Interaction of rifting and hot horizontal plume sheets at volcanic margins

Abstract

Abstract. We have explored the idea that observations of large igneous crustal thickness at volcanic rifted continental margins may be explained by the interaction of rift-driven flow in the lithosphere with an underlying, sublithospheric hot plume sheet. This concept is assessed by numerical modeling of the viscous flow caused by divergent plate motions, using a viscous, temperature-and pressure-dependent, nonlinear fluid. The plume sheet consists of an initially hot layer just below the lithosphere which responds to the overlying lithospheric motions. For reasonable values of input parameters we can successfully predict the observed thickness of igneous crust at the margin, its formation time, inferred average mantle melt fractions, and, importantly, the eventual transition to normal oceanic crustal thicknesses. Observations are selected from the North Atlantic region, with an emphasis on the recent results from the east Greenland volcanic margin. The best fitting models suggest that relatively little small-scale convection is needed after the initial strong pulse of mantle upflow due to the presence of moderately hot (-1450øC) buoyant plume material under the rift. The results favor a thin (-50 km) plume sheet layer. This hot mantle reservoir is soon depleted and replaced by normal temperature mantle, unless the rift lies directly over the deep source of plume material.

Lithospheric controls on melt production during continental breakup at slow rates of extension: Application to the North Atlantic

“…Furthermore, the authors question the identification of magnetic cryptochrons [see Larsen and Saunders , 1998] on which this period of fast extension is based. Our previous model of the southeast Greenland margin reproduced the igneous thickness observed, however it also relied upon the same prethinning of the lithosphere as Keen and Boutilier [2000] and Nielsen and Hopper [2004].…”

“…These geophysical observations show that caution must be used when applying simple geodynamic models to rifted margin formation. When modeling the formation of a rifted margin, some prethinning (necking region) of the lithosphere is often assumed [e.g., Keen and Boutilier , 2000; Nielsen and Hopper , 2004]. This has been justified as it encourages small‐scale convection and focuses the melting at the ridge axis [ Boutilier and Keen , 1999].…”

The North Atlantic Igneous Province: A review of models for its formation

“…The model by Nielsen and Hopper (2002) thus requires the passive influence of a mantle plume and spread of a plume head for rapid emplacement of warm mantle material in lithospheric thin spots. However, the estimated melt volumes predicted in this study and in the studies of Boutilier and Keen (2000) of breakuprelated magmatism do assume that the LCB represent magmatic material; an idea that is presently challenged.…”

“…This suggests that vertical motions of the margins could be expected around breakup time. Because the plume sheet layer is soon depleted below the margins, Keen and Boutilier (2000) conclude that formation of the Greenland-Iceland and Iceland-Faeroe Ridges requires a deeper-sourced thermal anomaly. Nielsen and Hopper (2002) also use an upper-mantle convection model to study the interaction of a sub-lithospheric hot plume sheet with rift-driven flow.…”

“…Application of this model to the NAIP, and especially the mid-Norwegian Vøring margin, suggest that characteristic pre-separation features (extension, vertical movements, timing and duration of magmatism) are well explained by it (van Wijk et al, 2004), provided (pre-) rift conditions are favorable. Keen and Boutilier (2000) and Boutilier and Keen (1999) find that upwelling of upper mantle material by divergent plate motions is insufficient to explain observed melt volumes and post-breakup oceanic crustal thickness of the NAIP. A thin (~50 km) Iceland plume sheet layer, with a thermal anomaly of about 100˚C, is needed in these models to generate large thicknesses of post-breakup igneous crust on the North Atlantic volcanic margins.…”