The Benham Plateau, also known as the Benham Rise, is a seismically active undersea region and extinct volcanic ridge located in the Philippine Sea approximately 250 km (160 mi) east of the northern coastline of Dinapigue, Isabela.
Under the Philippine Sea lie a number of basins including the West Philippine Basin (WPB), inside of which is located the Central Basin Fault (CBF). The Benham Plateau is located in the CBF and its basement probably represents a micro-continent. Several scientific surveys have been made on the feature to study its nature and its impact on tectonic subduction, including one about its effects on the 1990 Luzon earthquake. The area is claimed, as part of its continental shelf, by the Philippines, which has lodged a claim with the United Nations Commission on the Limits of the Continental Shelf (UNCLCS) on April 8, 2009 and was approved by the UNCLCS in 2012.
Benham Rise is a submerged extinct volcanic ridge located at 16 degrees 30 minutes N, 124 degrees 45 minutes E off the coast of Luzon, with the size of about 250 km in diameter and rises over 2,000 meters above the sea floor, from below 5,000 meters below sea level to above 3,000 meters below sea level. Its area is close to the Benham Seamount, located at 15 degrees 48 minutes N, 124 degrees 15 minutes E. The precise location is somewhere near the east of the Philippine Trench and near the south of the East Luzon Trench, both of which absorb the subducting force of the Philippine Sea Plate under the Philippine Mobile Belt, a collage of large blocks of that crust that amalgamated prior to the collision of the Philippine Sea Plate with the Eurasian Plate.
The origin of the landform, along with a fellow landform, the Urdaneta Plateau (a remnant of mantle plume), is identified in one study as at least five sequences of propagating rifts, probably triggered by mantle flowing away from the mantle thermal anomaly. Its presence of the landform disrupts the continuity of this region (known as the Philippine-East Luzon Trench) by continuously colliding with the Sierra Madre mountain range of eastern portion of the island of Luzon. Though it is generally thought that the Philippine Sea Plate is being subducted under the Philippine Mobile Belt, under the rules of tectonic subduction, there appears to be a resistance to this because of the presence of the landform, and instead, the plate is being displaced into the northern portion of Luzon to the west.
The geophysical features of the plateau may have been the result of an early Miocene collision event between the Benham Rise and the eastern margin of Luzon, which may have also allowed the inception of the NW striking strand of the Philippine fault. These forces may have impacted the shape of the island of Luzon because of the basaltic sea floor resisting the subduction that may have also cause the bending of the Philippine Fault. The active basins in Central Luzon, which trace an asymmetrical V shape, is the best place to observe recent tectonic evolution of the fault system.
The landform is presumably named after Admiral Andrew Ellicot Kennedy Benham (1832–1905) by American surveyors who were the probable discoverers of the geological feature. He was a United States Navy officer, who served with both the South Atlantic and West Gulf Blockading Squadrons during the American Civil War. There has been speculation in the scientific community about the nature of the landform. Following the major 16 July 1990 Luzon earthquake, scientists reconsidered their fault models and decided it likely that Benham Plateau has similarly displaced the Philippine Fault System to the west. After analysing older models such as that of Pinet and Stephan (1989), scientists reconsidered their fault models. They thought that it is highly likely that the Benham Plateau is still displacing Central Luzon and the Philippine Fault System to the west, which may have had an impact in causing such a catastrophic earthquake. The 20 second to 50 second wave in the 1990 quake that developed a new east-west sub-fault was so strong that it terminated disastrously at the city of Baguio in Benguet, Cordillera. Several scientific surveys, conducted between 2004 to 2008, collected hydrographic data that determined the morphology of the seabed in the region. Additional data from international bathymetric surveys and an analysis of international research projects were collected to support the findings.
Despite its proximity to the archipelago, the plateau was previously not included in the territory of the Philippine Islands. On 8 April 2009, the Republic of the Philippines lodged a partial territorial waters claim with the United Nations Commission on the Limits of the Continental Shelf in relation to the continental shelf in the region of Benham Rise. It was submitted as part of petition expanding the archipelago's baselines and exclusive economic zone through a law that also included other claims involving disputed territories of the Kalayaan Islands (Spratly Islands) and Scarborough Shoal. Although the landform, in itself, is not disputed, the petition still received some criticism inside and outside the country because of its controversial nature. According to the government's claim, based on a set of guidelines by the Commission on the Limits of the Continental Shelf, the area satisfies the 350-mile constraint line since the outer limits of the continental shelf are located landward of the constraint line, which is located 350 miles from the baselines where the measurement of the breadth of the territorial sea begins.
The Congress of the Philippines enacted Republic Act No. 9522, also known as the Archipelagic Baselines Law, which is the basis of the claim. According to the document the region is bounded by the Philippine Basin on the north and east, and by Luzon on the west and south. It asserted that, according to scientific data based on seismic, magnetic, other geological features, the Benham Rise is an extension of the Philippines’ continental shelf. In summary, the baselines, the basis used for delineating the maritime territorial and jurisdictional zones (including the continental shelf), conform with the requirements of the UN Convention of the Law of the Sea (UNCLOS). The claim is only a partial claim since the law that allows the Philippines to expand its territorial boundaries also includes islands in the South China Sea.
The Philippines filed its claim for Benham Rise in 2008 in compliance with the requirements of the United Nations Convention on the Law of the Sea (UNCLOS). The UN has officially approved the claim in April 2012.
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