Formation mechanism of bedding-parallel antitaxial fibrous veins in shale: A review

Abstract

Antitaxial fibrous veins are commonly developed in low permeability shale in orogenic belts and sedimentary basins. We review work on antitaxial fibrous veins, provide a classification, and discuss a formation mechanism. On the basis of crystal morphologies within veins, generalized fibrous veins can be categorized into stretched, syntaxial and antitaxial veins, and the antitaxial veins are fibrous veins in the strict sense. Typically, individual antitaxial fibrous veins consist of a median zone and two fibrous zones. According to detailed petrographic observations, four different types of solid inclusions within antitaxial fibrous veins were distinguished. Only solid inclusions occurring within blocky or elongate-blocky crystals in the median zone reliably indicate crack events. Other types of solid inclusions do not show clear evidence of crack-seal mechanism. The initial opening of antitaxial fibrous vein is considered to result from hydrofracturing caused by fluid overpressure. The origins of fluid overpressure mainly include hydrocarbon generation, tectonic compression and mineral transformation (such as illitization of smectite and hydration of anhydrite to gypsum), or a combination of these factors. The crack-seal mechanism cannot fully explain the formation mechanism of fibrous veins, and it may be only responsible for the median zone of antitaxial fibrous veins. The growth of fibrous crystals occurs at vein-wall interface without fracturing. Petrographic observations show that fluid pressure, force of crystallization, and sometimes tectonic compression synchronously drive fibre growth. The goal of further work should be to quantitatively determine fluid pressure and force of crystallization during fibre growth.