HETEROGENEOUS DEFORMATION IN A DUCTILE FOLD-THRUST BELT - THE PROTEROZOIC STRUCTURAL HISTORY OF THE TUSAS MOUNTAINS, NEW-MEXICO
The presence of widely variable deformational styles in the Proterozoic terranes of northern New Mexico has led to conflicting interpretations of the geologic history of the region. Proterozoic rocks of the Tusas Mountains display a range of deformational styles typical of northern New Mexico and suggest a model for interpreting the heterogeneity. Three broad types of structural domains have been identified: (1) regions characterized by complex polyphase fold patterns, (2) regions dominated by a single intense schistosity, and (3) regions with nearly pristine primary sedimentary and volcanic structures. Domain boundaries range from continuous deformational gradients to localized faults or shear zones. Structural and metamorphic evidence suggests that all domains were deformed in a single regional shortening event. The deformation occurred during prograde metamorphism at upper greenschist- and lower amphibolite-facies conditions. Despite the metamorphic grade, kilometer-thick massive units were shortened by folding and thrusting, producing geometries similar to those of thin-skinned deformational belts. Regions located beneath the ramps of large-scale ductile thrusts preserve primary structures and simple apparent deformational histories because much of the regional strain was partitioned to the overlying faults. Regions structurally above ramps display complex multiple fold patterns with strain histories that progress from subhorizontal shearing to upright folding and shortening. Regions away from ramps have an intense multi-phase schistosity and a deformational history involving progressive simple shearing. The large scale of the domains (as much as 75 km 2 ) and the extreme structural heterogeneity from domain to domain may reflect, at least in part, the competence, homogeneity, and extreme thickness (>1 km) of the Ortega orthoquartzite. Spectacularly refolded and multiply sheared regions in the Tusas Mountains reflect complex local strain environments that cannot be correlated from domain to domain. The kinematic histories of these environments are locally important but are misleading on a regional scale. The regional kinematic history was relatively simple, involving a single shortening event. Deformational styles in other Proterozoic terranes in northern New Mexico are directly comparable to those of the Tusas Mountains and can be interpreted in terms of this large-scale heterogeneous shortening model.