Review of major shale-dominated detachment and thrust characteristics in the diagenetic zone: Part II, rock mechanics and microscopic scale

Abstract

Characterising large shale thrust zone behaviour down to the diagenetic-metamorphic boundary is both simple and complex. The task in critical taper, analogue and numerical models has been successfully approximated using simple material parameters. Yet the weakness of shales thrusts on a case-by-case basis show great variability in key factors across a wide range of scales. Understanding of these variables and their relationships to different tectonic settings is patchy due to the different objectives of workers studying the various systems (e.g. seismic hazards, hydrocarbon exploration, structural geology research), and the types of available geological and geophysical data. Early research recognized that megathrusts feature creeping and locked patches, which may be described using the rate and state formalism. In addition to temperature, differential stress and strain rate, key controlling factors on thrust weakness are mineralogy, amount of weak phase present, structure localization mechanisms, grain size and shapes, porosity, permeability and pore fluid pressure. The smectite-illite transition has been a focus of seismic hazard research due to its coincidence with the top of the seismogenic zone. While there may be a relationship, other factors such as pore-fluid pressure variations or structure localization are also important. The critical taper model is a simple means of determining whether basic rock mechanics data (e.g. frictional strength, pore fluid pressure) is appropriate for natural wedges. For example, aseismic basal detachments of gravity driven systems (smaller critical taper wedges) appear to retain higher pore fluid pressures along the basal detachment than seismogenic basal detachments (higher critical taper wedges). In many continental and deltaic fold and thrust belts high organic carbon content is a very important factor in shale weakness due to: 1) the overall more ductile and well-cleaved nature of the shale when organic content is high, 2) the presence of high overpressures due to maturation of organic material, and 3) metamorphism of carbon to low friction graphite. This multifaceted influence of organic carbon content is just one example of the diversity of potential influences on shale thrust zone weakness, which enable shales to be weak despite considerable lateral deposition-related and vertical burial-related changes in composition. Some key variations between different types of fold and thrust belt (gravity driven; accretionary prism; Andean/Himalayan type) lie in systematic variations in clay mineralogy, magnitudes and origins of overpressures, seismic versus aseismic detachments, and different structural localization mechanisms. Further research is required to explore the viability of such distinctions and their impact on structural styles.