Ores and Orogenesis - Tectonics Lunch Abstract

Tectonics Luncheon Presentation, Thursday, September 27, noon
Palm, Sand, and Reef: Tectonics and Island Shorelines in the Tropical Pacific
(A Discourse on Scenic Views)

Speaker: William R. Dickinson

Intra-Pacific tectonics may seem tame from a perspective on the Pacific rim, but has a grand sweep of its own. Either the United States or Australia would fit twice, coast to coast, between the western fringe of Micronesia and the eastern fringe of Polynesia. Yet across that whole sea of islands, the land area is less than the state of South Carolina. And more than half of that total lies within Hawaii and Fiji (all the rest combined barely top Pima County in net size). But the tropical Pacific islands make up in variety what they lack in size, and plate behavior gives the whole a coherence that justifies the famous Mark Twain dictum that science can derive surprisingly rich dividends from a trivial investment in facts. A little bit of land here and there provides the template needed to interpret the tectonics of vast sweeps of seascape.

There are hotpot volcanoes and island arcs, atolls and barrier reefs dotted with the tiny islets called motus, islands uplifting on trench forebulges, islands both uplifting and subsiding along forearc belts, islands gradually sinking from growing loads of erupting volcanoes, other islands flexed upward on the annular flexural arches surrounding the volcano loads, and even a fault-block island at a restraining bend in a transform fault. One overriding theme of intraplate behavior is slow subsidence as oceanic lithosphere cools with age, and that sinking feeling is enhanced wherever hotspot magmatism has thermally rejuvenated the lithosphere. Another theme is the regional hydro-isostatic drawdown in sea level by 1.5-2.5 m since mid-Holocene time throughout the tropical Pacific as a result of equatorial ocean siphoning. That mouthful denotes the process underway to achieve global isostatic adjustment after the post-Pleistocene transfer of mass from circumpolar ice sheets to meltwater augmenting the volume of the global ocean. As the landmasses formerly weighted down by glaciers bounce upward (glacio-isostasy), uplifted submarine arches that bulged up around the glaciated regions decay, and the water to cover them as they sink has been drawn from the tropical seas. In the absence of local tectonics to counteract relative change in sea level, island coasts have thus typically been emergent through late Holocene time.

All these wheels within wheels have led to an infinite variety of island coastlines, each type scenic in its own unique way. They never speak with forked tongue of the tectonic controls that govern their morphology, but you must learn to speak their language, and you cannot get a real ear for the vernacular from any extant textbooks. As usual in our business of geology, you have to let the landscape teach you its secrets, one by one. As our lunch digests, we will view some images of paradise that display the kaleidoscopic nature of the shorelines hidden out there in the watery Pacific realm.

[Reference: Dickinson, W.R., 2004, Picture Essay of Pacific Island Coasts: Journal of Coastal Research, v. 20, p. 1012-1034 – just a sampling of the wonders we will explore]

Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits A symposium in honor of William R. Dickinson Tucson, Arizona 24-30 September, 2007 Sponsored by the Arizona Geological Society
Menu: Home William R. Dickinson Sponsorships Paper Submissions Program Tectonics Economic Geology Search: Powered by Website Baker	Tectonics Luncheon Presentation, Thursday, September 27, noon Palm, Sand, and Reef: Tectonics and Island Shorelines in the Tropical Pacific (A Discourse on Scenic Views) Speaker: William R. Dickinson Intra-Pacific tectonics may seem tame from a perspective on the Pacific rim, but has a grand sweep of its own. Either the United States or Australia would fit twice, coast to coast, between the western fringe of Micronesia and the eastern fringe of Polynesia. Yet across that whole sea of islands, the land area is less than the state of South Carolina. And more than half of that total lies within Hawaii and Fiji (all the rest combined barely top Pima County in net size). But the tropical Pacific islands make up in variety what they lack in size, and plate behavior gives the whole a coherence that justifies the famous Mark Twain dictum that science can derive surprisingly rich dividends from a trivial investment in facts. A little bit of land here and there provides the template needed to interpret the tectonics of vast sweeps of seascape. There are hotpot volcanoes and island arcs, atolls and barrier reefs dotted with the tiny islets called motus, islands uplifting on trench forebulges, islands both uplifting and subsiding along forearc belts, islands gradually sinking from growing loads of erupting volcanoes, other islands flexed upward on the annular flexural arches surrounding the volcano loads, and even a fault-block island at a restraining bend in a transform fault. One overriding theme of intraplate behavior is slow subsidence as oceanic lithosphere cools with age, and that sinking feeling is enhanced wherever hotspot magmatism has thermally rejuvenated the lithosphere. Another theme is the regional hydro-isostatic drawdown in sea level by 1.5-2.5 m since mid-Holocene time throughout the tropical Pacific as a result of equatorial ocean siphoning. That mouthful denotes the process underway to achieve global isostatic adjustment after the post-Pleistocene transfer of mass from circumpolar ice sheets to meltwater augmenting the volume of the global ocean. As the landmasses formerly weighted down by glaciers bounce upward (glacio-isostasy), uplifted submarine arches that bulged up around the glaciated regions decay, and the water to cover them as they sink has been drawn from the tropical seas. In the absence of local tectonics to counteract relative change in sea level, island coasts have thus typically been emergent through late Holocene time. All these wheels within wheels have led to an infinite variety of island coastlines, each type scenic in its own unique way. They never speak with forked tongue of the tectonic controls that govern their morphology, but you must learn to speak their language, and you cannot get a real ear for the vernacular from any extant textbooks. As usual in our business of geology, you have to let the landscape teach you its secrets, one by one. As our lunch digests, we will view some images of paradise that display the kaleidoscopic nature of the shorelines hidden out there in the watery Pacific realm. [Reference: Dickinson, W.R., 2004, Picture Essay of Pacific Island Coasts: Journal of Coastal Research, v. 20, p. 1012-1034 – just a sampling of the wonders we will explore]
Ores and Orogenesis, organizers@agssymposium.org Arizona Geological Society, Tucson, AZ Contact: Webmaster, organizers@agssymposium.org

Circum-Pacific Tectonics, Geologic Evolution, and Ore Deposits

A symposium in honor of William R. Dickinson

Tucson, Arizona 24-30 September, 2007

Sponsored by the Arizona Geological Society

Tucson, Arizona
24-30 September, 2007