Cryptic suture in the eastern Grenville Province

Abstract

IN order to explain palaeomagnetic data from the Precambrian Shield of North America (Laurentia), Irving et al.¹ have suggested that Grenvillia was separated from Interior Laurentia 1,150 Myr ago and then rejoined it about 1,000 Myr ago to form the present day Canadian Shield. This hypothesis requires that a suture exists within the Grenville Province south of a region of Grenville rocks which are considered to be metamorphosed equivalents of rocks within the adjacent, older structural provinces, and north of sampling sites used to determine Grenvillia poles (Fig. 1); the Grenville Front has been discounted as the site of the suture¹. A weakness of this hypothesis is that there is no known geological evidence for a suture in the appropriate location. It has been suggested¹, however, that the crust now exposed as the Grenville Province originally occupied deep crustal levels (10–20 km) and that any traces of former intervening oceans may have been destroyed. We present here an interpretation of gravity data in the region which is in accord with the presence of a cryptic sutuire within, or proximal to, the eastern part of the suture zone as delineated by the palaeomagnetic investigations.