CONSTRAINING THE CENOZOIC TOPOGRAPHIC EVOLUTION OF A PASSIVE MARGIN BY COMBINING THERMAL MODELING OF APATITE (U-TH)/HE AGES AND APATITE 4HE/3HE THERMOCHRONOMETRY, NW SCOTLAND

Passive margins’ characteristically slow background exhumation rates may result in low temperature thermochronometry ages that, individually, are too old to be useful in investigating Cenozoic topographic evolution. However, where Plio-Pleistocene glaciation and far-field tectonic forces have affected passive margins their modern topography is likely to have been partially shaped during the Cenozoic. One such region, the highlands of Northwest Scotland, maintains kilometer-scale relief in the absence of a classically active tectonic margin and has been extensively glaciated since the Plio-Pleistocene. Most previous thermochronological investigations of Scotland’s exhumation have used apatite fission track analysis, however the lower closure temperature of apatite (U-Th)/He and apatite ⁴He/³He are better suited to investigate Cenozoic cooling and topographic evolution. We have thirteen apatite (U-Th)/He ages from two age elevation profiles in the central highlands of Scotland, ranging from 15 Ma to 350 Ma, and two apatite ⁴He/³He diffusion profiles. We modeled possible temperature histories for the apatite (U-Th)/He profiles using the software QTQt (Gallagher, 2012) and modeled the apatite ⁴He/³He using Berkeley Geochronology Center’s search code. The age elevation profiles and the ⁴He/³He data indicate slow exhumation rates, <0.01 mm/yr, from the late Paleozoic through the mid-late Cenozoic when exhumation rates increased by two to three orders of magnitude. QTQt modeling shows that the distribution of much older ages (200-300 Ma) on peaks and much younger ages (15-60 Ma) in deep valleys can only be achieved through differential exhumation since the mid-late Cenozoic, responsible for removing <1 km of rock from peaks and 2-3 km from valleys. Glacial topography and offshore evidence of basin inversion in the mid-late Cenozoic may indicate that both tectonics and glaciation are responsible for Scotland’s modern relief.