(a) Discuss the importance of Remote Sensing (RS) and Geographic Information System (GIS) in geological mapping. Elucidate the…

(a) Remote sensing and GIS in geological mapping and mineral exploration (20 marks)

Remote sensing in geological mapping. Satellite and aerial data give a synoptic view of large and inaccessible terrain, saving field time and cost. Tone, texture, drainage and relief separate rock units, and multispectral bands discriminate lithology. Lineaments, faults, folds, dykes and geomorphic features are seen directly, so that a photogeological base map guides the field traverses and reduces the ground work. Repeat coverage helps in mapping landslides, floods and active faults.

GIS in geological mapping. A geographic information system stores, integrates and analyses spatially referenced data as layers: geology, structure, topography, drainage, geophysics, geochemistry and remote-sensing images. Digital maps can be edited and updated, queried, overlaid and printed at any scale, and the database is shared through portals such as GSI's Bhukosh. It also supports 3D models and DEM-based analysis, such as slope and structural attitudes.

Role in mineral exploration. Remote sensing maps favourable host rocks, structures controlling mineralisation, and hydrothermal alteration zones, such as iron-oxide gossans and clay or alunite halos, from their spectral signatures in the visible, SWIR and thermal bands. GIS then integrates these layers with geophysical and geochemical anomalies and drill-hole data; buffer and overlay analysis, weights-of-evidence and fuzzy-logic models yield a prospectivity map, so that targets are ranked and drilling is directed to them at lower cost.

(b) Types of folds and classification by mechanism of folding (15 marks)

Types of folds. Folds are classified geometrically as follows.

• By shape: anticline and syncline (antiform and synform), dome and basin, monocline, and forms such as chevron, box, fan and isoclinal folds.
• By attitude of the axial plane and hinge line: upright, inclined, overturned and recumbent (figure); horizontal or plunging.
• By interlimb angle: gentle (180–120°), open (120–70°), close (70–30°), tight (30–0°) and isoclinal (0°).
• By symmetry: symmetrical and asymmetrical, and by layer thickness, Ramsay's classes 1A, 1B, 1C, 2 and 3, which use dip isogons, distinguishing parallel folds (1B) from similar folds (2).

Classification by mechanism.

• Buckling (flexural folding): a competent layer shortened parallel to itself, either by flexural slip between layers or by flexural flow within them, giving parallel folds.
• Passive (shear) folding: layers act only as markers and are folded by slip along closely spaced planes, or by flow, giving similar folds.
• Bending: forces perpendicular to the layering, as in folds over uplifted blocks.
• Drape (forced) folding: cover strata folded over a faulted basement block.

(c) Homogeneous and heterogeneous deformation, and progressive deformation (15 marks)

Homogeneous deformation. The strain is the same at every point of the body. Straight lines remain straight, parallel lines remain parallel, and a circle becomes an ellipse (the strain ellipse) of the same shape everywhere. Pure shear and simple shear are examples, and a small part of a rock can often be treated as homogeneous.

Heterogeneous deformation. The strain varies from point to point. Straight lines become curved, parallel lines are no longer parallel, and a circle is distorted into different shapes at different places. Folds, boudinage, shear zones and most real rock bodies show it.

Contrast. In homogeneous strain, one strain ellipse describes the whole body and the strain can be calculated simply; in heterogeneous strain, the strain has to be described point by point. The distinction depends on scale.

Progressive deformation. Deformation builds up as a series of small incremental strains along a strain path. The finite strain is their cumulative result and is not a simple sum when axes rotate. In coaxial deformation (pure shear) the axes stay fixed, and in non-coaxial deformation (simple shear) they rotate relative to the material. Consequences include the growth of foliation and lineation, the rotation and folding of veins, boudinage, sigmoidal fabrics and porphyroclast rotation, shear-zone development and the amplification of folds.