A Model for Evolution of Laramide Axial Basins in the Southern Rocky Mountains, U.S.A.

Abstract

The structural, stratigraphic, and sedimentologic development of Laramide axial basins of the southern Rocky Mountains is inconsistent with previous models relating them to transpressional tectonics. Axial basins are better explained as broad synclinal troughs in the hanging walls of large pop-up structures, in contrast to perimeter and ponded basins formed in the footwalls of these same structures. Laramide faults in the southern Rocky Mountains can be divided into the E-dipping Park Range thrust system and the W-dipping Sangre de Cristo thrust system. The W-dipping Front Range fault is the backthrust of the Park Range system, and the E-dipping Nacimiento fault is the back-thrust of the Sangre de Cristo system. The broad synclinal troughs formed between these two pairs of oppositely verging thrust systems were the locations for sediment deposition in axial basins, as exemplified by the Galisteo-El Rito basin of northern New Mexico. The Eocene Galisteo and El Rito formations were deposited on a broad syncline of Phanerozoic strata with crystalline basement along the basin margins. Most sediment was derived from the northeast, north, and northwest, except near the NE-trending Tijeras-Canoncito transfer zone along the southeast margin of the basin. Uplift of the Sangre de Cristo Range occurred first, followed by backthrusting to uplift the Sierra Nacimiento. This model explains Laramide uplifts and basins as the direct response to rapid NE-SW convergence between the North American and Farallon plates, without multiple reorientation of Laramide stress fields, and without major strike-slip along the eastern side of the Colorado Plateau.