Kinematics of primary contacts between low- and relatively high-pressure rocks in orogens

Primary contacts among low- and relatively high-pressure rocks in convergent orogens have been mainly interpreted as a result of extensional tectonics, or reimbrication of high-pressure thrust units already resting on low-pressure ones and by consequent inversion of their stacking order. Our work in northern Calabria allows us to ascertain the nature and kinematics of several of these contacts, and to work out a general model that considers primary pressure break contacts as resulting from the growth of a deep passive roof duplex within a subduction-accretion system. Estimates of pressure differences across these contacts may be obtained for a simple passive roof duplex geometry as a function of the aspect ratio of the structure and of the angle of subduction. Our results suggest therefore that there may be a further possibility, in addition to extension or reimbrication, which explains pressure break contacts in orogens.