The basement rocks of the eastern San Gabriel Mountains
have been the subject of many studies in the last several decades. The
Pelona Schist in this region is disrupted by both NE-striking and NW-striking
fault systems that intersect one another. The nature of intersections of
such faults in the eastern San Gabriel Mountains is enigmatic. Does one
set of faults turn and blend into the other, or does one set of faults
truncate and post-date the other? Detailed geologic mapping was conducted
at one such intersection in the North Fork of Lytle Creek where three faults
are in proximity to the Pelona Schist and Tertiary granite in the hopes
of constraining motion along the faults. Structure and stratigraphy of
the Pelona Schist and the intrusive contact of Tertiary granite were used
to palinspastically reconstruct a crude, Late Miocene paleogeology with
respect to these mapped rock units. Since 5-2 Ma motion along the San Antonio
Canyon fault was constrained to 2.9 km based on alignment of a metabasalt
layer and two post-metamorphic arches within the Pelona Schist, as well
as alignment of the San Gabriel and Icehouse Canyon faults. The NE-striking,
left-lateral motion of San Antonio Canyon fault is hypothesized to predate
movement on the NW-striking, right-lateral Scotland, San Jacinto, and Glen
Helen faults, where Quaternary slip was constrained to 1.9 km, 5.9-8.1
km, and 4.4 km, respectively. Testing of the hypotheses embodied in the
reconstruction is possible by further field mapping and investigation.
I would like to thank Jonathan Nourse for recommending this project to me, accompanying me in the field for strenuous geologic mapping on occasion, and offering expert advice with field work and manuscript preparation. Had it not been for his support and enthusiasm, I probably would have chosen a more accessible study area and denied myself the challenges I feel privileged to have met. Many thanks to the USDA Forest Service, Lytle Creek Ranger District for allowing easy access to the study area by furnishing a key to the locked gate and for granting permission to conduct field studies on Forest Service land.
LOGISTICS AND PHYSIOGRAPHY
Geologic map of the North Fork of Lytle Creek
Foliations and large-scale folds
Stratigraphy deduced from gross foliation patterns
DISCUSSION AND INTERPRETATION
SUMMARY OF EXISTING REGIONAL MAPPING
Structure of the Pelona Schist
Folding styles within the Pelona Schist
Metamorphism and the Vincent Thrust
Late Cenozoic faults
Cucamonga fault zone
San Antonio Canyon and other NE-striking faults
San Andreas, San Jacinto, and related fault zones
Restoration of right-lateral faults
Restoration of left-lateral faults
Problems in the reconstructions
Field foliation data used in stereonet plot
The Pelona Schist is a dominant rock type of the eastern San Gabriel Mountains that has been the subject of many studies in the last few decades. Geologic mapping by Dibblee (1971), Morton (1981), Ehlig (1957, 1975), Nourse (unpublished mapping, 1986-97), Jacobson (1983), LaMascus (unpublished mapping, 1991), and Jones (unpublished mapping, 1993) has contributed much to the basic understanding of the origin, structure, and stratigraphy of the Pelona Schist and associated geologic features, such as faults and other cross-cutting intrusions. The Pelona Schist consists of oceanic rocks that were metamorphosed during Late Cretaceous-early Tertiary time in a subduction zone, later to be called the Vincent thrust fault, that since has been exhumed (Jacobson et al., 1996). In the eastern San Gabriel Mountains the Pelona Schist is disrupted by two Late Cenozoic fault systems that intersect one another. One system is east-to northeast striking with left-lateral slip and contains the San Antonio and associated faults. A second is the northwest striking, right-lateral slip system of faults of the San Andreas, San Gabriel, and San Jacinto fault zones. The location of this study is at one of these intersections.
Figure1. Location map (modified from Morton and Matti, 1993).
The study site is a 1.6 km by 4.8 km area of Pelona
Schist (Fig.1) bordered on three sides by faults: the San Antonio Canyon
fault to the west-northwest, the San Jacinto fault zone to the north-northeast,
and the Scotland fault to the south-southwest. The left-lateral motion
of the San Antonio Canyon fault appears to be truncated by large, right-lateral
offset on the San Jacinto and San Andreas fault system. As shown on the
1:250,000 San Bernardino Quadrangle, this area was previously unmapped
in detail (perhaps due to the dense vegetation overgrowth, or that for
many years, the surrounding area was a shooting range!). Thus, medium-to
large-scale geologic mapping (1:12,000) of Pelona Schist structure and
stratigraphy was the primary objective of this study. A second, and possibly
more important, objective of this project is what the study area can tell
us about regional fault reconstruction. That is, do the northeast striking
faults turn and blend into the northwest striking faults (as suggested
by Morton and Matti, 1993)? Or, do the northwest striking faults truncate
the northeast striking faults, or vice versa? Study of potential piercing
points in this area gives insight to the kinematic evolution of such fault
The study area is located on the southern side of the North Fork Lytle Creek Drainage. Field work was conducted over sixteen days between May, 1996 and March, 1997. The drainage has a paved access road that gave access to a public shooting range located about a mile up the road where the pavement ends. The shooting range was closed by the Forest Service a couple of years ago in the interest of public safety. As the area is located on Forest Service land, the Lytle Creek Ranger District granted permission to conduct field studies here and a key was issued to open the locked gate restricting access.
The mapped area consists of two main blocks or domains
of Pelona Schist (Fig. 7). The northwestern domain is separated from the
southeastern domain by a large, northeast trending drainage. The terrain
is steep, rugged, and covered with areas of dense vegetation, all of which
made mapping difficult, and sometimes, impossible in certain areas. This
vegetation included manzanita and sticker bushes with heights of up to
6 m. Mapping traverses had to be planned that maximized mappable outcrop
while avoiding impenetrable vegetation. Such traverses covered distances
of up to 5 km (one-way), vertical climbs of up to 1 km, and slopes with
gradients up to 31 degrees. Mapping was accomplished during cool temperatures
of the year. Occasionally, the ground was covered with snow, but that did
not prevent mapping as the majority of the mapping was of distinct outcrops.
Other than the vegetation problem, the only other irritation was the increased
presence of ticks after the thawing of snow in the Spring.
Rock type symbols are shown in parentheses as identified in Figure 2 and Plate 1, Geologic Map of North Fork Lytle Creek, California.
The Pelona Schist is the dominant rock type of the study area. Mapped subunits of the Pelona Schist are named for their likely protolith as used by other workers. Pelona Schist is a Late Cretaceous-early Tertiary (Ehlig, 1968) well-foliated, greenschist facies, metamorphic rock. The most abundant subunit in the area is a gray, well-foliated, albite-quartz-muscovite schist (mg) with common accessory minerals of chlorite, epidote, and graphite. The albite is usually porphyroblastic with grain sizes up to 2 mm. Gray schist comprises about 60 percent of the map area. The suspected protolith is a marine turbidite assemblage of immature sandstone, siltstone, and shale. Finer-grained protoliths form muscovite-chlorite-graphite schists or phyllites. A weakly foliated, greenish gray quartz-feldspar meta-sandstone with microscopic muscovite is also present in the area near gray schist rocks. This is a sandstone form of graywacke.
Metabasalt (mb) is a greenschist rock that accounts for about 10 percent of the study area. This metabasalt is a foliated, green to black chlorite-epidote-actinolite-albite schist. The presence of fine-grained chlorite, epidote, and actinolite gives the rock its dark green color and contributes to the schistose texture. Rounded albite grains, 1-4 mm in size, occur as porphyroblasts.
Minor amounts of metachert (mc) are found in the area, usually in association with metabasalt. This rock is ferromanganiferous in nature and ranges from almost a pure, non-foliated quartzite to a well-foliated quartz schist. In both cases a high quartz content (60-90 percent) is common. It can be a dark gray rock with lighter white or gray bands that represent original sedimentary textures. Bands may occur as 1-5 mm laminations. The ferromanganiferous minerals may give it a red, brown, or purple color. Metachert is thought to have originated from impure cherts (Jacobson, 1983).
A single, 1-2 m thick, rock layer of a foliated, calcium carbonate-rich schist was also found interlayered with metagraywacke and metabasalt. Foliation or layering occurs in millimeter to 1 cm bands. I am calling the rock a reddish brown, calc-silicate schist (cs).
Mylonitic rocks (myl) occur in the upper plate above the Vincent thrust fault. In the map area, these rock are a dirty yellow to brown, cataclastically deformed, quartz diorite gneiss. Quartz and plagioclase crystals, 2-5 mm in size, occur as stretched and sheared porphyroblasts. Structural characteristics of this unit were studied by LaMascus (1991) and Nourse (1991).
Middle Tertiary granitic rock (Tgr) occurs as a pluton and as intrusive dikes and sills into country rock, usually Pelona Schist, but also the mylonitic rock of the Vincent thrust fault. These granitic rocks range in composition from quartz monzonite through granite to granodiorite and in texture from a medium-grained granite to a rhyolite porphyry. The granite is a fine- to medium-grained, leucocratic hornblende granite with grain sizes of 1-4 mm. The rock has been referred to as Telegraph Peak granite by other workers, probably due to its abundance in proximity to Telegraph Peak. This rock has been dated at 26 Ma (Walker and May, 1986). The rhyolite or quartz latite porphyry is a gray to gray-green rock with quartz and feldspar phenocrysts up to 5 mm in size and minor amounts of fine-grained hornblende crystals. Granitic rocks account for up to 30 percent of the study area.
Middle Miocene mafic intrusive rock (Ta) of various andesitic and dioritic compositions and textures occurs as dike or sill intrusions into older rock. Andesite compositions include three varieties: a dark gray, fine-grained andesite with no apparent phenocrysts, a rock with a light gray groundmass and small, acicular phenocrysts of hornblende 1-5 mm long (hornblende andesite), and a blue-gray groundmass with plagioclase phenocrysts 1-5 mm long (plagioclase andesite). Minor amounts of quartz diorite with equant grains, 1-3 mm in size, of quartz, plagioclase, and hornblende are present. The naming of andesite versus quartz diorite is largely by textural comparison as both names represent the same chemical compositions. These rocks invariably intrude the Late Oligocene Telegraph Peak granite and associated sills and are brittly faulted by various strands of the San Antonio fault system (Hazelton and Nourse, 1994).
Quaternary rock types mapped in the study area include alluvium, talus, and landslide materials. Quaternary talus (Qt) covers much of the area of the lower slopes below outcrops. Most of the talus mapped in the area originates from cobble-sized, angular fragments of Pelona Schist, granitic rocks, and mafic intrusive rocks (in order from most to least abundant, respectively). Much of the talus shows fresh fracturing and a lack of lichen to indicate recent deposition. Quaternary landslide deposits (Qls) shown on the northwest corner of Plate 1 are inferred to be of Pelona Schist origin (compiled from mapping by Jones, 1993).
Quaternary alluvium (Qal) fills the drainage channels of the mapped area to varying degrees. This alluvium is up to 800 m wide and 150 m thick where Lytle Creek goes underground. The rounded particles range in size from silt to boulders, but the vast majority lies in the cobble to small boulder sizes. All of the rock types of the area are represented in the alluvium, but each drainage branch contains different rock types depending on the composition of parent rock material being eroded. The alluvium of Pelona Schist origin is more elongate and flat-shaped than alluvium of granitic origin which is more spherical.
Field mapping concentrated on the dominant structures of the Pelona Schist in the map area. As the Pelona Schist has likely undergone several generations of folding and transposition during metamorphism, primary bedding plane features are seldom seen in the field. However, axial-planar schistosity is well developed and easily seen in the Pelona Schist. Thus, primary field data included 280 axial-plane foliation attitudes--115 attitudes from the southeastern domain and 165 attitudes from the northwestern domain. Spatial variations in these attitudes define several(?) large-wavelength open folds that record late stage, post-metamorphic deformation. Domains of uniformly dipping schistosity are also helpful in constraining stratigraphic thickness and locating major faults.
Figure 3. Stereonet plot of poles to axial-plane foliation -- entire map area.
Foliations and large-scale folds
Poles to axial-plane foliation were plotted on a stereogram using an equal-area stereographic net that included both domains of Pelona Schist (Fig. 3). Contouring of these data points reveals a broad, evenly distributed great-circle girdle ( -girdle) with two maxima (Fig. 4). The maxima represent two generalized fold-limb planes with an interlimb angle of 90 degrees. A theoretical fold axis ( -axis) is found by taking the pole to the -girdle. This -axis is horizontal and trends N52oW. Thus, these foliation data points represent a normal, horizontal, nonplunging, slightly asymmetric, upright fold with a broad, rounded fold hinge with an interlimb angle of 90 degrees (Marshak and Mitra, 1988).
Figure 4. Density contour plot of poles to axial-plane foliation -- entire map area.
In looking at stereonet plots of southeastern and northwestern foliation domains individually, one can see the origin of the composite stereonet plot of above. Contour density plots of both domains show distinct point maximum distributions. The northwestern domain has a southwestern pole maximum (limb) that dips 40 degrees with the hint of a slight great circle girdle to the northeast (Fig. 5).
Figure 5. Pole and density contour plot -- northwestern domain.
This is due to the presence of both northeast- and southwest-dipping foliation planes within the domain. The southeastern domain has a northeastern pole maximum (limb) that dips 50 degrees (Fig. 6). Thus, the northwestern domain contains primarily northeastern and secondarily southwestern axial-plane foliation attitudes. The southeastern domain contains almost exclusively southwest-dipping axial-plane foliations.
Figure 6. Pole and density contour plot -- southeastern domain.
Schistose structures of the Pelona Schist may be shown as gross or average foliations within the mapped domains (Fig. 7) as derived from Plate 1 (or Fig. 2), Geologic Map of North Fork Lytle Creek.
Figure 7. Gross metamorphic foliation of the Pelona Schist within the map area.
Stratigraphy deduced from gross foliation patterns
Pelona Schist in the map area is exposed in section thickness up to 1900 m in the northwestern domain and up to 1000 m in the southeastern domain. In the southeastern domain Pelona Schist is mostly composed of sections 50 m to hundreds of meters thick of gray schist and quartzite layers centimeters to a few meters thick with minor layers of subordinate green schist up to 3 m thick. Green schist seems to become more abundant higher in the section. The northwestern domain north of the Scotland fault contains similar stratigraphy to the eastern domain with a few differences. There is a 1-2 m thick band of a limestone-like, foliated calcium-silicate rock (cs) found interbedded with gray schist and green schist about halfway up the section north of the Scotland fault. Also, there is more greenish metasandstone (gray schist variant) that contains less or very fine-grained muscovite. This metasandstone has a weak schistosity with a crenulated texture in some spots. South of the Scotland fault the northwestern domain begins with 5-50 m of gray schist with minor, interbedded quartzite layers 1-2 m thick. A transition zone where green schist rock is interbedded with occasional gray schist layers 1-2 m thick gives way to a large section of green schist up to 700 m thick interbedded with minor quartzite and gray schist layers centimeters to 1-2 m thick. The section returns to gray schist with a few beds of green schist and quartzite 50-200 m below the Vincent thrust fault. A section of folded, mylonitic rock was traversed at the edge of the map area before going down section into gray schist again.
Geologic field mapping revealed the basic structure and stratigraphy of the Pelona Schist. Various types of small-scale folds were seen in the Pelona Schist. Isoclinal folding was especially dominant in the gray schists (mg) and quartzites (mc). Limbs on such folds often were nearly parallel and separated by 2 to 30 cm. Folds were best seen where muscovite was in high contrast to darker, more pelitic, minerals, or to lighter minerals such as quartz and albite. Deep red to purple metacherts isoclinally folded with gray schist also gave a good contrast. Parasitic folding of quartz or albite on a millimeter to centimeter scale sometimes occurred above or below isoclinal folding. Chevron or kink folding on a centimeter scale usually was seen only in gray schists. In many cases where schistosity was poorly developed in the gray schist (usually due to less muscovite), small 1-5 mm crenulations were observed. Open, larger-scale (0.5-2 m) folding as well as boudinage structures were occasionally present in the gray schists.
Common to the entire study area is the intrusion of both felsic and mafic dikes and sills. While it appears that the majority of the granitic intrusions are sills and most of the mafic intrusions are dikes, this is not always the case. Most granitic and mafic rocks occur as 1-5 m wide intrusions, whether sill- or dike-like. Granitic sills are found in both domains interbedded with both green schist and gray schist, often in alternating bands. In the eastern domain, midway up the section, a large body of granitic rock intrudes gray schist to form alternating light and dark bands. This intrusion appears to carry across a drainage that cleaves this domain into two sub-equal blocks. Mafic dikes and sills are only seen cutting across the granitic rock and even older Pelona Schist. Thus, the mafic rocks are younger than the granitic rocks which in turn are younger than the Pelona Schist and mylonitic rocks of the Vincent thrust fault.
Several instances of recent faulting were mapped in the study area, mostly in the western domain. A smooth fault plane striking N73o W and dipping 78o in granitic rock in the western block of the eastern domain was noted with possible striae along strike. This was the only fault mapped in the eastern domain. Several faults were mapped in the western domain. On the north side two small faults in gray schist with attitudes of S85oW/77o and S65oW/77o show slickenside lineations raking 10o NW and 38o NW, respectively. These faults are 2 m apart and probably are part of a fault splay. In the same general area a large, vertical fault plane (3 m tall by 6 m wide) of gray schist striking N14oE has well-developed slickenside lineations raking 15o NW. Quartz veins (3 cm wide) in the area show right lateral offset (along strike) of 15-30 cm. Thus, this fault has apparent right lateral separation. The most interesting fault(s) mapped were three northeast-striking faults with attitudes of S85 oW/64o (lin. raking 55o NE), S66o W/90o (lin. raking 14o NE), and S39oW/65 o. These three faults all strike approximately along the same line of sight and cross the Scotland fault.
Figure 8. Right-lateral offset of quartz vein in metagraywacke at fault in NW domain.
The northernmost fault displays 3 cm of right lateral offset of a quartz
vein (Fig. 8) although it is possible that this may be slope creep. In
the same line of sight, a 10-20 m wide, vertical vegetation band has a
strong color contrast to the adjacent vegetation and soil, giving more
evidence for the presence of a fault here. A crushed rock, fracture zone
of granite occurs in the southernmost drainage channel bordering the contact
between the granite and adjacent green schist. The fracture zone, small
fault surfaces, and talus fragments all look "fresh", i.e. rock edges and
fracture surfaces are sharp, angular, and have little lichen build-up.
The area appears to be geologically active during Holocene time. Numerous
rock slides were heard while in the upper confines of the southern area
near the Vincent thrust fault.
Structure of the Pelona Schist
Geologic mapping by several workers has contributed much to the understanding of the basement terrane of the central and eastern San Gabriel Mountains. On the 1:250,000 scale San Bernardino Quadrangle, a 90 to 130 square-km area of Pelona Schist is exposed southwest of the San Andreas fault and east to northeast of three segments of the Vincent thrust fault. These segments generally strike northwest and are offset left-laterally by northeast striking faults. Within the Pelona Schist on this map, two northwest-trending antiforms are shown separated by the Punchbowl fault. Recent, unpublished mapping by Jones (1993) and Nourse et al., (1994) shows the presence of two, northwest-trending antiforms separated by a synform in the region southwest of the San Jacinto and Punchbowl faults and west of the San Antonio faults. Additionally, a possible antiform is shown further north but east of San Antonio fault near its possible intersection with the inferred continuation of the Scotland fault.
Ehlig and Jacobson have been particularly prolific in their collection and interpretation of data regarding the Vincent thrust and associated mylonitic rocks of the upper plate and the underlying lower plate rocks of the Pelona Schist. Widespread distribution of several bodies of the Pelona, Rand, and Orocopia Schists exists in the vicinity of the San Andreas and Garlock faults from the Rand Mountains in the north to the Chocolate Mountains near the Salton Trough in the south (Ehlig, 1968).
Folding styles within the Pelona Schist
The Pelona Schist is presently exposed in section
up to 4 km thick and contains several types of metamorphic folding. Jacobson
has separated metamorphic folding into three groups
based on style. Style 1 folds occur throughout the section and include early, synmetamorphic, tight isoclinal folds as well as later, isoclinally refolded folds. Such folds are easiest to see in metagraywacke and metachert with a well-developed axial-planar schistosity. Style 2 folds occur in the upper 700 m below the Vincent thrust fault and are exemplified by the Narrows Synform and associated minor folding that overprint style 1 folding. Style 2 folding is defined by open folding of both compositional layering and schistosity of style 1 folds. The "Narrows Synform" is a macroscopic feature whose bedding and axial-planar schistosity poles lie on a great circle with an axis that trends N58oW and plunges 5o (Fig. 9).
Figure 9. Diagrammatic cross section near the Narrows of the East Fork of San Gabriel River (Jacobson, 1983).
Style 3 folds are characterized by kink folds and a broad, post-metamorphic arch of Pelona Schist and the overlying mylonites. Style 3 folds may be important in that they may show that the orientations of style 1 and style 2 folds have changed over time. These styles of folding do not imply discrete pulses of metamorphism that predate one another. Rather, it is suggested that style 1 and style 2 folds occurred as a continuous process during thrusting. Such varied styles of folding reinforce the idea that the Pelona Schist has undergone a complex structural history. (Jacobson, 1983).
Metamorphism and the Vincent thrust
Metamorphism of the Pelona Schist is associated with movement on the Vincent thrust fault on the basis of several structural features in the San Gabriel Mountains. Orientations of bedding-plane schistosity within the Pelona Schist are approximately parallel to the base of the Vincent thrust and the foliation of the overlying (upper plate) mylonitic rocks. Upper plate rocks near the thrust have undergone retrograde metamorphism to similar mineral assemblages as the lower plate, prograde-metamorphosed Pelona Schist. Metamorphic deformation is stronger and metamorphic grain size is larger in the Pelona Schist close to the thrust than it is further away (down section). Based on these points, metamorphism of the Pelona Schist is thought to have occurred at the same time as movement on the Vincent thrust fault. It is suggested that this movement took place during Paleocene time (Ehlig, 1982).
The Vincent thrust fault is the most significant feature exposed in the San Gabriel Mountains west of the San Andreas fault. The thrust (Fig. 9) is marked by a thick zone of deformed mylonitic rocks that are overlain by gneissic-plutonic basement rocks of Precambrian to Mesozoic age and underlain by the Pelona Schist of probable late Mesozoic age (Ehlig, 1982). Although it is generally accepted that the Vincent thrust fault represents a paleo-subduction zone, there has been a long-standing controversy on the direction of thrusting. Northeast movement of the upper plate is suggested by overturning of the Narrows Synform and the assumption that movement was perpendicular to fold axes of the upper and lower plate rocks (Ehlig, 1982). Such conditions may be accommodated by a collision model where the protolith of the Pelona Schist was deposited in an oceanic, back-arc basin followed by thrusting of a outboard, continental arc across the basin. This model proposes a southwest-dipping subduction zone and requires a suture zone to the east of the Pelona Schist, for which there is no evidence (Jacobson, Oyarzabal, and Haxel, 1996). A competing model, proposed by Burchfield and Davis (1981) and supported by many others, suggests that the Pelona Schist is an extension of the Franciscan terrane that was subducted eastward beneath the continental margin, and that northeast transport of the upper plate occurred during late deformation and does not represent the original subduction direction but rather the exhumation direction (Jacobson, Oyarzabal, and Haxel, 1996). The origin and ramifications of the Vincent thrust have been and may continue to be enigmatic.
Late Cenozoic Faults
Cucamonga fault zone
The eastern San Gabriel Mountains are largely dissected and bordered by Late Cenozoic faults. The Cucamonga fault zone is the eastern part of the frontal fault system and borders the southern edge of the San Gabriel Mountains. The Cucamonga fault zone is made up mostly of east-striking, north-dipping thrust faults that separate crystalline basement rocks from alluvium of the Santa Ana Valley to the south (Morton and Matti, 1993). Faulting has occurred since Quaternary time and continues actively today. Compression rates are estimated to be 3-5 mm per year. Morton and Matti hypothesize that the Cucamonga fault zone merges with the San Andreas fault zone 13 km downdip to the north.
San Antonio Canyon and other NE-striking, left-lateral faults
As mentioned earlier, the Vincent thrust fault is dissected into three segments by two northeast-striking, left-lateral faults, the Weber fault to the west and the San Antonio Canyon fault (SACF) to the east. The Weber fault is a nearly straight, left-lateral, strike-slip fault that apparently offsets the Vincent thrust fault by 5.1 km as shown on the San Bernardino (S.B.) Quad 2 o sheet. This offset may be mostly erosional as Jones has demonstrated offset of only 200 m on a continuous, post-metamorphic arch within the Pelona Schist that crosses the Weber fault (Jones, unpublished mapping, 1993).
The SACF appears to left-laterally displace the Vincent thrust by 5.2 km as seen on the S.B. Quad 2o sheet. This offset also may be exaggerated due to subsequent erosion of the fault surface. Jones argues that offset is only 2.8-3.0 km on the basis of metabasalt layers and style 3 arches mapped within the Pelona Schist. The Icehouse Canyon fault (IHCF) is a continuation of the Late Miocene-Early Pliocene San Gabriel fault (SGF) that predates movement on the SACF. A young episode of movement on the SACF constrains offset of IHCF and SGF to 3.5 km (Nourse et al., 1994). However, Nourse speculates that a block of Pelona Schist is missing and has been translated northeastward as shown by a misalignment of basement terranes. Thus, to accommodate this disparity of lithologies requires 6.5 km of offset on the SACF during Middle Miocene time, totalling 10 km of offset for both episodes of movement.
San Andreas and San Jacinto fault zones and related NW-striking faults
The San Andreas fault zone (SAFZ) is an 1100-km long, right-lateral, transform fault system that plays a key role in the relative motion between the Pacific and North American plates. By 1981 a classic, plate-tectonic model of the San Andreas fault had three main points: (1) ~300 km of right-lateral displacement across the entire San Andreas fault system, (2) slip is distributed in southern California into ~240 km along San Andreas fault proper and ~60 km across the San Gabriel fault, (3) total displacement equates to ~300 km of rifting since 5 Ma in the Gulf of California (Powell and Weldon, 1992). Powell and Weldon feel that the classic model inadequately describes the complex nature of the San Andreas fault system. Their model proposes that movement on the fault began at 17 to 20 Ma and evolved in three phases of faulting activity including development and abandonment of ancestral faults of the San Andreas system, the last of which is the modern fault that emerged at 4-5 Ma with slip rates of 20-35 mm per year. It is worth noting that the San Gabriel fault is thought to be an ancestral San Andreas fault that had accumulated 42-45 km of displacement between 13 and 4 Ma at a slip rate of 5-10 mm per year (Powell and Weldon, 1992).
The San Jacinto fault zone (SJFZ) is an active member of the larger San Andreas fault system in southern California (Thatcher et al., 1975). It differs from the continuous break of the San Andreas fault in that it is comprised of a series of en echelon faults which strike N45oW throughout most of its length from Borrego Valley to the east side of the San Gabriel Mountains (Sharp, 1975). It appears to lose its identity as a mappable fault as it enters the alluvium of the Lytle Creek drainage. The SJFZ is often shown as joining the SAFZ or merging with the inactive Punchbowl fault in the North Fork of Lytle Creek (Morton and Matti, 1993). The S.B. Quad 2o sheet shows a southern branch of two SJFZ splays within the North Fork of Lytle Creek. This branch is shown as turning to the left between the NW and SE domains of the study area and dies out or is truncated by the Scotland fault to the south. Dibblee (1982) feels that the SJFZ continues its NW-strike until it dies out, while Morton and Matti (1993) suggest that the SJFZ coalesces with three north-dipping faults, each within the South, Middle, and North Forks of Lytle Creek, respectively. At this junction, these faults progressively rotate or bend counter-clockwise until they are northeast-striking faults. Morton and Matti argue that northwest-striking fault separation changes from oblique-right-reverse to northeast-striking faults with oblique-left-reverse, and also that most east-striking faults appear to be thrust (e.g. the Cucamonga fault). Slip rates of 20 mm per year are estimated since SJFZ activation at 1.5 Ma. Accelerated uplift of the eastern San Gabriel Mountains in the last 1.5 m.y. is hypothesized by transference of slip from the SAFZ to the SJFZ at a structural knot in the San Gorgonio Pass area (Morton and Matti, 1993). The Glen Helen and Lytle Creek faults are right-lateral faults north and south of the SJFZ, respectively, that parallel the latter.
The Scotland fault is a right-lateral fault that
displaces Tertiary granite and Pelona Schist and lies between the Middle
and North Forks of Lytle Creek. It appears to be a branch of the SJFZ northwest
of the confluence of the South, Middle, and North Forks of Lytle Creek.
Displacement measurements of Tertiary granite on the S.B. Quad 2o
sheet reveal a possible, maximum right-lateral separation of 3.1 km on
the Scotland fault. It is speculated that this is a Quaternary fault associated
with the San Jacinto fault zone.
The obvious question arises as to how the study area fits into the geologic setting and complex system of faults mentioned above. As a starting point I will attempt to use known fault ages and displacements to reconstruct a simplified paleogeology with respect to the rock types of the mapped area, particularly the Pelona Schist and Tertiary granite. Using new data acquired in the field (Fig. 10), I will see if new constraints on local fault displacements are warranted or possible, or if this data simply reinforces existing fault-interaction models of other workers. I offer two main models in the reconstruction with some alternatives, but other fault solutions may exist. These reconstructions are supported but not required by the data.
Figure 10. Present position of main rock types with new data added from study area (Adapted from S.B. Quad 2o sheet).
Restoration of right-lateral faults
The NW domain of the study area lies approximately at the junction of the left-lateral SACF and the right-lateral SJFZ. Restoration of the right-lateral, NW-striking faults will proceed first as it is assumed that they post-date movement on the left-lateral, NE-striking faults. Tertiary granite contacts with Pelona Schist are the primary constraints for movement of these faults. Reversing 4.4 km of right-lateral slip along the Glen Helen fault (GHF) is proposed to restore contacts of a small body of schist and granite to the southeast. I propose that the GHF deflects left to merge with the SJFZ. Restoration of right slip along the SJFZ in the North Fork of Lytle Creek is constrained to a minimum of 5.5 km and a maximum of 8.1 km in order to approximately line up the intrusive contact between the Tertiary granite and Pelona Schist on opposite sides of the fault. Likewise, right slip on the Scotland fault is constrained to a minimum of 1.5 km and maximum of 4 km to line up the intrusive contacts. With restored right slip of 4.4 km along the GHF, 5.9 km along the SJFZ, and 1.9 km along the Scotland fault, the intrusive contacts of the granite across the three faults are loosely lined up to form an elongate, intrusive granite body in contact with Pelona Schist. Additionally, the NE-trending drainage between the NW and SE mapped domains of Pelona Schist line up with drainages to the north and to the south and is coincident with the SACF. Thus, I am lining up drainages north of the Scotland fault with the SACF to the south. Stratigraphy and structure of the Pelona Schist seem to match as well. The SE domain is now stratigraphically above the block of Pelona Schist to the north that has also a SW-dipping gross foliation. The restoration of the Scotland fault places the thick layer of metabasalt of the NW domain of Pelona Schist stratigraphically above the metagraywacke of the SE domain as well as allowing for an inferred style 3 synform between the domains (Fig. 11).
Figure 11. Model 1 -- Restoration of right-lateral faults.
After restoration of right slip along the GHF, SJFZ, and the Scotland fault, a coherent block of folded Pelona Schist is reconstructed adjacent to a southwest-trending, elongate intrusion of Tertiary granite. I assume that the Punchbowl fault has since been abandoned. Thus, motion of the GHF deflects left to the SJFZ for a short distance which then branches left to a through-going Scotland fault. This requires at least 12.2 km of cumulative slip along a through-going Scotland fault that parallels the Punchbowl fault.
Restoration of left-lateral faults
Restoration of left-lateral slip along the SACF is more straightforward. Restoration of 2.9 km left slip along SACF with minor clockwise rotation moves the entire block of Pelona Schist and intruded granite mentioned above as a single body to a point that is compatible with Jones' constraints of 2.8 to 3.0 km of slip, i.e. to a point where the large metabasalt layer is in stratigraphic continuity across the SACF. While this is not as much as 3.5 km recommended by Nourse (1994), erosion may make up the deficit to still line up the SGF with IHCF. Two inferred style 3 arches east of SACF, an antiform and a synform, line up approximately with the southern arches to the west of SACF as mapped by Jones (1993). Jones used a different, more northerly antiform for constraining movement on the SACF. My mapping data and restoration of faults do not support the presence of this antiform east of SACF. Restoration of 2.9 km left slip brings together a coherent block of Pelona Schist across the SACF, the Vincent thrust fault trace (after erosional factors are considered), and upper plate mylonitic rocks as well as lining up the San Gabriel and Icehouse Canyon faults. Two inferred style 3 arches offer potential piercing points for the 2.9 km of movement along the SACF (Fig. 12).
Figure 12. Model 1 -- Restoration of 2.9 km on the San Antonio Canyon fault.
Restoration of 4.4 km of right slip along the GHF is as in Model 1. The GHF deflects left to the SJFZ which becomes a through-going fault that joins or reactivates late-phase movement on the Punchbowl fault. Movement along the SJFZ is increased from the 5.9 km of Model 1 to 8.1 km with movement deflecting around the granite pocket leaving it part of the main granite body. This increased movement will be necessary to allow SACF to pass on the west side on the NW domain of Pelona Schist. At this point 12.5 km has been restored on the reactivated San Jacinto/Punchbowl (SJ/PB) fault. I restore a total of 1.9 km along the Scotland fault east of SACF. To avoid placement of a through-going Scotland fault west of SACF, I propose that the Scotland fault steps right to the SJFZ as 2.2 km of rift basins are closed during restoration of motion along Scotland and the SJFZ west of SACF (Fig. 13). Total slip on the reactivated SJ/PB fault is 16.6 km.
Figure 13. Model 2 -- Restoration of right-lateral faults along a reactivated San Jacinto/Punchbowl fault.
Restoration of 2.9 km along the SACF follows as in Model 1 with some minor differences. There is no clockwise rotation during restoration of left-lateral slip. This creates more of a faulting void along the SACF (Fig. 14). Since more slip on the SJFZ is proposed, the SACF passes the NW domain on the west side instead of the east side as in Model 1. This through-going SACF deflects left after the SJFZ causing pull-apart basins during movement.
Figure 14. Model 2 -- Restoration of 2.9 km along San Antonio Canyon fault.
An alternative model resolves motion of the SACF into two fault splays. The main fault is west of the NW domain as above, but a minor fault splits the NW domain into two blocks that offset one another 0.5-1 km left-laterally (Fig. 15). This alternative allows for a better alignment of style 3 arches.
Figure 15. Model 2 alternative -- left-lateral slip is distributed along splay of San Antonio Canyon fault that cleaves the NW domain.
Problems in the reconstructions
An intriguing problem lies in the nature and sequence of fault displacements. In Model 1, I have proposed restoration of 4.4 km along the GHF, 5.9 km along the SJFZ, and 1.9 km along a through-going Scotland fault west of SACF and south of the inactive Punchbowl fault. That is fine for keeping the Punchbowl fault dormant, but that amounts to 12.2 km of slip on a Scotland fault west of SACF for which there is no direct field evidence within the Pelona Schist. Model 1 also results in an anomalous block granite to the northwest above the GHF. The NW domain of Pelona Schist ends up next to the SE domain resulting in an apparent stratigraphic and structural discontinuity. Stratigraphically, it places NE-dipping schist next to SW-dipping schist to the east resulting in a discontinuity of gross foliations. Structurally, the above placement of rocks disrupts continuity of style 3 arches across the SACF.
Model 2 helps correct the above problems. The anomalous block of granite is not separated by the SJFZ; it remains attached to the adjacent, southern block of granite during movement of the SJFZ. The lack of a through-going Scotland fault is accommodated by the Scotland fault stepping right to a reactivated SJ/PB fault that also accommodates movement of the GHF. The closure of rift basins during restoration and opening of the same basins during activation of the Scotland fault helps deal with the hypothesis that the Scotland fault begins to die out as it steps right to the SJFZ. The structural discontinuity introduced by the NW domain is mitigated by break up of the block by a branch of the SACF in the alternative to Model 2. This branch fault restores structural continuity of style 3 arches across the SACF, but the sense of motion required to offset the domain may be in the wrong direction (if, in fact, this fault is right-lateral).
These reconstructions are based on the assumption that movement on the S.J. and G.H. faults post-date movement on the SACF. If these faults have been active coevally during Quaternary time, one might not expect the blocks of granite and Pelona Schist to end up as a coherent body in the way they do as mentioned above. That is, as one simultaneously reverses motion on these NW- and NE-striking sets of faults, the blocks might tend to disperse or scatter to the south. Upon activation of faults, do the scattered blocks condense as they move northward? Or, should fault restoration and activation resolve into a north-south motion without any scattering or condensing of rock units? If so, how does one account for the preferred right-lateral separation of Tertiary granite and Pelona Schist present today in the eastern San Gabriel Mountains? Complicating the issue further, the faults may have been active coevally, but the faults "took turns" each moving in spurts alternating between NW- and NE-striking faults.
I have not considered rigorously the conservation of rock mass, nor rock area in these reconstructions. Restoration likely would have created extensional basins while fault activation would have tended to close these basins during compressional events and uplift. If there were basins before fault activation, what happened to the sediments deposited in them? Were they eroded first due to their sedimentary nature when compared to metamorphic and igneous basement rocks? I have assumed largely strike-slip displacements when these faults are thought to have oblique components to them as well, however minor they may be. Erosional offsets were considered only for the SACF as suggested by other workers (Nourse and Jones). I was not able substantiate the missing block of Pelona Schist nor the added 6.5 km of offset along the SACF as suggested by Nourse (1994). This motion probably predates fault movement that I am considering in my reconstructions.
The implications of my field work and paleo-reconstruction
of the Pelona Schist are several. I concur with Jones and Nourse that total
left-lateral slip on the SACF since Late Miocene-Early Pliocene time (5
Ma) is loosely constrained to 2.9 km based on metabasalt stratigraphy and
style 3 arches in the Pelona Schist that come together after restoration.
I argue that 1.9 km right-lateral offset on the Scotland fault largely
pre-dates 5.9 km right-lateral offset on the San Jacinto fault zone and
4.4 km right slip on the Glen Helen fault (since 1.5 Ma as proposed by
Morton and Matti, 1993). Continued movement on the Scotland fault stepped
right as it joined the SJFZ. I hypothesize that the GHF deflects left to
join a reactivated SJ/PB fault west of San Antonio Canyon and that this
fault should have 16.6 km of right-lateral displacement since 5 Ma. Lastly,
I feel that Morton and Matti's hypothesis that northeast-striking faults
turn and blend into northwest-striking faults is poorly justified. I propose
that the truncation of northeast-striking faults by northwest-striking
faults is more plausible due to the reconstruction of Pelona Schist and
granite in the mapped area. To test this hypothesis, one needs to look
outside the map area for better piercing points. The SACF is old, not seismically
active, and offset by seismically active faults, the SJFZ and GHF. NE-trending
valleys north of the SJFZ and GHF and north of the San Andreas fault should
have large (2-3 km) left-lateral offsets created by the SACF.
During this field and research project I learned many things about myself and geology. First and foremost, I learned that diligence and independence when doing field work are highly rewarding experiences. I remember some mornings when I found it difficult to get up and get motivated to go to the field and map geologic structures through vegetation and topography that seemed to zap one's strength. But, now I am glad that I did. It felt good to be in the field and to exert oneself on a physical level while collecting data for an intellectual endeavor on a mental level. On the academic side, I learned that field studies and the acquisition and recording of geologic data are a permanent record with many uses. With objectivity, good data may be collected that may continue to be of use through time. Inferences can be made based on this data to support scientific models.
On a geologic note, I learned many things. I learned good techniques for geologic field mapping. I learned how detailed mapping in a limited study area may be extended to regional geology. For example, in the study area I recorded about 300 attitudes of foliation planes of the Pelona Schist as well as rock type contacts, stratigraphy, and cross-cutting relationships. I was able to approximate gross foliations of mapped Pelona Schist in the area. By comparing these foliations and mapped contacts to existing, regional geologic mapping, I was able to reconstruct a paleogeology of a limited domain of Pelona Schist and a Tertiary granite body that intrudes it. This was accomplished by restoration of 4.4 km of right slip along the Glen Helen fault, 8.1 km right slip along the Early Pleistocene age San Jacinto fault zone, 1.9 km or right slip along the Pre-Early Pleistocene age Scotland fault, and 2.9 km of left slip along a Post-Late Miocene-Early Pliocene phase of movement along the San Antonio Canyon fault.
Although a crude paleogeology was reconstructed,
it was by no means a rigorous reconstruction, and may be better constrained
by further field work and analysis. Specifically, the exact nature and
position of the contact between the Pelona Schist and Tertiary granite
could use some refinement. However, the present, irregular contact may
be explained by intrusion of a thick sill of granite along previously folded
schist. Mapping of felsic and mafic dikes and sills within the Pelona Schist
of the study area and outside it may help define precise piercing points
for regional fault reconstruction. Detailed mapping of small-scale faults
in the study area as well as extension or continuation of faults outside
the area may offer more evidence to help answer the question regarding
interaction of right-lateral, NW-striking and left-lateral, NE-striking
Ehlig, P. L., 1968, Causes of distribution of Pelona, Rand, and Orocopia Schists along the San Andreas and Garlock faults, in Dickinson, W. R., and Grantz, A., eds., Conference on Geologic Problems of San Andreas Fault System, Proceedings: Stanford University Publications, Geological Sciences, v. 11, p. 294-306.
Ehlig, P. L., 1975, Basement rocks of the San Gabriel Mountains, south of the San Andreas Fault, southern California, in Crowell, J. C., ed., San Andreas Fault in Southern California, A Guide to San Andreas Fault from Mexico to Carrizo Plain: Sacramento, California, California Division of Mines and Geology Special Report 118, p. 177-186.
Ehlig, P. L., 1982, The Vincent thrust: its nature, paleogeographic reconstruction across the San Andreas fault and bearing on the evolution of the Transverse Ranges: Geology and Mineral Wealth of the California Transverse Ranges, South Coast Geological Society, p. 370-379.
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Jacobson, C. E., Dawson, M. R., and Postlethwaite, C. E., 1988, Structure, metamorphism, and tectonic significance of the Pelona, Orocopia, and Rand Schists, southern California, in Ernst, W. G., ed., Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Englewood Cliffs, New Jersey, Prentice-Hall, p. 976-997.
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Morton, D. M., 1975, Synopsis of the geology of the eastern San Gabriel Mountains, southern California, in Crowell, J. C., ed., San Andreas Fault in Southern California, A Guide to San Andreas Fault from Mexico to Carrizo Plain: Sacramento, California, California Division of Mines and Geology Special Report 118, p. 170-176.
Morton, D. M., and Matti, J. C., 1993, Extension and contraction within an evolving divergent strike-slip fault complex: The San Andreas and San Jacinto fault zones at their convergence in southern California, in Powell, R. E., Weldon, R. J., and Matti, J. C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution: Boulder, Colorado, Geological Society of America Memoir 178, p. 217-230.
Nourse, J. A., Hazelton, G. B., and Jones, R. K., 1994, Evidence of two phases of Late Cenozoic sinistral displacement on the San Antonio Canyon fault, eastern San Gabriel Mountains, California: Geological Society of America Abstracts with Programs, Cordilleran Section, p. 77-78.
Powell, R. E., and Weldon, R. J. II, 1992, Evolution of the San Andreas Fault: Annual Review of Earth Planetary Science, v. 20, p. 431-468.
Sharp, R. V., 1975, En echelon fault patterns of the San Jacinto fault zone, in Crowell, J. C., ed., San Andreas Fault in Southern California, A Guide to San Andreas Fault from Mexico to Carrizo Plain: Sacramento, California, California Division of Mines and Geology Special Report 118, p. 147-152.
Thatcher, W., Hileman, J. A., and Hanks, T. C., 1975, Seismic slip distribution
along the San Jacinto fault zone, southern California, and its implications:
Geological Society of America Bulletin, v. 86, p. 1140-1146.
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