GENESIS OF FAULT BRECCIA AT DEER CREEK: IMPLICATIONS FOR THE SLIP HISTORY OF THE PICURIS–PECOS FAULT

Abstract

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