New Mexico Geological Society Annual Spring Meeting — Abstracts

We present a new 1: 12,000 map and cross section of the Proterozoic rocks of the Tijeras quadrangle. The maps are based on field mapping at scales of 1 :9,000 and 1: 12,000, interpretation of 1:24,000 air photos, and compilation of previous work. Mapping and structural analysis has provided new insight into the deformational history of the region from the Tertiary to the Early Proterozoic.

The map depicts a complex network of brittle faults and fracture systems related to extension in the Rio Grande Rift The Tijeras fault is a NE-striking, subvertical zone several to tens of meters wide interpreted to display a scissors geometry with a hinge near the town of Tijeras. We interpret the Tijeras fault to have accommodated differential Tertiary block rotation between the Sandia block and the Manzanita block. Although the Tijeras fault parallels the strike of Proterozoic fabrics, it is vertical and separates domains of N and S moderately dipping fabric, and thus, in detail, does not appear to reactivate Proterozoic structures. Proterozoic metavolcanic sequences (Tijeras greenstone) occur on both sides of the fault, suggesting that the net lateral component of displacement is probably less than km-scale. We interpret NW striking faults and joints as related to movement on the Tijeras system. They probably accommodated shattering of the Sandia block and were important in transferring displacement between adjacent segments of the Tijeras fault in the hinge region.

Precambrian rocks of the Tijeras quadrangle preserve evidence for significant Middle Proterozoic tectonism during emplacement of the 1.42 Ga Sandia pluton. The pluton is bounded on the SE by a 1-2 km wide ductile shear zone whose development was synchronous with pluton emplacement. Field relationships and geochemical analyses suggest that the Cibola granite is genetically related to the Sandia pluton and probably formed as evolved liquids were drawn out of the crystallizing pluton and emplaced in the active shear zone. Magmatic layering the main phase of the Sandia pluton records sheet-like emplacement of the Sandia pluton in response to the same strain field as the shear zone. Pegmatite and aplite dikes in the Sandia record a kinematic regime involving E-W shortening and N-S extension during emplacement of evolved liquids. In summary, structural features of the pluton are all compatible with a regional strain field during emplacement involving W -WNW shortening and N extension. The Sandia pluton is not "anorogenic", but rather preserves the kinematic record of regional tectonism at 1.4 Ga.

South of the Tijeras fault, a discrete (m's wide) high strain zone (Vincent Moore thrust VMT) emplaces a km-scale overturned synclinorium of supracrustal lithologies (schists and quartzites) above mafic metavolcanics (Tijeras greenstone) and intrusive leucocratic phases interpreted to be part of the Sandia pluton, suggesting post -1.4 Ga movement on the shear zone. Microstructures in the VMT (core and mantle structures in quartz, reaction-softened feldspars) are consistent with fairly low temperatures during deformation. Kinematic indicators suggest top to the NW sense of shear. This deformation apparently postdates emplacement of the Sandia pluton and subsequent cooling through ~400°C. Although it is possible that folds and thrusts represent post-1.4 Ga regional deformation, we suggest that a large synclinorium of Early Proterozoic age in the Coyote Canyon was reactivated by later contractional deformation (Vincent Moore and Coyote ductile thrusts) and we tentatively correlate the Tijeras and Coyote metavolcanic successions.