Geometry of the El Fresnal basin, northern Chihuahua, Mexico, as inferred from three-dimensional gravity modeling

Abstract

A multidisciplinary study of the El Fresnal basin, Chihuahua, Mexico is being conducted to investigate the relationships between the structure, vegetation and geomorphology and the Quaternary paleoenvironment. As part of this study, 221 new gravity measurements were collected within the basin and combined with 506 preexisting measurements from the surrounding area to determine the basin geometry and the depth of the sediment infill. A basement-depth model calculated from these data indicates that the El Fresnal basin consists of two sub-basins separated by a basement high, which is presently buried by about 500 meters of sediments. The southern sub-basin, which lacks an axial playa lake, extends for 20 km in a N21oW direction and contains 800 meters of sediments. The geometry of the sub-basin is suggestive of a half-graben with the major normal fault located to the west. The northern sub-basin, within which lies the playa lake Laguna El Fresnal, extends for 20 km in a N10oE direction and contains 1500 meters of sediments. Like the southern sub-basin, its geometry is suggestive of a half-graben; however, unlike the southern sub-basin, the major normal fault is located to the east. In conjunction with previous studies of other rift basins comprising the Río Grande rift, the orientations of the two sub-basins suggest that the southern sub-basin formed first, during the late Oligocene to early Miocene, followed by the formation of the northern sub-basin sometime after the early Miocene when the orientation of the tensional stresses within the rift changed from WNW-ESE to E-W. Since the playa lake is now confined to the northern sub-basin, it appears that subsidence is presently occurring only in the northern sub-basin.