Diachronous Variscan late-orogenic collapse as a response to multiple detachments: a view from the internides in France to the foreland in the Irish Sea

Abstract

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.

Diatremes Act as Fluid Conduits for Zn-Pb Mineralization in the SW Irish Ore Field

“…Therefore, a close temporal association between volcanism, hydrothermal activity, and mineralization exists at Stonepark and raises the interesting possibility that volcanism is a key process in the localization of Zn-Pb mineralization. The temporal associations at Limerick are consistent with a scenario in which magmatism elevated heat flow and hydrothermal circulation in the Carboniferous Irish ore field (Praeg, 2004;Wilson et al, 2004;Davidheiser-Kroll et al, 2014;Hnatyshin et al, 2015;Wilkinson and Hitzman, 2015).…”

40 Ar/ 39 Ar geochronology of Carboniferous-Permian volcanism in the Midland Valley, Scotland

“…Ziegler 1990;Matte 1991). In the northern foreland of the orogen, Dinantian lithospheric extension and magmatism were widespread through Ireland, Britain, the North Sea, Germany and Poland during the closure of the Rhenohercynian Ocean (Leeder 1982;Ziegler 1990;Coward 1993;Warr 2000;Praeg 2004;. Fault-block basins with 3-5 km of syn-rift sedimentary rocks and small occurrences of largely mafic, withinplate alkaline, transitional and tholeiitic volcanic rocks characterized northern and central England and Ireland (Leeder 1982;Leeder & McMahon 1988;Chadwick et al 1995;.…”

“…After the Variscan orogeny, a StephanianEarly Permian system of NE-to NW-trending rifts developed across the northern European foreland, and a widespread magmatic phase related to post-Variscan extension and decompression melting took place from 300 to 280 Ma (Ziegler 1990;Coward 1995;Praeg 2004;Neumann et al 2004;. Regional stress patterns changed in response to the gravitational collapse of the orogen and a far-field dextral extensional stress (Arthaud & Matte 1977;Henk 1999;, and were subsequently influenced by proto-Atlantic rifting (Coward 1995).…”

The tholeiites of the Valaisan domain (Versoyen, western Alps): a Carboniferous magma emplaced in a small oceanic basin

“…This extension leads to crustal thinning and emplacement of within-plate calc-alkali and shoshonitic magmas [Ouazzani and Lapierre, 1986;Menot, 1987;Finger et al, 1990;Schaltegger, 1997]. The collapse of the orogen expended northward over time [Praeg, 2004].…”