2. 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.
3. 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.
4.
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.
5. 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.
8.
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.
Cogswell
Reservoi r
P!"
9.
10. 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
11. 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
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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.
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