Early weakening processes inside thrust fault

Abstract

Observations from deep boreholes at several locations worldwide, laboratory measurements of frictional strength on quartzo-feldspathic materials, and earthquake focal mechanisms indicate that crustal faults are strong (apparent friction μ ≥ 0.6). However, friction experiments on phyllosilicate-rich rocks and some geophysical data have demonstrated that some major faults are considerably weaker. This weakness is commonly considered to be characteristic of mature faults in which rocks are altered by prolonged deformation and fluid-rock interaction (i.e., San Andreas, Zuccale, and Nankai Faults). In contrast, in this study we document fault weakening occurring along a marly shear zone in its infancy (<30 m displacement). Geochemical mass balance calculation and microstructural data show that a massive calcite departure (up to 50 vol %) from the fault rocks facilitated the concentration and reorganization of weak phyllosilicate minerals along the shear surfaces. Friction experiments carried out on intact foliated samples of host marls and fault rocks demonstrated that this structural reorganization lead to a significant fault weakening and that the incipient structure has strength and slip behavior comparable to that of the major weak faults previously documented. These results indicate that some faults, especially those nucleating in lithologies rich of both clays and high-solubility minerals (such as calcite), might experience rapid mineralogical and structural alteration and become weak even in the early stages of their activity.

Mixed Brittle and Viscous Strain Localization in Pelagic Sediments Seaward of the Hikurangi Margin, New Zealand

“…Despite the possibility of overestimating mass loss due to initially higher concentrations of clay in stylolite‐forming volumes, these mass losses are consistent with previous estimates of bulk mass change of ‐16% to ‐44% and CaCO 3 change of ‐15% to ‐45% from early pressure solution in marl from Lacroix et al. (2015).…”

“…, stylolites in Cretaceous to Palaeocene chalks in the North Sea (34-58% Safaricz & Davison, 2005), pressure solution in 10.1029/2019TC005965Tectonics a young marly thrust fault in the Spanish Pyrenees (14-44%Lacroix et al, 2015)" and stylolites in carbonates in the Permian Zechstein Basin in Germany (25-39%Koehn et al, 2016).…”

Experimental Insights Into Fault Reactivation in Gouge‐Filled Fault Zones

“…Although considered planes at the crustal scale, mature faults are complex structures hosting fault cores up to several hundred meters thick (e.g., Ben‐Zion & Sammis, ; Caine et al, ; Faulkner et al, ; Shipton et al, ), which can be significantly weaker than the surrounding rocks (e.g., Collettini et al, ; Lacroix et al, ). Theoretical and experimental observations suggest that the presence of a weak gouge layer along a fault results in the rotation of the stress field in proximity to the fault (e.g., Byerlee & Savage, ; Gu & Wong, ; Lecomte et al, ; Lockner & Byerlee, ; Mandl et al, ; Rice, ), influencing fault reactivation and possibly enhancing slip on unfavorably oriented faults (Lecomte et al, ).…”

Mechanical Properties of the Sevier Desert Detachment: An Application of Critical Coulomb Wedge Theory

“…Fluids weaken faults in another way, by promoting the growth of clay‐rich coatings, which lower fault friction (e.g., Schleicher et al, ). Such clay gouges are common in carbonate‐dominated fold and thrust belts (such as the Sevier) where pressure solution preferentially dissolves carbonate and concentrates clay, resulting in shear zones and faults with very low frictional coefficients (Lacroix et al, ). For the LANF in the Panamint Valley, California, experimental studies have shown that fault gouges with greater than 50 wt % clay content are generally in a range of μ D = 0.2–0.4, and certain fault gouges have a friction coefficient less than 0.2 (Numelin et al, ).…”