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Electron spin resonance dating of fault gouge from Desamangalam, Kerala: Evidence for Quaternary movement in Palghat gap shear zone T K Gundu Rao, C P Rajendran∗, George Mathewy and Biju Johnz R.S.I.C., Indian Institute of Technology, Powai, Mumbai 400 076, India ∗Centre for Earth Science Studies, Akkulam, Thiruvananthapuram 695 031, India yDepartment of Geology, M.S. University of Baroda, Vadodara 390 002, India zGeological Survey of India, Northern Region, Lucknow 226 020, India The field investigations in the epicentral area of the 1994 Wadakkancheri (Desamangalam), Ker- ala, earthquake (M 4.3) indicate subtle, but clearly recognizable expressions of geologically recent fault zone, consisting of fracture sets showing brittle displacement and a gouge zone. The fracture zone confines to the crystalline basement, and is spatially coincident with the elongation of the isoseismals of the 1994 mainshock and a 10-km-long WNW-ESE trending topographic lineament. The preliminary results from the electron spin resonance (ESR) dating on the quartz grains from the fault gouge indicate that the last major faulting in this site occurred 430 43 ka ago. The experiments on different grain sizes of quartz from the gouge showed consistent decrease in age to a plateau of low values, indicating that ESR signals in finer grains were completely zeroed at the time of faulting due to frictional heat. The results show a relatively young age for displacement on the fault that occurs within a Precambrian shear zone. Discrete reactivated faults in such areas may be characterized by low degree of activity, but considering the ESR age of the last significant faulting event, the structure at Desamangalam may be categorized as a potentially active fault capable of generating moderate earthquakes, separated by very long periods of quiescence. 1. Introduction or lack of it cannot be considered as reflective of the real seismic potential. Many faults in this region Geological studies in southern peninsular India remain deceptively quiet for a very long time before have, in recent years, focused on the neotectonics it produces a damaging earthquake, without much and paleoseismicity (Rajendran et al 1996; Sub- warning. The 1993 Killari earthquake is a typical rahmaniya 1996; Valdiya 1998). While these stud- example. Such faults are primary targets of geo- ies have provided new neotectonic interpretations logical studies that aim at revealing the nature of and fault-specific data, little is known about the fault recurrence. absolute age of penultimate faulting. Lack of this However, recognizing active structures in the input seriously hinders proper evaluation of seismic cratonic areas is not easy. A major problem has potential of the faults in question. This problem been the poor development of surface rupture becomes acute in the cratonic hinterland because due to faulting complexities (Rajendran 2000). of the fact that the damaging earthquakes occur on In areas, categorized as stable continental regions pre-existing faults with a recurring period of tens (SCRs), where the recurrence interval is too long of thousands of years (Rajendran et al 1996; Crone and erosional processes too fast, the sedimentary et al 1997), and therefore, the historical seismicity layers containing fault-related structures are easily Keywords. ESR dating; fault gouge; neotectonism; paleoseismology; Palghat gap. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 111, No. 2, June 2002, pp. 103{113 © Printed in India. 103 104 T K Gundu Rao et al removed, thus destroying the surficial evidence of ESR age, T , can be calculated by applying the fol- faulting. Another problem is the subtle nature of lowing formula, the geomorphic indicators, which hinders recog- nition and dating of paleo-earthquakes. Lack of T = AD=D; well-developed sequence of overlying sediment/soil profile is another problem specific to exhumed where, AD is the accumulated dose of natural crystalline rocks within the Precambrian shear radiation in quartz (in Gy) that the sample has zones. Lateritization and the associated weather- accumulated since last resetting/faulting, and can ing processes can also obscure or camouflage most be determined by the additive dose method. The of the deformational features. accumulated dose AD is evaluated by the ESR Our recent studies in the reactivated zones intensity, since the intensity of ESR signal is within a Precambrian shear belt along the Pal- proportional to the number of lattice defects or ghat gap region, however, suggest that evidence of trapped electrons. D is the radiation dose rate geologically young faulting events might be found (Gy/ka) to which the sample has been exposed. in fault breccia and gouge within the fractures, The radiation dose rate D can be determined by present in the upper levels of exhumed crystalline measuring either the concentration of radioactive rocks (John and Rajendran 2002). These intrafault elements within the fault gouge or the radioactivity materials are apparently formed due to frictional by a thermoluminescence dosimeter (TLD) buried sliding of fault blocks, which is always accompa- within the fault gouge. nied by damage and erosion of slip surfaces, a The objective of the present study was to test process that is known as wear, which also gen- the electron spin resonance (ESR) method on erates frictional heat (e.g., see Sibson 1977 and fault gouge, collected from a fault outcrop near Scholz 1990, and references therein, for discussion Desamangalam, Thrissur District, Kerala State on various types of fault rocks and their defor- (figure 1). We discuss the preliminary results of mation mechanisms). Ideally, determining the age ESR experiments to constrain the possible age of of these intrafault materials by appropriate tech- the last faulting event at Desamangalam, and its niques should yield the date of the last faulting implications for neotectonic activity. event. Different methods including fission track dating, K-Ar and Rb-Sr methods have been employed to 3. Geology of the study area constrain the age of the intrafault material. But these methods are limited by their lack of resolv- Our study area lies in the southern flank of the ing power for rocks younger than 2 Ma (see Noller Palghat gap, a conspicuous E-W trending geo- et al 2000 and references therein). Zeller (1968) first morphic feature within in the southern gran- introduced electron spin resonance (ESR) method ulite terrain (figure 1). Both macroscopic and that could be used to determine the age of geolog- microscopic features suggest that this region is ically younger materials. Ikeya et al (1982) devel- a Precambrian shear zone, defined by a large E- oped this method further, and demonstrated that W-dextural oblique-slip component (D'Cruz et al the timing of fault movement with a maximum 2000). Thermobarometric studies indicate large- range of 1 Ma could be determined by electron scale tectonic exhumation of lower to mid-crustal spin resonance (ESR) using E0 (e-prime) and OHC rocks within this shear zone (Ravindrakumar and (oxygen hole-) centres in quartz. Chacko 1994). The present topography, consist- ing of a 250-km-long and 30-km-wide low land bordered by ridges and hills, is believed to have 2. ESR method resulted from the combined action of shearing and erosion. An antecedent river, called Bharathapuzha, In the dating of fault gouge material, it is essential occupies the middle part of the low land, whose that the ESR signals of inherent defect centres in course is apparently controlled by the linearity of the quartz grains reset to zero. The stress and the the shear zone. frictional temperature active on the fault plane at The major rock type at Desamangalam is the time of faulting are expected to reset the ESR charnockite, and it shows well-developed south dip- signals of several centres to zero level. After the ping foliation trending in WNW-ESE direction. faulting episode, the ionizing radiations regener- Extensive joint sets have also developed through- ate signals due to natural radioactivity that grows out the area; biotite-rich foliations serve as the monotonically with increasing dose in a known weaker planes for the development of these joints. manner. The final concentration of defects is pro- A few exposures show 3{5-cm-thick patches of portional to the total amount of natural radiation biotite schist, which are oriented parallel to the received by the sample after the faulting event. The regional foliation direction. Quartzo-feldspathic Electron spin resonance dating of fault gouge 105 Figure 1(a). Index map of study area. Lateral extent of the Palghat gap shear zone is shown by dashed line (after Subrahmaniam and Muraleedharan 1985). W XZY Y]\ [ ££ ¢¡§ Y Y ^ _ `ba c OQPRTV>PU )¢*,+-.+,/*+02143,*+6587 94:- Y d ¢¡¤£¥£¦¡¨§ ©¢ [ c d e E ££¦¡¨¤¡¨( ¡¨§ f%gih8jj¦k "#!$¡¨§%¡¨'&'¡( ¡¨§ jh lmnAm¢o np4q r hw s qut'v v¢tx OQPRTS>PU ;<+>=>+6?@?A+BDC,*:-¦7 587 94:- ¡¡¡¤¤ ¤! FGH.NNL FGH¦IKJ2L FG2H.MN2L Figure 1(b). Map showing linear structures of the area, prepared using satellite imagery, air photos and topographic maps. Solid arrows denote inferred direction of compressional stresses. veins have intruded into the parent rocks that Palghat gap region, dated at 484{512 Ma (Soman occur both parallel and perpendicular to the folia- et al 1990). We observed numerous semi-brittle tion direction. These veins may have been associ- offsets (0.5 m) in these veins. Most of them are ated with a major phase of emplacement in the oriented perpendicular to the foliation direction 106 T K Gundu Rao et al (WNW-ESE) and show a dextral strike-slip sense River, near Desamangalam (figure 1), is believed to of movement, indicating that the region has be a structural feature (Rajendran and Rajendran undergone several cycles of Precambrian tecton- 1996). The intensity survey conducted immediately ics. Alternatively, it may also suggest occurrence after the earthquake indicated that the activity of subsidiary faults in association with major E-W was concentrated along the southern bank of the trending fault zone.

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