Grain boundaries in a bubble raft

The PLATE Paradigm

Don L. Anderson

Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125

dla@gps.caltech.edu

Some explanations of melting anomalies at the Earth’s surface–so-called hotspots and hotspot tracks–do not involve high absolute temperatures or deep active upwellings. These have been criticized on various grounds. Propagating cracks do not provide a ready explanation for melting, and eclogite sources require large degrees of melting to generate basalts. However, a combination of lithospheric properties–architecture and stress–and asthenosperic conditions–variable lithology and fertility–overcomes these objections. The locations of magmatism are controlled by the lithosphere; the magnitude of the volcanism is controlled by the fertility and pre-eruption ponding. If ambient mantle is close to the solidus of peridotite then large amounts of melting can be generated from eclogite blobs or by adiabatic ascent of mantle at lithospheric discontinuities or thin zones. The volume of melting depends on the eclogite fraction in the melting domain, not the absolute temperature. A new parameter is involved–the scales of the fertile blobs. The fate of recycled and delaminated eclogite depends on its origin, composition and the amount of peridotite that accompanies it.

Plate reconstructions show that many so-called midplate volcanoes actually were formed at plate boundaries, such as abandoned ridges and triple junctions. On hindsight we know that many “hotspot tracks” are abandoned plate boundaries. Crustal delamination introduces warm fertile material into the source regions of basalts. This combined petrological and tectonic model is called the PLATE paradigm. It is a generalization of plate tectonics and top-down convection. It does not rely on concentrated jets of hot mantle.

The PLATE paradigm primarily involves the following elements and their interactions:

• Plates
• Lithosphere, Lithospheric Architecture
• Asthenosphere
• Tectonics
• Eclogite

PLATE involves passive mantle, heterogeneous shallow mantle, mantle heating, top-down processes, cooling from above, recycling, upper layer instabilities, stress and homologous temperature. Plate reorganizations, plate boundary migrations, no fixed reference system, and non-rigid plates are all implied by the hypothesis.

PLATE is a process (like thermal conductivity, or oxidation) and unlike plume, which is an object.

The PLATE hypothesis is basically athermal. It is based on geology and tectonics more than on fluid dynamics and thermal convection. It is not independent of plate tectonics, but a generalization of it. In addition to variable lithospheric stress and variable mantle fertility, ponding and lateral transport of magma may be involved. Magmatism may not be a steady state process, with melt being created and extruded simultaneously and at the same rate.

The PLUME paradigm involves thermal processes, absolute temperature, uniformity, homogeneity, chaotic stirring, active upwellings, core heat, bottom up, heating from below, vagueness about depth, deep thermal boundary layers, a fixed reference system and a pyrolite source. Magma volumes and seismic velocities are proxies for mantle temperature. The sources of heat, magma and force are all the same. A plume is a tangible object, like caloric, aether or phlogiston. The plume hypothesis is an entirely thermal process. It is based on a branch of fluid dynamics and is independent of plate tectonics; lithospheric properties are of secondary interest.
Both of the above are Paradigms. They are more than theories or hypotheses. A paradigm is a package of ideas, assumptions and auxiliary hypotheses and agreements about the potentially fruitful areas for research.

Plate tectonics is a more powerful concept than generally assumed. It is often considered to be a kinematic or descriptive theory, and plates are often assumed to be rigid and permanent. The PLATE hypothesis is more general than textbook plate tectonics and different from the propagating crack hypothesis, which many people erroneously assume is the main alternative to plumes. Just as the term “plume” implies many things, so the term PLATE implies a variety of tectonic and petrological processes.

Plates are not permanent rigid things, and the surface of the Earth is constantly “recrystallizing”. It thus resembles more a bubble raft or a granular aggregate than mudcracks or the patterns on the back of a turtle. Coherent assemblies of bubbles and grains form islands that are appropriate for the local stress system under which they find themselves. If stress changes, the whole system dissolves and reforms, just like a polycrystal that recrystallizes when stress changes. Plate reorganizations involve more than just rigid plates growing or shrinking and boundaries migrating and jumping. They involve self-organization and far-from-equilibrium processes. Once this is understood, it is recognised that many old “hot spot” tracks e.g., 90-E ridge and the Line Islands chain are previous plate boundaries and presently active “hot spot” tracks may be incipient plate boundaries e.g., the Samoa-Easter trend or faults under tension e.g., the Cameroon Line and Cenozoic volcanism in western North America.

If the underlying mantle is near its (variable) melting point, nothing more is needed than extension to release ponded melts. Plate reconstructions, coupled with the concept that plate boundaries are ephemeral, suggests a reinterpretation of volcanic chains and plateaus. The PLATE paradigm evolved during the first few years of the 21st century, but its roots extend back to the earliest days of plate tectonics. It is a synthesis of developments in petrology, lithosphere mechanics, sampling theory, understanding of stress and self-organized systems, and plate reconstructions.

References to shallow and athermal controls on melting anomalies:

http://www.mantleplumes.org/TopPages/GTofPT_Top.html
http://www.mantleplumes.org/Philosophy.html
http://www.mantleplumes.org/PTProcesses.html
http://www.mantleplumes.org/Eclogite.html
http://www.mantleplumes.org/Cracks&Stress.html
http://www.mantleplumes.org/Hawaii2.html

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