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Dike swarms on Seward Peninsula, Alaska, and their implications for the kinematics of Cretaceous

Summary: Dike swarms on Seward Peninsula, Alaska, and
their implications for the kinematics of Cretaceous
extension in the Bering Strait region
Jeffrey M. Amato, Elizabeth L. Miller, James E. Wright, and William C. McIntosh
Abstract: Late Cretaceous dike swarms on Seward Peninsula, northwestern Alaska, represent the youngest local
manifestation of a -115­75 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 87
Sr ranges from 0.7070 to 0.7077 and initial Nd ranges from ­0.85 to ­2.90. Field relations and
Ar geochronology bracket the dike ages between 90 and 84 Ma. Diabase dikes in the York Mountains are
associated with normal faults that strike east­west to east-northeast. Dikes in the Bendeleben Mountains are both mafic


Source: Amato, Jeff - Department of Geological Sciences, New Mexico State University


Collections: Geosciences