The Effect of Lithospheric Structure on the Lithospheric Stress Field

Abstract:

The state of stress in the lithosphere controls many geological processes from the local scale like earthquakes and mountain orogeny to plate tectonics. The sources of lithospheric stress range from mantle flow to lithospheric heterogeneity. The variations in thickness and density in the lithosphere can be a significant source of stress. In this study, we focus on understanding the sensitivity of the stress field to lithospheric heterogeneity by examining different models for lithospheric structure and assumptions regarding compensation and lithospheric mantle density. We use the crustal and mantle structure from Crust 1.0 and a thermodynamically determined lithosphere (TDL) to calculate the gravitational potential energy (GPE) and mean outward tractions. The gradients of deviatoric stresses that balance it are solved using the finite element package ABAQUS for an elastic and viscoelastic lithosphere. We compare our results for azimuth of the most compressive stress and inferred regimes to the observations from the World Stress Map 2016, and compare to previous work using Crust 2.0. Our results show that the crustal structure from Crust 1.0 and Crust 2.0 is different in both thickness and density especially in continental areas, resulting in a significant difference in the computed GPE and predicted stress field. Deeper compensation depths, such as may be found under continents, increase the role of the mantle, giving rise to larger magnitudes and changes in the stress regime from strike-slip to normal and trust. We find that using Pratt or Airy isostasy makes little difference to predicted stress fields, in contrast to previous results. Our study confirms however how important the uncertainty in lithospheric structure weighs on our understanding of the state of stress of the lithospheric plates. Our next step is to move away from GPE formulations and solve directly for the stress field resulting from lithospheric structures that vary laterally in thickness and density.