PALEOMAGNETIC DATA BEARING ON THE AGE OF SLIP ALONG THE PICURIS-PECOS FAULT OF NEW MEXICO, SOUTHWEST USA

The Picuris-Pecos fault system is the only well-exposed representative of a zone of structures that accommodate ~100km of net right-lateral slip offset of Precambrian piercing lines. Contributing ~37km of net dextral slip to this zone, the age of separation on the Picruis-Pecos fault is crucial to Precambrian, Paleozoic (Ancestral Rocky Mountain), and Cretaceous to Eocene (Laramide) tectonic models. Fortunately, the Picuris-Pecos fault system contains abundant material suitable for paleomagnetic analysis that may elucidate the timing of the principal phase of deformation. One-side of the fault system is exposed in Deer Creek Canyon near Santa Fe where well-lithified, brecciated and shattered Precambrian granitic gneiss is unconformably overlain by multiple ages of much less deformed Paleozoic limestones and siliclastic rocks. NW-striking, sub-vertical tabular fissure fills of limestone breccia and limestone up to 20 m in length crosscut the granitic breccia and the trend of the Picuris-Pecos fault zone. We have sampled over 30 sites in Deer Creek Canyon for paleomagnetic analysis including brecciated granitic gneiss, limestone and sandstone inclusions within the granite, and hematite bearing sandstone of the Permian Sangre de Cristo Formation. Thermal demagnetization of the granite breccia shows mixed results, but interpretable data from these materials yield a grand, tilt-corrected mean of 134°/10.4°/13.8/21° (Dec/Inc/k/α₉₅; n/N=6/12). Samples from the sandstone and limestone inclusions from within the granite show a grand, tilt corrected mean of 141°/-1.3/78/12.1° (Dec/Inc/k/α₉₅; n/N=4/4). Samples from the nearby exposures of the Sangre de Cristo Formation yield a grand, in situ mean of 139°/-10.2°/104/11.7° (Dec/Inc/k/ α₉₅; n/N=8/9). These mean directions are statistically indistinguishable and owing to the local tilt corrections are indistinguishable from the in situ directions. We interpret these data to suggest that the fault zone rocks were remagnetized in the late Paleozoic and that any subsequent deformation failed to significantly modify the observed paleomagnetic directions. Also, unconformable Paleozoic contacts indicate that the major brittle event is Precambrian in age thus making any hypothesis invoking post-Paleozoic, large-magnitude displacement along the Picuris-Pecos fault untenable.