Symmetry, Width, and Differential Thinning of Passive Margins, Insights from Dynamical Modelling: Implications for Heatflow and Subsidence History

Ritske S. Huismans¹ and Christopher Beaumont^2
1Dep. Earth Science, Bergen University, Bergen, Norway
²Dep. Oceanography, Dalhousie University, Halifax, Canada

Contrasting end members of volcanic and non-volcanic passive margin formation show a large variability in structural style and associated subsidence history that imply strong variability in the underlying thermo-mechanical conditions at the time of rifting. For instance the Iberia-Newfoundland non-volcanic conjugate margin system has evolved from initial wide to late stage narrow, most probably asymmetric rift, leading to exhumation of mantle lithosphere and sub-lithospheric mantle in a wide ocean-continent transition zone under essentially cold conditions. In contrast rifting in the non-volcanic Central South Atlantic conjugate passive margins resulted in very wide (> 250 km) strongly thinned crustal conjugates which remained close to sea level until break-up providing conditions for the late syn-rift shallow water salt basin, implying high thermal gradients at the time of rifting. Volcanic rifted margins such as in the North and South Atlantic show excess magmatic activity and shallow water conditions at the rift-drift transition implying even higher geothermal gradients.

We use thermo-mechanical finite element model experiments to investigate factors that are potentially important controls during volcanic and non-volcanic passive margin formation which may explain these characteristic differences. Our focus is on processes that create shear zones, on the rheological stratification of the lithosphere, and on processes that lead to differential thinning of upper and lower lithosphere during rifting. Dynamic modelling cases are compared where the crust is strong, weak, or very weak, and the mantle lithosphere is either strong or weak. Strain softening takes the form of a reduction in the internal angle of friction with increasing strain. Predicted rift modes belong to three fundamental types: 1) narrow, asymmetric rifting in which the geometry of both the upper and lower lithosphere is approximately asymmetric; 2) narrow, asymmetric, upper lithosphere rifting concomitant with narrow, symmetric, lower lithosphere extension; 3) wide, symmetric, crustal rifting concomitant with narrow, mantle lithosphere extension.

The different styles depend on the relative control of the system by the frictional-plastic and ductile layers, which promote narrow, localized rifting in the plastic layers and wide modes of extension in the viscous layers, respectively. A weak ductile crust-mantle coupling tends to suppress narrow rifting in the crustal layer. This is because it reduces the coupling between the frictional-plastic upper crust and localized rifting in the frictional-plastic upper mantle lithosphere. The simple strength variation may be taken to represent end-member thermal and/or compositional conditions in natural systems. Our results suggest that late stage rifting of the Iberia-Newfoundland margin system may be explained by slow extension of cold lithosphere resulting in asymmetric rifting and exhumation of mantle lithosphere. Rifting of warm lithosphere that includes a very weak middle lower crust allows strong differential thinning between upper and lower lithosphere leading to very wide crustal cross sections. This latter style may form an analog for the non-volcanic but ‘hot’ passive margin formation in the Central South Atlantic, where the associated strong differential thinning of the mantle lithosphere explains shallow water conditions during the late syn-rift and implies high geothermal gradients.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands