IMPLICATIONS OF NEW AGE CONSTRAINTS ON THE STRUCTURAL AND PETROLOGIC EVOLUTION OF MIDDLE PROTEROZOIC ROCKS IN THE CENTRAL TUSAS MOUNTAINS OF NORTHERN NEW MEXICO

Recent field mapping, geochronology, and petrologic analysis and have provided constraints on the early tectonic evolution of Proterozoic rocks in the Tusas Mountains. The area contains an east-west striking enigmatic discontinuity that is suspected to be a late mesoproterozoic ductile fault. This fault juxtaposes higher grade complexly deformed supracrustals to the south against lower grade mafic volcanics to the north. Ductile deformational features can be grouped into three generations. D1 produced a strong bedding parallel foliation, with few folds. M1 metamorphic conditions reached lower greenschist facies. D2 produced reclined tight to isoclinal folds with a strong SW dipping S2 foliation, and SW plunging stretching lineations (L2), sub-parallel to fold axes. Kinematic indicators suggest transport to the northeast. These structures are more pronounced south of the discontinuity. M2 reached greenschist facies conditions. D3 produced east-west trending open folds, and either a newer crenulation clevage, or reactivated S2. M3 is characterized by a metamorphic field gradient from upper greenschist to the north, to sub-amphibolite to the south. All three of these deformational events were traditionally correlated with the 1.67-1.65Ga Mazatzal orogeny. However more recent work has suggested that a significant portion of the total deformation occured at 1.40-1.45Ga. The weakly deformed Tusas granite, refined to 1.67-1.69Ga, cuts a strong fabric in the Moppin group. This suggests that the earlier fabric in the Moppin group was produced before 1.69Ga. (Yavapai Orogeny?) A refined age for the Tres Piedras Granite and syn-D1 dike of 1.67-1.69Ga is interpreted to suggest that the first two fabrics in the granite directly correlative to regional fabrics are indeed the result of Mazatzal tectonism. D3, which increases in strength from north to south across the Tusas Mountains, is constrained in the southern Tusas to 1.45-1.40Ga. Therefore the three separate tectonic events exposed vary across the discontinuity from preserved early events in the north, to an almost complete reconstitution of the crust by the third event in the south.