Florida Mountains, Southwest New Mexico: Part of Cordilleran Overthrust Belt or Foreland Block Uplift?: ABSTRACT

Several workers have proposed during the past 12 years that the Florida Mountains, about 15 mi (24 km) southeast of Deming, New Mexico, were involved in regional Cordilleran overthrusting. Consequently much oil and gas exploration is now based on the premise that the south Florida Mountains fault is a thrust fault that marks the northeastern edge of the belt of southwestern New Mexico. The overthrust model requires large-scale horizontal movements, versus dominantly vertical movements of the block uplift model. Stratigraphic separations in the southern Florida Mountains indicate a minimum of 4,000 ft (1,219 m) vertical displacement. There is no evidence for more than about 1,000 ft (304 m) of horizontal displacement on the south Florida Mountains fault, and perhaps 2,000 ft (610 m) of horizontal displacement on some of the small thrust faults in the 1-mi-wide area along the northeast side of the south Florida Mountains fault. The south Florida Mountains fault steepens at depth whereas overthrusting requires that the fault flatten at depth. Overthrusting generally involves thick, 25,000 to 50,000 ft (7,620 to 15,240 m) geosynclinal sequences. In contrast, basement-co ed uplifts involve crystalline basement rocks and typically, a thin sedimentary sequence. Precambrian plutonic rocks are against, and locally over, about 4,000 ft (1,219 m) of Paleozoic carbonate rocks along the south Florida Mountains fault. Regional surveys indicate that the Florida Mountains area has been relative high since Pennsylvanian time. Foreland block uplift areas typically have shown a long history of structurally positive tendencies evidenced by thinning of units and the presence of unconformities over positive areas. Thrust faults formed during overthrusting due to extensive horizontal compression should, if curved, be concave upward. Faults produced by vertical basement uplift should be concave downward. The south Florida Mountains reverse fault is steeper at depth and con ave downward. The imbricate thrust faults to the northeast are either concave downward or nearly horizontal. Regional overthrusting should produce telescoping of facies and stratigraphic anomalies yet to be observed or reported in publications on southwest New Mexico geology. Our current study demonstrates that Laramide deformation in the Florida Mountains is probably not a continuation of the Cordilleran overthrust belt. Evidence suggests that the deformation resembles the basement-cored block uplifts of the Rocky Mountain foreland. End_of_Article - Last_Page 432------------