Large-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterise the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new “Sif Island”. The island comprises of granite, zircon U-Pb dated to ~177-174 Ma and characterised by initial εNd, 87Sr/86Sr and εHf isotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block’s boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100-90 Ma, and then remained close to the Earth’s surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous.