Characterizing some major Archean faults at depth in the Superior craton, North America

Abstract. The geometry of ancient faults at depth can only be mapped by high-resolution geophysical surveys such as seismic reflection profiling. Recent deep (35–48 km) reflection profiles acquired across the Archean southern Superior craton of North America provided such data with which to map in 3-D some major shear zones, many of which are associated with significant orogenic gold or VMS deposits. Most faults are (re)interpreted as thrusts; a few appear as sub-vertically aligned breaks in prominent reflectors. Sub-vertical faults possibly originated as syn-volcanic transform faults. Thrusting probably relates to the dominant phase of folding and horizontal shortening strain that occurred during the regional crustal deformation, mineralization and peak metamorphism at 2.72–2.66 Ga, associated with the Kenoran orogeny. Most deformation after this orogenic event resulted in modest lateral movement. Coincident magnetotelluric (MT) surveys indicate pervasive conductive minerals such as graphite/carbon and sulfides, exist within the mid-crust and in near-vertical channels within the more brittle and resistive upper (greenstone) crust. Many such channels, but not all, coincide with fault zones and mineral deposits. Palinspastic and paleomagnetic-based reconstructions suggest many faults had multiple periods of activity with changing vertical to horizontal offsets. Some faults appear paired, partitioning normal and oblique strains on vertical shear zones and dipping thrust zones, respectively.