Caledonian extension in the north Irish Dalradian: implications for the timing and activation of gravity collapse

Following Caledonian crustal thickening and prograde metamorphism of the Dalradian rocks on the SE Laurentian margin in mid-Ulster, down-dip (gravity controlled) extension due to orogenic collapse was concentrated in the inverted limbs of the major ESE-facing fold nappes. Structural and metamorphic studies show that this extensional collapse, which can be correlated along strike for at least 100 km, occurred immediately after the crustal thickening episode. Such a relationship is compatible with dynamic wedge models for orogenic belts. Our data also imply that (a) collapse was triggered by the release of fluids during the prograde metamorphism and (b) late stage mobilization of the extensional zone created the gold mineralization in the Sperrin Mountains.