The formation and distribution of major tectonic features on Venus are closely linked to the dominant mechanism of lithospheric heat loss. Among the most spectacular and extensive of the major tectonic features on Venus are the Chasmata, deep linear valleys generally interpreted to be the products of lithospheric extension and rifting. Systems of chasmata and related features can be traced along several tectonic zones up to 20,000 km in linear extent. Mechanical and thermal models for terrestrial continental-rifting are applied to the rift systems of Venus. The models are tested against known topographic and tectonic characteristics of Venus chasmata as well as independent information on the physical properties of the Venus crust and lithosphere

Head, J. W.

Solomon, S. C.

English

NASA Technical Reports Server

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RIFT SYSTEMS ON VENUS 
Solomon, S.C. and Head J.W. 139 
simpl e s t  o f  these model s  are 1 ocal l y  one-dimensional and invo l  ve e i t h e r  
uni form 1 i thospher ic  s t r e t c h i n g  o r  d i f f e r e n t  degrees o f  s t r e t c h i n g  f o r  t h e  
c r u s t  and t h e  thermal 1 i thosphere. Accordi ng t o  t h e  two-1 ayer s t r e t c h i n g  
model o f  Tu rco t te  E131, f o r  instance, a c r u s t  o f  th ickness Co and dens i ty  pc 
and a 1 i thosphere o f  th ickness Lo ( i n c l u d i n g  t h e  c r u s t )  i s  s t re tched under 
extension t o  new c rus ta l  and 1 i t hospher i c  thicknesses C and L. There i s  a 
resul  t i ng subs i dence 
where B c  = C/Co and BL = L/Lo are  t h i n n i n g  f a c t o r s  f o r  t he  c r u s t  and 1 i t h o -  
. - sphere, Tm and pm a re  t h e  temperature and dens i ty  o f  t h e  asthenosphere, To i s  
t h e  ru r face  temperature, and a i s  the  volumetr ic  c o e f f i c i e n t  o f  thermal 
expansion f o r  t h e  1 i thosphere. The f i r s t  term i n  (1) represents t h e  i s o s t a t i c  
', 
r e s u l t  o f  c rus ta l  th inn ing;  t h e  second term represents the  thermal r e s u l t  of 
1 i thospher i  c  th inn ing.  
One simpl e t e s t  o f  t h i s  model i s  t o  examine t h e  imp1 i c a t i o n s  of t he  
topographic re1 i e f  f o r  t he  p r e - r i f t  th ickness o f  t h e  crust .  I f  e i t h e r  t h e  
present  r i f t  s i g n i f i c a n t l y  postdates the  episode o f  extension [12,131 o r  t h e  
w id th  o f  th inned l i t h o s p h e r e  beneath a Venus r i f t  9s considerably greater  than 
t h e  width o f  t h e  region o f  th inned c r u s t  [14], then t h e  t o t a l  r im- to - f l oo r  
re1 i e f ,  excl ud i  ng vol canic construct ion,  across a r i f t  on Venus i s  a measure 
of c rus ta l  th inn ing.  S e t t i n g  t h e  f i r s t  term i n  (1) equal t o  3.5 t o  4 km, and 
assuming pm = 3.4 g/cm3 and pm-pc = 0.4 g/crn3, gives Co(l-Bc) = 30-34 km. 
Since B~ > 0, a minimum th ickness f o r  t h e  p r e - r i f t  h igh l  and c r u s t  on Venus i s  
30 km, a r e s u l t  a t  1 eas t  cons is ten t  w i t h  ava i l  ab le  g r a v i t y  and topographic 
data f o r  Venus h i  ghl ands, p a r t i c u l  a r l y  h i  ghl ands removed f rom the regions w i t h  
t h e  s t rongest  s i  gnature o f  mantl e  dynamics i n  t h e  1 ong-wave1 ength g r a v i t y  and 
topography f i e l d s  [15,16]. A r i f t  developed i n  a region o f  s i g n i f i c a n t l y  
t h i n n e r  crust ,  perhaps i n c l u d i n g  t h e  Venus low1 ands and mid1 and p l a i n s  C71, 
woul d be expected t o  d i  spl ay 1 esser re1 i e f  than do t h e  h i  ghl and chasmata. 
On the  O r i g i n  o f  Raised Rims. A key issue f o r  mechanisms o f  r i f t  
--
formation and evol u m o n  i s  t h e  o r i g i n  of t h e  ra ised r ims adjacent t o  Venus 
r i f t  v a l l  eys. Whil e 1 ocal i z e d  topographic highs i n  Beta Regio may be 
i d e n t i f i e d  f rom radar images as vol cani c  const ruc ts  C81, t h e  const ruc t iona l  
component o f  r i m  topography e l  sewhere a1 ong Venus chasmata i s  d i f f  i c u l  t t o  
ascerta in.  A s i g n i f i c a n t  component o f  r i m  up1 i f t  appears t o  be genera l ly  
present  i n  t e r r e s t r i  a1 cont inenta l  r i f t  formation, an observat ion t h a t  has 1 ed 
t o  two- layer  extensional models w i t h  t h e  thermal l i t h o s p h e r e  th inned over a 
zone o f  g reater  w id th  than t h e  region o f  c rus ta l  t h i n n i n g  [14]. S t r i c t l y  
k inemat ic  models o f  t h i s  type do not  conserve 1 i t h s p h e r i c  mass; removal o f  t he  
lower  l i t h o s p h e r e  by some mechanism (e.g., s top ing)  must be postulated. More 
complete thermal and dynamical model s, however, y i e l d  r i m  up1 i f t  as a natura l  
consequence o f  the  1 a t e r a l  t ranspor t  o f  heat f o l l  owing i n i t i a l  r i f t i n g ,  e i t h e r  
by l i t h o s p h e r i c  conduct ion [ I 7 1  o r  by t h e  onset o f  convect ion i n  t h e  heated 
1 ower 1 i thosphere C181. 
The simpl e two-1 ayer s t r e t c h i n g  model o f  equation (1)  may be tes ted  
aga ins t  t he  imp l ied  th ickness o f  t he  thermal l i t h o s p h e r e  under the  assumption 
t h a t  t he  ra i sed  r i m  o f  a Venus chasma i s  a response t o  1 i thospheri  c  t h i n n i n g  
and associated thermal up1 i f t  C19). S e t t i n g  t h e  second term i n  (1)  equal t o  1 
t o  1.5 km, and adopt ing a = 3 x 1 0 - s O ~ - l  and Tm-To = l O 3 O K ,  g i  ves L o ( l - 6 ~ )  = 
70 t o  100 km. Since BL 2 0, the  p r e - r i f t  thermal l i t h o s p h e r e  i s  a t  l e a s t  
70 km th i ck .  Adding t h e  th ickness o f  t h e  presumably unthinned h igh l  and c r u s t  
RIFT SYSTEMS ON VENUS 
140 Solomon, S.C. and Head J.W. 
external  t o  t h e  r i f t  v a l l e y  would g ive  a lower  bound o f  100 km t o  t h e  p r e - r i f t  
thermal l i thosphere .  I t  should be noted t h a t  t h e  u p l i f t  a t t r i b u t e d  here t o  
1 i t h o s p h e r i c  t h i n n i n g  i s  i n  a d d i t i o n  t o  any thermal c o n t r i b u t i o n  t o  t h e  broad 
topographic r i s e  o f  h igh l  and topography i n  areas o f  a c t i v e  r i f t i n g  [20]; i.e., 
t h e  average th ickness o f  t h e  thermal 1 i thosphere  woul d have t o  exceed 100 km, 
perhaps by a considerable amount, i f  a p o r t i o n  o f  t h e  topographic r i s e  of a 
r i f t e d  h igh l  and i s  a t t r i b u t e d  t o  a broad area o f  heat ing  and t h i n n i n g  o f  t h e  
thermal 1 i thosphere. I f  t h e  physical  p roper t i es  governi ng convect ion i n  t h e  
Venus mantle are  s i m i l  a r  t o  those i n  t h e  Ear th ' s  mantle, t h i s  requirement of a 
t h i c k  thermal 1 i thosphere beneath t h e  Venus h i  ghl ands may be s u f f i c i e n t  t o  
r u l e  out  t h i s  simple s t r e t c h i n g  model, a t  l e a s t  f o r  models not  i n c l u d i n g  t h e  
e f f e c t s  o f  1 a t e r a l  heat t ranspor t .  
I t  i s  a1 so important  t o  consider non-thermal explanat ions f o r  up1 i f t  of 
Venus r i f t  v a l l e y  rims. I n  t e r r e s t r i a l  r i f t s ,  e a r l y  thermal u p l i f t  o f  r i f t  
margins may be susta ined we1 1 a f t e r  t h e  hea t ing  event by the  f l e x u r a l  r i g i d i t y  
o f  t he  th i cken ing  e l a s t i c  l i t h o s p h e r e  [21]. Such a process i s  not  l i k e l y  t o  
be important  f o r  Venus, however, because o f  t he  small value of e l a s t i c  
1 i thosphere th ickness expected even i n  t h e  absence o f  extension [1,11]. 
Dynamic model s  o f  extension o f  a d u c t i l  e  1 i thosphere can a1 so y i e l  d up1 i f t e d  
r ims by mater ia l  f l o w  [22]; t he  r e l a t i v e  con t r i bu t i ons  o f  thermal up1 i f t  and 
f l  ow up1 i f t  remain t o  be expl ored f o r  such model s, however. 
Concl u s i  ons. L i  thospher i  c  s t r e t c h i n g  model s  can account a t  1 eas t  broadly 
f o r  t he  topographic c h a r a c t e r i s t i c s  o f  Venus r i f t  s t ruc tures .  With add i t i ona l  
assumptions, such model p rov ide  bounds on the  thicknesses of t h e  c r u s t  and 
thermal 1 i thosphere i n  t h e  Venus h igh l  ands. Unresol ved i s  the  causat ive 
mechanism f o r  r i f t  formation, bu t  t h e  great  1 ength o f  r i d g e  systems po in ts  t o  
gl obal -scal e t e c t o n i c  processes. We f a v o r  t h e  view t h a t  the  up1 i f  t o f  t h e  
margins o f  Venus r i f t  systems invo l  ves a s i g n i f i c a n t  thermal component, which 
would i n d i c a t e  t h a t  these gl obal processes are c u r r e n t l y  act ive.  The 1 ack of 
major r i f t  s t r u c t u r e s  i n  I s h t a r  Terra provides add i t i ona l  support f o r  t h e  
hypothesis [ll] t h a t  t h e  most recent t e c t o n i c  a c t i v i t y  i n  t h a t  h igh l  and area 
has been domi nan t l y  compressi onal . 
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Rift systems on Venus: An assessment of mechanical and thermal models

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