ARE THERE OTHER TECTONICS THAN TIDAL DESPINNING, GLOBAL CONTRACTION AND CALORIS RELATED EVENTS ON MERCURY - A REVIEW OF QUESTIONS AND PROBLEMS

Mercury's tectonic activity was confined to its early history. A synth esis of classical references indicates that its tectonic activity was principally related to (1) a small change in the shape of its lithosph ere by tidal despinning, (2) a small change in radius and area by shri nkage due to secular cooling, and (3) the Caloris related events. Thes e activities produced the ancient tectonic grid, the lobate scarps, an d the Calorian ridges, scarps and grabens, respectively. This low degr ee of spite of this apparent simplicity, some features are still intri guing. Detailed compilation of lineaments on the entire planet indicat es that the grid is not similar to the theoretical despinning grid. So me trends are explained by despinning, but only with unusual mechanica l 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-t ectonic subsidence (caldera?). Some exibit narrow and particularly str aight grooves which cannot be explained as impact related features, an d may be interpreted as open tectonic cracks. The Tolstoj area exhibit s hills and grooves which cannot be interpreted as Tolstoj impact rela ted features. Morphological and chronological studies indicate that th ese features would consist of the extensional tectonic features (horst s and grabens) developed on the convex top of a tectonically uplifted bulge. The tectonic development of this area over a long period of tim e, and is probably due to a deep and long-lived internal source. These examples show the existence of large and small-scale internal activit ies which affect Mercury's surface independently from global or impact related tectonics. Such activities must be taken into account in furt her models of Mercurian internal structure and history, and must be se arched in data of future missions. A new Mercurian mission with a comp lete coverage of image and altimetric/gravimetric data is thus necessa ry to understand the geology and the tectonic of Mercury. (C) 1997 Pub lished by Elsevier Science Ltd.