Deformation of ultrahigh-temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica

The deformation history of ultrahigh-temperature metamorphic rocks from Tonagh Island, Napier Complex, East Antarctica is divided into nine stages, namely D_1 to D_9. The D_1 structure would have been formed under non- or weakly-deformational condition during thermal peak of prograde metamorphism. The D_2-D_6 structures would have been produced under retrograde granulite facies conditions. Subsequently D_7-D_9 brittle faulting modified the structures in part. The D_1 to D_8 deformation corresponds to first to third tectonometamorphic episodes, previously reported, of the whole Napier Complex before the intrusion of the Amundsen dyke. The structural geology of the Island is mainly characterized by NE-SW trending D_2 foliation (S_2) with WNW plunging mineral lineation, WNW-ESE to E-W trending D_5 folds and NE-SW to E-W trending D_6 mylonite zones. S_2 is the main foliation of this area, but was highly folded and faulted during the D_5-D_6 stages. The S_2 and S_2-parallel D_3 shear zones are the products of layer-parallel shearing due to a top-to-the-SE (dextral-reverse) displacement. The D_3 and D_6 fault rocks show that seismic faulting and plastic deformation alternated both during the D_3 stage and during the D_6 stage under retrograde granulite facies conditions. Multiphase faulting in different shear senses occurred during the D_6 stage