11642_McCoss

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A 400-km-scale strike-slip zone near the boundary of Thetis Regio, Venus

G.W. Tuckwell a;, R.C. Ghail b

a School of Earth Sciences and Geography, Keele University, Keele, Sta¡ordshire, ST5 5BG, UK b Department of Earth Science and Engineering, Imperial College, London SW7 2BP, UK

Received 13 August 2002; received in revised form 19 February 2003; accepted 28 February 2003

Abstract

We present a structural analysis of an area of Venus’ surface centred on 3‡S, 116‡E. The area is dominated by a

wide zone of deformation striking 050‡N, distinguished from the terrain on either side of it by markedly different

structural and morphological characteristics. The deformation zone contains two principal fault sets defined by form,

position and orientation. Members of the first set are present along the length of the deformation zone. They are

discontinuous, consistently right-stepping, and are interpreted as Riedel shears in a sinistral strike-slip regime.

Members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional

grabens. These faults are most prominent in the central region of the deformation zone, and coincide with a deflection

in strike of both the first fault set and the boundaries of the deformation zone. A detailed kinematic analysis of all

fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory, supportingthe hypothesis that this sinistral strike-slip zone contains an extensional jog. This interpretation is further supported

by analysis of the interaction between faults belonging to the two sets. Variation in intercept geometry, kinematic

linking of the two fault sets, and rotation of fault strike between the two fault sets all strongly suggest simultaneous

development of these structures, and therefore a common tectonic origin. A driving mechanism for such large-scale

horizontal deformation on Venus has yet to be established.

2003 Elsevier Science B.V. All rights reserved.

Keywords: Venus; strike-slip; faulting; transtension; tectonics

1. Introduction

Western Aphrodite Terra consists of two con-

tinental-sized upland areas, Ovda and Thetis Re-

giones, that straddle the equatorial region of Ve-

nus for nearly a quarter of the circumference (Fig.

1a). These two regions are both dominated bytesserae terrain [1^4], the origin of which remains

controversial [5,6]. The relationship between the

two highlands is not clear; they are separated

by a narrow belt of lowland plains, basins, and

fracture systems which have not previously been

mapped in detail.

A 2500-km-long, 200-km-wide, more or less lin-

ear deformation zone, oriented at 050‡N, sepa-

rates the Ovda and Thetis highlands. The area

0012-821X / 03 / $ ^ see front matter 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0012-821X(03)00128-6

* Corresponding author. Tel.: +44-(0)1782-583176;

Fax: +44-(0)1782-583737.

E-mail addresses: [email protected]

(G.W. Tuckwell), [email protected] (R.C. Ghail).

Earth and Planetary Science Letters 211 (2003) 45^55

www.elsevier.com/locate/epsl

mailto:[email protected]


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mapped in this study, located between 4‡S, 114‡E

and 2‡S, 118‡E, lies on a bend of this lineationtowards its northeastern end, 200 km directly

north of the westernmost limit of Thetis Regio.

In earlier work, Davis and Ghail [7] identi¢ed

Riedel shears, antithetic Riedel shears, P-shears

and releasing-bend folds, along a V500-km-long

section of this deformation zone, and interpreted

it as a sinistral strike-slip fault network. Detailed

structural analysis of a sub-section of their

mapped area is presented in this paper to achieve

three principal objectives: (1) to test the hypoth-

esis of sinistral strike-slip motion, (2) to establishthe regional kinematics of deformation, and (3) to

constrain the magnitude of relative motion.

2. Observations

The general features of the mapped area are

outlined in Fig. 2a. The fault system is bounded

to the north by an isolated block of tesserae and

a

100 km

Tesserae

S t r i k e

- s l i p

z o n e

Basinplains

Volcanicplains

Partly buried

earlier structures

b

b

c

c

FaultedContact

OnlappingPlains

BuriedFaults

OnlappingPlains

Fig. 2. (a) Sketch image of strike-slip zone (see Fig. 3), outlining features referred to in text and the locations of the full resolu-

tion insets. (b) An example of the onlap relationship between the plains that underlie the fault zone and the tesserae northwest

of it. (c) Detail illustrating the complex relationship between the fault zone and the volcanic plains north of it. There is evidence

of a faulted contact, an onlap contact, and some indication of faults inundated by the volcanic plains.

G.W. Tuckwell, R.C. Ghail / Earth and Planetary Science Letters 211 (2003) 45^55 47

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G.W. Tuckwell, R.C. Ghail / Earth and Planetary Science Letters 211 (2003) 45^5548

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to the south by a plains-¢lled basin. The mapped

faults cut (and are therefore younger than) un-

di¡erentiated plains units that onlap the isolated

tesserae block to the northwest (Fig. 2b). This

portion of tesserae is at a lower elevation,2900 m (elevations are referenced to the 6051

km radius level and are taken from revised Mag-

ellan altimeter data (Ford, pers. commun.)), than

the main body of Thetis Regio (which has an

average elevation greater than 4000 m) to the

southeast, and it is also separated from the main

body of Ovda Regio by another fracture system

to the west. The fault system itself is at an inter-

mediate elevation (1450 m), while the basin that

extends between it and the main body of Thetis

Regio is 900 m lower, at an elevation of 550 m.

Faults associated with the mapped system cut

the plains in¢lling the basin, and are younger than

it. The plains can be variously described as mot-

tled plains [8], smooth plains [9] and slightly lo-

bate plains [10]. The mean age of such plains is

(1.01 0.19)T , (1.21 0.16)T and between 0.1 T

and T , respectively, where T is the mean surface

age (crater retention age) of Venus, currently esti-

mated at 700^800 Ma [11]. Caution must be ex-

ercised in applying these mean ages to such a

speci¢c small locality as this basin but, nonethe-

less, it is likely that it is old. Another area of plains units, to the north, is more problematic.

The boundary between the fault system and these

plains is sharp and distinct but not clearly faulted.

Small shield volcanoes can be identi¢ed in this

area but £ow boundaries are di⁄cult to identify.

It is possible that the volcanism and faulting are

syntectonic, but their age relationship cannot be

stated with con¢dence. The region immediately to

the south of the fault system is of uncertain ori-

gin; it appears that there has been more than one

episode of deformation before a ¢nal partial buri-al by the plains material through which the fault

zone cuts. Hence these structures are older and

are not likely to be related to the fault zone ana-

lysed in this paper.

Within the fault zone are two distinct fault sets

(Fig. 3a,b). The principal set, termed Fault Set 1,

extends throughout the deformation zone. Faultscarp traces are often irregular along strike, mak-

ing measurements of orientation problematic. We

have made all measurements from tip-to-tip

rather than breaking the fault into linear seg-

ments. This has the e¡ect that the distribution

of orientations will have a variation about a max-

imum re£ecting the processes of fault interaction

and linkage during fault growth [12,13]. In a num-

ber of areas, faults are clearly arranged en eche-

lon. In every case such arrangements are right-

stepping (Fig. 4b.i). The predominant orientation

of this principal fault set is not constant through-

out the mapped area. In the central region, the

predominant orientation of the faults is rotated

by V10‡ anticlockwise to that in the eastern

and western regions (Figs. 3 and 5). This rotation

coincides with a change in orientation of the

boundaries of the deformation zone.

Faults of Set 2 are distinguishable by their ori-

entation and position. Faults in this set are most

prominent in the central region, and coincide with

the change in orientation of the boundaries of the

deformation zone. The predominant orientation isoblique to the trend of the deformation zone

(V40‡). Faults in this set are extensional, and in

some cases occur in antithetic pairs to form long

grabens (Fig. 4b.ii).

The detail of the intersection and linkage geom-

etry of these two identi¢ed fault sets is shown in

Fig. 4, together with both the right and left look-

ing Magellan data from which the interpretation

was derived. This area has been chosen as being

representative of the relationship between the

principal fault sets (see Fig. 3 for location), andfor its coverage by both radar look directions.

Although individual intercepts and fault traces

6

Fig. 3. (a) Magellan right-looking data for the area between 5‡S, 113‡E and 1‡S, 119‡E. The white box indicates the location of

the data in Fig. 4. (b) Structural lineaments interpreted as fault scarps. Dashed lines indicate dip-slip extensional faults, rose dia-

gram is length weighted. (c) Schematic interpretation indicating the geometry and kinematics of the deformation zone. The pre-

dicted orientations of Riedel shears, extensional faults and compressive structures are shown.


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are often complex, clear generalities may be

drawn from the map which provide insights into

the regional kinematics and relative timing. Geo-

metrically, Fault Set 1 appears to dominate.

Faults in Set 2 often perform a physical linking

role between Set 1 faults (Fig. 4b.iii). In some

instances, however, faults in Set 1 can be seen

to terminate against those in Set 2 (Fig. 4b.ii).

Furthermore, Set 1 faults can be seen to curveand/or splay such that their terminations are ori-

ented parallel to Fault Set 2 (Fig. 4b.iv). The var-

iation in intercept geometry, the linkage of the

two fault sets, and the rotation of fault strike of

individual faults, from parallel to Set 1 to parallel

to Set 2, point very clearly to simultaneous devel-

opment of these structures, and therefore a com-

mon tectonic origin within a single regional kine-

matic regime.

3. Interpretation

The apparent linear zone of deformation, and

the orientation and nature of the faults, are con-

sistent with sinistral strike-slip deformation. Ana-

logue models, using clay, and driven by displace-

ment boundary conditions to simulate shear and

transtension, have been presented by a number of

workers (e.g. [13^15]). In the models presented byClifton et al. [13], two fault sets, one with pre-

dominantly shear displacement and the other set

with predominantly extensional displacement, de-

veloped when the displacement vector was at an

angle of less than 45‡ to the trend of the defor-

mation zone boundary. Shear faults often formed

in en echelon arrays which were right-stepping for

models with a component of sinistral shear mo-

tion. For pure strike-slip motion between the de-

a b c

25 km

b.i

b.ii

iv

iii

ii

i

Fig. 4. Magellan imagery and structural interpretation of a section of the study area between 116.2‡E, 2.7‡S and 116.6‡E, 2.2‡S.

(a) Magellan Cycle 2 (right-looking) data. (b) Structural lineaments interpreted as fault scarps. The boxes highlight examples of:

(i) en-echelon right-stepping faults of Set 2 (enlarged separately below), (ii) fault abutment and a graben of Fault Set 1 (sepa-

rately enlarged below), (iii) fault linkage, and (iv) curvature and splaying of faults. See main text for details. (c) Magellan Cycle1 (left-looking) data. Black areas indicate data gaps.


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formation zone boundaries with no component

of extension, shear faults formed at a low angle

(6 20‡) to the trend of the deformation zone

boundary. Dauteuil and Mart [14] observed that

shear faults in their models initiated at an angleof V15‡ to the deformation zone boundary, and

rotated to steeper angles as the strain in the

model was increased. They further noted that in-

dividual shear fault traces were irregular along

strike, and fault scarps recorded complex slip be-

haviour. Shear faults also accommodated vertical

displacements with some adjacent segments of the

same fault having opposite senses of dip-slip mo-

tion.

The orientation and form of faults in Set 1 are

consistent with those of Riedel shears in a sinistral

strike-slip regime (Figs. 3 and 4). Fault Set 1 is

orientated at a low angle to the trend of the de-

formation zone boundary, and where en echelon

arrays are visible they are consistently right-step-

ping (Fig. 4b.ii). If the displacement vector is con-

sistent along the entire length of the deformation

zone in the study area, the central region, where

there is a change in the orientation of the defor-

mation zone boundaries, should form a gentle ex-

tensional jog, forming an oblique rift, and there-

fore a region of transtensional deformation.

Given the orientation of the deformation zoneboundaries, and the displacement vector inter-

preted from the eastern and western regions, pre-

dictions can be made about the nature and orien-

tation of the faults in the central region against

which the structural interpretation and regional

kinematic model can be tested.

Sanderson and Marchini [16] have shown that

the in¢nitesimal strain, and therefore the principal

stress axes, within a deformation zone can be pre-

dicted from the angle between the zone and the

displacement vector of the zone boundary. This

simple quantitative analysis rests on three as-sumptions: (1) volume is conserved, (2) lateral

extrusion or intrusion of material from or into

the deformation zone is prohibited, and (3) mate-

rial is allowed to thicken and thin vertically.

These assumptions have been shown to be valid

in the majority of terrestrial examples of oblique

rifting, including those in which there is signi¢-

cant magmatic input to the deforming volume

[17]. Following these assumptions, McCoss [18]

demonstrated that for a (initially undeformed)

unit square within the deformation zone (Fig.

6), the strain across the zone (K 31) and the shear

strain parallel to the zone (Q ) can be related to the

zone’s displacement vector S by:

K 31 ¼ 13S cos A ð1Þ

and

Q ¼ ðS sin AÞ=ð13S cos AÞ ð2Þ

where A is the angle between the normal to the

tectonic zone boundary and S, and S = jSj. There-

fore the triaxial deformation D is given by:

D ¼1 K 31Q 0

0 K 31 0

0 0 K

: ð3Þ

Furthermore, it was shown by McCoss [18] that

the angle, g , between the maximum principal hor-

izontal compressive stress SH, and the boundary

of the tectonic zone is given by:

Fig. 5. (a^c) Length weighted rose diagrams for faults in the western, central, and eastern sections of the deformation zone re-

spectively. Orientations of predicted principal compressive stress (SH), predicted Riedel (R), and antithetic Riedel (RP) shear

faults, and the observed deformation zone boundary (B) are shown.


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g ¼ A=2 ð4Þ

This prediction is of particular use in areas of

transtension where extensional faults or fractures

are often well developed, and are oriented parallelto SH.

Within our area of study, the orientations of

the deformation zone boundaries are di⁄cult to

map precisely, but are estimated to vary by 10^

15‡, in an anticlockwise sense, in the central re-

gion (Fig. 2). The absolute error in predictions of

fault orientations through the application of

transtension theory is half of the absolute error

in the measurement of deformation zone bound-

ary orientation (a 10‡ error in the measurement of

the zone boundary orientation translates to a 5‡

error in the prediction of SH orientation). Clearly,

there is also some uncertainty in the comparison

of predicted fault orientations with observations;

however, the maxima in the observed orientation

data are well de¢ned, and can be identi¢ed within

an estimated error of 5‡. Taking these measure-

ment errors into consideration, a consistent ¢t

between predicted and observed fault orientations

for the eastern, central and western regions,

coupled with a systematic change in fault orienta-

tion of the correct polarity, and of magnitude

consistent with that predicted from theory, re-mains a powerful test of our kinematic interpre-

tation. Assuming that the displacement vector is

parallel to the deformation zone boundaries for

the eastern and western regions, and is oriented

10‡ oblique to the trend of the boundary in the

central region, SH, and therefore extensional

faults, are predicted to trend at 40‡ to the bound-

ary in the central region. This orientation ¢ts

Fault Set 2 (Figs. 3b and 5). If the region to the

southeast of the deformation zone is taken to be

stationary, the vector of relative displacement, S,is oriented at 238‡N, with an estimated maximum

error of 10‡. Following the assumptions of An-

dersonian faulting theory [19], shear faults should

form at an angle of 30‡ to SH. The orientations

of SH calculated from estimates of the displace-

ment vector and the orientation of the deforma-

tion zone boundary, together with predicted ori-

entations of Riedel and antithetic Riedel shears,

are shown in Fig. 5 for the eastern, central and

western regions. All observed fault trends agree

well with predictions (Figs. 3 and 5), and strongly

support the interpretation of a sinistral strike-slip

zone. Extensional faults and Riedel shears in the

central region ¢t to an SH orientated between 5‡

and 10‡ anticlockwise compared with the faults in

the eastern and western regions, supporting the

interpretation of a change in orientation of theboundaries to the deformation zone generating

an extensional jog in the central region.

The magnitude of the displacement vector is

less certain. Given the orientation of S, a value

for either Q or K 31 would be enough to de¢ne the

vector. In the absence of clear horizontal markers,

the measurement of cumulative shear displace-

ment across the zone to yield a value for Q is

not possible. The measurement of cumulative ex-

tensional strain to yield a value for K 31 is also

problematic. The most suitable area in which tomake the estimate from the observed deformation

is within the central transtensional zone. How-

ever, there are two di⁄culties. Firstly, K 31 cannot

be measured directly from observed extensional

structures since these accommodate a component

of the simple shear deformation as well as exten-

sion. It is possible, however, to use observed esti-

mates of extension to give a value of V 1, the prin-

cipal quadratic elongation of the strain ellipse.

Fig. 6. A unit area with boundaries parallel and perpendicu-

lar to a zone of transtensional deformation. The relationship

between the boundary displacement vector S, the component

of stretching across the zone (K 31) and shear parallel to the

zone (Q ), and the maximum and minimum horizontal com-

pressive stresses (SH and Sh, respectively) are shown. The an-gle A is measured from the perpendicular to the deformation

zone boundary, and g is measured from a line parallel to

the deformation zone boundary.


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McCoss [18] derived the analytical relationship

between V 1, K 31 and Q :

V 1 ¼ p=2 ¼ 1=2ð p234q2Þ0:5 ð5aÞ

where p ¼ 1 þ K 32Q 2 þ K 32 ð5bÞ

and

q ¼ K 31 ð5cÞ

The second di⁄culty in estimating the magnitude

of S is that the spacing of the faults and the res-

olution of the imagery are such that it is not pos-

sible everywhere to distinguish unambiguously

whether a bright lineation is a single normal fault

or a pair of closely spaced normal faults forming

a graben. Given these constraints, we follow the

methodology employed by Koenig and Aydin [20]

to obtain upper and lower estimates for extension.

An upper estimate of extension is obtained if it is

assumed that each radar bright line represents a

fault that accommodates 500 m of extension. A

minimum estimate is provided by the assumption

that each lineament is a 500-m-wide graben with a

200-m-deep £at £oor bounded by normal faults

dipping at 60‡, giving an extension of 230 m per

lineament. No allowance is made for deformation

accommodated on structures below the resolutionof the imagery. Minimum and maximum values of

extension across the zone are thus estimated at 8.5

and 18.5 km, respectively, giving maximum and

minimum values of V 1 of 1.08 and 1.2, respec-

tively. These translate to a minimum estimated

strike-slip displacement of 7 km and a maximum

of 18 km. Given the measurement errors inherent

in the assumptions made, these ¢gures are only an

indication of the likely magnitude of strike-slip

displacement, which is of the order of a few tens

of kilometres.

4. Discussion

Terrestrial ¢eld observations [21^23] and labo-

ratory experiments [24^26] indicate that a strike-

slip fault generally consists of discontinuous fault

strands. This discontinuous property has been ob-

served at all scales, from lithosphere [27] to small-

scale shear fractures at the sub-centimetre scales

[28]. The stepping direction of such fault disconti-

nuities is determined by the shear sense of the

main fault: if the shear sense is right-lateral, fault

strands step left, and vice versa. Terrestrial exam-ples of structures formed within extensional and

compressional jogs are many (see [26] and refer-

ences therein). Comparison of such examples in

the light of transtensional theory strongly suggest

that the same structural processes are acting as in

the area of Venus lithosphere described here.

The driving mechanism for large-scale horizon-

tal deformation is yet to be established, and will

become better constrained by further mapping,

interpretation and kinematic analysis of this and

other regions. It is possible that it is associated

with horizontal block motion in a ‘sluggish-lid’

mantle/lithosphere system [29], or as part of het-

erogeneous-lithosphere response to vertical defor-

mation driven by mantle upwelling or downwel-

ling. Since these structures appear to be the most

recent at this location, and the kinematics are well

constrained, it is possible that this deformation

zone may help to elucidate the overall kinematics

of the region. The most obvious large-scale struc-

ture with which the strike-slip zone may be asso-

ciated is the Thetis tessera block. If the deforma-

tion zone were related to tesserae formation,sinistral strike-slip displacement in this locality

would be consistent with late-stage extensional

spreading of Thetis. This interpretation ¢ts the

predictions of the downwelling model [6] in which

tesserae are formed by compressional thickening

of the crust, which later deforms by gravitational

collapse. However, this interpretation remains

controversial in the light of competing generic

models of tessera formation [5], which are sup-

ported by speci¢c observations of Ovda Regio

[30,31]. Furthermore, the tesserae block immedi-ately adjacent to the strike-slip zone is not part of

the Thetis plateau, so it is equally possible that

these structures are unrelated to plateau forma-

tion. Without additional stratigraphic constraint

on the timing of deformation in the strike-slip

zone, and its spatial and temporal relationship

to tessera formation, it is not possible to directly

support or refute either model for tessera forma-

tion.


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5. Conclusions

The structures observed in this area imply a

tectonic and kinematic origin consistent with a

large-scale sinistral strike-slip zone containing anextensional jog. The details of the interaction be-

tween the identi¢ed fault sets demonstrate simul-

taneous development, and support the hypothesis

of a common tectonic origin. An along-strike ir-

regularity in the boundary of the deformation

zone has generated a zone of transtensional defor-

mation from which relative kinematics between

the two sides of the deformation zone can be es-

tablished with con¢dence. If the region to the

southeast of the deformation zone is taken to be

stationary, the vector of relative displacement is

oriented at 238‡N, with a magnitude of the order

of tens of kilometres.

The methodology of structural analysis applied

here highlights the value of identi¢cation and

analysis of zones of oblique deformation in pro-

viding well constrained regional kinematic indica-

tors. Future analyses of both geology and struc-

ture may provide valuable data to include in a

regional and perhaps global framework of relative

kinematics, and timing, of tectonic events.

Acknowledgements

We thank Amy Clifton, Laurent Montesi, Jay

Melosh, and an anonymous reviewer for helpful

and thorough reviews that have greatly improved

the manuscript. We acknowledge PDS Map-A-

Planet for providing the Magellan SAR data.

We also thank Craig Hutton for in£uential dis-

cussions early in this work.[SK]

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