bla

blb

ps

ps

ps

ps

blb

t

?

?

?

?

?

?

?

?

?

?

prb

prb

prb

prb

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bla

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t

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coc

coc

coc

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cob

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cob

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flb

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flt

c

cf

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pr

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c

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pra

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pr

pr

a

a

prb

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ps

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coa coa

coa

coa

blc

c

cf

coa

ps

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pra

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prc

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pra

prc

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fd flt

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cob coc

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t

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c cf

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prc

pr prc c

pra

Plains

Materials

Flow

Materials

Corona

Materials

Materials of

Linear Belts

Densely

Lineated

Material

Tessera

Material

Impact

Crater

Materials

75°

50°

25°

75°

25°

–25°

0°

50°

0°

90° 120° 150° 180° 210° 240° 270°

–25°

–50° –50°

–75° –75°

QUADRANGLE LOCATION

Photomosaic showing location of map area. An outline of 

1:5,000,000-scale quadrangles is provided for reference.

V–1

V–26 V–27 V–25

V–37

V–62

V–38

V–24

V–23

V–12

V–11

V–13

V–4

V–36 V–39

V–15

V–48 V–51

V–14

V–49 V–50

V–5

V–58 V–59

V–3 V–6

V–57 V–60

V–16

V–47 V–52

V–28

V–35 V–40

SCALE 1:4 711 886 (1 mm = 4.712 km) AT 25° LATITUDE

1:5 000 000 AT 34° and 73° LATITUDES

LAMBERT PROJECTION

205° 210°

220° 225° 230° 235° 240°

210° 205°

215°

180° 185° 190° 195° 200°

55°

60°

65°

70°

75°

180° 50°

200°

195°

190° 185°

75°

70°

65°

60°

55°

50° 240° 235° 230° 225° 220°

215° INTERIOR —GEOLOGICAL SURVEY, RESTON, VA—2001

Prepared on behalf of the Planetary Geology and Geophysics Program, Solar System 

Exploration Division, Office of Space Science, National Aeronautics and Space 

Administration

Edited by Derrick D. Hirsch; cartography by Hugh F. Thomas and Darlene A. Casebier

Manuscript approved for publication March 1, 2000

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Prepared for the

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

GEOLOGIC INVESTIGATIONS SERIES I–2721

ATLAS OF VENUS: PANDROSOS DORSA QUADRANGLE (V–5)

CORRELATION OF MAP UNITS

–25

20 25 30 35 40 45 50

INCIDENCE ANGLE IN DEGREES

–20

–15

–10

–5

5

BACKSCATTER COEFFICIENT IN DECIBELS

70.7, 182.0

67.4, 191.5

63.1, 211.1

64.6, 216.4

58.8, 211.0

70.5, 180.0

61.0, 211.5 61.0, 211.5

52.3, 203.6 52.3, 203.6

A

66.0, 186.1

61.0, 204.0

63.5, 212.5

56.6, 204.4 

B

6051.6

6052.8

0

0

0

0

0

110

160

260

80

320

RADIUS IN

KILOMETERS

RADIUS IN

KILOMETERS

RADIUS IN

KILOMETERS

RADIUS IN

KILOMETERS

RADIUS IN

KILOMETERS

DISTANCE IN KILOMETERS

DISTANCE IN KILOMETERS

DISTANCE IN KILOMETERS

DISTANCE IN KILOMETERS

DISTANCE IN KILOMETERS

V.E. = 50X

V.E. = 57X

V.E. = 61X

V.E. = 26X

V.E. = 73X

A A Lukelong Dorsa '

GEOLOGIC MAP OF THE PANDROSOS DORSA

QUADRANGLE (V–5), VENUS

By

Elizabeth Rosenberg and George E. McGill

2001

Figure 2. Relief map of Pandrosos Dorsa quadrangle.

Figure 1. Backscatter coefficient versus radar incidence angle for areally extensive map units. Unit 

symbols correspond to those used in table 1. Symbols connected by lines are backscatter coefficients 

for five lava flows on Kilauea Volcano, Hawaii (Campbell, 1995). Roughness of these flows ranges 

from aa (triangles), to pahoehoe (squares), to very smooth lava ponded in crater floors (circles). 

Dashed line represents average scattering law for Venus as defined by Muhleman (1964).

Figure 3. Synthetic aperture radar image of Pandrosos Dorsa quadrangle showing transects of topographic 

profiles in figure 4.

Figure 4. Full resolution synthetic aperture radar images and topographic profiles across major linear belts (fig. 3). 

A, Lukelong Dorsa; B, Lauma Dorsa; C, Anpao Dorsa; D, northern Pandrosos Dorsa; E, southern Pandrosos Dorsa.

Figure 7. Schematic diagram of regional plains emplacement between Bathkol Tessera and Pandrosos Dorsa. A, Preexisting long-wavelength deformation. B, Emplacement of radar dark regional plains material (unit prc). C, Continued 

long-wavelength deformation forms low-lying region between Bathkol Tessera and Pandrosos Dorsa. D, Infilling of 

low-lying region by radar-bright regional plains material (unit pra).

Figure 6. Structural features within southern Pandrosos Dorsa. Schematic trend of folds is shown by green lines, pervasive linear features 

by red lines. Note the generally parallel north south trend of both 

types of structures. Younger linear features trending northwest-southeast are shown in yellow; volcanic shields and domes are shown as 

blue dots.

Figure 5. Geologic sketch map of southern Pandrosos Dorsa. Contacts are solid where certain, dashed where approximate. Large patch 

of smooth plains material (unit ps) in center of belt contains dozens 

of small shields and domes, which defines region as a shield field. 

bla, blb: Linear belt materials a and b, respectively.

TABLE 1. Physical properties of mapped units.

[Symbols of units listed in this table are the same symbols used on the geologic map. At least one sample was taken for each unit. Regionally 

widespread units (pra, prb, prc, and t) and some local units (fd and flb) that outcrop over a large enough range of latitudes for the incidence angle 

to change significantly were sampled in two or three places. In these cases, sample numbers are indicated. Table displays information on 

sample locality, pixel scale (∆), number of pixels used in calculating backscatter (N), incidence angle (ι, angle between SAR beam and a 

normal to an assumed spherical surface where beam hits), radius, normalized backscatter cross section (σo), root mean square slope (θrms) for 

meter and larger scale slopes, Fresnel reflectivity (ρ), emissivity, and dielectric constants calculated for smooth-surface (εs) and rough surface 

(εr) cases. For σo, values in parentheses represent ±1 standard deviation about the mean. For all other data, numbers in parentheses represent 

minimum and maximum values of each parameter within confines of chosen sample boundaries. All mapped units are represented in these 

tables with one exception: linear belt material c is not represented because its material type is unresolved. Sizes of sample boxes represent 

maximum size admissible to accurately represent unit; for instance, sample box for a unit that contains a dense population of radar-bright 

structures must be small enough to fit between structures, which are not inherent properties of the material. Sample box for a widespread 

regional material, however, may be much larger because it is possible to define a larger area without any structural interferences or presence of 

other material units.]

 Unit Latitude; Longitude ∆ (m) N ι Radius (km)

t 1 56.980–57.530 N; 201.230–202.020 E 225 45232 29 6051.387 (6049.708, 6052.135)

2 59.430–60.690 N; 229.390–231.160 E 225 217175 28 6051.229 (6050.416, 6051.691)

prc 1 51.740–52.540 N; 232.330–233.480 E 225 105183 32 6051.378 (6051.254, 6051.629) 

2 62.550–63.180 N; 209.730–212.880 E 225 181621 27 6051.229 (6050.892, 6051.417)

prb 1 69.920–70.720 N; 223.260–226.040 E 225 254562 24 6051.535 (6051.440, 6051.670)

2 72.120–73.450 N; 182.640–187.100 E 225 130788 23 6051.743 (6051.447, 6051.896)

pra 1 53.360–54.420 N; 200.630–203.690 E 225 404000 31 6051.473 (6051.290, 6051.780)

2 58.830–60.950 N; 182.770–184.860 E 225 440674 28 6050.960 (6050.729, 6050.993)

 3 67.300–68.830 N; 209.450–212.890 E 225 418458 25 6050.973 (6050.758, 6051.174)

pm 55.940–56.610 N; 212.020–214.330 E 225 183226 30 6051.986 (6051.742, 6052.230)

pt 65.060–65.140 N; 225.840–226.110 E 225 1591 26 6051.208 (6051.134, 6051.395)

bla 48.360–48.730 N; 218.380–218.550 E 225 7875 33 6051.742 (6051.695, 6051.847)

blb 55.950–56.080 N; 208.390–208.610 E 225 3564 30 6052.175 (6051.737, 6052.305)

ld 56.960–57.160 N; 200.500–200.890 E 225 8448 29 6051.107 (6050.180, 6051.607)

ps 55.620–55.730 N; 207.710–207.780 E 225 1071 30 6052.392 (6052.372, 6052.405)

fd 1 55.390–56.050 N; 218.280–219.370 E 225 75746 30 6051.901 (6051.816, 6052.177)

 2 72.020–72.300 N; 199.250–200.340 E 225 18221 23 6051.394 (6050.987, 6051.545)

fs 70.530–70.860 N; 198.170–199.050 E 225 17538 24 6050.767 (6050.339, 6051.125)

flb 1 48.580–48.850 N; 212.080–212.560 E 225 17024 33 6051.026 (6050.450, 6051.197)

 2 48.900–49.400 N; 213.980–214.880 E 225 56165 29 6051.107 (6050.180, 6051.607)

c 60.070–60.570 N; 223.570–224.710 E 225 10400 28 6051.132 (6051.003, 6051.294) 

cf 60.070–60.570 N; 223.570–224.710 E 225 49533 28 6051.132 (6051.003, 6051.294)

coa 64.990–65.090 N; 208.200–208.460 E 225 2548 26 6051.300 (6050.396, 6051.403)

cob 64.130–64.760 N; 208.290–209.260 E 225 57424 26 6050.948 (6050.795, 6051.011)

coc 65.050–65.230 N; 205.950–206.330 E 225 5994 26 6050.655 (6050.265, 6050.912)

flt 72.910–73.060 N; 238.330–238.830 E 225 3692 23 6051.232 (6051.159, 6051.287)

 Unit σo (dB) θrms (deg) ρ Emissivity εs, εr

t 1 –7.621 (–12.952, –5.299) 5.95 (2.90, 10.40) 0.089 (0.045, 0.170) 0.906 (0.896, 0.914) 3.0, 3.5

2 –7.393 (–14.532, –4.824) 4.57 (1.20, 10.40) 0.070 (0.045, 0.175) 0.902 (0.888, 0.909) 3.1, 3.6

prc 1 –14.791 (–16.359, –13.641) 2.16 (0.70, 4.10) 0.127 (0.080, 0.160) 0.859 (0.850, 0.873) 3.8, 4.8

2 –12.432 (–14.253, –11.153) 2.47 (0.90, 5.10) 0.115 (0.075, 0.175) 0.868 (0.861, 0.877) 3.9, 4.6

prb 1 –12.389 (–13.774, –11.341) 2.36 (1.40, 2.90) 0.125 (0.105, 0.150) 0.857 (0.853, 0.860) 4.3, 4.9

2 –11.723 (–13.214, –10.615) 1.54 (0.60, 2.60) 0.108 (0.075, 0.130) 0.857 (0.854, 0.860) 4.3, 4.9

pra 1 –12.122 (–13.424, –11.105) 3.12 (1.50, 4.60) 0.130 (0.085, 0.185) 0.872 (0.862, 0.884) 3.6, 4.4

2 –11.917 (–13.427, –10.798) 2.67 (1.40, 4.30) 0.126 (0.105, 0.155) 0.867 (0.859, 0.877) 3.8, 4.6

3 –10.799 (–12.279, –9.697) 2.89 (1.80, 4.20) 0.100 (0.065, 0.130) 0.874 (0.867, 0.881) 3.8, 4.4

pm –11.715 (-14.545, –10.017) 1.69 (0.80, 3.20) 0.140 (0.105, 0.175) 0.849 (0.838, 0.869) 4.1, 5.1

pt –8.205 (–10.941, –6.540) 6.06 (5.50, 7.20) 0.097 (0.080, 0.110) 0.890 (0.890, 0.891) 3.4, 3.9

bla –9.971 (–12.594, –8.347) 2.38 (1.90, 3.10) 0.095 (0.085, 0.105) 0.889 (0.887, 0.892) 3.1, 3.9

blb –7.050 (–12.718, –4.672) 3.45 (2.40, 6.00) 0.113 (0.100, 0.140) 0.892 (0.890, 0.894) 3.2, 3.9

ld –9.208 (–12.142, –7.473) 3.04 (2.20, 3.60) 0.070 (0.055, 0.075) 0.896 (0.895, 0.899) 3.1, 3.8

ps –11.499 (–12.897, –10.443) 2.48 (1.80, 2.90) 0.096 (0.090, 0.100) 0.870 (0.870, 0.871) 3.7, 4.5

fd 1 –10.129 (–14.426, –8.012) 2.44 (1.30, 3.70) 0.120 (0.095, 0.145) 0.863 (0.856, 0.868) 3.8, 4.7

2 –7.888 (–9.187, –6.890) 4.20 (1.50, 7.50) 0.084 (0.070, 0.135) 0.887 (0.881, 0.891) 3.6, 4.0

fs –7.244 (–12.041, –5.020) 4.66 (2.30, 7.30) 0.102 (0.075, 0.170) 0.894 (0.892, 0.895) 3.4, 3.9

flb 1 –6.971 (–8.265, –5.976) 4.46 (2.90, 6.50) 0.119 (0.095, 0.155) 0.889 (0.881, 0.892) 3.1, 3.9

2 –12.281 (–14.325, –10.897) 2.50 (1.00, 4.10) 0.126 (0.100, 0.160) 0.859 (0.847, 0.865) 3.7, 4.8

c –7.198 (–9.589, –5.665) 2.26 (1.00, 3.90) 0.081 (0.045, 0.125) 0.895 (0.884, 0.906) 3.2, 3.8

cf –11.557 (–14.218, –9.920) 2.26 (1.00, 3.90) 0.081 (0.045, 0.125) 0.895 (0.884, 0.906) 3.2, 3.8

coa –9.467 (–10.799, –8.449) 5.03 (2.10, 6.10) 0.113 (0.070, 0.135) 0.891 (0.888, 0.892) 3.4, 3.9

cob –11.803 (–13.319, –10.681) 2.62 (2.00, 4.10) 0.106 (0.080, 0.130) 0.876 (0.870, 0.882) 3.7, 4.3

coc –6.757 (–10.182, –4.866) 6.08 (5.20, 10.30) 0.117 (0.090, 0.220) 0.892 (0.890, 0.895) 3.4, 3.9

flt –5.525 (–14.432, -2.804) 1.74 (1.40, 2.50) 0.102 (0.100, 0.110) 0.859 (0.858, 0.860) 4.3, 4.8

0

fd2

fd1

flb2

flb1

pm

pra3

prb1 prb2

prc2

prc1

pra2

pra1

t2 t1

6048 6049 6050 6051 6052 6053

PLANETARY RADIUS IN KILOMETERS

A A'

B

B'

D D'

C C'

E E'

6052.2

6050.6

6052.0

6050.8

6053.0

6051.4

6053.6

6052.2

B B Lauma Dorsa  ̄ '

C C Anpao Dorsa '

D D Northern Pandrosos Dorsa '

E E'

D

C

E

A

B

C

D

Southern Pandrosos Dorsa

Pandrosos Dorsa

Pandrosos Dorsa Pandrosos Dorsa

Pandrosos Dorsa

Bathkol Tessera Bathkol Tessera

Bathkol Tessera Bathkol Tessera

75

75

°

°

240°

240°

180°

180°

50°

50°

75

75

°

°

240°

240°

180°

180°

50°

50°

 ̄

Printed on recycled paper

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Government

For sale by U.S. Geological Survey, Information Services, Box 25286, 

Federal Center, Denver, CO 80225, 1–888–ASK–USGS

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75°

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500 400 300 200 100 0 100 200 300 400 500

The Magellan Mission

The Magellan spacecraft orbited Venus from August 10, 1990, until it plunged into the venusian atmosphere on October 12, 1994. 

Magellan had the objectives of (1) improving knowledge of the geologic processes, surface properties, and geologic history of Venus by 

analysis of surface radar characteristics, topography, and morphology and (2) improving knowledge of the geophysics of Venus by analysis of 

venusian gravity.

The Magellan spacecraft carried a 12.6-cm radar system to map the surface of Venus. The transmitter and receiver systems were used 

to collect three datasets: synthetic aperture radar (SAR) images of the surface, passive microwave thermal emission observations, and 

measurements of the backscattered power at small angles of incidence, which were processed to yield altimetric data. Radar imaging 

and altimetric and radiometric mapping of the venusian surface were done in mission cycles 1, 2, and 3, from September 1990 until 

September 1992. Ninety-eight percent of the surface was mapped with radar resolution of approximately 120 meters. The SAR 

observations were projected to a 75-m nominal horizontal resolution; these full-resolution data compose the image base used in 

geologic mapping. The primary polarization mode was horizontal-transmit, horizontal-receive (HH), but additional data for 

selected areas were collected for the vertical polarization sense. Incidence angles varied from about 20° to 45°.

High-resolution Doppler tracking of the spacecraft was done from September 1992 through October 1994 (mission cycles 

4, 5, 6). High-resolution gravity observations from about 950 orbits were obtained between September 1992 and May 1993, 

while Magellan was in an elliptical orbit with a periapsis near 175 kilometers and an apoapsis near 8,000 kilometers. 

Observations from an additional 1,500 orbits were obtained following orbit-circularization in mid-1993. These data exist as 

a 75° by 75° harmonic field. 

Magellan Radar Data

Radar backscatter power is determined by the morphology of the surface at a broad range of scales and by the 

intrinsic reflectivity, or dielectric constant, of the material. Topography at scales of several meters and larger can 

produce quasi-specular echoes, with the strength of the return greatest when the local surface is perpendicular to the 

incident beam. This type of scattering is most important at very small angles of incidence, because natural 

surfaces generally have few large tilted facets at high angles. The exception is in areas of steep slopes, such as 

ridges or rift zones, where favorably tilted terrain can produce very bright signatures in the radar image. For 

most other areas, diffuse echoes from roughness at scales comparable to the radar wavelength are responsible 

for 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 or volcanic ash, can absorb the incident energy and produce a 

lower observed echo. On Venus, a rapid increase in reflectivity exists at a certain critical elevation, 

above which high-dielectric minerals or coatings are thermodynamically stable. This effect leads to 

very bright SAR echoes from virtually all areas above that critical elevation.

The measurements of passive thermal emission from Venus, though of much lower spatial 

resolution than the SAR data, are more sensitive to changes in the dielectric constant of the surface 

than to roughness. As such, they can be used to augment studies of the surface and to discriminate 

between roughness and reflectivity effects. Observations of the near-nadir backscatter power, 

collected using a separate smaller antenna on the spacecraft, were modeled using the Hagfors 

expression for echoes from gently undulating surfaces to yield estimates of planetary radius, 

Fresnel reflectivity, and root-mean-square (rms) slope. The topography data produced by 

this technique have horizontal footprint sizes of about 10 km near periapsis and a vertical 

resolution of approximately 100 m. The Fresnel reflectivity data provide a comparison to 

the emissivity maps, and the rms slope parameter is an indicator of the surface tilts, 

which contribute to the quasi-specular scattering component.

 ̄

DESCRIPTION OF MAP UNITS

PLAINS MATERIALS

Smooth local plains material—Homogeneous radar-dark plains material having intermediate radar 

backscatter coefficient (–11.499). Contains few to no radar-bright linear features. Locally 

embays tessera material and deformation belt topographic rises. Occurs as small outliers within 

zones of high deformation. Interpretation: Young volcanic material

Mottled local plains material—Locally distributed plains material characterized by small-scale 

(20 km) blotchy pattern of moderately bright to moderately dark patches having intermediate 

radar backscatter coefficient (–11.715). Mottled appearance commonly associated with smallscale shields. Contains few to no radar-bright linear features. Appears to be superposed upon 

regional plains materials, although contacts are commonly gradational. Interpretation: Concentrations of small shield volcanoes and flank flows

Textured local plains material—Moderately radar-bright to radar-bright material with a generally 

diffuse, grainy, or textured appearance and high radar backscatter coefficient (–8.205). Commonly occurs as small patches tens of kilometers across within highly deformed regions. 

Embays regional plains materials. Interpretation: Outliers of rough material; radar brightness 

possibly caused by extremely dense fractures finer than resolution

Radar-bright regional plains material—Broadly distributed material having intermediate radar 

backscatter coefficient (–11.61) that is lower than average scattering law for Venus but high 

with respect to other regional plains units. Mostly homogeneous although locally blotchy with 

materials of slightly less brightness. Some wrinkle ridges visible at highest resolution. Locally 

infills fractures within unit prc. Interpretation: Lava flows

Intermediate regional plains material—Broadly distributed radar-dark to radar-bright variegated 

plains material with an overall diffuse appearance. Radar backscatter coefficient between that 

of units prc and pra (–12.06). Contacts are commonly gradational. Interpretation: Lava flows 

with superposed aeolian features

Radar-dark regional plains material—Broadly distributed material having low radar backscatter 

coefficient (–13.61). Locally embays tessera material and infills grabens. Locally contains high 

concentration of radar-bright fractures. Interpretation: Lava flows

FLOW MATERIALS

Slump material—Radar-bright to radar-dark material that contains an abundance of curvilinear 

radar-bright linear features. Has high radar backscatter coefficient (–7.24). Interpretation: 

Slump material

Digitate flow material—Material of radar-bright to radar-dark digitate flow fields having radar 

backscatter coefficient ranging from –10.13 to –7.89. Commonly associated with regions of 

highly concentrated shield volcanoes. Interpretation: Lava flows; probably basaltic

Lineated flow material—Radar-bright material having intersecting sets of fractures with very high 

radar backscatter coefficient (–5.53). Interpretation: Relatively old volcanic flow with subsequent deformation

Lobate flow material—Material of radar-bright and radar-dark lobate flow fields with a range of 

radar backscatter coefficients from –12.28 to –6.97. Contains a few digitate flows. Interpretation: Flow materials related to montes and coronae outside of quadrangle boundaries; probably 

basaltic

CORONA MATERIALS

Corona material a—Radar-bright material within concentric linear features around corona structures; high radar backscatter coefficient (–9.47) and homogeneous texture. Superposed on 

some corona structures. Interpretation: Volcanic deposit associated with corona development

Corona material b—Intermediate to radar-dark flow material areally associated with corona structures; intermediate radar backscatter coefficient (–11.80). Occurs as both digitate and lobate 

patterns. Superposed on regional plains materials. Interpretation: Lava flows associated with 

corona development

Corona material c—Radar bright material with intersecting sets of fractures similar to tessera terrain; high radar backscatter coefficient (–6.76). Superposed on all surrounding materials. Interpretation: Flows associated with corona development that either become deformed with corona 

evolution or have intrinsically rougher surfaces than surrounding materials

MATERIALS OF LINEAR BELTS

Linear belt material a—Moderately radar-bright material associated with linear belts. Has an 

intermediate radar backscatter coefficient (–9.971). Contains ridges and sparse radar-bright linear features. Embayed by surrounding plains materials. Most contacts are distinct, a few are 

either gradational or ambiguous. Interpretation: Deformed material of uncertain origin

Linear belt material b—Radar-bright to radar-dark material associated with linear belts. Has a 

high radar backscatter coefficient (–7.05). Contains ridges and a penetrative and dense pattern 

of linear features. Linear features more abundant than in unit bla. The densely spaced linears 

cross the ridges and are present in most of the material between individual ridges. Interpretation: Deformed material of uncertain origin

Linear belt material c—Material containing pronounced topography, radar-bright linears, ridges, 

and lobate margins. Radar backscatter coefficient due predominantly to structural deformation. 

Interpretation: Deformed material of uncertain origin, but may include deformed regional or 

local plains materials

DENSELY LINEATED MATERIAL

Densely lineated material—Radar-bright to radar-intermediate material having a radar backscatter 

coefficient of –9.208. Commonly occurs within topographic high tessera blocks and linear 

belts. Contains abundant densely spaced, parallel linear features. Embays tessera material. Also 

occurs as small patches, commonly with wispy edges, embayed by surrounding plains materials. Interpretation: Deformed plains? material older than regional plains materials

TESSERA MATERIAL

Tessera material—Radar-bright material having high radar backscatter coefficient (–7.507). Contains at least two sets of fractures, ridges, and grabens that intersect at high angles. Material 

and all structures are embayed and truncated by all surrounding materials. Present as large 

blocks parallel to north-south-trending linear belts and as scattered small inliers. Interpretation: Old, highly deformed material of unknown origin 

IMPACT CRATER MATERIALS

Crater material, undifferentiated—Radar-bright material having a high radar backscatter coefficient (–7.20). Includes floor, rim, ejecta, and central peak materials. Hummocky texture. 

Superposed on all surrounding materials. Some floor materials are radar-dark (for example, 

Barsova crater). Interpretation: Deposits and structures caused by bolide impact; radar-dark 

floor material either ponded impact melt or younger lava 

Crater flow material—Radar-bright to radar-dark, mottled flow material associated with impact 

craters. Intermediate radar backscatter coefficient (–11.56). Interpretation: Melt generated by 

bolide impact event; lavalike flow deposit

Contact—Dashed where approximate; dotted where buried; queried where uncertain

Impact crater rim—5–20 km diameter

Impact crater rim—≥20 km diameter

Depression

Corona annulus

Flat-topped volcanic dome—Diameter ≥10 km

Volcanic shield with flow

Flow direction

Shield field

Aeolian feature

Ridge

Wrinkle ridge

Lineament

Graben—Dotted where buried

Channel—Dotted where buried

Splotch feature

INTRODUCTION

The Pandrosos Dorsa quadrangle (V–5), Venus, lies between latitudes 50° N. and 75° N. and longitudes 

180° E. and 240° E. and includes the northern part of Vinmara Planitia. The quadrangle encompasses approximately 10 million square kilometers of the northern low-lying plains of the venusian surface and sits at an average radius of 6051.30 kilometers, approximately 500 meters below the mean planetary radius of 6051.84 

kilometers.

Vinmara Planitia is dominated by a highly complex pattern of long, narrow, apparently deformed belts and 

their surrounding regional plains. These belts are defined as large-scale linear zones that contain and confine 

smaller scale ridges and radar-bright linear features; they are termed "linear belts" in this report. Vinmara Planitia 

contains one of the two highest concentrations of linear belts on Venus; the second being within Lavinia Planitia 

in the southern hemisphere. Unlike the belts of Lavinia Planitia, the linear belts of Vinmara Planitia contain a 

wide variety of morphologic styles, and the material within these belts spans a significant age range. Furthermore, the belts within Vinmara Planitia cover a surface area three times greater than the belts in Lavinia Planitia.

All or parts of twelve linear belts are within the Pandrosos Dorsa quadrangle: Lukelong, Lauma, Okipeta, 

Iris, Surupa, Ahsonnuti, Akuanda, Tikoiwati, Aida-Wedo, Anpao (the linear belt in the longitudinal center of the 

quadrangle), an unnamed belt, and Pandrosos, the linear belt for which the quadrangle is named. Other small 

concentrations of ridges are present but are not substantial enough to be cataloged as major linear belts. Topographically, the belts commonly reach heights of approximately one kilometer above the surrounding plains, 

although portions of the belts are not elevated or even occur within troughs. Together, the twelve linear belts create a fanlike geometric pattern in plan view and form the majority of what has been referred to as the "Ridge Belt 

Plains-Fan Assemblage" (Frank and Head, 1990). As this name implies, some of the features within the belts are 

ridges. However, nearly half of the features within the belt boundaries are either recognizable grabens or radarbright linear features, which are generally interpreted as extensional fractures. Therefore, the more general term 

"linear belt" is preferable to ridge belt. The linear belts of the Pandrosos Dorsa quadrangle continue northward to 

the pole and southward into the regions of Nemesis Tessera (V–14) and Bellona Fossae (V–15) where they lose 

their intensity and eventually die out (Zimbelman, 1995).

In addition to the linear belts, the quadrangle includes the easternmost part of the regional plains of Atalanta 

Planitia and the northern part of Vinmara Planitia. These widespread plains lie approximately 500 meters below 

the mean planetary radius and show little to no topography. Atalanta and Vinmara Planitiae are similar to other 

regional plains covering nearly 80% of the global surface (Masursky and others, 1980), except that the plains of 

Vinmara contain far fewer wrinkle ridges.

The quadrangle also includes two large and elongate patches of tessera: Virilis on the east edge of the quadrangle and Bathkol farther west. These blocks of tessera materials mimic the nearly north-south orientation of the 

linear belts at elevations approximately one kilometer above the surrounding regional plains. Other major features within the quadrangle include twelve impact craters, two of which are large and include outflow material; 

nine coronae and coronalike features; seven volcanic shield fields; numerous small tessera inliers; and part of 

one lava channel. 

This region is of geologic interest because it contains the most widespread and largest concentration of 

extensive linear belts on Venus, and thus it is an ideal area to study the temporal and genetic relations between 

linear belts and regional plains. The variety of morphologies, structural styles, and temporal relations within the 

Pandrosos Dorsa quadrangle provides a unique opportunity for establishing stratigraphic relations not only 

among material units but also among tectonic episodes. Because the preserved record of tectonic processes on 

Venus is different from that of the oceanic plate tectonics we recognize on Earth (Solomon and others, 1991, 

1992), the Pandrosos Dorsa quadrangle is a key region of study for illuminating past and present tectonic styles 

of Venus. Interpretations extracted from this study may apply to other venusian linear belts and may help explain 

crustal and upper mantle properties and processes.

METHODS AND DATA USED

The accompanying 1:5 million-scale geologic map was prepared using Magellan radar imagery at several 

resolutions. The base map is a controlled mosaic of Magellan Synthetic Aperture Radar (SAR) data prepared by 

the U.S. Geological Survey. The data were collected during the cycle 1 imaging with a west-to-east look direction. Initial mapping was based on Compressed Once Mosaicked Image Data Records (C1-MIDR's) having a 

resolution of 225 meters/pixel. A more detailed, higher resolution analysis was completed using digital Full Resolution Mosaicked Image Data Records (F-MIDR's) and F-maps (hard copies of 12° by 18°, 12° by 24°, and 12°

by 36° regions at full resolution) which have an oversampled resolution of 75 meters/pixel. Altimetry, emissivity, 

root mean square slope, and Fresnel reflectivity also provided useful mapping information, especially with 

respect to delineating volcanic flows and surficial deposits. Synthetic parallax stereo images also were utilized.

SAR incidence angles within the Pandrosos Dorsa quadrangle range from 33° at 50° latitude to 23° at 75°

latitude. Below an incidence angle of about 25°, smoother surfaces have greater backscatter than rougher surfaces (Farr, 1993). A moderately rough surface, with wavelength approximating the 12.6 cm wavelength of the 

radar beam, has backscatter of –12 dB at 23° and –20 dB at 33°, the incidence-angle range within the Pandrosos 

Dorsa quadrangle. In general, and independent of wavelength, surfaces appear smoother as incidence angles 

increase; therefore, materials toward the southern part of the quadrangle appear smoother than similar materials 

in the northern part of the quadrangle. This difference in appearance may lead to erroneous interpretation of 

material units if incidence angles are not taken into account when mapping.

Stratigraphic material units were defined primarily on the basis of relative radar brightness, fabrics, and 

cross-cutting and superposition relations as displayed in the cycle 1 SAR images. Ancillary radar properties (radar backscatter coefficient, emissivity, Fresnel reflectivity, and root mean square slope) were calculated using 

programs provided by Campbell (1995). Further explanation of Magellan data collection and general methodology for SAR analysis can be found in Ford and others (1989).

STRATIGRAPHY

The stratigraphic units within the boundaries of the Pandrosos Dorsa quadrangle are categorized according 

to type location and material characteristics. Twenty units are defined: one tessera unit, one densely lineated unit, 

three materials confined to linear belts, six plains units, four flow units, three corona units, and two impact crater 

units. Units are defined primarily according to their relative radar brightness on the SAR image, homogeneity of 

the radar brightness, and texture. Other defining factors are cross-cutting relations and truncation of structures 

and fabrics. Whereas crater densities are useful in determining relative ages among underlying units on other 

planets, the small number of craters on Venus and their globally random distribution (Schaber and others, 1992; 

Strom and others, 1994) make this dating tool essentially useless for mapping at this scale.

TESSERA MATERIAL

The oldest material unit present within the Pandrosos Dorsa quadrangle is comparable in appearance to that 

described from Venera images as tessera by Basilevsky and others (1986). Tessera material (unit t) is characterized by its bright backscatter coefficient (averaging –7.507 dB) and high density of closely spaced linear features, which have at least two orientations at high angles to each other. The bright backscatter is inferred to result 

from high surface roughness at centimeter scale combined with the abundance of linear features, most of which 

are inferred to be fractures. Two distinctly different sizes of tessera exposures occur in the Pandrosos Dorsa 

quadrangle: two large blocks, Virilis and Bathkol Tesserae, are thousands of kilometers long, whereas the abundant small patches are tens of kilometers across. Virilis Tessera is on the east edge of the quadrangle and has no 

distinct trend. Bathkol Tessera is centered at 198.0° E. longitude and has a general north-south trend that mimics 

that of the linear belts. Each of these large blocks of tessera spans approximately 20° in latitude. The dozens of 

smaller patches of tessera material are widely distributed throughout the quadrangle.

Both the large blocks and the small patches are inferred to be inliers, poking through the surrounding plains 

material from beneath. Commonly, tessera material is cut by grabens as much as 8 kilometers wide, which generally are filled by the surrounding plains material. The materials in contact with tessera material consistently truncate all of the linear features present within the tessera, supporting the inference that tessera consists of the oldest 

material within Vinmara Planitia.

DENSELY LINEATED MATERIAL

Densely lineated material (unit ld) occurs within linear belts, at the edges of large blocks of tessera, and as 

small scattered inliers within the regional plains materials. Unit ld is characterized by moderate radar backscatter 

(–9.208 dB), which appears to result from a unidirectional linear fabric having a characteristic spacing of tens of 

meters. The dominant trend of this fabric in the quadrangle is northwest-southeast, but locally it is northeast 

southwest, implying some regional variation in principal stress orientation. Densely lineated material is consistently embayed by all material units with the exception of tessera, making it the second oldest unit in the quadrangle. 

MATERIALS OF THE LINEAR BELTS

Although numerous material units are located within the linear belts, only three materials are exclusive to 

the belts: linear belt materials a, b, and c, hereafter referred to as belt materials a, b, and c. The origins of these 

materials are unknown.

Belt material c (unit blc) is characterized by bright radar backscatter and high density of structural features 

such as ridges and radar-bright linear features. Unlike belt materials a and b, belt material c lacks undeformed 

portions that might provide information for stratigraphic correlation. Two relatively small regions of belt material c are mapped; they represent approximately five percent of the area covered by materials of the linear belts. 

A backscatter coefficient has not been measured for belt material c because it would reflect only the radar brightness due to structural and topographic features. The material is possibly a correlative of belt material a or b, but 

topographic and structural masking prevents unambiguous correlation.

Belt material b (unit blb) is characterized by a relatively bright backscatter coefficient (–7.050 dB), which 

indicates the surface is slightly rougher at centimeter scale than that of belt material a. As with belt material a, 

belt material b contains ridges that follow the trend of the belt itself, but the trend of the dense and pervasive 

radar-bright linear features varies from subparallel to approximately 30° from the trend of the belt. Belt material 

b is embayed by all surrounding material units with the exception of tessera material and densely lineated material. Most of the linear belts in the study region are composed exclusively of belt material b.

Belt material b is inferred to be older than belt material a from the single region within the Pandrosos Dorsa 

quadrangle in which the two units are in direct contact (near 55°–58° N., 206.2°E.). The relative ages are based 

primarily on cross-cutting relations and topography (see Structural Geology section). 

Belt material a (unit bla) has moderately bright backscatter (–9.971 dB) and contains ridges and narrow 

radar-bright linear features. The trend of the ridges within belt material a generally parallels the trend of the belt 

itself, but the few radar-bright linear features within the material are consistently oriented approximately 50° to 

the trend of the ridges. Belt material a is embayed by all surrounding regional plains materials.

PLAINS MATERIALS

The Pandrosos Dorsa quadrangle contains three regionally widespread plains units and three local plains 

units. The regional plains materials are defined by their spatial extensiveness and relative brightness on SAR 

images. Each regional plains unit has characteristic homogeneity in radar brightness and is interpreted to be the 

result of extensive flooding by basaltic materials. As indicated by their 1.5°–3.0° root mean square slopes (table 

1), the regional plains materials are relatively smooth at hundred meter scale. At centimeter scale, however, they 

differ in roughness (fig. 1; table 1). From smoothest to roughest, the regional plains have been defined as radar 

dark (unit prc); intermediate (unit prb); and bright (unit pra).

The radar-dark regional plains material covers approximately 20% of the quadrangle's surface area. Radardark regional plains material (unit prc) is characterized by a homogeneous, relatively low backscatter coefficient 

and a 2.32° root mean square slope, which are indicative of a smooth surface at both centimeter and hundred 

meter scales (fig. 1; table 1). The backscatter coefficients of the radar-dark regional plains plot between terrestrial very smooth ponded lava flows and pahoehoe flows (fig. 1). Locally, radar-dark regional plains material 

embays tessera material and fills grabens that cut the tessera, and it also embays and truncates structures within 

densely lineated plains, indicating that the radar-dark regional plains material is younger than both. Where radardark plains material is in contact with linear belts, the plains material is younger than both the linear features and 

the materials within the belt.

The scarcity of wrinkle ridges in the radar-dark plains material is puzzling. All other characteristics of radardark regional plains material suggest that it is related to the global plains inferred to have formed by extensive 

resurfacing approximately 300 million years ago (Strom and others, 1994), and most of the regional plains inferred to be the result of this resurfacing are characterized by common to abundant wrinkle ridges (for example, 

Squyres and others, 1992; Basilevsky and Head, 1995).

The absence of clear embayment relations (for example, indistinct unit boundaries) prevents determining a 

relative stratigraphic age for intermediate regional plains material (unit prb). The irregular and indistinct nature 

of many contacts suggests subsequent aeolian reworking of materials. Therefore, the difference in radar backscatter as seen on the SAR image alone distinguishes this unit from the other regional plains units.

Radar-bright regional plains material (unit pra) covers roughly 40% of the surface area of the Pandrosos 

Dorsa quadrangle. Most of the radar-bright regional plains are concentrated in the western half of the quadrangle, 

from approximately 85° E. to 205° E. Although the radar bright regional plains material is characterized as 

bright, the backscatter coefficients plot between very smooth ponded lava flows and pahoehoe flows as measured 

on Earth (fig. 1). Regardless of the local splotchy patterns within lobate flow forms, the sampled portions of 

radar-bright regional plains material follow the general trend of the Muhleman curve (fig. 1). This trend indicates 

that these plains have the consistent homogeneous properties characteristic of regional plains materials on Venus. 

Radar-bright regional plains material consistently fills narrow grabens within the radar-dark regional plains 

and embays Bathkol Tessera and parts of Pandrosos Dorsa, indicating that it is younger. However, in some places 

along Lauma Dorsa the radar-bright regional plains material appears to be deformed into short-wavelength ridges 

and wrinkle ridges which, along with the absence of embayment relations in Lauma Dorsa, implies that emplacement of the radar-bright regional plains predates the latest deformational episode associated with Lauma Dorsa.

A few structural features are present within the three regional plains units, including a small number of eastwest trending long linear features, grabens, and some wrinkle ridges. Wrinkle ridges are subtly present in small 

groupings within the radar-bright regional plains materials, although they are not as apparent as on regional 

plains elsewhere on Venus. Most structural features within the quadrangle appear to be concentrated within the 

linear belt regions and surrounding coronae and coronalike features.

 Isolated patches of material are correlated with the various regional plains units on the basis of similar 

radar properties even though the correlation cannot be proven due to lack of outcrop continuity.

The local plains materials differ from regional plains materials primarily in their limited area and very localized distribution. Local plains materials include textured plains material (unit pt), mottled plains material (unit 

pm), and smooth plains material (unit ps).

Textured plains material (unit pt) is present both within and outside of linear belts. Within linear belts it 

occurs as small, elongate patches. It is devoid of any resolvable structural patterns. The material has a textured 

appearance which is either the result of a small-scale dense structural fabric below the radar resolution or homogeneous roughness. The textured plains material is interpreted to be younger than both the regional plains and 

linear belt materials.

Mottled plains material (unit pm) also is found both within and outside of the linear belts. It is characterized 

by a small-scale (20 km) blotchy pattern of moderately bright to moderately dark patches inferred to be due to 

abundant small shield volcanoes and associated flows. The scalloped edges of the unit boundaries are interpreted 

as juxtaposed shield flows. Owing to the consistent roundness of the scallops and the absence of cross-cutting 

structures, the mottled plains material is interpreted to be younger than both adjacent regional plains and linear 

belt materials. 

The smoothest and least widespread of all material units within the quadrangle is smooth plains material 

(unit ps). Small patches of smooth plains material are concentrated in the southern part of the quadrangle near 

southern Pandrosos Dorsa and Neyterkob Corona. It is characterized by both its radar-dark backscatter (–11.499 

dB) and its jagged edges, which embay all of the surrounding materials. Smooth plains materials are superposed 

on digitate flows, dark regional plains, and all linear belt materials.

FLOW MATERIALS

Within the Pandrosos Dorsa quadrangle are four types of flow materials defined by their morphology on the 

SAR images: four lobate flows (unit flb); a lineated flow (unit flt); seven digitate flows (unit fd); and two flowlike 

deposits (unit fs) inferred to be due to slump associated with a volcanic feature. Each of the mapped flow units 

has a separate source, and thus it is unclear if the compositions of the materials are similar. Where distinctly separate units of flows share a contact, an age relation cannot be determined. The map shows dashed contacts within 

units where this occurs.

Lobate flow material (unit flb) is characterized by overall homogeneity with regard to texture and radar 

brightness. The unit contains faint traces of lobate forms throughout and has moderate radar backscatter. In some 

places, generally at the leading edges of the material unit, the lobate forms are more easily recognized due to 

their highly variable backscatter coefficients (–6.971 dB to –12.281 dB), which generally contrast greatly with 

the surrounding materials. Most of the mapped lobate flows originate from regions outside the quadrangle boundaries. Flow directions are evident from lobate forms within the unit and are consistent with present topographic 

slopes. Locally, the lobate flow material appears to be superposed on the regional plains material, but this distinction is too unclear to be definitive.

Also present in the quadrangle is a single deposit of lineated flow material (unit flt) located at 73.0° N., 

238.7° E. The backscatter coefficient is very bright (–5.525 dB) but the origin of this brightness is different in the 

northern and southern parts of the mapped unit. The southern half of the lineated flow has a fan-shaped morphology and a knobby or hummocky texture that is responsible for its radar brightness. In the northern half, the material is concentric to its source, and the radar brightness is created by sets of orthogonal radar-bright linear 

features. Due to morphologic differences, the two flow forms are interpreted to have been deposited at different 

times. The apparent source is outlined by the shape and direction of the flows, but the actual source is no longer 

visible. The lineated flow material is superposed on lobate flow material.

Digitate flow material (unit fd) is characterized by elongate individual flows that intermingle with fingerlike 

morphology. The backscatter coefficients vary greatly from very bright (–7.888 dB) to moderately dark (–10.129 

dB), indicating highly variable surface roughness. Of the seven digitate flows, four appear to originate in regions 

that are dotted with volcanic shields and where the material flow direction is clearly outward from the approximate center of the shield fields. The remaining three mapped flows originate either from a single volcanic source 

or from a source that is no longer visible. Each of the digitate flows is younger than regional plains. Where one 

of the digitate flows (Turgmam Fluctus) comes in contact with Lonsdale crater, the crater outflow embays the 

digitate flow, thus indicating that the crater outflow material is younger. The digitate flow material is superposed 

on the lobate flow material, as is indicated east of Nyterkob Corona where the digitate flow material is controlled 

by wrinkle ridges in the lobate flow material.

Slump material (unit fs) occurs as two small patches near 71°–72° N., 198° E. It is characterized by a curvilinear pattern of radar bright linear features and appears to be associated with digitate flows that surround it. Its 

radar brightness (–7.244 dB) is inferred to result from the radar bright linear features that are in turn inferred to 

result from slump activity along the edge of the small volcanic center east of the patches. The slump material 

unit is younger than the regional plains, but its upper stratigraphic limit is indeterminable.

CORONA MATERIALS

Portions or all of nine coronae are present within the quadrangle. Concentric annuli and radial structures 

that define the coronae are discussed in the section "Structural Geology". Three separate material units are 

related to these coronae, the relative ages of which are unknown. Not all coronae have associated corona materials; some of the coronae within the Pandrosos Dorsa quadrangle are structural features alone, with no evidence 

of associated deposits (for example, Aspasia Corona). Corona materials are superposed on all regional plains 

materials.

Corona material c (unit coc) lies within and near annuli of Neyterkob, Cassatt, and Muzamuza coronae, and 

an unnamed corona centered at 69.9° N., 198.9° E. This material is differentiated from corona material b in that 

corona material c has a brighter backscatter coefficient (–6.757 dB) and a prominent structural pattern that 

resembles tessera material. It is differentiated from tessera material by its distinct association with coronae features. 

Corona material b (unit cob) is characterized by a moderately dark backscatter coefficient (–11.803 dB) and 

digitate and lobate patterns that appear to originate at corona features. The corona flow materials of Cassatt, 

Muzamuza, Ninkarraka, and Schumann-Heink appear to mostly surround the coronae, which indicates little 

topographic variation across the affected region at the time of emplacement. Other corona material b flows do 

not entirely surround the associated coronae structures. 

Corona material a (unit coa) is characterized by a homogeneous textured appearance and a moderately 

bright backscatter coefficient (–9.467 dB). The unit is within coronae annuli and tends to follow the concentric 

pattern of the coronae structures. This material is associated with Neyterkob, Cerridwen, Cassatt, and Muzamuza 

coronae, and an unnamed corona centered at 69.9° N., 198.9° E.

IMPACT CRATER MATERIALS

Deposits of twelve impact craters are present within the quadrangle. These include Barsova, Dolores, Evangeline, Lonsdale, Orlova, Zoya, Ahava, Batya, Elizabeth, Ezraela, Kelila, and Yetta. Impact crater deposits 

include undifferentiated material of central peaks, floors, rims, and ejecta blankets (unit c), and fluid flow material (unit cf). Most of the materials included within the undifferentiated crater material have high radar cross sections due to the surface roughness. As with digitate flows, some of the flow materials have lower radar cross 

sections.

All of the craters are younger than the regional plains. Flow material extending westward from Lonsdale 

appears to embay eastwardly flowing digitate material of Turgmam Fluctus which originates at a shield field 

within Iris Dorsa. This relation indicates that Lonsdale is demonstrably younger than Turgmam Fluctus. Lack of 

direct contact between other craters and digitate flows prohibits further stratigraphic correlation. The flows from 

the crater Barsova follow the broad rise of Iris Dorsa and flow in a southeasterly direction as indicated by the 

digitate pattern. This relation implies that Iris Dorsa was elevated prior to the impact that formed Barsova.

Also of interest is the asymmetric crater Dolores, at 51.5° N., 201.6° E. The southern part of its rim is missing or obscured. If the rim is missing because it is covered by the surrounding regional plains materials, then the 

crater would be older than these plains. However, a zone of ejecta avoidance south of the rim of Dolores suggests 

a low angle impact from the south (Schultz, 1992), which also could result in an incomplete rim. The low angle 

impact is further supported by the butterfly ejecta pattern (Gault and Wedekind, 1978). Wrinkle ridges superposed on regional plains southeast of the crater rim appear to constrain flow material from Dolores, indicating 

that the wrinkle ridges, and thus the regional plains, must be older than the crater. Therefore, the regional plains 

material cannot be superposed on the southern part of the crater rim, and thus the crater clearly is younger than 

regional plains.

The lack of contacts with any material other than regional plains leaves the upper stratigraphic age limits of 

the remaining impact craters unconstrained. Also present are two "splotch" features centered at 51.5° N., 215.2°

E. and 54.3° N., 196.0° E. Splotches are believed to be the result of near-surface explosions of bolides that 

deposited fragmented material without actually reaching the surface (Campbell and others, 1992). These 

splotches also are younger than the regional plains.

SURFICIAL MATERIALS

Several local regions within the quadrangle appear to have undergone at least some aeolian activity, represented on the SAR images by 1) feathery, diffuse swaths bordering individual ridges toward the southernmost 

part of the quadrangle, or 2) diffuse patches overprinting regional plains materials near 53° N., 183° E., 69° N., 

180° E., and 73° N., 195° E. Both types probably represent very thin veneers of reworked older materials.

STRUCTURAL GEOLOGY

The Pandrosos Dorsa quadrangle contains numerous structural features: large linear belts, small structures 

confined to the large belts, wrinkle ridges, radar-bright linear features, and coronae and coronalike features. The 

dominant structural features are the twelve linear belts and their contained ridges and radar-bright linear features. 

This section includes a general overview of these belts, including a detailed analysis of the southern part of the 

most complex belt, Pandrosos Dorsa. This discussion is followed by descriptions of the other structural features.

LINEAR BELTS

Linear belts are defined as large-scale linear zones that contain and confine smaller scale structures. Owing 

to the combined effects of topography and roughness on radar backscatter, linear belts tend to be much brighter 

on SAR images than the surrounding plains materials. Structures related to linear belts have two different scales: 

long wavelength, representing the spacing between belts; and short wavelength, representing the spacing 

between the smaller structures confined to the individual belts. Belts are generally categorized by the type of 

short wavelength structures that they contain (for example, ridges or radar-bright linear features). Linear belts 

commonly are elevated relative to surrounding plains (Solomon and others, 1992; Squyres and others, 1992); 

however, due to topographic variations along the lengths of each belt, elevated topography is not a defining characteristic within the Pandrosos Dorsa quadrangle. 

Linear belts occur in low-lying plains, specifically within Lavinia, Atalanta, and Vinmara Planitiae (Solomon and others, 1992). Initial studies of linear belts were based on Venera 15 and 16 images (Barsukov and others, 1986; Basilevsky and others, 1986; Kryuchkov, 1988; Sukhanov and Pronin, 1989; Sukhanov and others, 

1989; Frank and Head, 1990) and Arecibo images (Campbell and others, 1991; Senske and others, 1991). The 

low resolution of these datasets made distinction between different styles of short-wavelength structures within 

the belts difficult, if not impossible. In most cases, the higher resolution Magellan data allow for clearer distinction between radar-bright linear features and ridges.

The apparently very low rate of deposition and erosion on Venus leads to the inference that most ridges and 

other narrow linear features are primary structures. These structures are inferred from their morphologies, as 

seen on the SAR images. Ridges are identified on cycle 1 SAR images by brightness along the west edge and 

darkness along the east edge, separated by a transitional region of intermediate radar brightness. This gradual 

transition from radar bright to radar dark across the crest indicates that the ridges have gentle curvatures. If these 

ridges are primary fold structures, as is generally inferred, then the folds are gentle and open, consistent with a 

simple buckling genesis. Radar-bright linear features are generally interpreted as fractures or faults, an inference 

supported by places where they are wide enough to resolve bright and dark scarps defining a grabenlike depression. On the basis of Venera 15 and 16 images, the belts in the Vinmara Planitia region were referred to as the 

Ridge Belt Plains-Fan Assemblage (Frank and Head, 1990). All but one belt were classified by Frank and Head 

(1990) into Class II, defined as belts that generally consist of a multitude of closely spaced smaller ridges 

(Kryuchkov, 1988). The one exception is Lukelong Dorsa, which was classified as a Class I belt, defined as 

swells with broad summits (Kryuchkov, 1988).

In comparison to the clearly defined belts of Lavinia Planitia, the belts of Vinmara Planitia are not exclusively ridge or fracture belts. Nor do the belts of Vinmara Planitia display the orthogonal pattern observed in 

Lavinia Planitia, where ridge and fracture belts are about normal to each other (Squyres and others, 1992). 

Rather, in Vinmara Planitia, most of the linear belts are composite or hybrid belts consisting of ridges, fractures, 

and grabens. Hence the more general and nongenetic term "linear" belts is preferred to "ridge" or "fracture" belts.

Although analysis of Magellan images indicates that many of the short-wavelength structures confined to 

the linear belts are not ridges, the previously described closeness and regularity of spacing between structures 

holds true. As noted by Zuber and Parmentier (1990), the distance between linear belts also tends to be uniform. 

The structures within the belts deform a variety of material units, including older material of unknown origin, 

younger regional plains materials, and patches of tessera.

Long-Wavelength Deformation

In plan view, the twelve linear belts of the Pandrosos Dorsa quadrangle bifurcate and merge along strike, 

thereby forming an anastomosing pattern having an overall north-south orientation. Belt lengths range from 

1,000 to 2,500 km; widths range from 20 to 230 km. Characteristic spacing between the belts is 300–400 kilometers, increasing southward because of the general north-south orientation of the belts, which creates the fanlike 

pattern noted by earlier workers. Each of the belts is surrounded by regional plains material, and one of the belts 

comes in direct contact with a major block of tessera material present in the quadrangle. 

Although the spatial resolution of altimetry data from Magellan is superior to that of prior missions, the 

altimetry footprints in the quadrangle range from 209 km2 in the south to 345 km2 in the north, areas that are 

large relative to the widths of short-wavelength structural features. Thus the resolvable topographic expressions 

of linear belts reflect only the belt-scale morphologies of these features. Ten of the twelve belts have elevated 

topography relative to both the surrounding plains materials and the mean planetary radius, although some parts 

of these belts show no relief at all (fig. 2). Whereas Lukelong and Lauma Dorsa are each associated with linear 

troughs, only Lauma Dorsa is topographically depressed in its entirety relative to the surrounding plains. A variety of linear belt styles is revealed from topographic profiles along transects drawn perpendicular to the trend of 

each belt (figs. 3, 4). Of the twelve linear belts of the Pandrosos Dorsa quadrangle, Lukelong, Lauma, Anpao, 

Pandrosos, Iris, and unnamed Dorsa are most illustrative of the variety of styles present among linear belts and 

provide the most important information for determining geologic history in the region.

The southern third of Lukelong Dorsa is in the northwest corner of the Pandrosos Dorsa quadrangle. The 

total length of Lukelong Dorsa is 1,560 km, and it has a relatively constant width of 50 kilometers. The belt 

trends approximately northwest. Unlike most linear belts on Venus, which are generally elevated relative to surrounding plains, Lukelong is contained within a trough approximately 100 km wide and 600 m deep relative to 

the average elevation of the surrounding plains (fig. 4A). The belt also is structurally simpler than most of the 

other belts within the Pandrosos Dorsa quadrangle, because the trends of the included ridges and linear features 

are generally parallel to the trend of the belt and lack the complex anastomosing patterns seen in the other belts.

In the western part of the quadrangle, Lauma Dorsa merges with Lukelong Dorsa at 65.5° N., 186° E. 

Lauma Dorsa is 2,480 km long and ranges in width from 32 to 89 km. Lauma Dorsa is oriented north-south 

throughout its entire length, and it is the longest and straightest belt within the quadrangle. Together, Lukelong 

and Lauma define the boundary between Atalanta Planitia to the west and Vinmara Planitia to the east.

In addition to the abundance of radar-bright linear features and short-wavelength ridges, Lauma contains 

Aspasia Corona and an abundance of ovoid structures elongated parallel to the trend of the belt. Although these 

ovoid structures are concentrated in the south, they are present along the entire length of the belt. Fractures and 

grabens are not observed anywhere within Lauma Dorsa.

Similar to Lukelong Dorsa, Lauma Dorsa occurs within a linear trough reaching depths of approximately 

600 m below the surrounding plains material and over 1 km below the mean planetary radius (fig. 4B). The 

trough is asymmetric, reaching greatest depths along the eastern edge of the belt.

Anpao Dorsa, near the longitudinal center of the quadrangle at 205° E., forms the northern extension of the 

wide southern part of Pandrosos Dorsa. The belt is oriented north-south, is 920 km long, and has a maximum 

width of 78 km. It contains radar-bright linear features, fractures, ridges, and grabens, which categorize the belt 

as a hybrid linear belt. Topographically, Anpao Dorsa is asymmetric with a deep trough on its west edge and 

broad ridge along its crest (fig. 4C).

Pandrosos Dorsa is near 205° E. and is primarily oriented north-south, but its orientation changes to N. 10°

E. at 58.3° N., 207° E. where the belt splits to form Anpao Dorsa to the west and the continuation of Pandrosos 

Dorsa to the east. The length of the belt is approximately 2,190 km, and its width changes from 62 km in the 

north to 229 km in the south. The belt contains linear features, fractures, ridges, and grabens, thus categorizing it 

as a hybrid linear belt. Details of the internal structures of Pandrosos Dorsa are addressed as a separate analysis 

in the "Discussion" section. 

Contained within the southern part of Pandrosos Dorsa is a shield field from which the digitate flows to the 

east of the belt emanate. Topographically, Pandrosos is asymmetric and quite complex, having an undulating pattern of ridges and troughs across its crest (fig. 4D, 4E). Figure 4E shows a profile across the widest part of southern Pandrosos Dorsa. From west to east the transect reveals a low, broad ridge, approximately 100 km across, 

followed by two ridges; one narrow and low, the other broad and high. At its apex, Pandrosos reaches 6053.3 km, 

the highest elevation of any belt in the quadrangle.

Iris Dorsa, near 58° N., 217° E., extends as far north as 65.5° N. and as far south as 48° N., just beyond the 

quadrangle boundary. Iris is 1,790 km in long and ranges from 57 to 92 km wide. Its strong anastomosing pattern 

creates a belt orientation that varies from the dominant north-south to N. 10° E. and N. 10° W. Whereas the 

northern part of the belt is dominated by short-wavelength ridges, the southern part resembles a hybrid belt, containing both ridges and radar-bright linear features. As with Pandrosos Dorsa, digitate flows emanate from the 

center of Iris Dorsa. Topographically, Iris has the smoothest profile of any of the belts in the quadrangle, as it 

forms a single broad ridge as opposed to a series of parallel ridges. A trough along the west edge of Iris is present 

only in the northern parts of the belt.

An unnamed hybrid linear belt is present in the northeastern part of the quadrangle, centered at 68° N., 231°

E. It is oriented N. 10° W., is approximately 1,600 km long, and has a relatively constant width of 85 km. 

Whereas the northern part of the belt is surrounded by regional plains materials, the southern part of the belt terminates within Virilis Tessera. Evidence that formation of the belt postdates formation of Virilis Tessera material 

is observed where tessera material contained within the belt is cut by belt structures and elongated parallel to the 

trend of the belt. The topographic expression of this linear belt resembles that of Pandrosos Dorsa in that it consists of multiple ridges and troughs, and the highest elevation is reached at the east edge.

Short-Wavelength Deformation

Each of the long-wavelength linear belts contains numerous short-wavelength deformation features. Two 

types of structures are observed: ridges, interpreted as folds resulting from contractional strain; and radar-bright 

linear features, interpreted as fractures and faults resulting from extensional strain (Solomon and others, 1991). 

The short-wavelength structures that are resolvable as ridges reach lengths of 200 km and widths of 30 km and 

average approximately 150 km and 15 km, respectively. The linear features range in length from tens to hundreds of kilometers. The ridges and linear features have a regular spacing of 25 km. Locally, the linear features 

are wide enough to be recognized as grabens. In plan view, the individual ridges are somewhat sinuous, whereas 

the fractures are straight to gently curved.

Linear belts are structurally complex. Some linear belts change along their lengths from being dominated by 

ridges to being dominated by fractures and grabens. Other belts are hybrids or composites of ridges, fractures, 

and grabens. In many places within the hybrid belts, ridges and fractures are oriented subparallel to each other 

and to the long axis of the belt. The clearest example of this relation is seen in the southernmost part of Pandrosos Dorsa, where individual ridges and fractures are oriented within 10° of north-south and within 5° of each 

other.

Elsewhere, ridges and fractures are not parallel to each other. For example, fractures present in the central 

part of Iris Dorsa (58° N., 217° E.) are oriented 55° east of the north northeast to south-southwest trending 

ridges. Farther north, where Iris Dorsa merges with Akuanda Dorsa, pervasive fractures are oriented approximately 90° from the ridges. Here the fractures can be followed across the broad arches of the north-northwest to 

south-southeast trending ridges indicating that either the fractures are superposed on the preexisting ridges or 

that the fractures formed before the ridges and are undisturbed due to the extremely gentle folding process.

In addition to the grabens recognized within the linear belts, a second set of grabens is observed bounding 

the edges of Pandrosos Dorsa and extending into the regional plains materials. These grabens are as long as 600 

km and tend to take many sharp turns along their lengths. Their presence in the regional plains material indicates 

that they are younger than grabens restricted to the deformation belt materials.

Also of significance are relations between the short-wavelength ridges and the tessera material. As discussed in the previous section, the unnamed dorsa in the northeast corner of the quadrangle appears to deform 

large blocks of tessera material that now occur as elongate strips with orientations parallel to that of the belt. In 

other places, tessera material contains short-wavelength ridges that parallel those of a nearby linear belt. An 

example of this relation occurs near 56° N., 198° E., where a block of tessera material contains broad north-south 

trending ridges spaced 30 to 40 km apart.

WRINKLE RIDGES

Wrinkle ridges are sinuous, radar-bright linear features that range in length from the lower limit of resolution (about 1 km) to hundreds of kilometers, and generally are less than 1 km wide. They are abundant on most 

of the plains of Venus (Solomon and others, 1992; Squyres and others, 1992; Bilotti and others, 1993) and are 

widespread, visible, and bright in the neighboring quadrangles within the same latitude range (Atalanta Planitia 

to the west and Metis Regio to the east). But wrinkle ridges are rare within the Pandrosos Dorsa quadrangle and 

only locally approach the density and brightness of wrinkle ridges on regional plains elsewhere. Where wrinkle 

ridges are present, they appear faint on the SAR images and follow only a single orientation within a defined 

region. However, on a quadrangle scale there is no consistent orientation of wrinkle ridges.

Some of the wrinkle ridges are adjacent to or included within linear belts. For example, wrinkle ridges west 

of Ahsonnutli Dorsa (73.0° N., 201.5° E.) parallel short-wavelength ridges within the belt. Along Lauma Dorsa, 

wrinkle ridges appear to mimic the trends of the radar bright linear features and ridges that comprise the belt. 

The similarity in spacing and orientation of wrinkle ridges and short-wavelength structures within the deformation belts implies similar stress orientations and suggests the possibility that linear-belt ridges could have 

evolved from wrinkle ridges.

In other parts of the quadrangle, however, wrinkle ridges and short-wavelength structures are not parallel. 

For example, in the region near 50°–53° N., 195°–205° E., wrinkle ridges within regional plains materials intersect ridges within the nearby linear belts at an angle of approximately 30°. The absence of cross-cutting relations 

between the wrinkle ridges and the ridges in this region prohibits determination of a sequence of events.

RADAR-BRIGHT LINEAR FEATURES

Radar-bright linear features too narrow (<1 km wide) to be resolved clearly by Magellan radar are abundant 

in the Pandrosos Dorsa quadrangle, as they are elsewhere on Venus. These narrow features occur within linear 

belts, across sections of regional plains, and surrounding coronae and coronalike features. Locally, small sections 

of the longer radar bright linear features are wide enough to be recognized as grabens, thus implying that the linear features are extensional along their entire lengths and that parallel members of the same set of linear features 

also are extensional. Where geometric evidence for extension is not visible, however, some of these radar-bright 

linear features may be very narrow ridges.

The radar-bright linear features within the quadrangle range in length from tens to hundreds of kilometers 

and define various patterns. Between the linear belts, sets of long, parallel, widely spaced linear features cut 

across large sections of the regional plains. Examples can be seen as east-west-trending features near 61.0° N., 

217.0° E. and 57.5° N., 226.0° E. Other linear features occur as radial and concentric sets around coronae and 

coronalike features. Aspasia Corona provides a good example of a concentric set, whereas Cassatt Corona provides a good example of both radial and concentric sets. A radiating lineament system centered at 63.7° N., 

195.0° E. is interpreted as the surface manifestation of shallow, laterally emplaced dikes that radiate from a 

magma reservoir beneath the central circular fracture pattern (Grosfils and Head, 1994). The curved lineaments 

radiating from the central structure extend to distances of 450 km from the central shallow depression. The radiating lineaments cross-cut surrounding regional plains and the large block of tessera to its east, implying the lineaments post-date these material units. With respect to the structures of Lauma Dorsa, however, the radiating 

lineaments are both throughgoing and deflected, implying that deformation of Lauma Dorsa started before formation of the lineament system and continued after its cessation (Grosfils and Head, 1994).

CORONAE AND CORONALIKE FEATURES

The nine coronae within the quadrangle exhibit various topographic and structural features that meet the 

corona classification of Stofan and others (1992). Those referred to in this section as "coronalike" features also 

meet the characteristic qualifications of coronae (Stofan and others, 1992; Squyres and others, 1992) but are 

presently uncataloged. The interiors of those coronae that are fully present within the quadrangle boundaries are 

relatively simple compared to larger coronae on the planet's surface. There is no spatial pattern identified in the 

distribution of the coronae within the Pandrosos Dorsa quadrangle as is apparently displayed along Parga 

Chasma in the southern hemisphere (Chapman, 1999) and elsewhere on Venus (Stofan and others, 1992; Squyres 

and others, 1993).

Corona diameters range from about 45 km to more than 200 km. Five coronae (Cassatt, Muzamuza, Ninkarraka, Nyterkob, and Cerridwen) fall into the concentric class of Stofan and others (1992), two (Nzingha and 

Aspasia) are asymmetric coronae, and Shumann-Heink is a domical corona. Four of the coronae occur as linked 

pairs: Cassatt and Muzamuza in the north-central part of the quadrangle, Cerridwen and Nyterkob straddling the 

south boundary of the quadrangle. Aspasia is astride the southern part of Lauma Dorsa, and the Cassatt/Muzamuza pair occurs at the north end of Anpao Dorsa. The other coronae do not appear to be spatially related to 

major structural features.

VOLCANIC FEATURES

Present within the Pandrosos Dorsa quadrangle are three distinct classes of volcanic features: flow materials, small shields, and lava channels. The flows are present as digitate, lobate, lineated, and slumped materials, 

and are described in detail in the stratigraphy section. The small shield volcanoes occur in three styles in the Pandrosos Dorsa quadrangle: as coalesced shields forming mottled plains (for example, unit pm), as clusters of 

shields forming volcanic shield fields, and as isolated shields. The following is a description of the variety of 

small shields and the single lava channel present in the quadrangle.

The first style of shield volcanoes, mottled plains, is considered a stratigraphic material unit because the 

flows emanating from the individual shields are extensive enough to completely cover the region in which they 

are situated. The description of this style is therefore found in the local plains materials section of the stratigraphy section.

The second style, volcanic shield fields, differs from the regions of mottled plains in that the flows of shield 

fields do not have contrasting radar coefficients and are not voluminous enough to merge with one another. 

Shield fields are defined by distinct clusters of volcanic domes and shields with diameters of 5-10 km. Some of 

these have central pits. In the Pandrosos Dorsa quadrangle, many of the shield fields are within linear belts, an 

occurrence that has prompted arguments for an extensional origin for these belts (Sukhanov and Pronin, 1989). 

Because the flows from shield fields do not completely cover the region in which they are situated, they are not 

considered a stratigraphic material unit and are delineated on the map by a stipple pattern.

The third style of volcanic shields includes a number of small, individual shield volcanoes not readily associated with other shield volcanoes or regions of intense deformation. These small shields appear to have low 

relief and some surrounding flank materials, but they are too small to be resolved on topographic maps and thus 

to distinguish them as volcanic edifices (Aubele, 1993).

The isolated shields are readily visible on the SAR images, because the radar backscatter properties of their 

flows contrast with those of the surrounding plains materials and because of the contrast in radar brightness of 

their west and east flanks. Genetically, they are not necessarily different from those small shield volcanoes that 

cluster to form mottled plains.

Part of a lava channel is present in the southwest corner of the quadrangle (50°–52° N., 180°–183° E.). This 

channel, the longest present on the surface of Venus, is nearly 6,800 km long and has a nearly constant width of 1 

to 3 km (Baker and others, 1992). It is classified as a canali-type simple channel and is inferred to have formed 

from lava that originated at a volcanic construct at 44.5° N., 185° E. (Baker and others, 1992). Owing to its enormous length, this lava channel provides relative age relations for many material units through which it passes. 

The section of the channel present in the study area incises radar-dark regional plains but is filled by radar-bright 

regional plains. This relation indicates that the channel formed while regional plains were being emplaced; after 

emplacement of radar-dark regional plains but before emplacement of radar-bright regional plains.

DISCUSSION

Puzzling issues related to the deformation within the Pandrosos Dorsa quadrangle include the origin of the 

linear belts, especially the dual-wavelength spacing, and the origin of the subparallel ridges and linear features 

within the belts. Geologic and structural mapping of the Pandrosos Dorsa quadrangle has revealed evidence that 

both supports and conflicts with some of the previously suggested origins for the latter. An important conclusion 

of this study is that tectonism is progressive on Venus, at least within Vinmara Planitia.

The characteristic dual wavelength spacing between linear belts and between individual ridges within these 

belts (referred to in this study as "long-" and "short-wavelength" deformation) implies buckling of flexurally 

strong upper crust and upper mantle separated by a zone of weakness in the lower crust (Banerdt and Golombek, 

1988; Zuber, 1987; Zuber and Parmentier, 1990). These models imply belt-normal contraction, and most models 

for the origin of the long-wavelength linear belts on Venus involve belt-normal lithospheric shortening and crustal thickening (Basilevsky and others, 1986; Zuber, 1987; Frank and Head, 1990; Phillips and others, 1991). 

 One of the more apparent and perplexing problems with the generally accepted contractional origin of the 

linear belts is the presence in most of the Vinmara Planitia belts of subparallel structures that are interpreted to 

result from both extension and contraction. Previous studies inferred that the "ridge" and "fracture" belts of Lavinia Planitia resulted from belt-normal shortening and crustal thickening based on the large-scale geometric similarities of the two types of linear belts and the observation that some belts transform from ridge to fracture 

morphology along their lengths (Squyres and others, 1992), albeit with approximately orthogonal orientations. 

The linear belts of Vinmara Planitia differ from those of Lavinia in that they are hybrid belts that contain interspersed structures interpreted to result from both extensional and contractional strains. The two types of structures are commonly oriented subparallel to each other and to the long axes of the belts themselves. The complex 

history of southern Pandrosos Dorsa, discussed in the following paragraphs, provides important data on the 

sequence of deformational events and thus provides a useful constraint on genetic models for linear belts.

 Pandrosos Dorsa contains four stratigraphic units; linear belt materials a and b, tessera material, and 

smooth plains material (fig. 5). Linear belt materials a and b are characterized by north-south-trending ridges 

having similar morphologies and similar spacing of 5–7 km, thus suggesting that ridge formation in both materials was the result of a single deformational event (fig. 6). However, only belt material b contains pervasive fractures, which are oriented from north-northeast-south-southwest to north-south, generally parallel to the ridges 

(fig. 6). Where radar-bright linear features are not exactly parallel to the ridges in belt material b, two crosscutting relations exist. In some localities the radar-bright linear features cut across the smaller ridges. This relation 

can form by preexisting fractures being folded during later contraction or from the fractures occurring after the 

folds. The resolution of the images is not sufficient to distinguish these two cases, and thus a sequence of events 

cannot be determined. Elsewhere, the radar-bright linear features appear to abut the flanks of the ridges. Whereas 

this relation may initially suggest that the folds formed prior to the fractures, it can be explained by younger 

flooding of only some of the topographic lows between ridges. This selective flooding will result in fractures 

apparently terminating against ridges because the fractures are present on unflooded sides of ridges but buried on 

flooded sides. The fractures and ridges were originally mutually intersecting and thus of ambiguous relative age. 

Consequently, crosscutting relations and truncation of structures are not useful tools for determining the 

sequence of deformational events in Pandrosos Dorsa.

The stratigraphic relations among the geologic units of Pandrosos Dorsa imply that the pervasive fractures 

formed before formation of the north-south ridges. The older belt material b was deposited and fractured before 

the deposition of belt material a. Following fracture formation but before ridge formation, the western half of the 

belt was flooded by belt material a, burying both material b and the fractures. Following deposition of belt material a, contractional ridges were formed in both units within the belt. This sequence is supported by topographic 

data (figs. 3, 4E) which show that exposed belt material b is elevated approximately 0.8 km above belt material 

a, a morphology that allows the pervasive fractures to remain present within high-standing belt material b while 

being obliterated by the deposition of the material forming belt material a within the lower part of the belt.

Most models for the origin of linear belts assume belt-normal contraction accompanied by crustal thickening, consistent with the dual wavelength spacing of structures. The stress responsible has been explained as due 

to convective downwelling centered beneath low-lying plains regions (Zuber, 1990; Bindschadler and others, 

1990; Phillips and others, 1991). A contractional origin was initially supported by the apparent morphological 

similarity of linear belts to lunar, mercurian, and martian wrinkle ridges (Barsukov and others, 1986; Frank and 

Head, 1990), which commonly consist of broad arches with a superposed single, narrow sinuous ridge. However, 

Magellan images indicate the ridges of linear belts are morphologically dissimilar to wrinkle ridges (Banerdt and 

others, 1997). Support for crustal thickening is found in the tendency for the belts to be elevated with respect to 

the surrounding plains materials (Solomon and others, 1992; Squyres and others, 1992), although not all belts or 

all parts of belts are elevated in the Pandrosos Dorsa quadrangle. An alternative extensional origin has been proposed by Sukhanov and Pronin (1989), who noted that some belts contain volcanic centers that are difficult to 

justify in a contractional environment. Sukhanov and Pronin (1989) interpret Lukelong Dorsa as a linear diapir 

and Lauma Dorsa as a region where structures are streamlined around preexisting domes. Supporting evidence, 

such as volcanic flooding in troughs of Lukelong Dorsa and volcanic domes along the entire length of Lauma 

Dorsa, is lacking. In addition, their model does not account for the broad archlike ridges that are characteristic of 

most belts and suggestive of contractional strains.

Three models have been suggested for the presence of extensional fractures within linear belts: extension 

due to local igneous intrusion and lithospheric heating (Solomon and others, 1992); stretching of a brittle surface 

layer over a contractional structure (Squyres and others, 1992); and gravitational collapse of the upper crust 

(Phillips and Hansen, 1994). Of these models, the first two were suggested for the origin of "fracture belts" of 

Lavinia Planitia in which only fractures are present within the belts. These models are not discussed further 

because they do not incorporate or explain the presence of ridges and thus do not explain the apparent shortening 

in the belts of Vinmara Planitia. Only the Phillips and Hansen model is applicable to the coexistence of subparallel ridges and fractures. In this model, the crust collapses when relief due to crustal thickening produces gravitational stresses sufficient for failure.

The geologic relations within the Pandrosos Dorsa quadrangle reveal some shortcomings of the gravitational relaxation model. The model predicts that the contractional ridges should predate the extensional linear 

features, whereas stratigraphic evidence, especially within Pandrosos Dorsa, indicates that the opposite is true. 

The extensional structures of Pandrosos Dorsa may be relics of a regionwide event predating the formation of the 

linear belt, but no evidence exists for extensive early fracturing of comparable relative age external to the belt. 

Lauma Dorsa also presents a problem for the gravitational collapse model because most of it is lower than surrounding plains, not higher. Therefore, if gravitational relaxation is the true mechanism of formation for Lauma 

Dorsa, a formerly elevated belt dominated by ridges must have undergone collapse that brought its elevation 

lower than the horizontal level at which the materials originated, a result more consistent with extension and 

crustal thinning than with the contraction and crustal thickening required to account for the ridges.

The geology of the linear belts indicates that Vinmara Planitia underwent extensive and prolonged tectonism. Embayment relations between most of the materials within the linear belts and the surrounding regional 

plains materials suggest that most belts began forming before emplacement of regional plains materials. However, Lauma Dorsa deforms the relatively young radar-bright regional plains material and therefore formed during or after emplacement of this plains material. Lancaster and others (1995) interpreted the radar-bright plains 

material near Lauma Dorsa as a post-regional plains sheet flow emanating from Lauma Dorsa and some nearby 

east-west-trending fissures. This alternate interpretation does not change the inference that linear belt deformation within the Pandrosos Dorsa quadrangle has been progressive, because both interpretations imply deformation within Lauma Dorsa after emplacement of most or all regional plains materials.

Additional evidence in support of prolonged long-wavelength deformation is the stratigraphic and morphologic relation between radar-bright and radar-dark regional plains materials. In the plains region between Pandrosos Dorsa and Bathkol Tessera the younger radar-bright regional plains material occurs in the longitudinal center 

of radar-dark regional plains material (approximately centered at 204° E.), implying that a linear depression 

formed after the dark plains material was deposited and before the bright regional plains material was emplaced 

(fig. 7). Thus, the stresses forming the long-wavelength deformation also were progressive with respect to 

emplacement of regional plains materials.

GEOLOGIC HISTORY

The geologic history of the Pandrosos Dorsa quadrangle begins with the formation and deformation of tessera material. Embayment and truncation relations indicate that tessera material is the oldest material within the 

quadrangle. However, tessera materials may not be the oldest materials planet-wide, and all tessera patches and 

blocks need not be stratigraphically correlated. The characteristics of tessera material before it was deformed are 

unknown.

Densely lineated material, present as radar-bright inliers, also is interpreted as relatively old. In places, 

densely lineated material occurs at the edges of the tessera blocks, embaying the tessera material and truncating 

structures within the tessera. The common azimuth of fractures in most patches of densely lineated material suggests regional continuity of deformation. These fractures are truncated by and thus older than surrounding 

regional plains materials. It is not possible to infer the elapsed time between the emplacement and deformation 

of tessera material and the emplacement of densely lineated material, or between either of these units and the 

emplacement of surrounding regional plains materials.

Most of the linear belts contain materials that are older than the surrounding regional plains materials, as 

indicated by the superposition of plains materials on belt materials and on the ridges and linear features deforming the belt materials. The origin of belt materials is inferred to result from an earlier episode of plains emplacement, although no direct evidence exists for correlative materials beneath presently exposed regional plains 

materials. Ridges within one linear belt, Lauma Dorsa, also deform the youngest regional plains material.

The emplacement of regional plains was approximately coeval with the deformational event that formed the 

linear belts. This scenario may imply rapid emplacement of regional plains or prolonged deformation, or both, 

but owing to the extensiveness of the regional plains materials beyond the boundaries of the study area and to 

indications of continued large-scale deformation after regional plains deposition (Lauma Dorsa), prolonged 

deformation is preferred. Radar-bright regional plains materials locally fill grabens and part of the lava channel 

cutting radar-dark regional plains, relations that suggest at least two episodes of widespread plains emplacement. 

Although the sparsity of pervasive wrinkle ridges on the regional plains tends to distinguish these plains from 

most regional plains elsewhere on Venus, these plains are otherwise similar to and areally continuous with 

regional plains in adjacent quadrangles. Thus they most likely formed during the near-global resurfacing event of 

approximately 200–600 million years ago (Strom and others, 1994). Formation of coronae and coronalike features was either coeval with or immediately followed regional plains emplacement, or both. 

Although geophysical models (Banerdt and Golombek, 1988; Zuber, 1987; Zuber and Parmentier, 1990) 

imply a similar age, the temporal relation between the two wavelengths of deformation characteristic of the linear belts cannot be resolved geologically. Resolution of this ambiguity will be important for understanding the 

causes of the structures confined within the belts.

Deformation of a lesser intensity continued after formation of the regional plains, further indicating progressive deformation in the region. The absence of radar-bright linear features or folds within digitate flows, lobate 

flows, and smooth local plains material indicates that these materials were emplaced after formation of the structures affecting regional plains. Although each of these flows is younger than the surrounding materials, they are 

not necessarily coeval; multiple episodes of volcanism are more likely. The embayment relation between Turgmam Fluctus and Lonsdale crater indicates that this digitate flow is older than the formation of Lonsdale. However, the stratigraphic relation from this single occurrence cannot be applied to the other digitate flows and 

craters in the quadrangle because they are not in direct contact with one another.

Impact of all craters and their associated flows followed regional plains emplacement. Flows from Dolores 

crater were diverted by and are thus older than nearby wrinkle ridges; the ages of other craters relative to these 

structures are not known.

The geologic history of the Pandrosos Dorsa quadrangle records progressive deformation, as indicated by 

the temporal interleaving of stratigraphic units and structures. The deformation of both the long- and short-wavelength scales of the linear belts appears to be prolonged, deforming materials before, during, and after emplacement of regional plains.

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Batya

Hikuleo

Fluctus

Corona

Neyterkob

Corona

Dolores

Ahava

Izumi

Patera

Hallgerda

Mons

Bathkol 

Bathkol 

Yetta

Ezraela

Evangeline

Zoya

Barsova

Orlova

Lonsdale

Elizabeth

Kelila

Pandrosos 

 Dorsa 

Pandrosos

Dorsa

Iris

Dorsa

Marake

Colles

Turgmam Fluctus

Tikoiwuti

Dorsa

Virilis

Tesserae

VINMARA

PLANITIA

ATALANTA

PLANITI A

Cerridwen

Akuanda Dorsa

Ninkarraka

Cassatt Corona

Corona Muzamuza

Corona

Lauma  ̄

Dorsa

Lukelong

Dorsa

Molpe

Colles

Nzingha

Corona

Ahsonnutli

Dorsa

Surupa

Dorsa

Schumann-Heink

Corona

A

i d

a - We d o

D o rs a

Okipeta

Dorsa

Tes sera

Tessera

Aspasia

Corona

Semele

Tholi

Nanen

Corona

Iris Dorsa

Anpao Dorsa