STRUCTURE OF THE VOLCANCITO DOME, VOLCAN FUEGO DE COLIMA, MEXICO, REVEALED IN HIGH RESOLUTION MAGNETIC ANOMALIES

The Colima volcano complex, Mexico, is one of the most active volcanic systems in North America and is currently experiencing dome building eruptions and Vulcanian explosions from its main vent, Volcan Fuego de Colima. We conducted a high-resolution magnetic survey on Volcancito, the parasitic set of domes on the NE flank of the main cone of Colima volcano that formed between 1869-1878. The goal of this survey is to better understand the process of dome growth and explosive vent-clearing eruptions at Colima and similar andesitic volcano complexes. A total of 2295 ground magnetic observations were obtained using a Geometrics magnetometer in a 6000m² area on the summit of Volcancito. These magnetic field measurements have a range of 2144 nT, reflecting the strong remanent magnetization of the Volcancito dome rock. Previously measured NRMs from Fuego's summit dome ranged between 1-6 A/m. The resulting magnetic map is dominated by a large-amplitude, long-wavelength magnetic anomaly generated by the topography of the dome itself, and short-wavelength (15-20 m) anomalies of up to 1300 nT. The long-wavelength topographic anomaly was removed from the data using a trend1d command in Generic Mapping Tools (GMT), by fitting a fifth degree Fourier series to the data. Contouring of the residual magnetic anomalies clearly shows two circular regions, 15-20 m in diameter. These residual magnetic anomalies were modeled by discretizing the subsurface into 5 x 5 m prisms of variable height and normal remanent magnetization. The best-fit solution was obtained using a downhill simplex method, with approximately 6 A/m magnetic contrast between the prisms and surrounding dome rock. Based on this model we interpret the magnetic anomalies as resulting from pit craters on the summit region that have been filled and buried by subsequent eruptions with a mixture of tephra from both Volcancito and Volcan Fuego de Colima. These 15-20 m diameter, ~5m-deep pit craters may represent the vents through which pyroclastic material erupted at the surface during the last stages of the eruptive sequence in the 1870’s. Assuming that the modeled anomalies do represent eruptive vents, they line up along a southwest trend that extends to the present day active summit crater of Volcan de Colima, consistent with structural control along which magma preferentially reaches the surface.