DISPLACEMENT PATTERNS IN THE CASCADES CORE DURING ARC MAGMATISM

The NW-trending Cretaceous to Paleogene Cascades core consists of 96- to 45 Ma plutons and amphibolite-facies host rocks, which are exposed over crustal levels ranging from ~ 8- to 40 km paleodepth. The most voluminous, dominantly tonalitic magmatism and extensive regional contractional deformation of the arc occurred from ~ 96 to 87 Ma and resulted in major crustal thickening. During this interval, host rocks outside of pluton aureoles experienced recumbent to gently inclined, tight to isoclinal folding and formation of associated transposition foliation. Continued NE-SW shortening formed mesoscopic to km-scale upright folds with gently NW-SE plunging axes. Layering was mechanically active during folding at all crustal levels and fold asymmetries indicate shear toward the hinges in both limbs of upright folds. Simultaneously, the host rocks experienced NW-SE, subhorizontal stretching subparallel to the fold axes. The 500 m to 1-km-wide structural aureoles of the plutons are dominated by vertical ductile flow with host rock generally moving downward relative to the pluton. Strong margin (and orogen)-parallel, subhorizontal stretching is also recorded in host rocks and margins of segments of a few plutons. Downward transport in the area now occupied by the plutons is indicated by stoped host rock xenoliths and truncated and presumably stoped earlier phases of the plutons. It is more difficult to evaluate the displacement field during a less voluminous pulse of plutonism at 78 to 65 Ma. For at least the latter part of this interval dextral strike slip in the Ross Lake fault zone accompanied NE-SW shortening, which continued to be dominated by folding. In aureoles to several steeply dipping sheeted plutons, lateral wedging of host rock was accompanied by downward flow reminiscent of the aureoles to the older plutons. Thus, the main features common to both magmatic pulses are the importance in the host rock of folding of mechanically active layers and downward transport in the aureoles and plutons at all crustal levels.

In comparison to the Sierra Nevada arc, both show the importance of mechanical anisotropy of host rock, downward flow in aureoles, and overall vertical displacement, but differ in that foliation and stretching lineation in the host rock are much gentler in the Cascades than in the Sierra Nevada.