The purpose of geologic mapping of a planet's surface is to characterize and classify areas and materials and to organize information for ready analysis in terms of geologic processes and history. In common with terrestrial geologic mapping, stratigraphic procedures are used to define the distribution and geometry of planetary rock bodies. Rock units are delineated on the basis of their composition and related lithic characteristics without regard to the mode of origin. Boundaries between units are placed at positions of change in physical properties [North American Commission on Stratigraphic Nomenclature, 1983]. The relative age of rock units is determined primarily from contact relations with adjacent units. Stratified rock units that commonly occur in tabular form are younger in the order of their superposition- younger units lie above older units. Nonstratified units that are commonly massive are generally younger than the host rocks they transect or deform.
In mapping the Moon, Wilhelms [1970 and 1972] and others used surface albedo, morphology, topography, and color to define units. The units can be thought of as "morphostratigraphic material units," with Ethology inferred from other physical characteristics. Lunar geologic units were mapped to reflect consistency in processes of formation and were subdivided into discreet time intervals [Wilhelms, 1972]. A variety of observational criteria define and characterize a unit, including texture (e.g., the presence of ridges, the presence of comical hills, and the degree of radar roughness), albedo, and topographic information. For the Magellan data, emissivity and radar reflectivity are also used to characterize geologic units. Standard symbols are used to show morphologic features such as ridges and troughs [Compton, 1962].
Previous mapping of Venus using Arecibo and Venera 15/16 data has produced geomorphologic/geologic maps of Venus with the definition of units based mainly on morphology and topography [Sukhanov et al., 1989; Campbell and Campbell, 1990; Senske et al., 1991a and 1991 b]. Analyses of Magellan radar images indicate that geologic terrains defined previously in Venera and Arecibo radar images have complex and variable characteristics. Magellan data can be used to compile maps based on geomorphology, as was done with the Venera 15/16 data [Sukhanov et al., 1989].
Geomorphologic maps do not necessarily represent stratigraphic rock units, but they are extremely useful in determining the regional- and global-scale distribution of features such as ridge belts and coronae. Other maps that concentrate on a particular type or particular types of features, such as structural maps, can be produced.
True geologic maps, that include the identification of rock-stratigraphic units, are difficult to construct and require the use of Magellan image, altimetry, emissivity, reflectivity, roughness, and, where possible, lander geochemical data.
This guide includes an example of a map compiled for Gula Mons in western Eistla Regio. The map combines both geologic and geomorphologic units. The area is centered at 22°N, 358°E, at a highland that rises over 2 km above the surrounding plains (Figure 10-1). Eistla Regio is composed of an east-west trending group of uplands that cover a distance of over 6000 km and link up with Aphrodite Terra to the east. Western Eistla Regio is dominated by two large volcanic constructs, Sif Mons and Gula Mons. These volcanoes are cut by extensional faults and graben and are surrounded by extensive radar-bright and -dark lava flows [Campbell and Campbell, 1990 and 1992; Senske et al., 1991b and 1992].
 Geologic Map of Gula Mons
A geologic map of Gula Mons and its surrounding area is shown in Figure 10-2. Thirteen units are identified on the basis of variations in surface texture, patterns of tectonic features, and an assessment of the degree of radar backscatter (ranging from bright to dark). The region was mapped at the Compressed Once Mosaicked Image Data Record (Cl-MIDR) scale and includes seven plains units (including units mapped on Gula Mons and surrounding low lying areas), two tectonic terrain units, one volcanic/tectonic unit, and three surficial materials units (crater materials and bright and dark diffuse deposits). In addition, structural features including depressions, faults, fractures, and small domes are identified.
Plains. Plains make up a large part of the lowland area surrounding Gula Mons; three classes are identified: (1) reticulate plains (Pr); (2) mottled plains (Pm); and (3) lobate plains (Pl) (Figure 10-2). Areally extensive reticulate plains are typically radar-dark and contain few identifiable volcanic centers, i.e., small shields (diameters of 2 to 10 km). Pervasive within this unit are the low, narrow, sinuous ridges responsible for its reticulate character. Mottled plains, located on the distal part of the highland in the northwest part of the area, contain abundant small volcanoes that often lie in clusters forming shield fields. The mottled texture is due to the presence of numerous lava flows that range from rough (bright) to smooth (dark) at radar wavelengths. The edge of this unit is embayed by reticulate plains, indicating that the mottled plains predate the reticulate plains. Localized areas of radar-bright lobate plains are made up of linear, long and narrow (500 km long and 10 to 20 km wide) lava flows. Source areas for these lava flows are associated with a corona or vents within the reticulate plains.
Lava Flows. Although numerous lava flows are observed at Gula Mons, the units at this volcano are simplified to show the major flow complexes. Three assemblages of lava flow units are identified: (1) mottled dark plains (Pmd); (2) bright/dark digitate plains (Pd); and (3) bright plains (Pb) [Senske et al., 1992]. Laterally extensive mottled dark plains radiate for distances of 300 to 600 km from the summit of the peak and are superposed on the surrounding regional plains units. This unit is composed of narrow (5 to 10 km) 150- to 200-km-long lava flows that are relatively smooth at radar wavelengths and is interpreted to represent an episode of large-scale lava emplacement early in the history of the volcano. The central part of the construct (bright/dark digitate plains) is made up of narrow, radar-bright lava flows 100 to 300 km long and interpreted to be associated with constructional volcanism at the central part of the volcano. The summit of Gula (bright plains) is cut by a northeast/southwest trending radar-bright linear zone of faulting (150 km long and 30 km wide). Other deposits, located in the western part of the mapped area, are associated with volcanic construction at Sif Mons and are collectively identified as a Sif Mons Unit (S).
Coronae. Adjacent to the northern part of Gula are two coronae (CO). These volcanic/tectonic features are circular to elliptical in plan view and are characterized by an annulus of concentric ridges or fractures/faults. The interiors of coronae are complex and contain lava flows, domes, and ridges and fractures that are arrayed radially or concentrically to the feature [Pronin and Stofan, 1990]. The first corona, IdemKuva (25°N, 358°E), is a nearly circular, 225-km-diameter structure 600 m high. This feature is surrounded by graben that are arrayed in a concentric pattern and lie within flanking U-shaped troughs. The troughs are source regions for lobate plains. A second, elliptical (300 km x 200 km), 360-m-high corona (Nissaba) is located to the west of Idem-Kuva. Lava flows radiate from a low, circular region, in the southeast part of this structure, suggesting the presence of a shallow caldera.
Tectonic Units. In addition to the volcanic features, two tectonic units are identified. To the north of Gula are several isolated regions of cross-lineated terrain (Tcl). This unit, characterized by locally elevated topography and multiple directions of deformation, is extensively embayed by surrounding reticulate plains. Areas of lineated terrain (Tl), made up of narrowly spaced (less than 1 km to 2.5 km) northsouth trending ridges, are identified on the southeast part of Gula Mons. Like the cross-lineated terrain, this unit is embayed by reticulate plains along with volcanic deposits that originate from a source on the east flank of Gula. An isolated occurrence of lineated terrain is located within Guor Linea where it is embayed by plains and crosscut by faulting. On the basis of these crosscutting and superposition relationships, this unit and the cross-lineated terrain are interpreted to be the oldest features on this part of the planet.
Impact Craters. Six impact craters (C), varying from 5 to 48.5 km in diameter, are mapped in the vicinity of Gula Mons. All of these craters are associated with a diffuse radardark halo [see Phillips et al., 1991, and Arvidson et al., 1991, for detailed discussions of radar-dark deposits surrounding impact craters]. Two of the dark deposits (D) extend for over 300 km, and both extend from west to east. The dark material and the craters superpose all other units, including plains, tectonic units, and deposits from Gula Mons, making them stratigraphically the most recent features in the region. A dark unit northwest of the summit of Gula Mons is very....
 ....linear and narrow, less than 40 km across. In comparison, a dark unit on the eastern part of Gula has a parabolic shape and is far more extensive, having a width of over 450 km. One diffuse radar-bright unit (B) is also mapped. The center may be an impact crater, but it is obscured by a data gap.
Superposition, crosscutting relationships, and the areal distributions of units lead to the following sequence of tectonic and volcanic events: (1) deformation forming the crosslineated terrain and the lineated terrain, and emplacement of the mottled plains; (2) volcanism forming the reticulate plains that embay the three earliest units; and (3) emplacement of coronae, lobate plains, and volcanic construction forming Gula Mons. Gula Mons appears to have had a complex evolution, undergoing multiple episodes of lava emplacement.
Geologic mapping shows that much of the early volcanic history of the area surrounding Gula Mons was characterized by the emplacement of mottled plains. These plains were subsequently subjected to lava flooding (formation of the reticulate plains) and uplifted to form the western Eistla Regio highland.
The tendency of many lava flows to change in brightness along their length often makes their mapping difficult. It is also difficult to establish temporal relationships between flows that are separated geographically. The age relationship between Gula Mons and Sif Mons to the west is not clear [Senske et al., 1992]. Superposition relationships between deposits are often ambiguous, indicating that the two constructs may have formed concurrently. Along with the construction of volcanic complexes, a great deal of extensional deformation is evident along a summit rift zone of Gula and at Guor Linea (Figure 10-2). Flows from Gula generally overlie Guor Linea, a rift valley formed in association with the uplift of western Eistla.
Lava flows from Gula appear to be deflected around both Idem-Kuva and Nissaba, indicating that at least some volcanism associated with Gula postdates formation of these coronae. Dark diffuse material associated with impact craters is superposed on a number of units (units both at Gula Mons  and Nissaba). This indicates that some of the most recent events in this part of western Eistla Regio are associated with impact cratering.
Detailed geologic mapping of this region and the rest of Venus using Magellan data is ongoing. The results presented here indicate that multiple episodes of tectonic and volcanic activity have occurred, with impact cratering and eolian processes also modifying the surface. Unraveling the complex interconnected history of these processes through detailed mapping is critical to understanding the evolution of Venus.
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