SEISMIC IMAGING OF A LOWER-CRUSTAL OCEANIC SLAB: EVIDENCE FOR NEOARCHEAN TECTONIC ACCRETION IN THE WESTERN SUPERIOR PROVINCE, CANADA

The western part of the Archean Superior Province is characterized by east-west-striking subprovinces of alternating granite-greenstone terrains, metasedimentary belts and plutonic terrains ranging. Drawing an analogy between the subprovinces and the belt-like superterranes of the Canadian Cordillera, the structural pattern of Western Superior Province was used as possible evidence for lateral accretion in early plate-tectonic models for the formation of Archean continental crust. Although subsequent geological mapping, geochronological and geochemical work appeared to be compatible with accretion of Neoarchean (<2.8 Ga) oceanic crust, island arcs and sedimentary prisms, with amalgamated Mesoarchean continental fragments, against the southern margin of the >3 Ga composite North Caribou superterrane, various aspects of field relationships, surface structure and age patterns remain equivocal. To test the model in the subsurface, the Western Superior Lithoprobe transect team acquired wide-angle seismic reflection/refraction data and near-vertical incidence, deep (32 s) seismic reflection data along a 600 km N-S corridor. The seismic section reveals two lower-crustal “sutures” along which the reflection Moho is offset by several kilometers, consistent with north over south displacement. Along the northern suture, a Mesoarchean continental fragment of the eastern Wabigoon Subprovince is thrust beneath the North Caribou superterrane. This fragment is underplated from the south by a basal crustal layer characterized by high sub-horizontal reflectivity, very high crustal p-wave velocities (7.6-7.7 km/s) and intermediate density (3.0-3.1 g/cc), that tapers northward, truncating the reflection Moho and extending into the upper mantle to at least 16 s TWT. Velocity and density constraints of this layer can be satisfied by amphibolitic rocks with a sub-horizontal N-S lineation, suggesting that the layer may represent a tectonically underplated slab of Archean mafic crust. Below the Moho, a north-dipping, 20 km thick layer at 50 to 60 km depth is characterized by a 5% p-wave anisotropy (fast direction approximately parallel to tectonic strike) that is interpreted as an Archean fossil fabric caused by LPO of olivine. The geometry and inferred compositions of the two layers are consistent with the previously predicted northward subduction of Neoarchean oceanic lithosphere.