LONG RETURN PERIODS FOR GREAT QUAKES MAKE CHARACTERISTIC GEOLOGICAL AND TOPOGRAPHIC SIGNALS OF GREAT IMPORTANCE

The linkage between advective cooling within a subduction zone and the partitioning of strain within an orogen results from the formation of a thin sliver of low-temperature material along the megathrust surface. We refer to this low temperature, brittle zone as the frictional sliver. This study focuses on the current tectonic regime of southern Alaska. Over the last ~10 Ma Yakutat microterrane has been subducting beneath the North American plate at the eastern margin of the Aleutian subduction zone, driving uplift of the Chugach-St. Elias Range. Three-dimensional thermo-mechanical numerical models were developed to study the effect of the transition from normal- to shallow-angle subduction on the partitioning of deformation within the evolving plate boundary. The cold down-going plate alters the thermal conditions of the subduction zone, offsetting the hot mantle wedge flow away from the fore-arc. This causes a net cooling that drives the formation of the low temperature frictional sliver thereby increasing the down-dip extent of the megathrust. This cooling, and resultant material strengthening, provides a severe control on large-scale strain partitioning within the orogen. Previous deformation associated with a long-standing subduction zone caused uplift within the Alaska Range. The formation of the frictional sliver initiates a spatially discontinuous shift of the deformation front southward to the St. Elias Range. The location of this secondary orogenic wedge is controlled by the down-dip limit of the frictional sliver. The low temperature zone is relatively strong and is capable of accumulating the high levels of strain responsible for large megathrust related earthquakes. The largest magnitude earthquakes in recorded history (1960 Chile, 1964 Alaska, 2004 Sumatra, and 2011 Japan) all are associated with shallow-angle subduction zones. These types of subduction zones exhibit enhanced advective cooling (i.e. tectonic refrigeration) and develop broad, low temperature megathrusts (i.e. frictional slivers).