Deep structure of an Archaean greenstone terrane

Abstract

MORE than half of the continental crust was formed before about 2,500 million years ago¹, yet there is still considerable uncertainty regarding the structure and evolution of the ancient Archaean cratons^2–7. Modern analogues proposed for the setting of Archaean greenstone belts include aborted intra-cratonic rifts, continental margins and various oceanic settings^2–7; alternatively, Archaean tectonics may have been quite different from the plate tectonics we see today^2–4,6. The Abitibi belt in Canada is the largest Archaean greenstone ternane³, and has been a touchstone for many models because its surface geology is relatively well known^8–10, it contains significant gold and base metal deposits¹⁰, and it is less deformed and has suffered less erosion than most Archaean ter-ranes. Here we present deep seismic reflection data from part of the Abitibi belt, which reveal extensive subhorizontal and gently dipping reflection zones, truncated by steeply inclined discontinuities which project to major fault zones at the surface. Our preliminary model of the Abitibi belt comprises a mostly juvenile magmatic crust, similar to that exposed in the nearby Kapuskasing uplift^11,12, dissected and imbricated by deep-penetrating discontinuities associated with thrust and strike-slipe faulting. In view of the similarity of greenstone belts worldwide^2–7,13, our seismic sections provide new constraints on the general processes that have affected Archaean metavolcanic terranes.