THARSIS SUPERPLUME(1): WHY SUPERPLUME?
A new superplume concept is proposed to explain low-V seismic anomalies of huge lateral extent located above the core/mantle boundary (CMB) and below the Pacific and Africa, referred to respectively as the Pacific and African superplumes (Maruyama, 1994; Fukao et al., 1994). Recent high-resolution global seismic tomography supports their presence. Moreover, a synthesis of stratigraphic, tectonic, geochemical, and geophysical data of the Pacific and African demonstrate that the stable, deep mantle superplumes are the dominant engines of the Earth that drive mantle convection and plate tectonism, dwarfing hotspots by an order of magnitude and persisting on much longer time intervals. The superplume hypothesis is also supported by combined S- and P-waves inversions and by rheology (Karato, 2001). The results show a compositional anomaly, rather than a temperature one, and are most consistent with the history of petro-chemical signatures of superplume rocks at the surface. Particularly, the superplume is characterized by its high water content and recycled MORB component in source mantle. These dynamic and geochemical aspects offer a new explanation, beyond conventional plate tectonics to explain Earths internal and external dynamics over long time scales, including plume pulsation, supercontinent cycle, local or whole-scale of mantle overturn, and climate change. We apply this superplume concept to the Tharsis magmatic complex, Mars, which has similar traits to Earths superplumes (see Dohm et al., this volume). Like Earth, the magmatic complex of Tharsis volcanism is characterized by abnormally long-lived and fixed deeper mantle plume activity, but it struggled to initiate supercontinent breakup. With that failure, the Tharsis superplume transported heat, material, and water from the deeper mantle/core to the surface, penetrating a thickened rigid lithosphere.