Intermittent Plate Tectonics
Abstract
Intermittent Plate Tectonics A basic premise of Earth Science is that plate tectonics has been continuously operating since it began early in Earth's history. Yet, plate-tectonic theory itself, specifically the collisional phase of the Wilson Cycle, constitutes a process that is capable of stopping all plate motion. The plausibility of a plate-tectonic hiatus is most easily illustrated by considering the expected future of the present-day plate-tectonic configuration. Since the opening of the Atlantic at ~200 ma, the area of the Atlantic basin has been growing at the expense of the Pacific. If this trend continues, relative plate motion models predict that in ~350 my, the Pacific Ocean basin will effectively close leading to widespread continent-continent collisions. Since a continent-continent collision represents the termination of subduction locally, the accumulated effect of all collisions is to stop subduction globally. In this scenario, ridges would then stop spreading and young oceanic lithosphere would cool, reaching a steady-state thickness of 100 km in about 80 my, based on the properties of oceanic lithosphere today. This would constitute the stoppage of plate tectonics. The presumption that plate tectonics never stops in the face of continental collisions is equivalent to requiring that subduction flux is approximately constant through time, such that subduction initiation roughly balances subduction termination. Such a balance then raises several questions about the subduction initiation process. When and how does subduction initiate? Is there a detectible relationship between subduction cessation and subduction initiation? We can gain some guidance into these questions by examining the plate motion history over the last 200 my. Subduction initiation has occurred over the last 80 my in three intra- oceanic subduction zones: Aleutians, Marianas-Izu-Bonin and Tonga-Kermadec in the Pacific basin. In these cases, however, subduction initiation would not prevent the ultimate closure of the Pacific basin and thus the cessation of subduction. More noteworthy is where subduction is not initiating. First, there is no evidence for subduction initiation anywhere within the Atlantic basin (excluding the Caribbean and Scotia), despite the mature 100-200 my age of passive-margin oceanic lithosphere. The formation of the Alpine-Himalayan chain represents the cessation of roughly 10,000 km of subduction at about 35-50 ma, Yet, no new subduction zones have initiated south of India or Africa, the two major continents that participated in the collision. These examples illustrate that subduction does not immediately initiate following a continent-continent collision, and may lag by 10s if not 100s of millions of years. The stoppage of plate tectonics, or even a dramatic reduction in subduction flux, would have significant thermal consequences for the mantle. It would effectively mark a temporary switch to "stagnant-lid" tectonics, analogous to that found on Venus, resulting in a significant increase in global mantle potential temperature (30- 100°C per 100 my) and a possibly widespread increase in magmatic activity. Such a hiatus may have occurred in the Mid-Proterozoic (1.1-1.6Ga), an era characterized by the virtual absence of orogenic activity, the longest-lived passive margin (600 My), and the production of enigmatic "anorogenic" granites found over thousands of kilometers in a belt presently stretching from southwestern to northeastern North America.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.U13B..08S
- Keywords:
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- 8120 Dynamics of lithosphere and mantle: general (1213);
- 8157 Plate motions: past (3040);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)