STRESS STATES AND DIKE EMPLACEMENT IN THE SAN FRANCISCO VOLCANIC FIELD AT SUNSET CRATER AND WUPATKI NATIONAL MONUMENT, ARIZONA

Developing a deeper understanding of the geometries of dike intrusions (creating a new path, following pre-existing faults, etc.) in the subsurface of the Colorado Plateau (CP), relies on good constraints on the 3D stress state as well as shallow geologic structures. The Southern CP preserves evidence of volcanism and faulting, most notably the San Francisco Volcanic Field (SFVF) and the monoclines. Using the Lacoste and Romberg model G Gravimeter, G-858 MagMapper Magnetometer, and simple Mohr-Coulomb stress analysis, we constrain the stress states and geometries of the subsurface features. Within the SE SFVF south of Sunset Crater, there is a 315^o alignment of volcanoes indicating a subsurface structure from which they are sourced. 22 km NNE of Sunset Crater, the Doney Mountain Cinder Cone follows the Wupatki section of the Black Point Monocline. Focusing on the Sunset alignment, and using the gravity and magnetic surveys, we are working to define the geometry of the inferred feeder dike. At Doney Mountain, the 030^o oriented monocline has late Quaternary cinder cones aligned along its trace, suggesting that magma followed the steeply dipping fault zone to the surface. Our geophysical investigation at Doney is pending; however, we have explored possible stress state relations to the fault-controlled magma pathway. For the stress analysis of Doney, we assume a normal faulting regime and made our calculations for 1 km depth, averaging the density of sandstone and limestone (2500 kg/m³). Vertical stress is the largest (25 MPa); we assume the horizontal principal stresses are 50% and 10% of that. We resolve the stress tensor components into the normal and shear traction components along the 210^o,60^o oriented fault. We assume that to inflate and intrude the fault, the magma pressure must exceed the normal traction magnitude. For a well-oriented fault (SHmax oriented 210 ^o), the pressure is at least 8.1 MPa. For a SHmax oriented 135^o (World Stress Map for the Southern CP) the minimum pressure is 15.1 MPa. This pressure is larger than the minimum horizontal stress (2.5 MPa in our simple model) which would be exceeded for a simple and well-oriented vertical dike. Developing these models for shallow igneous intrusions improves volcanic hazard assessment and understanding of monogenetic volcanic systems.