M N Mishra, Sr. Geologist, GSI, Bhopal

Geomorphology:concerned with the structure, origin, and development of the topographical features of the Earth's surface

Tectonic Geomorphology:competition between tectonic processes that tend to build topography and surface processes that tend to tear them down represent the core of Tectonic Geomorphology

Active crustal deformationLandforms in tectonically active areas: complex integration of effects of vertical and horizontal motion of crustal blocks and aggradational and degradational processes Thermal processes, heterogeneities of mantle, climate & erosion rates

Extraction of topographic info modeling surface processes- recognition of terrain morphologic elements understanding spatial relationships

DefinitionsMorphotectonics Geomorphology of landforms shaped by active tectonic movements faulting, folding and tilting of Quaternary sediments warping of planation surfaces off-setting of river valleys and streams abrupt variations in stream gradients & degree of incision segmentation of alluvial fans presence of linear alluvial scarps differential accumulation of recent sediments etc.

Quaternary landforms and drainage are extremely sensitive to active tectonics

Geomorphic analysis of the landforms and drainage has the potential for providing insights into active tectonic rates, styles, and patterns of deformationMorphotectonic studies

Himalayas: the abode of snow

The Eight Thousanders clubEverest 8848K28611Kanchanjunga8586Lhotse8516Makalu8462Cho Oyu8201Dhaulagiri8167Manaslu8156Nanga Parbat8126Annapurna8091KarakoramsA no. of peaks with >8000

Early Silurian (-444 ma)

Permian Trias Boundary (-253 ma)

Early Cretaceous (130-125 ma)

Convergence since Late CretaceousInitial contact ( Eocene 38 my)

Rate of movement 180 mm/a to 50 mm/a

Continent continent collision:

Indian & Eurasian plates

most tectonically active region of the world

moderate to large magnitude earthquakes

1905 Kangra (M 7.8)1934 Bihar (M 8.4)1950 Upp. Assam (M 8.5)

Prominent tectonically controlled geomorphic indicators

Tectonic SubdivisionsIndus suture zoneOphiolite melange : bedded cherts, scraped oceanic crust, Dras Volcanics : Upp Cret to Upp Jurassic basalts, dacites, volcaniclastics, pillow lavas, cherts Indus Molasse: continental clastics, fluvials, lacustrines, post-collisional, Eocene to post-EoceneCentral HimalayasTethys Himalayas: Folded, imbricated, weakly metamorphosed sediments, Upper Paleozoic to Eocene components, greenschists, eclogitesHigh Himalayan Crystalline Sequences: 30 km thick med to high grade meta- sed. Intruded by Ordo (500 Ma) to Lr. Mio (22 Ma) granites . Sed are Upp Proterozoic to Lr. Cambrian , thrust over Lesser Himalayas along MCTLesser HimalayasUpp Prot to Lr Cenozoic detritals from passive Indian margin, granites, rhyolites , are thrust over the Sub-Himalayas along the MBTSub-HimalayasFoothills of Himalayan range, Mio to Pleisto Molasses, Murees, Siwaliks, thrust over the Indogangetic plains along MBF / Main Frontal Thrust

Main Tectonic FeaturesCollision zone (Inactive at present)ISZ = Indus suture zoneMMT= Main mantle thrust

Tethys Himalayas & High Himalayas thrust over Lesser Himalayas along MCT (Main Central Thrust, largely inactive)Lesser Himalayas are thrust over Sub-Himalayas along MBT (inactive to reactivated)

Main Tectonic FeaturesHimalayan Frontal (Reverse) Fault / Thrust: Sub-Himalayas thrust over the Gangetic plainsMost active structure

Abrupt change in elevation from Gangetic plains to Sub-Himalayas Gradual change in elevation from Sub-Himalayas to Lesser Himalayas Abrupt Change in elevation from Lesser Himalayas to Great Himalayas (~ 5000 m) Greater Himalayas form an elevation front

Two Forelands & basement uplifts

The Kali Gandaki River: The worlds deepest gorge between Dhaulagiri & Annapurna

Steepest gradient: N of MCT

No deflection of rivers while crossing the MCT, but many rivers flow parallel to antiforms at HFF before breaching them

Hypsometric curve of Subansiri Basin in Tibet and India

Manifestations of the Himalayan Frontal Fault As antiformal ridges faulted on their flanks & development of duns: synformal valleys between growing anticlinesAs northerly dipping imbricate thrusts where antiformal ridges and intervening duns are absent

Absence of anticlinal ridges & duns, HFF as imbricate thrust planes

Antiformal ridges & Duns along HFFJanauri anticline: between Beas and SutlejMohand anticline: between Yamuna & GangaBetween Ghaghra & BagmatiJia BhareliItanagar

BeasSutlej

Fan Surface

Imbricate thrusts with piggy back structure

Structure and Tectonics LineamentsHimalayan structural trends and thrusts (ENE WSW & E - W, maxima at N60E)Conjugate set of Shear Fractures (Wrench Faults) N S (maxima at N 5 E) with sinistral sense of movementWNW ESE (maxima at N65W) with dextral sense of movementOrientation of max. principal horizontal stress N30W-S30EMaxima of thrusts occurs at right angles to this direction (i.e., at N60E)

Dextral Shear Model

The left-lateral Chaman Fault

Extensive terrace cultivation

1500 km of crustal shortening

Zone of deformation 3000 km wide

2400 km of missing cont crust subduction of cont crust squeezing of Indo China by the indenter Folding, thrusting, ramping

Linear morphologic features : structural discontinuitiesIntersection of structural discontinuities & land surface give rise to linear morphologic features

Linear valleys, ridge lines, linear break in slopes

Master joints, high-angle normal, reverse, strike-slip faults

Low-angle thrust faults: curvilinear

Intersection of folds with land surface

Planar morphologic featuresUniform hill sides along fractures / faults

Triangular-faceted spurs along faults

Geometry of planar surfaces is described by uniform aspect and constant slope values

Large elongated areas of uniform aspect and slope with linear boundaries can be associated with faults

Depressions, pressure ridges, tilt of surfaces, bulges with local maxima or minima present along fault zones

Morphometric Indices

Studies in Tectonic geomorphology & morphotectonicsShaded-relief / hill-shade modelsAccentuation of reliefExtraction of topographic lineaments, fractures, morphotectonic elements etc.

3-D Image drapeStudy of terrain features in 3-D

Digital cross-sectionsBetter comprehension of terrain features

Studies in Tectonic geomorphology & morphotectonicsDerivative maps:Slope & Aspect maps : regional morphotectonic evaluation

Trend analysis : to study tectonically induced tilt

Curvature maps: recognition and characterization of faults

Extraction of morphotectonic features from DEMSlope mapSlope is the first derivative of elevation and abrupt changes in slope across the landscape may be indicative of the presence of active faults in the terrain. Lineaments seen in slope maps may be fault scarps.

Extraction of morphotectonic features from DEMSlope map : escarpments have high slope

Extraction of morphotectonic features from DEMDigital topographic profiling : longitudinal / cross profiles of mountain ranges and faultsLongitudinal profiles in uplifted area may display young age of the landforms by highlighting the presence of V-shaped valleys. Transverse profiles across faults record significant topographic and slope breaks and facilitate measurement of escarpment heights

Hypsometric Index = (Mean EL Min EL)/ (Max EL Min EL)

Morphometric Indices as Indicators of Active TectonicsBasin Asymmetry FactorBasin asymmetry factor (Af) can be used to evaluate tectonic tilting at the scale of a drainage basin. Af is defined as:

Af = 100 (Ar / At)

Basin Asymmetry FactorShillong Plateau