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The structure of the Kohistan Arc terrane in northern Pakistan as inferred from gravity data—Reply
351
Ebblin, dus,
C., Marussi,
Boll. Geofis. Gansser,
Teor. Appl.,
Karakorum. terrane
G., Rahim,
measurements
1986. The structure
in Northern
Tectonophysics,
Pakistan
the Himalaya
Arc
from gravity
data.
delle zone orogenetiche Kush-Pamir.
del Kashmir
In: Geotettonica Naz. Lincei,
A., 1983. Geophysical
and evolution
R.A.K.,
Boll. Geofis.
granitization
1982. Geology
Hindukush
Teor. Appl.,
of bathohtic
dimen-
in Pakistan.
of the Himalaya Geol.
Karakorum
Bull. Univ.
Peshawar,
15: 51 pp. D.N., 1931. The syntaxis
Its rocks, tectonics
Rome,
Zeitler,
P.K.,
R.A.K., trends
and Karakorum.
443-461.
of the North-West
and orogeny.
Himalaya.
Geol. Surv. India Rec. 65:
189-220.
Himalaya-Karakor-
Proc. Accad.
pp. 131-137. Marussi,
and Wadia,
in the Karakorum.
syntaxis
sions. Am. J. Sei., 247: 211-245. Tahirkheli,
13: 6-9.
124: 297-307.
A., 1976. Gravity
urn-Hindu
and
of the Kohistan
as inferred
Pamirs
25 (99/100):
Miscb, P., 1949. Metasomatic
303-316.
between
Geol. Bull. Univ. Peshawar, L.L.,
S.M. and Richarin the Karakorum.
25 (99/100):
A., 1980. The division
Mahnconico,
Marussi,
A., Poretti,
P., 1983. Gravity
of the
Johnson,
N.M.,
1982. Fission-track
of the Nanga
Parbat
Naeser, evidence
region, Pakistan.
C.W.
and
Tahirkheli,
for Quatemary Nature,
uplift
298: 255-257.
The structure of the Kohistan Arc terrane in northern Pakistan as inferred from gravity data-Reply LAWRENCE L. MALINCONICO
(Received
February
23.1987;
Introduction I appreciate Dr. Ebblin taking the time to comment my gravity investigations conducted across the major structures associated with the Kohistan Arc terrane in the northwestern Himalaya. Many of his comments are very valid points that need to be addressed‘by further investigation. I would, however, like to reply to several specific points that were made in the discussion. The data I must respectfully disagree with Dr. Ebblin when he says that the spacing of the observations is not important. Modeling of gravity anomalies is very sensitive to small changes in the slope of the anomaly. This is especially true for large amplitude variations such as these which occur across the Main Mantle Thrust (M.M.T.). The figures reproduced in the original article do not do justice
accepted
Jr.
April 23,1987)
to the degree to which the observed and modeled anomalies were actually matched. While there may have been sufficient spacing of the data obtained by Ebblin (1982) along the Indus River, the data did not extend far enough to the south to allow the approximation
of a regional
trend
that could
be used to isolate the effects caused by the M.M.T. and the adjacent basic and ultrabasic rocks of the Jijal
Complex
(Jan
and
Howie,
(1982) has discussed the character high associated with the M.M.T.
1981).
Ebblin
of the gravity in the lndus
Valley and does compare that to the gravity field slightly to the east {near Nanga Parbat). He has not, however, actually modeled the effect, but simply speculated upon the possible causes. The density model presented (Malinconico, 1986, fig. 3c) shows that the gravity high is probably the result of the accumulation of the high density ultramafic and garnet granulite rocks (Jan and Howie, 1981) just north of the M.M.T. It is also very important
to construct
structural
models
of the M.M.T.
ble and not simply Indus
in as many places as possi-
on a single transect
River. How does the character
zone change
towards
along the
of the suture
the west? While
there
cer-
the river in less than This would
roughly
rane correction
900 m horizontal equate
component
sing from reductions
distance.
to a lo-15
mGa1 ter-
which would
performed
be mis-
using a 1 : 250,000
tainly have been gravity data collected by others (Ebblin et al., 1983) which was referred to in the
scale map. Since I had access to 1 : 50,000 scale maps, I was able to determine elevations in zone E
paper under
and then calculate
discussion),
the spacing
are far from adequate
to allow
very narrow
zones
M.M.T.
structural
modeling
associated
of the with the
I am forced to ask why. if adequate
have existed, been
of these data
done
hasn’t before?
density
modeling
The models
data
of the data
shown
the terrane
most zones using estimates tion recorded there
could
have
Ebblin’s been
in fig. 3
difference
caused
(Malinconico, 1986) were an attempt to determine if there was in fact a noticeable variation along the strike of the southern suture.
correction
techniques.
Earlier work
measurements which had been compiled (Ebblin et al., 1983) by the researchers at the Institute of Geodesy and Geophysics at Trieste University. Rather than have an anomaly created as the result of differences in collection and reduction methods, I chose to collect new data using the same gravimeter, base station and reference plane for all observations. I do stand corrected regarding the application of terrane Ebblin’s data, however,
corrections for some of I would like to point out
If I
data in my data set,
as much
simply
varia-
at every station.
as a 15 mGa1
by the different
terrane
Conclusions There
1 did not include gravity data from previous investigators in this study simply because I found it difficult to judge the quality of the various
of the elevation
in field notes
had tried to include
effect in the inner-
is no doubt
that
the detailed
structure
within the suture zones is more complex than that proposed by the gravity models. The models are, however, a good first approximation of the attitude of the structures below the surface. Regarding the Main Karakoram Thrust (M.K.T.), I agree that it may simply be a lithologic boundary. While its exact structural status requires much more extensive geologic field investigation, it is the northern lithologic boundary of the Kohistan Arc terrane. When the M.M.T. and M.K.T. models are combined (Malinconico, 1986, fig. 5) constraints on the gross structure
and tentative
thickness
the Kohistan Arc sequence can be obtained. I also must disagree with Dr. Ebblin’s
of
state-
that the reductions “. . in the innermost zones up to zone M were read off the 1 : 250,000 maps while
ment that a comparison between the Hunza and Chitral profiles is not warranted. A comparison is
those in zone N and 0 off the 1 : 500,000 maps”
completely valid and the fact is that the resulting density models are very different. The question that requires further investigation is “what has
(Ebblin, 1982). In Hammer’s tables for terrane correction (Telford et al., 1976), zone B extends from 1-16.64 m, zone C from 16.6-53.3 m, zone D from 53.3-170 m, zone E from 170-390 m and zone F from 390-895 m. At a scale of 1 : 250,000 where 1 cm equals 2.5 km, 895 m would equal approximately 0.4 cm. In other words, using a 1 : 250,000 scale map it would be impossible to determine the terrane corrections for any zone less than zone G. When this happens, especially where the relief is extreme, a very important component of the terrane correction term would be missing. In the Indus River Valley, there are many places where the terrane drops 150 m from the road to
caused the difference?” One of Dr. Ebblin’s themes throughout his discussion seems to be that the work presented was unnecessary because someone at some earlier time had already solved the problem. I would like to remind Dr. Ebblin that the word research is a combination of the prefix re-, meaning “repetition” or “again” and the word search. There is still a great deal of new data that have to be collected and interpreted before anyone can claim to understand the complex tectonic and structural evolution of the Karakoram.
353
References
chemistry
of the metamorphosed
of the Jijal Complex, Ebblin,
C., 1982.
Valley,
Gravity
Northern
profile
Pakistan.
along
the
Boll. Geofis.
Middle Teor.
Indus
Appl.,
24
Ebblin, ’ dus,
C., Marussi, P., 1983.
Boll. Geofis. Jan,
M.Q. and
A., Poretti, Gravity
G., Rahim,
measurements
Tear. Appl. 25 (99/100): Howie,
R.A.,
S.M. and Richarin the Karakorum. and geo-
L.L., Jr., 1986. The structure in Northern
Tectonophysics, Telford, 1976.
303-316.
1981. The mineralogy
N.W. Pakistan.
rocks
J. Petrol.,
22: 85-126. Malinconico, terrane
(93): 39-55.
basic and ultrabasic
Kohistan,
W.M.,
Cambridge,
as inferred
of the Kohistan from gravity
Arc data.
124: 297-307. Geldart,
Applied
Pakistan
L.P., Sheriff,
Geophysics.
860 pp.
R.E. and
Cambridge
Keys,
University
D.A., Press,
Ebblin, dus,
C., Marussi,
Boll. Geofis. Gansser,
Teor. Appl.,
Karakorum. terrane
G., Rahim,
measurements
1986. The structure
in Northern
Tectonophysics,
Pakistan
the Himalaya
Arc
from gravity
data.
delle zone orogenetiche Kush-Pamir.
del Kashmir
In: Geotettonica Naz. Lincei,
A., 1983. Geophysical
and evolution
R.A.K.,
Boll. Geofis.
granitization
1982. Geology
Hindukush
Teor. Appl.,
of bathohtic
dimen-
in Pakistan.
of the Himalaya Geol.
Karakorum
Bull. Univ.
Peshawar,
15: 51 pp. D.N., 1931. The syntaxis
Its rocks, tectonics
Rome,
Zeitler,
P.K.,
R.A.K., trends
and Karakorum.
443-461.
of the North-West
and orogeny.
Himalaya.
Geol. Surv. India Rec. 65:
189-220.
Himalaya-Karakor-
Proc. Accad.
pp. 131-137. Marussi,
and Wadia,
in the Karakorum.
syntaxis
sions. Am. J. Sei., 247: 211-245. Tahirkheli,
13: 6-9.
124: 297-307.
A., 1976. Gravity
urn-Hindu
and
of the Kohistan
as inferred
Pamirs
25 (99/100):
Miscb, P., 1949. Metasomatic
303-316.
between
Geol. Bull. Univ. Peshawar, L.L.,
S.M. and Richarin the Karakorum.
25 (99/100):
A., 1980. The division
Mahnconico,
Marussi,
A., Poretti,
P., 1983. Gravity
of the
Johnson,
N.M.,
1982. Fission-track
of the Nanga
Parbat
Naeser, evidence
region, Pakistan.
C.W.
and
Tahirkheli,
for Quatemary Nature,
uplift
298: 255-257.
The structure of the Kohistan Arc terrane in northern Pakistan as inferred from gravity data-Reply LAWRENCE L. MALINCONICO
(Received
February
23.1987;
Introduction I appreciate Dr. Ebblin taking the time to comment my gravity investigations conducted across the major structures associated with the Kohistan Arc terrane in the northwestern Himalaya. Many of his comments are very valid points that need to be addressed‘by further investigation. I would, however, like to reply to several specific points that were made in the discussion. The data I must respectfully disagree with Dr. Ebblin when he says that the spacing of the observations is not important. Modeling of gravity anomalies is very sensitive to small changes in the slope of the anomaly. This is especially true for large amplitude variations such as these which occur across the Main Mantle Thrust (M.M.T.). The figures reproduced in the original article do not do justice
accepted
Jr.
April 23,1987)
to the degree to which the observed and modeled anomalies were actually matched. While there may have been sufficient spacing of the data obtained by Ebblin (1982) along the Indus River, the data did not extend far enough to the south to allow the approximation
of a regional
trend
that could
be used to isolate the effects caused by the M.M.T. and the adjacent basic and ultrabasic rocks of the Jijal
Complex
(Jan
and
Howie,
(1982) has discussed the character high associated with the M.M.T.
1981).
Ebblin
of the gravity in the lndus
Valley and does compare that to the gravity field slightly to the east {near Nanga Parbat). He has not, however, actually modeled the effect, but simply speculated upon the possible causes. The density model presented (Malinconico, 1986, fig. 3c) shows that the gravity high is probably the result of the accumulation of the high density ultramafic and garnet granulite rocks (Jan and Howie, 1981) just north of the M.M.T. It is also very important
to construct
structural
models
of the M.M.T.
ble and not simply Indus
in as many places as possi-
on a single transect
River. How does the character
zone change
towards
along the
of the suture
the west? While
there
cer-
the river in less than This would
roughly
rane correction
900 m horizontal equate
component
sing from reductions
distance.
to a lo-15
mGa1 ter-
which would
performed
be mis-
using a 1 : 250,000
tainly have been gravity data collected by others (Ebblin et al., 1983) which was referred to in the
scale map. Since I had access to 1 : 50,000 scale maps, I was able to determine elevations in zone E
paper under
and then calculate
discussion),
the spacing
are far from adequate
to allow
very narrow
zones
M.M.T.
structural
modeling
associated
of the with the
I am forced to ask why. if adequate
have existed, been
of these data
done
hasn’t before?
density
modeling
The models
data
of the data
shown
the terrane
most zones using estimates tion recorded there
could
have
Ebblin’s been
in fig. 3
difference
caused
(Malinconico, 1986) were an attempt to determine if there was in fact a noticeable variation along the strike of the southern suture.
correction
techniques.
Earlier work
measurements which had been compiled (Ebblin et al., 1983) by the researchers at the Institute of Geodesy and Geophysics at Trieste University. Rather than have an anomaly created as the result of differences in collection and reduction methods, I chose to collect new data using the same gravimeter, base station and reference plane for all observations. I do stand corrected regarding the application of terrane Ebblin’s data, however,
corrections for some of I would like to point out
If I
data in my data set,
as much
simply
varia-
at every station.
as a 15 mGa1
by the different
terrane
Conclusions There
1 did not include gravity data from previous investigators in this study simply because I found it difficult to judge the quality of the various
of the elevation
in field notes
had tried to include
effect in the inner-
is no doubt
that
the detailed
structure
within the suture zones is more complex than that proposed by the gravity models. The models are, however, a good first approximation of the attitude of the structures below the surface. Regarding the Main Karakoram Thrust (M.K.T.), I agree that it may simply be a lithologic boundary. While its exact structural status requires much more extensive geologic field investigation, it is the northern lithologic boundary of the Kohistan Arc terrane. When the M.M.T. and M.K.T. models are combined (Malinconico, 1986, fig. 5) constraints on the gross structure
and tentative
thickness
the Kohistan Arc sequence can be obtained. I also must disagree with Dr. Ebblin’s
of
state-
that the reductions “. . in the innermost zones up to zone M were read off the 1 : 250,000 maps while
ment that a comparison between the Hunza and Chitral profiles is not warranted. A comparison is
those in zone N and 0 off the 1 : 500,000 maps”
completely valid and the fact is that the resulting density models are very different. The question that requires further investigation is “what has
(Ebblin, 1982). In Hammer’s tables for terrane correction (Telford et al., 1976), zone B extends from 1-16.64 m, zone C from 16.6-53.3 m, zone D from 53.3-170 m, zone E from 170-390 m and zone F from 390-895 m. At a scale of 1 : 250,000 where 1 cm equals 2.5 km, 895 m would equal approximately 0.4 cm. In other words, using a 1 : 250,000 scale map it would be impossible to determine the terrane corrections for any zone less than zone G. When this happens, especially where the relief is extreme, a very important component of the terrane correction term would be missing. In the Indus River Valley, there are many places where the terrane drops 150 m from the road to
caused the difference?” One of Dr. Ebblin’s themes throughout his discussion seems to be that the work presented was unnecessary because someone at some earlier time had already solved the problem. I would like to remind Dr. Ebblin that the word research is a combination of the prefix re-, meaning “repetition” or “again” and the word search. There is still a great deal of new data that have to be collected and interpreted before anyone can claim to understand the complex tectonic and structural evolution of the Karakoram.
353
References
chemistry
of the metamorphosed
of the Jijal Complex, Ebblin,
C., 1982.
Valley,
Gravity
Northern
profile
Pakistan.
along
the
Boll. Geofis.
Middle Teor.
Indus
Appl.,
24
Ebblin, ’ dus,
C., Marussi, P., 1983.
Boll. Geofis. Jan,
M.Q. and
A., Poretti, Gravity
G., Rahim,
measurements
Tear. Appl. 25 (99/100): Howie,
R.A.,
S.M. and Richarin the Karakorum. and geo-
L.L., Jr., 1986. The structure in Northern
Tectonophysics, Telford, 1976.
303-316.
1981. The mineralogy
N.W. Pakistan.
rocks
J. Petrol.,
22: 85-126. Malinconico, terrane
(93): 39-55.
basic and ultrabasic
Kohistan,
W.M.,
Cambridge,
as inferred
of the Kohistan from gravity
Arc data.
124: 297-307. Geldart,
Applied
Pakistan
L.P., Sheriff,
Geophysics.
860 pp.
R.E. and
Cambridge
Keys,
University
D.A., Press,