Anorogenic granites, magmatic underplating and the origin of intracratonic basins in a non-extensional setting

Abstract

We present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating. A case study of a deeply eroded continental rift belt in south Finland shows close association of anorogenic intrusives and remnants of cratonic basins. Lower-crustal thinning took place during anorogenic magmatism, whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies. Combined analyses of field data, petrological constraints, and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning. This model involves the formation of an incipient crustal dome by asthenospheric upwelling, associated with basaltic underplating, and migration of induced lower-crustal partial melts to upper-crustal levels. In this model, light lower-crustal “diorite” is replaced by a denser mafic layer. The consequences for the post-magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification. Subsidence occurs in the absence of crustal extension, suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin. Subsidence curves for this model show an incipient, nearly linear fast downwarp related to crystallization of gabbro at the Moho, followed by slow contraction induced by solid state cooling. The predicted subsidence is similar to the pattern of the vertical movement induced by passive extension. In the proposed model of continental rifting, the effects of extension and underplating largely cancel out. Concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion. The model provides a consistent explanation for a number of key aspects of cratonic basins, including their circular shape, crustal configuration and evolution, and their spatial distribution on continents.