SILURIAN TECTONICS OF WESTERN AVALONIA - STRAIN-CORRECTED SUBSIDENCE HISTORY OF THE ARISAIG GROUP, NOVA-SCOTIA

The Arisaig Group is a Silurian succession of predominantly shallow ma rine elastic sediments overlying volcanics, exposed in northern mainla nd Nova Scotia. Sediment accumulation provides a record of the subside nce of western Avalonia during the interval when terranes were being a ccreted within the Canadian Appalachians. To calculate the amount of s ubsidence, one must correct the measured thicknesses for the effects o f tectonic strain. Deformed fossils on bedding surfaces indicate strai n ratios mainly between 1.2 and 1.6. An empirical porosity-depth relat ionship is used to correct for compaction. The subsidence curves are t hen adjusted to allow for variations in water depth and eustatic sea l evel. The resulting curves show significant variations in subsidence r ate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian t ime, with deeper-water sedimentation during an early Ludlovian eustati c high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidenc e rate occurred during deposition of the Pridolian Stonehouse Formatio n. This episode was accompanied by a rapid increase in sediment supply , and a change in paleocurrent flow from southwest to northwest. The r apid Pridolian subsidence probably resulted from oblique collision bet ween Avalonia and the Meguma Terrane to the south.