JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B1, PAGES 533-544, JANUARY 10, 1996

Gravity field over the : Evidence for a composite basin along a transform fault Zvi Ben-Avraham,• Uri ten Brink,2 Robin Bell,3 and Margaret ReznikovTM

Abstract. The Sea of Galilee (Lake Kinneret) is locatedat the northernportion of the Kinneret-Bet Shean basin, in the northern transform. Three hundred kilometers of continuousmarine gravitydata were collectedin the lake and integratedwith land gravitydata to a distanceof more than 20 km around the lake. Analysesof the gravity data resultedin a free-air anomalymap, a variable densityBouguer anomaly map, and a horizontalfirst derivativemap of the Bougueranomaly. These maps,together with gravity modelsof profilesacross the lake and the area southof it, were used to infer the geometryof the basinsin this region and the main faults of the transformsystem. The Sea of Galilee can be dividedinto two units. The southernhalf is a pull-apart that extendsto the Kinarot Valley, south of the lake, whereasthe northern half was formed by rotational openingand transversenormal faults.The deepestpart of the basinalarea is locatedwell southof the deepestbathymetric depression. This impliesthat the northeasternpart of the lake, where the bathymetryis the deepest,is a youngfeature that is activelysubsiding now. The pull-apart basin is almostsymmetrical in the southernpart of the lake and in the Kinarot Valley southof the lake. This suggeststhat the basin here is boundedby strike-slipfaults on both sides.The easternboundary fault extendsto the northernpart of the lake, while the westernfault does not crossthe northern part. The main factor controllingthe structuralcomplexity of this area is the interactionof the Dead Sea transformwith a subperpendicularfault systemand rotated blocks.

Introduction zone at the Alpine orogenicbelt [Ben-Menahemet al., 1976; Nut and Ben-Avraham,1978]. The Sea of Galilee (Lake Kinneret) is locatedat the north- The structure of the Kinneret basin appears to be more ern portionof the Kinneret-BetShean basin (Figure la), in the complexthan that of other pull-apart basinsalong the Dead northern Dead Sea rift [Schulman,1962; Freund, 1978]. The Sea transform(e.g., Dead Sea basin,Hula basin).The com- Dead Sea rift is a plate boundaryof the transformtype which plexity of the area resultsfrom the fact that two fault systems connectsthe Red Sea, where seafloorspreading occurs, with intersectin the lake's area. The main fault systemtrends north- the Zagros zone of continentalcollision [Freund, 1965; Gaf- southand is part of the Dead Seatransform, and the secondary funkel, 1981]. The Dead Sea transformfollows a small circle fault systemtrends NW-SE on the western side of the main from its southernedge at the northern Red Sea to the Sea of fault and extendsinto the Galilee. Superpositionof vertical Galilee area [Gaffunkel, 1981]. North of the Hula basin the displacementsperpendicular or oblique to the transformcre- main trace of the transform changesits orientation to the ated complicated structures in this area. Because Plio- northeast(Figure lb) and forms the Yammunehfault, while Pleistocene basalt flows and intrusions of variable thickness other faults, mainly the Roum fault in Lebanon, continue cover the area [Neev, 1978;Mar, 1986], structuralinterpreta- along the small circle to the continentalmargin near Beirut tion of the Kinneret-Bet Shean basin is difficult. [Girdler, 1990]. At the region occupiedby the Kinneret-Bet The geometryof the plate boundaryin the Sea of Galilee Sheanbasin a secondaryNW-SE to W-E trendingfault system, area hasbeen the subjectof considerabledebate [Freund, 1978; which is composedof branchingfaults, existson the western Gaffunkel, 1981;Kashai and Craker, 1987;Rotstein and Bartar, sideof thetransform fault (Figure lb). Farthernorth, branch- 1989;Rotstein et al., 1992;Heimann and Ran, 1993]. The main ing faults were developedon the other side of the transform problemsin depictingthe geometry of the plate boundary fault in the PalmyraRange area [Walley,1988]. A diffusionof resulted from the lack of geophysicaldata from the Sea of slip from the main transformtakes place along the branching Galilee and the lack of surfaceexpressions of faults in the faultsas the Arabian and African Platesapproach the collision northern Sea of Galilee and in the area north of the lake. The Sea of Galilee is a body of freshwaterwhose surface is at about210 m belowmean sealevel (msl). Its lengthis about •Departmentof Geophysicsand Planetary Sciences, Tel AvivUni- 20 km, its maximum width is about 12 km, and its maximum versity, Tel Aviv, . 2U.S.Geological Survey, Woods Hole, Massachusetts. depth is 46 m (Figure 2). The Kinarot Valley southof the lake 3Lamont-DohertyEarth Observatory, Palisades, New York. is a narrow rift valley; the valley floor dips gently southward. 4Nowat the Institutefor PetroleumResearch and Geophysics, Ho- North of the Sea of Galilee is the Korazim Heights,an elevated lon, Israel. feature which is intensely deformed and covered mostly by Copyright 1996 by the American GeophysicalUnion. basalts[Heimann and Ran, 1993]. Farther north, anotherpull- Paper number 95JB03043. apart basinexists beneath the Hula Valley [Gaffunkel,1981]. 0148-0227/96/95JB-03043505.00 Previous studies of the subbottom structure of the lake have

533 534 BEN-AVRAHAM ET AL.' GRAVITY FIELD OVER THE SEA OF GALILEE

290-- 35•,50'E

•B (:zsi n / 280-

)'N

26( Komzim

250 Sea of Gatitee

(Lake Kinneret)

240

230

-32o30'N

180 190 200 210 220• I 230 0 5kin. I Figure la. Locationmap of the Seaof Galileeand its vicinity. Topographic contours 200, 0, and-200 m are shown. The locations of Zemah 1 and Notera 3 wells are also shown. The Kinneret-Bet Shean basin extends fromthe northern shore of theSea of Galileein thenorth to latitude32ø28'N in thesouth. Both geographic and Israeli coordinatesystem are shown. BEN-AVRAHAM ET AL.' GRAVITY FIELD OVER THE SEA OF GALILEE 535

35ø00' 55ø30'E

ß "•/•MUNAH- 0 ?5 KM i o

HULA VALLEY 33ø00'N

GALILEE

[.] ß

SEA OF GALILEE

SHEAN VALLEY

ALPINE OROSENIC BELT

_, .... -'"'"•,•1 • I _ MEDITERRANEANSEA • • 32000'

• - ,1• ARABIAN SUEZ.lET• •[/• PLATE ß i' DEADSEA

L I Figure lb. Major faultsof the Dead Seatransform system [after vanEck and Hofstetter,1990; Heimann and Ron, 1993].The box outlinesthe boundariesof the maps in Figuresla-6 and 8. JR] marks the area of pervasivefaulting accompaniedby block rotation. Inset showsthe configurationof tectonicplates.

usedvarious methods including seismic reflection and refrac- trusiveigneous rocks [Marcus and Slager,1985]. The valley is tion [Ben-Avrahamet al., 1981,1986], magnetics [Ben-Avraham coveredby the QuaternaryLisan Formation (Figure 3). et al., 1980; Ginzburgand Ben-Avraham,1986], bathymetry In order to learn more about the subbottom structure of the [Ben-Avrahamet al., 1990],and heat flow [Ben-Avrahamet al., Sea of Galilee, we conductedin October 1988 a detailed grav- 1978]. Although the seismicreflection profiles in the Sea of ity surveyon the lake. The marine data were combinedwith Galilee are of poor quality,they indicatethat the lake area is gravitydata on land aroundthe lake to constrainthe geometry tectonicallyactive. Active faults and folded structureswere and structureof the basin and understandits development. detectedin the uppermostsediments along the marginsand in the interiorof the lake [Ben-Avrahamet al., 1981].The seismic Methods reflectionprofiles were obtainedwith variousinstruments, all high-resolutionsingle channel with maximumpenetration of Gravity measurementsin the Sea of Galilee were made about 40 m. The maximumpenetration of the seismicrefrac- aboard R/V Hermona, a 6-m-longboat of the Kinneret Lim- tion profileswas 630 m. nologicalLaboratory, Israel Oceanographicand Limnological Severalseismic profiles were obtainednorth and southof ResearchLtd., usinga BGM-3 sea gravitymeter of Lamont- the lake [Rotsteinand Bartoy,1989; Rotstein et al., 1992].A few Doherty Earth Observatory.Positions were obtainedwith a holeswere drilled in the vicinityof the lake; the deepestone is Motorola Miniranger navigationsystem. the Zemah 1 well located at the center of the Kinarot Valley, The BGM-3 gravity meter system,manufactured by Bell south of the lake. It penetrated a 4249-m sequenceof graben Aerospace,consists of an inertial navigation-gradeaccelerom- fill consistingof clastic,evaporitic, and both intrusiveand ex- eter mountedon a gyrostabilizedplatform and a data handling 536 BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE

35' 30 35' 50'E

HU[Q Bc•sin

o ø

YN KorQzim Heights

26(

251

23( •.•1ley o •

n VOlley

180 190 200 210 220 230 Figure 2. Simplifiedtopographic map of the Seaof Galilee and itsvicinity. Contour interval is 100m on land and 10 m in the lake.

system[Bell and Watts, 1986]. The small size of the gravity north-southlines spaced 1-2 km apart.The total lengthof lines meter systemmade it possibleto use a relativelysmall boat for was about 300 km. The gravity data were collectedevery 1 s the measurements. and were averagedonce a minute. Sincethe averagespeed of The measurementswere made along a grid of east-westand the boat was 10 knots (18.5 km/hour), about 1000 new grav- BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE 537

35030 ' 35ø50'E

HULA VALLEY

33o00'N 'GALILEE

SE.4 OF 8.4 L IL EE

KINA t•0 T VALLEY

<:• 0 5 IOKM'J- 32030 '

Figure 3. Simplifiedgeological map of the land area surroundingthe Sea of Galilee (modifiedfrom Picard and Golani [1976] and Ron et al., [1984]). The patternsare grid, Plioceneto Pleistocenebasalts; inverted Ts, Jurassic-Eocenesediments (mainly carbonates);open areas,Neogene to recent clasticand lacustrinesedi- mentsof grabenfill. Arrowsindicate the locationsof gravityprofiles (Figure 7). The locationof Zemah 1 drill hole is shown.Abbreviations are AF, Almagor fault [Heimannand Ron, 1993]; JF, Jordanfault.

ity measurementswere collected, one every 300 m along the boat's velocity. The uncertaintyof the gravity measurementis lines. lessthan 1.5 mGal, and the uncertaintyof the navigationwas A moving averagefilter with a 200-s Gaussianwindow was estimatedat about 3 m. Bathymetricdata were availablefrom applied to the gravity data to remove the accelerationsof the an earlier survey[Ben-Avraham et al., 1990] in which measure- boat. The data were manually edited to remove bad measure- mentswere made on a grid of east-westand north-southlines ments. E6tv6s correctionwas applied to compensatefor the with line spacingof 100 m. An averageof the bathymetricdata 538 BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE

3•30'E 35 }'E

32' 3535'

50'

- 2

45'

_32ø3o,N 200 205 2•0 180 )0 220 Figure4. (a) Free-airgravity anomaly map of the Seaof Galileeand its vicinity and locations of gravity stations(dots on landand lines in thelake). The gravity values were gridded at 0.65-kmsquare cell size and smoothed.Contour interval is 5 mGal. Arrowsat the sidesof the map indicatelocations of gravityprofiles (Figure7). (b) Free-airgravity anomaly map of theSea of Galilee.The gravity values were gridded at 0.2-km squarecell sizeand smoothed. Contour interval is 4 mGal. wascomputed within a circlewith a radiusof 25 m around ciationwith topographicand bathymetricescarpments. Steep every gravity measurement. gradientsof 25 mGal/km exist in placesalong the western The gravitymeasurements over the Sea of Galilee were marginof the Bet Sheanbasin, including the KinarotValley, combinedwith gravitydata in the surroundingareas which and the southwestmargin of the lake. were obtained from the archives of the Institute of Petroleum An enlargementof the free-air anomalywithin the lake Researchand Geophysics,Israel. Relativelyfew data points (Figure4b) showsthat the gravityminimum, defined by the existeast of the lake, while a densecoverage exists south, west, -40-mGal contour,is displacedto the easternside of the lake and north of the lake. The tie with land data was accomplished andis southof the principalbathymetric depression of the lake by repeatedmeasurements at the KinneretLimnological Lab- (comparewith Figure2). In the southernpart of the lake a oratorydock and a knownreference station of the Israeli steepgradient in the free-air gravityanomalies is oriented gravitynetwork. The landvalues were rccalculatcdusing the north-south and is located between the western coastline and 1967 International Gravity Formula. the gravityminimum. It extendsabout half way up the lake from the southernedge of the lake. The steepesttopographic Free-Air and Bouguer Gravity Anomalies escarpmentis, on the otherhand, along the easternside of the lake. The free-airgravity anomaly map (Figure 4a) showsmany of the same featuresas the topography.The anomalyis quite Followingdensity measurements by Folkman[1976] and a variable: the lake coincideswith a negative anomaly (-40 gravitystudy of the DeadSea basin [ten Brink et al., 1993],two mGal) andthe surroundinghighlands with positive anomalies densitieswere chosenfor the Bouguercorrections of Figure5: (+ 120mGal). To thewest the anomalyis ratherflat anddrops 2550kg/m 3 for all the pointsshallower than 150 meters below by about 100 mGal into the basin.The Kinneret-BetShean sealevel (mbsl) and 2150 km/m 3 for allthe points deeper than basin,the KorazimHeights north of it, and the Hula basin 150 mbsl.The first densitycorresponds to the Mesozoicand farther north are well defined lows on the map. Severalzones early Cenozoiccarbonate cover outside the transformvalley of steepgradient of the gravityfield canbe observedin asso- and the secondto the Quaternarysediments inside the valley. BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE 539

35 32' 32 35'

50'

32 45'

.•o30• 21 180 220 200 205 2i0 Figure 5. (a) Bouguergravity anomalymap of the Sea of Galilee and its vicinity and locationsof gravity stations(dots on land and linesin the lake). The gravityvalues were griddedat 0.65-kmsquare cell sizeand smoothed.Density used for theBouguer correction is 2550 kg/m 3 above150 meters below sea level (mbsl) and 2150 kg/m3 below150 mbsl.Reference level for map and lake level duringthe experimentis 210.2mbsl. Contourinterval is 5 mGal. (b) Bouguergravity anomaly map of the Sea of Galilee. The gravityvalues were griddedat 0.2-km squarecell size and smoothed.Contour interval is 4 mGal.

Thewater density was replaced with a densityof 2150kg/m 3. A the -30-mGal contour, is located at about the same place as referencelevel of -210.2 m (the lake level duringthe experi- the free-air gravity minimum (Figure 5b). The northeastern ment) was chosenfor the Bouguercorrection. margin of the lake is not well definedby the contoursof either Bouguer gravity correction is an effectiveway of removing the free-air or Bouguergravity anomalies. the gravitationalinfluence of local (short wavelength)topog- The gravity maps (Figures 4 and 5) and first horizontal raphy. The Bouguer anomalymap (Figure 5a) showsthe re- derivativemap suggestthat Sea of Galilee is divided into two gional gradient which decreasesto the east acrossthe Dead subbasins.The center of the deepestone coincideswithin the Sea transform from values in excess of 40 regal to values gravityminimum at latitude 32ø46'N,and it extendssouthward around0 regal. West of the Dead Sea transform,the Bouguer into the Kinarot Valley. It also extendsnorthward up to the gravity map showsa gravity gradient decreasingeastward. A widest part of the lake, at latitude 32ø50'N, and has an elon- similar eastwardgradient was observedfarther south (along gatedshape in a north-southdirection. The secondmore subtle latitude 32ø00'N) and was interpretedto representthickening E-W trending subbasinoccupies the widestpart of the lake. It of the crust from -<12 km in the Mediterranean Sea to 35 km may extendto the northeastinto the Buteikha Valley on land in the [ten Brink et al., 1990]. The gradient is and to the west into the Ginosar Valley on land. steepestclose to the transform indicatingpossibly a step in The first horizontal derivative map of the Bouguer anoma- Moho thicknessacross the transform,due to the juxtaposition lies (Figure6) emphasizesthe shortwavelength features of the of crustsof different thicknessby the strike-slipmotion [ten Bougueranomalies. The Kinneret-Bet Sheanbasin is bounded Brink et al., 1990]. The gradient greatly reduces east of the on the western side by a lineament of high gravity gradient. transform,probably due to the fact that there is no thickening The crest of the maximum gradient in this area probablyrep- of the crust beyond the plate boundary. Two oblong-shaped resentsa fault separatingrocks of different densities.Within areasof relativelynegative gravity anomalyexist over the Sea the lake itself the westerngradient widens and appearsto split of Galilee and the Hula basin. away from the southwestcoast of the lake which is also asso- Within the lake the Bouguergravity minimum, denotedby ciatedwith high gravitygradient. On the east a lineamentof 540 BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE

35ø30'E 35050 '

33o00'N 33o00'N

Figure 6. Map of the magnitude of the first horizontal dcrivative of the Bougucr anomaly map. Map emphasizesthc shortwavclength features of the Bouguermap (Figure 5). Contourintcrval is 2 mGal/km. high gravitygradient borders the Sea of Galilee and the Kin- Gravity Models arot Valley, south of the lake. It probably extends farther south; however, only one profile of gravity stations at Three two-dimensionalgravity models were calculatedalong -32ø35'N extends east of the valley. The Hula basin is east-westprofiles across the basinand adjacentland areasand bounded, like the southern Kinneret-Kinarot basin, on the comparedwith free-air anomaliesalong the profiles(Figure 7). easternand westernsides by lineamentsof high gravitygradi- Two are located across the lake and one south of the lake ents.The first horizontalderivative map (Figure 6) alsoshows (Figures3 and 4). No gravitydata exist east of the Kinarot a high gravity gradient along the northwesternmargin of the Valley south of the lake along the easternpart of profile 3. lake. This suggeststhat a subsurfacefault probably separates Therefore data from the singleprofile of gravity stationslo- the Korazim Heights from the Sea of Galilee basin. catedsouth of the profile at -32ø35'N and data from the area BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE 541

north of it were interpolatedto fill the gap. A linear trend was W E removedfrom the free-air gravityprofiles prior to the model- ing. The densitiesthat were used are of Lake Kinneret water (1000kg/m3), the sedimentfill (2150kg/m3), the Mesozoic- earlyCenozoic carbonates (2550 kg/m3), the crystallinebase- •o ment(2670 kg/m3), and the Tertiary basalt flows (2750 kg/m3). o ',,,.•J The thicknessof the basaltflows in the modelsranges between -40 ß ß 2.75• 150 and 200 m in accordance with the observation of Mor [1986] on basaltflows in the Golan Heights. 5 • 2.15 2.55 The modelsused available geological and geophysicalinfor- 10 2.55 2.67 mation including topographyand bathymetry, seismicdata [Ben-Avrahamet al., 1981;Rotstein et al., 1992], stratigraphic 15- informationfrom the Zemah 1 well [Marcusand Slager,1985], 180 200 220 and the depth to crystallinebasement [Ginzburg et al., 1979] along the west margin of the Dead Sea transform.The latter is 2 E 80 . unknown under the Kinneret-Bet Shean basin. The modelsindicate that the width and depth of the basin •- 40 vary along its length. The deepestpart of the basin reaches n- 0 more than 6 km. The width varies from 8 to 18 km. The models -40 for the southernpart of the lake (profile2) and for the Kinarot . 2.75• Valley (profile 3) suggestthat the narrow, deep basin in this •, 5 2.55 • 2.55 region is probably bordered by faults on both sides.In the E 2.15 northern part of the lake the main basin extendswestward 'r •0 toward the Ginosar Valley on land with reduced thicknessof 2.67 2.55 2.67

the sedimentaryfill. In this part (profile 1), only the eastern • •5 ' I I I I side is probablybordered by a fault. 180 200 220

3 The Structure of the Sea of Galilee 80 The analysisof the gravity data together with previously obtainedbathymetric, seismic, and magneticdata providesin- o formation about the structure of the Sea of Galilee. The lake can be divided into two distinct units. South of latitude -40 2.75• Z 32ø49'N, the lake is narrow and bordered by two north-south •. 5 trendingboundary faults (Figures 6 and 7). In this area the E basin is the deepest.North of latitude 32ø49'N, the basin is 2: 10 2.55 wider. The westernboundary fault at the southernpart of the a. 2.67 lake probablydoes not continueinto the northern part. The main basinin the southernpart of the lake is shownas a valley ' in the first horizontal derivativemap (Figure 6). It has an I.ONGITUDF (km.) elongatedshape, typical of pull-aparts.The westernboundary Figure 7. Three east-westgravity profiles (dashed lines, see at this part of the basincontinues southward into the Kinarot Figures 3 and 4a for location) extracted from the gridded and Bet Shean Valleys. free-air gravity data shown in Figure 4 with linear trend re- The floor of the lake is quite smoothprobably because of the moved, comparedwith calculatedgravity (solid lines) from considerablesedimentation rate. A notableexception is a small two-dimensionaldensity-depth models (below the profiles). WNW trendingbathymetric scarp at water depthsof 13-21 m Densityis 1 x 103kg/m 3. Values for theeastern part of profile alonglatitude 32ø44'N (note the contours-225 m and -230 m 3 were interpolatedfrom data north and south of it. Dark in Figure2). Seismicreflection and magneticdata suggestthat layers are basalt flows. Distance in kilometers is measured the scarpis a surfaceexpression of a fault [Ben-Avrahamand ten from longitude 180, Israel coordinate system. Location of Zemah 1 drill hole is shown on profile 3. Inverted triangles Brink, 1989].It couldmark the locationof a transversefault that mark the boundaries of the lake. separatesthe Kinarot Valley from the southernpart of the lake. However,this fault is not associatedwith a high gravitygradient. The gravity models suggestthat the basin in the southern part of the lake is a full grabenor closeto a full grabensimilar Althoughthe situationin the Kinarot Valley, southof the lake, to the Dead Sea basin [tenBrink et al., 1993]. On the basisof is lessclear becauseof the lack of gravitydata eastof the valley earlier studiesin the Gulf of Elat [Ben-Avraham,1985] and exceptfor a singleprofile of gravitystations, evidence from other continental transforms [Ben-Avraham, 1992; Ben- seismicreflection profiles suggests that a full grabenmay also Avrahamand Zoback, 1992], this situationprobably indicates existhere. Rotsteinet al. [1992]have proposedon the basisof that strike-slipmotion is taking place along both the western high-resolutionmultichannel seismic profiles en echelonarrange- and easternboundary faults in the southernpart of the lake mentsof the Dead Seatransform segments in the Kinarot Valley. (Figure 8). The westernboundary fault in the deeppart of the The Bouguergravity map (Figure5b) andthe gravitymodels lake is expressedin the bathymetry[Ben-Avraham et al., 1990] further suggestthat the basinin the northern part of the lake as a step in the otherwise smooth slope of the lake floor. is asymmetricto the east. However, strike-slipmotion along 542 BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE

35ø30 ' 35ø50'E ß I ' I

PULL APART BASIN

- 33ø00'N

ß

OPENING DUE TO ROTATIONS

PULL APART BASIN

5 I0 KM I i 32030 ' - BetShean I Figure 8. Schematicmap showingthe main structuralelements in the Sea of Galilee and its vicinity.Heavy lines are faults. Faults with arrowsmark our interpretationof strike-slipfault strandsof the transform.The lowsin the Seaof Galilee and Hula basinare from the firsthorizontal derivative map (Figure6), andthe high in the KorazimHeights is the centerof the elevationin the Bougueranomaly map (Figure 5). Dotted lines are crestsof the maximumgravity gradients. Hatched areas mark fault zonesobserved in seismiclines [after Rotsteinand Bartoy,1989; Rotstein et al., 1992].North of the Seaof Galilee, the barbedline marksthe surface trace of a reversedfault which dips under the Korazim Heights.Arrows in the Korazim Heights and the easternGalilee mark rotations[after Ron et al., 1984;Heimann and Ron, 1993]. BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE 543

the easternmargin probablydoes not extendnorth of latitude Jordan fault extends from the western side of the Sea of Ga- 32ø49'N. Otherwise,a pull-apart basin shouldexist in the Bu- lilee to the easternside of the Hula Valley, then the fault trace teikha Valley (Figure l a), becausethe location of the left- is obliqueto the directionof plate motion,which will result in lateral transform north of the lake is shifted to the left com- shortening. pared with the fault along the eastern margin of the lake. Seismicreflection data [Rotsteinand Bartoy, 1989] showthat Seismicreflection profiles and gravitydata north of the lake do the main strike-slipfault east of the Korazim block dips west- not showthe existenceof a pull-apart basin there. Strike-slip ward at about70 ø and presentstructural evidence for compres- motion thus is taking place along the eastern margin of the sion acrossthe fault. The Jordan fault plane at a depth of 4-5 Kinarot Valley and the southernpart of the easternmargin of km lies beneaththe surfacetrace of the Almagor fault; thusthe the lake,while in the west,strike-slip motion is probablytaking Almagor fault is probably a secondaryantithetic fault. We place along a boundary fault that extends from the western suggestthat compressionacross the fault and the uplift of the marginof the Kinarot Valley northwardinto the southernpart of Korazim Heights is the result of block rotation on the inclined the lake. The westernfault doesnot extendthrough the entire Jordan fault. There can be two reasonsfor the shortening. lengthof the lake, unlike an earliersuggestion that wasbased on First, if the uplifted block consistsof NW-SE elongatedslivers gravitydata from around the lake [Kashaiand Croker, 1987]. [Heimannand Ron, 1993], the sliversmust undergoshortening during counterclockwiserotation. Second, rotation over an inclined fault surface,particularly when the fault surface de- Synthesis creasesin dip to the north [Rotsteinand Bartoy, 1989], will The Sea of Galilee basin is divided into two distinct units. force the edge of the block to climb the fault surfacein order The southernhalf forms a continuationof the Kinarot Valley. to fit the smaller available space.Therefore we interpret the The northern half was probablyformed by two setsof faults, uplift of the Korazim block not as a push-upstructure but as a the N-S trendingDead Sea rift and the NW-SE to E-W trend- result of shorteningforced by the rotation of the block. ing easternGalilee branchingfaults (Figures 3 and 8). The A key kinematicquestion is the continuityof motion along branchingfaults break the regionsadjacent to the transformin the differentsegments of the Dead Sea transformin the area. the Sea of Galilee area [Ben-Avrahamet al., 1980]. As a result, Although a gradualright stepfrom the westernboundary fault two subbasinswith different trendswere formed and superim- of the Sea of Galilee to the southeasternpart of the Hula posedwithin the area coveredby the lake. The southernbasin Valley [Rotsteinand Bartoy, 1989] is a simple solutionto this is probably a pull-apart which was formed as a result of the question,it will lead to an uplift centeredaround the right step. transformmotion. The deepestpart of thisbasin is locatedwell In other words,the uplift will be centeredin the northern part southof the deepestbathymetric depression, which is probably of the Sea of Galilee, an area currentlyunder subsidence.On a young feature that is activelysubsiding now. The center of the other hand, if the majority of motion along the Dead Sea the bathymetricdepression is located in the northern basin transform is taken on the easternboundary fault of the Sea of which was formed by a complexdeformation, and it is not a Galilee, then a small left step northward toward the Jordan pull-apart. The existenceof the two units gives the lake its fault is expected to create a small pull-apart basin. Seismic unusualshape, which is quite different from a typical rhomb- reflection [Rotsteinand Bartoy, 1989] and our gravity data do shapedbasin. not supportthe existenceof a basinunder the NE shoreof the In the north the Hula is a classicalpull-apart basinwith a left Sea of Galilee. Hence we are forced to conclude that the steppingjog occurringaround latitude 33ø06'N.The two faults motion in the northern Sea of Galilee is accommodatedby may overlapby as much as 5 km and are separatedby 6-7 km. either nonrigiddeformation (e.g., crushed zone [Sibson,1985]) The eastern fault extends from the Sea of Galilee northward or block rotations and numerousoblique and perpendicular parallel to the JordanRiver gorge(the Jordanfault [Belitzky, normal faults (Figure 8). 1987]). At the south margin of the Hula basin the strike-slip A possibleby-product of the counterclockwiserotation is motion is probablyshifted to the westernboundary fault [Rot- opening along the northern coast of the Sea of Galilee, as steinand Bartov, 1989]. The easternboundary fault along the suggestedby the gravitydata. The gravitymap showsgradual easternmargin of the Hula basinis mainly a normal fault. The increasein the gravityvalues toward the central and southern gravityanomaly delineates the spatialdimensions of the basin partsof the Sea of Galilee. This indicatesthat the northern Sea to be 20 km by 6 km (Figure5a). Drilling at the Notera 3 well of Galilee graduallydeepens southeastward. Large-scale coun- in the Hula basin shows a sediment thickness of-2.8 km terclockwiserotations (53ø) of rockswere also ob- [Heimann and Steinitz 1989]. The negative gravity anomaly servedwest of the lake (Tiberiasprovince IRon et al., 1984]). associatedwith the basin is about 25 mGal, which gives an These rotationscould result in openingof the Ginosar Valley averagesediment density of 2150kg/m 3. The sedimentcom- northwestof the lake. The structureof the northern part of the position as revealed in the drill hole consistsof alternating lake is probably further complicatedby a NW-SE trending basalt and layers of peat, lignite, marl, sand, freshwaterlime- normal fault systemwhich terminates eastward in the lake. stone, and conglomerate[Heimann and Steinitz,1989]. Thus the Sea of Galilee basin is perceived as a composite South of the Hula pull-apart basin is the Korazim Heights depressioncaused by a possible combination of pull-apart block.Heimann and Ron [1993] inferred 11ø counterclockwise opening,rotational opening,and transversenormal faults. rotation of this block from paleomagneticdata. They attribute the uplift of this block to a compressioncomponent which was Conclusions introducedby a secondarystrand of the Dead Sea transform (the Almagor fault) which strikes020 ø, while the main strand The resultsof the gravityanalysis over and aroundthe Sea of of the transform(the Jordanfault) trendsnorth-south (Figure Galilee indicate that two subbasins exist within the lake. The 3). Rotsteinand Bartoy [1989] suggestedthat the Korazim southernsubbasin was formed as a pull-apart and is bordered Height is a "push-up" structure. They reasoned that if the on its east and west sidesby segmentsof the Dead Sea trans- 544 BEN-AVRAHAM ET AL.: GRAVITY FIELD OVER THE SEA OF GALILEE form. This is the deeper subbasinwith more than 6 km of Freund, R., The conceptof the sinistralmegashear, in Lake Kinneret, sediments.The northernsubbasin is bathymetricallythe deep- Biogr. Biol., vol. 32, edited by C. Serruya, pp. 27-31, Junk, The Hague, 1978. est. It is probablythe most activelysubsiding area in the Sea of Garfunkel,Z., Internal structureof the Dead Sealeaky transform (rift) Galilee and was probably formed as a result of the counter- in relation to plate kinematics,Tectonophysics, 80, 81-108, 1981. clockwise rotation of the Korazim block north of the Sea of Ginzburg, A., and Z. Ben-Avraham, Structure of the Sea of Galilee Galilee and by branchingfaults. graben, Israel, from magnetic measurements,Tectonophysics, 126, Thus the Sea of Galilee is a compositedepression that was 153-164, 1986. Ginzburg, A., J. Makris, K. Fuchs, C. Prodehl, W. Kaminski, and formed by a possiblecombination of pull-apart opening,rota- U. Amitai, A seismicstudy of the crust and upper mantle of the tional opening,and transversenormal faults. We suggestthat Jordan-Dead Sea rift and their transition toward the Mediterranean the structuralcomplexity that led to the existenceof two sub- Sea, J. Geophys.Res., 84, 1569-1582, 1979. basins within the relative small area of the Sea of Galilee is the Girdler, R. W., The Dead Sea transformfault system,Tectonophysics, 180, 1-13, 1990. result of the breakup of coherentplates in front of the plate Heimann, A., and H. Ron, Geometric changesof plate boundaries collision zone to the north. alongpart of the northernDead Seatransform: Geochronologic and paleomagneticevidence, Tectonics, 12, 477-491, 1993. Heimann,A., andG. Steinitz,4øAr/39Ar total gas ages from Notera #3 Acknowledgments. This studywas supported by the Earth Sciences well, Hula Valley, Dead Sea rift: Stratigraphicand tectonicimpli- Administration, Israel Ministry of Energy and Infrastructure and by cations,Isr. J. Earth Sci., 38, 173-184, 1989. the Dead Sea Research Center, Tel Aviv University. We thank Kashai,E. L., and P. F. Croker, Structuralgeometry and evolutionof Y. Rotstein for permissionto publishthe land gravity data; G. Amit the Dead Sea-Jordanrift systemas deducedfrom new subsurface and A. Golan for their help in obtainingand reducingthe navigation data, Tectonophysics,141, 33-60, 1987. data; N. Schoenbergand the Institute of Petroleum Research and Marcus,E., and J. Slager,The sedimentary-magmaticsequence of the Geophysics,Israel, for measurementsof the tie-in gravity station; Zemah-1well (Jordan-DeadSea rift, Israel) and its emplacementin M. Ribakov for the use of his two-dimensionalgravity modeling pro- time and space,Isr. J. Earth. Sci., 34, 1-10, 1985. gram;J. Zwinakisand O. Paran for draftingmany of the figures;hnd Mor, D., The volcanismof the Golan Heights(in Hebrewwith English the captainof R/V Hermona and the staffof the Kinneret Limnological abstract),Ph.D. thesis, 159 pp., Hebrew Univ., Jerusalem,Israel, Laboratory,Israel Oceanographicand LimnologicalResearch Ltd. for 1986. their technicalassistance. We also thank K. 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Vered, Tectonics,seismicity and ences, Tel Aviv University, Tel Aviv, 69978, Israel. (e-mail: structureof the Afro-Eurasian junction--The breaking incoherent zri@jupiter 1. t au.ac. il) plate, Phys.Earth Planet.Inter., 12, 1-50, 1976. R. Bell, Lamont-DohertyEarth Observatory,Palisades, NY 10964. Folkman, Y., Magnetic and gravityinvestigations of the crustalstruc- M. Reznikov, Institute for Petroleum Researchand Geophysics,P. ture in Israel (in Hebrew), Ph.D. thesis,Tel Aviv Univ., Tel Aviv, O. Box 2286, Holon 58122, Israel. Israel, 1976. U. ten Brink, U.S. GeologicalSurvey, Woods Hole, MA 02543. Freund, R., A model of the structural developmentof Israel and adjacentareas since Upper Cretaceoustimes, Geol. Mag., 102, 189- (ReceivedNovember 9, 1994;revised September 25, 1995; 205, 1965. acceptedSeptember 26, 1995.)