Laramide Faults and Stress Distribution in Front Range, Colorado: ABSTRACT

Abstract

The Front Range of Colorado is a large uplift about 180 miles long and 40 miles wide. Precambrian rocks along the crest of the range are three to five miles above the basement rocks of the adjacent Denver basin. The eastern flank of the Front Range is marked by faults with large vertical displacements or by steep monoclinal folds, so that the change in elevation of the Precambrian surface takes place in a relatively narrow belt. South of Denver, large Laramide faults, upthrown to the west, place Precambrian rocks in contact with sediments as young as Tertiary in age. Stratigraphic displacement ranges up to 15,000 feet. An analysis of sandstone dikes in the upthrown blocks leads to the conclusion that the stress distribution causing the injection of the dikes is governed by dip-slip movement along steeply westward dipping, convex upward fault surfaces. Therefore, the major structures outlining the flank of the range south of Denver are high-angle reverse faults whose dips steepen with depth. Other large reverse faults whose dips probably also steepen with depth are found at Golden and Boulder and along the west flank of the range. These faults are bordered in the downthrown block by a narrow belt of steeply dipping or overturned faulted and fractured sediments. Any undiscovered petroleum accumulations associated with the Front Range are probably limited to this narrow belt, but the structural complex ty makes the location of such reserves a difficult task. If elastic theory and model experiment work can be extrapolated to a large crustal block of complex composition, faults of the type observed may be formed only by vertical normal stresses arranged to create a step-like displacement or an unbroken upwarp along the bottom surface of the block. Horizontal normal and End_Page 413------------------------------ shear stresses are only incidental to maintaining equilibrium conditions. The Front Range, judged from the configuration of the associated major faults where they are best known, is more likely the result of vertical uplift of the crust than of horizontal compression. Other ranges in the eastern Rocky Mountains appear to have similar structural origins. An understanding of stress distributions related to vertical uplift may aid in the interpretation of potential oil-producing structures related to these ranges. End_of_Article - Last_Page 414------------