THE NORTH AMERICAN CORDILLERA AS AN ARCHETYPICAL EXAMPLE OF ALONG STRIKE-VARIATIONS RELATED TO MULTIPLE STRIKE-SLIP EPISODES (Invited Presentation)

The dominant process generating along strike-variations in orogenic belts is strike-slip systems, yet their significance is often missed or misidentified in the geologic record due to geology’s inherently poor resolution of horizontal displacements, particularly when motion is nearly parallel to the orogenic strike. In convergent margins this includes margin-parallel motion of forearc elements, aka forearc slivers, as well as broader scale transcurrent motion from oblique collision to distributed transtension. The North American Cordillera shows numerous examples of this issue at a variety of spatial and temporal scales. Prominent among these Cordilleran elements is the Paleogene shuffling of terranes in the northern Cordillera by dextral fault systems and the late Paleozoic-early Mesozoic sinistral disruption of the southwest margin of Laurentia, both of which fundamentally reshaped the margin. A modern analog to the development of these Cordilleran strike slip systems is the linked transcurrent-contractional system of the southern Alaskan orogen where contraction in interior Alaska and southern Alaska are directly linked to strikes-slip systems (Denali and Fairweather faults) while those strike-slip systems in turn are linked to a collisional driver, the flat-slab subduction of the Yakutat oceanic plateau. Were the southern Alaskan orogen an ancient system, 50 m.y. or more in age, it is unlikely that we would be able to recognize the interrelationships among the kinematically distinct elements of the orogen. Examples of these kinds of ambiguities are abundant in Cordillera and are the source of numerous controversies. For example, although the northern Cordillera is known to be laced with a system of major Paleocene-Eocene strike-slip systems, linkages to other events are controversial ranging from Paleogene extension in southern British Columbia to contraction in Alaska. Similarly, although there is no doubt that significant sinistral motion occurred between North America and Gondwana during north Atlantic breakup, the search for the structures associated with that motion remain controversial, primarily because younger structures have thoroughly overprinted the system, presumably destroying most of the evidence.