PERSPECTIVE ON LATERAL TERRANE TRANSLATION FROM SYNTECTONIC EMPLACEMENT OF THE HICKS BUTTE PLUTON IN THE CENTRAL CASCADES OF WASHINGTON STATE

Late Jurassic to early Cretaceous sequence of phyllitic schists and greenschist with rare intercalated Ep-blueschist outcrops in the south-central Cascade Range near Cle Elum, WA have been correlated to rocks 140 km to the north across the regional north-south striking Straight Creek Fault as a laterally continuous convergent zone. The age of the plutonic arc rocks in the Cle Elum segment are problematic to this correlation because new age constraints from 150-157 Ma are close to the ~160 Ma Ar/Ar dated sodic-amphibole ages in blueschist in adjacent country rock that suggests that the plutonic rocks were emplaced syn-tectonically, making opposite subduction vergence for the two subduction zone segments bisected by the straight creek fault.

This was tested by Rf/Phi strain analysis on samples from several transects across the contact between the Hicks Butte diorite to tonalite orthogneiss into the Ep-amphibolite orthogneiss host. Orientation of plagioclase and hornblende grains relative to the host matrix measured as rigid deformation markers yielded an average finite strain ratio (Rs) of between 1.4 and 1.6 across all samples. This is a minimum value due to microstructual evidence of grain size reduction. Rf/Phi data interpretation also supports a single-phase of strain for the pluton. Metamorphic constrains on the P-T paths of plutonic and host rocks include: symplectite in the Hicks Butte margin Opx + Hbl + Sp (hercynite) + Mt show rapid cooling from 1100˚C to 700-800˚C at pressures between 6-8 kbars. Blueschist samples within 500m of the margin display intense planar fabrics with a full spectrum of subduction and higher structural conditions: 1) pristine Gln+Ep+Cz+Q+Ti (rare Omph inclusions) interpreted to show peak pressure conditions, 2) Act + Ep + Ab at the expense of Gln + Ep + Pl modeled to have occured at ~ 550˚C at pressures between 5-7 kbars interpreted as peak temperature conditions. Greenschist with Chl+Ab+Ep+Q+Mt that shows a range of planar fabric intensity.

Rf/Phi and metamorphic petrologic analysis of ductile fabrics displayed across rocks of radically different P-T paths and compositions show that the Hicks Butte was emplaced syntectoncially under lower crustal conditions while host rocks were being exhumed possibly during active subduction near the plate interface.