Diachronous cleavage development in the Robertson Bay Terrane, Northern Victoria Land, Antarctica: Tectonic implications

The Robertson Bay terrane is the easternmost of three regional lithotectonic units comprising Northern Victoria Land. It is largely represented by a thick succession of Cambrian‐Ordovician distal turbidites (Robertson Bay Group), which has been folded into upright structures with development of an associated, regionally penetrative cleavage during maintenance of regionally uniform, low‐grade regional metamorphic conditions. Whole rock 40Ar/39Ar incremental‐release analysis of slate/phyllite documents diachronous cleavage development between ∼500 Ma (west) and 460 Ma (east) during the Ross orogeny. This systematic regional variation in age is consistent with tectonic models for the Ross orogeny that involve southwest directed subduction of the outboard Robertson Bay terrane beneath the Bowers terrane. Convergence rates of ∼0.4–1.0 cm/yr are suggested.