GEOINFORMATICS IN EVALUATING MORPHOTECTONICS OF INDO-BURMAN FRONTAL FOLD BELT

Abstract

Morphotectonics combines topography and tectonic activity to explain landforms and allows studying the direct effect of earth movements on landform evolution. A valuable means in evaluating the morphotectonics of a region is to characterize the assemblage of landforms present. Concerned landforms are those produced by tectonic processes, such as faulting, folding, tilting, uplift, or subsidence. The earth's land surface exists in a sensitive balance between tectonic and erosional forces. Wherever the tectonic uplift and deformation are absent, erosional processes often predominate. After a sufficiently long period of time, the result is a subdued landscape with low elevation and modest relief. If uplift and deformation predominate over erosion then there is increase in elevation and relief. This competition between the tectonic processes liable to built up the topography and surface processes shredding them is easily manifested by the morphometric indices. A small part of the earth's surface falling in northeastern India and partly in Bangladesh represents unique rock deformations in the form of westerly convexing northsouth oriented alternating anticlines and synclines. This fold belt is the outcome of E-W oriented compressional tectonic forces that prevailed in the region during the eastward subduction of the Indian plate beneath the Burmese plate and still experiencing tectonic events. Owing to such process the westerly laying sedimentary wedge got deformed into such folded features and now represents fold-and-thrust belt. Intensity of folding gradually increases eastward in the Tertiary sedimentary sequence (sandstones and shales). These folds are also affected by several longitudinal and transverse faulting. Considering the tectonic significance of the region morphotectonic study of frontal part of Indo-Burman fold belt, northeast India encompassing Surma Basin has been attempted employing geoinformatics comprising mainly integrated approach of Remote Sensing-GIS and digital elevation modeling to bring out for the first time the pertaining morphotectonic characteristics. The objective of the present research work is to derive morphotectonic parameters by fold geometry analysis towards understanding the complexity of fold pattern and associated faulting and thrusting, detection of drainage anomalies, change detection analysis and estimation of several morphometric indices. Remote sensing and GIS techniques provide information of landforms and surface geological structures convincingly and efficiently. Satellite images acquired by Indian Remote Sensing Satellite (e.g. LISS-III, PAN) and Landsat-MSS, TM, ETM+, PAN provided very important information with regard to the several deformational features present in the study area. Digital elevation models (DEMs) proven to be a significant platform in morphological and tectonics studies. DEMs can also be used for identification of tectonic features such as faults, folds easily besides satellite images. All the processing steps were carried out using ArcGIS-9.3 and ERDAS-lmagine-9.3 software packages. Since DEMs were used intensively, DEMs of the Bhuban anticlinal hills of the study area have been generated from topographic maps (named Contour-DEM) with contour interval of 20m and Cartosat-1 stereo orthokit images (named Cartosat-DEM) forming an important component of the thesis. As DEM accuracy and quality are two major requirements for their applicability therefore, these generated DEMs have been analyzed for accuracy and compared with the available SRTM-DEM and ASTER-DEMs. Both vertical and horizontal accuracies have been calculated, along with topographic and drainage derivatives. A detailed comparative analysis has been carried out for all the DEMs. Cartosat-DEM emerged as the most reliable DEM and show least horizontal and vertical discrepancies. Cartosat-DEM also shows greater details about the topographic and drainage parameters. Based on overall analysis it could be assessed that for regional studies low resolution SRTM-DEM can be used but for the drainage analysis and hydrological studies high resolution Cartosat-DEM is more preferable. Satellite images and DEMs are capable of displaying prominently the geometry and morphology of the folds especially when these have large spatial extent. Frontal part of the Indo-Burman fold belt falling in parts of Bangladesh and Northeast India have developed into exemplary linear N-S oriented doubly plunging folds in Late-Tertiary II sedimentary rock sequences and are arranged in a set of alternating ridge forming anticlines and valley forming synclines. Fold types present in the area although appear to be simple but the existing fold geometrical arrangement reveal some interesting features. Digital processing of satellite images and DEMs enhancement were performed to understand better the geometry and morphology of the folds covering considerably large area. Morphotectonic information gathered in this study are: identification of multi-folds within the visibly single entity; fold overlapping nature and interrelation between the anticlines; geometry of fold noses; fault affected fold limbs causing bulging; anticline bifurcation and formation of elliptical and cuspate synclines, effect of deformation and faulting etc. Derived morphometric parameters are: drainage basin symmetry, basin elongation ratio, river sinuosity, stream length gradient index (SL-lndex), slope gradient index (S-lndex) and hypsometric integral (lhyP). It has been observed that the Bhuban anticlinal hills represent highly deformed and deviated topography from a normal anticline. This N-S trending curved ridge displays very steep scarp on the western side and gentle slope on the east and is affected by several transverse and longitudinal faults. This anticlinal hill has been identified as typical example of fault propagation folding. Different morphotectonic units (six in number) have been identified which are marked by the transverse faults. The northern segment comprising three morphotectonic units (MU1 to MU3) is unique in the sense that this part exhibits effect of both the longitudinal and transverse faulting, uplifted narrow ridge with downthrown eastern side (MU2). Fourth morphotectonic unit MU4 shows normal anticlinal topography with the upliftment. The fifth morphotectonic unit (MU5) is subsided part and bounded by transverse faults TF3 and TF4, whereas, the sixth unit (MU6) exhibit bulging effect as this part experienced differential fault movements. Stream profiles, longitudinal and transverse topographic profiles extracted from various DEMs also seems to support the topographic variations and deformation of the Bhuban Hills. Rivers are sensitive to intensity of tectonic deformation, adjusting over different periods of time depending on the physical properties of the host rocks, climatic effects and tectonic activity. Thus, the drainage system of a region records the evolution of tectonic deformation. Barak is the major river of the study area and is highly meandering III and changed its course with time at various places along with anomalous behavior at some places. Change detection analysis of Barak, Kushiara river and Surma rivers have been carried out using satellite images and topographic maps of different years spanning from 1955 to 2000. Major tributaries of Barak river have also been investigated for their responses to active tectonics. It has been observed that majority of them are active rivers and shows various meandering loops and also crossing various faulted anticlines through the wind gaps. The western most part of Tripura (India) region adjoining to Bangladesh has been investigated in the light of its geomorphological characteristics. Quantitative parameters such as stream sinuosity, drainage basin asymmetry, basin elongation ratio have been computed. Main rivers of this part, namely the Gomti and Khowai follows extremely meandering path and crosses through the transversely faulted anticlinal ridges. Fluvial anomalies viz. shift in stream channel and the abandoned meandering loops have been inferred and mapped. Development of drainage system towards north and south from the drainage divide along the latitude 23°45'N indicated up arching of the region which is also corroborated by the extracted topographic profiles. Derived morphometric parameters of Agartala slope and adjoining synclines remained useful to understand the nature of topographical modification attributed to the possible tectonic activity. A detailed morphometric study of the Lengpui region has also been carried out to understand the morphotectonics. This area lies near Aizwal in the central part of the Indo- Burman frontal fold belt. This area is marked by anomalous topographic expressions which are manifested through several gently sloping large surfaces. Dhaleshwari river is the major stream of the area and is highly active and shows intense down cutting, resulting into a deep V-shape valley also revealed by topographic profiles. Drainage basins show deviation from the normal concave morphometry and slopes are flatter with a tilting which indicate that area is under tectonic influence. Computed morphometric indices also corroborate the above facts.