geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...

, a forebulge is a flexural bulge in front as a result of a load on the

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

, often caused by

tectonic Tectonics ( via Latin ) are the processes that result in the structure and properties of the Earth's crust and its evolution through time. The field of ''planetary tectonics'' extends the concept to other planets and moons. These processes ...

interactions and glaciations. An example of forebulge can be seen in the Himalayan foreland basin, a result of the Indian-

Eurasian Eurasia ( , ) is a continental area on Earth, comprising all of Europe and Asia. According to some geographers, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents dates back to antiq ...

(continent-continent) plate collision, in which the Indian plate subducted and the Eurasian plate created a large load on the lithosphere, leading to the

Himalayas The Himalayas, or Himalaya ( ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the Earth's highest peaks, including the highest, Mount Everest. More than list of h ...

and the Ganges foreland basin.

Background

Forebulge is most commonly found with continent-continent convergent collisions, in which the formation of mountain ranges as the plates collide places a large load on the lithosphere below. The lithosphere flexes in response to the load on the mantle, causing depression and subsidence ( foredeep) followed by the forebulging in the front. The forebulge area is lifted by a height that is 4% of the depression height caused by the load. It takes roughly 10,000 to 20,000 years for forebulge to fully develop when the mantle flexure to reach isostatic equilibrium, a process that is controlled by mantle

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

Tectonic

Forebulge can be seen during the formation of a mountain range, which creates a large load and crustal thickening that leads to lithospheric flexure. Part of the land sinks under the load ( Foredeep) while part of the outer land forebulges, leading to the creation of these

foreland basins Foreland may refer to: * a landform projecting into the sea, such as a headland or a promontory * an area of land in front of something **Foreland basin, in geology, the zone that receives sediment from an adjacent mountain chain **Glacier foreland ...

. Forebulge associated with the formation of these basins is most commonly a result of convergent collision. Foreland basins can occur in convergent subduction, but this is rare. These basins are linked to fold-thrust belts, which are divided into three main types: collisional (peripheral), retroarc, and retreating collisional subduction. Collisional and retroarc thrust belts form in collision convergent plates whereas retreating collisional forms when the subduction rate exceeds the convergence rate of the collision. The Persian Gulf foreland basin and forebulge was created as a result of the collision of the

Arabian The Arabian Peninsula (, , or , , ) or Arabia, is a peninsula in West Asia, situated north-east of Africa on the Arabian plate. At , comparable in size to India, the Arabian Peninsula is the largest peninsula in the world. Geographically, the ...

and Eurasian plates around 13 million years ago. The

Zagros The Zagros Mountains are a mountain range in Iran, northern Iraq, and southeastern Turkey. The mountain range has a total length of . The Zagros range begins in northwestern Iran and roughly follows Iran's western border while covering much of s ...

mountains that formed, as a result, created a load on the lithosphere that led to the creation of the modern-day Persian Gulf. Collisions between

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

s and

island arc Island arcs are long archipelago, chains of active volcanoes with intense earthquake, seismic activity found along convergent boundary, convergent plate tectonics, tectonic plate boundaries. Most island arcs originate on oceanic crust and have re ...

s lead to loading and flexure of the lithosphere. The flexure produces a significant forebulge, which divides a

forearc A forearc is a region in a subduction zone between an oceanic trench and the associated volcanic arc. Forearc regions are present along convergent margins and eponymously form 'in front of' the volcanic arcs that are characteristic of convergen ...

basin and a back arc basin. Cross section of mariana trench

The

Ganges basin The Ganges ( ; in India: Ganga, ; in Bangladesh: Padma, ). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international which goes through India, Bangladesh, Nepal and China." is a trans-boundary rive ...

and Himalayan Basin roughly 50 to 70 million years ago as a result of the load of the Himalayan mountains after the collision of the Eurasian and Indian plates. The Andean foreland basins were created as a result of the lithospheric flexure due to the load of the Andean mountain (fold-thrust belt) and resulting foredeep, forebulge, and backbulge. Forebulge can also be observed in the flexural plate of the Mariana arc, which was formed when the Pacific plate subducted under the Philippine plate.

Glacial

Chesapeake Bay Bridge at Sandy Point MD1

One cause for forebulge formation is the loading of the continental lithosphere by

ice sheet In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacier, glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice s ...

s during continental

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...

s. Due to

recession In economics, a recession is a business cycle contraction that occurs when there is a period of broad decline in economic activity. Recessions generally occur when there is a widespread drop in spending (an adverse demand shock). This may be tr ...

of the ice sheets, the formerly-glaciated areas are currently rising in a phenomenon known as

post-glacial rebound Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound an ...

. Forebulge collapse due to post-glacial rebound is greatest along the east coast of the United States, which lay just under the North American ice sheet. As a result of forebulge collapse due to post-glacial rebound, most of the coast of the

Eastern US The Eastern United States, often abbreviated as simply the East, is a macroregion of the United States located to the east of the Mississippi River. It includes 17–26 states and Washington, D.C., the national capital. As of 2011, the Eastern ...

has slowly been sinking; it is estimated that the area around the

Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula, including parts of the Ea ...

will sink a foot (0.305 m) over the next hundred years. Due to the coupling of the mantle with the plates, data from

are used as a direct probe of the viscosity of the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

. As an ice sheet collapses, the land that was previously depressed rises in isostatic recovery followed by subsidence of forebulge, causing sea levels to rise. Forebulge collapse is also the reason why the

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

and parts of

southern England Southern England, also known as the South of England or the South, is a sub-national part of England. Officially, it is made up of the southern, south-western and part of the eastern parts of England, consisting of the statistical regions of ...

are still slowly sinking. One estimate is that the center of the

North Sea The North Sea lies between Great Britain, Denmark, Norway, Germany, the Netherlands, Belgium, and France. A sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian Se ...

rose by about 170 m (558 ft) during the

Ice Age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages, and g ...

because of forebulging.{{Cite journal, last=Devoy, first=Robert J. N., date=1995, title=Deglaciation, Earth crustal behaviour and sea-level changes in the determination of insularity: a perspective from Ireland, url=https://doi.org/10.1144/GSL.SP.1995.096.01.14, journal=Geological Society, London, Special Publications, volume=96, issue=1, pages=181–208, doi=10.1144/gsl.sp.1995.096.01.14, bibcode=1995GSLSP..96..181D , s2cid=130211641 , issn=0305-8719, url-access=subscription

References

Geodynamics Glaciology Geomorphology