PENROSE MEDAL: OVER THE HORIZON: CONNECTING CONTINENTS IN THE GIANT FOOTSTEPS OF DU TOIT AND HOLMES

I was born the year Alex. Du Toit published "Our Wandering Continents". Much of my childhood was in the remote Scottish Highlands. In the mid-1900s I attended the University of Edinburgh where GSA Penrose Medalist Arthur Holmes was Professor and continental drift considered a serious hypothesis. The Highlands drove an urge to explore, Du Toit and Holmes the curiosity to look over the geological horizon. Opportunities came at the University of Wisconsin where my colleague Robert H. Dott, Jr. ran a research program in the southernmost Andean foothills. I wondered what was out there towards the Pacific in the roadless archipelagic hinterland; we both marvelled at the continental character of South Georgia, the isolated island 2000 km to the east. Logistics from the US Antarctic Program permitted us to determine that the key to understanding the Cordillera is the Jurassic-Early Cretaceous opening and mid-Cretaceous closing and inversion of the Rocas Verdes basin, and that the South Georgia microcontinent is a displaced Andean fragment.

Looking again over the horizon I focused on other islands of the Scotia Ridge, and widely scattered nunataks of West Antarctica. These studies with international colleagues improved understanding of the evolution of the Scotia Arc, of implications for the onset/development of the Antarctic Circumpolar Current, and of West Antarctica as a 'collage' of displaced continental blocks that included Zealandia, significant for the history and future of the least stable part of the ice sheet.

In 1989 I Ied a field trip to the Scotia Arc. Among the participants was Eldridge Moores, and shipboard discussions indirectly evolved into his SWEAT hypothesis: that the SW United States and East Antarctica were formerly juxtaposed. The idea, elaborated upon by Paul Hoffman and myself, led to thousands of papers on pre-Pangea paleogeogrphy, supercontinents and the supercontinental cycle. My own contributions have been based largely on looking over the horizon – now from the southern United States to the Argentine Precordillera and Coats Land in Antarctica. Those 'tectonic tracers' suggest a 'southern' location for Laurentia until the opening of Iapetus, a Pacific-Iapetus seaway as the cause of the Cambrian transgression, a narrow Iapetus, and a potential Laurentia-Gondwana collision in the Ordovician.