The varied effect of the third deformation is an important result of this study. The D3 effects vary from folding to the south, to narrow shear zones and a reverse fault interpreted from offset strata in the central portion of the field area. The interpreted change in character of D3 structures combined with a significantly diminished 1.4 Ga metamorphic overprint is unique in the Tusas Mountains, and rare in Northern New Mexico. The transition in structures appears along the fundamental lithologic boundary, which suggests that the boundary was a zone of weakness reactivated by strain at 1.4 Ga. This reactivation suggests that the lithologic boundary may be an important structure in the crust. This structure may have been a basin bounding fault that focused the deposition of the Vadito Group to the southern portion of the study area. Focus of D3 strain onto the lithologic boundary is interpreted to have preserved D1 and D2 structures to the north that are thought to be the result of only Yavapai and Mazatzal orogenic tectonism.
A Spatial Tectonic Timeline - Figure 8.1
The complex stratigraphic relationships make visualization of the tectonic history of the study area difficult to understand Figure 8.1 summarizes the study area as a cartoon lithologic cross-section from southwest to northeast through time. The diagram is separated vertically with rocks that are currently found to the southwest on the left, and rocks found to the northeast on the left.
Lithologic units appear on the diagram at the times they were deposited or intruded. The shape of the polygons that represent the units is not significant, except for the height, which corresponds to a duration of time. The grey area represents time before the existence of rocks in that area. The earliest rocks in the field area, the Moppin Complex that was intruded of the Maquinita Granodiorite at approximately 1.75 Ga. The Vadito Group and Ortega Formation were deposited around 1.70 Ga, just before the emplacement of the Tres Piedras and Tusas Granites. The Vadito Group and Ortega Formation were probably deposited to the southwest in what may have been a significant basin, but only as a thin veneer on the Moppin complex. It is possible that the Vadito Group and Ortega Formation are completely thrust into place, however no significant structure has been identified. Thrusting during D1 did shorten the crust enough to juxtapose the basic units of the study area together. This is represented by the termination of the grey region above the syn-D1 Tres Piedras Granite. The Tusas granite is shown above this termination, because it crosscuts S1. D2 is thought to have occurred shortly after D1 based on its relationship to the Tusas Granite; however is definitely crosscut at 1.633 Ga. D3 is constrained to around 1.48-1.42 Ga.
Pressure Temperature Deformation Time Path (P-T-X-D-t) Figure 8.2
The tectonic evolution of the study area is summarized in Figure 8.2. This diagram illustrates the interpreted changes in pressure and temperature as time progressed for each side of the discontinuity. The path these rocks took is defined by the thick gray to black line on the diagram that is shown in progressively darker grey for each successive deformation. Both diagrams assume a starting position of very low metamorphic conditions. During the inception of D1, it is assumed that thrusting increased pressures faster than temperature until temperatures increased to peak conditions for this event around 1.699-1.695 Ga. Conditions for rocks to the south on the left-hand diagram may have reached garnet grade, while conditions to the north stayed below biotite grade conditions. There is no real constraint on pressure at this time.
The second tectonic event is interpreted to have occurred very shortly after the first, possibly at 1.693 Ga, so the beginning conditions for this event are thought to be close to the peak conditions seen for D1. Again the path is interpreted to have clockwise curvature because of the characteristics of thrusting. Rocks to the south developed staurolite and kyanite. The pressure is only constrained to be in the kyanite field. Rocks to the north are only constrained by the presence of biotite. Rocks of the field area in general are interpreted to have cooled back to a normal geothermal gradient based on the 200 Ma between D2 and D3.
The third tectonic event, interpreted to have happened around 1.49 Ga to 1.42 Ga, began near a typical geothermal gradient, and may have again risen in pressure before temperature; however, peak metamorphic conditions are tightly constrained. Conditions to the south were approximately 580 OC and 5.5kb, while conditions to the north were 580 OC but 4.6 kb. The path on both diagrams then returns to a normal geothermal gradient isobarically. These two diagrams in Figure 8.2 illustrate that the two sides of the discontinuity share a fundamentally similar tectonic history that has been offset. This is interpreted to be the result of a late stage reverse fault that moved the south side of the field area up by approximately 3 kilometers.