JOURNAL OF GEOPHYSICALRESEARCH, VOL. 95, NO. B12,PAGES19,973-19,987, NOVEMBER 10, 1990 Tertiary Structure and Thermal History of the Harquahala and Buckskin Mountains, WestCentral Arizona: Implications for Denudation by a Major Detachment Fault System S•.Pm• M. Pd• Institute for Crustal Studies, University of California, Santa Barbara U. $. Geological Survey, Flagstaff, Arizona Jom• F. Sum U.S. Geological Survey, Reston, Virginia The Harquahala and Buckskin mountains lieinthe footwall ofthe Whipplc-Buckskin-Bu!lard detachment sy stem. In the Harquahala Mountains, Mesozoic fabric and stmcutm areprogressively moreintensely overprinted by penetrative Tertiary deform. ation toward the northeastern pan ofthe range. Teniary mylonitic deformation isrecog- nized bythepresence of deformed Miocene marie dikes and characteristic textural features. Lineations in the mylo- nite trend 0400-060 ø, and megascopic kinexnatic indicators mostly indicate top-to-the-northeast sense of shear. The crest of an antiform der'reed by Tertiary mylonitic fabric does notcoincide withthetopographic crest of therange. Reorientation of Mesozoic foiletlon in the vicinity of this antiform suggests that it originated as twoshear zones, as opposed to a single zone that was then bent. The northeastern part ofthe Brown's Canyon granite acts as a large, low- strain lozenge in thesoutheast limbof the foliation arch. Mylonitic foliation along thesoutheast side of therange, above this lozenge, isstrongly oblique tothe trend of thedetachment fault, but the relative orientation is consistent withleft-oblique normal shear. Evidence of Tertiary plastic deformation is absent southwest of Sunset Pass. Minor low-angle normal faults in thesouthwestern Harquahala Mountains and LittleHarquahala Mountains dip northeast. Thelow-angle normal faults are cut byNW trending high-angle, right-oblique faults. In the central and southwestern Harquahala Mountains, •øAr-$9Ar age spectra from K-feldspar, muscovite, and hornblende and total gas ages from biotite indicate Late Cretaceous to early Tertiary cooling to argon closure temperatures. Biotite and K-feldspar from thearea northeast of.. Sunset Pass record rapid early to middle Miocene cooling. Reconstruction of theoriginal geometry of the detachment system, based on thermal differences indicated by contrasting cooling histories, and the orientation of anearly Miocene dikeswarm indicate that the initial dipofthe detachment faultwas most probably be- tween 30 ø and 40 ø. Thus the Harquahala Mountains are atilted block exposing ofthe order ofa 10-km section through the pre-Tertiary crust. Heterogeneous Proterozoic gneiss, sparse Paleozoic and Mesozoic metasedimentary rocks, and anOligocene plutonit complex are extensively overprinted by Tertiary mylonitic fabrics in the Buckskin Moun- tains. Hornblende •øAr39Ar cooling ages suggest that most ofthese rocks were below homb!ende closure temperature byearly Tertiary time, except inthe vicinity ofthe Oligocene plutohie complex. Feldspar and biotite •øAr39Ar cooling ages suggest that the footwall of the Whipple detachment system experienced a more uniform cooling history inthe Buckskin Mountains than in theHarquahala Mountains; cooling ages between about 13 and20 Ma arerecorded throughout therange with no consistent spatial pattern of ages. The Eagle Eyedetachment fault,southeasternmost extension of the Whipple-Buckskin-Bullard fault system, becomes atransfer fault along thesoutheastern side ofthe Harquahala Mountains. This fault separates two distinct extensional terranes. Onthe northwest, approximately 50 km of crustal extension has been accommodated ontheWhipple faultsystem, whileon•hesoutheast a similar total magnitude of crustal extension is accommodated by a series of smaller normal faults in the adjacent BigHorn-Lake Pleasant extended terrane. As the deformation becomes more distributed, maximum slip on any one normal faultde- creases tothe point where nosingle faulthas enough slip todenude crustal levels atwhich Tertiary plastic deforma- tion has occurred. I2qTRODUC•ON Geologic setting The Hareuvar complex [Rehrig andReynolds, !980] and Whipple Mountains [Davis et al., 1980,1987]together form a major extended terrane in westcentral Arizona and southeastern .... •Now at Deparunent of Geology, California State University, San Bemar- Copyright 1990 by, the American Geophysical Union. Paper nmnber 89JB03308. 0148-0227/90/89JB-0330850S.00 California. Northeastward movement of the upper plate of the regionally NE dipping Whipple-Buckskin-Bullard detachment system (referred to as theWhipple detachment system) has denuded middle crustal rocks over alarge area [Davis andLister, !988; Davis et el., 1980; Spencer et el., 1987; Reynolds and Spencer, 1985; Howard and John, 1987].Discontinuously ex- posed segments ofthe Whipple detachment system along this belt include the Whipple,Buckskin, Rawhide, Bullard, and Eagle Eye detachment faults (Figure !). The Whipple-Harcuvar extended terrane is bounded on the northeast by ahanging wall breakup zone, beyond which the hanging wallof the detachment system is not significantly extended, and on the southwest byafootwall breaka- way that corresponds to the surface breakout ofthe original upward extension of the detachment system. The hanging wallbreakup zone 19,973