PRE-RIFT EXTENSION DEFINED BY NEW MAPPING AND GEOCHRONOLOGY IN THE CIMARRON AND TAOS RANGES OF NORTHERN NEW MEXICO

New geologic mapping and U-Pb zircon ages from areas to the southeast of the Questa caldera complex in northern New Mexico provide strong evidence for extensional faulting prior to eruption of the Amallia Tuff at 26 Ma. Recent mapping within the Sangre de Cristo Range of northern New Mexico and southern Colorado has clearly identified early-rift flank structures, but no structure has previously been identified as pre-rift (26 Ma) and post-Laramide (Cretaceous to Eocene) in the region. New mapping in the Cimarron Range and eastern Taos Range has identified a prominent west-northwest-striking (~300°) fault system which extends for at least 40 kilometers from the Tooth of Time on the Philmont Boy Scout Ranch to the upper Red River Valley just east of Relica Peak. Previous mapping correlated the Cimarron Canyon stretch of the fault with the Laramide Fowler Pass fault, but newer mapping relationships make this correlation unlikely. The Fowler Pass fault is a high-angle reverse fault which trends north-south, parallel to other faults and folds flanking the Laramide Raton basin and places Proterozoic crystalline rocks over steeply dipping to overturned late Paleozoic to Mesozoic rocks which trend parallel to the fault. The younger high-angle fault places Proterozoic crystalline rocks against shallow to moderate dipping Paleozoic and Mesozoic rocks which trend at high angles to the fault trace with no indication of significant folding adjacent to the fault. The younger fault cuts the Fowler Pass fault and associated folds at a sharp 60° angle and continues to the east into the footwall. Along the exposed trend of the fault, large volumes of rhyolite to trachydacite were emplaced as epizonal sills, laccoliths, and plugs. The geometry and chemistry of these bodies suggest that they were fed through vertical open fractures along the length of the fault from two distinct magma sources. Magma spread laterally outwards into the Paleozoic and Mesozoic sedimentary cover rocks. New U-Pb zircon ages from the intrusions range from 34 to 30 Ma, indicating latest Eocene to Oligocene age. The fault does not cut any of these intrusions. Similar relationships in the Baldy Mountain area to the north suggest another possible pre-rift fault.