Structural and geochronological constraints of early Ross orogenic deformation in the Pensacola Mountains, Antarctica

• Published in: Geological Society of America Bulletin
• Main Authors: Curtis, Michael L, Millar, Ian L, Storey, Bryan, Fanning, Christopher
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Association of Engineering Geologists
• Subjects: Keywords: East Antarctica; Mountains; Orogenic deformation; Ross orogeny; Deformation; Geochronology; Granite; Landforms; Limestone; Microstructure; Shrinkage; Stratigraphy; Tectonics; Structural geology; Gondwana; orogeny; Proterozoic; Antarctica; East Antarctica; Polyphase deformation; Transantarctic Mountains; Transpression; Arctic; Hannah; Pensacola Mountains; Hannah Ridge; Serpan Peak; Antarc*
• Online Access: http://hdl.handle.net/1885/80336; https://doi.org/10.1130/B25170.1

During end-Neoproterozoic to early Paleozoic time, the southern margin of Gondwana was affected by widespread subduction-related orogenesis. In East Antarctica this event is known as the Ross orogeny, whose manifestation is exposed along the 3500 km length of the Transant-arctic Mountains. In the Pensacola Mountains, the Ross orogeny is characterized by two contractional deformation events, which bracket a brief period of latest Middle Cambrian to early Late Cambrian extension and sedimentation. The oldest and most intense of these deformation events effected a succession of latest Neoproterozoic metagraywackes (Hannah Ridge Formation), producing tight to isoclinal F1 folds that possess a slaty axial-planar cleavage. Steeply plunging asymmetric F2 folds and their associated cleavage are superimposed upon the NNE-SSW D1 structural grain, forming a consistent clockwise oblique angle. Together with several shear-sense indicators, the angular relationship of D2 and D1 suggests that the F2 folds formed as a result of sinistral reactivation of D1 structures. At Serpan Peak, new meso- and microstructural observations of the Serpan Peak granite suite reveal two phases of emplacement: an early granodioritic phase emplaced prior to the regional D1 deformation event, and a later phase of syntectonic biotite granites emplaced during D2. New U-Pb SHRIMP ages for three phases of the Serpan Peak granite suite constrain their emplacement-and, by structural relationship, the early Ross D1 and D2 deformation events-as having occurred at 505 ± 5 Ma. The ages of D1 and D2 are identical within error, and we interpret them as being the product of a single progressive sinistral transpressional deformation event. Our new structural and geochronological constraints necessitate that deformation of the Hannah Ridge Formation occurred in conjunction with exhumation from lower greenschist conditions prior to deposition of the upper Middle Cambrian Nelson Limestone (< 10 m.y.). Exhumation was closely followed by ca. 500 Ma bimodal ...