PALEOMAGNETIC ESTIMATE OF THE EMPLACEMENT TEMPERATURE OF THE LONG-RUNOUT NEVADO DE COLIMA VOLCANIC DEBRIS AVALANCHE DEPOSIT, MEXICO

Stoopes and Sheridan have mapped a volcanic debris avalanche of Nevado de Colima which has an exceptionally long runout (120 km) and low fall-height to length ratio (H/L = 0.04). We present paleomagnetic results from this volcanic debris avalanche deposit which provide evidence that this avalanche was emplaced at elevated temperatures. The majority of samples, collected from lithic clasts in the volcanic debris avalanche deposit, exhibit two-component remanent magnetizations with a low-temperature component (25-350 degrees C) which is well grouped about the geomagnetic field direction at Colima and a high-temperature component (350-580 degrees C) which is randomly oriented. Although the temperature of the deposit most likely varied with distance from the volcanic source and the thickness of the deposit, our results suggest an emplacement temperature of approximately 350 degrees C at intermediate distances (18-26 km) from the source. In order for the rock clasts (20-40 cm diameter) to be heated to these temperatures, the avalanche was most likely the result of a magmatic, Bezymianny-type eruption. The mixing of hot, juvenile gases with the clasts provides an explanation for the high degree of fluidization of this material, as evidenced by the long runout of this avalanche deposit.