Regional metamorphism of the Mathinna Group, northeast Tasmania

Regional variations in illite crystallinity of the Ordovician-Devonian Math inna Group of northeast Tasmania indicates that metamorphic grade is domina ntly anchizonal (200-300 degreesC, sub-greenschist facies) over an area of 80 km x 100 km. The b(0) white mica variation (b(0) 9.020 +/- 0.008 Angstro m) is characteristic of intermediate pressure Barrovian-type metamorphism. Peak metamorphism does not correlate with stratigraphic position and does n ot change across most of the large faults, indicating that peak metamorphis m occurred after east-directed thrusting and as a result of crustal thicken ing. Most Devonian granites in northeast Tasmania intruded at much lower pr essure than the peak metamorphic conditions, suggesting very rapid unroofin g in the Middle Devonian. These results are very similar to the metamorphic characteristics reported from the Lachlan Fold Belt in Victoria. The metam orphic history is used to support a thin-skinned model for the deformation in a tectonic wedge that tapered to the east and was subsequently exhumed b y backthrusting to the west.