RECONSTRUCTION OF CRETACEOUS TO RECENT CRUSTAL STRUCTURE OF THE SOUTHERNMOST COAST PLUTONIC COMPLEX: NORTH CASCADES, WA

The southern end of the Coast Plutonic Complex, North Cascades Core (CC), provides an excellent opportunity for reconstruction of Cretaceous to recent crustal structure. Structural, metamorphic, and geochronologic data for lower crustal rocks are compatible with three tectonic models that are best illustrated in cross sections. Well-documented metamorphic textures and P-T-t paths indicate that a 4 to 6 km thick thrust? wedge loaded the presently exposed rocks ca. 88 Ma. The base of all sections includes a master thrust which may be folded by late shortening. No data constrain the timing of youngest displacement on this fault; therefore, part or all of the 96-91 Ma Mount Stuart batholith and younger magma may have intruded across it. Near the structural top of the currently exposed crust, the Mount Stuart batholith is shown as thin sheets. Model I shows the structural base, Wenatchee Ridge Gneiss (WRG), as a rootless antiform above a folded thrust. The thickness and geometry is unknown at depth; however, it contains foliation and rotated porphyroclasts indicating intense deformation. Therefore, it is shown as a thin folded sheet containing a shear zone. In this shear model, the WRG could represent the lowermost section of this terrane above and/or within a detachment. Model II shows the WRG within a horst block separated from overlying rocks of the Nason Ridge Migmatitic Gneiss (NRMG) by ductile faults. The geometry is compatible with high pressure minerals in veins within the WRG. This horst model implies distinct shear zones with opposite sense of shear flanking the WRG, and ductile extension at high temperature. Model III illustrates the WRG as a tabular igneous body that intruded surrounding Chiwuakum Schist and was subsequently folded. This intrusive model interprets the NRMG as a contact zone of dikes and sills around the WRG, and high strain at the margins could be accounted for by rheological contrast during ductile folding. Based on paleobarometric and structural data, amphibolite of the Napeequa complex is shown at the base. A cogenetic intrusive relationship between the NRMG and WRG implies a ca. 93 Ma, pre M3, age for some of the dikes and sills in the NRMG. Additional structural and geochronological data are needed to fully test these models and constrain the present crustal structure and history of crustal development.