USING NONSEISMIC GEOPHYSICAL METHODS TO DETERMINE STRUCTURAL CONTROLS WITHIN THE SOUTHERN MESILLA BOLSON, WEST TEXAS AND SOUTHERN NEW MEXICO

The southern Mesilla Bolson is a region of structural complexity within the Rio Grande rift where rift related faults interact with the Laramide age Cristo Rey laccolith, southern end of the Franklin Mountains and the complexly folded/faulted rocks of the Sierra de Juarez. We have been using microgravity and magnetic studies to help identify changes in bedrock geometry of the southern Mesilla Bolson. This is essential to understanding the recent tectonic history of the southern Rio Grande rift and how structures influence the present day regional hydrology. Prior gravity studies have shown the presence of at least 3 faults that parallel the north-south strike of the Franklin Mountains producing a series of down dropped blocks between the mountains and the present course of the Rio Grande. Well logs, deep resistivity studies, and ground magnetic studies, in addition to recent microgravity surveys, suggest the presence of several northwest-southeast striking cross faults as well as at least one dike related to the emplacement of the Cristo Rey laccolith that complicate this simple geometry. The dike outcrops on the western bank of the Rio Grande near Sunland Park, NM, and we are using this outcrop to calibrate our geophysical methodology and assess its effectiveness in locating these important cross-cutting features in an urban environment.