New Mexico Geological Society Annual Spring Meeting — Abstracts

Spectacular fault breccia up to 250 m wide is exposed at Deer Creek along the Picuris– Pecos fault (PPf), ~18 km southeast of Santa Fe. There, the PPf juxtaposes Middle Pennsylvanian–Permian strata (Alamitos Fm and Sangre de Cristo Fm) with Proterozoic granitegneiss on the west. In addition to the ~38 km dextral separation documented elsewhere along the fault, the PPf at Deer Creek shows stratigraphic evidence for ~300 m of east-down separation. The breccia consists of two zones: (1) An eastern zone, 10–50 m wide, directly west of the PPf, within which clasts of Proterozoic gneiss exhibit diverse foliation orientations within individual outcrops. Also in this zone are sparse, meter-scale blocks of indurated Mississippian– Pennsylvanian sedimentary rocks and significant fine-grained cataclasite. We interpret this breccia zone as recording dilation and shear in a high-strain zone adjacent to the PPf. (2) A western zone up to 200 m meters wide has low dispersion of foliation orientations within individual outcrops. This zone exhibits relatively minor fine-grained cataclasite, is strongly indurated by red jasperoid, and locally is cut by less-brecciated pods of Mississippian carbonate (probable fissure fills) and Pennsylvanian–Permian arkosic sandstone. We interpret this breccia zone to have formed by weak dilational strain. In both breccia zones, the average orientation of foliation is rotated ~20-30° anticlockwise relative to that in undeformed gneiss to the west. This rotation occurred before widespread Permian remagnetization in the area, as no significant vertical-axis rotation has occurred since then (Wawrzyniec et al., 2007).

The inferred sequence of geologic events was: (1) Pre-Permian (probably Proterozoic, based on regional deformation patterns) sinistral shear caused anticlockwise rotation of foliation near the fault. (2) Pre-Middle Mississippian (early Ancestral Rocky Mountains orogeny (ARM) or older) dilation and brecciation. (3) Opening and filling of fissures during Middle Mississippian carbonate sedimentation and karst development. (Local post-brecciation, prejasperoid chlorite cement is also present, but its age relationship with the fissure fills is uncertain). (4) Late ARM injection of unlithified arkosic sand from the Alamitos Fm or Sangre de Cristo Fm into unindurated fault breccia. (5) Late ARM or Laramide deformation (sans vertical-axis rotation) caused mixing of clasts of lithified Upper Paleozoic strata into breccias along the PPf, folding and faulting in Paleozoic strata east of the PPf, and enhanced the permeability in breccias. (6) A major Oligocene hydrothermal system was localized by the highpermeability fault breccias, causing jasperoid cementation (microquartz after chalcedony) and reheating (>110° C) that reset regional Laramide apatite fission-track cooling ages to 32–26 Ma near Deer Creek. (7) Development of numerous minor faults that cut jasperoid-cemented breccias, presumably during Neogene rifting. In contrast to recently published interpretations, geological relationships at Deer Creek do not require major Proterozoic dextral slip (and instead suggest some Proterozoic sinistral slip), nor do they disallow major ARM or Laramide dextral slip on the PPf.