Twenty-four volcanic centres are located throughout the quadrangle. Each unit
consists o f an intermediate sized central volcanic edifice (typically 10 to 50 km in
diameter) surrounded by an apron o f flows that varies in extent from the source vent
(the average flow apron diameter is approximately 80 km). Radar backscatters vary
amongst individual units and sometimes within a unit itself. Individual flow s can be
seen within some units allowing the mapping o f flow direction. The central edifice may
display a summit pit or a caldera o f variable size.
Large volcanoes are classified as volcanic edifices with flow apron diameters o f
100 km or more (Head et al., 1992a) (see Chapter 1). V 39 contains three named large
volcanoes (Mbokomu Mons, Fedchenko Patera and Gwen Mons), although some other
volcanic centres also have flow aprons wider than 100 km. Mbokomu Mons sits on the
Jokwa Linea branch o f the Parga Chasmata system (Figures 4.18 and 4.19). Its main
edifice is 240 km wide and at its highest reaches 1.2 km above the surrounding plains.
Its shape is not typical o f other classic venusian large volcanoes (e.g. Sif, Sapas, or Gula
Montes) but does show some similarities to features such as Sappho and Anala Montes.
The central edifice has slopes o f a few degrees and is surmounted by a circular
depression that drops to 400 m below the height o f the rim. This depression does not
show the characteristic steep inner walls o f a typical caldera but may be as a result o f
T H E T A U S S I G Q U A D R A N G L E 113
the e d ific e are m o s t e x te n s iv e on the n o rth -e a s te rn a n d s o u th - w e s te r n sides, tre n d in g w ith the direc tio n o f J o k w a Linea.
A n a p r o n o f flo w s (u n it f M b ) e x te n d s o u t to 3 0 0 k m fro m th e c e n tr e o f the v o lc a n o an d d isp la y s s h a llo w flank slo p es o f g e n e r a lly less th an 1°. S o m e individual flow s tra v e l up to 6 0 0 k m fro m th e c e n tre . T h e a p r o n is c o n s t r u c te d o f v a r ia b le b a c k scatter, but p re d o m in a n tly ra d a r dark lava flo w s th at h a v e a d en d ritic m o rp h o lo g y . T h e o u ter, m o r e e x te n s iv e flo w s h a v e a v e r y d a r k , u n i f o r m b a c k s c a t t e r a n d c o u ld re p re s e n t an e a rlie r e r u p tiv e e p iso d e . M o s t flo w s a p p e a r to h a v e e m a n a te d from the c e n tre o f the v o lc a n o , but th ere c o u ld be a d d itio n a l flo w s e r u p te d from seve ral large g r a b e n lo c a te d a r o u n d the fla n k s o f the m a in e d ific e . T h e s u m m it o f M b o k o m u is c h a ra c te rise d by radial fractures and g raben c o n f in e d m a in ly to the central edifice. T he o u te r ap ro n s h o w s v ery little te c to n ic d e f o rm a tio n a n d p o s td a te s all o th e r local units in a d d itio n to the rift. T h e sh a p e an d s u m m it c h a r a c te r is tic s o f th e v o lc a n o has led to M b o k o m u b e in g classifie d as both a large v o lc a n o ( H e a d et ah, 1992a; C r u m p le r et ah,
1997) and a c o r o n a (S tofan et ah, 1992).
1
F ig u re 4.18. M b o k o m u M o n s s u p e r p o se d o n J o k w a L in ea . I m a g e c e n tr e d on 15.5°S, 2 15.5°E, scale b ar 100 km. (F ro m F -M a p s 0 6 S 2 1 0, 1 8 S 2 1 0, 18S222).
THE TAUSSIG QUADRANGLE 114
m
. -V
%
Figure 4.19. Perspective views o f Mbokomu Mons. Many o f the outer flows have been controlled by local topographic structures. These 3D images proved a valuable tool when interpreting the relationships between individual units. See also overly4.2.psd on the CD-ROM.
T H E T A U S S I G Q U A D R A N G L E 15
F e d c h e n k o P a te ra is lo cate d a lo n g the s o u th e r n b o u n d a r y o f the a r e a (F ig u re 4 .2 0 ). It h a s a b ro a d , low re lie f, ris in g o n ly 5 0 0 m at its h i g h e s t p o in t a b o v e the su r r o u n d in g plains. T h e s u m m it structure consists o f a 46 km w id e c a ld e ra m a rk e d by a c o n c e n tr ic rin g o f fractures. T h e c a ld e ra a p p e a rs to be filled b y ra d a r d a rk lavas that h a v e a lm o s t b re a c h e d th e so u th -w e ste rn rim. A ste e p sid e d d o m e an d a s e m i-c ir c u la r s h a p e d c o lla p s e fe atu re lie w ith in a ring o f fr a c tu re s th a t s u r r o u n d the m a in caldera. T h i s la rg e r rin g m a y r e p r e s e n t an o ld e r, f i l e d c a ld e r a c o m p l e x . T h e flo w ap ro n s u r r o u n d in g F e d c h e n k o is m a d e up o f both lobate a n d dig ita te flow s. Sm all edific es are s c a tte r e d t h r o u g h o u t the ap ro n an d m a y be r e p r e s e n ta tiv e o f fla n k e r u p tiv e vents. A ra d a r bright flo w to the north o f the ca lde ra is to p o g ra p h ic a lly c o n tro lle d b y an adjac en t a r e a o f u p s ta n d in g pl i plains. F e d c h e n k o flow s {fF) p o s td a te the e m p l a c e m e n t o f the
pli, p r T , p i i and p s plains units.
M
F ig u r e 4 .2 0 . T h e larg e v o lc a n o F e d c h e n k o Patera. I m a g e c e n tr e d on 2 6 .0 °S , 2 2 7 °E , sca le b a r 80 km . (F ro m F -M a p s 18S222, 3 0S 222).
G w e n M o n s ( 2 I . 2 ° S , 2 3 8 . 7°E) lies in the far s o u th - e a s t c o r n e r o f the quad. It is a s im p le c o n e -s h a p e d ed ific e re a c h in g a m a x im u m h e ig h t o f 800 m a b o v e the s u rro u n d in g p lain s. T h e m ain to p o g ra p h ic e d iflc e is 70 km in d i a m e t e r an d is s u rro u n d e d b y a flow a p r o n e x te n d in g up to 100 km from the centre. T h e v o lc a n o is s u p e r p o s e d on a set o f
THE TAUSSIG QUADRANGLE 116
east-west trending fractures most likely associated with the Parga rift system 200 km to
the north. The flow apron has an intermediate backscatter, similar to the surrounding
regional plains {prT), but some individual flows can be recognised. Part o f the summit
region and the northern flank is obscured by a data gap, but an 8 km w ide circular
collapse feature can be seen offset from the centre, possibly indicating the presence o f a
caldera complex.
The quad contains eight paterae that vary in caldera size from 40 km to 160 km.
The m ost prominent o f these are Ledoux, Jotuni and Villepreux-Power. Ledoux
consists o f a 40 km wide circular caldera located within the main Parga Chasmata rift
zone (Figure 4.28). It is surrounded by an irregularly shaped radar bright flow apron
extending on average up to 400 km from the central vent. Some flow s, to the far east
and south-west o f the centre, have a radar dark appearance and show a sinuous
m orphology. Som e show evidence o f being channel fed. The flow apron both
postdates, and is cut by, through going fractures associated with the rift. Ledoux
materials (fL) postdate all other surrounding units including Maram Corona materials
ifM), the p r T and p i plains, and undifferentiated corona materials {ucol,2).
The calderas characterising Jotuni and Villepreux-Power Paterae are both oval in
shape and approximately 160 x 120 km in size (Figure 4.21). They have similar
m orphologies displaying flat, uniform backscatter crater floors surrounded by
concentric rings o f fractures that characterise the w alls and rim o f the caldera.
Intermediate backscatter lava flows are visible to the north o f Jotuni and are interpreted
to have been erupted from the northern rim. Although o f little relief, the caldera
controls flow s from Ya-Yerv Corona, which subsequently embay the Jotuni flows. The
floor o f Villepreux-Power Patera is covered by radar dark lava flows that have embayed
the north-western rim (seen to the west o f the mapping area) and also postdate the local
plains iprT). Several small edifices are seen in the centre o f the patera.
The other paterae vary in size up to 140 km across but they laek the large-scale
associated flow aprons o f Ledoux. They are typically circular or near circular, with flat
floors and caldera walls often marked by concentric rings o f fractures. They show very
little relief, but can sometimes be seen to control younger lava flow s. A ll the other
paterae are located within the plains and from em baym ent and cross-cutting
T H E T A U S S I G Q U A D R A N G L E 17 . :.ik r W h # y ' A %
I M
i.. !
M - '
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J - ; ■ 'F igure 4.21. F o u r p a te ra e in V39. (C lo c k w is e from top left) J o tu n i, V ille p re u x -P o w e r, R o g n e d a a n d C h e r s k u y a P atera e. All sca le b a r s 100 km . I m a g e s c e n tr e d on; 6.5°S , 2 1 4 .0 °E ( F - M A P 0 6 S 2 1 0 ); 2 2 .0 °S , 2 1 0 .0 °E ( C l - M I D R 15S 215); 2 .7 °S , 2 2 0 .4 ° E ( C l - M I D R 0 0 N 2 1 5 ); 5.3°S , 2 3 2 .6 °E ( C l - M I D R 0 0 N 2 3 2 ).