The Uplift of The Lithosphereas

Andy Saunders et al. critically review the geological evidence for regional uplift in five continental flood basalt provinces (Emeishan Traps, North Atlantic, Deccan Traps, Siberian Traps, Yellowstone) as case study. Each of these five provinces are associated with surface uplift.

The authors believe that models which we try to explain for the formation of LIPs without hot mantle plumes have difficulties in explaining surface uplift, rifting and magmatism. They conclude that start-up plume models remain the most convincing way of explaining the formation of LIPs. Saunders suggested that in Iceland plume is the cause of the large volumes of basalt, and it the only reason why oceanic crust is thick (30 km) despite its presence in MOR. The newly developed Al-in-olivine geothermometer was used to find the olivine-Cr-spinel crystallizatioin temperature of a suite of picrites spanning the spatial and temporal extent of the North Atlantic Igneous Provinces (NAIP), which is widely accepted to be the result of deep-seated mantle plume that presently underlies Iceland. Iceland is the only part of the NAIP that remains active.

Here the Al-in-olivine thermometer calibrated by Wan et al. (2008) and extended by Coogan et al. (2014) is used. The Al content of olivine phenocrysts was measured using the Cameca SX100 electron microprobe at the University of Edinburgh. The geothermometer is based solely on the temperature dependence of Al exchange between olivine and Cr-spinel and is largely independent of pressure and melt composition, and so an estimate of these parameters is not required in order to calculate olivine crystallization temperatures. It is found that large-scale changes in the crustal architecture of the North Atlantic region can be explained by long-term cooling of the Iceland plume to 35 Ma, followed by a gradual increase in temperatures until the present day. This suggests that mantle plumes are susceptible to long-term (107 year) fluctuations in temperature throughout their lifetime.

The Al-in-olivine thermometry data thus confirm that start-up plumes are associated with a pulse of anomalously hot mantle over a large spatial area before becoming focused into a narrow upwelling. These conclusions are independent of traditional petrologic approaches to mantle temperature estimation, and provide another strong piece to the growing body of evidence indicating that the temperature of the mantle source region of LIPs is substantially higher than that beneath the mid-ocean ridges. However, some authors have argued that a mantle plume is not required to produce the enhanced melting beneath Iceland (Foulger and Anderson, 2005; Foulger et al., 2005).

For LIPs Saunders suggested that their initial bursts are due to the sudden increase in mantle flux as the plume head starts. Their sudden bursts could also be the result of initial decompression associated with the lithospheric extension in the overlying plate. Related process is the Reunion hotspot which was beneath India in the Late Cretaceous forming Deccan traps.