PROTEROZOIC METASEDIMENTARY SUCCESSIONS AS INDICATORS OF LITHOSPHERIC STABILIZATION AND REACTIVATION IN SOUTHWESTERN LAURENTIA

Thick metasedimentary successions containing abundant orthoquartzite are exposed throughout Proterozoic accretionary provinces of southwestern Laurentia. They were deposited in relatively short-lived basins following major deformational and metamorphic events during the southward growth of Laurentia ca. 1.8–1.6 Ga. Similar successions were deposited, deformed, and metamorphosed again during intracontinental orogenesis ca. 1.4 Ga. Paleoproterozoic successions are typically underlain by voluminous rhyolite and were deposited during granitic magmatism at deeper crustal levels in adjacent regions. Unimodal detrital zircon populations suggest local sources and a first-cycle origin for the orthoquartzites within short time intervals (ca. 20 m.y.) during unroofing of local basement. Basins to the south likely formed in an intra-arc setting, and basins to the north likely formed along the continental margin or continental interior. Deformation and metamorphism of the successions occurred in response to renewed convergent or collisional activity as the locus of accretion shifted southward through time. The spatial and temporal association of sedimentation and post-orogenic magmatism suggests that regional crustal extension occurred at approximately the same time, exhuming recently formed basement assemblages and accommodating voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface. The close temporal link between these events and major periods of orogenesis suggests that these two processes might have been critical for stabilization of heterogeneous, newly accreted lithosphere within the broad accretionary orogen. Recently discovered Mesoproterozoic metasedimentary successions in New Mexico and Arizona were derived from reactivated portions of the Paleoproterozoic accretionary provinces, and deformation and metamorphism of these successions support regional arguments for ca.1.4 Ga orogenesis in a transpressional tectonic setting. These successions are exposed along the trend of the Jemez lineament, a candidate for suture zone between Paleoproterozoic accretionary provinces. Thus, they provide a unique and important record of thrust belt development and reactivation of a major lithospheric structure in the southwestern United States.