Fluid flow during accretion in sediment-dominated margins: evidences of a high-permeability fossil fault zone from the Internal Ligurian accretionary units of the northern Apennines, Italy

We report here a detailed structural study carried out in the Internal Ligurian Units of the Northern Apennines, Italy, formed during the building of the Alpine accretionary complex through subduction of the sediment-filled Ligure-Piemontese oceanic basin. The deformation mechanisms associated with fluid migration across an accretion-related fault zone have been studied through a detailed analysis of different generations of syn-tectonic veins. Hydrofracturing occurred mainly sub-parallel to bedding in unlithified to semi-lithified sediments. Transient, upward-directed fluid injection locally connected the de´collement-parallel veins through bedding-normal hydrofractures of lithified sandstone layers. A third vein system comprises fibrous hydrofractures developed on the limbs of accretion-related folds. Crosscutting vein sets and the peculiar features of each identified vein set suggest that deformation was intricately associated with lithification and diagenetic processes. Dehydration-produced fluids transiently injected the lithifying sediments leading to local stress permutations. The proposed model provides a ‘‘ramp-flat’’ migration of fluids in which fluid flow is enhanced along high permeability, less cohesive layers, leading to the development of regional dilated hydrofracture channels like those recognized along the de´collement zone of modern margins. The more competent layers are truncated by high angle fractures representing the transient connectivity that existed between horizontal conduits

Fluid-rich damage zone of an ancient out-of-sequence thrust, Kodiak Islands, Alaska

The Uganik Thrust is a fossil out-of-sequence thrust fault which was active over a period of 3 Ma during the early Tertiary until activity ceased with the subduction of the Kula-Farallon spreading ridge at 57 Ma. During this period the fault experienced at least 1 km of throw and developed a strongly asymmetric damage zone. The brittle damage zone in the footwall of the fault acted as a conduit for fluid advection during the active faulting. A similar asymmetrical footwall damage zone has been interpreted as a fluid conduit at the Nobeoka Thrust, Shimanto Belt, SW Japan. Thermal indicators in the uppermost footwall give similar maximum paleotemperatures to those in the hanging wall (280C), while previous work elsewhere in the footwall formation suggests maximum burial temperatures of 240C. In this case, similar to the Irish Canyon thrust in the Franciscan accretionary complex, the location of the thermal anomaly is spatially offset from the structural fault which caused it owing to thermal overprinting in the vicinity of the fault

Deformation history of the eclogite- and jadeitite-bearing mélange from North Motagua Fault Zone, Guatemala: insights in the processes of a fossil subduction channel

In Guatemala, along the northern side of the Motagua Valley, a me ́lange consisting of blocks of eclogite and jadeitite set in a metaserpentinitic and metasedimentary matrix crops out. The metasedimentary rocks display a complex deformation history that includes four tectonic phases, from D1 to D4. The D1 phase occurs only as a relic and is characterized by a mineral assemblage developed under pressure temperature (P–T) conditions of 1.00–1.25GPa and 206–2638C. The D2 phase, characterized by isoclinal folds, schistosity and mineral/stretching lineation, developed at P–T conditions of 0.70–1.20 GPa and 279–4098C. The following D3 and D4 phases show deformations developed at shallower structural levels. Whereas the D1 phase can be interpreted as the result of underplating of slices of oceanic lithosphere during an intraoceanic subduction, the following phases have been acquired by the me ́lange during its progressive exhumation through different mechanisms. The deformations related to the D2 and D3 phases can be regarded as acquired by extrusion of the me ́lange within a subduction channel during a stage of oblique subduction. In addition, the structural evidences indicate that the coupling and mixing of different blocks occurred during the D2 phase, as a result of flow reverse and upward trajectory in the subduction channel. By contrast, the D4 phase can be interpreted as related to extension at shallow structural levels. In this framework, the exhumation- related structures in the me ́lange indicate that this process, probably long-lived, developed through different mechanisms, active in the subduction channel through time

The coupling of high-pressure oceanic and continental units in Alpine Corsica: Evidence for syn-exhumation tectonic erosion at the roof of the plate interface

The subduction of continental crust is now a matter of fact but which are the mechanisms and the factors control- ling the exhumation of continental units and their coupling with oceanic units are still a matter of debate. We herein present the tectono-metamorphic study of selected continental units belonging to the Alpine Corsica (Corte area, Central Corsica, France). The tectonic pile in the study area features thin slices of oceanic units (i.e. Schistes Lustrés Complex) tectonically stacked between the continental units (i.e. the Lower Units), which record a pressure–temperature-deformation (P-T-d) evolution related to their burial, down to P-T-peak conditions in the blueschist facies and subsequent exhumation during the Late Cretaceous – Early Oligocene time span. The metamorphic conditions were calculated crossing the results of three different thermobarometers based on the HP-LT metapelites. The continental units only recorded the P-peak conditions of 1.2 GPa-250 °C, up to the T-peak conditions of 0.8 GPa-400 °C, and the retrograde path up to LP-LT conditions. The metamorphic record of the oceanic units includes part of the prograde path occurring before the peak conditions reached at 1.0 GPa-250 °C followed by the last metamorphic event related to LP-LT conditions. The results indicate that each unit experienced a multistage independent pressure–temperature-deformation (P-T-d) evolution and sug- gest that the oceanic and continental units were coupled during the rising of the last ones at about 10 km of depth, where the oceanic units were stored at the base of the wedge. Subsequently they were deformed together by the last ductile deformation event during exhumation. We propose a mechanism of tectonic erosion at the base of the wedge, by which slices of Schistes Lustrés Complex were removed at the roof of the plate interface during the exhumation of the Lower Units

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

Ultrafine-grained black fault rocks (BFRs) in the Pasagshak Point Thrust of the Kodiak accretionary complex are examples of fault rocks that have recorded seismicity along an ancient subduction plate boundary. Trace element concentrations and 87Sr/86Sr ratios of BFRs and surrounding foliated/non-foliated cataclasites were measured to explore the nature of fluid-rock interactions along a subduction thrust. Foliated and non-foliated cataclasites do not show significant geochemical anomalies, suggesting that they were formed by slowly distributed shear. BFRs are characterized by Li and Sr enrichment, Rb and Cs depletion, and a low 87Sr/86Sr ratio. These geochemical signatures can be explained by fluid-rock interactions at >350°C, which result in preferential removal of Rb and Cs and formation of plagioclase under the presence of fluids with high Li and Sr concentrations and low 87Sr/86Sr ratios. Geochemical anomalies recorded by the BFRs indicate both frictional heating and external fluid influx into the subduction thrust

Mixed brittle and viscous strain localisation in pelagic sediments seaward of the Hikurangi margin, New Zealand

Calcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ~20 km seaward of the Hikurangi megathrust toe at Site U1520 during IODP Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at 150°C where frictional (possibly seismic) slip likely predominates

Quartz vein formation by local dehydration embrittlement along the deep, tremorgenic subduction thrust interface

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara belt. Here, subduction-related deformation occurred at temperatures ≤550 °C, and vein geometry is consistent with plate interface shear, low effective normal stresses, and mixed-mode deformation. Quartz vein δ18O values relative to Standard Mean Ocean Water (SMOW) range from 9.4‰ to 17.9‰ (n = 30), consistent with precipitation from metamorphic fluids. A dominant subset of quartz veins away from long-lived high-strain zones and basaltic slivers have δ18O values in a smaller range of 14.9‰ ± 1‰, requiring precipitation from a fluid with δ18O of 12‰ ± 1‰ at 470–550 °C. This uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime. In active subduction zones, brittle deformation within the plastic regime is inferred from observations of tectonic tremor, a noise-like seismic signal including overlapping low- and very low-frequency earthquakes, which occurs below the seismogenic zone. Both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure, could represent a mixture of shear and dilatant failure, and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime

Natural attenuation can lead to environmental resilience in mine environment

Four streams flowing in the Iglesiente and Arburese mine districts (SW Sardinia, Italy), exploited for zinc (Zn) and lead (Pb) extraction from sulphides and secondary non-sulphide mineralization (calamine ores), have been studied combining investigations from the macroscale (hydrologic tracer techniques) to the microscale (X-ray powder diffraction, scanning electron microscopy, X-ray absorption spectroscopy). In the investigated area, concerns arise from release of metals to water during weathering of ore minerals and mine-waste. Specifically, Zn is observed at extremely high concentrations (10s of mg/L or more) in waters in some of the investigated catchments. The results from synoptic sampling campaigns showed marked differences of Zn loads, from 6.3 kg/day (Rio San Giorgio) to 2000 kg/day (Rio Irvi). Moreover, natural attenuation of metals was found to occur i) through precipitation of Fe compounds (Fe oxy/hydroxides and “green rust”), ii) by means of the authigenic formation of metal sulphides promoted by microbial sulphate reduction, iii) by metal intake in roots and stems of plants (Phragmites australis and Juncus acutus) and by immobilization in the rhizosphere, and iv) by cyanobacterial biomineralization processes that lead to formation of Zn-rich phases (hydrozincite and amorphous Zn-silicate). The biologically mediated natural processes that lead to significant abatement and/or reduction of metal loads, are the response of environmental systems to perturbations caused from mine activities, and can be considered part of the resilience of the system itself. The aim of this study is to understand the effect of these processes on the evolution of the studied systems towards more stable and, likely, resilient conditions, e.g. by limiting metal mobility and favouring the improvement of the overall quality of water. The understanding of how ecosystems adapt and respond to contamination, and which chemical and physical factors control these natural biogeochemical barriers, can help to plan effective remediation actions

Evidence of seismic slip on a large splay fault in the Hikurangi subduction zone

The Hikurangi subduction zone is capable of producing moderate to large earthquakes as well as regularly repeating slow slip events. However, it is unclear what structures host these different slip styles along the margin. Here we address whether splay faults can host seismic slip at shallow (1 m as observed in the 1947 Poverty and Tolaga Bay earthquakes