LATE NEOTECTONIC EVOLUTION OF CENTRAL ALASKA, AND A REINTERPRETATION OF THE DENALI FAULT SYSTEM

The Denali fault of Alaska is considered by the majority of the geologic community to be an active dextral strike-slip feature cutting across the entire width of the State for over 1200 km with a prominently northward-convex surface trace. The two major earthquakes in 2002 at the apex of the fault in the central Alaska Range, and the associated dextral offsets in eastern Alaska appear to lend credence to the concept of a statewide dextral strike-slip fault.

The authors various geologic investigations and reconnaissance mapping for the last 30 years in large tracts of central Alaska, including a roughly 240 km segment of the Denali fault, indicate a different character for the Denali fault. Our concept of the fault, for which we have incorporated results of seismic studies of the 2002 earthquakes along the fault, is that the recently active Denali fault is a strike-slip feature in eastern Alaska, but going westward, along its central segment it progressively changes into primarily a dip-slip feature, and it is only one of a swarm of subparallel, dominantly dip-slip faults which converge downward and outline a number of upthrust wedge-shaped crustal blocks of regional dimensions. First-motion studies and ground-motion analyses at the epicenter of the largest M7.9 earthquake indicate the presence of a strong compressional component, including a newly developed thrust fault. These types of structural features are what one would expect in a transition zone where a strike-slip fault radically alters its surface course and changes into a number of dip-slip faults.

The changing character of the Denali fault in central Alaska we interpret to be the manifestation, in the relatively thin upper plate, of a subducted junction of trench and transform in the lower plate. We further interpret the wedge-shaped fault blocs in central Alaska to be analogous to the structural features described and named composite wedges by Migliorini (1948) from the Apennines of Italy. The inclined compressional stress field required for the development of these features we interpret to have been caused by the changing, that is upwardly moving and shallowing position of the subducting plate beneath central Alaska within the last approximately 6 million years.