Dike swarms on Seward Peninsula, Alaska, and their implications for the kinematics of Cretaceous extension in the Bering Strait region

Abstract

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.

Geologic map and summary of the evolution of the Kigluaik Mountains gneiss dome, Seward Peninsula, Alaska

“…The Kigluaik pluton has a U-Pb zircon age of 90 ± 1 Ma (Amato and Wright, 1998), and the fact that the mafi c rocks of the pluton have nearly the same age as the peak metamorphic ages in the country rock led to the suggestion that mafi c magmatism was the heat source for the second, high-temperature metamorphic event (Amato et al, 1994;Amato andWright, 1997, 1998). The age of the diabase dike swarm is within the interval 90-84 Ma based on 40 Ar/ 39 Ar dating of whole-rock samples from the dikes and fi eld relationships (Amato et al, 2003).…”

“…A diabase dike swarm cuts across all units within the Kigluaik gneiss dome (Plate 1), including the Kigluaik pluton (Amato et al, 2003). These steeply northwest-dipping dikes are generally 1-3 m thick and strike approximately N40°E within a corridor ~30 km wide centered just southeast of the center of the pluton.…”

“…We have proposed that both regional and local relationships suggest that magmatism and extension during Cretaceous time appear to have been particularly important in the evolution of the Bering Strait gneiss complexes. (1997) and Amato et al (2003) The color bedrock geologic map on the CD-ROM (Plate 1) that accompanies this discussion provides details of the distribution of units and structural fabrics within this gneiss dome. It was compiled by the authors with the assistance of Dean Miller (Stanford University) and Yvette Lopez (New Mexico State University), who helped digitize the map.…”

“…The geologic mapping used for this compilation was carried out over fi ve fi eld seasons between 1989 and 1994 by the authors and those listed as making a contribution to this publication (see page 1 footnote). The summary of geologic relations in the Kigluaik gneiss dome is based on Miller et al (1992), Amato et al (1994), , Hannula and McWilliams (1995), Dumitru et al (1995), , Calvert et al (1999), Amato et al (2002), and Amato et al (2003). The Kigluaik Mountains were originally mapped in reconnaissance by Brooks et al (1901), with more detailed maps of selected areas produced by Moffi t (1913), Hummel (1962), Sainsbury (1972Sainsbury ( , 1974, Sainsbury et al (1972), Till (1980), Till et al (1986), Bundtzen et al (1994), and others listed in the mapping credits (Plate 1).…”

“…GEOCHRONOLOGY DATA FROM THE KIGLUAIK MOUNTAINS, KEYED TO PLATE 2.References: 1-Amato and Wright (1998); 2-Amato et al (1994); 3-Amato et al (2003); 4-Dumitru et al (1995); 5-Calvert et al (1999).…”