Yigang Xu
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
September 1, 2004
A significant prediction of mantle-plume theory is crustal uplift of 1000 +/- 500 m prior to flood volcanism over an area of 1500–2000 km across (Campbell and Griffiths, 1990). This uplift would leave recognizable effects on the sedimentation, such as localized shoaling, thinning of strata over the uplifted area and erosional unconformity between the basalts and underlying stratigraphic sequence (Rainbird and Ernst, 2001) and on radial drainage patterns (Cox, 1989). The lack of such evidence, on the other hand, is an argument against the involvement of plumes in the formation of large igneous provinces (LIPs) (Anderson, 1994; Czamanske et al., 1998; Sheth, 1999).
Recent field investigations yield encouraging evidence of pre-volcanic uplift in some LIPs (Rainbird, 1993; He et al. 2003). Rainbird (1993) found that in the upper part of the Neoproterozoic Shaler Supergroup in Northern Canada, marine carbonates are abruptly overlain by coarse fluvial sediments that thin to the northeast and eventually are completely eroded. The flood basalts of the Natkusiak Formation overlie the marine carbonates. These facies relationships imply extensive and rapid uplift towards the northeast, where a mantle plume is thought to have been centered. Perhaps, the best-documented examples of plume-related sedimentation is described by He et al. (2003) in the late Permian Emeishan LIP in SW China. The Middle Permian Maokou Formation that immediately underlies the Emeishan flood basalts are variably thinned and are capped by a subaerial unconformity, which is generally manifested by karst paleotopography, a paleoweathering zone, or locally by relict gravels and basal conglomerates. Iso-thickness contour of the Maokou Formation delineates a subcircular uplifted area, in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling. The duration of this uplift is estimated to be less than 3 m.y. and the magnitude of uplift is greater than 1000 m. More importantly, systematic spatial variations are observed across this domal structure in the distribution and thickness of clastic and carbonate sediments, the extent of erosion, the thickness and chemistry of volcanic rocks, and the crust-mantle structure (Xu et al. in press). These features, which are best explained by a fossil mantle plume, highlight the role of field investigation as a reliable and independent means of identifying the role of mantle plumes in the generation of LIPs (Rainbird, 1993; Dam et al., 1998; Williams et al. 2000; He et al. 2003). It is thus anticipated that investigation using similar approaches would help clarify the relationship between uplift, deformation and volcanism in other LIPs.
References cited
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