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Oblique subduction

Oblique subduction is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Oblique subduction rather than just read about it. In short: 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. Most convergent boundaries involve oblique subduction, particularly in the Ring of Fire including the Ryukyu, Aleutian, Central America and Chile subduction zones.

Oblique subduction — main illustration
Oblique subduction — illustration

Key takeaways

  • Oblique subduction belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Oblique subduction to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Oblique subduction from memory before moving on to harder problems.

Reference excerpt

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. Most convergent boundaries involve oblique subduction, particularly in the Ring of Fire including the Ryukyu, Aleutian, Central America and Chile subduction zones. In general, the obliquity angle is between 15° and 30°. Subduction zones with high obliquity angles include Sunda trench (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 seaward) and slab fold (i.e. buckling of subducting slab due to phase transition) may also occur. 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. 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 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.

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 Mw 8.6 earthquake occurred in the subducting plate (i.e. the Indo-Australian Plate). 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 structures. These thrusts are generally discontinuous and their geometries change progressively.

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). 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 forearc 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.

Example: North Island oblique subduction zone

… excerpt ends here. Continue reading the full article.

Illustrations

Oblique subduction: Simplified model of oblique subduction. The oblique subduction motion is composed of motion vectors that are parallel and orthogonal to plate boundary.[1] The obliquity of plate convergence is compensated by the relative motion between forearc sliver and the remaining overriding plate.[1] In this way, the relative motion between the overriding plate and the subducting plate is almost perpendicular to the plate boundary.[1] Adapted from Westbrook, 2005.[1]
Simplified model of oblique subduction. The oblique subduction motion is composed of motion vectors that are parallel and orthogonal to plate boundary.[1] The obliquity of plate convergence is compensated by the relative motion between forearc sliver and the remaining overriding plate.[1] In this way, the relative motion between the overriding plate and the subducting plate is almost perpendicular to the plate boundary.[1] Adapted from Westbrook, 2005.[1]
Oblique subduction: Oblique subduction model with the development of forearc sliver and margin parallel strike slip fault. Forearc sliver is a microplate bounded by the oceanic trench and strike slip fault.[14] Trench parallel strike slip fault develops when the forearc sliver moves away from stable continent.[14] Adapted from Haq and Davis, 2010.[14]
Oblique subduction model with the development of forearc sliver and margin parallel strike slip fault. Forearc sliver is a microplate bounded by the oceanic trench and strike slip fault.[14] Trench parallel strike slip fault develops when the forearc sliver moves away from stable continent.[14] Adapted from Haq and Davis, 2010.[14]
Oblique subduction: A vertical strike slip fault model. The red line indicates the vertical fault. The fault extends from surface down to the subducting slab.[10]
A vertical strike slip fault model. The red line indicates the vertical fault. The fault extends from surface down to the subducting slab.[10]
Oblique subduction: A mega splay fault system model. The strike slip fault is suggested to be one of the branches in the mega splay fault, which also links thrust faults in the forearc.[21] The mega splay fault is subparallel to the subducting plate at depth.[21] Modified from Tsuji et al., 2014.[21]
A mega splay fault system model. The strike slip fault is suggested to be one of the branches in the mega splay fault, which also links thrust faults in the forearc.[21] The mega splay fault is subparallel to the subducting plate at depth.[21] Modified from Tsuji et al., 2014.[21]
Oblique subduction: A curved strike slip fault model. Adapted from Ormeño. et al., 2017[19]
A curved strike slip fault model. Adapted from Ormeño. et al., 2017[19]

Worked examples

Example 1 — a first encounter with Oblique subduction

Start with the simplest possible case. Write down what Oblique subduction claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Oblique subduction before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Oblique subduction ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Oblique subduction

In research
Oblique subduction appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Oblique subduction in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Oblique subduction is common in secondary-school and first-year university syllabi. It links to neighbouring topics Subduction, so understanding it makes those chapters shorter.
In everyday life
Look for Oblique subduction outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Oblique subduction in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Oblique subduction means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Oblique subduction out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Oblique subduction in simple terms?

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. Most convergent boundaries involve oblique subduction, parti…

Why does Oblique subduction matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Oblique subduction?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Oblique subduction.

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  • Subduction

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