Sinistral transpression during accretion of the Trinity Peninsula Group of the Antarctic Peninsula and evidence for large tract-bounding faults.

A.P.M. Vaughan1, R.A.J. Trouw2, J.D. Bradshaw3

1British Antarctic Survey, High Cross, Madingley Rd, Cambridge, CB3 0ET, U. K.

2Departamento de Geologia, I. Geo, UFRJ, 21910-900, Rio de Janeiro, Brazil 3Department of Geological Sciences, University of Canterbury, Christchurch, N.Z.

a.vaughan@bas.ac.uk

New fieldwork strongly suggests that the Trinity Peninsula Group is an accretionary complex, consisting of tracts, which formed by accretion during oblique sinistral subduction along the palaeo-Pacific margin of Gondwana. The Triassic Trinity Peninsula Group is the main tectonostratigraphic unit at the northern end of the Antarctic Peninsula. It consists of a 500 x 100 km complex of sub-greenschist facies, turbiditic sandstones and pelites, with minor conglomerates, pillow basalts, and other volcanic rocks. The current study set out to resolve any ambiguities over palaeo-subduction direction and to examine a distinctive suite of conglomerates with Archaean- and Ordovician clasts exposed at View Point. New structural data shows unequivocally that accretion progressed from modern-day northwest, i.e. from the palaeo-Pacific. Clear evidence of soft sediment deformation was identified, supporting a subduction accretion origin for the Trinity Peninsula Group. Palaeocurrent data from laminated units at View Point indicate sediment transport from modern day south, i.e. towards the palaeo-trench. Two phases of deformation are recognised affecting Trinity Peninsula Group rocks prior to deposition of the Lower Jurassic Botany Bay Group. Bedding generally dips steeply north to northwest. D1 is interpreted here to be associated with subduction accretion, and forms tight to isoclinal asymmetric folds with weakly developed slaty cleavage. Cleavage ranges from parallel to bedding and isoclinal fold axes to transecting F1 fold axes up to 30° clockwise in plan, suggesting sinistral transpression during deformation. Fold plunges are variable, but vergence is generally to the north, i.e., towards the Pacific. D1 is associated with development of minor, soft-sediment deformation-related, broken formation. D2 represents an episode of large-scale episode of ductile to brittle-ductile sinistral shear, overprinting D1 structures and associated with development of km-wide zones of broken formation. D2 structures include steeply plunging gentle to open folds of D1 structures, with a penetrative cleavage, oblique to bedding, developed in pelitic layers. Zones of broken formation appear to form boundaries between Trinity Peninsula Group tracts of different lithological composition. The zone at View Point is characterised by intense cataclastic to brittle-ductile deformation in belts exceeding 1 km in width. Neither bedding nor regular cleavage can be measured. Lenticular and rounded remnants of sandstone layers, varying in size from few centimetres to 50 metres diameter, are randomly distributed in a mudstone-dominated matrix. Only locally can a rough anastomosing east–west cleavage be observed, surrounding less deformed lenticular zones. Overall, identification of these zones suggests that the Trinity Peninsula Group can be subdivided into Southern Uplands-like tracts, although solely based on lithological composition: palaeontological and age information are sparse for the Trinity Peninsula Group.