Metamorphism of the Palaeozoic rocks of the Torquay district, Devon, 1:50k sheet 350

The pattern of regional metamorphism indicated by illite crystallinity has been used to interpret the tectonic and thermal history of the Torquay district. The metamorphic pattern is primarily related to the fold-and-thrust geometry but has been modified by later NW-trending faults. Areas of lower grades are associated with limestone and volcanic rock formed on the Torquay and Brixham basin highs that were generated by listric fault block rotation during passive margin rifting. Anchizonal grade associated with these rocks indicates burial under an overburden thickness of at least 5 km. The source of the overburden was the frontal portion of a thrust stack that advanced northwards as the rift system contracted and inverted. The tectonically thickened rear of the stack is represented by epizonal rocks cropping out in the southwest of the district. These rocks formed at temperatures of 300-350ºC under a tectonic overburden of at least 7 km.