Tolstoj Quadrangle - Structure

Structure

The circumbasin scarps around Caloris, Tolstoj, and Mozart are the most prominent structural features in the quadrangle. The main Caloris Montes scarp is thought to approximate the edge of the basin of excavation of Caloris and is probably a structural and stratigraphic counterpart of the Montes Rook scarp around the Orientale Basin on the Moon. A subdued outer scarp is present around most of the visible part of Caloris, better seen in the Shakespeare quadrangle to the north. This scarp is generally coincident with the transition between the massifs of the Caloris Montes Formation and the lineated facies of the Van Eyck Formation. The roughly rectilinear outlines of massifs within the Caloris Montes suggest structural control by a prebasin fracture pattern. The much lower, discontinuous outer scarp is considered to be the feeble equivalent of the Montes Cordillera scarp around Orientale. Like the Cordillera, it probably lies outside the limit of the crater of excavation. Its poor development and spacing much closer to the edge of the basin may be due to the greater mercurian gravity, as described by Gault and others. The Van Eyck Formation is characterized by an extensive radial ridge-and-valley system with minor concentric scarps and lineaments. These features are considered for the most part as gouges and depositional plumes from secondary cratering within the Van Eyck; the remarkably straight ridges and steep walls, however, suggest formation by fracturing.

Only a small part of the ridge and fracture system that characterizes the floor of Caloris is within the quadrangle. The ridges in the floor of Caloris, which are like those within the smooth plains, do not appear to be as complex as lunar mare ridges and are cut by numerous open grabenlike gashes. This area and its antipode in the Discovery quadrangle are the only two on Mercury where tensional forces can now be seen to have shaped the surface.

The Tolstoj Basin is encompassed by parts of at least three ragged and discontinuous inward-facing scarps. Lineated ejecta is best developed in the vicinity of and beyond the outer scarp, whereas blocky materials occur between the inner and outer scarps. These relations are similar to those around Caloris, although Tolstoj is less than half its size and is much more severely degraded by later impact cratering.

The sharpness of the single rim-scarp of Mozart reflects the youth (younger than the smooth plains) of this large impact. The position of Mozart at the west terminator of the Mariner 10 image data precludes visibility of its floor and thus hides any evidence of a possible central uplift or inner structural ring.

Lobate scarps or ridges, which are best seen within the smooth plains material and vary locally within the intercrater plains material, are generally steep on one side and gently dipping on the other. Some, like the lunar mare ridges, appear to mark the outlines of subjacent craters. Most workers, particularly Strom and others, Melosh, and Melosh and Dzurisin, have ascribed these ridges to compression and a slight shortening of the crust of Mercury after formation of most of the present surface. Some ridges, however, may represent flow fronts, but their estimated heights of several hundred meters would require formation by extraordinarily viscous lavas.

Numerous faint lineaments are visible within the quadrangle, especially in the area between the Tolstoj Basin and the large crater Zeami to the northeast. Many of these lineaments may be faint secondary-crater chains or gouges; others may represent traces of an ancestral structural pattern that partly controlled the excavation of the craters and basin. The lineaments may have been enhanced or preserved by the gentle upwarping of this region of Tolstoj ejecta discussed above. The largest lineament, which marks the northwest limit of recognizable Tolstoj ejecta, is a subdued scarp some 450 km long. Rejuvenation of earlier faults or fractures by subsequent impacts probably occurred throughout the history of the planet. Thus, except for the lobate compressional scarps, it is difficult to separate internally produced structures from those of the complex impact history of Mercury. The azimuthal trends of all lineaments mapped within the quadrangle are, however, dominantly northwest (315°) and northeast (35°–40°). A minor, almost north-south trend is also observed. This situation is reminiscent of the so-called lunar grid on the Moon, which is generally ascribed to planetwide internal causes.