2010

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of the Artemis Chasma Quadrangle (V–48), Venus

By Roger A. Bannister and Vicki L. Hansen

Pamphlet to accompany

Scientific Investigations Map 3099

75°

50°

25°

75°

25°

–25°

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50°

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0° 30° 60° 90° 120° 150° 180°

–25°

–50° –50°

–75° –75°

V–1

V–23 V–24 V–22

V–34

V–62

V–35

V–21

V–20

V–9

V–8

V–10

V–33 V–36

V–12

V–45 V–48

V–11

V–46 V–47

V–57

V–3

V–2

V–13

V–44 V–49

V–25

V–32 V–37

V–4

V–56 V–58

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Contents

The Magellan Mission ..................................................................................................................................1

Magellan Radar Data ..........................................................................................................................1

Introduction ....................................................................................................................................................2

Venus .............................................................................................................................................................2

Terminology: Artemis Chasma, Artemis Corona, and Artemis ....................................................2

Data and Methods ........................................................................................................................................3

Radar Interpretation ............................................................................................................................3

Mapping Methodology ........................................................................................................................4

Background ....................................................................................................................................................4

Previous Work ......................................................................................................................................4

Geologic Mapping ......................................................................................................................4

Gravity Analysis ...........................................................................................................................5

Modeling .......................................................................................................................................5

Geology of Artemis Chasma Quadrangle ..................................................................................................5

Geologic Relations ...............................................................................................................................5

Primary and Secondary Structures ..................................................................................................5

Impact Features ...................................................................................................................................7

Exterior Region .....................................................................................................................................7

Artemis Chasma ...................................................................................................................................8

Interior Region ......................................................................................................................................9

Pentetrative Tectonic Fabric ............................................................................................................10

Temporal Relations and Geologic History ...............................................................................................12

Hypothesis Evaluation ................................................................................................................................14

Subduction Hypothesis .....................................................................................................................14

Metamorphic Core Complex Hypothesis .......................................................................................15

Bolide Impact Hypothesis ................................................................................................................15

Plume Hypothesis ..............................................................................................................................16

A Hybrid Hypothesis ..........................................................................................................................17

References Cited..........................................................................................................................................18

Figures

Figure 3 .........................................................................................................................................................23

Figure 4 .........................................................................................................................................................24

Figure 5 .........................................................................................................................................................25

Figure 6 .........................................................................................................................................................26

Tables

Table 1. Crater data for the Artemis Chasma quadrangle (V–48), Venus .........................................27

Table 2. Penetrative fabric wavelenths measured across Artemis quadrangle .............................28

1

The Magellan Mission

The Magellan spacecraft orbited Venus from August 10, 

1990, until it plunged into the Venusian atmosphere on October 

12, 1994. Magellan Mission objectives included (1) improving 

the knowledge of the geological processes, surface properties, 

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

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 data sets: (1) synthetic aperture radar 

(SAR) images of the surface, (2) passive microwave thermal 

emission observations, and (3) 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 accomplished in 

mission cycles 1, 2, and 3 from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with 

radar resolution on the order of 120 m. The SAR observations 

were projected to a 75-m nominal horizontal resolution, and 

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 between about 20° and 45°.

High-resolution Doppler tracking of the spacecraft took 

place from September 1992 through October 1994 (mission 

cycles 4, 5, 6). Approximately 950 orbits of high-resolution 

gravity observations were obtained between September 1992 

and May 1993 while Magellan was in an elliptical orbit with a 

periapsis near 175 km and an apoapsis near 8,000 km. 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 (1) the morphology of the surface at a broad range of scales and (2) the intrinsic 

reflectivity, or dielectric constant, of the material. Topography at 

scales of several meters and larger can produce quasi-specular 

echoes, and the strength of the return is 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 thought to 

be present. This 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. 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 topographic data produced by this technique 

have horizontal footprint sizes of about 10 km near periapsis 

and a vertical resolution on the order of 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.

Introduction

Artemis, named for the Greek goddess of the hunt, represents an approximately 2,600 km diameter circular feature on 

Venus, and it may represent the largest circular structure in our 

solar system. Artemis, which lies between the rugged highlands 

of Aphrodite Terra to the north and relatively smooth lowlands 

to the south, includes an interior topographic high surrounded 

by the 2,100-km-diameter, 25- to 200-km-wide, 1- to 2-kmdeep circular trough, called Artemis Chasma, and an outer 

rise that grades outward into the surrounding lowland (fig. 1, 

on map sheet). Although several other chasmata exist in the 

area and globally, other chasmata have generally linear trends 

that lack the distinctive circular pattern of Artemis Chasma. 

The enigmatic nature of Artemis has perplexed researchers 

since Artemis Chasma was first identified in Pioneer Venus 

data. Although Venus’ surface abounds with circular to quasicircular features at a variety of scales, including from smallest 

to largest diameter features: small shield edifices (>1 km), large 

volcanic edifices (100–1,000 km), impact craters (1–270 km), 

coronae (60–1,010 km), volcanic rises and crustal plateaus 

(~1,500–2,500 km), Artemis defies classification into any of 

these groups. Artemis dwarfs Venus’ largest impact crater, 

Mead (~280 km diameter); Artemis also lacks the basin topography, multiple ring structures, and central peak expected for 

large impact basins. Topographically, Artemis resembles some 

Venusian coronae; however Artemis is an order of magnitude 

larger than the average corona (200 km) and about twice the 

size of Heng-O Corona (which is 1,010 km in diameter), the 

largest of Venusian coronae. In map view Artemis’ size and 

shape resemble volcanic rises and crustal plateaus; however, 

both of these classes of features differ topographically from 

Artemis. Volcanic rises and crustal plateaus form broad domical regions, and steep-sided regions with flat tops, respectively; 

furthermore, neither rises nor plateaus include circular troughs. 

So although it seems clear what Artemis is not, there is little 
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consensus about what Artemis is, much less how Artemis 

formed.

Debate during the past decade has resulted in the proposal 

of at least four hypotheses for Artemis’ formation. The first 

(herein referred to as H1) is that Artemis Chasma represents a 

zone of northwest-directed convergence and subduction (McKenzie and others, 1992; Brown and Grimm, 1995; Schubert 

and Sandwell, 1995; Brown and Grimm, 1996). The second 

hypothesis (herein referred to as H2) is that Artemis consists of 

a composite structure with a part of its interior region marking 

the exposure of deformed ductile deep-crustal rocks analogous 

to a terrestrial metamorphic core complex (Spencer, 2001). 

The third (herein referred to as H3) is that Artemis reflects the 

surface expression of an ancient (>3.5 Ga) huge bolide impact 

event on cold strong lithosphere (Hamilton, 2005). The fourth 

hypothesis (herein referred to as H4) is that Artemis marks the 

surface expression of a deep mantle plume (Griffiths and Campbell, 1991; Smrekar and Stofan, 1997; Hansen, 2002; Ivanov 

and Head, 2003). Each of these hypotheses holds different 

implications for Venus geodynamics and evolution processes, 

and for terrestrial planet processes in general. Viability of H1 

would provide support that terrestrial-like plate-tectonic processes once occurred on Earth’s sister planet. The feasibility of 

H2 would require high values of crustal extension and therefore 

imply that significant horizontal displacements occurred on 

Venus—displacement that may or may not be related to terrestrial-like plate-tectonic processes. The possibility of H3 would 

suggest that Venus’ surface is extremely old, and that Venus has 

experienced very little dynamic activity for the last 3.5 billion 

years or more; this would further imply that Venus is essentially 

tectonically dead, and has been for most of its history. This view 

contrasts strongly with studies that highlight a rich history of 

Venus including activity at least as young as 750 million years 

ago, and quite likely up to the present (for example, contributions in Bouger and others, 1997). If H4 has credibility, then 

Artemis could provide clues to cooling mechanisms of Earth’s 

sister planet. Each of these hypotheses might be tested through 

geologic mapping aimed at unraveling the geologic history of 

Artemis and its surroundings. Artemis lies almost completely 

within the bounds of the Artemis Chasma quadrangle, V–48, 

which encompasses more than seven million square kilometers 

of Venus’ southern hemisphere, extending from 25° to 50° S. 

and 120° to 150° E. Thus, construction of a 1:5 million-scale 

geologic map of Artemis Chasma quadrangle allows for unraveling the geologic history of Artemis to test various hypotheses 

of Artemis’ evolution.

As a result of geological mapping, we propose a hybrid 

hypothesis for Artemis’ formation. The hybrid hypothesis, 

which represents a modification to the plume hypothesis, 

involves the formation of an interior spreading center and newly 

formed crust, similar to that formed at a terrestrial divergent 

planet-boundary, as a result of a mantle plume; shortening 

of this crust within the chasma, which moves outward with 

continued formation of interior crust, might represent a failed 

Venusian attempt at subduction; the surface crust did not recycle 

to the mantle. Interior tectonomagmatic centers form broadly 

synchronously with interior crust formation. It is unclear when 

Artemis formed within Venus’ evolution, although we postulate 

that it formed on thin lithosphere. Through this hybrid hypothesis we postulate that Artemis may record Venus’ failed attempt 

at terrestrial plate-tectonic-like processes.

Venus

Venus, commonly referred to as Earth’s sister planet, 

nearly equals the Earth in size, bulk density, and distance from 

the Sun; these similarities suggest that the bulk composition and 

heat budget of Venus should be broadly similar to the Earth (see 

Grimm and Hess, 1997). Despite these similarities, Earth and 

Venus evolved quite differently. Conditions at the Venusian surface are inhospitable to humans and life as we know it—with a 

temperature of approximately 760 K (~480° C), a caustic, dominantly CO2 atmosphere at a pressure of approximately 92 bars, 

and only trace amounts of water. The Earth’s crust differentiated 

into granitic continental crust and basaltic oceanic crust, which 

is reflected in the bimodal distribution of the hypsometric curve 

for the Earth. Limited geochemical evidence from a series of 

Soviet Venera lander missions indicates that the Venusian surface likely resembles basalt (Barsukov and others, 1982; Surkov 

and others, 1986). The hypsometric curve for Venus displays a 

unimodal distribution implying that Venus’ crust is not differentiated in the same manner as the Earth (Rosenblatt and others, 

1994). Terrestrial plate-tectonic processes—manifested in long 

curvilinear features such as orogenic belts, subduction zones, 

oceanic spreading centers, and transform fault zones—provide a mechanism for interior heat to escape to the surface on 

Earth. Venus lacks such globally pervasive curvilinear features; 

instead, circular features dominate its surface, and their presence suggests that Venus loses its heat in a different manner (for 

example, Solomon and others, 1991; Solomon, 1993; Phillips 

and Hansen, 1994, 1998; Nimmo and McKenzie, 1998).

Terminology: Artemis Chasma, Artemis Corona, 

and Artemis 

Artemis Chasma was first identified in low-resolution radar 

data from the NASA Pioneer Venus Orbiter mission. Stofan and 

others (1992) categorized Artemis Chasma, the interior region, 

and outer high as a corona after Magellan SAR data became 

available; however, due to considerations of size and possible 

different mechanism of formation, Artemis is no longer considered a corona (Stofan and others, 2001). Regardless, Artemis 

is referred to as “Artemis Corona” in some literature. The term 

corona (from the Latin word meaning “crown,”) originated as a 

descriptive term for any quasi-circular structure defined primarily by an annulus of concentric fractures and/or ridges. The 

term gained genetic connotations as the body of corona research 

grew. Most researchers favor diapiric mechanisms for the formation of coronae (Squyres and others, 1992; Stofan and others, 

1992; Koch and Manga, 1996; Smrekar and Stofan, 1997; 

Stofan and others, 1997; Hansen, 2003). However, the diversity of morphologies and range of sizes indicate that coronae 

may form by non-diapiric processes as well, including caldera 

collapse (Squyres and others, 1992; Roberts and Head, 1993; 
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DeLaughter and Jurdy, 1999) and/or bolide impact (for example, Barsukov and others, 1984, 1986; Basilevsky and others, 

1987; Campbell and Burns, 1979; Grieve and Head, 1981; 

Head and Solomon, 1981; Masursky and others 1980; Schaber 

and Boyce, 1977; Nikolayeva and others, 1986; Greeley, 1987; 

Nikolayeva , 1993; Schultz, 1993; Hamilton, 2005; McDaniel, 2005; McDaniel and Hansen, 2005; Vita-Finzi and others, 

2005). Although Stofan and others (1992; 1997) suggested 

that the wide range in corona morphology represents different stages of corona development, coronae display no obvious 

age progression where they occur in chains or clusters (see 

Hamilton and Stofan, 1996; Stofan and others, 1997; Hansen 

and DeShon, 2002). The vast size differences between Artemis 

and mean corona diameter (200 km), and between Artemis and 

Heng-O, raise concern over classifying Artemis as a corona. In 

this contribution we use the term ‘Artemis’ to refer to the entire 

large circular geomorphic feature centered at 33° S., 133° E., 

including Artemis Chasma, the raised interior region, and the 

adjacent exterior region (fig. 1, on map sheet), following terminology of Hansen (2002). This broad definition of Artemis helps 

to distance discussion from possible genetic connotations and/or 

assumptions associated with the term corona.

Data and Methods

The NASA Magellan mission (1991 – 1994) produced an 

amazing correlated digital geophysical data set for Venus with 

nearly global (approximately 98 percent) coverage (Saunders 

and others, 1992; Ford and others, 1993). The Magellan radar 

sensor acquired data in three modes: synthetic aperture radar 

(SAR), radiometer, and altimeter mode. The system used a 

3.7-m diameter parabolic high-gain antenna (HGA) fixed 25° 

off nadir perpendicular to the trajectory of the spacecraft in 

SAR and radiometer modes. The SAR operated with a 12.6-

cm wavelength at 2.385 GHz (S-band) with horizontal parallel 

transmit/receive polarization (HH) in order to penetrate the 

thick, CO2-dominated cloud cover. SAR images were produced 

in three cycles with varying look geometries, and covering 

approximately 98 percent of the planet surface. Owing to the 

elliptical orbit, the SAR incidence angle was varied with latitude to provide an optimum signal-to-noise ratio. In the Artemis 

region incidence angles range from approximately 38–23°, 

approximately 25°, and approximately 20–14° from north to 

south for cycles 1, 2, and 3, respectively. SAR resolution is 

approximately 100 m/pixel. The altimeter mode used a smaller 

altimeter horn antenna (ALTA) fixed in the nadir direction. The 

size of the altimeter footprint varies with spacecraft altitude and 

therefore varies with latitude. In the Artemis region footprints 

measure 8–11 km in along-track dimension and 19–24 km in 

cross track dimension (Ford and others, 1993). Combining the 

surface-to-spacecraft distance with the known position of the 

spacecraft relative to the planetary center produces a global 

topographic data record (GTDR) with horizontal resolution 

approximately 10 km and vertical resolution approximately 80 

m. Reflectivity and RMS-slope data sets were also derived from 

the altimetry data. Emissivity data were derived from radiometry measurements interleaved with the SAR observations. 

Gravity data were collected during cycles 4 and 5 before the 

spacecraft was intentionally crashed into the surface as a result 

of an effort to circularize the polar orbit to increase gravity data 

resolution in the polar regions of Venus.

Data for this study were provided by the USGS Astrogeology Team in the projection parameters for the Artemis Chasma 

(V–48) quadrangle (Lambert conformal conic, standard parallels at 34° S. and 73° S., central meridian 135° E., latitude of 

origin 90° S.). Left-look (cycle 1) F-MIDR (approximately 75 

m/pixel), left-, right-, and stereo-look (cycles 1–3) C1-MIDR 

(approximately 225 m/pixel) SAR imagery were used in TIFF 

format, as were elevation, emissivity, RMS-slope and reflectivity data. The data used in this study are available online from 

the USGS Map-a-planet website (http://pdsmaps.wr.usgs.

gov/) and the USGS Planetary GIS Web Server (PIGWAD, 

http://webgis.wr.usgs.gov/) in a variety of formats, although 

not necessarily in the projection parameters used herein. This 

study primarily uses SAR image mosaics from cycles 1, 2, and 

3, including both left-looking and right-looking images. Cycle 

1 left-looking SAR and cycle 2 right-looking SAR coverage 

of Artemis is nearly complete, whereas cycle 3 stereo data is 

extremely patchy. Compressed once (C1-MIDR) and compressed twice (C2-MIDR) SAR images are used for regional 

analysis, whereas full resolution images (F-MIDR) are used for 

detailed analysis of areas of interest (resolution of SAR images 

are 225, 675, and 75 m/pixel, respectively). GTDR data is used 

for analysis of regional topography and to assist in visualization 

in three dimensions.

Stereo imagery can greatly enhance the interpretation of 

landforms by displaying data in three dimensions. This technique works because parallax differences between two images 

with different viewing geometries produce a sense of depth 

when viewing the images with a stereoscope (or with red-blue 

glasses if the images are combined to form a red-blue anaglyph). In the case of Magellan SAR data, image pairs with 

opposite look directions are difficult to visually merge because 

the illumination direction changes drastically between the two 

images; however, stereo pairs with the same look direction but 

different incidence angles (for example, cycle 1 and cycle 3 

left-look SAR images) can be combined to great effect (Plaut, 

1993). Although true stereo coverage of Artemis is poor due 

to the paucity of cycle 3 data, we used synthetic stereo images 

(Kirk and others, 1992) constructed with C1 SAR images and 

altimetry data using a custom program and macros authored by 

Duncan A. Young. Although regional topographic trends are 

easily gleaned from synthetic stereo images, this data set does 

not resolve subtle topographic features that might be resolved in 

true stereo images.

Radar Interpretation

Radar image brightness is a function of the roughness, 

topography, and electrical properties of the imaged surface. 

Surfaces that are inclined towards the incident radar and/or 

are rough at or above the scale of the radar wavelength (12.6 

cm) appear radar-bright; surfaces that are inclined away from 
4

the incident radar and/or are smooth (below the scale of the 

radar wavelength) appear radar-dark (Ford and others, 1993). 

The opposite is true for inverted SAR images. Inverted SAR 

images are useful for structural analysis because structural 

elements (typically lineaments) tend to be more visible than in 

normal SAR images (Hansen, 2000). Due to the geometry of 

radar, echoes from areas of high elevation return to the antenna 

before echoes from areas of low elevation. This causes mountain peaks to image forward of their actual position, known as 

foreshortening; the opposite occurs on the back slope, known 

as elongation. If the echo from the peak returns before the echo 

from the forward toe, then the peak will be imaged on top of 

the base, an effect known as lay-over. A surface inclined away 

from the radar look direction at an angle greater than the incidence angle will lie in radar shadow and will not be imaged. 

These effects complicate SAR image interpretation but once 

understood, SAR images can provide a tool for determining 

short-wavelength topography. For a more complete discussion 

of the interpretation of Magellan SAR data, see Ford and others 

(1993).

Mapping Methodology

Geologic mapping was conducted according to guidelines 

set forth by Wilhelms (1990) and Tanaka and others (1994) 

with caveats highlighted by Hansen (2000), Zimbelman (2001), 

and Tanaka and others (2010). Within the Artemis quadrangle 

we differentiate primary and secondary structures, structural 

domains, terrain units, material units, and radar facies. Primary 

and secondary structures are commonly defined based on morphology and regional patterns. By definition, primary structures 

formed during material unit emplacement whereas secondary 

(tectonic) structures record post-emplacement deformation. 

Structural domains comprise regions marked by closely spaced 

tectonic fabrics, and they typically show spatial correlation 

with regional morphologic features. Terrain units are defined 

texturally by penetratively developed secondary structures, and 

they indicate a shared geologic history of materials after the 

emplacement/formation of secondary structures that combined 

potentially previously unrelated material units. Terrain units 

do not imply a shared history or genetic relationship prior to 

the deformation event(s) responsible for the imparting of their 

characteristic (structural/tectonic) textures. Material units are 

defined by morphology, primary structures, and radar characteristics; material units are interpreted as representing coherent units emplaced in discrete periods of geologic time. Radar 

facies units represent surface areas that we were not comfortable uniquely interpreting as either a terrain unit or a material 

unit, that is, the character of radar facies units reflect radar properties that might not be uniquely attributed to material properties 

or tectonic properties; in fact, the radar signature could reflect a 

combination of material and tectonic characteristics at the effective resolution of the current data. The character and distribution 

of primary structures, secondary (tectonic) structures, structural 

domains, terrain units, and material units all provide clues to 

surface material properties (for example, rheology), geologic 

history, and formative processes.

Background

Artemis lies almost completely within the V–48 quadrangle (fig. 1 and map sheet). The V–48 map area is bounded 

to the north by the rugged Aphrodite Terra highland, but it is 

separated from this highland region by Quilla Chasma. To the 

northeast, the east-northeast-trending corona-chasmata chain 

of Diana-Dali feathers out just outside the V–48 quadrangle. 

A second, northeast-trending corona chain, south of the DianaDali chain, barely kisses the east margin of the map area, and it 

ends with Teteoinnan Corona in V–48. Annapurna and Colijnsplaat Coronae, also part of this chain, lie mostly within the V–49 

quadrangle to the east. To the southeast, south, and southwest 

of Artemis, Zhibek, Laimdota, and Anio Planitiae, respectively, 

mark vast expanses of lowland. East-trending Juno Chasma 

occurs to the west of Artemis and the V–48 map area. Artemis 

dominates V–48, and understanding its formation represents the 

focus of this study.

Previous Work

Geologic Mapping

At least three independent groups have published regional 

scale geologic maps covering at least part of Artemis. Brown 

and Grimm (1995), who focused mapping efforts on Artemis 

Chasma, published a simplified tectonic sketch map. They used 

Magellan data, as we do, although their visualization techniques 

differ from ours. These workers used the “magic-airbrush” technique (Kirk and others, 1992) to create shaded-relief images; this 

technique operates on the premise that the surface roughness and 

dielectric constant contributions to radar signal strength should 

be similar between left- and right-look images, whereas the slope 

effects should be quite different. By calculating a weighted difference between left- and right-look images, which effectively 

removes the roughness and dielectric constant components, only 

the slope component of image brightness remains. Although this 

method produces shaded-relief images at the resolution of the 

SAR imagery, it preserves radar foreshortening and elongation 

effects, particularly in regions of high topographic relief or steep 

slopes, such as parts of Artemis’ interior and along/within Artemis Chasma. Hansen (2002) published a reconnaissance geologic map that includes a broad look at Artemis’ interior, chasma, 

and exterior regions; this work is less detailed than the current 

mapping effort. Hansen (2002) used Magellan SAR images and 

topography data; inverted SAR images were used to highlight 

structural lineaments, and synthetic stereo images that combined SAR and topographic relations allowed three-dimensional 

visualization (Kirk and others, 1992). The geologic maps of 

Brown and Grimm (1995) and Hansen (2002) broadly agree in 

the location and characterization of structural features, although 

the interpretations of these workers follow quite different paths. 

Hansen’s (2002) map covers a slightly larger region and is more 

detailed both in structural notation and in the consideration of 

geologic units, as expected for a later work. Spencer (2001) 

produced a sketch map of the “central interior deformation 

belt,” through the examination of secondary structures within a 
5

limited region within Artemis’ interior. The sketch map shows a 

relatively low level of detail, and although it was published after 

the efforts of Brown and Grimm (1995), the sketch map is not 

presented in the context of that earlier work.

Gravity Analysis

Schubert and others (1994) analyzed Magellan gravity data 

of large coronae and numerous chasmata. A semi-circular positive gravity anomaly of 20–40 mgal lies along Artemis Chasma, 

most prominently expressed in the southeast part of the interior. 

Schubert and others (1994) calculated a best-fit apparent depth 

of compensation (ADC) of 200±12.5 km for the large gravity anomaly and about 50±12.5 km for the Artemis region in 

general, a gravity/topography ratio of 0.056±0.008 mgal/m, and 

a geoid/topography ratio of 35±5 m/km for the Artemis region. 

Gravity analyses provide some insight into subsurface structure; however, interpretations of lithosphere structure based on 

gravity are inherently non-unique and dependent on analysis 

and embedded assumptions. For instance, the gravity/topography data for Artemis is consistent with partial dynamic support, 

thick lithosphere (>100 km), or a combination thereof (Schubert 

and others, 1994; Simons and others, 1997).

Modeling

Artemis has been the focus of, or referred to within, flexural modeling, fluid-dynamics experiments, and finite-element 

modeling. Inelastic and elastic flexural modeling (Brown and 

Grimm, 1996), which hinges on the proposal that Artemis 

Chasma is analogous to terrestrial subduction zones (for example, Sandwell and Schubert, 1992; Brown and Grimm, 1995; 

Schubert and Sandwell, 1995), aims at estimating local geothermal gradient and lithospheric rheology. Brown and Grimm 

(1996) focused modeling on the southeast part of Artemis 

Chasma because that is where the chasma trends perpendicular 

to proposed northwest-directed convergence; the northeast- and 

southwest-trending parts of Artemis Chasma were not addressed 

through modeling. Fluid-dynamics laboratory experiments 

aimed at modeling the interaction of terrestrial plume-lithosphere interactions (Griffiths and Campbell, 1991), serendipitously found application to Artemis. Griffiths and Campbell 

(1991) noted that as a plume head approaches a rigid boundary 

it flattens and spreads laterally; ‘mantle’ material squeezes out 

from between the plume head and the rigid layer (lithosphere), 

and a ring-shaped instability develops to form an axisymmetric trough. Noting the similarity of the experimentally formed 

trough with Artemis Chasma, which had only recently been 

imaged by Magellan SAR data, Griffiths and Campbell (1991) 

proposed that Artemis might represent the surface expression of 

a large thermal mantle plume (the term ‘plume’ infers thermal, 

as opposed to compositional buoyancy [Griffiths, 1986a, b]). 

Finite-element modeling of the interaction of a large thermal 

mantle plume with the lithosphere, aimed at modeling Venusian 

corona topography (Smrekar and Stofan, 1997), also resulted 

in the formation of an axisymmetric trough, and it may provide 

further support for a plume origin for Artemis.

Geology of Artemis Chasma Quadrangle

Geologic Relations

V–48 includes Artemis, the termination of a northeasttrending corona chain marked by Teteoinnan Corona, the northwestern part of Zhibek Planitia, the northern extent of Laimdota 

Planitia, and a tiny part of the easternmost part of Aino Planitia 

(fig. 2 and map sheet). A northeast-trending fracture zone trends 

toward Artemis from the southwest. East-trending Juno Chasma 

lies completely to the west of V–48, outside the quadrangle. 

Lava flows sourced from Inari Corona and Quilla Chasma to the 

north spill into the northwest corner of V–48 and Artemis’ interior. Lava flows from Ceres, Bona and Miralaidji Coronae spill 

into the northeast corner of V–48. As noted, this study focuses 

on the formation of Artemis.

Artemis is composed of an interior region, Artemis 

Chasma, and an outer high. The interior of Artemis hosts four 

spatially and temporally overlapping tectonomagmatic centers 

(TMa–d). Britomartis Chasma trends northeast through the 

northernmost tectonomagmatic center (TMd). A regionally 

extensive, penetratively developed, tectonic fabric characterizes much of Artemis’ interior. This interior penetrative fabric, 

marked by tightly spaced lineaments (average wavelength 

500 m), generally trends northeast, although it locally changes 

trend near the tectonomagmatic centers. Artemis Chasma hosts 

a trough-parallel lineament fabric marked by closely spaced 

lineaments; the tectonic chasma fabric differs from the interior 

penetrative tectonic fabric in trend and spacing, being generally more widely spaced. Outside and exterior to the chasma an 

extensively developed suite of wrinkle ridges broadly parallels 

the concentric trace of the chasma. A 300-km-wide fracture 

zone trends northeast toward the chasma along the southwestern 

margin, and a chain of coronae developed to the east of Artemis, 

mostly outside the V–48 quadrangle. Locally, flows sourced 

from Artemis’ interior extend across the western part of the 

chasma, and they are locally buttressed by topographic expression of the chasma tectonic fabric along the south and east 

margins; these relations indicate that evolution of the interior 

region and the chasma temporally overlapped, at least in part. 

Temporal relations between the chasma and the fracture zone 

are unclear, although our results suggest broadly synchronous 

evolution of the chasma trough and the fracture zone.

We present geologic relations by region, starting with the 

exterior and working inward. Prior to this discussion we introduce primary and secondary structures and impact features that 

occur across and within the V–48 map area; primary structures 

and impact features are also discussed in the context of geologic 

descriptions of the individual regions: exterior, chasma, and 

interior. In our discussion of geologic history, we reverse the 

order and discuss evolution from the interior outward.

Primary and Secondary Structures

Primary and secondary structures defined within the 

Artemis quadrangle are described herein; individual map units 

and structural domains are described within the regional context 
6

in which they are found. For discussion purposes we divide 

Artemis morphologically into the chasma, exterior, and interior 

regions. Artemis Chasma, defined topographically by radial 

clock position, extends from approximately 12:00 clockwise 

to 10:30, with its deepest and steepest topographic expression 

occurring from approximately 2:00 clockwise to 7:00. Secondary structures generally parallel the concentric trend of the 

chasma, although locally structures trend normal to chasma-tangent trends. The exterior includes (a) chasma-concentric wrinkle 

ridges, (b) chasma-normal radial fractures, (c) a northeasttrending fracture zone also marked by a northeast-trending 

topographic trough that occurs to the southwest of Artemis, and 

(d) a chain of coronae developed to the east of Artemis. Within 

its interior Artemis preserves four tectonomagmatic centers; 

interior map units roughly divide into two groups. These groups 

are radar-rough tectonic terrains (units tAa and tAb) and radarsmooth volcanic flow materials (units fcAa, fcAb, fsAa, fsAb, 

fsAc, fu, and flu). A penetrative tectonic fabric developed in 

units tAa and tAb occurs across much of the interior. Radial 

and concentric structural suites and associated flows define four 

centers of tectonic and magmatic activity (TMa–d). The three 

divisions of Artemis—chasma, exterior, and interior—host 

division-specific map units, and, commonly, tectonic structural 

patterns. Only two units, unit fu (undivided flows) and unit 

fcAa (a flow unit), occur across these divisions. Unit rf (radar 

facies) occurs within the exterior and crosses into the chasma 

locally. One suite of structures, shallow troughs, extends from 

the interior across the chasma to the exterior along Artemis’ 

western margin. Other suites of primary or secondary structures 

are generally spatially limited to defining patterns within one 

of the three regions, although exterior radial fractures extend 

slightly into the outer reaches of the chasma.

By definition primary structures form during the emplacement of geologic units, and their character may assist in 

defining the spatial limits of individual material units. SAR 

images of primary structures within V–48 are shown in figure 

3 and include shields, channels, pit chains, flow fronts, shallow troughs, and crater rim crests. Shields are small (<1–15 

km in diameter) circular to quasi-circular features with dome, 

cone, shield, or flat-topped shapes that may or may not contain 

a central pit (Guest and others, 1992; Crumpler and others, 

1997; Addington, 2001), interpreted as small volcanic edifices. 

Channels are narrow (generally about 1 km) steep-sided, shallow (on the order of tens of meters), sinuous troughs morphologically similar to terrestrial fluvial channels, interpreted to 

form by channelized fluid flow (Baker and others, 1992; Komatsu and Baker, 1994; Baker and others, 1997). The nature 

of the fluid is undefined, as is the type of erosion, whether 

mechanical or thermal, cutting downward from the surface, 

or upward from depth (for example, Gregg and Greeley, 

1993; Bussey and others, 1995; Williams-Jones and others, 

1998; Jones and Pickering, 2003; Lang and Hansen, 2006). 

Pit chains are a collection of small (<1–10 km diameter) 

sharply defined circular to elliptical depressions arranged in 

a line, interpreted as the result of stoping of material marking 

the surface expression of dilatational dikes or faults (Okubo 

and Martel, 1998; Ferrill and others, 2004). Pit chains can be 

considered either primary or secondary structures depending on the question at hand. Flow fronts are lobate margins of 

discrete lava flows (Chapman, 1999). Shallow troughs are a 

pair of closely spaced (<5 km) lineaments marking a shallow 

(tens of meters) flat depression bounded by steep sides; they 

are interpreted herein as the result of subsurface magmatic 

activity. Crater rim crest is the raised rim of an impact crater 

enclosing an interior basin. Flow direction, although not a 

primary structure but interpreted from primary structures 

(for example, channels, levees, and other flow constructs), is 

also shown. In addition we indicate local radar boundaries, 

marked by a sharp transition in radar brightness; these boundaries commonly show lobate character, although we are not 

confident that they represent flow fronts or necessarily primary 

structures. In fact, different radar boundaries might represent 

different features or have a range of implications for surface 

geology as well as geologic history.

Secondary structures, by definition, form after the 

emplacement of geologic units and typically record tectonic 

processes; accordingly, they provide clues for operative tectonic 

processes. In addition, the distribution and (or) character of 

secondary structures may provide clues for the delineation of 

material units, as well as temporal relations between different 

material units (Hansen, 2000; Tanaka and others, in press). SAR 

images of secondary structures within V–48 are shown in figure 

4 and include fractures, folds, small ridges, large ridges, interior 

penetrative fabric, and lineaments. Fractures are extremely 

sharp lineaments with a negative, or null, topographic signature, 

commonly grouped into suites based on orientation, pattern 

(namely, radial or concentric) and/or spacing (that is, widely 

spaced or closely spaced). Fractures are generally interpreted 

as extensional structures (Banerdt and others, 1997). Folds

are ridges with a gradational radar character, normal to their 

trend, and wave-like topographic expression; they are generally 

interpreted as contractional structures (Stofan and others, 1993). 

Small ridges are topographic ridges with low relief and width, 

similar in appearance to folds except that the nature of the lineaments is ambiguous—though possibly of contractional origin 

(marked by folds or thrust faults). Large ridges are topographic 

ridges with moderate relief and width. Interior penetrative 

fabric represents closely spaced (0.5–1 km) lineaments with 

slight gradation in radar brightness across strike. It is interpreted 

in some cases as short-wavelength low-amplitude folds, and in 

other cases as fracture-like structures; however, in many cases 

the fabric character is ambiguous (also see fig. 5). Lineament is 

a discrete feature with possible ambiguous topographic expression and undetermined origin, yet interpreted as likely tectonic 

in origin.

Four structural fabric domains are delineated on the map: 

(1) fracture zone penetrative fabric, (2) chasma penetrative 

fabric, (3) interior penetrative fabric, and (4a) short, closely 

spaced wrinkle ridges occur within Artemis, and (4b) outside of 

Artemis northeast of the northeast-trending portion of Artemis 

Chasma. The three penetrative fabrics (1–3) comprise areally 

extensive tectonic (?) fabrics developed at the scale of current 

SAR effective resolution; the spatial extent of each fabric does 

not necessarily correspond with a material unit (see Hansen, 

2000). Each fabric domain is discussed in the context of its 

regional location.
7

Impact Features

The V–48 quadrangle hosts seven to nine impact craters 

that range in diameter from 4.5 to over 87 km (table 1). Seven 

of these features are noted in the two Venus crater databases 

(Schaber and others, 1992; Herrick and others, 1997). Schaber and others (1992) interpreted Ivne, the eighth feature, as a 

double impact crater; however, Ivne is not included in Herrick 

and others’ (1997) crater database. We favor the interpretation 

of Herrick and others (1997)—that is, Ivne is not an impact 

crater. In left-look SAR images, Ivne appears to be marked by 

two overlapping circular features (7 km and 9 km in diameter, 

which are stacked north to south). However, in right-look SAR 

images, only the southern feature is visible, and it is characterized by a diameter of about 4 km, as well as poor ‘rim’ definition along the north and south regions. No clear ejecta blanket 

or halo is visible in either SAR data set. The ninth feature is a 

possible impact crater, although this feature is not represented in 

either crater database.

Essentially all of the confirmed impact craters show 

ejecta blankets, and two show clear halo deposits. The lack 

of crater halos might reflect the small size of the craters, or it 

could record halo removal (see Izenberg and others, 1994). The 

four largest craters have radar-smooth interiors, interpreted as 

interior fill; the small craters do not show clear radar-smooth 

interiors, although this may be a function of their small size, 

rather than direct evidence of the character of their interiors. 

Crater-density values range from 0.96 to 2.55 craters/106 km2

(Herrick and others, 1997). Crater density might reflect the relative age of a composite surface. Only one crater (O’Connor) has 

a crater density value that is higher than Venus’ global average 

(approximately 2 craters/106 km2; Phillips and Izenberg, 1995; 

McKinnon and others, 1997). The calculated crater density 

values do not, however, include Ivne or the newly identified 

feature, which could (but might not) be impact craters. In any 

case, Venus preserves far too few craters to robustly constrain 

the age of any individual surface, much less individual geologic 

unit, across the planet in general, or V–48 specifically (Hauck 

and others, 1998; Campbell, 1999).

Exterior Region

The exterior region of V–48 includes the topographically 

defined outer high. Clockwise from the east, this region encompasses a northeast-trending chain of coronae that just extends 

to the eastern boundary of V–48, the northwestern extent of 

Zhibek Planitia to the southeast, Biliku and Laverna Dorsa to 

the south, and a fracture zone that trends northeasterly toward 

the southwest margin of Artemis Chasma. The fracture zone lies 

along trend with northeast-trending Britomartis Chasma within 

Artemis’ interior. The outer high forms a broad ridge that parallels the concentric form of Artemis Chasma. Radial fractures 

trend normal to Artemis Chasma from about 1:00 (the map 

area limit) clockwise to about 8:00. Wrinkle ridges, developed 

concentric to the chasma (and perpendicular to radial fractures), 

increase in spacing away from the chasma. The regional fracture 

suite trends normal to the trace of the outer high, and it forms 

an impressive radial pattern with foci in the center of Artemis’ 

interior. The spacing and intensity of fractures varies across the 

exterior region with the most closely spaced fractures developed within the northeast-trending fracture zone. The fracture 

zone, as well as fractures within it, trends consistent with the 

radial pattern marked by the regional fractures. Wrinkle ridges 

also form an impressive concentric pattern across the exterior 

region. Like the regional fractures, the concentric center for the 

wrinkle-ridge pattern falls within the center of Artemis’ interior 

region. Artemis Chasma, regional radial fractures, and wrinkle 

ridges all share the same center point (Hansen, 2002). Wrinkleridge spacing and intensity also vary across the exterior region; 

short, closely spaced wrinkle ridges parallel the chasma along 

the southern, southwestern, and western margins (though they 

do not occur along the region of intense fractures within the 

fracture zone); wrinkle ridges are both longer and more broadly 

spaced to the southeast and east. Along the western margin of 

the chasma, wrinkle ridges within the chasma and the exterior 

define the same trend, and they define a gradation in spacing. 

Along the northeastern margin of the chasma, at the northern 

boundary of the quadrangle, wrinkle ridges are also short and 

closely spaced. Short and closely spaced wrinkle ridges are 

present adjacent to the outer limit of the chasma, whereas longer 

and more widely spaced wrinkle ridges occur farther from the 

chasma. Along the outer high, outboard from the southeastern 

margin of the chasma, regional fractures describe a chasmaradial pattern, but wrinkle ridges are absent.

The corona chain east of Artemis trends northeast across 

V–49, Mahuea Tholus quadrangle. Teteoinnan Corona (38.5° 

S., 149.5° E.; 125 km in diameter), which marks the end of the 

corona chain, lies almost completely within V–48; Annapurna 

(35.5° S., 152° E.; 300 km diameter) and Colijnsplaat (32° S., 

151° E.; 350 km in diameter) Coronae lie mostly within V–49, 

although associated fractures (and possibly flows in the case of 

Colijnsplaat) extend into V–48. Radial fractures define each of 

these three coronae; Colijnsplaat Corona also displays concentric fractures and possible flows. The corona chain defines the 

northwestern boundary of Zhibek Planitia, which occupies the 

southeastern part of V–48. Zhibek Planitia is relatively featureless here—it is marked by east-northeast-trending wrinkle 

ridges of the Artemis Chasma concentric suite and northwesttrending regional fractures that are radial to Artemis Chasma.

Biliku Dorsa (600 km long, 100 km wide) lies completely 

within Zhibek Planitia, whereas Laverna Dorsa (1,000 km 

long, 100 km wide) appears to form the boundary with Zhibek 

Planitia to the east and Laimdota Planitia to the west, although 

both planitiae lie mostly within V–58 and V–57, respectively, 

to the south of V–48. Biliku and Laverna Dorsa represent subtle 

topographic ridges that trend northwest and northeast, respectively. Laverna Dorsa extends into V–57 where the dorsa trend 

reverts to a northeast-trend, as marked by Sunna Dorsa (500 km 

long, 100 km wide) that broadly parallels Biliku Dorsa in V–48. 

The region west of Laverna Dorsa appears to be Laimdota 

Planitia, although Laimdota and Zhibek Planitiae are not clearly 

differentiated within V–48. The radial regional fractures and 

concentric regional wrinkle ridges occur across both Laimdota 

and Zhibek Planitiae. The northeast-trending fracture zone and 

the relatively radar-smooth region just northwest of the fracture 
8

zone lie within either Laimdota Planitia or within Aino Planitia, 

which lies dominantly within V–47 to the west of V–48.

Three units, fcCBM (Ceres, Bona, and Miralaidji Coronae 

flow material, undivided), fu (flows, undivided) and rf (radar 

facies), dominate the exterior region. Unit fcCBM spills into 

V–48 in the northeast corner of the map area from its source 

region in V–37 to the northeast. This unit is correlative with 

unit fcCBM described by Hansen and DeShon (2002). Unit fu

includes, as suggested by its name, a range of undivided, generally low radar-backscatter flows. In several locations (for example, 143° E., 40° S.) sharply defined radar boundaries occur. 

Elsewhere radar boundaries are more diffuse and gradational 

(for example, 140° E., 45° S.), but radar character definitely 

changes within and across unit fu. We do not interpret the various radar boundaries as necessarily marking distinct flow units 

given that distinct radar domains can occur within single geologic units (Tanaka and others, 2010). Given the current SAR 

resolution, the overall size of the map area, and the 1:5 million 

map scale, we cannot delineate individual material units with 

confidence, and thus unit fu, as shown within the study area, 

represents a composite unit that was almost certainly emplaced 

in a time-transgressive fashion and represents numerous varied 

material units. Unit rf includes the part of the radar-bright facies 

defined by Hansen (2002) associated with exterior coronae 

along the eastern margin of V–48. Hansen’s (2002) radar-bright 

facies (volcanic and tectonic) is divided into several units in 

this mapping effort—these include unit rf discussed herein and 

a fracture zone fabric (both of which occur within the exterior 

region), chasma tectonic fabric (within Artemis Chasma), and 

the interior penetrative fabric and interior flow units, which 

occur within Artemis’ interior. The chasma tectonic fabric and 

interior penetrative fabric together form ‘fine-scale fabric’ of 

Brown and Grimm (1995). Unit rf remains as a radar unit, as 

opposed to a geologic unit; it likely includes both tectonic fabrics and flows. The other ‘units’ are each interpreted herein as 

tectonic fabric facies, and they are discussed within the context 

of their areal domains. Unit rf shows high-radar backscatter, 

likely due to a combination of roughness due to fractures and 

surface layer roughness. Unit rf is spatially associated with 

Colijnsplaat Corona. Fracture terrain of Zhibek Planitia, unit

frZ, marked by tightly spaced fractures, underlies unit fu, but it 

lies exposed in a local topographic high. It is not clearly understood whether unit frZ includes several distinct material units or 

a single unit; only the presence of tightly spaced fractures allow 

us to delineate this unit spatially and temporally from the overlying, and younger, composite unit fu. The limited spatial extent 

of unit frZ might favor a singular unit origin prior to fracture 

formation. Unit frZ lies along the southern extent of Laverna 

Dorsa, which shows a slightly higher density of dorsa-parallel 

fractures than the surrounding areas. Biliku Dorsa is similarly 

marked by slightly higher fracture density and by a slight 

z-shaped bend in otherwise north-northwest-trending fractures. 

Laverna Dorsa trends generally parallel to the regional fractures, whereas Biliku Dorsa trends at a high angle to the same 

fracture suite; both dorsa are quite subtle.

The approximately 300 km wide northeast-trending fracture zone that abuts the southwest part of Artemis Chasma along 

strike is defined as a structural domain marked by fracture zone 

fabric, rather than a terrain unit or material unit. As with the 

chasma tectonic fabric, a zonal pattern marks the spatial limit 

of the fracture zone fabric, whereas lines describe lineament 

trends. Fracture spacing diminishes outward normal to the trend 

of the fracture zone. The fracture zone appears to fade outward 

to the east, as marked by a gradual decrease in fracture spacing; 

however subsequent local or regional cover cannot be ruled out. 

The fracture zone is locally covered along its western reaches 

by shield terrain (unit s), which generally includes 1- to 4-km 

diameter edifices and associated flow material. Delicate interfingering of shield material into topographic lineaments indicates 

negative topographic expression of at least some of the fractures within the fracture zone. Unit s and unit fu are both time 

transgressive; as such, we cannot determine robust temporal 

relations between these units and therefore they are shown with 

essentially no temporal constraints.

Impact craters O’Connor and Jalgunik occur in the 

northeastern and southwestern parts, respectively, of the V–48 

exterior region.

Artemis Chasma

Artemis Chasma forms a well-defined circular trough that 

encloses approximately three quarters of Artemis’ interior. The 

chasma ranges in width from approximately 25 to approximately 200 km and displays an average relief of 1 to 2 km. 

Artemis Chasma fades away in the northwest (~10:30) and 

increases in width and depth to its maximum in the southeastern 

to eastern part of the chasma. The chasma appears to be defined 

topographically first, and second in terms of secondary structures; no material units define the chasma, and no units obviously embay the chasma. The chasma hosts a tectonic fabric that 

consists of trough-parallel lineaments with increasing intensity 

(decrease in spacing) clockwise to the southeast, and decreasing intensity (increase in spacing) to the northwest (Brown 

and Grimm, 1995; Hansen, 2002). The lineaments appear to 

form smooth ridges within the trough and along the inward 

facing (outside) chasma wall. We interpret these lineaments to 

be folds, in concurrence with Brown and Grimm (1995) and 

Hansen (2002). Brown and Grimm (1995) and Hansen (2002) 

also described chasma lineaments that occur along the interior 

chasma (outside facing) wall as normal faults; however, we 

cannot robustly determine the nature of these structures. We 

simply refer to these structures as lineaments; they could represent a combination of fractures, faults, and folds. Regardless of 

character, the spacing of the lineaments varies within the trough 

as noted, although the lineaments everywhere generally parallel 

the trend of the trough. We defined a structural domain, chasma 

penetrative fabric, wherein the lineaments become so closely 

spaced relative to the radar resolution that mapping individual 

lineaments at 1:5 million-scale is not viable; a stippled pattern 

shows the extent of the domain, whereas lines indicate lineament trend. At approximately 10:30, where the chasma narrows 

in width and shallows and fades topographically, chasma lineaments appear to become wrinkle ridges, parallel in trend to the 

trough, but with slightly wider spacing, and presumably lower 

height, than lineaments preserved counterclockwise within the 
9

trough. There is no evidence that the trough or trough lineaments are buried by younger material; obvious embayment 

relations are lacking. For more detail about chasma structures 

see Brown and Grimm (1995) and Hansen (2002). The material 

that comprises the chasma, a homogeneous radar smooth unit 

(unit fu, flows undivided) appears contiguous with exterior 

material across much of the chasma, and it is contiguous with 

both the interior and exterior along the northwest region where 

the chasma lacks strong topographic definition. Although the 

chasma is marked by wrinkle ridges and folds with axes parallel 

to the concentric form of the chasmata, and by fractures that 

trend normal to the chasma, there is a slight deviation from 

this pattern at the exterior-chasma boundary at 6:00. Here the 

fractures mark a moderate radial pattern across approximately 

170°; wrinkle ridges, orientated normal to the fractures, are 

shorter and more tightly spaced than wrinkle ridges away from 

the quasi-radial structure. Hansen (2002) referred to this structure as a corona (C6); we do not follow a corona interpretation 

herein because a (half) radial pattern of fractures does not, by 

itself, make a corona.

No impact craters are recognized within Artemis Chasma.

Interior Region

Artemis’ interior makes up the bulk of the quadrangle and 

records the richest local geologic histories. The interior, which 

displays significantly more radar-rough facies as compared to 

the exterior region, includes four tectonomagmatic centers, 

defined by radial and/or concentric fractures or lineaments and 

discrete flows as well as the northeast-trending Britomartis 

Chasma, which extends from the near center of Artemis to the 

northeast chasma margin. Although Hansen (2002) described 

the tectonomagmatic centers as corona-like features, in the 

interest of objectivity we refer to them herein as tectonomagmatic centers, labeled TMa–d starting with the westernmost 

feature and proceeding in counterclockwise fashion. Britomartic Chasma appears roughly parallel to, and along trend with, 

the northeast-trending fracture zone within the exterior region, 

however feature TMa and Artemis Chasma separate the two 

zones. Additionally, the exterior fracture zone and Britomartic Chasma are each marked by quite different geomorphic 

features, as discussed below. Four major units occur within the 

interior region. Tectonic terrain units a and b of Artemis (units 

taA and tbA) occur across a northeast-trending outcrop that 

dominates the central region; composite flow material a and b

of Artemis (units fcaA and fcbA) occur to either side of this, 

and it dominates the northwestern and southeastern interior 

regions, respectively. In addition, various flows and shieldrelated materials occur in association with individual tectonomagmatic centers, as described herein. Unit flu, localized 

flow material, undivided, occurs in small outcrops typically 

filling local topographic lows, particularly within Britomartic 

Chasma. Composite unit fu, which also occurs within Artemis 

Chasma and the exterior region, occurs within the northwestern 

part of the quadrangle and within the interior region. Unit fcI, 

flow material from Inari Corona, and unit fchQ, flow material of Quilla chasma, spill into V–48 from source areas to the 

north in V–35 and V–36, respectively. Unit fcI correlates with 

unit fI1b of Bleamaster and Hansen (2005). Each of the units 

within the northwestern corner of the quadrangle show gradational contacts with one another, except for unit fchQ, which 

has a well-defined unit boundary and appears to be the youngest of the units in this region. Artemis’ interior also hosts a 

distinctive structural domain indicated on the map by a stipple 

pattern. Units taA and tbA each host a penetratively developed 

linear fabric, called ‘interior penetrative fabric.’ The fabric generally trends northeast across the interior with local deviation 

from this trend in spatial association with the tectonomagmatic 

centers. Locally, a fabric that consists of short, closely spaced 

wrinkle ridges occur within unit fcAb along the easternmost 

boundary of the interior and Artemis Chasma, and within unit 

fI1b in the northwest part of Artemis. Feature TMa (C4 and 

C3 of Hansen [2002]; southwestern interior deformation belt 

of Spencer [2001]) is defined by radial fractures, concentric 

ridges, weakly concentric fractures, radial flows, and the 

low-radar-backscatter material of unit fsAa (flows and shieldrelated material a of Artemis). A shallow topographic trough 

wraps from the northern margin counterclockwise to the southwestern margin of feature TMa (most of Brown and Grimm’s 

[1995] north-northwest-trending deformation belt). Unit fsAa

(flows and shield-related material a of Artemis) hosts numerous small shield edifices and surrounds kipukas of unit tAa

(tectonic terrain of Artemis, unit a). Unit fsAa interfingers with 

unit tAa, and it fills long, narrow, shallow valleys within unit 

tAa. Unit fsAa locally covers earlier formed tectonic fabrics, 

but it is also cut by northeast-trending fractures that may represent reactivated buried structures. We interpret these relations 

to indicate that unit fsAa is locally thin (tens of meters) and 

had a low emplacement viscosity due to the intricate interactions with preexisting topography. Impact crater Bonnevie 

(approximately 80 km in diameter) lies along the eastern 

margin of unit fsAa. Robustly constraining temporal relations 

between Bonnevie and unit fsAa proves difficult due to the 

degraded state of the crater ejecta. The flow units may have 

been emplaced both prior to and after formation of Bonnevie. 

The crater interior is clearly flooded by radar-smooth material, 

taken here as flood lava. An interior central peak rises above 

the flooded crater interior. The eastern part of feature TMa (C3 

of Hansen [2002]) hosts radial fractures that weakly define 

a secondary center. Flows emanate from the eastern margin 

and extend eastward and southeastward toward feature TMb. 

Veronica crater (17.9-km diameter) lies along the southern 

flank of feature TMa. Veronica appears to dominantly post-date 

the formation of the penetrative fabric, as indicated by radarbright flows or impact ejecta that show spatial association with 

Veronica and appear to fill topographic lows within the penetrative fabric. Veronica might show fractures parallel to the penetrative fabric, although this observation is difficult to confirm. 

The interior of Veronica shows radar-smooth material not cut 

by fractures. Feature TMb (C2 of Hansen [2002]) is defined by: 

radial fractures, distal concentric fractures, and the low-backscatter material of unit fsAb. Unit fsAb hosts numerous small 

shield edifices with one approximately 5-km diameter volcanic 

edifice near the center. Unit fsAb surrounds kipukas of unit 

tAa and delicately interfingers unit tAa; it fills long, narrow, 
10

shallow valleys, which implies a low emplacement viscosity 

for unit fsAb. A northwest-trending scarp forms the transition 

between unit tAa and fcAb where a distinct flow in unit fcAb, 

apparently sourced near the scarp, extends approximately 200 

km toward feature TMa. Northwest-trending fractures located 

northeast of unit fsAb cut a radial fracture suite. Widely spaced 

(tens of kilometers) arcuate fractures to the northeast define a 

circle centered approximately on unit fsAb.

Feature TMc (C1 of Hansen [2002]) is defined by radial 

fractures that trace back to the low-backscatter material of unit 

fsAc (flows and shield-related material c of Artemis). Unit fsAc

hosts numerous small shield edifices and interfingers with unit 

tAa; it fills long, narrow, shallow valleys, which implies low 

viscosity during emplacement. The penetrative fabric of unit 

tAa trends northwest and parallels a suite of quasi-radial fractures. Three moderately sized (tens of km) volcanic edifices lie 

to the north of feature TMc on unit tAa. The flows of composite 

unit fcAb fan away from feature TMc and locally cover the 

tectonic fabric of unit tAa.

Feature TMd (C5 of Hansen [2002]—the northeasttrending deformation belt of Brown and Grimm [1995] and the 

northeastern interior deformation belt of Spencer [2001]) dominantly consists of sub-parallel northeast-trending rounded ridges 

and flow-filled valleys. Feature TMd lies predominantly in unit 

tAb (tectonic terrain of Artemis, unit b), which grades into the 

adjacent unit tAa. Units tAb and tAa differ in radar brightness; 

unit tAa shows higher radar backscatter. Broad ridges parallel, and preserve, the penetrative fabric. Brown and Grimm 

(1995) identified northeast-trending flat-topped ridges, which 

they interpreted as a horst and graben. We did not observe 

flat-topped northeast-trending ridges in this area; in contrast 

we observed rounded ridges and angular peaked ridges. This 

discrepancy in observations likely arises from angular ridges 

appearing as flat-topped ridges due to radar image artifacts of 

the “magic airbrush” technique used by Brown and Grimm 

(1995). Northwest-trending fractures, scarps, and penetrative 

fabric in the northern part of feature TMd cut northeast-trending 

penetrative tectonic fabrics. Northwest-trending fractures in the 

southern part of feature TMd cut northeast-trending penetrative 

fabric. Roughly concentric fractures and scarps, which lie to the 

west of feature TMd, also cut the penetrative tectonic fabric and 

ridges. This region deserves detailed geologic mapping outside 

the limits of the current study; locally clear, but complex, crosscutting relationships likely exist among the several tectonic elements; it appears however, that layer extension likely dominates 

the local tectonic story, as suggested within the published record 

(Brown and Grimm, 1995; Spencer, 2001). Ivne, a possible 

impact crater (see “Impact Crater” section for discussion), lies 

within feature TMd.

Britomartic Chasma trends northeast across the northern 

interior. Britomartic Chasma differs from most other chasmata 

on Venus in that it is <1,000 km in length, is not associated with 

an obvious corona chain (for example, Diana-Dali Chasmata, 

Hecata and Parga Chasmata), and is marked by smooth fold-like 

ridges (and troughs) rather than chasma-parallel fractures (for 

example, Diana-Dali Chasmata [Hansen and DeShon, 2002], 

Jana, Kuanja and Ralkumgu Chasmata [Bleamaster and Hansen, 

2005], Devan Chasma [V–41 and V–29]). Britomartic Chasma 

hosts local flow units, and it fills local topographic valleys, as 

previously noted (see Brown and Grimm, 1995; Spencer, 2001; 

Hansen, 2002).

Penetrative Tectonic Fabric

A remarkably extensive penetrative tectonic fabric occurs 

within the stratigraphically lowest terrains of Artemis’ interior. 

The fabric consists of tightly spaced lineaments (fig. 5). The lineaments display a generally consistent northeast trend; however, 

locally lineaments change trend in apparent spatial association 

with local tectonomagmatic centers (fig. 2, on map sheet). SAR 

images provide clues about the cross-sectional topographic 

shape of the penetrative fabric and therefore fabric morphology. The fabric has a gradational backscatter character across 

strike in both left- and right-illumination SAR imagery; this 

observation leads to the interpretation that the cross-sectional 

topography is generally marked by smooth, likely symmetric, 

ridges and troughs. Low-amplitude rounded ridges best fit the 

observations, although fracture-like topographic expressions 

and ambiguous topographic expressions also occur. Penetrative fabric wavelength is generally consistent across Artemis 

based on detailed transect measurements at 19 locations shown 

on the geologic map (table 2). Fabric wavelength, determined 

along transects normal to lineament trend, was determined using 

SAR images stretched to enhance the difference between light 

and dark lineaments, and represent maximum values because 

the ridges approach the effective resolution (Zimbelman, 2001) 

for the penetrative fabric using Magellan full-resolution SAR 

data. Wavelength ranges from 300–950 m (540±160m average). 

Ignoring one high outlier yields a range of 300–730 m (520±125 

m average). The penetrative fabric was lumped together with 

other units as part of a single radar facies unit, radar-bright 

facies, which extended across the exterior, chasma, and interior 

regions (Hansen, 2002); it was part of the ‘fine-scale fabric’ unit 

occurring within both Artemis Chasma and the interior (Brown 

and Grimm, 1995). Herein, the penetrative fabric is delineated 

as a distinctive tectonic fabric marked by a short-wavelength 

fabric, which is penetrative at effective resolution.

The penetrative fabric primarily occurs in units tAa and 

tAb, however it locally extends into adjacent units. Penetrative 

fabric mapped outside of units tAa and tAb always occurs at 

gradational contacts determined primarily by a gradual decrease 

in radar backscatter character; fabric-parallel lineaments remain 

easily identifiable but appear slightly subdued. The subdued 

appearance may reflect weathering, varying degrees of burial 

by thin flows or aeolian fines, and/or reactivation of buried 

fabric structures. Low-backscatter material delicately interfingers with the penetrative fabric locally; this relation indicates 

that the low-backscatter material constitutes a thin veneer, 

marked by low viscosity during its emplacement, as would be 

required to fill the long, narrow, and shallow valleys without 

completely burying the penetrative fabric. The low-backscatter 

material is interpreted as consisting of local volcanic deposits. 

A sedimentary origin is unlikely owing to the present lack of 

significant erosion rates on Venus and the pristine appearance of 

adjacent rocks (and lack of evidence for erosion or dissection) 
11

that would be the most probable source of sediment. In addition, 

a sedimentary origin might be expected to be less sensitive to 

local topography. Penetrative fabric-parallel lineaments in the 

low-backscatter material likely reflect the reactivation of buried 

penetrative fabric structures. The nature of the penetrative fabric 

is worth considering further, given its distinctive character 

and extensive regional development across much of Artemis 

Chasma’s interior.

The morphologic characteristics of the penetrative fabric, 

summarized here, might provide clues to its formation, which 

could, in turn, impose constraints for models of Artemis’ evolution. Penetrative fabric morphology (1) consists of symmetric(?) 

ridges with short wavelength (520±125 m), (2) shows relatively 

consistent along-strike character across the interior, (3) does not 

seem to be associated with parallel long-wavelength topography (and similarly lacks periodically spaced gaps), and (4) is 

developed across much of Artemis’ interior, yet it has not been 

recognized within the chasma or exterior region. Observation 4 

indicates that either (a) the penetrative fabric formed across the 

region and has been subsequently covered within the chasma 

and exterior (and therefore embodies temporal implications) or 

(b) penetrative fabric formation was limited to the interior, and 

thus does not carry direct temporal implications with regard to 

interior-chasma formation. Given (1) the lack of evidence for 

burial and embayment within the chasma, (2) temporal relations 

implied by unit fcAa and shallow troughs, which extend from 

the interior across the chasma to the exterior, and (3) buttressing 

relations that indicate broadly synchronous evolution of interior 

flows and chasma structures, we favor the hypothesis that penetrative fabric formation was limited to the interior of Artemis.

The fabric could record deformation of a pre-existing 

layer, in which case the fabric wavelength would likely reflect 

layer strength. It this case layer thickness would be 1 to 6 times 

wavelength (see Lan and Huddleston, 1995), or approximately 

0.3 to 5 km, using average wavelength. It is difficult to envision 

a layer this thin across the entire interior of Artemis without 

associated longer wavelength features, indicating the presence 

of a strong subsurface support layer. In addition, the generally consistent northeast-trend of the fabric might have formed 

approximately normal to major layer shortening (if lineaments 

mark fold or thrust fault ridges) or extension (if lineaments 

represent fractures or normal faults), yet shortening or extension across an existing layer should lead to changes in layer 

thickness, and therefore the existence of multiple wavelengths. 

Furthermore, southeast-northwest shortening or extension of a 

pre-existing layer is difficult to reconcile with the rest of Artemis Chasma’s geometry.

The physical nature of a laterally extensive, thin, relatively 

strong layer is difficult to address. A compositional difference 

between layers might provide one explanation. A compositional 

difference would require a mechanism to emplace a relatively 

uniform and thin layer of material across a very large area. 

Widespread sedimentation analogous to terrestrial ocean basin 

accumulation is one possibility. This would require liquid 

water to enhance erosion rates to create and transport sediment. 

Venus’ surface presently lacks liquid water and significant erosion (Kaula, 1990). However, conditions on Venus could have 

been very different in the past. The high deuterium/hydrogen 

ratio of Venus’ atmosphere is consistent with past wet conditions (Donahue, 1999), which may have supported widespread 

erosion and deposition.

Flood-lava flows might also create a thin layer with a different composition than subsurface material. This would require 

widespread volcanism across the entire interior of Artemis 

Chasma, low-viscosity lava, and presumably magma storage 

across a large area. The presence of a large subsurface magma 

chamber might be expressed by variation in the local strain 

regime; however, the penetrative fabric is moderately consistent 

in character and orientation across the entire interior, in apparent contradiction with predictions. Numerous small magma 

chambers or point sources of magma distributed across Artemis 

could provide source material with less influence on the local 

accumulation of strain. It is possible and even likely that the 

development of penetrative fabric and later tectonic deformation overprinted the surface expression of the magma chambers. 

However, the challenge remains—that is, how to impart a relatively coherent tectonic fabric upon this layer developed across 

Artemis Chasma’s interior.

Alternatively, a rheologically defined thin layer might 

result from a locally elevated geothermal gradient that raises the 

regional brittle-ductile transition to shallow levels. Heating the 

shallow crust to high temperatures without melting could anneal 

the surface layer with time and thereby strengthen the layer. An 

elevated geothermal gradient might not result in a sharp décollement between a thin surface layer and the subsurface, however 

the importance of the width of this mechanical transition zone 

is unknown and a wider zone might be acceptable at the first 

order. Furthermore, this scenario retains the problem of the lack 

of a second long-wavelength fabric that might be expected as 

a result of a subsurface layer. In addition, recent finite-element 

modeling of short-wavelength folding by contracting and cooling of ultra-dry diabase at surface temperatures temporarily 

elevated to 1,000 K indicate that the high temperatures might 

threaten the competence of the thin layer, making formation 

and preservation of short-wavelength structures difficult (Ghent 

and others, 2005). However, the composition of Venusian crust 

is not well understood and could be much stronger, or weaker, 

than currently believed.

Another possibility is that Artemis’ penetrative fabric 

formed progressively as the ‘layer’ itself formed, much like the 

fabric that characterizes terrestrial ocean crust parallel to, and 

observed along, submarine spreading centers. Indeed Artemis’ 

penetrative fabric appears morphologically similar to surfaces 

along terrestrial mid-ocean ridges viewed in high-resolution 

shaded relief maps posted on the Ridge Multibeam Synthesis 

(RMBS) Data Portal website (fig. 6). The terrestrial RMBS 

shaded relief maps and Venusian SAR images might be comparable in that they each provide views of the detailed topographic 

character of planet surfaces. Unfortunately high-resolution 

views of this distinctive terrestrial tectonic fabric are not available across large regional areas away from the ridge axes; this 

lack of availability makes regional comparison with Artemis’ 

penetrative fabric difficult. Despite this shortcoming, we favor 

formation of Artemis’ penetrative fabric in a manner similar to 

terrestrial spreading center fabric herein—that is, marking the 

formation of new crust, with cross-strike time-transgressive 
12

evolution. In this case Artemis’ penetrative fabric would not 

mark deformation of a previously existing layer (hence a secondary tectonic fabric), but rather it would reflect the formation 

of crust, and as such, the penetrative fabric might more correctly be considered a primary tectonic fabric that formed with 

the evolution/emplacement of the material unit/crust. Within the 

context of this scenario, the layer need only be thin at the location of its formation, and it could thicken from below as a result 

of under-plating, as the surface/crust translates away from the 

spreading center with the formation of new crust, as is the case 

for Earth’s oceanic crust.

Artemis’ interior penetrative fabric shows a regionally 

coherent trend, yet it is also characterized by local variation, 

which might be similar to variations observed in highresolution shaded- relief images of mid-ocean ridge fabrics. 

Within the southwestern interior of Artemis, the penetrative 

fabric is marked by a somewhat curvilinear fabric (fig. 7, on 

map sheet) in which the fabric locally deviates along parallel 

northwest-trending linear zones, and along one sharply defined 

zone in particular. Brown and Grimm (1995) originally 

proposed this sharp boundary as a dextral shear zone that 

displaces pre-existing lineaments in a right-lateral fashion. 

Given that the boundary coincides with a steep topographic 

slope, the apparent deflection of lineaments could be a result 

of radar foreshortening. Radar foreshortening would displace 

areas of high topography (in this case, the north side of the 

slope break) toward the direction of the radar, resulting in an 

apparent, but not real, change in trend. That is, the apparent 

bend in lineaments at this location might be best interpreted as 

a radar image artifact, rather than indicative of strike-parallel 

displacement. The oblique nature of the lineaments within 

the zone (as observed in high-resolution SAR data) suggests 

that the deflections are probably not solely artifacts. If the 

deflections are not solely artifacts of radar imaging, then it is 

important to know both the relative timing of the lineaments, 

as well as the nature of the lineaments—whether formed by 

shear, shortening, or extension processes—to constrain robust 

kinematic interpretations. The fabric is deflected in an apparent 

left-lateral sense along gentle slopes assuming that the fabric is 

simply sheared across the northwest-trending zone. However, if 

the penetrative fabric marks fold crests, the asymmetric fabric 

might record distributed sinistral, rather than dextral shear. If 

the penetrative lineaments mark extensional structures, then the 

region might record distributed northwest-southeast extension 

with distributed dextral (right-lateral) shear. The current data do 

not allow us to robustly and uniquely define either the nature 

of the lineament fabrics or their relative timing. Furthermore, 

comparison with terrestrial mid-ocean ridge fabrics and 

transform fault fabrics indicate that a specific kinematic 

interpretation might be suspect. We suggest here, however, that 

the interior penetrative fabric might be similar to terrestrial midocean ridge fabrics, and it might record distributed extension 

that resulted in fabric and layer formation as well as lateral 

displacement (perhaps dextral in some places and sinistral at 

others) along sharp to distributed ‘transform-like’ zones. Within 

the context of this interpretation, the penetrative fabric would 

form generally perpendicular to the direction of extension 

(parallel to spreading center ridge trends). This suggestion 

should be treated as a working hypothesis and should be subject 

to scrutiny with further study using new and existing data. 

None of the scenarios outlined above can be eliminated with 

any certainty using currently available data, and other scenarios 

might indeed exist. Future missions with higher resolution 

imagery and topography data will help refine constraints on the 

penetrative fabric; stricter constraints might pose requirements 

that better constrain the possible options. Lander missions 

that provide geochemical data could also be helpful if the 

geologic context of their sampling is well constrained and 

understood. Detailed high-resolution structural mapping and 

analysis of the penetrative fabric might also provide critical 

clues to the formation of this distinctive fabric. Such a study 

is beyond the scope of this report. We favor the interpretation 

that Artemis’ penetrative fabric is analogous to terrestrial midocean ridge fabrics, and similarly reflects time-transgressive 

formation of new crust in a cross-strike (or high angle to strike) 

direction. The consideration of time-transgressive formation 

of this fabric, as opposed to formation indicative of a singular 

geologic ‘event,’ will play a critical role in understanding 

Artemis formation and, as such, has implications for Venusian 

geodynamics.

Temporal Relations and Geologic

History

Few units or structures occur within each of the three Artemis regions: exterior, chasma, and interior; yet it appears that 

at least parts of each of these regions record a shared history. 

Artemis suites of tectonic structures at all scales show a remarkable consistency in style and trend. Suites of chasma-parallel 

structures, radial fractures, and wrinkle ridges each define individual circles each with their centers near the center of Artemis. 

Hansen (2002) suggested that these geometric relations are 

strongly suggestive of a genetic relationship; we concur with 

that interpretation, as we find no evidence to contradict this 

premise based on geometry and spatial relations. Thus, at the 

heart of our analysis is an acceptance that Artemis likely formed 

as a tectonically coherent entity with interior, chasma, and 

exterior regions evolving broadly contemporaneously. For this 

discussion, we depart from our earlier organization and discuss 

features from the interior of Artemis outward.

Artemis’ interior units roughly divide into two groups. 

One group consists of radar-rough tectonic terrains (units tAa

and tAb). The other group consists of radar-smooth (volcanic?) 

flow materials (units fcAa, fcAb, fsAa, fsAb, fsAc, fu, and 

flu). The penetrative tectonic fabric that marks units tAa and 

tAb shows a regionally consistent northeast trend throughout 

the interior. Radial and concentric structural suites and associated flows define four centers of tectonic and magmatic activity. 

Penetrative fabric typically marks the basal exposed terrain at 

most locations. Accordingly, although the onset of penetrative 

fabric formation is unconstrained, penetrative fabric formation 

appears to have formed (locally) relatively early in Artemis’ 

interior history. However, this theory does not require that 

the penetrative fabric formed within a single geologic instant, 
13

but rather, could record a time-transgressive process, just as 

terrestrial ocean crust forms time-transgressively. Penetrative fabric formation seems most likely to have involved the 

(progressive) creation of a layer across much (and perhaps all) 

of Artemis’ interior. Penetrative fabric formation overlapped in 

time and space with tectonism of units tAa and tAb, and with 

the formation of the four tectonomagmatic centers (TMa–d). 

Cross-cutting relations suggest that the development of penetrative fabric broadly ended earlier (locally) than the development 

of the tectonomagmatic centers, or temporally overlapped with 

early stages of tectonomagmatic center evolution. We envision 

that the penetrative fabric records formation of new crust along 

a northeast-trend, with local modification of the crust at each of 

the tectonomagmatic centers.

Within Artemis’ interior each of the four tectonomagmatic 

features (TMa–d) preserve histories of locally centralized deformation and volcanism. Evolution of the four centers broadly 

overlapped in time, and in concert with, though generally later 

than, the formation of local penetrative fabric elements. All four 

centers preserve histories that temporally and spatially overlap 

between centers. Flows that emanated from feature TMa cover 

flows that emanated from feature TMb, and vice versa, indicating broadly contemporaneous evolution of these two centers. 

The flows also locally bury pre-existing structures related to tectonomagmatic center formation. Lineaments in the flows show 

similar trends to those in the penetrative fabric; these lineaments 

likely represent reactivation of penetrative fabric structures. 

Flows in unit fcAb, located between features TMc and TMd, are 

difficult to individually delineate, and thus, to attribute to one 

tectonomagmatic center over the other. Additionally, flows in 

unit fcAb appear to emanate from feature TMb and flow toward 

feature TMc, and vice versa; however, no discrete boundary 

exists between opposing flows, which appear to laterally grade 

into each other. We interpret these relations as evidence of 

broadly synchronous and time-transgressive emplacement of 

these units at the scale of our analysis. Based on the surface 

record, the bulk of the deformation associated with tectonomagmatic centers predated the bulk of composite flow formation 

(or at least the flows preserved at the surface). Because observations are limited to the surface, we have no record of earlier 

(now subsurface) flows, which is a limitation of the data. Shield 

fields at the centers of features TMa, TMb, and TMc (units 

fsAa, fsAb, and fsAc, respectively) locally bury pre-existing 

structures related to their respective tectonomagmatic center, 

and these shields are in turn deformed by structures of the same 

trend and character that they bury. These relations indicate that 

the bulk of the individual shield fields’ formation occurred as 

tectonomagmatic center deformation and evolution waned. 

No clear temporal relations exist between the evolution of the 

shield fields and the composite flows because these map units 

are not in contact with one another, and such relations would be 

below the resolution of the current SAR data.

The pervasive development of Artemis Chasma topography 

and chasma structures currently prohibits obvious correlations 

of interior units to exterior units. The evolution of individual 

tectonomagmatic centers can be related to other parts of 

Artemis’ evolution. Unit fcAa extends from the interior (TMa) 

westward across Artemis Chasma and to the exterior. Shallow 

troughs, secondary structures associated with this flow unit, also 

extend from the interior to the exterior along Artemis’ western 

boundary. We interpret that the flow unit and shallow troughs 

formed time-transgressively as flows covered trough and 

trough-cut flows, as also noted by Hansen (2002). In addition, 

the shallow troughs appear to both cut, and be cut by, chasmaparallel folds (fig. 8, on map sheet). We interpret these relations 

to indicate broadly contemporaneous, time-transgressive formation of both shallow troughs and chasma folds (and by association, the chasma trough). Unfortunately it is difficult to track 

individual shallow troughs across the chasma due to alternating 

left- and right-look SAR data at a high angle to shallow troughs 

and broadly parallel to the chasma and chasma structures. 

Local minor apparent right-lateral offset of the some shallow 

troughs likely reflects shortening across the chasma structures 

rather than true dextral translation; however some component of 

dextral displacement is certainly possible. The relations across 

the western part of Artemis trough indicate that feature TMa

evolved time-transgressively and broadly contemporaneous 

with the formation of the western part of the chasma. In addition, the chasma records moderate, but not extreme, shortening 

across itself or its’ interior; furthermore, no significant chasmaparallel translation is evident [or is recorded] across the chasma. 

The flows and fractures could have formed broadly prior to 

the chasma, and the chasma could have formed a topographic 

trough late during evolution of flows and fractures. There is 

no evidence that flows spilled into the topographic trough that 

marks the chasma. Along the eastern part of Artemis’ interior, 

flows associated with TMb and TMc appear to locally embay 

chasma-related topography being buttressed by chasma-parallel 

folds at the boundary between the interior and Artemis Chasma. 

Some chasma-parallel structures near impact crater Behn cut 

unit fcAb, whereas other chasma-parallel structures in the area 

appear locally embayed by unit fcAb. These relations are consistent with broadly contemporaneous formation of the chasma 

structures and the emplacement of unit fcAb, which forms a 

composite, and likely time-transgressive, unit. Along the outer 

part of the eastern chasma (at about 3:00), exterior (radial) 

fractures are affected by chasma-parallel folds; crosscutting 

relations clearly indicate that the exterior fractures predated 

the formation of the outermost folds within Artemis Chasma 

(Brown and Grimm, 1995; Hansen, 2002, fig. 5 therein). These 

relations indicate that these radial fractures formed early with 

respect to chasma folds (and presumably chasma topography) 

at this location. Similar crosscutting relations occur within the 

southern part of Artemis Chasma, with exterior fractures locally 

preserved along the fold crests that lie within the outer parts of 

the chasma. There is no evidence that fractures occur within the 

inner parts of the chasma or its structures. It is unclear whether 

the fractures simply did not initially extend farther inward, or if 

the fractures have been obliterated either as a result of tectonic 

activity or burial. Given that no obvious flooding relations occur 

within Artemis Chasma, burial seems unlikely. We favor the 

interpretation that the fractures did not originally extend inward, 

but disruption by chasma tectonism is possible.

Fundamentally, the current resolution of the data does not 

place temporal constraints on the formation of the geologic 

units (materials) relative to the chasma (a topographic feature). 
14

There are no units that appear to clearly flow into the topographic trough that marks the chasma, so all that can be stated, 

with guarded confidence, is that it is unlikely that the interior 

tectonomagmatic centers formed well after Artemis Chasma 

marked its current topography. We postulate that Artemis 

Chasma could have developed locally after flows associated 

with the various tectonomagmatic centers were emplaced, and 

that the chasma may have formed by local down warping of 

the surface; the trough could translate spatially in a direction 

normal to its trend.

As noted previously, structural suites that characterize 

the exterior region of Artemis—concentric wrinkle ridges and 

radial fractures—share a center point with Artemis Chasma, 

located within Artemis’ interior region. We interpret these 

shared geometries as consistent with the suggestion that these 

exterior structures and Artemis Chasma are genetically related. 

The northeast-trending fracture zone abuts Artemis Chasma at 

approximately 7:00 to 8:00. Artemis Chasma, defined here both 

topographically and structurally, appears to narrow slightly, and 

to bend to the northeast along this juncture with the exterior 

northeast-trending fractures zone; these relations could indicate 

that the fracture zone and chasma might have formed over the 

same time interval. In any case, the fracture zone neither obviously cuts the chasma, nor does the chasma obviously cut the 

fracture zone. We suggest that the fracture zone and chasma 

might have evolved broadly synchronously, although we certainly cannot prove such a suggestion. The suggested timing 

relations are consistent with the current data. If Artemis Chasma 

served as a buttress to the younger propagation of the fracture 

zone, then we might expect to see evidence that the fracture 

zone, with a width of >200 km at the juncture, disrupted the 

chasma and chasma structures. If Artemis Chasma completely 

post-dated formation of the fracture zone we might expect to see 

a sharp difference in chasma structures at the juncture—which 

is not observed. Thus the slight narrowing of Artemis Chasma, 

and general lack of obvious crosscutting structural relations at 

the juncture of the chasma and the fracture zone suggest broadly 

contemporaneous evolution of the chasma and the fracture 

zone. It is also notable that the maximum extension direction 

across the chasma (northwest-southeast) broadly parallels the 

interpreted extension direction recorded by the interior penetrative fabric. Thus, we submit that the exterior fracture zone, the 

interior penetrative fabric, and the chasma might have evolved 

in broad temporal concert with one another, with the difference 

in interior penetrative fabric and exterior fracture zone perhaps 

reflecting marked differences in crustal rheology on either side 

of the evolving Artemis Chasma.

The eastern corona chain with Teteoinnon Corona marking 

the final feature barely extends into the V–48 quadrangle. Temporal relations between the corona chain in general, and Teteoinnon Corona, specifically with Artemis, are unconstrained.

Hypothesis Evaluation

Four hypotheses have been proposed for Artemis’ formation: subduction, metamorphic core complex, bolide impact, 

and deep mantle plume (fig. 9, on map sheet). Each hypothesis 

is discussed briefly and evaluated based on constraints gleaned 

from this 1:5 million-scale geologic mapping effort.

Subduction Hypothesis

The subduction hypothesis highlights the cross-strike 

topographic similarity of Artemis Chasma to terrestrial subduction trenches (McKenzie and others, 1992; Brown and Grimm, 

1995; Schubert and Sandwell, 1995; Brown and Grimm, 1996). 

The subduction hypothesis suggests that northwest-directed 

convergence resulted in under thrusting of approximately 250 

km of ‘lower-plate’ exterior lowlands (from the southeast) 

beneath what is now Artemis’ interior (upper plate). If the postulated subducted slab lacked water for magmatic fluxing (for 

example, Kaula, 1990), then slab subduction would be expected 

to contribute to local depression of the regional geothermal gradient above the slab resulting in a lack of upper plate volcanic 

activity, as favored by proponents of the subduction hypothesis. 

If sufficient water existed (consistent with, but not required by, 

atmospheric isotopic data; for example, Donahue and Russell, 

1997; Donahue and others, 1997; Donahue, 1999; Lecuyer 

and others, 2000; Hunten, 2002) and lead to magmatic fluxing, 

then upper plate volcanism might be expected to result in an 

alignment of volcanic features above the proposed subducted 

slab that formed concurrently with upper-plate tectonism. Thus 

volcanic activity within the interior of Artemis would either predate chasma formation (dry), or form as a result of subduction 

(wet). Either way, within the context of the subduction hypothesis, topographic expression of Artemis Chasma would result 

from flexure of the down-going slab, represented by the exterior 

lowland. However, flexure does not appear to be an adequate 

explanation for the topographic expression of the entire chasma 

in plan view. Plates should slip past each other rather than 

over or under ride one another where their common boundary 

is sub-parallel to the direction of convergence. Accordingly, 

flexure should not occur where the chasma axis trends generally 

northwest or southeast. Anticipating this, yet favoring the subduction hypothesis, Brown and Grimm (1995; 1996) suggested 

that Artemis Chasma actually represents three distinct trough 

segments. One is the part of the chasma from approximately 

2:30 to 6:30 that denotes a subduction zone marked by approximately 250 km of under-thrusting of exterior lowlands to the 

southeast under Artemis’ interior. The second is the chasma 

segment from 12:00 to 2:30 that marks an associated trough 

dominated by focused left-lateral displacement (a sort of transform plate boundary). The third segment is the chasma between 

6:30 and 10:30 that corresponds to an older feature, genetically 

unrelated to the other two segments. In this scenario the chasma 

never existed between 10:30 and 12:00, which is consistent with 

geologic relations.

The subduction hypothesis embodies several predictions 

that can be evaluated based on the results of geologic mapping. The first prediction is that the southwest part of the trough 

formed prior to, and is unrelated to, the northeast-southeast 

segment. This prediction is contradicted by temporal relations 

that suggest the chasma formed as a coherent feature as evi-
15

denced by the marked continuity of chasma parallel structures 

along the entire trace of the chasma, as well as near orthogonal fractures. A second prediction is that the chasma segment 

from 6:30 and 10:30 should record right-lateral displacement 

that accompanied postulated northwest-directed subduction 

along the chasma segment from 2:30 to 6:30. In detail there is 

no evidence for focused right-lateral displacement across the 

chasma, and, in fact, shallow troughs cross the chasma along the 

southwest boundary with broadly overlapping temporal relations with chasma structures (fig. 8, on map sheet). The local 

minor apparent right-lateral offset of the some shallow troughs 

likely reflects shortening across the chasma structures rather 

than true dextral translation. The low amount of possible dextral 

offset falls far short of the 250 km of displacement predicted. 

In addition, Brown and Grimm (1995) also interpreted rightlateral shear in a northwest-trending deformation belt at the 

southwest margin of feature TMa, which lies within the interior 

(and therefore the postulated upper-plate) rather than within the 

chasma, and yet might have accompanied subduction zone formation. It is unclear what a dextral shear interpretation is based 

upon, and as noted previously, kinematic implications of real or 

apparent deflections of the lineaments deflections are debatable. 

The third prediction is that section of the chasma from 12:00 

to 2:30 should record structural fabrics indicative of left-lateral 

displacement. However, Hansen (2002) found no evidence for 

sinistral displacement, nor did the current study reveal evidence 

for the required kinematic picture.) A fourth prediction is that 

interior volcanism should pre-date chasma formation (subduction hypothesis proponents call for dry Venusian conditions), 

yet temporal relations seem more consistent with contemporaneous development of the chasma and interior volcanism. Composite flow unit fcAa, interpreted herein as genetically related to 

feature TMa, extends across the western margin of the chasma. 

Cross-cutting relations are consistent with broadly contemporaneous formation of feature TMa and at least a part of Artemis 

Chasma. In addition, unit fcAb, interpreted herein as genetically 

related to features TMb and TMc, appears to embay chasmarelated structures along parts of the inner southeastern chasma 

margin; chasma-parallel lineaments, near impact crater Behn, 

also deform unit fcAb and yet also variably display evidence of 

burial by unit fcAb. In addition, small chasma-parallel ridges 

in unit fcAb near the southeast margin may represent inversion 

structures (for example, DeShon and others, 2000; Hansen, 

2005) or reactivation of buried fractures. These relations indicate that the interior volcanic activity temporally overlapped 

to some extent with formation of the chasma, at least at these 

locations. A fifth prediction is that chasma-concentric normal 

faults might be expected to form in the outer rise out board of 

the chasma segment from approximately 2:30 to 6:30. However, 

no normal faults are documented, and instead, wrinkle ridges—

indicative of distributed layer contraction—form concentric 

to, and out board of, the chasma. Flexural modeling obtained 

reasonable fits for this southeastern part of Artemis Chasma, 

although it required a high in-plane force to prevent brittle failure in the outer rise (Brown and Grimm, 1996).

Each of these observations taken together with documented continuity of structures along the entire trough in a 

trough-parallel fashion clockwise from 12:00 to 10:30, and a 

shared central location of trough topography, trough structures, 

radial fractures and wrinkle ridges, support the interpretation 

that the various features of Artemis are genetically related. 

In addition to map relations, the angle of subduction required 

by the tight curvature of Artemis trough is not geometrically 

viable on a planet the size of Venus. If subduction did occur 

along the length of Artemis Chasma, the lower plate must have 

deformed in a cylinder-like fashion, or the plate would emerge 

on the northwest side of Artemis. We find no compelling 

evidence for either scenario. And even if we did find evidence 

supportive of either scenario, neither situation would constitute 

subduction.

Metamorphic Core Complex Hypothesis

The metamorphic core complex hypothesis suggests that 

Artemis records a composite structure in which the interior 

represents a metamorphic core complex that resulted from 

exposure of crust deformed in a ductile manner (Spencer, 2001). 

Spencer (2001) postulated that the center of Artemis represents 

the grooved surface of a 300-km-square molded footwall of a 

crustal extensional system exposing deep crustal rocks due to 

approximately 170 km of northwest-southeast directed extension. The northern interior part of Artemis would represent 

lower plate deep crustal rocks marked by ductile northwesttrending grooves formed as a result of plastic molding of the 

footwall by irregularities on the underside of the hanging wall. 

Despite a rather detailed and restrictive interpretation of specific 

structures, the metamorphic core complex hypothesis is not 

comprehensive. It does not place postulated crustal extension 

into a regional context, and it makes very few testable predictions. In addition, the lack of significant erosion on the surface 

of Venus (Arvidson and others, 1992) makes exhumation of 

deep crustal rocks unlikely and proves challenging for the 

metamorphic core complex hypothesis. On Earth, significant 

crustal extension can result in the translation of deep crustal 

rocks to shallow crustal levels; but erosion processes expose 

those rocks at the surface. Rocks that record ductile deformation can also become exposed as a result of extension along 

terrestrial spreading centers, although extension associated 

with spreading also requires recycling of other crust at subduction zones. Spencer (2001) does not suggest how analogous 

recycling might be accommodated on Venus. The metamorphic 

core complex hypothesis does not address the formation of: (1) 

the tectonomagmatic centers, (2) the interior penetrative fabric 

(which trends perpendicular to the predicted ductile elongation 

tectonic fabric), (3) Artemis Chasma, or (4) the concentric outer 

high. As proposed, the metamorphic core complex hypothesis is 

essentially untestable owing to lack of details.

Bolide Impact Hypothesis

The bolide impact hypothesis suggests that Artemis marks 

the surface expression of a huge bolide impact that formed on 

a cold, solid Venus, no later than 3.9 Ga (Hamilton, 2005). The 

impact hypothesis makes few specific predictions about Artemis’ morphology, structures, or possible related volcanic activ-
16

ity; as such, the bolide impact hypothesis requires speculation 

on many important details. Furthermore, the impact hypothesis 

does not address many first-order aspects of Artemis including 

topography and geologic relations, nor does it consider studies 

of large impact features and expected morphological features. 

Artemis’ topographic form, with a narrow (100–150 km) circular trough surrounding a raised interior, is opposite that of large 

impact basins on Mars and the Moon, which display circular 

rims surrounding huge interior basins. For example, Mars’ 

Hellas Planitia, widely accepted as impact in origin, forms a 

2,000-km diameter, 6–8 km deep, circular basin surrounded by 

a modified outer rim (fig. 10, on map sheet). Hamilton (2005) 

infers that when Artemis formed, early Venus would have 

been rheologically similar to Mars during the formation of the 

Hellas impact basin. Therefore, within the context of the impact 

hypothesis, these two huge impact features (Artemis and Hellas) 

should show similar first-order character. Although Venus and 

Mars differ geologically, huge impact basins on both planets, as 

well as on Earth, should be negative topographic features unless 

strong geophysical or rheological reasons suggest otherwise 

(see Jones and others, 2005). Models of large bolide impact that 

result in massive partial melting of the mantle and possible formation of interior highs require thin lithosphere (~10 km or less; 

allowing the bolide to penetrate the crust) (for example, Jones 

and others, 2005; Elkins-Tanton and Hager, 2005). Large impact 

basins commonly show multiple- ring morphology (Wilhelms, 

1973; Hartmann, 1998), and Artemis lacks such morphology. In 

addition, contrary to the assertion by Hamilton (2005), there is 

no evidence that a northwest part of Artemis Chasma (10:30 to 

12:00) is buried beneath other constructs (Brown and Grimm, 

1995, 1996; Hansen, 2002; this study). Results presented herein 

indicate that Artemis Chasma narrows and shallows tracing 

clockwise from the southeast; chasma-related structures fade 

along the chasma with the diminishing topographic expression. 

We found no evidence of embayment of the chasma structures within this region, which would be expected in the case 

of burial. Additionally, the interaction of shallow troughs and 

chasma lineaments do not show any obvious strain partitioning or deflection that might be expected if a significant crustal 

anisotropy such as a buried chasma were present. Accordingly, there is no evidence that Artemis Chasma once formed a 

complete circular trough that experienced late burial along its 

northwest trace. Finally, the impact hypothesis does not address 

the formation of the interior tectonomagmatic features or formation of the penetrative tectonic fabric developed across the 

interior, despite the hypothesis inference that Artemis represents 

a coherent set of features formed within a very short geologic 

timeframe.

The bolide impact hypothesis is currently poorly developed 

and lacks such salient predictions as: (1) the size of bolide, (2) 

the thickness and rheology of the lithosphere at time of impact 

(except that the lithosphere was ‘cold’ and hence thick), (3) 

specific structures and structural patterns associated with large 

bolide impact, or (4) possible role of impact-induced melting. 

The bolide impact hypothesis does not address many first-order 

observations including: (1) Artemis topography, (2) the lack of a 

complete circular feature, (3) formation of interior tectonomagmatic centers, or (4) formation of the interior penetrative fabric 

with a regionally consistent northeast-trend. If significant 

impact melt were generated during Artemis’ formation by 

bolide impact, then a thin mechanical layer might have formed 

(although it is not clear how this could happen), and perhaps a 

penetrative fabric could develop (for example, Hansen, 2006). 

However, the consistent northeast structural trend would not be 

predicted, and it is difficult to reconcile with the bolide impact 

hypothesis. Ultimately the bolide impact hypothesis, as published, is not a viable explanation for the formation of Artemis 

Chasma and would require further development in light of the 

geologic constraints presented herein as well as elsewhere in the 

literature.

Plume Hypothesis

The plume hypothesis suggests that Artemis represents 

the surface expression of a deep mantle plume (Griffiths and 

Campbell, 1991; Smrekar and Stofan, 1997; Hansen, 2002, 

2007). This hypothesis initially arose from laboratory plume 

experiments aimed at modeling the interaction of a thermally driven mantle plume with the lithosphere (Griffiths and 

Campbell, 1991), and from later numerical modeling aimed at 

modeling corona formation (Smrekar and Stofan, 1997). Both 

physical and numerical models resulted in the formation of a 

circular trough, postulated as analogous to Artemis Chasma. 

Griffiths and Campbell (1991) also noticed that their physical plume experiments developed small-scale convection cells 

that compartmentalized the interior of the flattening plume 

head. The tectonomagmatic centers might represent subsurface 

compartmentalization contributing to the evolution of Artemis’ 

interior region. As an alternative to small-scale thermally driven 

convection cells, Hansen (2002) proposed the formation of 

compositionally driven diapirs above a flattening plume head 

to accommodate formation of the localized tectonomagmatic 

centers in Artemis’ interior. The formation of Artemis’ interior 

tectonomagmatic centers as diapirs is consistent with numerical modeling of diapirs (for example, Koch and Manga, 1996; 

Smrekar and Stofan, 1997). The plume hypothesis also accommodates crustal heterogeneity, which could influence strain 

partitioning and prevent map patterns idealized from physical 

and numerical models from developing.

Within the context of the plume hypothesis, Artemis’ 

topography should reflect a raised interior region, consistent 

with the observed topography of a raised interior region surrounded by a 1- to 2-km-deep chasma. Furthermore Artemis 

Chasma would have formed a coherent entity broadly synchronous with interior volcanic deposits and deformation, and deformation in the exterior. This prediction is consistent with the 

continuity of chasma structures, radial fractures and concentric 

exterior wrinkle ridges that strongly suggest a genetic relationship and broadly contemporaneous formation. As noted above, 

interior deformation and volcanic deposits formed broadly 

synchronously with the chasma.

The interpretation that Artemis represents the surface 

manifestation of a mantle plume on thin lithosphere, is consistent with Artemis’ large size and circular planform. Gravitytopography analysis, though not unique, is consistent with at 
17

least partial dynamic support for Artemis (Simons and others, 

1997). However, contemporary dynamic support requires a 

contemporary age for Artemis—a prediction that we can neither 

support nor reject based on currently available data. As a deepmantle plume rises toward the lithosphere, the lithosphere will 

be uplifted, and, if the strength of the lithosphere is exceeded, 

radial fractures could form above the plume head. An alternate 

scenario is if the lithosphere were sufficiently heated it might 

develop a penetrative tectonic fabric. A circular trough could 

also form, as illustrated in laboratory experiments aimed at 

modeling the interaction of thermal plumes with the lithosphere 

(Griffiths and Campbell, 1991). In Griffiths and Campbell’s 

(1991) experiments, as a plume head approached the rigid 

horizontal boundary, it collapsed and spread laterally. A layer of 

surrounding ‘mantle,’ squeezed between the plume and the surface, resulted in a gravitationally trapped asymmetric instability, 

and this process led to the formation of an axisymmetric trough. 

In addition, the interior squeezed layer might lead to convection 

on a scale much smaller than that of the original plume. These 

smaller scale instabilities could interact with the lithosphere 

inside the trough, and become manifested as the tectonomagmatic centers. It was on the basis of these laboratory experiments that a plume model for Artemis formation was proposed 

following initial release of Magellan SAR data (Griffiths and 

Campbell, 1991). Finite-element models of the interaction of a 

large thermal plume with lithosphere, aimed at modeling corona 

topography, also show development of an axisymmetric trough 

above large thermal mantle plumes (Smrekar and Stofan, 1997). 

In this case the trough results from delamination of the lower 

lithosphere. Delamination might contribute to a hybrid model 

that incorporates aspects of plume-lithosphere interactions with 

signatures that some workers propose might be better addressed 

through subduction, although subduction vergence might differ.

A Hybrid Hypothesis

Although the plume hypothesis seems the most consistent 

with geologic relations, it does not address the formation of the 

penetrative fabric. We suggest a hybrid hypothesis that builds 

on the plume hypothesis, but which might incorporate aspects 

of each of the four existing hypotheses. The hypothesis embodies two important postulates. One is that Artemis represents a 

singular geomorphic-tectonomagmatic feature, and, as such, the 

interior, chasma, and exterior evolved together. A second postulate is that various parts of Artemis formed transgressively, both 

spatially and temporally. We suggest that Artemis marks the 

surface expression of a large mantle plume that emerged within 

a broad mantle upwelling; the interaction of the upwelling, the 

plume, and the lithosphere resulted in significant rheological 

differences across the exterior, interior, and chasma regions. The 

exterior preserves a distal record of the initial plume-lithosphere 

interactions, as reflected in radial and concentric structures; 

the interior records the formation of new crust at a terrestriallike spreading center and associated tectonomagmatic activity 

during plume evolution; the chasma preserves parts of crust 

modified directly above the plume early in plume-lithosphere 

interactions, and subsequently shortened during the evolution 

of the plume. A series of time-step cartoons illustrates salient 

features of this new hypothesis, beginning with the arrival of a 

deep mantle plume at the base of relatively thin lithosphere (fig. 

11, on map sheet). Part of the crust resides above a broad quasicylindrical mantle upwelling (marked by the exterior fracture 

zone) where the crust is thinned, thermally and/or mechanically, 

relative to its global counter part (fig. 11, time 0). With arrival 

of the plume, the lithosphere/crust was uplifted due to thermal 

buoyancy, resulting in the formation of radial fractures and 

concentric wrinkle ridges, now preserved in the region outside 

Artemis Chasma (which forms later). The area above the plume 

thins (light-gray region forming the interior of Artemis to be), 

and there is a fundamental rheological boundary between the 

interior crust affect by the plume, and exterior crust, which lay 

outside the spatial limits of thermal-rheological influence of 

the plume. As the plume head flattens and spreads the plume 

tail delivers hot mafic magma to the surface, forming a nascent 

spreading center, also likely fed by pressure-release melting in 

the mantle (time 2); the penetrative fabric preserves a record 

of this newly formed crust, geomorphically similar to the crust 

formed at terrestrial mid-ocean ridges. With time, the ‘spreading 

center’ propagates along strike, influenced by both the plume 

and the broad mantle-upwelling (time 3). As new crust forms 

at the spreading center the previously formed crust is displaced 

outward from the spreading center; a trough develops within the 

modified crust (light gray), possibly along its boundary with the 

newly formed crust, marked by penetrative fabric. The trough 

moves outward with continued spreading and propagation of 

the spreading center, which propagates parallel to the trend of 

the broad mantle upwelling. The translation is driven both by 

spreading and new crust formation at the surface, and dynamics 

at the plume-lithosphere interface (from below; as in the case of 

laboratory experiments). Tectonomagmatic centers form locally 

within the region thinned by initial plume-lithosphere interaction (time 4, time 5), presumably as a result of localized magmatic activity related to the plume. Once the thinned/modified 

crust and the newly formed crust (marked by the penetrative 

fabric) meet the relatively unmodified crust (that region exterior 

to the current chasma), the trough (chasma) can no longer move 

outward as the crustal rheology is simply too strong and resists 

deformation (time 6). Spreading need not have occurred at a 

single spreading center, but rather could have occurred across 

various parts of Artemis’ interior.

Within the context of this hybrid hypothesis the interior, 

exterior, and chasma formed as genetically related features, 

each evolving through time, and related to the evolution of a 

plume-lithosphere interaction. The plume’s interaction with the 

lithosphere (or lack thereof) would account for strong rheological variation at a crustal-scale across the region as shown in 

figure 11 (on map sheet). The exterior fracture zone abuts the 

chasma as a result of rheological factors, with fracture zone and 

chasma evolving synchronously. The trough (chasma) represents a sort of nascent, but failed, subduction zone with the 

exterior region lying within an upper plate position relative to 

the interior lower plate. This proposal differs from the subduction hypothesis, which calls for northwest-directed subduction 

of the lowland under the Artemis interior (which would lie 

in an upper plate position). The hypothesis differs from the 
18

metamorphic core complex hypothesis in that interior extension 

results in the formation of new crust, rather than the exposure 

of mid- to deep-level crust within Artemis’ interior. The hybrid 

hypothesis is similar to each of these in that it calls for shortening of the surface crust within Artemis Chasma—which moves 

outward with time. And it calls for northwest-southeast directed 

extension across the interior of Artemis. With regard to the 

third hypothesis, it is possible that the mantle plume formed 

in response to a large bolide (>20–30 km diameter) impact on 

thin lithosphere, with the interior region representing a region 

of greatly thinned and modified crust as a result of the impact 

event and subsequent massive partial melting in the mantle (for 

example, Jones and others, 2005; Elkins-Tanton and Hager, 

2005). In this case the plume called for in time steps 1–6, could 

have formed in response to large bolide impact with a thin 

global lithosphere (for example, Wichman and Schultz, 1995), 

rather than the result of strictly endogenic processes. At this 

point we cannot favor plume formation via one mechanism over 

another (for example, endogenic rise of a deep mantle plume, or 

in response to an exogenic catastrophe); however, a thin lithosphere would be required in either case.

Several relations fall out of the hybrid hypothesis. (1) The 

exterior fracture zone and the interior region of Artemis formed 

broadly synchronously, although they developed quite different characteristics due to both different rheological characters 

and different spatial locations within the postulated tectonic 

framework. (2) Artemis Chasma could have moved outward 

through time; as such, the chasma topography could modify 

features previously formed at the surface. The chasma might 

move in a fashion similar to the ring-trough observed in experiments (for example, Griffiths and Campbell, 1991), and the 

subsurface delamination front developed in numerical models 

(Smrekar and Stofan, 1997). Artemis’ structures, volcanic units, 

and topography each record complex (but predictable) temporal 

relationships with one another. Within the context of the hybrid 

hypothesis, structures (and surface units) could both predate 

and postdate the formation of the trough, which would actually 

change spatial location (and size) through time. (3) Artemis 

Chasma might be somewhat akin to terrestrial subduction zones 

in that it marks a zone of shortening, however, the proposed 

vergence would vary from the published subduction hypothesis 

in that the exterior region would form the upper plate whereas 

the interior region, marked by newer crust and penetrative 

fabric, would form the lower plate. (4) Artemis might represent 

Venus’ failed attempt to develop plate-tectonic processes. (5) 

Reasons that Artemis may have failed to develop into a divergent plate boundary (along the nascent spreading center) and 

subduction zone (Artemis Chasma) could be a function of the 

lithospheric strength, perhaps tied specifically to the postulated 

rheological profile of dry versus wet lithosphere (for example, 

Regenauer-Lieb and others, 2001). Venus’ crust is currently 

dry and strong (Mackwell and others, 1998), which might have 

rendered its lithosphere too strong for the nascent spreading 

center to propagate outside the region of lithosphere modified 

above the plume head. In addition, water might play a critical 

role in subduction initiation (and hence plate tectonics) due to 

the mode of failure of silicate lithospheres (Regenauer-Lieb 

and others, 2001). Convergence across a dry lithosphere (such 

as Venus) results in focused brittle failure of the upper lithosphere, while the bottom of the lithosphere deforms in a diffuse 

fashion; the bottom of the lithosphere delaminates and recycles 

to the mantle—leaving the surface crust relatively intact. In 

contrast, wet lithosphere (as is the case on Earth) fails across 

its entire mechanical thickness, resulting in the formation of a 

narrow fault-like zone through the lithosphere; in this case the 

entire lithospheric thickness recycles to the mantle (RegenauerLieb and others, 2001). Once initiated, subduction of the entire 

lithosphere could provide a means of recycling water to the 

mantle, triggering a double feedback mechanism (thermoelastic 

and thermal-rheological) to promote plate tectonics (RegenauerLieb and others, 2001). In the case of a dry lithosphere (Venus), 

any surface water or hydrated surface crust would remain at the 

surface, and water would not be returned to the mantle.

At this point in the study of Artemis, it seems to us most 

likely that Artemis resulted, at some first-order level, from the 

interaction of a deep mantle plume with relatively thin lithosphere, along the lines outlined above. Clearly there is room 

for much discussion and debate about the formation of Artemis. There is also room for numerous additional studies of this 

wonderfully puzzling circular feature. High-resolution geologic 

mapping of existing datasets, as well as new even higher resolution datasets will shed much new information on the detailed 

character of Artemis’ surface features, as well as on the temporal and rheological evolution of the various surfaces. Such 

studies will provide critical clues to Artemis’ formation and 

evolution and the operative dynamic processes responsible for 

the formation of this apparently unique tectonic feature.

The current 1:5 million-scale geologic map does not provide enough detail to determine how Artemis formed with significant confidence, and it thus remains an unexplained feature. 

Did the unique morphology of Artemis result from serendipitous 

formation from the spatial and temporal overlap of genetically 

unrelated events or processes? Does Artemis’ unique character call for a unique process, or a relatively common process 

active under unique (rheological) conditions? Or does Artemis 

record a relatively unique (rheological) time in the evolution 

of this dynamic planet? Settling on a single hypothesis for the 

formation of Artemis seems premature at this time; it is entirely 

possible that the most likely mode of Artemis’ formation has not 

yet been put forward.

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Figure 3. Examples of primary structures in V–48 (center coordinates): A, channel (124°00' E., 25°07' S.); 

B, shields (137°39' E., 32°43' S.); C, pit chains (127°52' E., 26°09' S.); D, impact crater (127°02'E, 36°04' S.); 

E, flow fronts (133°11' E., 37°31' S.); and F, shallow troughs (126°34' E., 31°46' S.). All images are left-look 

inverted SAR; white rectangles in image F are data gaps. North toward top of image, except where noted.

B 0 30 KILOMETERS C 0 20 KILOMETERS

A 0 20 KILOMETERS D 0 50 KILOMETERS E 0 50 KILOMETERS

F N 0 50 KILOMETERS
24

Figure 4. Examples of secondary structures in V–48 (center coordinates). A, fractures (136°16' E., 36°59' 

S.); B, lineaments (139°07' E., 40°58' S.); C, folds (130°28' E., 34°36' S.); D, ridges (134°32' E., 28°18' S.), 

black arrows point to large ridges, white arrows indicate small ridges; E, penetrative fabric (130°20' E., 

30°44' S.), white line with arrows parallels fabric trend. All images are inverted left-look SAR.

0

0

A 20 KILOMETERS

C 20 KILOMETERS

B 0 20 KILOMETERS

D 0 50 KILOMETERS E 0 20 KILOMETERS
25

A 0 20 KILOMETERS B 0 20 KILOMETERS

Figure 5. Representative inverted left-look SAR images of Artemis’ penetrative fabric to indicate the range 

of character; white lines parallel fabric trend; (center coordinates): A, (127°41' E., 30°38' S.); B, (136°29' E., 

27°59' S.); C, (130°16' E., 30°42' S.); D, (129°39' E., 32°38' S.). Fabric appears fracture-like in A and D, and 

ridge-like in B and C (more common). Note the delicate interfingering of radar-smooth (light in color, 

low-backscatter) material with penetrative fabric (A and D) indicating: (1) the topographic expression 

associated with the penetrative fabric, (2) that the cover material forms a thin layer emplaced after fabric 

development, which (3) appears to have been low viscosity during emplacement; local cutting of the cover 

material by penetrative fabric trends likely represents reactivation of the buried penetrative fabric, and, in 

turn suggests that penetrative fabric structures were locally reactivated following the emplacement of the 

cover material.

C 0 30 KILOMETERS D 0 10 KILOMETERS
26

Figure 6. High-resolution shaded-relief images of terrestrial spreading centers from the Ridge Multibeam 

Synthesis (RMBS) Data Portal website (http://www.marine-geo.org/rmbs/). A, A part of the northern 

Mid-Atlantic Ridge centered at 29°30’ N. and 43° W.; the ridge trough trends from the upper right to lower 

left, with a left-lateral transform fault to the right of the ridge trough and a fracture zone to the upper left. B, 

A part of the northern Mid-Atlantic Ridge centered at 22°30’ N. and 45°30’ W.; the ridge trough trends NNE 

along the right side of the image. C, A part of the southern East Pacific Rise centered at 8°50’ S. and 108°40’ 

W.; NNE-trending ridge lies along the right side of the image; a complex left-lateral transform zone trends 

WNW in the lower part of the image; volcanic constructs occur individually and as short chains. In each case 

high-resolution data exists only near the ridge axis. Ridge axes indicated by dashed black lines; volcanic 

constructs marked by black arrows; relative displacement across ridges and transform faults shown by white 

arrows. Compare with penetrative fabric in figures 5 and 6. For more images see website listed above.

A 0 50 KILOMETERS

C 0 50 KILOMETERS

B 0 50 KILOMETERS
27

Table 1. Crater data for the Artemis Chasma quadrangle (V–48), Venus.

Name

Latitude

(° S)

Longitude

(° E)

Diameter

(km)

Unit

location

Ejecta

blanket

Impact

halo

Central

peak Rim

Interior

flooding M

Behn –32.4 142.0 25.8 fcAb Y N Y Y Y FBonnevie –36.1 127.0 87.6 tAa/fsAa Y Y Y Y Y Ivne (crater?) –27.0 132.8 9.0 tAb Y N N Y ? F

Jalgurik –42.4 125.1 7.7 fu Y N N Y N 

Janyl –28.0 138.8 5.4 tAa/fcAb Y N N Y N 

O’Connor –26.0 143.9 29.6 fu Y N Y Y Y 

Veronica –38.1 124.6 17.9 tAa Y N N Y Y FYomile –27.3 138.7 13.6 tAa Y Y N Y N? 

Unnamed –41.56 142.9 4.5 tAa Y N N Y N? *Crater density at a crater’s location. Value is the density of craters represents the number of craters (including the specified crater) within a 1,000-km radius circle, and normalized to give the number of craters per 1 x 106 km2
28

Table 2. Penetrative fabric wavelengths measured across

Artemis Chasma quadrangle, Venus.

Latitude Longitude Transect

(km)

Wavelenth

(m)

Orientation

–25.58 134.74 9.4 303 NE

–30.35 129.70 8.6 345 NE

–31.63 133.75 20.9 380 NE

–34.34 132.10 18.9 402 NE

–30.95 137.09 18.9 449 SE

–29.59 130.09 30.8 453 NE

–32.46 130.61 12.0 462 NE

–30.80 134.42 35.6 468 N

–27.57 136.65 21.7 494 NE

–32.15 130.93 27.2 495 NE

–35.38 133.62 32.1 526 NE

–30.71 130.64 45.8 572 NE

–30.78 129.99 58.2 588 NE

–36.21 132.48 44.3 607 NE

–28.12 136.09 85.9 641 NE

–35.83 132.13 48.0 657 NNE

–30.22 128.17 91.1 729 NE

–28.68 130.33 75.3 731 NE

–40.69 130.57 44.7 952 NE

Average 540±157 (1σ)

excluding

outlier

517±125 (1σ)