 REPORT |
When Did Plate Tectonics Begin?
Lander, Wyoming, USA
13-18 June 2006
Conveners:
- Kent C. Condie
- Department of Earth & Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA
- Alfred Kröner
- Institut für Geowissenschaften, Universität Mainz, D-55099 Mainz, Germany
- Robert J. Stern
- Geosciences Dept. (FO2.1), University of Texas at Dallas, Box 830688, Richardson, Texas 75083-0688, USA
This Penrose Conference, addressing the question of when plate tectonics began on Earth, took place in Lander, Wyoming, on 14–18 June 2006. At the onset of the meeting, we addressed the question of what constitutes plate tectonics. Although participants agreed that plate tectonics involves horizontal plate motions on Earth’s surface, including sites of plate formation and recycling into the mantle, there was considerable debate about how best to track plate tectonics back in time. Part of the disagreement was related to whether the mechanism of recycling lithosphere into the mantle evolved as Earth cooled. One way to partially bypass this issue is to consider “modern-style” subduction (steep slab subduction) and “premodern-style” subduction, which may have been different (e.g., symmetric lithosphere foundering, flat subduction, or eclogite-driven delamination of oceanic or lower continental crust).
Although geodynamic models support the existence of coherent lithospheric plates throughout earth history, just when and how these plates became negatively buoyant is not yet clear. This is partly due to our uncertainty about the early thermal history of the mantle. Most participants agreed that the Archean mantle must have been hotter, but it is not clear how much hotter. A closely related and unsolved question asks how thick the early Archean oceanic crust was. Although it is commonly thought to have been thicker than now, thus making Archean plates more buoyant, Geoff Davies presented a model whereby the early oceanic crust may have been no thicker than at present if the upper mantle was strongly depleted. Another interesting question raised by Paul Silver was the possibility that plate tectonics may have stopped and restarted more than once during Earth’s history.
Perhaps the most controversial subject at the meeting was that of tracking plate tectonics back into the Archean using modern rock associations and subduction P-T regimes. An arc-like petrotectonic assemblage (basalt-andesite-dacite-rhyolite-graywacke and associated minor rock types) is widespread to at least 2 Ga, common to 3 Ga, and is found locally in crust older than 3 Ga. We visited one such Archean greenstone (South Pass greenstone, Wyoming) on our first field trip.
Subduction-related ore deposits are common to at least 2.7 Ga, and sedimentary basins related to plate tectonics (passive margins, foreland basins, strike-slip basins, etc.) occur to at least 2.5 Ga, with possible examples as old as 3 Ga. Also, dual thermal regimes (low-P–high-T and high-P–low-T) suggestive of plate tectonics are identified at least to 2.8 Ga and possibly to 3.3 Ga. In addition, collisional orogens appear to exist to ca. 2 Ga and accretionary orogens to ³3.5 Ga in the West Pilbara (Australia). Both types of orogens contain terranes with distinct terrane boundaries.
In contrast to subduction-related rock packages, blueschists, ultra-high pressure (UHP) metamorphism, and ophiolites do not become common in the geologic record until after 1 Ga, suggesting to Bob Stern that “modern-style” plate tectonics did not begin until about 1 Ga. However, as pointed out by John Percival, more efficient subduction in the Archean may have prevented blueschists and UHP metamorphic rocks from returning to the surface. Another possibility is that subduction geotherms may have been too steep in the Archean to pass through the blueschist stability field, although the relationship of mantle temperature to subduction geotherms is not well understood. Although complete ophiolites older than 2 Ga have not been found and are rare before 1 Ga, many Archean greenstones may represent fragments of upper oceanic crust, as in the section we examined on our second field trip to the Tin Cup Mountain area in central Wyoming. If the Archean oceanic crust was thicker than at present, perhaps only the upper part was obducted and preserved.
Paleomagnetic data, which clearly record differential motions (probably plate motions) to 1.9 Ga and possibly to 2.6 Ga, contribute to answering the question of when plate tectonics began. Also consistent with plate tectonics is a long-term history of depletion in the upper mantle and the recycling of ancient lithosphere, as recorded by various radiogenic isotopes. Xenoliths of Archean continental mantle lithosphere that have stable isotope signatures for surficial processes can only be explained by the recycling of sediments into the mantle, again consistent with the early onset of plate tectonics.
Jean Bédard presented a testable non–plate tectonics model for the Archean involving oceanic plateau production from mantle plumes in which plateau root zones melt to produce trondhjemite-tonalite-granodiorite granitoids and eclogitic restite sinks into the mantle. Although other alternatives to plate tectonics in the Archean were considered (such as mantle plume tectonics and “drip” tectonics, whereby drips of hot oceanic lithosphere sink into the mantle), they were not discussed in detail.
We took a straw vote before and after the meeting as to when participants thought plate tectonics began. Although in both cases about 70% voted that plate tectonics began sometime between 2.5 and 4 Ga, 20% of the participants changed their vote between the two ballots. One participant favored the idea that “modern-style” plate tectonics began about 1 Ga. Regardless of the fact that a consensus was not reached, most of the evidence at hand suggested that “modern-style” plate tectonics “evolved” from an earlier form of proto–plate tectonics sometime in the Archean. The group did agree that answering the question of when plate tectonics began is critical for understanding the evolution of the solid earth system, and participants appeared to leave the meeting rejuvenated and challenged by a greater appreciation of the multidisciplinary nature of the question as well as the many other questions that remain unanswered.
Participants:
Don Anderson |
Mark Harrison |
Hugh Rollinson |