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

Source-depth estimations based on analysis of gravity data enabled us to es tablish the basement topography in the area of the Mexicali Valley (Mexico) . Analysis of the radial power spectrum from all the Bouguer gravity anomal y 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 correspon ds to the sedimentary basin. The gravity spectrum was analyzed to estimate the depth to the basement in different square sectors (windows) of the stud y area. Linear regression analysis was used to calculate the slopes of the respective power spectrums, to subsequently estimate the depths to the base ment in each sector. The basement topography obtained in this way ranged fr om 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 t he studied area. An independent estimation of the mean depth to the basement was obtained ba sed on the ideal body theory. In particular trade-off curves relating the l ower bound of the density contrast to the depth to the top of the geologica l interface were computed. If we assume that the sediments outcrop (as is a ctually the case), the minimum lower bound on the density contrast is 0.070 0 g/cm(3). This result would imply a maximum thickness of 13.5 km for the s edimentary infill. Seismic velocities of 5.83 and 4.9 km/s for the basement and the sedimentar y infill, respectively, indicates densities of 2.86 and 2.56 g/cm(3) accord ing to the Nafe and Drake's relationship between seismic velocities and den sities. The corresponding density contrast of 0.3 g/cm(3) helped us to cons train the analysis of the trade-off curves accordingly; the sedimentary thi ckness is of approximately 3.5 km. This result is in agreement with that ob tained from our spectral analysis. (C) 1999 Elsevier Science Ltd. All right s reserved.