Aug 10, 1998 - structure, let alone the processes occurring beneath some re- gionâ¢s, the task for ... along their southern margin, a possibility that gravity anoma- lies, discussed ... 1993] and Cenozoic [Thomas et al., 1993] sedimentary rock.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B8, PAGES 18,137-18,152,AUGUST 10, 1998
Gravity anomaliesover the Ferghana Valley (central Asia) and intracontinental
deformation
EvgeneB. Burov1 Directionde la Recherche,Bureaude Recherches G6ologiqueset MiniSres,Orl6ans,France
Peter Molnar 2 Department of Earth,Atmospheric, andPlanetarySciences, Massachusetts Instituteof Technology,Cambridge
Abstract. GravityanomaliesovertheFerghanaValley, a deepsedimentary basinin the westem Tien Shan,differ from thoseexpectedfor localAiry isostaticequilibriumby 120 mGal andimplya deficitof massbeneaththebasinequivalentto a Moho deeperby 10 km than prevailswhereisostasy holds.AlthoughtheFerghanaValley is underlainby thickMesozoic andCenozoicsediment,measurements of densityshowthatthismaterialcannotaccountfor the deficitof mass.Gravity anomaliesoverboththe surrounding mountainbeltsandthe KazakhPlatformfarthernorthimplythatlocalAiry isostaticequilibriumis approached; flexureof a relativelythin,effectivelyelastic,plate(thickness - 10-15km) loadedby thepresent topography canaccountfor therangeof Bougueranomaliesof -50 to -450 mGal overthese regions.We inferthatapproximately north-south shortening of therelativelythinlithosphere hascreatedmountainsnorthandsouthof thebasin,haswarpedthebasementof the immediatesurroundings of thebasinup by foldingthemantlelithosphere, andhasforcedthebasin floor downbeneaththe FerghanaValley. It appearsthatthepreexistingthermalstructureand variationsin crustalthicknesshavedictatedthe stylesanddistributionof deformationin this region. of the Andes [e.g., Jordan et al., 1983; Molnar and LyonCaen, 1988], and the erodedrangessurrounding the late PaAlthoughboththe highestandmostactivemountainrange leozoicAmadeusBasinin centralAustralia[e.g.,Shaw,1991; of the world, the Himalaya, and the beststudied,the Alps, oc- Shawet al., 1991a,b]. A logicalquestionposedby presence cur whereone plateof continentallithospherehasunderthrust of these intracontinentalregionsis what processesdictate the margin of another, a comparablyimportant class of wheresuchintracontinental mountainbuildingoccurs. mountainbelts developsin intracontinental settings.In Asia, Two classesof answerpresentthemselvesnaturally. In 1. Introduction
intracontinentalmountainbuilding accountsfor a large fraction of India'spenetrationinto Eurasia. Convergenceoccurs acrossthe Tien Shan,the highestrangeoutsidethe HimalayaKarakommchain, at a rate (-20 mm/yr [Abdrakhmatovet al., 1996]) that is comparableto that deducedfor the Himalaya (20-23 mm/yr [e.g., Bilham et al., 1997; E. Lavd and J.-P. Avouac, Abandonedfluvial terracesacrossthe Siwalik hills
one, processesoccurringbeneaththe regions,such as some
(Nepal):PartI, Fluvialresponse to climaticandtectonicforcing, submittedto Journal of GeophysicalResearch,1998]). Similarly,much of the otherhigh terrainnorth of the Himalaya seemsto havebeencreatedby intracontinental mountain building,suchas in the Nan Shan[e.g.,Meyer, 1991], Altay of Mongolia [e.g., Burov et al., 1993], and Gobi-Altay[e.g., Tapponnierand Molnar, 1979]. Moreover, the Rocky
another.Yet, givenourdifficulties in determining the deep structure, let alonethe processes occurringbeneathsomere-
form of small-scale convection, exert forces on the base of the
lithospherethat cause, or at least contribute to, the con-
vergence.In the other,forcesappliedfrom afar areresponsi-
ble for the convergence, andweaknesses in the lithosphere dictatewheredeformation occurs.Clearly,one classof explanationmight work in one area,whereasthe otherworksin
gion•s,the task for the moment seemsto be to test one or the
otherclassof mechanisms in a few regionsandlet suchtests eliminate
the other.
If intracontinental mountainbuilding occursbecauseregionsare stressedfrom afar, and weak areasdeform,thenthe Mountains of the western United States formed in an innext questionmust be, What aspectof a region makesit tracontinentalsetting,muchlike that of the easterncordillera weak? Againtwo possibilities presentthemselves.Hot areas arelikelyto be weaker,but chemical,or morepreciselymin1OnleavefromCenterof Geophysical DataStudies/United Insti- eralogical, differencescan also cause heterogeneityin strength.The greatstrengthof oceaniclithosphere seemsto tute of Physicsof the Earth, Moscow. 2Temporarily atLaboratoire deDynamique desSyst•mes G6oloderivefromits mineralogy beingdominated by olivine,which giques,Instimtde Physiquedu Globe,Paris. has beenmeasuredto be strongerthan the dominantcrustal mineralsin the continents (calcite,quartz,feldspars,micas, •opyright 1998 bytheAmerican Geophysical Union. andmaybepyroxene)[e.g.,Braceand Kohlstedt,1980], and from oceaniccrestbeing so thin as to play no role in the Papernumber98JB01079. 0148-0227/98/98JB-01079509.00 strength profile,compared withthatof thickerandpresumably 18,137
18,138
BUROV AND MOLNAR: GRAVITY ANOMALIES OVER FERGHANA VALLEY
weaker continental crust [Burov and Diament, 1996]. Thus
we might expecta cold regionto be strongerand moreplatelike than a hot region,and, similarly,a regionof thin crustto be strongerthanoneof thickercrust,for a similarthermalprofile.
With the goal to test the idea that lateral variationsin strengthcan dictate where intracontinentaldeformationoccurs, we studied the westernmostTien Shan, which includes
the FerghanaValley (Tadjik and Uzbek "Farghona,"hereinafter also referred to as FerghanaBasin, since both names, FerghanaValley andFerghanaBasin,are equallyusedin the Russian/Sovietgeologicalliterature)and the Chatkal Ranges to its north and the Alai (SouthTien Shan) Range to its south (Figure 1).
2. Tectonic Setting and Geological Evolution of the Ferghana Valley, Chatkal Ranges, and South Tien Shan
The Tien Shan terminateswest of the right-lateralTalasFerghanafault by splayinginto two narrowmountainchains that surroundthe FerghanaValley. The Central Tien Shan farther east consists of a more uniform structure that includes
an alternationof rangesandbasinsseparated by reversefaults [e.g., Makarov, 1977; Sadybakasov, 1990; Yudakhin,1983]. The dominantly east-northeasttopographictrend of ranges and basins terminates at this fault.
The northwest end of the
Talas-Ferghanafault also boundsthe northeastend of the Chatkal Ranges,a zone of northeasttrendingnarrow ranges and basins similar to those east of the fault, but oriented
somewhatdifferently[e.g.,Sadybakasov, 1990]. Slip on theTalas-Ferghana fault of 180 km sincePaleozoic time [Burttnan,1963, 1964, 1975] appearsto includeonly a relatively small fraction (~60 km) in Cenozoic time [e.g., Burtman et al., 1996]. Yet becausethis fault separatesthe FerghanaValley from a deformingregionto its east,neither the amountnor the currentrate of slip on the fault canbe constantalongit. For our purposeshere,it sufficesto note that thisfault seemsto separatetwo areaswith verydifferentstyles of deformation,different topography,and perhapsdifferent crustalanduppermantlestructures [e.g.,Kosarevet al., 1993; Roeckeret al., 1993]. A narrowbelt of mountainsparallel to the central sectionof the fault slopesdownwardto the FerghanaValley, underlainby a deepbasinwith asmuchas 8 km of Cretaceous-Cenozoic sediment [Cobbold et al., 1993, 1996; Yudakhin,1986; Yudakhinet al., 1991].
The geologic structureof the Chatkal Rangesrequires north-northwest-south-southeast crustalshortening,by reverse
faulting and basementfolding [e.g., Sadybakasov, 1990]. Apparentlyrelativelynarrow,east-northeast trendingvalleys mark loci of reversefaultsand tightlyclosedsynclines.Inferences of crustal thickness from seismic refraction lines and
gravityanomaliesimply that the crustthickensfrom roughly 40 km north of the ChatkalRangesto as muchas 60 km beneaththem [e.g., Ulomov,1974]. Yet no major thrustfault seemsto bound the rangeson the north side. Thus if one major thrust fault boundsthe ChatkalRanges,it must lie alongtheir southernmargin,a possibilitythat gravityanomalies, discussedbelow, seemto rule out. Thus crustalshortening seemsto have occurredby distributeddeformation acrosstheranges,asit doeswithintheTien Shanfarthereast. Paleomagnet.icdeclinations of Cretaceous [Bazhenov, 1993] and Cenozoic [Thomaset al., 1993] sedimentaryrock
demonstrateapproximately20o-30ø of counter-clockwise rotation of the FerghanaValley with respectto Eurasia. This rotation seems to have occurred about an axis at the southwest
end of the Chatkal Ranges. Thus it calls for convergence betweenthe FerghanaValley and Eurasiathat increasesfrom southwest to northeast and could exceed 100 km at the north-
eastend of the Ranges. Southof the FerghanaValley, the east-westtrendingSouth Tien Shan separatesthe apparentlyrigid block beneaththe FerghanaValley from the Pamir to the south. Fault plane solutionsof earthquakesshow activeunderthrusting of the FerghanaValley crust beneaththe South Tien Shan [Nelson et al., 1987]. The rate of underthrustingis difficult to estimate, but intenseQuaternarydeformation[e.g.,Nikonov,1977; Nikonov et al., 1983; Trifonov, 1983], plus geodeticmeasurementsfarther west [e.g., Guseva, 1986], imply that most of the convergencebetweenthe Pamir and the FerghanaValley occurswithin the Alai Valley, which separates the Pamir and SouthTien Shan[e.g., Burtmanand Molnar, 1993]. Although deformationclearly occurswithin the Chatkal Rangesandjust southof the SouthTien Shan,the Ferghana Basin can be consideredas a compositeramp basin. Overthrustingon two principalreversefaultsof oppositevergence, on the northern and southernsides of the FerghanaValley, may have depressedthe basin. What is mostpuzzlingabout the basin,as revealedby gravity data, is its greatdepth,much deeperthan would be expectedsimply from the loading by rangeson its north and southsides. Thus the basinmight be the result of flexure of the footwalls
of the reverse faults or
perhapsof compressional bucklinginstabilityassociated with the roughlynorth-southcompression of theregion. The valley surfaceis lessthan 500 m high, with the surroundingChatkal, Ferghana,and Alai ranges each reaching4000 - 5000 m height.On its westernside,accordingto seismicandborehole data, the basin is filled with more than 8 km of, mainly Neogene,sediment[Cobboldet al., 1993, 1996; Yudakhin, 1986].
3. Gravity Anomalies A completeBouguergravitymap was constructedat a scale of 1:2,000,000(Figure2) using ~ 15' by 15' averagevalues from datamadeavailablethroughthe Centerof Geophysical ComputerData Studiesof the JointInstituteof Physicsof the Earth RussianAcademy of Sciencesand from the gravity mapsusedby Burov and Kogan [1990], Burov et al. [1990], Burov et al. [1993], Beekman[1994], Artemjevet al. [1994]. Topographyis from the GETECH (GeophysicalTechnology Ltd.) Global DTM5 (5' x 5') database.Separate onedimensionalgravity profiles (Figure 3) and the isostaticresiduals(Figure2c), preparedin 1991 from 5' x 7.5' data,have 10 km interval, which providessufficientresolutionfor the purposesof this study.Becauseoriginalgravitymeasurements were made with a closerspacing(about 1-2 pointsper 1 km), thereshouldbe no artifactsdue to interpolation.High-quality datasets(griddedat spacingsof 5' x 5', 1' x 1', andsmaller) for the former Soviet Union (FSU) do also exist at Interna-
tional Scientific EnvironmentalCenter of RussianAcademy of Sciences, the Russian Military Geodetic Service, or GETECH.
We analyzegravity anomaliesin termsof deviationsfrom Airy isostasyusingflexed and buckled(or folded)elasticand inelasticplates. Suchplatesmay be loadedboth by vertical
BUROV AND MOLNAR:GRAVITY ANOMALIESOVERFERGHANAVALLEY
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BUROV AND MOLNAR: GRAVITY ANOMALIES OVER FERGHANA VALLEY
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