THE EFFECTS OF SUPERIMPOSED DEFORMATION EVENTS ON STRIKE-SLIP TRANSLATION IN THE CORDILLERA: AN EXAMPLE FROM THE DENALI FAULT SYSTEM

Despite paleomagnetic data and plate circuit reconstructions requiring major strike-slip displacement in the northern Cordillera, geologic piercing points on major Cordilleran strike-slip faults remain cryptic, controversial, and yield low estimates compared to other datasets. The Denali fault is a key structure accommodating northward transport of outboard (Wrangellia and Chugach) terranes. Geologic estimates of total late Cretaceous-Holocene strike-slip displacement on the Denali fault range from ~450 to ‘0’ km. The widely cited correlation between the Maclaren schist (S side of the Denali fault in Alaska) and Kluane schist (NE side of the Denali fault in Yukon) yields ~450 km of dextral slip on the Denali fault since late Cretaceous time. Although recent work on the Kluane schist aimed to refute the correlation, our new detrital zircon U/Pb age spectra from the Maclaren schist and U/Pb zircon crystallization ages from cross-cutting intrusive rocks correlate strongly with equivalent datasets from the Kluane schist. The new age data support metamorphic studies on both schist bodies and confirm that these rocks create a viable piercing point to restore late Cretaceous-Cenozoic slip on the Denali fault system.

The southern margins of the schist bodies are presently separated by ~460 km along the Denali fault. Whereas the Kluane schist appears to have experienced no Cenozoic deformation, late Cretaceous accretionary structures bounding the Maclaren schist have been reactivated as an Oligocene-Neogene SE-vergent imbricate thrust system. The Maclaren schist is carried in the hanging wall of the thrusts and, as a result, has been transported to the SE- back toward the Kluane schist. Therefore, the distance between the schist bodies at the surface provides only a minimum estimate of total dextral slip on the Denali fault. Restoring the Oligocene-Neogene shortening in the thrust system adds ~30-80 km of dextral slip to the Denali fault, thus pushing the geologically-determined estimate of post- mid Cretaceous dextral slip to >490 km.

Because continental strike-slip fault systems often contain dip-slip faults that are active during translation, accounting for shortening or extension of piercing point rocks during translation is an important, but often overlooked, step in reconstructing strike-slip fault systems.