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

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 Transantarctic 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 F-1 folds that possess a slaty axial-planar cleavage. Steeply plunging asymmetric F-2 folds and their associated cleavage are superimposed upon the NNE-SSW D-1 structural grain, forming a consistent clockwise oblique angle. Together with several shear-sense indicators, the angular relationship of D-2 and D-1 suggests that the F-2 folds formed as a result of sinistral reactivation of D-1 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 D-1 deformation event, and a later phase of syntectonic biotite granites emplaced during D-2. New U-Pb SHRIMP ages for three phases of the Serpan Peak granite suite constrain their emplacement-and, by structural relationship, the early Ross D-1 and D-2 deformation events-as having occurred at 505 +/- 5 Ma. The ages of D-1 and D-2 are identical within error, and we interpret them as being the product of a single progressive sinistral transpressional deformation event. Our new structural and geo-chronological 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 volcanism inferred to represent an extensional continental backarc setting. During the latest Cambrian-Early Ordovician, a second, less intense contractional deformation event deformed and inverted the backarc region. Our new data reveal that, along the Pensacola Mountains sector of East Antarctica, the Ross orogeny was manifested as a condensed sequence of rapidly changing tectonic events along a continuously subducting margin, characterized by switches in deformation mode within the continental margin. We interpret such tectonic switching to be the result of the intermittent arrival of buoyant oceanic plateaus at the Ross subduction zone, resulting in short-lived periods of flat-slab subduction and contractional deformation that punctuated the neutral or extensional strain regime generally associated with "normal" subduction. Given the limited length of plate margin likely to be affected by oceanic plateau subduction, specific correlation of stratigraphic and tectonic events along the entire length of the Ross orogen are unlikely. Nonetheless, similar patterns of punctuated tectonic events associated with sinistral transpressional deformation during the Ross orogeny are observed elsewhere along the orogen.

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