Abstract

Mercury's tectonic activity was confined to its early history. A synthesis of classical references indicates that its tectonic activity was principally related to (1) a small change in the shape of its lithosphere by tidal despinning, (2) a small change in radius and area by shrinkage due to secular cooling, and (3) the Caloris related events. These activities produced the ancient tectonic grid, the lobate scarps, and the Calorian ridges scarps and grabens, respectively. This low degree of activity was ultimately due to Mercury's small size. In spite of this apparent simplicity, some features are still intriguing. Detailed compilation of lineaments on the entire planet indicates that the grid is not similar to the theoretical despinning grid. Some trends are explained by despinning, but only with unusual mechanical properties of the Mercurian lithosphere, while some other trends are not explained at all by despinning. Examples of unexplained tectonic features in the same region are presented in this paper. Some circular depressions may be interpreted as the result of tectonic or volcano-tectonic subsidence (caldera?). Some exibit narrow and particularly straight grooves which cannot be explained as impact related features, and may be interpreted as open tectonic cracks. The Tolstoj area exhibits hills and grooves which cannot be interpreted as Tolstoj impact related features. Morphological and chronological studies indicate that these features would consist of the extensional tectonic features (horsts and grabens) developed on the convex top of a tectonically uplifted bulge. The tectonic development of this area occurred over a long period of time, and is probably due to a deep and long-lived internal source. These examples show the existence of large- and small-scale internal activities which affect Mercury's surface independently from global or impact related tectonics. Such activities must be taken into account in further models of Mercurian internal structure and history and must be searched in data of future missions. A new Mercurian mission with a complete coverage of image and altimetric/gravimetric data is thus necessary to understand the geology and the tectonic of Mercury.

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