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Ovda Regio is a crustal plateau located near the equator in the western highland region of Aphrodite Terra and stretches from 10oN to 15oS and 50oE to 110oE. Known as the largest crustal plateau in Venus, the Regio covers an area of approximately 15,000,000 km2 and bounded by regional plains to the north, the Salus Tessera to the west, the Thetis Regio to the east, and Kuanja as well as Ix Chel chasmata to the south. Its geologic feature significance of crustal plateau serves as a place to hold the localized tessera terrains in the planet, which makes up roughly 8% of Venus' surface area. [1] The tectonic evolution of crustal plateaus on Venus has been a highly debated topic in the planetary science community. To be able to understand its complex evolution is believed to hold the keys to unlock the fundamental knowledge of geodynamic history of Venus.

Structural Geology

Extensive research has been conducted to describe the structural geology of Ovda Regio. Synthetic aperture radar (SAR) images from the NASA Magellan mission has been continuously analyzed to recognize the distribution of its structural features. The distribution were then mapped to find its temporal and spatial relation to find insight into the Regio's deformation and formation mechanisms. [1] The challenge in this process is to find the ideal temporal and spatial relationship, which holds 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.

Western Ovda

Folds and its distinct compositional layering generally describe the western part of Ovda Regio. Compositional layering means that the structural layers are different in terms of its chemical composition. [2]Particularly, the layers are differentiated based on its 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 a east-west fashion.[3] Another feature that is observed in this part is ribbon 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. [4] In contrast to the folding structures, the ribbons in the western part are randomly distributed. [3]

Central Ovda

The central Ovda is distinguishable by ridges exhibiting similar east-west fashion as in the 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 stack and duplex formation on the southern margin. [3]

Eastern Ovda

In the eastern part of Ovda, the structural setting is define by wide folds with amplitudes up to 25 km and several hundred km in length. [4]

Crustal Plateau Modelling

There are several models that have been debated for its precision in relation to crustal plateau formation in Venus, particularly in Ovda Regio:

The Downwelling Model

This model describes that the mantle downwelling flow assisted the development of crustal thickening because of compression and accretion of thin lithosphere. However, this model needs a lot amount of time of crustal thickening (1-4 billion years). [5] 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.[6]

The Upwelling Model

This second model describes that the mantle upwelling (plume) flow accommodated the formation of crustal thickening by magmatic underplating and volcanic activities associated with the thin lithosphere.[6] Planetary scientists that supports this model identify two categories of extensional structures: a long-narrow graben or also known as ribbons and a wide spaced graben. The sequence of formation for these structures is still debatable. Some group of scientists believed that the ribbons were formed first, followed later on by the wide spaced graben. But there is also another group of scientists believed the reverse sequence. [1][6]

The Impact Model

Under this 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 because the mantle underneath the lava ponds are depleted with residual melts as compared to the neighboring undepleted mantle.[6] However, there are a few issues accompanying this model. The first issue is that scientists are not confidence that meteor impacts have the capabilities to melt a significant portion of the planet's lithosphere and generate enough magma that would cause isostasy. [7] The second issue is large folds need a lot amount of stresses to pass the thin brittle layer, but the underlying magma is not capable to transfer enough stresses through the brittle layer.


References

  1. ^ a b c Ghent, Rebecca; Hansen, Vicki (6 January 1999). "Structural and Kinematic Analysis of Eastern Ovda Regio, Venus: Implications for Crustal Plateau Formation". Icarus (139): 116–136. Retrieved 13 February 2015.
  2. ^ Kroeger, Glenn C. "Exploring Earth". Trinity University. Retrieved 1 March 2015.
  3. ^ a b c Chetty, T.R.K.; Venkatrayudu, M.; Venkatasivappa, V. (24 May 2010). "Structural Architecture and a New Tectonic Perspective of Ovda Regio, Venus". Planetary and Space Science (58): 1286–1297. Retrieved 13 February 2015.
  4. ^ a b Ghent, R.R.; Hansen, V.L. "Structural Analysis of Central and Eastern Ovda Regio, Venus" (PDF). Lunar and Planetary Institute. Lunar and Planetary Science XXVII. Retrieved 13 February 2015.
  5. ^ Kidder, J.G.; Phillips, R.J. (1996). "Convection-driven subsolidus crustal thickening on Venus". Journal of Geophysical Research: 23181–23294. {{cite journal}}: |access-date= requires |url= (help)
  6. ^ a b c d Romeo, I.; Capote, R. (13 June 2011). "Tectonic evolution of Ovda Regio: An example of highly deformed continental crust on Venus?". Planetary and Space Science (59): 1428–1445. Retrieved 13 February 2015.
  7. ^ Ivanov, M.A.; Head, H.J. (2003). "Impacts do not initiate volcanic eruptions". Geology (31): 869–872. {{cite journal}}: |access-date= requires |url= (help)
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