The Mesozoic LeMay Group accretionary complex of Alexander Island, Antarctica, contains thrust-bound slices of accreted ocean floor, ocean islands and seamounts. They represent fragments of proto-Pacific oceanic crust, of which only a tiny remnant (the Phoenix plate) remains off northern Antarctic Peninsula. They therefore provide an excellent opportunity to sample the ancient oceanic crust that formerly occupied the southern Pacific Ocean.All the basalts experienced sea-floor and subduction/accretion metamorphism ranging from zeolite to transitional blueschist facies. On the basis of rare-earth and other immobile trace element characteristics, the basalts are divided into depleted MORB, N-MORB, E-MORB, and tholeiitic and alkaline OIB.Oceanic basalts occur within two rock associations on Alexander Island, basalt-volcaniclastitechert and basalt-volcaniclastite-tuff. The basalt-volcaniclastite-chert rock association is dominated by pillow lavas which have light REE-depleted N-MORB geochemical characteristics, and is interpreted as representing ocean floor formed at spreading centres. Locally, sills of tholeiitic OIB intrude the sequence. The basalt-volcaniclastite-tuff rock association exposed in the Lully Foothills was formed in shallow water during the Early Jurassic. It is geochemically varied, consisting of basalts with N-MORB, E-MORB and tholeiitic OIB characteristics. The association is interpreted to have been formed on a large seamount or ocean island.
A new and well-preserved.asteroid, Protremaster uniserialis (gen. & sp.nov.) is described from the Lower Jurassic (Sinemurian) of Antarctica. This find extends the fossil record of the family Asterinidae and the subfamily Tremasterinae considerably and lends support to the idea that asteroids underwent an important morphological diversification in the late Triassic-early Jurassic.
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.
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