|
Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland daniel.praeg{at}ucd.ie
Models of the collapse of orogenic belts imply diachronous tectonism in which crustal uplift and extension may be compensated by peripheral compression. This first-order prediction is tested against published data on Varsican late-orogenic extensional and compressive structures along a 1500 km transect from the Variscan central internides in France to the foreland in the Irish Sea area. The collapse of the orogen is shown to have expanded northward over time, via three main stages: (i) collapse of the central internides (late Visean-mid-Westphalian, c. 335–310 Ma) — crustal thinning took place by NW-SE extension within a relatively narrow (< 500 km) central axis, accompanied to the north by passive infill of basins spanning a broad seaway inherited from extension during closure of the Rheic Ocean; (ii) reorientation and expansion of collapse (mid-Westphalian-late Stephanian, c. 310–300 Ma) — in the mid-Westphalian, a 90° rotation to NE-SW extension in the central internides was accompanied by changes across the northern internides (episodic basin formation and deformation), the externides (onset of thrust propagation) and the foreland (Westphalian C inversion), while from the early Stephanian, basins began to form in the central internides and expanded to the externides, coeval with final nappe emplacement along the orogenic front; and (iii) collapse of the foreland (late Stephanian-Early Permian, c. 300–290 Ma) — km-scale uplift and erosion of the foreland took place, prior to widespread basin formation in the Early Permian (<290 ± 5 Ma). These three stages are argued to support a model of Variscan late-orogenic collapse in response to three successive detachments of negatively buoyant lithospheric material: of a collisionally thickened orogenic root, and of two (Rheic) oceanic slabs subducted, previously, southward (beneath the orogen) and northward (beneath the foreland). Multiple detachments are a predictable consequence of ocean closure and continental collision, so that episodic collapse may be a common process in the rise and fall of orogenic belts and the tectonic evolution of their forelands.
