Introduction to the Northern Elkhorn Hillsweb.gps.caltech.edu/~mpl/Ge121_Web/NEHGuidebook… · Web...

PGS – Spring 2005 Field Trip FIELD DRAFT

“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 HillsSituated 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 fault-propagation 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 near-fault 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 RidgeThe 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-for-time 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 300-500 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 low-slope 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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8

Figu

re 1

: G

eolo

gic

map

of

the

Nor

ther

n El

khor

n H

ills

show

ing

rela

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hip

betw

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ong

the

SAF,

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Fig

ure

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Arr

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995)

.


9

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).


10

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.


11

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).


12

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.


13

Figu

re 6

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ure

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re ta

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14

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).


15

Figu

re 8

: Ph

otog

raph

s of

Gul

lies

A, B

, and

C.

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lies

with

in th

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(typi

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ully

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and

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16

Figu

re 9

: Geo

mor

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surf

ace

trans

port

proc

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ap o

f the

pre

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oorly

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17

Figu

re 1

0: L

ocat

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of s

tops

out

lined

in te

xt. 1

= S

an A

ndre

as F

ault

in th

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and

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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. 883-896.

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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