Abstract
Ba, F, Pb, Ag, Zn mineral deposits are widespread at the northern and eastern boundaries of the Aquitaine Basin. In most cases, they are hosted within high permeability carbonates that rest over the Hercynian basement and below an impermeable layer. Such a position suggests a Mississippi Valley Type (MVT) model for the formation of these deposits. This model is characterized by the lateral flow of sedimentary fluids expelled from the deeper part of the basin and mixed with other sources of water as they reach the basin boundaries. In the Jard-sur-Mer area, which sits in the north of the Basin, these deposits are also found higher in the sedimentary series suggesting that fluids have flown through the impermeable layer. Our field observations demonstrate that a brittle deformation episode, compatible with an upper-Jurassic N-S direction of extension, occurred as the mineralizing fluids were over pressured. The overpressure was the result of a large input of hydrothermal water ascending along inherited faults affecting the Hercynian basement and released at the onset of the tectonics event. When compared with the rest of the basin, these new results at the northern boundary suggests that the Aquitaine Basin recorded several stages of fluid overpressure both at the onset and during the opening of the Bay of Biscay.
Highlights
Fluid migration in sedimentary basins is a first order process that controls the formation of hydrological, geothermal, mineralGay and Migeon, 2017)
Because these structures are limited to the Hettangian to Pliensbachian unit, we propose that fluid overpressure is partly responsible for the rupture of the sedimentary cover in a state of stress compatible with the N-S to NNE-SSW direction of extension
Within fractures of the Callovian unit, they obtained temperature and salinities ranging from 120 to 220 °C and from 2–3 to 3–5 wt.% eq NaCl (Fig. 8B). Assuming this and considering fluids mixing as already proposed on the northern part of Aquitaine basin (Boiron et al, 2002; Fourcade et al, 2002; Cathelineau et al, 2012), we propose that the fluids of highest salinity values were probably present within the Hettangian to Pliensbachian units prior to faulting event, as this is not documented above in faults and veins affecting the Mesozoic series
Summary
Hydraulic fracturing can be initiated in a wide range of geological contexts: 1) as a consequence of a porosity decrease during burial (Bureau et al, 2013; Frazer et al, 2014; Suppe, 2014) or sea level rise (Gay and Migeon, 2017), 2) during fluids production associated with devolatilisation reactions (Aarnes et al, 2012), ice melting (Lelandais et al, 2016) or hydrocarbon maturation (Zanella et al, 2015), 3) as the result of heat advection or the build-up of a hydraulic head (Rice, 1992; Laurent et al, 2017), or 4) deformations (Beaudoin et al, 2014). Widely accepted geological evidence for fluids overpressures can be 1) breccias characterized by in-situ angular fragments organized in a jigsaw puzzle pattern (Sibson 1986; Jebrak, 1997; Tartarotti and Pasquaré, 2003; Chi and Xue, 2011), 2) coexisting vertical and horizontal veins (Cobbold et al, 2013; Zanella et al, 2015) and 3) decreasing fault dip angles induced by the lowering of the friction angle (Davis et al, 1983; Dahlen 1984; Mourgues and Cobbold, 2006; Collettini, 2011)
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