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We thank Nazario Pavoni for pointing out his previous observations of approximate bipolar symmetry in present-day global tectonics and deep mantle seismic velocities (Pavoni, 1991). The latter observation has been generally confirmed, especially by more recent analyses of teleseismic S-waves (Grand et al., 1997). However, in his comment, Pavoni also suggests this bipolarity extended back to at least the Middle Jurassic and that associated deep mantle plume activity (“up-welling in the underlying mantle”) caused the near simultaneous initial breakup of the Pangea supercontinent and the formation of the Pacific microplate. This interesting speculation contrasts with ours, which is that the Pangea breakup caused the formation of the Pacific microplate by transmitting stress changes through the plates themselves to the plates beneath the Pacific superocean.

Because the time of the initial Pangea supercontinent breakup (the separation of North America from Africa) is imprecisely known and very little remains unsubducted from the former plates beneath the Middle Jurassic superocean, it is nearly impossible to either disprove or support our speculation or Pavoni's. However, we argue by analogy with the Early Cretaceous breakup of the Gondwanaland supercontinent in the Southern Hemisphere and the near simultaneous changes in plate motions in the Pacific that changes in stresses transmitted through the tectonic plates are also likely to have been the case for the Jurassic events.

The Gondwanaland breakup and roughly antipodal changes in plate velocities and plate boundary configurations in the Pacific occurred during a time of well-known and dated magnetic reversals (Channell et al., 1995), which left characteristic magnetic anomaly patterns (Cande et al., 1989) marking these tectonic events in three ocean basins. Gondwanaland breakup began with the separation of South America and Africa off South Africa at magnetic anomaly M11 time (131 Ma). This was followed closely by the separation of India from Australia off western Australia at M10 time (129 Ma). In the Pacific, near simultaneous velocity changes occurred at various boundaries of the Pacific plate. The Pacific-Farallon spreading ridge rotated clockwise about 20° at M10N time (130 Ma) and at the same time the Pacific-Phoenix spreading ridge propagated an additional 700 km into what is now the Central Pacific Basin (Nakanishi et al., 1992). Sometime between M14 and M11 the Magellan microplate was born near the Pacific-Farallon-Phoenix triple junction (Tamaki and Larson, 1988). It then lived out its short tectonic life and died with the cessation of spreading at the Magellan Trough at M9 time (128 Ma).

The nearly synchronous timing of the above Early Cretaceous tectonic events suggests they were somehow related. Although the Gondwanaland breakup probably was triggered beneath Brazil by the Parana mantle plume, there is almost no evidence for a corresponding pulse in mantle plume activity forming oceanic plateaus on the Pacific plate at that time. This lack of evidence for mantle plume activity beneath the Pacific is not absolute proof there was none, as it all might have occurred on surrounding plates, which are now mostly subducted. However, that seems unlikely to us. Conversely, the nearly identical ages of these events suggest that the forces causing changes in plate velocities and plate boundary configurations in the Pacific were rapidly transmitted from the newly fragmented Gondwanaland supercontinent. The internal rigidity of the plates would allow this change in driving forces to be transmitted rapidly and globally through the plates themselves. We speculate that the same type of force transfer in the Middle Jurassic also caused the formation of the Pacific microplate in response to the breakup of Pangea by separation of North America away from Africa.