A tectonic history of northwest England

Abstract

Northwest England has been affected by the Caledonian, Variscan and Alpine orogenies no one of which is entirely unrelated to the others. Each successive phase is partially dependent on earlierones, whilst structures in older rocks became modified by succeeding events. There is thus an evolutionary structural sequence, probably originating in a preCambrian basement and extending to the present.

The Caledonian episodes are subdivided into pre-Borrowdale Volcanic, pre-Caradoc and end-Silurian phases. The recent suggestions of a severe pre-Borrowdale volcanic orogeny are rejected but there is a recognizable angular unconformity at the base of the volcanic rocks. The ‘pre-Borrowdales’; trends and those of the pre-Caradoc movements are variable but are generally northerly and could be posthumous upon a pre,Cambrian basement. The end-Silurian structures include early n–s and later ne to ene folding.

The Variscan structures are in part determined by locations of the older massifs and in part they are likely to be posthumous upon older structures with important n–s and ne. elements. Caledonian wrench faults were reactivated, largely with dip slip movement.

The more gentle Alpine structures also follow the older trends with a n–s axis of warp or tilt and substantial block faulting. The latter was a reactivation of older fault lines and resulted in uplift of the old north Pennine massifs relative to the downwarped Irish Sea Basin and the Vale of Eden.