STYLE OF EXTENTIONAL UPLIFT ON A RIO GRANDE RIFT RANGE RECORDED BY UPLIFTED STREAM AND LAKE TERRACES

GPS measurements of ancient fluvial terraces associated with the Rio Grande that have been uplifted by the Franklin Mountains show record the deformation during the last 4 million years of uplift. The Rio Grande flowed into lakes in the El Paso region and filled the basins to nearly level plains (slope less than .001). The alluvium buried the Franklin Mountains, surrounding it with a nearly level plain. Remnants of the basin floor are now preserved as terraces on both flanks of the range. Because the surface was nearly level, it records long-term differential uplift and tilting of the range.

The oldest Rio Grande river sediments in the region have been dated using paleomagnetism reversals by Greg Mack at 3.8 million years old. These oldest sediments have been deformed the most and now form the highest terraces along the mountains. Most high terraces date to being around 4 million years old but there are several sets of lower and younger terraces. The oldest terraces on the West side of the mountains are ancient Rio Grande channels. On the east side, there was a corresponding playa lake.

On the east and west side of the Franklins’, two sets of terraces are seen. The lowest terraces on the east side have elevations of approximately 1320m and the highest sets of terraces with maximum elevations around 1385m. The lowest terraces on the west side average 1315m and the highest terraces have maximums of 1375m. The east side terraces were 30-35 meters higher than the west side. This elevation difference indicates a tilt of between .5 and 1.5 degrees. Based on a 1.5 degree tilt between the east and west terraces and the West Mesa points, an axis of rotation can be calculated at 4904m from the eastern side of the range. At the northern end of the 35 km long range, the terraces are buried under younger terraces. However, 8 km from the northern end, the terraces emerge because uplift was greater than subsequent sedimentation. The terraces gradually rise forming an arch that is highest in the middle of the range and are slightly higher on the east side. The uplift corresponds to topography and is greatest where the topography in the range is highest.