Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue4.2000:

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Ž .

Journal of Applied Geophysics 44 2000 353–367

www.elsevier.nlrlocaterjappgeo

3-D gravity and magnetic interpretation for the

ž

/

Haifa Bay area Israel

M. Rybakov

)

_{, V. Goldshmidt, L. Fleischer, Y. Ben-Gai}

( )

The Geophysical Institute of Israel GII , P.O. Box 2286, Holon 58122, Israel Received 22 September 1998; accepted 28 March 2000

Abstract

Ž .

Recently observed features in the subsurface geology of the Haifa Bay area northern Israel have been evaluated using 3-D forward gravity and magnetic modeling and inversion schemes. The interpretation is based on updated petrophysical data of the Jurassic, Cretaceous and Tertiary sedimentary layers and volcanics. It has been shown that the Bouguer gravity anomalies correspond mainly to thickness variations in the Senonian to Tertiary sediments. The gravity effect of these sediments was calculated using their actual densities and structural setting as interpreted from seismic reflection data. This effect was removed from the Bouguer gravity in order to study the pre-Senonian geological structures. The pattern of

Ž .

residual gravity anomalies named ‘‘stripped gravity’’ is essentially different from the pattern of the Bouguer gravity. The prominent Carmel gravity high, clearly seen on the Bouguer gravity map, completely vanishes on the ‘‘stripped’’ gravity map. That suggests that this relatively positive anomaly is caused by the considerable thickness of the low-density young sediments in the surrounding areas and does not correspond to high-density magmatic rocks or crystalline basement uplift as previously suggested. The average densities of the Jurassic and Cretaceous volcanics are generally lower then those of the background sedimentary rocks. Volcanics are the main cause for magnetic anomalies onshore and offshore northern Israel. The magmatic root of the Asher volcanics is, most probably, located close to the Yagur fault. A large, deep-seated gabbroic intrusion is assumed to be located under the Mediterranean abyssal plain in the NW part of the study area. The Atlit marine gravity low appears to be caused by a thick Mesozoic and Tertiary sedimentary accumulation. The results presented should be of considerable assistance in delineating some aspects of hydrocarbon exploration in the area.q2000 Elsevier Science B.V. All rights reserved.

Keywords: Gravity; Magnetics; Subsurface structures; Magmatism; Northern Israel

1. Introduction

Ž The Carmel structure in northern Israel Fig. .

1 is an elongated, tilted block extending from the onshore into the shelf of the Haifa Bay area.

)_{Corresponding author. Tel.:}

q972-3-557-6050; fax: q972-3-557-2925.

Ž .

E-mail address: [email protected] M. Rybakov .

This structure comprises a prominent NNE trending folding system, traversed by several NW trending faults. The folding is probably part of the Late Cretaceous to Tertiary ‘‘Syrian

Arc’’ compressional phase Picard and Kashai, 1958; Arad, 1965; Sass, 1980; Ginzburg et al., 1975; Neev et al., 1976; Ron et al., 1984; Rotstein et al., 1993; Ben-Gai and

Ben-Avra-.

Ž .

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Fig. 1. Magnetic anomalies of the Eastern Mediterranean. The anomalies named after the major features, i.e., Eratosthenes

Ž .E , Carmel C and Hebron H are the largest anomalies of the region. The regional tectonic map is shown in the insert.Ž . Ž .

separated from the Qishon graben to the north

Ž .

by the Yagur fault Figs. 2, 4 and 6 with a vertical displacement of more than 1000 m Ž_{Picard and Kashai, 1958; Ginzburg et al., 1975;}

. Neev et al., 1976 .

Our present knowledge of the subsurface ge-ology, both onshore and offshore, is based on

Ž .

the deep oil wells Fig. 2 and on limited and scattered seismic reflection data. The Triassic and Jurassic sequences penetrated in the area are, in general, carbonates with minor facies changes. The Cretaceous strata are characterized by an abrupt change from shallow platform carbonates to open marine, shaly–marly slope facies. The Upper Cretaceous and Cenozoic rocks have been eroded from the elevated areas

of Mount Carmel and its offshore extension. Thick Cenozoic sequences of chalks and marls are only present in the Qishon graben and in the Ramot Menashe–Hadera syncline located north and south of the Carmel structure. The geologi-cal data on both sides of the Yagur fault suggest that it was non-existent until the mid-Cenozoic and its onset probably coincides with the left-lateral motion along the Dead Sea Transform

fault de Sitter, 1962; Freund, 1970; Ben-Gai .

and Ben-Avraham, 1995 .

Mount Carmel is characterized by intense volcanic activity which began in the Early Jurassic and continued during the Cretaceous and Tertiary. The Liassic Asher volcanics of the olivine basalt with some gabbroid magmatic

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M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 355

Ž . Ž .

Fig. 2. Bathyorographical bottom, after Hall, 1993 and magnetic top maps of the Haifa Bay area. Insert: location map of the referred wells corresponding to a bigger area.

intrusions are intercalated with the Triassic and the Jurassic carbonates and exceed a thickness

of 2500 m in the Atlit-1 well Gvirtzman and . Steinitz, 1983; Dvorkin and Kohn, 1989 . Simi-lar basaltic rocks, 200 m thick, were penetrated in the Yagur-1 well, while the alternations of tuffs with basaltic lava flows of 270 m were encountered in the Deborah-2A well located 30

Ž .

km east of the Carmel structure Fig. 2 . The

Ž .

Tayassir volcanics Mimran, 1972 overlie the regional Base Cretaceous unconformity and are widespread in northern Israel. These volcanics consist of basaltic lava and tuffs, alternating occasionally with sedimentary rocks, have been

penetrated in a number of wells. The youngest volcanics, composed mainly of a series of tuffs, are interbedded in the Cenomanian–Senonian rocks and mapped on Mount Carmel and south

Ž .

of it Sass, 1980; Arad, 1965 .

The magnetic anomaly corresponding to the Mount Carmel structure is one of the largest magnetic anomalies in the Eastern

Mediter-Ž .

ranean Fig. 1, Rybakov et al., 1994 . This Ž anomaly was the subject of many studies Ginz-burg, 1960; Domzalski, 1967, 1986; Folkman, 1976; Avraham and Hall, 1977; Ben-Avraham and Ginzburg, 1986; Gvirtzman et al.,

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findings in the Atlit-1 well in 1981 suggested a highly elevated crystalline basement while, af-terwards, the concept of an Early Jurassic shield volcano spreading over the area was adopted by

Ž .

researchers. Garfunkel and Derin 1984 sug-gested that this volcanic phase belongs to an early Mesozoic rifting of the Levant margin. For the moment, a pile of Asher volcanics, 2500 m thick penetrated in the Atlit-1 well, is a unique feature that bears on the evolution of the complex east Mediterranean, so that even its crustal composition is still in dispute.

As noted above, the evolution of the Carmel structure is not fully understood. An extensional tectonic regime, associated with widespread volcanism, is suggested for the Early Mesozoic, followed by the development of a shallow car-bonate platform during Mesozoic and Early Cenozoic times. Later on, in the Middle Ceno-zoic, the left lateral Dead Sea transform fault produced the modern faulted block of Mount Carmel and the Qishon Graben.

Several questions relating to the cycled vol-canic activity in the Carmel area could be for-mulated as follows.

Ø Are there more intrusive bodies in the area?

Ø Was the Asher Volcano fed from a single neck or from a zone of weakness?

Ø If such a zone of weakness is indeed present, is it associated with the modern Yagur fault? Two main problems may be defined with regard to the technical aspects of previous interpreta-tions: the three-dimensionality of the structures was not accounted for and the densities of the volcanics were overestimated. A new interpreta-tion of the gravity and magnetic data, based on 3-D routines with updated density values and magnetic vectors, has been performed and is presented in this paper.

2. The data

The gravity and magnetic data used in this Ž

study are part of the GII Geophysical Institute

. Ž .

of Israel database Rybakov et al., 1997 . The

magnetic data were composed of the aeromag-netic measurements at a constant flight level of

Ž .

about 1 km line spacing — 2 km and marine measurements. The marine magnetic data, con-tinued upward to an elevation of 1 km, are in agreement with the aeromagnetic data. This magnetic data set was checked for erroneous values, gridded and slightly smoothed using the inverse distance method. The International

Geo-Ž .

magnetic Reference Field IGRF was removed and, therefore, the magnetic data used represent magnetic anomalies, provided that the magnetic core field was adequately removed. The

result-Ž .

ing grid Fig. 2 was used for reduction to the pole and upward magnetic continuation, pseudo-gravity calculation and gravity–mag-netic correlation. 3-D maggravity–mag-netic modeling and inversion were also based on this grid.

All the graÕity data were reduced to a

Bouguer density of 2670 kgrm3_{. This value}

Ž was chosen using the Nettleton technique

Net-.

tleton, 1971 applied to a number of typical topographical sections. The terrain corrections for all land gravity stations were calculated up to a 20-km distance using a model with a 25-m grid adopted from the Digital Terrain Model ŽDTM compiled by the Geological Survey of.

Ž .

Israel Hall, 1993 . The gravity data have a reliability and accuracy that allows interpolation to a 2-mGal contour interval. The data set was gridded and gently smoothed using similar tech-niques and parameters. This grid was used for regional–residual gravity separation, horizontal and vertical gravity derivatives calculation and gravity–magnetic correlation. The 3-D gravity modeling and resulting map compilation were based on this grid.

3. Petrophysics

All the available density and magnetic rock susceptibility data were collected and

incorpo-Ž .

rated in a data bank Rybakov et al., 1999 . Fig. 3 presents a generalized petrophysical model of the Mesozoic and Cenozoic rocks.

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M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 357

Fig. 3. Generalized petrophysical model of the Haifa Bay area as inferred from a number of deep boreholes Rybakov et al.,

. Ž 3 3_.

1999 , only four typical density logs are shown. The logging density variation 10 kgrm is shown along the stratigraphic

Ž y5 _.

units drilled by the boreholes. Magnetic susceptibility K in 10 SI is assessed only for volcanics.

The average magnetic susceptibility values Ž

for all igneous rocks Early Jurassic Asher vol-canics, Late Jurassic Deborah volvol-canics, Early Cretaceous Tayassir volcanics and Late

Creta-.

ceous Carmel volcanics are assumed to be

Ž .

0.02–0.03 SI units Rybakov et al., 1999 . The relation between remanent and induced magne-tization, measured from samples of the Jurassic volcanics, was defined as 0.03–0.3. This

im-plies that the magnetic anomalies caused by such bodies depend mainly on induced zation. The parameters of the induced magneti-zation vector were computed using the IGRF program.

The densities for the stratigraphic sequences were calculated using borehole log density data. Fig. 3 shows that the main density contrast

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3_.

2100 kgrm Senonian–Tertiary rocks and the predominantly carbonate, older Mesozoic rocks.

Ž 3_.

The densest rocks up to 2850 kgrm are scattered anhydrite and dolomites of Late Trias-sic occurring in the Deborah-2A well. The den-sity of rocks older than the Triassic was esti-mated at 2670 kgrm3, corresponding to the Bouguer density. The Early Jurassic Asher vol-canics have an average density of about 2550–

3 _Ž _.

2600 kgrm Fig. 3 . This means that volcanic rocks have a negative density contrast relative to the Mesozoic carbonate sequence, which have an average density of about 2750 kgrm3_.

4. Analysis and interpretation of the gravity and magnetic data

A generation of initial subsurface geological model corresponding to the geophysical obser-vations is an important interpretation stage. All additional investigations mainly check and re-fine the results revealed during this stage which includes an analysis of both the potential fields and their transformations in an attempt to em-phasize the various frequency components of the gravity and magnetic fields. A variety of filtering techniques was employed in order to enhance the data sets prior to interpretation ŽCordell et al., 1992 : regional–residual gravity. separation; horizontal and vertical gravity and magnetic derivatives; reduction to the pole and upward magnetic continuation; pseudo-gravity and gravity–magnetic correlation.

The Bouguer graÕity values in the area vary

from 40 to 110 mGal and they generally de-crease to the southeast. The cause of the re-gional gravity trend could be the transition from oceanic crust of the Eastern Mediterranean to the continental crust of the Arabian plate, which

may occur under the Levant margin Makris and .

Wang, 1994 . For interpretation purposes, the regional–residual separation of the gravity anomalies was conducted using the regional trend that was calculated as a third order

poly-nomial surface. Removing this trend, the resid-ual Bouguer gravity map has been compiled ŽFig. 4 . The main features of this map are a. NW trending high with a magnitude of about 50 mGal and two wide lows. The gravity high

Ž .

consists of three separate anomalies Fig. 4 : a. The southern anomaly, located south of Mount Carmel, is about 20 km in length along its NNW oriented axis; its southern boundary coincides with the northern edge of the Tertiary sedimentary embayment.

b. The central and most intense part of the positive gravity ridge is steeply bounded on all sides.

c. The northern part, oriented NNE and per-pendicular to the central part, consists of a few local highs. A small, but clearly observed, local gravity low coincides with the Carmel high elevation near Haifa. The wide gravity low in the southwest has a magnitude of about y10 mGal. No significant gravity anomalies have been observed in the northwest side of the study area.

As mentioned above, the lateral changes in the thickness of low-density young sediments are the main reason for the residual anomalies in the area. The thickness of the young sedi-ments was deduced from seismic reflection data as the difference between the sea floor and the

Ž .

structural depth to the top Turonian Fig. 4 . These values were used to calculate and subtract the gravity effect of the low-density sediments. This stage of ‘‘gravity stripping’’ was per-formed using the PFGRAV3D program

devel-Ž .

oped by Blakely Cordell et al., 1992 . This program calculates the gravity effect using three rectangular grids that define the source: the top surface, the bottom surface and the density con-trast. In this case the first surface corresponds to the sea floor and the second to the top Turonian

Ž .

carbonates Fig. 5 . The density contrast was

Ž .

derived from density well logs Fig. 3 . The negative gravity effect of the low-density sedi-ments ranging from y60 to y10 mGal was removed from the observed gravity values. The gravity effect of the Asher volcanics was

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calcu-( )

M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 359

Ž .

Fig. 4. Residual Bouguer anomalies of the Haifa Bay area contour interval — 5 mGal . Schematic structural map on top of

Ž . Ž

the Turonian carbonates contour interval — 500 m . Residual ‘‘stripped’’ gravity map of the study area contour interval

— 5 mGal . Densities of the young sediments and Asher volcanics were replaced by average Bouguer density. Ž

lated using the GRAVPOLY program Godson, .

1983a .

The geometry of the volcanic bodies was taken from the 3-D magnetic interpretation

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de-Fig. 5. Stages of ‘‘gravity stripping.’’

scribed below. The density contrast was as-sumed from density logs. The negative gravity effect of the volcanic rocks, ranging from y0 to y15 mGal, was also removed from the Bouguer gravity values. Replacing the actual densities of the young sediments and volcanics

Ž 3_.

to an average Bouguer density 2670 kgrm , we cleaned the Bouguer gravity anomalies from the influence of the above mentioned geological bodies. The gravity anomalies obtained should be named ‘‘stripped’’ gravity anomalies. Re-moving the regional trend from the ‘‘stripped’’ gravity anomalies, we compiled the residual ‘‘stripped’’ gravity anomalies. The stages of the ‘‘gravity stripping’’ are illustrated in Fig. 5. Inasmuch as the final results are strongly de-pending on the accuracy of the calculated grav-ity effect we estimated this parameter by using realistic limitations of the top Turonian and density data as well as the accuracy of the 3-D gravity calculation. The total error does not exceeding of 4 mGal.

The pattern of the residual ‘‘stripped’’

grav-Ž .

ity anomalies Fig. 4 is essentially different from that of the residual Bouguer gravity. The studied area consists of a number of local grav-ity lows and highs that are not apparent on the Bouguer data. The elongated positive anomaly, with a magnitude of about of 15 mGal, is extended NW to about 20 km from the Qishon graben to the Haifa Bay area. The SW boundary of this anomaly coincides with the present Yagur fault, while its NE boundary appears to be a newly discovered lineament. The high gravity gradients are related to these boundaries. An elongated negative gravity anomaly with magni-tude of about y5 mGal, located SW of the above-mentioned positive anomaly, is about 15 km long. Only the SE part of this anomaly is seen inland as a small gravity low on the

non-Ž .

stripped data Fig. 4 . A rounded gravity high with a magnitude of about 5 mGal is located between the Atlit-1 and Foxtrot-1 wells. The Foxtrot-1 well is located in a complex gravity

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M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 361

saddle. A high gravity anomaly occupies the NW corner of the study area. The magnitude of this wide anomaly reaches about 25 mGal. A wide gravity low is located in the SW corner of the area with a magnitude of abouty25 mGal.

Ž .

A relatively small, elongated N–S positive gravity anomaly with a magnitude of about 10 mGal, oriented southward, is delineated in the near shore close to the Atlit-1 well.

Gravity features, possibly fault or structure related, were drawn in the interpretation map ŽFig. 6 using the residual gravity maps Bouguer. Ž

and stripped and horizontal gravity gradients. Magnetic anomalies are a distortion of the total geomagnetic field caused by local changes in the rock magnetization. In contrast to the gravity, the sedimentary strata are ‘‘transparent’’ and the magnetic anomalies are caused only by basic magmatism. The parameters of the Earth’s

total field vector for the area of study inclina-tions458, declinations38 and total field

mag-.

nitude about 43,500 nT were calculated using a program developed by the US National Space Science Data Center. The theoretical magnetic anomaly, caused by a body magnetized by this normal geomagnetic field, contains the conju-gated maximum and minimum, the latter lying to the north. This signature is important for geological understanding of a pattern of the magnetic anomalies. The central part of the studied area is occupied by a few magnetic anomalies as shown in Fig. 2. The easternmost

Ž intensive Carmel magnetic anomaly Ml peak to

peak is about 240 nT , oriented WNW, is about 40 km long. The northern edge of this anomaly is marked by a sharp magnetic gradient. Its southern edge shows moderate gradients.

Ž .

The next magnetic anomaly M2 is located offshore west of the Carmel anomaly. It is elongated in shape and trends NNE, perpendicu-lar to the strike of the main direction of the Carmel anomaly. Its peak to peak reaches 210 nT. This anomaly, extending over 20 km, is bounded to the south and north by steep hori-zontal gradients. The peak to peak of the M3

anomaly reaches 120 nT. Its marginal gradients are more moderate than those of the other anomalies mentioned above. The magnetic anomaly M4, shown in the northeastern corner of the studied area, is only a small part of a high magnetic anomaly located in the southern

off-Ž .

shore of Lebanon Fig. 1 . No magnetic anoma-lies are present in the southwestern part of the studied area.

High frequency magnetic anomalies, located offshore, have been mapped by the marine mag-netic survey. We speculate that these anomalies were caused by shallow basic volcanics. The location is marked on the interpretation map as

Ž .

M5 Fig. 6 . These volcanic rocks probably belong to the Cretaceous volcanic formation that outcropped in Mount Carmel. The

deep-Ž .

seated magmatics M3 are marked in the north-west part of the area using the pseudo-gravity transformation.

The results obtained from potential field data are inherently non-unique. The present interpre-tation should also be considered a member of the class of possible solutions that could pro-duce magnetic and gravity patterns matching the observations. The appraisal values of the den-sity, magnetic susceptibility and the calculated depths should be regarded as rough estimates.

In the first stage of the quantitative interpre-tation scheme, magnetic data were interpreted

Ž using the Werner deconvolution technique the USGS potential fields software package, Cordell

et al., 1992 . This 2-D program computes depths associated with magnetic basement dikes or faults using the input magnetic profile for the

Ž .

depth solutions dike model and the horizontal derivative of the input profile for depth

solu-Ž .

tions fault model . In spite of the sprays of Ž solutions are widely recognized features Fig.

7 , these data appeared to be useful to compile an initial iteration for the 3-D magnetic model-ing. The modeling was carried out using the

Ž .

MAGPOLY program Godson, 1983b which calculates the magnetic effect of polygonal bod-ies bounded by two horizontal planes and a

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number of intersecting vertical planes Talwani, .

1965; Plouff, 1976 . The magnetic effect of an

assemblage of polygonal prisms is calculated for all grid locations and should be compared

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M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 363

with the interpolated values. The following as-sumptions were made for the quantitative inter-pretation.

1. The total magnetization vector coincides with the vector of the Earth’s total field.

2. Magnetic susceptibility is the same as the sample measurements.

3. The anomalies examined are caused by the magmatic bodies, which are similar to the

Asher shield volcano Gvirtzman et al., .

1990 .

The MAGPOLY program was combined with interactive PC programs that permit digitization of polygon corners as well as imaging of the

observed and calculated fields. The resulting 3-D model was obtained after some experimen-tation. The modeling iterative process was stopped when the main features of the calcu-lated anomaly had been adjusted to the observed one. Based on the 3-D modeling, it appears that concealed magnetic bodies located inside as well as outside the investigated area cause the magnetic anomalies. The plane view of these bodies is shown on Fig. 6. The best fit for the magnetic anomalies that were observed in the area was obtained using the parameters shown in Figs. 6 and 7.

It is important to estimate the reliability of the model suggested. The deeper parts of the

Ž . Ž .

Fig. 7. 3-D modeling results for the Carmel magnetic anomaly. Crosses dike model and triangles fault model show the 2-D depth solutions obtained by using the Werner deconvolution technique.

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model give rise to the small anomalies and, hence, limit the resolution. The plane projections of the magnetic body are more reliably defined than the upper and lower limits. The relatively simple geometry obtained can be altered using a more complicated

assemblage of polygonal prisms as in a pyra-.

mid which could lead to bodies with smoother slopes. The difference between the anomalies of the simple and complicated models is negli-gible.

The top of the simple model was determined, after some experimentation, with an accuracy of about 0.5 km for the Carmel magmatics and about 1 km for the western bodies. The Asher volcanics were penetrated by the Atlit-1 and

Yagur-1 wells at a depth of 2.9 km estimated

. Ž .

as 3.5 km and 2.4 km estimated as 2 km , respectively. It should be noted that, by using realistic limitations for the initial parameters of the causative body, we could average the misfit between the observed and calculated anomalies to less than about 25%. The magnetic data for the deepest body has been interpreted by using

Ž .

2-D magnetic modeling Rybakov, 1991 . The magnetic effects of a large magmatic body with thick roots best fit the observed Carmel mag-netic anomaly. This root is probably located

Ž .

close to the Yagur fault Fig. 7 .

The difference between magnetic anomalies caused by basic volcanic layers intercalated within the sedimentary rocks and a solid

mag-Ž

matic body gabbro intrusion with a similar .

geometry and depth extension is small. The assumption that a magnetic body is located below the Phanerozoic strata at a depth of about 8-km was also checked. Modeling showed that a magmatic body could produce a magnetic anomaly of the same magnitude as the measured one, if an unrealistic magnetic susceptibility of about 0.15–0.2 SI units is used. However, even in this case, the gradients of the observed and calculated anomalies cannot be adjusted; there-fore, we suggest that a crystalline basement does not cause the magnetic anomalies in the studied area.

The magnetic effect of the Early Cretaceous Tayassir volcanics and the Late Cretaceous vol-canics, which outcrop on Mount Carmel, was calculated for various levels using 2-D model-ing. The magnitude of the calculated anomalies was less than 8–10 nT for the 1-km elevation; therefore such bodies cannot be effectively ob-served using the available magnetic data.

5. Discussion

Three main directions can be seen on the

Ž .

interpretation map Fig. 6 . The first, which strikes NW, generally coincides with the Yagur fault and the Carmel ridge and is probably enhanced by young tectonic dislocations. The second, oriented northward and parallel to the

Ž .

coastline and the continental slope Fig. 2 , reflects the thickness variations of the young sediments. The third, oriented E–W and mainly observed in the southern part of the study area, probably corresponds to a density heterogeneity in the Mesozoic sequence. An areal distribution of the basic magmatics has a similar direction

corresponding to an ancient probably

Meso-. Ž .

zoic weakness megazone. Ginzburg 1960 suggested that the density contrast between the Cenozoic and the Mesozoic sequences is one of the main causes of gravity disturbances. The compiled ‘‘stripped’’ gravity map shows, for the first time, the pattern of gravity anomalies

Ž .

after removal of this effect Fig. 4 .

The peak-to-peak of the gravity anomalies in the area decreased from about 90 mGal on the Bouguer gravity map to about 60 mGal on the ‘‘stripped’’ gravity map. The maximum hori-zontal gradients are also less for ‘‘stripped’’

Ž gravity. For example, a strip of the highest up

to 20 mGalrkm gravity gradients oriented northward and located on the Atlit offshore, is virtually invisible on the ‘‘stripped’’ map. The ‘‘stripped’’ gravity map shows a simple pattern and consists of fewer features than the Bouguer

Ž .

map Fig. 4 . This reflects the deep position and large size of the causative bodies. The

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M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 365

nent, positive Carmel gravity anomaly, clearly

Ž .

seen on the Bouguer gravity maps Fig. 4 , completely vanishes on the ‘‘stripped’’ gravity map, suggesting that this anomaly is produced by the great thickness of the low density rocks in the surrounding areas. In contrast, an elon-gated positive gravity anomaly appears north of the Yagur fault after replacing the low-density sediments of the Qishon graben. In our opinion, this positive anomaly most probably expresses high-density gabbroic rocks of the magmatic

Ž .

root of the Asher volcanics Fig. 7 .

A large positive, rounded anomaly,

occupy-Ž .

ing the whole NW corner of the area Fig. 4 emerged after gravity stripping. Good correla-tion of this anomaly with the magmatic body interpreted from the magnetic data leads us to suggest a deep-seated gabbroic intrusion here. The top of the intrusion reaches a depth of about 8-km, as estimated by the quantitative interpretation of the magnetic data. It is impor-tant to emphasize that a number of features interpreted from ‘‘stripped’’ gravity and located near previously delineated features, now show a different attitude as demonstrated, for example, by the Foxtrot-1 well. Its location coincides with the maximum of the NW oriented positive

Ž .

anomaly on the Bouguer map Figs. 4 and 6 , while a negative NE oriented anomaly appears in the same location in the ‘‘stripped’’ gravity map. We believe that this demonstrates different tectonic configurations for the Tertiary and Mesozoic formations.

A large rounded gravity low, occupying the SW corner of the study area, is the only one that did not vanish in the ‘‘stripped’’ gravity map. We assume that this anomaly corresponds to the subsurface geological structure, the Atlit em-bayment, with a series of thick Mesozoic and Tertiary sediments. This structure was probably inherited from the Mesozoic through the Ter-tiary.

The prominent geophysical features of the area are the gravity and magnetic highs of the Carmel area. These anomalies were interpreted

Ž .

by Gvirtzman et al. 1990 as being caused by

an Early Jurassic shield volcano below Mount Carmel. Our interpretation, presented in Figs. 6 and 7 is not very different apart from two exceptions.

1. Analysis of the gravity and magnetic maps clearly shows an inconsistency between the gravity and magnetic anomalies in the plane location and strike direction. The correlation between the pseudo-gravity and the gravity anomalies was computed and the results ob-tained clearly suggest a lack of correlation be-tween these data and confirm that the causes of the gravity and magnetic anomalies are essen-tially different.

2. The possible location of a magmatic root under the Carmel ridge: the first hint for the existence of such a root was obtained from automatic inverse programs that showed a few deep-seated magnetic heterogeneity located north of the Yagur fault. An elongated positive gravity anomaly, also appearing north of the

Ž Yagur fault in the ‘‘stripped’’ gravity map Fig.

4 , probably reflects the high-density gabbroic root of the Asher volcanics. 2-D and 3-D mag-netic modeling confirm the existence of such a

Ž .

root at this location Fig. 7 .

6. Conclusions

The main results of the interpretation of the updated gravity and magnetic data are as fol-lows.

1. This study confirms, as previously

sug-Ž .

gested by Gvirtzman et al. 1990 , that the Early Jurassic Asher shield volcano, consisting of basaltic lava flows, is the causative body of the Carmel magnetic anomaly onshore and off-shore. This body oriented NNE is about 30=20 km in size and its top reaches a depth of 2 km in the central part and 4 km in the northern part. The thickness of the body is estimated at about 2–3 km.

2. The root of the volcano is located immedi-ately north of the Yagur fault and not below Mount Carmel as previously suggested.

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3. The rocks of the Asher volcanics have, in general, relatively lower density values in com-parison to the Mesozoic rocks. This implies that this sequence cannot be the causative body for the Carmel gravity high.

4. The second magmatic body, lying about 5 km west of the Foxtrot-1 well and oriented N–S, is an elongated shape about 20 km long. These deep-seated magmatics, reaching a depth of about 4-km, are about 3.5 km thick. The westernmost magmatic body, expressed on the magnetic data, as well as a ‘‘stripped’’ gravity

high, probably correspond to a large 15–20 km

. Ž .

in diameter , deep-seated about 8 km deep gabbroic intrusion. Another shallow magmatic body about 20 km long is suggested south of the Foxtrot-1 well. The western and eastern bound-aries of this magmatic event are not defined.

5. Magnetic susceptibility of 0.02–0.035 SI units and an effective vector of magnetization parallel to the Earth’s present magnetic field were measured for the above mentioned mag-matics. These bodies should be joined in a common occurrence of deep-seated magmatic events, which cover almost all the study area.

6. Most of the Bouguer gravity anomalies in the area express the gravity influence of the low density Senonian to Tertiary sediments. The features of the gravity field, such as the gradi-ents and elongated anomalies, can be divided into three groups according to direction. The first, NW oriented group, corresponds to the direction of the Yagur fault and the Carmel ridge and is expressed by the young tectonic dislocations. The second group, oriented N–S parallel to coastal line, corresponds mainly to the thickness variations of the Tertiary sedi-ments. The third group, oriented E–W, probably corresponds to the density heterogeneity of the Mesozoic sequence.

7. The gravity effects of the young sediments and of the Asher volcanics were calculated and removed from the observed gravity. The ‘‘stripped’’ gravity map reflects, in general, the Mesozoic subsurface geology. The structural

Ž pattern of shallow geological formations

Ter-.

tiary does not coincide with the pattern of the

Ž .

deeper Mesozoic formations. The ‘‘stripped’’ map should be of considerable help in delineat-ing some aspects for hydrocarbon exploration in the area.

8. The Atlit marine embayment is suggested. A thick series of Mesozoic and Tertiary forma-tions appears to be deposited in this basin.

9. The present Yagur fault appears to be a rejuvenation of an ancient weakness zone, based on similar trend found on the ‘‘stripped’’ grav-ity and magnetic maps. These anomalies are interpreted to be caused by a deep-seated basic intrusion of Early Jurassic age.

Acknowledgements

The authors are grateful to the Earth Sciences Administration of the Ministry of National In-frastructures for their kind permission to use the gravity, magnetic and well data from Israel and for their support of this study. We also thank Drs. G. Tsokas, A. Camacho and a third referee for their valuable suggestions for improving the original version of this paper. We are grateful to the authors of the IGRF model and NASA, USA, for their kind cooperation. We would like to thank Drs. Y. Rotstein, M. Goldman and I. Bruner for useful discussions as well as Mr. I. Goldberg, Ms. R. Gapsou and Mr. O. Siman-Tov for their assistance.

References

Arad, A., 1965. Geological outline of the Ramot Menashe

Ž .

region northern Israel . Isr. J. Earth Sci. 14, 18–32. Ben-Avraham, Z., Hall, J.K., 1977. Geophysical survey of

Mount Carmel and its extension into the Eastern Mediterranean. J. Geophys. Res. 82, 793–802. Ben-Avraham, Z., Ginzburg, A., 1986. Magnetic

anoma-lies over the Central Levant continental margin. Mar. Pet. Geol. 3, 220–223.

Ben-Gai, Y., Ben-Avraham, Z., 1995. Tectonic processes in offshore northern Israel and the evolution of the

Ž .

(15)

( )

M. RybakoÕet al.rJournal of Applied Geophysics 44 2000 353–367 367

Cordell, L., Phillips, J.D., Godson, R.H., 1992. US Geo-logical Survey Potential Field Geophysical Software, Version 2.0. Department of the Interior, US Geological Survey, Boston, MA.

De Sitter, L.U., 1962. Structural development of the Ara-bian shield in Palestine. Geol. Mijnbouw 45, 116–124. Domzalski, W., 1967. Aeromagnetic survey of Israel:

in-terpretation. IPRG Report SMAr482r67, 62 pp. Domzalski, W., 1986. Review and Additional

Interpreta-tion of Selected Magnetic Data in Israel and Adjoining

Ž .

Areas. Oil Exploration Investments Ltd., 55 pp. Dvorkin, A., Kohn, B.P., 1989. The Asher volcanics,

northern Israel. Isr. J. Earth Sci. 38, 105–123. Folkman, Y., 1976. Magnetic and gravity investigation of

the crustal structure of Israel. PhD Dissertation. Tel-Aviv University.

Freund, R., 1970. The geometry of faulting in the Galilee. Isr. J. Earth Sci. 19, 117–140.

Garfunkel, Z., Derin, B., 1984. Permian–Early Mesozoic tectonism and continental margin formation in Israel and its implications for the history of the Eastern Mediterranean. In: Dixon, J.E., Robertson, A.H.F.

ŽEds. , The Geological Evolution of the Eastern.

Mediterranean. Blackwell, Oxford, pp. 187–201. Ginzburg, A., 1960. Geophysical studies in the central and

northern coastal plane and the western Emeq. PhD Thesis. Hebrew University, Jerusalem.

Ginzburg, A., Cohen, S., Hay-Roe, H., Rosenzweig, A., 1975. Geology of the Mediterranean shelf of Israel. AAPG Bull. 59, 2142–2160.

Godson, R.H., 1983. GRAVPOLY: a modification of a three-dimensional gravity modeling programs. Open-File Report 83-346, US Geological Survey, 53 pp. Godson, R.H., 1983. MAGPOLY: a modification of a

three-dimensional magnetic modeling programs. Open-File Report 83-345. US Geological Survey, 62 pp.

Ž .

Hall, J.K., 1993. The GSI Digital Terrain Model DTM

Ž .

completed. In: Bogoch, R., Eshet, Y. Eds. , GSI Curr. Res.. pp. 47–50, Jerusalem.

Gvirtzman, G., Steinitz, G., 1983. The Asher volcanics — an early Jurassic event in northern Israel. GSI Curr. Res., 28–33.

Gvirtzman, G., Klang, A., Rotstein, Y., 1990. Early Juras-sic shield volcano below Mount Carmel: a new inter-pretation of the magnetic and gravity anomalies and implications for early Jurassic rifting. Isr. J. Earth Sci. 39, 149–159.

Makris, J., Wang, J., 1994. Bouguer gravity anomalies of the Eastern Mediterranean Sea. In: Krasheninnikov,

Ž .

V.A., Hall, J.K. Eds. , Geological Structures of the Northeastern Mediterranean. pp. 87–99, Jerusalem. Mimran, Y., 1972. The Tayassir volcanics and Lower

Cretaceous formation in the Shomron, central Israel. GSI Bull. 58, 1–52.

Neev, D., Almagor, G., Arad, A., Ginzburg, A., Hall, J., 1976. The geology of the southern Mediterranean Sea. GSI, 68.

Nettleton, L.L., 1971. Elementary gravity and magnetic for geologists and seismologists. SEG Monogr. Ser. 1, 121. Picard, L., Kashai, E., 1958. On the lithostratigraphy and tectonics of the Carmel. Bull. Res. Counc. Isr. 7G, 1–18.

Plouff, D., 1976. Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections.

Ž .

Geophysics 41 4 , 727–741.

Ron, H., Freund, R., Garfunkel, Z., Nur, A., 1984. Block rotation by strike slip faulting: structural and paleomag-netic evidence. J. Geophys. Res. 89, 6256–6277. Rotstein, Y., Bruner, I., Kafri, U., 1993. High resolution

seismic imaging of the Carmel fault and its implica-tions to the structure of Mt. Carmel. Isr. J. Earth Sci. 42, 55–69.

Rybakov, M., 1991. Interactiverautomated crustal model-ing usmodel-ing multiple data sets and multiple optimized intuitiverautomated inversion techniques. In: Israel Ge-ological Survey, Annual Meeting, Abstracts. p. 90. Rybakov, M., Goldshmidt, V., Folkman, Y., Rotstein, Y.,

Ben-Avraham, Z., Hall, J., 1994. Magnetic anomaly map of Israel, scale 1:500,000. IPRG and Survey of Israel.

Rybakov, M., Goldshmidt, V., Rotstein, Y., 1997. A new compilation of gravity and magnetic data from the Levant and their preliminary interpretation. Geophys.

Ž .

Res. Lett. 24 1 , 33–36.

Rybakov, M., Goldshmidt, V., Rotstein, Y., Fleischer, L., Goldberg, I., 1999. Petrophysical constraints on grav-ityrmagnetic interpretation in Israel. Leading Edge 18

Ž .2 , 269–272.

Sass, E., 1980. Late Cretaceous volcanism in Mt. Carmel, Israel. Isr. J. Earth Sci. 29, 8–24.

Talwani, M., 1965. Computation with help of a digital computer of magnetic anomalies caused by bodies of

Ž .

(1)

number of intersecting vertical planes Talwani,

1965; Plouff, 1976 . The magnetic effect of an

assemblage of polygonal prisms is calculated for all grid locations and should be compared

(2)

with the interpolated values. The following as-sumptions were made for the quantitative inter-pretation.

1. The total magnetization vector coincides with the vector of the Earth’s total field.

2. Magnetic susceptibility is the same as the sample measurements.

3. The anomalies examined are caused by the magmatic bodies, which are similar to the

Asher shield volcano Gvirtzman et al.,

1990 .

The MAGPOLY program was combined with interactive PC programs that permit digitization of polygon corners as well as imaging of the

It is important to estimate the reliability of the model suggested. The deeper parts of the

Ž . Ž .

Fig. 7. 3-D modeling results for the Carmel magnetic anomaly. Crosses dike model and triangles fault model show the 2-D depth solutions obtained by using the Werner deconvolution technique.

(3)

assemblage of polygonal prisms as in a

pyra-.

mid which could lead to bodies with smoother slopes. The difference between the anomalies of the simple and complicated models is negli-gible.

Yagur-1 wells at a depth of 2.9 km estimated

. Ž .

Ž .

2-D magnetic modeling Rybakov, 1991 . The magnetic effects of a large magmatic body with thick roots best fit the observed Carmel mag-netic anomaly. This root is probably located

Ž .

close to the Yagur fault Fig. 7 .

The difference between magnetic anomalies caused by basic volcanic layers intercalated within the sedimentary rocks and a solid

mag-Ž

matic body gabbro intrusion with a similar

5. Discussion

Three main directions can be seen on the

Ž .

interpretation map Fig. 6 . The first, which

strikes NW, generally coincides with the Yagur fault and the Carmel ridge and is probably enhanced by young tectonic dislocations. The second, oriented northward and parallel to the

Ž .

coastline and the continental slope Fig. 2 ,

reflects the thickness variations of the young sediments. The third, oriented E–W and mainly observed in the southern part of the study area, probably corresponds to a density heterogeneity in the Mesozoic sequence. An areal distribution of the basic magmatics has a similar direction

corresponding to an ancient probably

Meso-. Ž .

zoic weakness megazone. Ginzburg 1960

suggested that the density contrast between the Cenozoic and the Mesozoic sequences is one of the main causes of gravity disturbances. The compiled ‘‘stripped’’ gravity map shows, for the first time, the pattern of gravity anomalies

Ž .

after removal of this effect Fig. 4 .

gravity. For example, a strip of the highest up

to 20 mGalrkm gravity gradients oriented

northward and located on the Atlit offshore, is virtually invisible on the ‘‘stripped’’ map. The ‘‘stripped’’ gravity map shows a simple pattern and consists of fewer features than the Bouguer

Ž .

map Fig. 4 . This reflects the deep position and large size of the causative bodies. The

(4)

promi-nent, positive Carmel gravity anomaly, clearly

Ž .

seen on the Bouguer gravity maps Fig. 4 ,

completely vanishes on the ‘‘stripped’’ gravity map, suggesting that this anomaly is produced by the great thickness of the low density rocks in the surrounding areas. In contrast, an elon-gated positive gravity anomaly appears north of the Yagur fault after replacing the low-density sediments of the Qishon graben. In our opinion, this positive anomaly most probably expresses high-density gabbroic rocks of the magmatic

Ž .

root of the Asher volcanics Fig. 7 .

A large positive, rounded anomaly,

occupy-Ž .

Ž .

The prominent geophysical features of the area are the gravity and magnetic highs of the Carmel area. These anomalies were interpreted

Ž .

by Gvirtzman et al. 1990 as being caused by

an Early Jurassic shield volcano below Mount Carmel. Our interpretation, presented in Figs. 6 and 7 is not very different apart from two exceptions.

2. The possible location of a magmatic root under the Carmel ridge: the first hint for the existence of such a root was obtained from automatic inverse programs that showed a few

deep-seated magnetic heterogeneity located

north of the Yagur fault. An elongated positive gravity anomaly, also appearing north of the

Yagur fault in the ‘‘stripped’’ gravity map Fig.

4 , probably reflects the high-density gabbroic root of the Asher volcanics. 2-D and 3-D mag-netic modeling confirm the existence of such a

Ž .

root at this location Fig. 7 .

6. Conclusions

The main results of the interpretation of the updated gravity and magnetic data are as fol-lows.

1. This study confirms, as previously

sug-Ž .

2. The root of the volcano is located immedi-ately north of the Yagur fault and not below Mount Carmel as previously suggested.

(5)

high, probably correspond to a large 15–20 km

. Ž .

7. The gravity effects of the young sediments and of the Asher volcanics were calculated and

removed from the observed gravity. The

‘‘stripped’’ gravity map reflects, in general, the Mesozoic subsurface geology. The structural

pattern of shallow geological formations

Ter-.

tiary does not coincide with the pattern of the

Ž .

deeper Mesozoic formations. The ‘‘stripped’’ map should be of considerable help in delineat-ing some aspects for hydrocarbon exploration in the area.

8. The Atlit marine embayment is suggested. A thick series of Mesozoic and Tertiary forma-tions appears to be deposited in this basin.

Acknowledgements

References

Arad, A., 1965. Geological outline of the Ramot Menashe

Ž .

region northern Israel . Isr. J. Earth Sci. 14, 18–32. Ben-Avraham, Z., Hall, J.K., 1977. Geophysical survey of

Mount Carmel and its extension into the Eastern Mediterranean. J. Geophys. Res. 82, 793–802. Ben-Avraham, Z., Ginzburg, A., 1986. Magnetic

anoma-lies over the Central Levant continental margin. Mar. Pet. Geol. 3, 220–223.

Ben-Gai, Y., Ben-Avraham, Z., 1995. Tectonic processes in offshore northern Israel and the evolution of the

Ž .

(6)

Cordell, L., Phillips, J.D., Godson, R.H., 1992. US Geo-logical Survey Potential Field Geophysical Software, Version 2.0. Department of the Interior, US Geological Survey, Boston, MA.

De Sitter, L.U., 1962. Structural development of the Ara-bian shield in Palestine. Geol. Mijnbouw 45, 116–124. Domzalski, W., 1967. Aeromagnetic survey of Israel:

in-terpretation. IPRG Report SMAr482r67, 62 pp. Domzalski, W., 1986. Review and Additional

Interpreta-tion of Selected Magnetic Data in Israel and Adjoining

Ž .

Areas. Oil Exploration Investments Ltd., 55 pp. Dvorkin, A., Kohn, B.P., 1989. The Asher volcanics,

northern Israel. Isr. J. Earth Sci. 38, 105–123. Folkman, Y., 1976. Magnetic and gravity investigation of

the crustal structure of Israel. PhD Dissertation. Tel-Aviv University.

Freund, R., 1970. The geometry of faulting in the Galilee. Isr. J. Earth Sci. 19, 117–140.

ŽEds. , The Geological Evolution of the Eastern.

Mediterranean. Blackwell, Oxford, pp. 187–201. Ginzburg, A., 1960. Geophysical studies in the central and

northern coastal plane and the western Emeq. PhD Thesis. Hebrew University, Jerusalem.

Ginzburg, A., Cohen, S., Hay-Roe, H., Rosenzweig, A., 1975. Geology of the Mediterranean shelf of Israel. AAPG Bull. 59, 2142–2160.

Godson, R.H., 1983. GRAVPOLY: a modification of a three-dimensional gravity modeling programs. Open-File Report 83-346, US Geological Survey, 53 pp. Godson, R.H., 1983. MAGPOLY: a modification of a

three-dimensional magnetic modeling programs. Open-File Report 83-345. US Geological Survey, 62 pp.

Ž .

Hall, J.K., 1993. The GSI Digital Terrain Model DTM

Ž .

completed. In: Bogoch, R., Eshet, Y. Eds. , GSI Curr. Res.. pp. 47–50, Jerusalem.

Gvirtzman, G., Steinitz, G., 1983. The Asher volcanics — an early Jurassic event in northern Israel. GSI Curr. Res., 28–33.

Makris, J., Wang, J., 1994. Bouguer gravity anomalies of the Eastern Mediterranean Sea. In: Krasheninnikov,

Ž .

V.A., Hall, J.K. Eds. , Geological Structures of the Northeastern Mediterranean. pp. 87–99, Jerusalem. Mimran, Y., 1972. The Tayassir volcanics and Lower

Cretaceous formation in the Shomron, central Israel. GSI Bull. 58, 1–52.

Neev, D., Almagor, G., Arad, A., Ginzburg, A., Hall, J., 1976. The geology of the southern Mediterranean Sea. GSI, 68.

Plouff, D., 1976. Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections.

Ž .

Geophysics 41 4 , 727–741.

seismic imaging of the Carmel fault and its implica-tions to the structure of Mt. Carmel. Isr. J. Earth Sci. 42, 55–69.

Ben-Avraham, Z., Hall, J., 1994. Magnetic anomaly map of Israel, scale 1:500,000. IPRG and Survey of Israel.

Rybakov, M., Goldshmidt, V., Rotstein, Y., 1997. A new compilation of gravity and magnetic data from the Levant and their preliminary interpretation. Geophys.

Ž .

Res. Lett. 24 1 , 33–36.

Rybakov, M., Goldshmidt, V., Rotstein, Y., Fleischer, L., Goldberg, I., 1999. Petrophysical constraints on grav-ityrmagnetic interpretation in Israel. Leading Edge 18

Ž .2 , 269–272.

Sass, E., 1980. Late Cretaceous volcanism in Mt. Carmel, Israel. Isr. J. Earth Sci. 29, 8–24.

Talwani, M., 1965. Computation with help of a digital computer of magnetic anomalies caused by bodies of

Ž .

Directory UMM :Data Elmu:jurnal:J-a:Journal Of Applied Geophysics:Vol44.Issue4.2000:

3-D gravity and magnetic interpretation for the

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M. Rybakov

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