New Mexico Geological Society Annual Spring Meeting — Abstracts

The Tijeras fault zone extends from north of the Manzanita Mountains northeast to the southernmost Sangre de Cristo Mountains. It cuts rocks ranging from Precambrian to Quaternary in age. Previous workers have proposed that faulting in this regionally extensive zone was · initiated in Precambrian time and that the fault system was reactivated in Pennsylvanian time, during the Late Cretaceous -early Tertiary Laramide Orogeny, and during the Quaternary. In addition, it has been suggested that the Tijeras fault zone aceommodated right lateral transtensional motion during the Laramide Orogeny, resulting in the formation of the Eocene Galisteo basin. On the basis of this model, sediment transport was predicted to initiate in uplifts to the west, north, and south and proceed through the basin toward the southeast. These hypotheses are currently being tested through detailed mapping of selected areas; collection of paleocurrent data in the Galisteo basin, and investigation of sense-of-slip indicators. Preliminary data from this study are presented for discussion.

The number of faults that form the Tijeras fault zone varies along strike; all of the faults dip steeply to vertically. Slip may be accommodated by a single fault or partitioned among numerous faults along the length of the zone. The youngest movement along the fault system is clearly Quaternary, as previously established; Quaternary sediments are unquestionably faulted. Laramide age motion is also supported by data from this study; more than 300 new paleocurrent measUrements from sandstone beds in the Galisteo basin, in addition to previously collected data, are consistent with the paleotopography previously proposed. Cross-sections through selected regions of the fault zone suggest variable amounts of dip-slip displacement of Paleozoic and Mesozoic sedimentary rocks along individual faults. Slickenside striae on other faults.record strike-slip movement, but the magnitude of strike-slip displacement has yet to be determined.

Fault motion older than Laramide in age, if any, has not yet been established, but several relevant observations have been made. Evidence for Pennsylvanian movement includes facies variations previously interpreted to indicate synchronous faulting and sedimentation. Preliminary data collected for this study suggest that this evidence is better interpreted as the result oflaterally abrupt facies variations or by strike-slip motion subsequent to sedimentation. Evidence for Precambrian movement has not yet been found. Slickenside striae indicating strikeslip motion are present where the Tijeras fault cuts Precambrian rocks, but this deformation is not constrained in age.