Geometry and kinematics of the Cañones Fault and the effects of lithology on the distribution of strain within the Cañones Fault damage zone
O'Keeffe, Kevin A. 1980-
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Understanding the structural history of a region is critical in the interpretation of fault system geometry and strain distribution. This study investigates deformation along the Cañones normal fault which marks the western boundary of the Rio Grande Rift in north-central New Mexico. Geologic mapping at a scale of 1:6,000 and structural reconstructions show that the Cañones fault formed in the forelimb of a west-vergent shortening-related monocline. Its footwall strata are sub-horizontal. Folded Jurassic strata in the hanging wall are eroded and unconformably overlain by Cenozoic rift deposits. The Cañones Fault and the monocline are sub-parallel. The amount of extension and shortening across the Cañones Fault and Laramide monocline decreases from south to north. Structural reconstructions indicate 170 meters of extension and 76 meters of shortening in the south, and 58 meters of extension and 16 meters of shortening in the north. Beyond termination of the monocline to the north, the trace of the Cañones fault trends nearly E-W and extension decreases to 42 meters. Although temporally separated, these structures are geometrically and spatially similar, suggesting Rio Grande Rift structures exploited Laramide structures. Field mapping, fracture scanlines and structural modeling are used to investigate lithologic controls on fault damage zone attributes at the Cañones Fault in the Entrada Sandstone and overlying Todilto Limestone. These lithologies exemplify end-member type damage zones, deformation banded and fractured. Density of damage zone structures in the Todilto Limestone and the Entrada Sandstone reach background levels at nearly the same distance from the fault core (101-105 m), and are consistent with similar data from other faults. Although the lithologic control on damage zone width is negligible, the distribution of fault damage zone structures differs by lithology. Deformation band and fracture distribution in softer rocks such as eolian sandstone and siliceous mudstones, tend to be clustered near the fault core and exponentially decay to background levels. Fracture distribution in more brittle lithologies such as limestone and porcelanite tends to be distributed throughout the damage zone with some clustering at the fault core ending abruptly at the outer margin of the fault damage zone.