TECTONIC SIGNIFICANCE OF CA. 1.6 GA DEFORMATION IN SOUTHWESTERN LAURENTIA - PROTRACTED MAZATZAL OROGENY OR PREVIOUSLY UNRECOGNIZED EVENT? A PROGRESS REPORT

The Mazatzal orogeny, generally thought to represent the culmination of the protracted period of Paleoproterozoic crustal growth and assembly in SW Laurentia, is considered to have been over by 1630 Ma. 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 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 SW Laurentia from southern Wyoming to Sonora, Mexico, a broader extent than the conventional Mazatzal orogeny. 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 two end-member hypotheses for this event: (1) It represents intracratonic deformation of newly accreted crust that may be linked to changes in distal plate-boundary dynamics and that may have Phanerozoic analogues in the Laramide orogeny or the Tibetan Plateau. This hypothesis would suggest rapid stabilization of, and efficient stress transfer within, newly accreted Paleoproterozoic crust. (2) It represents deformation associated with “delayed” subduction of a long-lived remnant ocean basin followed by final accretion (~ 150 m.y. post Cheyenne belt) at the southern edge of part of the Archean Wyoming craton. This ocean basin may be the N-dipping, 150-200 km long reflector imaged by the CD-ROM experiment. This model implies that Paleoproterozoic crustal assembly was not fully complete until after 1.60 Ga and it requires a 1000 km-long, roughly NNW-striking, transform fault or shear zone from southern Wyoming to northern New Mexico. The postulated structure coincides with a series of topographic lows that may be the northern extension of the Rio Grande rift into Colorado and it lies on strike with the 1.59 Ga Battle Lake thrust-tear fault system in southern Wyoming that truncates the Cheyenne belt, the Archean-Proterozoic suture, at depth. Our results strengthen the evidence for ca. 1.6 Ga tectonism throughout the Paleoproterozoic of Colorado and may have identified deformation associated with a NNW-striking transform fault that is consistent with delayed subduction.