West Antarctica record of oblique convergence along the Cretaceous proto-Pacific margin of Gondwana

In Marie Byrd Land, West Antarctica, mafic alkalic dike arrays and A-type granitoids of 115 to 95 Ma age are typically viewed as a record of continental extension and as a direct precursor to orthogonal breakup between New Zealand and Marie Byrd Land at circa 70 Ma. New kinematic data for a mafic dike array in the Ford Ranges, an approximately 1000 km2 region dominated by plutonic and metamorphic bedrock, provide a mean dike trend of N16W. This corresponds to a maximum finite strain axis oriented N74W, highly oblique to the Marie Byrd Land margin and to major fault trends in the Ford Ranges. The stretching direction is sub-parallel to the N65W strain axis determined from structural analysis of folds and fabrics in a migmatite dome in the northern Ford Ranges. 40Ar/39Ar dolerite dike emplacement ages are 146-97 Ma, broadly coeval with emplacement of the dome and with development of the eastern Ross Sea rift. The oblique orientation of maximum finite strain with respect to large faults, geophysical lineaments and the rifted margin of western Marie Byrd Land suggests that transcurrent tectonics were in effect along this segment of the Gondwana margin in the middle Cretaceous, prior to breakup. The Ford Ranges result contrasts with that from a contemporaneous dolerite dike array in central Marie Byrd Land, 300 km to the east, which records stretching orthogonal to the rifted Marie Byrd Land margin, but is compatible with kinematic results from the Antarctic Peninsula. Consequently West Antarctica fits the pattern of along-strike variations in kinematics and tectonics related to oblique convergence along the Cretaceous circum-Pacific margin.