The complex tectonic evolution of the Malvern region: crustal accretion followed by multiple extensional and compressional reactivation

Pharaoh, Tim. 2019 The complex tectonic evolution of the Malvern region: crustal accretion followed by multiple extensional and compressional reactivation. Proceedings of the Open University Geological Society, 5. 35-50.

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Abstract/Summary

The Malvern Hills include some of the oldest rocks in southern Britain, dated by U-Pb zircon analysis to c. 680Ma. They reflect calc-alkaline arc magmatic activity along a margin of the Rodinia palaeocontinent, hints of which are provided by inherited zircon grains as old as 1600Ma. Metamorphic recrystallisation under upper greenschist/amphibolite facies conditions occurred from c. 650–600Ma. Subsequently, rifting of the magmatic arc (c.f. the modern western Pacific) at c. 565Ma led to the formation of a small oceanic marginal basin, evidenced by basaltic pillow lavas and tuffs of the Warren House Formation, and Kempsey Formation equivalents beneath the Worcester Graben. By early Cambrian time this juvenile crust had stabilised sufficiently for thick quartz arenite-dominated sequences to accumulate, followed by mudstones in mid- to late-Cambrian time. In earliest Ordovician time, subsidence accelerated in a rift basin east of the Malverns, but was terminated by accretion of the Monian Composite Terrane to the Gondwana margin. Rifting led to a microcontinental flake (‘East Avalonia’) breaking away, eventually to impact with Laurentian terranes on the other margin of the Iapetus Ocean in early Silurian time. Minor inversion of the floor of the Worcester Graben might have occurred during the Acadian (early Devonian) deformation phase, but more significantly, during the Variscan (end Carboniferous) Orogeny, when a ‘Rocky Mountain Front’-type uplift was generated opposite a pinch-point within the orogen. Extensional reactivation of the earlier compressional structures in Permian, and particularly, during Triassic time, resulted in the Worcester Graben as we know it today. The structure of the Malvern Hills and the graben is thus the result of a surprisingly long and complex history of crustal accretion, compression and extension.