Cambrian palaeomagnetic data confirm a Natal Embayment location for the Ellsworth-Whitmore Mountains, Antarctica, in Gondwana reconstructions

Abstract

S U M M A R YThe Ellsworth-Whitmore Mountains (EWM) are one of five terranes that form West Antarctica. Constraining the positions of these terranes in pre-break up Gondwana is crucial to understanding the history of the palaeo-Pacific Gondwana margin. We report the results of a detailed palaeomagnetic investigation of the EWM, which comprises some 150 sites in six formations, ranging in age from Cambrian to Permian. Five of the studied units yield only viscous remnant magnetizations of recent age, or unstable natural remanent magnetizations. The remaining unit, the mid-late Cambrian Frazier Ridge Formation, yielded stable magnetizations at 16 of 35 sites. This component passes a fold test at the 95 per cent confidence level, indicating that it pre-dates Permian deformation, and we argue that it is of primary origin. The resulting palaeopole (9 • N; 293 • E; A 95 = 5.1 • ) is in good agreement with two previously published palaeopoles from similarly aged rocks in the EWM. Collectively these data indicate that the EWM were located in the Natal Embayment prior to Gondwana break-up, and underwent 90 • of anticlockwise rotation during break-up. All three studies, however, yield inclinations that are slightly too shallow when compared with coeval Gondwana reference poles.

The Gondwana Plate margin in the Weddell Sea sector: Zircon geochronology of Upper Paleozoic (mainly Permian) strata from the Ellsworth Mountains and eastern Ellsworth Land, Antarctica

“…1) (the Nash Hills Formation of Storey and Macdonald, 1987) have a thermally stable component of magnetization giving a paleopole position similar to that for the Ellsworth Mountains. Reinvestigation of the Ellsworth sequence (Randall and Mac Niocaill, 2004) reconfirmed previous studies. Rotation of the Ellsworth block is demanded by the paleomagnetic data, but, based on those data alone, there is uncertainty in the amount of translation.…”

A seismic transect across West Antarctica: Evidence for mantle thermal anomalies beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome

“…The inferred position of the Ellsworth‐Whitmore mountains crustal block within Gondwanaland [ Schopf , ; Grunow et al ., ; Randall and Mac Niocaill , ] and the underlying Precambrian basement observed in the outcrop geology [ Millar and Pankhurst , ] permits the possibility of a Precambrian keel for this region. Globally, unmodified Precambrian lithosphere typically ranges in thickness from > 200 km to as thin as ~100 km [ Artemieva and Mooney , ] with shear wave velocities in excess of 2–3% above the global average [ Lebedev et al ., ].…”

“…The breakup of Gondwanaland, beginning in the mid‐Jurassic with the opening of the Somali Basin, Mozambique Basin, and Weddell Sea, was accommodated by the counterclockwise rotation and translation of the Ellsworth‐Whitmore mountains crustal block [ Grunow et al ., ; Randall and Mac Niocaill , ; Dalziel et al ., ]. This process may have been set in motion by a mantle plume interacting and ultimately thermochemically breaking the subducted Phoenix slab, leading to slab retreat along the central Pacific margin of Gondwanaland and extreme extension along low‐angle detachment faults within the back arc [ Dalziel et al ., ].…”

Seismic Structure of the Antarctic Upper Mantle Imaged with Adjoint Tomography

“…Three of these crustal blocks, Marie Byrd Land, the Thurston Island‐Eights Coast, and the Antarctic Peninsula (Figure ), are forearc, and magmatic arc terranes that along with Zealandia and the Campbell Plateau formed the convergent central Pacific margin of Gondwana (Grunow et al, ; Mukasa & Dalziel, ). In contrast, the Ellsworth‐Whitmore Mountains crustal block (Figure ) may be a fragment of East Antarctica that underwent counterclockwise rotation and translation to its present‐day relative position during the opening of the Weddell Sea (Grunow et al, ; Randall & Mac Niocaill, ; Schopf, ).…”