(a) Describe fold geometry. Illustrate various types of folds on the basis of their symmetry, orientation of axial plane and the…

(a) Fold geometry A fold is a bend in rock layers produced by ductile compression. Its geometry is defined by the hinge line, the limbs, the axial plane that bisects the interlimb angle, and the fold axis, the line which, moved parallel to itself, generates the folded surface. Wavelength, amplitude, limb dip and interlimb angle quantify its shape; Fleuty's classes by interlimb angle are gentle (180–120°), open (120–70°), close (70–30°), tight (30–0°) and isoclinal (about 0°, limbs parallel). Symmetrical folds have limbs dipping equally; asymmetrical folds have one steeper limb; overturned folds have one limb dipping past vertical; recumbent folds lie nearly horizontal. By axial-plane orientation, folds are upright (axial plane vertical), inclined (axial plane tilted) or recumbent (axial plane horizontal). By symmetry, they are symmetrical, asymmetrical, overturned or recumbent. By trend of the fold axis, the axis may be horizontal, inclined or nearly vertical; trend is the map direction of the axis, and plunge is its angle below the horizontal. By plunge, folds are horizontal (0–10°), gently plunging (10–30°), moderately plunging (30–60°), steeply plunging (60–80°) or vertical (80–90°). Non-plunging folds have horizontal axes and map out as parallel, open curves; plunging folds have inclined axes with a definite trend, e.g. north- or south-plunging, producing closed or teardrop map patterns. The Aravalli fold belt shows asymmetric and overturned folds with regional NE–SW trends.

(b) Plate boundaries and motion Plate boundaries are where lithospheric plates interact. Divergent boundaries, such as mid-ocean ridges and continental rifts, separate as mantle upwelling creates new crust, forming rift valleys and axial troughs; a sketch shows plates moving apart over rising asthenosphere. Convergent boundaries occur where plates collide: oceanic-continental or oceanic-oceanic subduction produces trenches, volcanic arcs and deep-focus earthquakes; continental-continental collision produces thickened crust and mountain belts; ocean–ocean subduction builds island arcs (Andaman–Nicobar type) and ocean–continent subduction gives Andean-type arcs. Transform boundaries are lateral strike-slip faults, such as the San Andreas, accommodating parallel motion. Transform boundaries do not create or destroy crust. Plate motion is driven mainly by slab pull, the sinking of dense oceanic lithosphere, and ridge push, the gravitational sliding of elevated ridge material; mantle drag and convection in the asthenosphere provide basal traction. The Himalayan orogeny records the convergent boundary between the Indian and Eurasian plates, where the Indo-Australian plate underthrusts and crustal shortening thickens the Himalaya.

(c) Isostatic hypotheses The idea arose from the Great Trigonometrical Survey of India: plumb-line deflection towards the Himalaya between Kaliana and Kalianpur was far smaller than the mountains' visible mass predicted, implying a mass deficiency beneath them. The statement expresses isostatic equilibrium: at a depth of compensation, the vertical pressure of overlying columns is equal. The depth of compensation is commonly placed in the upper mantle. Airy’s hypothesis explains this by vertical thickness variation: crust of uniform density floats in denser mantle, so mountains have deep roots; pressure equality is achieved when the weight of the root balances the elevation. Airy predicts roots beneath mountains. Pratt’s hypothesis explains it by lateral density variation in crust of uniform thickness; high terrain has lower average density, and equality at depth D below sea level is written ρ(D+h)=constant, where h is surface elevation. Pratt predicts low-density crust beneath high terrain. Hayford-Bowie refined these by defining a finite depth of compensation below which thickness and density variations are averaged out, so columns of different thickness and density can be in equilibrium; Bowie further upheld local, column-by-column Pratt-type compensation, without flexure of the crust. Vening Meinesz’s regional hypothesis treated the crust as an elastic plate over a viscous mantle; loads cause local flexure, and pressure equalisation occurs through elastic rebound and lateral transmission of stress. Vening Meinesz's flexural model explains why sedimentary basins can remain depressed. Geodetic evidence, such as deflection of the vertical and isostatic residuals, and gravimetric evidence, such as Bouguer anomalies and isostatic reductions, show that compensated regions have small residual gravity anomalies, supporting equal pressure at compensation depth. Ongoing post-glacial rebound of Fennoscandia (about 1 cm/yr) shows that adjustment continues long after a load is removed. The Ganga basin illustrates sediment loading causing isostatic depression, with subsidence balancing the added weight.