Dynamics of the Yakutat collision and its impact on the Northern Cordillera

Stéphane Mazzotti, Roy D. Hyndman, Paul Flück, and the GSC Seismology and Geodynamics Group

Geological Survey of Canada, National Earthquake Hazards Program, Ottawa (ON)

As stated in the workshop's rationale, “Southern Alaska is an exceptional natural laboratory for studying a range of geologic problems”. We would like to extend this perspective by suggesting that the Yakutat collision system needs to be studied within the larger scale Northern Cordillera-Eastern Alaska tectonic picture. This proposition is based on our recent work that indicates a strong relationship between the dynamics of the Yakutat collision and that of the Northern Cordillera orogen. In this poster, we describe the kinematics and seismotectonics of this relationship, with an emphasis on the issues raised by our model and some ways to address them.

A strong kinematic model of the Yakutat collision system is an essential framework for any further investigation (as it provides boundary conditions to the system). Our GPS measurements in the Yukon Territory indicate a small (< 5 mm/yr) NNE motion of the Northern Cordillera relative to stable North America. On the basis of these results, we propose that a small part (< 10%) of the Pacific/North America (PA/NA) convergence is transferred from the Yakutat collision to the Northern Cordillera. Our hypothesis is in apparent contradiction with GPS measurements along the southeastern Alaska margin suggesting that most of the PA/NA convergence is taken up offshore along the Transition Zone [Fletcher and Freymueller, 1999, 2003]. The existence (or absence) of a transfer of motion and strain across the Chugach-St. Elias Mountains would provide some significant constraints to tectonic models of the collision zone. Integration of seismicity data needs to be a critical component of such models. Earthquakes provide a qualitative outline of where and how the contemporary strain associated with the collision tectonics is released. We have developed a more quantitative approach that converts seismicity statistics into seismic deformation rates with robust estimates of uncertainties. Using this method, we derive deformation rates for the main regions around the Yakutat collision (e.g., Denali Fault, Chugach Mountains). Our preliminary map of seismic deformation rates can be used to constrain models of strain partitioning (from the collision to the Northern Cordillera). It also raises issues regarding the seismic budget and potential aseismic deformation in different regions.

These kinematic and seismotectonic considerations highlight the relationship between the Yakutat collision zone and the Cordillera orogen. Our orogenic float model could explain the dynamics of this link (i.e., through a 600-800 km long lower crust decollement) [Mazzotti and Hyndman, 2002]. An integrated Ocean Drilling-Continental Dynamics proposal would provide the necessary data to refine (or invalidate) our model and its implications for the dynamics of the Yakutat collision system.