Late Cretaceous dike swarms on Seward Peninsula, northwestern Alaska, represent the youngest local manifestation of a ∼115–75 Ma magmatic event in the Bering Strait region. Magmatism accompanied and followed high-grade metamorphism and ductile deformation. A Late Cretaceous extensional tectonic setting for the region is suggested by the thickness and seismic-reflection characteristics of the crust, regional basin development, formation of high-strain tectonites with subhorizontal foliations, bimodal magmatism, and dike swarms. The orientation of the dike swarms is used to address the kinematics of extension. A diabase dike swarm in the Kigluaik Mountains consists of dikes that strike northeast (040°) and dip steeply. Phenocrysts include plagioclase, clinopyroxene, orthopyroxene, and hornblende. Geochemical data indicate that SiO2 ranges from 48% to 56%, and K2O from 1.2% to 4.0%. The dikes are geochemically similar to the mafic to intermediate root of the 90 Ma Kigluaik pluton. Sr- and Nd-isotope data show that initial 87Sr/86Sr ranges from 0.7070 to 0.7077 and initial εNd ranges from –0.85 to –2.90. Field relations and 40Ar/39Ar geochronology bracket the dike ages between 90 and 84 Ma. Diabase dikes in the York Mountains are associated with normal faults that strike east–west to east-northeast. Dikes in the Bendeleben Mountains are both mafic and felsic, but their orientations are unknown. Alkalic dikes in the Darby Mountains strike 030°–050°, similar to those in the Kigluaik Mountains. Regional relationships including the orientation of dikes, normal faults, mineral stretching lineations, and other shear-sense indicators suggest that between 110 and 90 Ma extension on Seward Peninsula was generally oriented north-south to north-northwest–south–southeast.

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