Deformation mechanism paths for oceanic rocks during subduction and accretion: the Mesozoic forearc of Alexander Island, Antarctica
The LeMay Group of Alexander Island, Antarctica, is a Mesozoic accretionary prism that contains slivers of ocean floor and ocean island material, accreted under a range of conditions and depths. It provides a rare opportunity to compare the deformation mechanism paths between oceanic and trench-fill lithologies during subduction and accretion by offscraping or underplating. Ocean floor slivers consist of a bedded basalt–volcaniclastite–chert rock association, overlain by trench-fill sedimentary rocks. The oceanic lithologies initially deformed by cataclasis, combined with particulate flow in the volcaniclastic rocks. At shallow levels the clastic trench-fill sedimentary rocks deformed by independent particulate flow, which changed to cataclastic flow at depth. At those depths crystal plasticity affected the cherts and jaspers. At the deepest levels achieved in the subduction zone, crystal plasticity was the dominant deformation mechanism in the clastic sedimentary rocks, whilst cataclasis continued in the lavas. Deformation was synchronous with metamorphism up to transitional greenschist–blueschist facies. All lithologies were affected by pressure solution after accretion, with the formation of a bedding-parallel or sub-parallel cleavage. The deformation mechanism paths indicate that (1) there is a crystal plastic deformation field at deep levels in the subduction zone, and (2) that the base of the pressure solution field is concordant with the decollement. A frontally accreted ocean island is exposed in the Lully Foothills, and consists of a basalt–volcaniclastite–tuff rock association. Deformation is represented only by a patchily distributed bedding-sub-parallel pressure solution cleavage in the tuffs and volcaniclastites, and is probably related to loading. Metamorphism does not exceed prehnite–pumpellyite facies. The accreted ocean island indicates that such features can be incorporated into accretionary prisms at shallow levels without significant segmentation.