Fold & Fault in Geology, Fold Mountains and Block Mountains

Fold Mountains

• Fold mountains are formed when sedimentary rock strata in geosynclines are subjected to compressive forces.
• They are the loftiest mountains, and they are generally concentrated along continental margins.

Geosyncline: a large-scale depression in the earth’s crust containing very thick deposits. E.g. Tethys geosyncline.

‘Fold’ in geology

• A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers).
• The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards.
• In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.

• Limbs: The limbs are the flanks of the fold.
• Hinge line: the where the flanks join together (the line of maximum curvature).
• Axial plane: plane defined by connecting all the hinge lines of stacked folding surfaces (the plane in which hinge lines of various strata lie).

Types of folds

• A symmetrical fold is one in which the axial plane is vertical.
• An asymmetrical fold is one in which the axial plane is inclined.
• An isoclinal fold has limbs that are essentially parallel to each other and thus approximately parallel to the axial plane.
• An overturned fold has a highly inclined axial plane such that the strata on one limb are overturned.
• A recumbent fold has an essentially horizontal axial plane.

Classification of fold mountains

On the basis of period of origin

• On the basis of the period of origin, fold mountains are divided into very old fold mountains, old fold mountains and Alpine fold mountains.

Very Old Fold Mountains

• They are more than 500 million years old.
• They have rounded features (due to denudation).
• They are of low elevation.
• Some of the examples are Laurentian mountains, Algoman mountains, etc.

Old Fold Mountains

• Old fold mountains had their origin before the Tertiary period (tertiary period started 66 million years ago).
• The fold mountain systems belonging to Caledonian and Hercynian mountain-building periods fall in this category.
• The Appalachians in North America and the Ural Mountains in Russia are the examples.
• They are also called thickening relict fold mountains because of lightly rounded features and medium elevation.
• Top layers are worn out due to erosional activity. Example: Aravalli Range in India.
• The Aravalli Range in India is the oldest fold mountain systems in India.
• The range rose in post-Precambrian event called the Aravalli-Delhi orogeny.

Alpine or young fold mountains

• Alpine fold mountains belonging to the Tertiary period (66 million years ago to present) can be grouped under the new fold mountains category since they originated in the Tertiary period.
• Examples are the Rockies, the Andes, the Alps, the Himalayas, etc.

Characteristics

• Rugged relief.
• Imposing height (lofty).
• High conical peaks.

On the basis of the nature of folds

Simple fold mountains

• Simple fold mountains with open folds in which well-developed systems of synclines and anticlines are found, and folds are of wavy patterns.

Complex fold mountains

• Complex fold mountains in which the rock strata are intensely compressed to produce a complex structure of folds.
• In the Himalayas, over folds and recumbent folds are often found detached from their roots and carried a few hundred kilometres away by the tectonic forces. These detached folds are called ‘nappe.’

Characteristics of Fold Mountains

• Fold mountains belong to the group of youngest mountains of the earth.
• The presence of fossils suggests that the sedimentary rocks of these folded mountains were formed after accumulation and consolidation of silts and sediments in a marine environment.
• Fold mountains extend for great lengths whereas their width is considerably small.
• Generally, fold mountains have a concave slope on one side and a convex slope on the other.
• Fold mountains are mostly found along continental margins facing oceans (C-O Convergence).
• Fold mountains are characterized by granite intrusions (formed when magma crystallises and solidifies underground to form intrusions) on a massive scale.
• Recurrent seismicity is a common feature in folded mountain belts.
• High heat flow often finds expression in volcanic activity (Himalayas is an exception, because of C-C convergence).
• These mountains are by far the most widespread and also the most important.
• They also contain rich mineral resources such as tin, copper, gold etc.

Block Mountains

• Block mountains are created because of faulting on a large scale (when large areas or blocks of earth are broken and displaced vertically or horizontally).
• The uplifted blocks are termed as horsts, and the lowered blocks are called graben.
• The Great African Rift Valley (valley floor is graben), The Rhine Valley (graben) and the Vosges mountain (horst) in Europe are examples.
• Block mountains are also called fault-block mountains since they are formed due to faulting as a result of tensile and compressive forces.

There are two basic types of block mountains:

1. Tilted block mountains have one steep side contrasted by a gentle slope on the other side.
2. Lifted block mountains have a flat top and extremely steep slopes.

‘Fault’ in Geology

• When the earth’s crust bends folding occurs, but when it cracks, faulting takes place.
• A fault is a planar fracture (crack) in a volume of earth’s crust, across which there has been significant displacement of a block/blocks of crust.
• The faulted edges are usually very steep, e.g. the Vosges and the Black Forest of the Rhineland.
• Faults occur due to tensile and compressive forces acting on the parts of the crust.

• Large faults within the Earth’s crust result from the action of plate tectonic forces, such as subduction zones or transform faults.
• Energy release associated with rapid movement on active faults is the cause of most earthquakes.
• In an active fault, the pieces of the Earth’s crust along a fault move over time.
• Inactive faults had movement along them at one time, but no longer move.
• The type of motion along a fault depends on the type of fault.

Types of faults

Types of faults (Actualist, from Wikimedia Commons)

Strike-slip fault

• In a strike-slip fault (also known transcurrent fault), the plane of the fault is usually near vertical, and the blocks move laterally either left or right with very little vertical motion (the displacement of the block is horizontal).

Transform fault

• A special class of strike-slip fault is the transform fault or transform boundary when it forms a plate boundary.
• A transform fault is the only type of strike-slip fault that is classified as a plate boundary.
• Most of these faults are hidden in the deep ocean, where they offset divergent boundaries in short zigzags resulting from seafloor spreading.
• They are less common within the continental lithosphere. The best example is the Dead Sea transform fault.
• The transform boundary ends abruptly and is connected to another transform, a spreading ridge, or a subduction zone.

Dip-slip faults

• Dip-slip faults can be either normal or reverse.
• In a normal fault, the hanging wall (displaced block of crust) moves downward, relative to the footwall (stationary block). In a reverse fault (thrust fault) the hanging wall moves upwards.
• Reverse faults occur due to compressive forces whereas normal faults occur due to tensile forces.
• A downthrown block between two normal faults is a graben.
• An upthrown block between two normal faults is a horst.
• Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary.
• Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary.

Rift Valley system

• Tension causes the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls.
• The East African Rift Valley system is the best example.
• In general, large-scale block mountains and rift valleys are due to tension rather than compression.

Block Mountains

• Block mountains may originate when the middle block moves downward and becomes a rift valley while the surrounding blocks stand higher as block mountains.

Plateaus

• Sometimes, the surrounding blocks subside leaving the middle block stationary. Such cases are found in high plateau regions.

Oblique-slip faults

• A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault.
• Nearly all faults have some component of both dip-slip and strike-slip.
• Many disastrous earthquakes are caused along the oblique slip.