CRUSTAL MAGMATIC PLUMBING AND ITS INFLUENCE ON THE GEOMORPHIC FORM OF THE MCMURDO DRY VALLEYS, ANTARCTICA

Earth abounds in volcanism and in vast terranes of intrusive igneous rock. But it is unusual to find magmatic plumbing systems where the field relations clearly reveal the overall nature of its geometry and inner-connectedness. Exceptional exposures coupled with deep erosional incising of the crust make the Ferrar-DV magmatic system special in this respect. The areal distribution of a slurry of large opx crystals in the Basement Sill indicates the central filling vent was in the area of Bull Pass. Yet, it has proven elusive to recognize the feeder conduit itself on the ground. 3D mapping shows all sills to be remarkably flat across the region, but locally in east central Bull Pass the contacts from aerial mapping appear vertical or even overturned. Thus the conundrum: clear evidence of horizontal regional and vertical local contacts at the same location. Coupled with this are many ‘climbing contacts' where the Basement Sill locally cuts steeply upward.

We have found key locations where the sill-granite contacts are, indeed, both horizontal and vertical at the same location. The sills are more or less everywhere horizontal and at the upper contact of the Basement Sill the dolerite climbs upward over the granite forming the dip slope then re-injects, forming a minor sill, and then continues again up the dip slope to the Peneplain Sill. This duo-natured contact may well be a common occurrence in this region. The key feature is the consistent rolling over and upward climb of the dolerite at upper contacts and conversely at lower contacts. The walls of Bull Pass may be original dip-slopes once coated by quenched magma traversing upward, supplying magma to the upper sills. Similar relations are found in parts of Wright and Victoria Valleys. These magma-coated dip slopes represent a major through-crust fracture system along which erosion was concentrated, perhaps forming a template for at least part of the Dry Valleys morphology. Extending the present slopes upward another 2-3 km suggests that these fractures formed basins or volcanic calderas on the original surface, the size of which is similar to some of the smaller basins in the Eastern North America fossil rift system. This may be a fundamental process in crustal evolution and sculpting.