Maya Block

The Block is commonly delimited by the continental margin in the Gulf of Mexico to the north, in the Caribbean Sea to the east, and in the Pacific Ocean to the southwest, and further, by the Motagua–Polochic Faults to the south-southeast, and by the Isthmus of Tehuantepec to the west.^[1] The Motagua–Polochic Faults divide the Maya Block from the Chortis Block, while the Isthmus of Tehuantepec divides it from the Oaxaquia Block (i.e. the Juarez, Cuicateco, or Oaxaquia Block, Terrane, or microcontinent).^{[2][n 1]}

The Block's precise subaerial limits are not widely agreed upon, in contrast to its relatively exact submarine borders.^{[n 2][citation needed]} Furthermore, it has been recently suggested that the Block's western extreme may rather extend past the Isthmus of Tehuantepec, along the Gulf of Mexico, and into Louisiana.^{[3][n 3]}

A broad arching fold belt of 'morphological distinct mountain ranges separated by deep fault-controlled canyons and occasional broad alluvial valleys' extends along the south-southeasterly limit of the Block.^[4] The most prominent of said mountain ranges are the Northern Chiapas Mountains and the Sierra Madre de Chiapas in Mexico, the Cuchumatanes, Chama, Santa Cruz, and Lacandon Ranges in Guatemala, and the Maya Mountains in Belize.^[5]

The 'most extensive karstlands of the North American continent' extend northwards from the Block's southern extreme.^[6] The Block's most prominent karstic landform is the Yucatán Platform to its north.^{[citation needed]} Relatively less prominent karstic formations occur in the Block's southern portion, including an unnamed formation in northwestern Peten to northeastern Belize, the Belize Barrier Reef, the Lacandon Range, the Cuchumatanes Range, and various formations to the north and south of the Maya Mountains.^{[7][n 4]}

The most prominent topographic features of the Block's Caribbean coast are extensive seagrass beds and coral reefs, with the Belize Barrier Reef forming a notable example of the latter.^[8] Its Pacific coast, in contrast, is predominated by extensive mangrove forests.^[9]

The terrestrial portion of the Block encompasses all six districts of Belize, five northerly departments of Guatemala (Huehuetenango, Quiche, Alta Verapaz, Izabal, Peten), and five southeasterly states of Mexico (Chiapas, Tabasco, Campeche, Yucatán, and Quintana Roo). Its submarine portion encompasses the continental shelf which abuts the coastal districts.

Mean thickness of the continental crust constituting the Block increases southwards, ranging from 20–25 kilometres (12–16 mi) in the northern Yucatán Peninsula to 30–40 kilometres (19–25 mi) in the Peninsula's south.^[10] The crust's i.e. Block's crystalline basement is composed mainly of Silurian–Triassic metamorphic and igneous rocks, and is exposed in at least five formations, namely, the Mixtequita Massif, Chiapas Massif, Cuchumatanes Dome, Tucuru–Teleman, and the Maya Mountains.^[11] Elsewhere, the basement is overlain by a thick sedimentary cover of Upper Palaeozoic clasts and carbonates, Upper Jurassic continental redbeds, and Cretaceous–Eocene carbonates and evaporites.^[4]

It has been suggested that the Block's continental basement is stretched, since its sedimentary cover reaches a thickness of up to 6 kilometres (3.7 mi), this being considered impossible on an unstretched basement at isostatic equilibrium.^{[10][n 5]}

The Block is thought to fully or partially incorporate between two and thirteen geologic provinces.^[12]

The Block is believed to fully or partially comprehend some four or five sedimentary basins.^[13]

A number of faults or fault zones have been identified within the Block, the most prominent of which include various boundary faults abutting the Maya Mountains, various offshore faults east of the Yucatán Peninsula–Belize, the Ticul Fault, the Malpaso Faults, and the Rio Hondo Faults.^{[14][n 6]}

The Block is thought to experience significant counterclockwise rotation and a north-northwest down tilt, which gradually lowers the northern portion of the Yucatán Platform, thereby lifting its southern extreme in the Maya Mountains.^[15] It is nonetheless tectonically rigid or stable, experiencing an absolute west-southwest motion of 1.8 centimetres (0.71 in) per annum.^{[16][n 7]} Central America, including the southern portion of the Maya Block, 'is very well-known and characterised by numerous, medium size earthquakes preceded and followed by damaging shocks,' with the Middle America Trench in the Pacific deemed the main source of such quakes.^{[17][n 8]} Of thirty-three earthquakes of M_s ≥ 7.0 in Central America during 1900–1993, the epicentres of at least two of these were located within the Block (in its southwestern quadrant), though a further nine were located near it (in the Motagua–Polochic Faults or the portion of the Middle America Trench bordering the Block).^[18]

Middle America, including the Maya Block, is thought to have taken shape sometime after 170 million years ago.^[19] Its formation is thought to have 'involved [the] complex movement of [various] crustal blocks and terranes between the two pre-existing continental masses [ie North and South America].'^[20] Details of the pre-Cenozoic portion of this process (170–67 million years ago), however, are not widely agreed upon.^{[20][n 9]} Nonetheless, it has been proposed that the Block formed before or during the opening of the Iapetus Ocean.^[21] It, together with the Oaxaquia, Suwannee, and Carolina Blocks, are thought to have constituted a peri-Gondwanan terrane on that continent's western, northwestern, northern, or eastern edge during the Appalachian–Caledonian or Ouachita–Marathon–Appalachian orogeny (that is, during the formation of Pangaea from the collision of Gondwana and Laurentia).^[22] It is thought to have been displaced away from the Laurentian craton by clockwise rotation, translation, or anticlockwise rotation, during the Middle Jurassic opening of the Gulf of Mexico and subsequent northwesterly drift of North America away from Pangaea.^{[23][n 10][n 11]}

Details of the Cenozoic (66–0 million years ago) geologic history of Middle America, including that of the Maya Block, are relatively more widely agreed upon.^[20] In broad strokes, the Chortis Block is thought to have reached its present-day position by at least 20 million years ago.^[24] The northern and eastern coasts of the Block are not thought to have been fully subaerially exposed until some 5–2 million years ago.^[25] The Block's coastlines, which were initially more expansive than its present-day ones, are thought to have reached modern dimensions due to rising sea levels some 11–8 thousand years ago.^[26]

The Block was discovered in 1969 by Gabriel Dengo, a Guatemalan geographer.^[27] It was quickly adopted in scholarship, and remains 'accepted by many as a valid subdivision of Central America's geology, especially of its crystalline basement.'^[28]

• Carolina terrane, a similar peri-Gondwanan block