New Mexico Geological Society Annual Spring Meeting — Abstracts

The Sandia Mountains are an eastward-tilted fault block of Precambrian rocks situated along the eastern margin of the Rio Grande Rift in central New Mexico. They are considered a "classical" example of tilt-block mountain building, but represent an enigma typical of mountain ranges formed in extensional environments. Most of the uplift of the Sandias has taken place in the last 15 my, coincident with the opening of the Rio Grande Rift. The present elevation and uplift rate are both much higher than that for surrounding terrain. The Sandias are too small to be the result of an upper mantle driving force, nor has there been significant emplacement of Cenozoic magmatic rocks to build a recent crustal root. We propose that the uplift of the Sandia Mountains can be explained by flexural isostasy.

We propose a three part geologic history for the sandias: (a) Precambrian magmatism resulted in a locally compensated proto-Sandia mountain range. The batholith cooled at a rate higher than the erosional rate, and eventually the underlying lithosphere flexurally suppresses the excess mountain root. (b) Late Paleozoic and Mesozoic sediments on the top of the Sandias indicate that the Sandias were a low relief, regional topographic high until at least mid-Cenozoic times. The low relief is consistent with a flexural thickness of 15 km (or greater). (c) The lithospheric thickness of 15 km was intact until approximately 15 mya. The onset of extension along the Rio Grande rift resulted in thinning, and finally breaking the lithosphere along the Sandia and Rincon faults, allowing the expression of the Precambrian root. The Sandias could have a root as thick as 7 km, but because the mountain range is spatially small it does not have a significant Bouguer gravity signature.