RAMPS, FLATS AND DUPLEXES IN THE PROTEROZOIC HONDO GROUP, PICURIS MOUNTAINS, NORTHERN NEW MEXICO

Previously unrecognized Ramps, flats and duplexes in the Hondo Group on the limbs of the Copper Hill Anticline coincide with the earliest deformational event and may characterize it. At least 3 deformational episodes are recognized, all associated with cleavages. The first cleavage (S1) is obscure, and when observed, it parallels bedding but rarely transposes it. There is no evidence of transposed folds associated with S1 at a map scale, and S1 is thought to coincide with bedding parallel ductile shear zones. Folds at both the outcrop and map scale are associated with S2 and the second deformation episode (D2). D2 is approximately coaxial with D1. The Copper Hill Anticline is an asymmetric D2 fold plunging SSW and overturned to the North. A third deformational episode is associated with a crenulation cleavage transecting S2, but it has little effect at the map scale. The ramps, flats and duplexes can be interpreted using the geologic map of the Copper Hill Anticline (Bauer & Helper, 1994, Map GM-71, NMBMMR). The map accurately shows the lithologies, but does not make the ramp-flat-duplex interpretation. Both a footwall and hanging wall ramp occur on the south limb of the Copper Hill Anticline. The lower flat is near the base of the R2 schist, the ramp crosses the R3 quartzite and the upper flat is at the top of the R3 quartzite. On the geologic map, the R2 schist occurs between two, right-side-up R3 quartzites. Duplexes occur on the overturned North limb. On the geologic map, R3 quartzites are separated by schists that are included in the R3, although as the map authors point out, those schists resemble the R2 and cannot be distinguished from it. Structurally, those schists can be interpreted as R2 and not part of the R3 formation. If that is correct, the R3 quartzite would be duplicated and those segments of the R3 quartzite become horses in a forward dipping duplex with a roof and floor thrust that can be connected back to the ramps and flats on the south limb of the anticline. Because the ramps, flats and duplexes are folded by the D2 event, they are clearly associated with D1, and represent significant thin-skinned shortening prior to the D2 episode.