Answer the following in about 150 words each: (c) What geological and tectonic processes lead to the formation of nappes in orogenic belts?

Geological and Tectonic Processes Leading to Nappe Formation

Nappes form primarily in compressional tectonic settings, most notably during continental collision zones or on the overriding plate in active subduction zones. The main processes involved are:

• Plate Collision and Continental Convergence: The primary driver for nappe formation is the collision of tectonic plates, especially continent-continent collision. Unlike oceanic crust, which readily subducts, buoyant continental crust resists subduction. This resistance generates immense compressive stress, causing the crust to buckle, shorten, and thicken.
• Compressional Forces and Crustal Shortening: Intense horizontal compressional forces during orogeny cause existing rock layers to deform significantly. This leads to crustal shortening, where a broad area of crust is compressed into a narrower, thicker belt.
• Thrust Faulting: A critical mechanism is the development of low-angle thrust faults. These are reverse faults where the fault plane dips at a shallow angle (less than 45 degrees). Along these faults, large slices of rock (the hanging wall) are pushed horizontally over other rock masses (the footwall) for many kilometers. These large displaced rock masses are often referred to as thrust sheets, which constitute nappes.
• Recumbent Folding: Accompanying thrust faulting, rocks undergo intense folding. Recumbent folds are characteristic features, where the fold's axial plane is essentially horizontal, and one limb is overturned. Extreme compression can cause these folds to shear along their lower limbs, leading to the detachment and transport of the rock mass, effectively forming fold nappes.
• Ductile and Brittle Deformation:
  • Ductile Deformation: At greater depths within the crust, where temperatures and pressures are high, rocks behave plastically, bending and flowing without fracturing. This ductile behavior facilitates the formation of large recumbent folds.
  • Brittle Deformation: At shallower depths, rocks tend to fracture. Thrust faults are a manifestation of brittle deformation under compression. The combination of brittle faulting and ductile flow is essential for the large-scale displacement characteristic of nappes.
• Décollement Surfaces: Nappes often detach and slide along mechanically weak layers, known as décollement surfaces or detachment planes. These layers, often composed of incompetent sedimentary rocks like shales or evaporites, act as lubricating horizons, reducing frictional resistance and facilitating the movement of vast rock sheets.
• Gravitational Spreading: In some cases, once a mountain belt has thickened significantly due to compression, gravitational forces can also play a role in the outward spreading and emplacement of nappes, particularly along low-angle faults.

The resulting nappe structures often feature complex geometries, including stacked thrust sheets (imbricate thrust stacks), and can expose older rocks over younger ones due to the significant horizontal displacement. Erosional features like klippes (isolated remnants of a nappe) and fensters (windows into the underlying autochthonous rock) further reveal the extent of nappe formation.