Análisis de las Condiciones Estructurales de Estabilidad en el ...

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Abstract This work represents a new proposal in relation to volcano flank instability. Furthermore, one of the proposals is that sector collapse on extinct or inactive structures is possible and new evidences of sector collapses not related to eruptive activity are presented here. This work also proposes that the volcano could have been slowly destabilized enough to become prone to collapse, supported along with documented historic cases which are also reviewed in the present work. Cofre de Perote volcano (19° 30’N; 97° 10’ W, peaks at 4,650 m.a.s.l) conforms the northern end of a volcanic chain known as Citlaltépetl-Cofre de Perote volcanic range (CCPVR). Aligned with Cofre de Perote volcano, the southern end is represented by Citlaltépetl volcano. The volcanic range forms an important physiographic barrier which separates the central altiplano, also known as Cerdán Oriental, from the coastal plains of Golfo de México. Contrary to common descriptions, Cofre de Perote volcano (CP) is not a stratovolcano as it has been suggested in most references, but in reality an entire volcanic complex conformed by a series of superimposed domic structures characterized by a soft morphology which resemble a shield volcano. Its last activity has been determined as 0.2 My which implies that the structure can be considered as extinct. Despite the mentioned conditions of the volcano, which suggest a stable structure with no possibility of presenting any type of volcanic hazard to nearby populations, geologic evidence represented by prominent fresh scarps at its summit along with avalanche and debris flow deposits on its eastern slopes indicate that the volcano has suffered repeated collapse events on recent times long after its ceased activity. This scenario suggests the possibility that events of this nature, can recur in the future in spite it is an extinct structure. This work presents an analysis of instability conditions of the volcanic massif, attempting to evaluate the possibility of a recurrent event. The analysis is based on the knowledge of the geologic characteristics of the volcano and its structural behavior. furthermore, the work considers the loss of structural strength due to exposure to hydrothermal fluids, being this one of the determinant factor favoring instability at the volcanic massif and in absence of a magmatic component. Mineralogy identified at the volcano and the debris flow and avalanche deposits suggest that the structure may had been weakened due to interaction of the rocks with hydrothermal fluids. It is also evident that weakening has not been thoroughly, showing zones of intense alteration contrasting with areas relatively fresh. main structural difference of hydrothermal alteration from that provoked by atmospheric erosion, is that this is more intense affecting the structure to an undetermined depth. ix
Alteration minerals have been zoned throughout techniques based on multispectral and hyperspectral high resolution imagery analysis, the work is supported by ground truth samples collected at the field analyzed under laboratory conditions using visible-infrared reflectance spectroscopy (VIS/IR) and X-ray diffraction. Other factors analyzed are erosion due to glacial advancement, and increased neutral pressure because of a hydrostatic water column, seismic events and some other considerations like volcano stratigraphy are included in the analysis. To analyze the mechanical behavior or the volcanic structure, different models were constructed by the use of different numerical algorithms such as finite element analysis and limit equilibrium analysis. Mechanical conditions of the slopes were achieved through the use of geophysical exploration techniques and a fracture analysis models by the use of field data and aerial photo interpretation of structures. The work proposes two models: the first is a reconstruction of the ancestral geometry of the volcano for which the mechanisms that could have facilitated the structure to collapse are evaluated. A second model evaluates structural stability of present volcano summit. Furthermore, the work includes a geotechnical zonification of the volcano summit and evaluates the maximum volume of material which continues being unstable. Keywords Hydrothermal alteration; volcano instability; sector collapse; Cofre de Perote. x
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