May 9, 2021
The Ellsworth Mountains, West Antarctica, consist of two mountain ranges; the Sentinel, and Heritage ranges. The more southerly Heritage Range is composed of a lower Paleozoic sedimentary and volcanic rock sequence, deformed during a single major deformation event in the early Mesozoic. This Gondwanian Orogeny possibly resulted from Andean-style convergence along the southern margin of Gondwana, prior to break-up of the super-continent, and the subsequent translation of the Ellsworth Mountains from a position close to the Natal embayment of southern Africa to that of the present day. Rocks of the Heritage Range are intensely folded, with close to tight, upright to inclined folds, plunging gently about a horizontal axis trending NNW-SSE. Locally, folds plunge moderately to subvertical toward the NNW, possessing asymmetries consistent with a dextral sense of shear. Cleavage is generally axial planar displaying downdip and strike-parallel stretching lineations that are frequently associated with domains of reverse, and dextral shear striking parallel to the regional structural grain. The spatial and temporal relationship of fractures developed within these domains as a result of noncoaxial shear, in addition to the progressive incremental strain histories derived from mineral fibres in strain shadows, indicate the contemporaneous nature of these shear domains. Strain analysis of deformed tuffaceous diamictites and oncolithic limestones reveal k-values <1 (mean 0.59) throughout the Heritage Range. The coexistence of strike-parallel dextral, oblique and reverse-shear domains, abrupt reorientation of progressive strain axes, steep cleavage dips, and k-values <1 are all consistent with a dextral transpressive deformation regime, previously unrecognized in the Ellsworth Mountains. A model of highly oblique (pure-shear dominated) transpression, associated with efficient spatial partitioning of the strike-slip component of shear is proposed to describe the structural relationships observed.