Journal of Conference Abstracts

Compaction-driven Fluid Flow Through a Viscoelastoplastic Matrix: Porosity-wave Morphology and Flow Localization

James Connolly (jamie@erdw.ethz.ch)¹ & Yura Podladchikov (yura@erdw.ethz.ch)²

¹IMP-ETHZ, Zurich, 8092, Switzerland

²Geology-ETHZ, Zurich, 8092, Switzerland

Deformation of the earth's lithosphere is dominantly elastic at surface conditions but becomes increasingly viscous with depth due to thermal activation of creep processes. This rheological variation, together with plasticity at low effective pressures, has profound effects on the character of porosity waves generated by compaction driven fluid flow. To assess these effects we introduce a mathematical formulation applicable to the entire range of rheological behaviors realized in the lithosphere (Geodinamica Acta, v 11, 1998). Stationary solutions to the viscoelastic compaction equations for finite background porosity are porosity shock waves with wave-like structures that resemble porosity waves in the simple viscous limit. Excepting cases of pathological degeneracy, this result implies that true solitary porosity-waves do not exist in natural environments with hydraulic connectivity. For normal geothermal gradients, thermal activation of viscous deformation results in a stabilization of one-dimensional sill-like porosity waves, a geometry that was thought to be unstable on the basis of constant viscosity models. Implications of this stabilization are that: the vertical length scale for compaction processes is constrained by the activation energy for viscous deformation rather than the hydrodynamic viscous compaction length; lateral fluid flow in viscous regimes may occur on greater length scales than anticipated by earlier studies. Decreasing temperature toward the earth's surface induces an abrupt transition from viscous to elastic deformation propagated fluid flow. Below the transition flow is accomplished by short wave-length, large amplitude porosity waves; above the transition flow is by high velocity, low amplitude surges. Vertical flow channelization can be induced by inverted gradients in the homologous temperature of the matrix or by plasticity.