J. Phys. Earth, 45, 167-176, 1997

Uplift and Tectonics of the , Northeast

N. Purnachandra Rao* and M. Ravi Kumar

National Geophysical Research Institute, , India

Different views prevail regarding the uplift and tectonics of the Shillong Plateau in northeastern India. In light of these, we discuss the cause of the current uplift, the time of initiation of the uplift, and the current tectonic scenario. Various geophysical results indicate that neither a thermal anomaly nor isostatic compensation could have caused the uplift as suggested by many workers in the past. Several coinciding factors point towards a combination of tectonic forces in this unique thrust environment, comprising the Himalayan thrust and the Burmese thrust, to be responsible instead. Stratigraphic evidence suggests that the initiation of the current uplift was during Mio-Pliocene. While most tectonic models attribute the Shillong Plateau uplift to a N-S compressive stress regime, it appears that an E-W compressive stress owing to the Indo-Burman subduction active during that period also aided the uplift by providing the necessary and timely impetus. Even at present, a small component of stress in the E-W direction seems to be operational as inferred from a computation of strain rates and velocity vectors. The predominance of thrust type focal mechanisms and their P axis orientations possibly indicate that the uplift is sustained by compressive forces acting on the plateau from all sides.

1. Introduction The Shillong Plateau, with a mean elevation of 1,600 m, stands on the northeastern flank of the Indian shield, to the south of the eastern Himalayan range and west of the Indo-Burman range. The plateau is bound on the northern and western sides by the Brahmaputra Valley, and on the southern side by the E-W trending Dauki fault (Fig. 1), which has been described as a major right-lateral fault by Evans (1964). However, others have suggested a vertical displacement across the fault (Murthy et al., 1969; Desikachar, 1974; Das Gupta, 1977; Ranga Rao, 1983). The plateau is characterized by numerous lineament structures and high seismic activity (Chandra, 1978; Khattri et al., 1983, 1988; Kayal, 1987; Kayal and De, 1987; Chen and Molnar, 1990; Mukhopadhyay, 1990; Mukhopadhyay et al., 1993). At present, the Shillong Plateau appears to be rising. Otherwise, such a steep and elevated feature would not have survived the onslaught of erosion in one of the rainiest regions of the world. Direct evidence for the plateau uplift comes from the results of

Received November 1, 1995; Accepted October 14, 1996 * To whom correspondence should be addressed .

167 168 N. P. Rao and M. R. Kumar

repeated geodetic leveling conducted over the plateau by the Geodetic and Research Branch of the Survey of India, which indicate a rise of 2.5 cm during 1910-1977 (Kailasam, 1979). The uplift is further evidenced by straight-edged scarps of older Brahmaputra alluvium within recent alluvium in (Kailasam, 1980). A major event in the geological history of the Shillong Plateau occurred in the late Jurassic period, when apparently a major hotspot caused uparching of the plateau with subsequent rifting (Talukdar and Murthy, 1970; Geological Survey of India, 1974; Kailasam, 1979). Subsequently, the development of major E-W trending fault systems like the Ribah and Dauki facilitated the eruption of flood basalts, known as the traps.

2. Mechanism of the Plateau Uplift Different views have been expressed regarding the cause of the current uplift of the Shillong Plateau. It has been suggested that the plateau is a horst initiated by a major thermal disturbance during the Jurassic period, and has continued to rise to its present elevation (Kailasam, 1979; Khattri et al., 1983; Kayal and De, 1987; Nayak, 1989). However, this requires that the mantle beneath the plateau continue to be hot, whereas several geophysical results indicate that the mantle beneath the Shillong Plateau has ceased to be hot. Seismic results from several workers confirm a high-velocity, cold upper mantle. A study of the Pn velocity of the uppermost mantle shows an anomalously high value of 8.57 km/s (Kayal and De, 1987). Similarly, modeling of the P-wave velocity structure by Kharshiing (1985) yielded a normal to high velocity of 8.08 km/s. Seismic travel time residuals at the Shillong station also show an anomaly of -0.57 s, indicating a medium faster than normal (Dziewonski and Anderson, 1983). The various crustal models proposed for the Shillong Plateau region (Khattri et al., 1983; Kayal and De, 1987; Mukhopadhyay, 1990) also suggest a high velocity for both the crust and upper mantle. Based on hypocentral depths greater than 40 km for earthquakes beneath the plateau, Chen and Molnar (1990) suggest an especially cold upper mantle. Gravity modeling using Bouguer and isostatic anomalies indicates the existence of high-density crust/upper mantle beneath the Shillong Plateau (Verma and Mukhopadhyay, 1977; Kailasam, 1979; Kayal and De, 1987). All these arguments point toward a high-velocity, cold, upper mantle. Therefore, a thermal source due to an earlier hotspot or plume activity, can perhaps be ruled out as the cause of the current uplift. Isostatic adjustment of the Shillong Plateau block has also been suggested as a possible cause of the current uplift (Kailasam, 1979). However, the positive Bouguer anomaly (up to + 40 mGal) and the strong, positive isostatic anomaly (up to +100 mGal) seen over the plateau (Verma and Mukhopadhyay, 1977) do not concur with uplift of the plateau due to isostatic adjustment. That the plateau continues to rise, although gravity results suggest that it should sink, is an indication that the uplift is a consequence of great external forces which can only be tectonic in nature. The location of the plateau in the vicinity of two major thrust zones provides for such a possibility. In understanding the tectonic history of any uplift, important clues can be derived from the stratigraphic sequence of the region (Table 1). The Shillong Plateau is underlain by an Archean gneissic complex, exposed in the central and northern parts of the

J. Phys. Earth Uplift and Tectonics of the Shillong Plateau 169

Table 1. Stratigraphic sequence on the Shillong Plateau (modified after Geological Survey of India (GSI), 1974, Misc. Publ., 30).

Fig. 1. Geology of the Shillong Plateau region (legend in Table 1). plateau. Overlying the eroded Precambrian basement, are the Sylhet traps (basalts) on the southern side of the plateau. Thick and extensive Cretaceous-Tertiary sediments are present mostly to the south of the plateau, continuing further down into the Bengal Basin. These include the Khasi group (upper Cretaceous), Jaintia group (Eocene), and Garo group (Eocene to Mio-Pliocene). The Garo group consists of a series of formations, the younger sediments appearing progressively towards the southwestern fringe of the plateau, with a similar disposition seen towards the southeastern fringe as well (Fig. 1). These formations, as well as the younger Dupitila formations, are identified as deltaic to fluviatile (Geological Survey of India, 1974). This fact constrains the period of initiation of the uplift towards the end of Mio-Pliocene, as suggested also by Chandra (1978) and Johnson and Alam (1991). This period, therefore, assumes great importance as it marks the initiation of a new phase when the Shillong Plateau as a whole was

Vol. 45, No. 3, 1997 170 N. P. Rao and M. R. Kumar uplifted (Geological Survey of India, 1974; Krishnan, 1960). It is interesting to note that, by this period, the Indo-Burman thrusting was well developed (Daly et al., 1991). One should not, therefore, ignore the possibility of the influence of the E-W directed Indo-Burman thrusting on the initiation of the Shillong Plateau uplift during that period.

3. A Neotectonic Model: Present Evidence The current uplift of the Shillong Plateau has been attributed by some workers to the India-Eurasia collision, and the resulting N-S to NNE-SSW directed stresses generated within the Indian shield (Chen and Molnar, 1990; Khattri et al., 1992; Mukhopadhyay et al., 1993). Even subduction of the Indian plate in the N-S direction beneath the Shillong Plateau along the Dauki fault has been proposed as a cause of the current uplift (Chen and Molnar, 1990; Khattri et al., 1992). We argue in favour of a combination of N-S and E-W directed stresses, with the latter being fairly active during the Mio-Pliocene period and providing the necessary and timely impetus for the uplift. The coinciding factors discussed in the previous section support such a possibility. Even at present, there is evidence for a small component of E-W directed stress influencing the current uplift, which is presented and discussed in this work:

3.1 Computation of strain rates Strain rates in the neighboring Burmese arc region have been computed using the approach of Kostrov (1974), for 50 Harvard CMT solutions of earthquakes occurring between 1977 and 1992 (Table 2), to estimate the component of E-W directed stress on the plateau. The method assumes that the strain in a region of volume V is all seismic and computes the average strain rate tensor for the region over time period T, by summing up the moment tensor elements of the earthquakes in that region, using the relation:

where U is the rigidity modulus generally taken as 3.0•~1011 dyn/cm2, Mij are the moment tensor elements of an earthquake, and N is the total number of earthquakes. The seismogenic thickness is normally taken as the maximum depth of seismicity (Holt et al., 1991) to compute volume V. As a standard practice, the rectangular coordinate system is chosen in such a way that X, Y, and Z coincide with the E-W, N-S and vertical directions, respectively. A negative sign for the strain in the X or Y directions indicates convergence while a positive sign indicates divergence. Similarly, in the Z direction, a negative sign implies crustal thinning while a positive sign implies crustal thickening. The strain rates obtained for the N-S, E-W and vertical directions are 3.02•~1-17, -0.74•~10-17, and 3.76•~10-17s-1, respectively, indicating horizontal - compression in both N-S and E-W directions and thickening in the vertical direction at the ratio of 4:1:5. This indicates a significant contribution of the Indo-Burman thrusting on the plateau.

J. Phys. Earth Uplift and Tectonics of the Shillong Plateau 171

3.2 Velocity vectors from NUVEL-1 model Velocity vectors have been predicted using the Eurasia-India Euler pole of the NUVEL-1 model (DeMets et al., 1990) at selected points on the plate boundary in the vicinity of the Shillong Plateau (Fig. 2). The components of these velocity vectors in the direction of the plateau are 5.1 cm/year in the N-S and 1.6 cm/year in the E-W directions. A first order picture of the forces acting on the plateau, assumed propor- tionate with the velocities in the northern and eastern directions, indicates a ratio of 3.2:1. This seems to be quite comparable with the ratio of 4: 1 obtained from the computation of strain rates, and therefore supports the idea of stress acting on the plateau in both N-S and E-W directions. However, the comparison has the following limitation that the strain rates might include some internal deformation while the NUVEL-1 velocities, in general, apply to rigid plate motions.

3.3 Focal mechanisms on the Shillong Plateau Data describing the stress scenario on the plateau is very sparse and only a few focal mechanisms are available (Fig. 3). Some of these mechanisms indicate P-axis directions closer to E-W (solutions 5, 6, and 10), although the overall pattern does not appear very consistent. One interesting observation is that the focal mechanisms are predominantly of the thrust type and occur all over the plateau. Their P-axis directions, although not very consistent, are found to be mostly normal to the boundaries of the plateau on all sides (solutions 1, 5, 6, 7, and 10). This indicates compressive forces acting on the plateau from all directions, aiding its uplift. One can therefore envisage a combination of N-S and E-W directed compressive forces causing the uplift by a "pop-up mechanism" facilitated by the pre-existing weak fault systems around the plateau (Fig. 2), such as the Dauki in the south, Brahmaputra in the north, Jamuna in the west, and Disang in the east. The resistance offered from both north and east to the crustal mass in this region, as the Indian plate drifts in the NNE direction, is seen as the cause of the uplift as the plateau is squeezed from all sides. The initiation of the uplift during the Mio-Pliocene period and the unique location of the plateau in the vicinity of two major thrust zones are the coinciding factors which support this model. Further acquisition of accurate geodetic and stress data, and detailed modeling studies can give a more quantitative picture of the uplift and tectonics of the Shillong Plateau.

4. Conclusions 1) The upper mantle beneath the Shillong Plateau appears to have ceased to be hot as evidenced by various geophysical results, suggesting that the current uplift is not due to a thermal cause. 2) The uplift may not be due to isostatic adjustment, as indicated by gravity results. 3) The stratigraphic sequence on the plateau constrains the initiation of the current uplift to the Mio-Pliocene period. Well developed Indo-Burman thrusting at that time and the conductive tectonic setup suggest the possible role of E-W directed Indo-Burman thrust forces in initiating the plateau uplift. 4) Computation of strain rates and velocity vectors indicate that, even at present,

Vol. 45, No. 3, 1997 17 2 N. P. Rao and M. R. Kumar

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Table 2. Harvard CMT catalog of 50 earthquakes in the Burmese arc region cast of the Shillong Plateau, along with the Moment Tensor elements (listed elements•~ 10Exp). Uplift and Tectonics of the Shillong Plateau 173

Vol. 45, No. 3, 1997

r,s, and e correspond to radial, south, and east directions, respectively. 174 N. P. Rao and M. R. Kumar

Fig. 2. A schematic diagram depicting the possible mechanism for the current uplift of the Shillong Plateau (modified after Johnson and Alam, 1991). The velocity vectors computed using plate motion model NUVEL-1 are indicated as dotted lines while their components are shown in bold.

Fig. 3. Fault plane solutions over the Shillong Plateau. 1-4, Mukhopadhyay (1990); 5 and 6, Kayal (1987); 7 and 8, Chen and Molnar (1990); 9, Chandra (1978); 10, Khattri et al. (1988).

J. Phys. Earth Uplift and Tectonics of the Shillong Plateau 175 a small component of E-W directed stress acts on the plateau. The abundance of thrust-type focal mechanisms and their P-axis orientations indicate uplift due to compressive forces from all sides of the plateau.

We are grateful to Dr. Harsh K. Gupta for his constant guidance, encouragement and a critical review of the manuscript. We thank Prof. D. Guptasarma for very useful discussions. The critical comments by Dr. Charles DeMets and an anonymous reviewer helped in improving the manuscript considerably.

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