Gravity anomaly, lithospheric structure and seismicity of Western Himalayan Syntaxis

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1 J Seismol (2009) 13: DOI /s ORIGINAL ARTICLE Gravity anomaly, lithospheric structure and seismicity of Western Himalayan Syntaxis V. M. Tiwari & R. P. Rajasekhar & D. C. Mishra Received: 29 June 2007 / Accepted: 31 March 2008 / Published online: 16 May 2008 # Springer Science + Business Media B.V Abstract A compiled gravity anomaly map of the Western Himalayan Syntaxis is analysed to understand the tectonics of the region around the epicentre of Kashmir earthquake of October 8, 2005 (Mw=7.6). Isostatic gravity anomalies and effective elastic thickness (EET) of lithosphere are assessed from coherence analysis between Bouguer anomaly and topography. The isostatic residual gravity high and gravity low correspond to the two main seismic zones in this region, viz. Indus Kohistan Seismic Zone (IKSZ) and Hindu Kush Seismic Zones (HKSZ), respectively, suggesting a connection between siesmicity and gravity anomalies. The gravity high originates from the high-density thrusted rocks along the syntaxial bend of the Main Boundary Thrust and coincides with the region of the crustal thrust earthquakes, including the Kashmir earthquake of The gravity low of HKSZ coincides with the region of intermediate deep-focus earthquakes, where crustal rocks are underthrusting with a higher speed to create low density cold mantle. Comparable EET ( 55 km) to the focal depth of crustal earthquakes suggests that whole crust is seismogenic and brittle. An integrated lithospheric model along a profile provides the crustal structure of the boundary zones with crustal thickness of about 60 km under the Karakoram Pamir regions V. M. Tiwari : R. P. Rajasekhar : D. C. Mishra (*) National Geophysical Research Institute, Hyderabad , India dcm_ngri@yahoo.co.in and suggests continental subduction from either sides (Indian and Eurasian) leading to a complex compressional environment for large earthquakes. Keywords Syntaxis. Effective elastic thickness. Gravity anomalies. Indus Kohistan and Hindu Kush Seismic Zones 1 Introduction The Himalayan collision zone is one of the most active plate boundaries, which is characterised by several large and great earthquakes. Convergence across the Himalayas is mainly accommodated along major thrust faults in the southern section of the Himalayan collision zone and causes thrust earthquakes while the northern and southern parts of Tibet show evidence of normal earthquakes (Ni and Barazangi 1984). The Western and Eastern parts of the Himalayas, i.e., the Western and Eastern Syntaxial Bends record more earthquakes and are structurally more complicated. Geological Complexities in the Western Syntaxial Bend arise due to presence of Kohistan arc between the Indian and the Eurasian plates causing two sutures among them viz. Main Mantle Thrust (MMT) between the Indian and the Kohistan arc, synonymous with the well known Indus Tsangpo suture zone and Northern suture between the Kohistan arc and the Eurasian plate (Fig. 1). Another important characteristic of this part DO09102; No of Pages

2 364 J Seismol (2009) 13: Fig. 1 Tectonics of the Western Himalayan Syntaxis with major thrusts and tectonic units marked on the topographic map. L S R Lahore Sargodha Ridge. SRT Salt Range Thrust also called Main Frontal Thrust; MBT Main Boundary Thrust, MMT Main Mantle Thrust or Indus suture, LA Ladakh Baltholith, KO Kohistan Arc; NS Northern Suture; KF Karakoram Fault; Karako Karakoram Range and TB Tarim Basin. Fault Plane solutions of thrust earthquakes of magnitude>6 since 1973 are plotted in red and deep-focus earthquake in yellow (USGS). K Kashmir Earthquake of IKSZ Indus Kohistan Seismic Zone and HKSZ Hindu Kush Seismic Zone is several mafic/ultramafic units along suture zones; also in the Kohistan arc and the occurrence of Nanga Parbat in the inverted U turn of the MMT, which shows maximum rate of uplift (Treloar et al. 2000). The major exposed thrusts are the Main Boundary Thrust (MBT), which is considered to be coupled to a large decollment plane known as Main Himalayan Thrust (MHT; Zhao et al. 1993) in a depth range of km, and the southern most thrust, the Main Frontal Thrust (MFT) or Salt Range Thrust (SRT), which coincides with the Salt Range in the western part of this section. Gravity data in the Western Himalayan Syntaxis (WHS) are meagre due to the rugged terrain and political boundaries; nevertheless, several gravity studies (Duroy et al. 1989; Caporali 1995, 2000a, b) in the different parts of the WHS have provided information on crustal structure, flexural rigidity, etc. Large wavelength gravity anomalies are interpreted due to crustal thickening and short wavelength gravity anomalies are interpreted due to folding/bulging or mid-crustal density heterogeneities (Duroy et al. 1989; Caporali 2000b). Mishra and Rajasekhar (2006) based on a gravity profile across this section suggested a crustal thickening from 40 km south of Lahore Sargodha ridge to about 70 km under the Karakoram range and reducing to km under the Tarim basin and attributed the Lahore Sargodha ridge to crustal bulge due to lithospheric flexure by the Himalayan load. It has also been suggested that crustal bulge introduces an extension in the upper crust and compression in the lower crust resulting in thrust faulting in the lower crust, where the hypocenter of most of the earthquakes in this region, including the Kashmir earthquake of 2005, are located. The Kashmir earthquake of 2005 was one of the most devastating earthquakes that occurred at a depth of 26 km with a dominant thrust type of focal mechanism (USGS). This crustal bulge, reflected in the short wave length gravity high, is a prominent feature of fore land basins. It is closest to the Himalayan front along the Western (West of Chandigarh) and Eastern Himalayan fronts, while in the central part (Central Nepal), it is located away from the front almost along the central axis of the Ganga basin (Mishra et al. 2004), suggesting comparatively low flexural rigidity of the western and eastern foreland. It is interesting to note that the western and eastern regions of the Himalayas are more seismogenic, recording several large and great earthquakes compared to the Central Himalayas. The Western Syntaxial Bend (Fig. 1) indicates two seismic zones, which are characterised by large and great earthquakes, viz., Indus Kohistan Seismic Zone (IKSZ) and Hindu Kush Seismic Zone (HKSZ). The HKSZ is commonly known for intermediate deepfocus earthquakes (Pegler and Das 1998). IKSZ follows the syntaxial bend of the Main Boundary Thrust (MBT) and is dominated by crustal thrust earthquakes. The hypocenters of earthquakes observed in the IKSZ are normally concentrated in the depth range of 5 15 km to km (Seeber et al. 1981), similar to the entire Himalayan front (Mishra and Rajasekhar 2006). The Kashmir earthquake of 2005 occurred in the IKSZ region, where MBT takes an inverted U turn (Fig. 1) with underthrusting on three sides. The rupture of this earthquake is observed

3 J Seismol (2009) 13: along western flank of the MBT forming the Syntaxis (Avouac et al. 2006). Figure 1 also shows a thrust type of seismic activity of magnitude greater than 6.0 since 1973, which are primarily concentrated along Salt Range Thrust (SRT) and Main Boundary Thrust (MBT; IKSZ) in South and Pamir thrust in the north along the Asian plate. Fore-deep in this section is marked by a basement high known as Lahore Sargodha ridge that extends southwards up to Delhi (Fig. 1). Deep-focus earthquakes occurring in mantle are confined to the Hindu Kush Range. In the present study, we combined tectonic interpretation of gravity anomalies observed in this region and tried to link the siesmicity of the region with interpreted tectonics. 2 Gravity anomalies, analysis and modelling A Bouguer anomaly map of this region (Fig. 2) is compiled from Caporali (2000a), UNESCO (1976) and GSI-NGRI Figure 2 shows relative gravity Fig. 2 Bouguer anomaly map showing major thrusts and epicentres of thrust earthquakes (black dots) and deep-focus earthquakes (red dots). H1 and H2 are gravity highs over thrust belts of Indian and Asian plates, respectively. L1 and L2 are gravity lows over Karakoram Hindu Kush ranges. NP Nanga Parbat highs, H1 and H2 towards SW and NE corners respectively and a gravity low, L1 in the central part with sharp gradients on either side related to various thrusts. These relative highs and lows primarily reflect thin and thick crusts in these regions as is the case with present-day continental collision zones. However, gravity anomalies also reflect low- and high-density crustal rocks, which, however, can be delineated only after the effect of the crustal thickening and thinning is separated out as the regional field from the observed field that presently dominates the observed field. Therefore, the gravity anomalies are analysed using suitable wavelength filter based on coherence analyses of Bouguer anomaly and topography. 2.1 Effective elastic thickness Effective Elastic Thickness (EET) is an effective part of the lithosphere that respond elastically for any topographic or geologic load over a geological time period and considered as a proxy of intrinsic property (strength) of the lithosphere, discussed in several literature (Watts 2001 and reference therein). This implies that in this part of the lithosphere, strain is proportional to the stress and beyond certain limit of stress; the rocks fail to produce seismic activity. Estimates of continental EET have been a matter of discussion (McKenzie and Fairhead 1997). Some scientists have refuted the long prevailing view of the jelly sandwich model (i.e., weak lower crust sandwiched between strong upper crust and upper mantle) and believe that the strength of the lithosphere lies in the crust only, based on correlation between focal depth of the earthquakes and EET (Jackson 2002). However, from numerical modelling, it has been demonstrated that the rheological model of the lithosphere varies over the geological scenario and the sandwich model is the more general case; however, it may not be fitting in the case of the Himalayan region (Burov and Watts 2006). EET is estimated by different approaches (Watts 2001). The most popular approach to estimate EET is based on coherence between Bouguer anomaly and topography accounting both surface and subsurface loads (Forsyth 1985). We have utilised a similar approach of computing coherence between the Bouguer gravity anomaly and topography over a grid of 600 km 600 km encompassing the whole WHS using the MEM method (Lowry and Smith 1994). Estimate of EET from coherence analysis is based on the

4 366 J Seismol (2009) 13: assumption that long wavelength topography is isostatically compensated and, therefore, topography and the Bouguer gravity anomaly are coherent; whereas at short wavelengths, topography is uncompensated and, thus, Bouguer gravity anomaly is incoherent with topography (Forsyth 1985). The roll off, on which coherence increases, is marked as the wavelength of isostatic compensation. This transition wavelength defines the rigidness of the lithosphere as well and can be used to define isostatic gravity anomalies. Figure 3 shows the estimated EET=57 km along with observed and best fitted coherence curve. Our estimate is almost similar to Caporali (2000b) but less than the earlier values of Caporali (1995). Our estimate is an average figure for the entire region extending from the Indian plate to the Asian plate, which could explain the discrepancy from earlier results. The difference with earlier results may also arise due to the technique of estimations; nonetheless, it is unlikely that the values would be more than 100 km as proposed earlier (Caporali 1995). 2.2 Isostatic residual gravity anomalies We followed two approaches to derive isostatic gravity anomalies. In the first approach, gravity field is computed due to flexure of topography load assuming EET of 50 km. This calculated regional gravity field is subtracted from the observed field to Fig. 3 Effective Elastic Thickness (Te) based on coherence between the Bouguer anomaly and topography. The best fit is obtained for Te=57 km obtain the residual field, which can be attributed to the shallow sources. The other approach is wavelength filtering using the cut off wavelength suggested from coherence analysis, i.e km. The two regional fields derived from the above-mentioned approaches are comparable (Fig. 5) and, thus, any of them can be utilised to define isostatic residual. A high-pass filtered map corresponding to wavelength smaller than 450 km is presented in Fig. 4, which represents the isostatic residual anomaly and can be attributed to the shallow sources in the crust. This figure also shows the various thrusts described in Fig. 1 and the thrust earthquakes of magnitude greater than 6, which have occurred in this region since They are plotted in two groups with their focal depths (USGS 2007) as shallow, crustal (<70 km) and deepfocus (>70 km) earthquakes. Two clusters of epicentres can be easily noticed related to IKSZ and HKSZ. It is also interesting to note that the IKSZ, which are primarily confined to the crust, correspond to the gradient of gravity anomaly related to syntaxial bend and thrusted high-density rocks. While the HKSZ are deep-focus earthquakes corresponding to the gravity low of Hindu Kush ranges and may be related to low density underthrusted rocks in the upper mantle (Chatelain et al. 1980) of the Indian Plate where subduction might be faster compared to the other sections (Negredo et al. 2007). There are some seismic activities in Pamir section which are related to thrusts of the Asian plate. These are primarily crustal earthquakes concentrated in the depth range of less than 50 km. The residual gravity field shows a gravity high (H1) over the Lahore Sargodha ridge and low (L1) over the Tertiary sediments along the SRT and the MBT, which partly occupied the syntaxial bend of the MBT. This gravity high due to Lahore Sargodha ridge is reflected in the Bouguer and free air anomaly maps of India (GSI-NGRI 2006) extending to Delhi but is considerably away from the Himalayan front in this section. The gravity highs, H2 and H3 appears to be related to thrusted high-density mafic and ultramafic assemblages such as Dras volcanics and ophiolites along the MMT and the Kohistan arc including Chilas complex of mafic/ultramafic rocks. The extent and amplitude of these gravity highs suggest much wider extent of mafic/ultramafic assemblages compared to what is exposed. A major gravity low, L2 coincides with the Karakoram Range

5 J Seismol (2009) 13: Fig. 4 Residual gravity anomaly based on high-pass filters of wavelength <450 km. It shows gravity highs, H1 related to Lahore Sargodha ridge (LSR) and H2 and H3 related to thrust belts. Gravity lows L1, L2 and L3 are primarily related to Tertiary sediments and crustal thickening under Karkoram Hindu Kush ranges besides other sources. Gravity high H4 is associated with thrust belts of Asian plate (Pamir ranges) and L3 with the Hindu Kush range indicating low density rocks in these sections. The gravity high, H4 along Karakoram fault and Pamir thrust on the Asian plate indicate high-density thrusted mafic and ultramafic rocks. It is to be noted that the thrusts are mostly related to gravity highs due to their association with thrusted high-density crustal rocks. 2.3 Lithospheric model The Bouguer anomaly along Profile A B from Fig. 2 is shown in Fig. 5, which shows a major gravity low over Karakoram Pamir region almost coinciding with maximum elevation and relative highs on either side. Coincidence of lowest gravity field with highest elevation along this section suggests some form of isostatic compensation. It also shows the gravity field for Airy s model of compensation based on topography, which runs almost parallel to the observed field showing a major mis-match mainly in the section of maximum elevation indicating that the crustal thickness will be less than that corresponding to Airy s model of compensation. This can be used as a constraint while modeling this profile. Figure 5 also shows the residual field along this profile based on wavelength filter (Fig. 4) and, secondly, the gravity and the residual fields based on flexural model for effective elastic thickness of 50 km, which are similar. The gravity field along this profile are modelled using the constraints from isostasy as discussed above; following the approach of Tiwari et al. (2006) and seismic investigations in the section (Kumar et al. 2005). Shallow bodies are primarily constrained from the residual field and local tectonics (thrusts) and deeper bodies from the regional fields given in Fig. 5 based on the flexural model. Kumar et al. (2005) have provided the configuration of underthrusting and Asian plates which is used in the initial model to constrain the upper Mantle structures. While crustal structures are constrained from nearby seismic profile (Bhukta et al. 2006). The computed crustal and lithosplheric section with their densities in g/cm 3 are given in the figure, which provide underthrusting of lithospheric mantle of Indian and Asian plates in depth range of km and km, the latter thrusting below the former. Crust thickens to about 60 km under Karakoram Pamir ranges, which is almost the same as reported by Bhukta et al. (2006) based on seismic studies. Another interesting feature is the association of high-density rocks with almost all major thrusts of both Indian and Asian plates. This figure also shows hypocenters of earthquakes projected on this profile. Most of the deep-focus earthquakes of the Asian plate are associated with underthrusted lithospheric mantle similar to Wadati Benioff Zone. 3 Discussion and conclusion Analysis and modelling of compiled gravity anomaly map of WHS have suggested that long wavelength gravity anomalies are produced due to compensation of long wavelength topography and short wavelength topography and gravity anomalies are supported by the strength of lithosphere. There are two distinct zones of isostatic gravity anomalies and their sources are of different nature. The relative gravity high is coincident over local topographic high and marked by

6 368 J Seismol (2009) 13: Fig. 5 Bouguer anomaly and elevation along profile AB (Fig. 2). Residual anomaly from Fig. 4 along the same profile is plotted along with the regional and residual fields for Te= 50 km. Airy isostatic anomaly is also plotted. Hypocenters of thrust earthquakes (Figs. 1 and 2) projected on this profile coincide with thrusts and their probable extensions depth wise. Hypocenters of some deep-focus earthquakes coincide with underthrusted slab crustal thrust earthquakes. The Kashmir earthquake of 2005 was located in the vicinity of the gravity high, which corresponds to the IKSZ. The other gravity anomaly, a low over Hindu Kush Seismic Zone (HKSZ), is characterised by intermediate deep-focus earthquakes. The region, which are marked by isostatic anomalies would be undergoing isostatic adjustment, which takes place over tens to hundred thousands of years. Thus, the area of isostatic anomalies is the possible area of present-day tectonic forces. Furthermore, positive gravity anomalies are noticeable over local topographic highs (thrusts faults) and topographic contrast, which induces differential shear stresses, can exert a profound influence on deformational processes in the lithosphere (England and McKenzie 1982). Thus, it appears that the area of gravity highs related to thrusted high-density rocks are possibly active, producing thrust type of earthquakes as in the case of IKSZ. IKSZ is even more complex where rocks are underthrusting on three sides viz. north, east and west that may be causing more strain and are therefore more prone to failure. Further, rocks underthrusting along the Main Frontal thrust in this section are related to crustal bulge due to the Himalayan load as well as the Lahore Sargodha ridge, which are characterised by gravity highs and high-density rocks as described above. High-density rocks tend to underthrust faster due to slab pull forces and, therefore, produce more strain, making this region vulnerable to seismic activity. Focal depth of most of the earthquakes in IKSZ is less than 50 km, which approximately coincides with the effective elastic thickness in this section. This implies that whole crust is strong and brittle and capable of producing large earthquakes. Similar effective elastic thickness of 40 km (Tiwari et al. 2006) has also been reported for Sikkim Himalaya where again seismic activity coincided with this zone. The other seismically active section in western Himalaya is HKSZ, which is characterised by the earthquakes originating in the upper mantle (Chatelain et al. 1980; Pegler and Das 1998). We propose that underthrusted crustal slab in the upper mantle, producing gravity low, might cause earthquakes in the mantle by lowering the isotherm and strength. Seismic tomography also suggests the northward subducting Indian plate in the upper mantle and transition zone under the Hindu Kush (Koulakov and Sobalev 2006). Hypocenters of some of the deep-focus earthquakes of the Asian plate, when projected on the crustal model computed along a profile (Fig. 5), coincide with the

7 J Seismol (2009) 13: underthrusted lithospheric mantle confirming their association with underthrusted rocks. Furthermore, faster subduction under the Hindu Kush region may also result into a deeper brittle region in the upper mantle (Negredo et al. 2007), which is consistent with our interpretation. The computed lithospheric model constrained from isostatic studies and seismic investigations in this region, suggest underthrusting of the Indian and Asian lithospohere, in a depth range of km to km. Subduction from either side would also cause cold mantle and may facilitate earthquakes in the mantle. Several thrusts in the Indian and Asian plate imply intense crustal shortening in this region. It shows the association of high-density thrusted rocks with almost all thrusts of Indian and Asian plates. Hypocenters of thrust earthquake mainly coincide with these high-density rocks and their presumed extensions depth wise. 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