Basement topography of the Mexicali Valley from spectral and ideal body analysis of gravity data

doi:10.1016/S0895-9811(99)00041-3

Journal of South American Earth Sciences

Volume 12, Issue 6, November 1999, Pages 579-587

https://doi.org/10.1016/S0895-9811(99)00041-3 Get rights and content

Abstract

Source-depth estimations based on analysis of gravity data enabled us to establish the basement topography in the area of the Mexicali Valley (Mexico). Analysis of the radial power spectrum from all the Bouguer gravity anomaly data indicates that the intermediate wave number interval ranging between 0.025 km⁻¹ and 0.112 km⁻¹ with a mean source depth of 3.5 km corresponds to the sedimentary basin. The gravity spectrum was analyzed to estimate the depth to the basement in different square sectors (windows) of the study area. Linear regression analysis was used to calculate the slopes of the respective power spectrums, to subsequently estimate the depths to the basement in each sector. The basement topography obtained in this way ranged from 2.1 to 4.5 km. Our basement topography is consistent with the depths to the basement reported from wells drilled in the study area. The basement is formed by granites to the northeast, dikes to the southwest, and shaped by structural lows and highs, with graben-horst structures at the center of the studied area.

An independent estimation of the mean depth to the basement was obtained based on the ideal body theory. In particular trade-off curves relating the lower bound of the density contrast to the depth to the top of the geological interface were computed. If we assume that the sediments outcrop (as is actually the case), the minimum lower bound on the density contrast is 0.0700 g/cm³. This result would imply a maximum thickness of 13.5 km for the sedimentary infill.

Seismic velocities of 5.83 and 4.9 km/s for the basement and the sedimentary infill, respectively, indicates densities of 2.86 and 2.56 g/cm³ according to the Nafe and Drake’s relationship between seismic velocities and densities. The corresponding density contrast of 0.3 g/cm³ helped us to constrain the analysis of the trade-off curves accordingly; the sedimentary thickness is of approximately 3.5 km. This result is in agreement with that obtained from our spectral analysis.

Introduction

The province of the Imperial and Mexicali valleys on both sides of the Mexico–USA international border has been the subject of extensive geological and geophysical studies to assess its geothermal potential. The first commercial geothermal plant generating electric power within that region has been tapping energy from the Cerro Prieto geothermal field. It is located on a complex boundary between the North American and Pacific plates (Elders et al., 1972). Right-lateral relative motion is distributed among a series of active en-echelon faults, which are linked by extensional basins at spreading centers (Fig. 1). Within the Mexicali Valley, the Salton Trough and the Cerro Prieto faults belong to this echelon fault-basin system.

The Mexicali Valley (Fig. 2) has been formed by a combination of rifting, rapid Cenozoic deltaic sedimentation, marine intrusions and large-scale strike-slip faulting, as well as alteration of the sedimentary layers. Puente and de la Peña (1979) have divided the deltaic sedimentation into two lithostratigraphic units: (a) unconsolidated Quaternary deltaic sediments comprised of clays, sands, and gravels, and (b) consolidated Tertiary deltaic sediments formed by siltstones, shales and sandstones. However, Lippman (1983) argued that such a division is misleading, and suggested an induration boundary cutting across the sedimentary layer. This implies a post-depositional alteration, at least near the Cerro Prieto geothermal field.

Fuis and Koheler (1984) based on seismic and gravity studies were able to establish a geophysical model for the Imperial Valley, north of the Mexico–USA boundary. In contrast, from a regional point of view, very little is known about the buried topography of the basement beneath the Mexicali Valley. As a contribution to the regional study of the Mexicali Valley sub-province, we have established the basement topography of the Mexicali Valley from Bouguer gravity data. The spectral factorization method (Spector and Grant, 1970) is employed to estimate the depth to the basement. Ideal body theory (Parker, 1974) constrained by seismic velocity analysis was also applied to compute bounds on the physical characteristics of the buried sediment–basement, as well as an independent estimate of the depth to the basement.

Section snippets

Geophysical background

De la Fuente and Sumner (1972) elaborated a preliminary basement model for a southern portion of the Mexicali Valley area from aeromagnetic data. They inferred lateral susceptibility changes (1.9×10⁻³ to 8.2×10⁻³ SI) within the basement, at depths from 1.5 to 5 km. Such authors associated magnetic variations with the presence of metamorphic rocks similar to those of the Cucapah Ranges to the west of our study area. Lower values of magnetic susceptibility might be associated with infill material

Gravity data analysis

The Bouguer anomaly map (Fig. 3) used in this study was elaborated from a set of different surveys carried out by Mexican and international organizations. This database was compiled by Espinoza (1988) and covers an area of 6400 km². The gravity data have all been referenced to the absolute gravity value of the station located in the international airport of the City of Mexicali.

This map shows a distinctive pattern (Fig. 3). Its central part presents a NW–SE trend with smooth gradients flanked

Conclusions

Spectral analysis based on window process was used to estimate average depths to the sediment–basement interface. The obtained basement topography indicates a series of graben-horst structures, characterized by low and high structures with a NW–SE trend, and a mean depth of 3.5 km, approximately. This trend corresponds to that found by Fuis and Koheler (1984) in the neighboring Imperial Valley. This result is in agreement with the extensional character of the sedimentary basin of the Mexicali

Acknowledgements

We are indebted to Dr. C. Flores-Luna (CICESE, México) for his support during this research. We thank M.Sc. J. M. Espinoza (CICESE, México) for allowing us to access his gravity database permitting the realization of this study. This project was partly funded by CONACyT (México) via an award program to O.L.M. while on sabbatical leave at Ensenada, and a DGAPA-IN103496 (UNAM, México) research project.

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¹: Currently at Division de Ciencias de la Tierra, CICESE, B.C.N., México 22890

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Basement topography of the Mexicali Valley from spectral and ideal body analysis of gravity data

Abstract

Introduction

Section snippets

Geophysical background

Gravity data analysis

Conclusions

Acknowledgements

Geothermics

Computers and Geosciences

Physics of the Earth and Planetary Interiors

Spectral Analysis in Geophysics

Estudio aeromagnético del delta del Rio Colorado, B.C., México

Geofı́sica Internacional

Crustal spreading in southern California

Science

Crustal structure and tectonics of the Imperial valley region, California

Analysis of the Nuevo Leon magnetic anomaly and its possible relation to the Cerro Prieto magmatic-hydrothermal system

Geothermics