BIRCH CREEK PLUTON, PAPOOSE FLAT PLUTON, LACCOLITHS, AND THE PROGRESSION OF STUDYING INTRUSIONS

Art Sylvester, Clem Nelson, and others worked on the Birch Creek and Papoose Flat plutons. Their models are still used as classic examples of forceful magma emplacement and their ideas can be linked to the progression of how we study the emplacement of intrusions today. One of their important concepts was that the magmas initially intruded as dike-like and then inflated outward. Their emplacement models still hold although the Birch Creek pluton is now modeled to contain many episodes of slightly different composition magmas emplaced in sheet-like increments.

These ideas have led us to work on laccoliths. Data from the Henry Mountains of Utah and the Shonkin Sag laccolith in Montana document how incremental emplacement of magma as sheets is now the rule, even though the evidence for these internal contacts is erased in the interior of these intrusions. Other studies suggest that there is a scale-independent mechanism that controls the dimensions of laccoliths. This relationship leads to another idea on how forces can be magnified from within the magma body, using simple hydraulic principles such as Pascal’s Principle. This model requires that the area of the initial sheet must increases in size, so that even though the magma pressure remains the same, the forces applied to the wall rocks increases relative to the area of the magma/wall-rock contact. Calculated magma pressures are similar to those determined for dome growth at Mt. St. Helens.

We can import these ideas from the laccoliths back to the emplacement of mid-crustal plutons, specifically the Birch Creek pluton. Sylvester and Nelson’s model - of initial magma that intruded along a fault and then expanded outward - can now be used as a force multiplier using Pascal’s Principle. This mechanism allows for the forces needed to cause intense wall-rock deformation in the mid-crust without relying on buoyancy as a driver.