3D Gravity and Magnetic Model of a Triassic Large Igneous Province Vent, Central Alaska Range, Alaska

The Amphitheater Mountains synform on the southern flank of the Alaska Range in south-central Alaska provides one of the most complete sections of a Large Igneous Province worldwide, revealing, in near continuous section: basal mafic and ultramafic sill complexes, associated mafic and ultramafic rocks, lower submarine lavas, and overlying subaerial lavas. Three-dimensional (3D) modeling of gravity and magnetic data shows details of the Amphitheater Mountains structure, flood basalt thickness, and geometry of ultramafic sills. Modeling is based on a compilation of existing regional and newly-acquired high-resolution profile gravity data, a compilation of regional and high-resolution aeromagnetic surveys, and numerous rock-property data (including density, susceptibility, and magnetic remanence measurements of outcrops, hand samples, and drill cores). The final 3D model includes 12 layers with up to 2 km of Nikolai basalts in the center of the asymmetric, west-plunging, 50 km long Amphitheater synform. The western end of the synform appears to be terminated by a steep N-S structure. The outcropping Fish Lake and Tangle ultramafic sills, apparently connected beneath the surface, are traceable to 5 km depth, and modeling indicates variations in thickness (1 to 3 km), shape, and orientation which may identify paths of magma flow within the system.