central part of the
Transverse Ranges Province. South of the Cucamonga and San Andreas Fault zones,
the extensive alluviated area in the south half of the quadrangle lies within
the upper Santa Ana River Valley, and represents the northernmost part of the
Peninsular Ranges Province.

There are numerous active faults within the quadrangle, including right-lateral
strike-slip faults of the San Andreas Fault system, which dominate the younger
structural elements, and separate the San Gabriel from the San Bernardino
Mountains. The active San Jacinto Fault zone projects toward the quadrangle
from the southeast, but its location is poorly constrained not only within the
quadrangle, but for at least several kilometers to the southeast. As a result,
the interrelation between it, the Glen Helen Fault, and the probable
easternmost part of the San Gabriel Fault is intrepretive. Thrust faults of
the Cucamonga Fault zone along the south margin of the San Gabriel Mountains,
represent the rejuvinated eastern end of a major old fault zone that bounds the
south side of the western and central Transverse Ranges (Morton and Matti,
1993). Rejuvenation of this old fault zone, including the Cucamonga Fault
zone, is apparently in response to compression in the eastern San Gabriel
Mountains resulting from initiation of right-lateral slip on the San Jacinto
Fault zone in the Peninsular Ranges.The structural grain within the San Gabriel
Mountains, as defined by basement rocks, is generally east striking. Within the
Devore quadrangle, these basement rocks include a Paleozoic (?) schist,
quartzite, and marble metasedimentary sequence, which occurs as discontinuous
lenses and septa within Cretaceous granitic rocks. Most of the granitic rocks
are of tonalitic composition, and much of them are mylonitic. South of the
granitic rocks is a complex assemblage of Proterozoic (?) metamorphic rocks, at
least part of which is metasedimentary. The assemblage was metamorphosed to
upper amphibolite and lower granulite grade, and subsequently remetamorphosed
to a lower metamorphic grade. It is also intensely deformed by mylonitization
which is characterized by an east striking, north dipping foliation, and by a
pronounced lineation that plunges shallowly east and west.

East of Lytle Creek and west of the San Andreas Fault zone, the predominant
basement lithology is Mesozoic Pelona Schist, which consists mostly of
greenschist grade metabasalt and metagraywacke. Intruding the Pelona Schist,
between Lytle Creek and Cajon Canyon, is the granodiorite of Telegraph Peak of
Oligocene age (May and Walker, 1989). East of the San Andreas Fault in the San
Bernardino Mountains, basement rocks consist of amphibolite grade gneiss and
schist intermixed with concordant and discordant tonalitic rock and pegmatite.
Tertiary conglomerate and sandstone occur in the Cucamonga Fault zone and in a
zone 200 to 700 m wide between strands of the San Andreas Fault zone and
localized thrust faults northeast of the San Andreas. Most of the conglomerate
and sandstone within the Cucamonga Fault zone is overturned forming the north
limb of an overturned syncline. Clasts in the conglomerate are not derived
from any of the basement rocks in the eastern San Gabriel Mountains. Clasts in
the conglomerate and sandstone northeast of the San Andreas Fault zone do not
appear to be locally derived either. The south half of the quadrangle is
dominated by the large symmetrical alluvial-fan emanating from the canyon of
Lytle Creek, and by the complex braided stream sediments of Lytle Creek and
Cajon Wash.

The San Andreas Fault is restricted to a relatively narrow zone marked by a
pronounced scarp that is especially well exposed near the east margin of the
quadrangle. Two poorly exposed, closely spaced, north-dipping thrust faults
northeast of the San Andreas Fault have dips that appear to range from 55? to
near horizontal. These hallower dips probably are the result of rotation of
initially steeper fault surfaces by downhill surface creep. Between the San
Andreas and Glen Helen Fault zones, there are several faults that have north
facing scarps, the largest of which are the east striking Peters Fault and the
northwest striking Tokay Hill Fault. The Tokay Hill Fault is at least in part
a reverse fault. Scarps along both faults are youthful appearing.

The Glen Helen Fault zone along the west side of Cajon Creek, is well defined
by a pronounced scarp from the area north of Interstate 15, south through Glen
Helen Regional Park; an elongate sag pond is located within the park.

The large fault zone along Meyers Canyon, between Penstock and Lower Lytle
Ridges, is probably the eastward extension of the San Gabriel Fault zone that
is deformed into a northwest orientation due to compression in the eastern San
Gabriel Mountains (Morton and Matti, 1993). At the south end of Sycamore Flat,
this fault zone consists of three discreet faults distributed over a width of
300 m. About 2.5 km northwest of Sycamore Flats, it consists of a 300 m wide
shear zone. At the north end of Penstock Ridge, the fault zone has bifurcated
into four strands, which at the northwest corner of the quadrangle are
distributed over a width of about one kilometer. From the northern part of
Sycamore Flat, for a distance of nearly 5 km northwestward, a northeast dipping
reverse fault is located along the east side of the probable San Gabriel Fault
zone. This youthful reverse fault has locally placed the Oligocene
granodiorite of Telegraph Peak over detritus derived from the granodiorite.

The Lytle Creek Fault, which is commonly considered the western splay of the
San Jacinto Fault zone, is located on the west side of Lytle Creek. Lateral
displacement on the Lytle Creek Fault has offset parts of the old Lytle Creek
channel; this offset gravel-filled channel is best seen at Texas Hill, near the
mouth of Lytle Creek, where the gravel was hydraulic mined for gold in the
1890s.

The Cucamonga Fault zone consists of a one kilometer wide zone of northward
dip-ping thrust faults. Most splays of this fault zone dip north 25 to 35.

The geologic map database contains original U.S. Geological Survey data
generated by detailed field observation and by interpretation of aerial
photographs. This digital Open-File map supercedes an older analog Open-File
map of the quadrangle, and includes extensive new data on the Quaternary
deposits, and revises some fault and bedrock distribution within the San
Gabriel Mountains. The digital map was compiled on a base-stable cronoflex
copy of the Devore 7.5 deg. topographic base and then scribed. This scribe
guide was used to make a 0.007 mil blackline clear-film, from which lines and
point were hand digitized. Lines, points, and polygons were subsequently
edited at the USGS using standard ARC/INFO commands. Digitizing and editing
artifacts significant enough to display at a scale of 1:24,000 were corrected.
Within the database, geologic contacts are represented as lines (arcs),
geologic units as polygons, and site-specific data as points. Polygon, arc,
and point attribute tables (.pat, .aat, and .pat, respectively) uniquely
identify each geologic datum.