PART 2: PLUTON SHAPES AS MARKERS OF INCREMENTAL ASCENT AND EMPLACEMENT PROCESSES IN TRANSCRUSTAL SYSTEMS

Plutons are records of incrementally grown transcrustal systems, requiring incremental changes in intrusive geometries and host rock removal. Studies of pluton areas and axial lengths have shown that those pluton traits follow power-law behavior. Incremental pluton growth has been inferred to proceed along those power laws, assuming that the shapes and structural aureoles of smaller plutons are equivalents to the earlier, overprinted growth histories of larger plutons. We analyze 14 basic pluton shapes that recur in nature and test existing power-law models in the context of transcrustal magmatism and incremental growth models from the petrologic, structural, and modeling communities.

We measure perimeter length, roughness, and shape as well as area and axial length on maps of 100 plutons and intrusive complexes. Points are traced around mapped pluton-host rock contacts in an R program. The points are then superimposed into cartesian coordinates for Procruste shape analysis. We compare the geometric properties with geologic properties such as length of magmatism, pluton compositions, host rock compositions, and tectonic setting. We also compare our geometric data with two existing power-law pluton growth models. Our assessment is that evolving pluton geometry and material transfer processes do not steadily proceed along the observed power laws with increased magma input. The non-steady-state pace of magma addition, the commonality of multiple coeval material transfer processes, and the existence of equivalent shapes at a wide range of sizes suggest that pluton shapes change relative to system-specific properties like thermal budget and shapes of earlier pulses, not the range shown by plutons of all sizes.

Our dataset provides the first profiling of pluton geometries beyond area and length and tests a new method for comparing the shapes of plutons that make up transcrustal systems. Challenges ahead include dealing with internal units, multiple magma and host rock compositions, variable internal contact types, and heterogeneous aureole deformation. However, it is clear that comparing geometric and geologic properties will allow the community to better assess why and where specific pluton shapes and their implied material transfer processes typically occur in developing transcrustal systems.