MOHORIVICIC DISCONTINUITY BENEATH MANNAR BASIN: A FAILED-RIFT

Transcription

1 MOHORIVICIC DISCONTINUITY BENEATH MANNAR BASIN: A FAILED-RIFT PASAN HERATH 1,*, JAGATH GUNATILAKE 1, DHAMSITH WEERASINGHE 2 1 Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka 2 Petroleum Resources Development Secretariat, Ceylinco House, Colombo, Sri Lanka * Corresponding Author pasansherath@gmail.com ABSTRACT (Received 30 th August 2016; accepted 24 th January 2017) A northeast - southwest trending gravity anomaly of about -20 mgal is observed from the marine gravity data derived from CryoSat-2 and Jason-1 satellites, in the Mannar Basin located offshore off the west coast of Sri Lanka where recent explorations for hydrocarbons have been conducted. Forward and inverse gravity modeling techniques aided with seismic interpretations have been used to determine the depth to the Mohorovičić discontinuity (Moho) beneath the Mannar Basin. A Wolfram Mathematica program was developed to calculate the gravity anomaly due to a 2D polygonal body having a density contrast with the surrounding. Seismic horizons in depth domain along a seismic line cross-cutting the gravity anomaly in the Basin were used as inputs to the program with the density of subsurface obtained from the log of Barracuda exploratory well to calculate the gravity anomalies caused by three subsurface layers, water column, sedimentary column and volcanic layer. The residual gravity anomaly (due to upper mantle) was obtained from the difference between the observed free-air gravity anomaly and the summation of the calculated gravity anomalies. The Moho was modeled with a trial-and-error sequence to match with the residual gravity anomaly, with an appropriate density contrast. The study revealed that the Moho is elevated beneath the Basin with a minimum depth of about 16 km below MSL and extending to a depth of about ~36 km below MSL on either ends of the Basin, providing evidence to classify the Basin as a failed-rift formed before or during Gondwana breakup. Key words: Mannar Basin, Failed-rift, Moho, Gravity Anomaly, Upper Mantle INTRODUCTION Geophysical techniques such as gravity, magnetic, electrical and seismic surveys are employed in the present scenario mainly in prospecting for natural resources buried underneath. Meanwhile, they also provide information on the existence of structurally and lithologically different subsurface bodies. Acoustic waves used in seismic surveys cannot penetrate deep into the Earth due to various obstructions in the subsurface. Exploratory drilling is also expensive and reaching large depths is practically impossible with the current technology. Therefore, to determine the Earth structure at great depths, supplementary techniques such as gravity modeling have to be used with the available seismic data. Talwani et al. (1959) have put forward an algorithm to compute the gravity anomaly caused by any polygonal body in the subsurface with a contrasting density with the surrounding. Forward modeling with this algorithm using the dimensions of a body obtained from a reflection seismic survey and density obtained from the well log will calculate the gravity anomaly caused by the object. Conventional trial and error approach can be followed to approximate the body causing a given gravity anomaly and its density using the same algorithm. The main objective of the study was to infer the depth to the crust - mantle boundary known as the Mohorovičić Discontinuity (Moho) using two - dimensional gravity modeling techniques constrained by available seismic data on subsurface horizons and the density logs of exploratory boreholes. The structure of the 77

2 Moho can provide vital evidence to interpret about the evolution of the Mannar Basin. GEOLOGICAL SETTING OF STUDY AREA Sri Lanka had been a part of the super continent East - Gondwana. It is supposed to have undergone at least two prominent rifting phases during the Gondwana breakup. The first rifting phase which is less prominent, initiated around 165 Ma ago has resulted in the formation of NE - SW and NW - SE discontinuities in the Mannar Basin. The more prominent second rifting phase has commenced around 142 Ma ago and had resulted in the formation of Cauvery and Lanka Basins. It was during this event that the separation of India, Madagascar and Sri Lanka from Antarctica had taken place. During this phase, the development of NE - SW, N - S and E - W discontinuities had caused more widening and mechanical subsidence of the Mannar Basin (Kularathna et al., 2015). Collision between India and Eurasia had occurred in the lower Eocene (~53 Ma), which resulted in uplift and subsequent erosion of the Himalayas. This resulted in a rapid deposition of the terrigenous sediments in the surrounding basins, Bay of Bengal, Cauvery and Mannar Basins. The Mannar Basin consists of sediments from Late Jurassic - Early Cretaceous to Recent sediments from which four distinct tectono - stratigraphic packages have been identified which are consistent with the tectonic setting of the area (Baillie et al., 2002). The lower Gondawana sediments can be considered pre - rift, in that they were deposited prior to the rifting phase which led to the breakup of eastern Gondwanaland. The upper Gondawana sediments, and perhaps some of the marine Cretaceous, Paleocene and even Eocene can be considered syn - rift. The present structure of the Mannar basin shows a northeast-southwest trending horst and graben system (Curray, 1984). On either sides of the Basin, are the continental landmasses of Sri Lanka and India (Figure 1). The southernmost region of the Indian peninsula, located in close proximity to the Mannar Basin is underlain by Precambrian pyroxene bearing granulites (charnockites), sillimanite granulites (khondalites), garnet-biotite gneisses and calc-silicates while Mesozoic and Cenozoic sediments cover up these rocks in the east along the coast (Meert et al., 2010). The northwestern region of Sri Lanka adjacent to the Mannar Basin consists of Precambrian Wanni Complex rocks, granitic gneisses, migmatitic gneisses, hornblende biotite gneisses and charnockites (Cooray, 1994) and are overlain by Miocene limestones along the coastline (Cooray, 1984). The Cauvery - Palk Strait - Gulf of Mannar Basin consists of two sectors; a passive continental margin sector facing the Bay of Bengal, and an aulacogen sector, or failed rift as the extension between India and Sri Lanka during the breakup of Gondwanaland failed to continue to the seafloor spreading stage with generation of oceanic crust (Curray, 1984). A prominent northeast-southwest trending free - air gravity anomaly can be observed in the satellite-derived gravity data in Mannar Basin located in between Sri Lanka and India (Figure 1). The study area was selected to cover the region where the gravity anomaly was observed in the Mannar Basin (Figure 1). A detailed seismic survey and a drilling program has been conducted in this region in view of prospecting for oil and gas under the mediation of the Petroleum Resources Development Secretariat (PRDS), Sri Lanka. DATA Gravity Data Satellite derived, free - air gravity data was obtained from Sandwell et al. (2014) which contained gravity measurements in 1 - minute grids. The data pertaining to the Mannar Basin was downloaded and interpolated using Kriging tool in ESRI ArcGIS software package with ordinary kriging method and linear semivariogram model. The interpolated gravity along the SL05-13 line was extracted for each point at 12.5 m intervals starting from the Sri Lankan end (Figure 1). Table 1 provides the gravity anomaly at 6.25 km intervals along the SL05-13 line starting from the Sri Lankan end. Depth Data of Subsurface Horizons Seismic section in the time domain available for the SL05-13 seismic line was demarcated for the different horizons with the aid of the log of the Barracuda exploratory well using SMT Kingdom software (Figure 3). These horizons in 78

3 Fig. 1. Free - air gravity anomaly observed in Mannar Basin. (Data obtained from Sandwell et al. (2014) Table 1. Free-air gravity anomaly along the SL05-13 seismic line at 6.25 km intervals Distance (km) Latitude (E) Longitude (N) Free-air Gravity Anomaly (mgal)

4 the time domain were then transferred to the depth domain using the subsurface velocity model of the region (Fugro, 2005). Horizons of water bottom, Oligocene, Eocene, volcanic top, volcanic bottom and the basement were thus interpreted. These depth data were available only within the maritime jurisdiction of Sri Lanka. In order to infer the continuation of the volcanic layer (volcanic top and bottom) in the Mannar Basin towards the Indian jurisdiction, raster seismic sections dating back to 1970s in that region were thoroughly studied. These seismic sections were not available for the same trend direction as the extended SL05-13 seismic line, but oblique to it. Since the continuation of the same volcanic layer in those seismic sections was observed, it was extended along the SL05-13 seismic line towards India upto a point where the highest flow of volcanics could have occurred with the observations made in seismic sections. Online map archives providing the satellite derived sediment thickness of the world were utilized to determine the depth to the basement towards India (Whittaker et al., 2013). However, they showed a sediment thickness slightly above 3,000 m for the Mannar Basin which was very reliably rejected as it was evident with the seismic sections, the sediment thickness was in excess of 9,000 m. The raster seismic sections in the Indian side were not clearly indicative of the basement since the record length of the seismic shot was insufficient to gather reflections from the basement. Therefore, the basement towards India was approximated considering isostasy and the regional gravity response. For proceedings of the study, the depth to the water bottom along the SL05-13 seismic line including the extended portion was required. For that, satellite derived topography data (Smith & Sandwell, 1997) was downloaded. The data was interpolated using the Kriging tool in ESRI ArcGIS software package with ordinary kriging method and linear semivariogram model. Oceanic depth data from admiralty charts of the Palk Strait region was also obtained since the satellite derived topographical data was found to be correct only for the deeper parts of the ocean. Since the shot point interval in the seismic survey was 12.5 m, the z - coordinate of all the subsurface horizons (water bottom, volcanic top, volcanic bottom, basement) at 12.5 m intervals along the x - direction, starting from Sri Lanka were extracted and tabulated. For all of these subsurface horizons, the mean sea level was used as the datum. The tabulated data for each subsurface horizon were arranged in an order such that two adjacent horizons including the free water surface were coupled together to form different subsurface layers defined by polygons. Starting from the Sri Lankan end, the x and z coordinates were ordered such that they defined the polygon in a counterclockwise sense. The subsurface layers thus obtained were, water column, sedimentary column and volcanic layer. Densities of Subsurface Layers The density of oceanic water was taken as 1.02 gcm -3 (Affholder & Valiron, 2001, Dziewonski & Anderson, 1981). The density of the sedimentary column and the volcanic layer were averaged to 2.40 gcm -3 and 2.75 gcm -3 respectively from the density log of the Barracuda Exploratory well (Ichron, 2012). The densities of the basement and the upper mantle were taken as 2.90 gcm -3 and 3.38 gcm -3 respectively (Dziewonski & Anderson, 1981, Jayawardena, 2012). It was assumed that all of these subsurface layers were homogenous, to reduce the complexity of the problem. Fig. 2. Free-air gravity anomaly along the SL05-13 seismic line extracted from Sandwell et al. (2014) METHODOLOGY Computer Program for Forward Modeling of 2D Gravity Anomalies Talwani et al. (1959) have put forward an algorithm to compute the gravity anomaly caused by a two-dimensional body in the 80

5 Journal of Geological Society of Sri Lanka Vol. 18 (2017), subsurface having a contrasting density with its surrounding. The boundary of the twodimensional body can be closely approximated by a polygon by increasing the number of sides of the polygon. This method assumes that the two-dimensional body is infinitely long parallel to the strike of the structure. Mathematical expressions have been obtained for both vertical and horizontal components of the gravitational attraction due to this polygon at any given point (Talwani et al., 1959). The expression for the vertical component has been simplified as given in Equation 1. Here, G is the constant of universal gravitation, ρ is the density contrast of the polygonal body with the surrounding. All other variables are described by Figure 4. Fig. 3. Subsurface horizons along the SL05-13 seismic line interpreted on time domain. In order to execute this algorithm, a Wolfram Mathematica 10 program (2Dgrav.nb) was coded. The x - and z - coordinates of the vertices of the polygonal body (in km) in a counterclockwise sense had to be introduced to -3 the program. The density contrast (in gcm ), number of vertices in the polygonal body, number of field points for the calculation of the anomaly and the interval between adjacent field points (in km) were the other required inputs. Density contrast was calculated as the difference between the density of the material that had replaced the originally existed material and the density of the originally existed material. The output of the program provided a table of calculated gravity anomaly caused by the polygonal body at each field point along the mean sea level. The variation of the gravity anomaly caused by the polygonal body along a horizontal axis on the mean sea level was plotted on a graph. Checking the Mathematica Consistency and Reliability Program for The consistency and reliability of the output provided by the coded Mathematica Program 2Dgrav.nb was checked with a Java applet designed by the University of California, Berkeley to calculate the gravity anomaly due to a polygonal body ( olygon/index.html) based on the algorithms and Fortran subroutines by Won and Bevis (1987). Two polygonal bodies, one with the crosssectional shape of a trapezium having a density -3 contrast of -0.1 gcm with the surrounding (Figure 5a), and the other with an erratic cross-3 section having a density contrast of +0.3 gcm (Figure 5b) were used. The comparison between the results from the two approaches is given in Figure 5 and Table 2. Figure 5 and Table 2 indicated that the gravity anomalies computed from 2Dgrav.nb program were closely comparable with those from Won and Bevis (1987) and therefore was considered consistent and reliable to be used in the forward modeling of gravity anomalies. 81

6 Fig. 4. Calculation of the gravity anomaly by a two - dimensional body with polygonal cross - section. (Modified after Talwani et al. (1959). (a) (b) Fig. 5. Comparison between gravity anomalies computed for bodies with (a) cross-sectional shape of trapezium (density difference of -0.1 gcm -3 with the surrounding) and (b) erratic cross section (density difference of +0.3 gcm - 3 with the surrounding), from the 2Dgrav.nb program and Won and Bevis (1987). Modeling the Structure of the Mohorovičić Discontinuity The individual subsurface layers in the Mannar Basin upto the basement were delineated with the aid of marine seismic data and other sources as mentioned under Depth Data of Subsurface Horizons. The densities of these layers were also obtained from different sources as mentioned under Densities of Subsurface Layers. Based on the assumption that these subsurface layers are homogenous and isotropic in density, the gravity anomaly due to each of these subsurface layers were computed using the 2Dgrav.nb program. The program was input with the x- and z- coordinates of the water column, sedimentary column and volcanic layer in a counterclockwise sense. The respective density contrasts for each of these subsurface layers were calculated as given in Table 3. Figure 6 shows the computed gravity anomaly due to the water column followed by the body of the water column. The anomaly 82

7 takes negative magnitudes due the negative density contrast induced with respect to the sedimentary column (Table 3). The negativity of the density contrast occurs as the less dense oceanic water is visualized to have displaced the high dense sedimentary rocks. Figure 7 shows the computed gravity anomaly due to the sedimentary column followed by its dimensions. The negativity of the density contrast (Table 3) is because the less dense sedimentary rocks are observed to have replaced the high dense basement rocks and is the reason for overall sign of the gravity anomaly being negative. Figure 8 shows the computed gravity anomaly due to the volcanic layer followed by its dimensions. The positivity of the density contrast (Table 3) is because the high dense volcanic rocks are observed to Table 2. Comparison between gravity anomalies computed for bodies cross-sectional shape of trapezium (density difference of -0.1 gcm -3 with the surrounding) and (b) erratic cross section (density difference of +0.3 gcm -3 with the surrounding), from the 2Dgrav.nb program and Won and Bevis (1987). Gravity Anomaly (mgal) x (km) Trapezium Erratic Body Won and Bevis 2Dgrav.nb Won and Bevis 2Dgrav.nb (1987) (1987)

8 have replaced the less dense sedimentary rocks and is the reason for overall magnitude of the gravity anomaly being positive. The individual gravity anomalies due to these subsurface layers were summed up to obtain the resultant gravity anomaly (Figure 9). However, it was evident that there should be another subsurface body causing a significant positive gravity anomaly, as the gravity anomaly in Figure 8 was not congruent with the observed free-air gravity anomaly along the same line (Figure 2). The residual gravity anomaly which is the difference between the observed gravity anomaly (Figure 2) and the summation of the gravity anomalies by the three known subsurface layers (Figure 9) was computed (Figure 10). Based on the assumption that this residual gravity anomaly is due to the upper mantle, its body was arbitrarily modeled in a trial and error approach using the 2Dgrav.nb program, in such a way to match the gravity anomaly due the arbitrary model to the residual gravity anomaly (Figure 10). The density contrast in this case was taken as gcm -3 as the higher dense upper mantle was visualized to have replaced the basement rocks (Table 3). Table 4 provides a comparison between the observed gravity anomaly and the calculated gravity anomaly that takes into account the composite gravity anomaly due to all the subsurface layers, along the SL05-13 line. RESULTS The resulting structure for the Mohorovičić Discontinuity derived using the trial and error approach using 2Dgrav.nb program (Figure 11) indicates that the mantle has upwelled into the Table 3. Density contrasts between subsurface layers used in computing gravity anomalies using the 2Dgrav.nb program. Density of Subsurface Layer (gcm -3 ) Host Density (gcm -3 ) Density Contrast (gcm -3) Water Column (1.02) Sedimentary Column (2.40) Sedimentary Column Basement (2.90) (2.40) Volcanic Layer (2.75) Sedimentary Column (2.40) Upper Mantle (3.38) Basement (2.90) Fig. 6 Gravity anomaly due to the water column along the SL05-13 seismic line followed by polygonal body of the water column as viewed in cross-section. 84

9 Fig. 7. Gravity anomaly due to the water column along the SL05-13 seismic line followed by polygonal body of the sedimentary column as viewed in crosssection. Fig. 8. Gravity anomaly due to the water column along the SL05-13 seismic line followed by polygonal body of the volcanic layer as viewed in cross-section. basement (crust) but has not continued until the formation of an oceanic crust. The depth to the Mohorovičić Discontinuity in the middle of the Basin (~ km) is approximately 16 km and descends to approximately 36 km towards the landmasses of Sri Lanka and India. The minimum thickness of the continental crust of about 6 km is also observed between approximately 70 to 100 km. DISCUSSION The results of the study indicate upwelling of the mantle into the crust, which has not progressed until the formation of an oceanic crust to initiate a new spreading center. Therefore, Mannar Basin could be considered as a failed-rift basin. Formation of a failed-rift is commonly attributed to a triple junction where two arms proceed to the stages of continental separation, whereas seafloor spreading fails to develop with a particular triple junction. seafloor spreading fails to develop in the other (Allen & Allen, 2005, Ingersoll, 2012). This implies that the Mannar Basin has been formed due to a tectonic event predating or synchronous with Gondwana Breakup (Dissanayake & Chandrajith, 1999, Gibbons et al., 2013). Fig. 9 Summation of the gravity anomalies seismic line due to the water column sedimentary column and the volcanic layer along the SL followed by the respective polygonal bodies of the volcanic layer as viewed in cross-section. 85

10 obtaining data from the recent hydrocarbon The thickness of the continental crust of approximately 36 km in the Sri Lankan end of the study area is quite consistent with the results from receiver function models computed using data from the IRIS broadband seismic station PALK in Sri Lanka that reveals a crust with a thickness of 34 km (Pathak et al., 2006). Depth to the Moho in this region according to Crust 1.0 Global Crustal Model is approximately 32 km (Laske et al., 2013). The sample points in Crust 1.0 are spaced in a 1 x1 grid and its resolution is not sufficient to make a judgement on the upwelled region of the mantle as proposed from this study. However, the depths to Moho on either sides of the basin from this study and Crust 1.0 model are closely comparable with each other. explorations conducted in the Mannar Basin. Fig. 11. Observed gravity anomaly (brown) and calculated gravity anomaly (blue), along the SL05-13 line followed by the resulting subsurface structure upto the upper mantle. Fig. 10. Residual gravity anomaly along the SL05-13 seismic line (blue) and the gravity anomaly due to the arbitrary model for the upper mantle (green). REFERENCES CONCLUSIONS The main conclusion arrived from the results of this study is that Mannar Basin is a failed-rift basin formed before or during Gondwana Breakup. The mechanism behind its formation is crustal thinning associated with continental rifting. ACKNOWLEDGMENT The authors would like to acknowledge Mr. Saliya Wickramasuriya - Director General and Mrs. Preeni Withanage Director Benefits of the Petroleum Resources Development Secretariat of Sri Lanka (PRDS) for the support extended in Affholder, M. & Valiron, F. (2001). Descriptive Physical Oceanography: Taylor & Francis. Allen, P. A. & Allen, J. R. (2005). Basin Analysis Principles and Applications: Blackwell Publishing. Baillie, P. W., Shaw, R. D., Liyanaarachchi, D. T. P. & Jayaratne, M. G. (2002). A New Mesozoic Sedimentary Basin, Offshore Sri Lanka. EAGE 64th Conference & Exhibition. Florence, Italy. Cooray, P. G. (1984). An Introduction to the Geology of Sri Lanka (Ceylon): National Museums of Sri Lanka. Cooray, P. G. (1994). The Precambrian of Sri Lanka: a Historical Review. Precambrian Research 66,

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