Tectonic history of the Queen Charlotte Islands and adjacent areas—a model

Abstract

The region including Queen Charlotte Islands, Hecate Strait, and Queen Charlotte Sound is underlain by two allochthonous terranes, Wrangellia and the Alexander terrane. The suture between them occurs in central Graham Island and central Hecate Strait and is coincident with the traces of the Sandspit and Rennell Sound fault zones, each of which developed in response to crustal rifting in Queen Charlotte Sound during mid-Tertiary time.The stratigraphic succession comprises four tectonic assemblages. (1) The allochthonous assemblages comprise Paleozoic rocks of the Alexander terrane and Upper Triassic and Jurassic rocks of Wrangellia, which on the basis of paleomagnetic and biogeographical data are clearly exotic. The distribution of these terranes beneath Queen Charlotte Sound and Hecate Strait is supported by geophysical information and subsurface data obtained from offshore wells. (2) The suture assemblage is represented by extremely coarse conglomerates, massive graywackes, and turbidites of Early Cretaceous age, and possibly by Upper Jurassic plutons. (3) The post-suture assemblage is expressed by the tripartite succession of the mid- to Upper Cretaceous Queen Charlotte Group whose middle component, the Honna Formation, comprises polymictic conglomerates that may have resulted from the final accretion of the amalgamated crustal fragments of the Alexander Terrane and Wrangellia to the continental margin. (4) The rift assemblage is expressed by mid- to upper Tertiary volcanics, epizonal plutons, and terrigenous clastics. Rifting is believed to have occurred in Queen Charlotte Sound above a mantle plume and resulted in crustal attenuation through development of listric, crustal-penetrative normal faults, and concurrent extrusion of subaerial volcanics and emplacement of high-level plutons. The attenuation caused northward motion of the Queen Charlotte Islands along the Louscoone Inlet – Sandspit fault zone and subsidence in Queen Charlotte Sound where Lower Miocene marine sediments were deposited within the rift zone. Later, additional rifting in southern Hecate Strait resulted in the reactivation of the old suture zone, manifest as the Rennell Sound fault zone. Concurrent with continued terrigenous deposition and volcanism, the Queen Charlotte Islands moved northwesterly along the Rennell Sound Fault, which disrupted the earlier fault trend. The final rotation of the islands to their modern position was accomplished through left-lateral motion along the Beresford Bay and Langara Faults.