JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. B10, PAGES 20,159-20,174, OCTOBER
10, 1995
Low-angle normal faults and seismicity: A review
Brian
Wernicke
Division of Geologicaland PlanetarySciences,California Institute of Technology,Pasadena
Abstract. Although large, low-anglenormal faults in the continentalcrust are widely
recognized,doubtspersistthat they either initiate or slip at shallowdips (<30ø), because
(1) globalcompilationsof normalfault focal mechanismsshowonly a smallfractionof
eventswith either nodal plane dippinglessthan 30ø and (2) Andersonianfault mechanics
predict that normal faults dippinglessthan 30ø cannotslip. Geologicalreconstructions,
thermochronology,
paleomagneticstudies,and seismicreflectionprofiles,mainly published
in the last 5 years,reinforcethe view that activelow-anglenormal faulting in the brittle
crustis widespread,underscoringthe paradoxof the seismicitydata. For dip-slipfaults
large enoughto break the entire brittle layer duringearthquakes(Mw "• 6.5),
considerationof their surfacearea and efficiencyin accommodatingextensionas a
function of dip 0 suggestsaveragerecurrrenceintervalsof earthquakesR' •c tan 0,
assumingstressdrop, rigiditymodulus,and thicknessof the seismogenic
layer do not vary
systematically
with dip. If the global distributionof fault dip, normalizedto total fault
length, is uniform, the global recurrenceof earthquakesas a function of dip is shownto
be R cr tan 0 sin 0. This relationshippred•icts
that the frequencyof earthquakeswith
nodalplanesdippingbetween30ø and60ø•'
will exceedthosewith planesshallower
than30ø
by a factor of 10, in good agreementwith continentalseismicity,assumingmajor normal
faults dippingmore than 60ø are relativelyuncommon.Revisionof Andersonianfault
mechanicsto includerotation of the stressaxeswith depth, perhapsas a result of deep
crustalshear againstthe brittle layer, would explainboth the commonoccurrenceof lowanglefaults and the lack of large faults dippingmore than 60ø. If correct,this resolution
of the paradoxmay indicatesignificantseismichazard from large, low-anglenormal faults.
ceeding three decades. Low-angle extensional structures,
thoughdocumentedby geologicalmappingstudies,were interIt is appropriate for the 75th anniversaryof the American preted as either peculiar thrust faults or surficiallandsliding
GeophysicalUnion that recognitionbe given to the 50th an- phenomena.Sliding and spreadingof rootless,internally coniversaryof a paper by Longwell[1945].Although not the first herent, extended allochthonsalong faults dipping only a few
descriptionof suchphenomena[e.g.,Ransomeet al., 1910],the degrees is well known. It includes caseswhere detachment
paper was remarkablein its documentationusingmaps,pho- occursalong incompetenthorizonsin sedimentssuchas shale
tographs,and crosssectionsof spectacularlyexposednormal or salt, as developedover thousandsof square kilometers in
faults in the Las Vegas region,with displacementsof 1-2 km the northernGulf of Mexico [Worralland Snelson,1989].Howand dips of 0-30 ø. In one large-scaleexposure,since partly ever, it also includesexampleswhere the slidingoccurswithin
drowned beneath the waters of Lake Mead, a fault was ob- competenthorizons,as in the Ordoviciandolostonesalongthe
served to flatten downward, from about 30ø to 5ø over a cross- Heart Mountain detachment [Pierce, 1957; Hauge, 1990].
sectionaldepth of 600 m.
These examplesgenerallyinvolveonly the upper few kilomeIt is perhaps a measure of a theoreticallybased prejudice ters of the crustand are not accompaniedby coevalextension
againstlow-anglenormal faults that Longwell[1945] excluded of the underlyingcontinentalbasement.In contrast,fault sysregional crustalextensionas a causefor faulting. He instead tems in the Basin and Range, such as those describedby
interpreted them to result from extension on the crests of Longwell[1945], clearlyinvolvecontinentalbasementand are
large-scalecompressionalanticlines.Mechanical arguments observedin some casesto cut structurallydownwardthrough
for downwardflattening (listric) normal faults date back at 10 km or more of the crust.
least to McGee [1883], but Hafner [1951], citing Longwell's
Beginningwith a handful of Basin and Range field studies
[1945] observations,showedthat certain loading conditions [e.g.,Anderson,1971;Wrightand Troxel,1973;Proffett,1977],it
along the base of an elasticplate induce curvature of stress was not until the late 1970s that ,the numerous documented
trajectoriesfavorable for the formation of low-angle normal low-anglenormal faults gained a measureof acceptanceas a
faults.
direct expressionof large-magnitudecontinentalextension.At
Despite both observationand theory, the assumptionthat about the same time, it was also realized that many metamorthe leastprincipalstressdirectionis horizontalthroughoutan phic tectonitesin the Basin and Range previouslythought to
extendingcrust [e.g.,Anderson,1942] held swayfor the suc- be Mesozoicor Precambrianin agewere actuallyTertiary [e.g.,
Davis and Coney, 1979]. In many casesthese rocks lay in the
Copyright 1995 by the American GeophysicalUnion.
footwalls of regionally extensivelow-angle normal faults or
Introduction
Paper number 95JB01911.
"detachments"
0148-0227/95/95JB-01911 $05.00
ters parallel to their transportdirections.By 1980, it was clear
20,159
that could be traced for several tens of kilome-
20,160
WERNICKE:
(a)
LOW-ANGLE
NORMAL
mities have since been documented to be low-angle normal
(b)
B
HISTORIES:
Depositionof A
Tilting & erosion
Depositionof B
Depositionof A
Depositionof B
Faultingof B on A
Erosion
Erosion
Figure 1. Contrast in geologicalhistory from interpreting a
contact between older sedimentarysequenceA and younger
sequenceB as (a) an unconformityand (b) a low-anglenormal
fault.
that numerous isolated exposuresof detachmentsand their
metamorphicsubstrateformed a nearly continuousbelt from
Sonora, Mexico, to southern British Columbia, referred to as
the Cordilleranmetamorphiccore complexes[Crittendenet al.,
1980;Armstrong,1982]. It was realized that the footwalls of
many exposeddetachmentswere not stronglymetamorphosed
in the Tertiary, raising the possibilitythat low-angle normal
faults formed and were active entirely in shallowcrust [e.g.,
Wernickeet al., 1985;Spencer,1985;Dokka, 1986;John,1987].
These
observations
ran
counter
FAULTS AND SEISMICITY
to Jackson
and
White's
[1989] descriptivesynthesisof some56 earthquakeson active
continentalnormal faults.They concludedthat (italicstheirs)
Among the mostimportant observationsthat now influencethe
debateare... that large earthquakesdo not occuron listtic faults
that flatten at shallowdepths(as originallythought:e.g. McKenzie, 1978a,b), but on faultsthat are steepthroughoutthe seismogenic upper crust...
Whether or not thisconclusionis correctis a first-orderproblem
in understanding
the structureand dynamicsof the lithosphere.
Geological Significance
The recognition of low-angle normal faults and the core
complextectonic associationis now global and includesoceanic lithosphere as well as the continents[e.g., Mutter and
Karson,1992]. The significanceof these structuresfor geology
as a whole may be illustrated by consideringan unexposed
low-anglecontactroughlyparallel to overlying,youngersedimentaryunit B but discordantto underlyingsedimentary(or
metamorphic)unit A (Figure1). Prior to 1980,manygeologists
would have interpreted such a contact as either an unconformity or a thrust fault. The possibilityof the contact being a
normal fault may have been overlooked on the basis that
known low-angle fault contacts were restricted to thrusts,
whichgenerallyemplaceolderrockson younger.The geologic
historiesfor these two casesare of coursemarkedly different
(Figure 1). The Basin and Range providesnumerouscase
historiesof the problem,where contactsbetweenTertiary and
underlyingpre-Tertiary strata, in some caseswith high angle
between the contact and Tertiary strata, were interpreted as
unconformities.For example, low-angle contactsmapped by
Kemnitzer[1937],Fritz [1968], and Dibblee[1970] as unconfor-
faults(Daviset al. [1980],Ganset al. [1989],andDokka [1986],
respectively).
Similarly,majorlow-angefault systems
interpreted
asthrustsbyNoble[1941],Misch[1960],andDrewesand Thorman
[1978]are now widelyregardedas normalfaultsrelatedto Cenozoicextension
(Wrightand Troxel[1984],Milleretal. [1983],and
Dickinson[1991], respectively).Reinterpretationscurrentlyunderwayin other mountainbeltsare similarlyprofound.
TheseBasinand Rangefield relationsrepresenteda classof
geologiccontactthat had not been previouslyrecognizedas a
fundamentaltectonicelement.Recognizingthem as suchis as
basicto accuratehistoricalinference in geologyas, for example, the knowledgethat rocks with igneoustexture intrude
their surroundingsin a molten state.
Mechanical Significance
The fact that low-anglenormal faults are not predictedby
Andersoniantheory is also fundamental to interpreting the
stressstate and physicalconstitutionof the crust.In the 1980s,
debate centered on the kinematics of generating the corecomplexassociation.Most current modelssuggestasymmetrical denudationalonglarge normal faults that transectthe upper 15-20 km of the crust at low angle, accompaniedby
isostaticrebound and flexure of the unloadedfootwall [e.g.,
Wernicke,1981;Howard et al., 1982;Allmendingeret al., 1983;
Spencer,1984; Wernicke,1985; Davis et al., 1986; Wernicke,
1992].Recently,controversy
hascenteredon the initial dip and
subsequentmodificationof thesefaults and the roles of footwall metamorphictectoniteand magmatism.
This paper addressesthe question: Are brittle low-angle
normal faults active while at low dip? A number of authors
have expresseddoubt that shallowlydippingnormal faults are
importantfeaturesin the extendingseismogenic
crust,pointing
to Andersoniantheory and a lack of seismicityon suchfaults
[e.g.,Buck, 1988;King and Ellis, 1990]. A large body of literature has nonethelessfocusedon non-Andersonianexplanations for active low-angle normal faulting [e.g., Xiao et al.,
1991;Forsyth,1992;Axen,1992;Parsonsand Thompson,1993].
If low-anglenormal faultsare indeedactivein the seismogenic
crust,why are there so few, if any earthquakesobservedon
them? Evidence summarizedbelow, mostly publishedin the
last 5 years,tendsto reinforcethis paradox.A simplemechanical model relating fault dip to earthquake recurrenceis developedthat may provide an explanation.
Observations of Low-Angle Normal Faults
Andersoniantheory predictsthat extensionof the crustresultsin faultsthat initially dip 60øbut providesno insightas to
how suchfaultswith largefinite slipdevelopkinematically.For
example,normal faults may rotate during and after their slip
history, as in the caseof a systemof "domino-style"or "bookshelf" fault blocks [Wernickeand Burchfiel, 1982], in which
case,dipslower than 60ø are generallyexpected[e.g., Thatcher
and Hill, 1991].The key questionsare whether a givenfault in
the seismogenic
part of the crustwasactiveat shallowdip, and
whether the fault initiated at shallowdip. Low-angle normal
faults present no conflictwith Andersoniantheory if, for example, they initiate at 60ø and rotate down to 30øwhile active
and are then further rotated to very low anglewhile inactiveby
a youngerset of domino-stylefaults [Mortonand Black, 1975;
Proffett,1977;Miller et al., 1983]. Clearly, manylow-anglenormal faults, including most of those described by Longwell
WERNICKE:
LOW-ANGLE
NORMAL
[1945], cut upper crustalsedimentarylayersat high angle and
therefore probablyhad steeporiginal dip.
A compilation of all well-determined focal mechanismsof
normal fault earthquakes (Mw > 5.2, using momentmagnitudescaleof Kanamori[1977]) in continentswith nearly
pure dip-slipmovement(56 events)showedthat most nodal
planesdip between 30ø and 60ø [Jackson,1987;Jacksonand
White,1989].A subsetof thoseeventswhere the fault plane is
resolvedby surfacerupture (15 events)showedno faultswith
dip less than 30ø. Based on this survey,many workers have
stressedthe uniformitarianinterpretation("the presentis the
key to the past") that all low-anglenormal faults dippingless
than 30ø are rotated while inactivefrom dipsgreater than 30ø,
either by youngerhigh-anglefaults or by isostaticadjustment
[e.g.,Buck, 1988; Gans et al., 1989;King and Ellis, 1990].
Others arguedthat althoughsuchrotationsmay be common,
initiationand slipon shallow(<15 km depth) normalfaultsare
required by geologicaland geophysicaldata [e.g., Wernickeet
al., 1985; John, 1987; Wernicke and Axen, 1988; Davis and
Lister, 1988; Yin and Dunn, 1992; Scottand Lister, 1992; Dokka,
1993;Axen, 1993].Thesedata includegeologicreconstructions
FAULTS AND SEISMICITY
20,161
and subparallelallochthonousstrata and (2) the autochthonousstrata below the thrust are definedwithin a few degrees
(a and/3, respectively,Figure 3). The anglesbetweenprerift
Miocenevolcanicand sedimentarystrataand (1) stratain the
thrustramp and (2) autochthonous
strataof the forelandjust
in front of the thrust plate are also well defined (•/ and /5,
respectively,Figure 3). Assumingwest dippingallochthonous
strataof the thrust ramp zone aboveand below the detachment
were parallel, the dip of the detachmentwith repect to the
prerift Miocene strata is
0,= 3/- a•20
ø.
Thrust loading presumablywould have deflected the autochthous strata to westward dip 4• relative to the undeformed
foreland (Figure 3). For undisturbedthin-skinnedforeland
thrust beltsworldwide and especiallythe Cordilleran belt, this
deflectionis generallyno more than about5ø [e.g.,Price, 1981;
Royseet al., 1975;Allmendinger,1992;Royse,1993]. Assuming
low &,
/5<27 ø.
and fault rocks associatedwith detachments, thermochrono-
(1) the detachment's
logic and paleomagneticinvestigations
of exposeddetachment Thereforetwo independentobservations,
footwalls,and seismicreflectionprofiles.
relationswith the thrustramp andoverlyingTertiaryand (2) its
relations with the thrust autochthonand overlyingTertiary,
both suggestan initial dip of the Mormon Peak detachmentof
Geologic Reconstructions
about 200-27ø [Wernickeet al., 1985].
A direct approachto resolvingwhether normal faults either
The initial dip of the Tule Springsdetachmentis alsoclearly
slip or initiate at low-angleis restorationof well-constrained defined[Axen,1993] (Figure 3). The detachmentruns subpargeologicsections.In the U.S. Cordillera, somelow-anglenor- allel to the thrust plane where it overrides autochthonous
mal faults cut abruptlydownwardthrough 10 km or more of strata for a horizontal distance of at least 10 km. Thus the
preextensionalstrata and crystallinebasement(e.g., Mojave detachmentinitiated at the dip of the decollementthrust and
Mountains, Arizona [Howard and John, 1987]; Egan Range, the authochthonousstrata prior to extension.In addition to
Nevada [Ganset al., 1989]; South Virgin Mountains,Nevada this constraint, the unconformity between synrift strata and
[Fryxellet al., 1992];and PriestLake area, Idaho [Harmsand allochthonousstrata is not markedly angular (Figure 3). DePrice, 1992]). These fault systemscut through uppermost tailed considerationof theseconstraints,includingreconstruccrustallevels(<1 km) at their shallowends.In other instances, tion of the detachment'shangingwall, suggestan initial dip in
however,the increasein footwall structuraldepth is small in the range 3ø-15ø [Axen,1993].
comparisonto exposeddowndip length of the footwall. This
The Mormon Mountains-Tule SpringsHills detachmentsysseemsespeciallytrue where detachmentsystemscut across tem is amongthe bestexposedupper crustal,low-anglenormal
wide (30-50 km) areasof deeper crustalrocks (•5-15 km fault systemsin the world, but it is not clear how typicalits low
paleodepth),as in most core complexes.Some examplesin- upper crustalinitiation anglesare comparedwith activeslip at
clude the Raft River Range, Utah [Compton et al., 1977; low angleon more deeplyexhumedstructures.The anisotropy
Malaveielle,1987;ManningandBartley,1994];the Ruby Moun- of shallowlywest dipping thrusts and bedding in the thintains-East Humbolt Range area, Nevada [Muellerand Snoke, skinnedthrust belt may have somehowplayed a role in gener1993];the Black Mountains,California[Holm et al., 1992];the ating the low initial dips. Seismicreflection data to the north
ChemehueviMountains,California [John,1987];the Harcuvar along the frontal Cordilleran thrust belt also suggestshallow
and Buckskin Mountains, Arizona [Spencerand Reynolds, crustalnormal faultswith low initial dipsdevelopedjust westof
1991];the SouthMountains,Arizona [Reynolds,1985];and the the frontal thrusts [e.g., Bally et al., 1966; Royseet al., 1975;
Catalina-Rincon Mountains, Arizona [Dickinson, 1991]. In Allmendingeret al., 1983; Smith and Bruhn, 1984; Planke and
Smith, 1991].The Mormon Mountains-Tule SpringsHills area
some instances,however,faults transecteven the upper 7-8
km of the crustat low averageinitial dip [e.g., Wernickeet al., lies at a point where these extensionalstructuresbegin to cut
southward well into the cratonic foreland of the thrust belt,
1985;Axen, 1993].
An example of the latter may be found in the Mormon therebyexhumingthe frontal mostthrustsfrom paleodepthsof
Mountains-Tule SpringsHills area of southernNevada [Wer- 7-8 km.
A secondexampleof shallowlydippingnormal faults in the
nickeet al., 1985;Axen et al., 1990;Axen, 1993]. Two Miocene
detachmentsare superimposedon the frontal decollement uppermost crust occurs in the Whipple Mountains area of
thrust of the Cordilleranfold and thrustbelt [e.g.,Burchfielet southeasternCalifornia and west central Arizona [Davis and
al., 1992],includingthe Mormon Peak detachment[Wernicke Lister, 1988; Scottand Lister, 1992]. There, severallarge areas
et al., 1985](Figure 2) and the Tule Springsdetachment[Axen, of hangingwall synriftstrata (either flat-lyingor cut by high1993].The Mormon Peak detachmentcutsdownwardfrom the anglenormalfaultsof opposingdips)are truncatedfrom below
hangingwall of the thrustinto its footwall(Figure2), suchthat by the very shallowlydipping Whipple-Buckskindetachment
the anglesbetweenthe detachmentand (1) the thrust ramp system. The depth to the active detachment system, con-
20,162
WERNICKE: LOW-ANGLE NORMAL FAULTS AND SEISMICITY
a)
b)
Figure 2. Photographs
of MormonPeakdetachment,
Nevada.(a) Lookingnorth,westernMormonMountains,fault (betweenarrows)emplaces
Carboniferous
strataoverCambrian.Cliff on rightsideis approxi-
mately50 m high.(b) Lookingsouth,westernMormonMountains,
detachment
(planartopographic
bench
between
arrows)
cutsat about5øacross
footwallCambrian
strata(lightanddarkbanding,
lowerleft).Hanging
wallcomprises
threeblocksof imbricately
normalfaultedOrdovician
throughCarboniferous
strata,variably
tiltedto the left. Thereis approximately
600m of relieffromvalleyin foreground
to highpeakon left.
strainedby the thickness
of synextensional
strata,waslessthan a biplanaror listricgeometryfor major normalfaults,with
2-3 km. Theserelationsarguestronglyfor a low initial dip for highlyvariabledepthof flatteningrangingfrom lessthan 5 km
depth [e.g.,Spencerand
the fault initiallycuttingthroughhangingwall strata,although to more than 10 km preextensional
it doesnot constrainthe trajectorythroughthe footwall,which Reynolds,1991; Wernicke,1992], a conclusionlargely reinlikelyhad a morecomplexhistory[Davisand Lister,1988].In forced by these additional data.
addition,the baseof a largesyntectonic
landslidemassderived
from the exposedfootwallwas depositedacrossthe detachmentsystemsubparallel
to the fault plane,offsetsome10 km Thermochronologic Data
An importanttool for addressing
the originalconfiguration
alongit, andlatercutby normalfaultswhichare in turn cutby
of crustal-scalenormal faults is the thermal history of their
the detachment[Yin and Dunn, 1992].
Field geologicrelationsare fundamentalto understanding footwalls,especiallywhere there are wide exposuresin the
of this
detachment
geometryandkinematics.
Additionaldata,includ- transportdirectionof the fault. Publishedapplications
mainlyin the centraland
ing thermochronology,
paleomagnetic
data,seismicreflection methodincludejust a few examples,
profiling,andseismicity,
arerequiredto testcompeting
models southernBasin and Range, and so the resultsmay be geobiased.Generally,the time of footwallunroofingis
for their evolution.In general,geologicreconstructions
suggest graphically
WERNICKE:
LOW-ANGLE
NORMAL
FAULTS AND SEISMICITY
20,163
thrust plane
0
0-10
ø
i
y = 40 ø
0-10
ø x• •<5ø
0 O•
000000
.....................................................
0i
13--17ø
Mormon
{<5 ø
Jle Springs detachment
Peak detachment
10--
Figure 3. Reconstructionof Mormon Peak and Tule Springsdetachments,slightlymodifiedfromaxen et al.
[1990]andaxen [1993]for clarity.Thick lineswith doubleticks,detachments;line with teeth, thrustfault; wavy
line with dots,sub-Tertiaryunconformity;other thin lines,variousstratigraphiccontacts.See text for discussion.
clearly expressedby rapid coolingeventsbetween 400øC and
100øC.The ambienttemperatureof most footwalls(excluding
coolingof synriftplutons) is usuallywell below the Ar retention temperaturein hornblende(450-500øC) and closeto that
for retentionin micas,or about300-400øC [e.g.,Richardet al.,
1990; John and Foster, 1993; Holm and Dokka, 1993; Dokka,
1993].A pattern emergingfrom thesestudiesin the Cordillera
is that deeperportionsof the footwallcool from thesetemperaturesto lessthan 100øC(fissiontrack annealingtemperaturein
apatite)in a periodof 1-10 m.y. [e.g.,Holm and Dokka, 1993].
In most examplesit is possibleto establishthe maximum
variation in temperature acrossthe exposedfootwall immediately prior to the thermal perturbation causedby unroofing.
Given the downdip temperaturevariation acrossthe footwall
prior to unroofing,the averagedip of the fault can be determined for variable assumptionsof the preextensionalgeothermal gradient.This techniquehasbeen employedfor a number
of extensional terrains in the Cordillera, where footwall strain,
includingelongationvia detachment-relatedshearingor postdetachmentnormal faulting,and transienteffectsfrom syntectonic intrusions,may be taken into account.The paleothermal
field gradient(preunroofing,downdipthermal gradientof the
exposedfootwall) betweentwo pointsA and B with temperature differenceA T is related to the paleogeothermalgradient
by the averagedip of the fault (Figure 4), which is
dT/dw
0= sin-•dT/dz
(1)
about 25-30øC/km at 15 Ma in the upper 3-4 km of the crust
[Fitzgeraldet al., 1991]. In the easternMojave Desert region,
rather highergradientsat about 18 Ma of 50 _+20øC/kmfor the
Piute Mountains and a range of 30-50øC/km for the Chemehuevi Mountains have been suggested[Fosteret al., 1991;John
and Foster, 1993]. In the Death Valley region, ambient temperatures at 10-15 km depth at 8-10 Ma were about 300350øC,suggestinga range of 25-35øC/km[Holm and Wernicke,
1990; Holm et al., 1992]. Possible gradients near or above
50øC/km in the eastern Mojave region are determined for a
time near the end of a major magmaticepisodeand are probably relatively transient. Thus a range in gradients of 2035øC/kmwould probably representthe averageupper crustal
paleogeothermal
gradientin mostareasof the Basinand Range
sincemid-Tertiarytime, in agreementwith the geothermsof Lachenbruch
and Sass[1978],with magmaticand extensionalstrain
locallyraisingit to 2 or perhaps3 timesthat amount.
A plot of field paleothermal gradient determined from Figure 5 versuspaleogeothermalgradient,contouredin initial dip
accordingto equation (1), is shownin Figure 6. In these examples,fault rocksshowevidenceof brittle extensionalfaulting and cataclasis,but major bulk elongationsof the entire
footwall block, particularly in the brittle field, are unlikely.
These data suggestthat although some sectionsyield dips as
high as 450-60ø at the extremesof their uncertainties,most of
the data suggestinitial dips of lessthan 30ø. The two examples
yielding the highest dips (SW Harquahala Mountains and
Piute Range) involve relatively short transectsacrossuppermost parts of the crust (Figure 6). The Gold Butte example
mayalsohavea highaveragedip (up to 45ø),but it too involves
uppermost crustal rocks in its shallow part (<5 km paleodepth) where the denudingfault originallydipped about 60ø
[Fryxellet al., 1992;Fitzgeraldet al., 1991], and hence the fault
probablyflattened downwardto its deepestexposuresin order
whered T/dz is the geothermalgradientjust prior to unroofing
and d T/dw is the measuredfield paleothermalgradient.
The overall range of field paleothermal gradient, with uncertainties,is 0-33øC/km, measuredacrossdowndip distances
of 6-40 km (Figure4). The two highestgradientsare from the
upper 5-10 km paleodepth(Piute and Harcuvar detachments,
shownas solid symbolsin Figure 5), while the other, deeper
examplesrange from 0 to 19øC/km.
The ambient geothermal gradient in the Basin and Range
prior to unroofinghasbeen determinedin severalareaswhere
the time-temperaturehistoryhas been determinedfrom rocks
of independently estimated paleodepth. For eastcentral
Nevada, the averagegeothermalgradient at 35 Ma was about Figure 4. Diagram showingvariablesusedto derive relation20øC/kmin the upper 10 km of the crust prior to unroofing ship between field paleothermal gradient, paleogeothermal
[Dumitru et al., 1991]. In the Gold Butte area of southern gradient,and fault dip betweenpointsA and B (equation(1)).
Nevada, an apatite fissiontrack study indicatesa gradient of See text for discussion.
(•T) •
"'"detachment
20,164
300
WERNICKE:
--
200
NORMAL
--
4
100
[
()
10
FAULTS AND SEISMICITY
zoic basementintruded by four groupsof intrusives,including
two discreteplutonsand two setsof youngerdikes [Reynolds,
1985]. Superpositionrelations of the intrusive suite indicate
unroofingand ductileshearingbeganshortlyafter intrusionof
the olderpluton [Reynolds,
1985].The older dikesintrudedlate
in the historyof ductile deformation,while the youngerdikes
intruded duringbrittle deformation,late in the unroofinghistory [Livaccariet al., 1993, 1995;Fitzgeraldet al., 1993].Thermochronologicdata indicate rapid cooling of footwall rocks
between22 and 17 Ma, from solidustemperaturesin the oldest
2
•o
LOW-ANGLE
20
30
40
Distancein TransportDirectionw, km
intrusion to 300øC between 22 and 20 Ma, then from 300øC to
below 100øCfrom 20 to 17 Ma [Fitzgeraldet al., 1993].
Paleomagneticdata indicateconcordanceof high-coercivity,
high unblockingtemperature magnetizationswith early Miocene expecteddirectionsfor all four intrusivesuites[Livaccari
et al., 1993, 1995]. These data suggestunroofingalong a fault
with initial dip of about 10ø.
The Black Mountains examplehas a more complexhistory.
In structurallydeep portionsof the detachmentfootwall, an
11.7 Ma mafic intrusivecomplexis locally ductilely deformed
and folded alongwith Proterozoiccountryrocks [Asmeromet
Figure 5. Maximum variation of paleotemperaturein downdip directionacrossfootwallsof Cordillerandetachments,
just
prior to unroofing.Solid symbolsindicate upper crustal sec- al., 1990; Holm and Wernicke, 1990; Mancktelow and Pavlis,
tions only. Locationsand sources:1, Piute Mountainsdetachment, easternMojave Desert, California [Fosteret al., 1991];2, 1994]. It is intruded by silicicplutonsand mafic to silicicdikes
rangingin agefrom --•9to 6.5 Ma whichlargelyescapedductile
southwestern Harcuvar Mountains, west central Arizona
[Richard et al., 1990]; 3, Garden Wash detachment,South deformation[Holm et al., 1992]. Rapid coolingand unroofing
Virgin Mountains,Nevada [Fitzgeraldet al., 1991;Fryxellet al., of the entire complex from over 300øC to less than 100øC
1992; J. E. Fryxell, unpublisheddata 1994]; 4, Chemehuevi occurredbetween --•8.5and 6.0 Ma [Holm and Dokka, 1993].
Mountains detachment,lower Colorado River trough,CaliforHigh unblocking temperature, high-coercivitymagnetizania [Johnand Foster, 1993]; 5, Newberry Mountains detach- tions from the youngergroup of intrusionsmay be restoredto
ment, centralMojave Desert, California [Dokka, 1993];6, Am- their Miocene expecteddirectionsby a 500-80ø counterclockargosadetachment,Death Valley region,California [Holm and wise rotation about a vertical axis,interpretedas deformation
Wernicke,1990;Holm et al., 1992;Holm and Dokka, 1993]; 7,
associatedwith postunroofingdextral-obliqueshear on the
Buckskin-Rawhidedetachment,lower Colorado River trough,
Death
Valley fault zone [Holm et al., 1993;Mancktelowand
Arizona [Richardet al., 1990;Spencerand Reynolds,1991].
Pavlis, 1994]. These plutonsdo not showa significantinclination anomaly.Subtractingthe vertical axis rotation from the
to maintain even a high extreme of averagedip at 45ø. The directionsin the early mafic intrusion,an additionaltilt of, in
remainingfour examples,all from relativelywide, deep expo- total, some200-40ø is required to restore the mean direction
from this intrusion into agreementwith a Miocene expected
sures,suggestaverageinitial dips of 30ø or less.
In summary, thermochronogythat allows comparisonof direction [Holm et al., 1993]. There is considerablebetweenfield paleothermal gradient with paleogeothermalgradient
prior to unroofingis a useful meansof constrainingthe initial
configurationof large normal faults. In general,the field gra/
/ q-•
/
dient is lessthan 1/2 the value of the paleogeothermalgradient,
corresponding
to initial fault dipsof 30øor less(equation(1)).
øø
/
I
300
Faults where the initial dip may be significantlyover 30ø seem
•
/
?
/
to be restrictedto high crustallevels.
I
/
Paleomagnetic Data
Paleomagneticstudiesare also a potentially useful method
for determiningthe initial dip of normal faults. If pretilt or
syntilt magnetizationscan be identified, they provide quantitative estimates,at relativelyhigh precision,of the original and
syntectonicdip of the detachment.To date, only two such
studies have been published for core complexeswith wide
downdip exposuresof midcrustalrocks, includingthe South
Mountains,Arizona [Livaccariet al., 1993,1995],andthe Black
Mountains,California[Holm et al., 1993].In both areas,largely
undeformed
intrusive
rocks from
•,
the
detachment
footwalls
• ••/
//
/
•'"
10
45ø
/'
•
//
//"
20
dT oC/km
/ 0:9oø•
30
dW'
span much of the historyof ductiledeformationand rapid unroofing.
Figure 6. Plot of paleogeothermal gradient d T/dz versus
The South Mountainsfootwall is exposedfor approximately field paleothermalgradientd T/dw for the sevendetachments;
20 km in the transportdirection and is composedof Protero- solid symbolsindicateupper crustalexamplesfrom Figure 5.
WERNICKE:
LOW-ANGLE
NORMAL
site dispersion(up to 90ø) in high-temperature,high-coercivity
magnetizationsfrom the mafic complex,possiblyresultingin
part from postintrusivefolding, and thus it is difficultto preciselydetermine the net tilt. However, since the oldest silicic
plutonspredate rapid coolingof the complex,little or no net
tilt occurredduring unroofingbetween 8.5 and 6.0 Ma. Thermochronologicdata suggestrapid unroofingis time transgressivein a downdipdirection,whichmay supportthe conceptof
a "rolling hinge" (discussedin more detail below) moving
through the footwall rocks during denudation, and thus it is
possiblethe detachmentmay have briefly had a steeper dip
duringunroofing[Holm and Dokka, 1993;Holm et al., 1993].
These two examples,while both suggestinglittle net tilt as a
result of unroofing,also demonstratethe potential of the approach,especiallyfor crystallinerocksthat characterizemany
detachmentfootwalls. Contrastsin the overall history of the
two examples,however,suggestsmany surpriseslie ahead for
paleomagnetic
studiesof detachmentcomplexes.
FAULTS
AND SEISMICITY
20,165
tachment in the Basin and Range province of west central
Utah [Allmendingeret al., 1983]. This profile revealeda strong,
continuous,multicyclicreflection that cuts from the surface,
alonga major rangefront, downto over 5 s two-waytravel time
(12-15 km depth) with an average dip of 12ø to the west
[Allmendingeret al., 1983, Figure 2]. As shown by a grid of
industryprofiles and well data along its shallow,easternportion, Cenozoichalf grabenabovethe reflectionare boundedby
relatively steep faults that do not offset it [e.g., McDonald,
1976;Planke and Smith, 1991]. These data also showthat the
detachment covers an area of at least 7000 km 2.
The positionof the reflectionwithin the east directed Cordilleran thrust belt led to the early interpretation that the
reflection was a thrust fault, reactivated as a Cenozoic exten-
sionalstructure[e.g.,McDonald, 1976].The geometricsimilarity of the seismicprofilesto exposedCordilleran detachment
systemsled to the suggestionthat the reflectionwas primarily
a Cenozoicnormal fault whichmay not havebeen a reactivated
thrust,since•any detachments
donot appearto reactivateold
thrusts[Wernicke,1981;Andersonet al., 1983;Allmendingeret
al., 1983;Wernickeet al., 1985;Allmendingeret al., 1986] (FigInterpretationsof seismicreflectiondata have played a ma- ure 2).
jor role in developingan awarenessof low-anglenormal faults,
This long-standinginterpretationof well and reflection data
particularlyin the geophysicalcommunity[e.g., Bally et al., hasrecentlybeen challenged,primarily basedon a comparison
from drill cuttingstaken near the reflection
1981;Wernicke
andBurchfiel,
1982;•!lmendinger
et al., 1983; of microstructures
Smithand Bruhn,1984].Hundredsof profiles,mostof them with thoseof the Muddy Mountain thrust,a major decollerrieht
unpublished,from a broad spectrumof extensionalenviron- thrust fault in southern Nevada landers and Christie-Blick,
ments show strong, shallowlydipping reflectionsfrom low- 1994]. In two wells,the reflectionis a contactbetweenTertiary
angle fault planes that bound asymmetrichalf graben, often sandstoneand Poleozoiccarbonate,while the Muddy Mounprojecting up to surface exposuresof the faults. These data tain thrust emplaCesPaleozoiccarbonateover Mesozoicsandstrongly
suggest
low-angle
'(<30ø) normalfaultsarecommon stone.Along the Muddy Mountain thrust, microfracturedenfeaturesin the upper 15 km of the continentalcrust.
sity in cataclasiteswithin a few meters of the fault is at least a
Because
the dataareu,sually
proprietary,
the exactlocation factor of three higher than in surroundingrocks [Brock and
of the line, velocitycontrol,and the possibleeffectsof migra- Engelder,1977].The cuttings,however,revealedno evidenceof
tion are often not presentedin publications.Thuswith muchof dense microfracturingnear the contact, which was therefore
the data, "sideswipe"of a steeperfault suchthat it appearsto interpreted as an unconformityrather than a fault landersand
be low-angle,"pull-down"of the shallowpart of the fault due Christie-Blick,1994].
to low-velocitybasin fill, and steepeningof the fault plane
The difficultiesin establishingany contactrelation from well
reflectionupon migration are important caveatsin evaluating cuttingsare considerable,sincea given set of cuttingssamples
whether any given fault is a low-anglenormal fault. However, a 10-m interval. It is not known what is being sampledin the
suchdata are normally acquiredperpendicularor parallel to size fraction preservedas cuttings.For example,prefractured
structuraltrends in the area, mitigating the problem of side- grainsof the cataclasitemay not survivepulverizationby drillswipe.Pull-downis alsonot usuallya major effecton fault dip. ing. It is alsopossiblethat cataclasiteson large detachmentsdo
For a typical section,the shallowpart of the normal fault is not develop microfracturesin the sameway as thrustsor that
imaged downdip for at least 10 km, structural relief on the thick cataclasticzoneson detachmentsmay be locally excised
basinfill-bedrockcontactin the hangingwall is lessthan 3 km, by faulting. Further tests,includinganalysison cuttingsrecovand basin fill velocityis on averagegreater than half that of ered from known fault zonesand on pulverizedand unpulverbedrock(e.g., parametersfor a typicalbasinin the Basin and ized samplesfrom surface-exposedlow-angle normal faults,
Range [Smithet al., 1989]). Using theseextremesfor a 10-km will be required to evaluate this technique.Other problematsegmentof fault, the apparentdip on a time sectionis no more ical aspectsof their interpretationsare discussed
by Allmendthan 10ø-12ø less than the true dip. Migration of reflections ingerand Royse[1995] and Orton [1995].
alsoservesto steependipsbut at largescalewith dipslessthan
Interpretations of the Sevier Desert detachment notwith30ø the dip of a given reflection is not significantlyincreased. standing,three examples,one from the Bohai Gulf in northern
Among the best documentedimagesof shallowlistric fault China, one from the Gulf of Oman, and one from the Basin
phenomena are from the northern Gulf of Mexico, where and Range, are typical of profiles from areas of basementlarge-scaleslumpingof passivemarginshelf stratatoward the involvedcontinentalextension(Figure 7) and includeintracraslopealonga saltdecollementis the underlyingcauseof fault- tonic rift, passivemargin shelf, and orogenic "collapse" tecing, rather than whole crustextension[e.g., Worralland Snel- tonic settings,respectively.
The Gulf of Bohai resides within the Sino-Korean craton,
son, 1989].
The mostspectacularseismicimageof a basement-involved, more than 500 km west of its boundary against the Pacific
uppercrustallow-anglenormalfault (or for that matter,of any plate. The imagedfault (Figure 7a) and associatedhalf graben
fault) is the Consortiumfor Continental Reflection Profiling is one of over 50 suchbasinsknown from the region [Zhang,
(COCORP) and related profilesacrossthe SevierDesert de- 1994]. The fault plane is listric,with an apparent dip of about
Seismic
Reflection
Profiles
20,166
WERNICKE:
LOW-ANGLE
NORMAL
FAULTS AND SEISMICITY
a)
=2km
b)
I
I
=2km
c)
' ß..... ' '.
ß'.
L'ø-?
.....
I
=1kin
•'•:.V,.;-.'."•
....;-•'• "'"
i
Figure 7. Seismicreflectionprofilesof low-anglenormalfaults.Vertical scalesare all •o-way travel times,
in seconds.
(a) Gulf of Bohai,eastof Beijing,China,fromZhang[1994];(b) Gulf of Oman,from Wernicke
and
Burchfiel[1982];(c) Lamoille Valley, Nevada,from Smithet al. [1989].See text for discussion.
WERNICKE:
LOW-ANGLE
NORMAL
35ø near the surface,flattening downwardto about 5ø [Zhang,
1994].Although the total depth of the sectionis not known,the
fault is imageddownto a two-waytravel time of 5.5 s, including
a few hundred meters of water. At 3-5 km/s averagevelocity,
this yields a depth range for the sectionof 9-15 km.
The Gulf of Oman example(Figure 7b) lies alongthe northeastern passivemargin of the Arabian Peninsula. Following
Late Cretaceousobductionof the Semail ophiolite, the Oman
Mountains and bordering shelf region experiencedbasementinvolvedextensionin Late Cretaceousand Tertiary time [e.g.,
Mann et al., 1990].The imagedfault is conceivablyassociated
with large-scaleslumpingtoward the trench rather than basement-involvedcontinentalextension,perhapsanalogousto the
Gulf of Mexico. However, evidencefor a protractedhistoryof
basement-involvedextensionnearbyon land, and the absence
of major evaporitesor diapirism in the Gulf of Oman [e.g.,
Mann et al., 1990; White and Ross,1979] suggestan analogy
with Gulf of Mexico is inappropriate.The fault plane is clearly
imaged to about 4 s two-waytravel time or a probable depth
range of 6-10 km.
The Basinand Rangeexample(Figure 7c) is from the center
of the provincealong the topographicallysharprange front of
the Ruby Mountains-East Humboldt Range core complex
[Smith et al., 1989; Mueller and Snoke, 1993]. Hanging wall
a)
20,167
50-
I Both
Nodal
Planes,
I
40-
I
Rake
='90+-30ø
60 events
>
ß
3o-
o
•
20-
10-
i
0
30
60
b)
•)
90
Rupture Plane
Known
o•
.c},,_ '• 5
Eo>eI •
sediments are nonmarine Cenozoic basin fill, while footwall
rocks are migmatitic gneissesof the core complex. Detailed
velocity analysisfor this example suggeststhe fault is a lowanglestructuredippingabout 10ø-22ø in the upper 4 km of the
crust[Smithet al., 1989].The fault projectstoward a fault scarp
in alluvium, suggestingactivity in late Quaternary time. Numerous other examples of either young or once-active lowangle normal faults have been describedfrom the Basin and
Range based on combined subsurfaceand neotectonic data
[e.g.,Effimovand Pinezich,1986;Burchfielet al., 1987;Johnson
and Loy, 1992;Bohannonet al., 1993].
It is difficult to argue that any of the above exampleshave
been passivelyrotated (i.e., while inactive) from a steep dip.
Hanging wall sedimentsand the topographic surface in all
examplesprecludesignificanttilting of the fault planesduring
their latest phasesof movement,which would require unrealistic paleotopographyand depositionalslope. In all examples,
however,it is difficult to constrainthe intial dip of the fault.
The apparent fault bed angle along the low-angle segments
suggestsrelatively modest net rotations of about 200-40ø.
However, becausethe faults are listric, these dips may be due
to rollover of an independentlydeforminghangingwall block,
rather than a measureof the rotation of the fault plane [e.g.,
Xiao et al., 1991].
It is emphasizedthat these three examplesare not particularly unique. Images from basement-involved,upper crustal
low-angle(0-30 ø) normal faults have been publishedfrom all
three tectonicsettingselsewhere(e.g., boundaryfaults of the
Rio Grande rift [Russelland Snelson,1990];Outer Islesfault in
the shelf region off Scotland[Brewerand Smythe,1984]; the
Slocan Lake fault in the Canadian Cordillera [Cook et al.,
1992]). As in the case of the Sevier Desert detachment,a
number of examplesshowfault plane reflectionscontinuously
traceable at shallow dip from near the surface to depths of
15-20 km [e.g.,Brewerand Smythe,1984;Cook et al., 1992]. It
is also stressedthat reflection data indicate there are a large
number of normal faults with moderate to steep dips through
the upper 10-15 km of the crust [e.g.,Andersonet al., 1983;
Okayaand Thompson,1985;Brunet al., 1991].
FAULTS AND SEISMICITY
0
30
16
event
60
90
c)
Mw> 6.5
6 events
i
30
0
60
i
i
90
dip
Figure 8. Frequency of earthquakes versus dip, crosshatchedeventsfrom Abets [1991]. (a) Both nodal planes,from
Jacksonand White [1989] and Abets [1991]; (b) eventswith
knownfocal plane, includingevent 1 of Abers[1991];(c) events
larger than moment magnitude6.5, from Doser and Smith
[1989] (Basin and Range events),Jacksonand White [1989],
and Abets [1991], including 1, Aegean Sea, 1970; 2, Aegean
Sea, 1969; 3, Hebgen Lake, 1959; 4, Borah Peak, 1983; and 5,
Italy, 1980.
Seismicity
The weight of evidencefrom field geology,thermochronologic studies, paleomagnetic studies, and seismic reflection
profilingsuggests
activeslipof major normal faultsdippingless
than 30 ø and in some cases initiation
of these faults at shallow
dip, especiallyalong their deeper parts. However, the majority
of focal planes from a compilationof all normal fault earthquakeswith a mechanismdefined by detailed waveform modeling dip between 30ø and 60ø (Figure 8). Three of the eight
shallowlydippingplanesare from focal mechanismstudiesfor
events
in 1982
and
1985
in the Woodlark-D'Entrecasteaux
extensionalprovinceof Papua New Guinea [Abets,1991], determined after Jacksonand White's[1989] synthesis.Of four
dip-slip events studied, two had nodal planes dipping about
15ø-20ø, and another two dipped about 30ø. Although no surface rupture is knownfrom theseevents,they are the only large
earthquakesknown to haveoccurredin a tectonicenvironment
of Plioceneand Quaternarymetamorphiccore complexes[Hill
et al., 1992;Baldwin et al., 1993]. The largestevent,with M.• -
20,168
WERNICKE:
LOW-ANGLE
D
NORMAL FAULTS AND SEISMICITY
C
B
A
..............
.:•:.
(• ..•:::.:.....•._•
ß
D
INGE
C
Figure 9.
Rollin• hingemodelof detachment
faultin•[fromWcmick½,
1992].Seetextfor discussion.
6.8, waspositioned
suchthat its shallownodalplaneprojects Discussion
into theyoungdetachment
described
byHill et al. [1992],and
thusthe shallowplanewassuggested
to be the morelikely Paradoxof Seismicityand the GeologicRecord
rupture plane [Abets,1991].
Manyfactorshavebeenproposed
to reconcilethe predomThe additionof the PapuaNew Guinea data to the earlier inanceof moderately
dippingplanesdefinedbyseismicity
with
compilation(Figure 8a), even for those eventsin which the the existence
of low-angle
normalfaults.Theseinclude(1)
rupture plane is known (Figure 8b), nonethelessrevealsa "rollinghinge"or "flexuralrotation"models,(2) a nonuniforpredominance
of moderateto steeplyinclinedplanes,as has mitarianlackof activelow-angle
normalfaults,(3) aseismic
beenreportedin a numbero,fpreviousreviews[Jackson,
1987; creepalonglow-anglefaults,and (4) longrecurrence
intervals
Jackson
and White,1989;Doserand Smith,1989].
betweenearthquakes
on low-angle
faults(e.g.,discussions
by
As emphasizedby Jackson[1987] and Jacksonand White Jackson
[1987],Buck[1988],DoserandSmith[1989],Kingand
[1989],largenormalfault earthquakes
nucleatenear the base Ellis [1990],and Wernicke[1992]).
of theseismogenic
layerandcutmostor all of thewaythrough Rollinghingemodels. Rollinghingemodelssuggest
that
it. They alsonotedthat the largestknownnormalfault rup- isostaticunloading during and after slip inducesshorttureshavestrikelengthsof the sameorderastheir diplenths, wavelength
flexureandtiltingof the footwall[e.g.,Buck,1988;
with few exceedingabout20 km. Thus if we considera 45øfault Wernicke
andAxen,1988;Hamilton,1988],so that manyancuttinga seismogenic
layer 15 km thick,we expecta seismic cientnormalfaultswithsubhorizontal
dipmayhavebeenmuch
moment [e.g.,Scholz,1990]
steeperwhile active(Figure9). For example,accordingto
Buck's[1988]model,basedon physical
reasoning,
all normal
M0-- /.L4D • 5 X 10•8N m,
faultsare essentially
planarand projectsteeplythroughthe
assuming
an averagefaultslipD of 2 m, a roughlyequantfault brittle,seismogenic
part of the crustwith moderateto steep
plane of areaA, and a rigiditypcof about6 GPa. This corre- dip,terminatingat the baseof the brittlelayer.Flexuralrotaspondsto a momentmagnitudeM w = -6.5.
tionof thefootwall
produces
a series
of sequentially
detached
In the compilation
of Jackson
and White[1989],whichin- faultblocks,all of whicharebounded
byhigh-angle
faults.The
cluded56 dip-slipnormalevents(rake within30ø of -90ø), Andersoniar
! theoryand seismicity
data are therebyresolved
only a dozenor so of theseare of M w _> 6.5, and these with the formation of subhorizontal detachments and core
dominati•the recordedmomentreleaseon normalfault earth- complexes,
asthe modeldoesnot requireactiveslipon lowquakes.Globally,thereareonlysixnormaldip-slipeventswith anglefault planes.A similarconclusion
wasreachedbyKing
M w = 6.5 or greaterwherethe planeis resolved
(Figure8c), andEllis [1990].
if the largeeventdescribed
byAbers[1991]is included.As can
In contrast,
themodelof Wernicke
andAxen[1988],basedon
be seenin Figure8c, nodalplanesdipping300-60ø are still geological
observations
alongthe boundarybetweenthe Basin
mostcommon,as in the larger samplethat includesmostly andRangeprovinceandColoradoPlateau[cf.KingandEllis,
smallevents.However,the PapuaNew Guineaeventrepre- 1990]stresses
a relationship
betweenthe dip of footwallbedsentsa muchmore substantial
fractionof the samplefor the ding
ofhørmal
faults
andtheirinitialdips.
Thefootwalls
of
largeevents,
whichis far moreevenlydistributed
withrespect initiallysteepnormalfaultsweredeformed
in abruptshortto dip.
wavelength
flexuresandlarge,subvertical
fractures(e.g.,the
WERNICKE:
LOW-ANGLE
NORMAL
FAULTS AND SEISMICITY
20,169
northernVirgin Mountains,Nevada),while thosewith shallow tation of stress axes favors creep for reasons currently uninitial dips resulted in broad footwall upwarps(e.g., western known. However, thrust earthquakesdisplaya wide range of
Mormon Mountains and Sevier Desert areas). Subsequent dip, with low-angle thrustsresponsiblefor the largest known
studies have documented both flexure and shear in a number
earthquakes.The fact that both thrust and normal fault earthof detachmentfootwalls,consistentwith the conceptof a roll- quakes occur argues against isolating stress orientation as
ing hinge [Bartleyet al., 1990;Manningand Bartley,1994;Selv- cause of aseismic behavior.
erstoneet al., 1995].
Long recurrence intervals. Another potential solution to
Wernickeand Axen [1988, p. 851] concludedthat the tran- the problem might be longer recurrenceintervals for shallow
sient steepnessof at least some ancient detachmentsin the faults and perhaps due to the greater efficiencyof low-angle
brittle crust may ameliorate the paradox with focal mecha- faults in absorbingelasticstrain that accommodateshorizontal
nisms but that this does not reconcile the seismic data with
extension.Since larger fault planeswould be able to accomthose faults active at low dip in the brittle crust, such as the modate more strain, low-anglefaults might fail more rarely,
SevierDesert, Mormon Peak, Whipple Mountains, and Pana- and in larger events,than steeperones,explainingthe dearth
mint Valley detachments[cf.Johnsonand Loy, 1992;Scottand of low-angleplanesin globalseismicity[Doserand Smith, 1989;
Lister, 1992]. Given the evidencesummarizedabovefor active Wernicke,1992]. In addition,Forsyth[1992] suggests
that finite
slip on low-anglenormal faults, rolling hinge models that ex- slip on low-anglenormal faults is favored by the fact that less
clude shallowfaulting seem not to provide a satisfactoryex- energy,and hence lessregional stress,is required for a given
planation of the seismicitydata.
amount of extensionin comparisonwith slip on high-angle
Paucity of active low-anglenormal faults. Another expla- faults.Geometrically,seismicslip on low-anglenormal faultsis
nation is that none of the currentlyactivezonesof continental more efficientlyinvestedin accommodatinghorizontal extenextensioninclude low-angle normal faults. Since most exam- sion than slip on high-angle faults, requiring fewer earthples of low-anglenormal faults in the literature are ancient,as quakes.
for phylum Trilobita,there may be no reason to suspectthey
One difficultywith this solution is that it does not explain
are activeat present.However, a number of examples,includ- why there are very few small- to moderate-sizedearthquakes
ing thosefrom PapuaNew Guinea [Hill et al., 1992];the Sevier (Mw < 6) which would be expectedif there are numerous
Desert, PanamintValley [Burchfielet al., 1987], and Lamoille active low-angle normal faults. The solutionto this difficulty
Valley (Figure 7c) in the Basin and Range; and the Gulf of mainly dependson whether seismicityis clusteredin time near
Oman (Figure 7a) appear to involve Quaternary deposits. infrequent mainshocksor occurrssteadilythrough the interHence unlike the trilobites, examplesfrom the most recent seismicinterval. The former seemsto be the most likely for
period of earth historydo not appear to be particularlyrare, large faults. For example, the two locked portions of the San
and so their suddendisappearance
would be rather fortuitous. Andreas fault, and perhapsthe Cascadiasubductionzone, are
A subsetof this explanationis that low-angle normal faults capableof generatinglarge earthquakes,but most of the seisare favored in certain tectonicsettingsthat are currently not mic moment release associatedwith them, including adjustactive [e.g., Burchfiel et al., 1992]. The examples discussed ments near the boundariesof coseismicslip, occurswithin a
above(e.g., Figure 7), however,seemto occurin a variety of few years of the mainshock,followed by long intervalswhere
tectonic environments,includingorogenic collapse,intracra- even microearthquakesare relatively uncommon.
tonic rift, and passivemargin settings,all of which are now
In the next section,these conceptsare integratedwith some
active globally. Thus the nonuniformitarianhypothesisthat simpleaspectsof earthquakemechanics,providinga quantitashallowlydippingnodal planesare rare becauselow-anglenor- tive basisfor empiricalrelationsof earthquakefrequencyvermal faults are simplynowhere currently active doesnot seem susdip describedby Jackson[1987],Doserand Smith [1989],
particularlyappealing.
Jacksonand White [1989], and Thatcherand Hill [1991]. In
Aseismicbrittle creep. Another way to explainthe seismic- general, this approach may offer a fairly simple resolution to
ity is that low-anglenormal faults tend to creep aseismically the paradox.
[e.g.,Jackson,1987;Doserand Smith, 1989]. This explanation
hasinterestingimplicationsfor the physicsof earthquakerup- Seismicity of Dip-Slip Faults
ture, althoughit is at presentnot obviouswhat the causemight
Model. Considera hypotheticalseismogeniclayer of thickbe.
nessh transectedby a fault dipping0 (Figure 10). The average
The major effectwould presumablybe the brittle constitu- stressdrop zXo-on the fault is proportionalto the averageslip
tive rheologyof the fault zone. Suchan effectwould presum- D and area of slipA [e.g., Scholz,1990],
ably be temperature dependentand therefore depth depenD
dent. For example, a transition from stick-slip to stable
frictionalslidingwith depth,hypothesized
for the SanAndreas
fault zone [Tse and Rice, 1986] may in some way apply to
The area of slip, assumingit about equant, is related to fault
normal faults, suchthat their flat segmentsare lessprone to
dip by
seismicslip than steepersegmentsin the upper crust.Sucha
rheologicaleffectwould haveto applyto a wide variety of rock
xf•: h/sin0
(2)
compositions,as detachmentsseem to be developedin every
major rock type [e.g.,Davis, 1980]. However, the observation which impliesthat for constantstressdrop, layer thicknessand
that large eventson steepfaults penetrateto 10-15 km depth rigidity modulusfor a given earthquake,
[Jacksonand White, 1989], well below the range of depths
D o• 1/sin 0.
(3)
discussed
abovefor shallowlydippingnormal faults,seemsto
In other words, large, low-anglefault planes may accumulate
argue againstsuchan explanation.
Alternatively,it may be that either the low dip or the orien- more strain between earthquakesthan small steep ones.For a
20,170
WERNICKE: LOW-ANGLE NORMAL FAULTS AND SEISMICITY
per unit time in the rift with steepfaultsthan in the rift with
low-angle faults.
v
g
Figure 10. Diagram showingvariablesusedto derive equations(2)-(6). See text for discussion.
The abovereasoningsuggests
that low-anglefaults should
fail lessoftenbut with largerearthquakes.Sincethe momentof
an earthquakeis defined as
M0:
/x,4D,
from (2) and (3) we have
M0 oc1/sin3 0.
constantrate of horizontal separationbetweenhangingwall
and footwallv, fewer earthquakesare requiredin a giventime
interval on shallow faults than on steep ones.
This relationshipassumes,
however,that strikelengthis free
to expand with decreasingdip. The question arises as to
whether the confinement of normal faults to relatively short
(7)
Again, givenconstantstressdrop, rigiditymodulus,thickness
of the seismogeniclayer, and extensionvelocity, low-angle
faults will have substantiallylarger earthquakesthan steep
ones.In terms of moment magnitudeM,,, faults dipping 10ø20øwill produceearthquakesaboutone magnitudepoint stronger than faultsdipping500-60ø. Thus if 50ø faultswould typically yield magnitude 6.0-7.0 earthquakes, 10ø-20ø faults
shouldproducemagnitude7.0-8.0 earthquakes.
Application to continental seismicity. Globally, earthquakestressdrop and the presumedrigidityof the crustmight
not be expectedto vary [e.g.,KanamoriandAnderson,1975],
but the thicknessof the seismogeniclayer and the horizontal
extensionvelocityprobablyvary from rift to rift. These and
other factorswouldproducea wide rangeof maximumearthquake magnitudes in extensional provinces, with rapidly
spreadingareasproducingmore frequent earthquakesfor a
givenfault dip. Of the five eventsstudiedbyAbets[1991],the
eventwith the shallowestnodal plane (--•17ø) wasM,, = 6.8,
segments[e.g.,Machetteet al., 1992]would limit their lateral
dimensionsand therefore their ability to slip accordingto (3).
As reviewedby Jacksonand White [1989], the largestknown
normal fault earthquakeshave strike lengthsrestrictedto the
rangeof a few tensof kilometers,about1-2 timestheir downdip rupture lengths.Thus a 15ø normal fault would have a
downdiplengthof about60 km and an along-strikelengthof
60-180 km. Shallowdip-slipruptureshave similardimensions
[e.g., Scholz,1990,p. 297]. As mentionedabove,the Sevier
Desert detachmenthasbeen imagedas a singlezone of reflectionsfor a downdiplengthof 60-70 km and for a strikelength
of at least 100 km [Plankeand Smith, 1991].Assumingit is
indeed a normal fault, it seemsto have an appropriatelylong
strike dimensionrelative to its dip dimensionand is substan- while the other events were all between 5.5 and 6.0. In other
tially longer than the steepfaults describedby Jacksonand words, 80% of the moment releaseoccurredduring the single
White [1989].
A secondconsiderationis the fact that for each earthquake
low-angle event.
Equation(6) maybe relatedto the globaldatasetof dip-slip
a greateramountof slip is transferredinto horizontalexten- normalfault earthquakes(Figure8), dependingon the global
sion for shallowfaults than for steep ones. Thus
distributionof fault dip over the total strike length of active
faults.The simplestsuchdistributionwould be uniform, such
v = D cos OR'
that the sametotal length of fault plane would existfor each
whereR' is the frequencyof eventsper fault. This impliesthat 10ø incrementof dip. This distributionwould not agree well
with the eventfrequencydata (Figure 8a), becauseit predicts
for constant v,
the vast majority of eventswould occur on planes dipping
D oc1/(R' cos 0).
(4)
600-90ø. In this case,considerationof both nodal planeswould
place
a minimum number of eventsin the 30ø-60ø interval
Equating(3) and (4) and solvingfor R,
rather than the observedmaximum(Figure 8a).
R'o• tan 0.
(5)
The simplestdistributionthat would explain the data in
Figure8a in termsof equation(6) is one that is evenfrom 0ø
Equation(5) allowscomparisonof earthquakefrequencyof to 60ø, greatly reducedfrom 60ø to 70ø (say, by an order of
two fault segmentswith contrasting0 but equal v, h, and
magnitude),and effectively
zero from 70øto 90ø (Figure11a).
For example,a fault dipping 10ø-15ø would be expectedto
Accordingto Figure 8a, the ratio of eventsin the 00-30ø dorupture about 7 times less frequently than a fault dipping main to that of the 300-60ødomainis about0.1. Integratingthe
55ø-60 o.
functionsin 0 tan 0 for thesetwo domainsalsoyieldsa ratio of
A third considerationis that for a giventotal strikelength of
shallowto steep eventsof about 0.1 (Figure 11b), in good
faults, there should be fewer faults in the case of low-angle
agreementwith the data.Addingthe conjugateplanesto such
versus high-anglefaults. The frequency of events per unit a model distribution doubles the number of events in the
length of fault is
300-60ø domain and adds whatever seismicitywould exist in
R-R'/•=R'
the 600-90ø domain to the 00-30ø domain, so the ratio of shal-
sin0,
low to steepeventsis not appreciablydifferentfrom the model
where1/V• is the numberof faultsper unitlengthof fault. without conjugateplanes(Figure l lb). The principaldifferThus
encebetweenthe model in Figure l lb and the data in Figure
R ocsin 0 tan 0.
(6)
8a is the ratio of events in the 300-40 ø domain to events in the
400-50ø domain, which is about 1 in the model and 2 in the
is perhapsmitigatedby the fact that the
For two rift zones of equal strike length with multiple fault data.The discrepancy
segments,
one characterizedby 10ø-15øfaultsand the otherby uncertaintyin dip is as largeas the 10øbin size[e.g.,Thatcher
550-60ø faults, we would expect about 28 times more events and Hill, 1991], and the total number of eventsis relatively
WERNICKE:
a) •
LOW-ANGLE
NORMAL
to be in need of substantial
0
60
90
dip, degrees
Model,
56 Events
4O
Conjugate Planes
30
..o
20
or abandonment.
attracted the attention of fault mechanists, in the tradition of
50
©
modification
One of its main assumptions,
that the principalstressaxesin
the brittle crust are orthogonalto the Earth's surface,is likely
to be the major problem. Over the last 5 years,the problem has
30
b)
20,171
predictsthat normal faultsform with a dip of 60ø,would appear
20
10
•-
FAULTS AND SEISMICITY
o< tanOsinO
10
o
o
30
60
90
dip, degrees
Hafner [1951]. Solutionsto the problem have includedrotation
of stresstrajectoriesthroughflexure[Spencerand Chase,1989],
igneousdilation at depth [Parsonsand Thompson,1993] or
viscousflow of deep crust againstthe seismogeniclayer [Yin,
1989;Melosh,1990], rotation of stresstrajectoriesin the vicinity of the fault zone via high fluid pressure[Axen, 1992], and
considerations
of the energyefficiencyof low-anglefaults [Forsyth, 1992]. As yet, there is no consensuson which if any of
these mechanismsare correct, but they do provide a framework for major progressin understandingfault mechanicsand
earthquakes.For example,the hypothesisthat low-anglenormal faults confinelocally high fluid pressureand rotated stress
trajectories[Axen, 1992] may be testableby moderate-depth
drilling (5-6 km) into the Sevier Desert detachmentof west
central Utah [Zobackand Emmetmann,1994].
The fact that progressiveextensiontendsto decreasethe dip
of fault planes reconcilesAnderson theory with the preponderanceof earthquakeson faults dipping much lessthan 60ø
with there being relatively few faults steeper than 60ø [e.g.,
Thatcherand Hill, 1991]. To the extent that rotation of stress
trajectories is common in continental rifts, this distribution
may be substantially"smeared"well below 30ø (the cutoff for
frictional slidingif stresstrajectoriesare not rotated), consistent with the model distributionin Figure 11a. In this case,60ø
would representthe maximuminitial dip, but lower initial dips
and active slip not predicted by Anderson theory would be
Figure 11. (a) Model for dip distributionof active normal common.
faults that involve the entire seismogeniclayer, discussedin
text. (b) Number of earthquakesas a functionof dip for 56
events (unpatterned areas) and conjugate planes (cross- Conclusions
hatching),accordingto equation(6).
Geologic reconstructions,thermochronology,paleomagnetism,and seismicreflectionprofiling indicate that initiation
small.The principalpoint is that the model predictsthe correct and slip on low-angle normal faults in the upper continental
crust are common in the geologicrecord. The paradoxically
overall proportionsof low-angleand high-angleplanes.
The 16-eventsamplewith resolvedfault planes(Figure 8b) low ratios of shallow and steep dipping focal planes to modis perhapstoo smallto make a meaningfulcomparisonwith the erate ones in global seismicitymay be resolvedby a simple
model, but nonethelessis in goodagreement.It is clear, how- recurrencemodel, where the larger size and greater efficiency
ever, that a 16-eventsampleover a few decadesis not neces- of shallow dip-slip faults cause them to fail much less fresarily sufficientto observea large earthquakeon a low-angle quently.This conclusionis perhapsnot surprisingwhenviewed
normal fault. Even if the large Papua New Guinea event oc- in comparisonwith compressionaldip-slip earthquakes.Apcurredon the steepplane, the model predictsonly one or two proximately80% of global seismicstrain releaseover the last
of the events would be less than 30 ø and none less than 20 ø. For
four decadesoccurred during two events, the 1960 Chilean
the even smaller sampleof eventswith M w > 6.5, the same earthquakeand the 1964 Alaska earthquake,both of which
conclusion holds.
occurredalong shallowlydippingthrust faults.
Of course, there are distributions other than the one shown
The most probable reconciliationof this model with Anderin Figure 11a that couldreconcilethe data with equation(6). sonianfault mechanicslies in rotation of stresstrajectoriesat
For example, an even distributionin the 300-60ø domain with depth in a significantfraction of active zones of continental
a smaller fraction from 60ø to 90ø,with no faults from 0ø to 30ø,
extension.
would also be consistent with the data. Unlike
The recognitionof low-anglenormal faults,and the prospect
that they fail in large earthquakes,hassignificantimplications
for seismichazard. Active low-angle normal faults may be
difficultto detect on the basisof surfacerupture patterns and
paleoseismicity(e.g., the Sevier Desert detachment),as are
low-anglethrust faults [e.g.,Haukssonet al., 1987]. Sincemany
geophysicists
have expresseddoubtthat large seismogenic
lowangle normal faults even exist [e.g., Jacksonand McKenzie,
the distribution
shownin Figure 11a, however,sucha distributionis not successfulin reconcilinggeologicalobservationsof brittle lowangle normal faults with the seismicity.
Mechanical implications. If distributions of the type
shownin Figure 11a do indeed representthe global distribution of a "major" ctivenormal faultsin continents,how do they
bear on Andersonian
fault mechanics?
The
existence
of low-
angle normal faults suggeststhat Andersoniantheory, which
1983; Stein et al., 1988; Buck, 1988; Jackson and White, 1989;
20,172
WERNICKE:
LOW-ANGLE
NORMAL
King and Ellis, 1990], hazardsin extendingareas suchas the
Basinand Rangeprovince,westernTurkey, and China may be
seriouslyunderestimated.
FAULTS AND SEISMICITY
Bally, A. W., D. Bernoulli, G. A. Davis, and L. Montadert, Listric
normal faults, Oceanogr.Acta, SP, 87-102, 1981.
Bartley, J. M., J. M. Fletcher, and A. F. Glazner, Tertiary extension
and contractionof lower-plate rocks in the central Mojave metamorphic core complex,southernCalifornia, Tectonics,9, 521-534,
1990.
Acknowledgments. Discussions with D. L. Anderson and H.
Kanamori helped clarify my thinkingon seismological
aspectsof this
paper. Reviewsby G. A. Davis, R. K. Dokka, J. W. Geissman,T. A.
Hauge, B. E. John, and JGR reviewersD. Davis, J. Oldow, and an
anonymousreferee contributed substantiallytoward improving the
presentation. This research was supported by NSF grants EAR9219939
and EAR-9316797.
Bohannon, R. G., J. A. Grow, J. J. Miller, and R. H. Blank Jr., Seismic
stratigraphyand tectonicdevelopmentof Virgin River depression
and associatedbasins, southeasternNevada and northwestern Arizona, Geol. Soc.Am. Bull., 105, 501-520, 1993.
Brewer,J. A., and D. K. Smythe,MOIST and the continuityof crustal
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Brock,W. G., and T. Engelder,Deformationassociated
with the movement of the Muddy Mountain overthrustin the Buffingtonwindow,
southeasternNevada, Geol. Soc. Am. Bull., 88, 1667-1677, 1977.
Brun, J.P., F. Wenzel, and ECORS-DEKORP Team, Crustal-scale
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(ReceivedAugust 26, 1994; revisedJune 13, 1995;
acceptedJune 20, 1995.)