In geology, a
basin is a region where
subsidence
Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...
generates accommodation space for the deposition of sediments. A pull-apart basin is a
structural basin
A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat-lying strata. They are geological depressions, the inverse of domes. Elongated structural basins are also known as syncline ...
where two overlapping (en echelon)
strike-slip fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
s or a fault bend creates an area of
crustal extension
Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere.
Deformation styles
The types of structure and the geometries formed depend on ...
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.
[Frisch, Wolfgang, Martin Meschede, and Ronald C. Blakey. ''Plate tectonics: Continental drift and mountain building''. Springer, 2010.] Pull-apart basins are also referred to as overlapping-tension-zones (OTZ).
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
In geology, transpression is a type of strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally ve ...
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
Two famous localities for continental pull-apart basins are the
Dead Sea
The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Ban ...
and
Salton Sea
The Salton Sea is a shallow, landlocked, highly saline body of water in Riverside and Imperial counties at the southern end of the U.S. state of California. It lies on the San Andreas Fault within the Salton Trough that stretches to the Gul ...
.
Pull-apart basins are amenable to research because sediments deposited in the basin provide a timeline of activity along the fault. The
Salton Trough
The Salton Trough is an active tectonic pull-apart basin, or graben. It lies within the Imperial, Riverside, and San Diego counties of southeastern California, United States and extends south of the Mexico–United States border into the sta ...
is an active pull-apart located in a step-over between the dextral
San Andreas Fault
The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal) ...
and the
Imperial Fault.
[Brothers, D. S., N. W. Driscoll, G. M. Kent, A. J. Harding, J. M. Babcock, and R. L. Baskin. "Tectonic evolution of the Salton Sea inferred from seismic reflection data." Nature Geoscience 2, no. 8 (2009): 581–584.] Displacement on the fault is approximately 6 cm/yr.
The current transtensional state generates normal
growth fault
Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward t ...
s 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
Throwing is an action which consists in accelerating a projectile and then releasing it so that it follows a ballistic trajectory, usually with the aim of impacting a remote target. This action is best characterized for animals with prehensile ...
ranging from 0.2–1.0 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
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
butte
__NOTOC__
In geomorphology, a butte () is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word ''butte'' comes from a French word me ...
s 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
The Matzen oil field is an oil field located in Vienna Basin. It was discovered in 1949 and developed by OMV. It began production in 1949 and produces oil. The total proven reserves
Proven reserves (also called measured reserves, 1P, and reserv ...
has been recast as extensional
graben
In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic conte ...
s produced by pull-apart basins of the
Vienna Basin. The
Dead Sea
The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Ban ...
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
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, wh ...
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
s with high
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
mineralisation. It is believed to be the main structural control on the giant
Escondida deposit in
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 eas ...
.
Geothermal fields are located in pull-aparts for the same reason due to the high heat flow associated with rising magmas.
[Monastero, F. C., A. M. Katzenstein, J. S. Miller, J. R. Unruh, M. C. Adams, and Keith Richards-Dinger. "The Coso geothermal field: A nascent metamorphic core complex." Geological Society of America Bulletin 117, no. 11–12 (2005): 1534–1553.]
References
Further reading
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{{Structural geology
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Tectonics
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