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 ᭧ 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 N35Њ± ¯uence of changing geothermal gradients on tinez Ranch study site, the pre-offset depth of 40ЊE and bounded by normal faults that dip crustal rheology probably supplemented the exposed footwall rocks was 8±12 km (Dick- 70Њ±75Њ 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 ϳ30 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 ϳ20 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 (65Њ±85Њ) 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 10Њ±30Њ. 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 Ͻ1 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 the master fault and Basin and Range faulting (12±6 Ma) delimited southern Arizona was a response to east-west the other antithetic and northwest-dipping (Fig. the western margin of the Santa Catalina regional extension (Menges and Pearthree, 2C). These two faults, in combination with the Mountains along the ¯ank of Oro Valley 1989). master fault, create a fanning pattern. Higgs et where the preoffset depth of exposed footwall Multiple faults exposed in a cliff face near al. (1991) described a similar fault-fanning pat- rock was 3±4 km. Cascabel offset subhorizontal conglomeratic tern associated with a change in the dip of a Two generations of older low-angle normal horizons of the Quiburis Formation (of post± prominent splay of the Sevier (Basin and faults are present within the Catalina-Rincon middle Miocene age), which forms the basin Range) fault near Mount Carmel in southern region (Dickinson et al., 1987, 1995; Dickin- ®ll of the San Pedro trough (Dickinson, Utah. The dip change in the graben-bounding son, 1991, 1993, 1998a, 1999; Force et al., 1998a, 2003). Displacements of a pink, sand- fault at the Cascabel site coincides with the 1995; Force, 1997). The Catalina detachment rich layer (ϳ3 m thick) within an otherwise base of the 3-m-thick pink sandstone marker fault, which is gently inclined and forms the homogeneous succession of buff, pebble to bed in the footwall and probably re¯ects a southwest front of the Catalina-Rincon meta- cobble conglomerate record fault offsets clear- change in mechanical properties of the domi- morphic core complex, controlled tectonic de- ly. None of the faults in the cliff face offset nantly conglomeratic Quiburis succession. nudation of mylonitic core rocks in its foot- bedding more than 20 m, but we assume that In the footwall beneath the southeastern wall (Davis, 1980). Rotated normal faults of the physical characteristics of the exposed (northwest-dipping) graben-bounding fault at somewhat younger age that dip beneath the faults are similar to those of unexposed, Cascabel, there are two synthetic footwall core complex and locally displace strands of greater-displacement faults mapped nearby by faults, striking N45Њ±50ЊE and dipping 75Њ± the detachment system are exposed within the Lingrey (1982). The faults near Cascabel 85ЊNW, which display normal offsets of ϳ3 San Pedro trough, a synformal downwarp strike northeastward, at an angle to the overall m. Variations in the attitudes of these footwall (Spencer, 1984) in the detachment system ly- north regional trend of Basin and Range fault- faults stem from grooves in the fault surfaces ing between the uplifted core complex and its ing. We interpret this departure from the ex- with wavelengths of ϳ10 m. Slickenlines and breakaway zone. pected orientation as the expression of an grooves on both the master graben-bounding The other two study sites focus on the two oblique transfer zone between en echelon faults and the subsidiary faults record normal

Geological Society of America Bulletin, January/February 2004 129 DAVIS et al.

scale normal faulting is not pervasive within the Quiburis Formation, and the total exten- sion accomplished by Neogene faulting was small. Areal mapping of widely spaced faults cutting the Quiburis Formation of the San Pe- dro trough (Dickinson, 1998a) indicates that Basin and Range normal faulting internal to the Quiburis Formation accommodated east- west extension of Ͻ5%. The depth of faulting at the Cascabel study site was Ͻ750 m, the approximate combined maximum thickness of exposed and buried Quiburis Formation (Dickinson, 1991, 2003). Remnants of a prominent paleosol that was developed on the aggradational surface at the stratigraphic top of the Quiburis Formation just prior to its dissection by post±middle Pli- ocene erosion stand only 350±450 m above river level at the base of the cliff-face fault outcrops (Dickinson, 2003), and suggest a depth of faulting of Ͻ500 m. As the residual remnants of the paleosurface capping the Qui- buris Formation lie along the ¯anks of the San Pedro trough, and slope at an angle of ϳ1Њ toward the San Pedro River (Dickinson, 2003), the burial depth of the faults at the Cas- cabel study site was probably Ͻ250 m. The age of the faulting is dif®cult to constrain as closely, but ca. 6 Ma tuffs and mammalian faunas are present in Quiburis ¯uvial and la- custrine facies correlative with the conglom- erates cut by the faults, and dissection of fault- ed Quiburis basin ®ll was under way no later than middle Pliocene time (Dickinson, 2003). These constraints imply faulting mainly dur- ing the interval 6±4 Ma; the younger age bracket is uncertain.

DEAD HAWK GULCH STUDY SITE

The Pirate fault, which dips 50Њ±65Њ at the Figure 1. Enhanced Landsat image of study area showing major physiographic features, surface (Dickinson, 1994; Spencer and Pear- key faults, and study areas (circles) near Tucson, Arizona (A±A؅ is line of topographic three, 2002), delineates the western ¯ank of pro®le of Fig. 7). bedrock exposures in the Santa Catalina Mountains (Fig. 1). The fault strikes N10ЊEat oblique slip, with net extension approximately Davis and Reynolds, 1996) as a combination its northern end, curving abruptly to a strike east-west, as expected for Basin and Range of mode I (jointing) and mode II (shear frac- of N45ЊE to the south (Fig. 3). The fault trace deformation (Menges and Pearthree, 1989). turing). To assume that faulting was controlled crosses modern topography, including moun- by strict Coulomb failure would require an an- tain spurs, without offsetting any geomorphic Fault Interpretations gle of internal friction for Quiburis conglom- features, although preferential erosion along erate of ϳ50Њ (coef®cient of internal friction the fault zone locally controls the positions of We interpret the conjugate oblique-slip nor- Ͼ 1), which far exceeds reasonable values for some stream segments. Where projected mal faults as an expression of faulting in a natural rock materials (Byerlee, 1978; Suppe, southward beneath alluvial cover, the fault ap- three-dimensional strain ®eld (Reches, 1978; 1985; Davis and Reynolds, 1996). parently truncates the Catalina detachment Krantz, 1986; Watterson, 1999) along a No intensive fracturing, brecciation, or fault as exposed along the southern ¯ank of displacement-transfer zone linking two larger- veining is apparent in wall rocks of the faults the Santa Catalina Mountains. displacement faults. The conjugate angle be- at Cascabel. Deformation mechanisms were Inferred ages of sedimentary units within tween the major graben-bounding faults in the restricted to fault breakage and frictional slid- Oro Valley suggest that displacement along cliff face is 30Њ to 40Њ, suggesting that faulting ing accompanied by minor grain-scale cata- the Pirate fault occurred during the interval occurred in the transtensile ®eld (Suppe, 1985; clasis and perhaps fault-zone dilation. Small- 12±6 Ma (Dickinson, 1994). At the northern

130 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA

modern range front, which marks the limit of headward incision into the footwall block by pedimentation after faulting slowed or ceased (Bull et al., 1990). The Pirate fault separates syntectonic but subhorizontal Neogene basin ®ll of Oro Valley in the hanging wall from crystalline bedrock in the footwall (Dickinson, 1994). Proximal facies of basin ®ll adjacent to the fault trace contain megaclasts as coarse as 1±5 m in di- ameter (Dickinson, 1994). The fault contact is marked by a thin seam of clay-rich gouge (1± 5 cm thick) between variably but moderately dislocated basin ®ll and highly fractured to brecciated crystalline rock in the immediate footwall. Mapping of the entire fault trace (Dickinson, 1994) indicates that the outcrops that most fully display the physical character of the fault zone occur at Dead Hawk Gulch (Figs. 1, 3), a tributary to CanÄada del Oro where erosional exhumation of the sediment- bedrock contact has produced a local fault-line scarp ϳ5 m high. Dead Hawk Gulch breaches the scarp and dissects fault rocks lying within the adjacent footwall to expose the internal geometry of footwall deformation in fractured bedrock (Fig. 4A) for a distance of ϳ200 m along strike. The footwall is Catalina Granite (ca. 26 Ma), intrusive into Middle Proterozoic Pinal Schist exposed nearby (Fig. 3).

Outcrop Observations

The fault surface at the sediment-bedrock interface (Fig. 4B) is slightly curved and scored by abundant slickenlines and grooves. Some of the slickenlines occupy Riedel shears that dip westward at 60Њ±70Њ, slightly steeper than the main fault surface (dipping at 50Њ± 55Њ). Exposed fault-zone features at Dead Hawk Gulch can be subdivided into ®ve struc- tural domains (Fig. 4A), in order downward into the footwall as follows (Davis et al., 1994): (1) the fault surface itself, (2) a thin sheet (ϳ10 cm) of compact breccia lying par- allel to the fault surface, (3) a thick zone (ϳ180 m) of cohesive breccia (brecciated Figure 2. Basin and Range faults near Cascabel (views to southwest). Location shown in granite) displaying semibrittle cataclastic fo- Figure 1. (A) Graben (ϳ70 m wide at top of outcrop), with offset pinkish marker bed (see liation throughout and cut by abundant cata- text) delineated by lines drawn parallel to subhorizontal bedding. (B) View upward along clasite dikes, (4) a diffuse zone (ϳ40 m) of the northwestern graben-bounding fault. (C) Diverging antithetic (left) and synthetic (cen- ``fracture grid work'' (Hancock, 1994) con- ter) branches of northwestern graben-bounding fault (major fault surface, upper right to sisting of orthogonal fracture sets, invaded lo- lower left). cally by cataclasite dikes, within unbrecciated granite, (5) undeformed Catalina Granite. The compact breccia sheet adjacent to the end of its exposed trace, the Pirate fault is (McFadden, 1978, 1981) standing 125±250 m fault surface (Fig. 4C) is resistant to erosion overlapped by a dissected alluvial fan built higher in elevation than modern wash ¯oors. and forms ¯atirons along the fault-line scarp. across the fault trace by the ancestral CanÄada Farther south, a dissected pediment carved Clasts within it range upward to ϳ10 cm in del Oro (Fig. 3). Fan remnants form bench- into granitic bedrock east of the Pirate fault diameter and are set in a matrix of massive lands of the Pleistocene Cordones surface trace is delimited uphill by steep relief at the cataclasite without throughgoing fractures. By

Geological Society of America Bulletin, January/February 2004 131 DAVIS et al. contrast, the cohesive breccia structurally be- mode I (tension) fractures containing at least low the compact breccia sheet is internally one, and in some instances two or three, gen- ruptured and contains throughgoing fractures erations of cataclasite ®ll (Davis et al., 1998a). up to 10 m in length. Individual clasts of gran- The contacts between cataclasite dikes and un- ite, equant but faceted or elongate, range from deformed Catalina Granite are everywhere 1 to 10 cm in diameter. Although the Catalina sharp. Different generations of cataclasite Granite protolith for the cohesive breccia is dikes are discernible from crosscutting rela- gray, the breccia is uniformly red to red- tionships, and the different generations of ca- purple, in part because most of its fractures taclasite ®ll commonly display empirical con- are coated with hematite and also because per- trasts in the sizes of residual clasts, 0.25 to vasive cataclasite dikes (Fig. 4D) are rich in 2.5 mm in diameter, which include angular to hematite as well. The variably oriented cata- subangular fragments of K-feldspar, quartz- clasite dikes crosscut protolith and fault-rock feldspar aggregates, and biotite ¯akes set in a fabrics that were dilated but not offset laterally dark, ®ne-grained matrix. The domain of co- across the dikes. In addition to fractures and hesive breccia showing traces of shear folia- cataclasite dikes, the cohesive breccia is cut tion contains similar hematitic cataclasite frac- by hematite-coated fault surfaces scored by ture ®llings separated by shear surfaces from slickensides and slickenlines and by foliated microbrecciated and cataclastically foliated shear zones formed by entrainment of ®nely granite (Fig. 4E). Fragments of intact but in- comminuted granite protolith (Fig. 4E). Dis- ternally deformed Catalina Granite are in part placements across discrete faults and more brecciated and in part cataclastically foliated, diffuse shear zones are recorded by offset of and their grain sizes have been reduced by mi- (1) markers such as aplite dikes, inclusions, crofracturing. The edges of some granite frag- and schlieren in the granite protolith and (2) ments are sharp, whereas others are diffuse, fault surfaces, shear zones, and cataclasite marked by mixing with breccia matrix. dikes formed earlier during progressive defor- mation by faulting and brecciation. Fault Interpretations Deeper into the cohesive breccia, shear- zone foliation ultimately disappears, although The combined petrographic and outcrop ob- brecciation and cataclasite dikes persist down- servations lead us to interpret deformational ward structurally for another 30±40 m (Fig. mechanisms during Pirate faulting as brittle to 4A). Brecciated rock passes eastward over a semiductile; they include cataclastic ¯ow, narrow (Ͻ1 m) zone of transition into Catalina Coulomb faulting, frictional sliding, and ten- Granite laced by orthogonal joints ®lled with sion fracturing accompanied by cataclasite thin (up to 2 cm), hematite-rich cataclasite dike formation. Repetitive motions along the dikes. In general, these joints strike both par- fault zone may have formed the band of com- allel and perpendicular to the trace of the Pi- pact crush breccia adjacent to the fault rate fault, isolating blocks of Catalina Granite surface. We regard the band provisionally, that are ϳ1 m on a side. Stewart and Hancock however, as the cohesive counterpart of shallow- (1994) described similar structures in lime- level noncohesive breccia belts formed where stone of the footwalls of normal faults in Tur- synfaulting fragmentation, brecciation, and key and Greece as ``grid-work fracturing.'' frictional sliding of fault-precursor shatter The contact between the zone of grid-work zones are produced by stress imposed on rock fracturing and undeformed Catalina Granite is ahead of propagating fault tips (Stewart and sharp, marked by an abrupt face of resistant Hancock, 1994). In this view, deformation granite standing ϳ2.5 m in steep relief for at spatially ahead of a developing fault plane is least 150 m parallel to the Pirate fault trace. accommodated by intense local fragmentation In detail, the con®guration of the prominent step in local topography is controlled by throughgoing joints that strike N10ЊE and dip Figure 3. Pirate fault trace (Dickinson, 55ЊW, parallel to the Pirate fault surface. 1994; J.E. Spencer, 2002, personal com- mun.) along west ¯ank of Santa Catalina Microstructural Features Mountains. Location shown in Figure 1. Area west of fault underlain by downfault- Microstructures of Pirate fault rocks as ed syntectonic basin ®ll capped by terrace viewed in thin section provide information on gravels deposited at multiple levels during the deformational mechanisms that accom- progressive stream dissection of basin ®ll. modated fault displacement. Cataclasite bod- Arrows denote fault dip (bars with balls on ies collected from the domain of grid-work downthrown side). fractures and from cataclasite dikes display M

132 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA

surface. Station spacing on the transect is ϳ330 m, and the observed gravity data were reduced by using meter drift, latitude, free air, Bouguer, and terrain corrections (Budden, 1975). The resultant Bouguer anomaly asso- ciated with the Oro Valley half graben, after removal of the regional gradient, is ϳ14 mgal, with steep gradients on both sides of Oro Val- ley. Modeling calculations were performed with a version of the standard two-dimensional program (Talwani et al., 1959). Subsurface data from two deep wells in the Tucson basin were used to establish the vari- ation of density with depth in basin-®ll sedi- mentary rocks of the Catalina-Rincon region. Subsurface data were derived from compen- sated neutron density (FDC-CNL) logs for Phillips Petroleum Company Redondo State A#1 and Humble Oil and Re®ning Company State 32#1. Both wells show a gradual in- crease in density with depth, from 2.2±2.3 g/ cm3 at shallow depths (Ͻ1000 m) to ϳ2.6 g/ cm3 at a depth of ϳ2500 m. The latter density closely approaches the density of underlying granitic bedrock and re¯ects the effect of com- paction on coarse basin ®ll derived from gra- nitic parent rocks. Subsurface densities inferred from the Tuc- son basin well logs compare closely with those derived from surface rock samples and borehole data from Tertiary grabens in west- ern Montana (Constenius, 1988). Density de- terminations for surface samples of Tertiary strata in western Montana indicate that sand- stones and conglomerates have average den- sities of 2.3 g/cm3 and 2.5 g/cm3, respectively. In Oro Valley, the density of basin ®ll com- posed of coarse granitic detritus is accordingly Figure 4. Pirate fault zone at Dead Hawk Gulch. Location shown in Figure 1. (A) Line inferred to be 2.3 g/cm3 or greater near the drawing of fault-zone features (see text) exposed in stream transect normal to fault trace surface, with higher densities prevalent at (Fig. 3). (B) Fault surface with ®gure for scale (view south). (C) Close-up of fault surface depth (Fig. 5). The background density used (view east) showing cataclasite (light-colored clasts of Catalina Granite 5±10 cm across) for modeling was 2.65 g/cm3, a value repre- forming compact breccia sheet (®eld of view ϳ0.5 m wide). (D) Close-up (®eld of view ϳ5 sentative of granitic bedrock on the basis of cm wide) of cut slab showing cataclasite dike intruding fractured granite of cohesive brec- the well control. Figure 5 depicts the structural cia zone. (E) Photomicrograph (®eld of view ϳ5 mm wide) of foliated cataclastic fabric con®guration of the Oro Valley half graben in sheared granite (comminuted fragments white). derived from our best-®t gravity model. Al- though the con®guration of the structural keel of the half graben, where sediment densities and brecciation of a precursor shatter zone. Oro Valley Gravity Modeling approach bedrock density, is not well con- Later propagation of the fault plane through strained, the Pirate fault is interpreted as a fault-precursor breccia concentrates deforma- Bouguer gravity modeling was used to de- west-dipping listric normal fault, ¯attening tion along the fault plane and reconstitutes the lineate the subsurface geometry of the Pirate from a dip of 55Њ on the outcrop to a dip of fault-precursor breccia into attrition breccia or fault system as re¯ected by the thickness and 25Њ at a depth of 2.5 km below the ground ®ne gouge. Where exposed by erosion, attri- con®guration of basin ®ll in the Oro Valley surface. The minimum dip-slip displacement tion breccia forms an indurated carapace half graben (Fig. 5). A single basin transect inferred from the model is 4.4 km. The min- (compact breccia sheet) adjacent to the fault was modeled by using the gravity data of Bud- imum fault throw is ϳ2.6 km, and it is prob- plane and provides a measure of protection for den (1975) combined with an assessment of ably ϳ4.0 km if the average bedrock relief of underlying fault-precursor breccia that is less expected basin-®ll density and the mapped at- ϳ1.4 km on the adjacent range front is taken resistant to erosion. titude of the Pirate fault at the modern land into account.

Geological Society of America Bulletin, January/February 2004 133 DAVIS et al.

rotational tilting of the San Manuel Formation and the San Manuel fault about the strike of the latter during or after faulting. Although the amount of net rotation is still controversial (Guilbert and Lowell, 1995; Dickinson et al., 1995), Force et al. (1995) showed from both ®eld relationships and paleomagnetic data that a rotation of 33Њ satis®es all known geologic constraints. Back-tilting by that amount would restore bedding in the San Manuel Formation to subhorizontal and steepen the present out- crop dip (25Њ±35Њ) of the San Manuel fault to an initial dip of 60Њ±65Њ, an attractive attitude for initiation of extensional slip. The fault is therefore inferred to have rotated domino-style to its present shallow dip during fault displacements. Along Cottonwood Wash, 2250±2500 m of San Manuel Formation are truncated by the Figure 5. Interpretation of Pirate fault geometry and displacement based on gravity mod- San Manuel fault (Fig. 42 of Dickinson, 1991, eling of Oro Valley basin ®ll. Shadings in Oro Valley basin ®ll denote different model 1993). Conglomerates of the San Manuel For- densities ␳; ␦ values are density contrasts between basin ®ll and bedrock basement. mation record syntectonic early Miocene growth-fault deposition of alluvial fans and braidplain aprons shed from uplands in both Flexural Uplift COTTONWOOD WASH STUDY SITE the footwall and the hanging wall (Dickinson, 1991). Approximately 3 km north of Cotton- We infer that isostatic footwall uplift ac- The San Manuel fault, best exposed at the wood Wash, uppermost horizons of the San companying Pirate faulting contributed to the Cottonwood Wash study site (Fig. 1), is a low- Manuel Formation lying 2750±3000 m strati- upward translation of rock masses lying with- angle normal fault of early Miocene age, graphically above its base overstep the fault in the Catalina-Rincon metamorphic core slightly younger than the detachment fault that to rest depositionally on the footwall (Dick- complex relative to surrounding basins. King de®nes the Catalina-Rincon metamorphic core inson, 1993) and place an upper limit on the and Ellis (1990) described other examples of complex, which lies within the hanging wall amount of fault displacement. From these stra- large relative uplifts in the immediate foot- of the San Manuel fault. The fault, which tal relationships, we infer that footwall rock at walls of large-displacement high-angle normal strikes approximately N30ЊW and dips the Cottonwood Wash study site lay 2±3 km faults, and attributed the phenomenon to iso- ϳ25ЊSW at Cottonwood Wash, separates tilt- below the ground surface when faulting and static response of elastic upper crust as foot- ed conglomerate beds of the lower Miocene deposition of the San Manuel Formation wall blocks are unloaded by displacements San Manuel Formation in the hanging wall began. along dipping faults. The apparent effect of from highly fractured and hematite-stained The age of the unfossiliferous San Manuel isostatic ¯exure is seen in the topography of Precambrian Oracle Granite (ca. 1425 Ma) in Formation is poorly known, but from subre- the Santa Catalina and Rincon Mountains the footwall (Fig. 8A). The homocline con- gional relationships is younger than 22±21 Ma where the loci of highest elevations occupy taining the San Manuel Formation at the study and older than 15±12 Ma (Dickinson, 1991). proximal footwalls of the Pirate and Martinez site occupies the uppermost stratigraphic lev- An air-fall tuff bed intercalated with conglom- Ranch faults (Fig. 6). The crestal region of the els of a half-graben tilt panel that exceeds 6 erate near the top of the exposed section in Santa Catalina Mountains, composed of foot- km in northeast-southwest outcrop breadth Smelter Wash (ϳ1 km south of Cottonwood wall core rocks lying structurally below the (Creasey, 1965, 1967; Hansen, 1983; Dickin- Wash) has yielded an apatite ®ssion-track age Catalina detachment fault, widens and rises in son, 1991, 1993). The trace of the San Manuel of 18.5 Ϯ 3.5 Ma (Dickinson and Sha®qullah, elevation westward toward the Pirate fault. fault is repeated at Cottonwood Wash by off- 1989). We infer that movements along the San The crestal region of the Rincon Mountains set of the fault surface across a steeper Basin Manuel fault at the Cottonwood Wash study has a similar con®guration, but in reverse, ris- and Range normal fault also striking N30ЊW site occurred mainly during the interval 19± ing toward the Martinez Ranch fault, another (Fig. 8A). 16 Ma, in the immediate aftermath of slip Basin and Range structure with at least 1.5 km Less than 5 km northwest of Cottonwood along the Catalina detachment fault. of throw downward to the east (Dickinson, Wash, Lowell (1968) showed that the San Ma- 1998b). We infer that the broad topographic nuel fault beheaded the San Manuel porphyry Outcrop Observations saddle (Redington Pass of Fig. 6) between the copper orebody and displaced its detached ex- Santa Catalina and Rincon segments of the tension (the Kalamazoo orebody) ϳ2.5 km in Undeformed Oracle Granite exposed in Catalina-Rincon metamorphic core complex is the direction S50Њ±55ЊW. Lowell (1968) and nearby unfaulted areas is broken only by the passive product of relative footwall uplifts Lowell and Guilbert (1970) further indicated joints spaced at intervals of 2±5 m. In the im- related to movements on the Pirate and Mar- that the San Manuel and Kalamazoo orebodies mediate footwall of the San Manuel fault, tinez Ranch faults on opposite ¯anks of the have been tectonically rotated, about a sub- however, Oracle Granite is broken for hun- core complex (Fig. 7). horizontal axis, in a manner compatible with dreds of meters by randomly oriented but

134 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA chment ed contours ng (Dickinson, acent to Pirate and 300 m; metric contour maps unavailable at appropriate scale). Major peaks of Santa ϳ ؍ Figure 6. Morphology ofMartinez Catalina Ranch core (MRf) complex faults(erosional showing (HVÐHappy incision highest Valley elevations restored) Neogene and on basin steepest exposed ®ll). relief Fault core of traces rocks lower-plate dashed at core where intervals rocks masked of (below by 1000 detachment alluvium. feet fault) Topographic adj ( form lines are generaliz Catalina (ML, MB)Banco and Ridge; Rincon BSfÐBuehman Spur; (MM, CLfÐCountysynform Line; RP) PCfÐPaige of Canyon; Mountains: Redington RPfÐRomero Pass: MBÐMount Pass; BRaÐBellota1993, SRfÐSoza Bigelow; Ranch; Ranch. 1994, Extensional ITaÐItalian MLÐMount 1998a, allochthons Trap. within Lemmon; 1998b, Modi®ed compound after deta 1999). MMÐMica Dickinson Mountain; (1991, his RPÐRincon Fig. Peak. 13) Other with information faults: from BRfÐ more recent mappi

Geological Society of America Bulletin, January/February 2004 135 DAVIS et al.

.(Figure 7. Topography along pro®le A±A؅ (Fig. 1) obliquely across Catalina-Rincon metamorphic core complex (no vertical exaggeration

closely spaced fractures and diffuse breccia- tion. Few fracture-bound blocks of internally unfractured granite exceed 20 cm in maximum dimension. Typical red-orange coloration re- ¯ects pervasive hematitic alteration. Tilted hanging-wall strata of the San Ma- nuel Formation are less disrupted than foot- wall granite where exposed in cliffs forming the north wall of Cottonwood Wash immedi- ately adjacent to the San Manuel fault (Fig. 8B). Conglomerate beds of the San Manuel Formation strike on average N25ЊW and dip 15Њ±55ЊNE (average, 30Њ±35ЊNE). Extensional duplex structure is evident across a structural thickness of ϳ8 m (Fig. 8C). Well-polished and slickenlined ¯oor and roof faults, which parallel the San Manuel fault itself, are inter- linked by more steeply dipping subsidiary faults bounding extensional duplex panels ϳ2 m thick and ϳ8 m long (Fig. 8C). Faults bounding the extensional duplexes strike N30Њ±60ЊW and dip 45Њ±75ЊSW, with slick- enlines trending S35Њ±S40ЊW. Cleavage that dips more gently than the faults, but in the same direction, is present in places within clayey gouge along fault strands. Hanging-wall deformation of the San Ma- nuel Formation is not restricted to the imme- diate vicinity of the fault. Over a structural thickness of 50 m or more, there are spaced extensional faults, of modest displacement (generally Ͻ1 m), which helped accommodate distortion of the hanging-wall strata and cre- ated local variations in bedding dip within the San Manuel Formation. Similar multiple faults are exposed in roadcuts ϳ1 km along bedding strike northwest of Cottonwood Wash (Hansen, 1983). Many such faults are marked Figure 8. San Manuel fault in Cottonwood Wash. Location shown in Figure 1. (A) Local by gouge zones with cleavage similar in style geologic map, with squares on hanging wall of low-angle San Manuel normal fault and and orientation to the gouge cleavage ob- barbells on hanging wall of Basin and Range high-angle normal fault offsetting San Ma- served at Cottonwood Wash (Hansen, 1983). nuel fault (strike-and-dip symbols denote bedding in San Manuel Formation and arrows At greater distances from the San Manuel denote fault dip). (B) North side of Cottonwood Wash in the northeast corner of Figure fault, normal faults are uncommon within the 8A (view north), showing tilted Miocene conglomerate (thin white lines, bedding; heavy San Manuel Formation. white lines, fault surfaces) in fault contact (half arrows) with Precambrian granite along Fault Interpretations San Manuel fault (topographic relief in view is ϳ75 m). (C) Close-up of extensional duplex faulting in hanging wall of San Manuel fault (Fig. 8B, center), with bedding in San Manuel At several places within the San Manuel Formation inclined downward to right. fault zone at the Cottonwood Wash study site,

136 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA

25% of their original stratigraphic thicknesses by multiple imbricate normal faults that inter- nally disrupt the formational units. The Paleo- zoic strata typically dip to the north or northeast, back-tilted with respect to the southwestward direction of tectonic transport along the Catalina detachment fault (Dickin- son, 1991). Oligocene±Miocene red beds (conglomerate and sandstone) of the Pantano Formation (Drewes, 1977) form part of the upper plate; they rest in fault contact on upper Figure 9. Schematic pro®le of Catalina brittle-ductile shear zone at Martinez Ranch study Paleozoic sedimentary rocks and form an un- site. Location shown in Figure 1. disrupted tilt panel that strikes east and dips north. Deposition of the Pantano red beds ac- companied detachment faulting in both time within the ®eld of view of Figure 8B, steeply formed footwall against the upper horizons of and space (Dickinson, 1991). dipping small-scale reverse faults are associ- the San Manuel Formation that are now in ated with small-scale normal faults and appear fault contact with the footwall at the study Outcrop Observations incompatible with extensional deformation, site. Net displacement of the footwall was but are interpreted as rotated faults that ϳ2.5 km, but stratal relationships along strike formed originally as high-angle normal faults to the north, where the uppermost San Manuel At the Martinez Ranch study site, the Cat- during concomitant layer-parallel extension of Formation depositionally oversteps the San alina detachment fault strikes east and dips untilted strata. Rotation through the vertical to Manuel fault, indicate that the San Manuel 10Њ±20ЊS, separating nonmylonitic and non- the present anomalous attitudes occurred as strata juxtaposed against the footwall at the cataclastic upper-plate rocks from structurally tilting of the San Manuel fault proceeded. As- study site were displaced only ϳ0.5 km. underlying cataclastic and mylonitic rocks suming that the reverse faults were initially (Fig. 9). The detachment fault crops out as a normal faults cutting and displacing essential- MARTINEZ RANCH STUDY SITE smooth surface capping an indurated ledge of ly horizontal bedding, the normal faults orig- brown or gray cataclasite 1±2 m thick. Gouge inally dipped 70Њ±80ЊSW, comparable to the The oldest structures associated with Ce- zones as much as 10 cm thick are locally pres- dip of shallow-level Basin and Range normal nozoic tectonic denudation of the Catalina- ent directly above the detachment surface faults at the Cascabel study site. Rincon metamorphic core complex are well where they formed at the expense of upper- From the nature of mesoscopic structures in exposed for Ͼ3 km along strike within the plate rocks. Typical gouge is cohesionless outcrop, we infer that deformational mecha- Martinez Ranch study site (Fig. 1). A brittle powdery silt, and clay. Gouge formed from nisms at the Cottonwood Wash study site were detachment fault structurally overlies and Precambrian granite locally contains residual Coulomb faulting and frictional sliding ac- overprints the upper structural levels of a my- faceted clasts of granite as much as ϳ10 cm companied by grain-scale cataclasis along nar- lonitic shear zone ¯anking the core rocks. My- in diameter. Despite overall structural thinning row gouge zones. Assuming Coulomb failure, lonites and microbrecciated mylonites derived of the upper plate by internal extensional the angle of internal friction for the San Ma- from granitic rocks of the core are capped, faulting, some Paleozoic units of the upper nuel Formation at the time of faulting was across the master detachment fault, by a tec- plate display mesoscale overturned folds ver- 25Њ±30Њ (coef®cient of internal friction, ϳ0.5). tonically thinned upper-plate succession of gent to the south or southwest, in the direction We view gouge cleavage dipping more gently Precambrian basement and Paleozoic to Mio- of relative tectonic transport of upper plate than fault surfaces as a record of ¯attening cene cover (Drewes, 1977; Spencer et al., over lower plate, with axial surfaces inclined strain during normal-sense shear. 2001b). Transformation of granitic protolith to the north or northeast. We attribute pervasive fracturing of the ®rst to mylonite and ultramylonite and then to The indurated cataclasite along the Catalina footwall Precambrian granite, as compared to cataclasite and ultracataclasite along the de- detachment surface was derived from mylon- the relatively minor fracturing and faulting tachment fault is recorded by microstructures itic rocks of the lower plate and has a random within the hanging-wall San Manuel Forma- displayed in thin section (Davis et al., 1998b). internal fabric that ranges in detail from pro- tion, in part to greater mechanical strength of The Martinez Ranch fault, a major Basin and tocataclasite to ultracataclasite. The intensive the granite and in part to greater net slip of Range high-angle normal fault (Fig. 6), trun- superposed fracturing that formed the cata- the footwall rocks, with consequent greater net cates the detachment fault and associated clasite was accompanied by hydrothermal al- intensity of brittle shearing. Whereas fault off- structures on the east. teration to produce thin yellow, green, and set of the Precambrian rocks began as soon as The upper plate of the detachment system black seams of ®ne-grained limonite, chlorite- the San Manuel fault was initiated, the ex- consists of internally deformed, fault-bounded epidote, and pyrolusite, respectively. Residual posed upper horizons of the San Manuel slices of Precambrian basement and Paleozoic feldspar porphyroclasts, derived from parent FormationÐdeposited within the half graben cover that are discontinuous along strike be- mylonitic rock and set in random orientations, adjacent to the faultÐwere not deposited until cause of stratal truncations by younger-over- are locally as large as ϳ5 mm in diameter in later in the history of faulting. We infer ac- older displacements along subsidiary faults protocataclasite, but are more commonly Ͻ2 cordingly that Precambrian granite in the foot- subparallel to the master detachment. Individ- mm in diameter; those porphyroclasts within wall was already strongly fractured before ual Paleozoic formations in the upper plate are narrow bands of ultracataclasite are Ͻ0.5 mm continuing fault movements placed the de- thinned to structural thicknesses only 15%± in diameter. Relict mylonitic foliation and lin-

Geological Society of America Bulletin, January/February 2004 137 DAVIS et al.

Ranch fault surface over a span of 25±50 m adjacent to the fault; observed dips range from 50Њ to 75Њ toward the fault.

Fault Interpretations

Structural features and geologic relation- ships at the Martinez Ranch study site are con- sistent with lateral displacement of the de- tached upper plate by 25±30 km across the Rincon Mountains (Dickinson, 1991). Defor- mation mechanisms within the detachment system ranged from ductile to brittle, re¯ect- ing the large-magnitude displacement accom- modated along the Catalina brittle-ductile Figure 10. Down-plunge view of mapped structural relationships at the Martinez Ranch shear zone. The deepest levels of shearing are study site (cl, cataclasite below subdetachment fault). represented by mylonites and ultramylonites, whereas upper-plate gouge zones re¯ect the shallowest levels of faulting and intermediate eation are also preserved locally within the ca- quartzose mylonites containing local enclaves levels of faulting and brittle shear represented taclastic rocks. of protomylonite and bands of ultramylonite by cataclasites display fabrics that overprint Less intense cataclastic deformation dis- were derived, as elsewhere within the core mylonitic foliation and lineation. We interpret rupts mylonitic rocks for a structural thickness complex (Dickinson, 1991; Force, 1997), from discrete fault surfacesÐsuch as the subdetach- of ϳ25 m below the Catalina detachment two main protoliths: (1) porphyritic granite of ment faultÐwithin the zone of cataclasis to fault. Approximately one-third of the distance Precambrian Oracle type; (2) ®ner-grained and represent some of the youngest shear-zone (5±10 m) through the zone of more subdued more voluminous, muscovite-bearing peralu- ruptures that survived without overprinting by cataclasis lies another discrete fault surface minous granite of Eocene Wilderness type. further cataclasis when movements along the that we term the ``subdetachment'' fault (Fig. The mylonitic rocks all display strong folia- Catalina detachment system ceased. Vertical 9). Structurally above the subdetachment tions and lineations distinguished by feldspar displacements associated with lateral transla- fault, cataclastic rocks tend to be relatively porphyroclasts set in a foliated matrix of tion of upper-plate rocks placed middle Ce- coarse grained, and their protoliths are dis- stretched quartz, biotite, muscovite, and ®ner- nozoic basin ®ll against mid-crustal granitic cernible as mylonite or ultramylonite over- grained feldspar, the latter granulated by brit- basement. Mylonites and cataclasites were lat- printed by cataclasis. Directly beneath the sub- tle fracturing. Protomylonites contain feldspar er carried farther toward the ground surface by detachment fault, over a distance of several porphyroclasts up to 10 mm in diameter, but Basin and Range Martinez Ranch faulting. meters, disrupted rock is reduced to ®ne- counterparts are Ͻ5 mm in diameter in my- grained cataclasite and ultracataclasite similar lonites and ultramylonites. Mesoscopic min- CORE-COMPLEX EVOLUTION to the fault rocks lying directly beneath the eral lineation in the mylonites and ultramylon- Catalina detachment surface itself. Detach- ites plunges gently S50ЊW, and strongly Combining our observations for postdetach- ment and subdetachment faults were evidently oriented mica ¯akes associated with S-C fab- ment phases of faulting in the Catalina-Rincon twin loci of intense cataclasis during Catalina rics, best developed in the mylonites derived region with previous analyses of detachment fault displacements. As structures within the from muscovite granite, record top-to-the- faulting and associated mylonitic deformation zone of cataclasis include discrete fault sur- southwest sense of shear. Within the interior allows appraisal of the contributions of suc- faces that interconnect to de®ne extensional of the Catalina-Rincon metamorphic core cessive episodes of extensional deformation to duplexes, the subdetachment fault may con- complex, mylonitic fabrics structurally below net core-complex evolution. Figure 11 shows nect downdip with the main Catalina detach- the zone of cataclasis give way downward in schematically the inferred time vs. elevation ment surface as part of an anastomosing array turn to nonmylonitic protoliths (Force, 1997), path followed by mylonitic rocks of the of fault surfaces forming the detachment and small pockets of nonmylonitic rock are Catalina-Rincon metamorphic core complex system. present within the Martinez Ranch study area during late Oligocene to early Miocene The thickness of cataclasite lying structur- where granitic rocks locally escaped mylonitic detachment faulting and subsequent post± ally below the subdetachment fault varies shear. middle Miocene structural and erosional along strike, but the base of cataclastic defor- At the eastern extremity of the Martinez development. mation is marked by a comparatively sharp Ranch study area, the Catalina detachment Thermochronological analysis of isotopic transition, over distances of 2±3 m, to mylon- system is offset downward below the ground and ®ssion-track dating indicates an episode itic rocks not overprinted by cataclasis. The surface by the Martinez Ranch fault. De- of rapid tectonic denudation, by an estimated base of the zone of cataclasis is sinuous in formed quartzite and marble in the upper plate 6±8 km (Spencer and Reynolds, 1989), during outcrop and undulose in down-plunge view of the detachment system are in sharp contact detachment faulting within the interval 28±20 (Fig. 10), normal to the fault-slip vector, to with fault breccia along the Basin and Range Ma in Oligocene±Miocene time (Dickinson, de®ne a surface geometrically analogous to an structure. Both mylonitic foliation in lower- 1991). Slower post±middle Miocene erosional undulating fault with mullions and grooves. plate rocks and bedding in upper-plate rocks exhumation, by 1.7±1.8 km, accompanied Ba- Structurally beneath the zone of cataclasis, are dragged into parallelism with the Martinez sin and Range normal faulting (Davy et al.,

138 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA

Figure 11. Summary diagram of incremental (left to right) exhumation and uplift of mylonitic rocks in Catalina-Rincon metamorphic core complex (see text discussions for controls on changing elevations of ground surface and burial depths of mylonitic rocks); sediment thickness in Tucson basin after Eberly and Stanley (1978) and Houser and Gettings (2000).

1989; Fayon et al., 2000). The direction along the core-complex belt of the Basin and Following tectonic denudation of the core of crustal extension during detachment Range province in southern Arizona is ϳ27.5 complex by Oligocene±Miocene detachment faultingÐand during postdetachment but pre- km (McCarthy and Parsons, 1994; Spencer et faulting (Fig. 11), we infer gradual reduction middle Miocene domino faultingÐwas N55Њ± al., 2001a); (2) because southern Arizona un- in the elevation of the local ground surface as 65ЊE (Dickinson, 1991). The direction of derwent net post±middle Cenozoic extension erosion and resultant isostatic uplift of under- crustal extension during post±middle Miocene of ϳ100%, the crustal thickness before de- lying rock masses proceeded during Miocene Basin and Range normal faulting, however, tachment faulting was ϳ55 km (Spencer et al., time (Fig. 11). Basin and Range faulting along was approximately east-west (Menges and 2001a); (3) for mean crustal and mantle the Pirate fault (Figs. 6, 11) offset mylonitic Pearthree, 1989). speci®c-gravity values of 2.78 and 3.35, re- rocks that are structurally contiguous with Structural mapping of the core complex in- spectively, a crustal pro®le 55 km thick would bedrock of the Tortolita Mountains (Fig. 6) dicates that mylonitic rocks (Fig. 11) of the stand isostatically ϳ2.5 km in elevation above from the ¯oor of the Oro Valley half graben Catalina-Rincon brittle-ductile shear zone the Colorado Plateau, which has a crustal to the crest of the Santa Catalina Mountains. were formed initially at a burial depth of 8± thickness of only ϳ40 km (McCarthy and Par- The keel of the Oro Valley half graben is al- 12 km (Dickinson, 1991); the most intense sons, 1994; Spencer et al., 2001a); (4) the Col- most as deep structurally as the ¯oor of the mylonitic deformation took place at depths of orado Plateau lay near sea level in middle Ce- Tucson basin, which covers the downfaulted 8±10 km (Force, 1997). The estimated depths nozoic time (Spencer, 1996), implying that the upper plate of the detachment system (Fig. were derived from inspection of multiple cross surface elevation of as-yet-unthinned Lara- 11). sections across the core complex revealing the mide crust along the core-complex belt was From the curves of Egan (1992) for iso- paleodepth of mylonitic rocks below a Lar- ϳ2.5 km (Fig. 11). High-standing Laramide static deformation adjacent to normal faults amide (Late Cretaceous to Eocene) erosion elevations in southern Arizona are con®rmed dipping 30Њ±60Њ, we infer ϳ1.25 km of ¯ex- surface. The elevation of the overlying Eocene by coarse clastic Eocene strata exposed along ural footwall uplift affecting the crest of the ground surface (Fig. 11), before any displace- the Mogollon Rim at the southern edge of the Santa Catalina Mountains adjacent to the Pi- ments along the Catalina brittle-ductile shear Colorado Plateau where sediment was trans- rate fault (Fig. 11). Comparable footwall up- zone, is estimated from the following line of ported northward from uplands that lay to the lifts are observed along the ¯anks of grabens reasoning: (1) the present crustal thickness south (Potochnik, 1989). and half grabens of the East African rift sys-

Geological Society of America Bulletin, January/February 2004 139 DAVIS et al. tem (Ebinger et al., 1991). Flexural isostatic Range faults exposed at the Cascabel locality and Range faults cutting the post±middle Mio- uplift may have been assisted by the buoyancy dip in excess of 70Њ. cene Quiburis Formation near Cascabel, with of a local crustal root beneath the Santa Cat- Changes in fault dip from steep to shallow no evidence for superposed fabrics or struc- alina Mountains (Wallace et al., 1986; Holt et thus re¯ect either greater depth of faulting or tures in the footwalls, nor any contrast in al., 1986; Myers and Beck, 1989). Flexural bulk rotation of fault surfaces through time. structural style between footwalls and hanging analysis implies that locales more than ϳ20 Tilt of stratigraphic sections in the hanging walls. km from the Pirate fault (Fig. 6) would not be walls of the normal faults may occur either We conclude that close attention to styles affected by ¯exural footwall uplift (Egan during translation along fault surfaces with lis- and degrees of deformation producing fault (1992). Accordingly, the broad topographic tric curvature or through domino-style tilting, rocks of varying nature can aid in the recog- swale at Redington Pass occupies a synform but observations at individual fault outcrops nition of relative amounts of vertical displace- with the ground surface at the same elevation cannot distinguish between the two modes of ment across normal faults of different gener- as the Tortolita Mountains (Fig. 11). The to- behavior. The systematic correlation of fault ations within an extensional domain. Fault pographic crests of the Santa Catalina and dip with depth of faulting suggests that the systems with displacements insuf®cient to Rincon Mountains rise higher to either side upper plate of the Catalina detachment system juxtapose rocks derived from different depth because of ¯exural footwall uplift adjacent to was cut by supradetachment faults, now un- levels in the crust display congruent structures the Pirate and Martinez Ranch faults, respec- exposed beneath basin ®ll of the Tucson basin in their footwalls and hanging walls, whereas tively (Fig. 6). or removed by erosion, which dipped as steep- faults that displace rocks from deep regimes ly as Basin and Range normal faults dip today. to shallow levels display contrasting structures CONCLUSIONS The geometry of postdetachment faulting sug- in their footwalls and hanging walls. gests that such upper-plate faults, dipping steeply near the middle Cenozoic land surface, ACKNOWLEDGMENTS Our case studies shed light on the geometry may have curved downward in listric fashion of faulting and the deformational mechanisms to merge with the master detachment surface We thank the Department of Geosciences at the associated with different phases of the incre- University of Arizona for funding ®eld trips, class or may have subsequently been rotated to projects, and student participation that supported mental exhumation and uplift of mylonitic shallower dips during continuing extensional data collection, and we also appreciate multiple ®eld rocks now exposed in the Catalina-Rincon deformation. visits by several University of Arizona colleagues metamorphic core complex. Present elevations Footwalls were cumulatively uplifted to during the course of our investigations. Golder As- sociates kindly granted access to the Dead Hawk of high-standing mylonitic core rocks are not higher and higher structural levels during se- a function of detachment faulting alone, but Gulch locality, and Roy Johnson provided borehole quential episodes of extensional deformation logs from the Tucson basin. Susie Gillatt of Terra stem in part from postdetachment normal (Fig. 11). The amounts of footwall uplift, rel- Chroma prepared most of the ®nal illustrations, and faulting, ¯exural footwall uplifts associated ative to hanging walls, are re¯ected by (1) Jim Abbott drafted the remainder. Norman Meader with Basin and Range faults, and isostatic up- contrasts among sequentially overprinted fab- coordinated submittal of the manuscript. We appre- lift during prolonged erosion. The present ciate detailed reviews and editing suggestions by rics in footwall rocks and (2) the degree of J.M. Bartley, L.B. Goodwin, R.D. Law, J. Lee, A.J. morphology of the core complex stems in part contrast between structures of the hanging- McGrew, and E.L. Miller that improved both text from detachment faulting, but also in part wall or upper plate and the footwall or lower and ®gures. from the later effects of Basin and Range plate. Fault-rock indicators of greatest net dis- faulting after a change in the prevailing direc- placement are observed for the detachment REFERENCES CITED tion of crustal extension. fault bounding the Catalina-Rincon metamor- In general, older faults exhumed from great- phic core complex where contrasting footwall Budden, R.T., 1975, The Tortolita±Santa Catalina Moun- er depths are more gently inclined than youn- fabrics range from ultramylonites to ultraca- tains complex [M.S. thesis]: Tucson, University of Ar- izona, 133 p. ger faults formed at shallower levels. Strands taclasites and where contrasting structures Bull, W.B., McCullough, E.F., Jr., Kresan, P.L., Woodward, of the middle Cenozoic detachment system, range from mylonitic shear zones in the foot- J.A., and Chase, C.G., 1990, Quaternary geology and active at a reconstructed paleodepth of ϳ10 geologic hazards near Canada del Oro, Tucson, Ari- wall to gouge-lined faults in the upper plate. zona, in Gehrels, G.E., and Spencer, J.E., eds., Geo- km at the Martinez Ranch study site, dip 20Њ Signi®cant but lesser translation by a combi- logic excursions through the Sonoran Desert region, or less. The postdetachment but pre±Basin and nation of brittle shearing and faulting is re- Arizona and Sonora: Arizona Geological Survey Spe- cial Paper 7, p. 1±8. Range San Manuel fault exposed at the Cot- corded along major Basin and Range faults, Byerlee, J.D., 1978, Friction of rocks: Pure and Applied tonwood Wash study site dips 25Њ±35Њ, al- notably the Pirate fault, where contrasting Geophysics, v. 116, p. 615±626. though its dip was probably 60Њ±65Њ before footwall fabrics range from cataclastic folia- Constenius, K.N., 1988, Structural con®guration of the Kishenehn Basin delineated by geophysical methods, domino-style tilting. Gravity modeling sug- tion in sheared protolith to mode I tensile frac- northwestern Montana and southeastern British Co- gests that the Basin and Range Pirate fault at tures ®lled with cataclasite dikes. The con- lumbia: Mountain Geologist, v. 25, p. 13±28. the Dead Hawk study site is a listric structure Creasey, S.C., 1965, Geology of the San Manuel area, Pinal trasting structural regimes in contact at the County, Arizona: U.S. Geological Survey Professional decreasing in dip from ϳ55Њ at the surface to Pirate fault range from thick breccia zones in Paper 471, 64 p. ϳ25Њ at a depth of 2.5 km. The Basin and the footwall to largely unfractured strata of a Creasey, S.C., 1967, General geology of the Mammoth quadrangle, Pinal County, Arizona: U.S. Geological Range Martinez Ranch fault also dips ϳ55Њ juxtaposed, growth-faulted sedimentary basin Survey Bulletin 1218, 94 p. where exposed along the eroded eastern face in the hanging wall. Similarly along the San Davis, G.H., 1980, Structural characteristics of metamor- of the basement-cored Rincon Mountains, but Manuel fault, intensely fractured footwall phic core complexes, southern Arizona, in Crittenden, M.D., Jr., Coney, P.J., and Davis, G.H., eds., Cordil- steepens to ϳ70Њ at a shallower structural lev- granite is in contact with only moderately leran metamorphic core complexes: Geological Soci- el farther south where it places Lower Creta- jointed and disrupted hanging-wall strata. In- ety of America Memoir 153, p. 35±77. Davis, G.H., and Reynolds, S.J., 1996, Structural geology ceous strata against Neogene basin ®ll (Dick- signi®cant translation by faulting is re¯ected of rocks and regions (2nd edition): New York, Wiley, inson, 1998b). The shallow-level Basin and by fabrics and structures along minor Basin 776 p.

140 Geological Society of America Bulletin, January/February 2004 CENOZOIC FAULT AND FAULT-ROCK CHARACTERISTICS, SOUTHEASTERN ARIZONA

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MANUSCRIPT RECEIVED BY THE SOCIETY 18 SEPTEMBER 2002 system along the range front of the southwestern Gal- Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical REVISED MANUSCRIPT RECEIVED 16 MAY 2003 iuro Mountains in southeastern Arizona: Geology, alteration-mineralization zoning in porphyry copper MANUSCRIPT ACCEPTED 23 JUNE 2003 v. 15, p. 727±730. deposits: Economic Geology, v. 65, p. 373±408. Dickinson, W.R., Force, E.R., and Hagstrum, J.T., 1995, McCarthy, J., and Parsons, T., 1994, Insights into the ki- Printed in the USA

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