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The San Gabriel Fault: A Summary of its Structural History and Neotectonics
Eldon Gath — Earth Consultants International, 1642 E. 4th St., Santa Ana, CA 92701
Structural History
The San Gabriel fault is a major right‐lateral strike slip fault that has played a long and complex role in the structural development of southern California. It extends about 140 km, arcing through the San Gabriel Mountains, terminating near the San Andreas fault in Tejon Pass and near the San Andreas fault in Cajon Pass (Figure 1). The fault trends approximately east‐west along its eastern trace north of the San Gabriel Valley, bending to more northwesterly through the Castaic region. From the north, the fault trends generally NW‐SE to Big Tujunga Canyon where it bifurcates into the SE‐trending Vasquez Creek fault and the more easterly‐trending San Gabriel fault, proper (Fig. 2). The Vasquez Creek fault appears to merge into the Sierra Madre frontal fault system (Fig. 2) while the San Gabriel strand appears to be terminated in the east by the San Antonio Canyon fault (Fig. 1).
Figure 1: Index map to the central and western Transverse Ranges, showing the principal faults and generalized geology. Abbreviations: C-SF—Clamshell-Sawpit fault; CH—Coyote Hills; RF—Raymond fault; SACF—San Antonio Canyon fault; SAR—Santa Ana River; SFF—San Fernando fault; SJF—San Jose fault; SJcF—San Jacinto fault; SMF—Santa Monica fault; VCF—Vasquez Creek fault; WCF—Walnut Creek fault. Figure from Yeats (2004).
Estimation of the total right slip on the fault has evolved from 24‐40 km (Crowell, 1952) to 40‐60 km (Ehlig et al., 1975) to, most recently, 42 km (Powell, 1993). All of these displacement estimates make major assumptions within the contacts being measured, and all rely on summing displacement across multiple faults that may have played a
role in the San Gabriel fault. Powell (1993) completed a comprehensive review of the displacement history of the San Gabriel fault, and concluded that: a) there is agreement that the fault has 22 km of displacement on the segment east of the Vasquez Creek fault; b) displacement on the northwest segment of the San Gabriel fault is substantially greater than on the eastern segment; c) no studies show large displacements (5 km maximum) on the Vasquez Creek fault; and d) no good explanation exists to explain the 20+ km difference in displacement between the two segments. It is clear that younger structural or geomorphic features are displaced less than older features, and that the rate of slip along the fault must have decreased by the Pliocene.
Figure 2: Simplified geologic map showing the area in Big Tujunga Canyon where the San Gabriel fault bends to the east, and the Vasquez Creek fault bifurcates from the San Gabriel fault (circle) to connect to the Sierra Madre fault at the range front. C-S—Clamshell-Sawpit fault; SG—San Gabriel fault; SM—Sierra Madre fault; VC—Vasquez Creek fault. Figure modified from Beyer et al. (2009).
Powell (1993) proposes three explanations to accommodate this missing 20‐22 km of displacement on the eastern segment: a) late Miocene left‐oblique extension in the LA and/or Ventura basins; b) the missing slip was accommodated along an unknown and now buried fault between the Santa Monica and Verdugo mountains; c) the missing right‐slip is accommodated on coeval left‐slip faults in the western San Gabriel Mountains and Soledad Basin. All three of these explanations have evidence in support, and all three have difficulties.
Palinspastic reconstruction of southern California is also evolving with respect to the San Gabriel fault. Once commonly assumed to be a proto‐San Andreas fault, this model is called into question by large (100‐110 km) displacements on the Clemmens Well‐Fenner‐San Francisquito fault up until about 11‐13 Ma (Powell, 1993). After abandonment of that fault system, the San Gabriel fault developed as the active strand of the San Andreas fault system, but some of the plate margin strain continued to step westward to the continental margin (Powell, 1993). From about 5‐4 Ma, the San Gabriel fault was largely bypassed by development of the modern San Andreas fault, and became relatively inactive, with only 1‐2 km of displacement post 4 Ma (Powell, 1993). Weber (1982) maps 1.2 km of displacement within the base of the 5.0‐3.5 Ma Hungry Valley Formation at the northwestern terminus of the fault, but the displacements apparently die out within the upper part of the unit.
Based on Powell’s analysis (Powell, 1993), the long‐term average slip rate for the San Gabriel fault, assuming 42 km of displacement between 13 and 5 Ma, is 5.3 mm/yr. Using other displacements and shorter intervals he calculated the range at 4.3 mm/yr (13‐10 Ma), 5.5 mm/yr (10‐6 Ma), and 4 mm/yr (6‐4 Ma). Using different relationships, Yeats proposed a 6.6‐9.2 mm/yr rate determined from offset Mio‐Pliocene units, and a 2.5‐3.0 mm/yr rate from offsets on the Pleistocene Saugus Formation (Yeats et al., 1994). In no case could the full plate margin rate be assigned to the San Gabriel fault, and they are significantly less than the 12 mm/yr required by the 60 km offsets between 10‐5 Ma of Ehlig et al. (1975).
Evidence for Holocene Activity
In general, the San Gabriel fault is assumed to be largely inactive in the Quaternary, other than a small segment to the northwest in the Santa Clarita – Newhall area. This portion of the fault was confirmed to have multiple Holocene displacements when the fault was exposed in trenching excavations conducted initially as geotechnical studies, then enhanced under two NEHRP grants (Cotton et al., 1988). Their work was the first to document Holocene displacements on the San Gabriel fault, constraining the most recent event (MRE) to between 1550 ybp and 3500 ybp. Examination of their trench log (Figure 3) permits a second event after 3590 ybp, but well before 1550 ybp. Based on the offset of a buried ridge and channel pair (Figure 3), they (Cotton et al., 1988) also determined a Holocene slip rate of about 0.6 mm/yr. This rate that is less than 10% of the geologic rates, strongly suggesting a dramatic reduction in the San Gabriel fault slip rate after the development of the current San Andreas fault. The active segment of the fault through Santa Clarita is presumed to die out both northwest and southeast. Crowell (2003) presents the San Gabriel fault dying out in the early depositional history of the Hungry Valley Formation in the northern Ridge Basin, and Weber (1982) ended his mapping at the Big Tujunga River. Yeats and Stitt (2003) proposed that the Holocene activity on the San Gabriel fault within Santa Clarita may be accommodating the folding of the Ventura Basin to the west, or accepting strain from the Holzer fault, though both interpretations have weaknesses. Additional geotechnical investigations (summarized by Yeats and Stitt, 2003) have confirmed Cotton’s findings, but have not added quantitatively to the San Gabriel fault’s seismic hazard understanding.
Figure 3: Set of figures from Cotton et al. (1988) showing their paleoseismic investigation of the San Gabriel fault in the Castaic - Santa Clarita area [Above].
[Left] A 9 ka buried ridge and channel pair were right-laterally displaced 21 meters across the fault.
[Below left] Graphic log of their north-facing trench wall shows the most recent event constrained between 3500 and 1550 ybp, but it is permissible to interpret the log as showing a second, penultimate event having occurred also between this same time interval. This event is constrained by the right-most fault splay’s termination by subsequently faulted units. Mismatch of stratigraphy across the faults demonstrates that lateral slip is dominant.
No studies of the eastern segment have shown similar evidence for Holocene activity, although no actual trenching studies are known. For this paper, I did an aerial flyover of the fault using Google Earth imagery, and was struck by the strong geomorphic expression of youthful faulting seen on those images east of the Cogswell Reservoir (Figures 4‐7). The imagery shows the lineament trending almost due east for 17 km to the San Gabriel River where it begins to curl more northerly, following the bend in the river. Farther east of the river, it is still possible to infer the presence of a major fault zone, but the freshness of the lineament is not visible. To the west of Cogswell Reservoir, the lineament is clearly visible for at least another 25 km, almost to the Big Tujunga River. It is not as fresh as the 17 km segment east of Cogswell, but it is more visible than exists east of the San Gabriel River and is directly on trend, implying that there is no structural complexity separating them.
The sense of slip appears to be right‐lateral based on the larger stream deflections, but with a south‐side up component resulting in upslope facing scarps. It is possible that the lineament could simply be a fault‐line scarp created by differential rock hardness and resistance to erosion. However, on the geologic map portion of Figure 8 there is a part of the fault that is separating the same geologic unit (Trlh, shown in red) immediately east of the eastern end of the Cogswell Reservoir. This is the same area shown on Figure 5 (P‐1b) and Figure 6 (P‐2) where the lineament is the sharpest. Furthermore, the lineament cuts across both geology and topography with about equal visibility, which implies to me that it is a youthful tectonic feature. No field reconnaissance has yet been performed, but that is the obvious next step.
Assuming that this lineament does reflect a late Holocene surface rupture of at least this portion of the San Gabriel fault, the question is what may be driving this reactivation (or continued activation) of the fault. One possibility is that this small fault segment is accommodating displacements from the active Clamshell‐Sawpit fault (Figure 4). This model has potential because it can explain the termination of the lineament where the Clamshell‐Sawpit fault intersects the San Gabriel. It also makes an interesting segmentation model for the eastern San Gabriel whereby the fault’s activity is controlled from, and segmented by, the Clamshell‐Sawpit, San Dimas Canyon, and San Antonio Canyon faults (Figure 4). These northeast‐trending faults appear to be accommodating some of the clockwise block rotations of the southern San Gabriel Mountains (Luyendyk, 1991) and the San Gabriel fault would be serving to constrain the northerly margins of these blocks. In this case, the fault rupture would presumably be limited to the 15‐20 km long block margin formed between the cross‐cutting faults.
Alternatively, the eastern San Gabriel fault may be serving to partition slip from the Sierra Madre frontal fault because of a small obliquity to the LA Basin and Transverse Ranges convergence. In this case the cross‐fault segmentation is more of a shallow hanging wall structure, and multi‐segment ruptures could more easily cascade across them along the deeper San Gabriel fault.
Figure 4: Index Map for P-1 to P-6 Google Earth Images
Figure 5 (previous page): P-1a an oblique view, looking east, along the trace of the San Gabriel fault from above the Cogswell Reservoir. The fault is moderately well defined at this distance by a series of upslope-facing scarps along all of the ridges. P-1b is a close-up of the fault immediately east of the reservoir where a sharp lineament is clearly defined across the landscape.
Figure 6 (above): Vertical image of the fault east of Cogswell Reservoir. The striking lineation that is affecting all of the ridges and drainages strongly suggests that a late-Holocene displacement has occurred on this portion of the San Gabriel fault.
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Figure 7 (previous page): P-3 to P-6 are a series of images along the fault showing the continuity of the youthful looking lineament trending east of the Cogswell Reservoir for 17 km to at least the San Gabriel River.
Figure 8: The top image is a montage made from P-2 through P-6, extending from Cogswell Reservoir on the west to the San Gabriel River on the east. The lower image is a section of the geologic map (Morton and Miller, 2003) for the same approximate area as the Google Earth imagery. The two map sections are approximately the same scale, though no actual scale is intended. The purpose of this figure is to contrast the linear nature of the inferred fault across the landscape with the complexly stepped nature of the fault as shown on the geologic map. There would appear to be at least three alternatives: 1) the landscape lineament is not the San Gabriel fault; 2) the geologic map is incorrect; or 3) the lineament reflects a late Holocene fault rupture that has cut across some of the older structural complexities.
Summary
The San Gabriel fault has had a long and complex role in the tectonic development of southern California. The fault began about 13‐11 Ma as a major portion of the proto‐San Andreas fault system, and at about 5 mm/yr, it accommodated 42 km of right slip before the modern San Andreas formed and the San Gabriel was essentially cut off. Although still active at a reduced rate through the Pliocene, the San Gabriel was assumed to be inactive until the 1980’s when geotechnical trenching in the Santa Clarita area showed displaced late Holocene deposits. No similar trenching studies are known from the eastern half of the San Gabriel fault, and it is still considered as an inactive fault. However, the presence of a youthful‐appearing lineament on Google Earth imagery, trending 17 km east from the Cogswell Reservoir, appears to indicate that this portion of the San Gabriel fault has also had a late Holocene surface rupture. The reactivation of the eastern San Gabriel may be the result of strain partitioning from the Sierra Madre fault due to a slight oblique convergence. However, a preferred model is
proposed that segments the eastern San Gabriel along a series of northeast trending hanging wall faults, and in which the San Gabriel accommodates the clockwise block rotations imposed by these faults.
References
Beyer, L.A., McCulloh, T.H., Denison, R.E., Morin, R.W., Enrico, R.J., Barron, J.A., and Fleck, R.J., 2009, Post‐Miocene right separation on the San Gabriel and Vasquez Creek faults, with supporting chronostratigraphy, western Can Gabriel Mountains, California; US Geological Survey Professional Paper 1759, 44 p.
Cotton, W.R., Fowler, W.L., and Hay, E.A., 1988, Late Pleistocene and Holocene paleoseismicity of the San Gabriel fault; William Cotton and Associates, Final Technical Report to the US Geological Survey, Contract No. 14‐08‐0001‐G1196, 21 p.
Crowell, 1952, Probable large lateral displacement on San Gabriel fault, southern California; American Association of Petroleum Geologists Bulletin, v. 36, p. 2026‐2035.
Crowell, J.C., 2003, Tectonics of Ridge Basin region, southern California, in Crowell, J.C., ed., Evolution of Ridge Basin, southern California: An interplay of sedimentation and tectonics: Geological Society of America Special Paper 367, p. 157–203.
Ehlig, P.L., Ehlert, K.W., and Crowe, B.M., 1975, Offset of the upper Miocene Caliente and Mint Canyon formations along the San Gabriel and San Andreas faults; in Crowell, J.C., ed., San Andreas fault in southern California; California Division of Mines and Geology Special Report 118, p. 83‐92.
Luyendyk, B.P., 1991, A model for Neogene crustal rotations, transtension, and transpression in southern California; Geological Society of America Bulletin, v. 103, p. 1528‐1536.
Morton, D.M. and Miller, F.K., 2003, Preliminary geologic map of the San Bernardino 30’ x 60’ quadrangle, California, Sheet 4 of 5, Faults; US Geological Survey OFR 03‐293, digital v. 1.0, http://pubs.usgs.gov/of/2003/of03‐293/
Powell, R.E., 1993, Balanced palinspastic reconstruction of pre‐late Cenozoic paleogeology, southern California: Geologic and kinematic constraints on evolution of the San Andres fault system; in Powell, R.E., Weldon R.J., II, and Matti, J.C., eds., The San Andreas Fault System: Displacement, Palinspastic Reconstruction, and Geologic Evolution; Geological Society of America Memoir 178, p. 1‐106.
Weber, F.H., Jr., 1982, Geology and geomorphology along the San Gabriel fault zone, Los Angeles and Ventura counties, California; California Division of Mines and Geology Open File Report 82‐2LA.
Yeats, R.S., 2004, Tectonics of the San Gabriel Basin and surroundings, southern California; Geological Society of America Bulletin, v. 116, p. 1158‐1182.
Yeats, R.S., and Stitt, L.T., 2003, Ridge Basin and San Gabriel fault in the Castaic Lowland, southern California, in Crowell, J.C., ed., Evolution of Ridge Basin, southern California: An interplay of sedimentation and tectonics: Geological Society of America Special Paper 367, p. 137–156.
Yeats, R.S., Huftile, G.J., and Stitt, L.T., 1994, Late Cenozoic tectonics of the east Ventura Basin, Transverse Ranges, California: American Association of Petroleum Geologists Bulletin, v. 78, p. 1040–1074.