Topographic and Faults Control of Hydrothermal Circulation Along Dormant Faults in an Orogen

Abstract

Hydrothermal systems involving dormant faults within orogenic belts are rarely targeted for geothermal exploration, partly because of the complexity of the 3‐D topography, the unknown permeability of the fault zones and the basement lithology, and the lack of deep‐level data. This study brings together various types of surface information (spring features, geological data, topography, and hydrochemistry) to explain the alignment of 29 hot springs (29–73 °C) along the dormant Têt fault (Eastern Pyrénées, France). Water ion concentrations, stable water isotopes, and lithium isotopic ratios indicate that (i) fluids originating from meteoric water infiltrate above an altitude of 2,000 m, (ii) the rocks interacting with the fluids are similar for all the springs, and (iii) the maximum fluid temperatures at depth show similar variations along the fault and at the surface. A 3‐D numerical model of the system, assembled from field structural data and from a digital elevation model, explores the permeability combinations for the basement and for a three‐fault network. The models indicate that for a relatively permeable basement (10−16 m2), fluids are topography‐driven down to thousands of meters (until −3,700 m) before being captured by the more permeable Têt fault. Hot spring temperatures can be numerically reproduced when fault permeability is around 10−14 m2, a value slightly lower than the critical permeability for which free convection would occur within the Têt fault. Our study shows that thermal anomalies are possible along dormant faults close to elevated topography in the core of an orogenic belt, thereby opening new perspectives for geothermal exploration.

Mapping a geothermal anomaly using apatite (U‐Th)/He thermochronology in the Têt fault damage zone, eastern Pyrenees, France

“…Significant geothermal anomalies (up to 90°C/km) are interpreted as resulting from the combination of three main factors, (a) high rock permeability, (b) substantial adjacent topography and (c) prolonged fault activity, which may transport rock and heat from depth and also generate frictional heat (Jordan et al, ; Saffer, Bekins, & Hickman, ; Sutherland et al, ). Even for a weakly active fault but adjacent to high relief (about 1.5 km escarpment), a geothermal loop can be active over a period >10 6 year (Taillefer et al, ).…”

Tracking geothermal anomalies along a crustal fault using (U-Th)/He apatite thermochronology and REE analyses, the example of the Têt fault (Pyrenees, France)

“…Stable isotope analyses of hot spring waters indicate a meteoric origin, with altitude of infiltration above 2000 meters for Thuès and St-Thomas, above 1800 meters for Vernet and in between for Llo hot springs (Petit et al, 2014). Various geothermometers (silica, chalcedony, Na/K, Na/Li, Na-K-Ca) show a range of maximum water temperature at depth between 70°C and 130°C consistent with the surface temperature of emerging water of each cluster (Krimissa, 1992;Taillefer et al, 2018). Hot springs are mainly located in the footwall DZ of the Têt fault except for Thuès-les-Bains area where 3 hot springs occur in the hanging wall too (Canaveilles, Tw=52-60°C).…”

“…Hot springs are mainly located in the footwall DZ of the Têt fault except for Thuès-les-Bains area where 3 hot springs occur in the hanging wall too (Canaveilles, Tw=52-60°C). The location of hot springs mainly in the footwall may be related to the occurrence in surface of impermeable metasediments in the hanging wall compartment (Taillefer et al, 2018) or to the fault CZ which can act as a barrier of permeability for hydrothermal fluids (e.g. Fisher and Knipe, 1998;Ballas et al, 2012).…”

“…Prats fault near St Thomas) cross-cutting or linked to the Têt fault can also increase the permeability and localised channelized fluid upflows. and Planès, that takes into account the combined effects of permeability, topography and structural discontinuities but do not consider a potential fluid contribution from the hanging wall (Taillefer et al, 2018). It shows the presence of a ~7 km-long surface thermal anomaly along the Têt fault.…”

“…Forster and Smith, 1989;McKenna and Blackwell, 2004;Taillefer et al, 2017;Sutherland et al, 2017;Volpi et al, 2017;Jordan et al, 2018;Wanner et al, 2019). Even in poorly active tectonic context, high topography gradient and fracture permeability inherent to normal faults provide good conditions to allow efficient hydrothermal flow, as shown by multiple hot springs located along faults (Taillefer et al, 2017(Taillefer et al, , 2018Wanner et al, 2019). In this case, meteoric fluids infiltrate from high reliefs in the footwall, heat up at depth and raise in areas of high permeability around the faults (Grasby and Hutcheon, 2001;Craw et al, 2013).…”