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The Cuddapah Salient: a tectonic model for the Cuddapah Basin, India, based on Landsat image interpretation
Tectonophysics, Elsevier
136 (1987) 237-253
Science Publishers
237
B.V., Amsterdam
- Printed
in The Netherlands
The Cuddapah Salient: a tectonic model for the Cuddapah Basin, India, based on Landsat image interpretation RAMESH VENKATAKRISHNAN Department * Department
and FIROZ E. DOTIWALLA *
of Geological Sciences, Old Dominion
of Geology, Civil Engineering
Department,
(Received
March
University, Norfolk,
Indian Insiitute
17.1986;
VA 23508 (U.S.A.)
of Technology, Bombay,
revised and accepted
August
400 0766 (India)
13.1986)
Abstract Venkatakrishnan, India,
Analysis pattern
of geologic,
of tectonic
middle-late
basin
Cuddapah
as structural
transverse orogeny
tear
fronts.
thrust
translation.
clockwise
and greater, resisted
fronts,
Cuddapah
confined
East-west
movement
paralleled
derived
address:
lineament
Veldruti-Guntur
by the east coast gravity
Basin,
on deep crustal
tear faults. The major
faults
the Eastern
lateral
ramp
Ghats
tear fault.
buttress
Steep imbricate
to the west, arcuate (Kaila
(latest
buttressed
with relative
and Bhatia,
and
represent which were are
Cuddapah-Nellore Proterozoic)
the west-verging
acted as a boundary
down”
a
of the
zones recognized
the northwest-trending
during
high anomaly
margin
The lineaments
were “pinned
convex
has revealed
eastern
faults in the Archean
in the basin.
zones that effectively
transverse
The resulting
interpretation
thrusted
The lack of a lateral ramp or basement
Cuddapah-Nellore
for the Cuddapah
trends
and
compression
to east to west
displacement
being
to the south resulted thrust
NalIamalai
ramps
in the
and Vellikonda
1981) is herein named
the
Salient.
Oil and Natural
Gas Commission,
Dehra
Dun, 248195 (India).
oo40-1951/87/$03.50
image and
to have nucleated
to the north
ends of folds and thrusts
model
set by deep crustal
and transverse
The first interpretations of the tectonics, structure and stratigraphy of the Cuddapah Basin in south India date back from the latter part of the last century (King, 1872). Since then, numerous excellent field studies have been conducted notably those by Narayanaswami (1966), Balakrishna et al. (1967), Sen and Narasimha Rao (1967) and recently available Deep Seismic Sounding (DSS) * Present
from Landsat
pattern
that appear
directed
a tectonic
by the folded
and tectonic
ramps,
these converging
westward.
Salient:
136: 237-253
elements
ramp
to the southwest.
of the left-lateral
broadly
alignments and frontal
along the NE-trending
increasing
further
structural
lateral
within
The northeastern
in the development foreland
lateral
to the south.
Block uplift
and lineaments
in both sedimentary
Veldruti-Guntur
fault
Tectonophysics,
Basin in India. A structural
and parallel
uplifts,
was apparently
thrust
data
of Archean
fabric is apparent
the northeast-trending
thrust
geophysical
zones of en echelon
reactivated
F.E., 1987. The Cuddapah
image interpretation.
inheritance
Proterozoic
intracratonic broad
R. and Dotiwalla,
based on Lansat
0 1987 Elsevier Science Publishers
B.V
studies (Kaila and Bhatia, 1981; Kaila and Tewari, 1985), have greatly enhanced our understanding of the tectonics of the area. Unfortunately no systematic attempt appears to have been made to reconcile the different geologic interpretations and the geology of this region to the kinematic processes involved in the formation of the arcuate basin pattern and the convex to the west thrusted eastern margin of the Cuddapah Basin. The eastern part of the basin is located in an area of complex folding and thrust faulting of the Cuddapah sediments and Archean basement that have been transported westward over the
238
basin. Specifically, the basin lies at a major change in the azimuth of the fold axial traces to northeast from a regionally
consistent
Accommodation
for the rotation
ing fold-thrust made
mainly
individual
north-northwest
complexes
appears
by variation
thrust
planes
trend.
of the west vergto have
in displacement
and by oroclinal
been along
bending
ural elements
and nature are poorly
westward
of these complex understood
struct-
due to abun-
vanishes
the deformational
intensity
Nallamalai
fronts
abruptly
the northern
Further all but
dip gently
the north-south
thrust
the thrusts
generally
(Fig. 1,
sediments
sediments.
as the sediments
form
decreases
Front
Kumool
the east. Upon tracing latitude,
Thrust
up older Cuddapah
over younger
to the west
and
of deformation
to the Nallamalai
1) that has brought directly
and
of folds in the cover rocks. The extent
pah Basin. The intensity
north,
Vellikonda past
thrusted
margins
Cuddapah Basin, the Vamikonda Thrust (Fig. 1, 3). On the footwall to these thrust
Ghats erogenic belt composed of complexly deformed Early Proterozoic and Archean crystalline-supracrustals thrust over little deformed middle to late Proterozoic platformal rocks
two major
of the Cuddapah Basin. The basement structures appear to have controlled the convex to the west arcuate thrust fronts and this feature is herein called the Cuddapah Salient. Based on a detailed analysis of computer edgeenhanced Landsat images and available geologic and geophysical data, a kinematic model is deveioped. The arcuate geometry probably reflects foreland buttressing of the advancing west-verging thrust sheets as they piled up along a northeast trending lateral ramp resulting in transpression and refolding of the fold-thrust complex. To the south, lack of buttressing massifs is reflected by northwest trending transverse changes in the tectonic fabric.
tear-faults
and
Geologic setting The Cuddapah Basin is an arcuate, convex to the west basin covering an area of about 35,000 km’ in the peninsula of south India. it is about 350 km north-south and 150 km wide along 15”N latitude. The widest section of the basin lies just south of the major swing in the structural fabric of the area {Fig. 1). Along its eastern margin, the sediments are strongly deformed into a convex to the west oroclinal fold and thrust complex that verges west and northwest. The eastern marginal thrusts, Vellikonda Thrust Front (Fig. 1, 2) have brought up Archean gneisses, granulites and gneisses that in most places directly overlie the Proterozoic sediments that comprise the Cudda-
15 o N
swing northeastward
dant Mesozoic and Cenozoic cover. In general the pre-Mesozoic subcrop consists of the Eastern
foreland
to
sub-basins,
the Kurnool
of the Front fronts and
Palnad sub-basins comprised of little deformed younger supracrustal Proterozoic rocks have been recognized. Stratigraphic studies conducted by Narayanaswami (1966) imply a thickness of at least 10 to 6 km of Proterozoic sediments within the Cuddapah Basin. More recent geophysical studies (Kailasam, 1976; Kaila et al., 1979; Kaila and Bhatia, 1981; Kaila and Tewari, 1985) appear to indicate that at least some of this vast thickness of sediments may be amplified by the westward-directed basement crustal shortening with a component of vertical tectonism. Narayanaswami (1966) and Sen and Narasimha Rao (1967), subdivided the basinal sediments into two major sequences, the lower Cuddapah System separated by a profound unconfor~ty from the upper Kurnool System. Since these studies, both systems have been elevated to Super Group and Group status, respectively (Geological Survey of India, 1981). The Cuddapah rocks are comprised zites, graded pellitic and sea-pellitic
of quartturbidites
and stromatolitic carbonates. Lateral facies variations, local unconformities of various sorts and, a general deepening of the basin to the east is indicated by progressive off-lap to successively younger rocks to the east. The younger (upper Proterozoic) Kurnool rocks overstep the featheredge of the older (upper to lower Proterozoic) Cuddapah Supergroup all along the western margin of the Kurnool and Palnad sub-basins indicating progressive uplift (and possibly west vergent thrusting?) during the Kurnool Group foreland basin development. It is indeed interesting that the Kurnool Group is entirely devoid of
239
........................... ...........................
...........................
........................... ....................... ....................
....................
KURNOOL
SUD;BASIN
MB0
.15O
. ......... 0~HARMAVARAM . . ................. . . ................... . . .................... .. ..
.........
Ido N
;O”E
I-NALLAMALAI 2- VELLIKONDA 3- VAMIKONDA ,
1 q&J I
THRUST FRONT THRUST FRONT THRUST FRONT
EXPlANATION
-I COASTAL PROTEROZOIC
PLAIN
HEAN
SEDIMENTS
SUPRACRUSTALS
SUPRACRUSTALS KHONDALITES
Major I-c/r-l
thrusts faults
ARCHEAN (PENINSULAR) GNEISSES/GRANITES
b other
Fig. 1. Generalized
geologic map of the Cuddapah
magrnatism. The lower part of the Cuddapah Supergroup is, however, characterized by an extensive volcanic and plutonic suite of basalts, gabbros, dolerites, and a few felsites. Crawford (1969a,b) and Crawford and Compton (1973) ob-
Basin area.
tained a Rb-Sr date of 980 + 110 Ma from one of the youngest post-Cuddapah sills, and an age of 1583 k 147 Ma from one of the oldest sills. These ages provide respectively a minimum and maximum age for Cuddapah Supergroup sedimen-
240
tation. Crawford and Compton (1973) also obtained a 1225 Ma Rb-Sr mineral age from a
Kaila et al., 1979, pp. 326-327).
kimberlite dike that intrudes the Cuddapah sediments. This clearly indicates at least some deep
to the north, west and south is intruded by extensive swarms of tholeiite dikes. These indicate a
crustal fracturing during basining events (see also
model age of 1700 Ma-so
LANDSAT
Fig. 2. Landsat ERTS:
image mosaic compiled
E-2054-04321-6,
E-1218-04325-7,
MOSAIC
The Archean basement surrounding the basin
c;:L
from 1 : 1,000,000 scale black and white prints. (Scene identification E-1183-04382-7,
must be entirely pre-
E-2035-04264-7.
E-1184-0441-6,
E-1218-04334-7
numbers
NRSA
820404,
and E-1218-04334-7.)
241
Cuddapah formed
(Drury Archean
the Cuddapah clearly
et al., 1984). The intensely basement-fabric
Basin
indicates
the Dharwar
is convex
the antiquity
Craton
unlike
de-
that
of The abundance
to the east-this of the structures
to the west (Drury
Interpretation of lineament map
in
and Holt,
of lineaments
makes it an impossible ment
with respect
to known
1980). The basement rocks of the study area are, for the most part, covered by a thick sequence of
more efficient
sedimentary
area, however, the distributions
prevent
direct
knowledge and
rocks of the Cuddapah
their
understood,
observation
and
of the characteristics structure
is only
especially
Basin which
study.
A better
of these
now beginning
with the recent
is rather
simple
readily
to be
statistically
of Deep Seismic Sounding (DSS) (Kaila et al., 1979; Kaila and Tewari, 1985). More specifically, a better knowledge is needed to allow us to conceive or to build models upon which to base our exploration for mineral or petroleum resources. Information pertaining to the character and structure of the basement rocks is obtained from a variety of sources: rock cuttings and cores from wells; aeromagnetic, seismic and gravity surveys; structure and stratigraphy of the supracrustal sedimentary cover; and remote sensing derived lineament maps. Information obtained by any one of these methods is of limited use; however, by integrating these bits of information, much can be learned. This paper attempts to develop a tectonic model for the Cuddapah Basin extracting information from computer edge-enhanced Landsat images (Fig. 2) and combining this data with information from published sources. The identification and interpretation of the Cuddapah Salient linked to basement structures is based on several factors, such as structural patterns interpreted from Landsat MSS images, including lineaments and deep crustal faults previously postulated by others. In addition, the Precambrian tectonic model for south India developed by Drury et al. (1984) is extended to the north to include the Cuddapah Basin. The model is not entirely different from the one proposed by Katz (1976) but is appealing because of its simplicity. Katz (1976) had suggested transform movement along the coastal Bouguer gravity-anomaly (Fig. 6,f) to create the basin as a pull-apart and subsequent deformation.
or available
data.
A
would be the one developed
by Sawatsky and Raines (1981) and recently applied by Maars and Raines (1984). In this study
rocks
availability
method
on satellite images
task to discuss each linea-
discerned
and
trends
patterns
can be
distinctive
(hence
analyze
of lineament
there is no real need to
the lineament
patterns).
discussions will therefore be restricted sion groups of en enchelon or parallel sets that lineament
Our
to discuslineament
would define a lineament zone. Each zone is then discussed as one composite
feature without validating individual lineaments within the zone. The striking difference in lineament density between the Cuddapah Basin and its surroundings is immediately apparent (Figs. 2 and 3a). The eastern portion of the map also has a reduced lineament density because of the masking effects of the almost featureless coastal plain sediment cover. The Precambrian terrain is, however, intensely fractured and among the welter of a large number of lineament out known structural
trends, by carefully filtering fabric, those lineaments that
may correspond to cross-strike fractures other than foliation and cross-cutting dike trends, a viable interpretation is possible. (This procedure is imperative because the post-Cuddapah deformation cross-cuts earlier fabric.) For this reason, Fig. 3a and b will be discussed together. Figure 3a is a map of all mapped lineaments within the eight Landsat frames used for mapping. Figure 3b is a compilation of the tectonic “fabric” elements in the same area. For ease in comparison, fabric elements within the Cuddapah Basin are differentiated from the preexisting Archean basement fabric and cross-cutting pre-Cuddapah dike swarms that infest the basement. The pre-Cuddapah dike swarms occur in three main groups: with ENE-WSW, NW-SE and NNE-SSW trends. Only northwest of Veldruti and Kumool do they locally follow the Archean fabric. Only a selection of these dikes has been drawn on Fig. 3b for the sake of clarity in presen-
242
tation. It is highly likely that the majority of the N75-80° E trending lineaments south of Anantapur (AN) and the Cuddapah Basin margin are entirely occupied by these dikes (Fig. 34. The large swarm of NlO-15 ‘E trending lineaments north of Srisailam (S on Fig. 3a) are also probably related to the NNE-SSW trending dike swarms in that area. However, unlike the N75-80°E trend-
Fig. 3. a. Lineament
map identifying
dashed line: combination ~-M~bubnag~, image
interpretation.
within basin. Arrows
of topographic
N-Nandyal, “Fabric”
major
lineament
identify
mark major hneament-zones
Dome south of Srisailam.
zones and locations
VE-Veldruti, planar (stippled
Boxed area north of Vinukonda
fabric
discussed
in text. Solid line: topographic
Agnigundala, AN-Anantpur,
A-
and tonal alignments.
S-Srisailam, elements
ing swarms, these lineaments are also expressed within Palnad sub-basin possibly due to fracture propagation and reactivation during the Eastern Ghats orogeny. The N50-70* W trending dike swarm (see Fig. 3a, lineament zone e) is also expressed within the Kurnool sub-basin; again possibly due to reactivation.
VI-Vinukonda. in the Archean
pattern)
recognized
is for Fig. 6b.
b. Tectonic basement
D-Dharmavaram, fabric
map compiled
lineament; G-Guntakal, from Landsat
rock and strike of Cuddapah
by combining
sediments
Figs. 3a and 3b. Note Eswarakuppam
243
kmi
CUDDAPAH
BAS IN
“FABRIC”
Fig. 3. (continued)
Cross-structural lineament zones The following is a discussion of lineament zones that cannot obviously be related to preexisting fabric elements. These lineament zones are considered features that cut across the Precambrian fabric and as will be shown below have played a crucial and hitherto unrecognized role in the structural development of the Cuddapah Basin area. Within the Cuddapah Basin the most prominent trends are a N65-70°E corridor of en echelon and parallel lineaments that can be traced from Guntakal (G on Fig. 3a) past Nandyal (N), Vellikonda (T/I) to Guntur. This lineament zone is
confined to a 50&m swath between Agnigundala (A) and Cumbum. Another similar trending lineament set can be traced from Anantapur (AN) across the basin to Ongole. These two northeast trending lineament zones have been identified as lineament zones (a) in Fig. 3a and b. A lineament swarm between Cumbum and Nandyal (N) in Fig. 3a is related to intense fracturation within the ~warakuppam Dome complex (see for example, Ramaswamy et al., 1983) possibly due (1) to the brittle nature of the outcropping rocks, and (2) this also marks the zone of oroclinal bending of the folds and thrusts (see Fig. 3b). Lineament zone a between Veldruti (VE) and Nandyal (N) also corresponds with the
244
mapped
extent
of the Veldruti-Gunnayal
zone of Narayanaswami Lineament the margins N60-70
OE and
forms
Cuddapah.
These unmapped
they are recognizable Lineament strong
zone
lineaments
and encloses zone
c trends
a discontinuous
that can be traced
across
southwestward
probably basement
band
that trend
have been delineated
only to express their
in the development
model for the Cuddapah
of the
tectonic
Salient.
the Cuddato south of
correspond fabric
with
elements
as a broad N20°E,
Basement of the Cuddapah Basin
of
as
on the gravity maps. i occurs
eaments
importance
Palar Mesozoic basin
Lineament
pah Basin from Nellore hitherto
N40”E
of the subsurface
et al., 1981).
lineaments
(1966, p. 44).
zone b trends
(Sastri
fault
The Precambrian Cuddapah simple
depression
mately
centered
The zone
of
south from
the town of Cuddapah and continue beyond the margins of the basin. Farther to the south and east this trend is parallel to the Eastern Ghats tectonic fabric but southwest of Cuddapah it entirely cuts across the fabric (compare Figs. 3a and b). Northwest trending lineaments are not as well developed as northeast trending sets. Several narrow lineament zones (e, Fig. 3a) can be defined in somewhat equally spaced bands from Nandyal (N) to Veldruti (I%) and Kurnool and beyond. Similar trending lineament swarms extending from Srisailam (S) to Mahbubnagar (M) are locally parallel with the Archean fabric and numerous pre-Cuddapah dikes also exhibit similar trends. It is indeed interesting to note that many lineament zones belonging to this trend appear to continue well into the interior of the Cuddapah Basin and similar lineaments can be traced between Cudda-
depth
shallow Until
basement
Basin was depicted with
configuration
a central
over Cumbum
to basement
of the
as an arcuate high
by Glennie
is known
and
approxi(1951).
from
a few
drill holes on the very edge of the basin. the recent
availability
of DSS-data
(Kaila
and Tewari, 1985), the basement architecture was not well known. Glenie’s (1951) map depicted a basement that generally deepened to the east from the western basin margins devoid of topographic relief except for the Cumbum high-wherein Glennie wrongly considered the core of the Eswarakuppam Dome to be comprised of basement rock. This map did, however, show a marked steepening of the basement surface toward the eastern margin of the basin. The Cuddapah Basin is
a
composite
in-
pah and Nellore. The Penner River locally parallels this lineament zone. This lineament zone is
tracratonic basin (Fig. 4a) and features indicating plate margin tectonism (Sarkar, 1982) such as the presence of volcanics, acid intrusives, ophiolites and high-pressure, low-temperature metamorphic suites are entirely absent; only retrograde metamorphism of the Archean granites, gneisses and grant&es of the basement appears to have occurred during the Eastern Ghats orogeny. The basin can be divided into four somewhat separate
distinctive largely because of variations in the tectonic fabric alignment; northwest oriented topographic features are subdued in Landsat images rather than enhanced by the similarly ori-
depressions and ridges bound by major basement faults. The younger Kumool sub-basin is located entirely west of basement fault 6. It is bound by faults 8 and 7 to the north and a series of possibly
ented solar azimuth (approximately N130 ’ ). The sense of rotation of the Cuddapah Basin fabric across the Cuddapah-Nellore lineament zone suggests left-lateral shear (Fig. 3b). If these lineaments and lineament zones represent the surface manifestations of deep-seated (basement) faults, they should then be depicted on the surface geology as variations of the fabric trend, localized increase in the intensity of deformation, offset fabric elements, etc. Figure 3b is one such compilation. Major cross-structural lin-
strike-slip faults to the south near the town of Cuddapah (Fig. 1). The basement in this sub-basin generally dips gently to the east and attains a depth of about 9000 m near fault 6. To the east the basement rises up to within 2000 m of the surface near fault 14 in the south and is within 500 m of the ground surface near fault 7 to the north (Kaila and Tewari, 1985). Lack of depth control in the central part of the Kumool sub-basin precludes better estimates of basement depth. The Palnad sub-basin containing Kumool
245
the south and to about 7000 m in the north. Faults 4 and fl are both high-angle normal-slip faults, hence the basement ridge may be considered an upthrown horst bound by fault 6 to the west. Outside the basin margins, the basement (Kaila and Tewari, 1985, Fig. 1) information is better especially for the Krishna-Godavari Mesozoic basin and in the Palar Mesozoic basin north of Madras (Sastry et al., 1981). These basins are bound by low-angle normal faults (1, 2 7 and IO)
Group rocks lies entirely north of fault 5 and is bound by fault 9 to the west. No basement data is availabIe beneath the Painad sub-basin. The central part of the Cuddapah Basin bound by faults 6 and 3 is essentially the deepest part of the basin. The basement surface rises up to about 5000 m just north of Cumbum in an arcuate, NE-trending ridge culminating beneath the Eswarakuppam Dome. The basement slopes rather steeply to depths of over 10,000 m near fault 3 in
i
“G-PALAR
‘I
SASI~
L Fig. 4. a. Basement-tectonic map of the Cuddapah Basin and surroundings. (Redrawn from Kaila and Tewari (1985) and Sastri et al. (1981).) b. ~~~ion~
stereoscopic block-diagram of the Cuddapah Basin basement topography. View is from the southeast
and aligned for viewing with pocket stereoscope. Small depression in foreground in Palar Basin and depression to the north of it is where the basement has been downdropped east of basement faults 10 and 17. The Bay of Bengal has been drawn at 3500 m below sea level. (Vertical scale exaggeration is 2 X .)
Fig. 4 (continued)
and obviously are controlled by normal-slip reactivation of the Precambrian thrust faults during Mesozoic rifting events. To help visualize the basement architecture, Fig. 4b was developed. It is presented as a stereoscopic view suited for viewing with a pocket stereoscope. In the areas where basement depth was unknown, crude estimates were calculated using the gravity and aeromagnetic data available for the area. The computer program extrapolated contours where coverage was not available. The figure clearly illustrates the arcuate, intracratonic character of the Cuddapah Basin surrounded by the Archean basement. The deep “holes” east of the main basin are essentially Mesozoic down-dropped blocks. A comparison of the lineament map (Fig. 3a) with the basement map (Fig. 4a) will show that parts of lineament zone a correspond clearly with basement fault 17 north of Ongole and basement fault I north of Guntur. Basement fault 8, northeast of Km-no01 corresponds with lineament zone e, and basement fault 5 corresponds with lineament zone a. The strong fracturation north of
Srisailam corresponds with basement fault 9. Basement fault 6 parallels the arcuate geometry of the Nallamalai Thrust Front (fabric map, Fig. 3b and Fig. 1). Interpretation of the gravity map The gravity map for the Cuddapah Basin (Fig. 5) has been reproduced here with auxilliary contours also drawn in to accentuate gravity gradients. Photographic reduction decreases the spacing between contour lines producing dark-zones wherever a steep gradient exists. Thus the most prominent “gravity lineaments” and gradients appear on the figure as dark zones of closely packed gravity contours. Gravity map derived lineaments were interpreted using the following criteria used by Gay (1972): (1) abrupt termination of linear highs and/or lows; (2) abrupt changes in linear gravity gradient trend; (3) linear close-packed contour patterns; and (4) any combination of the above.
247
Fig. 5. Gravity map of the Cuddapah Basin area. Redrawn from Qureshy et al. (1968) with auxilliary contours drawn at intervals of 2 mGai. Stippled zones correspond to gravity-lineaments discussed in text in conjunction with Landsat liieament zones from Fig. 3a.
The Bouguer gravity anomalies range from - 110 mGal to - 60 mGa1 within the margins of the basin. Higher values (up to + 10 mGa1) are generally obtained in the surrounding areas. The eastern margin of the Cuddapah Basin is marked by a strong, gently-convex to the west gravity gradient (and high) that has been explained as being due to the presence of massive anorthositic bodies emplaced during the Eastern Ghats orogeny (Katz, 1976; Kaila and Bhatia, 1981). Kailasam (1976) attributed the gravity anomalies within the Cuddapah Basin to basement topography, whereas Qureshy et al. (1968) considered the anomaly patterns to be due to
intrusive granitic bodies. Seismic sounding has shown that both the above conclusions may be partly applicable to explain the gravity anomalies in the basin (Kaila et al., 1979). The following discussion interprets the gravity map (Fig. 5) further. The gravity map reveals trends corresponding, in part, to lineaments (Fig. 3a) in both the Archean basement surrounding the Cuddapah Basin to the west, to the thrusted basement around the eastern margin of the basin, and within the basin where the basement lies deeply buried by the sediments (Fig. 4a). That one part of a lineament may parallel a strong gravity gradient, whereas another seg-
248
ment of the same lineament shows no expression in the gravity contours, suggests that the basement relief or htholo~c~ character may vary considerably along the trend of the lineament zone. Consequently many of the mapped lineaments parallel local changes in the gradient and, in places, cut across the steep gradients. If the lineament zones obtained from Landsat images represent surface expressions of basement faults, it is likely that they should be in evidence in various aspects of the basin structure and geology. The gravity map provides the much needed additional data to speculate on the basement architecture. Gravity lineaments in the Cuddapah Basin
The most conspicuous feature of the gravity anomaiies over Cuddapah Basin is the broad oval gravity high anomaly (- 60 mGa1) on the western part of the basin. It is effectively surrounded by narrower low anomalies (- 110 mGal to - 100 mGa1). To the east, the north-south trending low-anomaly corresponds to the deepest part of the basin bound by the Nallamalai and Vellikonda thrust fronts (Fig. 1). However, in detail this anomaly is consistently offset in a right-handed sense by northeast trending gravity lineaments (a, Fig. 5). These lineaments correspond extremely well with the northeast trending lineament zones (a, Fig. 3a) and abrupt changes in the tectonic fabric (a, Fig. 3b). These northeast-trending zones can be traced farther to the southwest and are observed as broad deflections of the gravity contours of the southwest Cuddapah Basin high anomaly just south of the Nandyal (Fig. 5). These zones can even be seen as northeast trending deflections in the gravity contours north of the town of Dharmavaram. This broad zone of northeast gravity lineaments probably corresponds to the northeastern extension of the Srisailam lineament recognized by Eremenko et al. (1969), and delineated by Qureshy (1982) as a coincidental magnetic-gravity trend. The sense of displacement and rotation of fabric elements as observed in Fig. 3b, and the offset of gravity gradients is consistently right-handed. It is therefore inferred that northeast trending struct-
ural zones in the Cuddapah Basin are due to right-lateral wrenching of the basement blocks i~ediately north and south of basement fault 5 (Fig. 4a). This deformation is reflected in the cover as lineament zones and folds. Narayanaswami (1966) also documented right-lateral faulting along the northeast trending Veldruti-Gynnayal fault south of Kurnool (see Fig. 1). Gravity lineaments c and b (Fig. 5) can also therefore be interpreted to have a similar sense of movement. Further, gravity lineaments 6, mark the boundaries of the northeast trending Mesozoic Palar Basin (Sastri et al., 1981). The northeastern limits of gravity lineament a near Guntur mark the position of the northern margin of the Krishna-Godavari Mesozoic basin and correspond with basement fault I (Fig. 4a); compare with lineament zone a in Fig. 3a. The gravity map also displays several other prominent gravity lineaments. Lineaments d, are N40”E trending gravity gradients just south of the town of Cuddapah. They mark the southern boundary of the southwest Cuddapah Basin high anomaly. Their orientation is orthogonal to the Archean fabric (Fig. 3b) but are represented by Landsat lineaments (d) in Fig. 3a. These structural zones are probably down-thrown to the northwest and may control the southern margin of the Km-no01 sub-basin (Fig. 1). Gravity lineaments e bound the Kurnool gravity low to the northeast and southwest. The northeastern lineament corresponds clearly with basement fault 8 of Kaila and Tewari (1985) and Fig. 4a. Qureshy et al. (1968) called this alignment the “M~bubnag~-~urnb~ low” and postulated that the turning of the fold axes near Cumbum (Fig. 3b) may be due to transcurrent faulting. They did not, however, suggest a kinematic interpretation. Gravity lineaments f are parallel with Archean basement fabric (Fig. 3b) and gravity lineament h is parallel to the Eastern Ghats erogenic fabric. Finally, gravity lineament g is locally parallel with Archean fabric but is truncated by a northeast-trending gravity gradient (i) that parallels the Eastern Ghats tectonic fabric south of the Cuddapah Basin (compare with Figs. 3a, b). Thus the striking parallelism of the Landsat
249
faults, and
the basement structures, and tectonic denudation
gravity lineaments strongly suggest a genetic inter-
of the inner thrust sheets exposed the underlying
lineaments, relationship.
tectonic
fabric, basement
These
broad
spatial
relationships
ment could be represented
frontal
thrust
system
in a series
of
tectonic
base-
windows such as those east of the Vellikonda
by a rather diffuse
Thrust Front (Fig. 1) exposing the Precambrian
suggest a sharp break in the Precambrian
zone (kilometers wide) of en echelon, and parallel zones of Landsat lineaments.
The tectonic fabric
high grade gram&es and gneisses. The stacking of thrust sheets and the presence of overturned Cud-
as exhibited by the folded “cover” rocks of the
dapah and Kumool
Cuddapah Basin would then faithfully mimic the
wall to the west-verging thrust fronts indicate the
structural discordance
probable buttressing effect of the “Cumbum base-
nucleated on the inhomo-
beds in the immediate foot-
geneities the basement faults provided.
ment-high” on the progressing Eastern Ghats de-
Basin
acted as the westernmost frontal ramp where the
formation. tectonics
and evolution
of the Cuddapah
Similarly, basement
fault 6 (Fig. 4a)
Salient
detachment cut through the thick Cuddapah and Kumool strata of the Kumool sub-basin.
The extent and internal geometry of the Cuddapah Basin was determined by the preexisting tectonic framework where structural elements were apparently reactivated during the Eastern Ghats orogeny (Narayanaswami, 1966; Kaila et al., 1979; Kaila and Tewari, 1985). The Archean structural trends also influenced the distribution of sediments in the Kurnool and Palnad Foreland subbasins which appear to have formed on the de-
The northern margin of the Cuddapah Basin is defined by the featheredge of the Cuddapah and Kumool sediments overstepping the basement fault 5 (Fig. 4a). This fault splays westward to the southwest and is mapped as basement fault 6. The sedimentary section just north of fault 5 (also defined by lineament zones a in Figs. 3a, b, 5) although thinner appears to be complete, (Narayanaswami, 1966), yet sediments deposited
pressed cratonic crust of the footwall to the west verging the Eastern Ghats erogenic belt. Although
to the north of it are relatively undeformed. The deformational complexity seen in the Cud-
the overall geometry of the Kurnool and Palnad sub-basins was set by the Proterozoic cratonic
dapah Basin south of fault 5 can be related thus to the thrusting mode of shortening and the proximity of the basin-edge to the northern faulted margin of the depression. Apparently, the block uplift (during sedimentation) along the basement fault 5 acts as a boundary to the horizontal translation of the thrust fronts and, as such, con-
subsidence (Drury and Holt, 1980; Drury, 1984), locally the sedimentation was affected by differential subsidence of preexisting crustal blocks. A good example is the Kumool sub-basin, which is generally confined to the area between the Nallamalai Thrust Front and basement fault 14 (Fig. 4a). The old structural pattern is even more clearly shown by the geometry of the Nallamalai and Vellikonda thrust fronts (basement faults 6 and 3, Fig. 4a). The Veldruti-Guntur lineament zone (lineament zone a in Figs. 3a, b and 5, and basement faults 7 and 5, Fig. 4a) acted as a structural buttress and lateral ramp for the west verging thrust fronts, and basement faults 6, 13, 14 and 15 in Fig. 4a probably acted as frontal ramps. Incidentally, faults 13 and 14 are located spatially where crustal thickness (to the Moho) decreases as seen in the Deep Seismic Sounding profiles of Kaila et al. (1979) and Kaila and Tewari (1985). Consequently thrusting, uplift of
centrates deformation to the south of it. Since the maximum horizontal stress direction is inferred to be in the east-west direction on the basis of the Nallamalai and Vellikonda fold axial traces throughout the basin and is thus oblique to the trend of the uplifted block, varying amounts of strain occur in the sedimentary section to the south (see Fig. 3b). In the sediments south of the lineament zone a this results in a complex geometry with increased displacement, imbricate thrusting, and decreased fold wavelength as one progresses along the structural trend toward the northeastern part of the Cuddapah Basin (Kaila and Tewari, 1985). A model for this structural setting is presented in Fig. 6a. Deformation inten-
250
sity increases where anticlines pile up against lineament zone c1 (along the Vamikonda Thrust Front). The folds and thrusts thus have a rotational component normal to the thrust plane, appearing as if the northeastern ends of the thrust were “pinned down” with relative displacement
being clockwise and greater, increasing to the southwest. Thus surface evidence for this major rotational component is observed in the interfering foid patterns observed in the Agnigundala area (King, 1872; Narayanaswami, 1959; Ziauddin, 1977). Figure 6b outlines the interference
Fig.
of
6.
a.
Tectonic
sketch
V-G-Veldt+&-Guntur
lateral
5.) C-N-Cuddapah-Nellore foreland);
6--Archean
map
illustrating
the
ramp,
half arrows
shows sense of transpressional
Transverse overthrust
in the west and Vamikonda comprised
of Kurnool
east-west
horizontal
Inset mq: kinematic
scenario
of the Cuddapah about
f-East
Coast during India
modified
and Cuddap~-Nellore
compatible
sense of shear:
fold-thrust
during
east-west
anomaly
a-Archean
thrust
complex
and Palnad
foreland
basement
block bound
bound
shear
by the Nallamalai
basins
from Kaila and Bhatia
(1981);
Thrust
Thrust
with minimal
and Holt (1984, fig. I). Cuddapah
zone under
compression
block bound
(stable cratonic
by Vellikonda
Front
deformation
g--arrows
east-west
of the southern
compression
indicate
block bound
Salient
by M-B
and P-CA
escapes
bound
is also illustrated.
by the Moyar-Bhavani
(A) shear zones. Note the convex to the east fabric of the Dharwar
Basin. The small wedge-shaped
compression.
zone a, Figs. 3a and
orogeny.
from Drury
(C-N)
with northeast(?)
and Anchankovil
Gravity Ghats
Salient
shear axis ramp. (Lineament supracrustal
e-Kurnool
Bouguer Eastern
Cuddapah
r-Dharwar
west-verging
in the northwest;
direction
the
zone) showing
block);
rocks showing
map of south
shear
(P-CA)
Front
rocks;
tectonic
is entirely
and Palghat-Cauvery
Thrust
Group
(V-G)
(supracrustal
Supergroup
compressional
Idealized
Veld~t~-Guntur
tear fault (lineament
basement
Front on the west; d-Cuddapah
kinematics
to the west. Dextral
by the This (M-B)
block to the west
offset on the M-B
is
90 km and 60 km on the P-CA.
b. Tectonic-fabric
map of the area shown in Fig. 3b. Northwest
causes
in the Cuddapah
f$ foiding
transpression
Basin rocks.
of F, folds that are refolded
fabric,
large blank
become
involved
areas are the Vinukonda in the progressive
Progressive
directed
thrusting
thrusting
into west verging north-northeast Domes
deformation.
(Narayanaswami,
across
and buttressing trending
1966) outliers
the Veldruti-Guntur
by the laterat
ramp
lateral results
ramp (V-G)
in ant-handed
Fz folds. Dotted
lines are the Cuddapah
of the Archean
basement
Basin
rock that also have
251
dapah
Basin sediments
the west, north convex
important
is exhibited anomaly Bhatia zone
thrust
by the Eastern
(Figs.
and northwest
geometry.
Ghats
positive
and Tewari
Veldruti-Guntur trending
It is
Salient.
gravity
by Kaila
and
(1985). Thus lineament
Cuddapah-Nellore
to have controlled
ment of the Cuddapah of the Cuddapah
to
giving rise to the
front
6a and b) mapped
trending
tear fault appear
farther
to note that the same geometry
(1981) and Kaila
the northeast I:
of the lineament
to the west
indeed
were transported
the develop-
In the deeper
Basin, all shortening
part
was accom-
modated in the thinned crustal zone of the basin, and basement thrusts and nappes(?) and cover folds developed. Towards the western margins 0
Fig.
VI NUKONDA
6 (continued)
patterns produced by the northwest verging (&) Vamikonda Thrust Front, refolded about the F,axis that was set up with right-lateral shear (transpression?) across the lateral ramp (lineament zone u, Figs. 3a, b and 5) forming the buried northern boundary to deeper parts of the Cuddapah Basin. In the shallower parts of the basin to the west, the wrench fault is expressed on the surface as a series of northeast trending faults between Veldruti and Srisailam. This system of faults has been named the Veldruti-Gunnayal fault system by Narayanaswami (1966) who also identified their strike-slip character (Venkatakrishnan, in prep.). To the south, however, a northwest trending lateral ramp cannot be suggested to explain the arcuate fold-thrust geometries of the Nallamalai and Vellikonda thrust fronts. However, on examining the tectonic fabric within the thrusts (Fig. 3b) a northwest trending lineament zone can be mapped between Cuddapah and Nellore. The counterclockwise rotation of the fabric’from NNW to NW across this lineament zone (the Cuddapah-Nellore lineament zone) suggests a left-lateral component of slip. The change in the trend of the fabric and closures of numerous folds at the lineament suggests that this lineament zone is a tearfault transverse to the erogenic fabric and Cud-
east-west compression was probably accommodate by transcurrent movement between the rigid Precambrian also appears
basement to control
massifs. The buttressing the attitude of the thrust
faults. The steep imbricate faults (basement fault 6) of the Nallamalai thrust front must have been caused by increased resistance to thrusting on the west. An examination of the seismic profiles presented by Kaila and Tewari (1985) shows that the imbricate thrusts (faults 3 and IO) merge to a nearly flat-lying sole thrust at depth to the east. The frontal thrusts (fault 6) are thus generally steeper and less imbricate than their extensions to the southeast (faults 3 and 2).
Conclusions
The Cuddapah
Salient,
which marks the eastern
edge of a zone of frontal breakthrough of the Eastern Ghats erogenic belt, changes in attitude and tectonic style along the northeastern margin of the Cuddapah Basin. Apparently the block uplift along a northeast trending lateral ramp acted as a boundary to the east-west horizontal translation of the Vellikonda and Nallamalai thrust fronts, and as such, concentrates deformation to the south of it along a zone of transpression, the Vamikonda Thrust Front. Differential westward transport of the thrust sheets farther into the Cuddapah embayment provided the frontal buttressing and resulted in the formation of the Cuddapah Salient.
252
Acknowledgements
Kaila,
K.L.
and
Bhatia,
Kavali-Udipi
R.V. wishes to thank Old Dominion University for granting a leave of absence to conduct field work in the Cuddapah Area in 1982-1983. F.D. was a graduate student at the Indian Institute of Technology when this work was undertaken. We wish to thank William E. Decker for developing the computer program to generate Fig. 4b. We also thank B.F. Windley and an anonymous reviewer for suggestions to improve the manuscript.
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The Cuddapah Salient: a tectonic model for the Cuddapah Basin, India, based on Landsat image interpretation RAMESH VENKATAKRISHNAN Department * Department
and FIROZ E. DOTIWALLA *
of Geological Sciences, Old Dominion
of Geology, Civil Engineering
Department,
(Received
March
University, Norfolk,
Indian Insiitute
17.1986;
VA 23508 (U.S.A.)
of Technology, Bombay,
revised and accepted
August
400 0766 (India)
13.1986)
Abstract Venkatakrishnan, India,
Analysis pattern
of geologic,
of tectonic
middle-late
basin
Cuddapah
as structural
transverse orogeny
tear
fronts.
thrust
translation.
clockwise
and greater, resisted
fronts,
Cuddapah
confined
East-west
movement
paralleled
derived
address:
lineament
Veldruti-Guntur
by the east coast gravity
Basin,
on deep crustal
tear faults. The major
faults
the Eastern
lateral
ramp
Ghats
tear fault.
buttress
Steep imbricate
to the west, arcuate (Kaila
(latest
buttressed
with relative
and Bhatia,
and
represent which were are
Cuddapah-Nellore Proterozoic)
the west-verging
acted as a boundary
down”
a
of the
zones recognized
the northwest-trending
during
high anomaly
margin
The lineaments
were “pinned
convex
has revealed
eastern
faults in the Archean
in the basin.
zones that effectively
transverse
The resulting
interpretation
thrusted
The lack of a lateral ramp or basement
Cuddapah-Nellore
for the Cuddapah
trends
and
compression
to east to west
displacement
being
to the south resulted thrust
NalIamalai
ramps
in the
and Vellikonda
1981) is herein named
the
Salient.
Oil and Natural
Gas Commission,
Dehra
Dun, 248195 (India).
oo40-1951/87/$03.50
image and
to have nucleated
to the north
ends of folds and thrusts
model
set by deep crustal
and transverse
The first interpretations of the tectonics, structure and stratigraphy of the Cuddapah Basin in south India date back from the latter part of the last century (King, 1872). Since then, numerous excellent field studies have been conducted notably those by Narayanaswami (1966), Balakrishna et al. (1967), Sen and Narasimha Rao (1967) and recently available Deep Seismic Sounding (DSS) * Present
from Landsat
pattern
that appear
directed
a tectonic
by the folded
and tectonic
ramps,
these converging
westward.
Salient:
136: 237-253
elements
ramp
to the southwest.
of the left-lateral
broadly
alignments and frontal
along the NE-trending
increasing
further
structural
lateral
within
The northeastern
in the development foreland
lateral
to the south.
Block uplift
and lineaments
in both sedimentary
Veldruti-Guntur
fault
Tectonophysics,
Basin in India. A structural
and parallel
uplifts,
was apparently
thrust
data
of Archean
fabric is apparent
the northeast-trending
thrust
geophysical
zones of en echelon
reactivated
F.E., 1987. The Cuddapah
image interpretation.
inheritance
Proterozoic
intracratonic broad
R. and Dotiwalla,
based on Lansat
0 1987 Elsevier Science Publishers
B.V
studies (Kaila and Bhatia, 1981; Kaila and Tewari, 1985), have greatly enhanced our understanding of the tectonics of the area. Unfortunately no systematic attempt appears to have been made to reconcile the different geologic interpretations and the geology of this region to the kinematic processes involved in the formation of the arcuate basin pattern and the convex to the west thrusted eastern margin of the Cuddapah Basin. The eastern part of the basin is located in an area of complex folding and thrust faulting of the Cuddapah sediments and Archean basement that have been transported westward over the
238
basin. Specifically, the basin lies at a major change in the azimuth of the fold axial traces to northeast from a regionally
consistent
Accommodation
for the rotation
ing fold-thrust made
mainly
individual
north-northwest
complexes
appears
by variation
thrust
planes
trend.
of the west vergto have
in displacement
and by oroclinal
been along
bending
ural elements
and nature are poorly
westward
of these complex understood
struct-
due to abun-
vanishes
the deformational
intensity
Nallamalai
fronts
abruptly
the northern
Further all but
dip gently
the north-south
thrust
the thrusts
generally
(Fig. 1,
sediments
sediments.
as the sediments
form
decreases
Front
Kumool
the east. Upon tracing latitude,
Thrust
up older Cuddapah
over younger
to the west
and
of deformation
to the Nallamalai
1) that has brought directly
and
of folds in the cover rocks. The extent
pah Basin. The intensity
north,
Vellikonda past
thrusted
margins
Cuddapah Basin, the Vamikonda Thrust (Fig. 1, 3). On the footwall to these thrust
Ghats erogenic belt composed of complexly deformed Early Proterozoic and Archean crystalline-supracrustals thrust over little deformed middle to late Proterozoic platformal rocks
two major
of the Cuddapah Basin. The basement structures appear to have controlled the convex to the west arcuate thrust fronts and this feature is herein called the Cuddapah Salient. Based on a detailed analysis of computer edgeenhanced Landsat images and available geologic and geophysical data, a kinematic model is deveioped. The arcuate geometry probably reflects foreland buttressing of the advancing west-verging thrust sheets as they piled up along a northeast trending lateral ramp resulting in transpression and refolding of the fold-thrust complex. To the south, lack of buttressing massifs is reflected by northwest trending transverse changes in the tectonic fabric.
tear-faults
and
Geologic setting The Cuddapah Basin is an arcuate, convex to the west basin covering an area of about 35,000 km’ in the peninsula of south India. it is about 350 km north-south and 150 km wide along 15”N latitude. The widest section of the basin lies just south of the major swing in the structural fabric of the area {Fig. 1). Along its eastern margin, the sediments are strongly deformed into a convex to the west oroclinal fold and thrust complex that verges west and northwest. The eastern marginal thrusts, Vellikonda Thrust Front (Fig. 1, 2) have brought up Archean gneisses, granulites and gneisses that in most places directly overlie the Proterozoic sediments that comprise the Cudda-
15 o N
swing northeastward
dant Mesozoic and Cenozoic cover. In general the pre-Mesozoic subcrop consists of the Eastern
foreland
to
sub-basins,
the Kurnool
of the Front fronts and
Palnad sub-basins comprised of little deformed younger supracrustal Proterozoic rocks have been recognized. Stratigraphic studies conducted by Narayanaswami (1966) imply a thickness of at least 10 to 6 km of Proterozoic sediments within the Cuddapah Basin. More recent geophysical studies (Kailasam, 1976; Kaila et al., 1979; Kaila and Bhatia, 1981; Kaila and Tewari, 1985) appear to indicate that at least some of this vast thickness of sediments may be amplified by the westward-directed basement crustal shortening with a component of vertical tectonism. Narayanaswami (1966) and Sen and Narasimha Rao (1967), subdivided the basinal sediments into two major sequences, the lower Cuddapah System separated by a profound unconfor~ty from the upper Kurnool System. Since these studies, both systems have been elevated to Super Group and Group status, respectively (Geological Survey of India, 1981). The Cuddapah rocks are comprised zites, graded pellitic and sea-pellitic
of quartturbidites
and stromatolitic carbonates. Lateral facies variations, local unconformities of various sorts and, a general deepening of the basin to the east is indicated by progressive off-lap to successively younger rocks to the east. The younger (upper Proterozoic) Kurnool rocks overstep the featheredge of the older (upper to lower Proterozoic) Cuddapah Supergroup all along the western margin of the Kurnool and Palnad sub-basins indicating progressive uplift (and possibly west vergent thrusting?) during the Kurnool Group foreland basin development. It is indeed interesting that the Kurnool Group is entirely devoid of
239
........................... ...........................
...........................
........................... ....................... ....................
....................
KURNOOL
SUD;BASIN
MB0
.15O
. ......... 0~HARMAVARAM . . ................. . . ................... . . .................... .. ..
.........
Ido N
;O”E
I-NALLAMALAI 2- VELLIKONDA 3- VAMIKONDA ,
1 q&J I
THRUST FRONT THRUST FRONT THRUST FRONT
EXPlANATION
-I COASTAL PROTEROZOIC
PLAIN
HEAN
SEDIMENTS
SUPRACRUSTALS
SUPRACRUSTALS KHONDALITES
Major I-c/r-l
thrusts faults
ARCHEAN (PENINSULAR) GNEISSES/GRANITES
b other
Fig. 1. Generalized
geologic map of the Cuddapah
magrnatism. The lower part of the Cuddapah Supergroup is, however, characterized by an extensive volcanic and plutonic suite of basalts, gabbros, dolerites, and a few felsites. Crawford (1969a,b) and Crawford and Compton (1973) ob-
Basin area.
tained a Rb-Sr date of 980 + 110 Ma from one of the youngest post-Cuddapah sills, and an age of 1583 k 147 Ma from one of the oldest sills. These ages provide respectively a minimum and maximum age for Cuddapah Supergroup sedimen-
240
tation. Crawford and Compton (1973) also obtained a 1225 Ma Rb-Sr mineral age from a
Kaila et al., 1979, pp. 326-327).
kimberlite dike that intrudes the Cuddapah sediments. This clearly indicates at least some deep
to the north, west and south is intruded by extensive swarms of tholeiite dikes. These indicate a
crustal fracturing during basining events (see also
model age of 1700 Ma-so
LANDSAT
Fig. 2. Landsat ERTS:
image mosaic compiled
E-2054-04321-6,
E-1218-04325-7,
MOSAIC
The Archean basement surrounding the basin
c;:L
from 1 : 1,000,000 scale black and white prints. (Scene identification E-1183-04382-7,
must be entirely pre-
E-2035-04264-7.
E-1184-0441-6,
E-1218-04334-7
numbers
NRSA
820404,
and E-1218-04334-7.)
241
Cuddapah formed
(Drury Archean
the Cuddapah clearly
et al., 1984). The intensely basement-fabric
Basin
indicates
the Dharwar
is convex
the antiquity
Craton
unlike
de-
that
of The abundance
to the east-this of the structures
to the west (Drury
Interpretation of lineament map
in
and Holt,
of lineaments
makes it an impossible ment
with respect
to known
1980). The basement rocks of the study area are, for the most part, covered by a thick sequence of
more efficient
sedimentary
area, however, the distributions
prevent
direct
knowledge and
rocks of the Cuddapah
their
understood,
observation
and
of the characteristics structure
is only
especially
Basin which
study.
A better
of these
now beginning
with the recent
is rather
simple
readily
to be
statistically
of Deep Seismic Sounding (DSS) (Kaila et al., 1979; Kaila and Tewari, 1985). More specifically, a better knowledge is needed to allow us to conceive or to build models upon which to base our exploration for mineral or petroleum resources. Information pertaining to the character and structure of the basement rocks is obtained from a variety of sources: rock cuttings and cores from wells; aeromagnetic, seismic and gravity surveys; structure and stratigraphy of the supracrustal sedimentary cover; and remote sensing derived lineament maps. Information obtained by any one of these methods is of limited use; however, by integrating these bits of information, much can be learned. This paper attempts to develop a tectonic model for the Cuddapah Basin extracting information from computer edge-enhanced Landsat images (Fig. 2) and combining this data with information from published sources. The identification and interpretation of the Cuddapah Salient linked to basement structures is based on several factors, such as structural patterns interpreted from Landsat MSS images, including lineaments and deep crustal faults previously postulated by others. In addition, the Precambrian tectonic model for south India developed by Drury et al. (1984) is extended to the north to include the Cuddapah Basin. The model is not entirely different from the one proposed by Katz (1976) but is appealing because of its simplicity. Katz (1976) had suggested transform movement along the coastal Bouguer gravity-anomaly (Fig. 6,f) to create the basin as a pull-apart and subsequent deformation.
or available
data.
A
would be the one developed
by Sawatsky and Raines (1981) and recently applied by Maars and Raines (1984). In this study
rocks
availability
method
on satellite images
task to discuss each linea-
discerned
and
trends
patterns
can be
distinctive
(hence
analyze
of lineament
there is no real need to
the lineament
patterns).
discussions will therefore be restricted sion groups of en enchelon or parallel sets that lineament
Our
to discuslineament
would define a lineament zone. Each zone is then discussed as one composite
feature without validating individual lineaments within the zone. The striking difference in lineament density between the Cuddapah Basin and its surroundings is immediately apparent (Figs. 2 and 3a). The eastern portion of the map also has a reduced lineament density because of the masking effects of the almost featureless coastal plain sediment cover. The Precambrian terrain is, however, intensely fractured and among the welter of a large number of lineament out known structural
trends, by carefully filtering fabric, those lineaments that
may correspond to cross-strike fractures other than foliation and cross-cutting dike trends, a viable interpretation is possible. (This procedure is imperative because the post-Cuddapah deformation cross-cuts earlier fabric.) For this reason, Fig. 3a and b will be discussed together. Figure 3a is a map of all mapped lineaments within the eight Landsat frames used for mapping. Figure 3b is a compilation of the tectonic “fabric” elements in the same area. For ease in comparison, fabric elements within the Cuddapah Basin are differentiated from the preexisting Archean basement fabric and cross-cutting pre-Cuddapah dike swarms that infest the basement. The pre-Cuddapah dike swarms occur in three main groups: with ENE-WSW, NW-SE and NNE-SSW trends. Only northwest of Veldruti and Kumool do they locally follow the Archean fabric. Only a selection of these dikes has been drawn on Fig. 3b for the sake of clarity in presen-
242
tation. It is highly likely that the majority of the N75-80° E trending lineaments south of Anantapur (AN) and the Cuddapah Basin margin are entirely occupied by these dikes (Fig. 34. The large swarm of NlO-15 ‘E trending lineaments north of Srisailam (S on Fig. 3a) are also probably related to the NNE-SSW trending dike swarms in that area. However, unlike the N75-80°E trend-
Fig. 3. a. Lineament
map identifying
dashed line: combination ~-M~bubnag~, image
interpretation.
within basin. Arrows
of topographic
N-Nandyal, “Fabric”
major
lineament
identify
mark major hneament-zones
Dome south of Srisailam.
zones and locations
VE-Veldruti, planar (stippled
Boxed area north of Vinukonda
fabric
discussed
in text. Solid line: topographic
Agnigundala, AN-Anantpur,
A-
and tonal alignments.
S-Srisailam, elements
ing swarms, these lineaments are also expressed within Palnad sub-basin possibly due to fracture propagation and reactivation during the Eastern Ghats orogeny. The N50-70* W trending dike swarm (see Fig. 3a, lineament zone e) is also expressed within the Kurnool sub-basin; again possibly due to reactivation.
VI-Vinukonda. in the Archean
pattern)
recognized
is for Fig. 6b.
b. Tectonic basement
D-Dharmavaram, fabric
map compiled
lineament; G-Guntakal, from Landsat
rock and strike of Cuddapah
by combining
sediments
Figs. 3a and 3b. Note Eswarakuppam
243
kmi
CUDDAPAH
BAS IN
“FABRIC”
Fig. 3. (continued)
Cross-structural lineament zones The following is a discussion of lineament zones that cannot obviously be related to preexisting fabric elements. These lineament zones are considered features that cut across the Precambrian fabric and as will be shown below have played a crucial and hitherto unrecognized role in the structural development of the Cuddapah Basin area. Within the Cuddapah Basin the most prominent trends are a N65-70°E corridor of en echelon and parallel lineaments that can be traced from Guntakal (G on Fig. 3a) past Nandyal (N), Vellikonda (T/I) to Guntur. This lineament zone is
confined to a 50&m swath between Agnigundala (A) and Cumbum. Another similar trending lineament set can be traced from Anantapur (AN) across the basin to Ongole. These two northeast trending lineament zones have been identified as lineament zones (a) in Fig. 3a and b. A lineament swarm between Cumbum and Nandyal (N) in Fig. 3a is related to intense fracturation within the ~warakuppam Dome complex (see for example, Ramaswamy et al., 1983) possibly due (1) to the brittle nature of the outcropping rocks, and (2) this also marks the zone of oroclinal bending of the folds and thrusts (see Fig. 3b). Lineament zone a between Veldruti (VE) and Nandyal (N) also corresponds with the
244
mapped
extent
of the Veldruti-Gunnayal
zone of Narayanaswami Lineament the margins N60-70
OE and
forms
Cuddapah.
These unmapped
they are recognizable Lineament strong
zone
lineaments
and encloses zone
c trends
a discontinuous
that can be traced
across
southwestward
probably basement
band
that trend
have been delineated
only to express their
in the development
model for the Cuddapah
of the
tectonic
Salient.
the Cuddato south of
correspond fabric
with
elements
as a broad N20°E,
Basement of the Cuddapah Basin
of
as
on the gravity maps. i occurs
eaments
importance
Palar Mesozoic basin
Lineament
pah Basin from Nellore hitherto
N40”E
of the subsurface
et al., 1981).
lineaments
(1966, p. 44).
zone b trends
(Sastri
fault
The Precambrian Cuddapah simple
depression
mately
centered
The zone
of
south from
the town of Cuddapah and continue beyond the margins of the basin. Farther to the south and east this trend is parallel to the Eastern Ghats tectonic fabric but southwest of Cuddapah it entirely cuts across the fabric (compare Figs. 3a and b). Northwest trending lineaments are not as well developed as northeast trending sets. Several narrow lineament zones (e, Fig. 3a) can be defined in somewhat equally spaced bands from Nandyal (N) to Veldruti (I%) and Kurnool and beyond. Similar trending lineament swarms extending from Srisailam (S) to Mahbubnagar (M) are locally parallel with the Archean fabric and numerous pre-Cuddapah dikes also exhibit similar trends. It is indeed interesting to note that many lineament zones belonging to this trend appear to continue well into the interior of the Cuddapah Basin and similar lineaments can be traced between Cudda-
depth
shallow Until
basement
Basin was depicted with
configuration
a central
over Cumbum
to basement
of the
as an arcuate high
by Glennie
is known
and
approxi(1951).
from
a few
drill holes on the very edge of the basin. the recent
availability
of DSS-data
(Kaila
and Tewari, 1985), the basement architecture was not well known. Glenie’s (1951) map depicted a basement that generally deepened to the east from the western basin margins devoid of topographic relief except for the Cumbum high-wherein Glennie wrongly considered the core of the Eswarakuppam Dome to be comprised of basement rock. This map did, however, show a marked steepening of the basement surface toward the eastern margin of the basin. The Cuddapah Basin is
a
composite
in-
pah and Nellore. The Penner River locally parallels this lineament zone. This lineament zone is
tracratonic basin (Fig. 4a) and features indicating plate margin tectonism (Sarkar, 1982) such as the presence of volcanics, acid intrusives, ophiolites and high-pressure, low-temperature metamorphic suites are entirely absent; only retrograde metamorphism of the Archean granites, gneisses and grant&es of the basement appears to have occurred during the Eastern Ghats orogeny. The basin can be divided into four somewhat separate
distinctive largely because of variations in the tectonic fabric alignment; northwest oriented topographic features are subdued in Landsat images rather than enhanced by the similarly ori-
depressions and ridges bound by major basement faults. The younger Kumool sub-basin is located entirely west of basement fault 6. It is bound by faults 8 and 7 to the north and a series of possibly
ented solar azimuth (approximately N130 ’ ). The sense of rotation of the Cuddapah Basin fabric across the Cuddapah-Nellore lineament zone suggests left-lateral shear (Fig. 3b). If these lineaments and lineament zones represent the surface manifestations of deep-seated (basement) faults, they should then be depicted on the surface geology as variations of the fabric trend, localized increase in the intensity of deformation, offset fabric elements, etc. Figure 3b is one such compilation. Major cross-structural lin-
strike-slip faults to the south near the town of Cuddapah (Fig. 1). The basement in this sub-basin generally dips gently to the east and attains a depth of about 9000 m near fault 6. To the east the basement rises up to within 2000 m of the surface near fault 14 in the south and is within 500 m of the ground surface near fault 7 to the north (Kaila and Tewari, 1985). Lack of depth control in the central part of the Kumool sub-basin precludes better estimates of basement depth. The Palnad sub-basin containing Kumool
245
the south and to about 7000 m in the north. Faults 4 and fl are both high-angle normal-slip faults, hence the basement ridge may be considered an upthrown horst bound by fault 6 to the west. Outside the basin margins, the basement (Kaila and Tewari, 1985, Fig. 1) information is better especially for the Krishna-Godavari Mesozoic basin and in the Palar Mesozoic basin north of Madras (Sastry et al., 1981). These basins are bound by low-angle normal faults (1, 2 7 and IO)
Group rocks lies entirely north of fault 5 and is bound by fault 9 to the west. No basement data is availabIe beneath the Painad sub-basin. The central part of the Cuddapah Basin bound by faults 6 and 3 is essentially the deepest part of the basin. The basement surface rises up to about 5000 m just north of Cumbum in an arcuate, NE-trending ridge culminating beneath the Eswarakuppam Dome. The basement slopes rather steeply to depths of over 10,000 m near fault 3 in
i
“G-PALAR
‘I
SASI~
L Fig. 4. a. Basement-tectonic map of the Cuddapah Basin and surroundings. (Redrawn from Kaila and Tewari (1985) and Sastri et al. (1981).) b. ~~~ion~
stereoscopic block-diagram of the Cuddapah Basin basement topography. View is from the southeast
and aligned for viewing with pocket stereoscope. Small depression in foreground in Palar Basin and depression to the north of it is where the basement has been downdropped east of basement faults 10 and 17. The Bay of Bengal has been drawn at 3500 m below sea level. (Vertical scale exaggeration is 2 X .)
Fig. 4 (continued)
and obviously are controlled by normal-slip reactivation of the Precambrian thrust faults during Mesozoic rifting events. To help visualize the basement architecture, Fig. 4b was developed. It is presented as a stereoscopic view suited for viewing with a pocket stereoscope. In the areas where basement depth was unknown, crude estimates were calculated using the gravity and aeromagnetic data available for the area. The computer program extrapolated contours where coverage was not available. The figure clearly illustrates the arcuate, intracratonic character of the Cuddapah Basin surrounded by the Archean basement. The deep “holes” east of the main basin are essentially Mesozoic down-dropped blocks. A comparison of the lineament map (Fig. 3a) with the basement map (Fig. 4a) will show that parts of lineament zone a correspond clearly with basement fault 17 north of Ongole and basement fault I north of Guntur. Basement fault 8, northeast of Km-no01 corresponds with lineament zone e, and basement fault 5 corresponds with lineament zone a. The strong fracturation north of
Srisailam corresponds with basement fault 9. Basement fault 6 parallels the arcuate geometry of the Nallamalai Thrust Front (fabric map, Fig. 3b and Fig. 1). Interpretation of the gravity map The gravity map for the Cuddapah Basin (Fig. 5) has been reproduced here with auxilliary contours also drawn in to accentuate gravity gradients. Photographic reduction decreases the spacing between contour lines producing dark-zones wherever a steep gradient exists. Thus the most prominent “gravity lineaments” and gradients appear on the figure as dark zones of closely packed gravity contours. Gravity map derived lineaments were interpreted using the following criteria used by Gay (1972): (1) abrupt termination of linear highs and/or lows; (2) abrupt changes in linear gravity gradient trend; (3) linear close-packed contour patterns; and (4) any combination of the above.
247
Fig. 5. Gravity map of the Cuddapah Basin area. Redrawn from Qureshy et al. (1968) with auxilliary contours drawn at intervals of 2 mGai. Stippled zones correspond to gravity-lineaments discussed in text in conjunction with Landsat liieament zones from Fig. 3a.
The Bouguer gravity anomalies range from - 110 mGal to - 60 mGa1 within the margins of the basin. Higher values (up to + 10 mGa1) are generally obtained in the surrounding areas. The eastern margin of the Cuddapah Basin is marked by a strong, gently-convex to the west gravity gradient (and high) that has been explained as being due to the presence of massive anorthositic bodies emplaced during the Eastern Ghats orogeny (Katz, 1976; Kaila and Bhatia, 1981). Kailasam (1976) attributed the gravity anomalies within the Cuddapah Basin to basement topography, whereas Qureshy et al. (1968) considered the anomaly patterns to be due to
intrusive granitic bodies. Seismic sounding has shown that both the above conclusions may be partly applicable to explain the gravity anomalies in the basin (Kaila et al., 1979). The following discussion interprets the gravity map (Fig. 5) further. The gravity map reveals trends corresponding, in part, to lineaments (Fig. 3a) in both the Archean basement surrounding the Cuddapah Basin to the west, to the thrusted basement around the eastern margin of the basin, and within the basin where the basement lies deeply buried by the sediments (Fig. 4a). That one part of a lineament may parallel a strong gravity gradient, whereas another seg-
248
ment of the same lineament shows no expression in the gravity contours, suggests that the basement relief or htholo~c~ character may vary considerably along the trend of the lineament zone. Consequently many of the mapped lineaments parallel local changes in the gradient and, in places, cut across the steep gradients. If the lineament zones obtained from Landsat images represent surface expressions of basement faults, it is likely that they should be in evidence in various aspects of the basin structure and geology. The gravity map provides the much needed additional data to speculate on the basement architecture. Gravity lineaments in the Cuddapah Basin
The most conspicuous feature of the gravity anomaiies over Cuddapah Basin is the broad oval gravity high anomaly (- 60 mGa1) on the western part of the basin. It is effectively surrounded by narrower low anomalies (- 110 mGal to - 100 mGa1). To the east, the north-south trending low-anomaly corresponds to the deepest part of the basin bound by the Nallamalai and Vellikonda thrust fronts (Fig. 1). However, in detail this anomaly is consistently offset in a right-handed sense by northeast trending gravity lineaments (a, Fig. 5). These lineaments correspond extremely well with the northeast trending lineament zones (a, Fig. 3a) and abrupt changes in the tectonic fabric (a, Fig. 3b). These northeast-trending zones can be traced farther to the southwest and are observed as broad deflections of the gravity contours of the southwest Cuddapah Basin high anomaly just south of the Nandyal (Fig. 5). These zones can even be seen as northeast trending deflections in the gravity contours north of the town of Dharmavaram. This broad zone of northeast gravity lineaments probably corresponds to the northeastern extension of the Srisailam lineament recognized by Eremenko et al. (1969), and delineated by Qureshy (1982) as a coincidental magnetic-gravity trend. The sense of displacement and rotation of fabric elements as observed in Fig. 3b, and the offset of gravity gradients is consistently right-handed. It is therefore inferred that northeast trending struct-
ural zones in the Cuddapah Basin are due to right-lateral wrenching of the basement blocks i~ediately north and south of basement fault 5 (Fig. 4a). This deformation is reflected in the cover as lineament zones and folds. Narayanaswami (1966) also documented right-lateral faulting along the northeast trending Veldruti-Gynnayal fault south of Kurnool (see Fig. 1). Gravity lineaments c and b (Fig. 5) can also therefore be interpreted to have a similar sense of movement. Further, gravity lineaments 6, mark the boundaries of the northeast trending Mesozoic Palar Basin (Sastri et al., 1981). The northeastern limits of gravity lineament a near Guntur mark the position of the northern margin of the Krishna-Godavari Mesozoic basin and correspond with basement fault I (Fig. 4a); compare with lineament zone a in Fig. 3a. The gravity map also displays several other prominent gravity lineaments. Lineaments d, are N40”E trending gravity gradients just south of the town of Cuddapah. They mark the southern boundary of the southwest Cuddapah Basin high anomaly. Their orientation is orthogonal to the Archean fabric (Fig. 3b) but are represented by Landsat lineaments (d) in Fig. 3a. These structural zones are probably down-thrown to the northwest and may control the southern margin of the Km-no01 sub-basin (Fig. 1). Gravity lineaments e bound the Kurnool gravity low to the northeast and southwest. The northeastern lineament corresponds clearly with basement fault 8 of Kaila and Tewari (1985) and Fig. 4a. Qureshy et al. (1968) called this alignment the “M~bubnag~-~urnb~ low” and postulated that the turning of the fold axes near Cumbum (Fig. 3b) may be due to transcurrent faulting. They did not, however, suggest a kinematic interpretation. Gravity lineaments f are parallel with Archean basement fabric (Fig. 3b) and gravity lineament h is parallel to the Eastern Ghats erogenic fabric. Finally, gravity lineament g is locally parallel with Archean fabric but is truncated by a northeast-trending gravity gradient (i) that parallels the Eastern Ghats tectonic fabric south of the Cuddapah Basin (compare with Figs. 3a, b). Thus the striking parallelism of the Landsat
249
faults, and
the basement structures, and tectonic denudation
gravity lineaments strongly suggest a genetic inter-
of the inner thrust sheets exposed the underlying
lineaments, relationship.
tectonic
fabric, basement
These
broad
spatial
relationships
ment could be represented
frontal
thrust
system
in a series
of
tectonic
base-
windows such as those east of the Vellikonda
by a rather diffuse
Thrust Front (Fig. 1) exposing the Precambrian
suggest a sharp break in the Precambrian
zone (kilometers wide) of en echelon, and parallel zones of Landsat lineaments.
The tectonic fabric
high grade gram&es and gneisses. The stacking of thrust sheets and the presence of overturned Cud-
as exhibited by the folded “cover” rocks of the
dapah and Kumool
Cuddapah Basin would then faithfully mimic the
wall to the west-verging thrust fronts indicate the
structural discordance
probable buttressing effect of the “Cumbum base-
nucleated on the inhomo-
beds in the immediate foot-
geneities the basement faults provided.
ment-high” on the progressing Eastern Ghats de-
Basin
acted as the westernmost frontal ramp where the
formation. tectonics
and evolution
of the Cuddapah
Similarly, basement
fault 6 (Fig. 4a)
Salient
detachment cut through the thick Cuddapah and Kumool strata of the Kumool sub-basin.
The extent and internal geometry of the Cuddapah Basin was determined by the preexisting tectonic framework where structural elements were apparently reactivated during the Eastern Ghats orogeny (Narayanaswami, 1966; Kaila et al., 1979; Kaila and Tewari, 1985). The Archean structural trends also influenced the distribution of sediments in the Kurnool and Palnad Foreland subbasins which appear to have formed on the de-
The northern margin of the Cuddapah Basin is defined by the featheredge of the Cuddapah and Kumool sediments overstepping the basement fault 5 (Fig. 4a). This fault splays westward to the southwest and is mapped as basement fault 6. The sedimentary section just north of fault 5 (also defined by lineament zones a in Figs. 3a, b, 5) although thinner appears to be complete, (Narayanaswami, 1966), yet sediments deposited
pressed cratonic crust of the footwall to the west verging the Eastern Ghats erogenic belt. Although
to the north of it are relatively undeformed. The deformational complexity seen in the Cud-
the overall geometry of the Kurnool and Palnad sub-basins was set by the Proterozoic cratonic
dapah Basin south of fault 5 can be related thus to the thrusting mode of shortening and the proximity of the basin-edge to the northern faulted margin of the depression. Apparently, the block uplift (during sedimentation) along the basement fault 5 acts as a boundary to the horizontal translation of the thrust fronts and, as such, con-
subsidence (Drury and Holt, 1980; Drury, 1984), locally the sedimentation was affected by differential subsidence of preexisting crustal blocks. A good example is the Kumool sub-basin, which is generally confined to the area between the Nallamalai Thrust Front and basement fault 14 (Fig. 4a). The old structural pattern is even more clearly shown by the geometry of the Nallamalai and Vellikonda thrust fronts (basement faults 6 and 3, Fig. 4a). The Veldruti-Guntur lineament zone (lineament zone a in Figs. 3a, b and 5, and basement faults 7 and 5, Fig. 4a) acted as a structural buttress and lateral ramp for the west verging thrust fronts, and basement faults 6, 13, 14 and 15 in Fig. 4a probably acted as frontal ramps. Incidentally, faults 13 and 14 are located spatially where crustal thickness (to the Moho) decreases as seen in the Deep Seismic Sounding profiles of Kaila et al. (1979) and Kaila and Tewari (1985). Consequently thrusting, uplift of
centrates deformation to the south of it. Since the maximum horizontal stress direction is inferred to be in the east-west direction on the basis of the Nallamalai and Vellikonda fold axial traces throughout the basin and is thus oblique to the trend of the uplifted block, varying amounts of strain occur in the sedimentary section to the south (see Fig. 3b). In the sediments south of the lineament zone a this results in a complex geometry with increased displacement, imbricate thrusting, and decreased fold wavelength as one progresses along the structural trend toward the northeastern part of the Cuddapah Basin (Kaila and Tewari, 1985). A model for this structural setting is presented in Fig. 6a. Deformation inten-
250
sity increases where anticlines pile up against lineament zone c1 (along the Vamikonda Thrust Front). The folds and thrusts thus have a rotational component normal to the thrust plane, appearing as if the northeastern ends of the thrust were “pinned down” with relative displacement
being clockwise and greater, increasing to the southwest. Thus surface evidence for this major rotational component is observed in the interfering foid patterns observed in the Agnigundala area (King, 1872; Narayanaswami, 1959; Ziauddin, 1977). Figure 6b outlines the interference
Fig.
of
6.
a.
Tectonic
sketch
V-G-Veldt+&-Guntur
lateral
5.) C-N-Cuddapah-Nellore foreland);
6--Archean
map
illustrating
the
ramp,
half arrows
shows sense of transpressional
Transverse overthrust
in the west and Vamikonda comprised
of Kurnool
east-west
horizontal
Inset mq: kinematic
scenario
of the Cuddapah about
f-East
Coast during India
modified
and Cuddap~-Nellore
compatible
sense of shear:
fold-thrust
during
east-west
anomaly
a-Archean
thrust
complex
and Palnad
foreland
basement
block bound
bound
shear
by the Nallamalai
basins
from Kaila and Bhatia
(1981);
Thrust
Thrust
with minimal
and Holt (1984, fig. I). Cuddapah
zone under
compression
block bound
(stable cratonic
by Vellikonda
Front
deformation
g--arrows
east-west
of the southern
compression
indicate
block bound
Salient
by M-B
and P-CA
escapes
bound
is also illustrated.
by the Moyar-Bhavani
(A) shear zones. Note the convex to the east fabric of the Dharwar
Basin. The small wedge-shaped
compression.
zone a, Figs. 3a and
orogeny.
from Drury
(C-N)
with northeast(?)
and Anchankovil
Gravity Ghats
Salient
shear axis ramp. (Lineament supracrustal
e-Kurnool
Bouguer Eastern
Cuddapah
r-Dharwar
west-verging
in the northwest;
direction
the
zone) showing
block);
rocks showing
map of south
shear
(P-CA)
Front
rocks;
tectonic
is entirely
and Palghat-Cauvery
Thrust
Group
(V-G)
(supracrustal
Supergroup
compressional
Idealized
Veld~t~-Guntur
tear fault (lineament
basement
Front on the west; d-Cuddapah
kinematics
to the west. Dextral
by the This (M-B)
block to the west
offset on the M-B
is
90 km and 60 km on the P-CA.
b. Tectonic-fabric
map of the area shown in Fig. 3b. Northwest
causes
in the Cuddapah
f$ foiding
transpression
Basin rocks.
of F, folds that are refolded
fabric,
large blank
become
involved
areas are the Vinukonda in the progressive
Progressive
directed
thrusting
thrusting
into west verging north-northeast Domes
deformation.
(Narayanaswami,
across
and buttressing trending
1966) outliers
the Veldruti-Guntur
by the laterat
ramp
lateral results
ramp (V-G)
in ant-handed
Fz folds. Dotted
lines are the Cuddapah
of the Archean
basement
Basin
rock that also have
251
dapah
Basin sediments
the west, north convex
important
is exhibited anomaly Bhatia zone
thrust
by the Eastern
(Figs.
and northwest
geometry.
Ghats
positive
and Tewari
Veldruti-Guntur trending
It is
Salient.
gravity
by Kaila
and
(1985). Thus lineament
Cuddapah-Nellore
to have controlled
ment of the Cuddapah of the Cuddapah
to
giving rise to the
front
6a and b) mapped
trending
tear fault appear
farther
to note that the same geometry
(1981) and Kaila
the northeast I:
of the lineament
to the west
indeed
were transported
the develop-
In the deeper
Basin, all shortening
part
was accom-
modated in the thinned crustal zone of the basin, and basement thrusts and nappes(?) and cover folds developed. Towards the western margins 0
Fig.
VI NUKONDA
6 (continued)
patterns produced by the northwest verging (&) Vamikonda Thrust Front, refolded about the F,axis that was set up with right-lateral shear (transpression?) across the lateral ramp (lineament zone u, Figs. 3a, b and 5) forming the buried northern boundary to deeper parts of the Cuddapah Basin. In the shallower parts of the basin to the west, the wrench fault is expressed on the surface as a series of northeast trending faults between Veldruti and Srisailam. This system of faults has been named the Veldruti-Gunnayal fault system by Narayanaswami (1966) who also identified their strike-slip character (Venkatakrishnan, in prep.). To the south, however, a northwest trending lateral ramp cannot be suggested to explain the arcuate fold-thrust geometries of the Nallamalai and Vellikonda thrust fronts. However, on examining the tectonic fabric within the thrusts (Fig. 3b) a northwest trending lineament zone can be mapped between Cuddapah and Nellore. The counterclockwise rotation of the fabric’from NNW to NW across this lineament zone (the Cuddapah-Nellore lineament zone) suggests a left-lateral component of slip. The change in the trend of the fabric and closures of numerous folds at the lineament suggests that this lineament zone is a tearfault transverse to the erogenic fabric and Cud-
east-west compression was probably accommodate by transcurrent movement between the rigid Precambrian also appears
basement to control
massifs. The buttressing the attitude of the thrust
faults. The steep imbricate faults (basement fault 6) of the Nallamalai thrust front must have been caused by increased resistance to thrusting on the west. An examination of the seismic profiles presented by Kaila and Tewari (1985) shows that the imbricate thrusts (faults 3 and IO) merge to a nearly flat-lying sole thrust at depth to the east. The frontal thrusts (fault 6) are thus generally steeper and less imbricate than their extensions to the southeast (faults 3 and 2).
Conclusions
The Cuddapah
Salient,
which marks the eastern
edge of a zone of frontal breakthrough of the Eastern Ghats erogenic belt, changes in attitude and tectonic style along the northeastern margin of the Cuddapah Basin. Apparently the block uplift along a northeast trending lateral ramp acted as a boundary to the east-west horizontal translation of the Vellikonda and Nallamalai thrust fronts, and as such, concentrates deformation to the south of it along a zone of transpression, the Vamikonda Thrust Front. Differential westward transport of the thrust sheets farther into the Cuddapah embayment provided the frontal buttressing and resulted in the formation of the Cuddapah Salient.
252
Acknowledgements
Kaila,
K.L.
and
Bhatia,
Kavali-Udipi
R.V. wishes to thank Old Dominion University for granting a leave of absence to conduct field work in the Cuddapah Area in 1982-1983. F.D. was a graduate student at the Indian Institute of Technology when this work was undertaken. We wish to thank William E. Decker for developing the computer program to generate Fig. 4b. We also thank B.F. Windley and an anonymous reviewer for suggestions to improve the manuscript.
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