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 In Mexico, Ordovician sedimentary rocks are exposed in the states of Baja California, Sonora, Chihuahua and Oaxaca, comprising approximately 30 stratigraphic successions ranging from Lower to Upper Ordovician. The ages of the sequences have been established primarily by utilizing conodonts and graptolites, which have also allowed us to differentiate between platform and oceanic basin environments. The State of Sonora has the most complete Ordovician stratigraphic sequences, ranging from Tremadocian to Hirnantian. The deposits in Baja California are Floian in age, while the sequences of Chihuahua range from Sandbian to Katian, and the deposits in Oaxaca are Tremadocian. The Ordovician deposits of northern Mexico (Baja California, Sonora, and Chihuahua) present a palaeogeographic relationship to the North American craton, mainly owing to faunal interspecific affinities, while the southern deposits (Oaxaca) are controversial owing to the high degree of endemism of the faunas; however, they show affinity with Gondwana, Baltica and Avalonia, with a possible insular origin. The biotic assemblages of the Ordovician of Mexico include a variety of taxa, including algae, poriferans, corals, bryozoans, brachiopods, molluscs, trilobites, echinoderms, graptolites and conodonts as predominant elements. Despite many years of field studies in Mexican Ordovician localities, biostratigraphic correlations are as yet insufficient and incomplete or are based on limited interpretations. Thus, the Ordovician biostratigraphic data from Mexico compiled in the present paper have great potential and significant value. The advancement in the knowledge of the Ordovician biostratigraphy of Mexico will contribute to a major understanding of the relationships with the Ordovician System to a continental scale. Future advances will come mainly through increasing the amount and quality of data as well as improving biocorrelations among the Ordovician sequences of Mexico.

ABSTRACT Jurassic northward migration of Mexico, which lay on the southern part of the North America plate, resulted in temporal evolution of climate-sensitive depositional environments. Lower–Middle Jurassic rocks in central Mexico contain a record of warm-humid conditions, indicated by coal, plant fossils, and compositionally mature sandstone deposited in continental environments. Paleomagnetic data for central Oaxaca and other regions of central and eastern Mexico indicate that Lower and Middle Jurassic rocks were deposited at near-equatorial paleolatitudes. In the Late Jurassic, the Gulf of Mexico formed as a subsidiary basin of the Atlantic Ocean when the Pangea supercontinent ruptured. Upper Jurassic strata across Mexico, including eolianite and widespread evaporite deposits, indicate dry-arid conditions. Available paleomagnetic data (compaction-corrected) from southern and northeast Mexico for Upper Jurassic strata indicate deposition at ~15°N–20°N. As North America moved northward during Jurassic opening of the Atlantic Ocean, different latitudinal regions experienced coeval Middle–Late Jurassic climatic shifts. Climate transitions have been widely recognized in the Colorado Plateau region. The plateau left the horse latitudes in the late Middle Jurassic to reach temperate humid climates at ~40°N in the latest Jurassic. Affected by the same northward drift, the southern end of the North America plate represented by central Mexico gradually reached the arid horse latitudes in the late Middle Jurassic as the Colorado Plateau was leaving them. As a result, Late Jurassic epeiric platforms developed in the circum–Gulf of Mexico region after a long period of margin extension and were surrounded by arid land masses. We propose that hydrocarbon source-rock deposition was facilitated by arid conditions and wind-induced coastal upwelling.

ABSTRACT The early to middle Pleistocene Colorado River Delta deposits exposed in the upper Gulf of California, Sonora, México, are host to a diverse paleo-fauna and paleo-flora (El Golfo local paleobiota) of Irvingtonian Land Mammal Age (Calabrian Stage). The fossiliferous exposures are found in badlands developed in fluvio-deltaic sediments that have been mildly deformed during late Pleistocene doming along the Cerro Prieto fault. The El Golfo Project is part of the resource inventory for the Upper Gulf of California and Colorado River Delta Biosphere Reserve. Through joint efforts by Arizona Western College, the La Brea Tar Pits and Museum, and the Centro de Geociencias, Universidad Nacional Autónoma de México, about 50% of the region has been prospected and mapped. To date, more than 13,500 mapped vertebrate fossil localities are documented, including important microvertebrate sites. New mammalian, avian, fish, and flora taxa have been recently added to a growing paleobiota list that now numbers more than 145 taxa. The preserved paleobiota suggests the existence of three ecologic communities: freshwater aquatic, shrub and brush woodland, and savannah-like grassland. Some of the fauna are presently endemic to geographic areas farther south, suggesting a more tropical to subtropical climate on the Colorado River Delta during this part of the Irvingtonian.

ABSTRACT We will embark on a five-day journey through northern, western, and central Sonora, in which we will see excellent examples of mostly Mesozoic to Cenozoic tectonics, sedimentation, and metallogeny. On Day 1, we will visit the porphyry copper deposit at Ajo, Arizona, and several Pleistocene cinder cones and maar craters in the Pinacate Biosphere Reserve. On Day 2, we will see L- and L-S tectonites at the type locality of the Mojave-Sonora megashear in Sierra Los Tanques, Noche Buena orogenic gold deposit, Ediacaran Gamuza beds in Caborca, and have an overview of the Carnero detachment fault on the south side of Sierra La Gloria. Day 3 will explore faults and related sedimentary and volcanic rocks associated with the late Miocene oblique opening of the Gulf of California rift and visit outcrops that record late Miocene timing constraints for flooding of the Gulf of California seaway, including several localities on southern Isla Tiburón accessible only by boat. Day 4 will visit exposures of Permian sedimentary to Paleogene igneous rocks in Hermosillo (Cerro La Campana); Puerto del Sol detachment fault zone; Aconchi batholith and a hot spring localized on a Basin and Range normal fault; Santa Elena low-sulfidation epithermal gold mine; and the Upper Jurassic Cucurpe Formation. On Day 5, we will visit several exposures of different crustal levels of the Magdalena-Madera metamorphic core complex, including the spectacular stretched pebble conglomerates in Arroyo Amolares.