LESSONS LEARNED FROM RESTORATION OF THE CENOZOIC DENALI FAULT SYSTEM

Despite decades of work, the timing and magnitude of terrane translation in the North American Cordillera remain controversial. Here we review our recent work restoring Cenozoic slip on the Denali fault between central Alaska and southwestern Yukon, and then discuss current research that results from asking the question– Where does the slip go along strike?

After restoring ~480 km of right-lateral displacement on the active strand of the Denali fault over the last ca. 50 Myr, the Late Cretaceous master suture between the Insular terranes and Intermontane terranes south of the Denali fault in Alaska (Valdez Creek shear zone) aligns with the Tatshenshini shear zone in southwestern Yukon and those structures project along strike to similar shear zones within the western metamorphic belt of the Coast Plutonic Complex. The restoration reveals continuity of a ~1500-km-long Late Cretaceous-Eocene contractional shear zone that has since been dissected by strike-slip motion on the Denali and Chatham Strait faults. Hypothesized Oligocene–Miocene motion on the Chatham Strait fault in southeast Alaska appears to be part of a multistranded dextral strike-slip fault system that transferred slip from the Queen Charlotte fault to the Denali fault as the Yakutat terrane migrated northward along the Canadian Cordillera. In southwestern Alaska, the Ahklun Mountains coincide with a restraining bend in the Denali fault system and offer the opportunity to reconstruct slip on the western-most on-land section of the Denali fault. Preliminary data from western Alaska suggest that Cenozoic slip dissipates to the west but persists into the Quaternary at a low rate. Offshore in the Bearing Sea, sediment accumulation rates likely outpace deformation rates, which obscures offset of the shelf edge. Our treatment of the Denali fault system highlights two important lessons pertaining to kinematic restorations of the Cordillera: 1) Sequential restoration of strike-slip fault systems is essential to understand the significance of older orogen-scale structures; and 2) Along-strike variability in slip magnitude on the strike-slip faults must be incorporated into restorations. Applying these lessons throughout the Cordillera will lead to more robust fault restorations that will hopefully quell controversy among Cordilleran geologists.