The Ordos Block is a

crustal block Fault blocks are very large blocks of rock, sometimes hundreds of kilometres in extent, created by tectonic and localized stresses in Earth's crust. Large areas of bedrock are broken up into blocks by faults. Blocks are characterized by relat ...

, that forms part of the larger North China Block (NCB). It is surrounded by

active Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * Active Records, a record label Ships * ''Active'' (ship), several commercial ships by that name * HMS ''Active'', the name of various ships of the British Royal ...

fault systems and has been a distinct block since at least the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

. It is bordered to the west by the Alxa Block, the westernmost part of the NCB, to the south by the

Qinling orogenic belt The Qinling orogenic belt is a tectonic feature that evolved throughout the Proterozoic and Phanerozoic eons due to a variety of tectonic activities. It is a part of the Central China Orogenic Belt, aligned in an east–west orientation across ...

, to the north by the Yanshan-Yinshan orogenic belt, part of the

Central Asian Orogenic Belt The Central Asian Orogenic Belt (CAOB), also called the Altaids, is one of the world's largest Phanerozoic accretionary orogens, and thus a leading laboratory of geologically recent crustal growth. The orogenic belt is bounded by the East Europea ...

and to the east by the

Taihangshan The Taihang Mountains () are a Chinese mountain range running down the eastern edge of the Loess Plateau in Shanxi, Henan and Hebei provinces. The range extends over from north to south and has an average elevation of . The principal peak is ...

mountain range, which forms part of the Trans-North China Orogen. The block is currently stable and large earthquakes are restricted to the bordering fault zones. It has been suggested that the block is currently undergoing anti-clockwise rotation with respect to the Eurasian Plate, as a result of the ongoing eastward spreading of the

Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...

, although this view has been challenged.

Extent

The Ordos Block is a roughly rectangular fault-bounded part of the larger North China Block, which itself closely matches the extent of the

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

North China Craton. It covers an area of about 250,000 km² and has a thick

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

of more than 200 km, suggesting that, unlike other parts of the NCB, it retains a fully cratonic character.

Geology

The shallow geology of Ordos Block consists of a thick sequence of

Phanerozoic The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period, when anima ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s in what is known as the Ordos Basin. There are three main sequences, of Lower

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

, Upper Paleozoic and

age. The Lower Paleozoic sequence consists of mainly

carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...

s ranging from middle

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

to lower

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

in age. Following a regional

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

associated with the

Caledonian orogeny The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occ ...

, sedimentation resumed during the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

and continued through the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

with a sequence of clastic sedimentary rocks, including significant thicknesses of coal. The overlying Mesozoic sequence consist of mainly

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

and

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

deposits. The youngest unit preserved in the basin is of Lower Cretaceous age, with any younger parts of the sequence having been eroded following uplift during the

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

. This erosion surface is covered in the southern part of the block by Upper Neogene to

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

deposits of red clay and

loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...

, part of the

Loess Plateau The Chinese Loess Plateau, or simply the Loess Plateau, is a plateau in north-central China formed of loess, a clastic silt-like sediment formed by the accumulation of wind-blown dust. It is located southeast of the Gobi Desert and is surroun ...

. The understanding of the deep geology of the Ordos Block is based on the interpretation of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

and

magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...

data, backed up by a limited amount of deep borehole samples, yielding geochronological and

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

data. On the basis of this dataset, the block appears to be divided into a northern and southern part with contrasting histories. They are juxtaposed across the northwest–southeast trending Datong-Huachi fault. The northern part consists mainly of partly migmatised

metasedimentary rock In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a naturally occurring material that is broken down by processes of weathering and er ...

s with granitic

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

es. They are partly of

Neoarchean The Neoarchean (; also spelled Neoarchaean) is the last geologic era in the Archean eon that spans from 2800 to 2500 million years ago—the period being defined chronometrically and not referencing a specific level in a rock section on Earth. ...

age, with reworking at several periods during the

. The southern part is thought be of

Paleoarchean The Paleoarchean (), also spelled Palaeoarchaean (formerly known as early Archean), is a Geologic time scale#Terminology, geologic era within the Archean, Archaean Eon. The name derives from Greek "Palaios" ''ancient''. It spans the period of time ...

Mesoarchean The Mesoarchean (, also spelled Mesoarchaean) is a geologic era in the Archean Eon, spanning , which contains the first evidence of modern-style plate subduction and expansion of microbial life. The era is defined chronometrically and is not r ...

in age, with some Neoarchean additions.

Margins

All the margins of the Ordos Block are tectonically active. The style of tectonics varies around the block, with

reverse fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

ing at its southwestern corner along the Liupanshan Fault.

Rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...

ing to the northwest and north on the Yinchuan, Jilantai and Hetao Basins and rifting within a zone of distributed right lateral

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

along its eastern and southern margin, forming the Weihe-

Shanxi Rift System The Shanxi Rift System or Fen–Wei Rift System is a zone of active extensional tectonics that forms the eastern margin of the Ordos Block in northern China. The zone extends for at least and runs south-southwest to north-northeast. The individ ...

Liupanshan Fault

This belt of NNW–SSE trending

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

and reverse faulting runs for about 180 km and forms the southwestern margin of the Ordos Block. This thrust belt began to form during the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58

Yinchuan Basin

This ~160 km long SSW–NNE trending rift basin has a

half-graben A half-graben is a geological structure bounded by a fault along one side of its boundaries, unlike a full graben where a depressed block of land is bordered by parallel faults. Rift and fault structure A rift is a region where the lithosphere ...

geometry. It has been active since at least the middle Oligocene and contains a thick sedimentary fill (>8 km). It has had a long tectonic history, starting with a phase of northwest–southeast directed extension from the Oligocene to the middle Miocene. This was followed by a short period of

inversion Inversion or inversions may refer to: Arts * , a French gay magazine (1924/1925) * ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas * Inversion (music), a term with various meanings in music theory and musical set theory * ...

related to NW–SE compression during the early part of the late Miocene, before a return to NW–SE extension for the rest of the Late Miocene into the Pliocene. From the Late Pleistocene to the present day, the basin has been in an overall

transtension Transtension is the state in which a rock mass or area of the Earth's crust experiences both ''extensive'' and ''transtensive'' shear. As such, transtensional regions are characterised by both extensional structures ( normal faults, grabens) and wr ...

al regime affected by a combination of NE–SW compression and NW–SE extension. A right lateral slip-rate of about 2 mm per year has been estimated across the basin, with about 1 mm per year of extension.

Jilintai Basin

The Jilintai Basin is arcuate in shape, following the main bounding fault zone to the north, consisting of the Langshan Piedmont and Seertengshan faults. It has an overall half-graben geometry. It has a similar tectonic history to the neighbouring Yinchuan Basin. A right lateral slip-rate of about 0.8 mm per year has been estimated, with about 1.6 mm per year of extension.

Hetao Basin

This basin trends WSW–ENE and also has a half-graben geometry. The main bounding fault zone lies to the north of the basin and consists of the Wulashan, Daqingshan and Helinggeer faults. It shares the early history of the Yinchuan and Jilintai basins, but the recent tectonics in this case appear to be approximately north–south extension. Estimated slip-rates are low, with high uncertainties, with small amounts of left-lateral strike-slip combined with a small component of either extension or shortening.

Shanxi Rift System

This group of rift basins forms the SSW–NNE trending eastern margin of the Ordos Block, over a distance of >900 km. The individual basins and their bounding high-angle normal faults have a WSW–ENE to SW–NE trend. They have an overall

en echelon An echelon formation () is a (usually military) formation in which its units are arranged diagonally. Each unit is stationed behind and to the right (a "right echelon"), or behind and to the left ("left echelon"), of the unit ahead. The name of ...

geometry, consistent with right lateral sense of displacement over the zone. The age of the 2.0–3.8 km thick sedimentary sequences in the basins indicates that they became active during the Miocene to Pliocene. GPS-derived slip rates on the various basins in the rift system show consistent small amounts of right lateral strike-slip combined with generally smaller amounts of extension.

Weihe Basin

The Weihe Basin forms the southern margin of the Ordos Block. It is regarded as part of the Shanxi Rift System by some geologists and as a distinct rift element by others. The basin which has a sedimentary fill of 4 km to 6 km in thickness, consists of several sub-basins with a half-graben geometry, controlled by major normal faults. The basins started to form in the Eocene as a result of NW–SE extension. After a brief period of NE–SW extension in the Pleistocene, the current tectonic setting began, which consists of NNW–SSE extension. This ongoing extension has been responsible for large historical damaging earthquakes, such as those in

1556 __NOTOC__ Year 1556 ( MDLVI) was a leap year starting on Wednesday (link will display the full calendar) of the Julian calendar. Events January–June * January 16 – Charles V, having already abdicated as Holy Roman Emperor, r ...

and

1815 Events January * January 2 – Lord Byron marries Anna Isabella Milbanke in Seaham, county of Durham, England. * January 3 – Austria, Britain, and Bourbon-restored France form a secret defensive alliance treaty against Prussi ...

. GPS data are unable to constrain the current displacement rates.

Current tectonics

The block remains a stable piece of cratonic continental lithosphere. However, it has been suggested that is currently rotating anticlockwise due to interactions with neighbouring blocks, particularly the continuing eastward spread of the Tibetan Plateau. This rotational model predicts the presence of right lateral shear along all of the block boundaries.

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

data have been interpreted to support this model. In another model, there is no rotation of the Ordos Block and right lateral shear only on the western and eastern boundaries and left lateral shear on the northern and southern boundaries. GPS data have also been interpreted to support the non-rotational model.

References

{{Reflist Geology of China