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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2096
An 1-2-1 Model for the Evolution of the Earths Mantle Structure and Their
Implications for Supercontinent Cycles and True Polar Wander.
Prof. Shijie Zhng
Department of Physics University of Colorado IASPEI
Nan Zhang, Zheng-Xiang Li, James H. Roberts
The present-day Earths mantle is predominated by long-wavelength structures including circum-Pacific
subducted slabs and Africa and Pacific super-plumes. These long-wavelength structures are largely
controlled by the history of plate tectonic motion. Although it dictates the evolution of mantle structure,
global plate tectonic history prior to 120 Ma is poorly constrained except for continental motions that
can be reliably traced back to >1 Ga. An important observation of continental motions in the last 1 Ga is
the two episodes of formation and breakup of super-continents Pangea and Rodinia. We formulated 3D
global models of mantle convection with temperature- and depth-dependent viscosity to study the
formation of mantle structure. We found that for the upper mantle with 30 times smaller viscosity than
the lower mantle and moderately strong lithosphere, in the absence of continents, mantle convection is
characterized by a hemispherically asymmetric structure in which one hemisphere is largely upwellings,
while the other hemisphere contains downwellings (i.e., degree-1 convection). We also found that the
lithosphere plays an essential role in producing degree-1 convection. This is the first study in which
degree-1 mantle convection is observed in mobile-lid/plate-tectonic convection regime at high Rayleigh
number. This result suggests that degree-1 convection may be a dynamically preferred state for the
Earth's mantle. We suggest that the evolution of mantle structure is controlled by a cyclic process of
formation and breakdown of degree-1 convection modulated strongly by continents. The formation and
breakup of supercontinents are surface manifestation of this cyclic process. During the degree-1
convection state, the upwellings in one hemisphere push all continents into the other hemisphere with
the downwellings to form a supercontinent. The non-subducting nature of continents dictates that
subduction in the downwelling hemisphere occurs along the edge of the supercontinent upon its
formation. The insulating effect of a supercontinent and return flow from the circum-supercontinent
subduction should heat up sub-continental mantle and lead to formation of another upwelling system
below the supercontinent (i.e., largely degree-2 platform) and eventually to breakup of the
supercontinent. After the breakup of a supercontinent, the mantle with two large upwellings, similar to
that for the present-day Earth, is then evolved back to degree-1 convection state. This cyclic process
also has implications for the Earths true polar wander. We found that after the sub-continental
upwelling is formed, the two upwellings should be centered at the equator due to their positive geoid
and true polar wander effects. This may explain the equatorial locations of the supercontinent Pangea
and Rodinia before their breakup.
Keywords: supercontinents, convection, true polar wander