Testing Models for the Formation of Archean Cratons: An Example from the Western Superior LITHOPROBE Transect, Canada

The Western Superior Lithoprobe transect provides a rare opportunity to integrate a multitude of petrological, geochemical, geochronological and structural studies within a geometric framework based on deep (32 s) vibroseis seismic reflection and refraction/wide-angle reflection data, acquired along a 600 km N-S corridor across the prominent E-W trending structural belts of the Western Superior Province. Previous models suggested accretion of Neoarchean (<2.8 Ga) oceanic crust, island arcs and sedimentary prisms with almalgamated Mesoarchean continental fragments against the southern margin of the more than 3 Ga composite North Caribou terrain during the ca. 2.7 Ga Kenoran orogeny. At surface, the southern margin of the North Caribou terrain shows Neoarchean convergent-margin magmatism. Outboard (south) from the protocraton margin, beneath the "accreted" terranes, two lower crustal sutures are identified, where 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 terrain. This fragment is underplated from the south by a basal crustal layer with high subhorizontal reflectivity, very high crustal p-wave velocities (7.6-7.7 km/s) and intermediate density (3.0-3.1 gm/cc), that tapers northward, truncating the reflection Moho and extending into the upper mantle to at least 16 s TWT. The velocity and density constraints of this layer can be satisfied by amphibolitic rocks with a sub-horizontal N-S lineation, and the layer may thus represent a tectonically underplated slab of Archean oceanic crust. Below the Moho, a north-dipping, 20 km thick layer at 50 to 60 km depth is characterized by 5% p-wave anisotropy (fast direction approximately parallel to tectonic strike). This anisotropy is interpreted as an Archean fossil fabric associated with LPO of olivine. All these observations are consistent with the previously predicted northward subduction of Late Archean oceanic lithosphere.