Abstract
It is well known that the existence of a low-strength level such as evaporites plays a fundamental role in the structural style of a folded belt and its evolution. However, although we know that there are other lithologies with also a low shear strength such as mudstones and shale sequences, there is some debate about their mechanical properties and about their role in tectonic processes. Shales can deform in a complex way and it is discussed under what conditions they can reach ductile deformation conditions or what role fluids play. Furthermore, we believe that under certain circumstances these formations may flow, forming complex diapir-like structures that may resemble salt diapirs.
Using various sources of information, such as borehole measurements, seismic images, and some in situ measurements, we will summarize the physical properties of shales, how they vary during burial and what sources of fluids they may have. Specifically, we will see the role played by two of the major diagenetic transformations that these formations undergo, such as mineralogical changes in the clay fraction or transformations in the organic matter generating different fluid phases. On these physical-chemical bases, we will also see how overpressure conditions can develop and more specifically, under what mechanical conditions shales can reach critical conditions that allow them to transform from a consolidated rock to a fluid or mobile shales, forming structures such as allochthonous shale sheets and mud volcanoes.
We will also present our latest results with experimental analog models, using a material that behaves like mobile shales, with a fluid phase and variable mechanical properties. These first analog models developed for systems under contraction show a marked difference with the model results obtained for salt systems, and allow us to address specific problems for shale tectonics, such as: under what conditions can their mechanical strength be overcome and flow?; how deformation is distributed inside shale sequences?; how deformation with thrusting and associated folds occurs when mobile shales are involved?; what is the resulting structural style when we have one or several shale detachments?, or; what are the structural differences between salt and shale systems in compressional settings?
This line of research explores the fascinating and still little-known world of mobile shales, because we intend to improve their seismic image, understand their contribution to the migration or trapping of hydrocarbons, not to mention how they behave under compression, to approach the role they may play in the origin and distribution of seismic ruptures in an active compressional setting such as the Taiwan orogen.
