A Hydromechanical Model for Lower Crustal Fluid Flow

Abstract

Metamorphic devolatilization generates fluids at, or near, lithostatic pressure. These fluids are ultimately expelled by compaction. It is doubtful that fluid generation and compaction operate on the same time scale at low metamorphic grade, even in rocks that are deforming by ductile mechanisms in response to tectonic stress. However, thermally-activated viscous compaction may dominate fluid flow patterns at moderate to high metamorphic grades. Compaction-driven fluid flow organizes into self-propagating domains of fluid-filled porosity that correspond to steady-state wave solutions of the governing equations. The effective rheology for compaction processes in heterogeneous rocks is dictated by the weakest lithology. Geological compaction literature invariably assumes linear viscous mechanisms; but lower crustal rocks may well be characterized by nonlinear (power-law) viscous mechanisms. The steady-state solutions and scales derived here are general with respect to the dependence of the viscous rheology on effective pressure. These solutions are exploited to predict the geometry and properties of the waves as a function of rock rheology and the rate of metamorphic fluid production. In the viscous limit, wavelength is controlled by a hydrodynamic length scale d, which varies inversely with temperature, and/or the rheological length scale for thermal activation of viscous deformation l A , which is on the order of a kilometer. At high temperature, such that d < l A , waves are spherical. With falling temperature, as d ! l A , waves flatten to sill-like structures. If the fluid overpressures associated with viscous wave propagation reach the conditions for plastic failure, then compaction induces channelized fluid flow. The channeling is caused by vertically elongated porosity waves that nucleate with characteristic spacing d. Because d increases with falling temperature, this mechanism is amplified towards the surface. Porosity wave passage is associated with pressure anomalies that generate an oscillatory lateral component to the fluid flux that is comparable to the vertical component. As the vertical component may be orders of magnitude greater than time-averaged metamorphic fluxes, porosity waves are a potentially important agent for metasomatism. The time and spatial scales of these mechanisms depend on the initial state that is perturbed by the metamorphic process. Average fluxes place an upper limit on the spatial scale and a lower limit on the time scale, but the scales are otherwise unbounded. Thus, inversion of natural fluid flow patterns offers the greatest hope for constraining the compaction scales. Porosity waves are a self-localizing mechanism for deformation and fluid flow. In nature these mechanisms are superimposed on patterns induced by far-field stress and pre-existing heterogeneities. IntroductionThe volume change associated with isobaric metamorphic devolatilization is usually positive, consequently devolatilization has a tendency to generate high pressure pore fluids. This generality...

Crustal Azimuthal Anisotropy Beneath the Southeastern Tibetan Plateau and its Geodynamic Implications

“…The consistency between the ϕ observations and the previously proposed southeast direction of the flow system (Clark & Royden, ) suggests that the LPO of amphibole in this area is Type II or Type III (Ko & Jung, ; Kong et al, ), for which the fast orientations are subparallel to the flow direction. In addition, some other possible mechanisms (Connolly & Podladchikov, ; Z. Liu et al, ) for generating anisotropy were proposed, such as fluid‐flow in the lower crust, facilitated by vertical vein structures that cut through the internal shear zones, or vertically aligned rock volumes with alternating hydration levels, specifically alternating amphibolite and granulite rock masses. Besides faults in the upper crust and mid‐lower crustal flow, shear‐related mineral lineation may be a possible contributor to the observed anisotropy in the southeastern Tibetan Plateau.…”

(De)compaction of porous viscoelastoplastic media: Model formulation

“…Brittle‐ductile deformation in rocks, whether due to dilatant brittle failure or microfracturing, leads to a difference in material response to compactive and dilating loads in the form of decompaction weakening. Such a difference in compaction/decompaction viscosities was identified before as a reason for generating tubular fast‐propagating high‐porosity regions (i.e., porosity waves) that are able to efficiently transport fluids in the crust and upper mantle [ Connolly and Podladchikov , ].…”

“…In particular, porosity waves are very sensitive to the functional dependence of effective viscosity on porosity and pressure. Depending on effective viscosity, they can take the form of spherical blobs, flattened ellipsoidal structures, or elongated tubular jets [ Connolly and Podladchikov , ; V. M. Yarushina, et al, (De)compaction of porous viscoelastoplastic media: Solitary porosity waves, submitted to Journal of Geophysical Research , 2015]. Their size and speed of propagation are strongly influenced by effective viscosity.…”

Activation of stylolites as conduits for overpressured fluid flow in dolomitized platform carbonates

“…The large faults, which may penetrate up to 15 km depth (Antolín-Tomas et al 2007), are the most likely conduits to rapidly bring such fluids to near-surface levels, as otherwise they would have quickly cooled to ambient temperatures (e.g. Bons 2001;Connolly & Podladchikov 2013).…”