On the origin of fore-arc basins: new evidence of formation by rifting from the Jurassic of Alexander Island, Antarctica

The Middle Jurassic-Lower Cretaceous Fossil Bluff Group of Alexander Island , Antarctica represents the fill of a fore-are ba sin unconformably overlyi ng an accretionary complex. Like most fore-are basins, this example had bee n considered to have a passive origin, as a topographic hollow between the are and the trench-slope break. Recent discoveries of igneous rock coeval w ith sedimentation have altered this view. Oxfordian-Kimmeridgian basaltic a nd rhyolitic sills and lava flows are found in a restricted area at the nor th of the basin, within a single formation. Chemically, most basalts are hi gh-Nb types, which cannot have originated in a supra-subduction zone settin g. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic are, it is conclude d that a late Jurassic pause in subduction led to active rifting to form th e fore-are basin.