Arc Boudinage, Basin Inversion and Obduction in an Evolving Subduction System of East Antarctica

Abstract:

The paleo-Pacific margin of Gondwana experienced protracted subduction and accretionary tectonics starting in late Neoproterozic-early Cambrian times. Northern Victoria Land (NVL), in East Antarctica, preserves a cryptic record of these active margin processes. Most models indicate that NVL contains three main terranes, namely the Robertson Bay, Bowers and Wilson terranes. Significant debate centres, however, on whether these are far travelled terranes with respect to the East Antarctic Craton, and on the tectonic and magmatic processes that affected its active margin and were ultimately responsible for the formation of the Ross Orogen.

Here we interpret new aeromagnetic, aerogravity and land-gravity compilations that enable us to trace the extent of major subglacial faults in the basement of NVL, examine crustal architecture, and propose a new evolutionary model for the active margin of the craton. Prominent aeromagnetic anomalies at the edge of the Wilkes Subglacial Basin delineate the extent of an early-stage magmatic arc (ca 530 Ma?). This arc may have accreted as an exotic element onto the former Neoproterozoic rifted margin of East Antarctica or (perhaps more likely) developed in situ upon a pre-existing suture.

Remnants of magnetic arc basement are also identified ca 150 km further to the east within the Wilson Terrane (WT). We propose that these were originally adjacent arc segments and that transtension triggered significant arc boudinage separating these segments. Transtension may have created accommodation space for the development of thick Cambrian sedimentary basins, which are marked by regional magnetic lows with an en-echelon geometry. Basin inversion likely occurred in a later traspressional stage of the Ross-Delamerian Orogen (ca. 490-460 Ma) that triggered the development of a major pop-up structure within the WT. Several buried thrusts of the pop-up can be traced in the aeromagnetic images and a prominent residual gravity high delineates its high-grade metamorphic core. High amplitude magnetic and gravity anomalies also delineate buried oceanic basement of the northern Bowers Terrane. Oceanic basement was likely uplifted and obducted onto to the margin either because of oblique subduction or because of docking of a microcontinent inferred to underlie part of the Robertson Bay Terrane.