APATITE FISSION TRACK EVIDENCE FOR REPEATED TERTIARY MOVEMENT ON THE TINTINA FAULT SYSTEM, ALASKA

The Tintina fault system cuts across Alaska and adjacent Canada with strong topographic and geologic definition along much of its trace. In central Alaska, fault definition becomes obscure. However, geophysical data indicate faults mapped in the area are major, crustal-scale splays of the Tintina system. Important among these is the Victoria Creek fault, which is shown on seismic reflection/refraction surveys as cutting the full thickness of the crust. North of and parallel to Victoria Creek is the Tozitna fault, marked by a strong linear aeromagnetic anomaly and adjacent Tertiary basins. The two faults bound a 35-km-wide zone between the continental Ruby geanticline and continental Yukon Tanana Upland underlain largely by the oceanic Tozitna terrane. This zone is also the site of 60 Ma plutonism, volcanism, metamorphism, and sedimentation.

An apatite fission track (AFT) survey of 25 samples collected across the zone documents multiple Tertiary uplift events. Track lengths are typically long and produce frequency vs. length plots with single, tight peaks, indicating rapid movement from crustal depths at which apatite is completely annealed to shallow levels at which no annealing occurs. The oldest AFT ages occur south of the Victoria Creek fault and record uplift at 70 and 60 Ma. Between the Victoria Creek and Tozitna faults, the oldest AFT ages record an event at 45 Ma, and periods of local exhumation as young as 18 Ma. A system of faults parallel to the two major splays accommodated these multiple events. No vertical movement has occurred on the Tozitna fault since approximately 25 Ma; samples on both sides of the fault yield ages of 24-28 Ma. However, a sliver of rocks immediately south of the Victoria Creek fault give 23 Ma AFT ages, indicating that fault has been active as recently as latest Oligocene/earliest Miocene.

Though the kinematic history of the involved faults is not known, we interpret the AFT ages to record pulses of deformation transmitted to the zone of fault splays from the Tintina fault system, and our work is not inconsistent with data from the Yukon documenting 400 km of Tertiary offset on the Tintina. Furthermore, uplift events in the eastern Brooks Range correspond in age to events we have documented in the Tintina system, suggesting a continental-scale linkage of crustal deformation during the Tertiary.