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Ovda Regio is a
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
ian crustal
plateau In geology and physical geography, a plateau (; ; ), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. Often one or more sides h ...
located near the
equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...
in the western highland region of
Aphrodite Terra Aphrodite Terra is one of the three continental regions on the planet Venus, the others being Ishtar Terra and Lada Terra. It is named for Aphrodite, the Greek equivalent of the goddess Venus, and is found near the equator of the planet. Aphrod ...
that stretches from 10°N to 15°S and 50°E to 110°E. Known as the largest crustal plateau in
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
, the regio covers an area of approximately and is bounded by regional plains to the north, Salus Tessera to the west, Thetis Regio to the east, and Kuanja as well as
Ix Chel Ixchel or Ix Chel () is the 16th-century name of the aged jaguar Goddess of midwifery and medicine in ancient Maya culture. In a similar parallel, she corresponds, to Toci Yoalticitl "Our Grandmother the Nocturnal Physician", an Aztec earth God ...
chasmata to the south. The crustal plateau serves as a place to hold the localized tessera terrains in the planet, which makes up roughly 8% of Venus' surface area. The kinematic evolution of crustal plateaus on Venus has been a debated topic in the planetary science community. Understanding its complex evolution is expected to contribute to a better knowledge of the geodynamic history of Venus. It is named after a Marijian forest spirit that can appear as both male and female.


Structural geology

Extensive research has been conducted to describe the structural geology of Ovda Regio.
Synthetic aperture radar Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...
(SAR) images from the NASA Magellan mission have been analyzed to recognize the distribution of its structural features. The distribution was then mapped to find its temporal and
spatial relation A spatial relationD. M. Mark and M. J. Egenhofer (1994), "Modeling Spatial Relations Between Lines and Regions: Combining Formal Mathematical Models and Human Subjects Testing"PDF/ref> specifies how some object is located in space in relation to s ...
to find insight into the Regio's deformation and formation mechanisms. The challenge in this process is to find the ideal temporal and spatial relationships, which hold a prominent role in comprehending the tectonic processes. In terms of structural setting, the Regio is characterized mainly by ribbons, folds, and a complex of
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 cont ...
.


Western Ovda

Folds and a distinct compositional layering generally characterize the western part of Ovda Regio. Compositional layering means that the structural layers differ from each other in terms of their chemical compositions. In particular, the layers are differentiated based on their tone and textural recognition from SAR images. The folds observed in this part of the Regio are concentric, associated with plunges, and share a common axis that is trending in an east–west fashion. Another feature that is observed in this part is ribbons structures. Ribbons can be described as structures that are steep with long depression of about 1–3 km in width and shallow depths of less than 500 m. In contrast to the folding structures, the ribbons in the western part are randomly distributed.


Central Ovda

Central Ovda is distinguishable by ridges exhibiting east–west orientations similar to those of western Ovda. These ridges are common on the northern margin and often share a common axis with the fold structures. Other structural features observed in this part of Ovda are
imbricate Aestivation or estivation is the positional arrangement of the parts of a flower within a flower bud before it has opened. Aestivation is also sometimes referred to as praefoliation or prefoliation, but these terms may also mean vernation: the a ...
stack and duplex formation on the southern margin. A more detailed analysis was conducted in this part indicate that central Ovda hosts a strike-slip tectonic regime where the deformation is accompanied by three different structures: folds, normal faults, and strike-slip faults.


Eastern Ovda

In the eastern part of Ovda, the structural setting is defined mainly by wide folds and ribbons structures. The wide folds are observed to have amplitudes up to 25 km and several hundred km in length. While the ribbons structures generally hold a radial pattern. Some of the ribbons structures on this part of Ovda are quite difficult to interpret due to the SAR images' limited resolution. A good number of grabens are also present in this part, although the grabens are not highly distinguishable and are limited to fold crests.


Kinematic evolution

There are a few ideas being continuously discussed in the planetary science community regarding the tectonic evolution of Ovda Regio:


Regional kinematic evolution

Regionally, there are two separate phases of tectonic evolution. Initially, the Regio was at a stable state where there are no stresses acting on the crustal plateaus. This state was then followed by the first phase in which the north–south-oriented compressional stresses acted on the Regio and produced an east–west folding pattern. This pattern provides the primary structural framework in Ovda Regio. Then, the second phase took place in which the compressional stresses intensified and developed significant mega
shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shea ...
s.


Marginal kinematic evolution

There are generally two different phases of structural evolution that describe the Regio's crustal plateau margins. The initial phase preceded the first phase and the last phase concluded the second phase. The initial phase was when all the material being set in place, which would then construct the tessera terrain. During the first phase, the thrust faults and fold belts started to develop parallel to the margins. At the beginning of the first phase, these faults and folds made an impact on the tessera terrain, but later on it made an impact on the intratessera volcanic plains. On second phase, all the thrust faults and fold belts experienced a perpendicular extension. Furthermore, the last phase occurred when the extensional events continuously carried out the deformed structures from the plateau and affecting the volcanic units.


Dynamic development

There are several models that have been debated to explain crustal plateau formation in Venus, particularly in Ovda Regio:


Downwelling model

This model describes that the mantle downwelling flow assisted the development of crustal thickening and shortening of the ductile crust because of compression and accretion of thin
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...
. However, this model needs a lot amount of time of crustal thickening (1-4 billion years). There are also a few constraints for this model. The first one is that this model provides no explanation for the contractional structures and the second one is that the timing of the extensional structures does not correlate well with the known cross-cutting relationships.


Upwelling model

This second model describes the upwelling of a mantle flow (plume) that accommodates the formation of crustal thickening by
magmatic underplating Magmatic underplating occurs when basaltic magmas are trapped during their rise to the surface at the Mohorovičić discontinuity or within the crust. Entrapment (or 'stalling out') of magmas within the crust occurs due to the difference in relat ...
and volcanic activities associated with the thin lithosphere. Planetary scientists that support this model identify two categories of extensional structures: long-narrow graben, referred to as a ribbons, and more widely spaced graben. The sequence of formation for these structures is still debatable. Some scientists believe that the ribbons were formed first, followed later on by the wide spaced graben. But there are other scientists who believe the reverse sequence.


Impact model

Under the impact model, the crustal plateaus were formed by lava ponds from mantle melting due to meteor impacts to the planet's thin lithosphere. Based on this model, the crustal plateaus would be uplifted by
isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on i ...
because the mantle underneath the lava ponds are depleted with residual melts as compared to the neighboring undepleted mantle. However, there are a few issues accompanying this model. The first issue is that scientists are not confident that meteor impacts have the capabilities to melt a significant portion of the planet's lithosphere and generate enough magma that would cause isostasy. The second issue is the planet's large folds need a high amount of stresses to pass the thin brittle layer, but the underlying magma is not capable of transferring enough stresses through the layer.


References

{{Venus Surface features of Venus