Tectonic history of the Ellsworth Mountains, West Antarctica: Reconciling a Gondwana enigma

Abstract

Early Paleozoic orogenesis has been recognized along the southern African (Saldanian orogeny) and East Antarctic (Ross orogeny) sectors of the Gondwana paleo- Pacific margin. However, the absence of a contemporaneous orogenic event in the Ellsworth Mountains of West Antarctica, once a contiguous part of this margin, has resulted in their geology being considered enigmatic. In this contribution, widespread, detailed structural studies from all stratigraphic levels of the Ellsworth Mountains allow their tectonic evolution to be reassessed. Geochemical, stratigraphic, and structural data indicate that the Middle to Upper Cambrian Heritage Group developed in a continental rift basin. Multiple observations of the crucial end-Cambrian contact between this rift sequence and the overlying Crashsite Group reveal the contact to be regionally conformable; there are localized outcrop-scale unconformities. Furthermore, structural continuity can be demonstrated across this key boundary, thus precluding the possibility of an end-Cambrian orogenic event. The entire stratigraphic succession was affected by two post-Permian phases of deformation. D1 structures are locally developed and superimposed by the main dextral transpressive D2 Permian−Triassic Gondwanian deformation event. D2 is succeeded by an episode of extension orthogonal to orogenic strike. Recent geochronological data on the early Paleozoic evolution of the Cape fold belt reveal a strong tectonostratigraphic correlation with the Ellsworth Mountains, indicating that this sector of the Gondwana paleo-Pacific margin was affected by Middle to Late Cambrian rifting. A new tectonic model is proposed for the southwestern paleo-Pacific margin of Gondwana that accounts for the presence of rifting along a margin otherwise dominated by active subduction.