Pacific microplate and the Pangea supercontinent in the Early to Middle Jurassic: Comment and Reply
COMMENT
Bartolini and Larson (2001) touch on a fundamental question in global tectonics, i.e., the relationship between the origin of the Pacific plate and the breakup of Pangea. The authors link the birth of the Pacific plate ca. 175–170 Ma with the start of Pangea's breakup ca. 190–180 Ma near Africa. They suggest that the initial plate separation of Pangea increased subduction rates at its outer margins and altered the plate boundaries in the Pacific superocean, leading to the formation of the Pacific plate. In this view, the formation of the Pacific plate is a consequence of the breakup of Pangea. Transfer of plate movements is mainly achieved by intralithospheric reorganizations.
The African plate came into existence, at low latitudes, as a consequence of the middle Mesozoic breakup and later disintegration of the supercontinent. The drift of North America and the Gondwana fragments were directed away from Africa (Fig. 1). Long-lived, large-scale diverging plate movements and widespread extensional deviatoric stresses are clearly documented by the evolution of the African plate.
Schematic representation of geotectonic bipolarity and paleogeographic evolution of lithosphere in Pacific and Eurafrican hemispheres (after Pavoni, 1991). World maps in cylindrical equidistant projection. Contour lines of continents and continental fragments are partially indicated. Single lines indicate divergent and transform boundaries. Lines with barbs indicate convergent boundaries. Arrows indicate large-scale, diverging movements of plates away from spreading centers P and A. (a) Stage of breakup of Gondwana and Pacifica ca. 130 Ma (see text). A north-south–trending zone of convergence, subduction, and downwelling between the two spreading centers is indicated. (b) Present configuration of Earth's lithosphere. Pacific plate and African plate are centered on equator in antipodal position.
The Pacific plate originated in the center of the lzanagi-Farallon-Phoenix ridge-ridge-ridge (RRR) triple junction ca. 175–170 Ma at equatorial paleolatitudes (Larson and Chase, 1972; Hilde et al., 1977). The triple junction was located in the midst of an assemblage of oceanic plateaus and continental fragments, the Pacifica Archipelago (Pavoni, 1991). Due to the growth of the Pacific plate and diverging movements of neighboring plates, the Pacifica Archipelago was dispersed and its fragments later incorporated into the circum-Pacific orogenic belts (Fig. 1).
At present, the Pacific plate and the African plate represent the two major plates of Earth. Both plates are of “circular” shape. They are centered in antipodal position on the equator (Pavoni and Müller, 2000). The center of the African plate is at 10°E and 0°N (pole A); the center of the Pacific plate is at 170°W and 0°N (pole P). Poles P and A mark the centers of long-lived lithospheric divergence in the Pacific and anti-Pacific hemispheres, respectively. The arrangement of the two plates reveals a fundamental hemispherical symmetry or bipolarity in global tectonics.
Geophysical investigations show that the same fundamental Pacific–anti-Pacific bipolarity is evident in the large-scale distribution of seismic velocity variations in the lower mantle (Richards et al., 1988). Regions of pronounced reduced seismic velocities are found deep in the mantle beneath the central Pacific as well as beneath Africa. Laterally reduced seismic velocities indicate upwelling flow. The long-lived lithospheric divergences in the Pacific and anti-Pacific hemispheres are thus fed by upwellings in the underlying mantle. However, laterally increased seismic velocities associated with the subduction of slabs in the circum-Pacific orogenic belt mark the downwelling flow in the mantle (Pavoni, 1991). A bicellular pattern of convection seems to govern the arrangement of plates at Earth's surface.
Considering the arguments presented here, a transfer of plate movements based primarily on intralithospheric reorganization, in conjunction with a pulse in subduction activity ca. 175–160 Ma, appears inadequate to explain the location of birth and the long-lived evolution of the Pacific and African plates. The formation of the Pacific plate is not just a secondary effect of the beginning breakup of the supercontinent near Africa. The breakup of the lithosphere of the ancestral Pacific region and the breakup of Pangea around Africa are manifestations of deep-seated, hemispherically symmetric processes, i.e., thermal convection, in Earth's mantle. The birth of the Pacific plate as well as the formation of the African plate are part of these manifestations.
- © 2003 Geological Society of America
