Ten stocks of the White Mountain Plutonic-Volcanic Series in New Hampshire have been sampled, and have yielded material for nine apatite and six zircon fission-track dates. The zircon ages (λ = 6.85 × 10−17 y−1) are slightly younger than isotopic ages (chiefly K-Ar) which have been previously determined on these same intrusives, but the apatite ages are considerably younger, averaging about 55% of isotopic age for intrusives older than 130 m.y., and 85% of isotopic age for intrusives younger than 130 m.y.
The differences between zircon and apatite ages have been used in conjunction with heat-conduction theory to show that 180 m.y. ago ambient wallrock temperatures of White Mountain intrusives (at the present erosion level) were ∼ 170 to 200°C. For intrusives 115 m.y. old, ambient wallrock temperatures were about 100°C. These numbers, assuming a 25 to 30 °C/km geothermal gradient, translate to depths of emplacement of ∼ 3 to 3.6 km (115 m.y.) and ∼ 5.3 to 7.6 km (180 m.y.), and indicate an average erosion rate, since Jurassic time, of 1 km per 32 m.y. (0.031 mm/yr). Jurassic isobaric surfaces are now tilted northeastward at an approximate gradient of 0.012 to 0.015 km per kilometre, Cretaceous isobaric surfaces at 0.007 to 0.008 km per kilometre and Devonian isobaric surfaces at an approximate gradient of 0.039 km per kilometre.