The effect of axial stress in maximum sustainable fluid pressure in Andersonian and non-Andersonian crust: A field-based numerical study from the Southern Andes (39°S)

Abstract

Fracture opening at low differential stress controls maximum sustainable fluid pressure (λ) within cohesive brittle crust. Standard Andersonian stress states occur when two conditions are met: (1) one of the principal stresses σ1≥σ2≥σ3 is vertical, and (2) failure occurs at optimal orientations so that the stress tensor shape ratio φ=(σ2-σ3)/(σ1- σ3) is irrelevant. Here we explore the role of φ-values (axial compression, triaxial stress and axial tension) on sustainable fluid pressure driving rock failure under general stress states. We analyzed two exposures representing tectonics of the Southern Andes. Calculated failure curves in λ-depth space indicate that the hydrostructural behavior of general stress states is governed by the steepest of the principal stresses and the φ-value. Generally, hydrostructural behavior falls within standard Andersonian λ-depth conditions. However, field examples suggest that non-Andersonian axial stresses may sustain fluid pressures that depart from the standard Andersonian condition: the lowest fluid pressures occur under subvertical axial compression and subhorizontal axial tension; and the highest fluid pressures occur under subvertical axial tension and subhorizontal axial compression. Since around 15% of global stress compilations correspond to one of these categories, it follows that a significant portion of tectonic regimes potentially define a hydrostructural infrastructure different from standard Andersonian crust.