Extension in geothermal fields between the Imperial and Mexicali Valleys revealed by 2D seismic imaging and joint gravity-aeromagnetic modeling
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.
Section snippets
Geophysical analysis
Seismic and aeromagnetic information available for the Mexicali Valley was compiled in a database and processed as discussed in the next paragraphs. Additionally, the gravity database and the analysis by Chanes-Martinez et al. (2014), Bouguer anomalies (Fig. 2), were considered in the elaboration of the geophysical model.
Seismic reflection
Seismic profile A-A' (Fig. 1, Fig. 3) show the sedimentary package in the northern Cerro Prieto basin. A general description of the inferred lithologies from these profiles are discussed in section 3.3. The reflectors exhibit a gradual increase in dip with depth, which suggests syntectonic deposition of the sediments. They are completely penetrated by a set of faults that form a negative flower structure between the Michoacán and the Imperial faults. Major faults (Imperial and Michoacán) are
Seismic data quality
The seismic data quality of profile A-A' have sufficient resolution to show the structure and configuration of the sedimentary package, but the ground penetration is not sufficient to observe the basement. Unfortunately, the quality of the seismic data in profile B-B' does not allow the structure of the sedimentary reflectors to be visualized. These could be due, among other causes, to technological issues (energy and bandwidth of the vibroseis, the bandwidth of the geophones), noise (from
Conclusions
A 2D gravimetric and magnetic model, supported by 2D seismic data, to a crustal depth of 10 km depth is shown for the first time for the Mexicali Valley. This approach was applied to identify zones of sediment densification due to hydrothermal alteration. These layers are probably the cause of the magnetic/gravity anomaly highs and seismic anomalies. The model reveals for the first time the Morelos Basin, the Paredones Intrusion, a set of faults in the north of the Cerro Prieto Basin and the
Author statement
I am Carlos Simón Reyes-Martinez, declare that I am the author of the research and manuscript here presenting in collaboration with Mario González-Escobar, Juan Carlos Montalvo-Arrieta, Fernando Velasco Tapia and Uwe Jenchen
Also, thank the Consejo Nacional de Ciencia y tecnología (CONACyT)_for the scholarship. Financial support of the PFCE 2019 from SEP. The Landmark University Grant Program (2008-UGP- to CICESE), OpendTect V5.0 provided licenses for the use of software and Google Earth Pro
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
A scholarship from Consejo Nacional de Ciencia y Tecnología, México funded this research. Financial support of the PFCE 2019 from Secretaría de Educación Pública, México. The Landmark University Grant Program (2008-UGP-008005 to CICESE), OpendTect V5.0, provided licenses for the use of software and Google Earth Pro grant Educators. We thank Artemio Alvarado for processing data support and Sergio Arregui for technical support. Constructive comments and suggestions by two anonymous reviewers
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