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CHAPTER-2
GENERAL GEOLOGY
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CHAPTER-2
GENERAL GEOLOGY
2:1.
REGIONAL GEOLOGICAL SETTING OF NAGALAND :
Morphotectonically,
northern
part
of
the
Indo-Burma
the
Naga
Mobile
Hills
Belt.
constitute
Indo-Burma
the
ranges
consist mainly of the Arakan Yoma and Chin Hills and the Naga Hills
in
Burma and
Yoma,
India
resp ectively.
Naga-Lushai
Hills
and
The
the
folded
block
Tripura
of
ranges
the
Arakan
separate
the
sediments of the Bengal basin and the Assam-Arakan Yoma basin. The
opinion
varies
stratigraphic
geosyncline
eastern
the
periods.
role
Some
of
or
£ssam,
early
disturbance
emerged.
The
subsidence
two
Bengal,
since
Eoceme
orogenic
and
time
thus
that
the
the
there
Permian
was
were
The
was
time.
a
single
During late
subjected
Arakan-Chin
formed,
different
part of Burma and the
basin
the
sedimentation.
basin during
greater
about
when
basins
matched
of individual
suggested
including Bay of
part
Cretaceous
about
Hill
never
geanticline
very
geanticline did
to the
deep and
not continue
north of Latitude 24° and thus did not constitute a permanent barrier
between
1964).
Indian and
A thick,
Burmese
grey
part
of
the
basin
(Mathur and Evans,
to dark grey concretionary
shales with
bands of siltstones and fine sandstones were deposited.
flysch
after
deposits
which
Mio-Pliocene
north
and
the
eastern
are
belongfcito the
area
part
raised,
of
the
Disang
partially
Assam
thin
This is the
Group.
During and
dismembered
Arakan basin,
of
the
and
the
sedimentary deposits were overthurst by orogenic movement to form a
part
of
the
sheild
(Mathur and
Evans,
1964)
.
The movement
was
controlled by the structure of the basement. The other school states
that
both
the
Bengal
basin and
the
independent developmental a c tivities.
Assam-Arakan
basin formed
Arakan
Yoma
basin
had
During Pre-Palaeogene time the
north-south
trending coastal
geosyncline.
It is evident that in the Naga Hills region and in the folded belt of
Tripura,
the
sediments
synclinal
valley
the Bengal basin
of
the
of
the
Arakan-Yoma
Upper Assam shelf
along the low thrust
basin
and the
crept
basinal
over
the
low of
planes (Desikachar,
1974).The
2
Palaeogene
shelf
extends
north-east
Brahmaputra
va lley.
The
Hills)
extends
north-eastward
also
northern limit
Upper
of
Assam
the
shelf,
Shillong
Lower
the
Karbi
alluvial
massifs
shelf
developed
of
Mikir
some extent
of
to
covered
(formarly
limiting to
Tertiary
mainly
Brahmaputra r iv e r and
to
Upper
the
the
Assam.
south
The
of
the
extending into the footshills of Naga Hills.
On the
basis
of
the
assumed tectonic
events,
the
Indo-Burma ranges are correlated with the Pre-Miocene Alpine orogeny
and the Arakan Yoma Naga Hills is now considered is a part of the
Himalaya-Indonesia erogenic belt (Khin and Win, 1969).
In the eastern Naga Hills, metamorphic rocks overlain
by molasse facies are thrust westward over the upper Cretaceous and
Eocene turbidite and ophiolites (Brunnschweiler, 1966). However, the
northern part of
the
Indo-Burma
ranges moved northwestward, eight
or possibly more over thrust, relative to the Foreland spur composed
of
Pre-cambrian
sediments.
rocks
This
has
(Brunnschweiler,
the
Assam
wedge
been
1966).
Valley
follows
covered
a
This
and
the
persistent
in
places
by low
designated
as
closely
follows
Naga
Hills.
thrust
dipping
the "Belt
the
Schuppen"
boundary
Towards
for about
of
younger
the
between
northwestern
350 Km known as
Disang thrust
. Disang thrust is the upper most member of the
of
(Mathur
schuppen
and
Evans,
1964).
The disposition
the
belt
of
the
younger group of rocks below the older ones in this belt is due to
the
presence
of
these
two
series
of
upthrusts
induced
by
Naga
thrust on one side and Disang thrust on the other.
The
Tertiary,
resulted
tectonic
local
basin
but also
on the
water
sediments
were
flysch
which
continued
activities
in
the
depressions and elevations
platform.
deposited
through
As a result
,
Eocene
to Middle
Nagaland and Manipur became landmass
during
not only
shallow
simultaneously. These
responsible for upllftment of the Barail range.
of
basin
early
in the
and
deep
depostis
Oligocene
and
are
is
In the Oligocene most
. Widespread unconformity
occurred
between
Barail
were
deposited
and
Pliocene
when the
Surma and Tipam Groups of
sediments
Middle
and
Surma
Groups.
continued
By
this
from
time
Upper
molasic
Oligocene
rocks
to
were
deposited.
A foredeep was formed along the south eastern margin
of the Himalayas due to major
upliftment of the whole
region during the Middle Miocene period. The
the
nearby
rep id ly.
young
mountain ranges
Consequently,
Dihing
were
and
northeastern
sediments derived
deposited
in the
Dupitila Group
from
foredeep
of rocks
were
deposited unconformably over the Tipam Group of rocks.
REVIEW OF THE STRATIGRAPHY OF NAGALAND :
In the Tertiary deposits of Assam- Arakan
sediments
of
distribution
shelf
and
geosynclinal
is more prominent
facies
are
Basin
developed.
the
This
in the Lower Tertiary than in
the
Upper.
The
established
incorporating
by
stratigraphy
Mathur
both
and
outcrop
as
Evans, (
and
developed
1964)
subsurface
is
in
shown
evidence
Nagaland
is
in
1.
Table
(d rillin g
and
seismic) which is generally accepted stratigraphic succession because
of its suitability for the study of stratigraphy of the north eastern
region.
4
TABLE-1
THE TERTIARY SUCCESSION IN ASSAM.
AGE
(APPROX.)
GROUPS
FORMATIONS AND LOCAL FACIES
GSOSYNCLINAL SEDIMENT
SHELF
SURMA VALLEY
SEDIMENTS
UPPER ASSAM AND
NAGA HILLS
Recent and
Pleistocene
Alluvium and
high level
terraces
Alluvium and
high level
terraces
Alluvium and
high level
terraces
300
Unconformity
Pliocene
DIHING
Mio Pliocene
DUPITILA
—
—
TIPAM
Miocene
SURMA
Oligocene
BARAIL
Not sub-divided Not sub-divided
400a
900a
Unconformity
Upper Dupi Tila Namsang
2800
Beds* 800
Lo.*er Dupi Tila
500
Unconformity
DISANG
Namsang
+
Beds 0-1000°
—
Girujan Clay
1500
Girujan Clay
1800
Girujan Clay
0-2300
Tipam Sandstone
1600
Boka Bil 1500
Bhuban 4000c
Tipam Sandstone
2300d
Not sub-divided
900
Tipam Sandstone
500-900
Not Sub­
divided
30-200
Unconformity
Renji 1000
Jenam 1200
Laisong 2400
Eocene^
—
2000b
Dhekiajuli
Beds*
Over 1500
Tikak Parbat
600
Baragolai 3300
Naogaon 2200
Probably over
3000
Not sub-divided
Jaintia Kopili
500
Alternation
Sylhet 500
Limestone
Thevria ?
100
Notes :
5
Figures give maximum thickness in metres *Local facies name :
a)
Total original thickness must have been much greater
b)
The
400 metres
of
Dihing
Group in the
extreme
north-east
of
Assam may belong to the Eastern Himalayan fore deep
c)
In the Garo Hills the Dupi Tila Group reaches 1000 metres
d)
The increased thickness is due to lateral passage of Upper most
Surma beds into Tipam sandstone
e)
The Bhuban Formation thickness southwards to exceed 6000 metres
in Arakan
f)
Including Palaeocene
g)
May range down into upermost Cretaceous.
The
stratigraphy,
given
above
lack--f
information regarding the Pre-Tertiary sediments as developed in the
eastern high h ill areas of Nagaland.
The
stratigraphy
generalised
in Nagaland as
sequence
of
the
lith o-
synthesised by the State Directorate of
Geology and Mining (1978) is shown in Table -2.
TABLE-2
The stratigrap h y
of
Nagaland
(A f t e r
the Directorate of Geology and Mining
Nagaland,1978).
Age ( A p p r o x . )
Litho-formation
Outer h i l l s
in the
L i tho-fama t bn
in
( ' B e l t of Sc- Eastern high h i l l
■'
huppen') dud Intermediate areas of Nagaland
Hills of Nagaland.
Recent and
Alluvium and high le v e l
Alluvium and high l e v e l
Pleistocene
terraces
terraces
Pliocene
Dihing
Unconformity------
Mio-Pliocene
Namsang Beds
Unconformity------
Miocene
Girujan Clay
Jupi Formation
(Molasse
Tipam
Formation
Conglomerate ,Greywacke
sediments)
Group
Tipam sandstone
Arkose (w ith
Formation
fo s s ils )
plant
Surma
Group
-Uuconfox u»ity---------- --------Oligocene
-----
-----
------
Barail
Tikak Parbat
Group
Formation
low er Barails
Baragolai Formation
Fine to medium grained
Naogaon Formation
sandstone (w ith
plant
fo s s ils )
Disang
Eocene to
Shale/slate/phyHite with
Upper C re­
calclenses in basal
taceous
Disang
sections and invertabrate
( Flysch
Group
and
sediments)
plant fo s s ils in
upper sections with
br i n e
s pr i ng s
7
Age (Approx.)
l.ltho-formatlnn in the
hltho-formation in
outer hills ('B elt of
Eastern high hill
Schuppen') and Inter-
areas of Nagaland.
mediate Hills of
Nagaland
- Fault/Thrust contactZepuhu Formation
Ophiolite
1.
Chert, «|uartzite, limestone, grey-
complex
wacks
,
tuffs,
(Emplaced
basic
during upper
major
Cretaceous to
suites noted under (2)
schists,
basic
volcanics,
blue schists
ultramafites
,
and
with
other
lower Eocene
2.
Gabbro , d io rite , quartz d io rite ,
and
ultramafites
with
minor
lenses.
- Fault/Thrust contactNimi Formation
Phyllites,
sericite
quartz
schist,
Pre-
feldspathic
Mesozoic (?)
orthoquartzite,
other
calc-clastic granite.
chloritite
schists,
quartzite,
limestone
and
8
NIMI FORMATION :
It is the oldest formation of the Nagaland exposed in
the eastern corner of the state . The Nimi Formation might represent
pTg-fi/SCVi
It
consists
unit belong to some older formation (pre-Cerrozoic).
mainly
carbonaceous
of
crystalline
phyllite,
limestone,
quart*.
sericite
quartzite,
schists
p h y llite ,
and
schistoze
granite.
Numerous
tuff
found along:
These
the
massive,
bands
whole
of
belt
coarsely
limestone
of
associated
ultramafites
crystallines
may
with
bordering
be
basic
Burma.
considered
as
equivalent to preflysch series of Pansat (Brunnschweile r , 1966 ).
The
limestone
phyllite or quartzite band.
white
bands
occur
alternately
with
either
The phyllite is grenish grey to silvery
in colour and well foliated and are intruded by innumerable
quartz veins along foliation planes. At some places, limestone bands
are found to enclose small bodies of intrusive schistose granite.
ZEPUHU FORMATION :
The
Zepuhu
Formation
occurs between
Formation on the east and Disang Group on the west.
the
ophiolitic
pyroxinite,
complex
gabbro,
of
Nagaland
dunite,
and
dolerita,
consists
the
It represents
of
harzburgite,
quartz-diorite,
serpentinite,
spilite, jasper, tuff, glaucophane-schists, basic schist, etc.,
at
places
with
oceanic
sediments
Nimi
such as chert,
mixed
tuffaceous chert,
greywacke, limestone, phyllite and cherty quartzite.
The ultramafites
occur as few meters to few kilometers thick bodies arranged in enechelon fashion within the flysch
pre-flysch
group
approximately
tectonic
1986) .
trend
of
rocks.
NNE-SSW
(
to
Srivastava,
(Chattopadhya and Roy,
The
NE-SW
general
trends
conforming
Mazumdar,
of
with
the
the
Chattopadhya,
Zepuhu Formation spreads over a strike
1975 ) and
rocks
is
regional
and
Roy,
length of about 90
9
Km. and width of 5 to 13 Km extending from Mollen-Molhe in the
south to Aniashu in the north.
In the ophiolitic suite
some of the basalts are
metamorphosed while most of the basic to acid volcanics retaining
their original mineralogy and textures. These characters suggest that
since they are not uniformly metamorphosed , the metamorphism did
not take place in the continental crust, where they have been
brought together ( but elsewhere, probably in the oceanic floor
( Chattopadhya et a l. ,1983). Ophiolities are tectonically emplaced
fragments of the oceanic lithosphere (Dietz, 19&3; Coleman, 1971).
Chert beds from various localities occurring within
the ophiolite suite have yielded rich radiolarian assemblage. From
the study of this assemblage the lower age limit of the ophiolite
suite
could
possibly
be
considered
as
Upper
Cretaceous
(Chattopadhya et al. ,1933).
DISANG GROUP :
A considerable part of the sedimentary rocks of the
Naga Hills has been grouped as Disang (Pascoe, 1912) and is confined
to the intermediate hill regions of the state, mainly to the east of
Disang
thrust
.
These
sediments
are
comparable
to
the
flysch
sediments of the Alps.
The
Disang
Group
comprises predominantly shale,
alternation of
represents
a
turbidite
sequence
sandy shale, sandstone, clay with
minor calcareous shale and
limestone. The proportion of arenaceous materials gradually increases
towards the top of the group with a lateral passage of Disangs into
Barails as can be seen from the predominates of sandstones than
shale near the top of the group and also an indication of gradual
shallowing of depositlonal basin. The shales of Disang Group changes
to hard glossy dark blue slate at places. These slates are low
10
grade
regional
metamorphic
products.
The
slate
exposures
generally argillaceous with thin bands of quartzite.
are
Due to tight
folding, the slates at places show phyllitic and schistose nature.
Numerous brine
springs and minor evaporite encrustations
occur in the upper part of this group. Pyrite
and fracture,
fillings are available in Disangs.
Pascoe
Disangs,
(L^ £ 0 , who
remarked
Cretaceous.
disseminations ■
The
that
assigned
it
limestone
might
bearing
a
Lower
possibly
beds
of
Tertiary
belong
Eastern
to
age
for
topmost
Manipur
was
considered by Oldnam (1883) to be older than Eocene, and included
in
the
between
'Axial
the
ascertained.
Group'.Evans
Disangs
Recent
and
works
(1964),
noted
that
Cretaceous Ukhrul
suggest that
the
relationship
Limestone
is
not
the Globotruncana bearing
Ukhrul Limestone indicate gradational relationship with the Disangs
shale and therefore, it may be proposed that the lower age limit of
the
Disangs
invertebrate
gastropods
could
fossils
be
considered
which
include
as Upper
species
Cretaceous.
of
Marine
lamellibranchs,
and Coralline algae indicate a Lower Tertiary age for
this group of rocks (Srivastav, e£ a£., 1986).
The
Disangs
have
a
maximum thickness
of
3003
metres
(Mathur and Evans, 1964).
BARAIL GROUP :
The Disangs gradually pass into the overlying Barail Group
of
rocks,
which
are
mainly
molassic sediments
(Directorate
of
Geology and Mining, Nagaland, 1978), and belong to Upper Eocene to
Oligocene age. Barails mainly occur within the "Belt of Schuppen"
and also exposed
at places
in the
intermediate hill
Barails vary in thickness from 4000 to 6000 metres
ranges.
The
11
(Mathur and Evans,
and
Tikak
Parbat
sandstone
Barails
with
in
is
Parbat
as
Borjan
Valley
Formation
Barails.
fossils
arc
The
thickness
well
Formations
the
of
the
are
not
Tne Tikak
coal
Valley
within
of
bedded
sub-division
Promising
Tiru
occur
of
Hills.
scam.
field,
belt
The
of Naga
coal
coal
coal
decreases from north
plant
in
mainly
Baragolai
part
rich
.1ban j i-l)osa i
of
and
southern
fairly
such
consisting
intercalations.
Tikak
recognizable
Nagaland
Formations,
shale
into
Pot mat ion
19G<1). It is sub-divided into Nagaon, Baragolai
belts
coal
the
coal
Parbat
belt,
Tikak
seams
of
and
Parbat
gradually
(Borjan area) to south (Paren area). Numerous
recorded
in
the
Barail
rocks
(Directorate
of
Geology and Mining, 1978).
SURMA CROUP
:
The widespread unconformity at the end of Oligocene
uetween
tool
Barails
place.
The
and
the
Surma
Groups,
molasse
state
continued
Middle pi iocene when
and molassic
from
lies
sedimentation
Upper
Oligocene
to
the Surma and the Tipam Groups of rock were
d eposited.
The Barials are followed by an unconformity over which the
Surma
Group
early
molasse
108)-
The mo lassie Soriuns co uprise of
The
Group
rapidly
of
is
rocks
stage
are
(Raju,
confined
towards
deposited.
to
north.
The
1968 , in
the
Shale
"Belt
Surma
Sinha
Group
and
Sastri,
sandstone,
of
Schuppen"
interlayered
belongs
with
shale
and
1973,
and
to
P.
r lay.
thins
out
sandstone
and
siltstone are conspicuous feature of
the Group which distinguishes
it from
Overlying
the
underlying
Barail
and
Tipam
Groups.
The
Surma Group attains a thickness of about 900 metres in Naga Hills
(Mathur and Evans, 1964).
TIPAM GROUP :
The
Tipam
Group
deposited conformably over
of
rocks
is molassic
in
nature
and
are
the Surmas and belong to Mio-Pl iocencage
1 2
T h e
b e d d e d
f e l d s p a t h i c
b e d d e d
I t s
T ip a m
a n d
m a s s s iv e
t h ic k n e s s
is
a b o u t
t h e
r o c k s
c l a y .
G ir u ja n
c o n s is t in g
T h e
o f
w it h in
t h e
g i v i n g
r i s e
F o r m a t io n .
t o
S c h u p p e n
G e o lo g y
a r e a s
b u t
T ip a m
a n d
w h e r e
O p h i o l i t e
u n a s s o r t e d
in t e r m o n t a n e
h a v e
b e e n
N A M S A N G
a g e
(
B E D S
:
N a m s a n g
c o n g lo m e r a t e s ,
lo c a l
f a c i e s
T h e s e
o f
S c h u p p e n '
r a n g e s
t h e
b e d
o f
b u t
N a g a
is
a b o u t
a r e
to
c l a y s .
o f
fe r r u g in o u s
a n d
le n s e s
a t
p la c e s
o f
e a s t
8 0 0
m e t r e s .
o f
t h e
t h e
t h is
B e lt
e a s t e r n
t o
o f
h i l l
D is a n g
a n d
b e d d e d ,
d e p o s i t e d
N u m e r o u s
a r e a
h ig h
p o o r l y
b e e n
o f
( D i r e c t o r a t e
p r e - M io c e n e
h a v e
r o c k s
o f
a
in
a r e
p la n t
in d ic a t e s
T i l a
n u m b e r
t h e
D is a n g
u n c o n fo r m a b ly
m o t t le d
l i g n i t e .
D u p i
fr o m
H i l l s ,
to
in
f o s s i l s
a
P o s t -
,1 9 8 6 ).
t h e
a b s e n t
in
m a s s iv e
o f
le n t i c u l a r
r e p o r t e d
a r e
t h i c k
f a c i e s
b e d s
fo r m a t io n
c o n d it io n s .
a l .
G r o u p
a n d
b e lo n g
b e e n
s h a le
t h e
to
r o c k s
s t
J o p i
p la c e s
r o c k s
c o n s is t in g
g r i t s
a n d
h a v e
t h e
T ip a m
B e d s
a n d
p o c k e t s
c o n fin e d
o v e r l i e
f l u v i a l
S r i v a s t a v
T h e
b y
in
-
a t
a p p e a r
u n d e r
p r e s e r v e d
P a la e o g e n e
r o c k s
T h e
a n d
b a s in s
a s
s a n d y
o f
u p w a r d
a r g i l l a c e o u s
c l a y
t h i c k
1 9 6 4 ).
is s u c c e e d e d
a n
m a in ly
1 9 7 8 ) o c c u r.,
r o c k s .
p e b b l e s
E v a n s ,
o c c u r
a n d
h o r iz o n s
s tic k y -m o ttle d
s a n d y
C la y s
is
u n c o n fo r m a b ly
o f
is
t o
d e p o s i t s .
e q u iv a le n t
t h e y
s u it e
G ir u ja n
G r o u p
M in in g ,
a n d
c l a y ,
s a n d s t o n e
e c o n o m ic
T ip a m
T h is
m o t t le d
P l a s t i c
f e l d s p a t h i c
g r e y ,
(M a t h u r
m a s s iv e
S o m e
S a n d s to n e F o r m a t io n
s h a l e ,
m o t t le d
b lu is h
m e t r e s
T ip a m
a r e
s a n d s t o n e .
c o n t a in
2 3 0 0
C la y
F o r m a t io n
fe r r u g in o u s
s a n d s t o n e
T h e
b y
s a n d s t o n e
c l a y s ,
T h e y
G r o u p
o f
o f
p la c e s
in t e r m e d ia t e
t h r u s t .
a r e
M
o v e r l a i n
s a n d s t o n e ,
n a m e d
io - P lio c e n e
w it h in
a n d
M a x im u m
a f t e r
a g e .
t h e
'B e l t
e a s t e r n
h i l l
t h ic k n e s s
o f
13
D1HING GROUP :
Over
sedimentation,
charaater
which
of
the
the
Namsang
Beds,
after
Dihing Group was deposited.
debris
flow.
These are
arc
sediments,, supported
a
break
in
Dihing exhibits
the
sediments of
by a cohesive
gravity
matrix,
mainly
flows
in
clayey
in
nature. Clasts to matrix ratio vary from bed to bed and as a result
some
beds
bedded,
appeared
varying
in the
conglomerate
in size
fine grained compact,
places
as
from
cobble
sandstones.
outer h ill
horizon
areas
which
to pebble.
are
poorly
Clasts are
mainly
They are reported from only a few
of
Nagaland
and
belong
to
pliocene
age.
ALLUVIUM AND HIGH LEVEL TERRACES :
High
unassorted clays,
level
sand and s ilt,
at various level all over
new and
low
terraces
north-eastern
side
of
alluvium
consisting
of
gravels and boulder beds are found
Nagaland.
level alluvium.
and
The
The alluvial deposits consist of
older
Naga-Patkai
alluvium
ranges
occurs
along
the
and
low
while newer
alluvium covers vast areas along the borders.
The
soil
cover
developed
over
the
ultramafites
in
ophiolite suite is seldom exceeds 3 metres in thickness.
3.
STRUCTURE :
The
belt,
produce
development.
v iz .
a complicated
being a part of the
structural
and
Tethyan orogenic
tectonic
history
of
its
At least three distinct tectonic divisions of Naga Hills,
the
Disang
Metamorphites,
Chattopadhya
continental
Naga Hills
with
Group,
the
independent
et_ a l . , 1983).
collision
and
a
Ophiolite
tectonic
These
process
are
suite
lineage
and
are
produced by
of StLbduction.
the
Naga
established
a
process
Based
on
(
of
the
14
studies
carried
out
by
Mathur
and
Brunnschweiler
( 1966),
Directorate
(1978),
(1978),
Sinha
Dutta
Evans
of
( 1964)
,
Geology and
et^ al.
,( 1932),
Evans
(1964),
Mining,
and
Nagaland
others
a
short
account of the structural history of Naga Hills is given here in this
text.
The
sheets
above
of
Schuppcn
forming an imbricate
and
south-eastern
limits
Synclinorium situated
Schuppen,
which
synclinorium
anticlines
composed
Disang
of
Thrust
the
towards
the
belt
the Disang
Group
the
It
is
having
wide
roughly
many
occurs
below
syncline ,
N-S axes.
Schuppen.
The
of
the
of
side of
into
folded
The
Belt
Barails.
merges
sharply
the north­
On either
the
thrust
one thrust
limit
south-east
Patkai Synclinorium
a
of
of
is broken up by strike faults.
south-south-west,
Synclinorium.
is
pattern due to overriding of
the another. Naga Thrust and
western
Patkai
Belt
To
the
Kohima
and
faulted
rim of
the
structure
lies
of
folding
have
been
scale
fold
mainly within the Disangs.
At
least three
generations
recorded in the litho-units of Naga Hills.
The
first
generation
of
with a N-S to NNE-SS'.V trending axial
steep
dips.
often
overturned
Chizami,
The
folds
are
towards
recumbent
fold
plane having sub-horizontal to
isoclinal,
west.
is a large
At
inclined
places
and
reclined
between
folds are observed and this
types
Pfutsero
and
related to the
Alpine-Himalayan Orogeny.
The
a N-S trending
axis
plunges
at
second
hills
of
folds
axial
plane
with
low
angle
towards
folds chiefly control
producing
set
and
dips
is broad upright folds with
towards
NNE-NE.
east
The
and
first
west.
and
The
second
the topographic features of the area, synforms
antiforms
producing
valleys.
The
second
generation of folds may be correlatable with the second phase of the
Alpine-Himalayan Orogeny of Oligocene-Miocene time.
15
The third generation of folds is broad and open with
low moderate
plunge and have
WNW-ESE to E-W trend.
Axial
plane
<
dipping steeply
towards N and S.
These folds may be linked with
the Pleistocene movements of the Himalayas.
Thefi are two set of distinctive faults in Naga Hills.
The
earlier
set
trending
nearly
regional trend of the early
NE-SW and
conformable
with
the
folds which makes the litho formations
in a longitudinal and imbricate manner. The later set of faults which
have offset the earlier ones, havixtgWNW-ESE to E-W trend, are mostly
reversecum
wrench
microfractures
has
type.
The
interference
given
rise
to
the
of
these
formation
of
two
types
large
of
tectonic
blocks similar to horst and graben like structures at certain places
of Nagaland, as for example Lacham lake area, Ghaspani Valley, etc.
Generally,
the
Nagaland is characterised by
ophiolite
zeolite
belt
facies,
Tectonically
suggesting
reveals
green-schist
the
and
metamorphic
facies
linear
pressure
of
litho-units
of
low grade regional metamorphism. The
a characteristic
significant
high
metamorphism
of
and
amphibolite
facies.
of
glaucophane
schists,
patches
low
assemblage
temperature
metamorphism,
have
been reported from a number of localities within the pphiolite belt
of Nagaland (Directorate of Geology and Mining, Nagaland, 1978).
The
phyllitic
towards
Mineralogical
shales
the
changes
montmorillonite
is
of
Disang
interior
are
not
considered
Group
of
the
fully
as a
are
known
basin
known
but
beginning
of
to
become
(Pascoe,
1945).
disappearance
the
of
metamorphism
(Winkler, 1976).
The
indicates a very
the
phylllte
mineral
low grade
suggests that
assemblage
of
the
Naga
metamorphites
metamorphism and absence of biotite in
of thethe temperature^ order of 450°C was not
attained or exceeded during metamorphism.
16
Metaultramafits of the ophiolite belt shows that they
are
not
possibly
uniformly
took
ophiolites
place
are
lithosphere
metamorphosed,
in
the
tectonically
(Dietz,
that
which
oceam
floors,
emplaced
1963,* Coleman,
indicates^ metamorphism
considering
fragments
1971).
of
that
the
the
oceanic
It is suggested that the
oceanic crust, below the thin (0.5-2.0 Km.) surface magnetised zone,
is
metamorphosed
to
green-schists
facies
caused
by
heat
flow.
Occurrence of various textures and similar grades of metamorphism in
the
thick
pile
of
very
low
grade
to
unmetamorphosed
sediments
indicates that the regional metamorphism has had no effect on these
rock
units
but
were
subjected
to
retrogression
during
their
subductton into the continental crust. The recent data on lithotectonic
framework alongwith the metamorphic phenomena are suggestive that
the rocks formed in a high-pressure low temperature regime , which
could
have
Burmese
been achieved
plate
(Chattopadhya
and
et
during
consequent
a l . , 1983).
The
the
collision
subduction
later
of
of
the
the
tectonism
Indian and
lithosphere,
g iv c i.
rise
to
Cenozoic development of Indo-Burman ranges along with the ophiolitic
complex of Naga Hills (Directorate of Geology and Mining, Nagaland,
1978).