Geochronologic investigations along the Alexander-Taku terrane boundary, southern Revillagigedo Island to Cape Fox areas, southeast Alaska

Abstract

The southern Revillagigedo Island to Cape Fox area, southeast Alaska, is critical in understanding the regional structural development of the Alexander-Taku terrane boundary in southeast Alaska and its implications for the tectonic history of the Insular suture zone, which extends from southern Alaska to northern Washington. The study area offers what currently appears to be the greatest structural relief through the Alexander-Taku terrane boundary. This study presents U-Pb zircon and supporting Sr-Nd isotopic data as a structural and stratigraphic tool to augment detailed field studies that are nearing completion. The principal relations established are as follows. (1) Ordovician-Silurian and apparently Late Proterozoic-Cambrian crystalline basement rocks and cogenetic volcanic arc rocks of the Alexander terrane tectonically underlie nearly the entire area west of the Coast batholith. Many of the more highly tectonized and metamorphosed rocks of this structural basement were originally included in the Taku terrane. (2) The Taku terrane consists of two distinct sequences that include a Silurian- Devonian and possibly older North American continental margin sedimentary sequence with interbedded and intruded arc volcanic and plutonic members (Kah Shakes sequence), and an upper Paleozoic-lower Mesozoic island arc-marginal basin sequence (Alava). Data presented in this chapter, along with detailed field relations, pose the possibility of a depositional link between these two sequences. (3) The Taku terrane occurs as an isolated folded nappe sequence above the Alexander terrane. (4) The Alexander terrane was thrust eastward beneath the Taku terrane prior to Late Jurassic time, at which point dikes and sills crosscut the original thrust boundary. (5) The Late Jurassic-Early Cretaceous Gravina basinal arc sequence lapped unconformably across the terrane boundary thrust complex. (6) The Alexander-Taku terrane thrust complex underwent high-magnitude horizontal flattening and stretching that may represent deep crustal flow within the suture zone or, alternatively, a distinct postsuturing extensional event commensurate with Gravina basin formation. (7) Mid-Cretaceous superposed deformation within a fold-thrust belt resulted in the reimbrication of the suture-zone assemblage and the destruction of the Gravina basin. (8) Steeply dipping sill-like magmatic epidote tonalite plutons were syntectonically emplaced into the foldthrust belt. (9) Fold-thrust deformation was west-directed along the western domain of the belt, upright along its axis, and east directed along its eastern domain. (10) Alexander terrane rocks from the lower plate of the original terrane boundary were thrust back up against the Taku terrane along the axis of the fold-thrust belt and, as a result, constitute a major gneiss complex to the east of the Taku terrane along the western wall of the Coast batholith. (11) Latest Cretaceous to Paleocene, mainly northnorthwest- trending elongate tonalitic plutons of the Coast batholith intruded the eastern margin of the fold-thrust belt and, at least locally, assimilated substantial quantities of Alexander terrane crustal materials. (12) The Alexander-Taku terrane boundary in the study area does not map as a linear contact, but has considerable topologic complexity, including large domains that have low dips. The complexity of the map pattern is a result of prolonged suture-zone deformation, a finding that contrasts sharply with commonly held ideas of the boundary having formed abruptly at mid-Cretaceous time in conjunction with fold-thrust belt deformation.

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