THE CASE FOR A TEMPORALLY AND SPATIALLY EXPANDED MAZATZAL OROGENY

The interval between 1.78 and 1.63 Ga was one of major crustal growth and assembly in southwestern Laurentia. The prevailing view is that the culminating event for this tectonism was the ca. 1.654–1.633 Ga Mazatzal orogeny. Many researchers consider the “Mazatzal front”, the locus of the most inboard effects of the orogeny, to be in central Colorado. Recent and ongoing work, however, has documented a continuum of 1.62-1.58 Ga (U-Pb zircon, titanite; in situ EMP Th-U-Pb monazite; ⁴⁰Ar/³⁹Ar hornblende, muscovite) deformational, magmatic, and thermal events throughout southwestern Laurentia from southern Wyoming to Sonora, Mexico. We suggest that the Mazatzal orogeny may have extended in time to ca. 1580 Ma and in space to the Cheyenne belt, southern Wyoming. If our interpretation for an extended Mazatzal orogeny is correct, the duration of the orogeny may have been ~70 m.y., similar to many orogenies in Earth’s history. A fundamental question is: does ca. 1.6 Ga tectonism represent the latest stages of a protracted Mazatzal orogeny or a previously unrecognized and separate event? We propose that the continuum in ages of deformational, magmatic, and thermal events provides evidence for a spatially and temporally extended Mazatzal orogeny rather than an arbitrarily defined separate event. The ca. 1.6 Ga tectonism appears to represent a shift in tectonic style from that typically associated with the Yavapai (ca. 1.70 Ga) and “classic” Mazatzal (1.65 Ga) orogenies to widespread intracontinental tectonism, similar perhaps to the Tibetan Plateau or the Laramide orogeny. If correct, a corollary to our interpretation is that newly accreted Paleoproterozoic crust stabilized rapidly and facilitated stress transfer far inboard of any active plate margin.