LATE EOCENE TO RECENT RECORD OF LONG-LIVED DEXTRAL SLIP ALONG THE TRANSITION FROM THE EASTERN TO CENTRAL DENALI FAULT IN THE ALASKA RANGE

Determining when horizontal slip begins on any one strand of a strike-slip fault system often relies on indirect evidence such as the timing of basin development or rock exhumation. The Denali Fault is an example of an active strike-slip fault with a poorly constrained pre-Holocene slip history. The published Holocene slip rate is ~ 12-13 mm/yr in the eastern Alaska Range, with decreasing rate of strike-slip (~7 mm) and inferred increasing rate of convergence as the fault bends into the central Alaska Range. Researchers have used a proposed offset of 38 km between two ~38 Ma plutons in the central Alaska Range to estimate the total late Eocene to recent displacement. Some conclude that the onset of strike-slip motion on the active trace of the Denali fault began no earlier than late Miocene based on a significant increase in the rate of exhumation at Mt. McKinley, in the central Alaska Range, at ~ 6 Ma.

Our recent research along the Denali fault in the central and eastern Alaska Range provides direct evidence that dextral strike-slip displacement on the current active trace was occurring during high T metamorphism, locally dated at > 35 Ma. This time frame overlaps with the emplacement of a plutonic suite ranging in age from ~35 to ~45 Ma that is restricted to a narrow band on both sides of the Denali fault from the Mt. McKinley region (-152° long.) to the Delta River (-146° long.). Numerous U-Pb detrital zircon dates from this time frame in glacial outwash sands in regions west of the Delta River also document the extensive nature of this late Eocene igneous event. This evidence of dextral slip synchronous with longitudinally extensive magmatism at ~38 Ma calls into question both the timing of initiation and the total slip along this section of the Denali fault.

Metamorphic rocks from this region contain oblique dextral and thrust kinematic indicators and have mica cooling ages of ~ 16 Ma, indicating dextral slip also was occurring during a known Miocene to recent rapid exhumation event. A minimum rate of dextral slip of ~ 3-4 mm/yr at ~16 Ma can be determined from the average pitch of the oblique lineation, combined with the rate of regional exhumation (~.9 km/Ma). These constraints on Miocene dextral slip provide additional data in support of piercing point relationships that point toward ~ 300 km of displacement along the eastern Denali fault since 25 Ma.