Determining the earliest history of a multiply reactivated fault is diffi cult, particularly when the rock record is incomplete and brittle deformation reorients strain markers. We present a method that incorporates strain analysis of an adjacent fold, inferred deformation temperatures, and published thermochronology to fi nd the initiation age of the north-striking Picuris-Pecos fault of the southern Rocky Mountains (USA). The Picuris-Pecos fault has accommodated at least 37 km of dextral

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... The older easttrending Hondo syncline (New Mexico) is refolded adjacent to the fault and has a map pattern consistent with dextral drag. We analyzed quartz grain shapes, deformation mechanisms, and fractures within the Hondo syncline to determine if the refold is genetically related to the Picuris-Pecos fault. Nine calculated grain-shape ellipsoids predate refolding and are rotated about a vertical axis subparallel to strike of the refold limbs. Healed fractures and semibrittle microstructures are most abundant in the hinge of the refold and adjacent to the fault. We interpret refolding and slip on the Picuris-Pecos fault to have initiated during unroofi ng through the brittle-plastic transition (~250-310 °C). Regional thermochronologic studies suggest that the rocks of the Hondo syncline were at this temperature range during the Grenville orogeny. The refold is probably a faultpropagation fold that predated throughgoing faulting, or possibly formed during dextral drag on the Picuris-Pecos fault. Refolding in the Picuris Range and the Truchas Mountains accounts for at least 2.6 km of the total dextral shear on the fault.