Dike intrusion and deformation during growth of the Half Dome pluton, Yosemite National Park, California

Abstract

Meter-scale mapping of the Late Cretaceous Half Dome Granodiorite of the Tuolumne Intrusive Suite (TIS) near Tenaya Lake, Yosemite National Park, defines an intricate internal structure that reflects a combination of incremental pluton growth by diking and internal deformation as the pluton grew. At least four ages of dikes of layered granodiorite are defined by crosscutting relations. Because dikes thicker than 1 m invariably contain multiple cycles of layering that field relations indicate record multiple intrusive increments, dozens of discrete intrusive events are likely. The kinematic pattern of dilation across dikes, offset lithologic markers across dikes, shearing of mafic enclaves and magmatic layering, and folding of dikes defines a synintrusive bulk strain field characterized by E-W extension and N-S contraction, with net volume increase in the extension direction. The geometric and kinematic pattern of the deformation are consistent with current understanding of Late Cretaceous Cordilleran tectonics and suggest that regional tectonic dilation played a significant role in making upper-crustal space for the growing pluton. Narrow shear zones offset lithologic markers and produced extreme strains, yet no rock fabric is preserved in the zones. This indicates that late-magmatic to subsolidus recrystallization, previously inferred in the TIS based on textural and mineralogical observations, greatly modified rock textures and obscured both the intricate internal structure of the pluton and the importance of synemplacement deformation.