THE 100 MA OBLIQUE COLLISION OF THE INSULAR TERRANE ALONG WESTERN NORTH AMERICA

The North American Cordillera experienced major contractional deformation at ~100 Ma, which we attribute to oblique collision of the Insular Superterrane with the North American margin. We use the term “strike-slip linked orogeny” to distinguish it from the more traditional orogenic belts with nearly orthogonal convergent motion. The manifestation of strike-slip linked orogeny is the occurrence of zones of intense deformation – in this case, where the North American margin impeded northward movement of the Insular terrane – separated by zones of strike-slip movement that preserve very little record of contractional deformation. The ongoing Yakutat collision in the inner corner of Alaska provides a modern analog. At 100 Ma, the continent formed a barrier to northward translation in two distinct places, western Idaho and southern California, both of which record major contractional deformation at 100 Ma. The irregularity of the North American boundary resulted from inherited Neoproterozoic rift-transform geometry and Permian-Triassic truncation, respectively.

The strike-slip/transpressional movement for the ~100 Ma deformation is localized into mélange belts and active magmatic arcs, because both form zones of lithospheric-scale, margin-parallel weakness. The record in the magmatic arcs is most clear, wherein a short (5-15 m.y.) period of intense dextral transpressional deformation initiates at ~100 Ma in the gneissic border of the Idaho batholith (western Idaho shear zone), northwestern Nevada, and the central part of the Sierra Nevada batholith. The Peninsular Ranges also show an intense period of deformation at this time, although it is interpreted as a steeply dipping reverse fault with a fanning foliation. This phase of deformation ceased by ~85 Ma, after which margin-parallel faults became increasingly active. This model is consistent with robust paleomagnetic data from the Canadian Cordillera. The proposed type of oblique orogeny requires a three-dimensional, time-dependent view of the deformation along an irregular and evolving continental margin.