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 Permian Rotliegend reservoir rocks are generally characterized by high net/gross (N/G) ratios, and faults in such sand-dominated lithologies are typically not considered likely to seal. Nevertheless, many examples of membrane sealing are present in Rotliegend gas fields in the Southern Permian Basin. This manuscript reviews examples of membrane sealing in the Dutch Rotliegend; it presents an extensive dataset of petrophysical properties of Rotliegend fault rocks and analyses two case studies using commonly used workflows. Fault (membrane) seal studies have been carried out on two Rotliegend fields to test the level of confidence and uncertainty of prediction of ‘across fault pressure differences’ (AFPD) based on existing SGR-based algorithms. From the field studies it is concluded that observable small AFPDs are present and that these are likely pre-production AFPDs due to exploration-time scale trapping and retention of hydrocarbons. Two shale gouge ratio (SGR)-based empirical algorithms have been used here to estimate AFPDs in lower N/G reservoir intervals with the aim of predicting membrane seal behaviour, and these results are compared to field data. It is concluded the selected SGR-based tools predict AFPD for Upper Rotliegend lower N/G reservoir rocks with reasonable results. Nonetheless, the core sample datasets show a much wider range of permeability and capillary entry pressure than predicted by the selected SGR transforms. This highlights the potential to modify existing workflows for application to faults in high N/G lithologies. Data sharing and collaboration between industry and academics is encouraged, so that in the long run workflows can be developed specifically for faults in high N/G lithologies.

Abstract The Holstein Field consists of poorly lithified turbidite sands deposited during the Pliocene Epoch. Dense arrays of cataclastic deformation bands have been observed in all cores from wells that penetrate the K2 reservoir sand, the highest density of which are located near the hinge of a monocline. The predominant set of deformation bands strikes parallel to the fold axis, and dips at both high and low angles with respect to bedding. Deformation band orientation and offset of marker beds indicate reverse shear and are consistent with a flexural slip origin during folding. Restorations suggest that the monocline and associated deformation bands formed early during the burial process with high pore pressure. Reservoir permeability estimates from well tests indicate a bulk permeability approximately one-third of the reservoir core permeability in regions with deformation bands, whereas other areas are unaffected. Bulk permeability estimated from the permeability of the reservoir and deformation band network is lower than the reservoir permeability alone, but exceeds the permeability observed in the well tests by a factor of 2. A reduction in permeability of oil relative to water for both the fault and host sand is required to match the well-test permeability with that measured from core.

Abstract Deformation bands are common subseismic structures in porous sandstones that vary with respect to deformation mechanisms, geometries and distribution. The amount of cataclasis involved largely determines how they impact fluid flow, and cataclasis is generally promoted by coarse grain size, good sorting, high porosity and overburden (usually >500–1000 m). Most bands involve a combination of shear and compaction, and a distinction can be made between those where shear displacement greatly exceeds compaction (compactional shear bands or CSB), where the two are of similar magnitude (shear-enhanced compaction bands or SECB), and pure compaction bands (PCB). The latter two only occur in the contractional regime, are characterized by high (70–100°) dihedral angles (SECB) or perpendicularity (PCB) to σ 1 (the maximum principal stress) and are restricted to layers with very high porosity. Contraction generally tends to produce populations of well-distributed deformation bands, whereas in the extensional regime the majority of bands are clustered around faults. Deformation bands also favour highly porous parts of a reservoir, which may result in a homogenization of the overall reservoir permeability and enhance sweep during hydrocarbon production. A number of intrinsic and external variables must therefore be considered when assessing the influence of deformation bands on reservoir performance.

Abstract Little is known about the effect of thrusting on lithological and petrophysical properties of reservoir sandstone. Here we use field observations, probe permeability measurements and thin-section analysis along ten transects from the Muddy Mountain thrust contact downwards into the underlying Jurassic Aztec Sandstone to evaluate the nature and extent of petrophysical and microstructural changes caused by the thrusting. The results reveal a decimetre- to metre-thick low-permeable (≤50 mD) and indurated (0–3% porosity) zone immediately beneath the thrust contact in which dominant microscale processes, in decreasing order of importance, are (1) cataclasis with local fault gouge formation; (2) pressure solution; and (3) very limited cementation. From this narrow zone the petrophysical and microstructural effect of the thrusting decreases gradually downwards into a friable, highly porous ( c. 25%) and permeable (≤2 D) sandstone some 50–150 m below the thrust, in which strain is localized into deformation band populations. In general, the petrophysical properties of the sandstone as a result of overthrusting reveal little impact in overall primary reservoir quality below some tens of metres into the footwall, except for the relatively minor baffling effect of deformation bands.

Abstract Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening. Supplementary material: Raman spectra of graphite from the Alpine Fault rocks is available at https://doi.org/10.6084/m9.figshare.c.3911797

Abstract: Many siliciclastic reservoirs contain millimetre-scale diagenetic and structural phenomena affecting fluid flow. We identified three major types of small-scale flow barriers in a clastic Rotliegend hydrocarbon reservoir: cataclastic deformation bands; dissolution seams; and bedding-parallel cementation. Deformation bands of various orientations were analysed on resistivity image logs and in core material. They are mainly conjugates, and can be used to validate seismically observable faults and infer subseismic faults. Bedding-parallel dissolution seams are related to compaction and post-date at least one set of deformation bands. Bedding-parallel cementation is accumulated in coarser-grained layers and depends on the amount of clay coatings. Apparent permeability data related to petrographical image interpretation visualizes the impact of flow barriers on reservoir heterogeneity. Transmissibility multiplier calculations indicate the small efficiency of the studied deformation bands on flow properties in the reservoir. Deformation bands reduce the host-rock permeability by a maximum of two orders of magnitude. However, host-rock anisotropies are inferred to reduce the permeability by a maximum of four orders of magnitude. The relative timing of these flow barriers, as well as the assessment of reservoir heterogeneities, are the basis for state-of-the-art reservoir prediction modelling.