New Mexico Geological Society Annual Spring Meeting — Abstracts

In the Picuris Mountains of northern New Mexico, the E-trending, Proterozoic N-vergent Hondo syncline is refolded adjacent the Picuris-Pecos fault (PPF) by a southwest-plunging synform that is <1.5 km wide E-W and ~8 km long N-S. The relationship between the PPF and the refold has not been studied systematically. Miller et al. (1963) interpreted the refold as a ductile dextral drag fold along the PPF, but it also may be due to PPF propagation or be an older, unrelated structure. Controversy over earliest PPF motion(s) continues: Precambrian and ductile (e.g. Miller et al., 1963; Fankhauser and Erslev, 2004, Wawrzyniec et al., 2007) versus Phanerozoic and brittle (e.g., Ancestral Rockies or Laramide; Karlstrom and Daniel, 1993, Daniel et al., 1995, and Cather et al., 2006). The focus of this study is to characterize the deformation due to folding and determine if there is a genetic relationship with the PPF.

Twenty four quartzite samples from the S- to SW-dipping Proterozoic Ortega and Rinconada formations in the S limb of the Hondo syncline were collected along strike through the refold, up to ~2 km from the PPF. Thin sections were cut in horizontal planes in order to document fabrics related to horizontal shear couples or vertical-axis rotations. A quartz microstructural analysis was done to test for a ductile strain gradient within the refold or with distance from the PPF; quartzite samples collected by Bauer (1987), mainly from the N limb of the Hondo syncline, provide ductile fabrics W of the refold for comparison. Those samples show mainly annealed foam texture, presumably relict from 1.4 Ga heating (Bauer, 1987). Minor faults and fractures were also measured and characterized along the same transects in order to relate brittle deformation to the folding.

Heterogeneous ductile strain in the refold overprints the foam texture and implies a Proterozoic (post-1.4 Ga) age for the refold, which may predate the PPF itself. Fabrics include mainly NE-SW elongate grains (NW-SE horizontal shortening) with undulose extinction, sutured grain boundaries, and subgrain development, suggesting low-T deformation. The highest strain is in the refold hinge zone but high-strain samples also come from both limbs ~1 km W and SE of the hinge. The deflection of the Hondo syncline can be explained by either E-side-up or dextral shear in a N-striking zone. However, such shear fails to explain the apparent lack of a systematic strain gradient approaching the PPF or the NW-SE grain-scale shortening direction, which implies sinistral shear on the PPF. Possibly, grain-scale fabrics were controlled by specifics of the fold mechanisms (e.g. flexural slip) rather than the regional tectonic strain patterns. Preliminary field and petrographic data suggest that brittle structures are also concentrated in the hinge zone of the refold. These structures may be related to the brittle-ductile folding event, or may be much younger.