Prepared for the National Aeronautics and Space Administration

Geologic Map of the Helen Planitia Quadrangle (V–52), Venus

By Iván López and Vicki L. Hansen

Pamphlet to accompany

Scientific Investigations Map 3026

2008

U.S. Department of the Interior

U.S. Geological Survey

i

Contents

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

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

Helen Planitia Quadrangle............................................................................................................................1

Introduction............................................................................................................................................1

Data and Methodology ........................................................................................................................2

Map Units...............................................................................................................................................4

Widespread Materials ................................................................................................................4

Sequence of Materials in the Parga Chasmata Region ........................................................6

Sequence of Materials in the Helen Planitia Region.............................................................9

Tectonic Structures ............................................................................................................................10

Structures Associated with Tessera Terrain.........................................................................11

Regional Deformation ...............................................................................................................11

Fold Belts and Wrinkle Ridges........................................................................................11

Regional Fracture Suites .................................................................................................12

Local Structural Suites Related to Tectonomagmatic Features ........................................12

Tectonomagmatic Structures Related to Parga Chasmata .......................................13

Tectonomagmatic Structures not Related to Parga Chasmata.................................13

Radial Fracture Systems..................................................................................................14

Localized Deformation ..............................................................................................................15

Impact Features ..................................................................................................................................15

Surficial Deposits and Aeolian Marks.............................................................................................16

Geologic History and Resurfacing Style .........................................................................................16

Parga Chasmata Region ...........................................................................................................16

Helen Planitia Region................................................................................................................17

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

Table 1. Crater data for the Helen Planitia quadrangle (V–52), Venus...............................................22

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.

Helen Planitia Quadrangle

Introduction

The Helen Planitia quadrangle (V–52), located in the 

southern hemisphere of Venus between lat 25° S. and 50° S. and 

between long 240° E. and 270° E. (fig. 1A), covers approximately 8,000,000 km2 (fig 1, map sheet). Regionally, the map 

area is located at the southern limit of an area of enhanced 

tectonomagmatic activity and extensional deformation, marked 

by a triangle that has highland apexes at Beta, Atla, and Themis 

Regiones (BAT anomaly) and is connected by the large extensional belts of Devana, Hecate, and Parga Chasmata (Crumpler 

and others, 1993). The BAT anomaly covers approximately 20 

percent of the Venusian surface (Crumpler and others, 1997).

Helen Planitia quadrangle (V–52), bounded to the east 

by Themis Regio, a corona-dominated rise, and to the west by 

Wawalag Planitia, represents an area of topographic transition 

between the highlands of Phoebe and Themis Regiones and 

the lowlands of Helen Planitia. Within the map area, another 

topographic transition occurs between the mesoland of eastern Parga Chasmata that dominates the north and the lowland 

of northern Helen Planitia in the south (fig. 1B, map sheet). 

Parga Chasmata, a northwest-trending 10,000-km-long fracture 

and trough system, connects Atla and Themis Regiones (for 

example, Stofan and others, 1992; Stofan and others, 1997). 

The Parga Chasmata system consists of 14 branching segments; 

two occur in the map area, associated with coronae and other 

tectonomagmatic structures. The southern map area hosts the 

volcanic plains of northern Helen Planitia, where the most 

prominent topographic features describe a network of broad 

warps and elevated areas that form the boundary with the rest of 

the map area (fig. 2A, map sheet). The central map area marks 

the transition between these two different geologic and topographic regions.
2

The location between the Parga Chasmata mesoland and 

the Helen Planitia lowland and the high concentration of features indicative of extensional tectonic processes and volcanic 

activity make Helen Planitia quadrangle an excellent region to 

test differing, and debated, models of Venus geological evolution (Basilevsky and others, 1997; Phillips and Hansen, 1998; 

Guest and Stofan, 1999).

Data and Methodology

The U.S. Geological Survey digital FMAP SAR imagery 

(75 m/pixel) and derived products form the map base. Adobe 

IllustratorTM 8, 9, and 10 were the primary mapping tools; 

image processing was accomplished using Adobe PhotoshopTM 

5. Ancillary data included Global Topographic Data Record 3 

(GTDR 3), as well as Fresnel reflectivity at wavelengths of 12.8 

cm, average meter-scale slope, and derived 12.8 cm emissivity 

data (GRDR, GSDR, and GEDR, respectively; figs. 2B, 2C and 

2D, map sheet). GTDR data were combined with SAR images 

to produce synthetic stereo anaglyphs (Kirk and others, 1992) 

using NIH-Image macros developed by D.A. Young (University 

of Minnesota Duluth). These images played a critical role in 

elucidating relations between geology and topography and, in 

particular, interaction of flows, primary and secondary structures, and topography.

The interpretation of features in Magellan S-band SAR 

imagery is key to developing a geologic history for the Helen 

Planitia quadrangle (V–52). The subject of radar image interpretation is explored in depth in Ford and others (1993).

Kilometer-scale slopes are resolved in radar images as 

systematic variations in radar backscatter; radar-facing slopes 

scatter more radiation than slopes facing away from radar. 

Converting the range data that SAR provides into a plan-view 

projection introduces geometric distortions that provide topographic information. Because SAR is sensitive to slight variations in slope along the spacecraft look direction, topography is 

highlighted, though difficult to constraint quantitatively.

SAR data is also sensitive to surface roughness at the scale 

of radar wavelength (12.8 cm); rough surfaces appear bright 

and smooth surfaces appear dark. These variations are textural, 

and a direct correlation with material type cannot be assumed. 

Variation in pahoehoe to aa surfaces within individual terrestrial 

lava flows presents an analogous problem.

A key assumption in the interpretation of the Venus rock 

record is that liquid water has not played a role in shaping the 

surface. Primary evidence for this assumption is rooted in the 

extremely dehydrated state of the lower atmosphere (Donahue and others, 1997) and lithosphere (Mackwell and others, 

1998) and the present surface temperature of approximately 

460°C (Crisp and Titov, 1997). The lack of surface water and 

its erosional processes argues against widespread occurrence 

of exposure of deep crustal metamorphic rocks (no erosion) or 

sedimentary rocks (no deposition). Exceptions relate to impact 

craters. Impact crater rim material presumably underwent shock 

metamorphism, and fine ejecta was presumably redistributed 

and deposited by wind, comprising limited aeolian sedimentary 

facies (Greeley and others, 1992). Bulk chemical analyses of 

the surface resulting from the Venera Mission may represent 

subsurface material excavated by impact craters and redistributed across the surface, providing subsurface compositional data 

(Basilevsky and Head, 2004). Although deep crustal metamorphic facies might not be typically exposed on the Venusian 

surface, current surface conditions lie within the metamorphic 

hornfels facies (~475°C, 92 bars). Furthermore, surface temperature could have been higher in the past (Bullock and Grinspoon, 2001; Phillips and others, 2001). Thus, material exposed 

at the surface is metamorphic by terrestrial standards, although 

metamorphism presumably occurred in situ.

Geologic map construction is a critical first step in unraveling geologic history, which provides critical relations for 

understanding the range of processes that contributed to planet 

evolution. Because geologic mapping is an interpretation that 

results in a database to be used for further interpretation of 

geological processes (Butler and Bell, 1988; Maltman, 1990), 

mapping methodology must not predetermine the geologic map. 

Secondary structures (strain) should be clearly differentiated 

from geologic units (materials), because materials and structures record different time slices in a surface evolution and they 

also reflect different aspects of past processes. Our methodology 

for the definition of geological units and structural fabrics builds 

on standard geological analysis detailed in Wilhelms (1990) 

and Tanaka and others (1994), with cautions of Hansen (2000), 

Zimbelman (2001), Skinner and Tanaka (2003), and McGill 

and Campbell (2004). Map units represent material emplaced 

within an increment of geological history, to which standard 

stratigraphic methods have some limited application. However, 

some units are most likely composite; they might not be stratigraphically coherent over their entire represented area, and (or) 

they may have been emplaced time transgressively in relation 

to other units and (or) the formation of secondary structures. 

To further complicate both temporal constraints and history, 

evidence for reactivation of secondary structures is common.

The term “terrain” is used to describe a texturally defined 

region. Characteristic texture could imply a shared history, 

similar to terrestrial gneissic terrain that is used to describe a 

particular shared history, such as a tectonothermal event responsible for melding possibly previously unrelated rock units (any 

combination of igneous, metamorphic, and sedimentary rocks) 

into a new package. As with gneissic terrain, we do not mean 

to infer any unique history prior to the event(s) that melded 

potentially separate units into a composite textural terrain. 

Events prior to terrain formation would be unconstrained unless 

specifically noted.

Criteria for distinguishing discrete geological (as opposed 

to radar) units include the presence of sharp, continuous contacts; truncation of, or interaction with, secondary structures 

and topography; and primary structures (for example, flow 

channels or edifice topography) that allow a reasonable geological interpretation and hint at a three dimensional geometry. 

Some mapped units fail to fit these constraints, limiting their 

use in constructing stratigraphic sequences. Composite units, in 

particular, cannot provide robust temporal constraints, even of a 

relative nature.

Both primary (depositional) and secondary (tectonic) 

structures are identified in the Magellan SAR data. Primary 
3

structures include channels, shields, lobate flow fronts, and 

impact crater haloes and rims. Channels represent sinuous 

low-backscatter features hundreds of kilometers long and a few 

kilometers wide; locally they lack apparent topographic relief 

(Baker and others, 1992). Because Venus is dry, low-viscosity 

lava is perhaps the most likely fluid to form these features. 

Shields are small (diameter generally 2–15 km, rarely 20 km), 

quasi-circular to circular, low- to high-backscatter features with 

or without topographic expression and with or without a central 

pit. The size of individual shields can be difficult to constrain 

because their bases are commonly poorly defined, and deposits 

can blend smoothly into a layer of coalesced deposits. Shields 

likely represent the manifestation of local volcanic flows (Guest 

and others, 1992; Crumpler and others, 1997; Addington, 2001). 

Shields and associated deposits form a composite unit that 

cannot be robustly treated as a timeline or marker unit (Hansen, 

2005). Pits, sharply defined depressions, or pit chains, which 

are linear arrays of pits, likely represent regions marked by 

subsurface excavation; they may mark the surface expression of 

dilatational faults or dikes (Okubo and Martel, 1998; Bleamaster and Hansen, 2001; Ferrill and others, 2004; Schultz and 

others, 2004).

Most radar lineaments represent secondary structures. 

Extremely fine, sharply defined, continuous, radar-bright lineaments, typically occurring in the lowland, are interpreted as 

fractures (for example, Banerdt and Sammis, 1992; Banerdt 

and others, 1997; DeShon and others, 2000). If the fracture is 

associated with pits, the fracture probably represents the surface 

expression of a subsurface dike or a dilational fault. Paired dark 

and light lineaments that describe linear troughs more than a 

few kilometers across are interpreted as graben.

In SAR imagery, the opposite of a groove (linear trough) is 

a ridge (positive linear topography marked by light and dark lineaments a few kilometers across with the light lineament closest 

to illumination direction). On Venus, ridges either have parallel 

edges with moderate sinuosity at the 10-km scale and have an 

across-strike gradation in backscatter (a form we specifically 

call ridges) or they have a more erratic plan view that contains 

frequent right-angle interruptions at the 10-km scale and variations in across-strike width (wrinkle ridges). Positive linear 

topographic features, hundreds of kilometers across and as 

much as thousands of kilometers long, are too subtle to appear 

prominently in SAR data, but they are visible in topographic 

data and are called warps.

Parallel bright and dark lineaments that defined alternating 

parallel ridges and troughs and have typical wavelengths of 2–5 

km are called ribbon fabric (Hansen and Willis, 1998). Ribbon 

fabrics are commonly associated with fold structures that trend 

at a high angle to the ribbon lineaments and with graben that 

commonly parallel ribbon trends but can be differentiated from 

the ribbons on the basis of smaller length:width ratios. Collectively referred to as ribbon terrain, this distinctive tectonic fabric 

commonly marks tessera terrain. The composite fabric may 

reflect a progressive increase in the depth to the rheological brittle-ductile transition (BDT) with time and fabric development 

(Hansen and Willis, 1998; Hansen and others 1999; Hansen, 

2006). For an alternative view of ribbon-terrain formation see 

Gilmore and others (1998).

Interpretation of geologic structures, material units, 

temporal relations among the structures and material units, 

and ultimately the geologic history using Magellan SAR data 

requires the same careful consideration that should be employed 

with any remote sensing data set (Wilhelms, 1990; Tanaka and 

others, 1994; Hansen, 2000). Radar data permit mapping of 

surface morphologies and structures (Daily and Stewart, 1979; 

Ford and others, 1989, 1993). Geologic unit boundaries can 

be defined by radar backscatter intensity and (or) by crosscutting relations among materials or by topographic features. 

Gradational contacts can be difficult to trace. The termination of tectonic structures could represent the spatial limit of a 

deformation front or the limit of younger onlapping material; 

whereas, structural facies boundaries typically represent a strain 

discontinuity related to a tectonic process (perhaps marked by 

a decrease in structural element density). Young burial is more 

likely if the truncation of structural elements occurs at a high 

angle to the structural trend. These determinations can help 

establish local sequences of events, but they are not applied to a 

composite unit. The degree of uncertainty of stratigraphic position is noted in unit descriptions.

Another problem in establishing regional sequences results 

from the absence of reliable time markers or marker units. Any 

marker unit must (1) be unique, (2) have formed quickly, and 

(3) cover a large region (Compton, 1985, p. 85). Terrestrial 

examples of a marker unit include air-fall tuff, sedimentary 

layers marked by unique fossils or mineral assemblages, or 

geomagnetic polarity. Terrestrial studies also typically include 

radiometric ages that constrain absolute time and enhance 

confidence in unit correlations. None of these methods are available for Venus. Correlation of map units in planetary maps is 

typically based on surface impact-crater statistics. Although this 

method has some utility for planetary bodies with old surfaces 

and high crater densities (for example, Moon and Mars), many 

problems arise on Venus because of crater-population characteristics. Fundamentally, Venus impact craters cannot place 

constraints on the age of surface units that cover the small areas 

within the Helen Planitia quadrangle (Hauck and others, 1998; 

Campbell, 1999).

As a result of these limitations, we establish local geological histories following geologic and geographic criteria, where 

units are in contact and (or) interact with the different structural 

suites. We define two regions in the map area (Parga Chasmata 

region and Helen Planitia region, fig. 1B) that are each separated into three sectors: western, central, and eastern. In these 

regions and subregions, we confidently define local temporal 

relations between the different units. These temporal constraints 

are only locally applicable and cannot be robustly extended 

across the map area, much less to adjacent regions. Geologic 

mapping is ultimately built on consistency arguments; observations, assumptions, and interpretations should always be open 

to question (Gilbert, 1886). Unfortunately, at this point in Venus 

geologic investigation, there is no robust means available to correlate regional surface units without operative assumptions that 

would deem any correlation circular. If we did not divide the 

map area into subregions, we believe no robust unit correlations 

could be made. Thus, the division of the map area into subregions allows us to propose limited unit correlation. Temporal 
4

relations between some units can be established in more than 

one of these regions and subregions, especially in the areas of 

contact/transition between them, although the unit is described 

in the area where the unit type locality is defined. Widespread 

materials that form undivided units are located across the map 

area.

Map Units

The topography and geology of the Helen Planitia quadrangle (V–52) is divisible into two major regions, the Parga 

Chasmata region (PCR) in the north and the Helen Planitia 

lowland/region (HPR) in the south. The extensional belt of 

Parga Chasmata dominates the northern map area and influences the type and distribution of the geologic materials and 

the trend and style of deformation. Many large tectonomagmatic features and their associated materials are geographically 

linked with Parga Chasmata. The southern map area displays 

different deformation trends and a multitude of large tectonomagmatic features, whose distribution and related materials differ from those of the chasmata. Widespread materials 

(undivided units) that occur throughout the map area are 

described separately.

Widespread Materials

Tessera terrain, undivided (unit tu) is exposed in a group of 

inliers and small outcrops distributed across the map area and 

contains intersecting suites of tectonic lineaments, high surface 

roughness, and positive relief relative to surrounding deposits. 

Individual inliers display different structural assemblages and 

deformational histories, and the relative timing among and 

between inliers is unconstrained.

In the western PCR (lat 26.0° S., long 241° E.), unit tu

displays northeast-trending, ribbon-like structures. Although the 

ribbon structures are highly degraded and locally embayed by 

surrounding volcanic material, the constant width, spacing, and 

high length:width ratio that characterizes ribbon fabric (Hansen 

and Willis, 1998) is preserved. A suite of troughs (interpreted 

as graben) and northwest-trending fractures cut the ribbon 

fabric and the surrounding materials. East-northeast-trending 

lineaments may represent fold crests, but their near-parallel 

orientation relative to radar illumination inhibits robust geometric characterization. The relative timing of ribbon fabric and 

the east-northeast lineaments is unconstrained. Unit tu in the 

western PCR also includes several small kipukas of extremely 

deformed material. Deformation fabrics mask the character of 

the predeformational terrain and even inhibit clear identification of the character of the tectonic structures. Individual linear 

structures are about 1 km wide and are arranged in subparallel 

sets that generally trend northwest, although some structures 

in isolated outcrops trend north-northwest. Similar materials 

have been named densely lineated plains material (for example 

unit pdf, Ivanov and Head, 2001); these workers state that their 

coresponding unit always occurs stratigraphically above tessera 

terrain (for example, Basilevsky and Head, 1995), and they 

assume that the precursor material was volcanic plains material 

composed of mafic lavas (Basilevsky and Head, 2000). Because 

penetrative deformation, or deformation fabric marked by very 

closely spaced lineaments relative to the scale of observation, 

precludes observation of precursory material and stratigraphic 

relations can only be locally extrapolated, we are not comfortable extending these previous assumptions to units in the map 

area.

A group of inliers of unit tu geographically related to Giltine Tesserae dominates the central and western part of the PCR. 

Some inliers display small isolated shields emplaced after tessera terrain deformation and are likely unrelated to the original 

formation of the unit materials. The degree of preservation of 

the different structures due to embayment by younger material 

varies among inliers. Crosscutting structures show evidence 

of tectonic reactivation and topographic modification, making 

the interpretation of some of the original characteristics of 

unit tu difficult. The inlier located at lat 39° S., long 248.5° E. 

preserves the best example of ribbon-like structures in the map 

area. These ribbon troughs, classified as tensile-fracture ribbons (Hansen and Willis, 1998), display a consistent northwest 

trend, an approximately 2 km width, and a regular spacing of 

approximately 2.5 km. Northeast-trending graben cut ribbons as 

illustrated in several inliers of unit tu; the graben are embayed 

by younger volcanic materials. Other tessera-terrain inliers display structures that could also be ribbons, but modification by 

later local tectonic structures or volcanic embayment frustrates 

efforts to quantitatively characterize these structures.

In the western HPR, unit tu comprises several large 

inliers and many small kipukas that crop out in Sopdet Tesserae, Tsovinar Dorsa, and Kastiatsi Dorsa and within Hlineu 

Corona—elevated areas that define long northwest- and 

northeast-trending topographic arches bordering Helen Planitia. 

In these locations, unit tu displays different suites of secondary structures that mostly trend parallel to the northwest trend 

of the inliers: closed-spaced ridge and fracture tectonic fabric 

and folds located preferentially along the margins of the large 

inliers. Another penetratively developed, subtle, northeast-trending lineament suite cuts some inliers (for example, lat 44.5° S., 

long 251.5° E.); SAR image resolution does not allow structural 

characterization. Late arcuate and north-northeast-trending 

graben cut northwest-trending ridge and fracture fabrics; graben 

are embayed by younger volcanic materials that surround unit 

tu.

In the central and eastern parts of HPR, along the border 

with the Godiva quadrangle (V–60), unit tu forms a group of 

large inliers and small kipukas named Norna Tesserae. Intratessera volcanic material occurs within unit tu in basins in the 

southernmost inlier. The inliers that form unit tu show differences in their deformation fabrics. In the southernmost limit 

of eastern HPR, an inlier located at lat 50° S., long 263° E. 

displays orthogonal suites of secondary structures: northeasttrending, fine-scale lineaments; northwest-trending, ribbonlike fabric with troughs spaced <2 km; northwest-trending, 

graben-like structures; and northeast-trending folds that have 

a wavelength of about 10 km. Temporal relations between 

ribbons and folds is unconstrained due to image resolution, 

although the longer wavelength of the folds relative to ribbons 

is consistent with younger fold formation (Hansen and Willis, 
5

1998; Ghent and Hansen, 1999; Brown and Grimm, 1997). A 

north-northeast-trending late graben that has a width of ~15 

km postdates ribbons and folds. The inliers located at lat 48.5° 

S., long 264.5° E. and lat 47.5° S., long 268° E. also display 

high radar backscatter and a blocky texture; the predominant 

deformation fabric is composed of a suite of northwesttrending fractures and ridges that lack the periodicity of the 

ribbon fabric. In some small kipukas, two intersecting sets of 

northwest- and northeast-trending orthogonal graben postdate 

the northwest-striking structures. Late flows and shield-related 

materials embay the graben. The inlier located at lat 48.5° S., 

long 264.5° E. is cut by Sinnan Vallis. Other outcrops of unit 

tu located around and in Enekeler Corona display a fine-scale, 

north-northwest-trending, tectonic fabric below image resolution. Small outcrops located in Enekeler Corona display 

east-trending graben that postdate the north-northwest-trending 

lineaments.

In eastern HPR a large inlier and a group of small kipukas 

of unit tu form Ustrecha Tessera. In local sections of the inlier, 

north-trending lineaments form a penetrative fabric with an 

indeterminate morphology and origin. A radial fracture suite 

also cuts this inlier, but temporal relations between the northtrending structures and the radial fracture suite are unclear due 

to embayment relations and possible structure reactivation. It is 

unknown if the radial fracture set is related to emplacement of 

unit tu or if it comprises a local fracture set that overprints unit 

tu. A set of east-northeast-trending graben postdate structures 

of unit tu and the surrounding fractures. Many shields and 

associated materials occur in spatial association with unit tu, 

but these primary structures are interpreted to postdate unit tu

formation.

Heterogeneous material, undivided (unit hu), is composed 

of undifferentiated materials distributed across the map area. 

Unit hu represents a composite unit without stratigraphic significance that combines materials of different origin, and probable different age, that cannot be confidently differentiated with 

available data. We use the unit name, heterogeneous materials, 

undivided, because unit hu includes a range of volcanic styles, 

including different corona-, volcano-, and shield-related materials of low to intermediate-high backscatter and homogeneous 

to mottled texture. Many volcanic and large tectonomagmatic 

features are located on unit hu. Large flow units are difficult 

to delimit, perhaps due to radar homogenization of flows with 

time (Arvidson and others, 1992). Primary structures are, 

however, identified in unit hu, providing evidence of the multiple genetic processes and distinct source locations implied in 

material emplacement. Individual lobate fronts are mapped in 

unit hu; some flows can be traced to a definite source, such as 

Chuginadak Mons and Darclée Patera, but the origin of others 

is uncertain. Helen Planitia contains a high concentration of 

channels (Baker and others, 1997); most channels in the map 

area occur in unit hu. One channel, Sinnan Vallis, extends for 

450 km eastward from southeastern V–52 into V–53. Other 

channels, some partially buried, show no recognized source, 

supporting the composite nature of unit hu. In addition, hundreds to thousands of small shields are scattered across unit hu. 

The point-sourced character of shield deposits makes it difficult 

to establish shield-related map units (Guest and others, 1992; 

Hansen, 2005). SAR relations indicate multiple individual 

shields instead of accumulations associated with individual 

flows. Shields and shield clusters are especially numerous in 

southern and southeastern Helen Planitia. Many of the shield 

fields in unit hu are spatially associated with large tectonomagmatic features such as coronae (Tangba Corona). Other isolated 

shields and shield-related material are related to the regional 

fractures that both postdate shields and are overprinted by 

shields, indicating time transgressive processes. Unit hu also 

displays medium-size volcanoes and steep-sided domes, but no 

distinct flow materials are mapped in relation to these volcanic 

features. The relation of unit hu with the surrounding material 

is difficult to establish due to the composite and undifferentiated nature of unit hu. Unit hu locally postdates local tessera 

terrain (unit tu). Unit hu appears to correlate with Chapman’s 

flow material (1999, unit f2), which underlies corona-related 

materials. Unit hu also appears to correlate with the regional 

plains material of Johnson and others (1999, unit pr, V–59). 

The character and regional significance of each of these 

composite units is defined and interpreted within the context 

of their local geologic settings, and direct correlation between 

these units is not assumed. Unit hu does not correlate with unit 

plains with wrinkle ridges of Basilevsky and Head (1998, 2000, 

units pwr1 or pwr2), because unit hu shows a wide range of 

contributing sources and was emplaced time-transgressively.

Different impact structures and associated materials are 

distributed across the map area (both PCR and HPR). Impactrelated primary structures (rim and central peak) are mapped 

as primary structures for each crater; whereas, crater-ejecta 

material and crater-floor material are mapped as crater material (unit c). Because each impact crater formed individually 

in space and time, we only consider the stratigraphic position 

of individual impact craters. Implications for material ages are 

not statistically robust (Campbell, 1999) and, therefore, are not 

considered.

Two craters, Rose and Wollstonecraft, show extensive 

distal outflow deposits mapped as crater flow material (unit 

cf). Rose displays a north-flowing, radar-rough, outflow material that locally interacts with preexisting northeast-trending 

fractures that channeled the flow. Schultz (1992) classified this 

crater as result of an oblique impact from the south. Apart from 

the direction of the outflows, Schultz (1992) used the blast 

pattern on the surrounding structures and established a change 

from turbulent flow near the crater to laminar and channeled 

flow through the fractures to the north. The other crater with 

an associated outflow, Wollstonecraft, displays butterfly-wing 

ejecta morphology, proposed as indicative of a low-impact angle 

(Schultz, 1992). The slightly elongated shape of the crater along 

an east-trending axis is also consistent with oblique impact. 

The west-flowing, extensive outflow of Wollstonecraft, which 

postdates unit hu, is formed by an intermediate-backscatter flow 

with a radar-rough inner section. The outflow displays bright 

edges along its perimeter, which indicates a rougher texture. 

The outflow covers portions of the radial fracture system (lat 

40.5° S., long 258° E.), although the fractures are locally visible 

through the outflow; these relations indicate that the outflow 

forms a thin layer. The outflow is also locally ponded along 

north-trending wrinkle ridges.
6

Sequence of Materials in the Parga Chasmata 

Region

The Parga Chasmata region (PCR) hosts several major 

units including Parga Chasmata materials, corona-related flows, 

shield-dominated units, and local volcano-related flows and 

heterogeneous materials (see Widespread Materials). Structurally, the PCR is dominated by the regional-scale, northwesttrending Parga Chasmata and numerous radial and concentric 

local structural suites associated with coronae emplaced within 

and near Parga Chasmata. Flows are locally cut by coronae 

and chasmata-related fractures; yet these flows also locally 

overprint the fractures, suggesting that tectonic and volcanic 

activity overlapped in space and time and may be genetically 

related. Each unit is discussed by temporal formation from old 

to young, development from more extensive to more localized, 

and location spatially from east to west. We end the discussion with two units that display gradational characteristics with 

adjacent units.

Parga Chasmata materials a, b, and c (units Pa, Pb, Pc) 

are basal units commonly associated with local highs (although 

not all basal units correlate with local highs and not all local 

highs correlate with the exposure of these basal units). These 

basal materials also underlie the corona- and volcano-related 

materials in the PCR. Emplacement mechanisms for the Parga 

Chasmata materials are not well constrained, but the presence of 

small shields and the flow-like, embayment relations with older 

materials suggest that polygenetic volcanic processes may have 

been important. We separate these three units in terms of backscatter, topography, and tectonic fabric, a practice that should 

be avoided if possible (Hansen, 2000). In this case, the different geographic locations, radar characteristics, and presence of 

primary structures and the different character and wavelength 

of the deformational structures allow us to divide Parga Chasmata materials into these three units; note, however, we have no 

evidence for temporal relations among the units.

Parga Chasmata material a (unit Pa) displays homogeneous 

texture and intermediate to high backscatter due to crosscutting 

fractures. In western PCR, unit Pa lies topographically above 

the surrounding materials that embay it (unit hu and coronarelated materials that dominate the PCR), although this relation is locally unclear due to gradual topographic transitions 

with surrounding materials. Unit Pa hosts numerous individual 

shields that locally cluster (for example, lat 25.5° S., long 242° 

E.). Unit Pa is cut by multiple sets of local fractures of various 

orientations; the fractures do not cut the surrounding materials and, thus, predated emplacement of those materials. Part of 

unit Pa (lat 26° S., long 241.5° E.) is cut by a set of regional, 

northwest-striking fractures that extend into the surrounding 

units. Locally the fractures do not extend far into adjacent units; 

this may be due to subsequent emplacement of local cover 

material. Unit Pa embays the structural topography of unit tu

and postdates deformation associated with the formation of unit 

tu at this location.

In the central PCR, unit Pa forms one large outcrop and 

three small kipukas. Most of unit Pa lies within a topographic 

high related to Parga Chasmata. Topographically, the contact 

of unit Pa with the surrounding materials is gradational and 

occurs within this elevated area. Closely spaced fractures and 

graben are penetratively developed across unit Pa and prevent 

characterization of the original material. The main fracture 

trends that affect unit Pa are (1) north-striking fractures and (2) 

regional northwest-trending fractures and graben that form a 

small branch of Parga Chasmata and postdate the north-striking 

fractures. In addition, a local suite of concentric fractures cuts 

unit Pa. Although the concentric fractures could be related to a 

local tectonomagmatic structure, for example, caldera or small 

corona, evidence of associated flow material is lacking.

Parga Chasmata material b (unit Pb) is composed of 

topographically elevated materials of high backscatter. Unit Pb

displays a tectonic fabric composed of fractures and troughs, 

including some ribbon-like structures; but embayment by 

younger flows inhibits classification. The outcrop centered at 

lat 30.5° S., long 268° E. shows a deformation fabric comprised of fractures and graben of Parga Chasmata and local 

radial structures of tectonomagmatic origin, such as coronae 

and novae. Younger material locally covers these fractures, 

yet the material is subsequently deformed along reactivated 

fractures. Material similar to unit Pb occurs in the Galindo 

quadrangle (V–40; Chapman, 1999), located north of the map 

area. Chapman (1999) maps this terrain following two different interpretations: tessera terrain (Tessera materials of Phoebe 

Regio, unit tpr) or volcano-related material (Mons materials 

unit a, unit ma).

Parga Chasmata material c (unit Pc) is a composite unit of 

volcanic origin located in the eastern limit of the PCR. Unit Pc, 

marked by heterogeneous texture and an intermediate backscatter signature, displays many volcanic structures and large 

tectonomagmatic features with associated flows. Clusters of 

shields and many isolated shields occur within unit Pc. It is not 

clear if the shields were emplaced during or after the emplacement of the materials that form unit Pc. A fine-scale reticulate 

tectonic fabric occurs in the southern boundary and center of 

unit Pc. The reticulate pattern is composed of two intersecting fracture trends, north-northwest and west-northwest, both 

formed by single lineaments with a constant interfracture spacing <2 km and lengths over 100 km. Another intersecting set of 

north-northeast- and northwest-trending fractures and graben 

cut unit Pc along its western boundary. These fracture suites do 

not cut the overlying materials and, therefore, provide temporal constraints among unit Pc, formation of the fractures, and 

emplacement of younger surrounding units. Local, ridge-like, 

northeast-trending lineaments with a regular spacing <2 km 

deform unit Pc. Although these structures appear to postdate the 

fractures, temporal relations cannot be robustly established.

Five corona-related units, four volcano-related, and four 

units formed by shields and shield-related materials overly the 

tessera terrain and Parga Chasmata materials.

Shield field and associated materials near Nordenflycht Patera (unit sfN,) are emplaced over basal unit Pc in an 

unnamed circular tectonomagmatic feature south of Nordenflycht Patera. Unit sfN, formed by low-backscatter and 

homogeneous-texture material, displays shields that may be the 

unit source. Unit sfN locally buries and, therefore, postdates 

northeast-trending fractures in the interior of the tectonomagmatic structure.
7

Flow material from Otohime Tholus (unit ftO,) is composed of intermediate backscatter flows arranged around 

Otohime Tholus. Otohime Tholus displays steep scarps but no 

associated debris apron, which is probably covered by some 

of the flows. Close-spaced, north-northwest-striking fractures 

deform unit materials, yet these fractures are also partly covered 

by flows to the east of the tholus. Flows of unit ftO postdate 

unit Pc but are, in turn, partly covered by younger fracture- and 

corona-related flows and individual shields.

Flow material from Mertseger Mons (unit fmM;) is composed of multiple overlapping flows of low to intermediate 

backscatter arranged radial to Mertseger Mons, an approximately 2-km-high, nova-like structure located on the east border 

with the Themis Regio quadrangle (V–53). The lobate, fingerlike morphology and the presence of flows that have leveed 

channels help to constrain the direction, maximum extent, and 

emplacement mechanisms of unit fmM flows. There is a transition from low-backscatter materials in the distal portion of unit 

fmM to intermediate-backscatter flows that present bright edges 

on the sides oriented toward the left-looking radar antenna; 

this radar facies may reflect volcanic flow facies related to unit 

emplacement. Unit fmM also hosts many individual, small 

shields, which are grouped near the summit of Mertseger Mons. 

In the summit region, a suite of radial fractures and graben 

occur with the extensive flow field at their termination. Some 

radial fractures display pits—evidence of dilational faults and 

(or) dikes and subsurface magma transport. Contacts with basement materials of unit Pc are clear; distal flows of unit fmM

postdate fine-scale deformation patterns that characterize the 

local basal unit. Flows of unit fmM locally overlap and, therefore, postdate materials of unit hu, but this relation cannot be 

regionally extrapolated.

Flows and shield-related materials around Zerine crater 

(unit fsZ;) embay unit Pa in northern Helen Planitia quadrangle 

(V–52). Unit fsZ shows variable backscatter with intermediate 

to high values and forms a composite unit comprising coronarelated flows, shields and shield-related flows, and materials of 

no assignable origin located in the vicinity of Zerine. Unit fsZ

displays flows of lobate morphology and high-backscatter flows 

disposed in different directions around an unnamed corona (lat 

28.5° S., long 258° E.). Shields within the unit cluster in two 

distinct areas, each with different characteristics. The first shield 

field (lat 28.5° S., long 256.5° E.) includes multiple <10-kmdiameter edifices with associated high backscatter material and 

postdates a local, subtle, northeast-trending fracture system 

and the local, radial, fracture suite associated with the unnamed 

corona that is the source of the flows. Graben of Parga Chasmata are postdated by this shield-related material; some shields 

are aligned with the northwest trend of the structures of Parga 

Chasmata, suggesting that these fractures and graben locally 

control shield emplacement. A steep-sided dome occurs close 

to the shield field; it postdated both the materials of unit fsZ and 

the regional fractures, likely representing a late volcanic episode 

of unit fsZ. The second shield field (lat 29° S., long 254° E.) 

includes shields that display channels and associated flows; 

some shields have pits elongated in the direction of preexisting northwest-striking fractures, suggesting structural control 

of shield emplacement. Temporal relations between the two 

described shield fields are unconstrained.

Corona-related flows dominate the PCR. Most of the 

coronae, where these flows originate, lie along Parga Chasmata, 

although some off-rift features that contain important volcanic 

flows are also present (for example, Oanuava Corona). Flow 

material from fractures and coronae of Parga Chasmata (unit 

fchP) is composed of materials related to Kulimina Corona, 

Hervor Corona, Xmukane Corona, and an unnamed nova (lat 

27.5° S., long 267.5° E.) located inside Parga Chasmata. Some 

of the flows are related to Parga Chasmata and flow out of 

graben and fractures of the fracture belt. Unit fchP covers most 

of the northern map area, forming a large flow field composed 

of multiple flows that have varying radar backscatter, flow direction, and origin. The western part of unit fchP is composed of a 

series of large flows containing different textural characteristics 

that could be explained by a change or evolution of material 

composition or by emplacement conditions. Large flows show a 

range of backscatter character and texture: (1) flows that contain 

dark margins, which provide evidence of changes in flow condition during emplacement or post-emplacement modification 

of the interior of the flow (inflation of the sheet flow; Byrnes, 

2002), (2) flows that have bright margins, which are considered the result of the deformation of the flow during advance 

(Byrnes, 2002), and (3) composite subparallel and polylobate 

flows that have hummocky texture. Some flows display a texture 

that appears related to flow emplacement and is perpendicular 

to the estimated flow direction. Other flows display a large ropy 

structure (lat 27.5° S., long 247° E.) that helps to constrain the 

flow direction to the west.

The eastern part of unit fchP is composed of flows that 

extend more than 1,000 km to the south and southwest. The 

flows display channels as much as ~170 km long parallel to flow 

direction. Some flows show a change in flow direction around 

previously formed structures (Oanuava Coronae and Achall 

Corona) or locally fill the inner lows of previously formed structures, indicating local topographic control of flow emplacement. 

High-backscatter flows that seem to emanate from fractures and 

graben of Parga Chasmata and flow for distances greater than 

350 km in the same direction are emplaced over the channel-fed 

flows. The youngest flows of unit fchP form a group of flows of 

high-backscatter and lobate morphology that surround the large 

tectonomagmatic features located in Parga Chasmata: Hervor, 

Xmukane, and Kulimina Coronae and an unnamed nova. Some 

of the flows display central leveed channels. Many of the flows 

related to these large features are intertwined, making it impossible to define individual units; however, one tectonomagmatic 

feature, Kulimina Corona, seems to be the main source for the 

materials that form unit fchP. In addition to these large flows, 

numerous medium and small volcanic features occur in unit 

fchP. Medium-size volcanoes are partially covered by unit fchP

flows, and small shields that occur across the unit and have 

steep-sided domes are concentrated in the interior of the coronae. Temporal relations between unit fchP and the surrounding 

materials are locally constrained, but these relations cannot be 

extrapolated across the map area due to the regional extent and 

composite nature of unit fchP. Local relations between unit fchP

and the basement are constrained, because basement units are 

located higher than the surrounding materials and host tectonic 
8

structures are covered by the younger flows. Distal flows of unit 

fchP locally embay unit tu, and flows of unit fchP embay and 

cut unit Pa, unit Pb, and unit Pc. Temporal relations with other 

volcanic units of the PCR are more difficult to determine. Unit 

fchP fills the interior and embays radial structures associated 

with an unnamed corona located in unit fsZ, indicating that local 

flows of unit fchP postdate the eastern part of unit fsZ. This 

relation does not apply to the whole unit, because shield-related 

materials in the western part of unit fsZ postdate unit fchP. 

Northwest-striking fractures, which dominate the northwestern 

map area, cut unit hu and are locally embayed by distal materials of unit fchP, indicating that part of unit fchP locally postdates part of unit hu. Due to the composite nature of both units, 

this temporal relation cannot be applied to the rest of the PCR. 

Unit fchP correlates with the unit flow materials unit 2 of Chapman (1999, unit f2, V–40). The materials of unit fchP associated 

with Hervor and Xmukane coronae and the unnamed nova in 

the map area are mapped in the Galindo quadrangle (V–40) to 

the north as a separate unit called corona material, undivided, 

of Chapman (1999, unit co). The intertwined character of the 

corona-related flows associated with Parga Chasmata makes 

correlating the unit from V–52 to V–40 difficult. In V–40, 

Chapman’s correlative unit is interpreted as regional materials 

that form part of the planum, which represents material that 

predates the emplacement of corona-related units related to 

Parga Chasmata; in V–52, we interpret unit fchP as corona- and 

chasma-related materials.

Flow material from Oanuava Coronae and Achall Corona 

(unit fcOA) consists of a composite unit formed by sheet flows 

of moderate to high backscatter. Unit fcOA completely surrounds Oanuava Coronae, Achall Corona, and other smaller 

volcanic features. West of Oanuava Coronae, unit fcOA shows 

digitate terminations and mottled radar texture in the proximity 

of the source. Most of unit fcOA shows a characteristic kilometer-scale, reticulate texture that marks the limit of the unit where 

radar contacts are not clear. East of Oanuava Coronae, unit fcOA

is more difficult to distinguish from its surroundings, although 

flow directions to the southeast help delineate the unit limit. 

Some small flows of variable backscatter southwest of Oanuava 

Coronae may originate from the corona annulus. Other volcanic features that might provide sources for unit fcOA include 

a steep-sided dome (lat 34.8° S., long 256.8° E.) southeast of 

Oanuava Coronae and a caldera-like structure (lat 34.9° S., 

long 253.4° E.) southwest of Oanuava Coronae. The calderalike structure displays concentric pit chains, possible evidence 

of subsurface dikes or dilational normal faults (for example, 

Okubo and Martel, 1998; Bleamaster and Hansen, 2001; Ferrill 

and others, 2004; Schultz and others, 2004). Shields occur as 

groups or in isolation within unit fcOA; clusters of small- to 

medium-size edifices in the interior of Oanuava Coronae and 

Achall Corona may represent a late volcanic episode associated 

with corona evolution. Unit fcOA postdates the local basement 

materials; units tu and Pa form kipukas. Unit Pa is in contact 

with the westernmost limit of unit fcOA; flows of fcOA overlap unit Pa, although, locally, contact relations are masked by 

small shields. Temporal relations between unit fcOA and other 

PCR units are difficult to establish. In northernmost unit fcOA, 

materials of fsZ are covered by the corona-related flows. These 

temporal relations cannot be extrapolated to the whole contact 

between unit fsZ and unit fcOA, because flows of unit fsZ related 

to the shield field located at lat 29° S., long 254° E. are locally 

emplaced over the materials of unit fcOA. The relations between 

unit fcOA and unit hu are constrained along the west boundary 

of unit fcOA where a long channel that cuts unit hu is clearly 

embayed by the distal edge of unit fcOA, indicating that, locally, 

unit hu predates unit fcOA. The extensive corona-related unit 

fcOA and unit fchP interact in different locations in the northern 

map area. In northeasternmost unit fcOA, flows of fchP locally 

fill the interior of Achall Corona and also embay local structures related to the corona. In other locations (for example, west 

boundary of unit fcOA), distal materials of this unit postdate 

flow fronts of unit fchP.

Flow material from Lalohonua Corona (unit fcL) is composed of a series of flows that originated in Lalohonua Corona, 

a 450-km-diameter asymmetric corona located in the Galindo 

quadrangle (V–40; Chapman, 1999). The longest flows of unit 

fcL are sheet-like flows that have homogeneous intermediate 

backscatter. The presence of a long channel (~120 km) that 

contains flows at its termination could indicate a channel-fed 

origin for these flows and could help to constrain a southwestern 

flow direction. Near Lalohonua Corona, unit fcL is composed 

of multiple, medium to high backscatter, lobate flows that flow 

radially from the corona towards the south and the southeast. 

Some lobes host central leveed channels, indicating that these 

flows are also channel fed. Flows of unit fcL postdate unit fchP. 

Unit fcL correlates with corona materials of Chapman (1999, 

unit m, com) in Galindo quadrangle (V–40), where the corona 

source of the materials is located.

Flow material from Hlineu Corona (unit fcH), located in 

the westernmost map area adjacent to Imdr Regio quadrangle 

(V–51), is a corona-related unit composed of materials of 

homogeneous intermediate to low backscatter that originated 

in Hlineu Corona. Some materials of unit fcH probably also 

emerged from small volcanic edifices near the corona. Unit fcH

includes a series of sheet flows of homogeneous texture that 

flowed to the east. We infer flow direction from the interaction 

of flows and a fracture set located east of the corona near Chuginadak Mons. The interaction of flows with fractures in some 

locations also indicate that unit fcH postdates part of unit hu; as 

with other previously described units, this relation is not applied 

to the whole unit due to the composite character of unit hu. A 

crater halo in the western map area further obscures delineation 

of unit contacts.

Flow material from Witte Patera (unit fpW), located in the 

northern map area along the boundary with the Galindo quadrangle (V–40), is composed of multiple flows of intermediate to 

high backscatter that are radial to the central, ~50-km-diameter, 

circular volcanic Witte Patera. Numerous, overlapping flows fed 

by leveed channels extend radially from the central source for 

50 to 100 km and have lobate fronts that delineate the contact 

between unit fpW and the surrounding materials. Lobate flows 

of unit fpW postdate unit fchP and the parts of unit hu where 

they are in contact. Temporal relations between unit fpW and 

unit fcL are best constrained in the Galindo quadrangle (V–40), 

where the materials of unit fcL (corona-related unit co of Chapman, 1999) postdate unit fpW.
9

Flow material from Destinnová Patera (unit fpD) is 

composed of materials of intermediate backscatter and homogeneous texture that flow from Destinnová Patera, a small (30 

km-diameter) caldera marked by a central depression and a set 

of concentric fractures. Flow lobes and lobate edges indicate 

flow direction and extent. Unit fpD flowed to the north for more 

that 140 km. Another suite of small radial flows (15–20 km 

long) surrounds the caldera. Concentric fractures of the caldera 

cut these radial flows, yet the flows also locally overprint the 

concentric fractures, indicating that the caldera formed contemporaneously with emplacement of the flows. The flows of unit 

fpD locally overlap units hu, fchP, and fcOA, but these local 

temporal relations cannot be extrapolated beyond this area.

Flow material around Azimua Tholi (unit ftA), located 

between impact craters Kanik and Rose, includes materials of 

high backscatter and reticulate to mottled texture. Two possible 

sources can be established for unit ftA: two nested, steep-sided 

domes (Azimua Tholi) around which the materials are disposed and a concentric set of fractures that forms a caldera-like 

structure located north of the unit. Flows of unit ftA embay the 

nested domes. The easternmost dome apparently experienced 

lateral flank collapse, which disrupted its original morphology. 

This dome hosts collapse-related deposits: (1) proximal facies 

composed of homogenous bright materials that have lineaments 

parallel to the original scarp and (2) distal hummocky terrain 

facies composed of blocks <5 km across. Many small shields 

also occur on unit ftA, so a contribution of point-sourced volcanism to unit formation should be considered. Unit ftA postdates 

a volcanic channel that formed in unit hu, suggesting unit ftA

locally postdates parts of unit hu.

Flow material from Monoshi Tholus (unit ftMo) is composed of materials of mottled texture and intermediate backscatter disposed around Monoshi Tholus, a flat-topped tholus with a 

summit caldera. Monoshi Tholus displays a steep western slope 

and a gradual eastern slope. Unit ftMo displays flow fronts to 

the northwest and south and hosts numerous small shields. Unit 

ftMo postdates the materials of unit tu, but the contact between 

units ftMo and hu is approximate.

Shield field and associated materials near Monoshi Tholus 

(unit sfMo), spatially related to unit ftMo, are composed of 

shield-related materials of medium backscatter that form an areally discontinuous layer due to the point-sourced nature of the 

shield-related processes. Shield characteristics include shields 

with or without summit pits and large edifices with summit 

calderas. Kipukas of unit tu appear within unit sfMo, predating the emplacement of the shield-related materials. Temporal 

relations with the surrounding units fcOA, ftMo, and hu are difficult to establish due to the discontinuous nature of unit sfMo, 

although the presence of shields on the surrounding materials 

could indicate that the emplacement of the shields that form unit 

sfMo also postdated the other units. The contact between unit 

sfMo and units fcOA, ftMo, and hu is mapped as gradational or 

approximate to express the uncertainties of these relations.

Shield fields and associated materials around Sitapi 

Coronae (unit sfSi), in the northwestern map area, consist of 

heterogeneous shield-related materials of different backscatter 

and texture. Shields form clusters with different characteristics; 

shield densities show no correlation with topography. Some 

shield clusters are related to Sitapi Coronae, but most show 

no obvious association with large tectonomagmatic features. 

Shields associated with Sitapi Coronae are parallel to a set of 

north-striking fractures. In the northwestern part of unit sfSi, 

shield-related material covers regional northwest-striking fractures; although, locally these fractures cut some shield-related 

materials, indicating time-transgressive relations of shields and 

fractures or structural reactivation of the fractures. The relations between shields and fractures suggest a structural control 

on shield emplacement. Individual shields within the clusters 

display different morphologies, including pits surrounded by 

flows of the same backscatter, mounds without pits, flat-topped 

shields with and without pits, and cone-like morphologies. 

Most edifices are ≤10 km in diameter, but some isolated shields 

are >10 km diameter. We consider unit sfSi a point-sourced, 

composite unit, comprised of different shield formation episodes of different age and nature. This point-sourced and locally 

discontinuous nature results in local gradational contacts with 

surrounding materials. Unit sfSi cannot be used as a temporal 

reference for surrounding units, although it embays basement 

units tu and Pa in the northwestern map area and locally postdates units hu and fcH.

Sequence of Materials in the Helen

Planitia Region

The HPR in southern Helen Planitia quadrangle (V–52) 

is dominated by regionally distributed, north-striking fractures 

and radial and concentric structures associated with coronae 

and radial fracture systems. Most of the coronae show spatial correlation with regional fracture systems, but large flow 

units—similar to those associated with PCR coronae (for 

example, Oanuava and Hlineu Coronae)—cannot be delineated 

with confidence. This apparent absence of large flows does not 

necessarily mean that these coronae lack large flows; flows 

could have homogenized due to weathering processes (Arvidson and others, 1992) to become part of the heterogeneous 

material. The HPR includes several major units, including basal 

Helen Planitia materials overlain by shield- and volcano-related 

flows and heterogeneous materials (pHu, sfSo, fmN, fmA, sfE

and fsU).

In western HPR, the basement of tessera terrain and Helen 

Planitia materials occurs topographically higher than surrounding materials and predates an assemblage of heterogeneous and 

shield-related materials. 

Helen Planitia materials, undivided (unit pHu), preserved 

in large outcrops located within broad topographic highs (for 

example, Tsovinar Dorsa) and in several kipukas, comprise 

materials of intermediate to high backscatter, which contain 

textures that vary from homogeneous to a kilometer-scale 

reticulate pattern. A bright parabolic halo related to impact 

crater Adaiah mantles much of southeastern HPR and obscures 

the original backscatter and texture, in part, of unit pHu. No primary structures are preserved within unit pHu, but many shields 

and associated materials lie stratigraphically above unit pHu. 

Whether shield-related volcanic activity contributed to unit formation or shields resulted from discrete shield formation events 
10

that postdated emplacement of unit pHu is unconstrained. The 

point-sourced nature of the shield-related materials emplaced in 

or on unit pHu and the surrounding units and the presence of the 

crater halo obscures some of the contacts with adjacent materials, as shown by approximate contacts. In the outcrop centered 

at lat 41.5° S., long 252° E., two local orthogonal fracture and 

graben suites cut unit pHu but do not cut the overlying material, which provides temporal constraints. Where unit pHu is in 

contact with tessera terrain material, clear embayment relations 

indicate that unit pHu postdated unit tu. All basement units are, 

in turn, embayed by unit hu.

In eastern HPR, the largest outcrop of unit pHu occurs 

in the interior of a large circular structure (lat 39.5° S., long 

262.5° E.). Other outcrops of unit pHu form isolated patches 

partially covered by young, shield-related materials. The relation of pHu with unit tu in eastern HPR is unconstrained due to 

the lack of clear contacts. In the transition area between PCR 

and HPR regions, flows of unit fmM postdate materials of unit 

pHu.

Although shields or groups of shields occur in almost all 

of the units across the HPR, there are areas where these volcanic features and their associated materials coalesce to form a 

distinctive unit. In these locations, the large number of edifices 

and the likelihood of additional shields below image resolution 

probably result in an underestimated number of shields. Shield 

field and associated materials near Sopdet Tesserae (unit sfSo) 

are geographically related to unit tu in the northwest-trending 

elevated swaths of basement terrain of western HPR. Unit sfSo

is composed of shields and shield-related flows of intermediate 

backscatter and reticulate texture that form a layer above the 

local tessera terrain. A subtle northwest-trending tectonic fabric 

in association with unit sfSo likely indicates the incomplete 

burial by shields or structures within the underlying unit tu, or 

it is a product of structural reactivation of these same structures. 

Unit sfSo surrounds small kipukas of unit tu and embays the 

large inliers of tessera terrain. The contact between units sfSo

and hu is difficult to establish in some locations due to the 

discontinuous character of the point-sourced shield material and 

the presence of shields in unit hu. In the western map area, unit 

sfSo postdated unit fcH along its southern limit, located in the 

transition between PCR and HPR.

Flows and shield-related materials around Ustrecha Tesserae (unit fsU), which cover much of eastern HPR, form a 

composite unit of corona- and volcano-related flows that are 

discontinuously covered by shields and shield clusters composed of intermediate to low backscatter materials that have 

a heterogeneous texture. Unit fsU postdated formation of the 

HPR basement. Some shield clusters are isolated, but others 

are spatially related to large tectonomagmatic features or to 

regional and local fractures sets. The point-sourced nature of the 

shield-related material makes unit fsU a locally discontinuous 

unit. The contact of unit fsU with surrounding units is approximate and (or) gradational. Units tu and pHu are embayed by the 

shield-related materials and individual shields also present in 

the basement materials. The locally discontinuous nature of unit 

fsU reveals underlying basal materials of unit pHu. Temporal 

relations between units fsU and hu are difficult to establish due 

to the composite and time-transgressive nature of both units, as 

well as the presence of shields in each unit. Along the HPRPCR boundary, materials of unit fmM were emplaced over unit 

fsU materials.

Shield fields and associated materials in Enekeler Corona 

(unit sfE), located in the interior of Enekeler Corona in eastern 

HPR, include multiple shields and shield-related flows that 

display intermediate backscatter and a heterogeneous local 

reticulate texture produced by the point-sourced nature of the 

unit. Shields display different morphologies, and pitted mounds 

dominate. Shield clusters and related flows coalesce to form 

locally continuous patches that cover the corona interior and 

part of the surrounding area. If unit formation is related to this 

large tectonomagmatic structure, it presumably represents a 

late episode of corona evolution. Unit sfE is emplaced over and, 

therefore, postdates units tu and fsU. Unit sfE embays small 

kipukas of unit tu inside Enekeler Corona. The contact of units 

sfE and fsU is shown as approximate due to the shield-related 

nature of both units. Where clear contact relations exist between 

these two units, the material of unit sfE postdates material of 

unit fsU; but, due to the composite character of unit fsU, this 

local relation is not extrapolated across the HPR.

Flow material fron Ne Ngam Mons (unit fmN), located in 

central HPR, locally postdated unit hu. Unit fmN shows long, 

intermediate-backscatter flows that form the outer apron of 

Ne Ngam Mons and low-backscatter flows that form the inner 

slopes of the edifice. Both sets of flows are radial to a central 

summit caldera. The longest flows extend to the southeast; some 

northwest-trending flows fill the interior of a nearby unnamed 

corona and embay two steep-sided domes that formed in the 

corona annulus. Mechanisms of emplacement for the intermediate-backscatter flows are unconstrained, but the low-reflectivity 

flows display a 50-km-long channel- or tube-like structure on 

the east flank of the volcano. Shields and a small dome on the 

summit of Ne Ngam Mons, probably part of the late evolution 

of the volcano, are also part of unit fmN. The materials of unit 

fmN interact with a suite of regional north-striking fractures; 

intermediate-backscatter flows predated the fractures, which are 

partially covered by younger flows.

Along the boundary with the Themis Regio quadrangle 

(V–53) east of the map area, low to intermediate backscatter 

materials that form the unit flow material from Abeona Mons 

(unit fmA) are composed of distal flows of Abeona Mons (lat 

44.8° S., long 273.1° E.), a 375-km-across, steep-sided dome 

located in southwestern V–53. Unit fmA embays unit tu. Temporal relations between unit fmA and unit fsU are unclear due to 

the heterogeneous character of the shield-related unit, similar 

backscatter character of both units along their contact, and the 

presence of scattered shields in both units.

Tectonic Structures

The Helen Planitia quadrangle (V–52) displays a variety 

of secondary structures that variably deform almost all units. 

Tectonic structures include (1) structures associated with tessera terrain, (2) regional deformation, (3) local structural suites 

related to tectonomagmatic features, and (4) localized deformation.
11

Structures Associated with Tessera Terrain

Tessera terrain inliers in the map area preserve different patterns of intersecting structures: ribbons, fractures, 

graben, and folds. Temporal relations of deformation between 

inliers are unconstrained due to the absence of mutual contact 

relations and the lack of similar structural fabric orientation. 

Ribbon-like structures, comprised of parallel, alternating, 

kilometer-wide ridges and troughs (Hansen and Willis, 1996, 

1998), occur in some inliers of unit tu. Ribbon trends vary from 

inlier to inlier, lacking coherent patterns across the map area. 

Northwest-trending ribbons deform inliers of unit tu, located 

at lat 39° S., long 248.5° E. and lat 50° S., long 263° E., that 

have a constant ribbon wavelength of 2–2.5 km, suggesting 

a shallow depth to the brittle-ductile transition (BDT) at the 

time of ribbon formation (Hansen and Willis, 1998) or a shallow depth to a solid-liquid phase transition (Hansen, 2006). In 

other tessera terrain inliers, ribbons are deformed, are embayed 

by younger flows, or show evidence of reactivation; original 

ribbon characteristics are indeterminate (for example, inlier 

of unit tu located at lat 35.5° S., long 258° E.). In the inlier of 

unit tu located at lat 50° S., long 263° E., northeast-trending 

5–10-km-wavelength folds trend orthogonal to ribbons. Fold 

wavelength indicates an increase in layer thickness as compared with the thickness for the time of ribbon formation 

(Hansen and Willis, 1998; Brown and Grimm, 1997; Ghent and 

Hansen, 1999; Hansen and others, 1999; Hansen, 2006). 

In Sopdet Tesserae, closely spaced northwest-trending 

fractures and ridges deform the materials of unit tu. Northnortheast-trending fractures and graben cut the northwesttrending fractures and ridges. All other tessera terrain inliers 

display an intricate pattern of intersecting lineaments (for 

example, local radial patterns in Ustrecha Tesserae). East- and 

northeast-trending graben and arcuate graben displayed in 

Sopdet Tesserae represent the last deformation events recorded 

in the tessera terrain inliers. Graben, embayed by younger 

volcanic materials of low backscatter, range in width from 2 

to 6 km and display coherent trends and characteristics across 

the map area, although temporal relations between inliers are 

elusive.

Regional Deformation

Fold Belts and Wrinkle Ridges

Two different styles of contractional deformation occur in 

Helen Planitia quadrangle (V–52): deformation belts (dorsa) 

that show concentrated strain and subparallel, regional wrinkle 

ridges that record distributed strain. Tsovinar Dorsa, a northwest-trending 1,100-km-long anastomosing Class II deformation belt (Banerdt and others, 1997), connects Tangba Corona 

with an inlier of unit tu and marks the north edge of Helen 

Planitia. The deformation belt varies in width along strike, 

displaying bottleneck morphology and a division into two different branches at lat 46.2° S., long 254° E. Fold characteristics 

include lengths from 10 to 100 km and wavelengths from 5 

to 10 km. Different fold morphologies are distributed along 

Tsovinar Dorsa: folds that show width variation along trend and 

complex, broad folds that have superposed, parallel, smallerwavelength folds. The other broad topographic ridge, Kastiatsi 

Dorsa, which extends about 1,200 km into V–60 to the south, 

lacks contractional structures like those developed in Tsovinar 

Dorsa. Kastiatsi Dorsa is a broad, northeast-trending warp that, 

combined with Tsovinar Dorsa, forms a circular topographic 

arch that dominates northern Helen Planitia.

Various suites of wrinkle ridges deform lowland materials, as well as corona- and volcano-related materials, that 

reside above the mean planetary radius (6,051.84 km) near 

Parga Chasmata (fig. 3, map sheet). In the southeastern map 

area, north-northeast- and north-northwest-trending wrinkle 

ridges reflect the main deformation trends; locally, this trend is 

modified around coronae-paralleling local annuli (for example, 

Enekeler Corona). The wrinkle ridges display variable spacing and length. This wrinkle-ridge suite is part of the regional 

circum-Themis trend, presumably formed in response to 

lithospheric loading of Themis Regio (Billoti and Suppe, 1999). 

In the western map area, northwest-trending wrinkle ridges 

mark the regional Helen Planitia trend (Billoti and Suppe, 

1999), where wrinkle ridges trend parallel to the geoid contours 

defined by Sandwell and others (1997) and are likely related to 

formation of Themis Regio. The wrinkle ridges, spaced 2 to 20 

km apart and having lengths of 5 to 50 km, deform a wide range 

of materials, including units pHu, sfSo, sfSi, fcOA, fmA, and hu. 

Locally, shield-related materials of unit hu pooled along wrinkle 

ridges, suggesting that the emplacement of some materials of 

unit hu and (or) wrinkle-ridge formation formed diachronously. 

North-trending wrinkle ridges extend over the central 

Helen Planitia quadrangle. Contrary to the north-northeast- and 

north-northwest-trending wrinkle-ridge suites that are related 

to regional trends (Billoti and Suppe, 1999), this wrinkle-ridge 

suite appears to be locally restricted to the map area. Although 

Billoti and Suppe (1999) described a regional change in 

wrinkle-ridge orientation west of the map area in Imdr Regio 

(V–51; lat 43° S., long 212° E.), we propose that Helen Planitia 

quadrangle displays two distinct trends of wrinkle ridges rather 

than a change in trend. Locally, north-trending wrinkle ridges 

parallel north-striking fractures and have a morphological transition from fractures to wrinkle ridges along strike. In locations 

where this transition occurs, flows covered the fractures prior to 

wrinkle-ridge formation. Similar observations led to a model for 

the formation of the wrinkle ridges that includes filling of previous fractures by flows and subsequent reactivation as inversion 

structures under the appropriate stress conditions (DeShon and 

others, 2000; Hansen, 2005). Because the existence of previous, covered fractures and wrinkle ridges is locally constrained, 

formation of the entire north-trending wrinkle-ridge suite as 

inversion structures cannot be established, although it might be 

possible. North-trending wrinkle ridges deform units Pa, fcH, 

fcOA, fchP, fpD, ftMo, sfSi, sfMo, and hu.

Another local northeast-trending wrinkle-ridge suite in 

the eastern map area deforms unit Pc. At lat 32.8° S., long 

269.4° E., two scales of ridges occur: closely spaced (<2 km), 

northeast-trending, small ridges and northeast-trending wrinkle 

ridges spaced 5–15 km apart. To explain the occurrence of both 

parallel ridge suites, mechanical considerations are taken into 

account. Assuming that the spacing of structural suites reflects 
12

the thickness of the deformed layer (for example, Ramberg, 

1955; Talbot, 1970; Price and Cosgrove, 1990; Pollard and 

Aydin, 1988), then the short- and long-wavelength ridges could 

indicate thin and thick layers, respectively, that were deformed 

within a single event. Both suites could also result from progressive thickening of a deformed layer through time, showing 

a temporal transition from short- to long-wavelength deformation. Whether ridge suites formed synchronously through 

deformation of thin and thick mechanical layers or as a result 

of a temporal change of layer thickness is unconstrained. At lat 

41° S., long 267.5° E., the long-wavelength, northeast-trending 

wrinkle-ridge suite is cut by the north-northwest-trending, 

circum-Themis trend and defines a local polygonal pattern.

Regional Fracture Suites

Northwest-striking, parallel fractures and graben cut units 

Pa, hu, fcH, and sfSi in the northwestern map area (fig. 4, map 

sheet). Fracture characteristics include 1–4 km spacing, lengths 

of 5–200 km, and widths <1 km. A spectacular case of fracture 

reorientation occurs around Sitapi Coronae, where the northwest fracture trend is completely reoriented to form a suite 

of east-trending fractures and graben that cuts units hu, sfSi, 

and ftMo. The length of the east-trending fractures and graben 

ranges from 40 to 500 km, and spacing between structures is 

from 2 to 35 km. The east-trending fractures display undulating morphology, shear component, and variation in width along 

strike. Other characteristics, such as en echelon terminations, 

pits, pit chains, and alignment of shields along the fractures, 

provide evidence for subsurface dikes or dilational normal faults 

(Okubo and Martel, 1998; Schultz and others, 2004; Ferrill and 

others, 2004). Local modifications of the northwest trend also 

occur around a shield cluster of unit sfSi, located at lat 33.8° 

S., long 244° E, where fractures bend around the shield cluster. 

Shields and shield-related flows locally cover the fractures, 

although fractures also cut shield-related materials, which indicates broadly contemporaneous shield and fracture formation.

A north-striking fracture suite, locally modified to northnortheast and north-northwest trends around large tectonomagmatic features, dominates the southern map area (fig. 4, map 

sheet). Most structures form single, radar-bright lineaments, 

interpreted as extension fractures, that are obvious in both 

left- and right-illumination images. Paired opposing lineaments (width <1 km) are interpreted as graben. Fracture 

length varies from 3 to 150 km. Fracture spacing varies in the 

southern map area from close spacing (<2 km) within fracture 

belts (for example, Ajina Fossae; lat 45° S., long 258° E.) to 

5- to 10-km-spaced fractures between fracture belts. Fracture 

belts connect and interact with different tectonomagmatic 

features (for example, Ajina Fossae connects Ne Ngam Mons 

and Tangba Corona), suggesting a possible genetic association of large tectonomagmatic features and fracture belts. The 

interaction between the regional stress fields responsible for 

regional fractures and the local stress fields associated with 

local tectonomagmatic features could produce deviations in 

fracture orientation adjacent to the tectonomagmatic features. 

The regional north trend of the fracture suite is modified along 

Kastiatsi Dorsa, where fractures parallel the arcuate northeast 

trend of the broad warp. Flows and shield-related materials 

of unit hu locally cover north-striking fractures; shields cover 

fractures and fractures cut shields, indicating that fracture and 

shield formation was diachronous. Shield clusters on units sfSi

and fsU are also related to fractures, which may control shield 

emplacement and produce local fracture reorientation around 

the shield clusters.

An approximately 2,000-km-long and 600-km-wide 

section of Parga Chasmata dominates northern Helen Planitia 

quadangle. This 10,000-km-long fracture belt includes twelve 

branches (Stofan and others, 2000), two of which—a main 

branch that dominates the northeast and a smaller branch centered at lat 28.5° S., long 254.5° E.—occur in the map area. The 

distribution of tectonic features that form the chasmata is not 

uniform; structures concentrate in the vicinities of large coronae 

and nova. The chasmata may represent the spatial and temporal 

interaction of different processes that contribute to tectonomagmatic feature formation. Tectonic features of Parga Chasmata 

include graben, fractures, and linear scarps interpreted as faults. 

Most lineaments follow the west-northwest trend of the chasmata, although some lineaments trend east and northeast around 

local tectonomagmatic features. A suite of north-northwesttrending, regularly spaced (<1.5 km), single, straight lineaments 

3–50 km long are interpreted as fractures. These fractures represent an early fracture suite, shown by crosscutting relations of 

corona-related flows and other chasmata-related structures. The 

predominant structures of Parga Chasmata are graben, marked 

by parallel-paired lineaments that display straight or sinuous 

traces and en echelon morphologies. The graben are generally <2 km wide, range from 3 to 40 km long, and are spaced 

1–3 km apart. Individual lineaments, interpreted as extension 

fractures, are associated with the graben that have similar length 

and spacing. Extension fractures are concentrated in chasmata 

troughs in the branch of the chasmata located at lat 28.5° S., 

long 254.5° E. Linear radar-bright scarps, interpreted as fault 

scarps, are located in central and eastern Parga Chasmata, 

around Kulimina Corona, and in an unnamed nova (lat 27.5° 

S., long 267.5° E.). These scarps, interpreted as normal faults, 

range from 2 to 50 km in length and typically face southwest 

parallel to the fracture zone. Faults around Kulimina Corona, 

spaced <2 km apart, merge with the corona annulus.

Parga Chasmata was an active fracture belt during most of 

the time represented by the materials in the northern map area. 

Fractures and graben of Parga Chasmata cut units Pa, Pb, Pc, 

ftO, fsZ, fcOA, fchP, fpW, and fcL. In most of these units, the 

materials postdate, but are also cut by, the structures of Parga 

Chasmata, suggesting that tectonism accompanied surface material emplacement.

Local Structural Suites Related to Tectonomagmatic Features

Magmatic and tectonic processes associated with large 

tectonomagmatic features such as coronae and novae played 

an important role in the geological evolution of Helen Planitia 

quadrangle (V–52). Fourteen well-defined coronae that have 

diameters of 100–375 km, five large structures whose clas-
13

sification is problematic, and two novae are distributed across 

the map area. The large tectonomagmatic structures in the map 

area show a strong spatial correlation with Parga Chasmata and 

other smaller deformation belts (for example, Ajina Fossae). 

We define two different groups of structures on the basis of 

their association with these fracture belts: structures spatially 

associated with Parga Chasmata and structures outside Parga 

Chasmata.

Tectonomagmatic Structures Related to Parga Chasmata

Six coronae and an unnamed nova are spatially associated 

with Parga Chasmata (fig. 4, map sheet): Oanuava, Achall, an 

unnamed corona (lat 28.5° S, long 258° E.), Kulimina, Hervor, 

Xmukane, and an unnamed nova (lat 27.5° S., long 267.5° E.). 

These tectonomagmatic structures interact among each other 

and with the chasmata structures, suggesting broadly synchronous formation of the coronae and the chasmata. Coronae associated with Parga Chasmata display three suites of structures: 

radial fractures, concentric fractures, and fault scarps. Most coronae host both radial and concentric fractures, but fault scarps 

form part of the annulus only in Xmukane Corona and appear 

restricted to chasma-related structures in the interior of Kulimina Corona. Radial fractures, presumably formed during domical uplift and (or) dike emplacement related to corona formation 

(Janes and others, 1992; Squyres and others, 1992; Stofan and 

others, 1992; Koch and Manga, 1996; Ernst and others, 1995) 

are preserved in Oanuava, Achall, Kulimina, and Xmukane Coronae. Xmukane Corona, a hybrid feature between a corona and 

a radial fracture system, displays radial fractures in its entire 

perimeter that postdated the adjacent Hervor Corona. In the 

unnamed corona and Achall Corona, portions of the radial fracture suites are covered by corona-related flows from Kulimina 

Corona. All coronae associated with Parga Chasmata display 

annuli marked by concentric fractures, although the morphology 

and degree of preservation varies between coronae. In Oanuava 

Corona, three different sections of concentric suites of fractures 

surround the tectonomagmatic feature and compound the complex annulus of the composite corona. Fractures of the complex 

annulus cut unit fcOA, although sections of the annulus are also 

postdated by part of unit fcOA, suggesting broadly synchronous 

formation of the annulus and corona-related volcanic activity. 

Hervor Corona annulus, marked by two concentric structural 

suites, displays a double-ring morphology. A third smaller 

concentric annulus, spatially coincident with a topographic low 

in the corona interior, postdated formation of the inner fracture 

ring. All structural suites postdated the materials of unit fchP, 

some which were emitted by Hervor Corona, and are cut by 

graben and fractures of Parga Chasmata. The annuli of Chanum, 

Achall, and Kulimina Coronae are partially flooded by chasmata- and corona-related flows of unit fchP and also are cut by 

fractures and graben of Parga Chasmata. Graben and scarps of 

Parga Chasmata merge with the annulus of Kulimina Corona.

Radial fractures of Xmukane Corona and the unnamed 

nova deform units Pb and fchP. The radial fractures cut the 

graben of Parga Chasmata but are also cut by fractures and 

scarps that form part of the chasmata-related fracture belt, 

indicating contemporaneous evolution of both structures and the 

chasmata. Radial fractures of the nova are interdigitated with 

the radial fractures of Xmukane Corona, and mutual temporal 

relations are unconstrained.

Tectonomagmatic Structures not Related to Parga 

Chasmata

Local fracture trends associated with tectonomagmatic 

features outside Parga Chasmata deform, and interact with, 

many different units. Contrary to relations in Parga Chasmata, 

these tectonomagmatic features are dispersed, inhibiting relative temporal interpretations. In addition, with the exception of 

Hlineu Corona, extensive corona-related flow units associated 

with coronae are not obvious—perhaps they are the result of 

homogenization processes that could mask evidence of extensive flows (Arvidson and others, 1992). These tectonomagmatic 

features are separated into two categories: features associated 

with north-trending regional fracture suites and isolated features 

(fig. 4, map sheet).

Hlineu, Sitapi, Nungui, Tangba, and an unnamed corona 

(lat 42° S., long 256.5° E.) are all associated with a suite of 

north-striking regional fractures. Two local structural suites 

relate to these coronae: radial and concentric fractures. Radial 

fractures occur in western Sitapi Coronae but are not recognized 

in the other coronae; they might never have formed, or they 

might be buried by subsequent corona-related volcanism.

Concentric fracture suites define the annuli of all the 

coronae, although each annulus is different and records 

histories not related to individual units. The annulus of Sitapi 

Coronae, slightly elongated along a northwest-southeast axis 

and marked by fractures and graben, postdates radial fractures 

and the regional east-trending fracture set, which is a result of 

the reorientation around the corona of the northwest-trending 

regional fracture system. North-striking regional fractures postdate the annulus, partially covered by the shield-related materials of unit sfSi. Hlineu Corona displays an asymmetric annulus 

that is open to the north and composed of graben and fractures. 

Annulus fractures postdate the materials of unit fcH, indicating that part of the process of annulus formation occurred after 

emplacement of extensive corona-related flows. North-striking 

regional fractures cut some annulus fractures but are also cut 

by other annulus-related structures, suggesting multi-stage 

annulus formation. Northwest-striking regional fractures in the 

western map area merge with the annulus of Hlineu Corona; 

temporal relations between both structural suites are difficult 

to establish. Nungui and Tanga Coronae display concentric, 

double-ringed annuli composed of fractures and graben, which 

formed in conjunction with north-trending regional fractures 

that deform the local materials of unit hu. These regional 

fractures postdate the outer annulus of Nungui Corona but do 

not affect the inner ring of fractures. Nungui Corona annulus is 

incomplete in the south; however, it is not clear if this relates 

to emplacement of, and burial by, shield-related material of 

unit hu or is the result of incomplete annulus formation. The 

interior of the corona, free of fractures, displays a shield cluster 

that locally buries the fractures, although fracture density due 

to strain partitioning around the corona cannot be ruled out. 

Tangba Corona displays a slightly elongated annulus along a 
14

north-south axis that is wider in the west than in the east. The 

western rim of the corona tilts to the east. Regional north-striking fractures cut concentric rings of structures north and south 

of the corona and merge with the annulus in the east and west. 

North-northeast-trending wrinkle ridges of the circum-Themis 

Regio trend (Billoti and Suppe, 1999) also cut the southern part 

of the annulus.

The unnamed corona (lat 42° S., long 256.5° E.) displays a 

concentric annulus that cuts unit hu and interacts with volcanorelated flows of unit fmN. Flows of unit fmN fill the corona 

interior, but some concentric fractures of the corona annulus 

cut the flows, which implies that the formation of the corona 

annulus occurred, at least in part, during emplacement of unit 

fmN. Two steep-sided domes that represent a late magmatic 

event presumably associated with corona evolution postdate the 

corona annulus, which is also cut by north-striking fractures and 

by radial fractures centered at lat 40.3° S., long 257.8° E.

Enekeler Corona, an isolated asymmetric corona characterized by an irregular topographic signature, displays a poorly 

defined annulus, composed of concentric ridges in the south 

and fractures in the northeast that deform shield-related materials of unit fsU, but is obscured by multiple clusters of shields 

and the associated materials of unit sfE. The northern part of 

the feature is cut by north-striking fractures that are covered 

in the interior by shield-related materials of unit sfE. Northnorthwest-trending wrinkle ridges of the circum-Themis trend 

are deflected around the corona and merge with the southern 

section of the annulus.

Chanum Corona, spatially associated with the northwesttrending regional fracture suite, displays a double concentric 

annulus of fractures in the south and a small section barely visible in the north. The fracture annulus cuts unit hu, but shieldrelated materials of unit hu postdate sections of the annulus. 

Northwest-striking fractures that cut the annulus are locally 

reoriented to north-striking fractures in the southern part of the 

corona. Some north-striking fractures south of the corona could 

be early-formed, radial fractures of the corona. A shield field in 

the corona interior postdated the northwest-striking fractures.

The group of tectonomagmatic features unrelated to 

regional fracture trends includes two coronae, Mertseger Mons, 

and three features of unclear classification. Three suites of 

structures, radial fractures, concentric fractures, and concentric 

ridges, are related to these features, although not all are related 

to the individual tectonomagmatic features.

Two structures, Mertseger Mons and a large unnamed tectonomagmatic structure (lat 40° S., long 263° E.), are characterized by a radial suite of fractures and graben. Mertseger Mons, 

a 450-km-diameter nova, displays a suite of radial fractures 

and graben that terminate in extensive flows of low backscatter. 

The association between fractures, graben, and flows suggest 

that these radial structures reflect a surface manifestation of 

dikes. The transition from graben to fissures and fractures, the 

presence of flows at the fracture termination, and the presence of northwest-trending pit chains that seem related to this 

large tectonomagmatic structure also support the presence of 

dikes associated with Mertseger Mons. The unnamed tectonomagmatic structure, whose topographic signature is a large 

topographic rim, displays radial and concentric fracture suites 

located within the topographic rim (unit pHu). Radial fractures 

and graben are distributed along the perimeter of the feature; 

whereas, concentric fractures are restricted to the northeastern 

part of the structure where they cut radial fractures and graben. 

The interior, and much of the annulus, is covered by unit pHu

and shield-related materials of unit fsU.

The unnamed corona (lat 41.6° S., long 265.5° E.) displays an incomplete circular annulus, formed by two concentric 

rings of fractures and ridges, that is absent in the northern and 

southern parts of the corona. North-northwest-trending wrinkle 

ridges of the circum-Themis trend (Billoti and Suppe, 1999) 

merge with the corona structures that form the western annuli. 

Shield-related materials of unit fsU cover annulus structures but 

are also cut by local corona-related fractures.

Three structures, Nordenflycht Patera and two other 

unnamed features (lat 32.8° S., long 268° E. and lat 36.5° S., 

long 267° E.) in the northeastern map area, display annuli of 

concentric fractures that characterize many coronae; these 

features are not included in the corona database of Stofan and 

others (1992) and are considered volcanic features (called 

patera). Parts of the annuli of these three tectonomagmatic 

features cut unit Pc, but materials of this unit also seem to 

embay the annuli of these tectonomagmatic features, suggesting synchronous development of unit Pc and formation of these 

three structures. In Nordenflycht Patera, young flows of unit 

fchP crosscut the annulus and fill its interior. Concentric fractures also postdate flows of unit fchP, indicating that annulus 

formation continued after emplacement of the youngest coronarelated flows.

Radial Fracture Systems

Radial fracture suites extend hundreds to thousands of 

kilometers across the surface of Venus. These radial systems, 

characterized by fractures and graben that radiate from a tectonomagmatic center (for example, caldera, corona), likely mark 

the surface manifestation of mafic dike swarms (McKenzie and 

others, 1992; Grosfils and Head, 1994; Ernst and others, 1995), 

although they could also represent cross-sectional hourglass 

structures (Schultz and others, in press). In the map area, radial 

fracture systems are mostly centrally located in the transition 

between Parga Chasmata and Helen Planitia.

A north-south-oriented fracture system centered on Darclée Patera dominates eastern V–52. The fracture system is not 

uniformly arranged around an interpreted source. It contains a 

350-km-long, densely fractured portion north of Darclée Patera 

and a less closely spaced fractured portion containing a fan morphology that extends more than 1,000 km to the south. Around 

Darclée Patera, volcanic flows cover the fractures. The absence 

of fractures east and west of the volcanic system could result 

from volcanic burial or from stress buttressing produced by 

basement units Pc and tu, located to the east and west, respectively. Stress butressing occurs in radial fracture systems on the 

Earth (for example, buttressing of the Spanish Peaks dike swarm 

by the Sangre de Cristo Mountain Range; Odé, 1957) and 

prevents dike propagation. The Darclée Patera system is defined 

by single lineaments interpreted as fractures, V–shaped fissures, 

and <2-km-wide, paired, dark and bright lineaments interpreted 
15

as graben. Some fractures display en echelon offsets, which are 

patterns considered in terrestrial dike-related fracture systems to 

result from the rotation of the underlying dike segment at shallow depth due to changes in the stress field or changes in host 

rock rheology (Pollard, 1987). The presence of shields aligned 

along fractures in the southern part of the fracture system is 

consistent with subsurface dikes. Flows at fracture terminations 

or other indications of magma beneath the fractures, such as the 

existence of pit chains, are not observed.

If we consider the radial fracture systems to be underlain 

by dikes that propagate laterally through the host rock (McKenzie and others, 1992; Grosfils and Head, 1994, 1995, 1996; 

Ernst and others, 1995), local stress related to the existence of 

a pressurized magma body would control the radial pattern. As 

dikes propagate, regional stresses are more important than local 

magma-source-related stresses, and the plan-view geometry of 

radial dyke swarms adopts patterns controlled by regional principal stress directions (Ernst and others, 1995). Fracture systems 

adjacent to chasmata of the Beta-Atla-Themis volcanic zone 

trend generally parallel to the regional fracture directions, which 

is consistent with fracture systems aligned with the regional 

maximum horizontal compressive stress direction. The fracture 

system centered on Darclée Patera is oblique to the northwesttrending fracture belt, suggesting that the fracture system of 

Darclée Patera was emplaced before fracture belt formation. 

The trend of fractures south of Darclée Patera, where fractures 

reach the farthest from the source and are, therefore, most sensitive to local modifications of the regional stress field, is locally 

modified around the unnamed corona (lat 41.6° S., long 265.5° 

E.). Fractures of the Darclée Patera radial system cut most materials in the eastern map area, including units tu, pHu, fsU, and 

hu. Radial fractures of the Darclée Patera system and materials 

of unit fsU seem diachronous; fractures cut unit fsU, yet shields 

and associated materials of fsU appear genetically related to 

fractures of the radial fracture system. Younger volcanic materials (unit fmA) in the southeastern map area and corona-related 

materials related to Parga Chasmata (unit fchP) postdate radial 

fracture formation.

Two other radial fracture systems occur in the center of 

the map area: one centered on Monoshi Tholus and the other 

centered on an unnamed patera (lat 40.5° S., long 258° E.). 

These fracture systems are arranged around the volcanic centers 

and are only absent where they are buried by flows and shield 

clusters. East of Monoshi Tholus, graben evolved to fissures, 

possibly indicating a decrease in dike width and (or) depth (Pollard and others, 1983). The radial fracture system centered at 

lat 40.5° S., long 258° E. exhibits pit chains, which is evidence 

of subsurface magma flow. The sharp radial disposition of both 

of these systems indicates a major influence by the local stress 

field that is produced around the volcanic center and minor 

modification in the trend of the distal fractures to the northeast 

and northwest of Monoshi Tholus. Shield-related materials 

of unit sfMo bury and, hence, postdate the fracture system 

centered on Monoshi Tholus. Some of the shields seem to be 

related to the radial fractures, so shield emplacement might be 

locally controlled by the fracture system. Radial fractures of the 

system centered at lat 40.5° S., long 258° E. are associated with 

materials of unit fsU. The alignment of shields with radial pits 

and fractures is consistent with a genetic relation between the 

shields and fractures.

Localized Deformation

Different suites of parallel, kilometer-scale lineaments, 

interpreted as extension fractures on the basis of their narrow, 

linear geometry and their character relative to radar illumination 

(Banerdt and Sammis, 1992), cut different materials in the map 

area. The consistent spacing and trend imply that these fractures 

represent single, genetically related fracture sets. In the northeastern map area, superimposed suites of parallel, fine-scale 

lineaments cut the materials of unit Pc in a reticulate pattern 

composed of intersecting north and west-northwest fractures 

that are both formed by constant-spaced lineaments (<2 km) as 

long as ~100 km. The lineament trends change locally around 

volcanic structures. Similar reticulate patterns are preserved in 

unit fchP, in the interior of Hervor Corona, and in unit fpW near 

Witte Patera. In each location, Parga Chasmata and the coronarelated structures postdate formation of the reticulate fracture 

suite.

Two models could explain these constant, regularly spaced 

fracture suites: (1) fracturing in a layer of constant thickness 

similar to fracture spacing, which is a situation observed in 

joints within terrestrial sedimentary layers (Pollard and Aydin, 

1988) or (2) a shear-lag model that occurs in a layer of variable 

thickness partially decoupled from a similar underlying material 

by frictional contact (Banerdt and Sammis, 1992). It is impossible to constrain the thickness of the deformed layer to favor 

one of the models, but it seems unlikely that two areas that are 

separated by more that 2,000 km (units Pc, fpW) and are found 

in different geologic settings would display the same layer 

thickness; thus, the shear-lag model seems to be a more likely 

hypothesis.

In the southern map area, two orthogonal northeast- and 

northwest-trending graben cut unit pHu at lat 41.7° S., long 251° 

E. but are covered by shield related materials of unit hu, helping 

to constrain the time relation between the units. Orthogonal 

graben, spaced 7–30 km apart, display about 2-km widths.

Impact Features

The Helen Planitia quadrangle hosts eleven impact structures that range from 6 to 44 km in diameter (table 1); most are 

pristine impact structures that lack obvious volcanic embayment, despite the numerous volcanic features and associated 

units represented in the map area. Xenia, Viola, and Wollstonecraft represent multiple impact structures composed of two 

or more craters. In Xenia and Viola, small impact structures 

overlap the main crater, but a flooded doublet crater plus a 

smaller crater located on the ejecta form the multiple impact 

structure of Wollstonecraft. Doublet craters, formed by simultaneous impact of two separated projectiles, are likely the result 

of well-separated binary asteroids or atmospheric disruption of 

a single asteroid (Cook and others, 2003). Moore, Xenia, Kanik, 

Steinbach, and Wollstonecraft display radar-smooth interior 

floor materials, which could be related to crater formation but 
16

more likely result from flooding long after crater formation 

(Izenberg and others, 1994; Herrick and Sharpton, 1999).

Local fracture suites cut the ejecta materials of two craters, 

Ustinya and Zerine. Ustinya, formed on unit pHu and partially 

embayed by shield-related materials of this same unit, is cut 

by local fracture sets. Zerine, located on unit fsZ, is cut by the 

radial fracture suite of Achall Corona.

Adaiah exhibits a parabolic halo of fine-grained material 

that forms a diffuse radar-rough layer to the west. This halo 

masks regional tectonic structures and the contact between units 

pHu and hu. Another radar-rough halo occurs in northwestern Helen Planitia quadrangle (V–52); four impact structures, 

Moore, Xenia, Kanik, and Rose, lie proximal to the halo, which 

partially mantles the contacts between units fcH, sfSi, and hu.

Three craterless splotches, interpreted as shock features 

formed by impactors that do not reach the surface (McKinnon and others, 1997), occur in the map area. Two features (lat 

49.3° S., long 265.3° E. and lat 46.5° S., long 241.7° E.) display 

smooth interiors and rough borders. The third feature (lat 39° 

S., long 257.5° E.) marks a radar-rough splotch that lacks other 

features.

Surficial Deposits and Aeolian Marks

Evidence of wind-related processes and deposits are 

preserved in western Helen Planitia quadrangle (V–52). Wind 

streaks are formed by the interaction of local topography, sediment, and prevailing winds and allow the characterization of 

surface wind patterns (Greely and others, 1997). Two different 

morphologies occur in the map area: linear and volcano-related 

wind streaks. Radar-smooth, linear wind streaks (>200 km long) 

are observed in stretched images at lat 34° S., long 244° E. 

The wind streaks trend northeast, but wind vergence cannot be 

determined. Wind streaks associated with small volcanoes in the 

eastern map area formed as wind tails on the northeast flanks of 

the volcanoes and indicate a predominant northeast wind across 

the eastern map area. Guest and others (1992) propose that similar deposits in other volcanic plains represent ash-fall deposits 

and record evidence of limited pyroclastic activity.

Geologic History and Resurfacing Style

The surface of the Helen Planitia quadrangle (V–52) 

preserves a diverse assemblage of structures and materials, but 

the absence of distinctive temporal markers and robust impactcrater dating techniques (Hauck and others, 1998; Campbell, 

1999) makes correlation of units across the map area difficult. 

Instead of a classical material description from older to younger, 

we follow geographic criteria describing relative time relations 

in the Parga Chasmata and Helen Planitia regions and subregions (western, central, and eastern). Within these regions and 

subregions, units that are in contact with and (or) interact with 

various structures or suites of structures allow the establishment 

of local temporal relations and, hence, a local history on the 

basis of reproducible observations.

Despite these mapping limitations, some general observations emerge about the evolution of the map area. Across 

Helen Planitia quadrangle, tessera terrain inliers and other 

basal materials (Parga Chasmata materials and Helen Planitia 

materals) form the substrate over which different volcanic units 

were emplaced. We cannot, however, determine the extent of 

tessera terrain and basal terrains beneath all units or the relative 

temporal evolution among various tessera terrain and basal terrain exposures. The various suites of secondary structures and 

deformation histories combined with the lack of contact relations between material units argues against a unique, coherent, 

synchronously formed basement layer for the entire quadrangle 

(for example, Basilevsky and Head, 1998).

The map area represents an area of transition between 

two topographic provinces, mesolands and lowlands, so the 

study of the resurfacing mechanisms is important to constrain 

whether changes in geologic style relate to this topographic 

transition. A varied assemblage of volcanic sources, including 

coronae, large volcanoes, rift-related fractures, medium-size 

edifices (volcanoes and steep-sided domes), and a large number 

of small shields contribute to resurfacing at scales from tens 

to thousands of kilometers across the map area. The regions 

adjacent to chasmata were resurfaced by extensive lava flows 

that originate from chasmata, coronae, and volcanic structures; 

individual flows extend more than 1,000 km from eruptive centers. Medium-size volcanoes and steep-sided domes scattered 

across the many corona-related units in the northern map area 

are diachronous relative to corona-related units. Local shield 

clusters associated with large tectonomagmatic features and 

individual groups of shields postdate the corona- and volcanorelated flows. South of Parga Chasmata, resurfacing is dominated by volcano- and shield-related processes. Heterogeneous 

materials are a mixture of corona- and volcano-related flows, 

fields of dispersed volcanic edifices, and materials of undetermined origin. A lack of temporal constraints disallows interpretation of stratigraphic significance for this spatial change in 

resurfacing style.

Parga Chasmata Region

On the basis of crosscutting and local superposition relations, the earliest events recorded in the western PCR are the 

emplacement and subsequent tectonic deformation of basement 

materials: tessera terrain and local Parga Chasmata materials. The original characteristics of tessera terrain prior to the 

acquisition of deformation fabrics are unconstrained; although 

many workers assume that these materials had volcanic origins, evidence of emplacement mechanisms are obliterated by 

subsequent tectonism. The spatial and temporal distribution of 

these basement materials is similarly unconstrained; there is 

no evidence to indicate that (1) tessera terrain occurs at depth 

beneath all other materials or (2) that either the host material 

for tessera terrain or tessera terrain deformation predated the 

formation of the host materials of other basal materials. Tessera terrain displays an extensional ribbon fabric that indicates 

a shallow depth to the brittle-ductile transition (BDT) or a 

shallow depth to a sharp decrease in viscosity contrast at the 

specific time and specific location of ribbon formation (Hansen 

and Willis, 1998; Hansen, 2006). In the northwestern map area, 

flows and shield-related materials of volcanic basal units embay 
17

ribbon-bearing tessera terrain preserved in local elevated areas. 

Heterogeneous materials, including flows and shield-related 

materials, then embay this assemblage of basement materials. 

Regional northwest-striking fractures cut the basement and 

heterogeneous materials and interact with local corona-related 

concentric fracture suites. Other regional deformation trends, 

such as northwest- and north-trending wrinkle ridges, also 

deform the heterogeneous materials. Corona-related materials of 

Parga Chasmata, locally emplaced over heterogeneous materials, embay regional and local fracture suites. Thus, northwestern 

Helen Planitia quadrangle records a sequence of events from 

basement terrain formation to corona-related materials related to 

Parga Chasmata. The formation of coronae and chasmata require 

the existence of a deep BDT (Hansen and others, 2000), so this 

sequence of materials also records a change in the local depth 

of the BDT from the shallow BDT conditions necessary to form 

ribbon-bearing tessera terrain to a deeper BDT condition necessary for Parga Chasmata formation. This relation requires a local 

change in the depth to BDT from shallow to deep wherever tessera terrain and coronae are spatially correlative and, therefore, 

across much of the map area, except central and eastern areas 

of the PCR where no tessera terrain units are exposed; although 

tessera terrain could occur at depth beneath the coronae and 

chasmata, the data do not require such a relation.

In the central map area, the basement material of Giltine 

Tesserae includes a set of individual inliers, including some that 

display ribbon-like structures and folds. The central and eastern 

areas of the PCR record formation and evolution of the regional 

extensional belt. The earliest events recorded in Parga Chasmata 

are emplacement and subsequent tectonic deformation of the 

Parga Chasmata materials. It is not clear if these materials are 

older terrains preserved inside the deformation belt or materials 

emplaced during the initial stages of chasmata formation. The 

different basal unit inliers, deformed by local suites of tectonic 

structures, are not in contact with one another; therefore, mutual 

temporal relations are unconstrained. Tessera terrain inliers and 

Parga Chasmata materials are postdated by flows originating 

from chasmata-related coronae and volcanic features, as well 

as from chasmata fractures. Corona-related flows and tectonic 

suites evolved in intimate spatial and temporal relation with the 

fracture belt, in an intertwined sequence of corona flows, local 

corona-related tectonic structures, and chasmata-related flows 

and structures that suggest coeval formation of coronae and 

Parga Chasmata. All the corona- and volcano-related flow units 

mapped in relation to Parga Chasmata are composite units comprised of diachronous flows as indicated by primary structures 

(for example, flow direction and flow fronts) and their relation 

to the fracture belt. After emplacement of corona-related materials, ribbon-like structures in Giltine Tesserae were reactivated, 

which probably related to the regional stress field responsible 

for Parga Chasmata; reactivated structures cut the corona-related 

materials.

Although volcano- and shield-related units bury extensive 

corona-related flows in the PCR, the composite nature of corona 

flows disallows extrapolation of locally established relative 

temporal relations between volcano-, shield-, and corona-related 

units across the map area. North-trending wrinkle ridges deform 

corona- and volcano-related flows. Temporal relations between 

the wrinkle ridges and chasmata fractures cannot be established. 

Point-sourced, shield-related volcanic deposits and medium-size 

volcanoes overlay all units. Shields are especially pervasive in 

the interior of large tectonomagmatic features and volcanoes, 

but they also occur in isolation or in small groups across the 

entire PCR.

Helen Planitia Region

Emplacement and subsequent tectonic deformation of 

tessera terrain in Sopdet Tesseare and Tsovinar Dorsa are 

the oldest events recorded in the western and central areas of 

the HPR. This tessera terrain exhibits a northwest-trending, 

pervasively developed tectonic fabric that may indicate a thin 

brittle layer in the area at the time of deformation. As with other 

tessera terrain, the original spatial distribution of this terrain 

is unconstrained. Basal Helen Planitia materials and shieldrelated materials locally bury the tessera terrain, as illustrated 

by embayment relations preserved along broad topographic 

warps in southwestern Helen Planitia quadrangle (for example, 

Tsovinar Dorsa). Basal materials are deformed by northwesttrending folds along the northwest-trending topographic rises 

of Sopdet Tesserae and Tsovinar Dorsa. Presumably folding/

warping occurred prior to the emplacement of the embayment 

materials—heterogeneous materials that occupy most of the 

western HPR. These materials host thousands of shields across 

most of the map area; shields are distributed across the unit 

and developed at the edges of and within the basal materials. 

Locally, the shields coalesce to form a discontinuous layer that 

covers the older materials in a patchwork style, or shield paint 

(Hansen, 2005), that locally buries or reveals substrate material. 

Northwest-trending wrinkle ridges of the Helen Planitia trend 

(Billoti and Suppe, 1999) deform the heterogeneous materials; 

north-striking fractures and fracture belts cut both the heterogeneous materials and the wrinkle ridges. Shields locally cover 

and postdate wrinkle ridges and fractures, yet the shields are 

also locally cut by, or interact with, regional structures, suggesting diachronous temporal relations between the shields and 

the formation and (or) reactivation of structures. In the western 

HPR, coronae correlate spatially with north-striking fractures 

and fracture belts. Unlike the coronae related to Parga Chasmata, these coronae lack evidence for large associated flow 

units.

In the central and eastern area of the HPR, tessera terrain 

inliers in Norna and Ustrecha Tesserae represent the oldest local 

surface materials. The presence of short-wavelength ribbon 

fractures and orthogonal longer-wavelength folds in Norna Tesserae may indicate a deepening of the BDT or depth to a sharp 

decrease in viscosity during tessera formation. Other tessera 

terrain inliers are cut by local tectonic structures, although in all 

inliers the youngest deformation is marked by a suite of graben 

that are embayed by younger volcanic materials. The individual 

tessera terrain inliers are not in mutual contact; therefore, temporal relations are impossible to constrain. Together with tessera 

terrain, dispersed kipukas of Helen Planitia materials form the 

basement, which is overlain by a heterogeneous materials unit 

and shield and associated flows units. Temporally both units are 

time-transgressive, and both are deformed by wrinkle ridges 
18

of the regional circum-Themis trend (Billoti and Suppe, 1999) 

and by north-striking regional fractures. Shields on these units 

postdate wrinkle ridges and fractures, yet they are also affected 

by these regional structures, suggesting that shield emplacement 

and regional deformation (or local structural reactivation) were 

diachronous.

Numerous coronae and large volcanoes are spatially 

associated with regional, north-striking fractures and fracture 

belts (for example, Ajina Fossae). As in the western HPR, 

these coronae lack obvious associated flow units. The absence 

of obvious flows could indicate a lack of an effusive stage, 

similarity of backscatter between possible flows and adjacent 

material, or surface backscatter homogenization caused by slow 

but time-persistent weathering (Arvidson and others, 1992). If 

homogenization is a function of time, slow weathering could 

suggest two temporally distinct coronae populations in the map 

area: older coronae related to north-striking fractures in the 

HPR and younger coronae located near or associated with Parga 

Chasmata. The general character of HPR coronae and PCR 

coronae are similar to old and young coronae of Chapman and 

Zimbelman (1998), respectively.

Locally younger materials in the HPR are volcano-related 

units, formed in relationship with north-trending fracture belts, 

shield clusters, and associated materials in corona interiors.

Interaction between materials of HPR and PCR occurs 

in the central map area. In this transition zone, corona- and 

volcano-related flows locally postdate basement and heterogeneous materials in both regions. Individual shields postdate 

these units, suggesting that shield-related, point-distributed 

volcanic activity was also diachronous during emplacement of 

these units.

In summary, geologic mapping reveals a range of resurfacing mechanisms (extensive volcano- and corona-related flows 

and regional resurfacing by shield-related coalesced material) 

and a geographic transition in the predominant resurfacing style 

from corona- and volcano-dominated large flows associated 

with Parga Chasmata to a patchy discontinuous shield terrain 

that characterizes much of southern Helen Planitia quadrangle 

(V–52). Coronae-, volcano-, and shield-related volcanic activity 

occurred across the map area and apparently was not restricted 

to a particular material or time. Similarly, local and regional 

tectonic suites that affect the different materials in the map area 

were not restricted to a particular material or time. The results 

of geologic mapping are inconsistent with global stratigraphic 

models (Basilevsky and Head, 1998), which call for synchronous development of the same units across globally extensive 

regions. In contrast, geologic relations within the Helen Planitia 

quadrangle record local complex histories that vary both spatially and temporally (for example, Phillips and Hansen, 1998; 

Guest and Stofan, 1999).

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22

Table 1. Crater data for the Helen Planitia quadrangle (V–52), Venus. Y, yes; N, no

Crater

Latitude

(°S.)

Longitude

(°E.)

Diameter

(km)

Underlying

unit/units

Ejecta

blanket

Impact

halo

Central

peak Rim

Interior

flooding Modification

Moore 30.35 248.40 21.1 hu Y Y Y Y Y Pristine

Xenia 30.35 249.42 13.5 hu Y Y N Y Y? Pristine Kanik 32.54 249.87 16.5 hu, fcOA Y Y Y Y Y Pristine

Viola 36.11 240.51 10 fcH Y N N Y N Pristine 

Rose 35.16 248.22 15.5 tu, sfSi, ftA Y N N Y N Pristine

Zerine 29.57 258.60 6.5 fsZ, fcOA Y N N Y N Fractured

Adaiah 47.28 253.35 18 hu Y Y Y Y N? Y

Ustinya 41.16 251.64 11.8 pHu Y N N Y N Y

Steinbach 41.42 256.94 20.3 hu Y N Y Y Y Y

Shasenem 44.05 258.92 9 hu Y N N Y N Y

Wollstonecraft 39.14 260.81 44.1 hu, fsU Y N Y? Y Y Y