New Mexico Geological Society Annual Spring Meeting — Abstracts

A major tectonic boundary separates two Proterozoic crustal blocks in northern New Mexico. The boundary is a zone of distributed shear that dips to the south at 30°. This tectonic zone separates mafic metavolcanic rocks comprising the structurally overlying, but older, Pecos terrane from underlying, younger rocks of the Truchas terrane. The rocks assigned to the Truchas terrane are isoclinally folded schists and quartzite of the Vadito and Ortega Groups which form a syncline that is overturned to the north. Kinematic indicators from rocks collected along a 5 km portion of this ductile shear zone document a north-directed shear sense. Asymmetric quartz grains in schist, elongate cobbles in highly deformed metaconglomerate and plagioclase fish-hook structures in amphibolite consistently give a northerly sense of shear at the macroscopic scale. Thin section observations, however, show only a few remnant asymmetric grains are preserved. Most of the grains in thin sections show recrystallization textures. We interpret this to mean that theI peak of the regional metamorphic heating postdated the north-directed shearing along this tectonic boundary. This interpretation is consistent with regionally mapped isograds that cut across the structure.