Articles written in Journal of Earth System Science
Volume 128 Issue 3 April 2019 Article ID 0051 Research Article
Roundness of survivor clasts (mineral/rock fragments) in fault rocks (e.g., pseudotachylyte and cataclasite/gouge) has been used by some workers to distinguish melt-origin from crush-origin of such rocks. Keeping in view the large overlap in the published data on the roundness of fault rocks, the reliability of such a discriminator appears somewhat uncertain. The present study attempts to reappraise the aforesaid criterion through roundness analysis of quartz and feldspar clasts in melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and fault-related cataclasite (F-Ct) collected from Sarwar–Junia Fault Zone in Rajasthan and from Gavilgarh–Tan Shear Zone in central India. Our analysis shows that roundness of clasts cannot reliably distinguish between fault rocks of melt-origin and crush-origin (especially M-Pt and F-Ct) as the roundness values overlap and a distinct limit of roundness value for each rock type cannot be established. While the roundness of clasts in M-Pt and C-Pt may be enhanced due to melt-induced rounding off of the initially angular clasts, rounding of clasts can also occur by abrasion during rolling of crushed material in F-Ct. Furthermore, anomalous thermal expansion of clasts in melt-origin pseudotachylyte may cause disintegration of larger clasts into smaller angular fragments, thereby increasing the percentage of angular clasts in melt-origin fault rocks. Therefore, roundness of survivor clasts cannot be solely used as a discriminator between melt-origin and crush-origin fault rocks.
Volume 129 All articles Published: 20 October 2020 Article ID 0216 Research article
Quartz/feldspar fragment (‘clast’) sizes were measured in thin sections of three types of fault zone rocks, e.g., melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and cataclasite (Ct), from two well-studied Precambrian shear/fault zones in the Indian craton (e.g., the Gavilgarh–Tan Shear zone in central India and the Sarwar–Junia Fault zone in western India). Logarithmic plots of clast area
$\bullet$ Clast size and cumulative frequency were measured in pseudotachylyte and cataclasite samples from two shear zones.
$\bullet$ Size-frequency relationship follows a power law in melting- and crushing-dominated pseudotachylytes.
$\bullet$ The power law does not hold good for the finest and coarsest size ranges in both types of pseudotachylytes.
$\bullet$ For cataclasite, a multi-fractal power law relationship exists between the size and cumulative frequency of clasts.
Volume 129, 2020
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