Bach Quadrangle - Structure

Structure

The map region displays a wide variety of structural features, including lineaments associated with ridges, scarps, and polygonal crater walls. Joint- controlled mass movements are most likely responsible for the polygonal crater- wall segments; segments as long as 100 km suggest that these fractures extend deep into the lithosphere. The most conspicuous trends of these lineaments are east-west, N.50° W., and N. 40° E. More trends are north-south, N.20° E., and N.70° E.

Large ridges and scarps are the most prominent structural features in the low-sun-angle Mariner 10 pictures of the Bach region. They are most numerous between long 0° and 90°, where they have no preferred orientation.

Ridges may have been formed by several processes, including tectonism and extrusion, or they may be buried crater-rim segments. Several large ridges may represent uplift of plains materials by normal faulting. Other ridges are arcuate to circular, which suggests that they are segments of old, subdued crater and basin rims. Near Boccaccio (centered at lat 81° S., long 30°), ridges are domical in cross section and have smooth tops with small irregular or rimless craters along their crests; they appear to overlap both a c3 and a c1 crater (FDS l66751). In turn, these ridges are superposed by c3 craters and c4 ejecta. The ridges may be volcanotectonic features, composed of extrusives along fissures. However, they are mapped only as ridges because we cannot determine if they are volcanic material that should be mapped as a separate unit or uplifted intercrater plains. These same structures may have been the source of older plains units.

Lobate scarps are the most common structural landforms in the Bach region. Almost all have convex slope profiles, rounded crests, and steep, sharply defined lobes. Three types are seen in the map region: (1) very small (<50 km long, ~100 m high), irregular scarps that commonly enclose topographically depressed areas; they are restricted to the intermediate and smooth plains units in the eastern part of the map region; (2) small (~100 km long, ~100 m high), arcuate or sinuous scarps, also confined primarily to the intermediate and smooth plains units in the eastern part of the map region; and (3) large (>100 km long, ~1 km high), broadly arcuate but locally irregular or sinuous scarps whose faces are somewhat steeper. Several of these scarps (lat 83° S., long 80°) deform craters and offset preexisting features vertically (FDS 166751). The morphology and structural relations of the scarps suggest that most result from thrust or reverse faults. However, an extrusive origin has been suggested by Dzurisin (1978) for a scarp more than 200 km long that extends from about lat 70° S. to the map border between long 45° and 52°; he based this interpretation on albedo differences between the two sides of the scarp and on partial burial of craters transected by it.

Age relations among structural features are not readily apparent. In the Bach region, the youngest craters cut by a scarp are of c4 age; the oldest crater to superpose a scarp is a c3. These relations suggest that scarp formation occurred in c3 to c4 time. Very smooth plains material flanks some scarps and ridges and, if the material is ponded extrusives or mass-wasted products, may postdate the structures. Scarps and ridges are abundant in intercrater, intermediate, and smooth plains units, but they are not embayed by intermediate and intercrater plains materials. These relations suggest that the structures began to form after emplacement of these two oldest plains units. Some of the oldest craters and basins, such as Cervantes, have polygonal shapes at least as marked as more recent craters, suggesting that some structural lineaments are older than c1 craters.