PEERING THROUGH THE ~1400 MA OVERPRINT: RECENT INSIGHTS INTO THE TECTONIC EVOLUTION OF THE TUSAS MOUNTAINS, NORTHERN NEW MEXICO

The Tusas Range in northern New Mexico documents increasing intensity of 1.4 Ga tectonism with depth in the crust and provide evidence that 1.4 Ga structures may have been influenced by older (Yavapai age?) structures.  Recent field mapping, structural analysis, and EMP in-situ dating of monazite reveal a transition in the character of 1.4 Ga (locally D₃) tectonism from discrete, EW-striking reverse faults in the northern Tusas Range to a penetrative cleavage and upright, EW folds in the south.  The increasing dominance of  ~1.4 Ga monazite ages from north to south within the range has also been documented. This is consistent with an increase in ~1. 4 Ga peak pressure and metamorphic grade (locally M₃) from north to south in the range, corresponding to an increase in preserved crustal depth.  A steep geothermal gradient associated with extensive ~1.4 Ga plutonism may be the cause for this observed increase in intensity.

Recent studies have also highlighted that ~1.4 Ga deformation in the Tusas Range may have been influenced by older structures.  EMP monazite geochronology implies substantial modification of EW upright synclines of the ~1.7 Ga Ortega quartzite (locally F₃) occurred at ~1.4 Ga.  Detailed mapping and structural analysis suggests the synclinal geometry may have been established during the early phases of deformation (D₁?), which may have occurred as early as ~1.69 Ga.  Discrete D₃ reverse faults (1.4 Ga) also coincide with major lithologic boundaries in the Tusas Range, including the southern terminus of exposed Yavapai basement, and a dramatic thickening of the >1.7 Ga supracrustal Vadito group.  The observations suggest a model in which the tectonic evolution of the Tusas range was controlled by pre-existing structures and topography on which the Vadito group was deposited.  The present-day geometry of Mazatzal-age rocks in the Tusas Range reflects a structural grain that may have been established as early as the Yavapai orogeny.