Fault textures in volcanic debris-avalanche deposits and transformations into lahars: The Pichu Pichu thrust lobes in south Peru compared to worldwide avalanche deposits
Résumé
Structures, lithofacies and semi-quantitative sedimentological analysis of volcanic debris-avalanche deposits areessential to constrain syn-emplacement processes along avalanche fault zones in the setting of mass-flowrunoutdistances. At a distance of 28 km from the extinct Pichu Pichu compound volcano, we show how internal structuresof thrust lobes, faulted as to form horst- and graben-like structures, have recorded the brecciation withgranular segregations. Cataclastic gradients have been recognized in more than eleven breccia structures withstatistical grain-size parameters, among them fractal D-values between 1.96 and 2.53 related to an extensionalfracturing in DADs with fractal D-values between 2 and 2.5 leading to granular disaggregation with fractal Dvaluesbetween 2.13 and 2.22. Primary fractures related to transport processes and the secondary cataclasis producedduring emplacement are quantified as having b50%matrix,while the avalanche transformations into lahardeposits occurwith N50%matrix. The hybrid lithofacies in graben-like structuresmay be the precursory stages ofthe matrix transformation into lahar deposits.Volcanic debris-avalanche structures of eleven worldwide volcanoes that belong to the Central Andes and theCascades volcanic arc have been compared using sedimentary and statistical methods (156 sieved samples andShape Preferred Orientationmeasured on 30,342 clasts).Median diameter, sorting index, skewness and kurtosisvalues help differentiate four deposits and processes: (1) the proximal gravitational collapse deposits; (2) the avalanchethrust lobes, located in inverted extensional fault zones, and (3) the hybrid and mixed deposits withpolymodal distributions; and (4) the bimodal, transformed matrix with clast sorting. The fractal D-values of avalanchematrix range between 2.3 and 2.7, suggesting that extensional fractures have led to granulardisaggregation.From the available statistical dataset, a few equations have been developed that help pointing to a synemplacementcataclastic gradient along avalanche fault zone. Three textural classes of avalanche fault zoneshave been identified based on the ellipse/a/b values of ~2.14 for the planar collapse in fault zone, 1.75 to 2 forthe crushing of volcanic avalanche breccias, and b1.7 due to the thermal effect of clast fragmentation. Inheritedclast shapes with a/b=2.5 and ellipse= 4 suggests a co-evolution of avalanche lithofacies. A sedimentologicalclassification of avalanche fault zones proposed here has been related to the syn-emplacement processes of DADstructural units. As a result, the semi-quantitative texture dataset helps constrain the different stages of avalancherunout and the transformation steps into lahar deposits.
Domaines
Volcanologie
Origine : Fichiers produits par l'(les) auteur(s)
