Three-dimensional structure of the Laguna Salada Basin and its thermal regime

A comprehensive reinterpretation of the available gravity, magnetic, geothe rmal, geological and borehole information has been made of the Laguna Salad a Basin to establish a 3D model of the basement and sedimentary infill. Acc ording to statistical spectral analysis, the residual gravity anomaly is du e to sources with a mean regional depth of 2.8 km. The topography of the ba sement was obtained from a three-dimensional inversion carried out in the w avenumber domain using an iterative scheme. The maximum density contrast of -300 kg/m(3) estimated from previous studies and the mean depth of 2.5 km finally constrained this inversion. The resulting model indicated that the sedimentary infill is up to 4.2 km thick at its deepest point. According to the gravity-derived basement topography, the basin presents an asymmetry ( i.e. it is of the half-graben type). It is deeper to the east, where it is delimited from the Sierra Cucapah by a step fault. By contrast, the limit w ith the Sierra de Juarez is a gently sloping fault (i.e. a listric fault). The basement is not even, but it comprises a series of structural highs and lows. N-S to NW-SE and E-W to NE-SW faults delimit these structural units. The magnetic modelling was constrained by (i) the gravity-derived basement topography; (ii) a Curie isotherm assumed to be between 7 km and 10 km; (ii i) assuming induced magnetization only; (iv) the available geological and b orehole information. The magnetic anomalies were interpreted successfully u sing the gravity-derived basement/sedimentary interface as the top of the m agnetic bodies (i.e. the magnetic modelling supports the gravity basement t opography). An elongated N-S to NW-SE trending highly magnetized body runni ng from south to north along the basin is observed to the west of the basin . This magnetic anomaly has no gravity signature. Such a feature can be int erpreted as an intrusive body emplaced along a fault running through the La guna Salada Basin. Treatment of the gravity and magnetic information (and o f their horizontal gradients) with satellite image processing techniques hi ghlighted lineaments on the basement gravity topography correlating with ma pped faults. Based on all this information, we derived detailed geological models along four selected profiles to simulate numerically the heat and fl uid flow in the basin. We used a finite-difference scheme to solve the coup led Darcy and Fourier differential equations. According to our results, we have fluid flow in the sedimentary layers and a redistribution of heat flow from the basin axis toward its rims (Sierra de Juarez and Sierra Cucapah). Our model temperatures agree within an error of 4% with the observed tempe rature profiles measured at boreholes. Our heat-flow determinations agree w ithin an error of +/- 15% with extrapolated observations. The numerical and chemical analyses support the hypothesis of fluid circulation between the clay-lutite layer and the fractured granitic basement. Thermal modelling sh ows low heat-flow values along the Laguna Salada Basin. Deep fluid circulat ion patterns were observed that redistribute such flow at depth. Two patter ns were distinguished. One displays the heat flow increasing from the basin axis towards its borders (temperature increase of 20 degrees C). The secon d pattern shows an increasing heat flow from south to north of the basin. S uch behaviour is confirmed by the temperature measurements in the thermomet ric boreholes.