Fault (Geology)

Fault behaviour is governed principally by frictional strength and the rigidity of the rocks forming the fault: where shear resistance is high, fault surfaces become locked and localised zones of mechanical resistance develop. These high‑resistance patches, termed asperities, inhibit relative motion and cause elastic strain to accumulate in the surrounding lithology until the applied stress exceeds the local strength. Failure of an asperity releases that stored elastic energy rapidly, part of which is emitted as seismic waves, and manifests as sudden displacement along the fault (an earthquake). The rheological contrast within the lithosphere controls how strain is stored and discharged: ductile domains (notably the lower crust and upper mantle) accommodate deformation by progressive flow or steady shearing, whereas the brittle upper crust fractures, converting accumulated elastic strain into instantaneous fault slip and seismic radiation. This brittle–ductile transition therefore produces strong depth dependence in earthquake behaviour. Nevertheless, ductile materials are not immune to abrupt rupture—if loading rates are sufficiently high they can fail faster than they can flow, producing sudden slip and seismic energy release even at greater depths.

Slip is the relative displacement between geological features on opposite sides of a fault and is most usefully described as a single vector that records both the magnitude and direction of that displacement. The sense of slip denotes the directional character of that vector—whether one side of the fault has moved up or down, left or right, or in an oblique sense—and thereby provides a qualitative description of fault kinematics.

This slip vector can be resolved into two orthogonal components: throw and heave. Throw is the vertical component of separation and heave is the horizontal component; the mnemonic “Throw up and heave out” summarizes this vertical/horizontal decomposition. Quantitative determination of these components requires locating matching markers (piercing points) that can be traced across the fault so their relative offset in the vertical and horizontal directions can be measured.

In many field situations continuous piercing points are absent or obscured, so practitioners often infer slip direction from structural indicators rather than obtain precise component magnitudes. Drag folds—localized folds adjacent to a fault produced by frictional resistance to slip—are a common qualitative indicator: the orientation and asymmetry of drag folding adjacent to the fault can reveal the sense of slip and give an impression of relative magnitude. Consequently, field studies frequently establish slip direction with confidence while reporting throw and heave as approximate values unless robust piercing points or other continuous markers enable precise measurement.

In non‑vertical faults the blocks on either side of the inclined fault plane are distinguished by relative position: the hanging wall overlies the plane while the footwall lies beneath it. These labels originate in mining practice, where miners stood on the block underfoot (the footwall) with the overlying block described as the hanging wall, a convention adopted in structural geology. Hanging‑wall and footwall kinematics are diagnostic for dip‑slip faults: upward displacement of the hanging wall indicates reverse faulting, whereas downward displacement indicates normal faulting, and identifying which block has moved is therefore central to determining slip sense. Because hanging‑wall motion reflects the prevailing stress regime, uplift of the hanging wall is associated with compressional tectonics and subsidence with extensional settings, so observations of relative block movement are routinely used to infer regional forces. Practically, concentration of stresses in the hanging wall has engineering and hazard significance—intense hanging‑wall loading can trigger failures such as rock bursts, a hazard documented at Frood Mine.

Faults are classified chiefly by two geometric and kinematic properties: the dip (the inclination of the fault plane relative to horizontal) and the slip vector (the direction of relative movement of the blocks on either side). These attributes together determine the fault’s spatial orientation, the pattern of its surface trace, and its likely seismic behavior.

In strike‑slip faults displacement is dominantly horizontal and parallel to the fault trace; surface expression commonly includes lateral offsets of linear features such as roads, streams or fences. The sense of horizontal movement is described as right‑lateral (dextral) or left‑lateral (sinistral) according to the apparent displacement of the opposite block as viewed from either side.

Dip‑slip faults are dominated by vertical motion along the dip of the fault plane. In extensional settings normal faults produce relative downward displacement of the hanging wall with respect to the footwall, whereas in compressional regimes reverse and thrust faults produce hanging‑wall uplift. The fault dip magnitude influences classification: steeply inclined faults are described as normal or reverse, whereas low‑angle surfaces with hanging‑wall thrusting are termed thrust faults.

Oblique‑slip faults contain both strike‑parallel and dip‑parallel components, so movement has simultaneous horizontal and vertical vectors. Identification requires documenting both along‑strike offsets and across‑strike changes in elevation or stratigraphy, and such faults indicate tectonic environments in which shear and either extension or compression operate together.

Strike-slip faults

Strike-slip faults are characterized by steep to near-vertical fault planes along which displacement is dominantly horizontal. Because the motion is lateral rather than vertical, these faults produce little overall vertical offset; alternative names in the literature include wrench, tear and transcurrent faults.

Kinematically, blocks on either side of a strike-slip fault move past one another in a strike-parallel sense. Observers describe this motion as left‑lateral (sinistral) or right‑lateral (dextral) according to the apparent direction in which the opposite side of the fault has shifted. Conventional footwall/hanging‑wall terminology still applies, but the primary relative movement of those blocks is side‑to‑side rather than up‑or‑down.

When a strike‑slip fault coincides with a plate boundary it is termed a transform fault. Transforms commonly link segments of divergent boundaries, accommodating offsets between spreading centres (for example between adjacent segments of a mid‑ocean ridge). Although most prominent in oceanic lithosphere as ridge offsets, transform systems also develop within continents; notable continental examples are the Dead Sea Transform and New Zealand’s Alpine Fault.

From a plate‑tectonic perspective transform faults are conservative boundaries: they transfer lateral motion between plates without the net creation or destruction of lithosphere, in contrast to divergent and convergent boundaries.

Dip‑slip faults

Dip‑slip faults are fractures along which the principal relative displacement of rock masses occurs parallel to the dip of the fault plane. They are conventionally classified into two end‑member senses of motion: normal (extensional) faults, in which the hanging wall moves down relative to the footwall, and reverse (compressional) faults, in which the hanging wall moves up relative to the footwall. This distinction reflects the prevailing tectonic stress: extension produces normal faulting, whereas compression produces reverse faulting.

Normal faults commonly form during crustal stretching and tend to display relatively steep fault planes; reverse faults accommodate crustal shortening and are frequently associated with low‑angle thrusts or local folding adjacent to the fault. Cross‑sectional illustrations made perpendicular to the fault plane are the standard way to depict these geometries, since such sections show both the dip of the fault and the up‑or‑down sense of block movement.

The terms “normal” and “reverse” derive from British coal‑mining usage, in which the downward‑moving hanging wall of extensional faults was the most familiar configuration. Importantly, faults are not immutable: changes in the regional stress field through geological time can reactivate a preexisting fault with the opposite sense of slip. Fault inversion—reactivation that reverses the original displacement sense—complicates structural interpretation and must be considered when reconstructing deformation histories and accounting for stratigraphic offsets.

Normal faults are dip‑slip fractures in which the block above the inclined fault plane (the hanging wall) moves downwards relative to the block beneath it (the footwall), producing measurable vertical offset between the two sides of the fault. This sense of displacement reflects crustal extension: as the hanging wall subsides, the footwall stands relatively higher.

The inclination of the fault plane (the dip, measured from the horizontal) strongly governs fault geometry and surface expression. Most normal faults are steep, with dips commonly around or exceeding 60°, though a subset are much shallower (<45°). In cross‑sectional representation the hanging wall lies above the plane and is shown with a downward sense‑of‑slip arrow relative to the underlying footwall, which is left in the relatively uplifted position after movement.

Kinematically, normal faulting accommodates horizontal stretching of the crust while producing vertical relief and block rotation. Steeper and shallower dips produce different deformation styles: dip angle affects the development of fault‑bend folds, the magnitude and visibility of surface offsets, and the geometry of linked extensional features such as tilted fault blocks and grabens.

Basin-and-range topography

In extensional crustal regimes, normal faults commonly bound discrete crustal blocks whose relative vertical motion produces the alternating basin-and-range landscape. A graben is formed when two normal faults dip toward one another so that the central block is downthrown; the inward-dipping fault geometry places the hanging walls on either side in relative subsidence, producing a linear, fault-bounded valley that commonly fills with sediment. By contrast, a horst is a block that remains relatively uplifted or stranded between adjacent faults that dip away from it; the outward-dipping faults isolate the horst as a linear ridge or mountain range elevated above neighboring basins.

The kinematics of these systems are controlled by fault orientation and dip: extensional stresses produce normal faulting and vertical displacement (throw) along fault planes, and whether a block becomes a graben or horst depends predictably on whether bounding faults dip toward or away from the block. At the surface this structural pattern yields arrays of parallel basins and ranges; grabens are expressed as sediment-filled valleys with steep fault scarps at their margins, while horsts form uplifted ridges. The cumulative throw and spatial arrangement of bounding faults govern basin depth, range elevation, and the linearity and symmetry of topography in extensional provinces.

Listric faults are a form of normal fault distinguished by a concave-upward fault surface that is steep near the Earth’s surface and progressively shallows with depth; kinematically they accommodate relative downward displacement of the hanging wall with respect to the footwall. Such geometry commonly arises when an originally planar normal fault curves into the subsurface, producing the characteristic change in dip from steep at shallow levels to near‑horizontal at depth.

In coastal or cliff settings where a hanging wall is missing or has been removed (for example by erosion or collapse), the exposed footwall may slump or collapse toward the free face; this mass movement promotes development of concave-upward fault surfaces as the footwall blocks rotate and translate. Rather than a single continuous plane, accommodation of large slump masses is often distributed across multiple listric surfaces: cross sections of these settings show stacked or en echelon concave-upward faults with displaced slump blocks stepping down the cliff.

For interpretive cross-sections, a single listric fault should be drawn with a steep near-surface segment that curves smoothly to a shallow dip at depth, illustrating the change in dip and the hanging‑wall downthrow. Multiple-listric cliff models should depict several concave-upward fault planes, the absence of a coherent hanging‑wall block at the cliff edge, and footwall slump blocks translated down the concave fault surfaces, thereby demonstrating how curvature and segmentation together accommodate progressive mass movement.

Listric faults are characterized by a curved fault surface that steeply dips near the surface but flattens with depth, commonly merging into a subhorizontal shear horizon or decollement. In extensional settings such decollements may widen and evolve into regional-scale detachment faults—low-angle normal faults that transfer and accommodate large amounts of horizontal extension by allowing slip to propagate dominantly along the near-horizontal shear horizon.

Kinematically, slip on a listric surface converts part of the hanging wall’s vertical displacement into horizontal extension along the decollement. Because the hanging wall and footwall follow different curvatures and relative motions, a geometric deficit or gap develops between them. How that gap is closed depends on the mechanical behavior of the hanging-wall rocks. Ductile or plastically deforming sequences tend to bend and flow into the space, producing continuous, convex-upward folds adjacent to the listric surface (rollover folds) that record progressive down-dip translation of strata. Conversely, more brittle hanging walls accommodate extension by fracturing: secondary, subparallel normal faults cut the hanging wall, producing stepped, overlapping blocks that rotate and stack in imbrication or domino-style arrays to progressively fill the extensional void.

Schematic cross-sections of listric–decollement systems therefore commonly portray either a curved fault with an associated rollover fold in the hanging wall or a suite of stepped faults and rotated blocks illustrating imbrication fans. These end-member responses—flexural rollover versus fault-bounded fragmentation—reflect the interplay of fault geometry, slip partitioning into the decollement, and rock rheology during the development of detachment systems.

Reverse faults

A reverse fault is a type of dip‑slip fault in which the hanging wall has been displaced upward relative to the footwall, producing the opposite sense of movement to a normal fault and commonly exposing formerly lower‑lying strata on the hanging‑wall block. This style of faulting records crustal shortening under a compressive stress regime: horizontal shortening is accommodated by vertical uplift of one block over another, produced where the maximum principal stress is approximately horizontal and the minimum principal stress approximately vertical.

Tectonically, reverse faults are characteristic of convergent plate margins, continental collision zones and fold‑and‑thrust belts, where regional shortening localizes strain into discrete reverse and thrust planes that transport rock masses toward the collision front. The low‑angle end of the spectrum—thrust faults, conventionally those with dips of roughly 45° or less—commonly develop as imbricate stacks, transport sheets and nappes that are fundamental to orogenic construction and crustal thickening.

At the surface and near surface, reverse faulting produces uplifted fault scarps, hanging‑wall topographic highs, folded sedimentary sequences and offset stratigraphic markers; when reverse faults do not break the surface (blind thrusts) they generate broad warping and folding without a clear scarp. Seismically, reverse and thrust faults are major sources of earthquakes: focal mechanisms with compressional quadrants indicate their activity, and rupture on these faults can produce strong vertical and horizontal ground motions, surface rupture and uplift that significantly reshape landscapes and drive mountain building.

Thrust faults

A thrust fault is a low-angle reverse fault in which the hanging wall is displaced upward relative to the footwall along a plane that dips typically less than 45°. Thrusting accommodates crustal shortening by transporting older stratigraphic units over younger ones along shallowly inclined slip surfaces, producing stratigraphic repetition and shortening of the section.

Thrust-fault geometry is commonly organized into alternating flats and ramps. Flats are segments that propagate subparallel to bedding within mechanically weak horizons, whereas ramps cut upward through the stratigraphic column where the fault climbs from one horizon to another. Displacement therefore accumulates by stepping along a succession of flats and ramps rather than by slip on a single continuous plane.

The change in dip between flats and ramps imposes bending of the hanging wall and generates fault-bend folds: as the hanging wall transits a footwall ramp onto a hanging-wall flat it is flexed, creating fold geometries that control the local style of folding and the distribution of repeated beds. Repeated sequences and fold shapes are thus predictable consequences of the flat–ramp geometry and the kinematics of slip.

At orogenic scales, repeated thrusting produces large tectonic assemblages such as nappes—extensive thrust sheets transported many kilometers over underlying units—and klippen, which are isolated erosional remnants of such sheets. On the largest scale, the plate interface in subduction zones is a megathrust: a broad, shallowly dipping thrust surface that can host the greatest earthquakes because of the enormous fault area and slip potential of interplate thrusting.

An oblique-slip fault is one on which net displacement combines substantial components of both dip-slip (vertical movement parallel to the fault dip) and strike-slip (horizontal movement parallel to the fault strike). Classification as oblique-slip implies that neither component is negligible; both dip- and strike-parallel displacements must be measurable and contribute materially to the total offset. Kinematically, the net slip vector lies oblique to the fault plane and can be resolved into dip-parallel and strike-parallel vectors. The dip-slip component may be normal or reverse/thrust, while the strike-slip component may be left- or right-lateral, so any combination of these end-member motions may occur on a single oblique-slip surface.

Oblique-slip faulting is widespread because most natural faults show some degree of mixed motion; nevertheless the term is applied only where the obliquity is significant and quantifiable. Such faults commonly form in transtensional and transpressional tectonic settings, and whenever the orientation of principal shortening or extension evolves during deformation while older faults remain active—producing simultaneous vertical and horizontal displacement on pre-existing structures.

Accurate geometric and kinematic characterization requires measuring both the strike and dip of the fault and quantifying the magnitudes and directions of displacement so that the dip- and strike-parallel components can be separated. Failure to account for obliquity impairs slip-partitioning analyses and leads to incomplete reconstructions of fault history and regional deformation. In geometric description, the hade angle is often used to express the inclination of a fault relative to vertical: hade = 90° − dip. The hade is the angle between the fault plane and a vertical plane striking parallel to the fault and quantifies how far the fault plane tilts away from vertical along strike.

Ring faults

Ring faults are circular to arcuate systems of normal (extensional, dip‑slip) faults that develop around a subsiding central block. They comprise multiple overlapping fault segments whose combined traces define a ring or concentric pattern. Such systems form where a major collapse or subsidence produces radial and concentric stress fields—most commonly in volcanic calderas after magma‑chamber evacuation and at large bolide‑impact structures—so that repeated normal faulting accommodates downward displacement of the roof or crustal block.

Mechanically, ring faults are the expression of extensional accommodation during collapse: as the central roof or weakened crust sinks, individual normal faults nucleate, propagate and rotate, yielding steeply dipping faults that encircle the subsiding area. The resulting subsurface geometry typically consists of stepped fault blocks and intervening grabens, and the surface expression includes circular depressions or scarps and concentric fault traces evident in outcrop or geophysical data.

The fracture network produced by ring faults also controls magmatic and hydrothermal behavior. Openings along the ring fractures commonly act as conduits for magma, producing ring dikes—tabular, often steeply dipping intrusions that mirror the fault geometry and may rim a caldera or crater. Despite differing triggers, endogenic caldera collapse and exogenic impact collapse produce comparable concentric normal‑fault patterns; the Chesapeake Bay impact structure provides a documented example of ring‑fault‑related architecture at an impact site.

Recognizing ring faults is important for reconstructing collapse histories, mapping volcanic plumbing and intrusive relationships, and interpreting subsurface structure in impact basins. Their influence extends to surface topography, patterns of sedimentary infill, groundwater flow, and localization of mineralization, making them a key structural element in both volcanic and impact geology.

Within fault zones a dominant master fault is commonly accompanied by numerous smaller subsidiary fractures that together constitute the zone’s internal architecture. Subsidiary faults are classified as synthetic or antithetic according to their dip sense relative to the master fault. Synthetic faults dip in the same direction as the adjoining master fault and typically form as part of the same displacement system within adjacent hanging‑wall or footwall blocks. Antithetic faults dip opposite to the master fault; their opposing orientation produces complementary geometries that can bound blocks or segments produced by differential movement between fault strands.

The spatial arrangement of synthetic and antithetic faults plays a key role in producing and partitioning hanging‑wall deformation. In particular, arrays of these subsidiary faults control the development and segmentation of rollover anticlines—convex‑upward folds in the hanging‑wall that arise from differential subsidence and rotation above a slipping master (often a growth) fault. Sedimentary basins such as the Niger Delta exemplify this style: a principal growth fault with attendant synthetic and antithetic minor faults generates pronounced rollover anticlines and thereby exerts first‑order control on basin architecture and fold patterning.

Fault rock

Fault zones are volumetrically measurable shear zones composed of highly deformed rock whose textures and mineralogy record the depth of deformation, the nature of the protoliths involved, and the presence and chemistry of mineralizing fluids. These controlling factors combine to produce a spectrum of fault‑rock types and fabrics; in outcrop a single planar surface can juxtapose contrasting lithologies (for example, a salmon‑coloured gouge separating dark‑gray and light‑gray rocks, as observed in the Gobi) and comparable inactive structures occur on regional scales (e.g. along routes between Sudbury and Sault Ste. Marie).

Classification of fault rocks rests primarily on texture and on the deformation mechanisms inferred from that texture. A single throughgoing fault that transgresses multiple crustal levels commonly displays several fault‑rock types along strike and down dip, and prolonged dip‑slip displacement tends to bring together material derived from different depths with varying degrees of overprinting—an effect pronounced in large detachments and major thrust systems.

Brittle comminution produces cataclasites: generally angular fragments set in a finer matrix of similar composition; cataclasites may be cohesive with weak or absent planar fabric or may be incohesive. When visible clasts exceed ~30% of the rock, the cataclasite is termed a tectonic (fault) breccia, indicating intense fragmentation and accumulation of discrete clasts. Fault gouge represents the extreme of grain‑size reduction: an incohesive, clay‑rich, very fine‑grained cataclasite that commonly shows a planar fabric but contains <30% visible clasts; when clay layers from sedimentary sequences are sheared into the fault, they form continuous or discontinuous clay smears that can act as seals along the fault surface.

At higher crustal levels and higher temperatures/strain rates, deformation becomes dominantly ductile and produces mylonites. Mylonites are cohesive, fine‑grained rocks with a pronounced planar fabric created by dynamic recrystallization and crystal‑plastic processes; they often preserve rounded porphyroclasts and fragments whose compositions match the recrystallized matrix. By contrast, pseudotachylytes are ultrafine‑grained, glassy‑appearing veins, injection fills or matrices in breccias formed by frictional melting during seismic slip. Because they require high slip rates to form, pseudotachylytes are valuable palaeoseismic indicators even on faults that are currently inactive.

Impacts on structures and people

Faults represent material discontinuities within soil and rock that materially change strength and deformation behavior of ground masses; for geotechnical design they therefore require explicit consideration in siting and engineering of tunnels, foundations, slopes and other earthworks. The degree to which a fault is currently active is a primary control on risk management: assessments of activity inform where to place buildings, storage tanks and buried pipelines and feed directly into estimates of seismic shaking and secondary hazards such as tsunamis that threaten people and infrastructure.

Regulatory frameworks reflect these concerns. For example, in California land‑use rules use activity–age criteria to restrict new construction directly on or adjacent to faults that have moved during the Holocene (≈11,700 years), and faults with evidence of displacement extending back into the Pleistocene (≈2.6 million years) are given heightened scrutiny when proposals involve critical facilities (power plants, dams, hospitals, schools).

Determination of fault age and activity combines surface, near‑surface and subsurface evidence. Field trenching and geomorphic analysis (landform patterns, offsets visible in aerial imagery) establish relative timing and map fault traces relevant to engineering. In the subsurface, older faulted horizons commonly show shear zones associated with secondary alteration—carbonate nodules, iron‑oxide staining and eroded clay textures—whereas their absence is consistent with relatively young, unweathered faulted deposits.

Absolute age control principally derives from radiocarbon dating of organic material stratigraphically associated with or overlying fault shears; calibrated 14C ages allow bounding of the most recent surface‑rupturing events. When stratigraphy, radiocarbon dates, geomorphic offsets and subsurface alteration are integrated, paleoseismologists can reconstruct the sizes and timing of past earthquakes over centennial to millennial timescales and produce first‑order projections of future fault behavior—information that is essential for long‑term infrastructure planning and public‑safety decision‑making.

Fault zones commonly localize ore because the extensive fracturing they produce enhances permeability, thereby facilitating two principal mineralizing pathways: upward transport of magma from deep crustal or mantle sources and the circulation of mineral-bearing hydrothermal fluids. Near-vertical faults and their intersections are particularly effective conduits, providing focused pathways for magmas and fluids and thus favoring the emplacement of large porphyry copper systems.

Regional examples in Chile illustrate this structural control. In northern Chile the Domeyko Fault system hosts a cluster of major porphyry copper deposits — including Chuquicamata, Collahuasi, El Abra, El Salvador, La Escondida and Potrerillos — indicating a district-scale linkage between fault architecture and copper endowment. Further south, Los Bronces and El Teniente occupy sites at the intersection of two distinct fault domains, demonstrating that intersections of multiple fault systems (not merely single fault traces) can concentrate large porphyry deposits.

Not all faults act as simple vertical conduits. Deep, misoriented faults can function as structural traps where ascending magmas stall, accumulate and evolve chemically during prolonged residence. Magmas that differentiate within these stagnation zones may later be released in relatively short, high-energy pulses that carry evolved melts and associated hydrothermal fluids upward, producing shallow-level porphyry mineralization.

A useful conceptual model therefore integrates two complementary structural roles in porphyry formation: (1) permeable, near-vertical faults and their intersections that directly channel magmas and hydrothermal fluids to shallow emplacement sites, and (2) deep misoriented fault traps that promote magma stagnation, differentiation and episodic breaching. Both mechanisms are critical for exploration and for understanding the distribution and genesis of porphyry copper deposits in settings such as Chile.

Fault zones constitute mechanically weakened, fractured volumes of the crust that increase rock surface area and connectivity to infiltrating water, thereby enhancing fluid–rock interactions. Infiltration into these fractured zones accelerates chemical weathering—mineral dissolution and alteration—progressively enlarging the surrounding weathered horizon and generating secondary porosity within the host rock. This development of additional void space gives faulted rock the capacity to store appreciable amounts of groundwater, allowing faults to behave as productive aquifers even where the intact lithology is otherwise impermeable. At the same time, the concentration of fractures and discontinuities along faults creates preferential flow pathways with higher permeability and connectivity than the surrounding country rock, promoting focused and more efficient subsurface flow. Consequently, fault zones perform a coupled hydrogeological function: they increase subsurface storage while simultaneously enhancing transmissivity, with important consequences for recharge and discharge dynamics, contaminant migration, and groundwater resource assessment and management.

The gallery presents fault features across scales, from millimetre‑scale microfaults to outcrop‑scale normal faults and associated fault rocks, illustrating how geometry and microstructure record displacement and kinematic sense. A photographed microfault, shown with an 18 mm (0.71 in) diameter scale object and a visible piercing point, demonstrates that measurable offset can occur on centimetre‑to‑millimetre structural elements; such piercing points provide precise markers for quantifying slip and determining motion direction on microstructural faults.

Outcrop examples of normal faulting emphasize classic dip‑slip kinematics and their tectonic implications. A steep, leftward‑dipping fault plane exposed in Morocco shows the block on the left displaced downward relative to the right, identifying the left block as the hanging wall and the right as the footwall and indicating extensional deformation during slip. A normal fault in the La Herradura Formation (Morro Solar, Peru) records a comparable sense of motion, with slip directed from the top left toward the bottom right of the exposure, so that the hanging wall moved downslope along the fault plane. Such geometries—steep dips, hanging‑wall subsidence and dominantly dip‑slip movement—are characteristic of brittle failure under extension, with the fault plane marking the surface of relative displacement.