130-7 Integrated Age, Deformational, Metamorphic and Biostratigraphic Evidence for Imbrication of Precambrian Crust and Marginal-Basin Successions during Ross Orogen Convergence

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part II

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Recently revealed geologic, age and biostratigraphic relationships in the central Transantarctic Mountains document tectonic linkages between extreme crustal thickening (resulting in HP-UHP metamorphism in cratonic basement) and overthrusting of a fringing carbonate platform (leading to extinction of marine fauna). These discoveries provide a refined understanding of tectonic development in a key portion of the Ross Orogen in Antarctica, a major segment of the plate-margin orogenic system active from Ediacaran to Early Ordovician time along the Terra Australis border of East Gondwana (ca. 470-610 Ma). A synthesis of geochronological and thermochronological data from Archean and Paleoproterozoic basement, Neoproterozoic to Cambrian autochthonous rift-margin strata, Cambro-Ordovician syn-orogenic deposits, and syn- to post-tectonic granitoid intrusions spanning the entire period of Ross activity reveals coherent plate-margin activity over a 140 m.y. timespan, including transformative periods of (a) HP-UHP eclogite-facies metamorphism related to deep-crustal thickening in crystalline basement at ca. 535 Ma, (b) rapid exhumation of cratonic basement and autochthonous supracrustal assemblages, (c) wide dispersal of forearc molasse deposits, and (d) a rapid pulse of contractional deformation in supracrustal assemblages at ca. 512-514 Ma that is linked to a Gondwana-wide Cambrian archaeocyathid extinction. Crustal thickening is marked by a wide crustal shear zone decorated with eclogitic tectonic blocks and abundant diatexitic leucosomes indicating synchronous deep-seated deformation, metamorphism and anatexis. Thickening is tied to a mid-Cambrian pulse of supracrustal contractional deformation that displaced older metamorphic basement over Lower Cambrian carbonate bioherms, drowning the reefs with alluvial fan detritus and burial by siliciclastic molasse. This deformation, recognized in Antarctica and Australia, may have triggered a Gondwana-wide biotic extinction (Sinsk Event) represented by loss of archaeocyathids (Myrow et al., Sci. Adv. 10, eadl3452, 2024). Geochronological and thermochronological data from crystalline basement, Ross magmatic intrusions, and detrital-mineral suites in syn-orogenic deposits thus link a variety of orogenic processes from source to sink, culminating in a major period of related crustal shortening, magmatism, basement exhumation, and sediment dispersal focused between 545-490 Ma. This orogenic phase does not mark the initiation of subduction, but rather a profound stage in the prolonged Ross Orogenic cycle that is exemplified in the central Transantarctic Mountains by craton-margin shortening and imbrication of crystalline and supracrustal rocks in a unique section of the Tasmanide orogenic system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5318

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