Melt generation at volcanic continental margins: no need for a mantle plume?

Melt generation in a rifting environment is studied using a dynamic 2-D finite element model. The lithosphere is extended to large, realistic thinning factors assuming a mantle temperature of 1333°C. The focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations. The timing of melt production (late synrift) and the amounts of melt are consistent with observations at volcanic margins. The dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins. Volcanic continental margins have been the subject of numerous modeling studies in which the large production of intrusive and extrusive melt was simulated [e.g. McKenzie and Bickle, 1988: Pedersen and Ro, 1992: Bown and White, 1995]. A main conclusion of most of these studies was that a higher than normal potential mantle temperature is required to explain observed melt volumes at volcanic rifted margins. A temperature anomaly in the range of 50-200°C has been suggested by e.g. Pedersen and Skogseid [1989], White and McKenzie [1989], Bown and White [1995]. These increased mantle temperatures are generally explained by the influence of mantle plumes [e.g. White and McKenzie, 1989; Skogseid et al., 1992; White, 1992]. There are several observations that require modifications of the plume model [King and Anderson, 1998]. For example, some volcanic margins including the northwestern of Australia and the eastern U.S. margin [Hopper et al., 1992; Holbrook and Kelemen, 1993], cannot be directly linked to mantle plumes or hotspots. Also, predicted plume head dimensions [Griffiths and Campbell, 1991; Bijwaard and Spakman, 1999] are not sufficient to affect entire volcanic provinces. Recently, Anderson [2000] proposed that these upper mantle temperature variations can also be caused by other processes, like small scale convection. Studies of small scale convection induced by either rifting or discontinuities in lithosphere thickness [Buck; 1986; Anderson, 1994; Boutilier and Keen, 1999; Keen and Boutilier, 1995; 2000; Mutter et al., 1988; King and Anderson, 1995; 1998] indeed show that the amount of melt produced is increased substantially by an enhanced upwelling of mantle material into the melting zone. In this study, a dynamic 2-D finite element model is used to study melt production in a rifting environment on lithospheric scale. The lithosphere is extended to breakup using various mantle temperature and extension rates. First order predictions of melt volumes that are generated using a mantle temperature of 1333°C are consistent with observations at volcanic continental margins