Nanocrystalline Principal Slip Zones and Their Role in Controlling Crustal Fault Rheology

Abstract

Principal slip zones (PSZs) are narrow (<10 cm) bands of localized shear deformation that occur in the cores of upper-crustal fault zones where they accommodate the bulk of fault displacement. Natural and experimentally-formed PSZs consistently show the presence of nanocrystallites in the <100 nm size range. Despite the presumed importance of such nanocrystalline (NC) fault rock in controlling fault mechanical behavior, their prevalence and potential role in controlling natural earthquake cycles remains insufficiently investigated. In this contribution, we summarize the physical properties of NC materials that may have a profound effect on fault rheology, and we review the structural characteristics of NC PSZs observed in natural faults and in experiments. Numerous literature reports show that such zones form in a wide range of faulted rock types, under a wide range of conditions pertaining to seismic and a-seismic upper-crustal fault slip, and frequently show an internal crystallographic preferred orientation (CPO) and partial amorphization, as well as forming glossy or "mirror-like" slip surfaces. Given the widespread occurrence of NC PSZs in upper-crustal faults, we suggest that they are of general significance. Specifically, the generally high rates of (diffusion) creep in NC fault rock may play a key role in controlling the depth limits to the seismogenic zone.In this paper, we aim to elucidate the significance of nanocrystalline PSZs in Earth's upper crust. We start with background on fault mechanics and upper-crustal seismogenesis, and summarize some key physical properties of nanophase materials which, when applied to fault rock, are expected to be of major importance in controlling fault strength and stability. We go on to review the micro-and nanostructural characteristics of natural and experimentally-formed nanocrystalline PSZs, and list reports from the literature of PSZs characterized by grains <100 nm in size. Our work demonstrates that nanocrystalline PSZs form under a wide range of conditions pertaining slow (a-seismic) and fast (co-seismic) upper-crustal fault slip. Also, we observe that they are frequently characterized by an internal crystallographic preferred orientation, and by the presence of amorphous materials and/or glossy fault plane interfaces known as "mirror-slip" surfaces. Given the abundant observations of nanocrystalline PSZs in field exposures of faults, as well as in experiments, we suggest that they are of general importance to upper-crustal fault deformation. The physical properties of nanocrystalline fault rock may play a key role in natural earthquake cycles, especially in controlling the depth distribution of upper-crustal seismicity. Fault Zones, Earthquakes, and the Seismogenic ZoneThe presence of long-lived, localized zones of shear deformation in the crust, or fault zones, implies that the fault rocks within are weaker than the surrounding country rocks and that their weakness is persistent [14,15]. The strength of the upper-crust is classically approximated using a Cou...

Mixed‐Mode Formation of Amorphous Materials in the Creeping Zone of the Chihshang Fault, Taiwan, and Implications for Deformation Style

“…However, we do find shear‐induced amorphization and the associated AMs in both the localized narrow band (PSZ) and along the boundaries of widely distributed scaly clay fabrics (with limited sample observations), likely marking the distribution of dissipated strain in the Lichi Mélange. Natural and experimentally formed PSZs are commonly treated as frictional interfaces embedded in host rocks, suggesting brittle behavior in the bulk fault rocks (Ikari, 2015; Sibson, 2003; Verberne et al, 2019). On the other hand, small amounts of slip occurring along the boundaries of abundant scaly clays allow the fault rock to deform with a ductile behavior.…”

The physics of fault friction: Insights from experiments on simulated gouges at low shearing velocities

“…By contrast, wet experiments showed i) an increase in apparent steady-state friction upon re-sliding after a hold period, and ii) a pronounced increase of (a-b) after the https://doi.org/10.5194/se-2020-85 Preprint. (Verberne et al, 2014a(Verberne et al, , b, 2019. Towards higher temperatures (400-550 °C) the shear band is composed of linear, cavitated arrays of polygonal grains (~0.3-1 μm in size), suggestive of incomplete grain boundary sliding and (possible posttest) grain growth ( Fig.…”

“…Adequate modelling of granular system dynamics brings with the additional complexity that the deformation properties of individual mineral particles can change with changing particle size. Constraining this is especially important in the case of nanometric gouge (grain size <100 nm), which, along with (partly) amorphized host rocks, are widespread in natural and experimentally-sheared fault gouges (Power and Tullis, 1989;Yund et al, 1990; for a recent review see Verberne et al, 2019).…”

“…Individual nanoparticles and nanocrystalline aggregates frequently exhibit dramatically different physical properties compared with their bulk counter-parts (e.g., Meyers et al, 2006;Hochella et al, 2019) -the room temperature ductile nanofibers encountered in calcite gouge being an example of this ( Fig. 5c-inset) (Verberne et al, 2014b(Verberne et al, , 2019. The implication is that extrapolation of data from compaction experiments using micron-sized crystals or larger, used to constrain parameter values appearing in the CNS model, may lead to large errors when applied to nanogranular or (partly) amorphous fault rock.…”

Characterization of Pseudotachylite and Fault Gouges in Drill Cores from Andong, Korea and Its Implications for Paleo-Earthquakes

“…Formation mechanisms for the amorphous silicates are complex and various, involving chemical reactions, mechanical deformation, and quenching after frictional melting [57][58][59][60][61][62].…”