Superscript

Joshua N. Distler
Paleomagnetic and Structural Analysis of the Wallowa Terrane, Northeastern Oregon and West-central Idaho: A Reevaluation of the Salmon River Suture Zone
Directors: Steven D. Sheriff and James W. Sears

Paleomagnetic evidence indicates that many Cordilleran terranes have experienced significant clockwise rotation since the Cretaceous. The rotation is often coupled with major dextral faults and evidence of coeval northward translation. Beck (1976, 1980) proposed a "ball-bearing model" to explain this data trend, whereby terranes are rolled northward along the North American cratonic margin after docking due to oblique subduction of the Farallon plate. The Wallowa terrane of northeastern Oregon and west-central Idaho provides an intriguing example of the allochthonous terranes which sutured onto the North American craton at this time. It experienced 60^o of clockwise rotation since the middle Cretaceous and exhibits a well-exposed suture zone against rocks of continental affinity along the Salmon River Canyon. However, because the terrane accreted in the restrictive Columbia embayment and shear sense on the suture has been unresolved, its tectonic history is unclear.

In order to test the ball-bearing model, paleomagnetic and structural examinations of the Salmon River suture zone (SRSZ) were conducted to determine whether strain evidence indicates transcurrent motion along the fault plane, and if so, whether it was dextral or sinistral. Results indicate that the SRSZ was indeed a left-lateral strike slip fault at one point. Paleomagnetic site means from the SRSZ locality near Slate Creek show pronounced streaking relative to an undeformed sampling locality on the Imnaha River (well outboard of the suture zone). The plane of streaking coincides with the strike of the foliation plane and suggests shear strain in that plane. East-west macroscopic fold axes measured in the Salmon River Canyon and the Seven Devils Mountains indicate large-scale left-lateral shear strain along the SRSZ. These results reveal that the Wallowa terrane did not undergo significant northward translation and associated dextral shearing after suturing. Such behavior appears anomalous among the Cordilleran terranes, and might be a result of this terrane's early entrapment in the Columbia embayment.