Fault and fault-rock characteristics associated with Cenozoic extension and core-complex evolution in the Catalina-Rincon region, southeastern Arizona George H. Davis Kurt N. Constenius William R. Dickinson² Edna P. RodrõÂguez Leslie J. Cox Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA ABSTRACT of younger structures formed at shallower volved motion on normal faults that originated depths display multiple generations of ca- at high dips but were progressively rotated to Cenozoic extensional deformation in taclastic features, including brecciation of lower dips as adjacent fault blocks tilted in southern Arizona included (1) a Neogene variable intensity and cataclasite dikes, but domino fashion during continued deformation. phase of Basin and Range deformation re- are juxtaposed against hanging-wall strata Additional post±middle Miocene extension corded by high-angle normal faults and (2) that are only moderately deformed by sub- produced high-angle Basin and Range normal an earlier phase of detachment faulting and sidiary faults. The shallowest fault zones faults that offset the basin ®ll of structural de- brittle-ductile crustal shearing associated lack either structural overprints in their pressions. Basin and Range extension was with tectonic denudation of metamorphic footwalls or any signi®cant contrasts be- most rapid before initiation of sea¯oor spread- core complexes. In the Catalina-Rincon re- tween footwall and hanging-wall deforma- ing in the nearby Gulf of California near the gion, exposed fault zones produced at dif- tion. Exposures of mid-crustal rocks within end of Miocene time, but continues at reduced ferent crustal depths during successive ex- the core complexes re¯ect successive exhu- rates. tensional episodes display differing fault mation and uplift of fault footwalls during Because all the younger generations of geometries and types of fault rocks formed sequential episodes of deformation. The faults offset middle Cenozoic detachment during progressive crustal extension. De- present high elevation of mylonitic rocks in faults, there has been a tendency to focus on tachment faults are associated with both the Catalina-Rincon metamorphic core a simple dichotomy in deformational style be- mylonites produced by ductile shear and complex re¯ects dip slip and isostatic foot- tween low-angle and high-angle normal faults. cataclasites produced by brittle shear. wall ¯exure during Basin and Range defor- We here call attention instead to progressive Younger faults formed at shallower depths mation as well as tectonic denudation dur- changes in fault behavior and fault-rock char- are associated with less intense cataclastic ing detachment faulting. Net uplift of core acter as crustal extension evolved through deformation and with brittle fracturing rocks resulted from multiple phases of multiple extensional episodes involving defor- that includes transtensile phenomena at the deformation. mation at different depths and varying exten- shallowest crustal levels represented. Qual- sion directions. Mylonitic rocks now exposed itative measures of net displacement along Keywords: core complex, crustal extension, 2±3 km above sea level within the Catalina- individual fault zones are provided by (1) detachment fault, fault rock, normal fault. Rincon metamorphic core complex were the nature of contrasts among successively raised in increments from mid-crustal levels overprinted fabrics and internal structures INTRODUCTION during a succession of deformational phases, in the footwall and (2) the degree of con- including isostatic and ¯exural responses to trast between fabrics and structures of foot- Cenozoic crustal extension in southern Ar- each. Net rock uplift during each stage of wall and hanging-wall rocks. Footwalls of izona was the product of superposed defor- structural evolution was a function of relative the oldest structures display varied brittle mational regimes separable in both timing and fault displacement in relation to changing el- overprints of ductile fabrics and are jux- style (Davis, 1980; Dickinson, 1991, 2002). evations of the ground surface as erosion taposed across gouge zones along detach- Late Oligocene to early Miocene extension proceeded. ment surfaces with stratal successions cut was accommodated at exposed crustal levels Our primary focus during this study was on by multiple brittle shear surfaces. Footwalls by subregional brittle-ductile shear zones, ex- postdetachment fault systems that to date have pressed as metamorphic core complexes and received much less study than the detachment ²Corresponding author e-mail: wrdickin@geo. associated gently dipping detachment faults. system and associated mylonitic rocks. We arizona.edu. Postdetachment early Miocene extension in- then assess the in¯uence of successive phases GSA Bulletin; January/February 2004; v. 116; no. 1/2; p. 128±141; DOI 10.1130/B25260.1; 11 ®gures. For permission to copy, contact [email protected] 128 q 2004 Geological Society of America CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA of extensional deformation on the exposed different generations of low-angle normal eastward-dipping normal faults striking more morphology of the Catalina-Rincon core com- faults. At the Cottonwood Wash study site, the nearly due north. plex. The fault geometries and fault rocks as- pre-offset depth of exposed footwall rocks ad- sociated with sequential phases of deforma- jacent to the rotated San Manuel fault north Outcrop Observations tion can be considered jointly to develop an of the Santa Catalina Mountains was 2±3 km integrated picture of crustal extension as it af- during pre±Basin and Range but postdetach- The main structure in the cliff exposures fected different levels within the crust. The in- ment normal faulting (19±16 Ma). At the Mar- near Cascabel is a graben, trending N358± ¯uence of changing geothermal gradients on tinez Ranch study site, the pre-offset depth of 408E and bounded by normal faults that dip crustal rheology probably supplemented the exposed footwall rocks was 8±12 km (Dick- 708±758 inward (Fig. 2A). Each of the graben- effects of simple depth control on the style of inson, 1991; Force, 1997) during mylonitic bounding faults is exposed for a height of ;30 faulting, but is more dif®cult to quantify with shear and detachment faulting (28±20 Ma) m (Fig. 2B), but their projected intersection con®dence. along the Catalina brittle-ductile shear zone at lies below the base of the cliff. The pink the southeast corner of the Rincon Mountains. marker bed (Fig. 2A) within the graben has STUDY SITES been down dropped ;20 m from its position CASCABEL STUDY SITE outside the graben on the cliff face. The faults Faults exposed around the periphery of the cut across, rather than curving around, rigid Catalina-Rincon metamorphic core complex clasts of the host conglomerate. All the faults (Dickinson, 1991) formed at various times and Along the San Pedro River at the east ¯ank at the Cascabel study site are marked by thin at a range of depths. Our knowledge of the of the Little Rincon Mountains (Fig. 1), mod- zones of powdery and clayey gouge up to 3 structural geometry of the core complex led erately to steeply dipping (658±858) normal cm thick. The gouge is silvery white, making us to select four key study sites (Fig. 1), dis- faults offset upper Cenozoic alluvial deposits the fault surfaces conspicuous in outcrop. The cussed in order of increasing age of faulting, of the Quiburis Formation and its correlatives gouge along each fault forms a layer of uni- where structural features associated with each (Dickinson, 1991, 2003), as well as older units form thickness, bounded on each side by par- successive phase of extensional deformation (Drewes, 1974). South of Cascabel (Fig. 1), allel planar fault walls bearing polish and can be examined to best advantage. Lingrey (1982) mapped a system of high- slickenlines. Penetrative strain of the clayey Two study sites involve high-angle Basin angle normal faults, striking north and spaced gouge during fault movement is recorded by and Range faulting, for which extant descrip- 100±400 m apart; individual trace lengths are closely spaced cleavage dipping more gently tions of physical characteristics are scanty be- up to 7 km. Along strike, the faults converge than, but in the same direction as, the fault cause exposures of Basin and Range faults are and diverge, creating trace intersections at an- surfaces. commonly masked by aggradational Neogene gles of 108±308. Most are locally east-dipping, Above a subtle bend in the fault surface, the alluvium deposited against range fronts. At but the overall system of Basin and Range uppermost exposed segment of the northwest- the Cascabel study site, intrabasinal Basin and faults in the Little Rincon Mountains and the ern (southeast-dipping) graben-bounding fault Range faulting (6±4 Ma) in the San Pedro San Pedro River valley includes important shifts to a slightly shallower dip than observed River valley east of the Rincon Mountains west-dipping normal faults as well (Dickin- lower in the cliff face. Immediately above the proceeded at a depth of ,1 km within dis- son, 1998a, 1998b, 2003). The north trend of angle in the fault surface, there are two rupted basin ®ll. At the Dead Hawk Gulch the system is consistent with the widely ac- hanging-wall normal faults, one synthetic but study site along the Pirate fault, range-front cepted view that Basin and Range faulting in dipping more steeply than