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Flexural Rotation of Normal Faults TECTONICS, VOL. 7, NO. 5, PAGES959-973, OCTOBER1988 FLEXURAL ROTATION OF NORMAL FAULTS W. RogerBuck Lamont-DohertyGeological Observatory of ColumbiaUniversity, Palisades, New York Abstract. A conceptualmodel is proposedfor the flat-lying abandonednormal fault (or "detachment") generationof low-anglenormal faults in Metamorphic below slicesof upperplate rocks and sedimentary infill; Core Complexes. The model is based on three (2) a strongcontrast in metamorphicgrade acrossthe assumptions:(1) the isostaticresponse to normalfault abandoned"detachment"; (3) rapid movementof lower motionis of regionalextent; (2) when a fault segment plate rocksfrom midcrustaldepths to shallowerdepths. is significantlyrotated from the optimumangle of slip, These results are qualitatively in agreement with relativeto the crustalstress field, it is replacedby a new geologicobservations for corecomplexes. planarfault orientedin the optimumdirection; and (3) the fault cuts the entire upper crust and fault motion alwaysnucleates in the sameregion at the baseof the INTRODUCTION uppercrust. The stressfield is consideredto be uniform throughthe crustand the regionalisostatic response of There has been much recent discussion about the the crust to loads is computedusing the thin plate origin of "Metamorphic Core Complexes." Faults flexureapproximations. Active low-angle normal faults within these complexes appear to have undergone a are difficult to reconcile with rock mechanics theories, normal sense of motion but are gently dipping or earthquakefocal mechanism studies, and geochronologic subhorizontal[e.g, Profett, 1977; Davis, 1980; Miller resultsindicating rapid coolingof corecomplex rocks. et al., 1983]. This is particularly interesting since The model does not require active fault slip on seismicallyactive normal faults are generallyobserved low-angle faults. The flexural response to normal to be dipping at steeperangles (>30 ø) [e.g., Jackson, faulting is shown to be significantly affected by 1987]. anelasticbehavior of the crust and by loading due to More than20 corecomplexes have been recognized sedimentation.The anelasticresponse to large bending in the Basin and Range province of western North stressesresults in significantreduction in the effective America, all of Tertiary age [Crittendenet al. 1980]. elasticrigidity of the uppercrust. This can explainthe Severalcharacteristics of metamorphiccore complexes observedshort wavelength of topographicresponse to are illustratedin Figure 1. The fundamentalrelation- normal fault loads. The model results in: (1) a nearly ship in core complexes is that low-grade or un- metamorphosedrocks overlie high-grademetamorphic Copyright1988 "core" rocks [Davis, 1983]. The boundary between by the AmericanGeophysical Union. rocksof contrastingmetamorphic grade, which is often subhorizontal,has been variously called a "low-angle Papernumber 8T0340. normal fault," a "detachment," or a "decollement." 0278-7407/88/008T-0340 $10.00 Although these terms imply that fault motion took 960 Buck: FlexuralRotation of NormalFaults (a) Harcuvar Mountains WSW ENE Miocene Sedimentary pC Gneiss & VolcanicRocks & _%•• • _ _ Granite Mylonitic IZll III I11 II Foliation "Detachment" (b) Whipple Mountains SW NE Older& VolcanicTertiary RocksSedimentary YoungerTertiary Sedimentary & Volcanic Rocks tic "Detachment" on Metamorphic & Intrusive Rocks i i i i i i 0 km 10 Fig.1. Interpretativecross sections oftwo metamorphic corecomplexes showing features common tomany core complexes:(a)the Harcuvar Mountains inArizona [after Rehrig and Reynolds, 1980] and (b) the Whipple Mountains in Californiawith inferred doming removed [after Davis, 1980]. placeon a nearly flat-lyingfault, thereis considerable thatthey were deformed in a temperaturerange where debateon the origin of suchstructures. Here the term quartz flowed but where feldspars fractured. "detachment"is usedin a nongeneticsense to describe High-temperaturerock mechanicsexperiments show themajor discontinuity in a corecomplex. that this could occur between about 400ø and 500 øC A normal senseof offset motion on the detachment [Caristan,1982]. The lower plate mylonitesoften is inferred from strain indicatorsand structuralrelations showoverprinting of brittledeformation on the ductile [e.g., Davis et al. 1983, 1986]. Upperplate rocks, fabric[e.g., Crittenden et al., 1980]. thoseabove the detachment, have typically undergone Geochronologicstudies of lowerplate rocks from onlybrittle deformation while lowerplate rocks have several core complexesindicate rapid uplift of experiencedconsiderable ductile deformation [e.g., midcrustalrocks [Davis, 1988]. Dallmeyer et al. [1986] Miller et al. 1983; Dokka et al., 1986]. The lower use40Ar/39Ar dating of lowerplate rocks from the platerocks frequently show mylonitization, indicating RubyMountain metamorphic core complex in eastem Buck: Flexural Rotation of Normal Faults 961 350 which are also thought to be greatly extended, are typically buried under deep piles of sediment. Mid-oceanridges are the majorareas of platedivergence, 300 - but becauseof the difficulty in working in deep water they are are not well mappedon a fine scale. Also, there is little or no erosionof mid-oceanridges, making it difficult to infer cross sectional relations among 250-coolingpath envelope faults. The basic problems to be explained in core complexes are twofold: (1) how to produce a • 200- subhorizontaldetachment surface separating rocks of • practicallimit of very differentmetamorphic grade and (2) how to rapidly • coolingpath envelope bring myloniticrocks from 10-15 km depthup closeto the surface. In this papera new model for the origin of detachmentsand associatedstructures is proposed. Before developingthis model a brief review of other 100-- ideasfor the originof detachmentsis given. PREVIOUS MODELS / Several models have been proposedto explain observationsin metamorphiccore complexes. They fall broadlyinto two categories,according to how mylonites 0 4 8 12 16 20 24 28 are broughtto the surface. The proposedmechanisms Time (rvla) for this are: (1) motion on a high-anglefault or set of Fig. 2. Tcml•ramr½-dm½curve inferred for rockswithin (UD) high-anglefaults and (2) motionon a low-anglenormal andbelow (LD) theRuby Mountains"detachment" in fault. For reasons discussedbelow, 30 ø is used here as b•e,.,don fissiontrack d•Ling [afiicr DoJd•.• ½t •.l., 1986]. the cutoffbetween low-angle and high-angle faults. In the first model the detachment is assumed to mark a brittle-ductile transition which starts out at a depth of 7-10 km depth in the crust [Profett, 1977; Nevada to infer rapid cooling from above 500øC to Rehrig and Reynolds,1980; Miller et al., 1983; Gans et below 300ø. In a fissiontrack studyof myloniticrocks al., 1985]. The crust above the detachmentdeforms as a from the same area, Dokka et al. [1986] find that set of closely spacedparallel normal faults, initially cooling from about 300øC to below 70øC occurred slippingat a high angle. As the faults slip, they must between about 25 and 23 Ma [see Figure 2]. If the be rotated to a shallower dip simply to conserve geothermalgradient is assumedto have been 25øC/km, volume. The detachment is considered to be the surface thenthis wouldgive a rate of upwardmotion of greater wherefaults end and more ductile(pure) shearbegins. than 4 km/m.y. Mylonites from the Whipple When the set of upperplate faults has been rotated to a Mountain core complex in California have also been shallowangle so that they canno longerslip, a new set studiedusing 40Ar/39Ar and fission track methods of high-anglefaults cuts them. [Davis, 1988;Dokka andLingrey, 1979]. Thesestudies This model is in accord with simple rock indicateupward rates of motionfor mylonitesbelow the mechanicstheory and seismicobservations discussed Whipple Mountain detachmentwhich are as great or below. However, it is difficult to produce the greaterthan for the Ruby Mountaindetachment. sharpnessof the contactbetween metamorphic grades Pseudotachylite,a rock which is thought to be which is observed.It is difficult to understandwhy the formedonly duringearthquake faulting, has been found detachmentshould continue to be a boundarybetween in association with detachments. This has led some areasdeforming differently as it is broughtclose to the authors to claim that low-angle fault slip may be surface.If the detachmentsurface began moving in the seismogenic[John, 1987]. depth range of the brittle-ductiletransition, then, as it Core complexeshave had a great influence on moves up, it should move out of the brittle-ductile discussionsof extension,since they are foundin one of transition. This is true even at extension rates of the few accessibleareas which appear to have been several cm/yr (see thermal calculationsof Buck et al. extended by large amounts. Continental margins, [1988]). Even if the detachment were to remain a 962 Buck: Flexural Rotation of Normal Faults boundarybetween faulting and more distributedpure Jackson[1987] reportson a worldwide survey of shear strain, there should be a gradational contact fault plane solutions for large continental normal between metamorphicgrades across the detachment. faultingearthquakes. He showsthat the overwhelming Simple shear along the upper plate set of faults and majorityof suchearthquakes nucleate in the depthrange rotation of the faults is equivalentto pure shearon a 6-15 km on faults dipping between 30 ø and 60 ø. grossscale [Jackson,1987]. Pure shear of a crustal Among all earthquakesstudied whose faulting was columnwill
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