NEW CONSTRAINTS ON THE WESTERN IDAHO OROCLINE: A PRIMARY FEATURE IN THE MESOZOIC COLLISION ZONE OR RESULT OF STRIKE SLIP MODIFICATION?

The Jura-Cretaceous Salmon River suture zone and the initial Sr 0.704/0.706 basement isotopic discontinuity coincide with the N-S contact of exotic Blue Mountains island arc terranes with North American metasedimentary assemblages south of the 46th parallel. There, at the western Idaho orocline, the discontinuity and island arc/continent contact trend abruptly westward; they can be bracketed for ~80 km between sparse basement exposures through Miocene cover; continental basement exposures continue ~300 km into NE Washington.

Rock assemblages from SW to NE across the suture zone near the orocline include: 1) ~157 Ma plutonic rocks of the Blue Mountains province; 2) hornblende gneiss, marble, and impure quartzite of the Orofino metamorphic series that has poorly known affinity and age because it straddles the isotopic discontinuity and contains detrital zircons whose cores yielded 280-80 Ma U-Pb ages; 3) pure quartzite, calc-silicate, and schist of the Proterozoic Syringa metamorphic sequence of continental affinity; and 4) feldspathic quartzite, calc-silicate, and schist of the Proterozoic Belt Supergroup of North American affinity. Several NW striking contractional or transpressional shear zones several km wide with steeply dipping mylonitic foliation and steeply pitching lineation span a broad region within and to the SE of the orocline. These deform all 4 assemblages described above as well as Late Cretaceous plutonic rocks, and appear to juxtapose the first 3 assemblages.

The origin of the orocline remains enigmatic. If the orocline results from ~300 km of strike-slip offset of the collision zone, then most of this offset must occur on a single NW shear zone that bends to the west to strike westward where it is fortuitously covered, and is intruded out to the east. Preservation of accretionary prism and passive margin assemblages at some distance NW and S of the orocline support this possibility. However, lack of piercing points across the E-W segment of the arc-continent contact to prove this, and lack of offset in the Syringa and Belt assemblages through the oroclinal region suggests that little E-W strike-slip offset has occurred. A compelling alternative is that the orocline is largely primary, the result of an E-W transform or accommodation zone that developed across the margin prior to suturing.