EOCENE SLAB DETACHMENT LEADS TO CENOZOIC ALKALI VOLCANISM IN THE SOUTHWEST PACIFIC

Cenozoic alkaline igneous rocks are inferred to underlie much of West Antarctica. The magmatism, which started at least 48 Ma in the Ross Sea region and about 36 Ma in Marie Byrd Land (MBL), continues today and has been linked to rifting or a mantle plume. However, plate reconstructions demonstrate that the last major episode of faulting in west Antarctica occurred during the Mesozoic, and geophysical data do not image large Cenozoic basins suggesting that the widespread magmatism cannot be explained by rifting. The geophysically-derived estimates of volumes of magmas erupted in west Antarctica, 1-2 x 106 km3, are low compared to areas associated with deep-seated plumes. Geochemical models and a low velocity zone imaged in seismic tomography data, indicate that the melts originate from a restricted zone between depths of 100-200 km, unlike areas underlain by deep-seated plumes. Restricted upper mantle low velocity zones overlain by Cenozoic alkaline igneous rocks with similar mantle geochemical signatures, also occur in eastern and southeastern Australia, New Zealand and the southwestern Pacific plate. To explain all of the volcanism, an unusually large plume would have to underlie the entire southwest Pacific or there would have to be hundreds of hot spots. Our alternative model links the sudden detachment and sinking of several dense, subducted slabs into the lower mantle in the early Eocene, with a slight rise of upper mantle material away from subduction zones. This ascent of warm mantle may induce melting of metasomatized mantle. When the crust in this region is put under minor extension, small batches of magma leak through pre-existing faults and fractures. Larger volumes of magmas erupt in areas with moderate extension such as documented near the Adare trough, Terror Rift (Mt. Erebus and Melbourne, for example), and the Balleny Islands regions.