New Mexico Geological Society Annual Spring Meeting — Abstracts

Along. the Lucero uplift, consistent transtensional fault geometries, steeply raking lineations on fault planes and rare ductile shear indicators, such as S-C fabrics and shear bands in bedded gypsum of the Permian Yeso and San Andres formations demonstrate the dextral transtensional nature of the deformation along the Comanche-Able Hill-Santa Fe fault zone for several miles from Able Hill, south of Carrizo Arroyo, to the Tijeras Accomodation zone to the north. Mafic hypabyssal rocks intrude this fault zone locally and have been dated with K-Ar whole rock isotopic methods to be about 27.1 Ma . Dikes of this rock intruding the Comanche fault display lineations on fault planes that rake 60° to 75° S on planes that strike N-S and dip 75° to 85°E. These show horizontal projection directions from 315° to 330°, showing motions to have a larger dextral-than dip-slip component to last movement on the fault.

Along the Able Hill Fault, between the Comanche and Santa Fe faults, that involves evaporitic sediments of the Permian San Andres Formation, bedded gypsum has undergone ductile deformation that locally displays both normal and reverse dextral shear textures.

In the area north of Carrizo Arroyo, the Santa Fe Fault, long thought to be the rift margin, exclusive of the Comanche fault to the west, has a steep (>700°) westward dip, making it a reverse fault, locally. The Comanche Fault zone in this area is less than 1/2 km wide and dips steeply east, indicating that they are part of the same system at a shallow structural level. The Santa Fe Fault here shows very weak steep dextral lineations where brecciated mudstones of the Santa Fe Group are faulted against the upper Cretaceous Mancos Formation, middle Triassic Moenkopi Formation, or upper Triassic Chinle Formation. Coarse sediments that cap the Santa Fe Group contain clasts of the <4 rna Carrizo Mesa basalt and have been locally deformed and uplifted by motion on the Santa Fe Fault. This would indicate that dextral motion has occurred at least locally, since the late Pliocene.

These features are geometrically identical to what seismologists have referred to as an extensional, or reverse, flower structure where steep faults dipping towards one another converge to a master fault or zone at depth. In this case the east dipping Comanche Fault is the best candidate for the master fault.