The Rock Physics of Fibrous Rocks: Chemo-Mechanical Interactions Controlling Rheological Properties

Abstract:

Traditionally, earthquake faulting portrays large earthquakes as sudden slip events propagating along a fault while releasing the strain energy accumulated over the years. At the other end of the spectrum, there are slow slip events, portrayed as swarms of tectonic tremor, which come from regions of the crust that slowly and benignly creep for long periods of time. Within the last couple of decades, slow slip events have been observed to occur regularly during the intervening time between large earthquakes within many seismogenic systems — from creeping segments of transform boundaries to downdip sections of subduction margins. A wide range of physical and chemical processes has been used to explain the occurrence of fault creep. However, the connection between the chemo-mechanics of the rock microstructure and its rheological properties remains baffling. Here, we start from the lessons learned from the common traits of creeping systems, namely the alkaline and hyper-alkaline chemistry of the host rocks and fluids at elevated temperature, to show that these traits are key to favor the diagenesis of (geo)polymerized micro- and nano-structures of mineral fibers — that is, long-chain polymer structures deriving from chain-building reactions. As an example, we focus on fibrous microstructures of palygorskite, a fibrous silicate mineral with tetrahedral silicate sheets joined by ribbons, rather than sheets, of (Al, Mg, Ca, Fe) octahedra, which is shown in the literature to exhibit higher frictional resistance compared to serpentine. We use SEM imaging, ²⁹Si- and ²⁷Al MAS NMR to study, respectively, the fine microstructure and level of polymerization. To study the macroscopic mechanical behavior of fibrous matrices, we use a statistical model that establishes a structure–mechanics interdependence of fibrous microstructures. Structural parameters such as fiber length, orientation, and degree of entanglement have a critical control on the rheological properties like strength, the fraction of fibrous threads bearing the load that resist against shear deformation, and the transition of the load-deformation relationship from brittle to ductile. Fluid-mediated polymerization can be a rapid process under hydrothermal conditions, which may control healing, the state and time-dependent evolution of fault friction.