PROGRESSIVE DEFORMATION IN MIDCONTINENTAL MINNESOTA FROM THE PENOKEAN (1.8 GA) THROUGH LARAMIDE OROGENS (~50 MA)

Radial intrusion of the Kenora-Kabetogama dikes (2.07 Ga) preceded the Penokean orogen (1.8 Ga) and the dike margins show evidence of strike-slip offsets related to dike-parallel horizontal shortening. Penokean deformation is also preserved in the Minnesota River valley along the reactivated Morton-Montevideo terrane boundary where pseudotachylite is found along a 30 meter section with complex kinematic relationships, as well as the Cuyuna thrust belt-Mesabi range foreland transition. Near Virginia, MN Mesabi BIF and underlying Pokegama Quartzite preserve a burial AMS fabric, while a calcite-quartz zone in the lower cherty horizon preserves a northwesterly, sub-horizontal shortening strain that is offset by top-to-the northeast (thrust) striations. The Mazatzal orogen (1.65 Ga) is represented by a thin-skin thrust belt where the northerly Barron and Sioux Quartzites are undeformed (1:1:1) and increasing deformation, using Fry, AMS, and EBSD techniques, is observed south to the Baraboo syncline. Underlying basement and dikes record this disturbance with coeval Ar/Ar ages. The Keweenaw province (1107-1094 Ma) quickly evolved into a Grenville back-arc thrust belt along the Douglas and Keweenaw thrusts and twinned calcite preserves two distinctive strains: rift-normal shortening (amygdule calcite; NW-SE thrusting), and rift-parallel shortening (veins, cements, clastic dike cements; SW-NE strike-slip motion). The Appalachian orogen (245 Ma) reactivated these same thrusts in the mid-continent, offsetting Paleozoic sediments as young as Devonian (Afton anticline, River Falls syncline, Limestone Mtn. syncline) and Permian (sub-surface). A SE-NW layer-parallel shortening strain is preserved in foreland Paleozoic carbonates (twinned calcite, AMS) from the Appalachian front to northwestern Minnesota. The Sevier-Laramide orogen caused calcite to twin in the Greenhorn Limestone, preserving a horizontal W-E layer-parallel shortening strain in the distal eastern foreland.