WHAT WERE THE MOST IMPORTANT PALEOPROTEROZOIC EVENTS IN THE ASSEMBLY OF LAURENTIA? A VIEW FROM SOUTH OF THE BORDER (Invited Presentation)

Laurentia is a collage of cratons that have been assembled during all of its history. Although much is now known about the several Archean cratons, the nature of tectonic processes during that eon remains somewhat obscure. In the mid-Paleoproterozoic, we begin to see decipherable tectonic processes that resemble modern plate tectonics. There are numerous examples of orogenies in the Paleoproterozoic terranes of Canada, but the best studied is the 1.9-1.8 Ga Trans-Hudson orogen of the Dakotas, Saskatchewan, Manitoba, and the Canadian arctic, and the coeval Penokean orogeny of the northern US. Here one sees rather clearly the results of continent-continent collision between the Superior craton and the Rae-Hearne craton, with the development of intervening arc terranes, inter-arc basins, and arc plutons. What is displayed seems analogous to the development of the arc terranes of the southwest Pacific as Australia inexorably collides with Asia.

The Paleoproterozoic assembly of southern Laurentia stands in contrast. The ca. 1.7-1.8 Ga Mojave and Yavapai terranes, and the ca. 1.6-1.65 Ga Mazatzal terrane, which extend from California through Arizona, New Mexico, and Colorado into the plains of Kansas and Nebraska, have been thought to represent accreted juvenile arcs. However, there is growing evidence that these terranes are underlain by older crust, likely 1.8-1.9 Ga, and are thus continental arcs. It has recently been suggested that these terranes are exotic to Laurentia, originally formed on the margins of the Mawson continent and accreted to Laurentia ca. 1.6 Ga, the Cheyenne Belt representing the suture. In any case, the tectonic mechanisms appear to be complex terrane accretion rather than simple arc accretion.

Subsequent Precambrian events are outside the scope of this talk, but clearly must include the widespread effects of ca. 1.4-1.0 Grenvillian orogeny and the assembly of Rodinia.