Previous geological studies have indicated that the major thrusting event in the San Juan Islands was of late-early to late Cretaceous age. Geochronological data are consistent with these prior inferences and confirm that thrusting occurred between approx 105 and 75 m.y. Reset zircon fission-track dates indicate the possible presence of a reversed geothermal gradient in the Decatur terrain, the uppermost preserved thrust plate. If present, this gradient was probably produced by conductive heating, and possibly by shear heating associated with a now-eroded overlying thrust plate and thrust fault. Thrusting in the southern San Juan Islands was accompanied by uplift and resetting of apatite dates. The Haro formation, the Spieden group, and their basement (probably the Wrangellia terrain) did not experience this uplift and probably acted as a 'backstop' to thrusting. Uplift of the southern San Juan Islands was, therefore, probably mainly accommodated on the Haro fault. The Nanaimo basin formed in the foreland of this advancing thrust system, probably as a response to thrust-related loading. Eastern exposures of the Nanaimo group were uplifted prior to deposition of the Middle and Upper Eocene Padden member of the Chucknaut formation, perhaps along a reactivated Haro fault system. -J.M.H.
Additional publication details
Fission-track dating of the tectonic development of the San Juan Islands, Washington.