- Email: [email protected]
Structural evolution of the Simla area, NW Himalayas: implications for crustal thickening
Journal of Southeast Asian Earth Sciences, Vol. 6, No. 1, pp. 41-53, 1991 Printed in Great Britain
0743-9547/91 $3.00 + 0.00 Pergamon Press plc
Structural evolution of the Simla area, NW Himalayas: implications for crustal thickening A. K. DUBEY and M. I. BHAT Wadia Institute of Himalayan Geology, 33 Gen. Mahadev Singh Road, Dehra Dun-248001, India (Received 6 June 1989; accepted for publication 6 November 1990) Abstract---Crustal thickening in the Himalayas is widely attributed to the two processes operational during continental collision in the region: firstly, internal deformation, folding and thrust-stacking in the upper crust, and secondly, underthrusting of the Indian plate crust. A significant contribution to the crustal thickening from the structural deformation is based on the notion that folding in the region was followed by thrusting. However, the geometry, orientation, and spatial distribution of folds and faults in and around the famous Sirnla Klippe in the NW Lower Himalayas indicate that folding and thrusting initiated and developed stimultaneously. Also, synsedimentary normal faults, indicative of a tensional regime, are the earliest structural features present in the area. Modelling of the structual evolution with these constraints suggests that the reactivation of rift-related, normal, listric faults was a key factor in controlling the orientation and spatial distribution of the structural elements in the area. While such a model is compatible with the pre-orogenic rift tectonics of the region, it puts severe limitations on the amount of possible crustal shortening, and in turn, crustal thickening through structural deformation. Furthermore, we argue that contribution to crustal thickening from underthrusting of the Indian plate crust needs re-evaluation in the light of known mafic magrnatic additions to the lower crust during lithosphetic rifting.
INTRODUCTION DOUBLE the normal crustal thickeness in the Himalayan region is generally accepted as a result of crustal shortening during the Tertiary orogenic compression. The two mechanisms suggested in achieving the thickening are: (1) folding and thrust-stacking in the upper part; and (2) underthrusting of the Indian plate in the lower part. The contribution through the first mechanism, though never quantified in net terms, hinges on the belief that folding was followed by thrusting. This view appears to be an off-shoot of a study by Ray and Naha (1971) on the structural development of the now bestknown klippe in the Himalayas--the Simla Klippe-wherein the authors suggested recumbent folding was followed by thrusting. This study acted as a typical example for application in other parts of the Himalayas and for generalising structural evolution for the whole Himalayan belt. Experiments on the deformation of physical models (Dubey and Behzadi 1981) indicate that folding takes at least 45% shortening before the initiation of thrusting. Therefore, if the concept of folding followed by thrusting is valid, it would suggest that structural deformation of the upper crust has accommodated large amounts of crustal shortening, and contributed substantially to the thickening of the crust in the region. As for the contribution through the second mechanism, it relies on the interpretation of the geophysical (especially palaeomagnetic) data, and on constraints imposed by plate tectonic reconstructions. Whether the earlier view, involving underthrusting of the whole normal Indian plate crust (Powell and Conaghan 1973), or the recent postulate, suggesting separate doubling by decoupling and thrusting of the upper and lower crustal layers (Hirn et al. 1984) is accepted, it is clear that the second mechanism has to be the major contributor to the crustal thickening in the region. $EAES6 / 1 ~
41
Implicit in this view is that pre-orogenic tectonic activity in the region has not played a role in either the structural evolution or the crustal thickening. Such a view appears too simplistic and is unique to the discussions on the crustal and structural evolution of the Himalayas. Studies in other orogenic belts (e.g. Zagros belt: Jackson 1980, Andean Foreland fold belt: Winslow 1981, external French Alps: Davies 1982, Welbon 1988, Scottish Dalradian: Soper and Anderton 1984, Bass Strait basins of southeastern Australia: Etheridge 1986, Proterozoic Davenport province, central Australia: Stewart 1987), theoretical considerations (Le Pichon and Sibuet 1981), and experimental results (Dubey and Bhat 1986a) have established that structural evolution of an orogenic belt is strongly controlled by pre-existing basement normal, listric faults that develop durng preorogenic extensional deformation. The present Himalayan region south of the IndusYarlung Zangbo Suture Zone is accepted to have been the southern passive margin of Tethys. Such regions have been described as having developed through lithospheric rifting, fundamental to which is the association of basaltic magma welling up into crustal levels (White 1988). The huge magnitude of such magmatic additions is emphasised by the estimated 107 km 3 of magma emplaced along the present North Atlantic passive margins within a period of a few million years during early Tertiary time (White et al. 1987). Studies of pre-orogenic mafic magmatism (chemistry, associated tectonic set-up, and changes in volume of erupted magma and geographic location of magmatic activity through time) in the Himalayas, and of the subsidence of the Tethys Basin during pre- and post-magmatism (using the back-stripping method to calculate the amount of basin subsidence due to the sedimentary loading and tectonic control) have shown clearly that the region has undergone repeated rifting episodes from the
4.
,~ K, [)UBEY and M t. BHAI
Precambrian until just before the Tertiary compressional phase (Bhat 1987 and references therein). This fact could have significant implications for the existing view about the mechanism and amount of crustal thickening in the Himalayas. Keeping these constraints in view, the primary objective of our study is to reassess the contribution of orogenic structural deformation to upper crustal shortening and thickening. We shall, however, also argue for a reassessment of the views on lower crustal thickening. For our study we have selected the Simla Klippe and its surroundings--the very area which has been fundamental to the framing of the existing view on the structural deformation and its large contribution to crustal shortening in the Himalayas. Our approach is to present and discuss the structural data from the area. As will be shown below, Ray and Naha (1971) have ignored as well as misinterpreted many of the structural features of the area. In order to avoid reliance on such a data base, we re-mapped the area during 1986-1989, and 42,000kin ~ on a 1:50,000 scale were mapped for geological and structural data. Before we present our results, however, some important details about the geology and structural set-up of the area, as well as necessary comments on earlier work, are given.
77*
GEOLOGY AND STRUCTURE: EXISTING KNOWLEDGE AND UNSOLVED PROBLEMS Our study area lies in the NW part of the Lower Himalayas (Fig. 1). The concepts of root-zone, klippe, and window were first introduced in the Himalayas as a result of work done in this area (Pilgrim and West 1928, West 1939) (Fig. 2). Subsequently, the geology and structure were discussed by Ray and Naha (1971), Srikantia and Sharma (1976), Chaterji and Swaminath (1977), Virdi (1979), and Sinha (1980). A brief description of the lithotectonic units is presented m Table 1. The highest grade of metamorphism in the area is observed in the Jutogh Formation which is characterised by a muscovite-biotite-garnet-staurolite mineral assemblage. Staurolite may be present, and its presence distinguishes the rock from the Chail Formation which in turn is distinguished from the Simla Group by the rare occurrence of garnet. A small variation in the lithology and metamorphic grade led Sinha (1980) to incorporate the Chail and Jutogh Formations into one unit--the undifferentiated crystallines. On the basis of extensive mapping and lithological studies in the area, Sharma (1977) reported that the contact between the Jutogh and the Chail Formations appears to be gradational in some parts of the area.
77°30
'
INDEX i---..-=1
8 I
J Siwalik Molasse
Tertiary Rocks (Undifferentiated) 6 ~ Blaini-lnfra Krol,-,-,4 KroI-Tol 5 ~ Mondhali-ChondpurNagthet 7
3P
;i,4 ~
DamtaSimla-~ B e r i n a g
3 ~
Shali-Deoban- Largi
2 ~i
s Chail
1~
t~..+~+ll'C'['CGranite T'q
Jutogh
-~
Strike and dip of bedding Strike and dip of foliation Thrust
~
Normal Lith~ogical Contact
"77.
Fig. I. A geological map of a part of the Himachel Lower Himalayas (modifiedafter Sharma 1977, Chaterji and Swaminath 1977, Virdi 1979, Sinha 1980).
Structural evolution of the Simla area, NW Himalayas
SW
SimloK!ippe
Sholi ^Window
,
-7-
~
U~ Alluvium
of
~ kt a ,
NE
i
~
_~
~'~
43
o, _ _
%,
o,
L t Y e t t'l~
g
\
Upper Terhory
/.'~. ~ " ~ ~ " =0,0oo feet~ / miles ~
VERTICAL HORIZONTAL
X X x~n'ru~lYteel
"
SCALE SCALE
Fig. 2. A geological cross-section of the Simla Himalayas (after West 1939). This is the first cross-section in the Himalayas that shows a number of parallel thrusts off-shooting from a root-zone and in the existence of klippe and window structure. (M.B.F. ffi Main Boundary Fault).
Pilgrim and West (1928) described the structure in the Jutogh Formation as a large-scale recumbent fold. Their conclusion appears to have been guided by the fact that the rocks with the higher grade of metamorphism lie at higher topographic levels, but neither the required structural data nor the inversion
of beds demonstrated by primary structures were provided to support the hypothesis. Naha and Ray (1972) interpreted the large-scale structure of the Jutogh Formation to be a "recumbent syncline'. However, the following points regarding their study are to be noted.
Table 1. Lithotectonic units in the Himachal Lower Himalayas (compiled after Azmi and Joshi 1981, Tewari 1984, Gururajan 1985) Name of litho-tectonic units Siwalik
Lithology Upper-Conglomerate Middle-Sandstone Lower-Sandstone, shale
Probable age PleistoceneMiddle Miocene
Foliations present
Folding episodes present
One set
Two (F: and F3)
One set
Two (F2 and F3)
One set
Two (F2 and F3)
Three sets
Three (Fl, F2 and F3)
Precambrian
Three sets
Three (Fl, F2 and F3)
Precambrian
Three sets
Three (FI, F2 and F3)
Precambrian
Three sets
Three (F:, F 2 and F3)
Main boundary fault Dharamsala Group
Kasauli Formation Dagrhai Formation Subathu Formation
Sandstone, few shales Sandstone and red shales Limestone, variegated shale
Middle Miocene -Eocene
Krol Thrust (Main Boundary Thrust) Krol Group
Tal Krol Infra-Krol Blaini Formation
Quartzites, shales with phosphorite Dolomites, subordinate limestone, black slates at lower levels Boulder slate, pink dolomite, sandstone and shales
? DevonianEocambrian
Gift Thrust Simla Group
Sanjauli Formation Slate, quartzite and sandstone Jaunsar Formation Chhaosa Formation Kunihar Formation - Inter tonguing relationship Basantpur Formation
Early-Late Riphaean
Shali Thrust Shali Formation
Cherty limestone and stromatolitic limestone
Middle Riphaean
Relationship not clear Rampur Formation
= Berinag Formation
Orthoquartzites with penecontcmporaneous volcanics Chail Thrust
Chail Formation
Low grade metamorphic rocks of greenschist to epidote amphibolite facies with associated granitoid and basic rocks Jutogh Thrust
Jutogh Formation
Medium to high grade metamorphic rocks of umphibolite facies with associated migmatites, granites and basic rocks.
44
A.K. DU~F.Y and M. 1. BHAT (1) A "single indistinct current bedding" observed in the northwestern part of the area (at Halog) was used to decipher the overall structure of the region. No overturning of beds was observed anywhere else in the region, although their crosssection of the area implies overturning of beds in the central region (i.e. Jutogh, Prospect Hill, and Taradevi) as well. (2) Three phases of folding movements were described from the Jutogh Formation of the area; of these three phases, the second phase was reported as missing in the Chail Formation. This implies that the Jutogh and Chail Formations were adjacent in their original position at the time of the first folding movement. Later, a thrust surface developed parallel to the axial surface of the isoclinal and recumbent F~ folds, and after translation along a sub-horizontal thrust surface, the Jutogh Formation now occupies a position on a different portion of the Chail Formation. The thrusting was described as simultaneous with the F2 folding movement, and it preceded the third folding phase. However, later studies in the Simla region by Raj (1983) and in the adjoining region by Virdi (1977) have shown clearly that the three important rock units of the area (i.e. Jutogh Formation, Chail Formation, and Simla Group) have undergone the same three phases of folding associated with the Himalayan orogeny. A detailed study by Raj (1983) in the southern part of the Simla Himalayas led to the identification of a regular pattern in the distribution of different generations of folds. Raj observed that the frequency of the F~ and ~ folds decreases near the thrusts, in contrast with the F3 folds, which are fairly uniformly distributed in the area. (3) The structural analysis was carried out mostly in the Jutogh Formation and only partly in the Chail Formation; the Simla Group surrounding the klippe was given only a cursory treatment. We believe that the study of Simla Group rocks is important because the fold structures are very well developed and primary structures, which act as younging indicators, are better preserved in these rocks.
PRESENT STUDIES
It is evident from the foregoing discussion that many of the structural features have not been explained satisfactorily. There also is confusion regarding the distinction of different rock units in the area. In order to improve our understanding, structural data were collected relating to fold geometries, the orientation of foliation planes and fault planes, not only from the Jutogh and Chail Formations but also from the Simla Group of rocks. The area was divided into 22 sub-areas (Fig. 3) in order to study the geometric pattern and variation of structural features. Part of the area not
covered by the sub-areas is either inaccessible or soil and vegetation covered. The relevant data are presented and discussed below. Folds Field observations indicate that the rock units of the Simla Group, Chail Formation, and Jutogh Formation are involved in the same three phases of folding. The plunge of the minor fold hinge lines and the dip of the axial surfaces in different sub-areas are shown in Fig. 3. (Data from sub-area Nos 19 and 21 are missing because of poor exposure of minor folds.) The first-formed F~ folds are mostly tight or isoclinal, upright, recumbent, or reclined. The trend of the fold hinge lines varies from NW-SE to E-W with a gentle plunge towards E or W. These folds have been folding coaxially into F 2 folds (type 3 interference pattern; Ramsay 1967, Chapter 10) which are open to tight, upright, overturned, and recumbent. The third phase of folding is characterised on a major scale by upright and asymmetric chevron folds, conjugate folds and kink bands. The hinge lines of these /73 folds trend from NE-SW to N-S (i.e. nearly perpendicular to the F~ and F2 folds) with a gentle plunge towards the N or S. Since F~ and F 2 are coaxial, they are grouped into one category as early folds. The F3 folds are a separate category and are designated as superposed folds in the subsequent discussion. Foliations
The formation of each of the fold generations was accompanied by the development of axial plane foliation. Thus three axial plane foliations exist in the area and help in the identification of different generations of folds. The orientation patterns of the foliation planes in different sub-areas are shown in Fig. 4. (Foliation data from sub-area Nos 1, 9, 10, 12, and 13 are missing because minor folds are predominant in these sub-areas; see Fig. 3.) It is evident that some of the sub-areas (Nos 3, 5, 6, and 7) incorporate data from all of the three important rock units of the area (i.e. Jutogh, Chail, and Simla Group), whereas some (e.g. Nos 14, 16, 17, 20, and 21) have data only from the Chail Formation and the Simla Group. Despite the fact that the data are from different rock units, the result is a simple single pattern, confirming that the rock units have undergone the same folding movements. The asymmetric concentration of pole points around a great circle in all of the plots indicates asymmetric folds with inclined axial surfaces (cf. Ragan 1985, p. 279). Since the pattern is characteristic of the whole region, the implication is that the regional structure is typified by second order folds. The trends of the early and the superposed fold hinge lines and the dip of the axial surfaces determined from the orientation diagrams (Figs 3 and 4) are shown in Fig. 5. Whereas the early folds in the western part of the area plunge due west, they have an easterly plunge in the eastern part [Fig. 5(a)]. Reversal of the plunge suggests
Structural evolution of the Simla area, NW Himalayas
45
N N
N
I X
•
~
A A
.+,
z •
",,,•
K~
"1-
oo
•
x i
A
•
•
._L... o A&
•
0 Q
4-1
==
A&
4
i
RI
O0 g
#, xl
N
X
x! ~ •
12
[
ox
I
N 0
~.
A
+
v
o"
A •
•
7
."
ba
it
tt
,.
A ~
~o
t,
°o~
#"
•
+
*~
o_/
. . . .
Q
18
O
14
20
22
17
Fig. 3. A n outline map of the area showing locations of the sub-areas and orientation of folds in different sub-areas. The sub-area number is indicated in each diagram. Dot = plunge of early fold hinge line; o p e n circle = plunge of superposed fold hinge line; triangle = pole to the early axial surface; cross = pole to the superposed axial surface.
refolding along a NNE-SSW axis--a trend congruent with the trend of the F 3 folds. The general trend of the superposed fold hinge lines [Fig. 5(b)] varies from N-S to NE-SW. A comparison of Fig. 5(a) with 5(b) reveals that the area has been affected by second order folds of both the early and superposed fold generations. The early folds produced a NW-SE trending synform which later was refolded into a NNE-SSW trending antiform during the superposed folding phase. While the greater variation observed in the early fold hinge lines is attributed to the refolding, the variation in the superposed fold hinge lines is thought to be due to initiation and development of these folds on initial undulating surfaces. Faults
Minor normal faults (with a throw of a few cm to a few m) are prominent in the southern part of the area (near the Girl, Chail and Jutogh Thrusts). The continuity of some, and the displacement of other, foliation surfaces across these faults indicate that the faults formed at different periods throughout the tectonic history of the region. The earliest faults are
synsedimentary faults. Their characteristic feature is that they do not displace the earliest foliation, which indicates that they were formed during post-rift subsidence and sedimentation (Dubey and Bhat 1986b). Since these faults are observed in the Simla Group, we infer that they formed during the first available record (which is of at least Late Archaean age) of the rift phase in the Himalayas (Bhat and Le Fort 1990). The youngest faults displace all the foliation planes present in the area (Fig. 6). The frequency of these faults gradually diminishes in a northerly direction. In the N and NW of Simla Town (Fig. 3; sub-areas 3, 5, 8, 9, and I0) these faults are absent; instead minor folds (mostly F2 and F3) are predominant. In summary, the following important observations and sequence of structural events may be emphasised: (1) synsedimentary normal faults are the first important structures to be formed in the area; (2) the large-scale, first formed structure of the Jutogh rocks is not a recumbent fold or a recumbent syncline as was thought earlier (Pilgrim and West 1928, Naha and Ray 1972), but a second order fold;
4
3.0 %
1.2%
t.8%
14
3.0% 4.5% 6.0%
I
7
+q
I
-?~--->
L__
3
0
2"3°1° 2.9%
0.6 1.2o7;o° - 2 0
2.1% 3.5%N
6%
1.5% 3.0 % 4.5 %
~ I ~
/
%.zJ
3.0 % 4.5 %
6.0%
,
22
//
3.6 ~o 4.8%
6.0%
2.4% 3.6 %
L2 %
~ " 19
,.,
,,~
4.8%
3.2%
~"~,
"
2.6% 5.2 %
e %
3.2% 4.5 %
21
"~-__2-J
16
L6 %
I-~J i 2 %
8
18
+
N
Fig. 4. An outline m a p of the area and contours of the poles of the foliation planes in different sub-areas• Dot := early fold hinge line (F~ and F~): oper circle (V~). The plots are characteristic of' asymmetric folds with inclined axial surfaces.
5.5%
1.2% 3.3%
• ~.9 %
2.7%
N
,-,
superposed fold hinge !m~
~.,~ :',:
,Qt;:o
N
N
= > ,.-1
-1
t'll ,<
ci,
;>
Structural evolution of the Simla area, NW Himalayas 77*
metry of the folds indicates a sense of shear towards the SW; (5) the early folds were followed by the development of NE-SW trending superposed folds (F3). These folds are fairly uniformly distributed throughout the area, irrespective of their distance from the thrusts; (6) the allochthonous Jutogh and Chail Formations and the parautochthonous Simla Group have undergone the same three phases of folding.
77" 30' ,
"~\
0
3L
47
I0 km
5L°
INTERPRETATION
It is now establised that folding initiates from irregularities, either in the layering or in the stress field (Dubey and Cobbold 1977). Normaly, fold initiation takes place Kolka ~ \ \ \ I ,Raj(:jq..~h // during an overall shortening of 3-19%. The actual amount depends mainly on the thickness of the layer, competence contrast between the layer and the matrix, initial irregularities on the layering, and the amount of 77* 77* .gO' Fig. 5(a). Trends of the early fold hinge lines and dips of the axial layer parallel slip, especially in a multilayered packet. surfaces determined from Figs 3 and 4. The pattern suggests refolding The amplification of folds is rapid during the initial of the Simla Synform along the NNE-SSW axis during the F3 folding stages of deformation and this is accompanied by a movement. decrease in the fold interlimb angles. At low interlimb angles, the folds acquire a stable geometrical shape; this I I 77 ° 77* 50' stage is defined as rotation hardening (Cobbold 1977, Dubey 1980). At the early stage of fold formation, the ~,N~N~~ 0 10 km I I axis of maximum extension remains parallel to the axial surfaces of the developing folds and normal to the fold hinge line; however, after reaching the stage of rotation . // " hardening, further shortening changes the axis of maximum extension, which becomes parallel to the fold hinge lines. This change in the axis of extension is accompanied by the formation of second order folds. Strike-slip faults may also develop at this late stage of fold formation. If the boundary conditions do not permit an extension parallel to the fold hinge lines, then thrust faults are likely to develop, initiating from the fold limbs (Dubey and Behzadi 1981). Considering the above facts, folding is normally followed by thrusting. However, the occurrence of the same phases of folding and the continuation of the same structural elements in the parautochthonous Simla .RoIgo Group and the aUochthonous Jutogh and Chail Formations suggest one of the following two possibilities: (1) the thrusting and the folding initiated simultaneously Fig. 5(b). Trends of the superposed fold hinge lines and dips of the and there is no significant effect of horizontal translation axial surfaces determined from Figs 3 and 4. The general trend of the on the fold geometries; and (2) the initiation of thrust fold hinge lines varies from N-S to NE-SW. and translation of the rock mass occurred before the folding and all three generations of folds were formed (3) the early folds (i.e. F] and F~) are most prominent when the rock units were in their present positions. An earlier study with model deformation experiments away from the major thrusts. Contemporaneous with the formation of the early folds, normal (Dubey and Bhat 1986a) revealed that folding and faults also developed, these faults are most thrusting initiate simultaneously in models where planes of discontinuity occur in multilayer models. However, it prominent near the thrusts; (4) the geometry of the early mesoscopic folds gradu- has never been observed that a significant displacement ally changes from predominently upright folds along a thrust occurs before the initiation of folding in in the southern part (Fig. 7) to overturned and the multilayers. Thus, the first probability appears to be recumbent folds in the north (Fig. 8); the asym- more appropriate in the present context. The weak \\\
•
X "-~
i .
S olan '~\.--'~ "~.So,, " ,u ~i()' Ir~
•
48
\ K. DUBEY and M. I BHA1
SW
~
~
:~
~
0
~=
NE.
! 0 Km I
Fig. 9. A revised geologicalcross-section of the Simla Himalayasalong the line X - Y in Fig. I. The various thrusts may be a part of the duplex system(M.B.T.= Main BoundaryThrust). planes required for the simultaneous initiation of thrusting and folding may have been provided by the listric, normal faults that formed during the tensional regime in the region (Bhat 1984a). The presence of synsedimentary normal faults supports this view. Our revised cross-section of the area is shown in Fig. 9. It is interesting to note that whereas the Chail and Jutogh Thrusts are distinctly spaced in the SW part of the klippe, they nearly merge in the NE part, thereby reducing the Chail Formation to the thickness of a few m. A likely explanation for this phenomenon is that the two observed thrust faults are termination splays of a single master thrust. We therefore propose to call this master thrust as the Chail-Jutogh Thrust. The geometries of various thrusts in the area suggest that they may be a part of a duplex system in which the Chail-Jutogh Thrust forms the roof thrust. The maximum horizontal translation took place along this thrust possibly because of the presence of a lubricant graphitic schist band at the thrust surface. The maximum horizontal translation along the Chail-Jutogh Thrust determined by the "klippe to fenster method" (Hobbs et al. 1976, p. 309) is approximately 40 km. No attempt has been made to balance the section for the following reasons: (1) there is no marker bed across the thrusts that could be employed to balance the section (cf. Elliott and Johnson 1980); (2) minor strike-slip faults have been reported in the Krot-Tal sequence around Solan (Fig. 1) by Bhattacharya and Niyogi (1971). They indicate an extension normal to the plane of the crosssection. Since these faults are the youngest structures, it is likely that the older rock sequences (i.e. Jutogh, Chail, and Simla) have undergone a similar extension (Dubey and Bhat 1986a, also cf. Brown et aL 1986). The total amount of fault displacement and the strain associated with these faults are still a matter of speculation. However, it is certain that the assumption of plane strain deformation is not valid for the region; (3) the depth of the basal d6collement and the dip of the sole thrust are unknown. It is to be noted that the only available deep seismic sounding (DSS) profile for the Indian part of the Himalayas (Kaila et al. 1978, 1984, Beloussov et al. 1980) depicts the major faults at a very steep angle and
does not show a basal thrust. Hence, no estimates can be made of the original area or length of the section restored. We therefore agree with Searle (1986) that "the amount of inference (fiddling) that has to be made is at present inacceptably great." The comment was made for the Ladakh and Zanskar regions; however, we consider it to be equally applicable to other parts of the Himalayas.
THE MODEL
In the light of the field data and their interpretation, we now attempt to model the tectonic events which appear to have determined the structural evolution of the area. Special emphasis will be placed on the well-marked variation in frequency and distribution of structural elements in the Simla Group of rocks. It is presumed that the fracture plates were formed during the tensional regime in the region as normal, listric faults and that they later reactivated as listric thrust faults during the compressional regime. The thrust movement was simultaneous with the formation of folds which initiated with a symmetric fold style (Fig. 10A). Since the faults had a listric geometry, the beds were tilted in the dip direction of the thrust faults. The oblique orientation of the rock units with reference to the thrust conforms to Zone 1 of the strain ellipse developed by progressive simple shear (Ramsay 1967, Fig. 3.6). The shear strain in the vicinity of the thrust surface resulted in extension of the layering and formation of normal faults near the thrust (Fig. 10A). Away from the thrust, layer parallel shortening (i.e. buckling), as a result of orogenic compression, was prominent, resulting in the formation of folds (Fig. 10A). Simultaneous folding and thrusting had a considerable influence on the geometry of the amplifying folds: a progressive increase in shear strain, associated with the roof thrust, modified the upright fold geometry to asymmetric and overturned fold patterns (Fig. 10B). This was accompanied by a decrease in the fold interlimb angles leading to the "rotation hardening", and finally the sequence was folded into second order folds (Fig. 10C). The roof thrust was also folded, resulting in the cessation of horizontal translation. The youngest F~ folds which formed after the locking of the thrusts therefore show a uniform occurrence throughout the
Structural evolution of the Simla area, NW Himalayas
Fig. 6. Normal faults in the Simla Group of rocks near the Girl Thrust. (Location--Kandaghat.) Such faults are prominent in the southern part of the area near the major thrusts. Fig. 7. A mesoscopic F2 fold with steep axial surface diping due NE in the Simla Group of rocks, approximately 1 km from the Gift Thrust. (Location--I km north of Kandaghat.) Such folds are prominent in the southern part of the area, away from the major thrusts.
49
Fig S Overturned fnld i~, the Sim!a Group of rocks, approximatel,, 8 km froil! the Shall Thrust. (Location --north of Simla town.) Mesoscopic, overturned and recumbent fold gcometr{e~ ;~rc char;tctcrislic of the northern part of the area near the contact of the Simla G r o u p with the klippe rocks.
,'v >
.,..-
r'~
7¢
J~
Structural evolution of the Simla area, NW Himalayas
51
A
)-
.-"!
o
"
"- " . . "
\
.......
--'2 ~
....
"%,
Fig. 10. A simplified model for the structural evolution of the Simla area. A: formation of uptight folds and simultaneous thrust movement along the fracture planes. Whereas the fractures are considered to have originated as listric, normal faults during the pre-orogenie tensional regime in the region, the upright folds developed as a result of horizontal orogenic compression. B: modification of the upright fold geometry to asymmetric and overturned forms as a result of increase in displacement along the subhorizontal Chall-Jutogh Thrust. C: formation of second order folds, steepening of the thrusts, and folding of the sub-horizontal part of the thrust, resulting into locking of the thrust. The structural features developing at locations A, B, and C are shown in Figs 6, 7, and 8, respectively.
area. The concept of the locking of the thrusts is further strengthened by the following arguments: (1) The fault surface gradually rotates away from the axis of maximum shortening with the result that the fault plane steepens and produces early locking (Dubey and Behzadi 1981). (2) The folded thrust surface was later refolded during the superposed folding movement and the resulting geometrical shape was obviously not favourable for further horizontal translation. (3) Thrust movement along a listric fault has an inbuilt restriction on the amount of total fault displacement because the fault has a non-planar geometry. A further significant point is that the component of vertical displacement on a steep fault exceeds that of the component of horizontal displacement. In view of these constraints, it is proposed that the minimum displacement of 40 km along the Chail-Jutogh
Thrust determined by the klippe-to-fenster method is very close to the maximum possible displacement along the thrust.
DISCUSSION Structural data presented here demonstrate that the pre-orogenic normal faults of this area must be taken into account to explain the geometry and evolution of the structural features as well as their spatial distribution--a feature neither considered earlier nor amenable to the explanations of earlier models. In addition, the two most critical facts that emerge from the study are that, (i) the initiation of fold and thrust movements were simultaneous; and (ii) no large scale recumbent fold structure is present in the area. Both these observations place severe limitations on the amount of crustal shortening and, in consequence, the crustal thickening in the area through orogenic structural deformation.
52
A K. DUBEY and M. I. BHAJ
Presence of material with a density of 3.05 cm ~ at a depth of 25 km below the surface and extending down to the depth of 55-70 km in the Himalayan region has been known for two decades (Gureshi 1969). Later Deep Seismic Sounding (DSS) studies also recorded a velocity of 6.9-7.5 km s-1 at nearly the same depth (Kaila et al. 1978, 1984). Whereas Gureshi (1969) interpreted the high density material as due to thickening of the basaltic layer (an interpretation supported also by Beloussov et al. 1980, 1984), the plate tectonic interpretation (with different variations, as noted at the outset) is that it represents the underplated/underthrusted Indian lithosphere. In either case, once the fact that the crustal column from about 2 5 k m down to 70km is "allochthonous" is accepted, it leaves only the upper 25 km for consideration of increase in crustal thickness through structural deformation and shortening. There is general consensus that the unfolded and unthrusted thickness of the Tethyan Zone sediments of the Himalayas is at least 12 15 kin, while the not fully exposed metasedimentary basement, also unfolded and unthrusted, is 7-8 km thick (K, K. Sharma, personal communication). Thus the total thickness of the sedimentary pile in the Himalayas is not less than 23 kin. It hardly leaves any space for a significant crustal thickening through structural deformation - a n inference compatible with (in fact, a prediction of) the pre-orogenic rift history of the region ~,is-a-~is thrust movement along the pre-existing listric faults. As for the high density layer, the interpretation usually provided is that it represents a high pressure transformed phase of the underthrusted Indian plate continental crust (Roecker 1982). While transformation of continental crust to high density and high seismic velocity phases cannot be denied--at least on theoretical g r o u n d s - - a n appeal to such a phenomenon having taken place in the Himalayan region appears to be a case of unnecessary special pleading. This is especially true if the distribution of this high density, high seismic velocity crustal layer is seen in the context of the world-wide occurrence of such a layer. The existence of material of this density and seismic velocity along oceam4continent transition zones and in extensional belts is an established fact (Keen et at. 1975, Bott 1980). Indeed, a recent survey of world-wide seismic data has shown the presence of such material ( V p = 7 . 0 - 7 . 8 k m s ~ at varying depths under b o t h extensional and compression belts (Meyerhoff et al. 1989). Under extensional belts, this material--variously called "intermediate layer", "rift cushion", "anomalous layer", "anomalous mantel", "anomalous lithosphere"--is ascribed to upwelling of basaltic m a g m a from the asthenosphere into the lower crust, resulting in deepending of the Moho (White 1988). If, as is accepted, an orogenic belt is the culmination point in the long chain of geodynamic processes beginning with lithospheric rifting through ocean development and finally to mountain formation, then there is no reason to consider this "anomalous layer" under fold belts as anything other than basaltic material intruded during the rifting phase that precedes each fold
belt formation. Even if we ignore for the moment the additions of basaltic m a g m a associated with old rifting events in the Himalayan region, we cannot possibly do so for the Late Palaeozoic-Early Mesozoic rifting event as it is now well established from several lines of evidence to have resulted in the birth of the Neo-Tethys along the present Himalayan region (Bhat 1987 and references therein). This rifting event is generally thought to have been completed by Triassic time. However, the timing of basin subsidence, calculated by employing the approach used in understanding the subsidence of passive margins following rifting and during ocean floor phases (e.g. Haworth and Keen 1979), shows that the region was in a rifting regime until the time of Late Jurassic-Early Cretaceous ophiolite formation (Bhat 1984a~b, 1987). Given these considerations, we suggest that crustal thickening in the Himalayas is primarily a result of magmatic additions into the crust, associated with characteristic pre-orogenic tensional tectonics rather than with hypothetical and unconstrained processes that are contrary to field geological data collected from the region. Acknowledgements--We are particularly grateful to A. A. Meyerhoff for his keen interest in the study and for thorough review of several drafts of the manuscript. A. J. Barber, V. C. Thakur and H.-U. Schwarz also provided many useful comments. Discussions with D. Mukhopadhyay, M. R. W. Johnson, A. K. Sinha, K. K. Sharma, N. S. Gururajan and B. K. Choudhury, at different stages of the study, were quite beneficial.
REFERENCES Azmi, R. J. and Joshi, M. N. 1981. Conodont and other biostratigraphic evidence on the age and evolution of the Krol Belt. Him. Geol. 11, 198-223. Beloussov, V. V., Belyaevsky,N. A., Borisov, A. A., Volvovsky,B. S., Volvovsky, I. S., Resvoy, D. P., Tal-Virsky, B. B., Khamrabaev, I. Kh., Kaila, K. L., Narain, H., Marussi, A. and Finetti, J. 1980. Structure of the lithosphere along the deep seismicsounding profile: Tien Shan-Pamirs-Karakorum-Himalayas. Tectonophysics 70, 193-221. Beloussov, V. V., Volvovsky,B. S., Volvovsky,I. S., Khamrabaev, 1. Kh., Kaila, K. L. and Marussi, A. 1984. Deep structure of Central Asia along the Tienshan-Pamir-Himalayasgeotraverse.In: Tectonics of Asia, 27th Int. Geol. Cong., Moscow, Colloquim 05 5, 29-39: Bhat, M. I. 1984a. Evolution of the southern passive margin of the Himalayan Tethys: model implications and constraints---I.Z Petrol. Geol. 7(3), 341-350. Bhat, M. I. 1984b. Evolution of the southern passive margin of the Himalayan Tethys: model implications and constraints--lI. J, Petrol. Geol. 7(4), 429-436. Bhat, M. I. 1987. Spasmodic rift reactivation and its role in the preorogenic evolution of the Himalayas. Tectonophysics 134, 103--127. Bhat, M. I. and Le Fort, P. 1990. Geochronology and petrogenesis of Rampur metavolcanic rocks, NW Himalayas: elemental and Ndisotopic evidence for penecontemporaneous mantle metasomatism and magma genesis. Precambrian Res. Commun. 0, 000--000. Bhattaebarya, S. C. and Niyogi, D. 1971. Geologic evolution of the Krol belt in Simla Hills, H.P. Him. Geol. 1, 178-212. Bott, M. H. P. 1980. Problems of passive margins from a view point of the geodynamic project: a review, Phil. Trans. R. Soc. London, Ser. A., 294-A, 5-16. Brown, R. L., Journeay, J. M., Lane, L. S., Murphy, D. C. and Rees, C. J. 1986. Obduction, backfolding and piggyback thrusting in the metamorphic hinterland of the south-eastern Canadian Cordillera. J. struc. Geol. 8, 255-268. Chaterji, G. C. and Swaminath, J, 1977. The stratigraphy and structure of parts of the Simla Himalaya--a synthesis. Mere. Geol. Surv. India 106(2), 408-488.
Structural evolution of the Simla area, NW Himalayas Cobbold, P. R. 1977. Description and origin of banded deformation structures. II. Rheology and growth of banded perturbations. Can. J. Earth Sci. 14, 2510-2523. Davies, V. M. 1982. Interaction of thrusts and basement faults in the French External Alps. Tectonophysics 88, 325-331. Dubey, A. K. 1980. Late stages in the development of folds as deduced from model experiments. Tectonophysics 65, 311-322. Dubey, A. K. and Bhat, M. I. 1986a. The role of reactivation of pre-rift basement listric faults in the structural evolution of the Himalaya: an experimental study. In: Current Trends in Geology, Himalayan Thrusts and Associated Rocks (Edited by Saklani, P. S.), Vol. 9, pp. 265-290. Dubey, A. K. and Bhat, M. I. 1986b. Experimental observations on the reactivation of normal listric faults of the pre-rift basement and the structural evolution of the Himalayas. In: Ophiolites and Indian Plate Margin (Edited by Ghose, N. C. and Vardarajan, S.), pp. 101-107. Patna University Press, Patna. Dubey, A. K. and Behzadi, H. 1981. Development of flexural slip folds, overlapping boudins and extension faults in multilayered materials: field evidence and experimental model. J. Geol. Soc. India 22, 274-284. Dubey, A. K. and Cobbold, P. R. 1977. Noncylindrical fiexural slip folds in nature and experiment. Tectonophysics 38, 223-239. Elliott, D. and Johnson, M. R. W. 1980. Structural evolution in the northern part of the Moine Thrust Belt, NW Scotland. Trans. R. Soc. Edinb. 71, 69-96. Etheridge, M. A. 1986. Reactivation of extensional fault systems. Phil. Trans. R. Soc. London Ser. A. 317, 179-194. Gururajan, N. S. 1985. Structure and metamorphism of the Chail Formation in the area between Baragaon and Nirath (Himachal Pradesh) in Simla Himalaya with a special reference to the genesis of the associated mylonite gneiss. Ph.D. Thesis (unpublished), Garhwal University, Garhwal. Haworth, R. T. and Keen, C. E. 1979. The Canadian Atlantic margin: a passive continental margin encompassing an active past. Tectonophysics 59, 83-126. Him, A. et al. 1984. Crustal structure and variability of the Himalayan border of Tibet. Nature 307, 23-25. Hobbs, B. E., Means, W. D. and Williams, P. F. 1976. An Outline of Structural Geology. Wiley, New York. Jackson, J. A. 1980. Reactivation of basement faults and crustal shortening in orogenic belts. Nature 283, 343-346. Kaila, K. L., Krishna, V. G., Roy Chowdhury, K. and Narain, H. 1978. Structure of the Kashmir-Himalaya from deep seismic soundings. J. Geol. Soc. India 19, 1-20. Kaila, K. L., Tripathi, K. M. and Dixit, M. M. 1984. Crustal structure along Wular Lake-Gulmarg-Naoshera profile across pir Panjal range of the Himalayas from deep seismic soundings. J. Geol. Soc. India 25, 706-719. Keen, C. E., Keen, M. J., Barrett, D. L. and Heftier, D. E. 1975. Some aspects of the ocean-continent transition at the continental margin of eastern north America. In: Offshore Geology of Eastern Canada (Edited by Van den Linden, W. J. M. and Wade, J. A.), Vol. 2, pp. 189-197. Geol. Surv. Can. Paper 74-30. Le Pichon, X. and Sibuet, J. C. 1981. Passive margins: a model of formation. J. Geophys. Res. 86, 3708-3720. Meyerhoff, A. A., Taner, I., Morris, A. E. L. and Martin, B. D. 1989. Surge tectonics. In: New Concepts in Global Tectonics (Abstract volume) (Edited by Chatterjee, S. and Hotton, N.), pp. 25-26. Texas Tech. University Press.
53
Naha, K. and Ray, S. K. 1972. Structural evolution of the Simla Klippe in the Lower Himalayas. Geol. Rundschau 61, 1050-1086. Pilgrim, G. E. and West, W. D. 1928. The structure and correlation of the Simla rocks. Mere. Geol. Surv. India 53, 1-140. Powell, C. Mc. A. and Conaghan, P. J. 1973. Plate tectonics and the Himalayas, Earth planet. Sci. Lett. 20, 1-12. Gureshi, M. N. 1969. Thickening of a basalt layer as a possible cause for the uplift of the Himalayas--a suggestion based on gravity data. Tectonophysics 7, 137-167. Ragan, D. M. 1985. Structural Geology. Third Edn. Wiley, New York. Raj, S. 1983. Variation of structural elements across the Jutogh Thrust and its implications: a study based on field evidence and model experiments. Geoseience J. 4, 157-168. Ramsay, J. G. 1967. Folding and fracturing of rocks. McGraw-Hill, New York. Ray, S. K. and Naha, K. 1971. Structural and metamorphic history of the "Simla klippe": a summary. Him. Geol. 1, 1-24. Roecker, S. W. 1982. Velocity structure of the Pamir-Hindu Kush region: possible evidence of subducted crust. J. Geophys. Res. 87, 945-959. Searle, M. P. 1986. Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture Zones of Zanskar and Ladakh, Western Himalaya. J. struc. Geol. 8, 923-936. Sharma, V. P. 1977. Geology of the Kulu-Rampur belt, Himachal Pradesh. Mem. Geol. Surv. India 106, 235-407. Sinha, A. K. 1980. Tectono-stratigraphic problem in the Lesser Himalayan zone of Simla Region~ Himachal Pradesh. In: Stratigraphy and Correlations of Lesser Himalayan Formations (Edited by Valdiya, K. S. and Bhatia, S. B.), pp. 99-116. Hindustan Publishing Co., New Delhi. Soper, N. J. and Anderton, R. 1984. Did the Dalradian slides originate as extensional faults? Nature 307, 357-360. Srikantia, S. V. and Sharma, R. P. 1976. Geology of the Shali belt and the adjoining areas. Mere. Geol. Surv. India 106, 31-166. Stewart, A. J. 1987. Fault reactivation and superimposed folding in a Proterozoic sandstone-volcanic sequence, Davenport Province, Central Australia. J. struct. GeoL 9, 441--455. Tewari, V. C. 1984. Stromatolites and Precambrian-Lower Cambrian biostratigraphy of the Lesser Himalaya, India. In: Proc. Fifth Indian Geophytol. Conf. (Edited by Tiwari, R. S.), pp. 71-97. Special Publication of the Palaeobotanical Society, Lucknow. Virdi, N. S. 1977. Stratigraphy and structure of the Narkanda area, district Simla, Himachal Pradesh, India. Recent Res. Geol. 3, 453-467. Virdi, N. S. 1979. Status of the Chail Formation vis-d-vis Jutogh~hail relationship in the Himachal Lesser Himalaya. Him. Geol. 9, 111-125. Welbon, A. 1988. The influence of intrabasinal faults on the development of a linked thrust system. Geol. Runds. 77, 11-24. West, W. D. 1939. The structure of the Shali "window" near Simla. Rec. Geol. Surv. India 74, 133-163. Winslow, M. A. 1981. Mechanisms for basement shortening in the Andean foreland fold belt ofsouthern South America. In: Thrust and Nappe Tectonics. Spec. Publ. Geol. Soc. London 9, 513-528. White, R. S. 1988. The Earth's crust and lithosphere. J. Geol. Special Lithosphere Issue, 1-10. White, R. S., Spence, G. D., Fowler, S. R., McKenzie, D. P., Westbrook, G. K. and Bowen, A. N. 1987. Magmatism at rifted continental margins. Nature 330, 439-444.
0743-9547/91 $3.00 + 0.00 Pergamon Press plc
Structural evolution of the Simla area, NW Himalayas: implications for crustal thickening A. K. DUBEY and M. I. BHAT Wadia Institute of Himalayan Geology, 33 Gen. Mahadev Singh Road, Dehra Dun-248001, India (Received 6 June 1989; accepted for publication 6 November 1990) Abstract---Crustal thickening in the Himalayas is widely attributed to the two processes operational during continental collision in the region: firstly, internal deformation, folding and thrust-stacking in the upper crust, and secondly, underthrusting of the Indian plate crust. A significant contribution to the crustal thickening from the structural deformation is based on the notion that folding in the region was followed by thrusting. However, the geometry, orientation, and spatial distribution of folds and faults in and around the famous Sirnla Klippe in the NW Lower Himalayas indicate that folding and thrusting initiated and developed stimultaneously. Also, synsedimentary normal faults, indicative of a tensional regime, are the earliest structural features present in the area. Modelling of the structual evolution with these constraints suggests that the reactivation of rift-related, normal, listric faults was a key factor in controlling the orientation and spatial distribution of the structural elements in the area. While such a model is compatible with the pre-orogenic rift tectonics of the region, it puts severe limitations on the amount of possible crustal shortening, and in turn, crustal thickening through structural deformation. Furthermore, we argue that contribution to crustal thickening from underthrusting of the Indian plate crust needs re-evaluation in the light of known mafic magrnatic additions to the lower crust during lithosphetic rifting.
INTRODUCTION DOUBLE the normal crustal thickeness in the Himalayan region is generally accepted as a result of crustal shortening during the Tertiary orogenic compression. The two mechanisms suggested in achieving the thickening are: (1) folding and thrust-stacking in the upper part; and (2) underthrusting of the Indian plate in the lower part. The contribution through the first mechanism, though never quantified in net terms, hinges on the belief that folding was followed by thrusting. This view appears to be an off-shoot of a study by Ray and Naha (1971) on the structural development of the now bestknown klippe in the Himalayas--the Simla Klippe-wherein the authors suggested recumbent folding was followed by thrusting. This study acted as a typical example for application in other parts of the Himalayas and for generalising structural evolution for the whole Himalayan belt. Experiments on the deformation of physical models (Dubey and Behzadi 1981) indicate that folding takes at least 45% shortening before the initiation of thrusting. Therefore, if the concept of folding followed by thrusting is valid, it would suggest that structural deformation of the upper crust has accommodated large amounts of crustal shortening, and contributed substantially to the thickening of the crust in the region. As for the contribution through the second mechanism, it relies on the interpretation of the geophysical (especially palaeomagnetic) data, and on constraints imposed by plate tectonic reconstructions. Whether the earlier view, involving underthrusting of the whole normal Indian plate crust (Powell and Conaghan 1973), or the recent postulate, suggesting separate doubling by decoupling and thrusting of the upper and lower crustal layers (Hirn et al. 1984) is accepted, it is clear that the second mechanism has to be the major contributor to the crustal thickening in the region. $EAES6 / 1 ~
41
Implicit in this view is that pre-orogenic tectonic activity in the region has not played a role in either the structural evolution or the crustal thickening. Such a view appears too simplistic and is unique to the discussions on the crustal and structural evolution of the Himalayas. Studies in other orogenic belts (e.g. Zagros belt: Jackson 1980, Andean Foreland fold belt: Winslow 1981, external French Alps: Davies 1982, Welbon 1988, Scottish Dalradian: Soper and Anderton 1984, Bass Strait basins of southeastern Australia: Etheridge 1986, Proterozoic Davenport province, central Australia: Stewart 1987), theoretical considerations (Le Pichon and Sibuet 1981), and experimental results (Dubey and Bhat 1986a) have established that structural evolution of an orogenic belt is strongly controlled by pre-existing basement normal, listric faults that develop durng preorogenic extensional deformation. The present Himalayan region south of the IndusYarlung Zangbo Suture Zone is accepted to have been the southern passive margin of Tethys. Such regions have been described as having developed through lithospheric rifting, fundamental to which is the association of basaltic magma welling up into crustal levels (White 1988). The huge magnitude of such magmatic additions is emphasised by the estimated 107 km 3 of magma emplaced along the present North Atlantic passive margins within a period of a few million years during early Tertiary time (White et al. 1987). Studies of pre-orogenic mafic magmatism (chemistry, associated tectonic set-up, and changes in volume of erupted magma and geographic location of magmatic activity through time) in the Himalayas, and of the subsidence of the Tethys Basin during pre- and post-magmatism (using the back-stripping method to calculate the amount of basin subsidence due to the sedimentary loading and tectonic control) have shown clearly that the region has undergone repeated rifting episodes from the
4.
,~ K, [)UBEY and M t. BHAI
Precambrian until just before the Tertiary compressional phase (Bhat 1987 and references therein). This fact could have significant implications for the existing view about the mechanism and amount of crustal thickening in the Himalayas. Keeping these constraints in view, the primary objective of our study is to reassess the contribution of orogenic structural deformation to upper crustal shortening and thickening. We shall, however, also argue for a reassessment of the views on lower crustal thickening. For our study we have selected the Simla Klippe and its surroundings--the very area which has been fundamental to the framing of the existing view on the structural deformation and its large contribution to crustal shortening in the Himalayas. Our approach is to present and discuss the structural data from the area. As will be shown below, Ray and Naha (1971) have ignored as well as misinterpreted many of the structural features of the area. In order to avoid reliance on such a data base, we re-mapped the area during 1986-1989, and 42,000kin ~ on a 1:50,000 scale were mapped for geological and structural data. Before we present our results, however, some important details about the geology and structural set-up of the area, as well as necessary comments on earlier work, are given.
77*
GEOLOGY AND STRUCTURE: EXISTING KNOWLEDGE AND UNSOLVED PROBLEMS Our study area lies in the NW part of the Lower Himalayas (Fig. 1). The concepts of root-zone, klippe, and window were first introduced in the Himalayas as a result of work done in this area (Pilgrim and West 1928, West 1939) (Fig. 2). Subsequently, the geology and structure were discussed by Ray and Naha (1971), Srikantia and Sharma (1976), Chaterji and Swaminath (1977), Virdi (1979), and Sinha (1980). A brief description of the lithotectonic units is presented m Table 1. The highest grade of metamorphism in the area is observed in the Jutogh Formation which is characterised by a muscovite-biotite-garnet-staurolite mineral assemblage. Staurolite may be present, and its presence distinguishes the rock from the Chail Formation which in turn is distinguished from the Simla Group by the rare occurrence of garnet. A small variation in the lithology and metamorphic grade led Sinha (1980) to incorporate the Chail and Jutogh Formations into one unit--the undifferentiated crystallines. On the basis of extensive mapping and lithological studies in the area, Sharma (1977) reported that the contact between the Jutogh and the Chail Formations appears to be gradational in some parts of the area.
77°30
'
INDEX i---..-=1
8 I
J Siwalik Molasse
Tertiary Rocks (Undifferentiated) 6 ~ Blaini-lnfra Krol,-,-,4 KroI-Tol 5 ~ Mondhali-ChondpurNagthet 7
3P
;i,4 ~
DamtaSimla-~ B e r i n a g
3 ~
Shali-Deoban- Largi
2 ~i
s Chail
1~
t~..+~+ll'C'['CGranite T'q
Jutogh
-~
Strike and dip of bedding Strike and dip of foliation Thrust
~
Normal Lith~ogical Contact
"77.
Fig. I. A geological map of a part of the Himachel Lower Himalayas (modifiedafter Sharma 1977, Chaterji and Swaminath 1977, Virdi 1979, Sinha 1980).
Structural evolution of the Simla area, NW Himalayas
SW
SimloK!ippe
Sholi ^Window
,
-7-
~
U~ Alluvium
of
~ kt a ,
NE
i
~
_~
~'~
43
o, _ _
%,
o,
L t Y e t t'l~
g
\
Upper Terhory
/.'~. ~ " ~ ~ " =0,0oo feet~ / miles ~
VERTICAL HORIZONTAL
X X x~n'ru~lYteel
"
SCALE SCALE
Fig. 2. A geological cross-section of the Simla Himalayas (after West 1939). This is the first cross-section in the Himalayas that shows a number of parallel thrusts off-shooting from a root-zone and in the existence of klippe and window structure. (M.B.F. ffi Main Boundary Fault).
Pilgrim and West (1928) described the structure in the Jutogh Formation as a large-scale recumbent fold. Their conclusion appears to have been guided by the fact that the rocks with the higher grade of metamorphism lie at higher topographic levels, but neither the required structural data nor the inversion
of beds demonstrated by primary structures were provided to support the hypothesis. Naha and Ray (1972) interpreted the large-scale structure of the Jutogh Formation to be a "recumbent syncline'. However, the following points regarding their study are to be noted.
Table 1. Lithotectonic units in the Himachal Lower Himalayas (compiled after Azmi and Joshi 1981, Tewari 1984, Gururajan 1985) Name of litho-tectonic units Siwalik
Lithology Upper-Conglomerate Middle-Sandstone Lower-Sandstone, shale
Probable age PleistoceneMiddle Miocene
Foliations present
Folding episodes present
One set
Two (F: and F3)
One set
Two (F2 and F3)
One set
Two (F2 and F3)
Three sets
Three (Fl, F2 and F3)
Precambrian
Three sets
Three (Fl, F2 and F3)
Precambrian
Three sets
Three (FI, F2 and F3)
Precambrian
Three sets
Three (F:, F 2 and F3)
Main boundary fault Dharamsala Group
Kasauli Formation Dagrhai Formation Subathu Formation
Sandstone, few shales Sandstone and red shales Limestone, variegated shale
Middle Miocene -Eocene
Krol Thrust (Main Boundary Thrust) Krol Group
Tal Krol Infra-Krol Blaini Formation
Quartzites, shales with phosphorite Dolomites, subordinate limestone, black slates at lower levels Boulder slate, pink dolomite, sandstone and shales
? DevonianEocambrian
Gift Thrust Simla Group
Sanjauli Formation Slate, quartzite and sandstone Jaunsar Formation Chhaosa Formation Kunihar Formation - Inter tonguing relationship Basantpur Formation
Early-Late Riphaean
Shali Thrust Shali Formation
Cherty limestone and stromatolitic limestone
Middle Riphaean
Relationship not clear Rampur Formation
= Berinag Formation
Orthoquartzites with penecontcmporaneous volcanics Chail Thrust
Chail Formation
Low grade metamorphic rocks of greenschist to epidote amphibolite facies with associated granitoid and basic rocks Jutogh Thrust
Jutogh Formation
Medium to high grade metamorphic rocks of umphibolite facies with associated migmatites, granites and basic rocks.
44
A.K. DU~F.Y and M. 1. BHAT (1) A "single indistinct current bedding" observed in the northwestern part of the area (at Halog) was used to decipher the overall structure of the region. No overturning of beds was observed anywhere else in the region, although their crosssection of the area implies overturning of beds in the central region (i.e. Jutogh, Prospect Hill, and Taradevi) as well. (2) Three phases of folding movements were described from the Jutogh Formation of the area; of these three phases, the second phase was reported as missing in the Chail Formation. This implies that the Jutogh and Chail Formations were adjacent in their original position at the time of the first folding movement. Later, a thrust surface developed parallel to the axial surface of the isoclinal and recumbent F~ folds, and after translation along a sub-horizontal thrust surface, the Jutogh Formation now occupies a position on a different portion of the Chail Formation. The thrusting was described as simultaneous with the F2 folding movement, and it preceded the third folding phase. However, later studies in the Simla region by Raj (1983) and in the adjoining region by Virdi (1977) have shown clearly that the three important rock units of the area (i.e. Jutogh Formation, Chail Formation, and Simla Group) have undergone the same three phases of folding associated with the Himalayan orogeny. A detailed study by Raj (1983) in the southern part of the Simla Himalayas led to the identification of a regular pattern in the distribution of different generations of folds. Raj observed that the frequency of the F~ and ~ folds decreases near the thrusts, in contrast with the F3 folds, which are fairly uniformly distributed in the area. (3) The structural analysis was carried out mostly in the Jutogh Formation and only partly in the Chail Formation; the Simla Group surrounding the klippe was given only a cursory treatment. We believe that the study of Simla Group rocks is important because the fold structures are very well developed and primary structures, which act as younging indicators, are better preserved in these rocks.
PRESENT STUDIES
It is evident from the foregoing discussion that many of the structural features have not been explained satisfactorily. There also is confusion regarding the distinction of different rock units in the area. In order to improve our understanding, structural data were collected relating to fold geometries, the orientation of foliation planes and fault planes, not only from the Jutogh and Chail Formations but also from the Simla Group of rocks. The area was divided into 22 sub-areas (Fig. 3) in order to study the geometric pattern and variation of structural features. Part of the area not
covered by the sub-areas is either inaccessible or soil and vegetation covered. The relevant data are presented and discussed below. Folds Field observations indicate that the rock units of the Simla Group, Chail Formation, and Jutogh Formation are involved in the same three phases of folding. The plunge of the minor fold hinge lines and the dip of the axial surfaces in different sub-areas are shown in Fig. 3. (Data from sub-area Nos 19 and 21 are missing because of poor exposure of minor folds.) The first-formed F~ folds are mostly tight or isoclinal, upright, recumbent, or reclined. The trend of the fold hinge lines varies from NW-SE to E-W with a gentle plunge towards E or W. These folds have been folding coaxially into F 2 folds (type 3 interference pattern; Ramsay 1967, Chapter 10) which are open to tight, upright, overturned, and recumbent. The third phase of folding is characterised on a major scale by upright and asymmetric chevron folds, conjugate folds and kink bands. The hinge lines of these /73 folds trend from NE-SW to N-S (i.e. nearly perpendicular to the F~ and F2 folds) with a gentle plunge towards the N or S. Since F~ and F 2 are coaxial, they are grouped into one category as early folds. The F3 folds are a separate category and are designated as superposed folds in the subsequent discussion. Foliations
The formation of each of the fold generations was accompanied by the development of axial plane foliation. Thus three axial plane foliations exist in the area and help in the identification of different generations of folds. The orientation patterns of the foliation planes in different sub-areas are shown in Fig. 4. (Foliation data from sub-area Nos 1, 9, 10, 12, and 13 are missing because minor folds are predominant in these sub-areas; see Fig. 3.) It is evident that some of the sub-areas (Nos 3, 5, 6, and 7) incorporate data from all of the three important rock units of the area (i.e. Jutogh, Chail, and Simla Group), whereas some (e.g. Nos 14, 16, 17, 20, and 21) have data only from the Chail Formation and the Simla Group. Despite the fact that the data are from different rock units, the result is a simple single pattern, confirming that the rock units have undergone the same folding movements. The asymmetric concentration of pole points around a great circle in all of the plots indicates asymmetric folds with inclined axial surfaces (cf. Ragan 1985, p. 279). Since the pattern is characteristic of the whole region, the implication is that the regional structure is typified by second order folds. The trends of the early and the superposed fold hinge lines and the dip of the axial surfaces determined from the orientation diagrams (Figs 3 and 4) are shown in Fig. 5. Whereas the early folds in the western part of the area plunge due west, they have an easterly plunge in the eastern part [Fig. 5(a)]. Reversal of the plunge suggests
Structural evolution of the Simla area, NW Himalayas
45
N N
N
I X
•
~
A A
.+,
z •
",,,•
K~
"1-
oo
•
x i
A
•
•
._L... o A&
•
0 Q
4-1
==
A&
4
i
RI
O0 g
#, xl
N
X
x! ~ •
12
[
ox
I
N 0
~.
A
+
v
o"
A •
•
7
."
ba
it
tt
,.
A ~
~o
t,
°o~
#"
•
+
*~
o_/
. . . .
Q
18
O
14
20
22
17
Fig. 3. A n outline map of the area showing locations of the sub-areas and orientation of folds in different sub-areas. The sub-area number is indicated in each diagram. Dot = plunge of early fold hinge line; o p e n circle = plunge of superposed fold hinge line; triangle = pole to the early axial surface; cross = pole to the superposed axial surface.
refolding along a NNE-SSW axis--a trend congruent with the trend of the F 3 folds. The general trend of the superposed fold hinge lines [Fig. 5(b)] varies from N-S to NE-SW. A comparison of Fig. 5(a) with 5(b) reveals that the area has been affected by second order folds of both the early and superposed fold generations. The early folds produced a NW-SE trending synform which later was refolded into a NNE-SSW trending antiform during the superposed folding phase. While the greater variation observed in the early fold hinge lines is attributed to the refolding, the variation in the superposed fold hinge lines is thought to be due to initiation and development of these folds on initial undulating surfaces. Faults
Minor normal faults (with a throw of a few cm to a few m) are prominent in the southern part of the area (near the Girl, Chail and Jutogh Thrusts). The continuity of some, and the displacement of other, foliation surfaces across these faults indicate that the faults formed at different periods throughout the tectonic history of the region. The earliest faults are
synsedimentary faults. Their characteristic feature is that they do not displace the earliest foliation, which indicates that they were formed during post-rift subsidence and sedimentation (Dubey and Bhat 1986b). Since these faults are observed in the Simla Group, we infer that they formed during the first available record (which is of at least Late Archaean age) of the rift phase in the Himalayas (Bhat and Le Fort 1990). The youngest faults displace all the foliation planes present in the area (Fig. 6). The frequency of these faults gradually diminishes in a northerly direction. In the N and NW of Simla Town (Fig. 3; sub-areas 3, 5, 8, 9, and I0) these faults are absent; instead minor folds (mostly F2 and F3) are predominant. In summary, the following important observations and sequence of structural events may be emphasised: (1) synsedimentary normal faults are the first important structures to be formed in the area; (2) the large-scale, first formed structure of the Jutogh rocks is not a recumbent fold or a recumbent syncline as was thought earlier (Pilgrim and West 1928, Naha and Ray 1972), but a second order fold;
4
3.0 %
1.2%
t.8%
14
3.0% 4.5% 6.0%
I
7
+q
I
-?~--->
L__
3
0
2"3°1° 2.9%
0.6 1.2o7;o° - 2 0
2.1% 3.5%N
6%
1.5% 3.0 % 4.5 %
~ I ~
/
%.zJ
3.0 % 4.5 %
6.0%
,
22
//
3.6 ~o 4.8%
6.0%
2.4% 3.6 %
L2 %
~ " 19
,.,
,,~
4.8%
3.2%
~"~,
"
2.6% 5.2 %
e %
3.2% 4.5 %
21
"~-__2-J
16
L6 %
I-~J i 2 %
8
18
+
N
Fig. 4. An outline m a p of the area and contours of the poles of the foliation planes in different sub-areas• Dot := early fold hinge line (F~ and F~): oper circle (V~). The plots are characteristic of' asymmetric folds with inclined axial surfaces.
5.5%
1.2% 3.3%
• ~.9 %
2.7%
N
,-,
superposed fold hinge !m~
~.,~ :',:
,Qt;:o
N
N
= > ,.-1
-1
t'll ,<
ci,
;>
Structural evolution of the Simla area, NW Himalayas 77*
metry of the folds indicates a sense of shear towards the SW; (5) the early folds were followed by the development of NE-SW trending superposed folds (F3). These folds are fairly uniformly distributed throughout the area, irrespective of their distance from the thrusts; (6) the allochthonous Jutogh and Chail Formations and the parautochthonous Simla Group have undergone the same three phases of folding.
77" 30' ,
"~\
0
3L
47
I0 km
5L°
INTERPRETATION
It is now establised that folding initiates from irregularities, either in the layering or in the stress field (Dubey and Cobbold 1977). Normaly, fold initiation takes place Kolka ~ \ \ \ I ,Raj(:jq..~h // during an overall shortening of 3-19%. The actual amount depends mainly on the thickness of the layer, competence contrast between the layer and the matrix, initial irregularities on the layering, and the amount of 77* 77* .gO' Fig. 5(a). Trends of the early fold hinge lines and dips of the axial layer parallel slip, especially in a multilayered packet. surfaces determined from Figs 3 and 4. The pattern suggests refolding The amplification of folds is rapid during the initial of the Simla Synform along the NNE-SSW axis during the F3 folding stages of deformation and this is accompanied by a movement. decrease in the fold interlimb angles. At low interlimb angles, the folds acquire a stable geometrical shape; this I I 77 ° 77* 50' stage is defined as rotation hardening (Cobbold 1977, Dubey 1980). At the early stage of fold formation, the ~,N~N~~ 0 10 km I I axis of maximum extension remains parallel to the axial surfaces of the developing folds and normal to the fold hinge line; however, after reaching the stage of rotation . // " hardening, further shortening changes the axis of maximum extension, which becomes parallel to the fold hinge lines. This change in the axis of extension is accompanied by the formation of second order folds. Strike-slip faults may also develop at this late stage of fold formation. If the boundary conditions do not permit an extension parallel to the fold hinge lines, then thrust faults are likely to develop, initiating from the fold limbs (Dubey and Behzadi 1981). Considering the above facts, folding is normally followed by thrusting. However, the occurrence of the same phases of folding and the continuation of the same structural elements in the parautochthonous Simla .RoIgo Group and the aUochthonous Jutogh and Chail Formations suggest one of the following two possibilities: (1) the thrusting and the folding initiated simultaneously Fig. 5(b). Trends of the superposed fold hinge lines and dips of the and there is no significant effect of horizontal translation axial surfaces determined from Figs 3 and 4. The general trend of the on the fold geometries; and (2) the initiation of thrust fold hinge lines varies from N-S to NE-SW. and translation of the rock mass occurred before the folding and all three generations of folds were formed (3) the early folds (i.e. F] and F~) are most prominent when the rock units were in their present positions. An earlier study with model deformation experiments away from the major thrusts. Contemporaneous with the formation of the early folds, normal (Dubey and Bhat 1986a) revealed that folding and faults also developed, these faults are most thrusting initiate simultaneously in models where planes of discontinuity occur in multilayer models. However, it prominent near the thrusts; (4) the geometry of the early mesoscopic folds gradu- has never been observed that a significant displacement ally changes from predominently upright folds along a thrust occurs before the initiation of folding in in the southern part (Fig. 7) to overturned and the multilayers. Thus, the first probability appears to be recumbent folds in the north (Fig. 8); the asym- more appropriate in the present context. The weak \\\
•
X "-~
i .
S olan '~\.--'~ "~.So,, " ,u ~i()' Ir~
•
48
\ K. DUBEY and M. I BHA1
SW
~
~
:~
~
0
~=
NE.
! 0 Km I
Fig. 9. A revised geologicalcross-section of the Simla Himalayasalong the line X - Y in Fig. I. The various thrusts may be a part of the duplex system(M.B.T.= Main BoundaryThrust). planes required for the simultaneous initiation of thrusting and folding may have been provided by the listric, normal faults that formed during the tensional regime in the region (Bhat 1984a). The presence of synsedimentary normal faults supports this view. Our revised cross-section of the area is shown in Fig. 9. It is interesting to note that whereas the Chail and Jutogh Thrusts are distinctly spaced in the SW part of the klippe, they nearly merge in the NE part, thereby reducing the Chail Formation to the thickness of a few m. A likely explanation for this phenomenon is that the two observed thrust faults are termination splays of a single master thrust. We therefore propose to call this master thrust as the Chail-Jutogh Thrust. The geometries of various thrusts in the area suggest that they may be a part of a duplex system in which the Chail-Jutogh Thrust forms the roof thrust. The maximum horizontal translation took place along this thrust possibly because of the presence of a lubricant graphitic schist band at the thrust surface. The maximum horizontal translation along the Chail-Jutogh Thrust determined by the "klippe to fenster method" (Hobbs et al. 1976, p. 309) is approximately 40 km. No attempt has been made to balance the section for the following reasons: (1) there is no marker bed across the thrusts that could be employed to balance the section (cf. Elliott and Johnson 1980); (2) minor strike-slip faults have been reported in the Krot-Tal sequence around Solan (Fig. 1) by Bhattacharya and Niyogi (1971). They indicate an extension normal to the plane of the crosssection. Since these faults are the youngest structures, it is likely that the older rock sequences (i.e. Jutogh, Chail, and Simla) have undergone a similar extension (Dubey and Bhat 1986a, also cf. Brown et aL 1986). The total amount of fault displacement and the strain associated with these faults are still a matter of speculation. However, it is certain that the assumption of plane strain deformation is not valid for the region; (3) the depth of the basal d6collement and the dip of the sole thrust are unknown. It is to be noted that the only available deep seismic sounding (DSS) profile for the Indian part of the Himalayas (Kaila et al. 1978, 1984, Beloussov et al. 1980) depicts the major faults at a very steep angle and
does not show a basal thrust. Hence, no estimates can be made of the original area or length of the section restored. We therefore agree with Searle (1986) that "the amount of inference (fiddling) that has to be made is at present inacceptably great." The comment was made for the Ladakh and Zanskar regions; however, we consider it to be equally applicable to other parts of the Himalayas.
THE MODEL
In the light of the field data and their interpretation, we now attempt to model the tectonic events which appear to have determined the structural evolution of the area. Special emphasis will be placed on the well-marked variation in frequency and distribution of structural elements in the Simla Group of rocks. It is presumed that the fracture plates were formed during the tensional regime in the region as normal, listric faults and that they later reactivated as listric thrust faults during the compressional regime. The thrust movement was simultaneous with the formation of folds which initiated with a symmetric fold style (Fig. 10A). Since the faults had a listric geometry, the beds were tilted in the dip direction of the thrust faults. The oblique orientation of the rock units with reference to the thrust conforms to Zone 1 of the strain ellipse developed by progressive simple shear (Ramsay 1967, Fig. 3.6). The shear strain in the vicinity of the thrust surface resulted in extension of the layering and formation of normal faults near the thrust (Fig. 10A). Away from the thrust, layer parallel shortening (i.e. buckling), as a result of orogenic compression, was prominent, resulting in the formation of folds (Fig. 10A). Simultaneous folding and thrusting had a considerable influence on the geometry of the amplifying folds: a progressive increase in shear strain, associated with the roof thrust, modified the upright fold geometry to asymmetric and overturned fold patterns (Fig. 10B). This was accompanied by a decrease in the fold interlimb angles leading to the "rotation hardening", and finally the sequence was folded into second order folds (Fig. 10C). The roof thrust was also folded, resulting in the cessation of horizontal translation. The youngest F~ folds which formed after the locking of the thrusts therefore show a uniform occurrence throughout the
Structural evolution of the Simla area, NW Himalayas
Fig. 6. Normal faults in the Simla Group of rocks near the Girl Thrust. (Location--Kandaghat.) Such faults are prominent in the southern part of the area near the major thrusts. Fig. 7. A mesoscopic F2 fold with steep axial surface diping due NE in the Simla Group of rocks, approximately 1 km from the Gift Thrust. (Location--I km north of Kandaghat.) Such folds are prominent in the southern part of the area, away from the major thrusts.
49
Fig S Overturned fnld i~, the Sim!a Group of rocks, approximatel,, 8 km froil! the Shall Thrust. (Location --north of Simla town.) Mesoscopic, overturned and recumbent fold gcometr{e~ ;~rc char;tctcrislic of the northern part of the area near the contact of the Simla G r o u p with the klippe rocks.
,'v >
.,..-
r'~
7¢
J~
Structural evolution of the Simla area, NW Himalayas
51
A
)-
.-"!
o
"
"- " . . "
\
.......
--'2 ~
....
"%,
Fig. 10. A simplified model for the structural evolution of the Simla area. A: formation of uptight folds and simultaneous thrust movement along the fracture planes. Whereas the fractures are considered to have originated as listric, normal faults during the pre-orogenie tensional regime in the region, the upright folds developed as a result of horizontal orogenic compression. B: modification of the upright fold geometry to asymmetric and overturned forms as a result of increase in displacement along the subhorizontal Chall-Jutogh Thrust. C: formation of second order folds, steepening of the thrusts, and folding of the sub-horizontal part of the thrust, resulting into locking of the thrust. The structural features developing at locations A, B, and C are shown in Figs 6, 7, and 8, respectively.
area. The concept of the locking of the thrusts is further strengthened by the following arguments: (1) The fault surface gradually rotates away from the axis of maximum shortening with the result that the fault plane steepens and produces early locking (Dubey and Behzadi 1981). (2) The folded thrust surface was later refolded during the superposed folding movement and the resulting geometrical shape was obviously not favourable for further horizontal translation. (3) Thrust movement along a listric fault has an inbuilt restriction on the amount of total fault displacement because the fault has a non-planar geometry. A further significant point is that the component of vertical displacement on a steep fault exceeds that of the component of horizontal displacement. In view of these constraints, it is proposed that the minimum displacement of 40 km along the Chail-Jutogh
Thrust determined by the klippe-to-fenster method is very close to the maximum possible displacement along the thrust.
DISCUSSION Structural data presented here demonstrate that the pre-orogenic normal faults of this area must be taken into account to explain the geometry and evolution of the structural features as well as their spatial distribution--a feature neither considered earlier nor amenable to the explanations of earlier models. In addition, the two most critical facts that emerge from the study are that, (i) the initiation of fold and thrust movements were simultaneous; and (ii) no large scale recumbent fold structure is present in the area. Both these observations place severe limitations on the amount of crustal shortening and, in consequence, the crustal thickening in the area through orogenic structural deformation.
52
A K. DUBEY and M. I. BHAJ
Presence of material with a density of 3.05 cm ~ at a depth of 25 km below the surface and extending down to the depth of 55-70 km in the Himalayan region has been known for two decades (Gureshi 1969). Later Deep Seismic Sounding (DSS) studies also recorded a velocity of 6.9-7.5 km s-1 at nearly the same depth (Kaila et al. 1978, 1984). Whereas Gureshi (1969) interpreted the high density material as due to thickening of the basaltic layer (an interpretation supported also by Beloussov et al. 1980, 1984), the plate tectonic interpretation (with different variations, as noted at the outset) is that it represents the underplated/underthrusted Indian lithosphere. In either case, once the fact that the crustal column from about 2 5 k m down to 70km is "allochthonous" is accepted, it leaves only the upper 25 km for consideration of increase in crustal thickness through structural deformation and shortening. There is general consensus that the unfolded and unthrusted thickness of the Tethyan Zone sediments of the Himalayas is at least 12 15 kin, while the not fully exposed metasedimentary basement, also unfolded and unthrusted, is 7-8 km thick (K, K. Sharma, personal communication). Thus the total thickness of the sedimentary pile in the Himalayas is not less than 23 kin. It hardly leaves any space for a significant crustal thickening through structural deformation - a n inference compatible with (in fact, a prediction of) the pre-orogenic rift history of the region ~,is-a-~is thrust movement along the pre-existing listric faults. As for the high density layer, the interpretation usually provided is that it represents a high pressure transformed phase of the underthrusted Indian plate continental crust (Roecker 1982). While transformation of continental crust to high density and high seismic velocity phases cannot be denied--at least on theoretical g r o u n d s - - a n appeal to such a phenomenon having taken place in the Himalayan region appears to be a case of unnecessary special pleading. This is especially true if the distribution of this high density, high seismic velocity crustal layer is seen in the context of the world-wide occurrence of such a layer. The existence of material of this density and seismic velocity along oceam4continent transition zones and in extensional belts is an established fact (Keen et at. 1975, Bott 1980). Indeed, a recent survey of world-wide seismic data has shown the presence of such material ( V p = 7 . 0 - 7 . 8 k m s ~ at varying depths under b o t h extensional and compression belts (Meyerhoff et al. 1989). Under extensional belts, this material--variously called "intermediate layer", "rift cushion", "anomalous layer", "anomalous mantel", "anomalous lithosphere"--is ascribed to upwelling of basaltic m a g m a from the asthenosphere into the lower crust, resulting in deepending of the Moho (White 1988). If, as is accepted, an orogenic belt is the culmination point in the long chain of geodynamic processes beginning with lithospheric rifting through ocean development and finally to mountain formation, then there is no reason to consider this "anomalous layer" under fold belts as anything other than basaltic material intruded during the rifting phase that precedes each fold
belt formation. Even if we ignore for the moment the additions of basaltic m a g m a associated with old rifting events in the Himalayan region, we cannot possibly do so for the Late Palaeozoic-Early Mesozoic rifting event as it is now well established from several lines of evidence to have resulted in the birth of the Neo-Tethys along the present Himalayan region (Bhat 1987 and references therein). This rifting event is generally thought to have been completed by Triassic time. However, the timing of basin subsidence, calculated by employing the approach used in understanding the subsidence of passive margins following rifting and during ocean floor phases (e.g. Haworth and Keen 1979), shows that the region was in a rifting regime until the time of Late Jurassic-Early Cretaceous ophiolite formation (Bhat 1984a~b, 1987). Given these considerations, we suggest that crustal thickening in the Himalayas is primarily a result of magmatic additions into the crust, associated with characteristic pre-orogenic tensional tectonics rather than with hypothetical and unconstrained processes that are contrary to field geological data collected from the region. Acknowledgements--We are particularly grateful to A. A. Meyerhoff for his keen interest in the study and for thorough review of several drafts of the manuscript. A. J. Barber, V. C. Thakur and H.-U. Schwarz also provided many useful comments. Discussions with D. Mukhopadhyay, M. R. W. Johnson, A. K. Sinha, K. K. Sharma, N. S. Gururajan and B. K. Choudhury, at different stages of the study, were quite beneficial.
REFERENCES Azmi, R. J. and Joshi, M. N. 1981. Conodont and other biostratigraphic evidence on the age and evolution of the Krol Belt. Him. Geol. 11, 198-223. Beloussov, V. V., Belyaevsky,N. A., Borisov, A. A., Volvovsky,B. S., Volvovsky, I. S., Resvoy, D. P., Tal-Virsky, B. B., Khamrabaev, I. Kh., Kaila, K. L., Narain, H., Marussi, A. and Finetti, J. 1980. Structure of the lithosphere along the deep seismicsounding profile: Tien Shan-Pamirs-Karakorum-Himalayas. Tectonophysics 70, 193-221. Beloussov, V. V., Volvovsky,B. S., Volvovsky,I. S., Khamrabaev, 1. Kh., Kaila, K. L. and Marussi, A. 1984. Deep structure of Central Asia along the Tienshan-Pamir-Himalayasgeotraverse.In: Tectonics of Asia, 27th Int. Geol. Cong., Moscow, Colloquim 05 5, 29-39: Bhat, M. I. 1984a. Evolution of the southern passive margin of the Himalayan Tethys: model implications and constraints---I.Z Petrol. Geol. 7(3), 341-350. Bhat, M. I. 1984b. Evolution of the southern passive margin of the Himalayan Tethys: model implications and constraints--lI. J, Petrol. Geol. 7(4), 429-436. Bhat, M. I. 1987. Spasmodic rift reactivation and its role in the preorogenic evolution of the Himalayas. Tectonophysics 134, 103--127. Bhat, M. I. and Le Fort, P. 1990. Geochronology and petrogenesis of Rampur metavolcanic rocks, NW Himalayas: elemental and Ndisotopic evidence for penecontemporaneous mantle metasomatism and magma genesis. Precambrian Res. Commun. 0, 000--000. Bhattaebarya, S. C. and Niyogi, D. 1971. Geologic evolution of the Krol belt in Simla Hills, H.P. Him. Geol. 1, 178-212. Bott, M. H. P. 1980. Problems of passive margins from a view point of the geodynamic project: a review, Phil. Trans. R. Soc. London, Ser. A., 294-A, 5-16. Brown, R. L., Journeay, J. M., Lane, L. S., Murphy, D. C. and Rees, C. J. 1986. Obduction, backfolding and piggyback thrusting in the metamorphic hinterland of the south-eastern Canadian Cordillera. J. struc. Geol. 8, 255-268. Chaterji, G. C. and Swaminath, J, 1977. The stratigraphy and structure of parts of the Simla Himalaya--a synthesis. Mere. Geol. Surv. India 106(2), 408-488.
Structural evolution of the Simla area, NW Himalayas Cobbold, P. R. 1977. Description and origin of banded deformation structures. II. Rheology and growth of banded perturbations. Can. J. Earth Sci. 14, 2510-2523. Davies, V. M. 1982. Interaction of thrusts and basement faults in the French External Alps. Tectonophysics 88, 325-331. Dubey, A. K. 1980. Late stages in the development of folds as deduced from model experiments. Tectonophysics 65, 311-322. Dubey, A. K. and Bhat, M. I. 1986a. The role of reactivation of pre-rift basement listric faults in the structural evolution of the Himalaya: an experimental study. In: Current Trends in Geology, Himalayan Thrusts and Associated Rocks (Edited by Saklani, P. S.), Vol. 9, pp. 265-290. Dubey, A. K. and Bhat, M. I. 1986b. Experimental observations on the reactivation of normal listric faults of the pre-rift basement and the structural evolution of the Himalayas. In: Ophiolites and Indian Plate Margin (Edited by Ghose, N. C. and Vardarajan, S.), pp. 101-107. Patna University Press, Patna. Dubey, A. K. and Behzadi, H. 1981. Development of flexural slip folds, overlapping boudins and extension faults in multilayered materials: field evidence and experimental model. J. Geol. Soc. India 22, 274-284. Dubey, A. K. and Cobbold, P. R. 1977. Noncylindrical fiexural slip folds in nature and experiment. Tectonophysics 38, 223-239. Elliott, D. and Johnson, M. R. W. 1980. Structural evolution in the northern part of the Moine Thrust Belt, NW Scotland. Trans. R. Soc. Edinb. 71, 69-96. Etheridge, M. A. 1986. Reactivation of extensional fault systems. Phil. Trans. R. Soc. London Ser. A. 317, 179-194. Gururajan, N. S. 1985. Structure and metamorphism of the Chail Formation in the area between Baragaon and Nirath (Himachal Pradesh) in Simla Himalaya with a special reference to the genesis of the associated mylonite gneiss. Ph.D. Thesis (unpublished), Garhwal University, Garhwal. Haworth, R. T. and Keen, C. E. 1979. The Canadian Atlantic margin: a passive continental margin encompassing an active past. Tectonophysics 59, 83-126. Him, A. et al. 1984. Crustal structure and variability of the Himalayan border of Tibet. Nature 307, 23-25. Hobbs, B. E., Means, W. D. and Williams, P. F. 1976. An Outline of Structural Geology. Wiley, New York. Jackson, J. A. 1980. Reactivation of basement faults and crustal shortening in orogenic belts. Nature 283, 343-346. Kaila, K. L., Krishna, V. G., Roy Chowdhury, K. and Narain, H. 1978. Structure of the Kashmir-Himalaya from deep seismic soundings. J. Geol. Soc. India 19, 1-20. Kaila, K. L., Tripathi, K. M. and Dixit, M. M. 1984. Crustal structure along Wular Lake-Gulmarg-Naoshera profile across pir Panjal range of the Himalayas from deep seismic soundings. J. Geol. Soc. India 25, 706-719. Keen, C. E., Keen, M. J., Barrett, D. L. and Heftier, D. E. 1975. Some aspects of the ocean-continent transition at the continental margin of eastern north America. In: Offshore Geology of Eastern Canada (Edited by Van den Linden, W. J. M. and Wade, J. A.), Vol. 2, pp. 189-197. Geol. Surv. Can. Paper 74-30. Le Pichon, X. and Sibuet, J. C. 1981. Passive margins: a model of formation. J. Geophys. Res. 86, 3708-3720. Meyerhoff, A. A., Taner, I., Morris, A. E. L. and Martin, B. D. 1989. Surge tectonics. In: New Concepts in Global Tectonics (Abstract volume) (Edited by Chatterjee, S. and Hotton, N.), pp. 25-26. Texas Tech. University Press.
53
Naha, K. and Ray, S. K. 1972. Structural evolution of the Simla Klippe in the Lower Himalayas. Geol. Rundschau 61, 1050-1086. Pilgrim, G. E. and West, W. D. 1928. The structure and correlation of the Simla rocks. Mere. Geol. Surv. India 53, 1-140. Powell, C. Mc. A. and Conaghan, P. J. 1973. Plate tectonics and the Himalayas, Earth planet. Sci. Lett. 20, 1-12. Gureshi, M. N. 1969. Thickening of a basalt layer as a possible cause for the uplift of the Himalayas--a suggestion based on gravity data. Tectonophysics 7, 137-167. Ragan, D. M. 1985. Structural Geology. Third Edn. Wiley, New York. Raj, S. 1983. Variation of structural elements across the Jutogh Thrust and its implications: a study based on field evidence and model experiments. Geoseience J. 4, 157-168. Ramsay, J. G. 1967. Folding and fracturing of rocks. McGraw-Hill, New York. Ray, S. K. and Naha, K. 1971. Structural and metamorphic history of the "Simla klippe": a summary. Him. Geol. 1, 1-24. Roecker, S. W. 1982. Velocity structure of the Pamir-Hindu Kush region: possible evidence of subducted crust. J. Geophys. Res. 87, 945-959. Searle, M. P. 1986. Structural evolution and sequence of thrusting in the High Himalayan, Tibetan-Tethys and Indus Suture Zones of Zanskar and Ladakh, Western Himalaya. J. struc. Geol. 8, 923-936. Sharma, V. P. 1977. Geology of the Kulu-Rampur belt, Himachal Pradesh. Mem. Geol. Surv. India 106, 235-407. Sinha, A. K. 1980. Tectono-stratigraphic problem in the Lesser Himalayan zone of Simla Region~ Himachal Pradesh. In: Stratigraphy and Correlations of Lesser Himalayan Formations (Edited by Valdiya, K. S. and Bhatia, S. B.), pp. 99-116. Hindustan Publishing Co., New Delhi. Soper, N. J. and Anderton, R. 1984. Did the Dalradian slides originate as extensional faults? Nature 307, 357-360. Srikantia, S. V. and Sharma, R. P. 1976. Geology of the Shali belt and the adjoining areas. Mere. Geol. Surv. India 106, 31-166. Stewart, A. J. 1987. Fault reactivation and superimposed folding in a Proterozoic sandstone-volcanic sequence, Davenport Province, Central Australia. J. struct. GeoL 9, 441--455. Tewari, V. C. 1984. Stromatolites and Precambrian-Lower Cambrian biostratigraphy of the Lesser Himalaya, India. In: Proc. Fifth Indian Geophytol. Conf. (Edited by Tiwari, R. S.), pp. 71-97. Special Publication of the Palaeobotanical Society, Lucknow. Virdi, N. S. 1977. Stratigraphy and structure of the Narkanda area, district Simla, Himachal Pradesh, India. Recent Res. Geol. 3, 453-467. Virdi, N. S. 1979. Status of the Chail Formation vis-d-vis Jutogh~hail relationship in the Himachal Lesser Himalaya. Him. Geol. 9, 111-125. Welbon, A. 1988. The influence of intrabasinal faults on the development of a linked thrust system. Geol. Runds. 77, 11-24. West, W. D. 1939. The structure of the Shali "window" near Simla. Rec. Geol. Surv. India 74, 133-163. Winslow, M. A. 1981. Mechanisms for basement shortening in the Andean foreland fold belt ofsouthern South America. In: Thrust and Nappe Tectonics. Spec. Publ. Geol. Soc. London 9, 513-528. White, R. S. 1988. The Earth's crust and lithosphere. J. Geol. Special Lithosphere Issue, 1-10. White, R. S., Spence, G. D., Fowler, S. R., McKenzie, D. P., Westbrook, G. K. and Bowen, A. N. 1987. Magmatism at rifted continental margins. Nature 330, 439-444.