Abstract:
The area discussed in this report, consisting of sixteen square miles, embraces a portion of the San Jose Hills approximately three miles south of the town of Covina, Los Angeles County, California. The area was mapped on a base map prepared from U. S. Geological Survey topographic maps, and the Brunton compass-pacing method of mapping was used.
The region is part of an upland that rises above the Los Angeles and San Gabriel basins, and consists of a group of rolling hills trending in an approximate east-west direction. The relief throughout the region is moderate, and as a result of the semi-arid climate vegetation in the area is sparse. Bedrock is well exposed, excepting in some of the areas underlain by shale, where slumping has distorted the rocks and soils and a dense grass growth further hinder exposure of the underlying formations.
All of the rocks exposed in the area are of sedimentary origin, and, with the exception of Recent and Pleistocene alluvium, are part of the Puente formation of upper Miocene age. The Puente formation is divided into three members--a lower member of shale, a middle member of sandstone and conglomerate, and an upper member of shale, sandstone, and conglomerate. The subsurface rocks, knowledge of which has been derived from wells drilled in the area, consist of the Topanga formation, the Glendora volcanics, the Mountain Meadows dacite porphyry, and the basement complex in that order with increasing depth.
There appear to have been two major periods of deformation in the San Jose Hills area during Tertiary time--one at the close of the Pliocene and one during and after the deposition of the Miocene Puente formation. The Puente deformation seems to have involved only gentle folding, but the post-Pliocene deformation was more severe, involving steep folding and some faulting. The general structural trend of the region is approximately N 60 E, and is parallel to the boundaries of the higher hills. Folding in the area has created a series of parallel anticlines and synclines, with the San Jose anticline in the northern half of the area being the major structure. The one fault of any magnitude in the area is the San Jose fault, which enters from the east and apparently dies out in the central part of the area. It is a vertical or steeply dipping reverse fault with considerable downthrow on the south side. There are other smaller faults in the southern half of the area.
The geologic history of the region largely involves erosion after the intrusion of the basement complex in Mesozoic time until the Miocene period, when the area became one of deposition. The Glendora volcanics were deposited in early middle Miocene time, followed by submergence of the area and deposition of the Topanga, Puente, and Pliocene formations. After the close of the Pliocene the region emerged from beneath the sea and again became an area of erosion.Keywords:
Conglomerate
Slumping
Basement
Bedrock
The west end of the San Gabriel Mountains is a relatively small block of pre-Cretaceous metamorphic complex faulted from the main mass of the San Gabriel Mountains by the San Gabriel fault. Their core of metamorphic rocks is flanked on three sides by Tertiary sediments represented by Eocene, Pliocene, and Pleistocene formations. The Eocene and Pliocene are characteristic off-shore and littoral marine deposits. The Pleistocene is principally of terrestrial origin.
The area is one of structural complexity. This can be accounted for, in part, by assuming that this end of the mountain block acted as a centre of rotation for north-south compressional forces that were active to the west.
The San Gabriel Range is believed to be a fault block, raised to its present elevation principally by movements along faults which parallel the north and south margins. The faulting is not restricted to the extreme margins, but often is located within the range itself.
The west slope of the range is characterized by depositional relations between the sediments and the underlying mountain mass. If any faulting has occurred, it is thought to be farther west than the contact between the sediments and the basement complex and to be concealed beneath the younger sediments.
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The San Gabriel fault zone, which trends northwesterly subparallel with the San Andreas fault for about 90 miles, appears to have a post-late Miocene right-lateral displacement of 15-25 miles. Southwest of this nearly vertical fault, 6 miles northwest of Castaic, Los Angeles County, upper Miocene coarse conglomerates grade southwest into finer sediments from their source, now disappeared, across the fault on the northeast. The conglomerates, which contain boulders of anorthosite and norite, were derived from a near-by basement terrane containing these rock types. A buried source is eliminated because at present, so far as known, all basement rock across the San Gabriel fault from the conglomerates for several miles northeast is covered by sedimentary rocks older than the onglomerates. Basement types exposed still farther northeast do not include anorthosite and norite. In fact, the only in situ occurrence of these rock types known in the entire region is in the San Gabriel Mountains 23 miles southeast of the conglomerates and on the other (northeast) side of the San Gabriel fault zone. It thus seems clear that this was their source area and that the conglomerates have been displaced 15-25 miles relatively northwest. Large right-lateral displacement is also suggested by the presence of upper Miocene sedimentary breccia now found northeast of the fault about a mile northwest of Castaic. This breccia, containing blocks of gneiss almost exclusively, has been offset from its source now at least 6 miles but probably 15 miles or more northwest. Again, sediments older than the breccia across the fault from its present exposure preclude noteworthy dip-slip displacement.
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Ridge Basin is a graben dropped between the San Gabriel and the San Andreas faults as a result of local tension or differential pressure where the Garlock and the San Andreas rift faults meet and westward movement causes the latter to be bent into a great arc. Following the inception of the graben in or about Middle Miocene time its point sank with unprecedented rapidity, forming an inland lake in which 21,000 feet of Pliocene and Lower Pleistocene(?) sediments were deposited to comprise the thickest post-Miocene section in California. Resting with marked unconformity on Eocene strata older than the basin is the basin sequence; reading upward, fragments of non-marine Mint Canyon (early Upper Miocene) yielding Hipparion, Merychippus, Protohippus, and Parahippus(?) near-by, then 2,000 feet of upper Monterey (late Upper Miocene) yielding a Neroly marine macrofauna at the base and grading upward to brackish strata, then 18,000 feet of Pliocene(?) lacustrine deposits more or less brackish at the bottom, and finally 3,000± feet of Lower Pleistocene(?) lacustrine beds. The 23,000-foot post-Mint Canyon succession is continuously exposed as a northwest-plunging syncline between Castaic and Gorman, the axis of which is essentially the course of the new Ridge Route highway. The southwest edge of the succession is everywhere angular conglomerate deposited along the San Gabriel fault scarp. The conglomerate grades northeasterly to shale, which, in turn, grades back to pebbly sandstone farther northeast. The vertical throw of the San Gabriel fault which bounds the basin on the southwest scissors near Castaic, and then increases from zero at the pivot to 23,000± feet 20 miles northwest. The particular throw occurred between Middle Miocene and Middle Pleistocene times concomitant with deposition. Following this extreme activity the deep-seated fault plane froze, and has now been essentially dead for a million or so years between Castaic and Beartrap Canyon. After deposition ceased the basin was deformed and degraded approximately 15,000 feet in the Coast Range revolution of Middle and Upper Pleistocene time. During this interval crystalline Frazier Mountain was dragged or thrust across the extreme northwest point of the basin from an original site computed to have been east of the junction of the Garlock and San Andreas rift faults, thus causing piracy of drainage systems to occur on a grand scale, and giving to Piru Creek its present anomalous upper course.
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Research Article| May 01, 1947 GEOLOGIC SECTION FROM THE SIERRA NEVADA TO DEATH VALLEY, CALIFORNIA RICHARD H HOPPER RICHARD H HOPPER KEBON SIRIH 52, BATAVIA, JAVA, NETHERLANDS INDIES Search for other works by this author on: GSW Google Scholar GSA Bulletin (1947) 58 (5): 393–432. https://doi.org/10.1130/0016-7606(1947)58[393:GSFTSN]2.0.CO;2 Article history received: 02 Jun 1939 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RICHARD H HOPPER; GEOLOGIC SECTION FROM THE SIERRA NEVADA TO DEATH VALLEY, CALIFORNIA. GSA Bulletin 1947;; 58 (5): 393–432. doi: https://doi.org/10.1130/0016-7606(1947)58[393:GSFTSN]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This paper describes the geology of a part of the region between the southern Sierra Nevada and Death Valley, one of the most rugged portions of the Great Basin. The topographic features of the mountain ranges in this region strongly suggest that each range owes its present height to faulting. In the western half of the area mapped, displacements of extensive basalt flows of probable early Pleistocene age support the topographic evidences of faulting. On the summits of the ranges are areas of low relief, believed to be remnants of a single old-age erosion surface which extended across the entire region before the beginning of the range-forming fault movements. The undisturbed erosion which produced this surface ended shortly before the deposition of the fossiliferous late Pliocene or early Pleistocene Coso formation; therefore, this surface is tentatively correlated with the Ricardo erosion surface of the Mohave Desert region, which bevels tilted early Pliocene strata and which is also dislocated by range-forming faults.The ranges are composed dominantly of pre-Tertiary rocks. The pre-Cambrian metasediments, chiefly mica schists and dolomites, have an exposed thickness of 15,000 feet. Limestones, dolomites, shales, and quartzites of Paleozoic age are more than 30,000 feet thick, and fossils collected in them indicate the probable presence of all the Paleozoic systems. During the late Jurassic Nevadian orogeny the pre-Mesozoic rocks were folded, faulted, and intruded by plutonic bodies ranging from granite to gabbro. All the post-Mesozoic rocks are believed to be Miocene or younger; they include a wide variety of volcanic and sedimentary types.Most of the faulting to which the region owes its present relief occurred in the early or middle part of the Pleistocene, probably after the first (McGee) glacial stage in the Sierra Nevada. The activity on one of the major fault zones, however, has continued into the Recent epoch. All the range-forming faults whose attitudes could be determined are high-angle normal faults. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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The area discussed is in the Le Brun and Mint Canyon Quadrangle which in turn form the southwest corner of the Elizabeth Lake Quadrangle. This area lies about 45 miles north west of Los Angeles and comprises a part of Sierra Pelona Ridge, Sawmill and Jupiter Mountains. These mountains form a portion of the transverse ranges.
The Pelona schists make up about two-thirds of the area, and are probably of Archeozoic age. A series of migmatite forming inclusions in the granitic country rock are probably of Pre-Cambrian age also.
These old metamorphics were intruded during Jura-Cretaceous time by batholith whose average composition is that of a monzonite although different facies of it vary from dioritic to granitic.
The Martinez formation was deposited in lowermost Eocene time and is made up of 9000+ feet of sandstones, shales and a few intercalated conglomerate beds. These are marine sediments.
Between Martinez and Mint Canyon times a thick series of fanglomerates, sands, silts, and muds were laid down in local basins. These continental beds are red in color, and make up the Le Brun formation and Vasquez series. Although this series contains no lava in this area, it contains large thicknesses of basic lava south and east of this locality.
The Mint Canyon formation is also continental in origin and lies on the truncated edges of the Vasquez series. It is composed of a basal conglomerate overlain by well bedded sandstones and shales.
Terrace materials of two different ages can be recognized. In addition there is reason to believe that this region was pen plained toward the end of Pliocene time.
The strata of the above formations strike east-west or slightly southwest-northeast, and with the exception of the Mint Canyon formation they dip very steeply, in places standing vertical. The foliation of the metamorphic rock also has the same general strike end steep dip of the sediments. The Mint Canyon beds dip off relatively gently to the south.
Faulting has been very active in this region and has taken place from middle Miocene time, or perhaps earlier, to the recent. The San Andreas Rift, which passes along just to the north of this area, provides the key to the structural history of the region. Adjacent to the San Andreas Rift large wedges of basement complex shoved up and over the Martinez formation and the Pelona schist, while the faults south of Sierra Pelona Ridge are of the normal type they are not tensional, but compressional faults due to the large horizontal displacement which has taken place along them. Compressional effects such as these are typical along the San Andreas Rift. Folding has played but a minor role in this area.
South of the Sierra Pelona Ridge, long slim wedges of igneous and metamorphic rock have been faulted into the Vasquez series; these horses are found sometimes half a mile from the nearest outcrop of igneous or metamorphic rock.
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The area described covers approximately ten square miles
in the Lang and Humphreys quadrangles, twenty-seven miles northwest of Pasadena. It comprises part of the drainage of Mint Canyon immediately southeast of the area previously described by R. H. Jahns, and continues eastward across Tick Canyon, west of an area previously described by R. P. Sharp.
A series of pre-Cretaceous gneisses outcrops in the northern part of the area as an uplifted fault block. Immediately to the south of the fault lies the Vasquez series, of doubtful Oligocene age. In this area, it is composed of two sections of coarse, colorful sandstones and gypsiferous shales, alternated with two thick flows of basaltic lava, the total thickness being about 3000 feet. Unconformably overlying the upper lava flow is the Miocene series. This series was deposited in a broad basin. The initial basinward dips have been accentuated by slight folding at a later date. The lower part of this series was named the Tick Canyon formation by Jahns. He described a section west of Mint Canyon, unconformably resting on the basin complex, which pinched out eastward. In this paper, the Tick Canyon formation is described from a section
measured in Tick Canyon. The map shows that it does not entirely disappear at any point, east of Jahns' area, except where it is cut off by a series of strike slip cross faults. The Tick Canyon formation is principally composed of green sandstones, reddish siltstones, and some conglomerates, with a total thickness of over 1000 feet. Overlying the Tick Canyon formation is the Mint Canyon formation; however, no clear evidence could be found in this area for an unconformity between the two formations, as reported by Jahns. The Mint Canyon formation is composed of continental deposits: light colored pebble to boulder conglomerates roughly interbedded with light sandstones and reddish siltstones. The total thickness of the Mint Canyon formation outcropping in the area mapped is about 3500 feet. The section is thicker than that in the area mapped by Jahns, possibly because it received coarser sediments more rapidly near the margin of the basin. A Pleistocene erosion surface, with accompanying terraces is shown on a separate map.
There is a normal fault of large but unknown displacement between the Tertiary sediments and the metamorphic complex. Several branches of this fault run into the sediments, but gradually die out to the southwest in the Mint Canyon formation. The largest branch, which causes a considerable horizontal displacement of the sediments at its northern end (west of Mint Canyon), gradually decreases in displacement to the southwest where it passes into an anticline. There are a number of northwest trending, normal, cross faults of relatively minor importance. Accelerated uplift of the San Gabriel mountains in late Miocene time is indicated by the increasing dominance of San Gabriel intrusive material in the upper members of the Mint Canyon formation. Middle Pleistocene uplift of the whole area is indicated by the stream terraces, but there is no evidence of recent faulting in this particular area.
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Research Article| July 01, 1971 Geology of Central San Clemente Island, California PAUL M MERIFIELD; PAUL M MERIFIELD Lamar-Merifield, Geologists-Geophysicists, 1318 Second Street, Suite 27, Santa Monica, California 90401 Search for other works by this author on: GSW Google Scholar D. L LAMAR; D. L LAMAR Lamar-Merifield, Geologists-Geophysicists, 1318 Second Street, Suite 27, Santa Monica, California 90401 Search for other works by this author on: GSW Google Scholar M. L STOUT M. L STOUT Geology Department, California State College at Los Angeles, Los Angeles, California 90032 Search for other works by this author on: GSW Google Scholar Author and Article Information PAUL M MERIFIELD Lamar-Merifield, Geologists-Geophysicists, 1318 Second Street, Suite 27, Santa Monica, California 90401 D. L LAMAR Lamar-Merifield, Geologists-Geophysicists, 1318 Second Street, Suite 27, Santa Monica, California 90401 M. L STOUT Geology Department, California State College at Los Angeles, Los Angeles, California 90032 Publisher: Geological Society of America Received: 11 Feb 1971 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (7): 1989–1994. https://doi.org/10.1130/0016-7606(1971)82[1989:GOCSCI]2.0.CO;2 Article history Received: 11 Feb 1971 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PAUL M MERIFIELD, D. L LAMAR, M. L STOUT; Geology of Central San Clemente Island, California. GSA Bulletin 1971;; 82 (7): 1989–1994. doi: https://doi.org/10.1130/0016-7606(1971)82[1989:GOCSCI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Central San Clemente Island is underlain primarily by nonmarine volcanic rocks. A 1,200-ft core hole, drilled near sea level on the west coast of the island, penetrated a homogeneous sequence of basaltic andesite flows varying in thickness from 11 to 169 ft. Whole-rock potassium-argon dates of samples taken near the top and bottom of the core hole indicate that the cored sequence was extruded in less than 1 m. y. during Miocene time. Unconformably overlying the andesitic flows are dacitic flows reaching a total thickness of about 300 ft. A distinctive volcanic breccia, which is in part of sedimentary origin, is commonly present at the base of the dacites. Miocene sediments and Quaternary beach sands overlie the volcanic rocks.The Tertiary rocks are folded into a northwest-trending anticline. The axis of the anticline corresponds approximately to the topographic crest of the island located about .5 mi inland from the eastern shoreline. North-northeast to northeast-trending faults cut the Miocene rocks but do not displace the prominent surf-cut terraces. Striations within well-exposed fault zones indicate that most movement has been oblique, with a principally horizontal component. The faults may be left-lateral, secondarily related to major right-slip movement along the northwest-trending San Clemente fault. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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With the exception of the Pleistocene terrace deposits, all of the sedimentary rocks exposed in the San Joaquin Hills are Tertiary in age. The Tertiary section has a thickness of about 5,000 feet and is correlated with rocks of Sespe, Vaqueros, Topanga, San Onofre, and Puente formations. The dip of strata is thought to be largely due to the action of normal faulting, rather than folding. Basic lavas are found intruded as dikes along or near the planes of many of the larger faults. While the age relationship between the faulting and the intrusion is not clear, it is thought that they are in part contemporaneous and that both are, at least, pre-Pleistocene in age. It is suggested that the present topography of the hills has resulted from a gentle northward tilting of the ar a occurring in Pleistocene and Recent time End_of_Article - Last_Page 1519------------
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Abstract Late Miocene strata occur on both sides of the San Andreas fault for several hundred miles in central California. The unlike facies and contrasting paleogeographic situations which are found across the fault along much of this distance must have been brought together by large-scale strike-slip movement on the fault. Clastic deposits west of the San Andreas need granitic-volcanic sources east of the fault, but the only such sources are to the southeast - in one probable case, more than 110 miles away. Likewise, voluminous coarse conglomerate beds east of the fault in the southern Temblor Range require the presence of a large, highly elevated granitic and metamorphic source immediately west of the fault during the later upper Miocene. The northern Gabilan Range, about 150 miles (240 km) to the northwest, was probably the only such basement area exposed at the time; the granitic and metamorphic bedrock here is very similar to the type of gravel found within the conglomerate of the southern Temblor Range. Thus, the Temblor Range clastic rocks were deposited adjacent to the Gabilan bedrock, and the two areas subsequently have been offset by about 150 miles (240 km) of right-slip on the San Andreas fault. The Gabilan source appears to have moved northwestward along the fault during the upper Miocene; together with tentative correlations between parts of the source and parts of the clastic deposits, this suggests post-Miocene offset of 145 ± 5 miles (234 ± 8 km), post-early upper Mohnian offset of 158 ± 3 miles (254 ± 5 km), and thus 50 to 21 miles (8 to 34 km) of slip during the later upper Miocene.
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In San Gorgonio Pass, 70 miles east of Los Angeles, a complex network of faults separates two of the highest mountain ranges of Southern California. The San Andreas fault, which forms a part of the network, exhibits several unusual features in this area. Among these are the absence of rift topography, absence of lateral stream offsets, an abrupt change in trend of the fault trace, seismic evidence for the predominance of thrusting over strike-slip movements, and a lack of great earthquakes in historic time.
Crystalline rocks of Mesozoic and earlier age crop out over most of the map area. North of the pass, the San Gorgonio igneous-metamorphic complex comprises an old metamorphic terrane of intermediate to basic composition and probable igneous parentage, and Mesozoic(?) plutonic rocks of quartz monzonitic composition. These plutonic rocks have intimately intruded and in large part reconstituted the older metamorphic rocks. The resulting migmatitic gneiss is the most widespread rock of the area, and includes flaser gneiss, green-schist, and piedmontite-bearing gneiss as distinctive varietal types. Rocks of the San Jacinto Mountains south of San Gorgonio Pass are distinctly different from those to the north, and comprise texturally uniform granodioritic and tonalitic rocks that contain sparse inclusions and septa of metasedimentary rocks.
Nearly all of the sedimentary rocks in the pass area are of alluvial-fan or flood-plain origin, and they reflect a Quaternary and late Tertiary history of recurrent deformation and deposition. The upper Miocene(?) Coachella fanglomerate is the oldest exposed sedimentary rock, and is overlain with marked angular unconformity by all younger units. Lower Pliocene(?) incursion of tropical marine waters into the Salton trough is represented by a thin stringer of Imperial formation which is conformably underlain and overlain by continental strata of the Hathaway and Painted Hill formations, respectively. All of these rocks are overlain with marked angular unconformity by Quaternary Cabezon fanglomerate, which probably is correlative with upper beds of the Pliocene-Pleistocene San Timoteo(?) formation in the western part of the map area. Other Quaternary deposits, each showing complex structural relationships to adjacent rocks, are the deformed gravels of Whitewater River, Heights fanglomerate, and Burnt Canyon breccia. Recent alluvium covers the floor of the pass. Flows and dikes of olivine basalt occur in the Coachella fanglomerate and Painted Hill formation. Lithology of clasts in the sedimentary rocks indicates derivation predominantly from rocks of the San Gorgonio igneous-metamorphic complex to the north.
Quaternary alluvial fans of Heights and Cabezon fanglomerate, which once buried a former rugged topography, are now being dissected along the north side of the pass. Surfaces of low relief and associated stream terraces resulting from this dissection are the Beaumont plain, Banning Bench, and Pine Bench surfaces. Upstream divergence of the Banning Bench surface from present stream levels is attributed in part to tilting. Farther east, the older Cabezon surface shows many effects of warping. This surface probably is correlative with an area of low relief at altitudes of 6500 to 8000 feet near Raywood Flat, and suggests Quaternary arching of the mountain range along a north-south axis.
Within San Gorgonio Pass, alluvial fans derived from areas to the north dominate those derived from the steeper San Jacinto scarp to the south. This unequal development of fans is attributed to greater flood-producing rainfall and larger drainage area on the north, together with more easily erodable rock in this area. Most of the faults that show Recent movement are well delineated by springs and vegetative contrasts. Other springs are caused by exposure of unconformities, and by superposition of streams onto the rugged pre-Cabezon topography.
The San Andreas fault is a continuous linear feature for a distance of more than 400 miles northwest from San Gorgonio Pass, but within the pass it curves abruptly southward and butts into the east-trending Banning fault at an angle of 45 [degrees] . Recent strike-slip movement on this part of the fault probably amounts to less than one mile, and post-Mesozoic displacement probably has not exceeded a few tens of miles.
The Banning fault, a major break that delineates the north side of the pass, extends for a distance of more than 50 miles eastward from a point near Redlands through the pass into the Coachella Valley. Within the pass, it is a steeply north-dipping reverse fault except for a zone of low-angle thrusting between Millard and Whitewater Canyons. At least 5000 feet of vertical displacement has taken place on this fault since San Timoteo time, and a right lateral offset of 5 miles is suggested. Recent displacement is limited to the segment of the fault east of Millard Canyon. Pre-Pliocene lateral displacement may have been great, but is not demanded by evidence in this area.
The Mission Creek fault branches from the San Andreas fault north of Banning, and is a major north-dipping fracture that is continuous for at least 40 miles to the southeast. The Pinto Mountain fault diverges from the Mission Creek fault at a low angle, and probably is continuous for more than 50 miles to the east; in this interval it forms the southern boundary of the northwest-trending fault system of the Mojave Desert. The Mill Creek fault branches from the San Andreas fault north of San Bernardino, and has guided erosion along deep linear valleys in the high mountains; this fault apparently dies out eastward.
Within the San Bernardino Mountains all of the faults north of the Banning fault separate crystalline rocks of the same family; these rocks are similar in their migmatitic structural features, remnants of amphibolite, intrusion by quartz monzonite, and high content of titanium minerals. Thus post-Mesozoic lateral displacements of hundreds of miles along these faults seem to be precluded. Although lateral displacements of a few tens of miles are possible, no observed evidence appears to demand such movements. Late Tertiary and Quaternary vertical movements are suggested by the physiography of the mountains. No Recent movements have occurred on parts of the Mission Creek and Mill Creek faults.
Recent movements on both the Banning and San Andreas faults probably were caused by a stress system involving a generally north-south maximum principal stress, with an east-west least principal stress of only slightly lesser magnitude than the vertical stress. In the vicinity of San Gorgonio Pass, an older east-west line of weakness causes the east-west stress effectively to become the intermediate stress, so that thrust faulting predominates over strike-slip faulting in this one local area.
San Gorgonio Pass is bounded by a reverse or thrust fault on the north, and indirect evidence suggests a similar fault on the south. Quaternary and late Tertiary displacement on these faults, rather than erosion, is primarily responsible for the present physiography of the pass. Local conversion of San Andreas-type lateral strain into vertical displacements on the bounding faults is a reasonable explanation of both the pass itself and the unusually high peaks adjacent to it.
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