EARLY TERTIARY EXTENSIONAL ZONE LINKING THE TINTINA AND KALTAG FAULT SYSTEMS, CENTRAL ALASKA

A zone of crust attenuated during the Early Tertiary links the NE-trending Kaltag fault system and NW-trending Tintina fault system; together these features girdle central Alaska from west to east, and mark a continental-scale crustal boundary. The zone of strongest attenuation is marked on its southern side by the Victoria Creek fault, a splay of the Tintina system that is thought to cut deeply into the crust based on seismic refraction/reflection data. The northwestern boundary of the zone is not distinct and involves Cretaceous(?) metamorphic rocks with a new biotite Ar/Ar plateau age of 60 Ma.

Metamorphic, plutonic, volcanic, and sedimentary rocks, all of Early Tertiary age, occupy the NE-SW trending zone of attenuation. Kyanite- and andalusite-bearing metamorphic rocks exposed in two NE-SW trending elongate bodies show well-developed NE-SW trending stretching lineations, subparallel to the trend of the adjacent Victoria Creek fault. The andalusite- bearing contact aureoles of two Early Tertiary plutons in the zone manifest the same NE-SW stretching direction. Ar/Ar cooling ages from one of the metamorphic bodies are Paleocene, while cooling ages from the other range from 66-90 Ma. Paleozoic-Mesozoic rocks of oceanic affinity (Tozitna terrane) that show no sign of Early Tertiary metamorphic effects, bimodal volcanic rocks similar in age to the metamorphic rocks, and small Maastrichtian and Paleocene nonmarine sedimentary basins appear to sit structurally above the metamorphic and oceanic rocks. Sediments in the basins were derived from local sources.

The exhumation of metamorphic rocks in close proximity to coeval plutonic, sedimentary, and bimodal volcanic rocks suggests crustal extension operated along the Victoria Creek Fault in the Early Tertiary. The oroclinal bending of Alaska and subduction of a spreading ridge along its southern margin are also thought to have occurred in Paleocene to Eocene time. The nature of the bending event and the far-field effects of slab windows are not known. However, the Early Tertiary extensional zone likely involved interaction between a high thermal gradient (related to the slab window?), a major strike-slip system, and oroclinal bending.