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: Interbedded shale and limestone successions in the Kilve Beach area, Bristol Channel Basin, UK, provide insights on fracture networks around normal faults in fine-grained lithologies. Fracture sets with distinct orientations are characteristic of both shale and limestone beds. Shear fractures (mode II) predominate in the shaly units, and they have typically more gentle dips and a larger spread in orientations than extension veins and shear fractures in the limestones. Fracture intensities decrease away from the fault core, but maximum intensities, total number of fractures and widths of the damage zones appear to be independent of throw for normal faults with offsets of less than 20 m. Thus, there is no clear systematic relationship between fault throw and damage zone width in the shales studied by us. However, an asymmetry in the fracture distribution is evidenced by a wider hanging-wall damage zone and differences in fracture orientations in some cases. We interpret the asymmetry and spread in fracture orientations to be the result of propagating fault-tip process zones and the tempo-spatial impact of fault-slip events.

Abstract: Rollover is the folding of the hanging-wall sedimentary record in response to slip on listric normal faults, and is a common feature of sediment-rich, gravity-driven tectonic provinces. Rollovers have been extensively studied by means of geometrical reconstruction, and numerical and analogue modelling. However, the detailed interaction between the kinematics of bounding listric normal faults and their hanging-wall deformation is not yet fully understood. In this study, we use 3D seismic-reflection data from the Forcados-Yokri area, western Niger Delta, Nigeria, to study the lateral linkage and landwards backstepping history of an array of listric normal faults, particularly focusing on their influence on the development and evolution of hanging-wall rollovers. Five individual, partly overlapping rollover structures have been studied with respect to their relative initiation and decay time, their spatial distribution, and their relationship to the tectonic history of their respective bounding faults. We demonstrate that the studied rollovers are highly dependent on the development of their bounding faults in terms of initiation time, lateral linkage, internal structural development and decay. Fault–rollover interaction is dynamic and changes through time depending on the temporal evolution of listric faults. Four genetic types of fault–rollover interaction were identified in this study: (1) the rotation of a rollover–crestal-collapse system, controlled by a changing lateral bounding-fault orientation during fault growth; (2) a stepwise shift of rollover–crestal-collapse systems associated with rollover abandonment, controlled by the initiation of a new fault in the footwall of an older structure; (3) a gradual shift of successive rollovers controlled by branching main faults; and (4) a general landwards and upwards migration of crestal-collapse faults within a rollover above stationary listric main faults.

Abstract The Anaconda and Bitterroot metamorphic core complexes are located in western Montana, along the eastern edge of the Cordilleran hinterland. This multi-tiered extensional terrain contains exceptional exposures that collectively exhibit a crustal cross section through orogenic continental crust (i.e., middle through upper crust). The core complex footwall rocks consist of Late Cretaceous arc-related plutons and Eocene granitic plutons intruded into deformed and metamorphosed Midproterozoic Belt Supergroup and Paleozoic to Cretaceous shelf-platform strata. Late Cretaceous shear zones and folds dominate footwall structure, representing significant thinning of the stratigraphic section. Eocene detachments, mylonites, and plutonic suites distinctly overprint the Late Cretaceous structures. A stark example of this Eocene overprint is the Anaconda detachment, which resulted in eastward translation of the Late Cretaceous, arc-related Boulder batholith. This field trip will cover a transect through the Anaconda core complex from the Philipsburg valley to Butte, Montana. Field trip participants will examine key locations that clarify the distinction between the timing and structural style of Late Cretaceous crustal thickening and/or collapse features versus those related to Eocene core complex development.

Abstract The Scottsville Basin in the central Virginia Piedmont forms one of the westernmost Mesozoic sedimentary basins in eastern North America. This small basin has received limited scientific attention during the past 50 years; this field trip focuses on recent stratigraphic and structural research concerning the Scottsville Basin and surrounding region. The ∼110 km 2 Scottsville Basin and adjacent ∼5 km 2 Midway Mills Sub-basin formed astride the boundary between the eastern Blue Ridge and western Piedmont. The Scottsville Basin is a half-graben, bound on its northwest margin by a segmented normal fault that places Neoproterozoic to early Paleozoic metamorphic rocks in the footwall against Triassic strata in the hanging wall. Basin strata dip to the northwest toward the boundary fault, and dip angles increase from west to east. The southeastern basin boundary, previously interpreted as a small displacement normal fault, is an unconformity with phyllitic rocks of the western Piedmont. Strata within the basin include 2–3 km of boulder to pebble conglomerate, breccia, arkosic sandstone, and siltstone. Sedimentary rocks in the Scottsville Basin were sourced primarily from Proterozoic rocks in the Blue Ridge province to the west of the basin. The age of Triassic strata in the Scottsville Basin is poorly constrained. The Midway Mills Sub-basin was originally contiguous with the Scottsville Basin, but now forms an erosional outlier. A suite of north-northwest–striking Jurassic diabase dikes crosscuts Triassic sedimentary rocks and is subparallel to the dominant extensional fracture set in basin sedimentary rocks.

Integrated structural, metamorphic, and geochronological data indicate that the evolution of the Southern Marginal Zone (SMZ) of the Limpopo Complex of southern Africa was controlled by a single Neoarchean high-grade tectono-metamorphic event. The exhumation history reflected by the high-grade rocks is determined by their location relative to the contact with the low-grade rocks of the Kaapvaal Craton. Exhumation of granulites far north from this contact is recorded by a decompression-cooling (DC) pressure-temperature (P-T) path linked to steep southward-verging thrusts related to the Hout River Shear Zone. This P-T path traverses from P ~8 kbar, T ~825 °C to P ~5 kbar, T ~550 °C and reflects exhumation of the SMZ in the interval ca. 2.68–2.64 Ga. P-T paths for granulites close to this contact are characterized by a distinct inflection at P ~6 kbar, T ~700 °C that exhibits near-isobaric cooling (IC) to T ~580 °C. The IC stage is linked to low-angle, out-of-sequence, southward-verging thrusts that developed in the interval 2.63–2.6 Ga. The thrust-controlled exhumation of the SMZ furthermore is demonstrated by the convergence at P ~6 kbar, T ~700 °C of DC P-T paths in the hanging wall with prograde P-T loops in the footwall of the steeply southward-verging Hout River Shear Zone, and by the establishment of a retrograde isograd and zone of rehydrated granulites in the hanging wall derived from the dehydration of the low-grade rocks in the footwall. A composite deformation-pressure-temperature-time (D-P-T-t) diagram provides evidence in support of a tectonic model for the evolution of the Limpopo Complex that involves early crustal thickening and peak metamorphic conditions followed by doming and diapirism related to gravitational redistribution mechanisms.

Heterogeneous strain commonly serves as an important natural instrument for unraveling complex tectonic histories in polyphase metamorphic terranes. We present key examples of multi-scale heterogeneous deformation from two classic deep-crustal granulite terranes, the Athabasca Granulite Terrane in western Canada and the Limpopo Complex in southern Africa. These examples are chosen to illustrate how localized strain and attendant metamorphism played a key role in the development and preservation of important records of deep-crustal processes. In addition, several common characteristics of these terranes are identified through this analysis and include heterogeneous deep-crustal flow, regional-scale tectonic heterogeneity, and multistage exhumation with high-resolution records developed in locally hydrated shear zones. Better recognition of the fundamental spatial and temporal heterogeneity in these and other similar polymetamorphic terranes may help to reconcile apparently conflicting interpretations and tectonic models.

The regional-scale salients and recesses in the Appalachian orogen are well accepted as being a product of tectonic inheritance. Smaller map-view curves are present in the slate belt of the Taconic allochthon, which lies between the New York recess and Quebec salient, and we investigate the possibility that these curves are also related to the geometry of the preexisting Iapetan rift margin. The orientation of the slaty cleavage (S 2 ) and mineral/stretching lineation (L 2 ) and the geometry of syntectonic fibers were used to identify along-strike differences in the nature of the main stage of deformation (D 2 ) in the slate belt. Where the axial traces of F 2 folds lie parallel to the overall trend of the Taconic allochthon, the strata are characterized by an approximately downdip L 2 , plane strain, monoclinic strain symmetry, and top-to-west-northwest noncoaxial flow. The strata also underwent noncoaxial flow where the axial traces of F 2 folds are oblique to the overall trend of the Taconic allochthon. However, L 2 rakes moderately from the south on S 2 , and syntectonic fibers record evidence for flattening strain, triclinic strain symmetry, top-to-northwest shear in the XZ plane, and sinistral shear in the XY plane. We infer that the deformation zone in which D 2 structures formed changed orientation along strike such that the region of obliquely oriented F 2 folds underwent transpression, and we suggest that the transpressional deformation was a result of reactivation of a northwest-striking Iapetan transform fault as an oblique ramp during the Taconic orogeny.

Three major low-angle normal faults in the eastern Lake Mead area, Nevada and Arizona, are segments of a regional, 55-km-long, detachment fault. This fault, the South Virgin–White Hills detachment, consists of the Lakeside Mine, Salt Spring, and Cyclopic Mine fault segments. All three segments dip gently west and record top-to-the-west displacement. Based on apatite fission-track and apatite and titanite (U-Th)/He thermochronology of footwall rocks, tilt relations, and 40 Ar/ 39 Ar dates on tuffs and basalts within hanging-wall synextensional sedimentary sequences, significant extension along the South Virgin–White Hills detachment occurred between 16.5 and 14 Ma. Minor extension continued until ca. 8 Ma. Displacement on the South Virgin–White Hills detachment decreases from a maximum of ~17 km at the Gold Butte block in the north to 5–6 km at the Cyclopic Mine in the south. The along-strike, southward decrease in displacement is accompanied by a change in type of fault rock from mylonite along the Lakeside Mine fault (northern segment), to chloritic cataclasite along the Salt Spring fault (central segment), to unconsolidated fault breccia along the Cyclopic Mine fault (southern segment). Differences in fault rock may reflect decreasing exhumation of footwall rocks as a result of decreased displacement to the south. About 40% of the displacement gradient can be accommodated along a series of left-slip faults in the upper plate of the detachment. The Golden Rule Peak lineament, an east-trending alignment of structural and topographic features, may be a transverse structure that accommodates differential displacement between the Salt Spring and Cyclopic Mine faults. The trace of the South Virgin–White Hills detachment is highly sinuous in map view and is marked by three prominent salients that define west-plunging antiformal warps in the detachment surface. We interpret the corrugations in the South Virgin–White Hills detachment to have formed by a process of linkage of originally separate en echelon fault segments followed by eastward tilting of the footwall. Depositional patterns, particularly between the Lakeside Mine and Salt Spring segments, support this interpretation. The Grand Wash fault forms the present-day physiographic boundary between the Colorado Plateau and the Basin and Range Provinces; however, based on greater amount of displacement and exhumation, we suggest that the South Virgin–White Hills detachment is the principal structure accommodating regional extension in the eastern Lake Mead extensional domain.

Geological and geochemical characteristics of banded iron formations (BIFs) suggest that they formed by mixing locally (or regionally) discharged submarine hydrothermal fluids with local seawater, rather than by upwelling deep ocean water. Submarine hydrothermal fluids typically evolved from local seawater by acquiring heat, metals, and sulfur during deep circulation through a variety of rocks (e.g., volcanics, evaporites) in greenstone terranes that developed under a variety of tectonic settings. In general, when the fluids were heated above ∼350 °C, they may have produced Cu- and Zn-rich volcanogenic massive sulfide deposits (VMSDs), whereas those heated less than ∼200 °C were generally poor in H 2 S and heavy metals, except Fe, and may have subsequently produced BIFs. Depending on the salinity contrast between discharging hydrothermal fluids (evolved seawater) and local seawater, hydrothermal fluids may (1) mix rapidly with local seawater to form smoker-type BIFs or (2) create a metal- and silica-rich brine pool, mix slowly with the overlying water body, and form brine pool-type BIFs. BIFs associated with VMSDs and volcanic rocks generally belong to smoker-type BIFs; many formed at seawater depths >2.5 km. Large BIFs, including the 2.6–2.4 Ga BIFs in the Hamersley Basin, Australia, the 2.5 Ga Kuruman IF in South Africa, and the 1.87 Ga BIFs in the Lake Superior region, United States-Canada, belong to brine pool-type BIFs. The Hamersley Basin and possibly other large BIF-hosting basins were probably land-locked seas (like the Black Sea) where river waters diluted the surface water zone and the underlying water bodies were anoxic. During the accumulation of a BIF sequence, the dominant Fe mineralogy frequently changed from ferric (hydr)oxides (oxide BIFs) to siderite (carbonate BIFs) and to pyrite (sulfide BIFs). Such changes were probably caused by changes in the relative amounts of dissolved O 2 (DO), ΣCO 3 2− , and ΣS 2− in local seawater. From the Fe 2+ -O 2 mass balance calculations for the formation of iron oxides in smoker-type BIFs, and the relationship between the atmospheric pO 2 and oceanic O 2 depth profile, we conclude that the atmosphere and oceans have been fully oxygenated since ca. 3.8 Ga, except in local anoxic basins. Thermodynamic analyses of the formational conditions of siderite and analyses of the carbon isotopic composition of siderite associated with major BIFs suggest that the pre–1.8 Ga atmosphere was CO 2 -rich (pCO 2 >100 PAL) and CH 4 -poor (pCH 4 ≈ 10 ppm); therefore, CO 2 , rather than CH 4 , was the major greenhouse gas throughout geologic history. After a decline of hydrothermal fluid flux, BIF-hosting basins generally became euxinic (H 2 S-rich) because of the increased activity of sulfate-reducing bacteria (SRB) and SO 4 2− -rich seawater, and thereby accumulated organic carbon-rich and pyrite-rich black shales (sulfide-type BIFs). The SO 4 2− contents and SRB activity in the oceans have been essentially the same since ca. 3.8 Ga. The Archean oceans were most likely poor in both Fe 2+ and silica, much like modern oceans. Our study also suggests that diverse communities of organisms, including cyanobacteria, SRB, methanogens, methanotrophs, and eukaryotes, evolved very early in Earth's history, probably by the time the oldest BIFs (ca. 3.8 Ga) formed. BIFs have been found in rocks of all geologic age. Therefore, they cannot be indicators of an anoxic atmosphere and/or anoxic oceans as suggested by many previous researchers. Instead, BIFs indicate that the atmosphere and ocean chemistry have been regulated at present compositions (except pCO 2 ) through geologic history by interactions with the biosphere. The general trend of declining size and abundance of BIFs with geologic time reflects the cooling history of Earth's interior.

A group of plutons were emplaced in the western Klamath Mountains province during the waning stages of the Late Jurassic Nevadan orogeny. Published U-Pb (zircon) ages indicate that the “western Klamath plutonic suite” was emplaced in the age range of 151–144 Ma. Crosscutting relationships, development of contact metamorphic aureoles, and the presence of distinctive inherited zircon populations indicate that the magmas intruded the footwall and hanging-wall rocks of the principal Nevadan thrust fault. The plutons are chiefly gabbroic to dioritic in composition, but commonly include ultramafic rocks and contain smaller volumes of tonalite and granodiorite. Hornblende is the most common mafic phase, except for some ultramafic rocks in which clinopyroxene ± olivine are locally distinctive, the two-pyroxene dioritic to monzodioritic rocks of the Buck Lake unit of the Bear Mountain pluton, and the most felsic rocks in which biotite is the most abundant mafic phase. Compositions of fine-grained mafic dikes suggest the presence of two principal parental, H 2 O-rich magmas: primitive basalt and evolved basalt/basaltic andesite. The former was parental to the ultramafic rocks of this suite. It was also parental to the basalt/basaltic andesite magmas by deep-seated fractional crystallization processes. The latter magmas were parental to the gabbroic and dioritic units. Many of the felsic rocks show evidence of origins by partial melting of metabasaltic crustal rocks, particularly their low heavy rare-earth element concentrations and high Sr/Y ratios. Mixing of crustal melts with primitive basaltic magmas was locally important (e.g., Pony Peak pluton). The mafic parental magmas show trace element features typical of an origin by partial melting of a subduction-modified mantle wedge. It is unclear whether subduction was coeval with western Klamath magmatism or whether the subduction signature developed as the result of Middle Jurassic subduction.

A well-exposed, 1-km-long section of the La Grange fault, a major detachment fault in the southern Klamath Mountains, California, is examined for the dual purposes of analyzing the processes that operated during faulting and evaluating the tectonic significance of this fault. Black foliated fault-related rocks form a 25-cm-thick layer that caps the fault surface and consists of finely interlayered ultracataclasite and cataclasite. Features such as parallel slickenside striations on ultracataclasite layers, veins perpendicular to slickenside striations, clasts of ultracataclasite in cataclasite, and clasts of vein material in cataclasite record prolonged, brittle, extensional deformation along the La Grange fault. This faulting resulted in both south-southeastward transport of hanging-wall rocks and uplift and exhumation of footwall rocks. The presence of Yreka terrane units in the Oregon Mountain klippe suggests on the order of 60 km of southward displacement of the hanging wall of the La Grange fault. This faulting post-dates the assembly of accreted terranes at a long-lived accretionary margin, for which the Klamath Mountains province is renowned, and the overprint of extensional faulting both influences the map pattern and explains anomalies in extent and distribution of some Klamath terranes. At the La Grange fault, the lithologic contrast between amphibolite in the foot-wall and siltstone, sandstone, chert, and mica schist in the hanging wall permits assessment of the relative contributions of footwall and hanging-wall rocks to the ultracataclasite. The ultracataclasite is composed of <10% single mineral grains ≤100 μm diameter in an ultra-fine-grained (<<1 μm) matrix. Single mineral grains are predominantly quartz, but also include calcite, pyrite or pyrrhotite, sphene, rutile, apatite, zircon, and barite. Matrix composition of the ultracataclasite is distinctly different from that of larger grains. Comparison of footwall and hanging-wall rock compositions indicates that most of the larger grains in the ultracataclasite are single crystals of mechanically resistant minerals, and most of these originated in the hanging wall. The less resistant grains in the cataclastic rocks (calcite and barite) are minerals that occur in veins; these were most likely introduced into the fault-related rocks relatively late in the faulting process. The preferential preservation of specific minerals as larger grains within the ultracataclasite shows that the mechanical properties of individual minerals play an important role in comminution processes. Micro-scale textures provide information about the processes that have operated during faulting and about the conditions and duration of faulting. The ultracataclasite is composed of rounded to subangular grains (1–100 μm diameter) in an extremely fine-grained (<<1 μm) matrix. The cataclasite contains ultracataclasite clasts (up to 500 μm) and angular to subrounded individual mineral grains (5–100 μm) in a fine-grained matrix. Texture appears to vary with depth in the fault-related rocks: the abundance of larger single-mineral grains increases upward, with larger grains occupying 3.3 ± 1.1 area% at 2 cm, 2.8 ± 1.1% at 17 cm, and 7.0 ± 1.3% at 24 cm above the base of the foliated ultracataclasite. Extremely small grain size in the ultracataclasite records extreme grain crushing, milling, and sustained cataclastic deformation. This extensive comminution is consistent with a very high degree of strain localization along the fault.