The Bristol-Mendip foreland thrust belt

Abstract

The thrusts and associated fold structures of the Bristol–Mendip area are interpreted in terms of a thin skinned, piggy-back foreland thrust belt of Variscan age with dominant northward transport. Strike-normal balanced sections and compatible strike-parallel sections have been constructed to illustrate the 3-D geometry of the linked thrust system. North–south shortening of c. 20 km (40%) is calculated from deformed and restored strike-normal sections. A thick and extensive thrust wedge of pre-Old Red Sandstone basement in the west of the area is responsible for culminations expressed at the surface as the Mendip periclines. Uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area. Strike-parallel sections and a hanging wall cut-off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps, or flats. The en-echelon pattern of the Mendip periclines may be related to the oblique NE climb of two of these thrusts. The angle of frontal ramp climb through the Dinantian stratigraphy is generally low, and this explains the presence of numerous klippen of Lower on Upper Carboniferous rocks, whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate. Early, pre-thrusting movement on basement lineaments in the Variscan foreland created uplifts, angular unconformities and a predominance of N–S structures in the north of the area.