(a) What is a Shear Zone ? Describe the different Shear Sense indicators. (20 marks) (b) Enumerate the Shepard's classification…

(a) Shear zones and shear-sense indicators

A shear zone is a tabular zone of concentrated deformation, much longer than wide, in which rocks on its two sides have moved past each other. It is brittle near the surface (fault zone) and ductile at depth, producing mylonite. The flow is non-coaxial, so strain axes rotate and the zone records a sense of shear, dextral (right-lateral) or sinistral (left-lateral), that is read from asymmetric structures.

(i) S–C fabric. The C-planes are shear-zone-parallel surfaces, and the S-foliation is oblique to them and curves into them. The acute angle between S and C points in the direction of shear. Shear bands (C′) cut the foliation at about 15–35° to C, synthetic with the shear.

(ii) Asymmetric porphyroclasts. σ-type clasts have recrystallised tails on opposite sides that step up and do not cross the reference plane. δ-type clasts have tails that wrap around the clast and cross it, forming when clast rotation is fast relative to recrystallisation. The stair-stepping of the tails gives the shear sense.

(iii) Mica fish are lenticular mica grains whose long axes are inclined to the foliation in the shear direction.

(iv) Oblique grain-shape fabric in dynamically recrystallised quartz, and asymmetric c-axis fabrics, show the sense of rotation.

(v) Asymmetric folds and sheath folds, and drag of markers near the zone, have vergence consistent with the shear.

(vi) Domino (bookshelf) structure, where fractured grains slide and rotate, and offset markers such as veins and dykes, are used.

(vii) Brittle indicators include slickenfibre steps, Riedel shears and striations.

Using several indicators together in a section parallel to the lineation and perpendicular to the foliation avoids wrong conclusions. Shear zones are important sites of mineralisation, and the Kolar and Hutti gold belts are examples; the Palghat–Cauvery shear zone is a major Indian example.

(b) Shepard's classification of coasts and the cycle on a submergence shore

Shepard (1948, 1963) divides coasts into two groups.

(i) Primary coasts are shaped chiefly by non-marine agents and their form is inherited. Land-erosion coasts: drowned river valleys (rias) and drowned glacial troughs (fjords). Subaerial-deposition coasts: deltas, glacial-deposit and wind-deposit coasts. Volcanic coasts. Coasts shaped by diastrophism: fault-scarp, fold and salt-dome coasts. Ice coasts.

(ii) Secondary coasts are modified by marine agents. Wave-erosion coasts: straightened cliffed coasts and headland–bay coasts. Marine-deposition coasts: barrier, spit and beach coasts. Coasts built by organisms: coral reefs and mangroves.

Cycle of erosion on a submergence shore. It follows Johnson's ideas, based on Davis.

(i) Youth. Drowning gives an irregular coast with headlands and bays. Waves attack headlands, forming cliffs, notches, caves, arches and stacks, and currents build spits and bars across bay mouths.

(ii) Maturity. Cliffs retreat, a wave-cut platform widens, and the coast is straightened as headlands are removed and bays are filled.

(iii) Old age. A wide, gently sloping platform weakens waves, and erosion nearly stops, leaving a straight shore with marshes behind it.

A new cycle starts with any change of sea level. The Konkan and Kerala coasts show some of these features.

(c) Volcanoes by eruption mode and their relief features

Volcanoes are grouped by the type of eruption, which depends on magma viscosity and gas content.

(i) Hawaiian eruptions are effusive, with fluid basalt. They give lava fountains, pāhoehoe and ʻaʻā flows, and broad shield volcanoes.

(ii) Strombolian eruptions are mildly explosive with bursts of scoria, bombs and lapilli, which build cinder cones.

(iii) Vulcanian eruptions are violent, with viscous andesitic magma giving ash clouds and bombs.

(iv) Peléan eruptions produce lava domes and glowing avalanches (nuées ardentes), as at Mont Pelée in 1902.

(v) Plinian eruptions are catastrophic, with silicic magma, tall ash columns, pumice and ignimbrites, as at Vesuvius in 79 CE.

(vi) Surtseyan eruptions occur where magma meets water.

Positive relief features are shield volcanoes, stratovolcanoes (composite cones), cinder cones, lava domes, lava plateaus such as the Deccan Traps, and volcanic necks left after erosion.

Negative relief features are craters, calderas formed by collapse after emptying of the magma chamber, maars, lava-tube collapse pits and fissures or rifts.

In India, Barren Island in the Andaman Sea is the only active volcano, and the Deccan Traps are a flood-basalt province.