HR: 0800h
AN: V21D-0648    [Abstracts]
TI: A GIS-based Spatial Analysis of Volcanoes in the Central Andes: Insights Into Factors Controlling Volcano Spacing.
AU: * Savant, S S
EM: ssavant@space.edu
AF: University of North Dakota, Department of Space Studies, Grand Forks, ND 58202-9008 United States
AU: de Silva, S L
EM: desilva@space.edu
AF: University of North Dakota, Department of Space Studies, Grand Forks, ND 58202-9008 United States
AB: Volcano spacing has received little attention since the mid-70's when studies undertaken by Vogt (1974; EPSL) and then Marsh (1979; J Geol) suggested a regular spacing of volcanoes in arcs that ranged from 50 to 75 km for different arcs. The spacing was thought to be influenced by the thickness of the lithosphere or gravitational (Rayleigh-Taylor) instabilities related to source layer thickness and viscosity respectively. We have revisited these ideas through a detailed study of volcano distribution in the Central Volcanic Zone (CVZ) of the Andes where volcano spacing was thought to be around 70 km. The CVZ was selected as it is the type example of continental arc volcanism, built on an extremely thick crust of up to 70 km. The availability of a comprehensive dataset describing the relative age, location, and geomorphic characteristics of each volcano (Volcanoes of the Central Andes, de Silva and Francis, 1990, Springer Verlag) made this a compelling case study. The ready availability of ARC GIS Geographic Information Systems software and the geospatial analysis tools therein, allowed a comprehensive spatial analysis of the volcanoes to be conducted. Of the 1,118 volcanoes of ages from 23Ma to active in the CVZ, we focused on the 106 active and potentially active large composite volcanoes that define the modern arc. These volcanoes are related in time and thus to a consistent set of tectonic factors. The frequency distribution of inter-volcano distances shows a peak frequency in the 10 - 30 km range (71%) with subordinate between 40-80 km (19%) and 80 - 120 km (10%). The characteristic spacing is thus much smaller than the characteristic spacing of 70 km found previously and is consistent with Baker (1974; EPSL). The primary cause appears to be clustering of volcanoes into groups. The density of volcanoes is variable along the arc with regularly spaced clusters of two to three volcanoes in northern and southern parts of the arc (13 r°S to 19 r°S and 24 r° to 27 r°S). The central part of the arc between 22 r° and 24 r°S shows the highest density of volcanoes. An interpolated continuous raster grid of volcano base elevation revealed a strong positive correlation with the density of volcanoes. Interestingly, the spacing of the peaks in density is fairly regular at about 75 to 110 km. We interpret the different orders of spacing as reflecting control by different levels within the plumbing system for the arc. The spacing of density peaks (clusters) is likely the primary spacing of diapirs rising from the source layer into the lithosphere. The dominant 10 to 30 km spacing of individual volcanoes probably reflects the influence of intra-crustal regions superimposed on the primary spacing. These regions may be mid-crustal, low-velocity, high-conductivity zones that have been imaged geophysically and/or the influence of older upper crustal batholiths that may have been emplaced into the thickest crust between 22 r° and 24 r°S.
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005