ABSTRACT The Santa Catalina and Rincon Mountains north and east of Tucson, Arizona, form one of the largest core complexes on Earth. Both ranges consist primarily of Eocene leucogranites that intrude Proterozoic and late Cretaceous granitoids, and two Oligocene plutons. Mylonitic fabrics are well developed on the southern flank of the Santa Catalina Mountains and the southwestern flank of the Rincon Mountains. The corrugated form of the two ranges reflects the grooved form of the ca. 15–30 Ma Catalina–San Pedro detachment fault exposed primarily at the foot of the ranges. Normal displacement on two younger high-angle normal faults is responsible for much of the substantial relief of the ranges. This field guide is focused on fault rocks and mylonitic fabrics in the footwalls of the detachment fault and the high-angle Pirate normal fault, and includes description and analysis of shear-zone kinematics and processes, U-Pb geochronology of leucogranites, and core-complex geomorphology.

Recent field work and accumulated Rb-Sr studies, when combined with previous U-Th-Pb and K-Ar investigations, allow a new synthesis of the crystalline terrane within the Santa Catalina–Rincon–Tortolita crystalline complex. When all the available data are integrated, it is apparent that the crystalline core is mainly a composite batholith that has been deformed by variable amounts of cataclasis. The batholith was formed by three episodes of geologically, mineralogically, geochemically, and geochronologically distinct plutons. The first episode (75 to 60 m.y. B.P.) consisted of at least two (and probably three) calc-alkalic, epidote-bearing biotite granodiorite plutons (Leatherwood suite). The Leatherwood suite is intruded by distinctive leucocratic muscovite-bearing peraluminous granitic plutons (Wilderness suite), which are 44 to 50 m.y. old. At least three Wilderness suite plutons are known, and their origin has been much debated. Leatherwood and Wilderness plutons are intruded by a third suite of four biotite quartz monzonite to granite plutons (Catalina suite) that mark the final consolidation of the batholith 29 to 25 m.y. ago. Much of the mylonitic (cataclastic) deformation of the plutonic rocks and recrystallization of the enclosing host rocks may be related to intrusion of the various plutons. At least three episodes of mylonitization (cataclasis) may be delineated by observing relations between mylonitic and nonmylonitic crosscutting plutons. The southern part of the Leatherwood pluton bears a moderate to strong mylonitic foliation that is cut by undeformed leucogranites and pegmatite phases of the Wilderness pluton. Elsewhere in the Santa Catalina–Rincon–Tortolita crystalline core, Wilderness suite plutons contain penetrative mylonitic foliation. Foliated Wilderness suite plutons are intruded by an undeformed portion of a Catalina suite pluton. In the Tortolita Mountains, however, intrusions of the Catalina suite themselves contain evidence for at least two events of mylonitic deformation. The most significant of these events is clearly constrained to the Catalina intrusive episode because it formed during or after the emplacement of Tortolita quartz monzonite (about 27 m.y. B.P.) but before the intrusion of postfoliation dikes (about 24 m.y. B.P.). All three episodes of mylonitization contain the distinctive and much discussed east-northeast–trending lineations. All events of mylonitization are constrained to a 50-m.y. interval of time from 70 to 20 m.y. ago. Although continuous mylonitization from 70 to 20 m.y. ago cannot be unequivocally disproved, the strong association of mylonitization with the three plutonic episodes suggests that deformation in the Santa Catalina-Rincon-Tortolita crystalline core, like intrusion, was episodic.

ABSTRACT Investigation of exhumed and well-exposed crustal-scale fault zones provides a rare window into the mechanics and timing of a broad range of deformation mechanisms, strain localization, and fault zone behavior. Here, we apply and integrate geo- and thermochronology analytics to carefully described brittle-ductile structural characteristics of the Catalina detachment zone as exposed in the Rincon Mountains domain of the Catalina-Rincon metamorphic core complex. This core complex is an exhumed extensional, broad-scale-normal-slip shear zone near Tucson, Arizona, USA. The Catalina detachment zone, as formulated here, is partitioned into a brittle-ductile fault-rock stratigraphy that evolved through progressive deformation. The Catalina-Rincon Mountains metamorphic core complex is one of the original type localities of Cordilleran metamorphic core complexes in western North America and has a long history of scientific study to document its structural characteristics and decipher its evolution in the context of Mid-Cenozoic extension. In this Memoir, we seek to provide a thorough accounting of the evolution of this shear zone, through integrating and synthesizing decades of previous research with new mapping, structural data, and geochronological analyses. The Catalina detachment zone stratigraphy is made up of the Catalina detachment fault, cataclasite, chloritic protocataclasite (referred to in most core-complex literature as “chlorite breccia”), subdetachment faults, and mylonites. When it was active, this zone accommodated a minimum of ~36 km of top-to-the-SW displacement. Characterizing the progressive evolution of this metamorphic core complex fault-rock stratigraphy requires a detailed accounting of the kinematic and temporal history of the detachment zone. Consequently, we first characterize and describe each structural unit and feature of this crustal-scale fault and shear zone network through the combination of previously published mapping, structural and microfabric analyses and newly collected structural data, thin-section analysis, large-scale mapping, and reinterpretation of stratigraphic and structural relations in the adjacent Tucson Basin. To improve our broad-scale mapping efforts, we employ multispectral analysis, successfully delineating specific fault-rock stratigraphic units at the core-complex scale. We then establish kinematic and absolute timing constraints by integrating results from well-log and seismic reflection data and with new and previously published zircon U-Pb, 40 Ar/ 39 Ar, 40 K/ 40 Ar geochronological, (U/Th)/He, 4 He/ 3 He, and apatite fission track thermochronological analyses. These temporal constraints indicate a deformation sequence that progressed through mylonitization, cataclasis, mini-detachment faulting, subdetachment faulting, and detachment faulting. This multidisciplinary investigation reveals that mylonitization occurred in late Oligocene time (ca. 26–22 Ma), coeval with rapid exhumation of the lower plate, and that slip on the Catalina detachment fault ceased by early Miocene, ca. 17 Ma. This temporal framework is consistent with results of our subsurface analysis of stratigraphic and structural relations in the Tucson Basin. Onset of metamorphic core complex deformation in southern Arizona slightly preceded that in central and western Arizona and southeasternmost California. Our compiled data sets suggest a shear-zone evolution model that places special emphasis on the transformation of mylonite to chloritic protocataclasite, and strain localization onto subdetachment, minidetachment, and detachment faults over time. Our model envisions mylonites drawn upward through a fluids-sourced brittle-ductile transition zone marked by elevated fluid pressures. This emphasis draws upon seminal work by Jane Selverstone and Gary Axen in analyzing structural-mechanical evolution in the Whipple Mountains metamorphic core complex. Progressive embrittlement and strength-hardening of the lower-plate rocks are manifest in intensive fracturing and minidetachment faulting, favored by the change in rheology produced by alteration-mineral products. Subdetachment faults, localized by earlier-formed ultramylonite and calc-silicate tectonite, coalesce to produce a proto-detachment fault, which marks the interface between mylonite and chlorite protocataclasite. Linking and smoothing of minidetachment faults within chloritic protocataclasite led to emergence of the Catalina detachment fault proper. All of this, from mylonite formation to final slippage on the detachment fault, kinematically conforms to top-to-the-SW shear. The macro-form of the antiformal-synformal corrugations of the Rincon Mountains began developing while mylonites were forming, continuing to amplify during proto-detachment faulting and detachment faulting. We emphasize and describe with examples how the timing and tectonic significance of mylonitization, cataclasis, and detachment faulting within the Catalina-Rincon metamorphic core complex continues to be hotly debated. Disagreements center today, as they have in the past, on the degree to which the structures and fabrics in the Rincons are Laramide products, mid-Cenozoic products, or some combination of both. In addressing tectonic heritage with respect to the Catalina detachment zone, it is hoped that the proposed model of progressive evolution of the Catalina detachment-zone shear zone will inform other studies of active and ancient metamorphic core complexes around the globe. In this regard, some new transferable emphases and methodologies emerged from this work, above and beyond what are now standard operating procedures for understanding crustal shear zones in general, and metamorphic core complexes particularly. For example, remote multispectral image analysis combined with ground-truth field analysis permitted mapping the full extent of chloritic protocataclasite, one of the best exposures of same globally, which is perhaps the most strategic fault rock in exploring the brittle-ductile transition. The added value of complete map control for chloritic protocataclasite is exploring, at its base in other metamorphic core complexes, for the presence of subdetachment faulting, i.e., proto-detachment faulting that influenced localization of detachment zones proper. Another example is the importance of continuously searching for certain mylonite protolith that yields opportunities for closely constraining timing of mylonitization. In our case, it is the Loma Alta mylonite that, more than any other protolith unit in the Rincon Mountains, permitted ‘locking’ the age of mylonitization as late Oligocene. We hope that insights from this detailed study will inform analyses of similar crustal-scale fault zones, both ancient and modern. Given its ready accessibility compared to most metamorphic core complexes, the Rincon Mountains present opportunities for others to use this contribution as part of the basis for exploiting this natural laboratory in research, teaching, and public science.

Abstract The first part of the route is southward from Tucson to Nogales. The geology and tectonic fabric is variable and includes from north to south the Santa Catalina-Rincon mountains metamorphic core complex, Late Cretaceous and Early Tertiary volcanic rocks of the Tucson Mountains, several porphyry-copper deposits associated with Laramide siliceous intrusion, and Precambrian units, imbricate fault slices of Paleozoic formations, and the Early Cretaceous Bisbee Group in the Santa Rita Mountains. The route follows the valley of the Santa Cruz River in which abundant Plio-Pleistocene gravel deposits are located to the international line. Further south, in northernmost Sonora, the route continues along the valley of the upper reaches of Rio Magdalena. This valley is largely surrounded by Jurassic volcanic flows and intrusions as far south as Magdalena de Kino where another core complex is situated. The route then turns eastward across the Magdalena extensional basin where borate and gypsum deposits have been discovered in Miocene age continental deposits. After crossing a drainage divide at Puerto Cucurpe, where ignimbrites are exposed in Sierra Torreón, the route turns northward through the valley-fills deposits exposed by Arroyo Las Rastras to the Santa Gertrudis, Carlin-type, disseminated gold deposit. 0.0 Buenos Dias! Turn right (west) on Speedway. The first part of the route (Fig. 1) is from Tucson to Nogales, parallel to the Santa Cruz River. Travel west towards 1-10. 2.1 2.1 To the north are the Santa Catalina Mountains. The Santa Catalina-Rincon metamorphic core complex is shown in Figure 2. The mylonite zone is 10-15 km wide and appears to have formed in the Middle Tertiary (37 ± 8 Ma) at a depth of 9.3 ± 1.9 km (Anderson et al., 1988). Lower plate rocks include Pinal Schist, Oracle Granite (1.4 Ga), Apache Group, Cambrian through Mississippian strata, and Lower Cretaceous Bisbee Group. Plutonic rocks include Tertiary quartz diorite and granites that form the bulk of the range except for the southwestern and northeastern flanks that comprise the upper plate (Naruk and Bykerk-Kauffman, 1990). 4.6 6.7 Cross Alvermon Way. Tucson Mountains straight ahead (12:00) on skyline. The Tucson Mountains form a low desert range on the west side of Tucson. They are underlain by Upper Cretaceous volcanic rocks that have been interpreted as parts of the fill of a large ash-flow caldera (Lipman, 1993). 1.6 8.3 Cross Country Club Road 1.8 10.1 Cross Campbell Street; University of Arizona football stadium and

The Catalina-Rincon metamorphic core complex (Tucson, Arizona, USA) is a type Cordilleran metamorphic core complex. This volume draws together decades of investigations into the geology of the Rincons, and presents results of multi-scale mapping and structural analysis of the Catalina detachment zone, a superbly exposed crustal-scale shear zone. A structural model for progressive incremental deformation synthesizes geological observations into a kinematic/mechanical framework. To this is added the first substantive application of multi-method geochronology and thermochronology, results of which place the evolution of the detachment zone (from mylonitization through cataclasis to exhumation) into a narrow time window, i.e., from ca. 26 to 17 Ma.

Reconnaissance mapping indicates that parts of nine mountain ranges previously considered to be Precambrian basement are instead variations of Tertiary metamorphic core complexes. From southeast to northwest, these ranges include the Pinaleno, Picacho, South Mountains, parts of the Buckeye, White Tank, Harquahala, Harcuvar, Buckskin, and Rawhide Mountains. Together with the already recognized Santa Catalina–Rincon–Tortolita complex, these ranges define a broad northwest-trending belt through Arizona. The northeast-trending Buckskin-Harcuvar-Harquahala Mountains are transverse foliation arches, the latest expression of a huge, northwest-elongated metamorphic area herein named the Harcuvar metamorphic core complex. This Tertiary phenomenon is superimposed on an ill-defined center of late Mesozoic metamorphism. Traverses into the complex from its unmetamorphosed southwestern margin reveal progressive Cretaceous conversion of Mesozoic sedimentary rocks into migmatites. Metamorphism just preceded intrusion of the Tank Pass batholith, an Upper Cretaceous pluton which itself became foliated and involved in early Tertiary migmatization and intrusion in the Harcuvar Mountains. A marginal zone of penetrative mylonitization, capped by a more brittlely deformed dislocation surface, flanks the Harcuvar complex on its upper and broadly arcuate northeastern margins. Resting on this tectonic surface are highly tilted, unmetamorphosed, layered rocks (Paleozoic to Tertiary). Geochronologic and geologic data place the time of mylonitization as Tertiary, perhaps as recently as 25 to 20 m.y. B.P. This deformation (flattening and northeast-southeast extension) was closely followed by development of chlorite breccia, the dislocation surface, thick wedges of coarse clastic sediment, and listric faulting. Finally, the core complex was arched and uplifted. A model for this sequence of events is predicated on mobile northeast-directed extension of a flat upper-crustal layer facilitated by intense mid-Tertiary plutonism in an actively tensile stress field. Tectonism in a brittle, surficial upper plate is governed by listric faulting and detachment as the plate fragments and extends “piggyback” style upon subjacent, ductilely stretched layer.