BASE AND TOP OF THE BOTWOOD GROUP, NEWFOUNDLAND: IMPLICATIONS FOR SLAB BREAKOFF, NET SLAB PULL, AND CLIMATE

The Silurian Botwood Group, comparable to the Old Red Sandstone of the British Isles, marks the onset of terrestrial conditions in the previously marine Iapetan realm. Locally, its base is a clear-cut angular unconformity as at Siccar Point in Scotland. On Change Islands, a transitional shale unit passes upward into an olistostrome and bimodal volcanic rocks, the latter features constituting evidence of slab breakoff following accretion of the main Gander block on the Dog Bay suture. At the top of the Botwood, a north-fed foreland basin delta includes increasing amounts upward of pyroclastic material, including the >80 m-thick Brimstone Head Formation, that may be co-magmatic with the bimodal Fogo Island intrusion. Folded sills attest to synchronous horizontal shortening, all of which might be attributed to collision and breakoff of the Avalon slab. Fogo lies at the northern termination of the >1500 km-long peri-Laurentian Coastal Arc, terminated in time by massive caldera eruptions (cause of end-Silurian biotic turnover?). Jumping to the late Cenozoic, a recent analysis of global plate motion, which is driven primarily by slab pull, raises the possibility that diminishing degassing rate was the immediate cause, rather than arc-continent collision in the southwest Pacific, of Neogene global cooling. Could syn- and/or post-collision slab breakoff, which must subtract from net slab pull, be an indirect but significant player in the larger earth system?