As one of the few accessible regions on Earth where rocks formed in the lower continental crust are exposed at the surface, the Kapuskasing structural zone within the Archean Superior Province was a natural early target for LITHOPROBE. The scientific program of the transect was designed as a study of the deep crust, and the geological processes that affected that crust, beneath a typical Archean craton. Much new information related to this objective has been derived. Unexpectedly, the transect also has provided many insights into the mechanics of later intracontinental deformation.

In south-central Superior Province, the east-west trending subprovince belts of contrasting granite-greenstone, metasedimentary and metaplutonic rocks are interrupted over a distance of 500 km by the northeast-trending Kapuskasing structural zone (KSZ) made of of high-grade metamorphic rocks and marked by associated strong positive gravity and aeromagnetic anomalies. Until 1980, the origin of the KSZ was enigmatic, to the extent that proposed interpretations ranged from an Archean continental suture zone, through a deep transcurrent fault zone of Paleoproterozoic age, to a Mesoproterozoic tensional horst. Percival and Card (1983) proposed that it is an Archean thrust uplift structure. The KSZ Transect scientific program began shortly thereafter, in 1984 as part of LITHOPROBE phase I. By the publication date of the KSZ Transect sysnthesis volume (Percival 1994) new geological and geophysical information had provided a relatively comprehensive understanding of the region's Archean and Proterozoic structural history. The significance of the KSZ Transect results continues to be amplified by the Abitibi component of the Abitibi-Grenville Transect and the Western Superior Transect, as these programs define the regional setting more clearly.

The KSZ exposes an oblique depth section, extending through the Abitibi-Wawa and Quetico subprovinces of the central Superior Province, into the lower crust (a "window" to the lower crust). Three mega-layers that are observed at the surface have been correlated with geophysical features of the crust beneath the Abitibi and Wawa belts:

• Greenstone assemblages: up to 10 km of volcano-associated supracrustal rocks deposited mainly between 2.75-2.70 Ga; deformed and metamorphosed to greenschist facies (up to 450^oC at 0.2-0.3 GPa) by 2.66 Ga; dominantly mafic compositions; upright fold structures; generally weak and localized seismic reflectivity; and moderate compressional wave velocities (Vp of 6.3-6.6 km/s in the mafic volcanics, lower in the rest of the assemblages). This mega-layer is deformed and intruded from beneath by tonalitic gneisses.
• Tonalitic gneiss: up to 10 km of gneiss and granite crystallized mainly between 2.71-2.66 Ga; deformed and metamorphosed to amphibolite facies (600-700^oC at 0.4-0.6 GPa) by 2.645 Ga; and with low-dipping extensional shear zones becoming more pervasive in deeper levels. This correlated with increasing reflectivity with depth and Vp up to 6.5 km/s in the undisturbed crust. This mega-layergrades with increasing depth into layered gneiss.
• Layered gneiss: mafic and parageneisses and anorthosite interlayered on a 0.1-10 km scale; crystallized mainly between 2.76-2.62 Ga and metamorphosed to amphibolite and granulite facies (700-850^oC at 0.8-1.2 GPa) at 2.70-2.60 Ga; and now mainly displaying low dips. This correlates with low dip, bright reflectivity and higher Vp (ca. 6.9 km/s) in the bottom half of the normal crustal section. In the Abitibi belt where the velocity near the base of the crust is only about 7 km/s and crustal xenoliths are of garnet granulite, material of this character could form even the lower most crust.

The mega-layering was in place by ca. 2.6 Ga. Cooling and modest erosion followed, which established the Superior Province as a craton by ca. 2.5 Ga. Rifting began shortly thereafter, with the Huronian basin forming along the Superior Province's southern margin (2.49-2.45 Ga). This was accompanied by emplacement into the craton of the north and northwesterly radiating Matachewan dyke swarm. Subsequeny dyking events at 2.22, 2.17 and 2.14 Ga likely relate to distal rifts and their mantle plumes.

Several lines of evidence indicate that all the dyke swarms were affected by the main Kapuskasing uplift events: geometric relations among dyke swarms and faults; paleomagnetic and geobarometric observations of dykes that crystallized at depth in the KSZ; and distortion of the Matachewan dyke swarm in the hanging wall of the Kapuskasing uplift that records dextral transpression. Thus the uplift occurred after the emplacement of the dykes. It has not been possible to date the Kapuskasing uplift event directly but a variety of constraints link it to collisions of the circum-Superior terranes with the Superior craton at ca. 1.9-1.8 Ga.

This Paleoproterozoic tectonic activity generated both uplift and lateral deformation. Surface geobarometric constraints, along with seismic, gravity and magnetic data support the model of a dextral transpressive zone within which the character changes along strike from a narrow dislocation in the northeast (Fraserdale block) to a broad band of deformation southwest of the Chapleau block. The zone involves about 30 km of horizontal (NW-SE) shortening, which in the central KSZ has been accommodated in the upper 20 km of the crust by brittle thrusting and at depth by ductile flow in a deep crustal root. This process has left a 70-km-wide, 15 km-thick zone with high seismic P-wave velocities at its base (7.0-7.8 km/s), above whick the former middle crust has been elevated to the present surface. Root formation is inferred to have occurred at ca. 600^oC in a "dry" granulite rheology. Apparently, this process also destroyed the original strong reflectivity of the lower crust (either by breaking it up finely or by annealing it metamorphically), since the relatively strong seismic reflectivity found at depth in adjacent undisturbed crust and in the Abitibi belt is much attenuated on profiles over the uplifted and thickened areas. Presence of anomalously dense material in the crustal root of the KSZ helps account for the fact that the long-wavelength negative gravity anomaly that normally should accompany a broad zone of thickened crust is of low amplitude. This material also may contribute locally to the 30-60 mGal positive anomaly which defines the Kapuskasing "high" over 500 km of strike length independently of the prescence of high-grade rocks at the surface. Although imbrication of lower crust and lithospheric mantle could yield P-wave velocities in the 7.0-7.8 km/s range, there is no evidence of strong wide-angle Moho reflections to indicate such a geometry. Alternatively, eclogite is stable at lower crustal conditions (600 ^oC, 1.5-2.0 GPa) and mixed mafic and felsic lithologies in the eclogite facies would have appropriate velocities. Conversion of garnet granulite to eclogite generally is prohibited kinetically unless fluids or strain are present. In the Kapuskasing root zone, the high-velocity material could be eclogite assemblages which formed through ductile strain during the crustal thickening event.

A full summary of the diverse geoscience studies associated with the KSZ Transect is included in the synthesis volume (Percival 1994). One result of the synthesis of these studies is a consistent and comprehensive 4-D (space and time) development of Archean (2.75-2.5 Ga) crustal evolution followed by Paleoproterozoic (2.5-1.7 Ga) cooling and uplift. Percival and West (1994) provided a brief summary of this development. Supracrustal rocks of the KSZ (2.75-2.7 Ga) were buried by younger supracrustal rocks and intrusion of mid-crustal tonalites (2.7-2.66 Ga). Metamorphism began during this period and continued in response to magmatic heat and crustal collapse (2.66-2.63 Ga), followed by slow cooling and intermittent deformation (2.63-2.59 Ga). Subsequently, the Superior Province was eroded by 10 km on average, elevating Kapuskasing levels from ca. 30-20 km. Incipient breakup of the Superior craton (2.5-2.45 Ga) was recorded as new mineral growth at deep structural levels and by Matachewan dyke injection. Additional erosion of a few km preceded intrusion of Kapuskasing dykes (2.04 Ga) into the crustal section. At ca. 1.9-1.8 Ga, stresses caused by plate collisions at the Superior margin were transmitted into the interior in the form of right-lateral transpression along NE-trending faults and NW-over-SE thrusting. These effects elevated Kapuskasing-level rocks from depths of ca. 18 km to 3 km along the NW-dipping Ivanhoe Lake fault zone, where about 30 km of shortening through dominantly brittle deformation is implied. Formation of a crustal root through ductile deformation accommodated shortening in the lowermost crust. Subsequently, northwest-dipping normal faults and a conjugate set of strike-slip faults broke the Kapuskasing structure into separate blocks with variable geometry. Later isostatic rebound reduced topography on the root, producing several km of uplift along steep structures not coincident with the Ivanhoe Lake fault.