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)[2] for which the convergence direction differs from 90° to the
plate boundary.[3] Most
convergent boundaries involve oblique subduction,[3] particularly in the
Ring of Fire including the
Ryukyu,
Aleutian,
Central America and
Chile subduction zones.[4] In general, the obliquity angle is between 15° and 30°.[5] Subduction zones with high obliquity angles include
Sunda trench (ca. 60°) and
Ryukyu arc (ca. 50°).[5]
Obliquity in plate convergence causes differences in
dipping angle and subduction velocity along the plate boundary.[6][7] Tectonic processes including
slab roll-back, trench retreat (i.e. a tectonic response to the process of slab roll-back that moves the
trench seaward)[8] and slab fold (i.e. buckling of subducting slab due to
phase transition)[9] may also occur.[6][7]
Moreover, collision of two plates leads to
strike slip deformation of the
forearc, thus forming a series of features including forearc slivers and
strike slip fault systems that are sub-parallel to
ocean trenches.[10] In addition, oblique subduction is associated with the closure of ancient ocean, tsunami and block rotations in several regions.[11][12][13]
Deformation features
Forearc slivers
Forearc slivers are partly detached continental blocks of the overriding plates.[14] They are bounded by the
trenches and trench parallel
strike slip fault systems.[14] The motion of forearc slivers depend on the obliquity of the subducting slabs.[14]
Moreover, some forearc slivers occur in the absence of well defined strike-slip fault systems, and sliver motions are not purely strike-slip.[15]
Trench parallel strike-slip fault systems
Trench parallel
strike-slip faults are deformational products contributed by trench parallel component of
strain partitioning.[10] They are located between the forearc slivers and the remaining overriding plates.[10]
Vertical
strike slip fault systems are generally accepted by the early literature of oblique subduction.[10] 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.[19][20]
Three hypothetical models of strike slip fault systems
Hypothetical models
Figures
Description
Vertical fault model
During oblique subduction, the convergence and coupling between two plates create horizontal
shear stress on the overriding plate.[10] Early studies suggested that horizontal shear is likely to concentrate in vertical planes.[10] Together with the field measurements on seismicity.[10] The trench parallel
strike slip fault is thought to be vertical from earth surface down to the subducting plate.[10]
Mega-splay fault system model
In
Nankai Trough (Formed by oblique subduction of the
Philippine Sea Plate),[22] seismic profiles reveal that the margin parallel strike slip fault and thrust structures are linked by the mega splay fault system, which align in a parallel manner with the subducting plate (i.e.
Philippine Sea Plate).[21]
Curved fault model
The
Liquiñe-Ofqui Fault is a trench parallel
dextralstrike-slip fault in
Andes. Based on analysis on shear stress distribution,[19] Ormeño et al., (2017) suggested that it is a curving strike slip fault.[19] The hypothetical geometry coincides with an curving reflector obtained in the seismic reflection profile of the
subduction zone.[23]
Slip accommodating mechanisms
Trench parallel slip component from oblique subduction may not be fully accommodated by the aforementioned trench parallel
strike slip faults.[24] Several models suggest that there are other slip accommodating mechanisms formed by oblique subduction as means to take up the remaining slip component.[24]
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.[24]
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.[24][25] The result shows that the trench parallel slip component of at least 32 mm per year is left.[24]
On 11 April 2012, a Mw 8.6
earthquake occurred in the subducting plate (i.e. the
Indo-Australian Plate).
Strike-slip seismicity was recorded in the earthquake.[24] This infers strike slip fault systems are present in the descending slab and they may potentially accommodate slip component from oblique subduction.[24]
Comparison between trench parallel strike slip faults[24]
Strain partitioning is a form of deformation. In oblique subduction zone, strain partitioning is initiated into trench parallel component and trench normal component.[26] 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.[27] Likewise, this component commonly leads to the formation of forearc slivers.[27] The trench normal component is taken up by
thrust structures.[28] These
thrusts are generally discontinuous and their geometries change progressively.[29][20]
Short-term deformation: Localized shear zone
Short-term deformation is mainly
elastic and acts at human time scale (i.e. perceptible during a human lifetime, unlike changes that take place on a
geologic time scale).[30] When the denser plate subducts beneath the upper plate, they are coupled at the interface (i.e. plate coupling).[31][32][33] The process of plate coupling thus generates tectonic force that follows the subduction direction.[27]
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.[27][32][34] In this way, only the frontal part, rather than the whole upper plate, is dragged by the subducting slab.[27]
Long-term deformation: Formation of forearc sliver and strike slip fault
Long-term deformation occurs at
geological time scale.[30] Under continuous oblique subduction, the aforementioned frontal part of the upper plate permanently accommodates the trench parallel component.[27][34] In this way, the orientation of tectonic force rotates gradually toward the trench parallel direction.[27]
Strong and continuing tectonic force in trench parallel direction leads to the development of trench parallel
strike slip fault system.[27] The fault thus separate a portion of the
forearc from the overriding plate, forming the forearc sliver.[27]
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.[11]
In the plate boundary, an approximately 80 km long and 30 km wide depression is observed.[11] It obscures trench parallel
strike slip fault and the topographic ridge of the wedge.[11]
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.[39] In these regions, the trench parallel
strike slip fault systems are less prominent.[12] This is because a portion of the trench parallel component is accommodated by the microblock rotation.[12]
The North Island oblique subduction zone in
New Zealand was established by the obliquely subducting
Pacific Plate beneath the
Indo-Australian Plate.[12] A trench parallel strike slip fault system,
North Island Dextral Fault Belt, was formed.[12] Based on geological and geodetic data, five tectonic blocks are identified in the region.[12] These blocks are separated by block-bounding faults.[12]
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
Indo-Australian Plate is observed.[12] This caused tectonic extension in
Taupo Volcanic Zone and tectonic shortening in northwestern
South Island, for example the
Buller region.[12]
In addition, the block rotation accommodates 25% to 65% of the trench parallel component from oblique subduction.[12] Therefore, high rate trench parallel
strike slip faults are absent in the
North Island.[12]
Rotation mechanism
In the oblique subduction zone, the sinking slab is characterized by the
Hikurangi plateau in the south.[12] The thickness of this
oceanic plateau ranges from 15 km to 10 km along the
oceanic trench.[12] The along strike thickness variation leads to differential subduction rate.[12] In the southern
trench, thick
oceanic plateau induces high collisional resistance forces that cripples the subduction process.[12] However, the thin
oceanic crust in the north is subducted. This activated the tectonic block rotations about a nearby axis.[12]
Tectonic features of oblique subduction, for example a right lateral strike-slip thrust belt are identified in the tectonic zone.[40] These evidence suggest that the
south China plate was obliquely subducted to the northwest beneath the
north China plate in the Early
Mesozoic and led to the closure of the northeastern
Paleo-Tethys Ocean.[40]
The
Peru–Chile Trench 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.[27] The current subduction direction is at east-north-east (see the summary below).[41] However, geological record shows southeast subduction direction in Late
Cretaceous period.[42]
Four major trench parallel
strike slip faults are identified in the oblique subduction zone.[27]Liquiñe-Ofqui Fault is a 1,200 km long fault that located in the southern Andes.[45] Left lateral strike slip motion was active during
Mesozoic period.[46] In
Pliocene period, strike slip motion of the fault system changed to right lateral motion to accommodate the trench parallel slip component from oblique subduction.[47][48]
The
El Tigre Fault is observed in the central part of the subduction zone.[27] It is a relatively short
strike slip fault (ca. 120 km) that located further landward.[49] The slip rate of the fault system is approximately 1 mm per year.[49]
The
Atacama Fault and the
Precordilleran Fault are located in northern
Chile. The
Atacama Fault extends more than 1,000 km.[50] It was formed during the Mid to Late
Jurassic period as a left-lateral fault due to oblique subduction of the
Phoenix Plate.[51] The fault system has been inactive since the
Miocene Period. The right lateral slip rate is estimated to be less than 1 mm per year since the
Pliocene.[52]
The
Precordilleran 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.[53]Precordilleran Fault was formed in the Late
Eocene.[54] In
Neogene period, the fault system changed from left lateral to right lateral motion along with the uplift of the
Precordillera.[55][56][57]
Forearc sliver
Two major forearc slivers are observed along the
Peru-Chile Trench.[59][60][58] 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.[59] It extends from the
Gulf of Guayaquil in the north to the
Altiplano in the south.[60] The continental boundary is located between the Western Cordillera and the Eastern Cordillera.[60]
Chiloe Microplate, also known as Chiloe Block, is a forearc sliver that detached along the
Liquine Ofqui Fault.[58] It is bounded by
Arauco Peninsula and
Chile Triple Junction.[58] 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.[58] 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.[58]
^Rosenau M (2004) Tectonis of the Southern Andean intra-arc zone
(38°–42°S), PhD thesis, Freie Universität Berlin
^Hervé, F. (1977) Petrology of the Crystalline Basement of the Nahuelbuta Mountains, South-Central Chile. In: Ishikawa, T. and Aguirre, L., Eds., Comparative Studies on the Geology of the Circum—Pacific Orogenic Belt in Japan and Chile, Japanese Society for the Promotion of Science, London, 1-52.
^Tomlinson AJ, Blanco N (1997b) Structural evolution and displacement history of the West Fault system, Precordillera, Chile: part II, postmineral history. 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, Universidad Catolica del Norte, pp 1878–1882
^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