TRIASSIC TO NEOGENE GEOLOGIC EVOLUTION OF THE QUEEN-CHARLOTTE REGION

A wealth of new geological and geophysical data from recent studies of the Queen Charlotte region are integrated into a coherent model. In this paper, we summarize these new studies and discuss possible correlations with other areas. Four tectonostratigraphic divisions are distinguished by stratigraphic, structural, and magmatic character, and each is separated by a major unconformity. The oldest division comprises widely distributed, upper Paleozoic through Middle Jurassic strata of Wrangellia that accumulated in volcanic-arc and stable shelf and basinal settings. No significant deformation occurred in the Queen Charlotte Islands region during the accumulation of these rocks. A Middle and Upper Jurassic assemblage comprises two plutonic suites and volcanic and epiclastic rocks. The unconformity below the Middle and Upper Jurassic assemblage marks a regional, southwest-vergent contractional deformation that is the most significant Mesozoic or Cenozoic deformation in the region. Jurassic plutons in the Queen Charlotte Islands are the oldest and most primitive members of an eastwardly migrating and evolving Jura-Cretaceous magmatic front recognized by other workers in the Coast Plutonic Complex. Widespread Late Jurassic block faulting led to differential uplift and erosion of northwest-trending fault blocks. A third assemblage consists of Cretaceous marine sedimentary rocks derived principally from subjacent Jurassic volcanic rocks as well as older strata. The present distribution of Cretaceous strata reflects a gradual eastward transgression, briefly interrupted in the Coniacian by progradation of conglomerate fans from the east. A second regional contractional deformation event in latest Cretaceous time was concentrated along a northwest-trending zone coinciding with Jurassic block faults. The early Tertiary marked another distinct shift in sedimentation style, with the inception of local nonmarine deposition on the present islands and widespread volcanism and plutonism on the southern islands. Syntectonic deposition in offshore extensional basins (Hecate Strait and Queen Charlotte Sound) may have commenced at this time. Later in the Tertiary, extensive deposition occurred in offshore regions, coeval with northward migration of plutonism and volcanism on the islands. Contractional structures in Pliocene sediments in Hecate Strait are the youngest deformational features observed.