Amagmatic geothermal systems within regional-scale orogenic faults are promising renewable resources for heat and possibly electricity production. However, their behavior needs to be better understood to improve exploration and assessment of their energy potential. To provide more insight, we report a geochemical, geological, and geophysical study of seven hot spring sites strung along a 90 km segment of the Agua Blanca Fault (ABF), which traverses a mountainous region of northern Baja California, Mexico. Our results show that topographic heads drive infiltration of meteoric water deep into basement rocks, where it is heated according to the local geothermal gradient (15–24 °C/km). Our diverse dataset provides strong evidence that the flow system, magnitude, and location of the thermal anomalies are primarily controlled by the permeability of the ABF system and the hydraulic head gradients. The hot water ascends along preferentially permeable zones, discharging at temperatures from 37 °C in inland springs to 102 °C at the Pacific coast. Higher temperatures correlate positively with the degree of extensional fault displacement (a proxy for fault permeability). Correlations between hydraulic head gradients, residence times, and 3He/Hetotal of the thermal waters show that the hydraulic head gradient controls the length and depth of the flow paths. Long paths to great depths lead to long water residence times and high 3He/Hetotal fractions. Optimal conditions at the coast allow the 120 °C temperature threshold for electricity production to be reached at relatively shallow depths (< 4 km), demonstrating the potential of orogenic geothermal systems for petrothermal exploitation.
<p>Non-magmatic, orogenic geothermal systems are recognized as significant energy resources for electricity production or direct uses. This study focuses on the non-magmatic geothermal system hosted by the Agua Blanca fault, Ensenada, Mexico. The Agua Blanca fault is a 140 km long transtensional structure with segments recording up to 11 km of dextral strike-slip displacement and normal throws of up to 0.65 km. We have identified at least seven geothermal areas manifested by hot springs discharging at temperatures ranging from 38 &#176;C to 107 &#176;C. These systems involve topography-driven infiltration of meteoric water deep into the Agua Blanca fault and exfiltration of the heated water at valley floors and along a local beach known as La Jolla.</p><p>For this contribution, we present recent and ongoing exploration activities aiming to (i) obtain a fundamental understanding of the governing thermal-hydraulic-chemical processes controlling the circulation of meteoric water in the hydrothermally active fault system and (ii) quantify the natural discharge rate and its respective advective heat output. Chemical and isotopic analyses of thermal springs and seismic epicenters' location reveal that meteoric water penetrates between 5 to 10 km deep into the brittle orogenic crystalline basement and thereby attains temperatures between 105 and 215 &#176;C. Interestingly, the deepest circulation and hottest reservoir temperatures occur where the extensional displacement along the fault shows maximum values. However, our data provide no evidence that meteoric water infiltrates beyond the brittle-ductile zone in the crust (12-18 km).</p><p>For the La Jolla beach thermal area, we have quantified the advective heat output from thermal images acquired with an unmanned aerial vehicle equipped with a thermal camera and from water flow and direct temperature measurements. The total thermal water discharge is 330 &#177; 44 L s<sup>-1</sup> and occurs over a surface area of 2804 m<sup>2</sup> at temperatures up to 52 &#176;C. At 20 cm depth, the temperature is as high as 93 &#176;C. These observations collectively imply a current heat output of 40.5 &#177; 5.2 MW<sub>t </sub>(Carbajal-Mart&#237;nez et al., 2020). We are currently estimating the shape and magnitude of the subsurface thermal anomaly at La Jolla beach by performing coupled thermal-hydraulic-chemical simulations using the code Toughreact.</p><p>We conclude that meteoric water circulation through the Agua Blanca fault system reflects the interplay between the permeability distribution along the fault system and the rugged regional topography. Under ideal conditions such as at La Jolla beach, such circulation generates rather large thermal outputs that could supply the thermal energy for a multi-effect distillation desalinization plant and contribute to cover the shortage of fresh water in Ensenada.</p>
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