Geografia

Geomorphology 123 (2010) 142–153

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Geomorphology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Geomorphic assessment of the debris avalanche deposit from the Jocotitlán volcano, Central Mexico
Sergio Salinas ⁎,1, Jorge López-Blanco
Depto. de Geografía Física, Instituto de Geografía, UniversidadNacional Autónoma de México, Ciudad Universitaria, C. P. 04510, México, D.F., México

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Edifice collapse of the Jocotitlán volcano produced a debris avalanche deposit whose morphology is characterized by conical hummocks and elongated ridges. We consulted aerial photographs, orthophotographs, and conducted field work to define field relationships between moundmorphology and stratigraphy. Based on field evidence and geomorphic and geologic interpretation we sub-divide the deposit into three sectors (north, northeastern, and eastern). We determine the emplacement mechanisms of the different sectors based on their distinct morphologic and lithologic features. In this context, we generate a geomorphometric database comprising 17 variables for each mound andapply multivariate statistical methods (principal component analysis and cluster analysis) to define the relationships between them. The principal components incorporated 73% of the total data variance and seven geomorphometric variables (perimeter, major axis, area, height, distance to the source, axis ratio and circularity index) defined two groups: hummocks and ridges. The circularity index andthe axis ratio best characterize the elongated form of hummocks composing the deposits. Contrasts in lithological characteristics, such as material strength and mobility, provide evidence for a transition from a sliding mass (debris avalanche deposit) to a debris-flow-like emplacement. Differences in deposit morphology suggest two collapse mechanisms: magmatic intrusion (Bezymianny-type sectorcollapse) and a tectonic mass-slide, an earthquake could provide the trigger for both collapses. The eastern lower flank of the volcano then collapsed gravitationally due to a movement along a fault (tectonic activity possibly related to the Acambay–Tixmadeje Fault System). This produced the northeastern sector of the debris avalanche deposit dominated by large elongated ridges. The spatial arrangementof both sectors (the N and NE) suggests that the two failure events occurred simultaneously. Finally, the eastern sector of the deposit was emplaced by posterior remobilization of material from the two other sectors through a debris flow. © 2010 Elsevier B.V. All rights reserved.

Article history: Received 25 July 2009 Received in revised form 8 July 2010 Accepted 9 July 2010 Available online 15July 2010 Keywords: Jocotitlán volcano Geomorphometry Principal components Cluster analysis Hummocks Ridges

1. Introduction 1.1. Volcanic debris avalanche deposits Major edifice collapse is a common phenomenon in the growth history of many composite volcanoes. It occurs mainly in andesitic to rhyolitic stratovolcanoes (e.g. Mt. St. Helens in 1980 — Lipman and Mullineaux, 1981; Siebert, 1984;Siebe and Sheridan, 1990). The instability and weakening of a volcanic edifice is the result of several factors, which include repeated magmatic intrusions and emplacement of the volcano in a tectonically active region prone to large earthquakes (Francis and Wells, 1988; Voight and Elsworth, 1997). Seismic activity not only directly affects the stability of the volcanic edifice but may also deform itsbasement, weakening the volcanic edifice (Borgia et al., 2000).
⁎ Corresponding author. + 52 5 6224119x20; fax: +52 5502486. E-mail address: sss@geofisica.unam.mx (S. Salinas). 1 Present address: Depto. de Volcanología, Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, C.P. 04510, México, D.F., México. 0169-555X/$ – see front matter © 2010 Elsevier B.V. All...