STRUCTURAL EVOLUTION OF THE SOUTHERN CADDO GAP QUADRANGLE, ARKANSAS: FIELD EVIDENCE FOR STRIKE-SLIP AND BACKTHRUST FAULTING IN THE OUACHITA MOUNTAINS

The Caddo Gap quadrangle is located on the southern margin of the Benton Uplift, the orogenic core of the Ouachita Mountains. Recent field mapping focused on delineating stratigraphic and structural relationships in the southern part of the quadrangle to improve existing reconnaissance-scale maps of the area. New structures were discovered that provide further information about the structural evolution of the Ouachita fold and thrust belt.

Field work included mapping the three informal members (Lower, Middle and Upper) of the Arkansas Novaculite. Major folds within the study area include the Nelson Mountain Anticline and two unnamed synclinal folds (“South Caddo Mountain” and “Arrowhead Mountain”) where south-dipping Novaculite has been folded around almost 180 degrees in map view, resulting in a “fishhook.” The backthrust interpretation for Strawn Mountain (Haley et al., 2009) was found to be more structurally complex than previously proposed.

Analysis of the Nelson Mountain Anticline shows it is steeply inclined and gently plunging: the calculated mean axial plane (n = 104) is 276°, 78°SW and mean fold axis is 22°, 101°, similar to the reconnaissance-scale findings of Evansin (1976). Two fishhook folds, both located at the west ends of E-W ridges of Arkansas Novaculite, have similar geometries. The mean axial planes are 290°, 71°SW and 289°, 81°SW and average fold axes are 37°, 125° and 40°, 117°.

Unmapped, NE-trending strike-slip faults in the Arkansas Novaculite were discovered on the E-W “South Ridge” immediately south of and adjacent to the South Caddo Mountain fold. This style of faulting differs significantly from previous regional interpretations (Haley et al., 2009) which instead show South Caddo Mountain juxtaposed against the north side of the South Ridge by thrust faulting. Field mapping found no evidence for this thrust faulting on South Caddo or Arrowhead Mountains.

An alternative hypothesis is that late-stage, NW-directed compression formed left-lateral strike-slip tear faults that cut obliquely through the regional structures of the Caddo Mountains. This style of faulting better explains the fishhook drag folds and the antithetic strike-slip faults, with the Strawn Mountain backthrusts accommodating the NW-directed compression on the north limb of the NMA.