FISSION-TRACK ANALYSIS OF DETRITAL ZIRCON

Detrital fission-track (FT) thermochronology involves using FT ages of single zircons for: (a) stratigraphic correlation; (b) provenance analysis; (c) dating unfossiliferous sediments; (d) exhumation studies; and (e) dating low-temperatures thermal events. While the concepts and techniques have been refined over the past ~15 yr, many of the fundamental studies originated in units of the Pacific Northwest. The utility of this technique is illustrated using several examples: (a) Stratigraphic correlation of widely separated flysch-like units in Kamchatka has been reinforced based on the similarity for FT age distributions as demonstrated between the Lesnaya Gp. and the Ukelayet Gp. in central Kamchatka (Russia). (b) Provenance of detritus has been amplified by the powerful addition of the cooling ages of zircon, as demonstrated in a number of studies. (c) Unfossiliferous strata with a partial volcanic source yield a fraction of young zircons with an age that can be used as a proxy for depositional age. FT ages of zircons from Cretaceous to Eocene flysch in Kamchatka, constrain the timing of deposition and has radically altered the interpretation of tectonic events in this area. The young population of fission-track grain ages from samples previously mapped as Cretaceous range from ~44 to 88 Ma demonstrating deposition was as young as Eocene. (d) Perhaps the most exciting advances have been made using the progression of grain ages from stratigraphic sequences deposited adjacent to exhuming mountain belts. Strata of the Tofino basin, exposed on the Olympic Peninsula, have detrital zircon with cooling ages that record the near continuous exhumation of the BC Coast ranges at rates of c. 200 m/Myr through the Tertiary. (e) Radiation-damaged grains in detrital suites can provide a sensitive indicator of low-temperature thermal events. FT ages of detrital zircons from Paleozoic strata in the Hudson Valley (NY) can be divided into those reset in the Jurassic (~185 Ma) and older unrest and partially reset grains. These data indicate that this area experienced an Jurassic thermal event that affected a larger area in NY and New England than is traditionally assumed.