THE ALEXANDER TERRANE BOUNDARY WITH NORTH AMERICAN MARGIN TERRANES LIES WEST OF THE COAST BATHOLITH IN SOUTHERN SOUTHEAST ALASKA

The Insular suture zone (ISZ) is perhaps the largest tectonic boundary in the North American Cordillera. It joins the outer allochthonous Alexander (AT) and Wrangellia terranes, running between southern Peninsular Alaska and Vancouver Island, to rocks of the early Mesozoic North American active margin. Much of the ISZ appears to be intruded out by the Paleocene-Eocene Coast Batholith (CB), although in southern southeast Alaska a relatively intact section through the zone is preserved. In this area the AT is in general tectonically beneath the Taku terrane (TT) which as defined here is correlative to North American margin rocks of the Yukon-Tanana terrane. Initial thrusting of the AT eastward beneath the TT occurred in Late Jurassic or earlier time with structural overlap on the order of ~100 km. In latest Jurassic to earliest Cretaceous time the crustal-scale structural stack underwent horizontal flattening, possibly in conjunction with the development of the overlying Gravina extensional or transtensional basin. Both AT and TT deformed rocks are cut by Late Jurassic dike and sill swarms, and both are overlapped by the Upper Jurassic-Lower Cretaceous Gravina Formation. At ~100 Ma the region underwent thick-skinned west-vergent reverse faulting synchronous with the emplacement of magmatic epidote tonalite plutons- marking intra-arc deformation above strongly coupled east-dipping subduction of the Farallon plate. Reverse faulting migrated eastward in time with the ultimate tectonic ascent of lower plate AT gneisses westward back over TT gneisses and schists in latest Cretaceous-early Paleocene time. In spite of the intensity of mid-Cretaceous to Paleocene crustal shortening these relations show that initial suturing was early Mesozoic, and not mid-Cretaceous as commonly cited; and that the CB is a younger superposed tectonic feature, and does not coincide with the ISZ.