Cenozoic Exhumation of the Western Antarctic Peninsula: Thermochronologic Results from Northern and Southern Graham Land

Several major tectonic changes affected the Antarctic Peninsula in the Cenozoic, including cessation of subduction south of the Bransfield strait, Scotia arc tectonism along the peninsula's northernmost reaches, and opening of the Drake Passage in the late Eocene. In order to reconstruct the exhumational history of Graham Land associated with these events, we analyzed fission track and (U-Th)/He ages of apatite and zircon from 14 sites from the northern tip of the peninsula to just north of the Antarctic Circle. Apatite He ages in two regions along the western coast of Graham Land show strongly differing ages, suggesting distinct tectonic domains. South of Anvers Island apatite He ages are relatively young, ranging from 8 to 14 Ma, whereas to the north they range from 25 to 58 Ma, with one exception at 12 Ma. In the southern region, apatite He ages are the oldest at the southernmost site and generally young towards the north, whereas the opposite is seen in the northern region (oldest ages at the northernmost site and younging towards the south). These same trends are seen in zircon He ages which range from 16 to 53 Ma in the south and 79 to 91 Ma in the north. Time-averaged erosion rates calculated from apatite He ages range from 0.04 to 0.1 km/Myr in the north and 0.15 to 0.3 km/Myr in the south. Variations in the degree of either glacial erosion or tectonic exhumation provide possible explanations for the thermochronologic differences between northern and southern Graham Land. The north appears to record predominantly Paleogene exhumation, possibly associated with opening of the Drake Passage, whereas the south records Late Miocene exhumation, possibly associated with back-arc extension accompanying subduction of the Phoenix plate south of Anvers Island.