U-PbAND FISSION TRACK DATING OF DETRITAL ZIRCON SUITES FROM JURA-CRETACEOUS (J3-K1) SYN-OROGENIC DEPOSITS OF CHUKOTKA, RUSSIA: IMPLICATIONS FOR BROOKIAN OROGENESIS IN THE ARCTIC

The J3-K1 Brookian orogeny created an archetypal fold and thrust belt in the Brooks Range of Alaska, replete with oceanic and ophiolitic allochthons. It's cause is presumed to be the collision of an arc with the continental margin but the internal portion of the orogenic edifice (the “collider”) is no longer there.

Deformation in the Chukotka fold belt began in the Late Jurassic (with arrival of orogenic debris from the S); Early Cretaceous strata (Valanginian fossils) are deformed, similar to timing constraints provided by the Okpikruak Formation of the Brooks Range. The “collider” in Chukotka is also not well-defined. Tectonic slivers of oceanic/ophiolitic and volcanic sequences occur within the topographically depressed South Anyui Zone (SAZ) to the south.

LA-ICPMS and SHRIMP U-Pb ages of detrital zircon grains and fission track ages of zircon from J3-K1 sandstones of Chukotka provide a clearer picture of the architecture of the orogenic belt during its development. Immature arkoses consisting of granitic, metasedimentary and volcanic debris yield zircon populations that are largely 1.8- 2.1 Ga (peak at 1.935 Ga) suggesting basement-involved thrust faults could have formed mountainous highlands to the south. A second part of the zircon population appears recycled from deformed Pz-Tr strata as they match age populations in older strata. A third population is represented by young zircons that range from ~175 to ~ 145 Ma. SHRIMP dating of zircons extracted from volcanic clast-rich conglomerate lenses yielded a coherent group (N=7) of zircons with a mean age of 148 Ma. Single grain ages of select euhedral zircons (a proxy for minimal transport) also yield Jurassic as well as Precambrian ages, supporting field and petrographic data for the proximal location of these sources next to the basin. Zircon fission track ages suggest that source regions cooled through 200°C during the time-span of deposition (peaks in age populations from 3 samples at 155±9, 150±10, 131±8 Ma).

Thus it appears that the Brookian orogeny involved shortening of units that lay to the north of a volcanic arc edifice with a magmatic history spanning at least ~175- ~145 Ma. The fragmented nature of the orogen today is related to rifting and strike-slip as the magmatic arc migrated Pacificward and the Arctic basins opened.