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The Influence of Pre-Existing Structures on the Initiation of an Oblique Rift: the Cañones Fault Zone in North-Central New Mexico


The earliest fault in an intracontinental rift tells us a lot: when the rifting initiated, where the weak zone in the crust was, how the upper plate moved relative to the lower plate, etc. Here we present our field mapping results and discuss its implications on the tectonic model of the initiation of an oblique rift. The Cañones fault zone in north-central New Mexico is a boundary between the Colorado Plateau to the west and the Rio Grande Rift to the east. Offsetting a 20-Ma basaltic dike and the Oligocene Ritito Conglomerate, it is one of the oldest faults in the western flank of Española Basin, Rio Grande Rift. It consists of a master fault, the Cañones Fault (CF), a transfer fault, the Las Minas Fault (LMF), and a family of synthetic and antithetic faults. The South CF is a SE-dipping high-angle normal fault, striking ∼040°; the North CF, however, dips NW while strikes NE, too. The South and North CF have a similar arcuate trace while the near E-W-striking LMF connects them in the middle. Detailed mapping and fault kinematic studies from the slickenlines on fault surfaces reveal that the relative movement orientations on the Cañones Fault range 070°∼130°, and average 100°, and thus characterized an oblique rift. The LMF is a left-lateral strike-slip fault, based on the sub-horizontal slickenlines, and hence accommodated the movement on the hanging-wall blocks of both South and North CF. A noteworthy phenomenon along the Cañones fault zone is that both South and North CF juxtapose a monocline in the hanging-wall block upon the sub-horizontal footwall strata. The monoclines are shortening structures, probably formed during the Laramide Orogeny. A series of right-stepping, en echelon thrusts and uplifts are preserved adjacent to our study area. Along the South CF, a monocline locates to the east of the fault and is west vergent; along the North CF, in contrast, the other monocline locates to the west of the fault and is east vergent. The polarity switch of the monoclines is consistent spatially with the dip direction change of the Cañones Fault. We also observed three reverse faults and two reactivated normal faults on one outcrop. Our model suggests that after the Laramide shortening weakened the core of monoclines, the following Rio Grande Rift extension initiated obliquely along these weak zones. Therefore, the pre-existing shortening structures strongly influenced the structure of the rift-bounding faults when the extension initiated.