Abstract. This paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small-scale convection in the upper mantle. This may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures. This concept is quantitatively assessed by numerical modeling of the flow, caused by divergent plate motions, in a viscous, temperature-and pressure-dependent, nonlinear fluid. Significant timedependent small-scale convection is generated at the lower model viscosities (and/or higher temperatures, and/or sharper rift geometries). These models can provide the required thick igneous crust observed at volcanic margins. However, they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable. An additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed. This increases its viscosity by 2 to 3 orders of magnitude in the melting zone, above -80 km. The high-viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness, while at the same time allowing for vigorous small-scale convection during the early history of the rift and the delivery of thick igneous crust against the margin. This factor, coupled to the models for flow described above, allow the main features of volcanic margins to be simulated. Besides the thick igneous crust predicted at the margins, these models suggest that time-dependent small-scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis. The periodicity of this time-dependent convection is primarily dictated by the model viscosities; the models suggest that episodicities of tens of million years are reasonable. These episodicities may be reflected in the record of vertical motions at rifted margins.

A model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed. Model predictions are then compared with geological and geophysical observations from a transect of the -185 Ma old rifted margin off Nova Scotia through the Scotian Basin.Three kinematic models of the rifting process are discussed. These are: (1) the uniform extension model, in which the amount of extension is uniform with depth but varies with position across the margin; (2) the uniform extension and melt segregation model which has similar properties, but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust, and; (3) the depth-dependent extension model in which the first-order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth, each of which undergoes differing amounts of extension.These rift models predict the form of crustal and lithospheric thinning, subsidence and temperature change once the amount of extension has been determined. This is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness, equal to that currently measured in the adjacent continental region, before rifting occurred. Rifting is also considered to occur instantaneously.A time-stepping thermo-mechanical model is used to predict the cooling of rifted margin, additional thermal contraction subsidence and its amplification by water and sediment loading. Thermal aspects are calculated using a finite difference model of time-dependent conductive heat transport, whereas regional isostatic response to loading is calculated by a finite element 66 8 C. Beaumont, C. E. Keen and R. Boutiliermodel. The models are coupled because the temperature distribution is used to define a rheological lithosphere (an elastic region with thickness that varies in time and space as the model evolves) for flexural calculations in the mechanical model. Secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment.The model predictions o f (1) sedimentary basin stratigraphy, (2) Moho position, (3) free air gravity anomaly, (4) age-depth relations for deep exploratory wells and (5) subsidence and temperature histories agree well with observations from the Scotian Basin. Additional effects due to sedimentary and crustal radiogenic heat production, lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered. It is concluded that a model in which rifting occurred by depth-dependent extension, and which includes a finite thickness for the rheological lithosphere, radiogenic heat production in the sediments...

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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.

Results from geodynamic models for lithospheric extension that includes one or two large-scale upper crustal faults are presented. The study does not address the origin of these faults, but the consequences if they exist. Model components include a solid mechanical lithosphere composed of thermoelastic-plastic material with viscous creep that is subject to extension, buoyant supporting forces and sedimentary loads, and a coincident, but thicker, thermal lithosphere that includes the effect of sediment blanketing and radiogenic heating. We have chosen for comparison purposes a "standard" reference model which minimizes creep in the crust; consequently, our results depend strongly on plastic deformation. Our models show that large faults can control the position and growth history of the mantle instabilities that can lead to rupture. We observe that these instabilities have secondary, "normal mode" like character. Models with two faults show that the normal mode behaviorcan interact and create an enhancement to the necking process, provided the faults are an ideal distance apart. We compare models with two rates of extension, 1.2 cm yr 4 ("fast") and 0.038 cm yr 4 ("slow"), which show remarkably little difference. We compare our reference model with models using "wet" and "dry" rheologies and observe that flow in the crust can attenuate the propagation effect created by the faults. Introduction This paper presents an analysis of dynamical models of lithospheric extension in the presence of large-scale upper crustal faults. Although the work has been inspired primarily by observations of the deep crustal structure beneath simple half graben basins off easternCanada [e.g., Keen et al., 1987] the basin setting and style are a common feature of basins globally [e.g., Manspeizer, 1988]. Thus our results are more generally applicable. Our models simulate the structural effect of a preexisting deep upper crustal fault and do not address the question of how these are formed. However, in asking what the consequences of these features are to the regional development of extension leading to lithospheric rupture, we have been able to gain important insights into the controlling thermomechanical processes. Other than the assumption of preexisting faults, no a priori, assumptions are made about the form of the deformation that results from the extension. We have chosen for comparison purposes a "standard" rheology dominated by plastic failure in crustal regions, which must be viewed as an "end member" case. Many of our results depend strongly on this assumption. Both thermal and mechanical aspects of the extending lithosphere are included in the model calculations. They predict deformational character, which includes elements of pure shear and simple shear, and which is similar to the model predictions of Braun and Beaumont [1987, 1989], Dunbar and Sawyer [ 1988, 1989], Keen and Boutilier [1990] Chery et al., [1990], Sawyer and Harry [1991], and Harry and Sawyer [1992a, b]. Our models also show some characteristics of lithospher...