Orogenic Earth Movements

2 years ago

Atiqur Rahman

9 minutes

Orogenic Movements are the ones which work horizontally to the surface of the earth like a tangent to the surface of the earth; therefore, they are also called tangential forces. These forces work in two ways 1) in the opposite direction, which means tensional force or divergent forces and 2) towards each other, which means, compressional forces or convergent forces.

Such tectonic geomorphology is associated with the construction of the landscapes. The rate of the crustal movement can be measured in millimetres per year (1 mm/yr = 1 m/1000 yr = 1 km/million yr). The results of these strain and stress of the rocks have produced great folded ranges of mountains, and therefore it is called ‘oros’, which in Greek means mountains.

There are many regions of the earth’s surface where the vertical crustal movements have persisted throughout the late Cenozoic time. The rate of cumulative uplift has outpaced erosional lowering. Such tectonic geomorphology is associated with the construction of the landscapes. There are regions of erosional landscapes where the tectonic origins are still obvious. The constructional landscape of such a region is called tectonic geomorphology.

Orogeny refers to mountain formation. Unfortunately, the term has assumed much more complex and contradictory implications in geologic usage. Gilbert (1890) formalized the term: “Displacements of the earth’s crust which produce mountain ridges are called orogenic.”

Compressional Forces

Compressional Forces Crustal Bending:

When due to horizontal movement, which is working towards each other crustal rocks are forced to bend, then it is called crustal bending.

It can be of two kinds:

(i) Warping: when the large area of the earth’s crust is affected by crustal bending. When the crust is bending upward, it is called up warping, and when the crustal part is bent downwards like a basin or depression, then it is called down warping.

(ii) Folding: When the crust is layered and bedded with sedimentary or igneous rocks and already has a preexisted rock structure. In such cases, a slight fold in any such structures would result in a wave-like structure. The up-folded rock strata are called anticline, and the down-folded strata are called syncline. Fig. Two sides of the fold are called limbs.

The axis of the syncline and anticline is called the axis of syncline and anticline, and the area in-between them is called the axial plain. Fig.

Different kind of folds depends upon different factors like the nature of the rocks involved (elasticity and rigidity) and the intensity and duration of the compressive forces (magnitude). Differences in elasticity and magnitude lead to differences in the inclinations of the limbs.

Symmetrical folds: both limbs inclined uniformly
Asymmetrical folds: both limbs inclined at different angles.
Monocline folds: one limb inclined moderately and one steeply inclined.
Isocline folds: both limbs become parallel to each other due to immense force but are not horizontal.
Overturned fold: limbs folded beyond vertical and turn so much that both the limbs bend in the same direction.
Recumbent fold: The compressive force is so strong that both the limbs bed and become parallel to each other.
Nappes: When the pressure of the compressive force is continuous, and as a result, the root of one of the limbs is uprooted and thrust on the opposite limb.

Tensional Force

Faults are formed due to fractures in the crustal block and their displacement.

Fault Plain is the plane along which the vertical or horizontal displacement takes place. Upthrown sides and downthrown sides are the two sides representing the upper block and the lower block, respectively. The angle between the fault plain and the horizontal plain is called fault dip. Fig.

The direction of the displacement of the crustal blocks defines the type of fault, which can be a dip-slip fault or strike-slip fault.

Normal Fault: when the tensional force pulls apart the crustal block in the opposite direction. Down dropped block is graben or rift, upthrust block is a horst.
Reverse Fault: when the movement of the fractured block is towards each other due to compressional forces, hence also called a compressional fault. Fig.

Lateral Fault: also called strike-slip fault, is when the displacement is horizontal. Fig.

Step Fault: a series of faults with their slopes in one direction.

Mountains and Orogeny

The process of mountain building runs in a cyclic motion, which reoccurs periodically throughout the geological time scale. Between two periods, there can be long periods where the earth’s crust remains relatively stable.

All orogenic belts are on or near converging plate margins. As in central Asia, plate boundaries can be wide if embedded within the continents (Gordon and Stein, 1992, p. 334). Mountain ranges embedded within continents, such as the Himalayas Mountains and the Ural Mountains of Asia, have been shown to have been near plate margins at their time of formation. The new continental terranes are accreted to them later.

In Orogenic movements, vertical movements don’t need to be more rapid than broad, regional movements. However, they seem to be more continuous through time. It is that not all orogenic mountain belts are randomly placed but are in some distinctive patterns over the earth’s surface. Even though there is no direct relationship between the earthquake zone area and the orogenic belt, we find the circum-Pacific belt associated with active volcanoes along subduction zones.

Three distinct types of orogenic belts can be identified on converging plate margins:

Island Arcs and Trenches:

Along thousands of kilometres of western Pacific margins along Indonesia, subduction zones exist between two portions of oceanic plates or lithosphere in the Caribbean Sea. The plate, which is drowning, flexes upward in a broad outer swell seaward of the trench, into which it descends beneath the overriding plate.

When the plate submerges, it rubs with the inner trench wall and scraps the accumulated sediments of deformed mudstone and sandstone. Many times such deformed sedimentary rocks rise above sea level to form a frontal arc of mountainous islands. Volcanic arcs of andesitic composite cones can follow island arcs. The upper plate spreads in response to the buoyancy of the subducted plate. Fig.

Cordilleran-Type Mountain Ranges:

On the cordilleran type of converging plate margin (like the Andes Mountains of South America), complicated mountain ranges evolve, where the oceanic lithosphere subducts under thicker continental lithosphere. Narrow coastal ranges may be formed on the continent’s edge due to the accretionary prism of trench sediments or to faulting and uplifting continental crustal blocks along with an elongate longitudinal valley parallel to the coast.

With older sedimentary and metamorphic rocks as the axial part of the belt, the volcanic peaks on a cordilleran type of mountain belt rise above a broad plateau. Due to heavy compression, the central mountain range becomes thick and continental crust makes it rise because of isostatic adjustment. The continental platform can later be folded and thrust toward the continental interior.

In the northwestern United States, the Olympic Mountains are part of the coastal ranges, the Puget Sound Lowland and Willamette Valley are part of the longitudinal valley, and the Cascade Range is the volcanic arc. Fig.

Collisional Mountain Belts:

Due to convergence and subduction, an island arc or another plate of continental lithosphere comes into contact with the overlying plate of the subduction zone and creates collisional mountain belts. Due to their low density, the island arcs and continental rocks, instead of going into the mantle, get thrust over each other, forming high mountain ranges. High mountain ranges, such as the Himalayas or Alps, with intensely deformed rocks of contrasting lithology, are good examples. Fig.

Extensional Mountains:

A fourth type of mountain is built by extensional and strike-slip plate motions in various tectonic settings. Rift Valley formation is relatively localized and is considered a special orogeny style. Continental breakups are characterized by an elongated swell or bulge along which the faulting generates, creating rift valleys. Strike-slip faults are the cause of some of the most complex tectonic mountain ranges. The Transverse Ranges of southern California have a similar origin in their relation to the San Andreas Fault.

Orogenic Earth Movements