GEOMETRY AND KINEMATICS OF THE CAñONES FAULT AND THE EFFECTS OF LITHOLOGY ON THE DISTRIBUTION OF STRAIN WITHIN THE CAñONES FAULT DAMAGE ZONE

1:6,000 scale geologic mapping was conducted along the Cañones Fault, a steeply dipping ~68°SE extensional structure which marks the western boundary of the Rio Grande Rift, located in north-central New Mexico. Mapping and restoration have revealed that the Cañones fault occurs in the forelimb of a west vergent Laramide aged monocline. Its footwall strata are sub-horizontal, while folded Jurassic strata in the HW are eroded and unconformably overlain by Tertiary rift fill conglomerates offset by the Cañones Fault. Analysis suggests that the two structures trend sub-parallel to one another ~N40°E. Fault throw decreases along strike from 450 m in the south to 256 m in the north. The amount of extension and shortening across the Cañones Fault and Laramide monocline decrease from south to north, coincidentally from 9% extension and 3% shortening in the south to 6% and 1% respectively. Beyond termination of the monoclonal structure to the north, the trace of the Cañones fault trends nearly E-W and extension decreases to 2%. While temporally separated, these structures are geometrically and kinematically similar.

Detailed field mapping, fracture scanlines (fault core into HW) and structural modeling are used to investigate lithologic controls on fault damage zone attributes at the Cañones Fault. Relationships have been determined for the Jurassic Entrada Sandstone and overlying Todilto Limestone. Both of which exemplify the two end-member type damage zones, deformation banded and fractured, respectively. 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 do not plot as outliers when compared with similar data from other faults, therefore lithological controls on damage zone width are negligible. However, distribution of fault damage zone structures has been shown to differ by lithology. Deformation banding and fracturing in softer lithologies such as eolian sandstone and siliceous mudstones, tend to be clustered more near the fault core and tapper off to back ground levels, while fracturing in harder lithologies such as limestone and porcelenite 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.