Jigsaw-fit blocks

Abstract

Jigsaw-fit blocks are highly fractured rocks of up to tens of meters wide, associated to volcanic debris avalanches and debris flows, traveling long distances (km) from the volcano edifice. Despite the mass movement long runout and agitated motion, jigsaw-fit blocks are found on their deposits with no apparent disaggregation and with occasional thin matrix facies filling the jigsaw cracks. The mechanisms behind the fragments constrained disaggregation and matrix infilling remain unclear and are limited to field observations. The rheology of granular flows suggests that segregation mechanisms should prevail and high fragmentation rates result from internal shearing and intense inter-granular collisions, challenging the theorized kinematics associated to the frustrated disaggregation of jigsaw-fit blocks. We study experimentally for the first time the segregation and disaggregation processes of analogue jigsaw-fit blocks within a granular flow as a function of fragmentation patterns and fragments density. We found that disaggregation appears regardless of the fragmentation pattern and its initiation is conditioned by the fragment density, occurring faster in fragments lighter than the moving granular media. Our results demonstrate that jigsaw-fit blocks remain united for a short period of shearing, but separate as a result of the fragments rotation and eventual particle infilling. We predict that this work sets a starting point for reviewing the interpretation of jigsaw-fit blocks in debris avalanche deposits, allowing the inference of kinematic features from the fragments configuration and its level of disaggregation.