PRIMITIVE PLATE TECTONICS IN THE CONTEXT OF PRESENT-DAY PLATE TECTONICS ON EARTH: A PLANETARY PERSPECTIVE

The operation of the present-day plate tectonics on Earth is well understood since its first formulation more than five decades ago. The process is characterized by the following three attributes: (1) horizontal motion of rigid plates on a spherical surface, (2) a kinematically linked global network of moving plates, and (3) recycling of the outer thermal boundary layer (=lithosphere) through subduction and seafloor spreading. Validating the above three diagnostic criteria for the present-day plate tectonics has proven to be difficult, or nearly impossible, when examining the geologic records of the early Earth (>3.5 Ga). For example, a large magnitude of horizontal motion (i.e., >100s km) of a crustal domain may be established, but the evidence alone does not demonstrate the operation of the present-day plate tectonics due to the lack of information for establishing the other two plate-tectonic attributes. Similarly, the existence of a global network of rigid and horizontally moving plates, or the occurrence of local thermal-boundary-layer recycling, also does not prove the occurrence of the present-day plate tectonics. It is conceivable that the three diagnostic attributes of modern plate tectonics may have evolved independently in space and time, and their intersection at a particular time in Earth’s history marked the beginning of the modern plate tectonics. In this context, I suggest the use of “primitive plate tectonics” when only one of the three attributes can be established, and the use of “present-day plate tectonics” when all three attributes are established. Examples of primitive plate tectonics in the solar system include (1) crustal-scale continental underthrusting as revealed by the studies of Hadean zircon, (2) local slab-rollback subduction and supra-subduction crustal accretion over a distance of 1000s km below the Tharsis rise on Mars, (3) large horizontal strike-slip motion (~ 150 km) along the Valles Marineris fault system on Mars, and (4) retrograde subduction on Venus at the rims of large (>1000 km in diameter) coronae. As primitive plate tectonics operates at a local scale as illustrated by the above examples, it is possible that a plate-tectonics process, such as subduction or large-scale strike-slip faulting, may operate simultaneously with other non-plate tectonic processes.