Ovda Regio

 

Extensive research has been conducted to describe the structural geology of Ovda Regio. Synthetic aperture radar (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 to find insight into the Regio's deformation and formation mechanisms.^[2] 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.

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.^[6] 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.^[7] 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.^[8][9] In contrast to the folding structures, the ribbons in the western part are randomly distributed.^[7]

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 stack and duplex formation on the southern margin.^[7] 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.^[10]

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.^[8]

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

 

 

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 zones.^[7]

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.^[11]

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

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. However, this model needs a lot amount of time of crustal thickening (1–4 billion years).^[12][13] 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.^[11]

This second model describes the upwelling of a mantle flow (plume) that accommodates the formation of crustal thickening by magmatic underplating and volcanic activities associated with the thin lithosphere.^[11][14] 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.^[2][11]

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 because the mantle underneath the lava ponds are depleted with residual melts as compared to the neighboring undepleted mantle.^[11][15] 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.^[16] 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.