Location, Age, and Tectonic Significance of the Western Idaho Suture Zone

The Western Idaho Suture Zone (WISZ) represents the boundary between crust
overlying Proterozoic North American lithosphere and Late Paleozoic and
Mesozoic intraoceanic crust accreted during Cretaceous time. Highly deformed
plutons constituted of both arc and sialic components intrude the WISZ and in
places are thrust over the accreted terranes. Pronounced variations in Sr, Nd,
and O isotope ratios and in major and trace element composition occur across
the suture zone in Mesozoic plutons. The WISZ is located by an abrupt west to
east increase in initial 87Sr/86Sr ratios, traceable for over 300 km from
eastern Washington near Clarkston, east along the Clearwater River thorough a
bend to the south of about 110° from Orofino Creek to Harpster, and extending
south-southwest to near Ola, Idaho, where Columbia River basalts conceal its
extension to the south. K-Ar and 40Ar/39Ar apparent ages of hornblende and
biotite from Jurassic and Early Cretaceous plutons in the accreted terranes are
highly discordant within about 10 km of the WISZ, exhibiting patterns of
thermal loss caused by deformation, subsequent batholith intrusion, and rapid
rise of the continental margin. Major crustal movements within the WISZ
commenced after about 135 Ma, but much of the displacement may have been
largely vertical, during and following emplacement of batholith-scale silicic
magmas. Deformation continued until at least 85 Ma and probably until 74 Ma,
progressing from south to north.

[Summary provided by the USGS.]