Active Tectonics on Venus:  Evidence from Coronae and Chasmata



     Venus, the twin-planet of Earth, was radar-mapped by  Magel-
lan  in  1990-92  with nearly complete coverage.  Although earth-
like plate tectonics is not evident and  horizontal  motions  are
limited,  Venus  exhibits  tectonic activity. Chasmata, linear to
arcuate troughs extending thousands  of  km,  show  the  greatest
relief,  as much as 7 km over a horizontal distance of 30 km, and
may be sites of  active  rifting.   Coronae,  perhaps  unique  to
Venus,  are  circular uplifts hundreds of km across surrounded by
ridges. The closest analogue may be Earth's hotspots.   The  dis-
tribution  of  Venus'  coronae  and impact craters are related to
chasmata.  Coronae are almost twice as dense near the chasmata as
a  random  set  of  the  same size.  Of the various morphological
types of coronae,  the  radial-concentric  (dominated  by  radial
faults)  and  multiple  (several  linked  features) are even more
highly concentrated near chasmata, whereas the concentric-caldera
(collapse)  type  are absent near the chasmata.  Venus has nearly
1000 impact craters, mostly unmodified, nearly  randomly  distri-
buted,  and  the density consistent with a mean planetary surface
age of 500 Ma.  When distributions are compared with random  sets
there  is a deficit of about 15-20 craters close to the chasmata.
Craters modified by tectonism and volcanism tend to be  near  the
rift  zones  and their distribution closely resembles that of the
coronae. The higher  proportion  of  tectonized,  and  especially
volcanically-embayed,  craters  within  5-108o9 of the chasmata, is
about the fraction that would be expected  from  modification  of
the  full set of craters within a band 100-200 km about the chas-
mata.  Craters and coronae fill disjoint regions  that  are  more
connected than regions of randomly-assigned points. This suggests
that the volcano-tectonic process creating  coronae  may  be  the
same one destroying craters.