HUNTING FOR CRYPTIC CRETACEOUS - EARLY CENOZOIC STRIKE-SLIP DISPLACEMENT IN THE NORTHERN CORDILLERA

Plate motion reconstructions and paleomagnetic studies require significant dextral displacement of terranes along the northern Cordillera margin, but identifying the timing and the culprits for major displacement remains a challenge. Using geophysical and geologic data three major fault systems stand out: the Tintina, Denali, and Border Ranges. Each of these have “known” offsets, based on matching specific units or structures, of over 450 km, but because significant vertical motion has occurred along many sections of these faults, finding matching units results in only the most recent, usually a minimum, displacement. A broader approach, considering the whole geologic history of terranes adjacent to the fault, can provide additional displacement estimates.

Using this approach, we present evidence for significantly greater dextral offset in the rocks immediately north of the Denali fault in the Hayes Range than has been previously recognized. This area has undergone vertical exhumation since 30 Ma, locally exposing greenschist – amphibolite facies rocks. We present detrital zircon data from 5 samples of qz-pl-bt schist, all within 2 km of the active strand of the Denali fault. Determining a precise maximum depositional age (MDA) for these rocks is complicated by multiple Cretaceous – Cenozoic intrusive and metamorphic events, but all estimated MDAs are Paleozoic, likely Permian – Devonian. The proportion of Precambrian aged grains is highest in the oldest samples, 50-60% of the total. Although most of the samples have a number of Devonian grains that could be derived from igneous belts in North American margin rocks to the north, all of them also have major peaks, including Pennsylvanian, Silurian, Ordovician and Neoproterozoic ages, that do not have an apparent North American source. The most similar rock units described in the literature are in the SE Yukon-Tanana terrane in SE Alaska 1200 km away. The Hayes Range rocks also record intrusive and/or peak metamorphic events at around 58, 66, and 92 Ma, similar to events recorded in SE Alaska. A cryptic dextral fault system along or between the Hines Creek and Denali faults may have initiated in the Late Cretaceous and eventually localized onto the modern Denali fault, resulting in L. Cretaceous-Paleocene displacement in addition to the well-documented 480 km since 50 Ma.