Patches o f isolated volcanic flow materials are found within the quadrangle
(Figure 4.15). They typically show little variation in their backscatters although
different flow s can appear radar bright, or dark, indicating variations in surface
roughness. Source vents for these materials are small pits or occasionally fissures;
som e flows are located within part o f the Parga Chasmata rift system. The materials are
usually less than 75 km in length, although som e longer flow s are observed in the
T H E T A U S S I G Q U A D R A N G L E 109
r e la tiv e ly y o u n g . S o m e s h o w v a ry in g a m o u n ts o f te e to n ie d e f o r m a tio n an d th e re fo re m a y n o t be lim ite d to the y o u n g e s t ep o c h . T h e m a te r ia ls are in te r p r e te d to be the m a n if e s ta tio n o f local a re a s o f m e ltin g b e lo w th e lith o s p h e r e , p r o d u c in g v o lc a n o e s l a c k in g e d ific e s . T h e lav as e r u p te d m a y h a v e a lo w v i s c o s ity t h e r e f o r e in h ib itin g e difice c o n stru c tio n .
I m
F ig u r e 4 .1 5 . Iso la te d flow m a te r ia ls (f) w ith in th e re g io n a l p la in s {prT^ s h o w in g a s tr e a m lin e d m o r p h o lo g y . Im a g e ce n tre d on 2 1 .7 °S , 2 3 7 ° E , sca le b a r 70 km . (F ro m F- M a p I8 S 2 3 4 ).
4.6.5 Edifice Fields, e f
C lu ste rs o f sm all ed ific e s are c o m m o n t h r o u g h o u t the q u a d r a n g le ( F ig u re s 4.16 an d 4.17). T h e s e ed ific e fields tend to h a v e d ia m e te r s o f 5 0 -1 0 0 k m but so m e are m o re e x t e n s i v e , u p to 3 0 0 k m a c r o s s . A ty p ic a l n u m b e r o f e d i f i c e s fo r a field is
T H E T A U S S I G Q U A D R A N G L E 10
a p p r o x im a te ly 50. In m a n y areas the field is s u r r o u n d e d by an ap ro n o f flo w m aterial w h ic h m a y ex te n d as far as 150 km from the ce n tre o f the field. T h e s e m a te ria ls sh o w a v a r ia b le r a d a r b a c k s c a t t e r but in m a n y a r e a s are r a d a r d a r k in d ic a tin g a re la tiv e ly s m o o th surfa ce texture.
Iff
P 0
F ig u re 4.16. A 100 k m w id e e d ific e field c h a r a c te r is e d b y ra d a r d a rk flo w m aterials. T h e field is cut b y E W a n d N W - S E t r e n d in g fr a c tu r e s b u t in p la c e s s e v e ra l sm all ed ific e s an d th eir a ss o c ia te d flow a p ro n s p o std a te th e s e lin e a m e n ts . Im a g e c e n tred on 6.5°S , 21 T E , scale b a r 50 km. (F ro m F -M a p s 0 6 S 2 1 0).
E d ific e field s are d e f in e d as a r e g io n o f s m a ll v o l c a n o e s in w h ic h th e r e is a c o n c e n tra tio n h ig h e r than the re g io n a l a v e ra g e n u m b e r d e n s ity r a n g in g from 4 -1 0 per 1 0 \ m ^ an d n u m b e r i n g > 10“ w ith in a g e n e r a l l y e q u a n t a r e a > 100 k m in d ia m e te r ( A u b e l e et al., 1992). V o l c a n ic c o n s tr u c ts w it h i n e d i f i c e fie ld s s h o w a ra n g e in m o rp h o lo g y (G u e s t et al., 1992) but are g e n e ra lly t o p o g r a p h ic a lly lo w (slo p e s less than 5°) a n d are f r e q u e n t l y s u r m o u n t e d by a pit ( A u b e l e a n d S ly u ta , 1990). E d ific e s
T H E T A U S S I G Q U A D R A N G L E II
a s s o c ia te d w ith a r e a s o f f r a c tu rin g o ften h a v e e l o n g a t e d s u m m i t pits o r ie n te d in a s im ila r d ire c tio n to that o f the fractures. G r u m p ie r et al. (1 9 9 7 ) h a v e id e n tifie d three m a jo r c la s s e s o f e d ific e fields. “ S im p l e ” fields are a r e a s o f c lu s te r e d e d ific e s on a p la in s unit, o ften w ith o u t a s s o c ia te d m aterials; “ A p r o n ” fields d is p la y a s s o c ia te d ra dar d a r k o r b rig h t m a te r ia ls in te r p r e te d to be v o l c a n i c flo w s ; “ C o m p a n i o n ” fie ld s are s p a tia lly a s s o c ia te d w ith a n o t h e r ty p e o f v o l c a n i c c e n tr e (e.g. la rg e v o l c a n o e s o r c oronae). T h e m ajo rity o f edifice fields in V 39 are th e re fo re interpre ted to be e ither o f a s im p le or ap ro n type.
F ig u re 4.17. A c lu s te r o f sm all ed ific es w ith w ell d e v e lo p e d pits w ith in p r T. Im a g e ce n tred on 2 3 .0 °S , 2 1 2 .5 °E , scale b a r 50 km. (F ro m F - M a p 18S210).
A u b e le an d G r u m p ie r (1 9 9 2 ) h av e s u g g e s te d th re e p o s s ib le m e c h a n i s m s for the fo rm a tio n an d orig in o f edifice fields: 1) th ey re p r e s e n t islan d s o f to p o g ra p h ic a lly high, old plains m ateria ls that h av e been e m b a y e d and isolated b y y o u n g e r v o lc a n ic plains; 2) a field r e p re s e n ts an are a o f a n o m a lo u s m e ltin g ; o r 3) a field re p r e s e n ts the are a o f a m a g m a reservoir. I f m odel 1 is correct, then p rio r to floo d in g , a re a lly e x te n siv e v olcano fields (sim ila r to the S hie ld P lains M aterials, d e s c rib e d ea rlier) m u s t h a v e existed.
THE TAUSSIG QUADRANGLE 112
Whilst some edifice fields in V39 are embayed by local plains units, the majority are
seen to postdate the surrounding materials indicating their relative youthfulness.
Similar observations are seen in other quadrangles (e.g. at Olosa Colles in V 3 1 [Copp
and Guest, 1997]). Crumpler et al., (1997), follow ing a study o f 647 shield fields,
found that although enhanced small volcano formation may have occurred at some time
or in some regions, small volcano formation did not occur planet wide as a single event.
M odels 2 and 3 imply that the fields represent areas o f melting anomalies. The area o f
the field is therefore controlled by the extent o f a shallow regional magma reservoir or
trap (Aubele et al., 1992; Guest et al., 1992). These models may explain the generally
equant shape and average diameter o f many shield fields.