A MICROGRAVITY SURVEY TO DETERMINE THE EXTENT OF AN ANDESITIC DIKE THAT INTRUDES ACROSS THE RIO GRANDE RIVER BASIN, RIO GRANDE RIFT VALLEY, SUNLAND PARK, NEW MEXICO

In Sunland Park, NM, there is an andesite outcrop near the bank of the Rio Grande (called the River Andesite) which does not match the surrounding sedimentary deposition. Studies of the River Andesite by Garcia (1970) indicate the outcrop is petrologically similar to the Muleros Andesite of Mt. Cristo Rey located several km to the south. A limited GPR and magnetic survey conducted by UTEP students in 2008 suggested the River Andesite is part of a dike, although Garcia mapped smaller outcrops of andesite ~700 feet west of the river that may be part of the same body. An attempt was made to locate these western outcrops in 2008, but urbanization appears to have erased all traces of the outcrops. With this in mind, we have conducted a series of microgravity survey to determine the length, width, and if possible, depth extent of the dike. Initial data collected in December 2011 showed that the andesite body was much larger than expected, and we increased our survey spacing from 2 meters to 10 meters. In February 2012, data were collected across the river in order to correlate with a magnetics survey conducted in the same area. These results suggest the dike extends across the river and is more than two meters wide. The goal of the survey is to determine how large this dike is and how it may impact future development, groundwater flow and flooding in the area.