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10.
11.
12.
13.
14.
15.
16.
International Congress on Plant Physiology, IARI, New Delhi, 8–12
January 2003.
Shashidhar, G., Sheshshayee, M. S., Shankar, A. G., Bindumadhava, H., Nadaradjan, S., Prasad, T. G. and Udayakumar, M., Genetic
variability in water use efficiency and transpiration rate based on a
stable isotope approach among diverse groundnut germplasm
lines. In Paper presented at the 2nd International Congress on
Plant Physiology, IARI, New Delhi, 8–12 January 2003.
Udayakumar, M., Sheshshayee, M. S. Bindumadhava, H., Anil
Koushik, Raju, Y., Janardhan, K. V. and Prasad, T. G., Assessment
of genetic variability in mean transpiration rate (MTR) based on
∆ 18O bm in field established cashew accessions. J. Plant. Biol. (in
press).
Anon., Tea Reports, 2000, pp. 3–7.
Tiaz and Zieger, Plant Physiology (3rd edn), Academic Press,
USA, 2002.
Bindumadhava, H., Sheshshayee, M. S., Shashidhar, G., Prasad, T.
G. and Udaya Kumar, M., The ratio of carbon and oxygen stable
isotopic composition (∆ 13C/∆ 18O) describes the variability in leaf
intrinsic carboxylation efficiency in plants. Curr. Sci., 2005, 89,
1256–1258.
Kramer, P. J., In Adaptation of Plant to Water and High Temperature Stress (eds Turner, N. C. and Kramer, P. J.), John Wiley and
Sons, New York, 2000, pp. 7–20.
Angus, J. F. and van Herwaarden, A. F., Increasing water use and
water use efficiency in dry land wheat. Agron. J., 2001, 93, 290–
298.
ACKNOWLEDGEMENTS. We thank Mr G. M. Hedge, General
Manager, HLL Tea Plantations, Valparai, and Assistant Managers, Dr
Chandramouli and Mr Uma Shanker, for research support to carry out
this work at Stanmore estate, Coimbatore. We also thank Dr M. S.
Sheshshayee, Department of Crop Physiology, UAS, GKVK, Bangalore
for help in analysing tea samples for oxygen isotopes. The research
grant provided by Hindustan Lever Ltd, Research Centre, Bangalore, is
acknowledged.
Received 14 March 2006; revised accepted 29 June 2006
Thickness estimation of Deccan
Flood Basalt of the Koyna Area,
Maharashtra (India) from inversion
of aeromagnetic and gravity data and
implications for recurring seismic
activity
G. K. Nayak*, P. K. Agrawal, Ch. Rama Rao and
O. P. Pandey
National Geophysical Research Institute, Hyderabad 500 007, India
Thickness estimation of volcanic suite and delineation
of underlying Achaean basement topography using
geophysical methods have always been a challengingproblem confronting the geoscientific community. In
most cases, their estimations are unsatisfactory due to
*For correspondence. (e-mail: [email protected])
960
lack of quality dataset or inverse geological situation,
where high susceptibility/velocity rocks at the surface
are underlain by low susceptibility/velocity rocks. In
order to circumvent the above situation, an inversion
scheme has been attempted to model aeromagnetic
and gravity datasets acquired over the seismically active
Koyna region situated over the Deccan Traps of western Maharashtra. Inversion of aeromagnetic data results into a Deccan basalt thickness of about 1500 m
below the Koyna region. Further, inversion of gravity
data indicates that the entire column of lava below
this region is made up of non-massive vesicular type of
basalts having a low density of 2.58 g/cm3 and a porosity
of about 17%. Presence of vesicles, faults and fractures within the porous basaltic column appears to facilitate the diffusion of fluid in the surrounding medium
and in the basement, thus causing the reactivation of
faults which may be responsible for recurring seismic
activity in this region.
Keywords: Aeromagnetic, gravity, inversion, induced
seismicity, Koyna.
THE Koyna region of Maharashtra (Figure 1) assumed great
importance globally among geoscientists after the occurrence of an earthquake with M ~ 6.5 on 11 December 1967.
This region, considered to be a part of hitherto believed
aseismic Indian peninsular shield, suddenly gained
prominence after this earthquake and resulted in the accumulation of vast quantity of geophysical and geological data
to (i) understand the nature and physical characteristics of
the Pre-Deccan Trap topography which existed before extrusion of Deccan volcanism, and (ii) delineate the subsurface
structural and tectonic configurations which may hold
clues to the occurrence of the devastating earthquake. Recent
analysis of geophysical datasets such as gravity, magnetic,
deep electrical resistivity, magnetotellurics, seismics, etc.1–7
has thrown significant light on the seismotectonics of Koyna
Seismic Zone (KSZ). However, there does not seem to be
any consensus on the cause of recurring seismic activity
so far in this region.
Recently, Pandey and Chadha8, based on pore fluid
pressure study, concluded that the diffusion process
within the volcanic lavas and to some extent within the
basement has been quite prevalent, which facilitates reactivation of pre-existing faults causing earthquakes. A detailed
magnetotelluric (MT) sounding study over this seismic
zone7 found a low apparent resistivity of 40 to 150 ohm-m,
which compares with the resistivity of non-massive basalts.
In contrast, the underlying basement is found to have
high resistivity range of 5000–20,000 ohm-m. Basaltic
thickness in this region was estimated to be 1.5 km.
Thickness estimation of such rock types and delineation
of basement topography from the potential field data have
always been difficult due to (i) high velocity and highly
randomly magnetized suite of basaltic rocks underlain by
low velocity, low magnetic susceptibility granitic-gneissic
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Figure 1. Residual total intensity aeromagnetic map of Koyna and its surrounding area recorded at 2134 m amsl.
Areal extent of the 65-Ma-old Deccan Traps is represented by dots. Study area is shown by rectangle.
Figure 2. Reproduced aeromagnetic map of the study area (contour
interval 10 nT) obtained after digitization of contour map within rectangular portion (Figure 1). AB is the profile along which anomaly is
plotted in Figure 5.
basement, (ii) high Qn ratio (ratio of remanant to induced
magnetization) varying from 1 to 100, and (iii) possibility
of several magnetic reversals during total extrusion of
lavas. To solve these problems, Negi et al. 4 used spectral
technique on the available aeromagnetic data over and
around the earthquake-affected Koyna region. They gave
a 3D block model for an area of 100 km × 50 km and inferred
a basaltic thickness of 1400 m in and around the KSZ,
which was more or less consistent with other studies1,9.
With recent advancements in the methodology, dataprocessing and interpretation techniques, we attempted
here to re-examine the available potential field data over
CURRENT SCIENCE, VOL. 91, NO. 7, 10 OCTOBER 2006
the KSZ using 2D and 3D inversion scheme, which sheds
new light on the recurring seismic activity of this region.
The aeromagnetic data have been acquired at a constant
flight height of 2134 m amsl during March 1974, along
13 short E–W traverses of 100 km length with a separation
of 4 km around the Koyna region by the National Geophysical Research Institute (NGRI), Hyderabad. A rubidium
vapour magnetometer was employed to record the data.
In order to record the time-varying magnetic field of the
earth, a proton precession magnetometer was used in the
base camp. The position location of the aircraft was achieved
by the visual navigation with aid of the Survey of India
toposheet with a scale of 1 inch = 1 mile. After diurnal
correction and regional separation, the residual map with
a contour interval of 20 nT was prepared (Figure 1) and
an area of 22 km × 16 km (~ 350 sq. km) in dimension was
selected from this residual map for detailed analysis, which
represents the seismically active Koyna region of western
India. The selected region was then digitized using such a
digitization interval that the data can faithfully represent
actual anomaly pattern without any noticeable distortion.
Figure 2 represents the re-plotted anomaly with a contour
interval of 10 nT obtained from digitized data. The finite
dimensional anomaly shows that the source of this type of
anomaly probably indicates 3D prismatic type of body
having limited width, length and thickness.
The gravity data used in the present study (Figure 3) is
derived from contour map given by Kailasam et al.1. Figure 3
shows that the entire Koyna area is characterized by highorder negative Bouguer gravity anomaly (–112 mGal).
This anomaly has been earlier ascribed to the presence of
low-density material in the upper mantle10. However, this
negative anomaly over the KSZ needs careful treatment as
it may be caused by cumulative effects of three components,
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Table 1.
Estimated thickness of Deccan Basalt over Koyna region using different geophysical methods
Deccan trap thickness (m)
1200
1250
1200
1436
1100
~ 1200
1500
Method
Deep electrical study
Deep electrical study
Gravity
Aeromagnetics
Flight height (2314 m)
DSS
Magnetotellurics
Inversion of aeromagnetic data
Figure 3. Bouguer gravity anomaly distribution over Koyna region1,31.
Contour interval is 1 mGal. CD is the profile along which gravity
anomaly is modelled (Figures 6 and 7).
viz. (i) the strong isostatic anomaly of about –90 mGal10 ,
(ii) the effect of strong negative gravity anomaly
(–16 mGal) arising due to the south Indian Ocean gravity
low as revealed by the satellite-derived gravity data11 ,
and (iii) due to the density contrast confined within the
shallow subsurface region. Thus the effects of isostatic
and south Indian Ocean gravity low components from the
observed gravity anomaly are removed appropriately to
arrive at the residual anomaly for further analysis.
Both, the aeromagnetic anomaly map as represented in
Figure 2 and the residual gravity anomaly obtained along
CD after applying appropriate correction to the anomaly
map shown in Figure 3, have been utilized for 3D and 2D
inversion respectively, following the inversion schemes
of Radhakrishna Murthy12. In case of aeromagnetic data
inversion the assumed palaeomagnetic parameters are: inclination (I) = –34°, declination (D) = 130° and K = 2.5 ×
10–3 CGS units, which are based on laboratory measurements13.
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Source
Kailasam et al.2 from deep electrical studies
Athavale and Indra Mohan13
Guha et al. 32
Negi et al. 4
Kaila et al. 9
Sarma et al. 7
Present study
After inversion of aeromagnetic anomaly shown in Figure
2, the acceptable solution yields the depth to the top and
bottom of the possible source causing this anomaly to be
1800 m and 3300 m from the sensor respectively. The
magnetic response calculated from the model parameter
is contoured and shown in Figure 4, which matches well
with the actual magnetic data (Figure 2). The amplitudes
of the observed and calculated anomalies are 180 and 170 nT
respectively. A graph showing observed and calculated
anomaly along the section AB (Figures 2 and 4) reveals
close conformity, as shown in Figure 5. Thus the present
result of aeromagnetic data inversion, using the abovementioned method, reveals the thickness of the body causing the magnetic anomaly to be 1500 m, which corresponds
to trap thickness below the KSZ. This estimate is in
agreement with the available findings from other methods
(Table 1).
Using this estimation of Deccan Trap cover, we derive
the subsurface basement topography under the traps from
the residual gravity data along the line CD shown in Figure 3.
The obtained result of 2D inversion provides basement
topography as shown in Figure 6. In the present case, a
satisfactory match between the observed and calculated
anomalies is obtained using densities of the overlying basalt
as 2.58 g/cm3 and that of the underlying basement as
2.76 g/cm3.
The KSZ has been the subject of numerous studies since
almost four decades. However till today, no viable mechanism
has been put forward for the recurring seismic activity.
Seismicity induced by impounding of water in Shivaji
Sagar lake, still remains one of the most accepted causes,
based on well-studied, long term seismic data14–18. Besides,
to understand the seismotectonics and crustal structure of
this region, several other studies were also undertaken3,7,19–21.
However, in many of these studies the problem of recurring seismic activity has not been dealt with in totality.
For example, based on 3D P-wave velocity study of this
region, the possibility of an igneous intrusion in the crust
was suggested19. In this analysis, residual gravity anomaly is said to be positive, but in that case it will be difficult to explain the high order negative gravity anomaly of
–112 mGal over Koyna. The positive residual gravity
anomaly, as referred by Srinagesh et al. 19 , is based on a
method where regional–residual separation is done using
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zero free air anomalies. This method has a drawback in this
region because there is no intersection between the zero
free air anomaly and Bouguer anomaly within 100 km of
this region. Their interpretation has also been questioned by
Rajendran22. The fact is that the travel-time modelling of the
subcrustal lithosphere beneath a deep seismic sounding
traverse over Koyna region, indicates the presence of several
low-velocity layers at crustal as well as subcrustal depths23.
Our finding of 1500 m thick basaltic column below
Koyna by inversion of aeromagnetic data is compatible
with the known estimates (Table 1). However, this thickness
estimate would satisfy the gravity data only when we
adopted a density of 2.58 g/cm3 for the overlying basaltic
column (Figure 6), which is much lower than the known
densities of basalts. Interestingly, recent bore-hole density
measurements24 across a 338 m thick basaltic column
within the Deccan Trap at Latur (18°10′N, 76°35′E) sug-
gests that 53% basalts are of massive variety, while 47% are
of vesicular and amygdaloidal (non-massive) variety. Average wet density of the non-massive basalt is measured at
2.56 g/cm3, which is close to our inferred density of
2.58 g/cm3 below Koyna (Figure 6). The prevalent ideas of
compact massive basalt consisting of the entire thickness
of individual flows, and amygdaloidal basalts only at the
top and bottom, has been argued25. In fact, in a thick section
from Bor Ghat (near Pune), out of 475 m exposed lava
flows, 314 m is of vesicular and amygdaloidal type25 .
Thus, the presence of amygdaloidal lavas is not a freak
occurrence. It is predominant over large areas of western
Maharashtra.
However, if we choose an average wet density of
2.72 g/cm3 for the entire lava pile 24 in the gravity model,
we get an unrealistically large basaltic thickness of more
than 9 km below the Koyna region (Figure 7), which does
not conform to known estimates (Table 1) as well as those
obtained in this study. Further, calculated porosity of basalt
is about 16.8%, which is close to the known average porosity of vesicular and amygdaloidal basalt26 .
There could be a possibility that the trap rocks below
Koyna region may be predominantly non-massive, lowdensity vesicular basalts with an average porosity of about
17% arising due to the presence of vesicles and fractures,
which facilitate migration of water in the surrounding
medium as well as in faulted and fractured basements below the KSZ. Several major lineaments/faults are known to
intersect each other near the KSZ (Figure 8), which is
also neotectonically uplifting 6. This inference is also supported by MT measurements, which reveal a resistivity of
40–150 ohm-m for trap rocks below Koyna 7, corresponding to 100% water-saturated vesicular basalts27 .
Figure 4. Calculated magnetic response obtained from model parameters. Contour intervals are 10 nT. AB is the profile along which the
anomaly is plotted in Figure 5.
Figure 5. Observed and calculated magnetic response of the body
(trap thickness) along profile AB.
CURRENT SCIENCE, VOL. 91, NO. 7, 10 OCTOBER 2006
Figure 6. Residual Bouguer gravity anomaly (after removing the effect of isostatic and south Indian Ocean gravity low) and basaltic thickness
along profile CD (Figure 3) beneath Koyna, using a density contrast of
0.18 g/cm3 between trap rocks (2.58 g/cm3 ) and granitic-gneissic basement (2.76 g/cm3 ). a, Observed anomaly; b, Fitted anomaly.
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Figure 7. Residual Bouguer gravity anomaly (after removing the effect of isostatic and south Indian Ocean gravity low) and basaltic thickness
along profile CD (Figure 3) beneath Koyna, using a density contrast of
0.04 g/cm3 between trap rocks (2.72 g/cm3 ) and granitic-gneissic basement (2.76 g/cm3 ). a, Observed anomaly; b, Fitted anomaly.
Figure 8. Tectonic and structural features around KSZ derived from
aeromagnetic data (solid line) and satellite imagery maps (broken line) 6 .
This may be the reason why the diffusion process is so
prevalent in the vicinity of the Koyna reservoir which
creates changes in pore fluid pressure causing failure of
pre-existing critically stressed faults8 and leading to recurring earthquake occurrences.
It is quite likely that the Koyna region may have been a
sagging rift28 or a basinal structure before the eruption of
Deccan lavas in which explosive volcanic material were
deposited in quick succession. According to Courtillot 29 ,
the major eruptive phase of Deccan Trap probably lasted
964
for only about 10,000 years and according to Negi et al.30 the
Deccan Trap suddenly erupted at the K–T boundary due
to an asteroidal impact near Mumbai on the west coast. In
either case, sudden cooling of basaltic material would
have been eminent thereby leading to the formation of
vesicles.
Hence, based on the application of inversion scheme to
available aeromagnetic and gravity data over the Deccan
flood basalts of Koyna region, a basaltic thickness of
1500 m is concluded. It also appears that the underlying
basaltic/volcanic rocks are made up of highly porous and
vesicular-type lavas having a much lower density of
2.58 g/cm3. Migration of water in these rocks is high,
which creates changes in pore fluid pressure enabling
failure of existing critically stressed faults, leading to recurring seismic activity. There are 25,000 reported events
of magnitude < 3.0 and several above this during the past
four decades.
1. Kailasam, L. N., Murthy, B. G. K. and Chayanulu, A. Y. S. R.,
Regional gravity studies of the Deccan Trap area of peninsular India.
Curr. Sci., 1972, 41, 403–407.
2. Kailasam, L. N., Reddy, A. G. B., Rao, J. M. V., Satyamurthy, K.,
and Murthy, B. S. R., Deep electrical resistivity soundings in the
Deccan Trap region. Curr. Sci., 1976, 45, 9–13.
3. Kaila, K. L., Reddy, P. R., Dixit, M. M. and Lazrenko, Deep
crustal structure of Koyna, Maharashtra indicated by deep seismic
soundings. J. Geol. Soc. India, 1981, 22, 1–16.
4. Negi, J. G., Agarwal, P. K. and Rao, K. N. N., Three-dimensional
model of Koyna area of Maharashtra State (India) based on spectral
analysis of aeromagnetic data. Geophysics, 1983, 48, 964–974.
5. Rastogi, B. K. and Mandal, P., Foreshocks and nucleation of
small-to-medium sized Koyna earthquakes (India). Bull. Seismol.
Soc. Am., 1999, 89, 829–836.
6. Agarwal, P. K., Pandey, O. P. and Chetty, T. R. K., Aeromagnetic
anomalies, lineaments and seismicity in Koyna–Warna region. J.
Indian Geophys. Union, 2004, 8, 229–242.
7. Sarma, S. V. S., Prasanta, P. B. K., Harinarayana, T., Veeraswamy, K., Sastry, R. S. and Sarma, M. V. C., A magnetotelluric
(MT) study across the Koyna seismic zone, western India: evidence
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8. Pandey, A. P. and Chadha, R. K., Surface loading and triggered
earthquakes in the Koyna–Warna region, western India. Phys.
Earth Planet. Inter., 2003, 139, 207–223.
9. Kaila, K. L., Reddy, P. R., Murthy, P. R. K. and Tripathi, K. M.,
Technical report on deep seismic soundings studies along Koyna I
and Koyna II profiles in Deccan Trap covered area, Maharashtra
State, India. Report, NGRI, November 1979.
10. Tiwari, V. M., Vyaghreswara Rao, M. B. S. and Mishra, D. C.,
Density inhomogeneities beneath Deccan Volcanic Provinces, India
as derived from gravity data. J. Geodyn., 2001, 31, 1–17.
11. Subba Rao, D. V., Resolving Bouguer anomalies in continents – A
new approach. Geophys. Res. Lett., 1996, 23, 3543–3546.
12. Radhakrishna Murty, I. V. (ed.), Gravity and Magnetic Interpretation in Geophysical Exploration, Geological Society of India
Memoir No. 40, Bangalore, 1998, p. 360.
13. Athavale, R. N. and Indra Mohan, A technical report on integrated
geophysical study in Koyna hydraulic project area of Maharashtra
State, India, 1976.
14. Rastogi, B. K. et al., Seismicity at Warna reservoir (near Koyna)
through 1995. Bull. Seismol. Soc. Am., 1997, 87, 1484–1494.
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15. Gupta, H. K. and Rastogi, B. K., Dams and Earthquakes, Elsevier,
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17. Talwani, P., Kumar Swamy, S. V. and Sawalwade, C. B., The revaluation of seismicity data in Koyna–Warna area, 1963–95.
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Verma, O. P.), DST’s Spl. Publ. Indian Geological Congress, 2001,
vol. 2, pp. 21–37.
19. Srinagesh, D., Singh, S., Reddy, K. S., Prakasam, K. S. and Rai, S.
S., Evidence for high velocity in Koyna seismic zone from P-wave
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20. Visweswara Rao, C. and Rajendra Prasad, An interactive computer package for modeling the crustal structure from Bouguer
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21. Naik, P. K., Awasthi, A. K., Anand, A. and Mohan, P. C., Hydrogeologic framework of the Deccan Terrain of Koyna river basin,
India. Hydrogeol. J., 2001, 9, 243–264.
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26. Evaluation of Aquifer Parameter, Central Ground Water Board,
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32. Guha, S. K., Gosavi, P. D., Krishna Nand, Padale, J. G. and Marwadi, S. C., Koyna earthquake 1963 to December 1973: Central
Water and Power Research Station, Khadakwasla, Poona, India,
1974.
ACKNOWLEDGEMENTS. We thank Dr V. P. Dimri, Director,
NGRI, Hyderabad for encouragement to take up this work and permission to publish the paper. Thanks are also due to Mr B. Vyaghreswarudu for drafting the figures and Mr V. Subrahmanyam for help in the
preparation of the manuscript. The anonymous reviewers are also
thanked for their positive suggestions which improved the manuscript
considerably.
Sex ratio, population structure and
roost fidelity in a free-ranging
colony of Indian false vampire bat,
Megaderma lyra
H. Raghuram*, Balaji Chattopadhyay,
P. Thiruchenthil Nathan and K. Sripathi
Department of Animal Behaviour and Physiology, School of Biological
Sciences, Madurai Kamaraj University, Madurai 625 021, India
We studied sex ratio, population structure and roost
fidelity in Indian false vampire bat, Megaderma lyra
for four years using mark-recapture method, in a freeranging colony at Pannian cave, Madurai, South India.
Jolly–Seber analysis of mark-recapture data showed
variable fluctuation in population size of both sexes.
The population size from 2001 to 2004 varied from
138 to 37 for males and 213 to 61 for females. In all the
years, females outnumbered males and sex ratio ranged
between 0.2 and 0.3. Compared to males, females exhibited low roost fidelity, and also showed high percentage of emigration across four years. However, there is
no significant difference in percentage of immigration
between the sexes. We predict that bats exhibit sexually
dimorphic dispersal behaviour that depends on time
and space, similar to other mammals and we hypothesize a few reasons for this dispersal, including population
density, habitat destruction and inbreeding avoidance.
Keywords: Dispersion, Megaderma lyra, population structure, roost fidelity, sex ratio.
E VOLUTIONARY theory predicts that most populations
should consist of roughly as many males as females. Females
in relatively good physiological condition should produce
offspring of the more expensive sex, if the increased allocation is likely to benefit the fitness of offspring more
than it would benefit the cheaper sex1. Thus, the ability to
control the sex of her offspring could be of survival value to
a mother. Sex ratio variation is observed in many mammals
such as red deer, soay sheep, rhesus monkeys and rodents.
In ungulates, the maternal condition is one of the important
factors determining the sex ratio of a population2,3. For
example, in polygynous red deer Cervus elephus, maternal
dominance governs the sex ratio of offspring, where the
dominant mothers produce significantly more number of
sons than their subordinates2. However, increasing population
size decreases number of males born to them4. Similarly,
in soay sheep Ovis aries, sex-ratio variations are due to
maternal condition and are independent of ecological variables such as population growth rate and weather conditions3.
Received 8 December 2005; revised accepted 13 June 2006
*For correspondence. (e-mail: [email protected])
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