Je. Viruete, One- and two-dimensional thermal modelling of orogenic crustal extension in the Tormes Gneissic Dome, NW Iberian Massif, Spain, INT J E SCI, 88(3), 1999, pp. 444-457
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.