Study of Late Triassic biofacies and associated paleoecology reveals new silicified shallow-water corals and other fossils from new and previously known localities within the Alexander terrane (Keku Strait and Gravina Island, southeast Alaska) and Wrangellia (Wrangell Mountains, Alaska, and Vancouver Island, British Columbia). Twenty-five species of coral are identified from eight localities within the Alexander terrane and 34 species are identified from four localities in Wrangellia. Distributions of silicified shallow-water marine fossils contribute to Late Triassic (Norian–Rhaetian) paleoecology, biotic diversity, and terrane paleogeography. Depositional environments establish the conditions in which these organisms lived as well as provide evidence for lithological correlation between tectonically separate fragments. This study also confirms the presence of biostrome reef buildups in the southern Alexander terrane (Gravina Island), indicating warm, clear, and nutrient-free water with lots of sunlight; this differs from the central Alexander terrane (Keku Strait) and northern Wrangellia (Wrangell Mountains), where corals grow as individual colonies, not in a structured, reef-like buildup, and are accompanied by filter- and detritus-feeding organisms indicating warm, cloudy and nutrient-rich water in a back-reef environment. Paleobiogeographic results from silicified Upper Triassic corals show faunal similarity between Gravina Island and Keku Strait (Alexander terrane) and no similarity between northern and southern Wrangellia. Likewise, no similarity was found between the Alexander terrane and either northern or southern Wrangellia.

The southern Alaska continental margin has undergone a long and complicated history of plate convergence, subduction, accretion, and margin-parallel displacements. The crustal character of this continental margin is discernible through combined analysis of aeromagnetic and gravity data with key constraints from previous seismic interpretation. Regional magnetic data are particularly useful in defining broad geophysical domains. One of these domains, the south Alaska magnetic high, is the focus of this study. It is an intense and continuous magnetic high up to 200 km wide and ∼1500 km long extending from the Canadian border in the Wrangell Mountains west and southwest through Cook Inlet to the Bering Sea shelf. Crustal thickness beneath the south Alaska magnetic high is commonly 40–50 km. Gravity analysis indicates that the south Alaska magnetic high crust is dense. The south Alaska magnetic high spatially coincides with the Peninsular and Wrangellia terranes. The thick, dense, and magnetic character of this domain requires significant amounts of mafic rocks at intermediate to deep crustal levels. In Wrangellia these mafic rocks are likely to have been emplaced during Middle and (or) Late Triassic Nikolai Greenstone volcanism. In the Peninsular terrane, the most extensive period of mafic magmatism now known was associated with the Early Jurassic Talkeetna Formation volcanic arc. Thus the thick, dense, and magnetic character of the south Alaska magnetic high crust apparently developed as the response to mafic magmatism in both extensional (Wrangellia) and subduction-related arc (Peninsular terrane) settings. The south Alaska magnetic high is therefore a composite crustal feature. At least in Wrangellia, the crust was probably of average thickness (30 km) or greater prior to Triassic mafic magmatism. Up to 20 km (40%) of its present thickness may be due to the addition of Triassic mafic magmas. Throughout the south Alaska magnetic high, significant crustal growth was caused by the addition of mafic magmas at intermediate to deep crustal levels.

The White Mountain granitoid suite represents an isolated window into Cretaceous age magma intruded into Wrangellia terrane basement. Although the total area of exposed granitoid at White Mountain is relatively small (∼1 km 2 ), substantial textural, chemical, and isotopic complexities exist. The granitoid suite consists of six surficially isolated bodies, all of which are calc-alkaline and metaluminous, ranging in composition from hornblendebiotite quartz diorite to biotite granodiorite. Three 40 Ar/ 39 Ar analyses provide cooling ages between 113.3 ± 1.3 and 117.38 ± 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Two of the bodies, comprising ∼20% of the total exposed granitoid, are enclave-bearing, with the hosts representing the most chemically evolved material at White Mountain and the enclaves among the least evolved. The enclaves typically are <15 cm in size and circular to oval in shape, are dominated by plagioclase and amphibole, and are intermediate in composition (∼54 wt% SiO 2 ). Enclave rare-earth element patterns and isotopic characteristics, and the lack of petrographic evidence for quenched margins, suggest that they are cumulates from liquids chemically similar to but isotopically distinct from their host materials. One granitoid hand specimen exhibits textural and geochemical evidence for mixing at the low MgO end of the compositional spectrum. Although the granitoid suite exhibits a narrow range in whole rock isotopic compositions (ε Nd(115 Ma) 7.2–9.1 and 87 Sr/ 86 Sr (i) 0.7032–0.7043) further suggesting open system differentiation, these compositions do not require an appreciable role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s). Rather, the dominant source reservoir was depleted mantle. This conclusion provides evidence that mid-Cretaceous magmatism in this region was generated in either an intraoceanic island arc or an “immature” (proto-) continental arc tectonic setting.