Fault interaction at the junction of the Transverse Ranges and

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Tectonophysics 379 (2004) 43–60

Fault interaction at the junction of the Transverse Ranges and

Eastern California shear zone: a case study of intersecting faults

James A. Spotila*, Kevin B. Anderson

Virginia Polytechnic Institute and State University, Blacksburg, VA, USA

Received 21 August 2002; accepted 29 September 2003

Abstract

A case study from southern California illustrates the value of detailed geologic data for understanding the kinematics of

complex fault systems. The neotectonic behavior of faults at the junction of the Eastern California shear zone (ECSZ) and

Transverse Ranges has implications for the interaction of intersecting, segmented fault systems and regional plate boundary

evolution. Paleoseismic observations indicate that the North Frontal thrust system (NFTS) has ruptured once in the Holocene

with 1.7-m displacement, despite previous speculations of inactivity based on its dissection by younger strike-slip faults. Simple

polyphase deformation, in which dextral shear has replaced and overprinted thrusting, is thus not a valid explanation for this

system of intersecting faults. This illustrates the limitations of inferring rupture behavior from mapped fault patterns alone.

Neotectonic and geomorphic observations along the thrust system also suggest that the thrust segment west of the intersection

with the dextral Helendale fault is significantly more active than the segment to the east. This is consistent with a simple block

velocity model, in which dextral slip on the Helendale fault is balanced by convergence on the western thrust segment, dextral

motion on the poorly studied Pipes Creek fault to the southeast, and inactivity on the eastern thrust segment. This divides the

San Bernardino Mountains into domains dominated by thrusting (west) and strike-slip (east), the union of which is a quasi-

stable triple junction. We speculate that this union has migrated to the west as the Mojave Desert has been translated southwards

along the San Andreas fault.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Eastern California shear zone; Transverse Ranges; Faults

1. Introduction

Complex fault systems are a common mechanism

of accommodating plate motion within the continental

lithosphere (Molnar, 1988). Suites of intersecting,

0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2003.09.016

* Corresponding author. Geological Sciences, 4044 Derring

Hall, Virginia Tech, Blacksburg, VA 24061, USA; Tel.: +1-540-

231-2109; fax: +1-540-231-3386.

E-mail address: [email protected] (J.A. Spotila).

conjoined, and otherwise spatially associated faults

can represent quasi-stable, least-work means of yield-

ing to distributed strain (e.g., Jackson and McKenzie,

1994; Nur et al., 1993). As a result, the kinematics of

active continental deformation can be perplexing and

associated seismicity patterns difficult to forecast.

Understanding the interaction of such complex fault

systems, with respect to the long-term evolution of

plate boundaries and the dynamic behavior of rupture

cycles, is an important challenge.

J.A. Spotila, K.B. Anderson / Tectonophysics 379 (2004) 43–6044

Faults may be spatially associated and interact in a

variety of ways, such as conjugate fault pairs, faults

produced by slip partitioning, and secondary faults at

bends or stepovers (Fig. 1). Such fault systems inter-

act dynamically and can result in complex rupture

patterns (Segall and Pollard, 1980; Philip and Megh-

raoui, 1983; Hudnut et al., 1989). Fault unions and

intersections are particularly important for rupture

mechanics, because they can act as segment bound-

aries that sometimes control rupture dimension (e.g.,

Sharp et al., 1982; Barka and Kadinsky-Cade, 1988;

Harris and Day, 1993; Magistrale and Day, 1999).

What will act as a segment boundary can be hard to

predict until observed in an actual earthquake event

(Sieh et al., 1993; Rubin, 1996). Complex groupings

of faults also occur when deformation patterns are

overprinted or represent transient ‘‘snap-shots’’ of

fault evolution, such as within nascent shear zones

or in response to changing plate boundary conditions

(Wilcox et al., 1973; Tchalenko, 1970; King and

Yielding, 1984). This also occurs on short time scales,

such as the dynamic interaction of individual faults

during rupture sequences (Wald and Heaton, 1994;

Spotila and Sieh, 1995; Harris and Day, 1999). Fault

groupings can often be so complex as to seemingly

violate simple rules of fault mechanics or to require

unusual arrangements of principal stresses (Anderson,

1905), such as with intersecting faults that form

quadruple junctions (Fig. 2). Alternatively, these fault

sets may represent polyphase deformation, in which

active faults cut previous generations of structures.

Mapped patterns of cross-cutting relationships be-

Fig. 1. Schematic examples of complex interacting fault systems. (a) Conjug

(Anderson, 1905), are common in the rock record (e.g., Nieto-Samaniego a

earthquakes (e.g., Jones and Hough, 1995; Pavlides et al., 1999). (b) Faults o

between pure and simple shear on oblique plate margins (e.g., Norris and Co

Faults of different slip type also result from stress-fieldmodification near fau

bends and strike-slip tear faults between dip-slip fault segments (e.g., Gaud

tween faults might even be indicative of what struc-

tures are inactive. In most cases, however, existing

geologic data do not contain the completeness re-

quired for kinematic interpretation of how such com-

plex fault systems evolve and dynamically interact.

In this paper, we explore the interactions of inter-

secting faults from a location where ample preexisting

data help to define fault behavior. The Transverse

Ranges of southern California provide examples of

interacting faults at several levels (Fig. 2c). This

system of convergent deformation occurs on both

sides of the southern San Andreas fault that is oblique

to plate motion (the ‘‘big bend’’), creating a complex

intersection near San Bernardino (Blythe et al., 2002).

The fold and thrust belt of the western Transverse

Ranges is, in turn, abutted by strike-slip faults that are

parallel to the San Andreas fault, including the Whit-

tier–Elsinore, Newport Inglewood, and San Jacinto

faults (Tsutsumi et al., 2001). These fault confluences

create mechanical problems that render the forecasting

of rupture scenarios dubious (Dolan et al., 1995). To

the east in the southern Mojave Desert, the North

Frontal thrust system (NFTS) is intersected by fault

strands of the Eastern California shear zone (ECSZ)

(Dibblee, 1975). We have focused on this region

because the intersection of these fault systems creates

quadruple junctions that may interact in a complex

dynamic fashion or may be the result of polyphase

deformation. By documenting fault activity using

geomorphic and paleoseismic observations, we test

whether mapped fault patterns are actually indicative

of active deformation and in turn explore the recent

ate fault pairs are predicted by basic Mohr–Coulomb fracture criteria

nd Alaniz-Alvarez, 1997; Ingles et al., 1999), and produce compound

f different slip type may be associated due to deformation partitioning

oper, 1995; Mount and Suppe, 1987; Jones andWesnousky, 1992). (c)

lt tips and stepovers (Sibson, 1986), such as dip-slip faults in strike-slip

emer et al., 1995; Bayasgalan et al., 1999).

Fig. 2. Examples of complex interacting fault systems that create unstable quadruple junctions and require a variable stress field in order to fit

with Anderson’s (1905) rules of faulting unless fault segments are inactive. (a) The intersection of the Bitlis thrust zone and sinistral East

Anatolian fault (after Barka and Kadinsky-Cade, 1988; Lyberis et al., 1992; Saroglu et al., 1992) may be explained if the western continuation of

the Bitlis thrust is inactive and the fault union near C�elikhan is a triple junction between westward-extruding Anatolia and colliding Arabia and

Eurasia. Changes in stress domain between vertical and horizontal r3 and the inferred direction of maximum horizontal compression are shown.

(b) The complex intersection of the dextral Nayband–Gowk fault and Shahdad–Kuhbanan thrust system in Iran (after Berberian et al., 1984)

may similarly represent a polyphase deformation pattern which is not now entirely active. (c) The intersection of the North Frontal thrust system

(NFTS) and Eastern California shear zone (ECSZ) (indicated by circle) is the case used in this study. The direction of relative plate motion

between Pacific and North America is shown, which is f 25–30j oblique to the ‘‘big-bend’’ segment of the San Andreas fault (SAF).

GF =Garlock fault, HF =Helendale fault, NIF =Newport– Inglewood fault, SB = city of San Bernardino, SJF = San Jacinto fault, SM–

CF=Sierra Madre–Cucamonga fault zone, TR=Transverse Ranges, WEF=Whittier–Elsinore fault.

J.A. Spotila, K.B. Anderson / Tectonophysics 379 (2004) 43–60 45

interaction and long-term evolution of two complex

fault systems.

2. The North Frontal thrust system and Eastern

California shear zone

The NFTS occurs along the northern range front of

the San Bernardino Mountains and has been largely

responsible for uplift of the Big Bear plateau over the

past few Myr (Dibblee, 1975; Meisling and Weldon,

1989; Spotila and Sieh, 2000). The 80-km-long thrust

system is complex, consisting of discontinuous, over-

lapping scarps and folds in Pleistocene alluvium and

older shear zones in bedrock across a >1-km-wide

zone (Miller, 1987; Bryant, 1986; Eppes et al., 2002)

(Fig. 3). The degree of the NFTS’s recent activity is

not well known, and there is no previous evidence for

major Holocene motion. The thrust has produced

minor seismicity, such as a 1992 M = 5.4 event that

occurred as an aftershock triggered by the modified

stress field following the Landers earthquake (Feigl et

al., 1995) (Fig. 3). Scarps in late Pleistocene alluvium

are common, but these are of uncertain age (Dibblee,

1967; Miller, 1987). Based on the total vertical offset

along the thrust of about 1.6 km, the average uplift

Fig. 3. Fault map of the San Bernardino Mountains region, showing the intersection of the NFTS and Helendale fault. Numerous strike-slip

faults of the ECSZ occur in both the hangingwall and footwall of the eastern thrust system. Focal mechanisms are shown for the 1992 Landers

(a) and Big Bear earthquakes (b), as well as a 1992 aftershock with a thrust mechanism that probably occurred on the thrust (Feigl et al., 1995)

(c). Earthquakes ofM>4 with focal mechanisms consistent with dextral motion on northwest trending planes are shown as filled squares (1983–

6/28/92), open circles (6/28/92–11/27/92), and filled circles (11/27/92–2/16/02) (Jones and Hough, 1995).


rate over the past 2–3 Ma has been >0.5 mm/year

(Spotila and Sieh, 2000). In contrast, uplift rates based

on soil–chronosequence age estimates of late Pleis-

tocene ( < 0.5 Ma) alluvium in 25- to 70-m-high

scarps are much slower, ranging from 0.05 to 0.3

mm/year (Meisling, 1984; Bryant, 1986; Meisling and

Weldon, 1989). Thus, the NFTS has decelerated and

may even have even become inactive.

The southernmost ECSZ is a broad array of

discontinuous right-lateral faults that are sub-parallel

to the San Andreas fault and have been active for at

least the past few million years (Savage et al., 1990;

Dokka and Travis, 1990a). It initiates near the

restraining bend at San Gorgonio Pass and continues

north for f 1000 km to interact with the Basin and

Range. The ECSZ accounts for about 11–14 mm/year

dextral shear, or f 25% of the relative motion

between North America and the Pacific plate (Miller

et al., 2001). Slip rates on individual faults in the zone

are generally undocumented, but would be f 1 mm/

year if slip is equally distributed from west to east

across the dozen or so parallel fault strands (e.g.,

Houser and Rockwell, 1996). Earthquake recurrence

times for these faults average several thousand years

(Rockwell et al., 2000). Nonetheless, the ECSZ has

produced significant earthquakes in the last several

decades (Sieh et al., 1993; Fialko et al., 2001). The

western limit of the southern ECSZ is defined by the

Helendale fault, one of the more continuous strands.

This fault bisects the NFTS into segments; a western

branch unbroken by the shear zone and an eastern

branch within the shear zone (Fig. 3). The eastern

segment is intersected and abutted by several other

dextral faults (e.g., Old Woman Springs and Lenwood

faults). Neither the Helendale fault nor the other

dextral faults have experienced historical rupture,

but paleoseismic investigations of dextral faults north

of the NFTS have demonstrated that they have been


active in the Holocene and have earthquake recur-

rence times of several thousand years (Bryan and

Rockwell, 1995; Houser and Rockwell, 1996).

The degree of recent activity along the ECSZ and

the apparent lack of Holocene motion along the NFTS

point to the possibility that the dextral system has

replaced and rendered inactive the thrust system

(Sadler, 1982; Meisling and Weldon, 1989). This

would make their intersection a cross-cutting relation-

ship between older and younger fault sets. In this case,

the M = 5.4 earthquake in 1992 would represent a

local fault patch triggered during an aftershock se-

quence, not indicative of major through-going activity

on the entire NFTS. This ‘‘cross-cut’’ model fits with

the younger geomorphic expression of the strike-slip

faults and recent seismicity that has been dominated

by strike-slip mechanisms. The 1992M = 6.5 Big Bear

sequence was produced by rupture of short conjugate

strike-slip faults (Jones and Hough, 1995). Subse-

quent to this sequence there have been numerous

dextral events of M>4 throughout the San Bernardino

Mountains, particularly between the San Andreas fault

and the Big Bear epicenter (Fig. 3). These coincide

with short, northwest trending fault strands, such as

the Dollar Lake and Deer Creek faults, which show

Fig. 4. Aerial photograph of the western segment of the NFTS (location

1:30,000, but shown here asf 1:65,000. Box denotes location of Fig. 5 a

evidence of Quaternary displacement and could rep-

resent a nascent dextral system (Spotila and Sieh,

2000). Dextral shear in the hangingwall of the NFTS

supports the idea that the ECSZ is growing westward

and has replaced thrusting.

The cross-cut model is attractive, because it

presents an escape from a structural conundrum of

intersecting strike-slip and reverse faults that require

different stress regimes in the same volume of crust

(Anderson, 1905). The model circumvents the me-

chanical difficulties of active, intersecting faults by

invoking polyphase deformation, which predicts that

the thrust fault predates the strike-slip system.

Unfortunately, fault activity data have failed to

validate this model by proving that the NFTS has

become inactive. Demonstrating that any fault is

inactive, rather than just slowly deforming with

long intervals between seismic activity, is difficult

along range fronts in which sedimentation and

erosion rates are rapid. Geologic mapping has also

yet to delineate a clear cross-cutting relationship at

the intersection of the NFTS and Helendale fault

(Miller, 1987; Sadler, 1982; H. Brown, Omya

(California) personal communication, 1999). The

cross-cut model is still testable, however, in that

shown in Fig. 3). Original scale of photograph is approximately

nd the Marble Canyon paleoseismic site.


it can be disproven or modified if evidence is

found for recent activity along segments the NFTS.

To evaluate this, and thus learn more about fault

interaction and regional seismic hazards, we thus

tested the cross-cut hypothesis using paleoseismic

and neotectonic techniques. We first used paleoseis-

mology to test whether the NFTS has been recently

active. We then used geomorphic observations to

evaluate the longer-term activity along each segment

of the NFTS, to see what effect the intersecting

ECSZ may have had and to develop a kinematic

model of this deformation system.

Fig. 5. Quaternary map of the thrust fault at location indicated by

box in Fig. 3. Sharp scarp is indicated by solid, heavy black line

(dotted where buried, dashed where inferred). The older segment in

late Pleistocene alluvium (Qa3) is 5.9 m high at aaV and 4.8 m high

at bbV. Younger 1.2-m-high scarplet in younger alluvium (Qa2) at

ccV was previously unrecognized and excavated (indicated by white

bar) in this study (Fig. 8). A second fault may be present at an

inflection in the base of the WSW-trending ridge in the south, which

consists of northward-tilted section of resistant, older Pleistocene

alluvium. Roads are indicated by parallel dashed lines. Contours are

from 1:24,000 scale topographic map with 40-ft interval. Stream

courses are light dashed lines. Irregular polygon defined by solid

black line indicates location of the detailed map in Fig. 6.

3. Paleoseismic observations

We searched for sites along the thrust system

with young tectonic landforms to maximize the

probability of identifying recent rupture. This search

was limited to the fault segment west of the Helen-

dale fault, to limit possible reactivation due to local

convergence or slip transfer within the ECSZ and

because reconnaissance revealed numerous sites

with apparently young tectonic geomorphology

(Spotila and Anderson, 2000). Well-defined thrust

fault scarps are common there, but most are inap-

propriate for assessing recent earthquake activity.

High scarps that record cumulative displacement of

old alluvium are difficult to date and are not ideal

for paleoseismology (Anderson et al., 2003). We

focused on small scarps in the alluvium with the

least degree of soil development, to capture the

most recent deformation.

About 3 km west of the Helendale fault at Marble

Canyon, the thrust system consists of several discon-

tinuous, overlapping scarps and folds in Pleistocene

alluvium (Fig. 4). The sharpest scarp is several kilo-

meters long, f 5–8 m high, and set within late

Pleistocene alluvium of mixed granitic and carbonate

composition between older structures to the north and

south. It is locally broken by stretches of alluvium

with less soil development and appears to have been

eroded and replaced by younger deposition (Fig. 5).

At one location, a subtle, 1.2-m-high apparent scarp in

the younger alluvial (Qa2) surface occurs along strike

of the larger, older scarp (Figs. 6 and 7). This feature

trends obliquely to the main scarp, allowing it to be

interpreted as either an erosional feature along the

grain of alluvial transport or an actual scarplet. We

excavated this feature, on the chance that it was

indeed a degraded fault scarp.

A 20-m-long, 3-m-deep bulldozer excavation across

this small alluvial ridge revealed the youngest known

displacement along the NFTS (Fig. 8). A coherent

stratigraphy of coarse, poorly indurated, f 0.5-m-

thick channel gravels is clearly offset by 1.7 m of

dip-slip along a N85jW, 23j south-dipping plane.

Two thin, poorly sorted, matrix-dominated paleosols

are more cemented and appear white due to pedogenic

carbonate. All horizons thicken downslope, but the

minimal thickness variation across the fault suggests

the fault experienced a minimal (if any) strike-slip

component. The precise slip vector could not be

measured, given the two-dimensional exposure and

lack of slickensides on the fault plane. We can thus

Fig. 6. Detailed topographic and geomorphic map of the Marble Canyon paleoseismic site. Elevations were determined with a laser theodolite

(total station). Points indicate surveyed locations. Contours are 50 cm interval and were hand drawn. The gray box indicates the location of the

excavation across the scarplet. Unit Qa4c is colluvium derived from Qa4 alluvium. Location of figure shown in Fig. 5.


not discount the possibility of minor oblique displace-

ment. Note that the apparent vertical separation of Bkb2

is slightly (f 25%) greater than Bkb along the fault.

Although this hints at two faulting events, the lack of a

colluvial wedge or evidence of a buried ground surface

between these paleosols argues this is unlikely. The

difference in vertical separation is thus apparent, due

most likely to thickness variations in the layers and the

fact that the trench, displacement direction, and depo-

sitional trend are not mutually parallel. An absence of

any cumulative deformation features, such as rotated

clasts or multiple shear planes, also argues that this

fault plane and offset were produced by a single rupture

event.

The fault can be clearly traced to within f 1 m of

the surface. In the upper portion of the trench, gravel

layers are less compacted, less cemented, and sand-

ier, and thus disaggregate easily. The fault is lost

where a sandy gravel (G1) is juxtaposed against

itself. The uppermost layer is a weak Bw horizon

and does not appear to be offset, although it has

probably experienced recent pedogenic reworking.

Post-faulting erosion and colluvial deposition has

also taken place, based on the absence of the highest

Fig. 7. Topographic profiles across the scarp and scarplet at Marble Canyon. Lines are indicated on Fig. 6 (bbV, ccV). Elevation data are from

laser theodolite (total station) survey. The 4.8-m-high scarp is thought to be late Pleistocene, while excavations argue that the 1.2-m-high

scarplet is Holocene in age.


paleosol (Bwk) and the thicker Bw horizon in the

footwall. Whether the missing Bwk horizon has been

incorporated into the colluvium or was eroded by a

channel that ran along the footwall ground surface is

not clear. If the footwall surface has been eroded, the

vertical separation across this fault scarp is actually

greater than the vertical fault offset. In this case,

erosional degradation of the scarp partly enhanced its

geomorphic expression.

Given the lack of a post-faulting depositional

horizon, no minimum age can be established. The

maximum age of the event, however, indicates

younger displacement than previously suggested

for the NFTS. The active soil (Bw +A) has minimal

redness (10YR5/3) and pedogenic clay and paleo-

sols are all less than stage II calcic (K) horizons,

despite being developed in alluvium that consists

almost exclusively of marble clasts and calcite

sand. Based on this soil chronosequence, the total

time required to develop all of the observed ped-

ogenic features is f 20 + 10 ka (M. Eppes, pers.

comm., 2002). This age estimate is corroborated by

radiocarbon dating. No datable charcoal was recov-

ered from the excavation, but a radiocarbon age

was determined for carbon from a bulk sample of a

sand lens from the lowest gravel layer (G3) in the

hangingwall. The sand sample was sieved to < 180

Am, treated to remove non-detrital and inorganic

carbon (carbonate dissolved in hot HCl baths,

roothairs and macrofossils manually extracted),

and combusted to yield 14 cm3 of CO2 (0.18%

carbon by weight). The resulting accelerator mass

spectrometer radiocarbon age of 9710 + 50 years BP

(1r) calibrates to 9220 BC (11,160 years BP)

(Talma and Vogel, 1993; Stuiver et al., 1998; Beta

Analytic Inc., written communication, 2001). Given

the lack of field evidence for groundwater fluctua-

tions, soil redeposition (depositional fabric of over-

lying gravels is intact), or transfer of humic acids,

and the similarity to the soil chronosequence, this

age should approximate the deposition of the low-

est gravel horizon. Subsequent to this, several

meters of alluvium were deposited and weakly

weathered prior to the rupture event, thereby indi-

cating that this event was likely Holocene in age.

This demonstrates that the NFTS, at least in part, is

active.

4. Neotectonic observations

Although we did not identify features suitable

for paleoseismic studies along the eastern NFTS,

some measure of the relative tectonic activity of

eastern and western segments is provided by com-

parison of neotectonic observations from airphoto

mapping and field inspection. The distribution of

apparent scarps and other tectonic geomorphic fea-

tures along the entire NFTS is illustrated in Fig. 9.

The proportion of range front that contains mappa-

ble fault scarps is significantly greater west of the

Helendale fault. East of the Helendale fault, the

NFTS is broken by multiple strands of the ECSZ

and is manifest mainly as folds or depositional

Fig. 8. (A) Log of the western wall of a N05jE-trending excavation across the NFTS at location indicated in Fig. 5. East wall of trench displayed

comparable stratigraphy and faulting and was not logged. Grid represents 1-m cells without exaggeration. Field classification of soil horizons is

indicated in abbreviated form (based on M. Eppes, personal communication, 2001). Layers can be clearly correlated across the thrust fault,

including two prominent paleosols (Bkb horizons) that are thinner and more cemented. Sand lenses and clasts shown as gray polygons were

drawn precisely, while patterns indicate the general depositional and pedogenic fabric of each layer. Gray-ruled pattern indicates a small area

that could not be logged because of a bench dug into the side of the trench wall. Location of bulk carbon sample dated as 9220 BC shown as C1.

(B) Photo of the offset Bkb2 paleosol along the thrust fault, from area shown in panel (A). Photo was taken after the log was completed, so minor

details may differ.


bedrock–alluvium contacts (i.e., fault inferred to be

buried). Less than 25% of the eastern range front

contains apparent scarps, and these are less sharply

defined than scarps present along the western

segment (Spotila and Anderson, 2000). Slopes of

alluvial surfaces along the range front are also less

steep along the eastern segment and show abrupt

changes at the intersections of major strike-slip

faults (Fig. 10). This suggests a lower degree of

activity for the eastern segment and demonstrates

the effect of intersecting faults on structural tilt and/

or depositional patterns.

This west-to-east change in tectonic expression

corresponds with a decrease in range-front bedrock

relief and total displacement along the NFTS, which

reach a maximum near Marble Canyon and taper to

zero at the eastern edge of the San Bernardino

Mountains (Spotila and Sieh, 2000). A slower rate

Fig. 9. Neotectonic map of the NFTS. The NFTS is broken into western and eastern segments at the intersection of the Helendale fault. Tectonic

features are illustrated by line weight and shading as fault scarps in alluvium, fold axes in alluvium, or faults manifest in bedrock as lineaments

or inferred along the bedrock–alluvium interface. Mapping was based on 1:30,000 airphotos and field observations. Dashed lines indicate

boundaries between alluvium dominated by limestone and marble clasts in the middle and alluvium with predominantly gneiss or granite clasts

to west and east.

Table 1

Soil and scarp data for observation sites along the NFTS shown in


of activity, shorter duration of activity, or greater

period since activity along the eastern NFTS segment

could explain this. However, the local geomorphic

expression of NFTS is also influenced by variable

erodibility, as tectonic landforms are better preserved

where indurated petrocalcic soil horizons have devel-

oped in alluvium sourced from carbonate bedrock

Fig. 10. Plot of alluvial fan slope from west to east along the NFTS

(AAV; shown in Fig. 9). Slopes were measured from fan apexes at a

distance of 1 km (near slope) and 2 km (distal slope) from the range

front, using 1:24,000 scale topographic maps. The steeper gradients

west of the Helendale fault probably reflect higher rates of

deformation as well as a more abundant and coarse supply of

alluvium (due to greater mountain front relief) and more resistant

alluvium due to presence of carbonate parent bedrock. The minima

that occur at the intersecting strike-slip faults may result from their

influence on deformation or their control on sedimentation patterns.

(Eppes et al., 2002). Yet, sharp changes in scarp

frequency and total displacement occur at the inter-

section of the Helendale fault, rather than where

Fig. 9

Site Scarp

height (m)

Lithologya % Clay Redness

(Torrent)

Redness

(Harden)

1 13.3 granite 15.3 4.4 40

2 33.6 granite 12.4 4.9 46.7

3 13.0 granite 10.9 0.8 20

4 45.6 granite 24.8 5.5 46.7

5 11.6 granite–gneiss

( +marble)

26.1 4.4 36.7

6 12.8 granite 11.9 0 6.7

7 6.6 granite 8.9 0 13.3

8 4.5 granite – – –

9 39.6 marble – – –

10 9.2 marble – – –

11 51.2 marble – – –

12 18.2 granite

( +marble)

3.5 0.8 20

13 54.5 gneiss,

granite

26.5 9.2 60

14 46.2 gneiss 11.4 4.9 43.3

15 3.6 granite,

gneiss

12.4 4.4 36.7

16 14.9 granite,

gneiss

5.8 5.3 46.7

a Lithology of parent alluvium listed in order of occurrence;

parentheses indicate only minor occurrence.


alluvium changes from carbonate to granitic parent

material (Fig. 9). This suggests that the intersection of

the ECSZ affects the tectonic activity of the NFTS.

Fig. 11. (A) Comparison of fault scarp height and clay content at 30 cm dep

the < 2-mm sized fraction as measured with standard settling techniques is

scarp height being the dependent variable (assuming uplift rates vary along

and level. Triangles represent scarps from the western NFTS segment and

were examined on the east, because they are so rare. A rough increase in sc

statistically not significant. (B) Comparison of scarp height and redness in

each step greater in hue and chroma above the unweathered parent alluvium

hue weights are 5YR= 5, 7.5YR= 2.5, and 10YR= 0; Birkeland, 1999). Bo

not significantly ‘‘older’’ or slower in uplift rate. (C) Clay content and Hard

to calibrate an age proxy. Clay content is shown versus known terrace age f

time is excellent, but predicts considerably younger ages for soils on NFTS

on soils from the San Joaquin River (Harden, 1982), Cajon Pass (McFadde

sites vary considerably in present and past environmental conditions, ye

calibration for redness as a proxy of age that is less sensitive to local climat

from Keller et al. (1982) and McFadden and Weldon (1987).

To further evaluate variations in tectonic activity,

we measured scarp heights and compared them with

characteristics reflecting scarp age along the length

th in Bt horizons developed on hangingwall surface. Clay content of

plotted as the x-axis, as would time (for which it should proxy), with

the fault). Scarp heights were measured in the field using stadia rod

squares represent sites from the eastern segment. Only four scarps

arp height with clay content is shown by the regression line, but it is

dices from average wet and dry soil color (Harden index = + 10 for

(Harden, 1982); Torrent index = hue weight� chroma/value, where

th exhibit better correlations than clay content, but eastern scarps are

en redness index versus time in other southern California soils, used

rom Cajon Pass (McFadden and Weldon, 1987). The correlation with

scarps than expected. The plot of Harden redness versus age is based

n and Weldon, 1987), and the Antelope Valley (Ponti, 1985). These

t the redness-age correlation is fairly uniform. It thus provides a

e. Similar results were found for the Torrent redness index using data


of the range front. In the absence of more quanti-

tative geochronology, we used soil development,

represented by clay content and rubification (red-

ness), as a relative proxy for age. Observations were

made of Bt horizons (f 30 cm depth) in soil pits

within granitic alluvium at consistent positions

along the tops of uplifted hangingwall surfaces.

These observations were reconnaissance in nature

and do not compose formal chronosequences. They

are also representative of relative time only if each

hangingwall surface has been preserved from ero-

sion, has experienced the same climate and vegeta-

tion cover, and consists of parent alluvium of

comparable lithology and grain size. We acknowl-

edge that these restrictions are not always met by

the sites investigated, but consider the degree of soil

development to be at least a qualitative indication of

scarp age.

Redness and clay content increase very irregu-

larly with scarp height and do not define clear

differences among eastern and western fault seg-

ments (Table 1, Fig. 11A,B). If the eastern segment

is less active, its scarps should display greater

pedogenic clay content and redness than those of

equal height along the western segment. Such a

distinction is not apparent from these data. Al-

though clay content and redness may be loose

proxies for duration of soil development, they may

be too imprecise due to local variations in boundary

conditions to resolve differences in fault activity.

Comparisons of these soil characteristics with

other regional data provides additional insight on

Fig. 12. Simple motion diagram of possible fault interactions in the San B

solution for horizontal shortening rate across the western NFTS segment

experiences an arbitrary shortening of 0.1 mm/year (holding plate A fixed).

fault (HF-n). The resultant vector (velocity of plate A with respect to plate

oblique normal– sinistral slip to balance this shortening across the eastern

rate on the western NFTS of 0.16 mm/year and the assumed rate of 1.0

Helendale fault (HF-s) is considered inactive. The resultant vector must

reverse motion of >1 mm/year. This scenario seems unlikely, given the ind

eastern NFTS. (C) The same vector solution as in part (B), when resolved

NFTS is considered inactive. The resultant vector requires >1 mm/year dex

by existing data. The direction of this motion should be 8j clockwise to t

more northerly orientation of the Pipes Canyon fault (Fig. 3). (D) Schematic

deformation pattern today, with an inferred separation (gray line along the tr

associated with the restraining bend at San Gorgonio Pass (SGP) and d

illustrate how the Mojave block moves laterally with respect to the restrain

with respect to the opposite side of the fault. Strike-slip faults (A, B, C) ar

portion of the thrust fault is translated into the domain of dextral shear an

the age of these scarps. Soils developed on alluvial

terraces of granitic parent rock in Cajon Pass have

higher clay content at a similar depth in Bt

horizons (McFadden and Weldon, 1987). The

resulting correlation between clay content and age

predicts that a soil with 26.5% clay, the highest

clay content we observed in our soil pits (site 13,

Table 1), would correspond to an age of only 137

ka (Fig. 11C). Although soil development should

be faster in Cajon Pass due to higher annual

precipitation (f 50–70 vs. 20–30 cm/year along

the NFTS), this comparison implies the fault

scarps may be younger than previously suggested.

Redness values for Bt horizons from a variety of

locations in California also show a correlation with

soil age (Harden, 1982; McFadden and Weldon,

1987; Ponti, 1985) (Fig. 11C). Using this relation-

ship, the predicted age of the scarp at site 13 is

0.51 Ma, considerably older than predicted from

clay content. Given that the increase in soil

redness with time should be fairly uniform for

the limited range in precipitation and consistent

temperatures found across these California sites,

this is probably a more reliable estimate of scarp

age (Birkeland, 1999). Using this redness–age

relationship for all of our sites, the corresponding

scarp uplift rates are 0.19 mm/year and 0.08 mm/

year for locations west and east of the Helendale

fault, respectively. This comparison is consistent

with the hypothesis that the NFTS east of the

Helendale fault has a lower degree of relative

tectonic activity.

ernardino Mountains (SBM) and Mojave Desert (MD). (A) Vector

(NFTS-w), assuming the eastern NFTS segment (NFTS-e) currently

A dextral rate of 1.0 mm/year is assumed for the northern Helendale

C; AVC) implies the western NFTS would need nearly 1 mm/year of

NFTS. (B) Vector solution using the maximum observed shortening

mm/year on the northern Helendale fault. In this case, the southern

be accommodated entirely by the eastern NFTS, requiring oblique-

ications that the western NFTS has a higher rate of activity than the

entirely onto the southern Helendale fault. In this case, the eastern

tral motion on the southern Helendale fault, that cannot be confirmed

hat on the northern Helendale fault, which is consistent with the 8jof proposed eastward migration of the ECSZ. Upper diagram shows

end of the southern San Andreas fault) between domains of thrusting

extral shear of the ECSZ (Du and Aydin, 1996). Diagrams below

ing bend, the rate of which depends on what degree the bend is fixed

e translated with the Mojave block and new fault D initiates, while a

d terminates.


5. Discussion

That the western strand of the NFTS is active has

practical implications for regional seismic hazard. A

rupture with 1.7-m displacement would be on the order

of 40 km long (Wells and Coppersmith, 1994), about

the length of the western NFTS. If such a rupture

extended to the f 15-km-deep base of the seismo-


genic crust (f 30 km down-dip width), a damaging

earthquake of moment magnitude Mw = 7.2 would

result (assuming rigidity of 3.3� 1011 dyn cm� 2).

Due to relative proximity (Fig. 2c), an NFTS rupture

could also affect the seismic cycle of the San Andreas

fault. Rupture of a similar thrust fault (Susitna Glacier

fault) initiated the November 3, 2002 rupture of the

Denali fault (M = 7.9) (Eberhart-Phillips et al., 2003),

suggesting NFTS rupture could precede failure of the

San Andreas’ Mojave segment to the northwest. NFTS

rupture would also reduce the normal stress on the San

Andreas’ San Bernardino segment to the southeast,

thereby bringing it closer to failure (e.g., Stein et al.,

1992). Whether such effects are likely to happen in the

near future is unknown, however, given that the recur-

rence history of the NFTS is not constrained.

Our observations also have implications for the

behavior of intersecting fault systems. Because the

western segment of the NFTS is active, the simple

cross-cut model is not valid. The ECSZ has not

fully replaced thrusting along the northern range

front of the San Bernardino Mountains. This illus-

trates that using mapped patterns of faulting alone is

inadequate for characterizing potential fault activity.

It would be incorrect to use the intersection of these

faults, which is an unstable quadruple junction that

appears to violate simple Mohr–Coulomb failure

laws for a uniform stress field (Anderson, 1905),

as evidence that one fault system had become

entirely inactive.

To fully characterize how these fault systems co-

exist, the degrees of activity along the NFTS to the

east of the Helendale fault and the Helendale fault

to the south of the NFTS must be known (Fig. 3).

Our neotectonic observations suggest that the east-

ern half of the NFTS is less active than the western

segment. It is possible that it is inactive, given the

lack of prominent fault scarps. This is not a robust

conclusion, however, given the complexities of

geomorphic and pedogenic processes (Eppes et al.,

2002). It is appealing, however, in that it avoids

having to explain co-active intersecting faults and a

quadruple junction with different stress regimes in

the same volume of crust or oscillating stress

regimes throughout the seismic cycle. It is also

reinforced by a simple model that illustrates how

the NFTS and ECSZ could be coactive if the

eastern segment of the NFTS is inactive (Fig. 12).

Treating the blocks between these faults as rigid

and two-dimensional, we plot known slip rates in

velocity space to resolve what slip the eastern

NFTS or southern Helendale fault must have. We

assume the northern Helendale fault has 1 mm/year

slip rate, based on roughly equal distribution of the

total strain of the ECSZ across the dozen main

faults (Sauber et al., 1994).

In the first example, we assign an arbitrary

horizontal shortening rate of 0.1 mm/year to the

eastern segment of the NFTS, to see what such a

minimal rate of activity would imply for the slip rate

on the western NFTS (Fig. 12A). The result indicates

that the western NFTS would need nearly 1 mm/year

oblique sinistral–normal motion to balance the mo-

tion to the east. In the next two cases, we assume a

shortening rate of 0.16 mm/year for the western

NFTS and evaluate what adjacent faults must have

to balance motions. This shortening rate is based on

the observed fault dip, offset, and the minimum

duration of our paleoseismic record (f 11 ka). It

could be less, if a substantial period of time passed

prior to f 11 ka without rupture. It could be higher,

however, if large displacements occurred prior to

f 11 ka (or on other fault strands) and were not

represented in this paleoseismic record. Although it

is inappropriate to calculate displacement rates from

offsets without closed time intervals, we adopt this

rate for the purpose of our model. Using this

shortening rate, a resultant slip vector of 1.07 mm/

year (S35jE) translates to either oblique dextral/

reverse motion on the eastern NFTS or dextral

motion on the southern Helendale fault (Fig. 12B,C).

Based on our neotectonic observations, it seems

improbable that the eastern NFTS could have a slip

rate an order of magnitude greater than the western

segment. It is more likely that the eastern segment is

inactive and that strain from the northern Helendale

fault transfers south onto the southern Helendale fault,

albeit without a through-going, mappable connection.

This would be consistent with the pattern of recent

seismicity, that shows a prominence of northwest-

trending, dextral fault planes in the eastern San

Bernardino Mountains (Fig. 3). The present activity

on the southern Helendale fault, however, is not

known. It traverses mainly eroded bedrock, so that

geomorphic indications of recent activity in young

deposits are not clear. Displacement could transfer to


the Pipes Canyon fault, which has a prominent 1.5-km

dextral offset of bedrock-incised Pipes Canyon. The

Pipes Canyon fault also trends 8j clockwise (N34jW)

to the northern Helendale fault, which is the identical

rotation predicted by the slip vector model (Fig. 12C).

This could be tested by future geologic study. If

correct, the model implies future earthquakes pro-

duced by the NFTS may be limited in size, given that

it is only partly active. However, complex scenarios of

rupture involving compound or induced earthquakes

between the thrust and strike-slip faults may be

possible.

This model is similar to previous interpretations of

regional tectonic evolution. Clockwise rotation of

dextral faults from north to south in the ECSZ, such

as the 20j difference in trend of the ruptures of the

1992 Landers earthquake, has been explained by

Unruh et al. (1994) as a result of convergent strain

in the San Bernardino Mountains. Our model echoes

this proposition, but moves the domain of convergent

strain and rotating faults further west. We predict that

the triple junction between the NFTS and ECSZ is

now at the junction of the NFTS and Helendale fault.

Previously it would have been further east, and its

westward migration may have been linked with ter-

mination of the eastern thrust segment and sequential

overprinting by new strike-slip faults. This is consis-

tent with Dokka and Travis (1990b) proposition that

deformation within the ECSZ has migrated westward

since the middle Pleistocene. The more northerly

orientation of strike-slip faults that lie south of the

NFTS but east of our proposed modern triple junction,

such as the southern Johnson Valley fault (Fig. 3),

may owe their orientation to initial formation when

the eastern thrust segment was still active. A test of

this is whether the southern ruptures of the Landers

earthquake involved a component of extension, as

they should if their orientations formed more north-

erly than what is now favored by the regional pattern

of strain. This is contrary to the idea that counter-

clockwise vertical axis rotation causes faults of the

ECSZ to rotate out of the north–northwest trend that

may be favorable for failure in the regional stress field

(Nur et al., 1993).

This model represents a transitional state of poly-

phase deformation. The local overprinting of dextral

shear on the eastern thrust system is analogous to

examples of polyphase deformation in microstruc-

tural fabrics from the rock record (e.g., Kurz et al.,

2000). A potential cause of our proposed westward

migration of the ECSZ is southwestwards translation

of the Mojave block and ECSZ along the San

Andreas fault, with respect to the local geometry of

the San Andreas fault. As the Mojave block is

advected along the San Andreas fault, it continuously

passes through a region that experiences local con-

vergence due to the restraining bend at San Gorgonio

Pass (i.e., north–northwest of the bend) and into a

domain that experiences dextral shear (i.e., east of

the northward projection of the bend at the present-

day location of the Helendale fault) (Du and Aydin,

1996) (Fig. 12D). The window of dextral shear

propagates west with respect to the Mojave block,

but is fixed with respect to San Gorgonio Pass. The

predictions and feasibility of this model could be

tested with additional geologic data and numerical

models of deformation associated with the San

Gorgonio Pass restraining bend.

6. Conclusion

The intersection of the Transverse Ranges and

Eastern California shear zone provides an interesting

structural setting for investigating fault interaction.

Our paleoseismic observations indicate that the North

Frontal thrust system has experienced surface rupture

in the Holocene and should be considered an active

fault and a potential source of future large earth-

quakes. This further indicates thrust system has been

coactive with individual strands of the Eastern Cal-

ifornia shear zone that intersect it, such as the Helen-

dale and Old Woman Springs faults. This suggests

that a simple cross-cut model, in which one fault

system postdates the other, is not valid. Based on

neotectonic observations, the segment of the thrust

system east of the Helendale is interpreted to be

considerably less active than the western segment.

This fits with a simple rigid block velocity model, in

which estimates of slip rate on the Helendale fault and

western thrust segment can be reconciled as a quasi-

stable triple junction that would be completed by the

Pipes Canyon fault to the southeast. We further

speculate that this system is transient, as the Eastern

California shear zone propagates to the west. If

correct, this model predicts that this system of inter-


secting faults is active in a complex, polyphase

fashion, which was not apparent from mapped fault

patterns alone. Detailed geologic investigations of

such complex fault systems are thus important for

understanding the kinematics of active deformation

and fault evolution.

Acknowledgements

Gratitude is extended to Specialty Minerals Inc.

and the Mitsubishi Cement Corporation for access to

field sites. We also thank Missy Eppes, John Hole,

Jamie Buscher, Jon Matti, Doug Yule, and Howard

Brown for constructive scientific exchange. This

research was funded by the National Earthquake

Hazards Reduction Program (USGS); external award

number 00HQGR0028.

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Fault interaction at the junction of the Transverse Ranges and

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