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A fan-shaped deposit formed where a sediment-carrying stream spreads and loses transport capacity. Offset or warped fan surfaces can preserve evidence of fault slip or folding.

A fault with reverse slip whose upper tip remains beneath the ground surface. It can deform the overburden through folding, localized fold scarps, and secondary faults.

Horizontal contraction of continental crust, commonly accommodated by folding and thrust faults and contributing to tectonic thickening.

Relative movement parallel to bedding during folding, allowing layers at different radii of curvature to accommodate different arc lengths. For two layer surfaces separated by thickness $h$ through bend angle $\theta$, their arc-length difference is approximately $h\theta$. Bedding-parallel slip can accommodate that mismatch without requiring each layer to stretch by the same amount. Shallow flexural slip can produce surface breaks above a blind reverse fault.

Bending of geological layers during deformation. A buried fault can produce folding without a throughgoing primary rupture at the surface.

A topographic step or steepened belt produced by localized folding. It can form above a blind reverse fault without being the trace of the principal fault plane.

A distinguishable layer of geological material. Deformation of layers of different ages can constrain the history of folding.

Depositional layers laid down while adjacent structures deform. Systematic thickness and dip changes can record progressive folding or fault slip.

Compare the scatter of remanent magnetization directions before and after correcting for folding. Improved agreement after restoring bedding supports acquisition before folding; agreement only in present coordinates supports a younger overprint. Partial unfolding can indicate acquisition during deformation or more complicated histories. The test constrains relative timing, rather than supplying an absolute age.

Choose an ocean–continent convergent margin. The essential relationship is recycling of oceanic lithosphere by subduction, together with production and modification of crust in a volcanic arc.

The incoming tectonic plate consists of thin oceanic crust, chiefly basalt and gabbro, above colder lithospheric mantle. As that oceanic lithosphere approaches the subduction zone, bending produces the subduction trench. Its cold interior can become denser than the surrounding mantle; the resulting slab pull helps sustain subduction. The descending slab carries both oceanic crust and lithospheric mantle into the interior. Thus the recycled unit is an entire portion of lithosphere, rather than just its crust.

At the subduction trench, some incoming sediment is scraped off and stacked by thrust faults into an accretionary prism. Material can also be added beneath the overriding plate by tectonic underplating. A forearc basin collects sediment between the accretionary prism and the volcanic arc. This is an accreting example: other convergent margins lose upper-plate material through tectonic erosion, so an accretionary prism is not universal.

The cold slab undergoes increasing pressure and metamorphism. Conversion of basaltic rocks and gabbro towards eclogite increases their density, while slab dehydration releases water from hydrated minerals. The rising fluid lowers the solidus of hot peridotite in the mantle wedge, causing flux melting. The usual source of arc magma is therefore the fluid-modified mantle wedge; extensive slab melting is not required. Thermal-petrological calculations show that the depth and amount of slab dehydration depend on the incoming slab's thermal structure.^[ref]

Buoyant magma rises through the overriding plate, freezes in igneous intrusions or erupts in the volcanic arc. Fractional crystallization, interaction with existing continental crust and repeated intrusion can produce more evolved arc magmas and add new crust. Some material merely reworks old crust, and simultaneous tectonic erosion can remove it: volcanism alone does not measure net continental growth. Tectonic shortening, folding and thrust faults can thicken and raise the upper plate.

The Wadati-Benioff zone marks earthquakes along and within the cold descending slab. Shallow interface earthquakes reflect unstable slip where the plates are coupled; deeper earthquakes require mechanisms such as dehydration embrittlement and are not simply dry shallow friction operating at every depth. In contrast, the hot mantle wedge deforms mainly by material creep. Together, the subduction trench, accretionary prism, forearc basin, volcanic arc and inclined seismicity connect surface features to the processes recycling and creating crust.

The network partitions deformation into oblique horizontal displacement on the strike-slip fault and more directly north–south crustal shortening on the approximately east–west reverse fault. On the oblique strand, northeastward motion of the southeastern side relative to the northwestern side has a northward component, reducing separation in the direction of ${\sigma }_1$. On the reverse fault, northward and upward motion of the southern hanging wall shortens and thickens the crust.

Sketch north–south opposing compression arrows around the plan view, left-lateral arrows along the oblique trace, and teeth on the southern side of the reverse fault trace. A section normal to the latter shows horizontal shortening converted into tectonic uplift. Where strands meet, folding, distributed strain or subsidiary faulting can maintain displacement compatibility; a strike-slip fault terminating abruptly in undeforming rock would not. This is an interpretation involving transpression, and the map does not by itself give a unique partition of slip rates between the strands.

An overlapping pair of basin-dipping normal fault segments can first accommodate displacement through an unbreached relay ramp. Fault throw tapers toward each tip and transfers between the strands; the intervening block tilts and bends while retaining a continuous ramp-like surface. Draw overlapping parallel traces in plan view and a tilted connecting surface between their different fault throws.

With continued extension, the ramp can be breached by a linking fault. An oblique connector joins the segment tips and takes up a combination of dip-slip and strike-slip displacement; the system becomes a more continuous, zigzag normal fault. Draw this connector diagonally through the overlap, with downthrown arrows on the basin side. Distributed small faults and folding can accompany either arrangement. The central bend around $u\simeq 0.6$ in Attachment B is the candidate transfer zone; the image alone does not uniquely establish the three-dimensional connection at depth.

The $31.7^{\circ }$ result predicts initial failure in a uniform material under the assumed principal-stress orientations. The actual Figure 3 is a present-day NW–SE cross section through a thick, heterogeneous Alpine stack, extending to about 50 km depth. It contains near-horizontal basal stretches, curved ramps, tightly bent contacts, and steep structures in the internal zones around Viso and the Dora Maira complex. It does not show a single undeformed conjugate pair newly formed in horizontal uniform rock.

Several processes explain the departures:

Finally, a section shows apparent dip unless it is perpendicular to fault strike. For an ideal plane, $\tan {\delta }_{\mathrm{app}}=\tan {\delta }_{\mathrm{true}}\sin \alpha$, where $\alpha$ is the angle between the section direction and fault strike. Obliquity can make a plane look shallower, but cannot on its own explain an apparent dip steeper than its true dip. Do not infer that each black curve is a currently active brittle earthquake fault.

A blind reverse fault can transfer displacement into folding above its buried tip. Look for an asymmetric growing anticline, often with a steep forelimb and gentler backlimb, aligned with the inferred fault strike. A fault-propagation fold is one possible geometry; the fold alone does not uniquely establish the fault's depth or shape. Exposed older geological strata may be more steeply tilted than younger deposits, while growth strata thin and onlap toward an uplifting crest or fan in dip as deformation accumulates. Progressive unconformities and warped young deposits help distinguish active folding from an inherited fold.

In the landscape, seek an elongate uplifted ridge, warped or tilted river terraces and alluvial fans, localized fold scarps, incision across the rising structure, and drainage deflection or abandoned channels. Compare landforms of known relative ages: increasing deformation with age records repeated motion. Opposite fold limbs may show contrasting uplift and tilting, while active axial surfaces can concentrate deformation into narrow belts even without the main fault reaching the surface. Combine geological mapping and dating with seismic reflection or evidence from geodesy where available; an erosional ridge alone is not proof of a buried active reverse fault.^[ref]

Reduction of distance between material markers during tectonic deformation. Folding, reverse faulting and distributed strain can accommodate it; the shortening direction alone does not determine all three principal stresses.