The Vanda Dike Swarm, Dry Valleys, Antarctica I: Field Relations and Tectonic Implications

Abstract:

The Vanda Dike Swarm is a set of >600 compositionally diverse dikes that are unusually well exposed in the Dry Valleys of Antarctica. New precise zircon age determinations indicate that they intruded between 491 and 495 Ma. The dikes range in composition from basaltic to rhyolitic, although they are arguably bimodal. They were emplaced into both foliated and massive Cambrian granitoids of the Ross Orogeny and their Proterozoic wall rocks. There is no apparent relationship between the major element composition and the dikes’ emplacement age or location. Most dikes have distinctly non-planar contacts, and there is no evidence that dike emplacement was controlled by basement structure, either on a meter scale or regionally. Instead, most dikes are sinuous, strike 28 to 44°, and dip eastward (many <70°), indicating tectonic controls on emplacement. The dikes are not distributed evenly: many are concentrated in km-scale areas, which may mark the infrastructure of central volcanoes. Chilled margins and glassy groundmass indicates that the dike swarm was emplaced at shallow levels. So far as we know, there are no coeval volcanic rocks in the region, where the Devonian to Triassic Beacon Standstone unconformably overlies the dikes and their wallrocks. Many of the dikes have strongly mylonitized margins with normal sense of shear, indicating that solidified dikes served as normal faults. Otherwise, so far as we are aware, there is no recognizable evidence for normal faulting in the area. Our interpretation is that the dikes were emplaced into the shallow crust at the end of the Ross Orogeny, at the culmination of uplift and synchronous with extension. They thus represent the transition from an Andean-type margin to a rifting environment, in all likelihood due to reconfiguration of subduction near the end of the Cambrian.