Geodynamic evolution of the Pacific margin of West Antarctica based on apatite thermochronology

Abstract

The West Antarctic continent formed during most of the Palaeozoic and Mesozoic era by accretion of crustal blocks to form the palaeo-Pacific margin of Gondwana. In the mid Cretaceous, Gondwana break-up was associated with a major change from convergence to extension along the palaeo-Pacific margin, which opened the West Antarctic rift system (WARS) and the Southern Pacific Ocean by separating Zealandia from West Antarctica. Further disintegration of Gondwana led to the separation of West Antarctica from South America by opening of the Drake Passage and the development of the Scotia Sea. Thus, Gondwana break-up and ensuing tectonic movements brought geographic isolation to Antarctica and potentially favoured the onset of glaciation. Extensive ice-cover of West Antarctica is itself one of the main challenges when trying to better constrain the geodynamic evolution since Gondwana break-up that remains poorly understood. This study focussed on western Marie Byrd Land (MBL) and the Thurston Island area which both form parts of the conjugate continental margins of Zealandia. Further investigation was carried out in the area of the South Orkney Islands and Elephant Island group (southern Scotia Sea). To comprehensively reconstruct the thermo-tectonic evolution of the upper 2-5 km of the crust, the main methods utilised in this study were apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) thermochronology. Thermal history modelling of both AFT and AHe data was also carried out. For a better understanding of the geodynamic history, selected samples were analysed by means of U-Pb zircon geochronology and hornblende thermobarometry. The oldest thermochronological record was obtained from Carboniferous to Early Cretaceous (meta)igneous rocks of the Thurston Island area. Their AFT ages of 145-92 Ma and AHe ages of 112-43 Ma were integrated in thermal models and show episodes of cooling and reheating since the Late Palaeozoic. These episodes refer to exhumation and back-arc basin development between the Permian and the Early Cretaceous along the active paleo-Pacific margin. They also record exhumation during Early to mid Cretaceous basin inversion associated with magmatism and crustal thickening. Extension-related exhumation from 95 to 70 Ma occurred in response to the rifting between Zealandia and West Antarctica and from 75 to 60 Ma to activity along the Bellingshausen-Antarctic Plate boundary. Final exhumation since the Late Eocene was most-likely caused by activity of the WARS. Western MBL samples were mostly taken from the mid Cretaceous Byrd Coast Granite suite. These samples yield AFT and AHe ages of 105-68 Ma and thermal models are compatible with rapid tectonic exhumation from 105 to 95 Ma during WARS-extension. This was followed by a phase of exhumation between 95 and 80 Ma in response to rifting and breakup between Zealandia and West Antarctic. The southern-most samples record exhumation from 80-60 Ma due to faulting in the Ross Sea region. Slow cooling during the Late Cretaceous-Early Cenozoic corresponds with the formation of the West Antarctic erosion surface and relative tectonic quiescence. Late Cenozoic WARS rifting, MBL-dome uplift, and onset of glaciation appears to have had little effect on erosion processes in western MBL. Pre-mid Cretaceous sedimentary and metamorphic samples of the South Orkney Islands yield AFT ages of 77-54 Ma and AHe ages of 61-5 Ma. These ages, interpreted with the help of thermal history models record Late Cretaceous-Early Cenozoic margin-wide uplift and erosion of high-standing topography. Relatively slow exhumation since the Late Eocene was associated with the opening of the Powell Basin. Metamorphic and igneous samples from the Elephant Island group reveal AFT and AHe ages of 30-23 Ma and 6-2 Ma. A diorite and a granodiorite samples yield U-Pb zircon ages of 28 and 10 Ma. The geo- and thermochronological ages and the identified cooling episodes probably refer to two periods of major plate reorganisations in the southern Scotia Sea. First, intrusion and exhumation during the Oligocene occurred probably in response to inception of the Shackleton Fracture Zone and the West Scotia Ridge. Second, rapid tectonic exhumation in the Late Miocene/Pliocene occurred due to cessation of two spreading ridges coupled with the opening of the Bransfield Strait.