National Geophysical Research Institute,
Email: [email protected]
National Geophysical Research Institute,
Global Positioning System (GPS) is a fast, accurate and cost-effective tool that became an integral part of any geological mapping. In the present study, we have used GPS for a precise and quicker geological & structural mapping of a gabbro pluton, emplaced along the Terrane Boundary Shear Zone (TBSZ) situated between the Eastern Ghats Belt and the East Dharwar craton, Andhra Pradesh, India. A hand-held GPS was employed during the mapping of Pasupugallu gabbro pluton to record the locations of several structural measurements, the extents of dolerite dykes within the pluton, and the pluton boundary. The real-time preliminary geological map, prepared from the GPS software, has been conveniently used for plotting the voluminous structural data and reduced a large amount of time and work. In addition, the application of remote sensing technique has greatly enhanced the existed data. The observed structural features of the pluton strongly suggest its syntectonic nature with the TBSZ kinematics.
Surveying has born out of the curiosity of the people to discover the unexposed lands on the Earth in pre-historic times. Basically, surveying aims at producing a map, a plan or an estimate of an area by recording the measurements such as lengths, directions, etc. of features on the Earth’s surface. Geological surveying/mapping, one of its offshoots, is the systematic examination of any region for geological and economic information (Lahee, 1961). It is the backbone for any geologic, exploration and analytical activity and involves identification of landforms, rock types and structures; and the portrayal of these geologic units and structures on a map with their correct spatial relationship with one another. These maps are needed by a broad spectrum of clients for locating the groundwater, mineral, hazard prone areas, etc. It essentially comprises three components, viz., (a) examination and the interpretation of geological and other related features in the real world, (b) determination of the location of features observed, and (c) plotting of this information on a map.
Along with the expansion of known world through navigation, the field of surveying also witnessed several phases of development in its methodology, equipment and map quality. It started initially with simple sketches on terracotta, skin (Mishra & Ramesh, 1989) and reached the stage of using theodolite to aerial methods in the recent times. Surveyors, in the last century, were very much successful in producing a wide range of thematic maps such as toposheets, cadastral maps, etc. by using conventional pain-staking surveying procedures of triangulation and traversing (Fig.1a). The Brunton compass, attached with a clinometer is indispensable for any geological field operation. The other mapping methods are gridding with tape, plane table, prismatic compass surveys, hand level, barometer, etc. The compass and clinometer method, traditionally used in reconnaissance mapping, is employed for measuring dips, strikes and other geological parameters; and distances are often measured by pacing and compass traversing. The grid and tape method is very much useful for small areas, but not for regional surveys. The hand level is employed for differences of elevations within low-dipping strata, and the distance is measured by pacing or by stadia. The barometer method of field surveying involves the measurement of relative elevations and vertical distances. The plane table survey (Fig. 1b) is a possible precise method among these to measure horizontal and vertical distances; and maps will be plotted in the field itself. It consists of a plane table, telescope, plotting scale, map sheets, stadia rod and stadia tables and the determination of location can be done by radiation, intersection, and traversing.
However, these laborious methods of mapping and collecting geological data, and the hard copy cartography have made the geological mapping a slow, long and cumbersome process. There are several problems associated with the triangulation, such as fixation of permanent markers in the field, and the bearings at large distances, etc. Many of these instruments are not easily portable, and needs more manpower. Moreover, all these methods need to have lot of measurements within short intervals for better accuracy and the data should undergo several correction procedures. The accuracy by these methods is also ambiguous, since it depends on the type of equipment used, the nature of survey and the skills of the people engaged in the complete operation. Inaccurate representation and misorientation of geological datasets generated by these methods can lead to the misunderstanding of the geological process and wrong interpretations.
The era of Geospatial technologies in geological mapping
The complications associated with convention mapping techniques were greatly reduced with the advent of geospatial technologies like Global Positioning System (GPS), Geographic Information Systems (GIS), Remotely Sensed systems (airborne and satellite-based multispectral and radar images) and digital surface systems like Ground-Penetrating Radar in surface and subsurface geological mapping. The introduction of GPS constituted by a constellation of 24 satellites established by U.S. Department of Defence in the 1970s is the onset of the technological revolution, which entered in almost all fields of human life, such as natural sciences, business, entertainment, environment, transportation, etc. The GPS is a space-based, highly accurate and all weather continuous navigation system that enable the user to access the position, elevation, velocity and time anywhere in the world (Thurston et al., 2003). The continuous upgradation and modification in the type of sensors and instrumentation have increased the efficiency of GPS functions, where it provides accurate positions at almost the touch of a button. As such, now-a-days, a hand-held GPS becomes an integral part of routine geological field surveying. It is light, portable, and allows the user to store the spatial (in terms of latitude, longitude and altitude) and aspatial (attributes located at the position). Thus the stored data tables and associated attributes can be constructed in a digital format using the GPS software ‘Mapsource’. Geographical Information Systems (GIS) is a collection of several interconnected programs designed for capturing, editing, transforming, retrieving and presenting the geographically referenced data. The vector data provided by the GPS can be imported to the GIS package to produce a geological or other thematic map. Another technique is the Remote Sensing, which is also widely in use as an additional tool of geological mapping. Apart from the facility of synoptic view than that of aerial photographs, the digital image data provides a high resolution, multi-scale, and multi-spectral readability in even inaccessible regions. It provides the real time data about the targets and detects all kinds of changes on the surface of the Earth. All these advances are fast, better and efficient to record, process, analyze and communicate the geological and other related data with a low cost.
Geologic significance of the study area
In the present study, we have made use of these state of the art techniques (GPS, GIS and Remote Sensing) for a precise and quicker geological and structural mapping of Pasupugallu layered gabbro pluton, Andhra Pradesh, India (Fig. 1c). The elliptical Pasupugallu pluton is one of several mafic, alkaline and granitic plutons emplaced along the Terrane Boundary Shear Zone (TBSZ) situated between the Eastern Ghats Mobile Belt (EGMB) and the East Dharwar craton (Fig. 2 inset). The site of this pluton is critical in the regional tectonic context, since the TBSZ is an easterly dipping thrust between the EGMB and the Dharwar craton, and represents an ancient cryptic suture with dismembered ophiolite occurrences. This zone consists of highly mylonitised amphibolites, migmatitic gneisses, metapelites and sporadic charnockites. The study of pluton can throw light on the genetic association between the emplacement of magma and TBSZ, and further understanding of the regional tectonics prevailing at the time of emplacement.
Fig.2. Preliminary geological map of Pasupugallu gabbro pluton prepared from the waypoints (around 410) stored in GPS, which have been transferred to the GPS software ‘Mapsource’. (Inset map showing the location of Pasupugallu pluton within the Terrane Boundary Shear Zone (TBSZ) at the western margin of the Eastern Ghats Mobile Belt (EGMB). Mapping the pluton with GPS
The Pasupugallu pluton (N 150 48’15” – E 790 48′) is an elliptical shaped mafic body measures 13×7 km in plan view, intruded into the Proterozoic high-grade gneisses and migmatised amphibolites of the boundary zone of the Eastern Ghats, Prakasam district, Andhra Pradesh (Nagaraju & Chetty, 2005). Gabbro is the dominant rock type along with a few lenses of anorthosite. The pluton was traversed by several dolerite, pegmatite, syenite dykes with sporadic isolated outcrops of olivine-pyroxenites. The pluton was mapped on 1:50,000 scale by on-foot survey and a hand-held GPS (Garmin eTrex Vista) was employed to get the accurate location of the observations. This eTrex Vista is a 12 parallel channel GPS receiver with an accuracy of 9-10 meters and is provided with an in-built base map, with a capability of importing the external maps. It is fitted with and internal electronic compass and a barometric altimeter and can store up to 500 waypoints. Since the outcrops of the pluton are at ground level; and are scarcely, irregularly distributed; the grid method is not suitable for mapping. Several hundreds of structural measurements such as attitude of foliations and lineations, fold axes, shear zones, etc. within and outside of the pluton have been collected along with lithological information. Locations of all these measurements are stored as waypoints in GPS and are transferred to the personal computer using GPS software (Mapsource). The GPS has aided significantly in lithological mapping. The pluton boundary with the country rock, wherever exposed, has been precisely measured. Few lenses of anorthosite and leucogabbro have been identified during the mapping, and their GPS locations have enabled to plot their shapes and contact with the gabbroic body. The GPS measurements were facilitated in measuring the length/extent and accurate plotting of several dolerite, pegmatite and syenite dykes traversing the pluton in varied directions. It is also used to measure the width and exposure lengths of marginal shear zones enveloping the pluton. Different symbols were assigned to the waypoints at different rock types to produce a real-time preliminary geological map in the Mapsource itself, and the aspatial data has been saved to all the waypoints with the help of the field notebook. This preliminary map (Fig. 2) has been conveniently used for plotting of geological and large amount of structural data, manually, and reduced a large extent of time and work along with the preservation of field relationships.
Application of Remote Sensing and GIS
Remote sensing technique, widely used for regional scale mapping, has also been applied in the present study, since the pluton boundary is concealed at many places, and there are hardly any outcrops in the central part of the pluton. The IRS-1D LISS-3+PAN merged satellite image of the area on 1:50,000 scale (with a resolution of 5.8 m) remarkably displays the pluton boundary and the structural trends within the pluton distinguished by the tonal variations. The trends inferred from this image perfectly matches with those obtained from field mapping (Fig. 3). It also enhanced the existing data by filling gaps wherever the field structural data is lacking. Based on the dark tonal character of gabbro, a two isolated small bodies of gabbro have been identified out side the pluton; and their presence have been confirmed in the field. This has proved that the remote sensing technique can effectively be used for the local large-scale geologic and structural mapping. A foliation trajectory map has been prepared manually by merging both the datasets from the field survey and satellite image for a comprehensive analysis of the plutonic fabrics. The waypoints stored in Mapsource in vector form have been imported into ArcGis (ver. 9), where a geological map of the pluton has been prepared.
Fig. 3. Lithology and structural trends interpreted from IRS-1D LISS3+ PAN merged satellite image of the Pasupugallu gabbro pluton on 1:50,000 scale. Anorthoite lenses are also shown in the pluton.
The structural map of the Pasupugallu pluton (Fig. 4) reveals a concentric, inward-dipping, helicoidal foliation pattern and a near circular distribution of lineations. The gabbros display magmatic layering in the central parts, which gradually transforms into mylonitic near the margins. Petrographic studies also confirm the magmatic fabrics in the central parts, characterized by the coarse-grained cumulate textures and euhedral nature of plagioclase and pyroxenes. The mylonitic fabrics near the margins characterized by the fine-grained aggregates and the development of intense foliation. The marginal dextral shear zones constitute a narrow structural aureole around the pluton, which is defined by the deflection of regional gneissic foliations into a margin parallel mylonitic with steep stretching lineations and elliptical enclaves of amphibolites. All the structural features, the shape and the parallelism of the long axis of the pluton with the regional trends strongly suggest the syntectonic emplacement with the TBSZ kinematics.
Fig. 4. Geological and structural map of Pasupugallu gabbro pluton, Andhra Pradesh.
The digital mapping in earth sciences
Though, involving the drawbacks like power loss and restriction of use in mountainous and forest areas, the GPS technique has been successfully used in the current study. However, the integration and interoperability among all geospatial technologies have paved path for the digital mapping, which can virtually bring the lab plotting into the field and thus producing real-time maps, rapid analysis and decision-making on-site systems. The modern field surveys are planned with the aid of an advanced Data Acquision System, which integrates both GPS, and GIS with a graphic user interface technique and the database and maps created in the field can be exported to other formats. When these technologies coupled with LIDAR and other in-situ laser instrumentations, the output will be a 3-D and 4-D visualization that brings a virtual reality environment before the spatial data users.
We sincerely thank Dr. V.P. Dimri, Director, National Geophysical Research Institute, Hyderabad for permitting us to publish the paper and DST for their financial support to take up the job. The second author gratefully acknowledges CSIR, New Delhi for providing the financial support under CSIR-SRF scheme.
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