Exhumation and incision history of the Torngat Mountains, northern Labrador and Quebec, Canada, using apatite (U-Th)/He thermochronology

Abstract

Apatite (U-Th)/He thermochronological data provide new insights into the exhumation history of the Torngat Mountains, which are located in Ungava Peninsula, northern Labrador and Quebec, Canada. Latest Jurassic to earliest Cretaceous rifting resulted in crustal extension and opening of the Labrador Sea, causing westward tilting and exhumation of the Labrador continental margin. Post-rift erosion and incision led to the formation of significant topographic relief in the Torngat Mountains. Thermal modeling of apatite (U-Th)/He thermochronological data from a vertical sample transect indicates that rapid cooling starting at ~140-150 Ma, which closely coincides in age with independent geological evidence for initial rifting in the area. Apatite (U-Th)/He ages along an E-W transect, which was collected perpendicular to the Torngat Mountains and the Labrador coast, record differential extensional and erosional exhumation diminishing westwards towards the interior of Ungava Peninsula. Ages near the Labrador coastline are as young as ~ 78 Ma and are significantly younger than the timing of onset of initial rifting (~140-150 Ma); this suggests significant amounts (~1-2 km) of post-rift erosion and incision along the eastern continental margin of the Labrador Sea. West of the Torngat Mountains, along the E-W transect, apatite (U-Th)/He ages also show evidence of substantial post-rift erosion likely related to Mesozoic and Tertiary fluvial erosion or to Pleistocene continental glaciation. Two constant-elevation N-S transects collected parallel to the Labrador coast show apatite (U-Th)/He ages that mimic the larger scale topographic features of the area, such as major rivers and fiords, indicating a deflection of shallow crustal isotherms due to post-rift development of highamplitude topography. Additional numerical modeling is required to refine the timing and spatial magnitude of post-rift erosion in more detail.