[S-MP46_28PM2] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-11] Possible tectonic models before, during and after mylonitization in the Sor Rondane Mountains, East Antarctica

*Tsuyoshi TOYOSHIMA¹, Akiko KANEYA¹, Yasuhito OSANAI², Sotaro BABA³, Tomokazu HOKADA⁴, Nobuhiko NAKANO², Tatsuro ADACHI² (1.Niigata University, 2.Kyusyu University, 3.University of the Ryukyus, 4.National Institute of Polar Research)

Keywords:S-tectonite, flattening, mylonitization, Sor Rondane Mountains, Gondwana, East Antarctica

The deformational history in the Sor Rondane Mountains (SRMs), eastern Dronning Maud Land (DML), East Antarctica, is divided into 13 stages (D1?D13). The tectonic regime varied frequently from extension (D3?D4) to layer-normal compression and layer-parallel extension (D5), to compression (D6), top-to-the S shearing (D7), top-to-the SE shearing and sinistral strike-slip (D8), compression (D9?D11), and ?nally extension related to dextral shearing (D12?D13). In this paper we discuss change in deformation and P-T conditions before, during and after the D7-D8 mylonitization, using mineral textures, assemblage, compositions and microstructures of D7 and D8 mylonites.Garnet porphyroclasts of the D7-D8 mylonites include high-Ca mantles and crenulation microfolds defined by sillimanite fibrolites. The high-Ca mantles of garnets and their plagioclase inclusions in the mylonites imply an increase in pressure before the D7-D8 mylonitization. S-tectonites having a dominant planar fabric were formed before the D7-D8 mylonites and after the high-Ca mantles of the garnets. The planar fabric (foliation) of the S-tectonites is produced by fan-shaped arrangement of sillimanite and biotite grains. The sillimanite and biotite grains were formed by breakdown of garnet. Most of the sillimanite and biotite grains have been rotated and folded by the D7-D8 mylonitization. The D7-D8 mylonite foliations are parallel to the planar fabric of the S-tectonites. The S-tectonites indicate a flattening type of strain and resulted from the layer-normal shortening after the increase in pressure and before the D7-D8 mylonitization. kyanite-quartz porphyroblasts and randomly oriented crystals of sillimanite/kyanite and biotite were formed after the D7-D8 mulonitization. The randomly oriented crystals of sillimanite/kyanite and biotite resulted from the breakdown of garnet porphyroclasts of the D7-D8 mylonites. The kyanite-quartz porphyroblasts accompany leucogranite veins cutting the D7-D8 mylonite foliations. The randomly oriented crystals and porphyroblasts imply non-deformational conditions after the D7-D8 mylnitization and D9 folding. Three possible tectonic models for D7 and D8 mylonite-forming events before the D9 deformation can be considered as follows: extensional tectonic model, positive flower structure model and rotated mylonite model. In the former model, D7 and D8 indicate major extensional tectonic activity in the southern part of the East African and Antarctic Orogen (EAAO) before the Pan-African compressional event, and after the 650-600 Ma peak of metamorphism. In the latter two models, D7 and D8 mylonites may have resulted from the compressional events. In the positive flower structure model, the SRMs are the southern half of the E-trending positive flower structure. The flower structure model needs top-to-the N shear zones to the north of the SRMs. In the rotated mylonite model, the present S-dip of the D7 and D8 mylonites results from the rotation and folding of originally N-dipping reverse (top-to-the S-SE, normal-sinistral shear, present day coordinates) mylonites. The Pan-African compressional event resulted in the formation of upright folds with horizontal axes that curve along the coastline in central to eastern DML during the D9 deformation that took place between 600 and 560 Ma. The coastline-parallel fold axes and subvertical axial-planes correspond to the X-axes and the XY-planes, respectively, of strain ellipsoids that were progressively rotated counterclockwise toward the central parts of a sinistral shear zone. Therefore, the curved fold axes and axial-planes suggest the EAAO acted as a zone of sinistral transpression during the collision of parts of East and West Gondwana.