Variscan very low‐grade metamorphism in southwest England: a diastathermal and thrust‐related origin

Abstract

Regional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low-grade metamorphism whilst in the southern region the very low-grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High-Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late-stage structural up-doming of the higher grade areas over the roof of the batholith.

Burial and uplift history of the Palaeozoic Flysch in the Variscan foreland basin (SE Bohemian Massif, Czech Republic)

“…A close relationship of the foreland propagating thrust wedge and the decreasing metamorphic grade and diagenesis from the rear to the frontal part and autochthonous underthrust substratum was documented in the external Variscides in Southwest England (Warr et al 1991). The above mentioned case histories and the area studied in this paper are consistent in several aspects and document a more general validity of the observed diagenetic patterns and the respective thermal models related to the Variscan orogeny.…”

The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation

“…Accretion of the Gramscatho Group succession resulted in deformation and intermediate pressure facies series anchizonal-epizonal regional metamorphism [96]. D1 deformation is characterized by ENE-WSW slaty cleavage, tight to isoclinal curvilinear folds, and a NNW-SSE mineral lineation that are compatible with a top sense of shear to the NNW [57].…”

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

“…However, unlike MPG, the oldest Cornubian granites (≥ 290 Ma) were emplaced > 65 Ma after initial Variscan collision in SW England at c. 360 Ma (Shail and Leveridge, 2009). Modest collisional crustal thickening, as evidenced by almost ubiquitous sub-greenschist facies regional metamorphism of the host rocks (Warr et al, 1991), would have raised lower crustal temperatures during the Carboniferous but, even if partial melting was initiated, the absence of felsic igneous rocks of this age do not suggest effective melt segregation. The low abundance of muscovite within a metagreywacke source would ensure that a critical melt fraction could not be achieved by muscovite melting alone.…”

“…The youngest deformed successions are late Carboniferous (Moscovian) in age (Edwards et al, 1997). Regional metamorphism of all Devonian-Carboniferous successions seldom exceeds very low grade (epizone-anchizone) (Warr et al, 1991).…”