Borealis Quadrangle - Structure

Structure

One of the major differences between the mercurian and lunar surfaces is “the widespread distribution of lobate scarps which appear to be thrust or reverse faults resulting from a period of crustal compression...” These scarps are unique structural landforms that were noted soon after the acquisition of Mariner 10 photographs. Murray and others (1974) described them as having a sinuous outline, a slightly lobate front, and a length of more than 500 km. A more detailed description is given by Strom and others. Dzurisin (1978) classified these scarps, differentiating between intercrater and intracrater scarps (a scheme adopted in mapping the Borealis region) in an attempt to understand the tectonic and volcanic history of Mercury. Melosh (1977) and Melosh and Dzurisin (1978) proposed a planetary grid composed of conjugate northeastand northwest-trending shear fractures formed by the stresses of tidal despinning early in mercurian history. They thought that these fractures were later modified, and predicted that east-trending normal faults caused by tensional stresses would be found in the polar regions. In a later report, Pechmann and Melosh (1979, p. 243) stated that “the NE and NW trends become nearly N-S in the polar regions.”

The northwest-trending component of the postulated global grid of fractures is markedly absent in the Borealis region. Northeast-trending scarps and troughs are conspicuous, however, across intercrater plains material and in crater fill (smooth plains material) between the 155° and 185° meridians, and from crater Van Dijck northward to crater Purcell and beyond. The scarps tend to be straight in intercrater plains material, but become notably lobate in crater fill (for example, within Saikaku). This set of northeast-trending scarps and troughs, and another set of north-trending scarps and troughs within and north of crater Van Dijck, probably follow zones of structural weakness in the mercurian crust. Ancient fractures that were reactivated by later impacts may have first provided the conduits for crater fill (smooth plains material) and later been propagated upward through the fill. That these ridges, scarps, and troughs are parts of a global grid of fractures cannot be stated conclusively because of their proximity to the terminator and the lack of photographic coverage beyond the 190° meridian. Some scarps probably were formed by normal faulting of the smooth plains material that covers some crater floors, as in the Kuiper quadrangle (Scott and others, 1980). We cannot, however, determine whether most lineaments are internal or are parts of a faulted and lineated facies associated with a nearby but unphotographed impact basin. Melosh (1977) predicted that normal east-trending faults would form in high Mercurian latitudes as a result of slight crustal shortening. His predicted faults may be represented by a generally east-northeast-trending scarp and a lineament that cut across intermediate plains material and the crater Jókai between the 125° and 155° meridians. The north pole is too close to the terminator to detect the presence or absence of a “polygonal arrangement without preferred orientation,” as predicted by Melosh and Dzurisin (1978, p. 233).

Arcuate and radial lineaments that might result from tectonic adjustments of the Mercurian crust, following excavation of very large multiring impact basins such as the one postulated under Borealis Planitia (Boyce and Grolier, 1977), were not unambiguously identified in the Borealis region. On one hand, some ridges on the surface of the smooth plains material in Borealis Planitia may be of structural (internal) origin; this type of ridge elsewhere on Mercury has been ascribed to compression and a slight shortening of the crust (Melosh, 1977; Melosh and Dzurisin, 1978). On the other hand, the wrinklelike sinuous ridge along the northeast border of the Goethe Basin, together with the outward-facing concentric scarps along its periphery, may represent the fronts of lava flows that are associated with the development of a structural moat between the basin fill and the wall. The latter interpretation supports the view that impact craters and basins on Mercury, as on the Moon (Schultz, 1977) and Mars, “have played a dominant role in controlling the surface expression of igneous activity” (Schultz and Glicken, 1979, p. 8033). Slow, long-lasting isostatic adjustment of the basin floor may have continued well after the emplacement of the basin fill, a structural situation similar to that of crater Posidonius on the Moon (Schaber and others, 1977, Schultz, 1977).

In Borealis Planitia, however, most of the ridges are of external origin. They appear either to outline the rim crests of subjacent ghost craters that are lightly mantled by smooth plains material or to be lava flow fronts. The map shows the rim crests of 20 ghost craters, ranging in diameter from 40 to 160 km, that are buried under the smooth plains material of Borealis Planitia, which material is coextensive with the fill covering the floor of the Goethe Basin. In addition, ejecta from the crater Depréz extend more than 40 km eastward beyond a circular scarp that may represent the rim crest of a buried crater 170 km in diameter (FDS 156, 160) or, more likely, the front of lava flows. The size and density of these ghost craters suggest that, prior to emplacement of smooth plains material, the original heavily cratered surface of Borealis Planitia—which may have been the cratered floor of a very large multiring impact basin—and the cratered floor of the Goethe Basin were similar in composition and age to the intercrater plains material of the highlands to the west. Many scarps in Borealis Planitia are subconcentric to the rim of the Goethe Basin and have steeper slopes that face away from it, suggesting that they represent the fronts of lava flows that resurfaced extensive areas of heavily cratered terrain (intercrater or older plains material).