Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created bytectonic and localized stresses inEarth's crust. Large areas of bedrock are broken up into blocks byfaults. Blocks are characterized by relatively uniformlithology. The largest of these fault blocks are calledcrustal blocks. Large crustal blocks broken off from tectonic plates are calledterranes.[1] Those terranes which are the full thickness of thelithosphere are called microplates. Continent-sized blocks are called variouslymicrocontinents, continental ribbons, H-blocks, extensional allochthons and outer highs.[2]
Because most stresses relate to the tectonic activity of movingplates, most motion between blocks is horizontal, that is parallel to the Earth's crust bystrike-slip faults. However vertical movement of blocks produces much more dramatic results. Landforms (mountains, hills, ridges, lakes, valleys, etc.) are sometimes formed when the faults have a large vertical displacement. Adjacent raised blocks (horsts) and down-dropped blocks (grabens) can form highescarpments. Often the movement of these blocks is accompanied by tilting, due to compaction or stretching of the crust at that point.
Fault-block mountains often result fromrifting, an indicator ofextensional tectonics. These can be small or form extensiverift valley systems, such as theEast African Rift zone.Death Valley inCalifornia is a smaller example. There are two main types of block mountains; uplifted blocks between two faults andtilted blocks mainly controlled by one fault.
Lifted type block mountains have two steep sides exposing both sides scarps, leading to thehorst andgraben terrain seen in various parts ofEurope including theUpper Rhine valley, a graben between two horsts – theVosges mountains (inFrance) and theBlack Forest (inGermany), and also theRila –RhodopeMassif inBulgaria,Southeast Europe, including the well definedhorsts ofBelasitsa (linear horst),Rila mountain (vaulted domed shaped horst) andPirin mountain – a horst forming a massiveanticline situated between the complexgraben valleys ofStruma and that ofMesta.[3][4][5]
Tilted type block mountains have one gently sloping side and one steep side with an exposed scarp, and are common in theBasin and Range region of the western United States.
An example of agraben is the basin of theNarmada River inIndia, between theVindhya andSatpura horsts.
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