Impact of Mafic Underplating and Mantle Depletion on Subsequent Extension: a Numerical Modeling Approach

Abstract:

Orogenic processes leave pervasive and long-lasting structural and compositional heterogeneities such as suture zones, faults and magmatic intrusions in both the crust and the mantle. Intuitively, rifts are expected to take advantage of inherited weaknesses, thus to localize at former orogenic structures. This theory known asthe Wilson Cycle is well-illustrated in the northern North Atlantic, where extension follows the structural grain of the Caledonian orogen. However, the Alpine and southern North Atlantic rift systems are striking counterexamples, since both circumvent the core of the Variscan orogen to the southeast and to the west, respectively. Yet, one major distinctive feature between the Caledonides and the Variscides is the amount of post-orogenic magmatic activity. Indeed, while the Caledonian range orogenic collapse was essentially a-magmatic, widespread acidic intrusions and mafic underplating were emplaced in the Variscan continental crust.

In this study we investigate how mafic underplating of the continental crust and associated upper mantle depletion may impact a subsequent extensional event. We design numerical models to compare the behavior of lithospheres with various distributions of lower crust and mantle heterogeneities under different thermal states.

We show that the existence of a mafic layer in and / or a region of depleted mantle beneath a quartzite crust bearing weak heterogeneities results in delocalization of extension outside this area, in the case of thermally re-equilibrated lithospheres. Furthermore, the existence of a strong heterogeneity within the lower crust and / or the upper mantle triggers a necking instability, which may result in the formation of ribbons of little- or un-thinned continental crust between regions of more intense thinning. The wavelength of these ribbons compares well with the scale of the Flemish Cap and Galicia Bank, both of which developed over underplated Variscan lithosphere.