The LITHOPROBE Abitibi-Grenville Transect is located in southern Ontario and Quebec. Seismic reflection line AG48 lies in the Archean Superior Province that forms part of the central core of the North American continent. This ancient cratonic lithosphere formed between 3,100,000,000 and 2,650,000,000 years ago.

Figure 1: Location of line AG48 relative to the general tectonic elements of North America. The continent is comprised of Archean age cratons (green-blue) sutured together by collisions between plates during the Proterozoic (b/w patterns). More recent collisions (Phanerozoic, red patterns) have added additional lithosphere to the continent.

The Superior Province itself was formed by the amalgamation of a number of "subprovinces" during Archean time. These form a sequence of belts (alternating granite-greenstone belts and metasedimentary subprovinces) that decrease in age from north to south. Line AG48 investigates the transition between the Opatica plutonic belt and Abitibi greenstone belt.

Plate tectonic processes have indisputable controlled the evolution of the lithosphere over the past 2 billion years. However, many continental cores (cratons) were formed during the Archean. There is considerable debate in the scientific community concerning what tectonic processes were most important during this early phase of the Earth's development. At that time, higher temperatures existed in the interior of the Earth and this may have had an significant effect on the formation of continental crust.

However, the seismic reflection data collected along line AG48 clearly image a reflective structure dipping beneath the Opatica belt (see below).

Figure 2: a) Migrated line drawing of seismic reflection data (line AG48). b) Interpretation of crustal structure. Source: Calvert et al., Nature, 1995

The combined seismic and geologic datasets (Benn et al., 1992) provide convincing evidence that the Abitibi/Opatica contact represents an Archean subduction-related suture zone. Calvert et al (1995) interpret the AG48 data as describing a continental-style collision between the Opatica and Abitibi belts. This occurred 2693 Ma and initiated as the northern Abitibi margin underthrust the Opatica, forming a north-dipping lower crustal decollement that displaced the Opatica lower crust and then continued down into the mantle. The collision resulted in the emplacement of the Abitibi greenstone belt (2693 Ma) and later, in the uplift of the central Opatica (2690 Ma; Benn et al., 1992; Davis et al., 1995). The line AG48 data have contributed to the LITHOPROBE evolutionary model of the Superior Province development.

The subduction zone imaged by the Abitibi-Grenville Line48 data is the oldest signature of plate tectonics that has been observed on Earth. The data provide the first evidence of plate tectonics operating in the Archean (more than 2.5 billion years old).

Fig 3: Cross-section of a modern subduction zone that is currently active beneath southwest British Columbia and the northwest USA.