One- and two-dimensional thermal modelling of orogenic crustal extension in the Tormes Gneissic Dome, NW Iberian Massif, Spain

Situated in the inner zone of the Variscan Iberian Massif, the Tormes Gneis sic Dome offers a good opportunity for thermal modelling of orogenic crusta l extension, because the P-T-t loops are well constrained by an extensive s et of thermobarometric, structural and geochronological data. As an example of feedback between forward and inverse methods, the aim of this study was to establish one- and two-dimensional thermal models that reproduce the co ntrasting petrological P-T paths of two structural units separated by an ex tensional tectonic contact in the metamorphic complex, and to explain the s patial and temporary development of the low-pressure metamorphism in the ro cks located just above this contact. In one dimension, the syn-extension pa th of the lower unit resulting from modelling is characterized by an isothe rmal decompression phase, followed by near isobaric cooling, which is typic al of exhumed rocks. The upper unit path records a syn-extension near isoba ric heating, more important in rocks just above the tectonic contact. Conde nsed isograds of low-pressure/high-temperature metamorphism in the basal up per unit are thus interpreted as a consequence of advective crustal extensi on and conductive upward heat transfer. In two dimensions, the delaminated simple shear geometric model of crustal extension explains the observed tem perature rise in excess of 500 degrees C in the basal upper unit and is con sistent with the spatial distribution of M2 low-pressure/ high-temperature isograds. This demonstrates the important role of extensional structures pr oduced during the collapse of the thickened crust in the thermal evolution. The heating phase. well explained with intermediate dip angle for extensio nal fault in the upper crust (45 degrees) and finite extension of 75 km, is followed by cooling, thus reflecting normal erosional process.