Nanometer quartz grains and rapid cooling melt in fault gouge during earthquake process - observed from the WFSD-1 drilling core sample

Abstract:

Drilling into active faults is an effective way to get data and materials at depth that help to understand the material properties, physical mechanisms and healing processes of the faults. The Wenchuan earthquake fault scientific drilling project (WFSD) was conducted immediately after the 2008 Wenchuan earthquake (Mw 7.9). The first borehole of the project (WFSD-1) penetrates the Yingxiu-Beichuan fault with a final depth of 1201.15 m and meet the principal slip zone (PSZ) of Wenchuan earthquake at depth of 589.2 m. About 183.3 m-thick fault rocks are recognized in the WFSD-1 drilling core from 575.7 to 759 m-depth, which was confirmed as the Yingxiu-Beichuan fault zone with a real thickness of about 100 m due to the borehole inclination of 11°.

In this research we got samples from WFSD-1 drilling core at the depth of 732.4-732.8 m, in which black gouge, gray gouge, fine-grained fault breccia and coarse-grained fault breccia layers can be distinguished clearly. Slickensides were developed in the surface of the black gouge layer. The protolith of this segment is sandstone. Based on detailed microstructural analysis using electron optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). An about 1 mm-thick amorphous material layer containing small quartz grains was observed. Circles with different densities were observed in the amorphous material indicate a melt-origin. Cracks are developed in the amorphous material, which are suggested to be caused by general volume reduction as a result of rapid cooling contraction. TEM-EDX analysis of the amorphous material indicates mainly feldspar composition, implying the melting temperature was >1230℃, while quartz grains did not melt indicating a temperature <1700℃. Nano-scale quartz grains were observed in a very small layer showing a different structure at the edge of the amorphous layer, indicating that nano quartz grains were formed by the comminution during earthquake, which is very important in earthquake energy budgets calculation.

These microstructural analysis results reveal that the amorphous layer may formed by rapid cooling of the frictional melt material caused by high-velocity slip during a large earthquake, and fluid flow may played an important role in the rapid cooling process.