Underplating of an accretionary prism: An example from the LeMay group of central Alexander Island, Antarctic Peninsula
The LeMay Group of Alexander Island, Antarctic Peninsula, is a Mesozoic accretionary prism that was constructed during subduction of Pacific and proto-Pacific oceanic crust. In central Alexander Island, several distinct lithologic associations can be identified, including two interpreted as underplated units: (i) a sandstone-mudstone association, consisting of thin- to medium-bedded non-channelled turbidite deposits, representing probable trench fill, and (ii) a basalt-chert association, representing oceanic crustal rocks, and its siliceous sedimentary cover. These two units are complexly deformed by dominantly westward-(oceanward-) directed thrusting. Structural relief introduced by later faulting reveals a wide range of structural styles and metamorphic grades representing different levels within the progressively deforming underplated units. Deformation ranges from thrust-related stratal disruption of poorly-lithified clastic sediment, achieved by independent particulate flow and cataclasis, through solution-dominated processes in clastic and siliceous rocks, to the development of pervasive cleavage fabrics at green-schist and transitional blueschist facies, with local crystal-plastic deformation. Later deformation (crenulation fabrics and isolated zones of folding) is of uncertain origin but probably resulted from further accretionary adjustments within the underplated units. The deepest levels may have been partially exhumed by syn-accretionary backthrusting or by transpression within a strike-parallel zone related to oblique convergence. Microstructural evidence reveals the importance of fluids in controlling deformation. Fluids were introduced with the underthrusting sediment and/or were generated during diageneis and metamorphism. In particular, evidence for locally elevated pore-fluid pressures is consistent with the rapid tectonic burial of a lithologically heterogeneous sequence and its subsequent evolution in a semi-closed system, with only limited fluid escape. Such microstructural criteria may be crucial to the identification of underplated units in other ancient accretionary prisms where the overall large-scale structural geometry cannot be reconstructed from fragmentary exposures.
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