Autogenic avulsion,channelization and backfilling dynamics of debris‐flow fans

Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy.     

Origin of fluids in iron oxide–copper–gold deposits: constraints from δ 37Cl, 87Sr/86Sri and Cl/Br

The origin of the hypersaline fluids (magmatic or basinal brine?), associated with iron oxide (Cu–U–Au–REE) deposits, is controversial. We report the first chlorine and strontium isotope data combined with Cl/Br ratios of fluid inclusions from selected iron oxide–copper–gold (IOCG) deposits (Candelaria, Raúl–Condestable, Sossego), a deposit considered to represent a magmatic end member of the IOCG class of deposit (Gameleira), and a magnetite–apatite deposit (El Romeral) from South America. Our data indicate mixing of a high δ ³⁷Cl magmatic fluid with near 0‰ δ ³⁷Cl basinal brines in the Candelaria, Raúl–Condestable, and Sossego IOCG deposits and leaching of a few weight percent of evaporites by magmatic-hydrothermal (?) fluids at Gameleira and El Romeral. The Sr isotopic composition of the inclusion fluids of Candelaria, Raúl–Condestable, and El Romeral confirms the presence of a non-magmatic fluid component in these deposits. The heavy chlorine isotope signatures of fluids from the IOCG deposits (Candelaria, Raúl–Condestable, Sossego), reflecting the magmatic-hydrothermal component of these fluids, contrast with the near 0‰ δ ³⁷Cl values of porphyry copper fluids known from the literature. The heavy chlorine isotope compositions of fluids of the investigated IOCG deposits may indicate a prevailing mantle Cl component in contrast to porphyry copper fluids, an argument also supported by Os isotopes, or could result from differential Cl isotope fractionation processes (e.g. phase separation) in fluids of IOCG and porphyry Cu deposits.     

Authigenic sericite record of a fossil geothermal system: the Offenburg trough,central Black Forest,Germany

A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K–Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of ¹⁸O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150–210 °C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.     

Geometry and Properties of the Excavation-Induced Fractures at the Meuse/Haute-Marne URL Drifts

The performance and safety assessment and technology demonstration are the main objectives of research programs for feasibility studies for deep geological repository of radioactive waste. In this context, the French national radioactive waste management agency (ANDRA) started to develop the Meuse/Haute-Marne underground research laboratory (URL) at Bure, nearly 300 km East of Paris. The host formation consists of a Callovo-Oxfordian claystone found between 420 and 550 m below ground, overlain and underlain by poorly permeable carbonate formations. One of the major concerns related to performance assessment is the excavation-induced fractures which may provide groundwater preferential pathway for radionuclide migration. The extent of the fractures possibly acting significantly in the radionuclide migration is known as the excavation damaged zone (EDZ). A scientific study on the EDZ characterization is performed at the main level of the URL (?490 m). Observations such as structural analysis on core, overcored resin-filled samples, geological survey of the drift face and sidewalls, were made to better understand the fracture network characteristics, extent and its generation. Pulse and constant head test hydraulic conductivity measurements were performed with multi packer system to estimate the extension of the EDZ hydraulic conductivity. Fractures exhibited high transmissivity near the excavation walls, but farther from the exaction walls, shear fractures showed hydraulic conductivity values reflecting values of undisturbed or slightly disturbed rock mass condition. The major findings in terms of geometry and properties of excavation-induced fractures are discussed in detail in this paper. For example, it is observed that the shape of the fracture network depends on the orientation of the drift in relation to the orientation of the in situ stress field.     

Preliminary magnetic susceptibility characteristics of the Bharati promontory (Grovness Peninsula), Larsemann Hills,Prydz Bay region,East Antarctica

The coastal tract of the Prydz Bay region in the East Antarctica exposes Archean to Late Proterozoic magmatic and medium- to high grade (amphibolite — granulite facies) metamorphic rocks. The para- and ortho gneisses from the Bharati promontory (Grovness Peninsula) forming a part of the Larsemann Hills in the southern segment of Prydz Bay were investigated for magnetic characterization. In this small peninsula the upper amphibolite facies gneisses occur as NE-trending bands. The para-gneisses show a range of mineral assemblages (± cordierite ± sillimanite ±garnet) while ortho-gneiss mineralogy includes quartz, feldspar, biotite, garnet. All the lithological units in Bharati promontory contain ubiquitous magnetite, however, with wide variation in the volume proportions. This has resulted in a wide range in magnetic susceptibility (10^?4 to 10^?2 SI). Magnetic foliations show a correspondence with the general trend of lithounits (050° NE) and define a resulting geometry of mainly D₁ and D₂ foliations. The magnetic lineations show a preferred orientation with moderate easterly plunge (mean vector 093/36). The findings have implications for the magnetic field survey because such fabrics would impart a strong horizontal component of induced magnetization.     

Fabric studies within the Cascade Lake shear zone, Sierra Nevada, California

The Cascade Lake shear zone occurs on the eastern margin of the Tuolumne Intrusive Suite, Sierra Nevada Batholith, California. Foliation in the zone is NNW trending and subvertical, and lineation is moderately south plunging. Deformation is syn-tectonic with emplacement of the Cathedral Peak granodiorite. A deformation gradient exists toward the NE margin of this pluton, with higher strains and lower temperatures of deformation found near the contact. We compare fabric data collected very densely in this shear zone using several techniques: field fabrics, 3D orientation of K-feldspar megacrysts, and AMS (anisotropy of magnetic susceptibility) analysis. In general, the results from the three different methods are in agreement. Deformation in this shear zone is part of a larger pattern of deformation within the Cathedral Peak granodiorite, as recorded by AMS analysis, and dextral shearing associated within the last stage of plutonism within the Sierra Nevada magmatic arc.     

Age and tectonostratigraphic significance of the Upper Carboniferous series in the basement of the Andean Frontal Cordillera: Geodynamic implications

The age and tectonosedimentary environment of the Palaeozoic sediments on the Frontal Cordillera is not well known and earlier studies have been unable to satisfactorily explain the geological history of the basement of the Andes.In the vicinity of the old Castaño Viejo mine crop out various levels of partially metamorphosed microbialite limestones, which alternate with thin marly–lutitic interstrata. These levels contain abundant palynomorph remains, which allow the series to be dated as Silurian–Devonian. These data, together with the presence of warm climate fossils, lend support to the hypothesis of a major allochtony of the Chilenia Terrane (of which the Frontal Cordillera formed part), relative to the Cuyania Terrane (which included the Precordillera), prior to their amalgamation.Upper Carboniferous palynomorphs found during this study occur in association with resedimented palynomorphs and chitinozoa, of possible Devonian age. This demonstrates the equivalence of both fossiliferous series and their location within the upper part of the Upper Carboniferous Agua Negra Fm. The Silurian–Devonian elements, deformed during a phase prior to the Gondwanic orogeny, were eroded and transported to the foreland basin during the Upper Carboniferous.The palynomorph associations found in all samples correspond to the Ancistrospora palynological zone and to the Raistrickia densa–Convolutispora muriornata Biozone, which are indicative of Upper Carboniferous times. Characteristic forms such as Ancistrospora verrucosa and C. muriornata, both indicative of an Upper Carboniferous age, were found in samples from the Castaño Viejo area.Earlier interpretations of the Frontal Cordillera attributing the sedimentation to a palaeo-latitude at some distance from Gondwana, were based on the presence of Silurian–Devonian hot water stromatolithic limestones. Our results suggest that Cuyania and Chilenia were not necessarily separated by a great distance before their amalgamation. This in turn means that a large ocean was not necessarily consumed in the process.