Extension in geothermal fields between the Imperial and Mexicali Valleys revealed by 2D seismic imaging and joint gravity-aeromagnetic modeling

Highlights

• The seismic reflection method has been little used in volcanic environments and the exploration of geothermal resources.

• 2D gravimetric and aeromagnetic models constrained by seismic data showing zones of sediments metamorphosed.

• three domains for future geothermal exploration in the Mexicali Valley.

Abstract

We present 2D seismic reflection images covering 64 km along with 62.5 km of gravity and aeromagnetic models. The modeled region extends from the Cerro Prieto volcano, across the Mexicali Valley, Baja California, Mexico, to the USA-Mexico border near Pilot Knob, California, USA. The seismic images illustrate the shallow structure to the north of the Cerro Prieto basin, and a seismic reflector in the northeastern part of the profiles corresponds to the metamorphic basement. Some major faults were identified through discontinuities in the reflectors (Dunes and San Luis faults), while others (Michoacán, Imperial, and Calipatria faults) are expressed as chaotic low-amplitude anomalies in the seismic images. 2D gravimetric and aeromagnetic models constrained by seismic data demonstrate the presence of zones of sediments metamorphosed by hydrothermal alteration fronts due to major faults and intrusions. We present for the first time a geologic model supported by geophysical data that cover transversely the entire Mexicali Valley. Finally, we identify three domains for future geothermal exploration in the Mexicali Valley, supported by the analytic signal of the aeromagnetic data after reduction to the pole and seismic reflection profiles from previous papers.

Introduction

The Mexicali Valley region is located within the Salton Trough, a structure that is part of the transtensional boundary between the Pacific and North American Plates (Ramírez-Ramos et al., 2015; Lin, 2020). The Imperial and Cerro Prieto dextral faults are the major faults in this region and have an NW-SE strike. The regional tectonic regime has been related to the opening of the Gulf of California over the last 12 Ma. This led to a transform fault system along the plate boundary, where overlap of the faults creates spreading centers in the southern part of the Gulf and pull-apart basins in the north of the Gulf and the Mexicali and Imperial Valleys. These basins have been filled with Colorado River sediments deposited in the last 6 Ma that was quickly buried and metamorphosed, creating a new crust (Dorsey, 2010; Chanes-Martínez et al., 2014; Han et al., 2016). Thus, the Mexicali and Imperial valleys share the same geological-geophysical framework, with a change in the name at the international Mexico-USA border (Fig. 1).

As a consequence of its tectonic regime, the Mexicali Valley is characterized by several geothermal manifestations closely related to the main fault systems (Arango-Galván et al., 2015), being the most important the Cerro Prieto Geothermal Field (CPGF) as well as Tulecheck and Riíto geothermal zones (Fonseca et al., 1981; Arango-Galván et al., 2015). On the other hand, the Imperial Valley contains Heber, and East Mesa geothermal fields, located close to the México-USA Internarial border (Fig. 1), and further north these fields, Brawley, Dunes, off the map of Fig. 1. Dobson (2016) classified the Imperial Valley fields as hidden geothermal systems, which lack surface thermal features and were identified accidentally during explorations for water, oil, and gas. Lack of surface thermal evidence implies that a combination of geophysical methods needs to be applied to locate and characterize these systems (Garg et al., 2010).

The Heber and East Mesa geothermal fields (Fig. 1) have been studied to a greater extent applying diverse geophysical methods. Geophysical characteristics related to these fields are a gravity high with no magnetic anomaly (Salveson and Cooper, 1979; Goldstein and Carle, 1986). In some cases, seismic reflection profiles show a poorly reflective zone (PRZ), which is well correlated with the gravity and temperature anomalies of 165 °C, and also suggesting the presence of sills or igneous intrusion (van de Kamp et al., 1978; Goldstein and Carle, 1986, Kell, 2016). The absence of reflection and amplitude in seismic profiles has been associated with fronts of thermal alteration and geothermal fluids (Sahakian et al., 2016; Sena-Lozoya et al., 2020). A PRZ is attributed in part to the densification what does it mean porosity loss of the sediments by hydrothermal alteration. This effect is observed in seismic reflection profiles in different places of Mexicali and Imperial Valleys (e.g., Lyons and Van de Kamp, 1980; Kell, 2014; Sahakian et al., 2016; González-Escobar et al., 2016). On the other hand, this effect may appear as a positive gravity and magnetic anomalies suggest basement highs, which may or may not be associated with the hydrothermal activity, but a positive gravity anomaly coincident with a negative magnetic anomaly, or absence of a magnetic anomaly, suggest densified sediments above the basement (Lyons and Van de Kamp, 1980). A heat flow >200 mW/m², has been proposed for this region (Prol-Ledesma and Morán-Zenteno, 2019). In the Mexicali Valley, some geophysical studies have been carried out outside the Cerro Prieto Geothermal Field (Fonseca et al., 1981). Gravity and magnetic data have been used to propose alternatives for geothermal exploration and to distinguish areas such as Paredones, San Luis Rio Colorado, Lazaro Cárdenas and Riíto as related to intrusions, while Aeropuerto Oeste, Aeropuerto Este, and Lagunitas Norte are alternatively related to sediment densification due to hydrothermal alteration (Fig. 2). Actually, available seismic reflection data in the Mexicali Valley is restricted to the Cerro Prieto Geothermal Field and the western part of the valley (Lyons and Van de Kamp, 1980; Fonseca et al., 1981; González-Escobar et al., 2020). In this paper, we present a 2D seismic profile across the main structures in the valley. We have used this information to propose gravity and magnetic models that contribute to increasing knowledge about the geological-geophysical structure and its geothermal potential.