Uplift and Deformation of Marine Terraces Along the San Andreas Fault; Duncan’s Landing to Fort Ross, California

Breanyn MacInnes

Senior Integrative Exercise March 10, 2004

Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota.

Table of Contents

Abstract

Keywords

Introduction 1

Figure 1 2

Geological Setting 3

Generation of Marine Terraces 3

Preservation of Marine Terraces 5

Figure 2 6

Figure 3 7

Mapping Marine Terraces ______8

Figure 4 9

Terrace Identification 10

Figure 5 11

Age Assignment and Uplift Rate 12

Figure 6 13

Figure 7 14

Figure 8 16

Discussion 17

Figure 9 18

Conclusion 20

Acknowledgements 20

References Cited 21

Uplift and Deformation of Marine Terraces Along the San Andreas Fault; Duncan’s Landing to Fort Ross, California Breanyn MacInnes March 10, 2004 Faculty advisors: Dorothy Merritts, Franklin and Marshall College Cameron Davidson, Carleton College

Abstract Marine terraces between Fort Ross and Duncan’s Landing, California were used to evaluate uplift and deformation of the coast. Six terraces were mapped using Global Positioning System equipment to record position and elevation. Terraces found at elevations of 25-30 m, 40-50 m, 50-70 m, 90-110 m, 100-120 m, and 150-170 m correlate respectively with 83, 108, 122, 194, 305, and 330 ka sea-level highstands. These results give uplift rates between 0.4 and 0.6 mm/yr along the coast. Because of the variation in uplift rates, originally horizontal terraces now appear to tilt down towards the north. Offset terrace inner edges at the San Andreas Fault indicate that the tilting is due to differential uplift on the fault and offset terrace inner edges at the Russian River suggest that there is an unmapped fault along the river channel. The tilting terraces and two locations of offset define three separate fault blocks isolated by the San Andreas Fault and the proposed Russian River Fault. As such, the northern Californian San Andreas Fault and associated splay faults, including the Russian River Fault, are predicted to form a positive flower structure in which the three fault blocks uplift and tilt at different rates.

Keywords: marine terraces, Northern California, San Andreas Fault, deformation, uplifts, GPS, sea level 1

Introduction

The staircase-like marine terraces that line the coast of northern California

illustrate the complex interaction between sea-level fluctuation and tectonism along an

active margin (Bradley and Griggs, 1976; Merritts and Bull, 1989; Anderson, 1990;

Muhs et al., 1990; Lajoie et al., 1991). The mechanisms for creating marine terraces have been in place since at least the beginning of the Quaternary (Rosenbloom and Anderson,

1994; Anderson et al., 1999) which provides a unique opportunity to study the dynamics of the northern San Andreas Fault.

This project, sponsored by the Keck Geology Consortium, is a continuation of a similar study to the north begun in 1999 (Merritts, et al., 2000). The field area is located along a 19 km-long stretch of coast from Fort Ross to Duncan’s Landing, in northern

California, just north of Bodega Bay (Fig. 1). The San Andreas Fault is located onshore in the northern part of the study area, but runs offshore south of Fort Ross, remaining a few kilometers from the coastline in the south (Fig. 1). The relative motion between the

North American and Pacific Plates is approximately 50 mm/yr (Irwin, 1990), and is predominantly accommodated by right-lateral strike-slip motion along the San Andreas

Fault, with a small component of dip slip motion due to compression (Anderson, 1990;

Wallace, 1990a). Previous studies have determined the slip rate for the northern San

Andreas Fault to be 25-30 mm/yr (Prentice, 1989; Brown, 1990).

Uplift rates north of the study area near Fort Ross and Point Arena are estimated to be between 0.24-0.77 mm/yr (Richardson, 2000; Tellinghuisen, 2000). To the south, uplift rates vary between 0.06-0.65 mm/yr near Bodega Bay (Landis, pers. com., 2004) and 0.03-0.3 mm/yr near San Francisco (Peirce, 1996). This study examines a section of 2

study area

Figure 1. Location and tectonic map of the study and surrounding areas. Modified from Merritts (2000). The field area is approximately 150 km north of San Francisco and is bisected by the San Andreas Fault. 3 emerging coastline in northern California to determine the local rate of uplift and deformation related to Late Pleistocene and Holocene motion on the San Andreas Fault system.

Geological Setting

The bedrock of the North American and Pacific Plates in the study area are lithologically similar. The Gualala block of the Pacific Plate is a group of formations from the accretionary wedge formed during the subduction of the Farrallon Plate underneath the North American Plate (Wentworth et al., 1998). The formation in the vicinity of Fort Ross is comprised mainly of sandstone, mudstone, and conglomerate

(Wentworth et al., 1998). The Franciscan Complex of the North American Plate is a slightly older group of accretionary wedge sedimentary rocks comprised primarily of mudstone, sandstone and chert (Irwin, 1990). Due to their similar genesis, history, and lithology, the bedrock is believed to erode at comparable rates across the entire field area.

Generation of Marine Terraces

Marine terraces in northern California are the imprint of a fluctuating eustatic sea level on an uplifting coastline. Sea level in the Quaternary is known to be highly variable and controlled by the glacial-interglacial cycle (Chen et al., 1991; Lajoie et al., 1991;

Gallup et al., 1994; Ludwig et al., 1996). During a glacial period, a significant quantity of the Earth’s water is stored as glacial ice thus lowering sea level. However, in interglacial periods, ice volume is reduced and water accumulates in the oceans (Lajoie et al., 1991). The oscillation between the glacial and interglacial periods creates at least one 4

sea-level maximum, or highstand, during interglacial periods and a minimum, or low-

stand, during glacial periods (Bradley and Griggs, 1976; Lajoie et al., 1991).

Rising sea level has created the modern shoreline by constantly eroding the

bedrock at the intersection of water and land. This erosion causes sea cliffs to retreat,

leaving behind a platform (Rosenbloom and Anderson, 1994; Anderson et al., 1999).

Impacting waves at high tide carry suspended sediment which can abrade the cliff and/or

remove blocks while at low tide the exposed surface is affected by subaerial weathering

(Anderson et al., 1999). The bedrock above high tide eventually collapses, resulting in an equilibrium cliff angle close to vertical (Bradley and Griggs, 1976). Cliffs can retreat slowly if sea level is stagnant, but during a transgression, the retreat rate is much faster

(Anderson et al., 1999).

The submerged surface created by the retreating cliff is the inshore or wave-cut

platform. The inshore platform over time will reach an equilibrium of approximately a 2-

4˚ seaward slope due to erosion by and deposition of wave-suspended sediment (Bradley and Griggs, 1976). A platform is typically flat, but topographic variations due to channel formation and preferential erosion may be up to five meters.

Modern sea level is currently high relative to sea level for most of the Quaternary, although the maxima for the present interglacial period may not have yet been reached.

Because the ocean erodes the coast in the field area at a similar rate (Wallace, 1990b), the profile of the active coastline is analogous to the profile of a paleo-coastline at a sea-level highstand. As such, every highstand has the potential to create comparable sea cliffs and inshore platforms.

5

Preservation of Marine Terraces

During a regression, the paleo-coastline of a highstand is abandoned (Bradley and

Griggs, 1976; Lajoie et al., 1991). However, in areas of regional tectonic uplift, the

landmass is constantly rising. By the next transgression thousands of years later, the

former cliff and platform may be elevated above the sea-level elevation of the next highstand and preserved as a marine terrace (Bradley and Griggs, 1976; Anderson, 1990;

Lajoie et al., 1991). However, not all highstand events record terraces. Terraces will be

erased during the next transgression if the uplift rate is low, the time interval between

highstands small, or the elevation of the next highstand is above the previous one

(Rosenbloom and Anderson, 1994). Also, a long duration highstand can erase older

terraces by causing sea cliff retreat to persist for enough time to pass through one or more

terraces (Anderson et al., 1999).

For terraces that are preserved, the paleo-cliff of a terrace is known as a riser and

the abrupt concave change in slope between the riser and the paleo-inshore platform,

referred to as the inner edge, marks the elevation of the highstand (Fig. 2). The uplifted

paleo-inshore platform, the tread, is mantled during the regression by marine sediments,

but still remains relatively horizontal (Bradley and Griggs, 1976; Lajoie et al., 1991).

The convex point of inflection, where the tread meets the riser, is known as the outer

edge (Fig. 2).

Once a terrace is abandoned, it is acted on by fluvial, colluvial, aeolian and

biological processes which degrade, disconnect, or erase their record. Erosion of

elevated sea cliffs is initially rapid as a result of land sliding and other mass-wasting

processes, but diminishes as the slope of the riser decreases and covers the inner edge Tread Outer Edge Outer Edge Riser Tread Inner Edge Tread Riser Inner Edge

Figure 2. View north from Portuguese Beach towards Duncan's Landing (headlands on left) showing the components 6 of a terrace. The dashed line is the modern sea cliff and inshore platform. The cliff is approximately 25 meters high. 7

terrace 3 riser inner edge

terrace 2 riser

inner edge

terrace 1

stream channels

modern modern cliff beach

Figure 3. Terrace dissection by stream incision. As stream channels evolve they erode through and dissect older terraces. Terrace risers also become less apparent over time as they reduce their gradient through erosion. Modified from Anderson et al. (1999). 8

with colluvial material (Bradley and Griggs, 1976; Wallace, 1990b; Lajoie et al., 1991;

Anderson et al., 1999). As a result, an older terrace has a less distinct change in slope at

the inner edge or outer edge and can be difficult to recognize in the field or on aerial

photographs.

Terrace discontinuity is created by stream incision as channels with large drainage areas erode through risers and dissect the terraces (Fig. 3) (Anderson et al., 1999). As uplift continues, more channels increase their drainage area to the extend that enables them to erode through risers. These channels also widen with age as streams respond to lowered base level, incise more and steepen their banks, causing mass wasting. The older the terrace, the more often base level has fluctuated, leading to the stream’s influence on the terrace widening with age (Anderson et al., 1999). The decrease in the gradient of

risers and isolation of intact inner edges onto more resistant ridges can make the oldest

terraces difficult to distinguish from other topographic variations.

Mapping Marine Terraces

Stereoscopic analysis of aerial photographs initially determined the location of

terrace inner and outer edges. Later fieldwork incorporated field mapping and the use of

Trimble Global Positioning System Pathfinder Pro XR receivers in recording the precise

elevation of specific terrace components. Transects utilizing these GPS units were

walked perpendicular to terraces in areas identified by preliminary mapping to have

multiple terrace exposure with minor stream erosion, or in areas where an inner edge was

identified from aerial photographs (Fig. 4). Each transect data point recorded the

Northing (± 0.5 m), Easting (± 0.5 m), elevation (± 1 m), location on the terrace, and the

depth to bedrock if known.

10

In the Muniz Ranch area (Fig. 4), between the Russian River and Russian Gulch,

the land east of Highway One was inaccessible for field work because of Private Property

concerns. To prevent a large data gap, virtual transects were created using elevation data

from the Arched Rock quadrangle topographic map. Inner edges were identified,

although vertical resolution is low because topographic variations less than forty feet

were missed.

To determine accurately the elevation of terraces, one must know the bedrock elevation. Sediment cover on the bedrock deposited after the sea-level highstand by

colluvial processes at the inner edge and riser and colluvial and marine processes on the

tread means that the ground surface elevation today does not accurately reflect the

uplifted terrace elevation. Bedrock exposure on these surfaces is rare, so most elevation

readings incorporate local sediment cover. If bedrock was exposed on the tread or outer

edge, then the bedrock elevation of the inner edge was estimated by projecting a line with

a 2-4° slope, the average gradient of a wave-cut platform (Bradley and Griggs, 1976),

from that point to where it intersects the position of the sediment-inner edge. The result

is a range for the bedrock-inner edge elevation. In situations where no bedrock exposures

existed, the calculated sediment thickness covering the inner edge bedrock surface on

terraces above and/or below gave a range of potential bedrock elevations.

Terrace Identification

Detailed fieldwork and subsequent analysis of GPS data identified a minimum of

six terraces near Duncan’s Landing but only one terrace southeast of Fort Ross (Fig. 5).

Bedrock elevations, combined with aerial photographs and field mapping, enabled the

correlation of terraces across the field area. However, this process was severely hindered 11

Fort Ross

Legend Terrace 1 inner edge h ulc ll G Mi Terrace 2 inner edge Terrace 3 inner edge N Terrace 4 inner edge Terrace 5 inner edge Unknown Terrace scale (km) 1 0 1

s

a

e

r t

l d n sia u s n Ru h a ulc A G F

n

a

S

r ve Ri n ia ss Ru

Duncan's Landing

Figure 5. Map showing terrace inner edge location. Some high terraces were not able to be correlated laterally. 12

by terrace degradation. Terraces 4, 5 and 6 are spatially discontinuous across the entire

field area due to stream erosion and in the Muniz Ranch and Russian Gulch areas Terrace

4 apparently eroded through Terrace 5. Minimal to no terrace preservation occurs

directly southeast of the San Andreas Fault Zone (Fig. 5). The bedrock in this location is

highly erodible because it has been sheared during motion along the fault (Wallace,

1990b) resulting in land-sliding and erosion from the present highstand, eliminating the

record of terraces.

Age Assignment and Uplift Rate

Potential ages of formation of the terraces in the field area were derived from the work of Darter (2000) Lambeck, et al. (2001) and Muhs, (pers. com., 2003) (Fig. 6).

Their sea-level curves are a compilation of global studies of uplifted marine terraces and coral reefs combined with radiometric dating of fossils. The timing and elevation of some highstands, like the elevation for the 120-124 ka highstand, are thought to be well constrained. Others are much more controversial, such as the 305 ka highstand, which

may have been equivalent to current sea level (Muhs, pers. com., 2003), 28 meters below today (Merritts, pers. com., 2004) or may not have existed (Darter, 2000). The combined sea-level curve (Fig. 6) aims to give the best assimilation of current understanding.

Terrace age assignments are based on the assumption that for any given transect,

the rate of tectonic uplift remains constant over time. The argument for this is that the

tectonic regime driving uplift has probably not changed substantially on the time scale of

terrace formation (Lajoie et al., 1991). In order to date the terraces, age assignments are

proposed and then analyzed to find dates that correspond to a uniform uplift (Fig. 7). For

-28 mbyDarter(2000)and0Muhs(2003). peaks aresea-levelhighstands,inka,thatrecordterracesthefieldarea. Figure 6.Eustaticsea-levelcurvemodifiedfromDarter(2000),Lambecketal.(2001)andMuhs(pers.com.,2003).Labeled

Sea Level (m) -140 -120 -100 -80 -60 -40 -20 20 0 0 50 82-84 100 108 119-124 Eustatic SeaLevel 150 193-195 200 Age (ka) 250 The 305hahighstandhasbeenrecordedat 300 305 305 330 350 410 400

450

3 1 A B Shell Beach area uplift: first terrace 62 ka Shell Beach area uplift: first terrace 83 ka 200 200 Terrace 1 Terrace 1 Terrace 2 Terrace 2 Terrace 3 150 Terrace 3 150

) Terrace 4 Terrace 4 )

m

m

(

Terrace 5 (

Terrace 5

n Terrace 6 100 Terrace 6 100 n

o

o

i

i

t

t

a

a

v

v

e

50 e

l

50 l

E

E 0.76 mm/yr 0 0.51 mm/yr 0.58 mm/yr 0.64 mm/yr 0.48 mm/yr 0.50 mm/yr 0 0.75 mm/yr 0.50 mm/yr 0.51 mm/yr 0.70 mm/yr 1.21 mm/yr 0.48 mm/yr -50 -50 400 350 300 250 200 150 100 50 0 400 350 300 250 200 150 100 50 0 Time (ka) Time (ka) C D Shell Beach area uplift: first terrace 108 ka Shell Beach area uplift: first terrace 122 ka 200 200 Terrace 1 Terrace 1 Terrace 2 Terrace 2 Terrace 3 150 Terrace 3 150 Terrace 4 ) Terrace 4 )

m m

Terrace 5 ( (

Terrace 5

Terrace 6 100 n 100 n

o o

i i

t t

a a

v v

50 e 50 e 0.42 mm/yr l l

E 0.26 mm/yr E

0.31 mm/yr 0 0.25 mm/yr 0 0.32 mm/yr 0.27 mm/yr 0.17 mm/yr 0.18 mm/yr 0.39 mm/yr 0.41 mm/yr 0.31 mm/yr -50 -50

400 350 300 250 200 150 100 50 0 400 350 300 250 200 150 100 50 0 1 Time (ka) Time (ka) 4 Figure 7. Potential age assignments for the Shell Beach transect. Uplift rates, given by the slope of the line connecting the elevation at formation and current elevation, only remain constant if Terrace 1 is 83 ka. 15 example, at Shell Beach, if Terrace 1 is assigned to the 83 ka sea-level highstand, then the uplift rate can be determined by equation 1.

Uplift = (Present Elevation – Elevation at Formation) (1)

Age of Terrace

Each preserved highstand is then sequentially assigned to Terrace 2, 3, etc. and uplift rates are calculated from the slope of the line connected the age and elevation of formation with the current elevation (Fig 7.). The results show that assuming an age of

83 ka for Terrace 1 results in relatively constant uplift rates of approximately 0.5 mm/yr.

Terraces recorded with this uplift rate are the 83, 108, 122, 194, 305, and 330 ka highstands.

Some of the mapped inner edge locations were not able to be correlated in the field or with aerial photographs. This is because these locations had become isolated by streams or were hidden in aerial photographs by concentrations of sea stacks. However, using the calculated uplift rates for a well known section, the age of some of these uncorrelated terraces could be inferred.

Once age assignments were made to as many of the mapped terraces as possible, six terrace forming events that correspond with the 83, 108, 122, 194, 305 and 330 ka sea-level highstands were able to be correlated across much of the field area, (Fig. 8). In the southernmost area, the uplift is calculated at 0.5 mm/yr. Following the terraces northward towards the Russian River, the inner edge elevations of the terraces decrease and the corresponding calculated uplift rate is lower (0.4 mm/yr). Just north of the

Russian River, the calculated uplift rate is 0.6 mm/yr. The calculated uplift rate near

Russian Gulch is 0.45 mm/yr. North of the gulch, the San Andreas Fault is closer to Inner Edge Bedrock Elevation San Andreas Fault Zone Russian River 250 225

200 6 175 6

) 5b 150 (m

n 5a

o 125

ti 4 a

v 100 4 e l e 75 3 3 3 50 2 2 2 1 25 1 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 N distance along coast (m) S

Figure 8. Inner edge elevations mapped along a north-south cross-section of the coast. Solid lines show lateral continuation of terraces evident in aerial photograph or field mapping. Dashed lines are possible correlations across areas where the inner edge no

longer exists. Terraces that are assigned ages are numbered 1-6. Inner edges above Terrace 6 were not used. Terrace 5a and 5b are 16 the 305 ka high-stands at either -28 m or 0 m (see Fig. 6). 17 shore and the bedrock erodes rapidly so few inner edges remain. Therefore calculated uplift rates in this area were obtained by projecting the slope of the terraces between the

Russian River and Russian Gulch. The closest terrace to the San Andreas Fault is designated Terrace 4, giving an uplift rate of 0.4 mm/yr. Across the fault at Fort Ross, the calculated uplift rate is relative well-constrained at 0.5 mm/yr.

Discussion

Mapping and terrace correlation along a north-south section of the California coastline from Ft. Ross to Duncan’s Landing identified six terraces that formed during the 83, 108, 124, 194, 305 and 330 ka sea-level highstands. The average calculated rate of uplift for these terraces is approximately 0.5 mm/yr, which is slightly greater than the observed average for the emerging northern California coast. For example, Peirce (1996) found rates of 0.03-0.3 mm/yr, 150 km south of the study area, and Landis (2004) found rates of 0.06-0.65 mm/yr immediately south of the study area. To the north, Richardson

(2000) found rates of 0.24-0.77 mm/yr in an adjacent study area, and Tellinghuisen

(2000) found rates of 0.47-0.61 mm/yr, 50 km north.

The elevations of terrace inner edges decrease northward, resulting in the tilting of an originally horizontal feature (Fig. 8). There are also noticeable breaks in the elevations of the inner edges at both the San Andreas Fault and the Russian River. The tilting and breaks in elevation result in the observed uplift rate varying between 0.4 mm/yr and 0.6 mm/yr from south to north (Fig. 9).

The break in elevation at the San Andreas Fault and the Russian River shows that terrace inner edges are offset (Fig. 8); the inner edges are higher on the northern side than 18

A 0.53 h mm/yr ulc ll G Mi N

0.40 mm/yr ian uss R lch Gu San Andreas Fault 0.60 mm/yr

r 0.40 ive R mm/yr an ssi Ru

0.53 Russian River mm/yr Fault

B h ulc Legend ll G Mi Block 1 Block 2 Block 3 N River Strike-slip fault Fault with ian San Andreas uss unknown R lch kinematics Fault Gu

r ive R an ssi Ru

Russian River Fault

Figure 9. The geometry of the San Andreas Fault and proposed Russian River Fault. A. Uplift rates for various sections along the coast. B. Proposed separate fault blocks. The proposed trace of the Russian River Fault is derived from the knowledge that the fault must offset the northern and southern banks of the river, the fact that there was an observed shear zone parallel to the coast at Russian Gulch, and that in aerial photographs there appears to be a linear feature south of the river, potentially due to faulting. 19

the southern. A method of offsetting terrace elevation is through vertical motion along a

fault. Therefore, the break in terrace elevation across the San Andreas Fault is due to

such differential uplift; the north side of the fault is uplifting faster relative to the south side. However, there is no documented fault at the Russian River to explain the break in terrace elevation across the river. The abrupt change is not an erosional feature because the primary eroding agent, the river, has been incised within its modern banks for at least the last 200 kyr. Because the terraces are offset on either side of the Russian River and rivers follow paths of least resistance (Wallace, 1990b), an undiscovered fault may exist along the river channel. The San Andreas Fault system is a complicated and broad deformation zone with many splay faults (Brown, 1990; Fuis and Mooney, 1990;

Griscom and Jachens, 1990). The presence of sag-ponds and sheared rock at Russian

Gulch and a newly discovered sag-pond at Eagle Rock show evidence of faulting off the trace of the San Andreas Fault. Local faulting associated with the Eagle Rock sag-pond could account for the anomalously high inner-edge reading of Terrace 2 on the Eagle

Rock transect (Fig. 8). Therefore, the existence of a fault along the Russian River is consistent with the prevalence of faulting in the field area.

Differential uplift implies that there are three distinct crustal blocks in the study area separated by the San Andreas Fault and the proposed Russian River Fault (Fig. 9).

Each block is an individual unit that moves both horizontally and vertically at different rates, causing the observed tilting of the terraces. Other northern California study areas also record tilting (Peirce, 1996; Grove, 2003; and Landis, 2004) and deformation due to faulting, whether from the San Andreas Fault or associated splay faults (Peirce, 1996 and

Tellinghuisen, 2000). 20

Because of the differential movement on the faults, the fault geometry proposed

in this paper is that the San Andreas Fault and associated fault splays form a positive

flower structure (Harding, 1985). A flower structure occurs when compression, and

associated crustal shortening, is accommodated by slivers of crustal material uplifting as independent crustal blocks rather than by large-scale folding or faulting. This model also explains why, in a compressive environment, uplift rates of similar magnitude are found on both sides of the San Andreas Fault.

Conclusion

Analysis of data gathered on the coast of northern California identified six marine

terraces, however, correlation between transects was difficult due to degradation and

erasure of terrace inner edges. Younger terraces were easily traced in aerial photographs

and in the field, but older terraces are no longer spatially continuous.

Assuming an age of 83 ka for the lowest terrace, uplift rates vary between 0.4 and

0.6 mm/yr along the coast. The originally horizontal terraces gradually tilt northward.

Variation in uplift rate suggests there are three separate fault blocks isolated by the San

Andreas Fault and the proposed Russian River fault. Further study could advance this

work by confirming the dating of terraces and/or finding more evidence of the proposed

fault.

Acknowledgments

This project was completed under the supervision of the Keck Research

Symposium. It was a joint project with six other students and three advisors. Each

student had their own field area but helped with others’ field work. Thanks are given to 21 those students, Graham Boardman, Annie Covault, Paul Landis, Sarah Leibson, David

Trench, and Andrey Voynov and advisors, Tom Gardner, Dorothy Merritts and Carol

Prentice. Primary advisors of the integrative exercise were Dorothy Merritts and

Cameron Davidson, but the additional help from Dave Bice, Clint Cowan, Bereket

Haileab, Jenn Macalady, Sara Mitchell, Mary Savina and Tim Vick has been greatly appreciated.

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