engineering

Pull-apart basin

Pull-apart basin is a engineering 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 Pull-apart basin rather than just read about it. In short: In geology, a basin is a region where subsidence generates accommodation space for the deposition of sediments. A pull-apart basin is a structural basin where two overlapping (en echelon) strike-slip faults or a fault bend create an area of crustal extension undergoing tension, which causes the basin to sink down.

Key takeaways

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

Reference excerpt

In geology, a basin is a region where subsidence generates accommodation space for the deposition of sediments. A pull-apart basin is a structural basin where two overlapping (en echelon) strike-slip faults or a fault bend create an area of crustal extension undergoing tension, which causes the basin to sink down. Frequently, the basins are rhombic or sigmoidal in shape. Dimensionally, basins are limited to the distance between the faults and the length of overlap.

Mechanics and fault configuration

The inhomogeneity and structural complexity of continental crust causes faults to deviate from a straight course and frequently causes bends or step-overs in fault paths. Bends and step-overs of adjacent faults become favorable locations for extensional and compressional stress or transtension and transpression stress, if the shear motion is oblique. Pull-apart basins form in extensional to transtensional environments along fault bends or between two adjacent left-lateral faults or two right-lateral faults. The step-over or bend in the fault must be the same direction as sense of motion on the fault otherwise the area will be subject to transpression. For example, two overlapping left lateral fault must have a left-step-over to create a pull-apart basin. This is illustrated in the accompanying figures. A regional strike slip fault is referred to as a principle displacement zone (PDZ). Connecting the tips of step over faults to the opposite fault are bounding basin sidewall faults. The tectonic subsidence of strike-slip basins is mainly episodic, short lived (typically less than 10 Ma), and end abruptly with commonly very high tectonic subsidence rates (greater than 0.5 km/Ma) compared to all other basin types. Recent sandbox models have shown that the geometry and evolution of pull-apart basins varies greatly in pure-strike slip situations versus transtensional settings. Transtensional settings are believed to generate greater surface subsidence than pure-strike slip alone.

Examples Famous localities for continental pull-apart basins are the Dead Sea, the Salton Sea, and the Sea of Marmara. Pull-apart basins are amenable to research because sediments deposited in the basin provide a timeline of activity along the fault. The Salton Trough is an active pull-apart located in a step-over between the dextral San Andreas Fault and the Imperial Fault. Displacement on the fault is approximately 6 cm/yr. The current transtensional state generates normal growth faults and some strike slip motion. The growth faults in the region strike N15E, have steep dips (~70 deg), and vertical displacements of 1–4 mm/yr. Eight large slip events have occurred on these faults with throw ranging from 0.2 to 1.0 meters. These produce earthquakes greater than magnitude six and are responsible for the majority of extension in the basin and consequently thermal anomalies, subsidence, and localization of rhyolite buttes such as the Salton Buttes.

Economic significance Pull-apart basins represent an important exploration target for oil and gas, porphyry copper mineralisation, and geothermal fields. The Matzen fault system in the Matzen oil field has been recast as extensional grabens produced by pull-apart basins of the Vienna Basin. The Dead Sea has been studied extensively and thinning of the crust in pull-aparts may generate differential loading and instigate salt diapirs to rise, a frequent trap for hydrocarbons. Likewise intense deformation and rapid subsidence and deposition in pull-aparts creates numerous structural and stratigraphic traps, enhancing their viability as hydrocarbon reservoirs. The shallow extensional regime of pull-apart basins also facilitates the emplacement of felsic intrusive rocks with high copper mineralisation. It is believed to be the main structural control on the giant Escondida deposit in Chile. Geothermal fields are located in pull-aparts for the same reason due to the high heat flow associated with rising magmas.

References

Further reading Bally, A.W.; Snelson, S. (1980), "Realms of subsidence", Canadian Society for Petroleum Geology Memoir, 6: 9–94 Kingston, D.R.; Dishroon, C.P.; Williams, P.A. (1983), "Global Basin Classification System" (PDF), AAPG Bulletin, 67: 2175–2193, retrieved 2017-06-23 Klemme, H.D (1980), "Petroleum Basins – Classifications and Characteristics", Journal of Petroleum Geology, 3 (2): 187–207, Bibcode:1980JPetG...3..187K, doi:10.1111/j.1747-5457.1980.tb00982.x, retrieved 2017-06-23

Worked examples

Example 1 — a first encounter with Pull-apart basin

Start with the simplest possible case. Write down what Pull-apart basin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, 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 Pull-apart basin 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 Pull-apart basin 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 Pull-apart basin

In research: Pull-apart basin appears in engineering 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 Pull-apart basin 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: Pull-apart basin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Pull-apart basins, Sedimentology, Structural basins, so understanding it makes those chapters shorter.
In everyday life: Look for Pull-apart basin 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 Pull-apart basin in 20 minutes

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

Frequently asked questions

What is Pull-apart basin in simple terms?

Why does Pull-apart basin matter?

Because it connects several engineering 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 Pull-apart basin?

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 Pull-apart basin.