Usgs scientific investigations map 2897

Prepared for the National Aeronautics and Space Administration Geologic Map of the Mead Quadrangle (V–21), Venus
By Bruce A. Campbell and David A. Clark Pamphlet to accompany Scientifiic Investigations Map 2897 U.S. Department of the Interior U.S. Geological Survey or volcanic ash, can absorb the incident energy and produce a lower observed echo. On Venus, a rapid increase in reflectivity THE MAGELLAN MISSION exists at a certain critical elevation above which high-dielec- The Magellan spacecraft orbited Venus from August 0, tric minerals or coatings are thought to be present. This leads 990, until it plunged into the Venusian atmosphere on October to very bright SAR echoes from virtually all areas above that 2, 994. Magellan Mission objectives included () improving critical elevation.
the knowledge of the geological processes, surface properties, The measurements of passive thermal emission from and geologic history of Venus by analysis of surface radar Venus, though of much lower spatial resolution than the SAR characteristics, topography, and morphology and (2) improv- data, are more sensitive to changes in the dielectric constant ing the knowledge of the geophysics of Venus by analysis of of the surface than to roughness. They can be used to augment Venusian gravity.
studies of the surface and to discriminate between roughness The Magellan spacecraft carried a 2.6-cm radar system and reflectivity effects. Observations of the near-nadir back- to map the surface of Venus. The transmitter and receiver sys- scatter power, collected using a separate smaller antenna on tems were used to collect three data sets: () synthetic aper- the spacecraft, were modeled using the Hagfors expression for ture radar (SAR) images of the surface, (2) passive microwave echoes from gently undulating surfaces to yield estimates of thermal emission observations, and (3) measurements of the planetary radius, Fresnel reflectivity, and root-mean-square backscattered power at small angles of incidence, which were (rms) slope. The topographic data produced by this technique processed to yield altimetric data. Radar imaging and altimet- have horizontal footprint sizes of about 0 km near periapsis ric and radiometric mapping of the Venusian surface were and a vertical resolution on the order of 00 m. The Fresnel accomplished in mission cycles , 2, and 3 from September reflectivity data provide a comparison to the emissivity maps, 990 until September 992. Ninety-eight percent of the sur- and the rms slope parameter is an indicator of the surface tilts, face was mapped with radar resolution on the order of 20 m. which contribute to the quasi-specular scattering component.
The SAR observations were projected to a 75-m nominal hori-zontal resolution, and these full-resolution data compose the image base used in geologic mapping. The primary polariza-tion mode was horizontal-transmit, horizontal-receive (HH), The Mead quadrangle (V–2) of Venus is bounded by lat but additional data for selected areas were collected for the 0° and 25° N., long 30° and 60° E. This quadrangle is one of vertical polarization sense. Incidence angles varied between 62 covering Venus at :5,000,000 scale. Named for the larg- about 20° and 45°.
est crater on Venus, the quadrangle is dominated by effusive High-resolution Doppler tracking of the spacecraft took volcanic deposits associated with five major coronae in east- place from September 992 through October 994 (mission ern Eistla Regio (Didilia, Pavlova, Calakomana, Isong, and cycles 4, 5, 6). Approximately 950 orbits of high-resolution Ninmah), corona-like tectonic features, and Disani Corona gravity observations were obtained between September 992 (fig. A). The southern extremity of Bell Regio, marked by and May 993 while Magellan was in an elliptical orbit with lava flows from Nyx Mons, north of the map area, forms the a periapsis near 75 km and an apoapsis near 8,000 km. An north-central part of the quadrangle. The shield volcanoes Kali, additional ,500 orbits were obtained following orbit-circular- Dzalarhons, and Ptesanwi Montes lie south and southwest of ization in mid-993. These data exist as a 75° by 75° harmonic the large corona-related flow field. Lava flows from sources east of Mead crater flood low-lying areas along the east edge of the quadrangle.
MAGELLAN RADAR DATA The predominantly smooth volcanic plains of Akhtamar Planitia form the background terrain. The highest locations Radar backscatter power is determined by () the mor- in eastern Eistla Regio are almost 4 km above the planetary phology of the surface at a broad range of scales and (2) the datum radius of 6,05 km, but the average elevation across intrinsic reflectivity, or dielectric constant, of the material. this broad rise is –2 km (fig. B). The surrounding plains are Topography at scales of several meters and larger can pro- typically within  km of the datum. Near the crater Orczy (lat duce quasi-specular echoes, and the strength of the return is 3.7° N., long 52.3° E.) is a network of anastomosing channels greatest when the local surface is perpendicular to the incident (Ganga Valles) that incise the regional plains. Mead crater, beam. This type of scattering is most important at very small approximately 270 km in diameter, lies near the east edge of angles of incidence, because natural surfaces generally have the map region. An additional 4 impact craters are identified few large tilted facets at high angles. The exception is in areas within the quadrangle.
of steep slopes, such as ridges or rift zones, where favorably Tectonic deformation within the quadrangle produced a tilted terrain can produce very bright signatures in the radar network of wrinkle ridges in the regional plains material. The image. For most other areas, diffuse echoes from roughness at density of these ridges varies across the map region (for exam- scales comparable to the radar wavelength are responsible for ple, Bilotti and Suppe, 999). Greater degrees of deformation variations in the SAR return. In either case, the echo strength is also modulated by the reflectivity of the surface material. The density of the upper few wavelengths of the surface can have a significant effect. Low-density layers, such as crater ejecta Name provisionally accepted by International Astronomical Union are associated with uplifted corona rims, a belt of ridges in Eastern Eistla Regio is of interest for the large-scale vol- the northwestern portion of the quadrangle (Metelitsa Dorsa), canism associated with the coronae and corona-like deforma- and a relatively small ridge complex, Ojuz Dorsa, that links tion features. The geologic history of this region appears to Mafdet and Gbadu Tesserae. The central regions of individual be significantly different from that delineated for Bell Regio coronae are characterized by densely lineated, stellate or wish- (Campbell and Campbell, 2002) in the lack of relatively bone-shaped features. Relatively small outcrops of heavily young, steep-sided volcanic constructs such as Tepev Mons deformed terrain occur within the plains units (Mamitu, Salus, and Otafuku Tholi in V–9. There are similarities, however, Vako-nana, Mafdet, and Gbadu Tesserae), and the southeast- between the initial stages of volcanism at Bell Regio, associ- ern portion of the quadrangle contains the outlying highlands ated with the corona-like Nyx Mons just north of the map area, of western Ovda Regio (Manatum Tessera).
and the sheet-like flow aprons of the coronae of eastern Eistla Previous work focused on the distribution and morphol- Regio. Despite the lack of steep-sided constructs, several of ogy of the coronae (quasi-circular features displaying a wide the coronae in the Mead quadrangle have associated radar- range of annular rim structures, exterior deformation, and asso- bright deposits that we interpret to be pyroclastic materials. ciated volcanism) within eastern Eistla Regio as part of global There is evidence in both regions for a broad range of volcanic statistical studies (Stofan and others, 992). Detailed geologic processes, perhaps including more evolved magmas over time, mapping of coronae elsewhere on Venus includes a study by but the differences between these two adjacent highland rises Copp and others (998), which shows that corona-related offers an opportunity to characterize the distinctive features of volcanism may have been punctuated by periods of tectonic corona- and shield-related materials.
deformation; our results support this broad premise. McGill (994) found that coronae in central Eistla Regio (quadrangle METHODS AND DATA
V–20, immediately west of the map region) are stratigraphi-cally older than nearby major shield volcanoes, which sug- This geologic map was compiled from Magellan radar gests a thickening of the local lithosphere with time. In studies images at a base resolution of approximately 260 m/pixel. The of Bell Regio (quadrangle V–9, immediately north of the map resampled (75 m/pixel) F-MAP data were used to check unit region), Campbell and Campbell (2002) and Campbell and contacts and to identify subtle or small features. Most of the Rogers (994) note a similar progression from coronalike fea- quadrangle lies within the superior conjunction gap of the first tures to steep-sided shield volcanoes, with inferred increasing Magellan mapping cycle, so the left-looking image data were lithospheric thickness (Rogers and Zuber, 998). Four of the collected primarily during cycle 2. These data have radar inci- major coronae in eastern Eistla Regio have distinguishable dence angles of 44°–46°. The eastern and northern parts of the gravity anomalies (Zimmerman and Johnson, 2000), as do the quadrangle were covered by Magellan cycle 3 right-looking two corona-volcano hybrid structures in central Eistla Regio, stereo observations, with incidence angles of approximately Irnini and Anala Montes (McGill, 998, 2000). These gravity 25°. These data were used to distinguish the relief of tectonic anomalies indicate that corona topography is not fully com- features surrounding Mead crater and to extend the incidence- pensated, implying possible dynamic support (for example, angle range of surface property descriptions. Altimetry data mantle plumes).
were derived from both resampled topographic maps having Mead crater was studied by Schaber and others (992) 5.4 km horizontal resolution and from analysis of individual and Herrick and Sharpton (996). These authors differ slightly Magellan altimeter footprint records (Ford and Pettengill, in their interpretation of the radar-dark materials surround- 992; Campbell and others, 999). Magellan emissivity (fig. ing Mead; Schaber and others (992) propose that the dark 2A), rms slope (fig. 2B), and Fresnel reflectivity data provide material is related to impact-triggered volcanism, while Her- valuable additional information on the presence and nature of rick and Sharpton (996) suggest that it is composed of plains surficial deposits.
volcanic material that embays the crater ejecta blanket. Our Unit boundaries in the geologic map were defined primar- mapping supports the latter hypothesis, though the sources for ily on the basis of changes in surface morphology inferred from the embaying material include sources east, west, and north the radar data. The mapped units are shown within a tentative of the crater. The Mead crater floor may consist of a layer of relative age sequence, based on superposition and cross-cutting impact melt emplaced immediately after crater excavation, relations, in the Correlation of Map Units and Major Events with subsequent modest tectonic deformation, or a post-impact (map sheet). Radar backscatter strength at incidence angles of volcanic deposit (Herrick and Sharpton, 996). Weitz (993) 44°–46° is controlled largely by the wavelength-scale rough- and Campbell (994) discuss the backscatter and emissivity ness of the surface. Below 6,054 km radius, variations in the properties of the Mead crater floor and note a minor but dis- bulk dielectric constant are a minor contributor to the surface tinct shift in dielectric constant from east to west. Mead impact backscatter coefficient, unless there is a significant depth of ejecta is also the likely source of fine-grained, radar-dark fine-grained mantling material. Above this elevation, however, materials that mantle large areas in the southeastern region of surface-atmosphere interactions produce highly reflective ter- the quadrangle. This material is preferentially concentrated rain for which roughness is a secondary factor in the back- in local areas of low elevation, and high-standing areas show scattered return (Campbell, 995; Campbell and others, 999). evidence for progressive stripping (presumably aeolian) of an Magellan ancillary data on surface emissivity and reflectivity originally more extensive mantling layer.
were used to further constrain surface unit properties.
slope) and the centimeter scale (low 2.6-cm-radar cross sec-tion: table ). We interpret the regional plains material to have The stratigraphic units in the Mead quadrangle are originated as the result of extensive flooding by low-viscosity grouped by major geographic province: western Ovda Regio, (for example, basaltic) volcanic material. The regional plains which forms the southern portion of the quadrangle; eastern material in quadrangle V–9 (Campbell and Campbell, 2002) is Eistla Regio; and southern Bell Regio, which forms the north- subdivided into two units based on the density of superposed central portion of the quadrangle. A separate group comprises tectonic deformation (wrinkle ridges). Because this division is widespread units: tessera (unit t), regional plains (unit pr), and not based on properties of the original rock units, we do not impact craters (units cu and co). For each province, we present make the same distinction in V–2.
a correlation chart of the major units. This approach avoids the implication that, for example, localized homogeneous plains WESTERN OVDA REGIO MATERIAL units across the map form a common time-stratigraphic hori-zon. Within the three provinces, we identify densely lineated We map as individual units the flow materials of Dzalar- materials (unit ld); flow materials of central volcanoes, local- hons Mons, including Nekhebet Fluctus (unit fD) and Ptesanwi ized sources, and coronae (unit names beginning with f); radar- Mons (unit fP), which form aprons of lobate flows that trend bright halo materials (unit hb); and localized plains materials radial to the respective volcano center, superpose the regional (unit names beginning with p). Materials related to Mead plains (unit pr), and, in some locales, embay tessera materials crater are mapped as distinct units (names beginning with cM) (unit t). Based on apparent topographic control of flow direc- within the eastern Eistla Regio province. Surficial materials tion, we infer that Ptesanwi Mons postdates the uplift of Cala- are shown as a stippled pattern. Unit names are chosen to iden- komana Corona.
tify each unit by its primary characteristics or to show associa- Homogenous plains material of Ovda Regio (unit phO) tion with a major geomorphic feature. Quantitative backscatter occurs in the southeast portion of the quadrangle, superposes and ancillary data for type areas of mapped units are presented the regional plains material (unit pr), and embays Salus and in table , and the relative backscatter properties of selected Manatum Tesserae. Unit phO likely comprises contribu- major units are presented in a comparison with radar data for tions from a number of sources, including Verdandi Corona terrestrial lava flows in figure 3.
(southeast of the map region), Disani Corona, and a region of abundant small shields and pit crater chains northeast of WIDESPREAD TESSERA AND PLAINS MATERIAL Orczy crater (fig. 4). Unit phO is characterized by homoge-neous radar image texture, low rms slope, and moderate to The oldest exposed unit at any locale in the quadrangle low radar backscatter (table ). These properties suggest a forms complex ridged terrain called tessera material (unit t). smooth surface at scales ranging from centimeters to hun- All tesserae are embayed by plains units and (or) corona/edi- dreds of meters.
fice lava flows. In general, there is not agreement as to whether Channel structures are common in the portion of Ovda or not tesserae constitute a global time-stratigraphic marker Regio within V–2. A narrow, sinuous, anastomosing channel (Basilevsky and Head, 998; Guest and Stofan, 999). In the system (Ganga Valles) extends northeast about 200 km from Mead quadrangle, however, continuity of structural patterns Orczy crater, which likely masks the source vent. A channel between outcrops of tessera material separated by as much as farther to the northeast of Orczy is wider and has been par- ,000 km strongly suggests a common period of deformation. tially flooded and buried by later volcanic materials (fig. 4). For example, the trend and deformation pattern within Mafdet We infer these channels were formed by thermal and mechani- Tessera are very similar to those of Mamitu Tesserae, which cal erosion caused by flowing lava and are incised only into extends northward into Bell Regio. Mafdet Tessera, in turn, regional plains material of unit pr. Materials of units phO and appears to be an extension of Gbadu Tessera. This suggests a fc truncate or embay the channels, filling areas of low eleva- possible early, more extensive tessera fabric across the regions tion. These later materials likely superpose any distal outflow that has subsequently been largely buried by volcanic materi- deposits associated with the channel-forming eruptions. A als. As a result, we have not differentiated the tessera materials channel that exploited pre-existing graben structures occurs in among the three geographic provinces, and we suggest that they the north-central portion of Disani Corona (fig. 5), and we infer are similar in at least the age of their last major deformational this channel formed contemporaneously with the surrounding period. Their actual age of formation as elevated terrain and the material of unit phO.
nature of the original surface material are indeterminate.
Regional plains material surrounding eastern Eistla Regio EASTERN EISTLA REGIO MATERIAL is mapped as a single unit (unit pr). A dense network of wrin-kle ridges and annular tectonic patterns deforms unit pr. We Densely lineated material (unit ld) forms primarily the also included the annular materials of Calakomana Corona center and annular bounding terrain of coronae and is char- within this unit, because the density of fractures and ridges in acterized by dense patterns of tectonic deformation with a the bounding structures is much less than that found at Didilia single dominant trend; for the annular features, this trend is and Pavlova Coronae and other coronae in eastern Eistla often radial to the corona center. While the tectonic defor- Regio. Where not affected by tectonic deformation, the sur- mation is often so pervasive as to preclude inference of the face is smooth at both the hundreds of meters scale (°–3° rms pre-existing surface properties, these outcrops are sufficiently distinct from tessera and plains materials to warrant mapping Lobate, radar-bright flows are typically associated with as a discrete unit. In the center of each corona is an area of ele- high-volume eruption rates (rapid overturn of lava surface vated, densely lineated material (fig. 6). These central features crusts) or increased magma viscosity. Sheetlike, low-return range from stellate (Pavlova Corona) to linear or wish-bone materials are likely produced by low eruption rates (tube fed shaped (Didilia and Isong Coronae). There are also apparent or slowly inflated deposits) or by less viscous magma. The pro- remnants of corona center structures southwest and east of gression in lava flow morphology observed for Pavlova and Pavlova Corona and southeast of Isong Corona. Materials of Didilia Coronae implies a complex history of magma sources, unit fc embay the densely lineated material and superpose the compositions, and (or) eruption rates.
regional plains, so we infer that the densely lineated materials Flow material of Kali Mons (unit fK) forms an apron of are older than the surrounding flows. Whether unit ld consists lobate flows that trends radial to the volcano center, super- of highly deformed regional plains material (unit pr) or older poses the regional plains, and, in some locales, embays tessera terrain embayed by the regional plains is uncertain. We indi- deposits. The radiophysical properties of most edifice flows are cate this uncertainty by an overlap of the relative age ranges of consistent across the quadrangle. Where the radar backscatter unit ld and unit pr in the Correlation of Map Units and Major is enhanced, we observe moderate to slightly enhanced emis- Events (map sheet). We also suggest that variations in extant sivity, consistent with a surface that is rougher at the wave- corona structures reflect varying degrees of subsidence and length scale than the surrounding plains (Campbell, 995). burial by unit fc. A correlation between structural preservation Where the backscatter is low, the emissivity is correspondingly and corona age is speculative, but we indicate a possible over- lower than the planetary average. There is, however, a distinc- lap in age for unit ld relative to flow materials of coronae (unit tive area of higher emissivity correlated with low-radar-return fc) in the Correlation of Map Units and Major Events.
lava flows from Kali Mons (fig. 2A, fig. 8). Small patches of In the plains surrounding eastern Eistla Regio, localized enhanced emissivity are also associated with radar-dark units flow materials (unit fl) of moderate to bright radar return sur- in Calakomana Corona. Enhanced emissivity is indicative of round low-relief shield volcanoes or caldera-like depressions. either an increased wavelength-scale roughness or a lower There is also a large flow field associated with Ojuz Dorsa. bulk dielectric constant. Since the lava flows are radar dark, While mapped as a single unit, we make no inference about the former explanation is untenable. The likely reason for such the relative age of these isolated flow fields. All of these flow behavior is a low bulk density, due either to a surficial mantling complexes superpose the regional plains (unit pr) and, in some layer or an intrinsic property of the lava flows (for example, locales, embay tessera materials (unit t) or densely lineated high vesicularity). While a low-density pyroclastic mantling material (unit ld). Where the contacts can be distinguished, deposit could also exhibit these properties, it seems unlikely north and northwest of Didilia Corona, corona flow materials that such a deposit would be confined to the area of a single (unit fc) appear to superpose the localized flow materials (unit flow complex.
Mead crater and its associated deposits represent a sig- The central rise of eastern Eistla Regio is covered by vol- nificant time-stratigraphic marker for the eastern portion of canic flows associated with the major coronae and a number the map region. Mead crater is considerably more complex of structural features that may be corona remnants. Because than other craters in the quadrangle. Following previous stud- the distal lobate margins of flow complexes are discontinu- ies (Schaber and others, 992; Herrick and Sharpton, 996), ous, we map the deposits as flow material of coronae (unit we mapped as separate units crater floor material (unit cMf), fc), which covers an area of approximately 2.2x06 km2. Two terrace material (unit cMt), and distal ejecta material (unit distinct flow complexes are mapped as flow materials of Cala- cMu) of Mead crater. The crater floor exhibits an interesting komana Corona (unit fC). Despite the lack of clear boundar- dichotomy in surface properties; emissivity data (fig. 9) sug- ies between most flow complexes associated with the major gest that the southeastern portion has a dielectric constant of coronae, the area north of Pavlova Corona offers an example 4–5 (close to the planetary average of 4.2), while the remain- of the general stratigraphic succession (fig. 7). There is a dis- der of the crater floor is characterized by values of 7–8 (Weitz, tinct contact between older Pavlova Corona flow material and 993; Campbell, 994). The crater floor material, which likely younger deposits from Didilia Corona that are deflected by formed as either an impact melt sheet or post-emplacement higher-standing flow margins. At least in this region, the older volcanic deposit, is characterized by a modest but abrupt shift flow units from both coronae are lobate, radar bright, relatively in chemical or physical properties. It is interesting that no sig- narrow, and frequently exhibit central channels. Younger flows nificant areas with decreased emissivity are observed in the are more sheetlike and radar dark and have diffuse distal con- Mead crater terrace region or proximal ejecta blanket. If the tacts with the surrounding plains. The younger flows super- low-emissivity floor material is composed of impact melt, it pose lower-standing areas of the radar-bright flow lobes and suggests some combination of minimal melt ejection and (or) appear to flood some channels. As a result, the distal margins rapid drainage of any exterior deposits into the crater cavity. of unit fc northeast of Didilia and Pavlova Coronae are char- The uncertainty in the duration of floor material emplacement acterized by a combination of feathery, radar-dark terrain and is reflected in the Correlation of Map Units and Major Events the remnant portions of partially buried, rough lobate flows. In (map sheet).
contrast, the later flows of Nyx Mons (unit fN3) are more radar The distal ejecta deposits of Mead crater are much less bright than the preceding sheet-like deposits (unit fN1).
radially extensive than might be expected for a pristine, large crater. Even in the dense atmosphere, primary and secondary rial in the Bell Regio quadrangle (V–9). In V–2, however, the ejecta features should form a more complete annulus around radar-dark materials do not have distinct lobate margins, and, the rim. This discontinuous primary ejecta pattern suggested in some locales, they form wind streaks associated with topo- to previous workers the embayment of the initial ejecta blan- graphic gaps in tessera units. The low-radar return in this por- ket by flow materials. We divide the radar-dark deposits sur- tion of the quadrangle appears to be due to a combination of rounding Mead crater into flow material of coronae (unit fc) surficial mantling material (possibly from Corinna crater) and and flow material near Mead crater (unit fM), which is likely relatively smooth plains-forming and corona flow materials.
related to a chain of small edifices northeast of the crater and The Homogeneous plains material of Bell Regio (unit a volcanic center near lat 4° N., long 64.5° E. We mapped phB) is characterized by relatively homogeneous radar-image a short north-trending channel structure just north of Mead texture, subdues pre-existing terrain features such as wrinkle crater, which may also be associated with the emplacement of ridges, and superposes unit pr. Unit phB is also characterized unit fM. Contacts between the two embaying units are difficult by enhanced backscatter and emissivity relative to unit pr (fig. to trace, and we used the local topographic relief and degree 2), suggesting a rougher texture at the 2.6-cm scale. McGill of burial of tectonic structures to infer a contact. The materials (2000) mapped similar plains-forming material as homoge- of both units superpose unit pr, as evidenced by the absence neous plains material in quadrangle V–20.
of wrinkle ridges in areas of low topography (fig. 0). High- Flow material of Nyx Mons (units fN1 and fN3) extend standing arcuate ridges surround the crater along all but the south into the topographic low between Bell and eastern Eistla southwest corner and confine the erupted material.
Regiones. The oldest (fN1) and youngest (fN3) of these depos- Within eastern Eistla Regio are at least seven distinct areas its occur within the Mead quadrangle; unit fN2 is present only of very high radar backscatter, mapped as bright halo material in quadrangle V–9. The lower flow material of Nyx Mons (unit (unit hb). These areas occur adjacent to densely lineated mate- fN1) forms a moderate- to low-radar-return field of flows that rial (unit ld), are characterized by feathery edges and minimal superpose unit pr, extending radially as far as 600 km from the internal features, and mantle pre-existing lava flows from the center of the edifice. The upper flow material of Nyx Mons coronae (fig. ). There is little evidence for spatial variability (unit fN3) forms a prominent aggregate of radar-bright flows in deposit depth or roughness, with the exception of small oval to the south and southwest of Tepev Mons (V–9). To the south or linear regions of enhanced backscatter. Based on backscat- and southeast of Tepev Mons, these bright flows are super- ter and emissivity data, unit hb deposits are interpreted to be posed on unit fN1.
coarse-grained (particle sizes greater than a few centimeters) mantling material emplaced by pyroclastic volcanic activity WIDESPREAD IMPACT CRATER MATERIAL (Campbell and others, 998). The lack of apparent sorting of the material precludes identification of a likely source for each A total of 5 craters occur within the V–2 quadrangle. deposit, though a surge driven by a collapsing eruption column We divide the impact crater material for all but Mead crater would travel primarily downslope. Radar-dark wind streaks into two categories: crater material, undifferentiated (unit cu), are observed along the margins of some deposits of unit hb, comprising material of crater rims, floors, and ejecta blankets suggesting that the total relief within the rough terrain is on and crater outflow material (unit co). The paucity of craters the order of perhaps tens of centimeters to permit burial by the across the quadrangle offers little useful information about thin wind-borne material. McGill (994) identified similar, but relative unit ages (Campbell, 999). Underscoring this point, a more extensive, deposits on Anala and Irnini Montes in the majority of the craters within V–2 occur on units fc and phO, adjacent V–20 quadrangle.
which are inferred to be younger than the regional plains (unit Within V–2, the radar-bright areas occur on the west side pr), and none occur on the presumably older tesserae (unit t). of densely lineated materials that form corona rim deposits There is also no evidence for a sub-population of embayed cra- (unit ld). This might suggest some control of deposition by ters other than Mead crater.
prevailing east-west winds, but the similar deposits in central Craters within the quadrangle do not exhibit large outflow Eistla Regio are concentric to Anala and Irnini Montes. It is deposits, but small outflows are mapped for Karo and Brad- also possible that the bright radar echoes are due to a micro- street craters. Outflows at Huarei and de Ayala craters are too dune structure on scales below the resolution of the Magellan small to be represented clearly at the :5,000,000 scale. Crater radar images rather than to a jumbled surface of small pyro- floors vary in radar return from very bright (for example, de clastic fragments. Where such deposits have been identified Ayala) to dark (for example, Orczy). The bright floors are on Venus, they are associated with a nearby impact crater (for likely rough due to emplacement of a fractured impact melt example, Guan Daosheng crater near lat 6.° S., long 8.8° sheet, whereas the dark floors may reflect post-excavation E.). The radar-bright materials in central and eastern Eistla flooding by volcanic materials or a smooth melt sheet.
Regio are not clearly linked to impact crater deposits.
Farida crater is of interest for its associated parabolic halo of fine-grained debris, which forms an arc to the south of the crater. The ejecta blanket of Farida is also concentrated to the SOUTHERN BELL REGIO MATERIAL south, suggesting an oblique impact event. Ayashe crater, which In the northwestern portion of the quadrangle, low-radar- straddles a boundary between older regional plains material return regions resemble terrain mapped as smooth plains mate- (unit pr) and younger, radar-bright homogeneous plains mate- rial (unit phB), appears at first to contradict this stratigraphy: In general, fine-grained deposits are expected to have lower the ejecta blanket of Ayashe is evident only on the younger bulk density than a rock of the same composition and, in turn, plains material. We suggest that this was an oblique impact a lower dielectric constant and enhanced emissivity. For the postdating both plains regions and leading to an asymmetric low-return material in V–2, we observe a slight decrease in ejecta pattern.
emissivity. While this is consistent with smooth terrain, it sug-gests that the bulk dielectric constant of the mantling material SURFICIAL MATERIAL also may be higher than that of the underlying plains (Camp-bell, 994). We speculate that the fine-grained ejecta and low- We infer that surficial deposits in the Mead quadrangle are emissivity material of the Mead crater floor are related in their predominantly of impact origin and represent thin (a plausible mode of formation (for example, impact melting).
range is on the order of 0 cm to a meter or more) layers of fine-grained material that reduce the radar echo by smoothing the surface and lowering the effective reflectivity. The primary source of these fine materials was the Mead crater impact. Tessera materials are characterized by multiple sets of sub- Major deposits of surface mantling material occur northeast perpendicular tectonic deformation patterns, including ridges, and southwest of Mead crater and mantle materials of units pr grabens, or ribbon structures. Within the Mead quadrangle, and fc. Quasi-circular deposits south and northwest of Mead the outcrops of tessera material are principally related to the crater appear to be due to younger impact splotches. The man- equatorial highlands (Ovda Regio) and to a possibly related set tling deposits are evident in the emissivity data as subtle lows of elongate outcrops that trend approximately northeast from relative to the average plains behavior (fig. 2A) and form dis- Dzalarhons Mons past Tepev Mons to the north in quadrangle tinct patches of low rms slope (fig. 2B).
V–9. As noted above, the deformation pattern within Mafdet A comparison of the Magellan radar, emissivity, and rms Tessera is similar to that of Mamitu Tesserae, which extend slope measurements with altimetry data suggests a high degree northward into Bell Regio. Mafdet Tessera, in turn, appears of correlation between the mantling materials and areas of low to be an extension of Gbadu Tessera. This suggests a possible elevation. In some of these low-lying areas, the radar-dark early, more extensive tessera fabric across both regions that material exhibits lobate margins that conform to subtle topo- has subsequently been largely buried by volcanic materials.
graphic relief, such as wrinkle ridges (fig. 2). This suggests The plains material (unit pr) surrounding eastern Eistla that the initial emplacement of the mantling material may have Regio is deformed by wrinkle ridges, ridge belts, and corona been characterized by fluid behavior rather than solely by an structures whose topographic expression limits the lateral airfall mechanism. Areas of locally high topography tend to extent of corona flows (unit fc). To the west, northeast, and be more radar bright, and radar-dark wind streaks or yardangs south of the corona cluster, belts of closely spaced ridges form trend downslope from small patches of high-standing mantled topographic highs against which these lava flows terminate. terrain. The prevailing winds also produce accumulations To the west, this high topography forms Metelitsa Dorsa. of radar-dark material along the east sides of many wrinkle The southern topographic high is associated with the annular ridges. We infer that the mantling material was originally tectonic deformation of Calakomana Corona. Virtually all of much more extensive and is being progressively stripped from this deformation appears to predate the emplacement of flow high-standing terrain and redeposited in low areas. This aeo- materials from Pavlova, Didilia, Isong, and Ninmah Coronae, lian transport of material, in many cases, appears to progress because only a few wrinkle ridges occur within unit fc. To the from east to west (consistent with the prevailing winds), but, north, the radial extent of unit fc is limited by low-relief wrin- in some locales, the wind regime appears to be controlled by kle ridges and the broad topographic low in Akhtamar Planitia local topography. For example, there are very distinct arcuate between Bell and Eistla Regiones. Southeast and southwest of radar-dark wind streaks on unit fM southeast of Mead crater the coronae, volcanic flows are less confined and spread onto that appear to arise from small, radar-dark accumulations of the ridged plains and into the topographic lows surrounding fine material on old, high-standing ridges to the north.
Mead crater.
Based on the evidence for lateral transport of the Mead- Bilotti and Suppe (999) mapped wrinkle-ridge pat- related mantling material, these deposits do not represent a terns across Venus and noted that the geoid high near Pavlova reliable stratigraphic marker, but we examined the implica- Corona appears to be the center of a circumferential defor- tions of their observed distribution. We inferred that materials mation pattern approximately 3,200 km in diameter. Their of unit fc and fM embay the Mead crater ejecta blanket. There analysis of other geoid highs suggests that ring-like patterns are no extensive occurrences of mantling deposits on unit fM, of wrinkle ridges are associated with compressional deforma- so our conclusions appear to be consistent. There are, however, tion in the lowlands surrounding regional uplifts. While this considerable mantling deposits on portions of unit fc located mechanism may explain some of the circumferential wrinkle southeast of Isong Corona. We must, therefore, leave open the ridges in the plains surrounding eastern Eistla Regio, gravity possibility that some of the corona-related flows predate the sliding at the margins of a broad –2-km-diameter rise seems Mead crater impact.
unlikely to produce the densely spaced ridge belts that sur- The physical nature, inferred from radar backscatter and round the corona complex. The origin of these ridge belts is emission properties, of the mantling material is intriguing. therefore uncertain, but we infer that they record a period of compressive tectonism (either regional or associated with rise Mamitu Tesserae appear to share many similar attributes and formation) that predates the formation of structures and flows may be associated with Gbadu Tessera, which suggests a pos- related to Didilia, Pavlova, Isong, and Ninmah Coronae.
sible early, more extensive tessera fabric linking Bell and east- The stress fields related to the deformation of the densely ern Eistla Regiones. Regional plains materials, interpreted as lineated material forming coronae center features (unit ld) sheet- or flood-like volcanic flows, embay and superpose the appear unrelated to those associated with nearby belts of ridges. For example, the outcrop of unit ld southwest of Pav- The regional plains material (unit pr) later underwent lova Corona has an approximately orthogonal trend to that of widespread tectonic deformation, creating belts of densely Metelitsa Dorsa. The densely lineated material is also charac- spaced wrinkle ridges that constrain the radial extent of corona terized by extensional fractures, as well as possible compres- flow materials. If this deformation reflects a response to the sional tectonic features. Coupled with the superposition of unit uplift of eastern Eistla Regio (Bilotti and Suppe, 999), then fc on the plains material and the evident topographic control any record of this period within the central corona-dominated of corona flow material margins, it appears that the ridge belts region has been obscured by later volcanism and tectonism. predate both the effusive volcanism associated with the coro- The ridge-forming deformation was contemporaneous with, or nae and the tectonic deformation of their central regions. If the closely followed by, the uplift of Calakomana Corona. Based ridge belts represent some combination of regional tectonism on its association with the circumferential ridge-belt structures and a response to the formation of the rise, then the current and the topographic subsidence of its central region, we infer corona features reflect later, and more localized, stress regimes. that Calakomana Corona predates the other large coronae of Evidence of the initial effects of the uplift in the center of east- eastern Eistla Regio. ern Eistla Regio are obscured by subsequent volcanism and The post-plains-emplacement geologic history of eastern Eistla Regio is similar to the early phases of volcanism in Bell The four major coronae of eastern Eistla Regio (Didilia, Regio. The volcanic deposits of some coronae in eastern Eistla Pavlova, Ninmah, and Isong) have relatively similar structures: Regio resemble the broad apron of flows surrounding Nyx an uplifted, concentric outer rim; a relatively flat interior floor; Mons, which also has a wishbone-shaped central feature (V–9) and a central dome or ridge (fig. B). In contrast, Calakom- similar to outcrops of unit ld in V–2. Effusive volcanism cen- ana Corona has a discontinuous, less deformed outer rim, and tered on the coronae in eastern Eistla Regio is interleaved in the floor comprises a group of depressions separated by low time with tectonic deformation associated with the formation domes or ridges. The deformation at Calakomana predates the of the rim materials, and remnant central structures and radial volcanism associated with the coronae of the central rise, so fractures suggest that earlier coronae have been buried by a the topography may reflect subsidence with increased age and combination of subsidence and volcanic embayment.
withdrawal of dynamic support.
These combined observations suggest multiple periods of Within the eastern Eistla Regio flow complex, swarms of local corona uplift, volcanism, subsidence, and burial. If the radar-bright lineaments, interpreted to be fractures, superpose average thickness of lava flows within unit fc is 0 m, a rela- and surround the highly deformed terrain of each corona (fig. tively low mass eruption rate of 0 m3/s could emplace these 3). Most are radial to the major topographic expression of materials in less than 05 yr. This is a minimum estimate for the nearest corona, though, in some instances, the deformation the duration of volcanism, because the corona-related depos- patterns of multiple coronae overlap. These fractures extend its may be considerably thicker. We cannot define a clear age into the plains, indicating that the associated stress regime progression for the major coronae in the region, but the latest postdates the formation of the regional wrinkle ridge patterns. surface flows of Didilia Corona appear to postdate those of Fractures associated with Pavlova Corona are concentrated in Pavlova Corona. Tectonic deformation, in the form of radial areas of high topography, suggesting that old fractures along fractures, also appears to be more recent at Didilia Corona.
the corona flanks have been buried by later lava flows. Frac- In eastern Eistla Regio, the period of corona-related vol- tures associated with Didilia Corona cut both the elevated canism is not followed by the development of progressively areas and the flank deposits. This suggests that corona uplift steeper shield volcanoes such as Tepev Mons and Otafuku Tholi and volcanism were contemporaneous, or interleaved in time, in V–9. Instead, the most recent mapped phase of volcanism at Didilia. Some fractures are associated with isolated outcrops includes material erupted from vents located along the corona of densely lineated material (unit ld), such as southeast of rims (unit hb). We infer that materials of unit hb formed as Isong Corona and southwest of Pavlova Corona. This suggests pyroclastic flows of coarse material produced during eruptions that these fractures mark the remnant structures of coronae that of volatile-rich magma along the margins of corona structures. have been largely buried by later lava flows.
There is little evidence for sorting of the debris with increas-ing distance from the apparent source (highest elevation), nor are there extensive associated low-radar-return, fine-grained deposits attributable to deposition by an eruption plume. Given The earliest geologic event within the Mead quadrangle, the high atmospheric pressure, even very explosive volcanic as elsewhere on Venus, is the development of tessera material events are unable to ballistically deposit coarse debris over dis- (unit t) that is preserved as isolated patches of originally more tances of more than approximately  km (Fagents and Wilson, extensive, possibly contiguous units. In particular, Mafdet and 995). The much longer run-out distances of the bright halo deposits (50–00 km) implies a ground-hugging flow regime, though the magmatic properties required to sustain an eruption column during deposition have not been established.
This work was funded, in part, by a grant from the With the exception of Mead crater, the impact craters National Aeronautics and Space Administration (NASA) Planetary Geology and Geophysics Program. The authors within the quadrangle appear to postdate the formation of the thank A.K. Johnston and C.M. Fortezzo for assistance with regional plains and the major corona flow units; we do not the digital mapping. Comments by D.A. Young, K.L. Tanaka, observe partially buried craters such as Gautier and Heloise and an anonymous reviewer were very helpful in revising the craters in the Bell Regio quadrangle (V–9). Mead crater post- text and map.
dates the formation of the regional plains (unit pr) but may predate the emplacement of some corona-related flows (unit fc). Later flows from the corona complex and sources to the north and east of the crater embay the distal ejecta of Mead but do not appear to be the source of the enigmatic low-emissivity material that forms the crater floor.
Basilevsky, A.T., and Head, J.W., 998, The geologic history of Venus—A stratigraphic view: Journal of Geophysical Fine-grained materials that smooth the surface and reduce Research, v. 03, p. 853–8544.
the backscattered radar echo mantle large areas in the eastern Bilotti, F., and Suppe, J., 999, The global distribution of and southeastern portions of the quadrangle, and at least some wrinkle ridges on Venus: Icarus, v. 39, p. 37–59.
component of this debris is readily moved by the regional Campbell, B.A., 994, Merging Magellan emissivity and SAR winds. We attribute this mantling material to impact commi- data for analysis of Venus surface dielectric properties: nution or melting of the target rock, but its mode of emplace- Icarus, v. 2, p. 87–203.
ment differs with location. Many such deposits have feathery ———995, Use and interpretation of Magellan quantitative edges and tend to drape local topography. In at least some data in Venus mapping: U.S. Geological Survey Open- areas, however, the radar-dark material appears to behave as File Report 95–59.
a fluid, exhibiting lobate margins and control by subtle topo- ———999, Surface formation rates and impact crater densi- graphic features. Given the large volume of impact-generated ties on Venus: Journal of Geophysical Research, v. 04, debris expected for a crater like Mead, such behavior may be p. 2,95–2,956.
analogous to that of basin-related light plains on the Moon (for Campbell, B.A., and Campbell, P.G., 2002, Geologic map of example, Howard and others, 974). We infer that the radar- the Bell Regio (V–9) quadrangle, Venus: U.S. Geologi- dark mantling material was initially much more extensive and cal Survey Geologic Investigations Series I–2743, scale is being progressively stripped from high-standing terrain and redeposited in local depressions.
Campbell, B.A., Campbell, D.B., and DeVries, C., 999, Given the lack of stratigraphic contacts between Bell and Surface processes in the Venus highlands—Results from eastern Eistla Regiones, inferences of their relative age are analysis of Magellan and Arecibo data: Journal of Geo- speculative. Studies of western Eistla and Bell Regiones imply physical Research, v. 04, p. 897–96.
a common progression of volcanic landforms with inferred Campbell, B.A., Glaze, L., and Rogers, P.G., 998, Pyroclastic increasing lithospheric thickness at Venusian rises (Campbell deposits on Venus—Remote-sensing evidence and modes and Rogers, 994; McGill, 998). As hypothesized, the early of formation [abs.], in Lunar and Planetary Science Con- stage of the rise is characterized by hot, thin lithosphere that ference, 29th, March 6–20, 998: Houston, Tex., Lunar permits rapid eruption of rising magma and the development and Planetary Institute [CD-ROM].
of annular tectonic deformation patterns (coronae). Over time, Campbell, B.A., and Rogers, P.G., 994, Bell Regio, Venus— the crust thickens, trapping magma at depth, with more infre- Integration of remote sensing data and terrestrial analogs quent eruptions producing steeper constructs (for example, for geologic analysis: Journal of Geophysical Research, Tepev Mons in V–9 and Gula Mons in V–20) and rougher v. 99, p. 2,53–2,7.
Copp, D.L., Guest, J.E., and Stofan, E.R., 998, New insights The gravity signature of eastern Eistla Regio and Nefertiti into Corona evolution—Mapping on Venus: Journal of Corona in Bell Regio (V–9) suggests ongoing dynamic support Geophysical Research, v. 03, p. 9,40–9,47.
for these features in contrast to the compensated topography of Fagents, S.A., and Wilson, L., 995, Explosive volcanism on Nyx and Tepev Montes (V–9; Zimmerman and Johnson, 2000). Venus—Transient volcanic explosions as a mechanism The lack of major shield volcanoes in eastern Eistla Regio may for localized pyroclast dispersal: Journal of Geophysical thus be attributed to several scenarios: () eastern Eistla Regio Research, v. 00, p. 26,327–26,338.
is younger than Bell Regio; (2) the supply of magma waned Ford, P.G., and Pettengill, G.H., 992, Venus topography and prior to development of steep-sided constructs; or (3) the kilometer-scale slopes: Journal of Geophysical Research, mechanism of plume rise and eruption differs between the two v. 97, p. 3,03–3,4.
regions. Our geologic mapping cannot differentiate between Guest, J.E., and Stofan, E.R., 999, A new view of the strati- these possibilities, though the presence of uncompensated graphic history of Venus: Icarus, v. 39, p. 55–66.
topography supports scenarios () or (3) over (2).
Herrick, R.R., and Sharpton, V.L., 996, Geologic History of the Mead impact basin: Geology, v. 24, p. –4.
Kirk, R.L., Chadwick, D.J., Dawson, D.D., Gaddis, L.R., Howard, K.A., Wilhelms, D.E., and Scott, D.H., 974, Lunar Boyce, J.M., and Russell, J., 992, Geology and distribu- basin formation and highland stratigraphy: Reviews of tion of impact craters on Venus—What are they telling Geophysics and Space Physics, v. 2, p. 309–327.
us?: Journal of Geophysical Research, v. 97, p. 3,257– McGill, G.E., 994, Hotspot evolution and Venusian tectonic style: Journal of Geophysical Research, v. 99, p. 23,49– Stofan, E.R., Sharpton, V.L., Schubert, G., Baer, G., Bind- schadler, D.L., Janes, D.M., and Squyres, S.W., 992, ———998, Central Eistla Regio, Origin and relative age Global distribution and characteristics of coronae and of topographic rise: Journal of Geophysical Research, v. related features on Venus—Implications for origin and 03, p. 5889–5896.
relation to mantle processes: Journal of Geophysical ———2000, Geologic map of the Sappho Patera quadrangle Research, v. 97, p. 3,347–3,378.
(V–20), Venus: U.S Geological Survey Geologic Investi- Weitz, C.M., 993, Impact Craters, in Guide to Magellan Data gations Series I–2637, scale :5,000,000.
Interpretation: Jet Propulsion Laboratory Publication 93– Rogers, P.G., and Zuber, M.T., 998, Tectonic evolution of 24, p. 75–92.
Bell Regio, Venus—Regional stress, lithospheric flexure, Zimmerman, S.B., and Johnson, C.L., 2000, Gravity signa- and edifice stresses: Journal of Geophysical Research, v. tures of coronae on Venus—Implications for models for 03, p. 6,84–6,853.
corona formation and evolution: Eos (American Geo- Schaber, G.G., Strom, R.G., Moore, H.J., Soderblom, L.A., physical Union, Transactions), v. 8, p. S300.
Table 1. Ancillary data for selected map units within the Mead
quadrangle (V–21), Venus [The table presents image location, average values for incidence angle (i), planetary radius, HH-polarization backscatter coefficient (σo), rms slope (θrms), Fresnel normal reflectivity (ρo), horizontal polarization emissivity (EH), and calculated bounds on the surface dielectric constant using smooth (εs) and rough (εr) surface cases (Campbell, 994, 995). Where Cycle 3 stereo images are available, these data provide a second incidence angle for each unit. Some mapped units are omitted where the microwave properties could not be meaningfully presented as a single value (for example, crater ejecta)] 22.63–22.88 N. 47.97–48.38 E. 26.29–26.67 N. 50.30–5.55 E.
30.7–3.05 E.
33.0–33.77 E.
44.5–45.06 E.
6.84–7.65 N. 39.02–39.73 E. 44.32–44.79 E.
49.5–50.5 E.
6.84–6.99 N. 36.69–36.89 E. 54.65–55.4 E. 3.5–32.0 E.
30.69–3.90 E.
55.45–56.3 E.

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