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

, 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 (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

, often caused by

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

interactions and

glaciations 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 betwe ...

. An example of forebulge can be seen in the Himalayan foreland basin, a result of the

Indian Indian or Indians may refer to: Peoples South Asia * Indian people, people of Indian nationality, or people who have an Indian ancestor ** Non-resident Indian, a citizen of India who has temporarily emigrated to another country * South Asia ...

Eurasian Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern Hemisphere, Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Ja ...

(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 planet's highest peaks, including the very highest, Mount Everest. Over 100 ...

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 ( fordeep) 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

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 A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere ...

) 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 Himalayas (46407126511)

The Persian Gulf foreland basin and forebulge was created as a result of the collision of the

Arabian The Arabian Peninsula, (; ar, شِبْهُ الْجَزِيرَةِ الْعَرَبِيَّة, , "Arabian Peninsula" or , , "Island of the Arabs") or Arabia, is a peninsula of Western Asia, situated northeast of Africa on the Arabian Plate. ...

and Eurasian plates around 13 million years ago. The

Zagros The Zagros Mountains ( ar, جبال زاغروس, translit=Jibal Zaghrus; fa, کوه‌های زاگرس, Kuh hā-ye Zāgros; ku, چیاکانی زاگرۆس, translit=Çiyakani Zagros; Turkish: ''Zagros Dağları''; Luri: ''Kuh hā-ye Zāgr ...

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 (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

s and

island arc Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle alon ...

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

forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

basin and a

back arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most of ...

The

Ganges basin The Ganges Basin is a part of the Ganges-Brahmaputra-Meghna (GBM) basin draining 1,999,000 square kilometres in Tibet, Nepal, India and Bangladesh. To the north, the Himalaya or lower parallel ranges beyond form the Ganges-Brahmaputra divide. ...

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 The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

subducted under the

Philippine plate The Philippine Sea Plate or the Philippine Plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of ...

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 glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at Las ...

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 betw ...

s. Due to

recession In economics, a recession is a business cycle contraction when there is a general decline in economic activity. Recessions generally occur when there is a widespread drop in spending (an adverse demand shock). This may be triggered by various ...

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 a ...

. 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 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 the parts: the ...

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 (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

. 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 ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...

and parts of

southern England Southern England, or the South of England, also known as the South, is an area of England consisting of its southernmost part, with cultural, economic and political differences from the Midlands and the North. Officially, the area includes G ...

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

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

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 gree ...

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, issn=0305-8719

References

Geodynamics Glaciology Geomorphology