INTRACRYSTALLINE DEFORMATION IN NATURAL EXPLOSION STRUCTURES RESULTING FROM EXPANDING VAPOR EXPLOSIONS IN HYDROTHERMAL SYSTEMS

We have been studying a newly identified geological structure formed by expanding vapor explosions, which occur when superheated fluids rapidly ascend through the crust and reach their boiling point in the subsurface (Chan and Alvarez, 2023). Deformation lamellae (DL) in quartz, resulting from intracrystalline deformation, were identified in rocks showing outcrop-scale explosion structures, indicating the involvement of high stresses and strain rates during their formation. The characteristics of DL in quartz grains from an explosion-affected rock sample (KO, Hong Kong) were studied and compared with intracrystalline deformation features in two samples: one from a meteorite impact structure (VR6, Vredefort; extreme high pressure and strain rates) and another from greenschist-facies metamorphism (BRG1127, Bergell Alps; moderate pressure and low strain rates). The petrographic study recorded the planarity, spacing, continuity, and morphology of the DL, and the proportion of grains exhibiting DL. KO revealed predominantly wavy, bifurcating DL with spacings ranging from 3 to 7 µm, and the proportion of DL-bearing grains varied with the sample position within the structure. BRG1127 showed a higher density of DL-bearing grains (320 grains/cm²) and an average DL spacing of 5.4 µm, while VR6 displayed fewer DL-bearing grains (41 grains/cm²) and an average DL spacing of 4 µm. The crystallographic orientations of the DL relative to the quartz C-basal plane (0001) also differed significantly. In VR6, the orientations of the DL deviated only slightly from (0001) while BRG1127 displayed a broader range (0°–35°), with the DL consistently intersecting the rock foliation at specific angles. In contrast, KO exhibited widely distributed angles between the DL and (0001). These findings indicate that the DL observed in the explosion structures are distinctly different from planar deformation features resulting from high-velocity impacts. Also, the characteristics and crystallographic orientation patterns of the DL can be used for identifying their formation mechanisms. The study has enhanced our understanding of the dynamic roles played by hydrothermal fluid in the crust, as well as the processes leading to formation of hot springs in non-volcanic regions.

Chan, L.S. & Alvarez, W., 2023, J. Struct. Geol., 174, 104933