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 magnetic modeling and
inversion were also based on this grid. All the graÕity data were reduced to a
Bouguer density of 2670 kgrm
3
. 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 Ž
. DTM 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.
Ž 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 volcanics, 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 magneti-
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 Ž
occurs between the least dense about 2000–
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 kgrm
3
, 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 kgrm
3
.
4. Analysis and interpretation of the gravity and magnetic data
