Structure, Composition and Evolution of the South Indian and Sri Lankan Granulite Terrains from Deep Seismic Profiling and other Geophysical and Geological Investigations: a legends initiative

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8. Work Plan and Methodology

The geophysical surveys will be carried out by scientists of the NGRI in cooperation with Cornell University, Geoforschungszentrum Potsdam and CSIRO, Australia. The accom­panying geological, petrological, geochemical, geochronological and palaeomagnetic work will be undertaken by an international consortium consisting of scientists from India, Sri Lanka, Germany, France, U.K., Australia, USA and China.

The project will try to implement a mixed seismic (termed SEISMIX) experiment. The core of the effort will be a series of deep seismic reflection profiles to provide the structural detail needed to relate surface geology to lower crust and upper mantle heterogeneity. The CMP style profiling will be complemented by wide-angle recording of both P and S waves to provide velocity estimates at depth. Offsets adequate to obtain Pn and Sn arrivals will be sought to allow unambiguous identification of the Moho. The controlled source effort will be matched by passive recording of earthquake sources in two modes: (a) high resolution passive profiles (using both broadband and short period instruments) across key features (e.g. shear zones, the gap between India and Sri Lanka) to provide detailed receiver functions to trace key markers such as the Moho across putative terrain boundaries and closely spaced estimates of crust and mantle anisotropy to evaluate kinematics of lithospheric deformation; and (b) a regional 2D array of broadband instruments to allow P wave and surface wave tomography delineation of lithospheric thickness in 3D.

A provisional deployment geometry for the active/passive seismic surveys is shown in Fig. 18. Instrumentation for these surveys will be derived from pools available to India, US and German participants. Key components include:

(a) Two 24-bit high dynamic range (144 db) seismic RF telemetry systems available at NGRI. This system is comparable to the industry reflection systems commonly used to collect multichannel deep seismic reflection data in other parts of the world This system is a substantial technical upgrade over the DFS IV used by NGRI in their previous deep seismic surveys. The large dynamic range and digital telemetry should greatly improve the signal to noise in reflection recording.

(b) Ca. 1000 portable single channel recorders (e.g. Texans) from the US IRIS instrument pool, to be provided through Cornell University (USA). These instruments can be used for both reflection and wide-angle recording, as well as special geometries such as wideline and crossline configurations for 3D control. They are especially useful in rough terrain.

(c) Ca. 100 three-component seismic recorders (REFTEKS and PDAS) from the IRIS (USA) and GFZ (Germany) instrument pools, to be equipped with short period (e.g. Guralp or Mark Products L-28) sensors for recording wide-angle/refraction data and teleseismic data for receiver functions.

(d) Ca. 50 broadband recording systems (e.g. REFTEK DAS with Guralp or STS2 sensors) from the instrument pools of IRIS (Cornell) and the GeoForschungsZentrum (GFZ), Potsdam, Germany, respectively. The broadband units will be used in both the detailed passive program and the 2D tomographic deployment.

(e) Explosive sources for the reflection/wide-angle/refraction program. NGRI has considerable expertise in supervising the drilling and shooting activities needed for this type of seismic work. It is expected that the seismic drilling will be carried out by local contractors under NGRI supervision. Since the quality of the seismic data depends heavily upon good source coupling, special attention will be placed upon source positions and quality control during source preparation.

Continuous high resolution seismic reflection coverage may not be feasible for the entire geotransect, so emphasis will be placed on key tectonic boundaries and shear zones. Source spacing of 100-200 m for 25-50 kg charges, with receiver spacing of 50-100 metres for 240 channels, is the nominal expectation for reflection recording. This corresponds to a nominal stacking fold of 30-60 (not including wide-angle contributions). These closely spaced sources will be augmented by larger shots (500 –2000 kg) at ca 50-100 km intervals to provide adequate signal at large offsets for refraction work. A recording spacing of ca 1 km is planned for the wide-angle component, with 3-component instruments interspersed with single-component Texans.

Given that many of the passive instruments (short period and broadband) will be located near coastal regions, where noise is known to be a serious problem, and that teleseismic source regions are limited, we expect that extended deployments will be needed to obtain satisfactory receiver function and tomographic results. Therefore we expect to use a minimum of 1 year for the high resolution passive profiles and 3 years for the 2D broadband array.

The channel between South India and Sri Lanka (Gulf of Mannar) suggests the possibility of using marine sources with the land stations to image (at least at wide angle) the crustal structure between these two landmasses. However, this marine zone is a sensitive area, and permissions for use of marine sources and receivers is currently uncertain. Likewise, possible instrument sites along Adam’s bridge, the thin strand of land between southeastern India and NW Sri Lanka, are an attractive option for spanning the zone between the countries. However, access to this region is currently limited due to political concerns. Deployment of ocean bottom instruments (including broadband) is also an option under consideration. These aspects will be pursued as circumstances allow. Details of instrument deployment is dependent upon careful scouting of routes, to be carried out during the preparatory phases of this work.

Gravity and magnetic data will be acquired simultaneously at a large number of stations (about 4000) within a ~100 km wide corridor. The station interval will be guided by the avai­lable gravity data. Magnetotelluric (100 stations) and resistivity (50 stations) experiments will be conducted along the transect, coincident with the seismic profile to derive the subsurface geoelectric structure. Sampling and determination of physical properties such as density, mag­netic susceptibility, electrical and thermal conductivity, compressional and shear wave velocities palaeomagnetic directions and heat flow on representative sets of rock samples along the corridor will facilitate a better interpretation of the geophysical data.

Lacoste-Romberg (Model-G) gravimeters with an accuracy of 0.01 mGal will be used for the 4000 new gravity measurements along the geotransect. In order to minimize the error due to drift in the instruments, secondary bases will be established through­out the corridor, which will be tied to the base already established by NGRI. The gravity data will be collected along a 300 km long seismic line at an average interval of about 0.5 km and in a 100 km wide corridor on either side of this transect at a spacing of approximately 2 km located along roads. The geodetic survey team provides the position location and elevation of the grav­ity stations along the seismic line. The 1:50,000 toposheets of the Surveys of India and Sri Lanka will be used for locating the rest of the gravity stations along the corridor. For elevation control most of the stations are located at the Bench Marks and the Spot elevations, whereas elevation for the other stations will be derived from measurements made by GPS. Total intensity magnetic measure­ments will be made along with the gravity at all stations.

Synoptic geological and structural interpretations will be prepared from satellite images and field mapping of key domains and preparation of geological and structural cross sections. Geological mapping of the corridor will be undertaken, where required, with detailed structural mapping of the shear zone systems along the corridor and at key locations outside the corridor. Emphasis will be placed on understanding the geometry and kinematics of fault/shear movements at major terrane boundaries.

Sampling, petrography and electron microprobe mineral analysis (EPMA) will be carried out to decipher P-T-t paths of important metamorphic units. The currently available data only pro­vide a broad overview and is limited to a few studies. The proposed study will employ EPMA and geothermobarometric studies involving metapelites, mafic granulites, charnockites and granitoids. Keeping in mind that the terrain has probably experienced more than one granulite-facies metamorphic event, the P-T determination and study on fluid migration should focus only on carefully time-calibrated samples. Combined dating and P-T determination on core and rims of single garnet crystals may reveal P-T-t ancestry of the rocks. The nature and composition of fluids in charnockites, gneisses and granitoids will be studied along the traverse in order to assess the role of such fluids during metamorphism and fluid-rock interaction during lower crustal processes as well as to understand the influence of fluids in localization and initiation of shear zones.

Major and trace element geochemistry of representative and dominant lithologies will be determined along the corridor, combined with isotopic studies (O-, Rb-Sr, Sm-Nd, Pb-Pb) and modelling of petrogenetic conditions. The focus will be on the charnockites and granitoid gneisses.

Age determinations by a combination of U-Pb, 207Pb/207Pb, Sm-Nd, Lu-Hf, Rb-Sr and 40Ar/39Ar isotopic methods with an objective to determine the timing of rock and protolith for­mation, deformation and metamorphism in the major crustal blocks along the corridor are required. Dating major faults, shear zones and crustal uplift by whole-rock and mineral analysis using U-Pb, Rb-Sr, 40Ar/39Ar, U-He and fission track methods will contribute significantly to a better understanding of the exhumation and uplift history of the terrain.

The crustal growth and reworking history will be determined from a combination of age and isotopic tracer studies. The tectonic setting of the major crust-forming events will also be evaluated using these data and other geochemical information. Where available, lower crustal and mantle xenoliths will be studied to determine the composition and thermal history of deep, inaccessible parts of the lithosphere.

The field program proposed here is designed not only to address the specific issues related to southern India and Sri Lanka but to provide a basis of comparison for future data acquired in other parts of East Gondwana. This is an important aspect of the LEGENDS initiative: indi­vidual surveys are to be acquired in the context of their relevance to understanding the evolution of Gondwana as a whole. In addition the geophysical and field data will provide an important base for exploration of unexposed mineral resources.

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