Integration of zircon color and zircon fission-track zonation patterns in orogenic belts: application to the Southern Alps, New Zealand

An exhumed crustal section of the Mesozoic Torlesse terrane underlies the Southern Alps collision zone in New Zealand. Since the Late Miocene, oblique horizontal shortening has formed the northeastern-southwestern trending orogen and exhumed the crustal section within it. On the eastern side, rocks are zeolite- to prehnite-pumpellyite-grade greywacke; on the western side rocks, they have the same protolith, but are greenschist to amphibolite facies of the Alpine Schist. Zircon crystals from sediments in cast-flowing rivers (hinterland) have pre-orogenic fission-track ages (> 80 Ma) and are dominated by pink, radiation-damaged grains (up to 60%). These zircons are derived from the upper similar to 10 km crustal section (unreset FT color zone) that includes the Late Cenozoic zircon partial annealing zone; both fission tracks and color remain intact and unaffected by orogenesis. Many zircon crystals from sediments in west-flowing rivers (foreland) have synorogenic FT ages, and about 80% are colorless due to thermal annealing. They have been derived from rocks that originally lay in the reset FT color zone and the underlying reset FT colorless zone, The reset FT color zone occurs between similar to 250 and 400 degreesC. In this zone, zircon crystals have color but reset FT ages that reflect the timing of orogenesis. (C) 2002 Elsevier Science B.V. All rights reserved.