Cretaceous-early Tertiary evolution of Northeast Russia

Phillip B. Gans1, Andrew T. Calvert1,
William S. Dinklage1, and V V Akinin2

1Department of Geological Sciences, UCSB, Santa Barbara, CA 93106
2 NEISRI, Magadan, Russia

Senyavin Synopsis

In late June and July, 1995, we conducted a geologic reconnaissance of the Senyavin uplift, an upper greenschist to amphibolite facies metamorphic terrane situated ~30 km north of Providenya on the southern Chukotka Peninsula. This metamorphic complex, interpreted to be Archean basement by previous investigators, is flanked by supracrustal Paleozoic marine deposits and late Cretaceous volcanic rocks. Our working group included Phil Gans, Andy Calvert and Bill Dinklage from UCÐSanta Barbara and Slava Akinin from NEISRI, Magadan, Russia.

The area had been mapped previously at 1:100,000 and we worked in the most complicated areas with hopes of understanding the relationship between metamorphic and volcanic sequences and the age and significance of faults. We mapped a 10km x 30km strip across the complex at 1:50,000 scale. Metamorphic rocks include marble, banded calcsilicate, calc-schists, pelitic schist, amphibolite, and rare lenses of ultramafic rocks. The amphibolite and associated graphitic schist overlie a predominantly carbonate section, in places, along a conspicuous low-angle, thrust(?) or normal(?) fault with top-to-the-north displacement. A pervasive high strain synmetamorphic fabric is characterized by a well developed, mainly SE-dipping foliation and mainly NE-trending stretching lineation. Metamorphic grade in pelites increases from NW to SE, from chloritoid-garnet-biotite to kyanite-staurolite-garnet-muscovite to finally sillimanite-garnet-biotite-muscovite.

The metamorphic rocks are depositionally overlain by a volcanic and volcaniclastic succession that includes, in ascending order, andesitic breccias, an extensive dacite ignimbrite, andesite lavas, breccias, and intercalated waterlain tuffs, and finally rhyolitic lavas and tuffs. Subvolcanic rhyolite and dacite porphyry dikes and stocks are ubiquitous in both the volcanic and metamorphic successions and produced extensive hydrothermal alteration. The volcanic rocks and underlying metamorphic basement are cut by a number of N-S trending, W-dipping normal faults with up to 1 km of displacement. These faults probably account for ~20-40% extension. Thus, field relations indicate that the metamorphic terrane was unroofed prior to deposition of the Late Cretaceous volcanic rocks and that the area was subsequently extended east-west by a significant amount. The mechanism of unroofing of the high grade metamorphic rocks and the timing/regional significance of post-volcanic east-west extension remain poorly understood. Geochronologic and petrographic work in progress should help address these issues and will better constrain the age of metamorphism, exhumation, magmatism, and post magmatic extension in this region.

40Ar/39Ar thermochronology shows that the metamorphic rocks cooled in the early Cretaceous from >500šC to <300šC between 132 and 128 Ma and were exhumed prior to deposition of the >95 Ma Yashka TuffÑa dacite ignimbrite just above the Cretaceous unconformity.

Project Funding

This project was funded by NSF EAR-9317142 to study the crustal evolution of the Bering Sea region in collaboration with workers at the USGS, Stanford, Rice and Western Washington universities. The various projects were funded by the Continental Dynamics program of NSF and involved a seismic transect across the Bering Shelf. Our work centers on mapping, structural studies and geochronology of deep-seated gneissic rocks and associated magmatic rocks on the Seward and Chukchi Peninsulas.

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