On the evolution of rifted continental margins: comparison of models and observations for the Nova Scotian margin

Abstract

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 model. 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 of: (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 and crust, and a brittle listric faulted crustal layer affords an accurate description of the first-order processes that occur during rifting and evolution of this margin.