PERSPECTIVES ON LITHOSPHERIC EVOLUTION IN SOUTHWESTERN LAURENTIA, AND PERSISTENT RESEARCH QUESTIONS

SW Laurentia remains a globally important field laboratory for understanding the history of continental accretion along an evolving 1.8-1.0 Ga convergent plate margin and its associated wide intracratonic orogenic zone. Hf analyses of zircon from the oldest rocks of various provinces reveal complex lower crustal architecture involving different aged domains; for example Mojave province is underlain by both Archean and 1.85 Ga crust. ~1.75 Ga juvenile granitoids are present in Grand Canyon, central AZ, and CO. Juvenile ~1.65 granitoids are present in AZ and NM. Suture locations remain poorly defined (except Cheyenne belt at the northern boundary) and assembly of blocks is expressed as wide zones of middle crustal imbrication and distributed transpression that may reflect transposed middle crustal accretionary complexes (e.g. ultramafic tectonic slivers in the Grand Canyon transect). Deformation and metamorphism were active from >1720 to 1350 Ma in what could be viewed as a regional Yavapai – Mazatzal- 1.4 Ga orogenic continuum, although discrete episodes are locally identifiable by pluton-enhanced tectonism, angular unconformities (1720, 1700 in AZ) and disconformities. Isobaric sections are well preserved in different areas, with clockwise P-T loops. Periods of tectonic quiescence (1.6-1.5 Ga and 1.3-1.1 Ga) separate >100 Ma -long orogenic intervals (1.75-1.60; 1.45-1.3, 1.1-1.0). Finite strain fabrics are dominantly contractional. Proposed but still unresolved roles of extension include potential orogenic collapse, back arc basin formation (slab role back), where quartzites were deposited at 1.7 and 1.65 Ga, and pluton-related transtensional strain. Identifying the relative roles of 1.6 and 1.4 deformations in shaping observed fabrics remains a major challenge. Initially considered “anorogenic”, 1.45-1.35 magmatism and tectonism is now considered to be the intracratonic expression of outboard plate margin events. A Cordilleran analogy is useful for the1.45-1.35 orogeny involving flat slab upper plate hydration, lithosphere delamination/ignimbrite flare up, upper crustal extension, and development of a wide orogenic plateau due to epeirogenic mantle-driven uplift. The Great Unconformity represents a billion years of isostatically driven denudation of the 1.45-1.35 Ga orogenic plateau.