A major geologic boundary has been proposed in the Southern Rocky Mountains separating Proterozoic crustal provinces with different ages and tectonic histories. These provinces probably correlate with the Yavapai (1.8–1.7 Ga) and Mazatzal (1.7–1.6 Ga) provinces of Arizona. Geologic, geochemical, geochronologic, and xenolith data suggest that the boundary lies within a ∼300 km-wide zone that trends northeastward through southern Colorado and northern New Mexico. This zone also seems to have focused later tectonic and thermal effects. However, no major shear zone that might represent a discrete tectonic suture has been identified in the area, and there is no agreement on precisely where the boundary is or what tectonic significance it may have.
We present a review of evidence supporting extrapolation of the Yavapai-Mazatzal boundary through the Southern Rocky Mountains. Limitations in the precision, quantity, and interpretation of available data probably contribute to disagreement over the location of the boundary. However, the disparity in boundaries defined by different data sets may partly reflect a complex or gradational transition between crustal domains. We propose a speculative model for the boundary based on a preliminary structural analysis. Tectonic fabrics appear to be consistent with the initial juxtaposition of arc terranes of the Yavapai and Mazatzal provinces on a low-angle thrust system with later modification and steepening of the boundary during continued crustal shortening. This model explains the diffuse isotopic boundary as a manifestation of a vertically heterogeneous crustal column that might promote isotopic mixing. The cryptic structural expression of the suture may result from a layer-parallel style of suturing and complex post-accretionary tectonic overprinting.