PARALLEL OR PERPENDICULAR FOLIATIONS? MAGMATIC PRESSURE, FLUID DYNAMICS, AND RESULTANT WALL ROCK DEFORMATION

In certain regions it can be demonstrated that regional tectonics played a minimal role during magmatism and therefore wall rock deformation can be interpreted to be largely due to the forceful emplacement of magma, which is a result of fluid pressure originally derived from the mantle or lower crustal site of melting. In these situations we use a hydrodynamics approach and therefore interpret the fabric of pre-existing crystals in magmas as resulting from motion related to gradients in fluid pressure. In these cases the application of simple fluid dynamics theory (Bernoulli’s and Pascal’s Principles) is consistent with the observation of the foliation in relation to wall rock contacts and deformation. The felsic mid-crustal Birch Creek pluton of California (70% SiO₂) and the intermediate to mafic upper crustal intrusions in the Henry Mountains of Utah (63% SiO₂) and Shonkin Sag Laccolith (50% SiO₂) in Montana can be interpreted in this light. In all three cases, foliations are observed to be either parallel to wall rock contacts or perpendicular. The differences are due to whether the magma pressure is decreasing and magma is being transported to a new location along a fracture opening (channelized), or has been effectively sealed and magma pressure is increasing and a new chamber is being created. In these three cases, we know the direction that the wall rocks were displaced and therefore we know the direction the magma was moving which aids in determining type of flow. Channelized, decreasing pressure regions exhibit foliations consistent with laminar flow and are parallel to displacement direction. In areas where pressure is increasing, magmas exhibit foliations that are perpendicular to displacement direction and are consistent with flattening. The structures observed are consistent with fluids that are connected throughout the larger intrusion and being driven by maintaining pressure and flowing toward lower pressure, regardless of magma composition and or crystallinity and therefore seemingly independent of magma viscosity. These interpretations are also consistent with recent Plaster of Paris physical modeling of intrusions by other authors.