Size distribution of survivor clasts in pseudotachylyte and cataclasite: Implications for crushing and melting processes
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Indian Academy of Sciences (0123456789().,-volV)(0123456789( ).,-volV)
Size distribution of survivor clasts in pseudotachylyte and cataclasite: Implications for crushing and melting processes in seismic fault zones ARINDAM SARKAR1, DIPANJAN BHATTACHARJEE2 and ANUPAM CHATTOPADHYAY1,* 1 Department 2
of Geology, University of Delhi, Delhi, India. Department of Earth Science, Indira Gandhi National Tribal University, Amarkantak, MP, India. *Corresponding author. e-mail: [email protected] MS received 10 October 2019; revised 17 June 2020; accepted 11 July 2020
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 vs. cumulative frequency in the pseudotachylytes demonstrate a fractal clastsize distribution (c.s.d.) for the intermediate size range, whereas the Bner and coarser clast size fractions clearly deviate from the fractal trend. Under-representation of the Bner size clasts in the pseudotachylyte samples may be attributed to their preferential melting and removal from the clast population. The relative paucity of coarse clasts, on the other hand, is possibly due to a sampling bias against coarse clasts. The c.s.d of the cataclastic rock shows a multi-fractal character with two different slopes (i.e., lower D-value for Bner clast sizes) and absence of the left-hand (Bner size) fall oA. This suggests less eDcient crushing in the Bner clast size fraction. The proportion of clasts, compared to the matrix, is very small in M-Pt, increases in C-Pt and is highest in Ct, suggesting that melting of rock/mineral fragments is a dominant process in forming M-Pt, whereas it is less significant in C-Pt, and is absent in Ct, which corroborates the microscopic observations. Keywords. Pseudotachylyte; cataclasite; fragmentation; melting; clast size analysis; fractal.
1. Introduction Fault zones are characterized by intense crushing and milling of host rocks, forming a variety of fault zone rocks, e.g., cataclasite, fault breccia and gouge (Sibson 1977; Wise et al. 1984; Woodcock and Mort 2008). In addition, frictional heating along the fault plane may lead to partial melting of the host rock to generate pseudotachylyte (Pt) – a dark coloured aphanitic rock in which lithic/mineral fragments are dispersed within an ultraBne-grained, glassy matrix (Sibson 1975; Maddock 1983). Presence of
glass is not ubiquitous in pseudotachylyte as quite often glass devitriBes and alters to secondary phases (Kirkpatrick and Rowe 2013) or recrystallize to new mineral phases during subsequent deformation and metamorphism of the host rock (Lin 2008; Chattopadhyay et al. 2008; Price et al. 2012). The absence of glass phase in most of the studied pseudotachylytes initially le
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