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

Abstract

The Middle Jurassic–Lower Cretaceous Fossil Bluff Group of Alexander Island, Antarctica represents the fill of a fore‐arc basin unconformably overlying an accretionary complex. Like most fore‐arc basins, this example had been considered to have a passive origin, as a topographic hollow between the arc and the trench‐slope break. Recent discoveries of igneous rock coeval with sedimentation have altered this view. Oxfordian–Kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin, within a single formation. Chemically, most basalts are high‐Nb types, which cannot have originated in a supra‐subduction zone setting. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic arc, it is concluded that a late Jurassic pause in subduction led to active rifting to form the fore‐arc basin.