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“Progressive deformation around a restraining bend in the San Andreas Fault,
Carrizo Plain, CA” contribution to PGS San Andreas Fault Field Trip Guidebook
George Hilley, Department of Geological and Environmental Sciences, Stanford
University, Stanford, CA
J Ramón Arrowsmith, Department of Geological Sciences, Arizona State University,
Tempe, AZ
Overview of the Northern and Southern Elkhorn Hills
Situated in the southerastern Carrizo Plain, the Northern Elkhorn Hills (NEH) are
an uplifted platform bounded on the southwest by the San Andreas Fault (SAF) and faultpropagation folds on the northeast (Figure 1) (Arrowsmith, 1995). Within the center of
the NEH lie a series of extensional grabens and half-grabens whose geometry indicates
both extension and SAF-parallel right-lateral shear (Arrowsmith, 1995; Arrowsmith et
al., 1998). The main geologic unit exposed within the NEH and Southern Elkhorn Hills
(SEH) is the Plio-Quaternary Paso Robles Formation, which locally consists of clay, silt,
sand, and gravel that is sourced from the Miocene rocks of the Temblor range to the
northeast. On the southwest side of the SAF, the formation contains three mapped
members in the area (Arrowsmith, 1995) including, in decreasing stratigraphic age, Pink,
Tan, and Gray members. The Paso Robles Formation overlies the mid-late Tertiary Santa
Margarita and Bitterwater Creek formations (Figure 1). The former comprises
sandstones and conglomerates, while the latter is a grey marin mudstone. An overlying
alluvial unit, which may be contemporaneous with the deposition of the Gray member of
the Paso Robles Formation, consists of channel and debris flow deposits that are
pervasively bioturbated by ground squirrels (Wallace, 1973). This unit and associated
geomorphic surface likely represents the Pleistocene range-front of the Temblor Rage to
the east. A younger, undisturbed alluvial unit is less bioturbated and generally occupies
the floors of many of the extensional structures in the area (Arrowsmith, 1995).
Deformation along the boundaries of the NEH juxtapose these different geologic
and geomorphic units (Figure 1). Along its southwestern margin, the SAF is defined by a
topographic break known as the Elkhorn Scarp. This feature is produced by lateral offset
along the SAF and probably a small component of northeast-side-up vertical surface
displacement. The uplift along this boundary exposes the underlying Paso Robles
Formation and juxtaposes it with the old, formerly undisturbed Pleistocene Temblor
rangefront. Along the northeastern boundary of the NEH, contraction accommodated
along a blind reverse fault separates uplifted and partially exhumed Paso Robles
Formation from the old alluvial fan surface that represents the active Pleistocene Temblor
Mountain rangefront. The largely monoclinal folding that defines the northeastern
margin of the NEH trends ~30 degrees counterclockwise of the strike of the SAF. In
most locations throughout the NEH, the underlying blind reverse fault responsible for the
folding does not reach the surface. However, to the southeast along the boundary
between the NEH and Southern Elkhorn Hills (SEH), a right-stepping, en echelon fault
whose orientation is similar to that of the folds of the NEH daylights (Arrowsmith, 1995;
Thomas and Sieh, 1981). To the south, this fault undergoes an ~30 degree clockwise
rotation and parallels the SAF for the remainder of the length of the SEH. The amplitude
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of the monoclinal folding within the NEH and relative offset of the Paso Robles
Formation suggest that near-fault contractional deformation generally increases from
northwest to southeast in the area.
In addition to contractional deformation, a series of grabens and half-grabens
whose location parallels the SAF occur within the hanging-wall of the thrust sheet
responsible for the uplift of the NEH and SEH (Figure 2). The right-stepping, en echelon
geometry of the grabens indicates that a component of SAF-parallel shear accompanies
their predominantly normal motion (Arrowsmith, 1995). The maximum relief between
the graben floor and shoulders increase systematically from ~18 m, ~28 m, and ~40 m in
the northwestern, central, and southeastern NEH, respectively (Arrowsmith et al., 1998).
In addition, the morphology of the graben-bounding normal fault scarps changes
systematically from northwest to southeast. Based on morphometric analyses of these
features, Arrowsmith et al. (1998) found that displacement along the northwestern-most
graben-bounding fault (labeled L&S for Long and Skinny graben, Figure 2) commenced
~12 ka and continued to slip at an average rate of 2 mm/yr. Within the central NEH
(labeled Big Graben, Figure 2), fault motion likely started much earlier, around 62 ka and
experienced an average slip rate of ~1 mm/yr (Arrowsmith et al., 1998). Therefore,
similar to the contractional deformation in the area, the extensional grabens in the
hangingwall of the NEH and SEH thrust sheets record increasing total deformation from
northwest to southeast.
The relationship between geomorphic features, the folding, and the grabens within
the NEH demonstrate that contraction along the margins of the NEH was approximately
contemporaneous with extension within it. In the northwestern portion of the NEH, a
small channel traverses its width, crossing both the northeastern-bounding fold and the
Long and Skinny graben (Figure 2). The current channel geometry allows it to maintain
a hydrologic connection throughout its course; however, an abandoned reach of the
channel has left an alluvial terrace that is located southeast of the current channel
location. Thus, the originally planar alluvial terrace of the abandoned channel serves as a
geomorphic marker that may be used to gauge the distribution and magnitude of
deformation since its abandonment. This older, abandoned surface is folded along the
northeastern margin of the NEH and faulted along the northeastern shoulder of the Long
and Skinny graben (Figure 3). Therefore, since the time that the alluvial channel
assumed its present course, both contractional folding and reverse faulting has occurred
in the area. Because this terrace surface, and the current channel gradient grades to the
Pleistocene Temblor range-front surface, it is likely that both these deformation regimes
coexisted contemporaneously.
The coexistence of normal and reverse faulting in the NEH likely results from a
down-dip steepening of the thrust fault geometry that is responsible for uplift of the NEH
(Arrowsmith, 1995). In this case, a steep reverse fault at the location of the SAF
shallows approximately one kilometer to the northeast of the SAF. The resulting flexure
of the hangingwall sheet creates extension above the change in fault geometry (Figure 4).
However, right-lateral motion transferred from the SAF to the thrust sheet resolves a
component of shear onto the normal faults formed in the hangingwall. Hence, local
extension with a component of right-lateral shear may be created in this overall
transpressive environment (Arrowsmith, 1995).
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Along the southwestern side of the SAF, a series of pressure ridges have
developed in response to transpression adjacent to the fault. The along-fault length of
features range from ~1-4.5 km, and have widths on the order of several hundred meters.
These features are generally located to the north of the NEH; however, the largest of
these features, the Dragon’s Back Pressure Ridge (discussed below) overlaps with the
northwestern-most portion of the NEH.
The geomorphology and structural geology of this portion of the Carrizo Plain
record progressive contraction adjacent to the SAF from northwest to southeast. North of
the NEH, pressure ridges line the southwestern side of the SAF, and accommodate
relatively small amounts of transpression. Within the NEH, increasing amplitude of
monoclinal folding and relative offset of the Pleistocene Temblor rangefront suggest that
deformation along the margins of the NEH and SEH increases systematically from
northwest to southeast in the area. In addition, progressive extension recorded by the
grabens nested in the NEH thrust sheet also indicate that slip increases along the blind
reverse faults responsible for uplift of the Elkhorn Hills towards the southeast. This
progressive deformation corresponds with the counter-clockwise rotation of the SAF as it
enters the Big Bend in Southern California. This spatial association with increased nearfault deformation within the NEH and SEH suggest a causal link between this
deformation and the development of the restraining bend in the SAF in Southern
California (Arrowsmith, 1995).
Geology and Geomorphology of the Dragon’s Back Pressure Ridge
The Dragon's Back pressure ridge is the southernmost pressure ridge expressed
along the SAF in the Carrizo Plain (Hilley, 2001). It is ~4.5 km-long and flanks the
southwestern side of the SAF (Figure 1). Systematic changes in topography and relief
along the Dragon's Back are present: the steepest basins and highest topography exist in
the southeastern portion, while lower relief and topography dominate the northwest. The
total relief from basin mouth to ridge crest along the landform is less than 80 meters.
Channels drain southwestward into the Carrizo Plain, building small alluvial fans at the
base of the landform. The semi-arid grassland plain to the southeast of the Dragon's
Back is a gently southwestward-sloping surface and receives ~23 cm of annual rainfall
each year.
Deformed Paso Robles Formation (Dibblee, 1973) is exposed and progressively
tilted along the northwestern portion of the landform (Arrowsmith, 1995; Figures 1 and
5). The stratigraphically lowest of the units exposed in the Dragon's Back is the Pink
member of the Paso Robles formation. Field mapping does not constrain the total
thickness of this unit; however, available exposure indicates a minimum thickness of 350
m. Conformably overlying this member, the Tan and Grey members of the Paso Robles
formation are ~60 and ~20 m thick, respectively. Rotation of bedding and fracture
orientations indicates that the units are not only uplifted, but folded into a monoclinal
structure (Figure 5).
Structural cross sections through the pressure ridge show that total uplift and
flexure increase northwestward along the landform (Figure 5). The contact between the
Pink, Tan, and Grey members of the Paso Robles formation in the southeastern-most
cross section (Section A-A'; Figure 5) are gently and moderately tilted at the SW edge
and NE center of the landform, respectively. Dips range from 0-30 degrees, generally to
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the southwest. Farther northwest, the contact between the Pink and Tan units of the Paso
Robles Formation increases 80 m in elevation, as the contact is progressively exposed
(Section B-B'; Figure 5). Bedding dips are similar to the previous section, but increase
towards the southwest edge of the landform, where a southwest-dipping normal fault
juxtaposes the Gold and Tan members of the Paso Robles formation. Finally, widespread
exposure of the Pink member of the Paso Robles formation in the northwestern section of
the pressure ridge results from erosion of the monoclinal structure (Section C-C'; Figure
5).
These observations of progressive deformation within the pressure ridge
apparently result from the movement of an initially gradually SE-sloping surface on the
southwestern side of the fault (Pacific Plate) though an uplift zone fixed to the North
American Plate/NEH (Figure 6). As the right-lateral motion of the SAF moves material
of the Pacific Plate over the uplift zone, it is deformed into a broad anticlinal structure.
We inferred from the exposed Pink/Tan Paso Robles Member contact that the
northwestern termination of the uplift zone is approximately 1700-1800 m to the
northwest of the beginning of the uplift zone. The SAF acts as a conveyor belt that
moves material of the Pacific Plate through the fixed uplift zone. The mechanism by
which material is uplifted along the strike-slip fault is unclear; however, we suspect that a
shallow, ~400-500 m southwestward offset of the SAF at depth may be responsible for
the uplift. Above this inferred subsurface fault offset, uplift would result as material of
the Pacific Plate laterally encounters it. Our interpretation of the subsurface fault
geometry is supported by magnetotulluric measurements (Unsworth et al., 1999) that
indicate the presence of a fault offset beneath the Dragon's Back, and from its possible
link with the southeast dipping reverse fault in the NEH (Figure 4). In any case, the
mechanism by which material is uplifted does not affect the inferred kinematic history of
the structure.
Assuming that the SAF has slipped at a constant rate of 35 mm/yr in the area for
the last 120 kyr (Sieh and Jahns, 1984), the length along the pressure ridge can be
converted to the time since material moved through the uplift zone. Using this space-fortime substitution with the rock uplift computed from the uplifted contacts of the members
of the Paso Robles Formation, we can calculate uplift rates along the landform and the
uplift rate history during the landform’s deformation (Figure 7). Using this uplift history
(Figure 7), we used field mapping, surveying, and DEM analysis to examine the effect of
uplift on the landform's topography. The clearest change in morphology along the
landform was basin geometry (Figure 7). At the southeast end of the pressure ridge,
basin widths and areas are noticeably smaller than those at the northwest. In addition,
basin width and area are least variable in the uplift zone, and most variable after uplift
has ceased. The transition between narrow, small basins within the uplift zone to wide,
large basins outside of it progresses by integration of small drainages consumed by their
larger neighbors. Prior to basin integration, basins are of similar size and shape;
however, the process of basin capture after uplift has ceased causes basin geometries to
be highly variable.
In addition to basin geometry, channels and tributaries systematically change in
response to the initiation and cessation of uplift. The three main channel and tributary
geometries are typified by three gullies along the pressure ridge (Figure 8; gullies are
labeled A, B, and C in Figure 6). The upper and lower rows of photographs in Figure 8
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show views from the base of the landform to the northeast, and the crest of the landform
to the southwest, respectively. Gully A is located within the beginning of the uplift zone
and is characterized by a well-established channel to which relatively steep hillslopes are
adjusted (Figure 8, first column). Channels northwest of Gully A show steep gradients as
uplift is sustained. During and after sustained uplift, threshold mass failures in the upper
reaches of the basins (Gully B) are adjusted to a channel that is steep in the upper
reaches. Occasionally, gullies to the northwest of Gully B appear to be aggrading slightly
in their lower reaches, due to increased sediment input from the hillslopes. After uplift
has ceased, relief is gradually reduced (Gully C; Figure 8, third column).
The distribution of surface processes shows systematic variations with uplift rate
as well (Figure 9). At the beginning of uplift (southeastern-most section of landform),
small channel systems develop. These networks grow as they become established 300500 m into the uplift zone. The hillslopes in this area are generally convex-up and
gradually sloping. With continued uplift, the gradual hillslopes give way to debris flow
scars and gullying. The density of these gullies is variable throughout the uplift zone and
increases dramatically at and northwest of the uplift zone. In these locations, the
uppermost sections of the hillslopes are convex; however, they are abruptly truncated by
landsliding near the ridge crest. About 400 m after uplift has stopped, the density of
landslide scars decreases as the convex portions of the hillslopes extend into the lower
parts of the ridge. At this point, landsliding along the northeast side of the landform
reduces the ridgeline elevation. Finally, at the northwest end of the landform, debris flow
scars along steep slopes become virtually absent and are replaced by continuous lowslope convex hillslopes.
Based on our study of the Dragon's Back, we propose the following sequence of
topographic responses to uplift. Where no preexisting topography exists, moderate uplift
results in slope steepening and establishment of a channel network. As uplift continues,
slopes are oversteepened and diffusive hillslope transport processes give way to
landsliding. Sustained uplift may steepen low order stream channels that have
insufficient transport capacity to remove uplifted material. While low-order stream
channels may be the most sensitive to uplift, sufficiently high uplift rates may steepen
correspondingly high-order stream channels. Upon cessation of uplift, stream channels
rapidly incise and reach their graded condition. Finally, continued denudation of the
landscape decreases slopes enough to allow diffusive transport to dominate hillslopes.
Field Trip Stops (see map in Figure 10):
Stop: San Andreas Fault in the northernmost Elkhorn Hills and associated near-fault
structures. To get to this stop, proceed southeast on Elkhorn Road until you leave the
biological preserve boundary. This boundary will be marked by a sign posted along one
of the barbed-wire fences through which you will drive. On the right, a 4WD dirt road
will branch from the Elkhorn Road at N35o 07.189’, W119o 37.779’. Proceed down this
road into the channel, and park when the road becomes impassable. Walk down the rest
of the road to the location that the gully debouches into the Carrizo Plain. On the drive
down towards the SAF, notice small graben structures along upper Elkhorn Plain. These
structures presumably have formed above the shallowing of the thrust that uplifts the
NEH. Drive down into gully, where the back side of the Dragon’s Back pressure ridge
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(on cover of Prof. Paper 1515) is defined by SAF. Inspect fault zone, and walk SE along
fault zone looking for springs. Note that here, displacement along SAF and uplift of
Dragon’s Back along its SE side have uplifted Paso Robles Formation. A relatively large
channel breaches the pressure ridge just north of this site and parallels the SAF, just to
the northeast of the fault trace. This channel undermines the back side of the pressure
ridge, resulting in large landslides directed northeastward that facilitate lowering of the
ridgeline of the pressure ridge. Return to vehicles and head back to Elkhorn Rd.
Stop: Morphologic expression of fold bounding northeast side of NEH. Stop is located
along Elkhorn Road at N35o 06.091’, W119o 36.139’. Inspect morphologic expression
of fold that is being created by movement along blind reverse fault. Here, a shallowly
dipping reverse fault folds the Pleistocene Temblor mountain rangefront. Also, note the
location of the active channel incising through this feature relative to its abandoned
counterpart located to the southeast. Also note small grabens forming on the edge of the
fold. These young, low-displacement grabens apparently cut offset the fold topography.
In addition, more depressions (presumably young grabens) are found to the northwest of
this area. These features are interpreted to result from the formation of a third blind
thrust sheet that is forming to the northwest of this location as material of the NEH moves
into the restraining bend of the SAF.
Stop: Long and Skinny Graben. Proceed to next fence boundary after Stop 2 fence.
Along the southern side of this fence, you will see a road that branches to the left and
right. The road to the left heads up to a water tank, while the road to the right heads
towards the SAF. The location of the intersection is N35o 06.543’, W119o 36.433’. Take
the right hand road onto a 4WD road towards SAF (along southeast side of fence; go past
water tank/salt lick area). Road will descend steeply into “Long and Skinny” graben.
Note the proximity of the active contraction with the active extension. Also, note the
smaller scarp formed in the interior basin of the Long and Skinny graben that suggests
deformation may be stepping into the graben with time. Inspect hillslope geometry of
graben shoulder for later comparison with the Big Graben.
Stop: The Dragon’s Back Pressure Ridge. Continue to end of road. Here, get out of the
car and proceed northwest on a hike along the Dragon’s Back pressure ridge. Note the
increase in ridgeline elevations that result from uplift within the southeastern portion of
the pressure ridge. Also, note steep channels with convex-up hillslopes and evidence of
debris flows and shallow landsliding near the mainstem channels. As you proceed to the
northwest, note changes in erosional process distribution: to the northwest, channels cut
down into the landform, exposing deeper portions of the Pink member of the Paso Robles
Formation and undermine hillslopes. Here, shallow landsliding dominates hillslope
transport. As you approach the southern end of the portion of the pressure ridge observed
at Stop 2, note the large landslides that undermine the back side of the pressure ridge and
lower the drainage divide. Finally, note the transition back to hillslopes in which
landsliding is not a dominant erosional agent in the landscape. Return to vehicles.
Stop: Big Graben. Head back out 4WD road to Elkhorn Road, turn right. Pass water tank
on left. Park car next to dirt pile on right, at N35o 06.737’, W119o 35.602’. Walk toward
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the southwest, in the direction of the San Andreas Fault. You will cross over the fold
above the blind thrust just 50 m west of the road. Note here the likely shallow
imbrication that has developed the double-bump in the topography on the east-facing
slope. Walk along the drainage to debauch into the Big Graben. You will see younger and
older fault scarps and collapse pits along the floor of the graben. We infer that both sides
of the graben are bounded by ~equally active faults. The differences in morphology may
be due to aspect effects on bioturbation and hillslope processes. Note the increasing
normal displacement relative to the Long-and-skinny graben. This records progressive
extension to the SE along the SAF, which is probably kinematically linked to motion
along reverse faults that steepen towards the SAF in the subsurface, creating a series of
hangingwall grabens. Also, compare hillslope angles within Big Graben to those
observed within the Long-and-Skinny graben. Note also that geometry of graben
boundaries implies right-lateral shear across the zone of distributed deformation.
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Figure 1: Geologic map of the Northern Elkhorn Hills showing relationship between strike-slip motion along the SAF, contractional deformation
associated with transpression, and extension within the hangingwall of the northeast-vergent thrust sheet. Figure modified from Arrowsmith (1995).
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Figure 2: Aerial photograph of the Northern Elkhorn Hills. Photograph courtesy of the
Fairchild Aerial Photography Collection at Whittier College. Original photograph scale
1:24,000; photograph date 26 February, 1936. Image taken from Arrowsmith (1995).
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Figure 3: Topographic profiles along a beheaded and active channel within the NEH showing
both contractional and extensional deformation of abandoned floodplain. Figure taken from
Arrowsmith (1995). See location on Figure 2.
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Figure 4: Conceptual isometric block diagram through a portion of the NEH showing
relationship between underlying fault geometry and development of grabens in a generally
transpressive tectonic regime. Note also the modeled offset of the SAF at shallow depth (A)
which may drive uplift of the Dragon’s Back. Figure taken from Arrowsmith (1995). 11
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Figure 5: Cross-sections through the Dragon's Back pressure ridge from southeast (A--A') to
northwest (C--C'). Between sections A--A' and B--B', material is uplifted and tilted. After
section B—B’, uplift ceases and erosion exposes the lower units along the monoclinal structure
(C--C'). Figure modified after Hilley (2001) and Arrowsmith (1995). See Figure 1 for
explanation of units.
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Figure 6: Inferred location of uplift zone along the Dragon's Back pressure ridge. Deformation recorded in the uplifted strata of the Paso
Robles formation constrain the area that is uplifted along the pressure ridge. We interpret the deformation pattern to result from the
movement of the Pacific Plate (top) though an uplift zone fixed to the North American Plate (bottom). The deposits are progressively
deformed as the material of the Pacific Plate is moved through the uplift zone. Letters A, B, and C denote the location of Gullies A, B, and
C (Figure 9). Figure taken from Hilley (2001).
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Figure 7: Uplift and basin characteristics along the Dragon's Back. (A) Orthorectified
image of the pressure ridge. (B) Rock uplift (solid line) measured from offset stratigraphic
contacts exposed within the pressure ridge and inferred rock uplift rate (dashed line)
assuming a 35 mm/yr slip rate along the fault and a relatively fixed uplift zone through
which material is advected. (C) Basin width plotted as distance along the fault. Solid line is
200 m interval average of basin width. Figure taken from Hilley (2001).
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Figure 8: Photographs of Gullies A, B, and C. Gullies within the uplift zone (typified by Gully A) are generally steep and
narrow. Hillslopes change from rounded, convex hillslopes to planar hillslopes with landslide scars. Gullies just after the
uplift zone (Gully B) show high relief, numerous side tributaries, and planar hillslopes. Gullies toward the NW end of the
pressure ridge (Gully C) consist of low-gradient channels and convex hillslopes. Note truck for scale in lower photos. Figure
modified from Arrowsmith (1995) and Hilley (2001).
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Figure 9: Geomorphic surface transport process map of the pressure ridge. At the SE end of the pressure ridge, small basins with poorly
developed channels exist. As material moves into the uplift zone, debris flow scars and landsliding dominate the hillslopes. Directly
after the uplift zone, extensive debris flow scars and landslides are seen. Finally, towards the NW end of the ridge, channels once again
become less well-defined and debris flow scars and landsliding are replaced by continuous, convex hillslopes. Figure modified from
Hilley (2001).
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Figure 10: Locations of stops outlined in text. 1 = San Andreas Fault in the northernmost Elkhorn Hills and
associated near-fault structures, 2 = Morphologic expression of fold bounding northeast side of NEH, 3 = Long and
Skinny Graben, 4 = The Dragon’s Back Pressure Ridge, 5 = Big Graben.
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References
Arrowsmith, J. R., 1995, Coupled tectonic deformation and geomorphic degradation
along the San Andreas Fault Zone [Dissertation thesis]: Stanford University, 346
p.
Arrowsmith, J. R., Rhodes, D. D., and Pollard, D. D., 1998, Morphologic dating of scarps
formed by repeated slip events along the San Andreas Fault, Carrizo Plain,
California: Journal of Geophysical Research, B, Solid Earth and Planets, v. 103,
no. B5, p. 10,141-10,160.
Dibblee, T. W., 1973, Regional geologic map of the San Andreas and related faults in
Carrizo Plain, Temblor, Caliente, and La Panza ranges and vicinity, California.
Hilley, G. E., 2001, Fault behavior and landscape development in tectonically active
areas [Dissertation thesis]: Arizona State University, 191 p.
Sieh, K. E., and Jahns, R. H., 1984, Holocene activity of the San Andreas Fault at
Wallace Creek, California: Geological Society of America Bulletin, v. 95, p. 883896.
Thomas, E. A. H. and Sieh, K. E., 1981, Quaternary development of the Elkhorn Hills
along the San Andreas fault: sequential development of folds and thrusts along a
strike-slip fault, Geological Society of America Abstracts with Programs, 13, 566.
Unsworth, M., Egbert, G., and Booker, J. R., 1999, High-resolution electromagnetic
imaging of the San Andreas Fault in Central California: Journal of Geophysical
Research, B, Solid Earth and Planets, v. 104, p. 1,131-1,150.
Wallace, R. E., 1973, Surface fracture patterns along the San Andreas fault, in
Proceedings of the Conference on Tectonic Problems of the San Andreas fault
system, Stanford University, p. 248-250.
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