Víctor Hugo Garduño-Monroy

Debris avalanches and debris flows transformed from collapses in the Trans-Mexican Volcanic Belt, Mexico – behavior, and implications for hazard assessment

Volcanoes of the Trans-Mexican Volcanic Belt (TMVB) have yielded numerous sector and flank collapses during Pleistocene and Holocene times. Sector collapses associated with magmatic activity have yielded debris avalanches with generally limited runout extent (e.g. Popocatepetl, Jocotitlan, and Colima volcanoes). In contrast, flank collapses (smaller failures not involving the volcano summit), both associated and unassociated with magmatic activity and correlating with intense hydrothermal alteration in ice-capped volcanoes, commonly have yielded highly mobile cohesive debris flows (e.g. Pico de Orizaba and Nevado de Toluca volcanoes). Collapse orientation in the TMVB is preferentially to the south and northeast, probably reflecting the tectonic regime of active E^W and NNW faults. The differing mobilities of the flows transformed from collapses have important implications for hazard assessment. Both sector and flank collapse can yield highly mobile debris flows, but this transformation is more common in the cases of the smaller failures. High mobility is related to factors such as water content and clay content of the failed material, the paleotopography, and the extent of entrainment of sediment during flow (bulking). The ratio of fall height to runout distance commonly used for hazard zonation of debris avalanches is not valid for debris flows, which are more effectively modeled with the relation inundated area to failure or flow volume coupled with the topography of the inundated area. = 2002 Elsevier Science B.V. All rights reserved.

Continental boundaries of the Jalisco block and their influence in the Pliocene-Quaternary kinematics of western Mexico

Extensional faulting observed in southwestern Mexico has been related to the incipient rifting of the Jalisco block from the Mexican mainland since the Pliocene. On the basis of new structural and geophysical data, we propose that (1) the continental boundaries of the Jalisco block are ancient structures reactivated since the Pliocene at a low (<1 mm/yr) rate of deformation, and (2) Pliocene-Quaternary extensional faulting at the edges of Jalisco block is a basement-controlled intraplate deformation related to plate boundary forces rather than to active continental rifting. The Jalisco block boundaries first developed in response to the uplift of the Puerto Vallarta batholith in pre-Neogene time and underwent a complex contractile deformation before the Pliocene. During Pliocene-Quaternary times north-northeast extension reactivated the northern boundary, forming the Tepic-Zacoalco rift, whereas east-southeast extension formed the northern Colima rift. South of the Colima volcano, active extension is found only west of the so-called southern Colima rift and partly reactivates old northeast-trending basement faults. The parallelism between the subducted Rivera-Cocos plate boundary zone and the eastern neotectonic boundary of the Jalisco block supports east-southeastward motion of the southern Mexican blocks induced by the differential motion and oblique subduction of the Cocos and Rivera plates. On the other hand, we envisage Pliocene-Quaternary extension along the northern boundary as an upper-plate response to the low convergence rate and the steep subduction angle of the Rivera plate.

Preliminary geologic studies of Sierra El Aguajito (Baja California, Mexico): a resurgent-type caldera

Abstract Geologic field studies conducted in the Quaternary volcanic field of Tres Virgenes (State of Baja California Sur, Mexico) revealed the existence of a resurgent caldera. The calderas eruptive products, the Aguajito sequence, overlie the products of the nearby Reforma caldera (Reforma sequence) whose youngest products have already been dated as Quaternary. The rim of the Aguajito caldera is inferred by the existence of an arcuate alignment of rhyolitic domes. The mean diameter of this subcircular feature is 10 km. The volume of its mapped acidic products is a minimum of 10 km3. Several horizons within the sequence contain shells. K/Ar dates of the ignimbrites and domes of El Aguajito formation confirm that the unit are Pleistocene. The detailed stratigraphy also shows the evolution of a marine regression partly related to the caldera.

Geology and Stratigraphy of the Cerro Prieto Volcanic Complex, Baja California Norte, México

New mapping and stratigraphy of the Cerro Prieto Volcanic Complex indicates that it was constructed by several events: (1) The beginning was characterized by the emission of dacitic lavas that interacted with water-saturated sandstones, producing brecciated lavas; (2) then, effusive activity formed a lava dome that was destroyed by a phreatic eruption, producing a lithic fallout and a 300-m-wide summit crater; (3) afterwards, the volcano recorded effusive activity with the emplacement of three domes and a fissural lava flow; and (4) subsequent erosion of the volcanic complex resulted in the emplacement of debris flows around the complex. These magmatic events occurred at around 80 ka over a relatively short period of time. All rocks of this complex are composed of porphyritic lavas with phenocrysts of opx + plg + qtz + iron oxides ≫ ghost of amphibole, embedded in a groundmass with plagioclase microlites and glass, with a dacitic chemical composition (67–69 wt % silica).