Plate Tectonic Processes – A shallow model for “hot spot” magmatism

G. R. Foulger

Plate Tectonic Processes (PTP) is a model for “hot spot” volcanism based on lithospheric extension and variable mantle fecundity. Earth’s tectonic plates move coherently but they are not rigid, stress is variable within them, and intraplate extension is common, e.g., the East African Rift and the Basin Range province. Extension also occurs at spreading plate boundaries, where many “hot spots” occur. Inhomogeneity of the upper mantle is maintained at plate boundaries by transient processes such as continental breakup and collision and equilibrium processes such as subduction of oceanic lithosphere and extraction of fusible components at mid-ocean ridges. These processes maintain variable fusibility in the mantle. When continents collide, young, late-subducting lithosphere, including overlying sea-floor sediments and back-arc basins is subducted. Such lithosphere is relatively hot and buoyant, does not sink deep into the mantle, and thus eclogitised slabs may lie in the upper mantle below sutured continents. Sutures are preferentially reactivated when continents break up, when delamination of the continental lithosphere may also recycle more fusible, enriched material into the shallow mantle beneath the new ocean. Continental breakup often occurs along sutures, may be accompanied by flood basalts, and enhanced volcanism may persist at some portions of new ridges, e.g., Iceland and the Azores. Some of such volcanism has ocean-island basalt (OIB) geochemistry. Transient large-volume magmatism can be explained by the remelting of subducted oceanic crust in sutures, which is more fusible than mantle peridotite. OIB geochemistry can be explained by large-degree melting of old, eclogitised subducted oceanic crust. Ridges migrate with respect to underlying mantle and thus where rifting occurs longitudinally along a suture, eclogite may be available only for a few Myr. Where a new rift crosses a suture, enhanced volcanism may persist longer as a result of the lateral migration. Where the rift is oblique to a suture, the locus of enhanced melting may migrate longitudinally along the rift. Where deep seismic structures and petrological and heat flow evidence for elevated mantle temperatures is lacking, but suggestive tectonic associations occur, models involving extension and high mantle fertility may fit the observations better than a thermal diapir or plume.