Structural geology — stress, strain, folds, and faults

Intuition Beginner

Rocks feel permanent, but given enough force and enough time they bend, break, and even flow. Squeeze a block of wax and it bulges outward; bend a thin ruler and it flexes back; bend it too far and it snaps with a sharp crack. Mountains, valleys, and the cracks we call faults are the results of these same three behaviors, played out across millions of years on a planetary scale.

The agent of all this change is stress, the push, pull, or squeeze acting on each unit area of rock. Stress has a magnitude, measured like pressure in pascals, and a direction. When stress acts on a rock, the rock changes shape: it is shortened, stretched, tilted, or sheared. That change in shape is called strain. Whether a rock bends, flows, or fractures depends on its type, its temperature, the pressure, and how fast the stress is applied.

Three behaviors matter. Elastic deformation is reversible: release the stress and the rock springs back, like a rubber band. Ductile (or plastic) deformation is permanent but smooth: the rock bends or flows without breaking, like warm tar. Brittle deformation is fracture: the rock cracks and breaks. Cold, shallow rocks tend to fail brittly; hot, deeply buried rocks tend to flow. One layer can fold at depth and fracture higher up, where it is cooler.

When layers are compressed they buckle into folds. An anticline arches upward; a syncline troughs downward. Push two edges of a rug together and the same wrinkles appear. When rock breaks under stress it slips along a fault. Pull a layer apart and one side drops down a normal fault. Push it together and one side rides up and over the other, a reverse fault (a gentle, low-angle one is a thrust). Slide two blocks past each other and you get a strike-slip fault, like the San Andreas.

Pile up enough thrust sheets and you have built a mountain range. The Himalaya, the Alps, and the Appalachians are stacks of folded and thrust-faulted crust, raised by colliding continents. The process of mountain-building is orogeny, the most visible work of structural geology on the planet's surface. The forces behind orogeny come from plate tectonics, the engine described in 27.01.01, which shoves continents together and tears them apart over geologic time.

This unit is about reading the deformations recorded in rock and reconstructing the stress field that produced them. Every folded layer and every fault plane is a frozen record of the forces that shaped a continent. The reward is a window into the deep past, written in geometry that the patient observer can learn to read.

Visual Beginner

Deformation mode	How the rock responds	Reversible?	Typical setting
Elastic	Springs back when stress is released	Yes	Shallow crust under small loads; seismic waves
Ductile (plastic)	Bends or flows, stays deformed	No	Warm, deeply buried rock; folds
Brittle	Cracks and fractures	No	Cold, shallow rock; faults

Structure	Stress regime	Motion on the fault or fold	Example
Anticline	Compression	Layers arch upward	Appalachian ridges
Syncline	Compression	Layers trough downward	Valley-and-ridge belts
Normal fault	Extension (largest stress vertical)	Hanging wall slides down	Basin and Range, USA
Reverse / thrust fault	Compression (smallest stress vertical)	Hanging wall pushed up	Himalayan thrusts
Strike-slip fault	Horizontal shear (middle stress vertical)	Blocks slide past horizontally	San Andreas, California

Worked example Beginner

A limestone bed originally 1000 meters long lies between two harder sandstone layers. Compression shortens the bed by folding until its end points, measured along a straight line, sit only 850 meters apart. How much has it shortened? The fractional shortening is the change in length divided by the original length: $(1000-850)/1000=150/1000=0.15$. The bed has been shortened by 15 percent, a value structural geologists call 15 percent strain.

If the same 1000-meter layer were stretched instead, to a straight-line length of 1200 meters, the extension would be $(1200-1000)/1000=0.20$, or 20 percent extension. The sign tells the story: negative for shortening, positive for stretching. Real mountain belts commonly record 20 to 50 percent shortening across their folded and thrust-faulted width; the Himalaya has absorbed well over 50 percent as India has pushed into Asia.

Now consider Anderson's theory of faulting, the simple but powerful rule that links a fault's type to its stress regime. Near the free surface of the Earth, one of the three principal stresses must be vertical: it is the weight of the overlying rock. Anderson showed that the placement of that vertical stress decides which kind of fault forms.

When the vertical stress is the largest, the crust is being pulled apart and normal faults form, with blocks dropping down. When the vertical stress is the smallest, the crust is being squeezed horizontally and reverse or thrust faults form, with rock pushed up and over. When the vertical stress is the middle value, the crust is being sheared and strike-slip faults form, with blocks sliding horizontally past each other.

Take a typical stress at 5 kilometers depth in continental crust. The vertical stress from the weight of overlying rock (density about 2700 kilograms per cubic meter) is roughly 135 megapascals. In an extensional region like the Basin and Range of the western United States, this vertical load is the largest stress, so normal faults develop. In a compressional region like the Himalayan front, the horizontal push exceeds this vertical load, making the vertical stress the smallest, so thrusts develop.

Check your understanding Beginner

Formal definition Intermediate+

Stress. The state of stress at a point in a continuous body is the symmetric second-rank Cauchy stress tensor $\mathbf{\sigma }=({\sigma }_{ij})$. Its eigenvalues are the principal stresses ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$, ordered largest to smallest (compression taken positive in geologic convention), and its eigenvectors are the principal stress directions. On any internal plane with unit normal $\hat{\mathbf{n}}$, the traction vector is $\mathbf{t}=\mathbf{\sigma }\hat{\mathbf{n}}$, which resolves into a normal component ${\sigma }_n=\mathbf{t}\cdot \hat{\mathbf{n}}$ and a shear component of magnitude $\tau =|\mathbf{t}-{\sigma }_n\hat{\mathbf{n}}|$.

Strain. The infinitesimal strain tensor is ${\epsilon }_{ij}=\frac{1}{2}({\partial }_i{u}_j+{\partial }_j{u}_i)$ for a displacement field $\mathbf{u}$. Its principal values describe shortening (negative) or extension (positive) along three mutually perpendicular axes. Finite strain, which matters once rotations and length changes are large, is described by the Lagrangian strain tensor or by the strain ellipsoid and its principal stretches $S_1\ge S_2\ge S_3$.

Mohr circle. For two-dimensional stress in the $({\sigma }_1,{\sigma }_3)$ plane, the set of all $({\sigma }_n,\tau )$ pairs on planes through the point lies on a circle of center $c=({\sigma }_1+{\sigma }_3)/2$ and radius $R=({\sigma }_1-{\sigma }_3)/2$. A plane whose normal makes angle $\theta$ with ${\sigma }_1$ has coordinates ${\sigma }_n=c+R\cos 2\theta$ and $\tau =-R\sin 2\theta$.

Mohr-Coulomb failure criterion. A brittle rock fails when the shear stress on a plane exceeds the sum of cohesion $C$ and frictional resistance:

$$|\tau |=C+{\sigma }_n\tan \varphi ,$$

where $\varphi$ is the angle of internal friction (typically $25^{\circ }$ to $35^{\circ }$). In the Mohr diagram this is a pair of straight lines tangent to the stress circle at the onset of failure.

Deformation regimes. Elastic deformation obeys $\sigma =E\epsilon$ (Hooke's law, with Young's modulus $E$) and is fully recoverable. Plastic or ductile deformation is permanent and proceeds by crystal-defect motion once a yield stress is exceeded. Brittle deformation is the loss of cohesion by fracture. Whether a rock is brittle or ductile at a given strain rate depends on its composition, temperature, confining pressure, pore-fluid pressure, and grain size.

Folds. A fold is a bend in a layer. The line of maximum curvature is the hinge; the flanks are the limbs; the surface bisecting the hinge angle is the axial plane. An anticline has older rocks in its core and arches upward; a syncline has younger rocks in its core and troughs downward. Folds are classified by interlimb angle (gentle, $180^{\circ }$–$120^{\circ }$; open, $120^{\circ }$–$70^{\circ }$; tight, $70^{\circ }$–$30^{\circ }$; isoclinal, $<30^{\circ }$) and by the attitude of the axial plane (upright, inclined, overturned, recumbent).

Faults. A fault is a fracture along which the two sides have slipped. The block above the fault plane is the hanging wall; the block below is the foot wall. Normal faults have the hanging wall displaced down (extension). Reverse faults have the hanging wall displaced up (compression); a reverse fault dipping below $45^{\circ }$ is a thrust. Strike-slip faults have near-horizontal slip; right-lateral and left-lateral describe the sense of offset seen from either side. The rake measures the slip direction within the fault plane.

Shear zones. A shear zone is a tabular volume of relatively high strain, ranging from a meter to many kilometers across, that accommodates relative motion between two less-deformed wall rocks. Brittle shear zones contain faults and gouge; ductile shear zones contain foliated mylonite produced by crystal-plastic flow.

Orogeny and the Wilson cycle. Orogeny is the set of processes that build mountains: crustal shortening, thickening, metamorphism, magmatism, and uplift. Most orogenies occur at convergent plate boundaries, where the stress fields of 27.01.01 concentrate strain into belts hundreds of kilometers wide. The Wilson cycle describes the opening of an ocean basin by rifting, its growth by seafloor spreading, its destruction by subduction, and its final closure by continental collision, after which the resulting mountain belt may rift again.

Key theorem with proof: Anderson's classification of faulting Intermediate+

Theorem (Anderson, 1905). Assume a homogeneous, isotropic rock mass with a horizontal free surface, in which one principal stress is vertical and the rock fails by the Coulomb criterion with friction angle $\varphi$. Then exactly three end-member fault regimes exist, one for each placement of the vertical stress in the principal triad:

1. ${\sigma }_1$ vertical $\Rightarrow$ normal faulting (extension).
2. ${\sigma }_3$ vertical $\Rightarrow$ reverse / thrust faulting (compression).
3. ${\sigma }_2$ vertical $\Rightarrow$ strike-slip faulting (horizontal shear).

In each case the two conjugate failure planes form at an angle of $45^{\circ }-\varphi /2$ to the ${\sigma }_1$ direction.

Proof. Because the surface is free (no shear traction), one principal stress must act along the vertical; call it ${\sigma }_v$, and call the two horizontal principal stresses ${\sigma }_H$ and ${\sigma }_h$. Failures occur on planes tangent to the Mohr circle, where the Coulomb envelope $|\tau |=C+{\sigma }_n\tan \varphi$ first touches the stress circle. By the geometry of the tangent (derived in the Full proof set), the radius to the tangent point makes an angle $2\theta =90^{\circ }+\varphi$ on the Mohr circle, so the normal to the failure plane lies at $\theta =45^{\circ }+\varphi /2$ from ${\sigma }_1$. The failure plane itself, perpendicular to its normal, sits at $45^{\circ }-\varphi /2$ from ${\sigma }_1$.

Enumerating the three placements of ${\sigma }_v$:

• ${\sigma }_v={\sigma }_1$. Both ${\sigma }_2$ and ${\sigma }_3$ are horizontal. The conjugate planes contain the ${\sigma }_2$ axis and dip at $45^{\circ }+\varphi /2\approx 60^{\circ }$, with the hanging wall sliding down dip. These are normal faults.
• ${\sigma }_v={\sigma }_3$. Both ${\sigma }_1$ and ${\sigma }_2$ are horizontal. The conjugate planes contain the ${\sigma }_2$ axis and dip at $45^{\circ }-\varphi /2\approx 30^{\circ }$, with the hanging wall sliding up dip. These are reverse (low-angle: thrust) faults.
• ${\sigma }_v={\sigma }_2$. ${\sigma }_1$ and ${\sigma }_3$ are horizontal, so their bisector is horizontal and the failure planes are vertical, with horizontal slip. These are strike-slip faults. $■$

This classification is a first-order model: real stress fields rotate, pore pressure shifts the effective stress, and pre-existing weaknesses reroute fractures. Yet as a mapping from three numbers (the principal stresses) to three structures, Anderson's theorem organizes nearly every map-scale fault on Earth.

Bridge. Anderson's classification is the foundational reason that every map-scale fault in the crust can be decoded back into a tectonic stress regime, and it is the central insight on which the rest of structural geology is built; it builds toward the rheology and fold-mechanics results of the Master tier, where the same principal-stress triad governs ductile buckling, and it appears again in the earthquake source mechanics of 27.03.01, where the stress drop on a fault controls seismic moment. Putting these together, this is exactly the reciprocal structure that defines the field: the stress field sets the structure, and the structure records the stress field, so geometry becomes a frozen strain gauge and the bridge is a two-way map between force and form.

Exercises Intermediate+

Lean formalization Intermediate+

There is no Lean formalization for this unit (lean_status: none). Structural geology is a branch of continuum mechanics coupled to empirical rheology, and the objects it manipulates are not present in Mathlib at the level this field requires. A faithful formalization would need, at minimum, the Cauchy stress tensor as a symmetric bilinear form over a continuous medium, the stress-to-trraction map and its eigen-decomposition into principal stresses, the Mohr-circle construction in real projective geometry, and the Coulomb yield function as a cone in $({\sigma }_n,\tau )$-space.

Mathlib today has linear-algebra and tensor-vocabulary machinery, and its differential-geometry layer is maturing, but it has no continuum-mechanics hierarchy, no physical units layer tied to the SI pascal, and no constitutive-law library. Anderson's theorem is essentially a classification of the orbits of $\mathrm{SO}(3)$ acting on a symmetric trace-form together with a yield cone, so it is formalizable in principle once a continuum layer lands; until then the content is reviewed as quantitative prose. The gap is tracked so that a future physics-continuum initiative can pick it up without re-deriving the motivating examples.

Advanced results Master

Rheology: friction, fracture, and flow laws

The crust is not a single material with one strength: it obeys different constitutive laws at different depths. Byerlee's law (Byerlee, 1978) states that frictional sliding on most rock surfaces obeys $\tau =0.85{\sigma }_n$ for normal stresses below 200 MPa and $\tau =50+0.6{\sigma }_n$ (in MPa) above 200 MPa, almost independent of rock type. This empirical law is the foundation of brittle strength estimates through the upper 10 to 15 kilometers of the crust.

Below the brittle-ductile transition, rocks flow by thermally activated creep. Two mechanisms dominate. Dislocation creep (power-law creep) follows $\dot{\epsilon }=A{\sigma }^n\exp (-Q/RT)$ with stress exponent $n\approx 3$ to $5$ and activation energy $Q$, where $R$ is the gas constant and $T$ the absolute temperature. It is grain-size insensitive and dominates the lower crust and mantle. Diffusion creep follows a Newtonian law $\dot{\epsilon }\propto \sigma /d^m$ (grain size $d$, exponent $m\approx 2$ to $3$) and dominates in fine-grained rocks under low stress.

The contrast is decisive: dislocation creep localizes strain (because higher stress means much higher strain rate), while diffusion creep distributes it. Shear zones, which concentrate kilometers of offset into meters of rock, are sites where grain-size reduction has switched the mechanism from dislocation to diffusion creep, weakening the zone and focusing even more strain into it.

Fold mechanics and the Biot-Ramberg dominant wavelength

When a stiff layer of viscosity ${\mu }_1$ is embedded in a softer matrix of viscosity ${\mu }_2$ and shortened parallel to its plane, it buckles into a train of folds. Linear stability analysis of the layer-matrix system yields a dominant wavelength ${\lambda }_d$ that grows fastest and therefore dominates the observed fold spacing:

$${\lambda }_d=2\pi t{\left(\frac{{\mu }_1}{6{\mu }_2}\right)}^{1/3},$$

where $t$ is the layer thickness. This is the Biot-Ramberg result. It explains why thick sandstone beds in a shale matrix form long-wavelength folds while thin beds in the same matrix form short-wavelength folds, and why the ratio of wavelength to thickness is roughly constant within a single formation. For viscosity contrasts of $10^2$ to $10^3$, typical of competent sandstone in weak shale, the predicted wavelength-to-thickness ratio of $10$ to $30$ matches field observations well.

Beyond the initial buckling stage, continued shortening amplifies folds, rotates their axial planes, and ultimately produces isoclinal and sheath folds in high-strain shear zones. The geometry of these end-state folds is a direct record of finite strain and can be inverted to recover the kinematics of the shear zone.

Critical-taper theory of orogenic wedges

A collisional mountain belt behaves like a wedge of sand in front of a bulldozer: it grows until its surface slope, internal strength, and the push from the converging plate are in balance. Critical-taper theory (Davis, Suppe, and Dahlen, 1983) treats an orogenic wedge as a thin, perfectly plastic sheet on a basal detachment, and predicts the surface slope angle $\alpha$ and basal slope $\beta$ at which the wedge is just at the verge of internal failure:

$$\alpha +\beta =\frac{(1-{\lambda }_b)\rho gH\tan {\varphi }_b-C}{\rho gH(1-\lambda )K},$$

where $\rho$ is wedge density, $g$ gravity, $H$ wedge thickness, $\lambda$ and ${\lambda }_b$ the pore-pressure ratios within and at the base of the wedge, ${\varphi }_b$ the basal friction, $C$ the cohesive strength, and $K$ a function of the internal friction angle. Despite its simplifications, critical-taper theory reproduces the observed surface slopes of the Himalaya, Taiwan, and the Andes to within a few degrees, and predicts that a wedge thickens until it reaches its critical taper, then advances forward over its basal detachment.

Shear-zone thermodynamics and strain localization

Ductile shear zones are not just strain concentrators: they are geochemical reactors. As grains are reduced in size and fresh surfaces are exposed, metamorphic reactions are driven out of equilibrium. Fluid influx along the zone hydrates and weakens the rock, lowering its viscosity and further localizing strain in a positive feedback. This coupling between mechanical deformation, chemical reaction, and fluid flow is what allows a shear zone to persist as a plane of weakness for tens of millions of years, reactivating under successive tectonic regimes.

The thermal state of a shear zone is governed by the balance between shear heating (which raises temperature at a rate proportional to $\tau \dot{\gamma }$) and conductive cooling. In fast-slipping faults, shear heating can raise the temperature enough to trigger melting, producing the glassy vein rock called pseudotachylyte, a fossil record of ancient earthquakes.

Synthesis. Putting these together — Anderson's stress triad, Mohr-Coulomb failure, and the temperature-dependent rheology of the lithosphere — yields the central insight that the same three principal stresses explain both a single $30^{\circ }$ thrust ramp and a 3000-kilometer mountain belt. The foundational reason is that rock strength is scale-invariant only within a given deformation mechanism, so this is exactly why orogenic wedges, fold trains, and ductile shear zones all obey a single critical-taper and dominant-wavelength scaling. The framework generalises to other planets, where the absence of water shifts the brittle-ductile transition and suppresses plate tectonics, it builds toward the planetary-tectonics end-members of Venus and Mars, and it appears again in the seismic-cycle models of 27.03.02 pending and the deep-time Wilson-cycle reconstructions of 27.08.01.

Full proof set Master

Proposition (Coulomb failure orientation). Let the state of stress at a point be given by the principal stresses ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$ (compression positive), and let the rock fail by the Coulomb criterion $|\tau |=C+{\sigma }_n\tan \varphi$ with friction angle $\varphi$. Then the two conjugate failure planes make an angle of $45^{\circ }-\varphi /2$ with the ${\sigma }_1$ direction, and the acute dihedral angle between the conjugate planes is $90^{\circ }-\varphi$.

Proof. Represent the stress state in the $({\sigma }_n,\tau )$ Mohr plane as a circle with center $c=({\sigma }_1+{\sigma }_3)/2$ on the ${\sigma }_n$-axis and radius $R=({\sigma }_1-{\sigma }_3)/2$. A plane whose normal makes angle $\theta$ with the ${\sigma }_1$ direction has stress coordinates

$${\sigma }_n(\theta )=c+R\cos 2\theta ,\tau (\theta )=-R\sin 2\theta .$$

The Coulomb envelope is the pair of lines $\tau =\pm (C+{\sigma }_n\tan \varphi )$. Failure begins when one of these lines is tangent to the circle, because at the moment of first contact the circle lies entirely below the envelope and only the tangent point reaches it.

Consider the upper envelope $\tau =C+{\sigma }_n\tan \varphi$, rewritten as ${\sigma }_n\tan \varphi -\tau +C=0$. The perpendicular distance from the center $(c,0)$ to this line is

$$d=\frac{|c\tan \varphi +C|}{\sqrt{{\tan }^2\varphi +1}}=\frac{|c\tan \varphi +C|}{\sec \varphi }=|c\sin \varphi +C\cos \varphi |.$$

At the onset of failure this distance equals the radius: $d=R$. The tangent point is the foot of the perpendicular from the center to the envelope line, so the radius drawn to the tangent point is perpendicular to the envelope. Since the envelope has slope $\tan \varphi$, the radius to the tangent point has slope $-\cot \varphi$, which makes an angle $90^{\circ }+\varphi$ with the positive ${\sigma }_n$-axis.

On the Mohr circle, an angle $2\theta$ at the center corresponds to a physical rotation $\theta$ of the plane normal. Thus $2\theta =90^{\circ }+\varphi$ gives $\theta =45^{\circ }+\varphi /2$ as the angle from ${\sigma }_1$ to the normal of the failure plane. The failure plane itself, being perpendicular to its normal, makes the complementary angle $90^{\circ }-(45^{\circ }+\varphi /2)=45^{\circ }-\varphi /2$ with ${\sigma }_1$.

The second tangent point lies symmetrically below the ${\sigma }_n$-axis, giving the conjugate plane at $-(45^{\circ }-\varphi /2)$ from ${\sigma }_1$. The acute dihedral angle between the two conjugate planes is therefore $2(45^{\circ }-\varphi /2)=90^{\circ }-\varphi$, containing the ${\sigma }_1$ axis, and the obtuse angle $90^{\circ }+\varphi$ contains the ${\sigma }_3$ axis. $■$

For a typical friction angle $\varphi =30^{\circ }$, the failure planes form at $30^{\circ }$ to ${\sigma }_1$, the acute dihedral angle is $60^{\circ }$ about ${\sigma }_1$, and the obtuse angle $120^{\circ }$ lies about ${\sigma }_3$. These are exactly the angles observed in conjugate fault sets in the field and recovered from earthquake focal mechanisms.

Connections Master

• The stress fields analyzed here originate in the plate-boundary forces catalogued in 27.01.01. Ridge push, slab pull, and basal drag set the far-field principal stresses; Anderson's theorem then converts each tectonic setting into its characteristic structures. A divergent margin yields normal faulting, a convergent margin yields thrusting, and a transform margin yields strike-slip motion, all because the boundary type fixes which principal stress is vertical.

• Faults are the structures that store and release elastic strain as earthquakes, so every result of this unit returns in the seismic source theory of 27.03.01 and the wave-propagation mechanics of 27.03.02 pending. The stress drop on a fault during rupture, the slip sense recovered from first-motion focal mechanisms, and the geometry of a seismic gap all map directly onto the Anderson regimes derived above. Earthquake seismology and structural geology are two readings of the same fault plane.

• Deformation rarely acts alone: the same pressure and temperature that fold and fault a rock also drive metamorphism, so the fabrics of 27.02.03 pending (foliation, lineation, porphyroblasts) are coeval with the strain recorded here. A mylonitic foliation in a shear zone is simultaneously a structural-gauge of finite strain and a metamorphic index of peak temperature, allowing the structural and metamorphic histories of an orogen to be read from the same outcrop.

• Orogenic cycles structure the geologic time scale of 27.08.01. Each Wilson-cycle closure deposits an orogenic belt whose folds, thrusts, and shear zones are then eroded into the sediments of the next basin. Supercontinents such as Rodinia and Pangaea assemble through these collisions and disassemble through the rifts that follow, so the structural geology of a single mountain belt is one chapter in a cycle that repeats on a 400-to-600-million-year cadence and is recorded across every continent.

Historical & philosophical context Master

The modern science of rock deformation was assembled in stages, each of which reframed how geologists read a landscape. The earliest quantitative step was Coulomb's 1773 study of the shear strength of soils, which introduced the failure criterion that still bears his name; Mohr's 1882 graphical construction then gave that criterion its circle, turning a list of principal stresses into a visible locus of failure.

The decisive conceptual move for structural geology itself was made by the Irish geophysicist E. M. Anderson. In a 1905 paper and his 1951 book The Dynamics of Faulting, Anderson argued that the existence of a free horizontal surface forces one principal stress to be vertical, so that only three fault regimes are possible ^{[Anderson 1905]}. His classification converted the chaos of mapped faults into three clean categories tied to three stress states, and it remains the first lesson taught in every structural geology course. The move is philosophically interesting because it is a symmetry argument: a boundary condition (the free surface) constrains the form of all admissible solutions, much as a boundary condition fixes the modes of a vibrating string.

The large-scale setting for Anderson's stress fields came from continental drift. Alfred Wegener's 1915 synthesis argued that continents move horizontally across the globe ^{[Wegener 1915]}, supplying the very horizontal stresses that structural geologists measure. Wegener's proposal was rejected for half a century for lack of a mechanism; once seafloor spreading supplied that mechanism in the 1960s, every fold belt and fault province on Earth could at last be located in a single global stress field.

J. Tuzo Wilson then closed the loop on time. In a 1966 paper he asked whether the Atlantic Ocean had closed and re-opened, arguing that the Appalachian mountain belt of North America and the Caledonian belt of Europe were the suture of a Paleozoic ocean that later reopened as the present Atlantic ^{[Wilson 1966]}. This was the seed of the Wilson cycle, the recognition that ocean basins open and close repeatedly, and that mountain belts are the scars of those closures. Structural geology thereby acquired not only a spatial framework (where stress comes from) but a temporal one (when and how often belts form), and the two together transformed the field from the cataloging of structures into the reconstruction of planetary history.

Bibliography Master

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}

@book{anderson1951,
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}

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@article{dahlensuppe1983,
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}