Ontong Java Plateau and late Neogene changes in Pacific plate motion [Review]

摘要

Aided by a new geometrical technique called hot spotting, we recently proposed a new model for the absolute motion of the Pacific plate. This model, called WK97, assumed the Hawaiian hotspot is under Loihi and that there was a change in plate motion at 3-6 Ma. The model located the Louisville hotspot close to the Hollister Ridge, a shallow volcanic ridge associated with a geoid high, seismicity, and recent volcanism. Most of the tectonic implications of WK97, however, were not addressed. Furthermore, although several recent studies suggest that the geochemistry of Hollister lavas may indicate a mixture between Louisville melt material and Pacific-Antarctic Ridge mid-ocean ridge basalt, these studies strongly disagree with WK97 and consider it seriously flawed. Here we discuss numerous tectonic predictions of WK97 and present a large amount of observational evidence in support of it. We find that a late Neogene collision between the Ontong Java Plateau and the northern margin of the Australia plate appears to have altered the motion of the Pacific plate, as inferred from hotspot volcanism, by forcing it to rotate counterclockwise. This rotation, about a low latitude pole, appears to have induced right-lateral sheer stress along the entire Pacific plate divergent boundary, which resulted in the formation of extensional transform faults, microplates, and propagating ridges. This change in motion also appears to have triggered concomitant circum-Pacific tectonism, including trench migration and back are rifting. The WK97 suggests that long-lived, plume-fed volcanism on the Pacific plate appears very limited, perhaps only to five to six hotspots: Hawaii, Louisville, Cobb, Caroline, and perhaps Marquesas and Bowie; the geometry of these chains are consistent with our model. Despite some evidence for volcanic age progressions, most other Pacific Neogene hotspots are most likely "crack spots", i.e., sites of extensional volcanism at preexisting zones of weakness reactivated by plate stresses. Indeed, several volcanic ridges, such as Hollister Ridge near the Louisville hotspot, appear to have formed in response to stresses following the change in absolute plate motion. WK97 also obviates the need for simultaneous, different directions of mantle flow in the south central Pacific. Transpression at the San Andreas and Alpine strike-slip faults and Aleutian Are explosivity since 6 Ma also appear to be consequences of the changes in Pacific plate motion. Furthermore, both the WK97 model and observed relative plate motions suggest that adjoining plates have changed their absolute motions as well and that large changes in absolute plate motion do not necessarily imply large changes in relative plate motion. Our findings support slab purl as the dominant plate tectonic driving force, highlight the rheological difference between strong oceanic lithosphere and weak continental crust, and suggest that WK97 presently provides a unifying, albeit preliminary, framework for relating both intraplate and circum-Pacific tectonism and volcanism in the late Neogene. [References: 167]