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
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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. q 2000 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: rybakovgii.co.il 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
Ž 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-
. ham, 1995 . The Carmel structure onshore is
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 9 2 6 - 9 8 5 1 0 0 0 0 0 1 3 - 6
Fig. 1. Magnetic anomalies of the Eastern Mediterranean. The anomalies named after the major features, i.e., Eratosthenes Ž .
Ž . Ž .
E , Carmel C and Hebron H are the largest anomalies of the region. The regional tectonic map is shown in the insert.
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 ŽPicard 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
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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
Ž 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
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
Ž 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; Ben-Avraham and Hall, 1977; Ben-
Avraham and Ginzburg, 1986; Gvirtzman et al.,
. 1990 . Interpretations performed prior to the
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
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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
