Stratigraphy and tectonic significance of Cretaceous volcanism in the Queen Elizabeth Islands, Canadian Arctic Archipelago: Discussion

Abstract

Embry and Osadetz discuss the stratigraphy, ages, and tectonic significance of Cretaceous volcanism in the northeastern part of the Queen Elizabeth Islands and draw various conclusions regarding the timing and formation of the Canada Basin and the Alpha Ridge and the spatial relationships between these features and the Sverdrup Rim and Sverdrup Basin. Although in agreement with interpretation of their onshore data, I disagree with some of their conclusions regarding the offshore regions. They conclude from their geological observations that volcanic activity increased northwards to an apparent centre north of Ellesmere Island identified as the site of a mantle plume or hot spot that was active throughout the Cretaceous. This activity, most of which occurred in late Albian time ( 100 Ma ago), was interpreted as coinciding with the main rifting phase of the Canada Basin and the formation of the Alpha Ridge as a hot-spot track (Forsyth et al. 1986). An analysis of gravity data, which provide an insight into the distribution of relatively light and dense masses within the crust (relative to a normal density of 2.67 Mg/m3), calls into question some of these conclusions. Figure 1 herein, a map of enhanced isostatic gravity anomalies taken from Sobczak and Halpenny (1989, and in press), outlines the areas of mass excesses and deficiencies within the crust. Theoretically, any variations in the observed gravity field related to continental-oceanic crustal boundaries, water rock interfaces, topography bathymetry, isostatic roots antiroots, crust mantle interfaces, and thermal and residual glacial loading have been removed in the process of computing the enhanced isostatic map. The zone of volcanism in northern Axel Heiberg and Ellesmere islands, as depicted by Embry and Osadetz, is well outlined by positive gravity anomalies, maximum amplitude 45 mGal (1 mGal = lo-= m/s2), shown in our Figs. 1 and 2. These anomalies are explained by the increased density (by as much as 0.17 Mg/ m3) of the sediments produced by thermal metamorphism associated with intrusion of mafic igneous rocks and an additional increase (0.10 Mg/m3) caused by the presence of the latter occurring as intrusions and related volcanic extrusions within the sedimentary column (Sobczak et al., in preparation). This belt of positive anomalies extends northeastwards