Comment on se-2022-11

Abstract

The input sediments of the North Sumatra subduction zone margin, drilled during IODP Expedition 362, exhibit remarkable uniformity in composition and grain size over the entire thickness of the rapidly deposited Nicobar Fan succession (sea-floor to 1500 mbsf depth), providing a unique opportunity to study the micromechanisms of compaction. Samples were prepared from dried core samples from sites (U1480 and U1481) by both Ar-ion cross-section polishing and broad-ion beam cutting, and imaged with a field-emission SEM. Shallowest samples (sea-floor to 28 mbsf) display a sharp reduction in porosity from 80 % to 52 % due to collapse of large clay-domain/matrix pores associated with rotation and realignment of clay-platelets parallel to the bedding plane. The deeper succession (28 mbsf to 1500 mbsf) exhibits less rapid reduction in porosity from 52 % to 30 % by the progressive collapse of silt-adjacent larger pores by bending and subsequent sliding/fracturing of clay particles. In addition, there is a correlated loss of porosity in the pores too small to be resolved by SEM. Clastic particles show no evidence of deformation or fracturing with increasing compaction. In the phyllosilicates, there is no evidence for pressure solution or recrystallization: thus, compaction proceeds by micromechanical processes. Increase in effective stress up to 18 MPa (~1500 mbsf) causes the development of a weakly aligned phyllosilicate fabric defined by illite clay particles and mica grains, while the roundness of interparticle pores decreases as the pores become more elongated. We propose that bending of the phyllosilicates by intracrystalline slip may be the rate-controlling mechanism. Pore size distributions show that all pores within the compactional force chain deform, irrespective of size, with increasing compactional strain. This arises because the force chain driving pore collapse is localized primarily within the volumetrically dominant and weaker clay-rich domains; pores associated with packing around isolated silt particles enter into the force chain asynchronously and do not contribute preferentially to pore loss over the depth range studied.