AGE-CONSTRAINTS ON FABRIC REACTIVATION IN THE TUSAS RANGE, NORTHERN NEW MEXICO, USING ELECTRON-MICROPROBE MONAZITE GEOCHRONOLOGY: IMPLICATIONS FOR THE NATURE OF REGIONAL ~1400 GA DEFORMATION

A key issue in constructing models for the southward growth of Laurentia during the Proterozoic is distinguishing the effects of ~1650 Ma and ~1400 Ma tectonism. These events share similar styles of deformation and metamorphism, making it difficult to assign structures, fabrics, and metamorphic phases to a particular event. The fundamental geometry of this orogen in the southwestern United States is defined in many areas by fold-fault pairs and isolated synclines of thick ~1700 Ma quartzite. In-situ EMP chemical dating of monazite, combined with detailed structural analysis, indicates that such synclines within the Tusas Range of northern New Mexico (locally F₃) were substantially modified, if not developed, during ~1400 Ma tectonism. Monazite grains from the Ortega quartzite in the central Tusas Range display a shape preferred orientation parallel to the axial-planar fabric of these folds (S₃), with overgrowth rims preferentially developed in the X direction of strain. These monazite grains have either >1700 Ma cores or ~1650 Ma cores with ~1400 Ma overgrowth rims, or are entirely ~1400 Ma in age. Field and microstructural observations show that the upright, east-west trending F₃ and S₃ are reactivations of older, northwest-trending fabrics and structures. The presence of ~1650 Ma overgrowth rims on monazite grains from the central and northern Tusas Range implies that these folds and fabrics may have nucleated prior to ~1400 Ma tectonism. Previous studies have shown an increase in ~1400 Ma monazite ages from north to south within the range, consistent with a similar increase in metamorphic grade. This gradient suggests that the central and northern Tusas may have been at progressively shallower crustal levels during ~1400 Ma tectonism, thus increasing the preservation of older fabrics, structures, and metamorphic monazite from south to north within the range. These observations support the hypothesis that ~1400 Ma tectonism locally reactivated and utilized pre-existing structures and fabrics, but had also profoundly shaped the geometry and metamorphic character of the orogen.