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Apatite (U–Th)/He and fission track data from profiles in western Scotland constrain the timing and magnitude of denudation during the early Cenozoic when the north Atlantic region was the site of intense magmatic activity related to the proto-Icelandic plume. Apatite helium ages vary from 77 ± 8 to 265 ± 27 Ma (± 2σ) at Sgorr Dhonuill, Ballachulish, and from 104 ± 10 Ma to 166 ± 17 Ma at Clisham, Outer Hebrides. At both locations apatite fission track (AFT) ages are older than the corresponding He ages; at Clisham they vary from 189 ± 28 Ma to 242 ± 26 Ma, and from 186 ± 6 Ma to 257 ± 12 Ma at Sgorr Dhonuill. Apatite He ages increase linearly with elevation suggesting that the cooling rate remained constant in the late Mesozoic. However, the apatite He age profile requires a period of rapid cooling after ∼ 100 Ma. Apatite He ages predicted from the AFT-derived thermal histories are indistinguishable from measured He ages for a rapid cooling event of 1 to 10 Myr duration between 61 and 47 Ma at Sgorr Dhonuill and 65 to 49 Ma at Clisham. The combined apatite FT- and He-derived thermal histories constrain the early Cenozoic geothermal gradient at 39 ± 9 °C/km at Sgorr Dhonuill and 19 ± 6 °C/km at Clisham. Amounts of denudation related to the rapid cooling event vary from 1330 ± 230 m at Sgorr Dhonuill to 2250 ± 750 m at Clisham, in agreement with models that predict greater amounts of denudation where magmatic underplating is thicker. However, the direct correlation between underplating-driven surface uplift and denudation may only be apparent and a more complex link between spatial variation of surface uplift and denudation is suggested. The integration of results from multiple low-temperature thermochronometers, combined with inverse and forward modelling, provides a convincing and quantitative method to deduce onshore erosional histories, and provides critical information about the spatial distribution of erosion that cannot be derived from the offshore record.

Apatite (U–Th)/He and fission-track thermochronometers are combined to constrain models of denudation and escarpment development of the high elevation Eritrean margin along the southern Red Sea. (U–Th)/He ages have been determined for apatites from two coast-perpendicular transects that were previously used for apatite fission-track analysis [E. Abbate, M.L. Balestrieri, G. Bigazzi, Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis, J. Geophys. Res., 107, B11, doi: 10.1029/2001JB001009, (2002)]. In central Eritrea near Asmara, He ages increase from c. 7 Ma at the coast to 152 Ma on the plateau along a transect. Further south, He ages from the margin border to the base of the escarpment span a narrower range (7 to 12 Ma). One sample from the top of a marginal high (2950 m) yields older ages (22–43 Ma). Forward modeling suggests that the He age distribution across the margin should allow scarp retreat and downwearing mechanisms to be distinguished in the case of a margin formed in the last twenty million years. The distribution of He ages suggests that the escarpment evolved by downwearing and that post-break up erosion of the escarpment was facilitated by in situ excavation rather than parallel retreat. This implies the existence of an inland drainage divide located seaward of the present day escarpment rim. We envisage that the inland divide formed during or immediately after continent break up, and that the eastern marginal belt represents a remnant of the apex structure. The short distance between the eastern marginal belt and the rim (c. 10 km) represents the total retreat of the escarpment. Comparison of the measured He ages from the coastal plain with those predicted from forward modelling indicates that the main phase of post-break up erosion started at about 15 Ma, closer to the initiation of seafloor spreading in the Red Sea than was thought previously.

The southeast Australian margin, like other high elevation passive margins, is characterised by a steep escarpment that separates a dissected coastal plain from a low relief inland plateau. Quantitative constraints on the generation of escarpments can be provided by apatite (U–Th)/He ages. Here we use a coast-perpendicular traverse across the coastal lowlands, escarpment and plateau to test the three prevailing models of SE Australian escarpment formation, namely retreat into a downwarped rift shoulder, escarpment retreat and down-wearing on high elevation rift shoulder with flexural rebound. Apatites from the coastal plain have He ages of between 87 and 112 Ma, suggesting that the coastal lowlands developed very rapidly after rifting and continental break-up at 85–100 Ma. The He age data are inconsistent with the erosion of a downwarped rift margin, and cannot be explained by a constant post-break-up rate of lateral escarpment retreat across the coastal plain or by constant down-wearing. The data require either rapid escarpment retreat or rapid in-place excavation of the escarpment soon after break-up, in response to rifting and the lowering of base levels on the margin of the new continent at break-up, followed by a period of landscape stability and low erosion. Combined with the existing apatite fission track record, the He data are consistent with erosion of 3–4 km within a maximum of 28 Myr of break-up, at a minimum vertical erosion rate of 130 m Myr−1 along the coast. The rapid denudation period across the coastal plain in this region took less than 48 Myr (from the coast to the escarpment base), which corresponds to an average vertical erosion rate of 45 m Myr−1. This is equivalent to a mean escarpment retreat rate of 5–10 km Myr−1. Apatite He ages from the plateau (183–247 Ma) indicate that the highlands remained stable throughout continental break-up, experiencing average erosion rates of less than 10 m Myr−1 since the late Palaeozoic/early Mesozoic.