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Low-Angle Normal Faults and Seismicity: a Review

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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 Geological and Planetary Sciences,California Institute of Technology,Pasadena Abstract. Although large, low-anglenormal faults in the continentalcrust are widely recognized,doubts persist that they either initiate or slip at shallowdips (<30ø), because (1) globalcompilations of normal fault focal mechanismsshow only a small fraction of eventswith either nodal plane dippingless than 30ø and (2) Andersonianfault mechanics predict that normal faults dippingless than 30ø cannot slip. Geologicalreconstructions, thermochronology,paleomagnetic studies, and seismicreflection profiles, mainly published in the last 5 years,reinforce the view that active low-anglenormal faulting in the brittle crust is widespread,underscoring the paradox of the seismicitydata. For dip-slip faults large enoughto break the entire brittle layer during earthquakes(Mw "• 6.5), considerationof their surfacearea and efficiencyin accommodatingextension as a function of dip 0 suggestsaverage recurrrence intervals of earthquakesR' •c tan 0, assumingstress drop, rigidity modulus,and thicknessof the seismogeniclayer do not vary systematicallywith dip. If the global distributionof fault dip, normalized to 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 nodalplanes dipping between 30 ø and 60ø•' will exceedthose with planesshallower than 30 ø by a factor of 10, in good agreementwith continentalseismicity, assuming major normal faults dipping more than 60ø are relatively uncommon.Revision of Andersonianfault mechanicsto include rotation of the stressaxes with depth, perhapsas a result of deep crustalshear againstthe brittle layer, would explain both the commonoccurrence of low- angle faults and the lack of large faults dippingmore than 60ø . If correct,this resolution of the paradoxmay indicate significantseismic hazard from large, low-anglenormal faults. Introduction ceeding three decades. Low-angle extensional structures, thoughdocumented by geologicalmapping studies, were inter- It is appropriate for the 75th anniversaryof the American preted as either peculiar thrust faults or surficial landsliding GeophysicalUnion that recognitionbe given to the 50th an- phenomena.Sliding and spreadingof rootless,internally co- niversaryof a paper by Longwell[1945]. Although not the first herent, extended allochthonsalong faults dipping only a few descriptionof suchphenomena [e.g., Ransome et al., 1910],the degrees is well known. It includes caseswhere detachment paper was remarkable in its documentationusing maps, pho- occursalong incompetenthorizons in sedimentssuch as shale tographs,and crosssections of spectacularlyexposed normal or salt, as developed over thousandsof square kilometers in faults in the Las Vegas region,with displacementsof 1-2 km the northern Gulf of Mexico [Worralland Snelson,1989]. How- and dips of 0-30 ø. In one large-scaleexposure, since partly ever, it also includesexamples where the slidingoccurs within drowned beneath the waters of Lake Mead, a fault was ob- competenthorizons, as in the Ordoviciandolostones along the served to flatten downward, from about 30 ø to 5 ø over a cross- Heart Mountain detachment [Pierce, 1957; Hauge, 1990]. sectionaldepth of 600 m. These examplesgenerally involve only the upper few kilome- It is perhaps a measure of a theoreticallybased prejudice ters of the crust and are not accompaniedby coevalextension againstlow-angle normal faults that Longwell[1945] excluded of the underlyingcontinental basement. In contrast,fault sys- regional crustal extensionas 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], clearly involvecontinental basement and are large-scalecompressional anticlines. Mechanical arguments observedin some casesto cut structurallydownward through 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,showed that certain loading conditions [e.g.,Anderson,1971; Wrightand Troxel,1973; Proffett, 1977], it along the base of an elastic plate 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-magnitudecontinental extension. At Despite both observationand theory, the assumptionthat about the same time, it was also realized that many metamor- the least principal stressdirection is horizontal throughoutan phic tectonitesin the Basin and Range previouslythought to extendingcrust [e.g.,Anderson, 1942] held sway for 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 Paper number 95JB01911. "detachments" that could be traced for several tens of kilome- 0148-0227/95/95JB-01911 $05.00 ters parallel to their transportdirections. By 1980, it was clear 20,159 20,160 WERNICKE: LOW-ANGLE NORMAL FAULTS AND SEISMICITY (a) (b) mities have since been documented to be low-angle normal faults(Davis et al. [1980],Gans et al. [1989],and Dokka [1986], B respectively).Similarly, major low-ange fault systemsinterpreted asthrusts by Noble [1941], Misch [1960], and Drewes and Thorman [1978]are now widelyregarded as normalfaults related to Ce- nozoicextension (Wright and Troxel[1984], Miller et al. [1983],and Dickinson[1991], respectively).Reinterpretations currently un- derwayin other mountainbelts are similarlyprofound. These Basinand Rangefield relationsrepresented a classof HISTORIES: geologiccontact that had not been previouslyrecognized as a fundamentaltectonic element. Recognizing them as suchis as Depositionof A Depositionof A basic to accuratehistorical inference in geologyas, for exam- Tilting & erosion Depositionof B Depositionof B Faultingof B on A ple, the knowledge that rocks with igneous texture intrude Erosion Erosion their surroundingsin a molten state. Figure 1. Contrast in geologicalhistory from interpreting a Mechanical Significance contact between older sedimentarysequence A and younger The fact that low-anglenormal faults are not predictedby sequenceB as (a) an unconformityand (b) a low-anglenormal fault. Andersonian theory is also fundamental to interpreting the stressstate and physicalconstitution of the crust.In the 1980s, debate centered on the kinematics of generating the core- complexassociation. Most current modelssuggest asymmetri- that numerous isolated exposuresof detachmentsand their cal denudationalong large normal faults that transectthe up- metamorphicsubstrate formed a nearly continuousbelt from per 15-20 km of the crust at low angle, accompaniedby Sonora, Mexico, to southern British Columbia, referred to as isostaticrebound and flexure of the unloadedfootwall [e.g., the Cordilleranmetamorphic core complexes[Crittenden et al., Wernicke,1981; Howard et al., 1982;Allmendinger et al., 1983; 1980; Armstrong,1982]. It was realized that the footwalls of Spencer,1984; Wernicke,1985; Davis et al., 1986; Wernicke, many exposeddetachments were not stronglymetamorphosed 1992].Recently, controversy has centered on the initial dip and in the Tertiary, raising the possibilitythat low-angle normal subsequentmodification of thesefaults and the roles of foot- faults formed and were active entirely in shallowcrust [e.g., wall metamorphictectonite and magmatism. Wernickeet al., 1985;Spencer, 1985; Dokka, 1986;John, 1987]. This paper addressesthe question: Are brittle low-angle These observations ran counter to Jackson and White's normal faults active while at low dip? A number of authors [1989] descriptivesynthesis of some 56 earthquakeson active have expresseddoubt that shallowlydipping normal faults are continentalnormal faults. They concludedthat (italicstheirs) importantfeatures in the extendingseismogenic crust, pointing to Andersoniantheory and a lack of seismicityon suchfaults Among the most important observationsthat now influencethe debateare... that large earthquakesdo not occuron listtic faults [e.g., Buck, 1988;King and Ellis, 1990]. A large body of liter- that flatten at shallowdepths (as originallythought: e.g. McKen- ature has nonethelessfocused on non-Andersonianexplana- zie, 1978a,b), but on faults that are steepthroughout the seismo- tions for active low-angle normal faulting [e.g., Xiao et al., genic upper crust... 1991;Forsyth, 1992;Axen, 1992; Parsons and Thompson,1993]. If low-anglenormal faults are indeed activein the seismogenic Whether or not this conclusionis correctis a first-orderproblem crust, why are there so few, if any earthquakesobserved on in understandingthe structureand dynamicsof the lithosphere. them? Evidence summarizedbelow, mostly publishedin the Geological Significance last 5 years,tends to reinforcethis paradox.A simplemechan- ical model relating fault dip to earthquake recurrenceis de- The recognition of low-angle normal faults and the core veloped that may provide an explanation. complex tectonic associationis now global and includesoce-
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