Sandstone Dikes in Relation to Laramide Faults and Stress Distribution in the Southern Front Range, Colorado

Abstract

The Front Range of Colorado is more likely the result of vertical uplift of the crust than of horizontal shortening. This mountain range is large, 40 miles by 180 miles, and Precambrian rocks exposed along the crest of the range arc 2–4 miles above equivalent basement rocks of the adjacent basins. The flanks of the range are formed either by reverse faults with large vertical displacement or by steep monoclines. Therefore, the change in present elevation of Precambrian rocks between the crest and basins is not gradual; rather, it is concentrated in a narrow belt girding the range. The interpretation of the Front Range as a product of vertical uplift is based mainly on the presence of numerous sandstone dikes, found along the southeastern flank, which intrude Precambrian crystalline rocks that form the hanging walls of Laramide reverse faults along the mountain border. The interpretation is as follows: (1) dikes seek extension positions; (2) convex-upward reverse faults must have extension zones in the hanging wall block, whereas concave-upward reverse faults do not; (3) reverse faults of the southern Front Range must have a convex-upward form because dikes are found in the hanging wall blocks; (4) convex-upward reverse faults result from vertical uplift rather than horizontal shortening; (5) therefore, the structure is caused by vertical uplift. Other structural features of the Front Range arc compatible with an origin by vertical uplift. These are: (1) the large and abrupt elevation differences of the present Precambrian surface, (2) the large reverse faults or narrow monoclines along its margins, (3) the symmetry of the range, (4) and the relative lack of deformation in adjacent basins. Postulating a horizontal shortening of the crust would complicate interpretation of these features. Perhaps all the ranges of the eastern Rocky Mountains reflect vertical uplift rather than horizontal shortening within the crust. The relationship of these uplifts to a more universal tectonic system remains uncertain.