Strain accumulation studies between Antarctica and India through geodetic tying of two continents from continuous GPS measurements

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1 Indian Journal of Marine Sciences Vol. 37(4), December 2008, pp Strain accumulation studies between Antarctica and India through geodetic tying of two continents from continuous GPS measurements N. Ravi Kumar, E.C. Malaimani* & A. Akilan National Geophysical Research Institute, Council of Scientific and Industrial Research (CSIR), Hyderabad India *[ ecm@ngri.res.in] The geodynamical and crustal deformation processes in the south of Indian peninsula between India and Antarctica had been examined. Two global networks (IND and ANT) that geodetically connect the two continents, the IGS Station at Diego Garcia (DGAR), as a common station between the two networks, had been chosen for the study. The data from 1997 to 2005 were used for above purpose. The present study elucidates the increase in baseline lengths between Kerguelen in Antarctic plate and other stations. Whereas the baseline lengths shorten between HYDE in Indian plate and other common stations. The GPS velocity fields of Maitri and other IGS stations in and around Antarctica in different plates relative to Kerguelen show the departure from rigid plate behaviour. This is in a manner consistent with the mapped intra plate stress field, deformations and seismicity in the region. The analysis shows lengthening of baselines for sites Yaragadee (YAR1) and Tidbinbilla (TID2) in Australian plate; and Seychelles (SEY1) and COCO in Indian plate. The above lengthening is at the rates of 5.3 cm/yr, 3.8 cm/yr, 5.6mm/yr and 5.5 cm/yr respectively. The station Hartebeesthoek (HRAO) in South African plate shows a trend of minimal shortening at the rate of 1.7 mm/yr. The velocities with uncertainties (±1σ) are in good agreement with geodetic and geologic models. The present results are with precision of approximately 3-4 mm (North), 5-6 mm (East), and mm (vertical). The accumulation of elastic strain, provide new information on the direction and rate of Indian plate motion, and intra plate seismicity of the Indian Ocean. The estimated elastic strain accumulation shows an increasing trend of yr -1. [Key words: Geodynamics; Crustal deformation; Kerguelen; strain accumulation] Introduction The India Eurasia collision zone has been the focus of intense studies by the Space Geodetic community for Indian Plate Motion. The velocity data for 2 sites (HYDE and IISC) fit the rigid plate model within uncertainties, estimated at about 3.7 cm/yr for Hyderabad and 3.9 cm/yr for Bangalore respectively. The Space geodesy can test many otherwise untestable predictions and boundary conditions. Hence GPS-Geodesy is extended to envelop the large Indian Ocean lithosphere and observations are extended to the Indian Ocean and Antarctica. The geographic distribution of the rigid Indian plate comprises the 2000 km long chain of islands atop the Chagos-Lakshadweep ridge. These include Lakshadweep islands, part of India, Maldives islands, and the islands that are located on the Indian plate proper (the southern most part of the chain includes Diego Garcia, which may lie on the Capricorn Plate). This unique geographic feature lies on the oceanic lithosphere, in the stable interior of the Indian Plate. In effect it constitutes a 1200 km long strain gauge, optimally oriented in the N-S direction. All these lead to the new hypothesis that it is possible that at least part of the problem in describing the plate kinematics of the Indian Ocean Basin relates to the unique set of forces on the boundaries. These boundary forces may have lead to frequent plate boundary reorganizations in the past, and generation of either small plates such as the Capricorn Plate or a diffuse boundary zone between India and Australia 1 complicating kinematic interpretations. These boundary forces may also contribute to non-rigid behaviour of the Indian Plate. In any event, the deformation of equatorial Indian Ocean lithosphere is not ephemeral, but long-lived 1. Despite the extensive studies on plate tectonics using geophysical investigations in the larger oceanic part of the Indian plate, major issues remain unresolved. Motions across some of the plate boundaries seem well constrained, as implied by good agreement between space geodetic and geologic models 2. In others, apparent discrepancies exist. Very few studies have been conducted on the larger oceanic part of the Indian plate using space geodesy. To holistically determine the kinematics of the Indian

2 RAVI KUMAR et al.: STRAIN ACCUMULATION STUDIES FROM GPS MEASUREMENTS 405 Ocean Basin between Antarctica and India, the data available are very sparse and characterization and the delineation of the plate boundaries especially in the Indian Ocean are poor. The present study consists the following: (1) How rigid is the Indian Plate and Indian Ocean Basin? (2) Does relatively high level of intraplate seismicity on the oceanic part of the Indian Plate indicates internal deformation in excess of other plates? and (3) Is this related to Indo-Eurasian collision and the uplift of Himalayas? GPS Data Acquisition NGRI s had established a permanent GPS Station at Maitri, Antarctica. It is continuously operational since 1997 and simultaneously has become a permanent geodetic marker. It started contributing to the Scientific Committee on Antarctic Research (SCAR) Epoch GPS Campaigns since Table 1 Coordinates of the sites in the first Network used in this study SITES LATITUDE LONGITUDE MAIT CAS DAV YAR TID COCO SEY DGAR HRAO Table 2 Coordinates of the sites in the second Network used in this study Site Plate Latitude Longitude COCO Indo-Australia DGAR Edge of Capricorn HRAO South Africa HYDE India IISC India IRKT Eurasia KIT3 Eurasia LHAS Eurasia MALD India POL2 Eurasia SEY1 Somalia WTZR Eurasia YAR1 Australia The global networks namely IND and ANT have been chosen to geodetically connect the two continents. The IGS Station at Diego Garcia (DGAR) is common to both the networks. Eight years of data from 1997 to 2005 were used. The stations included in the first network (ANT), is shown in Table 1 and second network (IND) in Table 2. The stations HYDE, SEY1, DGAR and COCO are shown in the Figure 13. Since the baseline length between HYDE, India and MAIT, Antarctica is more than 10,000 km, it is mandatory to form these two different networks to improve the accuracy of the baseline measurements by GPS. This approach is to circumvent the limitation in the estimation of baseline length by GPS, maximum of which is 6,900 km. This is due to the availability of less number of double difference observables for the GPS data analysis. GPS data processing and analysis The strategies used for the data processing is given in Table 3. Time series of MAIT site coordinates is shown in Fig. 1.The scatter observed in the MAIT time series is due to the change of receiver from Turbo- Rogue to Ash Tech and the WRMS values are within the acceptable limits. Most of the sites chosen except SEY1 in this study have continuous data almost 20 to 24 hours a day. Fig. 2 shows the baseline length evolution of MAIT. Figs 3 to 10 show all the other baseline lengths evolution. Table 5 depicts the minimal base line shortening of all the stations MAIT, DAV1, and CAS1 in Antarctica from KERG, which reveals minimum deformation in this part of Antarctica. The lengths of the baselines of present network are between 2000 and more than 6500 km, which makes the ambiguityfixing impossible. All the solutions presented in this study leave the ambiguity non-fixed. Table 3 Strategies adopted for GPS Data processing Parameters Description GPS Processing Software Bernese Version 4.2 Sessions 24 hours Earth orientation parameters IERS Bulletin B Elevation angle cut-off 15 Ambiguity Resolution Free-Not Resolved Tropospheric dry delay Model Saastamoinen Tropospheric Wet Delay Estimated every 4 hrs. Apriori station positions ITRF-2000 Station Position constrains Free Network Approach

3 406 INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008 Many stations between southern Indian peninsula and Maitri had been included to eliminate the errors due to non-availability of double difference observables. They have longer time series of site coordinates in the IGS network so that the short baselines between the stations estimated could be tied geodetically. Fig. 3 Fig. 4 Fig. 1 Time series of MAIT site coordinates. Site velocity is given by slope of weighted least squares line fit (solid line through the data (solid circles). Site velocity error (1 standard error) are based on weighted least squares straight line fit, accounting for white noise approximation, total time span of observation and total number of observation. Fig. 5 Fig. 2 to 10 Lengths of baselines estimated for stations Maitri, Davis, Casey, Yaragadee, Tidbin billa, COCO, Seychelles, and Hartebeesthoek relative to reference station KERG to the accuracy of sub centimeters. The baseline lengths in meters are shown against each baseline. Fig. 6

4 RAVI KUMAR et al.: STRAIN ACCUMULATION STUDIES FROM GPS MEASUREMENTS 407 Fig. 7 Fig. 9 Fig. 8 Fig. 10 Table 4 Estimated result of Geodetic tie up between Indian and Antarctica plate Station Baseline length in m Baseline change from Kerguelen Baseline length in m Baseline change from Hyderabad Seychelles 5,005, m/yr 3,476, m/yr Diego Garcia 6,797, m/yr 2,790, m/yr Coco 4,677, m/yr 3,783, m/yr Table 6 shows our estimated site velocities in North and East directions and their error ellipses. The velocity vector of each site has been estimated with the weighted least squares fit to the weekly solutions and with the 95% confidence error ellipse. Very long baselines have been estimated using ITRF-2000 from HYDE to other chosen IGS stations in and around India including DGAR. The baseline lengths from Kerguelen to all the other stations having DGAR as a common station for both these networks and the velocity vectors of each site were also estimated. The analysis and results show increase of baseline lengths between Kerguelen in Antarctic plate and other stations and shortening of baseline lengths between HYDE in Indian plate and other common stations which is depicted in Table 4. The stations HYDE and MAIT are geodetically tied through DGAR by the global network analysis. The geodetic tie up (shown in Fig. 11) had resulted in the geodetic signatures of the geodynamical processes between India and Antarctica. The continuous monitoring has enhanced the understanding of the crustal deformation processes between these two continents despite many plates, micro plates and ridges in this study region. The present analyses also indicate and confirm geodynamically the emergence of diffuse plate boundary between India and Australia and relates to the late Miocene Himalayan uplift. The newly emerging platelet Capricorn in the central-western Indian Ocean with a broad, diffuse boundary may be due to the result of frequent plate boundary reorganizations in the past and the geodynamical processes in the present. These boundary forces complicate the kinematic interpretations and may also contribute to the non-rigid behavior of the Indian plate and Indian Ocean Basin (Fig. 13). The inter

5 408 INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008 Table 5 Estimated baseline lengths between Kerguelen and other stations, rate of changes and their error (1 standard error) estimates Sl. No. Stations Baseline length from Kerguelen (m) WRMS (m) Baseline length change/year m/y 1. Davis 2,172, ± Casey 2,933, ± Maitri 3,742, ± Yaragadee 4,323, ± Hartebeesthock 4,391, ± Diego Garcia 4,564, ± Coco 4,677, ± Seychelles 5,005, ± Tidbinbilla 6,103, ± SITE Table 6 Estimated velocities of all the sites and their error (1 standard error) estimates N-VEL (mm/y) E-VEL (mm/y) N-ERROR (mm/y) E-ERROR (mm/y) MAIT CAST DAV YAR TID DGAR COCO SEY HRAO continental networks in our study and the data analysis suggest that the high rate of movement of DGAR at the edge of Capricorn plate and COCO could be the result of excessive strain accumulation due to the Indo-Australian diffuse plate boundary forces acting upon this region. The present study also conform to the genesis that the deformation of equatorial Indian Ocean lithosphere is not ephemeral, but long-lived. Results and Discussion Inspection of GPS velocity data in Tables 5 and 6, suggest several important points that bear on data quality and error estimation: Relative position estimates plotted as a function of time (Fig. 1) define a nearly linear set of points spanning almost 8 years. This suggests that a constant velocity model is appropriate and the present analysis procedures for treating the disparate data sets are Fig. 11 Map showing the geodetic tie-up of India and Antarctica through DGAR mainly and all the other GPS stations in between India and Antarctica forming two different networks with HYDE and KERG being the reference sites for two plates of India and Antarctica. The estimated baseline lengths from HYDE and KERG to all the other stations are also indicated. adequate. The estimated base line length and its rate of change from Kerguelen to Maitri are as shown in Table 5. The overall analysis is that the estimated

6 RAVI KUMAR et al.: STRAIN ACCUMULATION STUDIES FROM GPS MEASUREMENTS 409 baseline lengths between Kerguelen and Maitri and other IGS stations namely Casey, Davis in Antarctica Plate are shortening at the rates of 5.9 mm/yr, 4.4 mm/yr and 1.9 mm/yr respectively The rate of change in the order of mm in the base line lengths indicate no significant change at the one standard deviation level. It may be presumed that the stations in the same plate are moving together. When applying the weighted linear fit to the baseline length results of the individual epoch, several common features emerged throughout the estimation. Table 5 clearly shows the increase in weighted root mean square (WRMS) values with the increase in the baseline length, confirming the baseline dependent errors according to error relation 3 σ 2 = A 2 + B 2 * L 2 with A = 0.5 cm and B = The velocity vectors of the stations in the Australian plate indicate that they are moving away from the Indian plate conforming to the recent plate tectonic theory. The baseline lengths between Kerguelen and the stations Yaragadee (YAR2) and Tidbinbilla (TID2) in Australian plate are increasing at the rate of 5.3 cm/yr and 3.8 cm/yr, respectively (Figures 6 and 10). These estimated rates of changes in the baseline lengths of YAR2 and TID2, which are original contribution in this study, are in support of the theory that India and Australia lie in two different plates. The velocity vectors of these two stations, which corroborate this theory, are discussed later in the paper. Table 5. It reveals that the distance between Kerguelen (KERG) and the islands in the Indian Ocean Seychelles and COCO are lengthening at the rates of 4.9mm/yr and 5.5 cm/yr, respectively. The station Hartebeesthoek (HRAO) in South African plate shows a trend of minimal shortening at the rate of 1.7 mm/yr. The present estimated velocity vectors are in good agreement with the International GPS Services for Geodynamics (IGS) Global Velocity Map. The velocities and corresponding errors are defined by a weighted least squares line fit through the daily position data for a given period of 8 years from 1997 to 2005 (all uncertainties are quoted at 1 standard error, ±1σ). The WRMS scatter about the line gives an estimate of the average precision of a daily measurement, again assuming white noise. Precisions are approximately 3-4 mm (North), 5-6 mm (East), and mm (vertical), typical of performance at many other IGS sites in more benign environments. The velocity errors depend on the daily precisions, the number of measurements, the total time span of observations, and the nature of the other noise sources. The velocity vectors of the stations Yaragadee (YAR1) and Tidbinbilla (TID2) in the Australian plate clearly indicate that these stations are moving away from the Indian plate conforming to the recent Plate tectonic theory that India and Australia lie in two different plates with a diffused boundary separating them. The unique networks utilised in the present study and the GPS derived velocity vectors for these stations also corroborate the theory proposed by Gordon et al. 4 that the Australian plate is pivoting counter clock wise around 600 km south of the tip of India, pushing into the Indian plate to the east as the velocity vector of Tidbinbilla (TID2) shows and pulling away from it to the west as the velocity vector of Yaragadee (YAR2) shows. The present study provides new information on the overall direction and rate of Indian plate motion and on some tectonic processes in the intraplate seismicity region of the Indian Ocean, including the elastic strain accumulation. The effects of elastic strain on measured site velocities had been assessed as a part of this study. Elastic Strain Accumulation The strain accumulation had been computed using GPS-Geodesy in the global network using the following formula: 1 Change in baseline length (DL) Strain accumulation ( y ) = Baseline length L ( ) The strain accumulation in the intraplate region is around to yr -1. The present results of intraplate strain accumulation within Antarctica Plate covering three sites MAIT, CAS1 and DAV1 are yr -1, yr -1 and yr -1, respectively. The estimates of interplate strain accumulation between Antarctica and other plates such as Somalia (SEY1), Africa (HARO), Australia (YAR1), and diffuse plate boundary between India and Australia (COCO) are found to be yr -1, yr -1, yr -1 and yr -1 respectively (Table No 7). These estimates are in good agreement with the Global Strain Rate Map 5. The rate of change of COCO makes an interesting observation. The high rate of movement of COCO relative to all the other sites agrees with the global strain rate in the Indian Ocean near COCO, proposed by Gordon et al. 4 and Kreemer et al. 5. The SEY1 site

7 410 INDIAN J. MAR. SCI., VOL. 37, NO. 4, DECEMBER 2008 Table 7 Estimated strain accumulation of all the chosen sites Site Strain Accumulation MAIT /yr -1 CAS /yr -1 DAV /yr -1 SEY /yr -1 HARO /yr -1 YAR /yr -1 COCO /yr -1 Fig. 12 Boundaries in study area show relative knowledge of plate motion from geologic models. Our study, using GPS data beyond those used in REVEL together with longer time series at REVEL sites, will improve Euler vector estimates for all plates listed except Capricorn, which does not have islands. The plates are denoted as Ar=Arabia, Nu=Nubia, Eu=Eurasia,In=India,So=Somali,Cp=Capricorn, DBZ=Diffuse Boundary Zone, Au=Australia, An=Antarctica. in Somalian plate was very noisy and the continuous data was also not available for the time span of our study (Fig. 8). The increase in baseline length between KERG and SEY1 at the rate of 5.6 mm/yr clearly indicates that SEY1 is moving away from KERG. The velocity vector of SEY1 shows the movement towards the Indian peninsula and this also agrees with the plate model 6. If this rate of movement is verified upon, this may result in the increase in the strain accumulation in the southern Indian Peninsula. Conclusion The GPS data from 3 sites MAIT, CAS1 and DAV1 within Antarctica plate result in the accumulated strain of the order of 10-9 yr -1. Similarly an order of 10-8 to yr -1 for accumulated strain are Fig. 13 Study area showing the deformation in Oceanic Part of Indian Plate illustrated by seismicity (Red dots) and focal mechanism data. Anticlines, synclines show the N-S compression on land. Squares denote sites that form a Strain Gauge. Triangles are existing CGPS sites; solids were used in REVEL Model and open have become available since White lines are plate boundary, dashed were uncertain. Cp-Capricorn, DBZ- Diffuse Boundary Zone, In-India, So-Somalia the results between Antarctica and other plates SEY1, HARO, YAR1 and diffuse plate boundary. These estimates are in good agreement with global strain rate map 5. GPS data at COCO suggests the high rate of movement that could be the result of excessive strain accumulation due to the Indo-Australian diffuse plate boundary forms acting upon this region. The GPS velocity fields of Maitri and other IGS stations on different plates show the departure from rigid plate behaviour consistent with the observations of deformations and seismicity in the region. The GPS analysis confirms the emergence of diffuse plate boundary between India and Australia and relates to the late Miocene Himalayan uplift. The calculated stress field in the West of the Indian Peninsula has a roughly N-S directed tensional and E-W oriented compressional character (Figures 12 and 13) The velocity vectors of all other sites throw a significant insight into the plausible causes of the strain

8 RAVI KUMAR et al.: STRAIN ACCUMULATION STUDIES FROM GPS MEASUREMENTS 411 accumulation processes in the Indian Ocean and the northward movement of Indian plate. Acknowledgements The authors are grateful to Dr H. K. Gupta, former Secretary, Department of Ocean Development, Govt.of India for launching this scientific program and constant support. Authors are also thankful to Dr V. P. Dimri, Director, NGRI, Hyderabad for permitting to publish this paper. The data contributed by all the IGS stations are gratefully acknowledged. References 1 Gordon R G, The plate tectonic approximation: plate non rigidity, diffuse boundariesand global plate reconstructions, Ann.Rev.Earth Planet.Sci., 26 (1998) Stein S & Sella G, Fig. 3 and Fig. 7 of NSF proposal titled Investigation of Indian Ocean intraplate deformation and plate boundary evolution using GPS, Lichten S M, High accuracy global positioning system orbit determination: progress and prospects, in: Global Positioning System: An overview, IAG symposium No. 102, edited by Y Bock & N Leppard, (Springer, New York) 1990, pp Gordon R G, DeMetsand C, Royer J Y, Evidence of for long-term diffuse deformation of the lithosphere of the equatorial Indian Ocean, Nature, 395 (1998) Kreemer C, Holtand W E, Haines A J, An integrated global model of present day plate motions and plate boundary deformation, Geophysical Journal International, 154 (2003) Gordon R G & DeMets C, Present-day motion along the Owen fracture zone and Dalrymple trough in the Arabian Sea, J. Geophys. Res., 94 (1989)