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

Abstract

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 zone in Mesozoic plutons. The WISZ is located by an abrupt west to east increase in initial Sr/Sr 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 Ar/Ar 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 batholithscale silicic magmas. Deformation continued until at least 85 Ma and probably until 74 Ma, progressing from south to north. INTRODUCTION Geologic investigations of western Idaho and eastern Oregon and Washington have disclosed the presence of several tectonostratigraphic terranes, referred to here collectively as the Wallowa-Seven Devils (WSD) terranes, representing Late Paleozoic-Mesozoic island-arc complexes accreted to North America in Cretaceous time (Hamilton, 1976; Jones and others, 1977; Vallier, 1977; Fleck and Criss, 1985; Lund and Snee, 1988; Snee and others, 1995). The classic Sr-isotope and geochronologic study by Armstrong et al. (1977) demonstrated that plutons intruding these arc terranes are distinct from those intruding the Precambrian crust of North America. Subsequent studies by Fleck and Criss (1985), Criss and Fleck (1987), Fleck (1990), and Manduca and others (1992, 1993) generally confirm these relationships and provide abundant data on the isotope geochemistry and age of plutonic rocks of the region, as well as on the character of their sources and the terranes through which they were emplaced. This study focuses on the location, age, and character of the tectonic boundary between the North American craton and the WSD terranes. Although inferred to extend from southeastern Washington to the Snake River Plain, exposures of this boundary are recognized in western Idaho from near Ahsahka in the north to near Gross, ID in the south. Because the boundary is confined to western Idaho over its recognized length, but is not well located by any other geographic feature, the structure is referred to here as the western Idaho zone or WISZ (Strayer and Hyndman, D.W., 1987; Strayer and others, 1988; Fleck and Criss, 1988; Strayer and others, 1989; Fleck, 1990). The name River suture has also been used for the zone (Lund and Snee, 1988) in west-central Idaho, but because the Salmon River is one of several geographic features that cross the at almost 90°, we do not consider the name