Late Paleozoic extensional reactivation of the Rheic–Rhenohercynian suture zone in SW England, the English Channel and Western Approaches

Abstract

The Rheic Ocean is a persistent feature of Paleozoic palaeogeographies whose closure contributed to the development of the Variscan orogen and formation of Pangaea. Geological and geophysical data indicate repeated episodes of Paleozoic rifting and plate convergence around SW England and the adjacent offshore areas. SW England occupied a lower plate position during the Devonian-Carboniferous, on the northern passive margin of the short-lived Rhenohercynian Ocean that had formed near a recently closed segment of the Rheic Ocean. Variscan plate convergence resulted in the development of the composite southwardsdipping Rheic-Rhenohercynian suture zone by the latest Devonian and inversion of the lower plate basins during the Carboniferous. Early Permian NNW-SSE extensional reactivation of this suture zone controlled the development of the Western Approaches Basins in its hangingwall and provides an excellent example of Wilson Cycle structural inheritance. The onshore expression of this episode includes shear zones and detachment faults consistent with top-to-the SSE extensional reactivation of Variscan thrust faults. There is a progression to higher-angle brittle extensional faults that cutout earlier structures. Exhumation of the lower plate was accompanied by Early Permian mantle and concomitant crustal partial melting, the construction of the Cornubian Batholith, and W-Sn-Cu fracture-hosted mineralisation.

Magmatic, Metamorphic and Structural History of the Variscan Lizard Ophiolite and Metamorphic Sole, Cornwall, UK

“…Distinct expressions of the Rheic and Rhenohercynian sutures have been inferred, in the Spessart Mountains (Germany), but the two sutures are generally shown as coincident and described as the Rheic-Rhenohercynian suture zone (Franke, 2024;Franke et al, 2017). This suture, which is approximately 8 km wide, is imaged cutting the whole crust on reflection seismic profiles across the English Channel (Alexander et al, 2019;BIRPS and ECORS, 1986;Klemperer & Hobbs, 1992;Le Gall, 1990). The suture zone separates a lower plate (SW England) from the upper plate Normannian High/Mid-German Crystalline Rise, that formed the southern active margin of this ocean basin (Franke, 2024;Franke et al, 2017;Holder & Leveridge, 1986).…”

“…The detailed timing of Rheic ocean closure, and the nature and polarity of its associated subduction zones and suture(s) are not fully constrained and a "two-stage" collision model for the consolidation of Pangaea is increasingly recognized (e.g., Arenas et al, 2014). The Rhenohercynian Zone represents either a marginal ocean basin north of the Rheic Ocean during the Devonian, or a successor Rhenohercynian Ocean following the latest Silurian-earliest Devonian closure of the Rheic Ocean (e.g., Alexander et al, 2019;Eckelmann et al, 2014;Franke, 2000Franke, , 2024Franke et al, 2017;Shail & Leveridge, 2009). Distinct expressions of the Rheic and Rhenohercynian sutures have been inferred, in the Spessart Mountains (Germany), but the two sutures are generally shown as coincident and described as the Rheic-Rhenohercynian suture zone (Franke, 2024;Franke et al, 2017).…”

A single multi-scale and multi-sourced semi-automated lineament detection technique for detailed structural mapping with applications to geothermal energy exploration

“…structures formed during latest Carboniferous to Early Permian post-Variscan regional extension during which thrust faults were reactivated as top-to-the-SSE extensional faults and new higher-angle ENE-WSW striking extensional faults formed Shail, 1995, 1995;Shail and Alexander, 1997;Shail and Leveridge, 2009;Alexander et al, 2019).…”