Tectonic deformation is often accompanied and is influenced by fluid flow, e.g., subsurface migration of hydrocarbons, contaminants and ore rich fluids, and metamorphic and magmatic fluids. Traditionally these processes have been studied separately. Metamorphic petrology presume a lithostatic fluid and rock pressures distribution within the crust, an assumption that implies the crust is fluidized and has no strength. In the same depth range, structural geologists recognize complex stress patterns and development of instabilities, but assume hydrostatic fluid pressure distribution. As the result, there are numerous unresolved questions in petroleum, structural, magmatic, metamorphic and hydro- geologies due to lack of a consistent treatment of fluid flow and rock deformation. Tectonic deformation can change the directions of fluid/melt flows up to 180 degrees and increase the fluxes by orders of magnitude. In turn, the effect of fluids or melts on tectonic deformation ranges from stabilization (e.g. dilatation hardening) to destabilization (e.g. due to fluid overpressure).

Most of the complexity in studying rock deformation comes from rheology. An even greater challenge arises if a combination of several rheologies, such as brittle and ductile, is essential to describe a single process. The deformation of the Earth crust at the tectonic time scale is essentially the brittle-ductile process greatly affected by presence of fluids. Heterogeneity and non-linear rheology give rise to instabilities, which are "unexpected", rapidly developing, deformation paths triggered by small and normally negligible irregularities. There are several classical theories of instabilities developed for various rheologies. More recently, generalization has been made for combinations of rheologies. Similar results have yet to be developed if fluids are involved and actively affect rheology. There is a feedback between deformational instabilities and fluid flow, which is driven by fluid pressure gradients and controlled by permeability. The deformational instabilities cause sudden changes in both - permeability and fluid pressure distributions. New dynamic paths of the fluid flow are created as response to the rock matrix deformation; their prediction requires a combined treatment of fluid flow and rock deformation.

Recently a consensus has emerged that recognizes the essential role of rock matrix deformation in generation, segregation and transport of melts and metamorphic fluids. Numerous workers have argued that dynamic spatial and temporal focusing of pervasive fluxes is essential to geological mass and heat transfer processes in environments ranging from sedimentary basins to the asthenosphere. Typically such arguments are based on the observation that transport times, computed on the basis of pervasive fluid flow through a static porous medium, are inconsistent with those required by measurements or by indirect thermal or chemical evidence. However, it is not uncommon that geochemical patterns provide direct evidence of flow channelling or fluid trapping by structures that appear to develop dynamically as an intrinsic feature of the transport process.

The models of melt generation and fluid production during metamorphism will be briefly reviewed. An attention will be paid for the time scales and identifications of the limiting processes; e.g. melting versus "dissolving": former is controlled by the heat supply whereas the later is by mass flux of the reagents.

While models for large-scale dynamic features such as self-propagating cracks are well understood, the explanation for the transition from pervasive flow to large-scale features remains enigmatic. Porosity waves, which are self-propagating domains of fluid filled porosity that nucleate from flow instabilities in a deformable porous medium, are a possible transitional mechanism, but the popularity of the porosity wave model has declined because of the perception that the mechanism is too inefficient to operate effectively on geological time scales. This perceived failing is in large part due to oversimplification of the rheological models used to evaluate the relevance of the porosity wave mechanism. Recently, realistic rheological models, which take into account both the viscoelastic character of the rock matrix, its temperature dependence and yielding behavior, have revealed that the spectrum of compaction generated flow instabilities is much broader than hitherto anticipated. These instabilities are manifest as both sill- and dike-like self-propagating high-porosity structures that give rise to fluid fluxes that may be orders of magnitude larger or smaller than those predicted by classical Darcyian flow models. The challenge is to characterize the effective permeability of a deformable rock matrix due to these instabilities.

Once melt has been segregated by Darcyian pervasive or channeled fluxes, the mode of long-range transport is in question. The arguments "pro" and "contra" for diapirism versus dyking and their implications will be discussed in detail.