THE MESOPROTEROZOIC--A TIME OF CHANGE IN TECTONIC STYLE AND MAGMA TYPES IN THE NORTH AMERICAN CONTINENT

Mesoproterozoic time was an important milestone in the evolution of continental United States. After assembly of the continent in the Paleoproterozoic (ca. 1.6 Ga), the tectonic style changed abruptly from convergent tectonism involving accretion of oceanic plates to intraplate tectonism involving the formation and reactivation of a continental-scale northwest-trending left-lateral strike-slip fault system. Fault-bounded crustal blocks of varying dimensions, formed during this deformation, provided zones of weakness for later reactivation by long-lived transpressional-transtensional deformation. Boundary conditions between crustal blocks, rather than the regional stress field, controlled subsequent deformation of the continent. The change in tectonic style was accompanied by a change in magma type, from subduction-related calc-alkaline magmatism to A-type magmatism.

Major features in continental United States that can be attributed to Mesoproterozoic shearing and younger reactivation include: (1) the large Mesoproterozoic clastic sedimentary sequences (e.g., Belt Supergroup). (2) the 1.5-1.4 Ga granite-rhyolite terranes in the northern Midcontinent, and (3) the location of the Ancestral and present Rocky Mountains. Possibly, the Midcontinent rift (1.1 Ga) and the Redfoot rift (Cambrian) owe their existence to transtension along the fault system, as suggested by qualitative analysis. The widespread 1.45-1.35 Ga ductile shears of northeast trend and associated A-type granite plutons in Paleoproterozoic terranes of the southwest could be related to strike-slip transpressional deformation. If so, the shears were guided by pre-existing Paleoproterozoic crustal fabrics.