Fault textures in volcanic debris-avalanche deposits and transformations into lahars: The Pichu Pichu thrust lobes in south Peru compared to worldwide avalanche deposits

Abstract

Structures, lithofacies and semi-quantitative sedimentological analysis of volcanic debris-avalanche deposits are essential to constrain syn-emplacement processes along avalanche fault zones in the setting of mass-flow runout distances. At a distance of 28km from the extinct Pichu Pichu compound volcano, we show how internal structures of thrust lobes, faulted as to form horst- and graben-like structures, have recorded the brecciation with granular segregations. Cataclastic gradients have been recognized in more than eleven breccia structures with statistical grain-size parameters, among them fractal D-values between 1.96 and 2.53 related to an extensional fracturing in DADs with fractal D-values between 2 and 2.5 leading to granular disaggregation with fractal D-values between 2.13 and 2.22. Primary fractures related to transport processes and the secondary cataclasis produced during emplacement are quantified as having <50% matrix, while the avalanche transformations into lahar deposits occur with >50% matrix. The hybrid lithofacies in graben-like structures may be the precursory stages of the matrix transformation into lahar deposits.Volcanic debris-avalanche structures of eleven worldwide volcanoes that belong to the Central Andes and the Cascades volcanic arc have been compared using sedimentary and statistical methods (156 sieved samples and Shape Preferred Orientation measured on 30,342 clasts). Median diameter, sorting index, skewness and kurtosis values help differentiate four deposits and processes: (1) the proximal gravitational collapse deposits; (2) the avalanche thrust lobes, located in inverted extensional fault zones, and (3) the hybrid and mixed deposits with polymodal distributions; and (4) the bimodal, transformed matrix with clast sorting. The fractal D-values of avalanche matrix range between 2.3 and 2.7, suggesting that extensional fractures have led to granular disaggregation.From the available statistical dataset, a few equations have been developed that help pointing to a syn-emplacement cataclastic gradient along avalanche fault zone. Three textural classes of avalanche fault zones have been identified based on the ellipse/a/b values of ~2.14 for the planar collapse in fault zone, 1.75 to 2 for the crushing of volcanic avalanche breccias, and <1.7 due to the thermal effect of clast fragmentation. Inherited clast shapes with a/b=2.5 and ellipse=4 suggests a co-evolution of avalanche lithofacies. A sedimentological classification of avalanche fault zones proposed here has been related to the syn-emplacement processes of DAD structural units. As a result, the semi-quantitative texture dataset helps constrain the different stages of avalanche runout and the transformation steps into lahar deposits.