Oblique subduction is a form of subduction (i.e. a tectonic process involving the convergence of two plates where the denser plate descends into Earth's interior) for which the convergence direction differs from 90° to the

plate boundary 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 ...

. Most

convergent boundaries A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a ...

involve oblique subduction, particularly in the

Ring of Fire The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a region around much of the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur. The Ring o ...

including the

Ryukyu The , also known as the or the , are a chain of Japanese islands that stretch southwest from Kyushu to Taiwan: the Ōsumi, Tokara, Amami, Okinawa, and Sakishima Islands (further divided into the Miyako and Yaeyama Islands), with Yonagu ...

, Aleutian,

Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...

and

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

subduction zones. In general, the obliquity angle is between 15° to 30°. Subduction zones with high obliquity angles include

Sunda trench The Sunda Trench, earlier known as and sometimes still indicated as the Java Trench, is an oceanic trench located in the Indian Ocean near Sumatra, formed where the Australian- Capricorn plates subduct under a part of the Eurasian Plate. It is ...

(ca. 60°) and Ryukyu arc (ca. 50°). Obliquity in plate convergence causes differences in dipping angle and subduction velocity along the plate boundary. Tectonic processes including slab roll-back, trench retreat (i.e. a tectonic response to the process of slab roll-back that moves the

trench A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from ero ...

seaward) and slab fold (i.e. buckling of subducting slab due to

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states o ...

) may also occur. Moreover, collision of two plates leads to strike slip deformation of the

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 vo ...

, thus forming a series of features including forearc slivers and strike slip fault systems that are sub-parallel to ocean trenches. In addition, oblique subduction is associated with the closure of ancient ocean, tsunami and block rotations in several regions.

Deformation features

Forearc slivers

Forearc slivers are partly detached continental blocks of the overriding plates. They are bounded by the

trenches A trench is a type of excavation or in the ground that is generally deeper than it is wide (as opposed to a wider gully, or ditch), and narrow compared with its length (as opposed to a simple hole or pit). In geology, trenches result from erosi ...

and trench parallel strike slip fault systems. The motion of forearc slivers depend on the obliquity of the subducting slabs. Moreover, some forearc slivers occur in the absence of well defined strike-slip fault systems, and sliver motions are not purely strike-slip.

Trench parallel strike-slip fault systems

Trench parallel strike-slip faults are deformational products contributed by trench parallel component of strain partitioning. They are located between the forearc slivers and the remaining overriding plates.

Orientation of strike slip faults

Vertical strike slip fault systems are generally accepted by the early literature of oblique subduction. However, modern technology, such as seismic profiling, reveals that the faults are not necessarily vertical. Several other models concerning the orientations of the faults are proposed.

Slip accommodating mechanisms

Trench parallel slip component from oblique subduction may not be fully accommodated by the aforementioned trench parallel strike slip faults. Several models suggest that there are other slip accommodating mechanisms formed by oblique subduction as means to take up the remaining slip component. USGS map of the 2012 Indian Ocean Earthquake

USGS map of the 2012 Indian Ocean Earthquake

Margin parallel strike-slip faults in subducting plates

Ishii et al., (2013) suggested that the trench parallel strike-slip faults may appear in the obliquely subducting slabs to accommodate a portion of the trench parallel slip component. In the Sumatra subduction zone, the trench parallel slip component is measured to be approximately 45 mm per year, the motion rate of northern Great Sumatra Fault ranges from 1 to 9 mm per year with the maximum rate of 13 mm per year. The result shows that the trench parallel slip component of at least 32 mm per year is left. On 11 April 2012, a M_w 8.6

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

occurred in the subducting plate (i.e. the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

). Strike-slip seismicity was recorded in the earthquake. This infers strike slip fault systems are present in the descending slab and they may potentially accommodate slip component from oblique subduction.

Strain partitioning

Strain partitioning is a form of deformation. In oblique subduction zone, strain partitioning is initiated into trench parallel component and trench normal component. The trench parallel component is accommodated by localized shear zones (short-term deformation) or trench parallel strike slip fault systems (long-term deformation) in the overriding plates. Likewise, this component commonly leads to the formation of forearc slivers. The trench normal component is taken up by

thrust 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 ...

structures. These thrusts are generally discontinuous and their geometries change progressively.

Short-term deformation: Localized shear zone

Short-term deformation is mainly

elastic Elastic is a word often used to describe or identify certain types of elastomer, elastic used in garments or stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rubber used to hold objects togeth ...

and acts at human time scale. When the denser plate subducts beneath the upper plate, they are coupled at the interface (i.e. plate coupling). The process of plate coupling thus generates tectonic force that follows the subduction direction. The orientation of tectonic force gradually rotates toward the trench normal direction. This attributes to the decline of trench parallel component when the force leaves the plate coupling zone. In this way, only the frontal part, rather than the whole upper plate, is dragged by the subducting slab.

Long-term deformation: Formation of forearc sliver and strike slip fault

Long-term deformation occurs at geological time scale. Under continuous oblique subduction, the aforementioned frontal part of the upper plate permanently accommodates the trench parallel component. In this way, the orientation of tectonic force rotates gradually toward the trench parallel direction. Strong and continuing tectonic force in trench parallel direction leads to the development of trench parallel strike slip fault system. The fault thus separate a portion of the

from the overriding plate, forming the forearc sliver.

Tectonic events related to oblique subduction

The 1771 Great Yaeyama Tsunami

The tsunami occurred in the southwestern part of the Ryukyu arc. Yukinobu et al., (2018) suggested that oblique subduction was the primary reason leading to the occurrence of the tsunami.

Tectonic setting

In the plate boundary, an approximately 80 km long and 30 km wide depression is observed. It obscures trench parallel strike slip fault and the topographic ridge of the wedge.

Oblique subduction and tsunami

Block rotation

Oblique subduction has led to rotation of microblocks about nearby poles of rotation (See also: Euler poles) in some oblique subduction zones. In these regions, the trench parallel strike slip fault systems are less prominent. This is because a portion of the trench parallel component is accommodated by the microblock rotation. Examples of oblique subduction-induced block rotation are identified in North Island, Cascadia and

New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; id, Papua, or , historically ) is the world's second-largest island with an area of . Located in Oceania in the southwestern Pacific Ocean, the island is separated from Australia by the wide Torr ...

Example: North Island oblique subduction zone

= Tectonic setting

= The North Island oblique subduction zone in

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

was established by the obliquely subducting Pacific Plate beneath the

. A trench parallel strike slip fault system, North Island Dextral Fault Belt, was formed. Based on geological and geodetic data, five tectonic blocks are identified in the region. These blocks are separated by block-bounding faults.

= Microblock rotation

= Based on GPS measurement, a clockwise rotation of microblocks at a rate of 0.5° to 3.8° per million year relative to the

is observed. This caused tectonic extension in Taupo Volcanic Zone and tectonic shortening in northwestern South Island, for example the Buller region. In addition, the block rotation accommodates 25% to 65% of the trench parallel component from oblique subduction. Therefore, high rate trench parallel strike slip faults are absent in the North Island.

= Rotation mechanism

= In the oblique subduction zone, the sinking slab is characterized by the

Hikurangi plateau The Hikurangi Plateau is an oceanic plateau in the South Pacific Ocean east of the North Island of New Zealand. It is part of a large igneous province (LIP) together with Manihiki and Ontong Java, now located and north of Hikurangi respective ...

in the south. The thickness of this oceanic plateau ranges from 15 km to 10 km along the

oceanic trench Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...

. The along strike thickness variation leads to differential subduction rate. In the southern

, thick oceanic plateau induces high collisional resistance forces that cripples the subduction process. However, the thin

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...

in the north is subducted. This activated the tectonic block rotations about a nearby axis.

Closure of Northeastern Paleo-Tethys Ocean

Geological setting

The Qinling-Dabieshan orogen in central China consists of three separate plates, including the

north China plate The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The ...

, the Qinling-Dabieshan microplate, and the south China plate. Geological and geochemical analysis suggest that there was an ocean basin between the plates and it was part of the

Paleo-Tethys Ocean The Paleo-Tethys or Palaeo-Tethys Ocean was an ocean located along the northern margin of the paleocontinent Gondwana that started to open during the Middle Cambrian, grew throughout the Paleozoic, and finally closed during the Late Triassic; exi ...

Evidence of oblique subduction

Tectonic features of oblique subduction, for example a right lateral strike-slip thrust belt are identified in the tectonic zone. These evidence suggest that the south China plate was obliquely subducted to the northwest beneath the

in the Early

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 Cretace ...

and led to the closure of the northeastern

. = Example of oblique subduction =

Peru-Chile trench

The

Peru–Chile Trench The Peru–Chile Trench, also known as the Atacama Trench, is an oceanic trench in the eastern Pacific Ocean, about off the coast of Peru and Chile. It reaches a maximum depth of below sea level in Richards Deep () and is approximately long; ...

is part of the Andean oblique subduction zone that was formed as a result of oblique subduction between the sinking Nazca Plate and the South American Plate. The current subduction direction is at east-north-east (see the summary below). However, geological record shows southeast subduction direction in Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period.

Margin parallel strike slip faults

Four major trench parallel strike slip faults are identified in the oblique subduction zone.

Liquiñe-Ofqui Fault The Liquiñe-Ofqui Fault is major geological fault that runs a length of roughly in a NNE-SSW orientation and exhibits current seismicity. It is located in the Chilean Northern Patagonian Andes. It is a dextral intra-arc strike-slip fault. Most ...

is a 1,200 km long fault that located in the southern Andes. Left lateral strike slip motion was active during

period. In

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58El Tigre Fault is observed in the central part of the subduction zone. It is a relatively short strike slip fault (ca. 120 km) that located further landward. The slip rate of the fault system is approximately 1 mm per year. The

Atacama Fault The Atacama Fault Zone (AFZ) is an extensive system of faults cutting across the Chilean Coastal Cordillera in Northern Chile between the Andean Mountain range and the Pacific Ocean. The fault system is North-South striking and runs for more t ...

and the Precordilleran Fault are located in northern

. The

extends more than 1,000 km. It was formed during the Mid to Late

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

period as a left-lateral fault due to oblique subduction of the

Phoenix Plate The Phoenix Plate (also known as the Aluk Plate or Drake Plate) was a tectonic plate that existed during the early Paleozoic through late Cenozoic time. It formed a triple junction with the Izanagi and Farallon plates in the Panthalassa Ocean as ...

. The fault system has been inactive since the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

Period. The right lateral slip rate is estimated to be less than 1 mm per year since the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Precordilleran Fault, also known as the Domeyko fault, is composed of several anastomosing faults (i.e. branching and irregular faults) including Sierra Moreno Fault, West Fault and Limon Verde. Precordilleran Fault was formed in the Late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

. In Neogene period, the fault system changed from left lateral to right lateral motion along with the uplift of the Precordillera.Dilles J, Tomlinson AJ, Martin M, Blanco N (1997) The El Abra and Fortuna complexes: a porphyry copper batholith sinistrally displaced by the Falla Oeste. In: VIII Congresso Geológico Chileno, ACTAS Vol III – Nuevos Antecedentes de la Geologí a del Distrio de Chuquicamata, Periodo 1994–1995, Sessión 1: Geología Regional: pp 1878–1882, Universidad Catolica del Norte, Chile

Forearc sliver

Two major forearc slivers are observed along the Peru-Chile Trench. The Peruvian Sliver, also known as Inca Sliver, has a width of 300 to 400 km and a total length of over 1,500 km. It extends from the

Gulf of Guayaquil The Gulf of Guayaquil is a large body of water of the Pacific Ocean in western South America. Its northern limit is the city of Santa Elena, in Ecuador, and its southern limit is Cabo Blanco, in Peru. The gulf takes its name from the city of Gua ...

in the north to the

Altiplano The Altiplano (Spanish for "high plain"), Collao (Quechua and Aymara: Qullaw, meaning "place of the Qulla") or Andean Plateau, in west-central South America, is the most extensive high plateau on Earth outside Tibet. The plateau is located at the ...

in the south. The continental boundary is located between the Western Cordillera and the Eastern Cordillera. Chiloe Microplate, also known as Chiloe Block, is a forearc sliver that detached along the Liquine Ofqui Fault. It is bounded by

Arauco Peninsula The Arauco Peninsula (Spanish: península de Arauco), is a peninsula in Southern Chile located in the homonymous Arauco Province. It projects northwest into the Pacific Ocean. The peninsula is located west of Cordillera de Nahuelbuta. Geologicall ...

and Chile Triple Junction. The sliver moves northward with a motion rate ranges from 32 mm per year in the south to 13 mm per year in the north.{{Cite journal, last1=Melnick, first1=Daniel, last2=Bookhagen, first2=Bodo, last3=Strecker, first3=Manfred R., last4=Echtler, first4=Helmut P., date=January 2009, title=Segmentation of megathrust rupture zones from fore-arc deformation patterns over hundreds to millions of years, Arauco peninsula, Chile: EARTHQUAKE SEGMENTATION AT ARAUCO, journal=Journal of Geophysical Research: Solid Earth, language=en, volume=114, issue=B1, doi=10.1029/2008JB005788, doi-access=free This northward motion not only caused by the oblique subduction of the Nazca Plate, but also the oblique collision and spreading of the Chile Rise at the southern edge of the sliver.

References

Subduction