Abstract

Antarctica is characterized by a unique combination of active processes, including active crustal deformation, volcanism, and glacial loading and unloading, but little is known about the Neogene to contemporary geodynamic state of the Antarctic plate. This paper presents new data on the structure and timing of volcanism on the Mount Morning shield volcano, with the purpose of defining Pleistocene stress directions concomitant with volcanism and rifting in the southern portion of the Victoria Land rift basin. Elongate vents and vent alignments indicate parasitic volcanism has predominantly occurred along a primary set of NE fissures. Parasitic basaltic cinder cones yield 40Ar-39Ar cooling ages, presented herein, that range from essentially zero to ca. 3.5 Ma, though mostly of Pleistocene age. The systematic NE trend of the fissures on Mount Morning records magmatically induced fracturing of the volcano flanks controlled by a regional N31°E maximum horizontal stress (SH) that dominated the Mount Morning area during the Pleistocene and, probably, also the Pliocene. Minor volcanism occurred along shorter NW alignments that have no discernible age difference from the NE fissures. The Pleistocene age of the parasitic volcanism suggests that the stress direction documented at Mount Morning represents the contemporary differential stress field in the area. This NE SH direction differs from the N15°W SH direction measured in a borehole at Cape Roberts, 100 km to the north, and the SH orientations appear to track the changing trend of the segmented rift boundary. The exact cause for the variability in the SH directions is uncertain, but the change in SH could record different stress provinces within the rift system, or it could reflect stress reorientation along the major lithospheric boundary of the rift. The new contemporary stress datum at Mount Morning is consistent with a neotectonic normal-fault to strike-slip fault regime within the Terror Rift, which was active in Pliocene-Pleistocene times and could remain active today. This stress regime still dominates the contemporary geodynamic state of this sector of the West Antarctic Rift system.

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