Tectonic implications of minor folds of the Imphal Valley in the Indo-Myanmar Ranges.
The Imphal Valley, an integral part of the Indo-Myanmar Ranges (IMR), is located in the central part of the state of Manipur and lies approximately between 24[degrees]14'N & 25[degrees]00'N and 93[degrees]48'E & 94[degrees]07'E and at an elevation of about 782m above m.s.l. It is near oval in shape with a width of about 30-35 km (E-W) and a length of about 60-65 km (N-S). The Valley is connected with other parts of the sub-continent by NH-39, NH-53 and NH-150. Figure 1 represents the location as well as the geological and structural features of the Imphal Valley.
The Disang and the Barail Groups are the two major litho-stratigraphic units in the Imphal Valley region. The former is represented by a huge pile of dark gray splintery shales showing frequently fine laminations and the later group which lies conformably above the Disangs with a gradational transition is represented by light brownish gray, fine to medium grained sandstones often interbedded with brown to dark gray shales. Both groups of rocks have been deposited in a flysch basin. The Disang shales are dominant lithology in the Valley in comparison to the Barail sandstone, (Soibam, 1998; Hemanta, 2004).
Various forms of minor folds that are found associated with the rocks of the Imphal Valley are Coffer (Box) fold, Sharp (Angular) fold, Isoclinal fold, Harmonic and Disharmonic fold.
The present study is especially aimed at examining the tectonic importance of such folds, and could be put to use for evaluation of the regional fold geometry based on their orientations.
GEOLOGICAL SETTING OF THE IMPHAL VALLEY
The Imphal Valley has peculiar geomorphic and/or geologic features, for it occurs within the surrounding hilly terrain of the region. This nearly flat intramontane basin is filled with alluviums of fluvio-lacustrine origin. Almost all the hills and hillocks within the Valley are in the form of inselbergs/monadnocks.
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The hills and hillocks are synclines whereas the valleys are anticlines, suggesting that there was sufficient gap between the upliftment of the IMR which is believed to be culminated during Mio-Pliocene and subsidence of the Imphal Basin in the later phase (Pleistocene?) of the IMR tectogenesis. The Valley is bounded by two prominent thrusts, the Thoubal Thrust and the Churachandpur-Mao Thrust. The later thrust partly serves as the tectonic contact between the Disangs and Barails. Earlier studies indicated that rocks of the Imphal Valley experienced compression in the N75[degrees]W-S75[degrees]E (WNW-ESE) direction and extension in the N15[degrees]E-S15[degrees]W (NNE-SSW) direction, (Soibam, 1998; Hemanta, 2004).
REGIONAL FOLD ANALYSIS
Regional folds are usually not observable in a single outcrop. To accomplish the evaluation of the form and geometry of regional folds as many as 307 bedding dip-strike data from different parts of the study areas have been collected. And in order to observe any change in minor fold orientations in the different parts of the Valley, the study area is roughly divided into four sections as Northern, Eastern, Southern and Western. Rosettes, poles and contour diagrams are prepared with these data.
Study of the rosettes (figure 2) in the four sections yield mean resultant directions of N015[degrees] (Northern), N009[degrees](Eastern), N007[degrees] (Southern), and N005[degrees] (Western) respectively. The mean resultant direction of Northern part, N015[degrees], is almost parallel to the regional strike, N15[degrees]E-S15[degrees]W, which is also the tectonic elongation direction of the region (Soibam, 1998; Hemanta, 2004). However, the mean resultant directions in the Eastern and Southern parts become more or less closer to N000[degrees]. This could be related to the change in regional tectonic strike. That is, in Northern part of Manipur, regional strike is NNE-SSW while in the Southern part it is nearly N-S. Such a behavior could also be co-related with the arcuate nature of the IMR. Contrary to the above, the rosette of the Western part of the Valley show slight difference in orientation pattern. That is, a considerable number of beddings of the Western side orient in the N340[degrees]-N350[degrees] direction which is not common in the other parts. This particular deviation from the general trend may indicate that there are certain folds with axes trending in the NW/SE direction. The occurrence of such NW/SE trending folds could be of local extent and may be related with shearing along the Churachandpur--Mao Thrust, that lie along the western foothill region.
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Analyses of the pole diagrams (poles to beddings) in the Northern, Eastern, and Southern display nearly uniform distribution patterns where pole concentration lies in the WNW-ESE (E-W) direction with relatively less poles in the central parts of the projection (figure 3a, b & c). Such distribution patterns of the pole or [pi]-diagrams may reflect some typical fold geometry of narrow hinge folds. However, distribution of poles in the Western part indicates scattering pattern representing fairly good numbers of poles in the central part of the projection (figure 3d). Such a distribution pattern suggests slightly different fold geometry where hinge or fold closer may be broad and rounded.
Further, the contour diagrams prepared with the pole data (figure 4) show that the geometry of the folds in the Northern, Eastern, and Southern parts is usually angular with narrow hinge, but not necessarily of angular and sharp hinge. The folds are generally asymmetric in nature (figure 4a', b' & c') with one limb relatively short and dipping steeply while the other limb longer and dipping relatively gentle. But as evident from the pole and contour diagrams of the Western part (figures 3d and 4d) the fold could be of broad or rounded hinge similar to a concentric fan or box fold. Figure 5 a, b, c & d represents the overall rosette, pole plot, contour diagram and corresponding fold form as deduced from 307 bedding dip-strike data. It is clear from the analyses of the regional fold orientation and geometry in the four sections of the Valley that there is a significant change in the fold geometry in the Western region. Such a change in the geometry of the fold in this section might have been influenced by reactivation Churachandpur--Mao Thrust.
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MINOR FOLD ANALYSIS
Minor folds are commonly associated with large scale regional folds. In fact, minor fold analysis enables us to study the compatibility in the geometry, form, orientation, etc., between the minor folds and the regional folds of the region.
Within Imphal Valley and its adjoining regions, limited outcrops of minor (mesoscopic) folds are found, since major portion of the Valley is covered with alluviums. Above all, most of the rock exposures are of Disang shales that do not develop fair numbers of minor folds, which could be due to monotonous litho-character of the Disangs. However, occurrence of minor folds within the Disangs is common, wherever there is intercalation of shales and siltstones or fine-sandstones. As a result, minor folds are more abundant in the western part of the Valley, where intercalations of shales, siltstones and fine sandstones increase as the region is closer to the Disang-Barail boundary/transition zone. Because, such intercalation nature creates anisotropic multilayer complex producing competency contrast between the layers, thereby enabling to develop quite a good number of minor folds (Price and Cosgrove, 1990; Ghosh, 1993; Pluijm & Marshak, 1997). These minor folds have varied plunge amounts and can be summarized as in table-1.
[FIGURE 6 OMITTED]
MINOR FOLD ORIENTATION
A. NE and SW plunging co-axial folds
Minor folds which are sub-horizontal to moderately plunging in NNE-SSW to NE-SW directions are more or less coaxial with the regional folds. They may be correlated with the regional compression direction along WNW-ESE. Figure 6a&b represent the pole and contour diagrams of this category of minor folds. Comparison of attitudes of axial planes (009[degrees], 75[degrees]/099[degrees]) and [beta]-axis (5[degrees]/011[degrees]) of the regional fold with that of the NE /SW plunging folds (A/P: 014[degrees], 72[degrees]/104[degrees] and mean [beta]-axis 4[degrees]/015[degrees]) show that they are fairly compatible, which implies that the NE/SW plunging minor folds are coaxial with the regional fold(s).
B. NW and SE plunging non-coaxial folds
Another group of minor folds, which are sub-horizontal to moderately plunging towards NW and SE directions, are non-coaxial with the regional folds. Their occurrence may imply a secondary stress field having compression at angles of about 45[degrees] with respect to that of the regional compression and extension directions (Badgley, 1965). Figures 6c&d represent the pole and contour diagrams of this second category of minor folds. Such types of folds are found to occur abundantly along the western foothill regions like Thongjaorok Stream section, etc. The calculated axial plane (90[degrees]/068[degrees]) and mean ?-axis orientation (3[degrees]/338[degrees]) are not aligned with the regional fold axial plane and [beta]-orientations.
RESULTS AND DISCUSSION
Study of the minor folds in the rocks of the Imphal Valley and its surrounding areas has very significant outcomes. Some of the important observations made are as follows:
1. Imphal Valley rocks have experienced a regional compression in WNWESE direction, as evident from the coaxial minor folds that plunge towards NNE and SSW directions, which is also confirmed from the major fold analyses made above.
2. The NW and SE plunging minor folds imply that such folds which are very common along the western foothill, that more or less aligned with the Churachandpur-Mao thrust (C-M-T), are non co-axial and noncontemporaneous with the regional folds. Because, such folds cannot be produced by the regional stress field with WNW-ESE (E-W) compression and NNE-SSW (N-S) extension. Probably these groups of minor folds might have been produced by a secondary stress field, having a localized compression axis in the NE-SW direction and extension in the NW-SE direction, developed as a result of dextral shearing along the Churachandpur-Mao Thrust (C-M-T) zone. This compression direction produced by secondary stress field is at an angle of about 45[degrees] with the regional compression direction, that is, N30[degrees]W-S30[degrees]E and such mechanism have also been discussed by Badgley (1965) and Soibam (1998, 2001).
1. Badgley, P.C. 1965. Exploration methods for the geologist and a series of problems for the structural students. Oxford Book Company, Calcutta: p 280.
2. Ghosh, D.E. 1994. Structural Geology, fundamentals and modern developments. Pergamon Press: p 598.
3. Hemanta, R.K. 2004. Tectonic Significance of Minor Structures of the Rocks of Imphal Valley. Unpublished Ph.D. Thesis of Manipur University, Imphal: p 220.
4. Pluijm Van der B.A. and Marshak, S., 1997. Earth Structure: An introduction to Structural Geology and Tectonics, WCB/Mc Graw-Hill: p 495.
5. Price, N.J. and Cosgrove, J.W. 1990. Analysis of Geological Structures. Cambridge University Press, Cambridge: p 502.
6. Soibam, I.1998. Structural and Tectonic Analysis of Manipur with Special Reference to Evolution of the Imphal Valley. Unpublished Ph.D. Thesis of Manipur University, Imphal.
7. Soibam, I., 2001. Imphal Valley--A Transtensional Basin? (DST Project Completion Report), Department of Earth Sciences, Manipur University: p139.
R.K. Hemanta Singh *, Soibam Ibotombi, and E. Ranjitkumar Singh **
Department of Earth Sciences, Manipur University, Imphal (Manipur), India.
* Imphal College, Imphal, Manipur, India, Emailemail@example.com,
** United College, Chandel, Manipur, India.
Table-1 Class Plunge Percentage (%) Horizontal/ Sub-horizontal 0 [degrees]- 10 [degrees] 35 Gently Plunging 11 [degrees]- 30 [degrees] 20 Moderately Plunging 31 [degrees]- 60 [degrees] 35 Steeply Plunging 61 [degrees]- 80 [degrees] 10 Sub-vertical/ Vertical 81 [degrees]- 90 [degrees] -
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