Mid-crustal magmatic sheets in the Cascades Mountains, Washington: implications for magma ascent

Diking, diapirism, ascent along faults, and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust. We suggest that these mechanisms are end-members in a complex spectrum of ascent processes. In an attempt to evaluate which combination of ascent processes formed sheet-like bodies in the mid-crustal (20–25 km) Entiat pluton, Washington, we examined the tip regions of these sheets. The sheets have length/width ratios ranging from ∼6 to >75, with increasing ratios strongly correlated to decreasing sheet tip radii (from 850 to 100 m) and decreasing ratios of tip diameter/sheet width (from 0.66 to 0.33). Thus, these bodies have geometries falling between those of dikes and those associated with elliptical diapirs. The sheets are not associated with faults or fracture zones extending from their tips. Instead, sheet walls are oriented parallel to the axial planes of upright, syn-emplacement folds. In sheets with high length/width ratios, magmatic foliations in sheets are folded or parallel to axial planes of host rock folds. With decreasing length/width ratios, margin parallel foliations in both sheet and host rock are increasingly common. Our studies indicate that the sheets are emplaced at high angles to σ1, not σ3 as proposed in elastic dike models, and are always associated with complex, viscoelastic flow of host rock. These observations rule out elastic dike and fault models, and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock.