DETAILS OF SILL AND LACCOLITH GROWTH IN THE SHALLOW CRUST: COMPARING RESULTS FROM FIELD STUDIES, GEOPHYSICS, ANALOG MODELS, AND GEODESY

Detailed studies of the geometry of sills and laccoliths now exposed at the surface demonstrate many of these upper crustal intrusions do not have the simple, smooth outer surfaces predicted by analytical models. Instead, many intrusions grow from amalgamation of separate magma sheets and finger-like lobes into a contiguous sheet-like body. Evidence of amalgamation is best preserved at intrusion margins where contacts with host rock have complex geometries. The Henry Mountains of southern Utah include spectacular 3-d exposure of sills and laccoliths preserved at several different stages of growth. Observations from different intrusions there can be interpreted as snapshots of the progressive growth of a igneous body as pulses of magma were injected.

Similar complex geometries have been observed in subsurface sheet intrusions studied with seismic reflection. In numerous studies, individual sills and laccoliths are demonstrated to be comprised of numerous finger-like lobes radiating outward from a central feeder region. These lobes become amalgamated to form a contiguous sheet-like body. In some high-resolution 3-d studies, magma flow paths can be confidently inferred based on details of intrusion geometry.

Further, analog models of sill and laccolith emplacement reproduce many of these characteristics, including lobate sheet propagation and other complex behaviors. However, these realistic patterns of growth are only observed when analog magma propagation is governed by an interplay between solidification and crack propagation.

Finally, volcano geodesy provides constraints on spatial and temporal patterns of active shallow magmatism. Modeling of geodetic data (satellite radar topography, GPS, tiltmeters, etc.) demonstrates that, in some cases, sill-like bodies grow through propagation of lobes like those seen in the field, seismic data, and analog models.

Taken together, these disparate techniques for studying the spatial and temporal development of magma propagation in the shallow crust provide a rich, complex picture of sill and laccolith growth. Although the first-order geometry of these intrusions is relatively simple (e.g. a tabular sheet, or bell-like laccolith), the actual growth history involves injection of multiple magma pulses and complex propagation of each of those pulses.