Nano-scale earthquake records preserved in plagioclase microfractures from the lower continental crust

Seismic faulting causes wall rock damage driven by both mechanical stress and thermal energy. In the 8 lower crust, coseismic damage has important implications for wall rock permeability, the progress of subsequent 9 fluid-driven metamorphic reactions, and rock rheology. Wall rock microstructures reveal high-stress conditions 10 near the slip surface during lower crustal earthquakes, however, there is limited documentation on the thermal 11 effect. Here, we present a transmission electron microscopy study of coseismic microfractures in plagioclase 12 feldspar from lower crustal granulites from the Bergen Arcs, Western Norway. Focused ion beam foils are 13 collected 1.25 mm and 1.8 cm from a 2 mm thick eclogite facies pseudotachylyte vein. Dislocation-free plagioclase 14 aggregates fill the microfractures and record a history of recovery from a short-lived high stress-temperature (σ15 T) state caused by seismic slip and frictional melting along the nearby fault surface. The plagioclase aggregates 16 retain the crystallographic orientation of the host rock and shape preferred orientation relative to the fault slip 17 surface. We propose that plagioclase partially amorphized along the microfractures at peak stress conditions 18 followed by repolymerization to form dislocation-free grain aggregates within the timeframe of pseudotachylyte 19 formation. The heat from the slip surface dissipated into the wall rock causing a short-lived temperature peak. 20 Subsequent cooling led to exsolution of intermediate plagioclase compositions by spinodal decomposition within 21 a few millimeters distance to the fault surface. Our findings provide microstructural evidence for the high σ-T 22 conditions that are expected in the proximity of seismic faults, highlighting the importance of microand 23 nanostructures for the understanding of earthquakes ruptures. 24

showed that for a dynamic earthquake rupture propagating at 91% 39 of the Rayleigh wave speed wall rock stresses may approach 10 GPa within 3 mm of a propagating rupture. Such 40 conditions, although short-lived, are expected to drive irreversible processes within the rock record, such as 41 thermal shock fracturing (Papa et al., 2018) and dynamic pulverization (Reches and Dewers, 2005). Extensive wall

49
Here we present a microstructural study of co-seismic microfractures in plagioclase from granulites in the Lindås

59
The Lindås Nappe of the Bergen Arcs of Western Norway is host to a population of seismic faults identified by 60 the presence of mm-to cm-thick pseudotachylytes that cut through granulite facies anorthosite (Austrheim and 61 Boundy, 1994). The pseudotachylytes contain either an eclogite-facies or amphibolite-facies mineralogy, and the 62 wall rock damage adjacent to them are spatially related to fine-grained products of the same metamorphic grade.

63
The earthquakes took place within the lower crust during the Caledonian collision at 423-429 Ma (Jamtveit et al.,

71
Photomicrographs of the plagioclase microstructures were taken with a Hitachi SU5000 field emission electron 72 microscope (FE-SEM) at the Department of Geoscience at the University of Oslo. Chemical maps of the 73 plagioclase were obtained with a Cameca SX100 electron microprobe analyzer (EMPA) at the University of Oslo's 74 3 Department of Geoscience. The working conditions for EMPA were a beam diameter of 1 µm, an accelerating 75 voltage of 15 kV and a beam current of 10 nA. The EMPA maps were used to perform mass balance calculations 76 of three plagioclase microfractures. After segmenting the feldspar in the microfracture from their host, the average 77 composition of the feldspar grains was compared to the average composition of the surrounding plagioclase host.

78
All other phases were excluded in the mass balance calculation.

88
The grain sizes were extracted from the EBSD data to fit a probability density function (pdf) to their size

98
1d and e). The FEI Talos 200FX equipped with a high-sensitive 2D energy dispersive X-ray (EDX) system was 99 used to obtain bright-field (BF), dark-field (DF) and high angular annual dark-field (HAADF) images in scanning

114
The temperature evolution at the distance representing each microfracture was studied.

138
A few microfractures contain minor amounts of carbonates or phengite.

139
The distribution of plagioclase grain sizes from each microfracture are displayed in

144
A bright field TEM image shows that MF1 contains dislocation-poor and dislocation-free grains of dominantly 145 plagioclase and K-feldspar defined by straight grain boundaries with 120° triple junctions (Fig. 5a). Few grains 146 5 contain dislocations. In contrast, the host plagioclase contains a high density of dislocations that are locally 147 arranged to form a subgrain wall. Ankerite (Ca(Fe,Mg)(CO3)2), grossular-rich garnet and sphene are additional 148 phases in MF1, with apatite and rutile inclusions inside the grains, pinned along grain boundaries and concentrated 149 along the subgrain wall in the host (Fig. 5b).

150
The EDX map of MF1 displays K-feldspar grains with homogeneous composition and plagioclase grains that are 151 heterogeneous with respect to their CaAl and NaSi content (Fig. 5b). The K-feldspar grains are clustered together 152 creating a fabric dominated by grain boundaries instead of phase boundaries. The irregular composition 153 distribution of Na and Ca in the plagioclase grains contradicts the backscatter electron image that suggests Ca 154 zoning around the grains ( Fig. 1d and 5b). Instead, the Ca-rich domains locally overlie areas with submicron 155 lamellae ( Fig. 6a-f). The lamellae are discontinuous throughout the plagioclase grains and, locally, they are 156 superimposed by tapered mechanical twins (Fig. 6a). Other grains contain both lamellae and twins that are spatially 157 distinct but are parallel to each other (Fig. 6d). In some grains, the lamellae appear slightly curved (Fig. 6c)

169
The EDX map of MF2 shows clustered homogeneous K-feldspar grains and zoned plagioclase grains (Fig. 7c)

215
The power-law grain size distributions of the MF1 and MF2 grain populations (Fig. 4), also support relatively 216 rapid recovery as a slow steady state growth process is expected to lead to a log-normal distribution of grain sizes

225
They interpreted the amorphous material to result from shock loading during the propagation of a dynamic rupture.

226
Although their experiments involved a short recovery time (<1 hour) some of the amorphous material 227 recrystallized, creating idiomorphic garnet crystals with a size of ~20 nm.

228
Amorphization of plagioclase feldspar is dependent on pressure (P), temperature (T), composition (X), 229 compression rate (P/t) and pressure duration (t). Amorphization that is strongly dependent on temperature is 230 commonly referred to as heterogeneous amorphization or melting, and is a relatively slow process due to its 231 dependence on the diffusion of atoms (Wolf et al., 1990). On the other hand, amorphization that is strongly 232 dependent on pressure, pressure-induced amorphization, may be static or dynamic depending on the compression 233 rate (Sharma and Sikka, 1996). In the following, we will discuss pressure-induced amorphization.

277
To what extent this fluid production has contributed to the brittle failure of the overlying lower crust is still not 278 well constrained.