Fault Initiation in Serpentinite

Abstract

Serpentinite fault rheology is fundamental to tectonic and earthquake processes, yet links between deformation textures and strength evolution during fault initiation are poorly constrained. Here I present field and petrographic microstructural observations of unsheared and sheared serpentinite that demonstrate a progression of fault development. I compliment observations with a clast‐size distribution analysis to investigate the evolution of fault rigidity and a numerical model to query the stress distribution of a clast‐in‐matrix geometry. Unsheared microstructures reveal well‐aligned, elongate serpentine in the matrix and short, randomly oriented serpentine in clasts. Sheared matrix displays cataclastic textures, discrete brittle surfaces, dissolution bands, and ductile textures defined by anastomosing networks of well‐aligned, fine‐grained serpentine. During fault initiation, matrix serpentine anisotropy promotes slip on basal planes or fiber aggregates, and clast‐on‐clast interactions drive a high bulk viscosity prone to stick‐slip behavior. As deformation progresses, clast fracturing is focused at clast tips, and smaller clasts are preferentially removed by dissolution‐precipitation processes, increasing the relative abundance of matrix and promoting fault creep. Strain is continually focused in the matrix and as clast content reduces the bulk viscosity drops. This study reveals that viscosity contrasts formed by primary serpentinization textures are a major driver for the development and strength evolution of faults. On a continental‐scale, similar processes may govern earthquake distributions, fault growth, and segmentation patterns on young, serpentinite‐hosted faults.

The internal structure and composition of a plate-boundary-scale serpentinite shear zone: the Livingstone Fault, New Zealand

“…The slip surfaces that make up the exterior of these lenses are commonly coated in a shiny, polished serpentinite, although in some outcrops weathered coatings of fibrous chrysotile give the serpentinite a rougher, splintery appearance. The scaly serpentinite displays a "fractal-like" geometry in which each phacoid can be cleaved apart to create smaller phacoids of serpentinite, a common characteristic of scaly fabric fault rocks (Maltman et al, 1997;Vannucchi et al, 2003;Vannucchi, 2019;Melosh, 2019). This self-similarity in geometry is observed down to a scale of < 10 µm, at which lenticular domains of serpentinite are separated by shear planes along which seams of magnetite are concentrated (Fig.…”

“…Examples of important serpentinite-bearing shear zones include oceanic detachment faults and fracture zones (Cann et al, 1997;Escartín et al, 2003;Bach et al, 2006), oceanic transform faults (Melson and Thompson, 1971;Skjerlie and Furnes, 1990;Hekinian et al, 1992;Fagereng and MacLeod, 2019) and plate-boundary-scale continental transform faults such as sections of the San Andreas Fault in California, USA (Moore and Rymer, 2007;Moore et al, 2018), and a portion of the southern segment of the Alpine Fault in New Zealand (Barth et al, 2013). Additionally, serpentinitedominated mélanges are associated with ophiolites worldwide (Shackleton et al, 1980;Nicolas et al, 1981;Saleeby, 1984;Norrell et al, 1989;Guillot et al, 2004;Federico et al, 2007;Chen et al, 2014).…”

“…While specific processes and microstructures related to serpentinisation and deformation in serpentinite-bearing shear zones have been reported in a wide range of studies (e.g. Maltman, 1978;Williams, 1979;Twiss and Gefell, 1990;Alexander and Harper, 1992;Gates, 1992;Hirauchi and Yamaguchi, 2007;Bellot, 2008;Melosh, 2019;Fagereng and MacLeod, 2019), there are few detailed field descriptions of the internal structure and composition of large serpentinite shear zones (e.g. Norrell et al, 1989;Strating and Vissers, 1994;Hermann et al, 2000;Soda and Takagi, 2010;Singleton and Cloos, 2012).…”

Response to Telemaco Tesei

“…Examples of important serpentinite-bearing shear zones include oceanic detachment faults and fracture zones (Cann et al, 1997;Escartín et al, 2003;Bach et al, 2006), oceanic transform faults (Melson and Thompson, 1971;Skjerlie and Furnes, 1990;Hekinian et al, 1992;Fagereng and MacLeod, 2019) and plate-boundary-scale continental transform faults such as sections of the San Andreas Fault in California, USA (Moore and Rymer, 2007;Moore et al, 2018), and a portion of the southern segment of the Alpine Fault in New Zealand (Barth et al, 2013). Additionally, serpentinitedominated mélanges are associated with ophiolites worldwide (Shackleton et al, 1980;Nicolas et al, 1981;Saleeby, 1984;Norrell et al, 1989;Guillot et al, 2004;Federico et al, 2007;Chen et al, 2014).…”

“…While specific processes and microstructures related to serpentinisation and deformation in serpentinite-bearing shear zones have been reported in a wide range of studies (e.g. Maltman, 1978;Williams, 1979;Twiss and Gefell, 1990;Alexander and Harper, 1992;Gates, 1992;Hirauchi and Yamaguchi, 2007;Bellot, 2008;Melosh, 2019;Fagereng and MacLeod, 2019), there are few detailed field descriptions of the internal structure and composition of large serpentinite shear zones (e.g. Norrell et al, 1989;Strating and Vissers, 1994;Hermann et al, 2000;Soda and Takagi, 2010;Singleton and Cloos, 2012).…”