Structural geology

In short

Structural geology is the study of how rocks deform under stress — the geometry, kinematics and mechanics of folds, faults, joints, foliations and shear zones, and the maps and projections used to describe them in three dimensions. It explains why strata tilt, repeat or disappear, and so controls where coal seams, ore shoots, groundwater and hydrocarbons sit, and whether a dam, tunnel or slope will stand. In B.Sc and M.Sc courses it is examined through map interpretation, cross-section construction, three-point problems and stereonet exercises; in GATE (Geology and Geophysics, GG paper) it is a high-weightage section. Employers who test it directly include GSI, ONGC, Coal India and its subsidiaries, NMDC, Hindustan Zinc, CGWB, NHPC, NTPC, mineral exploration firms and geotechnical consultants.

Explained

Stress, strain and deformation behaviour

• Stress (σ) = force per unit area (Pa, MPa). Resolved into normal stress (perpendicular to a plane) and shear stress (τ) (parallel to it). At any point the state of stress is described by three mutually perpendicular principal stresses σ1 ≥ σ2 ≥ σ3, on which shear stress is zero.
• Lithostatic (confining) pressure is the hydrostatic part, σ1 = σ2 = σ3, roughly 27 MPa per km of depth; it causes volume change, not shape change. Deviatoric stress is the remainder and causes distortion.
• Regimes: compression (σ1 horizontal, σ3 vertical) → thrusts and folds; tension/extension (σ3 horizontal) → normal faults and rifts; shear/strike-slip (σ1 and σ3 both horizontal) → wrench faults.
• Strain (ε) = change in shape or size. Longitudinal strain e = (l − l₀)/l₀; stretch S = l/l₀; shear strain γ = tan ψ (ψ = angular shear).
• Homogeneous strain: straight lines stay straight, parallel lines stay parallel (a circle becomes an ellipse). Heterogeneous strain: folds, shear zones — analysed by dividing into homogeneous domains.
• Pure shear (coaxial, irrotational) vs simple shear (non-coaxial, rotational) — simple shear dominates shear zones; strain markers rotate progressively.
• Strain ellipsoid: three principal strain axes X ≥ Y ≥ Z. X direction → lineation; XY plane → foliation/cleavage. Flinn diagram (a = X/Y vs b = Y/Z) separates prolate/constrictional (L-tectonite, cigar) from oblate/flattening (S-tectonite, pancake); plane strain is Y = 1.
• Behaviour: elastic (recoverable, Hooke's law σ = Eε), brittle (fracture past yield — shallow, cold, high strain rate, low confining pressure), ductile/plastic (permanent flow — deep, hot, low strain rate, high confining pressure), viscous/creep (time-dependent). The brittle–ductile transition in continental crust is around 10–15 km.
• Mechanisms: fracturing and frictional sliding (brittle); pressure solution, dislocation glide and creep, dynamic recrystallisation, diffusion creep, twinning (ductile).

Primary structures and attitude of beds

• Primary (depositional) structures form at deposition — bedding, graded bedding, cross-bedding, ripple marks, mud cracks, sole marks, pillow structure in lavas, flow banding. They are the way-up (top-bottom) criteria used to recognise overturned beds.
• Attitude of a plane = strike + dip. *Strike*: bearing of the horizontal line in the plane. *Dip*: maximum inclination, measured perpendicular to strike, and always with the direction of dip (e.g. 35°/N40°E, or dip 35° towards S50°E). Written as N40°E, 35°SE or in dip-direction format 130/35.
• Apparent dip is any inclination measured oblique to the dip direction; it is always less than the true dip.
• For a line: trend (bearing) and plunge; pitch/rake is its angle measured within a plane.
• Measured with a Brunton compass / clinometer compass; corrected for local magnetic declination (about 0° to 2°E over much of peninsular India, higher in the northeast — check the current IGRF/NGRI value for the area).
• Outcrop pattern and the rule of Vs: horizontal beds follow contours; vertical beds run straight across valleys; dipping beds make a V in a valley that points in the dip direction (except where dip is less than stream gradient, in downstream-dipping cases).
• Unconformity = a buried erosion or non-deposition surface. Angular (nonconformity of attitude) — beds below are tilted/folded and truncated; disconformity — parallel beds, erosional surface; paraconformity — parallel, no visible break; nonconformity — sediments on igneous/metamorphic basement. Overlap and offlap mark transgression and regression. (Stratigraphic significance is treated separately under stratigraphy.)

Folds

• A fold is a bend or flexure in a planar structure. Antiform = convex-up, synform = concave-up; anticline has older rocks in the core, syncline younger. An antiformal syncline is an overturned pair.
• Geometry: hinge line (locus of maximum curvature), axial plane/surface (joins hinge lines of successive layers), fold axis, limbs, crest and trough, inter-limb angle, amplitude and wavelength.
• By inter-limb angle: gentle (180–120°), open (120–70°), close (70–30°), tight (30–0°), isoclinal (0°, parallel limbs).
• By axial plane dip: upright (vertical axial plane), inclined, overturned/overfolded, recumbent (horizontal axial plane), fan fold.
• By plunge: non-plunging (horizontal), plunging, reclined (plunge of hinge in an inclined axial plane), doubly plunging (canoe-shaped). A closed circular pattern = dome or basin.
• By layer thickness: parallel/concentric (Class 1B) — constant orthogonal thickness, so the fold must die out at depth, limited by a lower detachment; similar (Class 2) — constant axial-plane-parallel thickness, thickened hinges, typical of ductile rocks; Class 1A (thinned hinge), Class 1C (thinned limbs), Class 3 (thickened hinge, strongly thinned limbs) — Ramsay's dip-isogon classification. Chevron folds (sharp hinge, straight limbs), kink bands, ptygmatic and disharmonic folds, monocline, drag/drape fold.
• Mechanisms: buckling (flexural) under layer-parallel shortening with competence contrast; flexural slip (slip along bedding, so bed thickness conserved — parallel folds); flexural flow; passive/shear folding (layers are passive markers, slip along axial planes — similar folds); bending (load applied at an angle to the layer, e.g. drape folds over a fault block).
• Superposed (polyphase) folding — F1, F2, F3 generations; refolded folds, hook and mushroom interference patterns (Ramsay's Types 0–3). Dated against fabrics S0, S1, S2.
• Minor structures as way-up/vergence indicators: parasitic (drag) folds show Z-shape on one limb and S-shape on the other, M on the crest; cleavage–bedding intersection angles; vergence points towards the antiform.

Fractures, joints and faults

• Joint = fracture with no appreciable displacement; fault = fracture with displacement parallel to the surface; fissure = open fracture; shear zone = ductile equivalent of a fault.
• Joint types: systematic vs non-systematic; strike, dip, oblique and bedding joints; columnar (cooling) joints in basalt (Deccan Traps); sheeting/exfoliation joints in granite; mural joints; tension (extension) joints (rough, plumose, open) vs shear joints (conjugate sets each at about 30° to σ1, dihedral angle 2θ ≈ 60°, slickensided). A joint set is parallel; several sets form a joint system.
• Fault elements: fault plane (strike, dip), hanging wall and footwall, slip (net, strike, dip components), throw (vertical) and heave (horizontal), shift, separation, hade (angle from vertical = 90° − dip), slickenside and slickenlines/striae, drag folds, fault breccia, gouge, mylonite and cataclasite.
• Genetic classification (Anderson's theory, using the free surface so one principal stress is vertical):
  • Normal (dip-slip, hanging wall down): dip 60°±, extension, σ1 vertical. Listric, horst and graben, half graben, growth faults, detachments.
  • Reverse: hanging wall up, dip > 45°; a thrust dips < 45°, shortening, σ3 vertical. Overthrust, nappe, klippe, window (fenster), duplex, imbricate (schuppen) structure, ramp and flat, décollement, blind thrust.
  • Strike-slip (wrench/transcurrent): horizontal slip; dextral/right-lateral or sinistral/left-lateral, σ2 vertical. Transform faults at plate boundaries; Riedel shears, pull-apart basins (transtension) and pop-ups/restraining bends (transpression).
  • Oblique-slip, rotational/scissor faults, step and en-échelon patterns, radial and peripheral (ring) faults.
• Effect on outcrop: normal faults cause omission/gap in a stratigraphic column, thrusts cause repetition; a dip fault offsets strike lines; a strike fault may hide or duplicate beds. Older rocks over younger across a low-angle contact is diagnostic of a thrust.
• Recognition criteria: offset of markers, slickensides, drag folds, breccia and gouge zones, springs and alignments, scarps and triangular facets, truncated beds, repeated or missing strata, abrupt stratigraphic change, mineralised veins.
• Faulting mechanics: Coulomb failure criterion τ = c + σn tan φ; graphically a Mohr circle against the failure envelope. Byerlee's law gives friction coefficients of ~0.6–0.85 for most rocks. Effective stress: σn′ = σn − Pf; raising pore fluid pressure (Pf) shifts the circle left and triggers failure at lower differential stress — the basis of reservoir-induced and injection-induced seismicity.
• Active faulting in India: Main Frontal Thrust (HFT), Main Boundary Thrust, Main Central Thrust and Indus–Tsangpo Suture in the Himalaya; Narmada–Son lineament; Kachchh rift faults (Kachchh Mainland Fault, Allah Bund); Eastern Ghats and Godavari graben faults. Fault data feed the seismic zoning map in IS 1893 (Part 1) : 2016 (Zones II–V).

Foliation, lineation, shear zones and tectonites

• Foliation = any planar fabric: bedding (S0), cleavage, schistosity, gneissosity, mylonitic foliation, primary igneous flow layering.
• Cleavage types: slaty (fine, continuous, mica-rich, low grade), crenulation (microfolding of an earlier fabric), fracture/spaced cleavage, axial-plane cleavage (parallel to the axial plane of a fold — key to linking fabric with folding), transposition foliation.
• Origin of cleavage: mechanical rotation of platy grains, pressure solution with dissolution on faces normal to σ1, and recrystallisation of new micas in the XY plane. Cleavage therefore approximates the flattening plane, perpendicular to maximum shortening.
• Lineation: intersection lineation (cleavage × bedding, parallel to the fold hinge), crenulation lineation, mineral lineation (stretching), boudin and mullion structures, slickenlines, rodding, fold mullions. Pencil cleavage where two fabrics intersect.
• Shear zone: tabular ductile high-strain zone with foliation oblique to the shear plane. Shear-sense indicators: S–C fabrics, asymmetric porphyroclasts (sigma and delta clasts), mica fish, oblique grain-shape fabric, shear bands (C′), asymmetric boudins and folds, drag of markers.
• Tectonites: S-tectonite (planar), L-tectonite (linear), S-L tectonite. Mylonite series by matrix proportion: protomylonite (< 50%), mylonite (50–90%), ultramylonite (> 90%); phyllonite for mica-rich.
• Boudinage (competent layer stretched into sausages), pinch-and-swell, necking, tension gashes, en-échelon veins, augen structure, porphyroblast and porphyroclast distinction.
• Indian examples: Main Central Thrust mylonites, South Indian shear zones (Moyar–Bhavani, Palghat–Cauvery, Achankovil), Singhbhum Shear Zone (copper–uranium belt), Great Boundary Fault at Chittorgarh, Narmada–Son lineament.

Measurement, projection and representation

• Field kit: Brunton/clinometer compass, hand lens, hammer, GPS, topographic sheet (Survey of India 1:50,000), geological map, field notebook; data recorded as strike/dip or dip-direction/dip and plotted as strike-and-dip symbols (T-bar with dip amount, arrow for plunge).
• Geological maps show lithology, contacts, attitude symbols and structures; GSI publishes district and 1:50,000 quadrangle geological maps and the Seismotectonic Atlas (SEISAT). Cross-sections are drawn perpendicular to strike; use true scale (no vertical exaggeration) for structural work.
• Outcrop width (w) relation: for a bed of true thickness t and dip δ on flat ground, w = t / sin δ. Across a slope, correct for ground slope.
• Three-point problem: from three non-collinear points of known position and elevation on a planar surface, construct structure contours (strike lines) by interpolation, then compute strike and dip.
• Stereographic (equal-angle, Wulff) and equal-area (Schmidt, Lambert) projections on the lower hemisphere: a plane plots as a great circle, a line as a point, a plane also as its pole (normal). Used for intersections (two planes intersect along a line found where their great circles cross), angles between planes/lines, rotation of data, pi (π) diagram (poles of bedding lie on a great circle whose pole is the fold axis), beta (β) diagram (bedding great circles intersect at the fold axis), contoured pole density diagrams for joint sets, and kinematic analysis of rock slopes (planar, wedge, toppling failure). Lower-hemisphere equal-area is standard for density contouring because it conserves area.
• Rose diagrams for lineament and joint orientations; structural analysis software — Stereonet/OpenStereo, Dips (Rocscience), Move, GeoMapApp, and QGIS for lineament mapping from satellite data.
• Codes and practice in India: IS 1893 (Part 1) : 2016 seismic zoning; IS 13365 (geotechnical investigation / rock mass classification for river valley projects) and rock-mass systems RMR (Bieniawski) and Q (Barton) that use joint spacing, condition and orientation; IS 1892 and IS 1498 for site investigation and soil classification; IS 4464/coal practice for seam correlation; the OSH Code, 2020 (in force 21 November 2025; it repealed the Mines Act, 1952) with the Coal Mines Regulations, 2017 and Metalliferous Mines Regulations, 1961 continued under it (fault and barrier rules, DGMS oversight); MMDR Act, 1957 as amended and the Mineral (Auction) Rules for exploration reporting; NMET-funded exploration reported under the UNFC/Indian Mineral Resource codes (IMIC). For dams and nuclear sites, AERB and CWC guidelines require demonstration that no capable fault lies within specified distances — check the current version of each document before citing a clause.

Key points

• Stress vs strain — stress is the applied force per unit area (cause); strain is the resulting change in shape or size (effect).
• Principal stresses - σ1 ≥ σ2 ≥ σ3, no shear on their planes; their orientation decides whether a normal, thrust or strike-slip fault forms (Anderson).
• Strike and dip - strike is horizontal in the plane, dip is the maximum inclination perpendicular to strike; apparent dip is always less than true dip.
• Rule of Vs - a dipping bed's outcrop V in a valley points in the direction of dip; vertical beds are unaffected by topography.
• Anticline vs antiform - anticline is defined by older rocks in the core (needs stratigraphic order); antiform is purely geometric (convex up).
• Parallel vs similar folds - parallel (Class 1B) keeps orthogonal thickness and dies out with depth; similar (Class 2) keeps axial-plane-parallel thickness and continues.
• Flexural slip vs passive shear - bedding-slip buckling gives parallel folds in competent layered rocks; shear along axial planes gives similar folds in ductile rocks.
• Inter-limb angle classifies tightness: gentle > 120°, open 120–70°, close 70–30°, tight < 30°, isoclinal 0°.
• Axial-plane cleavage - cleavage parallel to the fold axial surface, and roughly perpendicular to maximum shortening; the cleavage–bedding intersection parallels the fold hinge.
• Throw and heave - vertical and horizontal components of dip separation on a fault; hade = 90° − dip.
• Normal fault omits, thrust repeats - in a borehole or traverse, extension deletes section while contraction duplicates it; older over younger means a thrust.
• Coulomb criterion - τ = c + σn tan φ; failure plane makes an angle of 45° − φ/2 with σ1, i.e. about 30° for φ ≈ 30°.
• Pore pressure weakens rock - effective normal stress σn − Pf, so fluid injection or reservoir filling can trigger faulting at unchanged tectonic stress.
• Brittle–ductile transition - around 10–15 km in continental crust; controlled by temperature, confining pressure, strain rate, fluids and mineralogy.
• Shear-sense indicators - S–C fabric, sigma/delta porphyroclasts, mica fish and C′ shear bands give the sense of movement in mylonites.
• Stereonet - lower-hemisphere plot; plane → great circle or pole, line → point; π and β diagrams recover the fold axis.
• Unconformity types - angular, disconformity, paraconformity, nonconformity; marks missing time and a deformation episode below.
• Indian structural framework - Himalayan MFT–MBT–MCT thrust stack, Narmada–Son lineament, Great Boundary Fault, Singhbhum Shear Zone, South Indian Precambrian shear zones, Kachchh rift faults.

Formulas and examples

Example 1 — Apparent dip. A limestone bed dips 40° towards N90°E (due east). A road cutting runs N30°E. What dip is seen in the cutting face?

• Angle between the dip direction (090°) and the section line (030°) is β = 60°.
• tan δ_app = tan 40° × cos 60° = 0.8391 × 0.5 = 0.4196.
• δ_app = arctan 0.4196 = 22.8° — the cutting shows only about 23°, well below the true 40°.

Example 2 — True thickness and outcrop width. A sandstone crops out across a horizontal plateau over a width of 250 m, measured perpendicular to strike; dip is 18°.

• t = w · sin δ = 250 × sin 18° = 250 × 0.3090 = 77.3 m true thickness.
• If the same unit were dipping 70°, the outcrop width would be w = 77.3 / sin 70° = 77.3 / 0.9397 = 82.3 m — steep dips give narrow outcrops.
• Depth to the top of the bed 400 m down-dip from its trace: d = 400 × tan 18° = 400 × 0.3249 = 130 m.

Example 3 — Three-point problem. A coal seam is struck in three boreholes: A at 220 m above MSL, B at 160 m, C at 100 m. On the map, the perpendicular distance between the A-level and C-level strike lines is 900 m (B lies on the interpolated 160 m line).

• Elevation drop from A to C = 220 − 100 = 120 m over a horizontal distance of 900 m.
• tan δ = 120/900 = 0.1333 → δ = 7.6°, dipping from A towards C; strike is the trend of the structure contours (perpendicular to that line).
• Seam elevation 1,500 m further down-dip beyond C = 100 − 1,500 × 0.1333 = 100 − 200 = −100 m MSL, i.e. a working depth of about 100 m plus the local surface elevation.

Example 4 — Fault throw and heave. A dolerite dyke is offset across a fault whose plane dips 55°; the dip separation measured on the plane is 46 m, and a 30 m left-lateral strike-slip component is also present.

• Throw = 46 × sin 55° = 46 × 0.8192 = 37.7 m (vertical offset).
• Heave = 46 × cos 55° = 46 × 0.5736 = 26.4 m (horizontal shortening/extension across the fault).
• Net slip = √(30² + 46²) = √(900 + 2,116) = √3,016 = 54.9 m; the fault is oblique-slip (sinistral-normal or sinistral-reverse depending on hanging-wall sense).

Example 5 — Coulomb failure with pore pressure. A rock at 3 km depth has cohesion c = 20 MPa, φ = 30°, overburden density 2,700 kg/m³.

• Vertical stress σv = ρ g z = 2,700 × 9.81 × 3,000 = 79.5 × 10⁶ Pa = 79.5 MPa.
• With dry rock and σ3 = σv, failure needs τ = 20 + σn tan 30°. The optimum plane lies at 45° − φ/2 = 30° to σ1, so its normal is at θ = 60° to σ1.
• If pore pressure Pf = 40 MPa (near-hydrostatic plus excess), effective σ3′ = 79.5 − 40 = 39.5 MPa — the Mohr circle shifts 40 MPa to the left, so a much smaller differential stress (σ1 − σ3) touches the same envelope. This is why water injection or reservoir impoundment can reactivate faults; the 1967 Koyna earthquake is the classic Indian case.

Where it is used

• Coal and lignite mining — Coal India subsidiaries (ECL, BCCL, CCL, SECL, MCL, WCL), SCCL and NLC use seam correlation, fault and dyke mapping, three-point problems and structure-contour plans for mine planning and DGMS statutory plans; a fault can cost a longwall panel.
• Metal and mineral exploration — GSI, MECL, NMDC, HZL, Hindalco and Vedanta target ore shoots along shear zones and fold hinges (Singhbhum copper–uranium belt, Rampura Agucha and Zawar lead-zinc, Kolar and Hutti gold in shear zones); reporting under UNFC/IMIC.
• Petroleum exploration — ONGC and Oil India map anticlinal traps, fault traps and growth faults in the Cambay, Assam-Arakan, Krishna–Godavari, Cauvery and Mumbai Offshore basins; structural interpretation of 2D/3D seismic is a core job role.
• Engineering geology and geotechnics — NHPC, NTPC, THDC, SJVN, NHAI, Metro Rail and DMRC need joint orientation data, RMR/Q classification and stereonet kinematic analysis for tunnel support, dam foundation and rock-slope design (Himalayan hydel and Char Dham road tunnels).
• Earthquake hazard and seismotectonics — NCS/IMD, NGRI, WIHG and the GSI SEISAT map active faults; outputs feed IS 1893 (Part 1) : 2016 zoning and AERB/CWC site clearance for nuclear and dam sites.
• Groundwater — CGWB and state ground water boards site bore wells on lineaments, fracture intersections and dyke contacts in hard-rock terrain (Deccan basalt, peninsular gneisses) using lineament maps from satellite imagery.

Interview questions

• Q: How do you distinguish an anticline from an antiform in the field? A: An antiform is any convex-upward fold; calling it an anticline requires knowing that the oldest beds lie in the core, which needs stratigraphic order or way-up criteria.
• Q: What is the difference between a reverse fault and a thrust? A: Both move the hanging wall up under compression; a thrust is the low-angle variety, conventionally dipping less than 45°, and often carries older rocks over younger for long distances.
• Q: A borehole shows the same limestone twice at different depths. What does that mean? A: Either a thrust (or reverse fault) has repeated the section, or the hole has passed through a recumbent fold limb; check for a fault zone, drag folds and whether the lower repeat is inverted.
• Q: Why does apparent dip matter in a road cutting or tunnel face? A: Any section not perpendicular to strike understates the dip, so thickness, depth and support design based on the face measurement will be wrong unless corrected by tan δ_app = tan δ · cos β.
• Q: How does a stereonet give you a fold axis? A: Plot poles of bedding — they fall on a great circle (π circle) whose pole is the fold axis; alternatively plot bedding as great circles and their common intersection (β axis) is the fold axis.
• Q: Why does raising pore fluid pressure cause faulting? A: It reduces the effective normal stress on the fault, so the Mohr circle reaches the failure envelope at a lower differential stress — the mechanism behind reservoir-induced earthquakes such as Koyna.
• Q: Distinguish parallel and similar folds and say where each occurs. A: Parallel (Class 1B) folds keep orthogonal bed thickness constant and must die out with depth; similar (Class 2) folds keep thickness parallel to the axial plane and can persist — typical of ductile, higher-grade rocks.
• Q: What does axial-plane cleavage tell you? A: That cleavage and folding formed in the same event, with cleavage approximately normal to the maximum shortening direction, and that the cleavage–bedding intersection lineation parallels the fold hinge.
• Q: How do you tell a joint from a fault in the field? A: A fault shows displacement — offset markers, slickensides, drag folds, breccia or gouge; a joint has no measurable displacement across it.
• Q: Which structures favour groundwater in Deccan basalt? A: Weathered and vesicular flow tops, columnar joints, fracture and lineament intersections, and the contacts and jointed margins of dolerite dykes — dykes can also act as barriers and dam up water on one side.
• Q: What shear-sense indicators would you look for in a mylonite? A: S–C fabric and C′ shear bands, asymmetric sigma and delta porphyroclasts, mica fish, oblique grain-shape fabric and asymmetric boudins or folds.
• Q: Why is joint orientation, not just joint count, critical for a tunnel or slope? A: Failure needs a kinematically free direction — a joint or wedge daylighting out of the face with friction exceeded; stereonet kinematic analysis and RMR/Q adjust the rating for orientation relative to the excavation.

Common mistakes

• Reporting dip without its direction, or measuring dip oblique to strike and recording it as true dip.
• Forgetting magnetic declination correction, or reading the wrong end of the compass needle so bearings are 180° out.
• Mixing up strike/dip format with dip-direction/dip format (N40°E, 35°SE vs 130/35) when plotting data.
• Applying the rule of Vs mechanically and ignoring the exception where the dip is downstream and gentler than the stream gradient.
• Using outcrop width as true thickness — thickness needs t = w · sin δ, and a further correction on sloping ground.
• Confusing throw with net slip, or quoting the dip separation measured on a map as the actual displacement.
• Assuming a normal fault always implies extension at depth without checking for listric geometry, reactivation or inversion; and plotting the upper hemisphere on a stereonet when the convention is lower hemisphere.

Related topics

Stratigraphy and palaeontology · Geomorphology · Plate tectonics and geodynamics · Engineering geology and rock mechanics · Petroleum and coal geology · Metamorphic petrology and fabrics