NASA Technical Reports Server (NTRS) 19930005201: Structural characteristics and tectonics of northeastern Tellus Regio and Meni Tessera

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by  NASA Technical Reports Server (NTRS)

LPI Contribution No. 789 125
TABLE 1. Potential vcnusian stratigraphicmarkers terrain (CRT [1]) or tessera terrain [2]: Tellus Regio and Meni
(fromyoungest tooldest).
Tessera areas and deformed plains areas between the highland
tesserae. The area was previously studied from the Vencra 15/16
• Relatively pristine lava-flow surfaces as indicated byradar
beekscaner characteristics data and typical characteristics of complex tesscra terrain of Tellus
• Preserved craterhalos andsurface splotches Regio were analyzed and a formation mechanism proposed [3].
• Widespread fractures, grabens, andridges Apparent depths of compensation of -30--50 km were calculated
• Plains material from Pioneer Venus gravity and topography data. These values
• Complex ridged terrain (tesserae) indicate predominant Airy compensation for the area [4,5]. Re-
gional stresses and lithospheric structures were defined from analy-
which local volcanic and tectonic features and units may be corre- sis of surface structures, topography, and gravity data [6]. In this
lated. Finally, very young time markers may be defined by the work weconcentrate onnortheastern Tellus Regio and MeniTessera,
degree of weathering or eolian modification of a surface. For which are situated north and west of Tellus Regio. Structural
example, lavas flows at Maat Mons show a decrease in radar features and relationships are analyzed in order to interpret tectonic
contrast and brightness with increasing age (based on superposition history of the area. Study area was divided into three subareas:
relations) [10]. Also, some impact craters retain radar-dark (less northeastern Tellus Regio, Meni Tessera, and the deformed plain
commonly radar-bright) halos perhaps consisting of impact debris; between them.
apparently related tothe halos are dark and light surface "splotches" Description of Areas and Interpretations of Structures:
that may represent relatively young debris and shock-induced Northeastern Tellus Regio. Northeastern Tel]us Regio is defined
surface roughness produced by impacting bolides of anarrow size here as aroughly triangle-shaped area between longitudes 84°and
range that disintegrated deep down in the dense venusian atmo- 92°E and latitudes 46° and 53°N. The ridgelike northern end of
sphere [1]. As these features age, they may become less distinct northeastern Tellus Regio iscut by afracture belt at530N, 85°E, but
relative to surrounding terrains. the overall trend of the CRT iscontinued by CRT of Dekla Tessera.
Initially, superl>osition relations weredifficult toascertain among Together these areas form a>2000-kin-long south-concave arc of
various geologic units on Venus because of the general difficulty in tesserae, which extends to Kamari Dorsa south ofAudra Planitia and
perceiving topography on the radar images. (Exceptions, such as west of Laima Tessera.
thick lava flows or domes, were relatively rare). Still, many The northernmost area of northeastern Tellus Regio ischaracter-
stratigraphic relations can be determined in plan view, because ized by wide, arcuate ridgelike features, which form the major
overlying materials tend to mute or embay the texture and structure structural element of the CRT in this area. The longest one of these
of underlying surfaces. More recently, Magellan has produced ridgelike features isoriented in anorthwest -southeast direction and
repeated radar images of selected areas, which permits it forms the curving northeastern edge of the CRT of Tellus Regio
stereoradargrammetry [11]. In addition, synthetic- parallax stereo- ("A" in Fig. 1). It is -660 km long, 4---40 km wide, and has gentle
images (produced from merged Magellan images and altimetry) slopes. The east-facing slope is steeper and more pronounced. The
commonly show the association between geologic/tectonic-terrain other two ridgelike structures of the CRT are situated southwest of
units and regional topography. the f'n-stone ("B" and "C" in Fig. 1). They are less distinct and
Magellan radar mapping shows that Venus has had acomplex shorter: .--40 km wide and 280 km long ("B") and -30--85 km wide
geologic history that can be unraveled to a large extent from and 230 km long C'C"). Their general orientation is west-northwest/
available data. Even though exposed rocks apparently record only east-southeast. These ridgelike features end abruptly in the north-
a small portion of the planet's history, stratigraphic markers are west, but in the south and southeast they merge tomore topographi-
sufficient to permit the development of a useful scheme of time- cally flat-looking tessera surface. The large ridgelike features of
stratigraphic units. Such ascheme should result from NASA's new northeastern Tellus Regio are composed of narrower (widths 1-2.5
Venus Geologic Mapping (VGM) Program, which will cover the kin), closely spaced, linear, and generally ridges tens of kilometers
entire planet. in length.
References: [1] Schaber G. G. et al., JGR, special Magellan The narrow ridges and the larger-scale ridgelike features com-
issue, in press. [2] Palmer A. R. (1983) Geology, 11, 503. prising them appear to be the oldest structural elements of the CRT.
[3] Wilhehns D. E. (1987) USGS Prof. Paper 1348, Ch. 7. In the topography data the widest ridgelike patterns can be distin-
[4] Tanaka K. L. (1986) Proc. LPSC 17th, in JGR, 91, E139. guished. Their heights are typically several hundred meters below
[5] Spudis P. D. and Guest J. E. (1987) In Mercury (F. Vilas et al., or about the mean planetary radius of 6052 km. The topography and
eds.), 118, Univ. of Arizona, Tucson. [6] Scott D. H. and Carr M. H. morphology of ridges support compressionai origin. There are oval-
(1978) USGS 1-Map 1083. [7] Ryder G. et al. (1991) Geology, 19, shaped intratessera plains depressions and troughlike features in
143. [8] Lucchitta B. K. (1992) personal communication. between the major ridgelike structures of the CRT in several places
[9] Squyres S. W. et al., JGR, special Magellan issue, in press. (e.g., 49°N, 87°E,"D"). These postdate the ridgelike features of the
[lO]Arvidson R. E.etai. (1992 )Eos, 73,161. [l l]Moore H. G., this CRT. The oval plains areas have smooth radar-dark surfaces and are
volume. obviously covered bylavas. There are narrow (1-3 krn wide) ridges
N93-14389 on the edges of the oval-shaped plains, which follow the curvature
of the edges of the plains. These ridges appear to have formed in a
STRUCTURAL CHARACTERISTICS AND TECTONICS OF later episode of compression in the area.
NORTHEASTERN TELLUS REGIO AND MENI TESSERA. There isa30-kin-wide belt of narrow linear ridges adjacent tothe
T. T6rm_inen, Department of Astronomy, University of Oulu. northeastern tessera border. The widths of these ridges are 1-2 km
SF-90570, Oulu, Finland. and are several tens of kilometers in length. These ridges apparently
formed due to compressional stresses oriented perpendicular to the
Introduction: Tellus Regio-Meni Tessera region is an inter- border of CRT (approx. northeast-southwest). These ridges are not
esting highland area characterized by large areas ofcornplex ridged
126 International Colloquium on Venus
Fig. 1. Sketchofamapof northeaslem Tellus Regio andMeni Tessera region. Mercator projection. CRT ortessera areas areoutlined byaslightly
thicker line. Lines with small cross lines represnt ridges, lines with crosses represent fractures andgraben, lines with circles are troughs.
part of the CRT because their spacing is larger and they donot form sional deformation occurs along the edges. (5) Valley networks and
a similar structural pattern to the ridges of the neighboring CRT. some extensional graben form. Medeina Chasma and other long
The ridges of the CRT are extensively cut by northeast-south- troughs of the area probably formed during phase 2. This sequence
west-oriented flat-floored valleys. These features are interpreted to agrees well with both earlier and other recent studies of the area
be graben and manifest moderate tensional strain in the area. The [3,11 ]and analyses of deformation of many other regions of CRT or
graben cut almost orthogonally both the ridges of the CRT and the tessera [1,12,I 31.
ridges bordering the CRT but they rarely cut across the intratessera The Deformed Area Between Northeast Tellus Reglo and
plains. Their typical widths are 1.5-3.5 km with lengths 20-150km. Menl Tessera: The troughlike area is characterized by several
Two large, linear depressions in the area follow the strike of the irregular patches of CRT, which seem to be remnants of an earlier
graben CE" and "F" in Fig. 1). Based on their distinct morphologi- continuous CRT connecting Meni Tessera and Northeastern Tellus
cal characteristics (wide scarp-bounded openings on the plain, Regio. The CRT areas near eastern Meni Tessera have very similar
sinuous edges, widenings along the strike and up to 20 km long structures as adjacent Meni Tessera. Linear, narrow fractures form
orthogonal to the strike-oriented segments) they are classified as up to45-krn-wide belts on the plain between the CRT areas. These
valley networks of subtype III [7,8]. Valley networks of Venus fractures are oriented northwest -southeast inthe northern part ofthe
resemble sapping valleys of the Earth and are intepreted to have trough and they follow the dominant strike of fractures to the
formed by the fluid motion through anunderground fracture system southeastern comer of northern Leda Planitia. They turn to anorth-
[9,10]. These two valley networks of northeastern Tellus Regio cut south direction inthe trough. Fractures directed almost perpendicu-
across the CRT, but are more pronounced on the plains within and larly against ascarp-like, linear edge of the eastern extension of the
adjacent to the CRT than on the CRT itself. They have similar Meni Tessera ("G" in Fig. 1) turn near the edge to the north-
orientation to the graben, which shows that the valley formation northeast and form abend in the fracture pattern. The linear edge
probably occurred along two large preexisting graben. The north- appears tobe afault (its strike can be traced also further tothe west-
eastern Tellus Regio is cut by a560-kin-long northwest-southeast- southwest) and the bend in the fracture belt can be due to right-
striking linear trough (Medeina Chasma). Most of the northeast- lateral shear. Fracture belts also cut into areas of CRT. Fractures are
southwest-oriented graben cross-cut it, which shows that the trough covered in some places by plains material, apparently lava flows.
formed earlier. Fractures were probably formed by extensional deformation of the
Inferred Sequence of Deformation: (1) The narrow linear area, but later plains formation has covered them extensively.
ridges and larger ridgelike structures of the CRT form in compres- Eastern and Central Meni Tessera: MeniTessera is situated
sional folding and thrusting. (2) The ridge belt along the northeast- between latitudes 45° and 50C'N and longitudes 67°and 83°E. It has
ern edge ofTellus Regio forms innortheast-southwest compression. narrow, topographically distinct extensions to the southeast and
(3) Extensional deformation results in widespread graben forma- east. Its central part is composed of more cquidimensional and
tion. (4) Formation of irttratessera plains and subsequent compres- topographically lower area of CRT. The northern part of Meni
LPI Contribution No. 789 127
Tesscraischaracterizedby troughs,which det"me-I10-370-kin- of Vcnera data [14,15]and more recentlyby a comprehensive
long and ~15-50-km-wide east-west-orientedsegments ofCRT. analysisofdistributioanndcharacteristicosftesserafromMagellan
Theseelongatedareashave some ofthehighesttopographyinthe images[16].Based onthiswork,however, itisveryhardtodefine
Meni Tessera.One ofthesegmentsformsthepreviouslymentioned exactlytheoriginalextentoftheCRT inthisregion.
easternextensionof theCRT C'G" inFig.l). Although these Tesseraarcproposedtoformlayhot-spot-relatevdolcanismand
elongatedpartsoftheMeni Tcsscradonothave asimilarstrikeas tcctonism[17,18]orby convection-driventectonicsabovemantle
theridgeliketypographyand morphology, andtheyarenotcorn- downweUings [1,12,19].The resultsof thiswork do notconclu-
posedofclearindividualridges,theydo haveastrikesimilartothe sivelyruleouteithermodel. Analysisofstructuresanddeformation
ridgelikecomponents innortheasternTellusRegio. shows thattheearliesdtistinguishabledeformationwas compres-
StructurallMycni TcsseraCRT ismore complex thannortheast- sion,which was followedby widespreadextensionandvolcanism
ernTellusRegio.The oldestunderlyingstructuresarecurvilinear (formationofintratesserpalains).Thisresultisinagreement with
ridges,buttheyarcwiderandshorter(2--8krnwide and 10-30krn otherstudies[e.g.,1,3,11,13]andsimilarresultshave beenusedto
long)andmore widelyspacedthaninnortheasternTellus.There supportthemantledownweUing model [I,12],butinouropinion
doesappeartobevery]'meridgelikestructuressupcrposedon these theydonotleaveoutotherpossibilities.
ridges,buttheycan notalways be distinguishedfrom scarpsand The arclikearrangement of topographicallyhigherridgelike
normalfaults.The dominant directionoftheridgesistobenorth- featuresinnortheasternTeUus Regioand northernMcni Tesserais
south/northeast-southwestb,utthisorientatiomnay bcductomore roughlysimilarinplanform,butsmallerthantheDckla Tcssera-
laterdeformationandtheoriginaldirectionsmay notbcobservable northcastcrnTellusRegioarchinthenorth.Thcscarcuatepatterns
anymore. Near theeasternedge ofcentralMcni Tcsscratheridges oftcsscraarctypicaltotheareabctwccn longitudes0°and 150°E
followthecurvingtcsseraborder.The centralpartsofMcni Tcsscra [16]andcouldtellusaboutthescalesofdcformationofthecrustin
arecharacterizedby areasof orthogonalterrainof intersecting theseareas.Observed complex dcformationalsequences inthe
northeast-southwestand northwest-southeastgrabcn ('H" inFig. northeasternTellusRcgio-Meni Tcsscraregiondo suplxrntheidea
l).This terrainhas been partlycoveredby lavasof intratesscra thattheCRT isprobablyarcsuitofrepeateddeformationthrough
plainsintheareaswhere itisvisible.Therearealsoplaceswhere differentmechanisms [20].We arccurrentlyanalyzinginmore
grabcn cutacrosstheborder between theCRT and theplain, dctailstructuresinMcni TcsscraandnorthernTcllusRcgioandthcir
especiallyaroundwcstcm and northernedgesofMcni Tcsscra. relationshipswith topography, intratcsscravolcanism, and the
The relationshipbsetween grabcnwithdifferentorientationiss deformationand volcanismon theadjacentplains.
complex:North-southstrikinggrabcnandindividualscarps(.prob- References: [I]BindschadlcrD. L.ctai.(1992)JGR, submit-
ablynormal faultsc)utotherfcaturcsextcnsivclyintheeasternand ted.[2]Barsukov V. L.ctal.(1986)Proc.LPSC 17th,inJGR, 9],
northernpartsofthecentralMcni Tcsscra.Thcrcarealsograben D378-D398. [3]BindschadlcrD.L.andHead J.W. 0991)JGR, 96,
orientedinthenortheast-southwestdirection,especiallynearthe 5889-5907. [4]SrnrekarS.and PhillipsR. J.0990) LPSC XXI,
northwesternandsoutheasternborders,whichcutridgesandnorth- I176-I177. [5]Smrckar S.and PhillipsR.J.0992) EPSL, 107,
west-southeastgrabcn. InthecentralpartsoftheCRT thcrcarc 582-597. [6] Williams D. R. and Gaddis L.(1991) JGR, 96,
northeast-southwest-orientegdrabcn thatcutothcrfeatures,but 18841-18859. [7]Bakcr V. R. ct al.0992) JGR, submitted.
thesegrabcnarcfrequentlycovcrcdbylavas.Therearcalsosmall [8]Komatsu G. and Baker V. R. (1992)LPSC XXIII,715-716.
areaswhere grabcn arcnot widcsprcad or atleastcan not bc [9] Gulick V. C. et al. (1992) LPSC XXIII, 465-466. [101 Komatsu
distinguishedfrom small-scaleridgesor closelyspaced faults. G. et at. (1992)LPSCXXIIl, 719-720. [11] Bindschadler D. L. and
Deformation seems tohave followed the same kind of basic Tatsumura M. J. (1992) LPSCXXHI, 103-104. [12] Bindschadler
sequenceasinnortheasternTcllusRcgiocxccptthatthcrchavebccn D. L. et al. (I 992) JGR, submitted. [13] DeCharon A. V. and Stofan
severaldifferentepisodesofgrabcnformationwithboth spatially E. R. (1992) LPSC XXIH, 289-290. [14] Sukhanov A. L. (1986)
andchronologicallymore complex relationshipsA.lso.diffcrcnccs Geotectonics, 20, 294-305. [15] Nikishin A. M. (1990) Earth Moon
inorientationand morphology of ridgesinMcni Tcsscra and Planets, 50151, 101-125. [16] lvanov M. A. et at. (1992) LPSC
northeasternTcllusRcgio may rcflcctdifferentoriginalstrcss XXIH, 581-581. [17] Herrick R. R. and Phillips R. J. (1990) GRL,
17, 2129--2132. [18] Phillips R. J. et at. (1991) Science, 252,
,regimes. Although no major strike-slifpaultswcrc idcntit'ieidn
Mcni Tcssera,thereisevidenceofprobablesheardcformationin 651-658. [19] Bindschadler D. L.and Parmentier E.M. (1990)JGR,
nearbyplainsareas. 95, 21329-21344. [20] Solo_ori $. C:et al. (1992) JGR. submitted.
Discussionand Conclusions: Similaritieisnthctopographic N93 -14390 '
trends,especiallythesimilartypesoflincarridgclikcfeaturesin
northeasternTcllusRcgioandcorrespondingelongatedsegmentsin EPISODIC PLATE TECTONICS ON VENUS. Donald Turcotte,
northernMenl Tcsscra,which togethcrform a roughly south- Department of Geological Sciences, Cornell University, Ithaca NY
concave arc of topographicallyhigherCRT, as well as some 14853, USA.
similaritiewsith structuresoftheCRT of theeasternmostMcni
Tcsscraandwcstcm edgeofnortheasternTcllusRegio,indicatcthat Studies of impact craters on Venus from the Magellan images
theseareasof CRT were probably earlierinterconnected.The have placed important constraints on surface volcanism. Some 840
troughlikeplainareabetwccn Mcni Tcsscraand TcllusRcgio is impact craters have been identified with diameters ranging from 2
probablyunderlainby CRT, which hasbeen disruptedandcovered to280 kin. Correlation sof this impact flux with craters on the Moon,
by lavas. The adjacentnorthern Lcda Planitiaisdefo_rmedby Earth, and Mars indicate amean surface age of 0.5 5:0.3 Ga. Another
complex intersectinsgystcrnsoffracturesandridges.Some ofthis important observation is that 52% of the craters are slightly frac-
deformationmay reflectapresenceofacoveredbasement ofCRT. tured and only 4.5% are embayed by lava flows. These observations
The arclikepatternoftcsscraebetween KamariDorsaandnortheast- led Schaber et al. [7] to hypothesize that apervasive resurfacing
ernTellusRegio may alsoreflectancarlicrlargerareaoftcsscra. event occurred about 500 m.y. ago and that relatively little surface
Similarconclusionswere carlicrpresentedonthebasisofanalysis volcanism has occurred since. An alternative hypothesis has been