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

Abstract

The Superior Boundary Zone (SBZ) forms the northwestern margin of the Archean Superior craton and constitutes a tectonic foreland of the ∼1.8 Ga Trans‐Hudson Orogen. The Superior Boundary Fault (SBF) separates the SBZ 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 constrain the deformation and crustal geometry resulting from Trans‐Hudson orogeny. Analysis and interpretation of spatially coincident regional seismic and magnetotelluric data acquired along the southern 200 km profile indicate the following: (1) the Reindeer Zone accretionary collage forms an east dipping, eastward steepening, crustal‐scale tectonic stack of moderately conductive rocks near the SBZ. (2) The SBZ is characterized at shallow depths (<5–6 km) by steep to moderately east dipping reflectivity that is associated 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 contiguous to the east with resistive crust beneath the Superior craton proper. (3) The SBF is recognized in the subsurface as an abrupt resistivity contrast between the Reindeer Zone and the SBZ, extending subvertically to 15 km depth. (4) Moderately conductive rocks of the Reindeer Zone extend eastward 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 crustal structure with some notable exceptions: (1) The SBF is recognized in the subsurface as a truncation of interpreted collisional fabrics and extends subvertically to ∼30 km depth. (2) There is no compelling evidence 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 ∼200 Myr convergent margin history from a lower plate collisional thrust belt setting at >1.88‐1.81 Ga, through lithospheric delamination at ∼1.82‐1.80 Ga to a steep transpressive plate boundary at 1.80‐1.72 Ga.