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Geologic evolution of Ishtar Terra

by
Lucia Marinangeli

 

Digital Elevation Model of Ishtar Terra.

The "Magellan" mission to Venus, ended in 1994, provided for the first time images of almost 70% of the Venusian surface. This has been possible by using radar techniques that penetrate through the thick cloud cover surrounding Venus. The new Magellan data have shown how complicated the venusian surface deformation style and geology are. Ishtar Terra is one of the most problematic areas. Ishtar Terra is a large highland in the northern hemisphere of Venus. Ishtar Terra is a complex feature because it is the site of both compression, along its border, and volcanism, at its interior. Four mountain belts (Maxwell, Freyja, Akna and Danu Montes) are present in Ishtar and they bound a large volcanic plateau, Lakshmi Planum. The outermost portion of this region is represented by highly deformed areas, Tesserae, such as Fortuna, Itzpapalotl, Atropos and Clotho Tessera. The presence of mountain belts makes Ishtar unique among the Venusian highlands and implies peculiar crustal mechanism for its formation. The compressional nature of the lineaments composing the mountain belt range, has been recognized since Venera 15-16 and Arecibo radar images data [1,2,3,4,5]. On the radar image, the mountain belts characterized by a very bright tone and it has been interpreted as the result of an altitude-depending weathering effect that has dramatically enhanced the electrical properties of the surface [6].

Our detailed study [7,8] on the geology, stratigraphic and structural analysis has shown that the geological evolution of the study area can be explained in four main steps:

1) formation of the older substrata of surrounding Tessera,
2) extensive plain emplacement,
3) orogenic phase and formation of Akna Montes,
4) local emplacement of younger plains.

The tectonic evolution of Ishtar Terra can be interpred as a sequence of contractional, shear and topographic relaxation events. This kind of sequence is interpreted as a consequence of the variation of crustal stresses and crustal thickenning during the orogenic event as observed for terrestrial high plateau associated with a mountain belt (i.e. Himalaya and Tibet, Ande and Altiplano). At the initial stage of convergence, the biggest stress is horizontal and allows the formation of compressional features.Subsequently, the crust starts to become thicker since the minimum horizontal stress is equal to the vertical one; this is the stage of shear deformation development. The last phase is extensional and arrives when the vertical stress become the higher value.
Using two end-members for structural style of one of the mountain belt, Akna Montes. We estimated the amount of crustal shortening associated with the mountain belt for symmetric folds model and fault-bend folds model. For the first model the amount of shortening is less than 1% whereas values ranging between 17-34% can be reached for the second one. The large difference between these values uderscores the importance of a geodynamic model of the mountain belt formation for strain estimates. These values, however, do place bounds on the amount of strain recorded by Akna.

 

REFERENCES:
[1] Pronin, A.A., The structures of Lakshmi Planum, an indication of horizontal asthenospheric flows on Venus, Geotectonics, 20, 271-281, 1986
[2] Basilevsky, A.T., Structures of central and eastern areas of Ishtar Terra and some problems of venusian tectonics, Geotectonics, 20, 282-288, 1986
[3] Crumpler, L.S., Head, J.W., Campbell, D.B., Orogenic belts on Venus, Geology, 14, 1031-1034, 1986
[4] Head, J.W, The formation of mountain belts on Venus: Evidence for large-scale convergence, underthrusting, and crustal imbrication in Freyja Montes, Ishtar Terra, Geology, 18, 99-102, 1990
[5] Vorder Bruegge, R.W., Head, J.W., Campbell, D.B., Orogeny and large-scale strike-slip faulting on Venus: Tectonic evolution of Maxwell Montes, J. Geophys. Res., 95,357-8,381, 1990
[6] Pettengill, G.H., Ford, P.G. and Wilson L., Venus surface radiothermal emission as observed by Magellan, J. Geophys. Res., 97, 15,923-15,948, 1992
[7] Marinangeli L., Geologia dell'Ishtar Terra Occidentale (Venere) ed evidenze geologiche del comportamento litosferico del pianeta, PhD Thesis, Universita' di Bologna, 1998
[8] Marinangeli, L. & Gilmore M.S., Geologic history of the Akna Montes-Atropos Tessera region, in preparation


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