ABSTRACT Calcite twinning analysis across the central, unbuttressed portion of the Sevier thin-skin thrust belt, using Cambrian–Cretaceous limestones ( n = 121) and synorogenic calcite veins ( n = 31), records a complex strain history for the Sevier belt, Idaho and Wyoming, USA. Plots of fabric types (layer-parallel shortening, layer-normal shortening, etc.), shortening and extension axes for the Paris thrust (west, oldest, n = 11), Meade thrust ( n = 46), Crawford thrust ( n = 15), Absaroka thrust ( n = 55), Darby thrust ( n = 13), Lander Peak klippe ( n = 5), eastern Prospect thrust ( n = 6), and distal Cretaceous foreland ( n = 3) reveal a W-E layer-parallel shortening strain only in the Prospect thrust and distal foreland. Calcite twinning strains in all western, internal thrust sheets are complex mixes of layer-parallel (LPS), layer-normal (LNS), and non-plane strains in limestones and synorogenic calcite veins. This complex strain fabric is best interpreted as the result of oblique convergence to the west and repeated eastward overthrusting by the Paris thrust.

ABSTRACT In a reconnaissance investigation aimed at interrogating the changing topography and paleogeography of the western United States prior to Basin and Range faulting, a preliminary study made use of U-Pb ages of detrital zircon suites from 16 samples from the Eocene–Oligocene Titus Canyon Formation, its overlying units, and correlatives near Death Valley. The Titus Canyon Formation unconformably overlies Neoproterozoic to Devonian strata in the Funeral and Grapevine Mountains of California and Nevada. Samples were collected from (1) the type area in Titus Canyon, (2) the headwaters of Monarch Canyon, and (3) unnamed Cenozoic strata exposed in a klippe of the Boundary Canyon fault in the central Funeral Mountains. Red beds and conglomerates at the base of the Titus Canyon Formation at locations 1 and 2, which contain previously reported 38–37 Ma fossils, yielded mostly Sierran batholith–age detrital zircons (defined by Triassic, Jurassic, and Cretaceous peaks). Overlying channelized fluvial sandstones, conglomerates, and minor lacustrine shale, marl, and limestone record an abrupt change in source region around 38–36 Ma or slightly later, from more local, Sierran arc–derived sediment to extraregional sources to the north. Clasts of red radiolarian-bearing chert, dark radiolarian chert, and quartzite indicate sources in the region of the Golconda and Roberts Mountains allochthons of northern Nevada. Sandstones intercalated with conglomerate contain increasing proportions of Cenozoic zircon sourced from south-migrating, caldera-forming eruptions at the latitude of Austin and Ely in Nevada with maximum depositional ages (MDAs) ranging from 36 to 24 Ma at the top of the Titus Canyon Formation. Carbonate clasts and ash-rich horizons become more prevalent in the overlying conglomeratic Panuga Formation (which contains a previously dated 15.7 Ma ash-flow tuff). The base of the higher, ash-dominated Wahguyhe Formation yielded a MDA of 14.4 Ma. The central Funeral Mountains section exposes a different sequence of units that, based on new data, are correlative to the Titus Canyon, Panuga, and Wahguyhe Formations at locations 1 and 2. An ash-flow tuff above its (unexposed) base provided a MDA of 34 Ma, and the youngest sample yielded a MDA of 12.7 Ma. The striking differences between age-correlative sections, together with map-based evidence for channelization, indicate that the Titus Canyon Formation and overlying units likely represent fluvial channel, floodplain, and lacustrine deposits as sediments mostly bypassed the region, moving south toward the Paleogene shoreline in the Mojave Desert. The profound changes in source regions and sedimentary facies documented in the Titus Canyon Formation took place during ignimbrite flareup magmatism and a proposed eastward shift of the continental divide from the axis of the Cretaceous arc to a new divide in central Nevada in response to thermal uplift and addition of magma to the crust. This uplift initiated south-flowing fluvial systems that supplied sediments to the Titus Canyon Formation and higher units.

ABSTRACT Rattlesnake Mountain is a Laramide uplift cored by Archean gneiss that formed by offset along two reverse faults with opposing dips, the result being an asymmetric anticline with a drape fold of Cambrian–Cretaceous sediments. Rattlesnake Mountain was uplifted ca. 57 Ma and was a structural buttress that impeded motion of upper-plate blocks of the catastrophic Heart Mountain slide (49.19 Ma). North of Pat O’Hara Mountain anticline, Rattlesnake Mountain anticline has a central graben that formed ca. 52 Ma (U-Pb age on vein calcite in normal faults) into which O- and C-depleted fluids propagated upward with hydrocarbons. The graben is defined by down-dropped Triassic Chugwater shales atop the anticline that facilitated motion of Heart Mountain slide blocks of Paleozoic limestones dolomite (i.e., the Ordovician Bighorn Dolomite and Mississippian Madison Limestone) onto, and over, Rattlesnake Mountain into the Bighorn Basin. Heart Mountain fault gouge was also injected downward into the bounding Rattlesnake Mountain graben normal faults (U-Pb age ca. 48.8 ± 5 Ma), based on O and C isotopes; there is no anisotropy of magnetic susceptibility fabric present. Calcite veins parallel to graben normal faults precipitated from meteoric waters (recorded by O and C isotopes) heated by the uplifting Rattlesnake Mountain anticline and crystallized at 57 °C (fluid inclusions) in the presence of oil. Calcite twinning strain results from graben injectites and calcite veins are different; we also documented a random layer-parallel shortening strain pattern for the Heart Mountain slide blocks in the ramp region ( n = 4; west) and on the land surface ( n = 5; atop Rattlesnake Mountain). We observed an absence of any twinning strain overprint (low negative expected values) in the allochthonous upper-plate blocks and in autochthonous carbonates directly below the Heart Mountain slide surface, again indicating rapid motion including horizontal rotation about vertical axes of the upper-plate Heart Mountain slide blocks during the Eocene.

Abstract The main metallic mineral resources of New Caledonia are hosted by the obducted Peridotite Nappe. Ni, Co, Cr and the Pt group elements (PGEs) are specific to this ultramafic terrane. Cr, as podiform chromitite in the uppermost mantle, is the only hypogene metal mined economically in the past. The largest chromitite deposits are located in the lherzolitic Tiébaghi Massif. Supergene Ni and Co deposits are concentrated by the tropical climate that has prevailed since the Miocene. New Caledonian lateritic Ni deposits account for 10% of the global Ni resources. Hydrous Mg silicate and oxide types coexist in a single deposit. A local genetic model based on geomorphological evolution is proposed. Sc is a prospective resource associated with these supergene deposits. The PGEs are a prospective resource associated with chromite, with potential in the hypogene, supergene and fluvio-littoral domains. Pt and Pd are the most significant elements. The transition zone between the upper mantle and crustal cumulates constitutes a regional Pt–Pd-enriched horizon. The concentrations are related to small disseminated chromite lenses in a pyroxene-rich lithology. The PGEs are concentrated in weathering profiles. The value of chromite-rich sands as placers or sand beach deposits might be enhanced by the occurrences of PGEs.

Abstract This review presents an objective account of metamorphic, microstructural and geochronological studies in the Greater Himalayan Sequence (GHS) and adjacent units in Nepal in the light of recent research. The importance of integrated, multidisciplinary studies is highlighted. A personal view is presented of strategies for determining pressure–temperature evolution, and of petrological processes at the micro scale, particularly in relation to departures from equilibrium and the behaviour of partially-melted rock systems. Evidence has accumulated for the existence within the GHS of a High Himalayan Discontinuity, marked by differences in timing of peak metamorphism in the hanging wall and footwall, and changes in P–T gradients and paths. Whether or not this is a single continuous horizon, it forms at each location the lower boundary to a migmatitic zone capable of ductile flow, and separates the GHS into an upper division in which channel flow may have operated in the interval 25–18 Ma, and a lower division characterized by an inverted metamorphic gradient, and by metamorphic ages that decrease downsection and are best explained by sequential accretion of footwall slices between 20 and 6 Ma. An overall model for extrusion of the GHS is still not resolved.

Abstract Crystalline klippen over the Lesser Himalayan Metasedimentary Sequence (LHMS) zone in the NW Himalaya have specific syn- and post-emplacement histories. These tectonics also provide a means to understand the driving factors responsible for the exhumation of the rocks of crystalline klippen during the Himalayan Orogeny. New meso- and microscale structural analyses, and thermochronological studies across the LHMS zone, Ramgarh Thrust (RT) sheet and Almora klippe in the eastern Kumaun region, NW Himalaya, indicate that the RT sheet and Almora klippe were a part of the Higher Himalayan Crystalline (HHC) of the Indian Plate which underwent at least one episode of pre-Himalayan deformation and polyepisodic Himalayan deformation in ductile and brittle–ductile regimes. The deformation temperature pattern within the Almora klippe records a normal thermal profile from its base to top but an inverted thermal profile from the base of Almora klippe down towards the LHMS zone. New fission-track data collected across the RT sheet and Almora klippe along Chalthi–Champawat–Pithoragarh traverse in the east Kumaun region document the exhumation of both units since Eocene times. Zircon fission-track (ZFT) ages from the Almora klippe range between 28.7 ± 2.4 and 17.6 ± 1.1 Ma, and from the RT sheet between 29.8 ± 1.6 and 22.6 ± 1.9 Ma; and the apatite fission-track (AFT) ages from the Almora klippe range between 15.1 ± 1.7 and 3.4 ± 0.5 Ma, and from the RT sheet between 8.7 ± 1.2 and 4.6 ± 0.6 Ma. The age pattern and diverse patterns of the exhumation rates reflect a clear tectonic signal in the RT sheet and the Almora klippe which acknowledge that the Cenozoic tectonics influenced the exhumation pattern in the Himalaya.