New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent mapping of the Placitas and Sandia Park Quadrangles sheds new light on the geometry of contractional features of probable Laramide age in the Sandia uplift. Although reactivated in the Tertiary, the Laramide history was a major factor in producing the present geometry of the Sandia uplift. Two main contractional zones control the geometry of the eastern margin of the Sandia uplift: 1. the N-S trending, faults and related small to amplitude folds of the Antonio fault system and 2. the NE-trending, NW-verging reverse faults and monoclines of the Tijeras-Gutierrez fault zone. The interaction of these two features oppositely verging contractional features produced the prominent Tijeras synclinorium, which is one of the few places on the east flank of the Sandia-Manzano Mountains where Mesozoic rocks are preserved. The San Antonio structures are an east-verging fault-system analogous to the Montosa fault of the Manzano Mountains to the south. The Sandia Park Quadrangle contains numerous small scale N-S to NW trending folds and incipient axial planar cleavage associated with a network of high angle east-side down, north-striking reverse faults and monoclinal warps. Small scale folds are disharmonic and most common in thinly-bedded Madera Formation near La Madera. This system can be traced north to the Placitas Quad, where east-vergent contracitonal deformation is preserved in one segment of the Tertiary-reactivated (west-side-down) East Las Huertas fault indicating a Laramide ancestry. Tertiary west-side-down movement along all these faults has diminished and often reversed the throw, making it impossible to estimate original extent of contraction along this system. Nevertheless, we think it was an important zone in controlling the geometry of Laramide Sandia uplift.

Although also overprinted by Tertiary movements, the geometry along the Tijeras-Gutierrez fault system was produced mainly by E-W contractional deformation in the Laramide. The bulk of the movement was NW -vergent with offset partitioned onto different fault segments linked by the SW-dipping monoclinal warp of the Frost Arroyo. To the south of the Frost Arroyo, the Gutierrez fault accommodates about one km of SE-side-up throw, and related large scale drape folding to produce the Tijeras synclinorium and its fault-truncated anticline pair. Throw on the Gutierrez fault decreases rapidly NE, towards Frost Arroyo, where it changes sense to (more minor) SE-vergent sense along the northern segment. Major SE-side-up displacement is transferred to the Tijeras fault across the Frost Arroyo monocline. SE-side-up throw of about 1 km across the Tijeras fault juxtaposes Proterozoic rocks with the San Pedro syncline. Both the Tijeras and Gutierrez are scissors faults, with associated folds that have axial plane and vergence consistent with an origin as "drape" folds associated with the contractional faults.

Possible strike slip displacement along the Tijeras-Gutierrez system remains controversial and evidence sparse. Shallowly-plunging slickenlines are present, but age and magnitude of slip is unknown. One of us (C.AF.) prefers the interpretation that Laramide deformation was entirely contractional and that small scale N-S faults adjacent to the zone represent sinistral R-shears formed during Tertiary strike slip. The other author (K.E.K) favors the interpretation that these are N-S contractional faults related to the San Antonio system and that the E-W contraction produced a component of Laramide dextral slip on the Tijeras, with the transfer zone across the Frost Arroyo as a restraining bend and the Monte Largo uplift as a flower structure. By either interpretation, magnitude of strike slip is probably minor (< a few km) as there is little strike-slip offset of markers; including the roof of the Sandia pluton and associated isograds, which can be mapped across the Tijeras-Gutierrez fault as shallowly SE- dipping surfaces.