Subduction zone fluids and arc magmas conducted by lithospheric deformed regions beneath the central Andes

Dehydration of the oceanic subducting slab promotes the formation of magmatic arcs, intra-slab intermediate-depth seismicity, and hydration of the overlying mantle wedge. However, the complex permeability structure of the overriding plate controls the magma and fluid migration and their accumulation at shallower depths. In this regard, mapping the inner structure of the overriding crust and mantle is crucial to understand the magmatic and hydrological processes in subduction zones. We integrate 3-D P-wave, $$V_p/V_s$$ V p / V s , and electrical resistivity tomographic models of the northern Chilean subduction zone to map the magmatic and fluids derived from the subducting oceanic Nazca plate. Results show a continental crust relatively thick (50–65 km) characterized by a lower zone of high $$V_p$$ V p values (7.2–7.6 km/s), which is interpreted as the presence of plutonic rocks. The mantle lithospheric wedge is weakly hydrated ($$V_p/V_s$$ V p / V s = 1.75–1.8) while the forearc continental crust is traversed by regions of reduced electrical resistivity values ($$< 10^2$$ < 10 2 $$\Omega m$$ Ω m ) interpreted as zones of relatively high permeability/fracturing and fluid content. These regions spatially correlate with upper plate trans-lithospheric deformation zones. Ascending melts accumulate preferentially in the back-arc, whereas hydrothermal systems form trenchward of the volcanic arc. The results highlight the complex permeability structure of the upper South American plate.

Architectural Diversity of Submarine Lobate Deposits

Lobate deposits in deep-water settings are diverse in their depositional architecture but this diversity is under-represented in the literature. Diverse architectures result from multiple factors including source material, basin margin physiography, transport pathway, and depositional setting. In this contribution, we emphasize the impact of differing source materials related to differing delivery mechanisms and their influence on architecture, which is an important consideration in source-to-sink studies. Three well imaged subsurface lobate deposits are described that display three markedly different morphologies. All three lobate examples, two from intraslope settings offshore Nigeria and one from a basin-floor setting offshore Indonesia, are buried by less than 150 m of muddy sediment and are imaged with high resolution 3D reflection seismic data of similar quality and resolution. Distinctively different distributary channel patterns are present in two of the examples, and no comparable distributaries are imaged in a third example. Distributary channels are emphasized because they are objectively recognized and because they often represent elements of elevated fluid content within buried lobate deposits and thus influence permeability structure. We speculate that the different distributary channel patterns documented here resulted from different processes linked to source materials: 1) a lobate deposit that is pervasively channelized by many distributaries that have branched at numerous points is interpreted to result from comparatively mud-rich, stratified, turbulent flows; 2) an absence of distributaries in a lobate deposit is interpreted to result from collapse of mud-poor, turbulent flows remobilized from littoral drift; and 3) a lobate deposit with only a few, long, straight distributaries with few branching points is interpreted to be dominated by highly viscous flows (i.e., debris flows). We propose a conceptual model that illustrates the relationship between the proportion of mud in contributing flows and the relative size and runout distance of lobate deposits. We conclude that reconciling 3D seismic morphologies with outcrop observations of channels, scours, and amalgamation zones, and simple application of hierarchical schemes, is problematic. Furthermore, when characterizing unconfined deep-water deposits in the subsurface, multiple models with significant differences in predicted permeability structure should be considered.

Development of spatial permeability variations in English chalk aquifers

The Cretaceous Chalk in England forms dual-porosity aquifers, with low-permeability matrix and high-permeability network of fissures, which are predominantly stress-relief fractures that have been enlarged by dissolution. This enlargement is a function of the volume of water that has passed along a fracture (the flowrate effect) and its degree of chemical undersaturation. Feedback effects result in the development of a distinctive permeability structure, with four particular characteristics: i) troughs in the water table with high transmissivity and convergent groundwater flow; ii) substantial increases in transmissivities in a downgradient direction; iii) downgradient decreases in hydraulic gradient; and iv) discharge from the high-transmissivity zones to the surface commonly at substantial springs. This distinctive self-organised permeability structure occurs throughout unconfined chalk aquifers. Early enlargement of fissures at a depth of 50 - 100 m below the water table is slow, but is much more rapid close to the water table and in the uppermost bedrock due to non-linear dissolution kinetics. A modelled dissolution profile shows that more than 95% of dissolution takes place in the top 1 m of bedrock, and that enlargement of fissures in the saturated zone results from progressive dissolution occurring over a period of a million years or more.

Influence of fluid-assisted micro-crack healing on fault permeability structure

&lt;p&gt;Micro-cracks in fault damage zones can heal through diffusive mass transfer driven by differences in chemical potential, with rates controlled by temperature and pressure. The diffusion of pore fluid pressure in fault damage zones accelerates mass diffusion and assists healing processes. In this work, we use fluid flow model coupled with heat transfer and crack healing to investigate, through different scenarios, the role of subsurface warm fluid migration, along damage zones, in enhancing healing and re-shaping the fault permeability structure. Our results show that if the flow communication exists between the bed and only one side of the damage zone and not the other side, it leads to an asymmetric permeability structure caused by healing in the side circulated by fluids (ex: Rapolano geothermal area, Italy). Another scenario is when the damage zone adjacent to the fault core is not the interval with the highest permeability, as conventionally expected, which is the case of the Alpine Fault, New Zealand. As shown by our simulations, this can be due to healing by diffusive mass transfer, favored by the localized high geothermal gradients and the upward fluid migration through the fault relay structure.&lt;/p&gt;