JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. B8, PAGES 15,249-15,266, AUGUST 10, 1995 The Mount Perkins block, northwestern Arizona: An exposedcross section of an evolving, preextensional to synextensionalmagmatic system JamesE. Faulds,• Daniel L. Feuerbach,1 Mark K. Reagan,1 Rodney V. Metcalf,2 Phil Gans,3 and J. D. Walker4 Abstract. The steeplytilted Mount Perkinsblock, northwestern Arizona, exposes a cross sectionof a magmaticsystem that evolved through the onsetof regionalextension. New 4øAr/39Arages of variably tilted (0-90 ø) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensionalintrusive activity includedemplacement of a compositeMiocene laccolithand stock,trachydacite dome complex, and east striking rhyolite dikes. Related volcanicactivity producedan -18-16 Ma stratovolcano,cored by trachydacitedomes and flankedby trachydacite-trachyandesiteflows, and -16 Ma rhyoliteflows. Similar compositionsindicate a geneticlink betweenthe stratovolcanoand granodioriticphase of the laccolith. Magmatic activity synchronouswith early regionalextension (15.7-14.5 Ma) generateda thick, felsic volcanicsequence, a swarmof northerlystriking subvertical rhyolite dikes,and rhyolite domes. Field relationsand compositionsindicate that the dike swarmand felsicvolcanic sequence are cogenetic.Modes of magmaemplacement changed during the onsetof extensionfrom subhorizontalsheets, east striking dikes, and stocksto northerly striking,subvertical dike swarms,as the regionalstress field shiftedfrom nearly isotropicto decidedlyanisotropic with an east-westtrending, horizontal least principal stress. Preextensionaltrachydacitic and preextensionalto synextensionalrhyolitic magmas were part of an evolving system,which involvedthe pondingof mantle-derivedbasaltic magmas and ensuingcrustal melting and assimilationat progressivelyshallower levels. Major extension haltedthis systemby generatingabundant pathways to the surface(fractures), which flushed out preexistingcrustal melts and hybrid magmas.Remaining silicic meltswere quenchedby rapid, uppercrustal cooling induced by tectonicdenudation. These processes facilitated eruptionof mafic magmas.Accordingly, silicic magmatism at Mount Perkinsended abruptly duringpeak extension-14.5 Ma and gaveway to mafic magmatism,which continueduntil extension ceased. Introduction extendedregions are essentiallyamagmatic (e.g., southern Nevada in the Basin and Range province; [Eaton, 1982]), The relationsbetween magmatism and the dynamic and whereas some areas of intense volcanism within extensional kinematic evolution of extensional terranes have been the settings(e.g., Mogollon-Datil volcanic field, New Mexico; subject of significant controversy. Many workers have [Elston, 1989]) have experiencedlittle mechanicalextension proposedrelationships between magmatismand extensionin (i.e., faulting and tilting). One approachto elucidatingthe which (1) the emplacementof large volumes of magma at interplaybetween magmatism and extensionis to examinean depth triggers extension at shallow crustal levels [e.g., individualmagmatic system that evolvedduring the onsetor Anderson, 1971; deVoogd et al., 1988; Gans et al., 1989; termination of extension. Meyer and Foland, 1991; Lister and Baldwin, 1993] or (2) Locating volcanic centers in extended terranes can be the geometryof major normal faults, particularlydetachment difficult, however, especiallyin the more highly extended faults, controlsthe distributionand type of magmatism[e.g., regions. For example,sources for much of the widespread Wernicke, 1985; Bosworth, 1987]. Others, however, have and voluminousTertiary volcanismin the Basin and Range noted diachroneities in the timing of magmatism and province have not been located [Lipman, 1984]. Moreover, extensionwithin extendedterranes [Bartley et al., 1988; Best mostof the well-documentedcenters occur in mildly extended and Christiansen, 1991; Taylor et al., 1989; Taylor and regions [Best and Christiansen, 1991], where tilts of fault Bartley, 1992; Axen et al., 1993]. For example, somehighly blocksare generally less than 30 ø . Althoughmuch can be gleanedfrom volcanic centersin the mildly extendedareas, •Departmentof Geology, University of Iowa,Iowa City. 2Departmentof Geoscience, University of Nevada,Las Vegas. the exposedstructural levels in such regionsare generally 3Departmentof GeologicalSciences, University of California, limited to the upperfew kilometersof crust,which precludes Santa Barbara. comprehensiveanalysis of magmaticplumbing systems and 4Departmentof Geology, University of Kansas,Lawrence. emplacementmechanisms. The difficulty in identifyingvolcanic centers, especially in Copyright1995 by the AmericanGeophysical Union. highly extended regions, is a function of structural Paper number 95JB01375. dismembermentby normal and strike-slip faults [e.g., 0148-0227/95/95 JB-01375 $05.00 Anderson,1,973], tilting, and erosion. Large-magnitude 15,249 15,250 FAULDSET AL.: EVOLVINGMAGMATIC SYSTEM, ARIZONA tiltingmay be the most effective agent for obscuring volcanic corridor,several volcanic sequences have been correlated edifices.Tilting induces burial of the downthrownportions withcogenetic plutons on thebasis of structuralrelations and of fault blocks and erosionof the upthrownparts and thus geochemistry[Weber and Smith, 1987]. greatlyreduces the "exposureprobability" of volcanic edifices. In some cases, however, tilting can reveal cross Mount Perkins Block sectionsof magmaticsystems from plutonic roots, through TheMount Perkins block is a steeply(-90 ø) westtilted hypabyssaldike swarms, to surficialvolcanic complexes [e.g., fault block that containsan apparentlycogenetic suite of Proffett,1977; Gans and Miller, 1983;Asmerom et al., 1990; Mioceneplutonic, hypabyssal, and volcanicrock [Faulds, Holm and Wernicke,1990; John, 1995]. Studiesof such 1993b](Figures 1 and 2). The across-strikewidth of the systemscan elucidate the interplay between magmatism and steeplytilted exposures suggests that as much as 9 km of crust extensionby permittinganalysis of the emplacementmay be exposed on end within the Mount Perkins block. The mechanismsof intrusions, structural controls on volcanic blockis borderedon the eastby the gently(5-30 ø) east- centers,and petrogenesis of magmas. northeastdipping Mockingbird Mine fault and on the west by The steeplytilted Mount Perkinsblock, northwesternthe moderatelyeast dipping Mount Davis faultø The 30-km- Arizona,affords a cross-sectionalview of the upperseveral long Mount Perkins block terminatesabruptly northwardat a kilometersof a magmaticsystem that was activebefore, major accommodation zone [Faulds et al., 1990] in during, and after the onset of major extension. We first conjunction with the along-strike termination of the east describe the field relations and geochronologicaland dipping normal fault system. The more diffuse, southern geochemicalevidence that indicate exposureof a tilted boundary of the block is related to a gradual southward magmaticsystem and genetic ties between different structural decreasein displacementon the Mockingbird Mine fault levels of the system. The evolutionof the systemis then (Figure 1). discussedwithin the contextof regionalextension. The Mockingbird Mine fault originated as a steeply dippingnormal fault and was rotatedto a gentledip by fault blocktilting, as shown by steeptilts (70-90 ø) withinboth its Geologic Setting hangingwall and footwall. Nucleationof the fault probably Northern Colorado River Extensional Corridor occurred shortly after the onset of extension and tilting, The Mount Perkins block lies within the central Black however,as 90 ø of rotationwould imply initiation as a steeply dipping reversefault. Middle to late Tertiary reversefaults Mountains in the northern part of the Colorado River are very rare within the region. Asymmetricmicrostructures extensionalcorridor (Figure 1), a 70- to 100-km-wideregion developedin breccia and gouge along the fault, such as R• of moderatelyto severelyextended crust situated between the Riedel shearsand rough facets [Angelieret al., 1985; Petit, relativelyunextended Colorado Plateau on the east and Spring 1987], indicate an east-northeasttrending slip line (i.e., Range-OldWoman Mountains region on the west[Howard essentiallydip-slip). Althoughpiercing points have not been and John, 1987; Faulds et al., 1990]. The extensional corridor terminates northward at the left-lateral Lake Mead observed,offset volcanic strataand the orientationof the slip line suggestthat the MockingbirdMine fault accommodateda fault systemand right-lateralLas VegasValley shearzone. maximumof 5-6 km of slip in the vicinity of Mount Perkins. Thick sections(generally > 3 km) of Miocenevolcanic and In the Mount Perkins region, west tilting increases sedimentarystrata rest directlyon Proterozoicand late eastward across many of the major, east dipping normal Cretaceousmetamorphic and plutonic rock within the faults. This suggeststhat fault block tilting was largely extensionalcorridor [Longwell, 1963; Anderson, 1971; Frost accommodatedby displacementon concave upward, east and Martin, 1982; Sherrodand Nielson, 1993]. dipping normal faults and dominolike tilting of intervening Calc-alkalicmagmatism and extensionswept northward fault blocks[e.g., Proffett, 1977]. The DupontMountain fault acrossthe northernpart of the corridorin early to late on the east flank of the southern Eldorado Mountains Miocenetime [Glaznerand Bartley,1984; Gans et al., 1989; accommodatedthe greatestamount of slip (>10 km) in the Armstrongand Ward,1991;