LPSC XXIII
REGIONAL NON-COAXIAL DEFORMATION ON VENUS: EVIDENCE FROM
WESTERN ITZPAPALOTL TESSERA; V. L. Hansen, Dept. of Geological Sciences,
Southern Methodist University, Dallas, TX 75275
The venusian surface displays regions of significant apparent crustal shortening and
extension (e.g. 1,2, 3), hence strike-slip displacement may be predicted. However, to date
no evidence for significant strike-slip displacement on Venus has yet been noted (3). This
raises the question of how horizontal displacement might be exemplified in surface
deformation structures. Because venusian lithosphere is interpreted to be hot and thin (4) it
may deform in both a brittle (by faulting) and ductile (by folding) manner (5,6). The lack of a
low-viscosity zone (4,7, 8) and the inability to transmit stress over great distances, which
should preclude the existence of large plates (8), is consistent with the view that deformation
is probably distributed rather than focused (9). Thus, at a regional scale the venusian surface
may deform in a continuous rather than discontinuous manner. We may, therefore, discover
structures similar to those observed within terrestrial ductile shear zones.
SAR imagery allows characterization of the structure within Ishtar terra deformed belts.
A regionally consistent, asymmetric structural fabric comprised of two principal sets of
structures characterize western Itzpapalotl tessera (Fig. 1). A penetrative fabric of tightly
spaced ridges(?), which occurs mainly in the interior of radar-bright blocks, trends NNW;
individual elements if this fabric are spaced less than 1 km apart. Throughgoing lineaments,
or flooded lows, that separate these blocks trend WNW; these spaced lineaments are separated
by 5 to 10's of km. As the penetrative ridge fabric approaches the spaced lineaments it is
rotated into parallelism with them. A third set of minor structures, developed only locally in
westernmost Itzpapalotl, is comprised of troughs that are 5-20 km long and -2 km wide, and
trend perpendicular to the trend of the ridge structures. I propose that the penetrative ridges,
spaced lineaments, and the troughs, which together comprise the structural fabric of
Itzpapalotl, represent crustal scale S-C fabrics formed as a result of distributed non-coaxial
sinistral shear across Itzpapalotl. Thus the structural fabric of Itzpapalotl tessera may record
crustal-scale sinistral transpression along the northeast boundary of Ishtar terra.
S (penetrative schistosity)-C (spaced shear planes) tectonite fabrics, originally
documented in orthogneiss of the South Armorican shear zone, Brittany, record the
displacement history of shear zones (10). These fabrics have since been documented
throughout the world, and have been reproduced in the laboratory (e.g. 11, 12). Material
undergoing non-coaxial strain initially develops a schistosity, S, at an angle to the shear zone.
S is formed by shortening and extension parallel to the principal axes of the finite strain
ellipse. With continued displacement, strain is partitioned into shear planes, (C = cisaillement,
French for shear), which parallel the shear zone boundary and are marked by high local strain.
C-planes are more spaced than the penetratively developed S fabric. With continued
displacement, S and C structures evolve further as strain is partitioned; deformation along S
persists as dominantly homogeneous (penetrative) shortening and extension, whereas
deformation along C is more brittle in character and is dominated by shear strain. C-surfaces
commonly fade along their length, or step into S-surfaces; the displacement along one Csurface is presumably transferred to a near by C-surface. The spacing between C-surfaces is a
function of the material properties of the deformed medium and total strain. With continued
deformation, S-surfaces rotate into parallelism with C and C-surfaces become more closely
spaced. The geometric relations between S and C reflect shear sense, whereas the absolute
angle between S and C, and the spacing between C-surfaces, can be used to qualitatively
estimate minimum displacement across a shear zone. The development of S-C fabrics
illustrates that an originally homogeneous and isotropic material can evolve into a mechanically
heterogeneous material through progressive deformation. In addition, the strength of the
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479
480
LPSC SXIII
N O N - C O A X I A L D E F O R M A T I O N ON VENUS:
Hansen, V.
L.
strained material may change during progressive deformation, undergoing work-hardening
that may ultimately cause the material to fail in a brittle fashion (12, 13). Cross-cutting
relations, changes in fabric asymmetry (in time or space), the distribution of fabrics, and the
relative penetrative versus non-penetrative character of these fabrics have been used
successfully to unravel complex deformation histories of terrestrial mountain belts.
Itzpapalotl tessera is much broader than many regions of non-coaxial strain on Earth (e.g.
<20 km, and commonly 1-5 km); however, Grimm and Solomon (9) predict that large-scale
motion on Venus should be reflected in distributed deformation. Therefore, the scale and
geometry of shallow crustal venusian structures may be closely correlative with deep terrestrial
structures which broaden with increasing depth (14). Crustal-scale shear fabrics, comparable
in scale and geometry to Itzpapalotl fabrics, have recently been described for mid-crustal
Proterozoic crystalline rocks in northeast Brazil (15). The Brazilian fabrics are similarly
interpreted to reflect crustal-scale transpression.
Crustal-scale S-C fabrics may record noncoaxial strain of the Venusian surface related to
differential translation of lithospheric blocks. The angular relationship between postulated S
and C surfaces over the -150 krn-wide belt of the Itzpapalotl tessera (generally 23-30') may
reflect a minimum of 250-350 km of distributed sinistral horizontal displacement across this
belt.
References. (1) Kaula, W. M., et al., 1991, Lunar and Planetary Science XXII, 699-700.
(2) Srnrekar, S. E. and S. C. Solomon, 199 1, Lunar and Planetary Science XXII, 128312844. (3) Solomon, S. C., et al., 1991, Lunar and Planetary Science XXII, 1229- 1300.
(4) Phillips, R. J. and Malin, M. C., 1983, Hunten, D.M., et al., 159-212. (5) Weertman, J.,
1979, Phys. Earth & Plant. Int., 19, 197-207. (6) Phillips, R. J., 1990, JGR, 95, 13011316. (7) Phillips, R. J., 1986, Geophys. Res. Lett., 13, 1141-1144. (8) Phillips, R. J., et
al., 1991, Science, 252, 65 1-658. (9) Grimm, R. E. and S. C. Solomon, 1989, JGR, 94,
12, 103-12, 13 1. (10) BerthC, D., et al., 1979, J. Struct. Geol., 1, 31-42. (1 1) Simpson, C.
and S. M. Schmid, 1983, Geol. Soc. Amer. Bull., 94, 1281-1288. (12) Shimamoto, T.,
1989, J. Struct. Geol., 11, 51-64. (13) Shimamoto, T., 1986, Science, 231, 7 11-714. (14)
Sibson, R. H., 1977, J . Geol. Soc. Lond., 133, 191-213. (15) Corsini, M., et al., 1991,
Geol., 19, 586-589.
Figure 1
STRUCTURAL MAP of
ITZPAPALOTL TESSERA
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penetrative ridges(?)
\\ \
....._..__
....,
... ..._ "..
'
. .
. .
throughgoing
lineaments
troughs
.. . .
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