We report on new gravity and aeromagnetic data and interpretations across four north-south transects through the southwestern part of the Mt Hayes quadrange, Alaska. This work is part of a study to characterize the tectonics and metallogenesis of south-central Alaska. Specific goals of this work include resolving the nature of major internal and terrane bounding structures (such as between Peninsular and Wrangellia terranes) and assessing the locations, size, and depth of buried sources of potential mineral targets. Particularly important sources are feeder zones to the Triassic Nikolai Greenstone -- an extensive accreted flood volcanic basalt province distributed throughout Alaska and British Columbia. Exposures of the Nikolai Greenstone in the Amphitheater Syncline are both thick and unmetamorphosed. It is thought that outcrops of the Fish Lake layered, ultramafic complexes, such as those at Fish Lake and Tangle Lakes, exposed on either side of the syncline, may represent the type locality and core of the Nikolai magmatic chambers. These features suggest that the Amphitheater Mountains preserve part of the central magmatic axis of the Nikolai system. Due to the high density and strong magnetic character of these ultramafic rocks, they are prominently expressed in the geophysical data. Potential-field modeling across the Amphitheater syncline indicates that it may be underlain by a several-kilometer thick keel of high density and highly magnetic ultramafic rocks that likely represents the roots of the magmatic system.