Constraints Mechanism of the Early Cenozoic Cooling and Denudation on the Outer Hebrides in the Northwest of Scotland From Joint Apatite Fission Track and (U‐Th‐Sm)/He Data

Abstract To constrain the mechanism driving early Cenozoic cooling and denudation in the Outer Hebrides, apatite fission track (AFT) analysis was conducted on 16 samples, with apatite (U‐Th‐Sm)/He (AHe) analysis on half of them. AFT ages range from 125.7 ± 13.2 to 245.7 ± 13.7 Ma, while the single‐grain AHe ages are highly dispersed, ranging from 13.9 ± 1.4 to 176.1 ± 17.6 Ma with values showing a general decrease toward the south. These data, together with grain fragment dimensions from AHe data, were incorporated into the generation of thermal history models. The combined models show that the northern Outer Hebrides started cooling during the Permian to Triassic, while the southern part cooled later, around the Late Triassic to Early Jurassic. Estimates of denudation predicted are approximately 1–2.5 km during the cooling episode from the Permian to the Early Jurassic, which coincides with Pangaea's breakup, triggering lithospheric thinning, rifting, fault reactivation, and removal of synrift topography. From the Middle Jurassic to the Cretaceous, the area experienced much slower, monotonic cooling, with approximately 0.1–1 km of denudation. Then the entire region underwent synchronous cooling and denudation in the early Cenozoic (∼65 ± 5 Ma), likely driven by mantle plume initiation and magmatic underplating which extends from east Greenland to St. Kilda and northwest Scotland inferred from geophysical studies rather than compressive stress from Alpine Orogeny or North Atlantic rifting. In addition, spatial denudation amount around 65 Ma along the Outer Hebrides suggests uneven underplating beneath the Outer Hebrides, decreasing from east to west and north to south.