Preliminary Apatite Fission Track Thermochronology of Wrangel Island, Arctic Russia

Wrangel Island is part of a regional structural high that forms the continuation of the offshore Herald Arch and Chukchi Platform of Alaska. It is flanked on the north by the deep North Chukchi Basin, which in addition to Paleozoic strata, is inferred to contain up to 12 km of Beaufortian and Brookian (Late Jurassic to Tertiary) sediments (Dinkelman et al., 2008). To the south, ~E-W trending faults bound the Longa Basin that separates Wrangel from mainland Chukotka. This basin lies along strike of the early Tertiary Hope Basin in the Alaskan offshore. Wrangel Island itself exposes a broad, doubly-plunging anticlinorium-like structure cored by Neoproterozoic basement and flanked by Paleozoic shelf successions and a thick section of Triassic turbidites, representing about 5-7 km of structural section. The structural geology of Wrangel Island has been interpreted to represent a north-vergent Mesozoic fold and thrust belt linked by seismic reflection to the Herald Arch and then to the Lisburne Hills and the Brooks Range foreland fold and thrust belt (e.g. Kos’ko et al., 1993). However, deformation differs considerably from typical foreland fold-thrust structures of the Brooks Range as it is penetrative, involves large strains, and occurred under greenschist facies metamorphic conditions. Parts of the sequence exhibit mylonitic fabrics. Apatite fission track thermochronology of rocks from Wrangel Island can establishes the age of cooling to temperatures below ~ 100° C, providing temporal constraints on the uplift and erosional history of rocks that form this regional structural high. We analyzed seven fission track samples from a 9-km long N-S transect along the Kishchnikov River, from Triassic strata on the southern flank of the anticlinal structure to Devonian(?)-Mississippian feldspathic grits, conglomerates, and underlying Neoproterozoic igneous basement rocks in its core. All samples yielded statistically indistinguishable fission track ages averaging about 95 Ma, with no discernable N-S variation in ages. The provisional interpretation is that these ages date cooling of the area below ≈100°C at about 95 Ma, probably during post-metamorphic erosional unroofing of the area. These preliminary data provide the first upper bound on the age of deformation and greenschist facies metamorphism of rocks on Wrangel Island (older than 95 Ma) and are compatible with the interpretation that uplift of the Wrangel Arch represents footwall uplift during North Chukchi Basin structuring and subsidence in the Late Cretaceous. The ages also indicate that structuring of the Wrangel Arch was younger than the structural stacking and cooling history of the Lisburne Hills fold and thrust belt (Moore et al., 2002), and that it is definitely a Mesozoic, not a Cenozoic structural feature.