OROGENIC FLOAT AND THE MONAZITE CONNECTION

New field mapping in the western Canadian Shield combined with in situ Th-U-total Pb electron microprobe monazite geochronology document simultaneous movement of three crustal-scale shear zones. All three shear zones exhibit similar SW-plunging stretching lineations and top-to-the-NE kinematics. The structures record oblique-slip and thrust-sense displacement during dextral transpressive strain and uplift of >20,000 km² of continental lower crust. Monazite grains in tectonites from all three shear zones display rims that grew syn-kinematically during fluid-mediated metamorphic and metasomatic reactions at 1849 +/- 6 Ma (MSWD = 2). The results provide a spectacular example of a hundreds of kilometer-scale “orogenic float” during detachment and uplift of lower crust, as documented by micrometer-scale rims on monazite. The style of strain partioning during uplift is indicative of discrete segmentation and translation of rheologically strong blocks of lowermost crust. This study provides geochronologic evidence for strain partitioning and contemporaneous displacement on strike-slip and thrust-sense shear zones.