On the Origin of Plate Tectonics

Abstract:

The emergence of plate tectonics was Earth's defining moment. How and when plate
tectonics started is shrouded in mystery because of the paucity of observations in the
Archean as well the challenge of understanding how plates are generated. The damage theory
of lithospheric weakening by grain-reduction provides a physical framework for plate
generation. This model builds on grain-scale processes to elucidate planetary-scale
tectonics, and is consistent with lab and field observations of polycrystalline rocks and
lithospheric shear zones. The grain-damage model accounts for the evolution of damage and
healing (by grain growth) at various planetary conditions, hence predicts plate boundary
formation and longevity, and how they depend on surface environment.

For example, the onset of prototectonics is predicted to require clement conditions to
keep healing from erasing weak zones; conversely, cool conditions possibly required
tectonics to draw down primordial CO₂. Thus whether tectonics preceded a cool climate (and
water) or vice versa is immaterial as they likely needed each other or neither would
exist. Sparse evidence that prototectonics co-initiated with liquid water hints at the
link between tectonics, water and surface conditions.

The establishment of wide-spread plate tectonics started between >4Ga and 2.7Ga, and may
have taken over a billion years to develop. Under Earth-like conditions, combining
grain-damage with intermittent Archean protosubduction produces persistent weak zones that
accumulate to yield well developed plates within 1Gyrs. In contrast, Venus' hotter
surface conditions promotes healing and prohibits weak zone accumulation, which explains
why plate tectonics failed to spread on our sister planet.

Damage and weak-zone inheritance may also influence plate evolution and reorganization in
the modern era. Changes in plate direction, such as reflected in the Emperor-Hawaiian
bend, leave weak zones misaligned with plate motion, causing oblique plate boundaries that
persist for the age of the plate. Grain-damage within a cold subducting slab may also
cause its very rapid detachment, and the abrupt loss of the slab-pull force could account
for precipitous changes in plate motion, such as for the Pacific plate at both 47Ma and
6Ma.