Cycle

The assembly and dispersal of the supercontinents reflect interactions between continental lithosphere and processes operating in the mantle. The first type of interaction involves the broad upwellings and down-wellings that define mantle convection cells (Section 12.9). The second is related to the possible impingement of deep mantle plumes (Section 12.10) on the base of continental lithosphere.

Numerical simulations have provided an important means of investigating the possible relationships between mantle convection patterns and plate motions. Gurnis (1988) suggested that, during periods of dispersal, the continents tend to aggregate over cold down-wellings in the mantle, where they act as an insulating blanket. The mantle consequently heats up, altering the convection pattern, and the supercontinent rifts apart in response to the resulting tension. The continental fragments then move toward the new cold down-wellings resulting from the changed convective regime. Gurnis emphasized the fact that the continents, except Africa, are currently moving to cold regions of the mantle, which are characterized by few hotspots and high seismic velocities. It appears that about 200 Ma ago, Pangea was positioned over what is today the upwelling beneath southern Africa. Since Africa has moved only slowly with respect to the hotspot reference frame, it seems that Pangea may have been situated over this upwelling prior to break-up, in accord with the model. It would thus appear that a positive feedback exists between patterns of mantle convection and the formation of the supercontinents.

The results of experiments also suggest that several mechanisms produce convection patterns that promote the growth and dispersal of supercontinents. The insulating properties of large masses of continental lithosphere create mantle upwelling beneath their interiors (Gurnis, 1988; Zhong & Gurnis, 1993; Guillou & Jaupart, 1995). Large plates also prevent the mantle beneath them from being cooled by subduction, which further promotes upwelling (Lowman & Jarvis, 1999). Sites of downwelling may be controlled by the intrinsic buoyancy of continental lithosphere, which tends to concentrate subduction zones along continental margins. This effect was illustrated by Lowman & Jarvis (1996, 1999) who showed that the collision of two continents at a site of downwelling can trigger a reorganization of the convection pattern, leading to downwelling at the margins and upwelling beneath their interiors (Fig. 12.15). These authors also showed that slab-pull and trench suction forces (Section 12.6) probably were as important as mantle upwelling in the break-up of the supercontinents.

Another important process that affects the relationship between patterns of mantle convection and plate motions is internal heating (Section 12.5.1). Lowman et al. (2001, 2003) showed that, in internally heated models, plate motion is characterized by episodic reversals in direction as mantle circulation patterns change from clockwise to counterclockwise and vice versa. These reversals are caused by the trapping and build-up of heat and buoyancy forces in the interior of convection cells, which destabilizes the convection pattern. The results of modeling suggest that the downwelling of cold material at one edge of a plate can entrain hot material that is trapped below the plate and drag it into the lower mantle. The hot, buoyant material then begins to ascend as the drag of the cold downwelling wanes. The ascent of hot material pushes the plate laterally and induces new cold downwelling on the other side of the plate, beginning a new cycle of upwell-ing and plate motion in the opposite direction. This type of feedback relationship between plate motion and internally heated mantle convection may explain why some plates suddenly change direction on timescales of some 300 Myr.

Many geologic investigations (e.g. Hill, 1991; Storey, 1995; Dalziel et al., 2000a) have demonstrated timespace relationships among LIPs, hotspots, and supercontinental fragmentation. Nevertheless, the role of hot spots or upwelling deep mantle plumes during continental break-up is uncertain. Thermal buoyancy forces due to mantle upwellings and tractions at the base of the lithosphere caused by convecting asthenosphere may contribute to a horizontal deviatoric tension that is sufficient to break continental lithosphere (Section 7.5). Lowman & Jarvis (1999) showed that tensile stresses in the interior of supercontinents depend on the size of the plate, the Rayleigh number of mantle

Figure 12.15 Temperature fields in a numerical model of whole mantle convection that incorporates two continents 5800 km wide (modified from Lowman & Jarvis, 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). Dark and light shading represent cool and warm temperatures, respectively; markings at the top indicate the locations of the continental margins. The continents collide at the model symmetry plane between panels (a) and (b), forming a supercontinent of width 11,600 km. As subduction (dark downwellings) between the two continents ceases, new subduction zones form along the continent margins. Eventually a central upwelling of warm material forms beneath the supercontinent. The supercontinent rifts between panels (k) and (l) some 600 Myr after its formation.

Figure 12.15 Temperature fields in a numerical model of whole mantle convection that incorporates two continents 5800 km wide (modified from Lowman & Jarvis, 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). Dark and light shading represent cool and warm temperatures, respectively; markings at the top indicate the locations of the continental margins. The continents collide at the model symmetry plane between panels (a) and (b), forming a supercontinent of width 11,600 km. As subduction (dark downwellings) between the two continents ceases, new subduction zones form along the continent margins. Eventually a central upwelling of warm material forms beneath the supercontinent. The supercontinent rifts between panels (k) and (l) some 600 Myr after its formation.

convection (Section 12.5.2), the viscosity profile of the mantle, and the amount of radioactive heat present. In addition, their models suggest that, given an internally heated mantle, stresses generated at subduction zones also may be sufficiently large to cause rifting in a stationary supercontinent.

Some geologic data suggest that plume-related magmatism coincided with the assembly, rather than the break-up, of the supercontinents. Hanson et al. (2004) showed that large-scale magmatic events occurred within continental interiors during the Pro-terozoic assembly of Rodinia (Section 11.5.3). These authors also concluded that the impingement of mantle upwellings on the base of continental lithosphere prob ably occurred independently of the supercontinent cycle. Isley & Abbott (2002) used a series of plume proxies, including massive dike swarms, high-Mg extrusive rocks (e.g. Section 11.3.2), flood basalts, and layered intrusions, to identify mantle plume events through time. At least two global scale events coincided with continental assembly in Late Archean and Proterozoic times. From these relationships, it seems that there may be two types of mantle plume events, those associated with supercontinental break-up and those associated with their formation (Condie, 2000). These studies highlight the intriguing but uncertain relationships between mantle plumes and the supercontinent cycle.

Implications of plate tectonics

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