Basement topography of the Mexicali Valley from spectral and ideal body analysis of gravity data R.E. ChaÂvez a, *, O. Lazaro-Mancilla b, 1 , J.O. Campos-EnrõÂquez a , E.L. Flores-MaÂrquez a a Instituto de GeofõÂsica, UNAM, Ciudad Universitaria, 04510 Me Âxico D.F., Mexico b Facultad de IngenierõÂa, UNAM, Division de Ciencias de la Tierra, 04510 Me Âxico D.F., Mexico 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 dierent 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-o 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/cm 3 . This result would imply a maximum thickness of 13.5 km for the sedimentary in®ll. Seismic velocities of 5.83 and 4.9 km/s for the basement and the sedimentary in®ll, respectively, indicates densities of 2.86 and 2.56 g/cm 3 according to the Nafe and Drake's relationship between seismic velocities and densities. The corresponding density contrast of 0.3 g/cm 3 helped us to constrain the analysis of the trade-o curves accordingly; the sedimentary thickness is of approximately 3.5 km. This result is in agreement with that obtained from our spectral analysis. # 1999 Elsevier Science Ltd. All rights reserved. 1. 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 geo- physical studies to assess its geothermal potential. The ®rst commercial geothermal plant generating electric power within that region has been tapping energy from the Cerro Prieto geothermal ®eld. It is located on a complex boundary between the North American and Paci®c plates (Elders et al., 1972). Right-lateral relative motion is distributed among a series of active en-eche- lon 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 sedimen- tation, marine intrusions and large-scale strike-slip faulting, as well as alteration of the sedimentary layers. Puente and de la PenÄa (1979) have divided the deltaic sedimentation into two lithostratigraphic units: (a) unconsolidated Quaternary deltaic sediments com- prised of clays, sands, and gravels, and (b) consoli- Journal of South American Earth Sciences 12 (1999) 579±587 0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(99)00041-3 * Corresponding author. E-mail address: exprene@tonatiuh.igeofcu.unam.mx (R.E. ChaÂvez). 1 Currently at Division de Ciencias de la Tierra, CICESE, B.C.N., MeÂxico 22890