Tectonic evolution of the Superior Boundary Zone from coincident seismic reflection and magnetotelluric profiles

The Superior Boundary Zone (SBZ) forms the northwestern margin of the Arche an Superior craton and constitutes a tectonic foreland of the similar to 1. 8 Ga Trans-Hudson Orogen. The Superior Boundary Fault (SBF) separates the S BZ from the adjacent Reindeer Zone, a collage of Paleoproterozoic juvenile intraoceanic rocks. Lithoprobe seismic reflection and magnetotelluric data were acquired along two profiles crossing the SBZ in an attempt to better c onstrain the deformation and crustal geometry resulting from Trans-Hudson o rogeny. Analysis and interpretation of spatially coincident regional seismi c and magnetotelluric data acquired along the southern 200 km profile indic ate the following: (i)the Reindeer Zone accretionary collage forms an east dipping, eastward steepening, crustal-scale tectonic stack of moderately co nductive rocks near the SBZ. (2) The SBZ is characterized at shallow depths (< 5-6 km) by steep to moderately east dipping reflectivity that is associ ated with the limbs of third generation folds (F3 and D3) and east-side-up shear zones. At greater depth, the SBZ crust is highly resistive and is con tiguous to the east with resistive crust beneath the Superior craton proper . (3) The SBF is recognized in the subsurface as an abrupt resistivity cont rast between the Reindeer Zone and the SBZ, extending subvertically to 15 k m depth. (4) Moderately conductive rocks of the Reindeer Zone extend eastwa rd for 30 km beneath the SBZ at depths of 15-45 km. Seismic reflection data from a second crossing located 100 km NE along strike indicate a similar c rustal structure with some notable exceptions: (1)The SBF is recognized in the subsurface as a truncation of interpreted collisional fabrics and exten ds subvertically to similar to 30 km depth. (2) There is no compelling evid ence for the eastward continuation of Reindeer Zone lower crust beneath the SBZ. To explain the present-day SBZ crustal structure, we propose that the nature of the SBZ evolved over an similar to 200 Myr convergent margin his tory from a lower plate collisional thrust belt setting at > 1.88-1.81 Ga, through lithospheric delamination at similar to 1.82-1.80 Ga to a steep tra nspressive plate boundary at 1.80-1.72 Ga.