Stopping Plate Tectonics

1. Stopping Plate Tectonics Paul G. Silver Carnegie Institution of Washington, DTM Mark Behn Woods Hole Oceanographic Institution

2. Intermittent Plate Tectonics on Earth: I. Demonstrating Plausibility • Assumption: Plate tectonics has never stopped. • This assumption has rarely been challenged. • There is, however, little compelling evidence to support it. • Explore alternative: Plate tectonics (i.e., subduction) is intermittent. • Significant hiatus possible during collisional phase of Wilson Cycle due to “trench lock”. • Length of hiatus depends on timing of subduction re-initiation. • Implications of plate tectonic hiatus: • A significant increase in sublithospheric mantle temperature while in stagnant-lid mode. • Implies temporal variations in mantle temperature tied to Wilson Cycle.

3. Example: When the Pacific Closes • At current rates, Pacific closes in about 350 my. • At this time all subduction stops, plate tectonics ceases. Today 100 my in future 200 my in future 300 my in future

4. Critical Issue: Subduction Death and Birth • Continent-continent collisions produce trench lock. • Commonly assumed plate tectonic conservation law: Conservation of Subduction-Zone Length. Subduction will spontaneously initiate elsewhere to maintain plate tectonics. • Plate tectonics is continuous in time only if subduction initiates elsewhere during or before the onset of trench lock. Otherwise plate tectonics stops. • Question: When and where does (does not) subduction initiation occur? Look at recent record. • Intra-oceanic in Pacific. • Not in Atlantic (neither at passive margin nor intra-oceanic). • Not in response to response to recent episodes of trench lock.

5. Subduction Birth and Death (Cont.) • Cases of subduction initiation in the last 80 my: Tonga-Kermadec, Izu-Bonin-Marianas, Aleutians. • All are in Pacific • All are intra-oceanic. • No subduction initiation in the Atlantic (200my). • No subduction initiation related to either the India-Eurasian or Africa-Eurasian trench lock. (Gurnis et al., 2004)

6. Consequences of a Hiatus in Plate Tectonics • A long lag between the stoppage and initiation of subduction has important geodynamic implications. • Based on present-day examples, plate tectonics could stop for 10’s to 100’s of millions of years. • This represents a transition from plate tectonics to stagnant-lid tectonics: much less efficient in removing heat from mantle. • Consequence: a significant increase in mantle temperature of up to several hundred degrees. Plate Tectonics Stagnant Lid (Reese et al., 1998)

7. Switching Between Plate Tectonics, Stagnant Lid • Assume heat flow q solely a function of potential temperature q=q(T) (large planet approximation). • Two modes: Plate Tectonics, Stagnant lid. • Trench lock leads to switch to stagnant lid. • Trench lock produced by continent-continent collision, governed by Wilson Cycle. (After Sleep, 2000)

8. Time(109 yrs) How Much, How Fast Does the Earth Heat Up in Stagnant Lid Mode? • where, • A = radioactive heat generation rate • C = mantle heat capacity. • Assume that in stagnant lid regime, heat generation rate is much greater than conductive heat loss. • Then is ~10oK/100my today and up to 100oK/100my in the early Earth. (Solomatov and Moresi, 1996)

9. Stopping Plate Tectonics • Plate tectonics may shut down during supercontinent assembly (trench lock). • There will be a hiatus in plate tectonics if subduction initiation is delayed. • Such a hiatus constitutes stagnant lid convection and may be accompanied by significant increase in sublithospheric mantle temperature (tied to Wilson Cycle). • Alternative: There is a subduction conservation law. • Outstanding issues: • Looking in geologic record. • Highlights importance of understanding subduction initiation.