OROGENESIS AND CRUSTAL GROWTH: A LITHOPROBE PERSPECTIVE

LITHOPROBE data and synthesis have provided new insight into the growth, modification and stabilization of the continental lithosphere through geophysical images and surface studies of Archean through active orogenic belts in Canada. Comparison of parameters including the nature and time scale of rifting, through accretion, collision and post-collisional processes, enhances understanding of present lithosphere structure and composition. Active volcanic (plume-driven) rift sequences characterize the Archean Superior, Slave, ca. 1.9 Ga Trans-Hudson and 1.1 Ga Keweenawan margins, in contrast to passive margins developed on the 2.45 Ga Huronian, ca. 2.0 Ga Paleoproterozoic, <1.8 Ga Cordilleran and ca. 0.6 Ga Appalachian margins. Accreted terranes, recognized on the basis of isotopic and/or faunal provinciality, range from oceanic to continental, and from proximal to exotic. Ophiolites and mélange are rare and true MORB absent, making sutures cryptic in most orogens. Tectonically underplated oceanic crust in the southern Cordilleran forearc could reflect active subduction erosion, however a similar slab at the base of southwestern Superior crust illustrates the potential for survival in the geologic record. Northern Cordilleran accreted terranes are mainly klippe, whereas stacked terranes comprise the entire Appalachian crust in central Newfoundland. Reflection profiles of Precambrian crust reveal similar, gently dipping panels of distinct reflectivity interpreted as accretion geometry modified by intrusions and younger ductile structures. For example in the Grenville Province, channel flow along low-dipping structures has exhumed lower crustal eclogites and granulites onto the craton. Post-collisional processes recognized in Archean through Cenozoic orogens include delamination, extensional denudation, transpressional strain, successor basin formation, granite “blooms” and “cratonization”. The refractory composition of cratonic lithospheric mantle was likely established during extraction of crustal components although its structure may have been modified by subsequent orogenic processes.