CONSTRUCTION OF HIGH-SYMMETRY SHEET-LIKE INTRUSIONS IN THE UPPER CRUST THROUGH AMALGAMATION OF LOW-SYMMETRY LOBES

Most igneous intrusions in the upper crust have a sheet-like geometry in cross section, with a relatively small thickness compared to their length. Many theoretical models of these intrusions have been developed to better understand magma propagation processes. Almost all of these models assume the intrusions grow through radial propagation of a pressurized magma sheet. We present evidence from a wide range of sources (field examples, geophysical studies of ancient intrusions, geodetic studies of active upper crustal igneous systems, and analog models) that this general model may apply to many intrusive sheets of meter- and decimeter-scale thickness, but that many thicker intrusive sheets grow through a distinct process of lobe amalgamation.

Detailed field studies of upper crustal intrusions demonstrate that, over a wide range of spatial scales, the final form of radially symmetric sheet intrusions (e.g. sills, laccoliths, etc.) in many cases is the product of amalgamation of numerous finger-like magma lobes. An early model of sheet propagation acknowledged the existence of lobes, and hypothesized that groups of lobes form the advancing outer margin of a sheet. This “fingered sheet intrusion” geometry has been observed in the field on many occasions in association with sheets ranging up to meter- or decimeter-scale thickness. However, new data demonstrate that not only do thicker sheet intrusions also grow from component lobes, but the original fingered sheet intrusion model is inadequate for these larger igneous bodies. Specifically, these thicker intrusions in many cases grow from sequential emplacement of a series of isolated finger-like lobes propagating far in advance of the sheet-like portion of the igneous body. Over time, these low-symmetry lobes collectively produce a high-symmetry, radially symmetric sheet intrusion like a sill or laccolith.

These observations call into question the applicability of theoretical models of sheet intrusion growth that presuppose the existence of a single pressurized magma throughout the entire emplacement history. Interestingly, the geometric predictions of the models (e.g. a bell-shaped upper laccolith surface) generally agree well with observed geometries of many intrusions, despite clear evidence that a fundamental assumption of the models is incorrect.