Abstract The recent shale gas revolution originated in the USA in the late 1990s with the exploration of the Carboniferous Barnett Shale in Texas. Success in a number of additional basins in North America, such as the Marcellus, Eagle Ford and Bakken basins, stimulated a search for similar opportunities elsewhere around the world. Among the shales and basins targeted by industry was the Carboniferous Bowland Shale (and equivalents) in northern England. The initial premise that the Barnett Shale represented an excellent analogue for the Bowland Shale led to over-optimistic reserve estimates that have since been shown to be largely incorrect. On the basis of visual inspection of wellbore cores, the Carboniferous Barnett and Bowland shales appear to be very similar. Unfortunately, it is there that the similarity ends. Research carried out for the UK Unconventional Hydrocarbons project has highlighted important differences adversely impacting prospectivity. These can be summarized as basin type/continuity and structural complexity. The total organic carbon, maturity, mineralogy and thickness of the Bowland Shale and equivalents are broadly similar to the successful US examples. Our conclusion is that the Bowland Shale in the UK does not represent a technically significant resource, and in hindsight did not merit the considerable industry and media attention that has been associated with it. One key learning is that fundamental research based on heritage data and modern analytical and modelling techniques should have preceded drilling and fracking operations in northern England.

Abstract Brittle structures are crucial for enabling several key natural processes in the Earth's upper crust. In addition, understanding the 3D characteristics and geological evolution of these features is equally important to support various developmental objectives, such as those, inter alia , linked to natural gas, groundwater, hydrothermal minerals and seismicity. In this study, we map various fractures of Gondwana based on the available geological information, satellite imagery and digital elevation data. The lengths and orientations of more than 10 000 fractures in their present-day position reveal four clearly defined patterns, with those striking NW being predominant. Archean–Paleoproterozoic domains are defined by fractures oriented north and NE, whereas the Mesoproterozoic has dominant NNW-striking fractures. In contrast, the Neoproterozoic has mostly NE-striking fractures and the Phanerozoic sequences are defined by a predominant NW and a subordinate west fracture pattern. The style and geometry of these structures can be linked to major geodynamic events that led to the formation of Gondwana building blocks during the Eburnean ( c. 2.2–1.8 Ga), Kibaran ( c. 1.4–1.0 Ga) and Pan African–Brasiliano ( c. 800–550 Ma) orogens, and amalgamation of Pangaea ( c. 350–250 Ma). Many structures were reactivated and new faults formed during opening of the Atlantic and Indian oceans ( c. 180–120 Ma), the India–Asia collision and rifting across East Africa since about 40 Ma. Although the changes in palaeogeography remain difficult to model with accuracy, major structural orientations are corroborated by the occurrence of major mineral deposits and seismicity. The spatial distribution of mapped patterns across the different continents also correlates well with large shale gas prospects and increased groundwater yields. Thus, Gondwana fractures need to be considered in more detail for informing future development related to water and energy use, especially across regions of Africa.

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.