The nature of the plumbing systems that feed major sill complexes has long been a mystery. Regional dike swarms in the near surface are common in dolerite provinces, and probably fed comagmatic flood basalts. Sills, which form with systematic downward development of the local magmatic system, generally cut all dikes and seem to be fed through local, equi-dimensional magmatic conduits. Feeder zones for the sills are speculated to be deep-seated regional plumes, local stocks, or massive fissures. Stacks of sills fill and interconnect to form highly integrated complexes like the Ferrar Dolerites of the McMurdo Dry Valleys (McDV). Even though there is good evidence for flow directions at small spatial scales (m), clear field evidence at intermediate spatial scales (km) is uncommon. The Basement Sill of the Ferrar Dolerites of the McDV may, however, offer an exceptional opportunity to trace the flow field of emplacement and especially the nature of the deeper magmatic feeding conduit. The Ferrar Dolerites magmatic system is unique in that it is complete, petrologically diverse, and well-exposed. Virtually the entire upper 4 km of the magmatic column is exposed, from basal layered ultramafic ortho-pyroxenites (the OPX Tongue of the Basement Sill) to regional flood lavas (Kirkpatrick Basalts) at the top. The regional distribution of the OPX Tongue of the Basement Sill indicates that the feeder zone of this igneous complex is located near Mt. Cerebus in the Bull Pass-Clark Glacier block of the Olympus Range. Although the ultimate deep feeder must exist nearby, its structure has been elusive. The Basement Sill's structure in the area of Bull Pass is complex with multiple bifurcations, climbing contacts, and internal chilled margins. Given these complications, we have turned to the next generation of geological field analysis: 3D geologic mapping and model interpretation. 3D modeling using the GOCAD system provides geologic insight in its truest sense; that is, seeing into the structure and petrology of the model through the rocks, lose rock debris, and ice, from any perspective, isolating any component at will. By combining the digital elevation model (DEM) and aerial photography of the USGS, geologic maps from the New Zealand Geological Survey as well as our own and our collaborators' with our detailed field observations, we find the geometry of the Basement Sill feeder to be a multi-lobed funnel or flower-shape feature centered roughly beneath Mt Cerebus. The Bull Pass-Clark Glacier block lies above the throat of the feeder, and Bull Pass itself may represent the geomorphic expression of the subvertical western edge of this structure. In particular, emanating from a common local source the many lobes of the Basement Sill, some very expansive, some only local, reach several stratigraphic levels, indicating the general style of filling for the whole complex. That is, a strong central conduit locally spawns lobe-like breakouts, spreading laterally and in places coalescing into massive sills through repeated infusions. Breakouts form only at convenient locations, forcing the sill-like mush column structure system to propagate downward with time. The formation of the central conduit itself may be due to early progressive localization at depth of the initial dike swarm. The detailed 3D structure of the feeder zone may have broad implications for the dynamics of igneous transport in this and other magmatic systems.
