Integrated 3D density modelling and segmentation of the Dead Sea Transform

Abstract

A 3D interpretation of the newly compiled Bouguer anomaly in the area of the 'Dead Sea Rift' is presented. A high-resolution 3D model constrained with the seismic results reveals the crustal thickness and density distribution beneath the Arava/Araba Valley (AV), the region between the Dead Sea and the Gulf of Aqaba/Elat. The Bouguer anomalies along the axial portion of the AV, as deduced from the modelling results, are mainly caused by deep-seated sedimentary basins (D > 10 km). An inferred zone of intrusion coincides with the maximum gravity anomaly on the eastern flank of the AV. The intrusion is displaced at different sectors along the NNW-SSE direction. The zone of maximum crustal thinning (depth 30 km) is attained in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal thickness of the region (38-42 km). Linked to the left lateral movement of approx. 105 km at the boundary between the African and Arabian plate, and constrained with

Full text

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Abstract
Bouguer anomaly in the area of the ‘‘Dead Sea Rift’’ is
presented. A high-resolution 3D model constrained
with the seismic results reveals the crustal thickness
and density distribution beneath the Arava/Araba
Valley (AV), the region between the Dead Sea and
the Gulf of Aqaba/Elat. The Bouguer anomalies along
the axial portion of the AV, as deduced from the
modelling results, are mainly caused by deep-seated
sedimentary basins (D > 10 km). An inferred zone of
intrusion coincides with the maximum gravity anomaly
on the eastern flank of the AV. The intrusion is dis-
placed at different sectors along the NNW–SSE
direction. The zone of maximum crustal thinning
(depth 30 km) is attained in the western sector at the
Mediterranean. The southeastern plateau, on the other
A 3D interpretation of the newly compiled
hand, shows by far the largest crustal thickness of the
region (38–42 km). Linked to the left lateral movement
of approx. 105 km at the boundary between the Afri-
can and Arabian plate, and constrained with recent
seismic data, a small asymmetric topography of the
Moho beneath the Dead Sea Transform (DST) was
modelled. The thickness and density of the crust sug-
gest that the AV is underlain by continental crust. The
deep basins, the relatively large intrusion and the
asymmetric topography of the Moho lead to the con-
clusion that a small-scale asthenospheric upwelling
could be responsible for the thinning of the crust and
subsequent creation of the Dead Sea basin during the
left lateral movement. A clear segmentation along the
strike of the DST was obtained by curvature analysis:
the northern part in the neighbourhood of the Dead
Sea is characterised by high curvature of the residual
gravity field. Flexural rigidity calculations result in very
low values of effective elastic lithospheric thickness
(te< 5 km). This points to decoupling of crust in the
Dead Sea area. In the central, AV the curvature is less
pronounced and teincreases to approximately 10 km.
Curvature is high again in the southernmost part near
the Aqaba region. Solutions of Euler deconvolution
were visualised together with modelled density bodies
and fit very well into the density model structures.
Keywords
Flexural rigidity Æ Euler deconvolution Æ Dead Sea
Transform
Bouguer anomaly Æ 3D-density modelling Æ
Introduction
The unique geological setting of the Dead Sea
Transform (DST) makes this region a main focus of
H.-J. Go ¨tze (&) Æ S. Schmidt
Institut fu ¨r Geowissenschaften, Abtlg. Geophysik,
Christian-Albrechts-Universita ¨t zu Kiel,
Otto-Hahn-Platz 1, 24118 Kiel, Gemany
e-mail: hajo@geophysik.uni-kiel.de
R. El-Kelani
An-Najah University, Nablus, Palestine
M. Rybakov
Geophysical Institute of Israel, Holon, Israel
M. Hassouneh
Natural Resources Authority, Amman, Jordan
H.-J. Fo ¨rster
GeoForschungsZentrum, Potsdam, Germany
J. Ebbing
Norges Geologiske Undersøkelse,
7491 Trondheim, Norway
Int J Earth Sci (Geol Rundsch) (2007) 96:289–302
DOI 10.1007/s00531-006-0095-5
123
ORIGINAL PAPER
Integrated 3D density modelling and segmentation of the Dead
Sea Transform
H.-J. Go ¨tze Æ Æ R. El-Kelani Æ Æ S. Schmidt Æ Æ
M. Rybakov Æ Æ M. Hassouneh Æ Æ H.-J. Fo ¨rster Æ Æ
J. Ebbing
Received: 6 May 2005/Accepted: 20 March 2006/Published online: 31 May 2006
? Springer-Verlag 2006

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interest for geoscientific research. The nature of the
crust underlying the shoulders east and west of the
Arava/Araba valley (AV) as well as the Dead Sea
depression have been controversial among researchers
for the past five decades. Consequently, the crustal
structure of the AV and its eastern and western
plateaus have been well studied and documented
(Ginzburg et al. 1979a, b; Garfunkel 1981; Makris
et al. 1983; Garfunkel and Derin 1984; El-Isa et al.
1987; Rotstein et al. 1987; Atallah 1992; Amiran et al.
1994; Frieslander 2000; Klinger et al. 2000; Ben-
Avraham et al. 2002). Based on both reflection/
refraction seismic experiments (Fig. 1) and gravity
data, a gradual transition from the continental crust of
the eastern part of the rift (Arabian Plate) with
thicknesses of 35–40 km (El-Isa et al. 1987; Makris
et al. 1983; Al-Zoubi and Ben-Avraham 2002) to the
crust of the eastern Mediterranean (Palestine Sinai
Plate) is interpreted. Here the crust is assumed to be
partly underlain by typical oceanic crust with thick-
nesses smaller than 10 km (Ginzburg et al. 1979a;
Makris et al. 1983; Ben-Avraham et al. 2002).
Major components of the investigations under the
framework of the international DESERT group (DES-
ERT Group 2000, 2004) were combined reflection/
refraction surveys across the territories of Palestine,
Israel and Jordan, crossing the DST in the Arava/Araba
Valley, where the geometry of the DST appeared to
be relatively simple. The main seismic results can be
described as: (1) the seismic basement is offset by
3–5 km below the DST, (2) the DST cuts through the
entire crust, broadening in the lower crust, (3) strong
lower crustal reflectors are only imaged on one side of
the DST, (4) the seismic velocity sections show a steady
increase in the depth of the crust–mantle transition
(Moho) from ~26 km at the Mediterranean to ~39 km
undertheJordanhighlands,withonlyasmallbutvisible,
asymmetric topography of the Moho under the DST.
These observations can be linked to the left-lateral
movement of 105 km of the two plates, accompanied by
strong deformation within a narrow zone cutting
through the entire crust. Comparing the DST and the
SanAndreasFaultsystem(SAF),astrongasymmetryin
sub-horizontal lower crustal reflectors and a deep
reaching deformation zone both occur around the DST
and the SAF. This suggests that these structures are
fundamental features of large transform plate bound-
aries (DESERT Group 2004; Haberland et al. 2003;
Maercklin et al. 2004; Sobolev et al. 2005; Mohsen et al.
2004).
A combination of seismic and magnetotelluric
experiments showed, that a strong contrast across the
Arava fault (AF, the name of the DST between the
Dead Sea and the Gulf of Aqaba/Elat) exists; low
seismic velocities and electric resistivities to the east
and high seismic velocities and resistivities to the
west (Ritter et al. 2003). It is therefore suggested that
the reflector imaged by Maercklin et al. (2004) is the
boundary between two blocks, offset from the pres-
ent day location of the DST at the surface, which
also acts as an impermeable seal isolating the two
blocks over at least several hundred-thousand years
(Maercklin et al. 2004). This is in marked contrast
to findings at the San Andreas Fault. On an even
smaller scale the analysis of fault-zone guided waves
in the DST provided evidence for an extremely
narrow region of reduced velocities (few meters)
(Haberland et al. 2003), indicative of a near surface
damage zone of the AF. This narrow occurrence
could point towards the fact that absolute motion on
the AF is much smaller than on the SAF, which has
larger offsets and a thicker fault zone. On a larger,
plate tectonic scale, Ru ¨mpker et al. (2003) were able
to demonstrate that a distinct narrow vertical zone in
the crust exists under the AF, where the Arabian and
Fig. 1 Location map of the study area and the seismic
experiments in the Middle East. The 260 km long wide-angle
reflection/refraction profile (WRR yellow circles) crosses Pales-
tine, Israel and Jordan. The near-vertical seismic reflection
profile (NVR) coincides with the inner 100 km of the WRR. A
white line indicates the Dead Sea Transform (DST) between the
Dead Sea and the Red Sea. The white arrows indicate the left-
lateral motion of 105 km between the African and Arabian
plates in the last 20 My. Background colours show topography
(gtopo30). The inset shows the regional features of the suture
zone
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the African plate are decoupled. This vertical zone
even extends through the entire lithosphere.
In the present work, a 3D gravity model of the
southern part of an area is presented, which is centred
around the DST, and complement the existing inter-
pretations of seismic surveys. The results of the
DESERT seismic reflection/refraction experiments in
the AV, which cross the eastern and western Jordan
Rift Plateaus, have been used to constrain the initial
3D density model of both the DST and adjacent
areas. The gravity database stems mainly from data-
sets of the Natural Resources Authority (NRA) of
Jordan and the Geophysical Institute of Israel (GII)
and was reprocessed and homogenised in the frame-
work of the PhD thesis of Hassouneh (2003). In
addition, the present study incorporates gravity data,
which were collected and processed by joint collabo-
ration of the DESERT Group from Germany (FU
Berlin); Jordan (NRA); Israel (GII) and Palestine
(An-Najah University).
Geological setting
General information from geology and tectonics of
the study area has been incorporated both in the
qualitative and quantitative interpretation stage of the
observed Bouguer gravity anomaly. The geological
maps of Jordan and Israel, compiled by the Natural
Resources Authority of Jordan and the Geological
Survey of Israel, were used to constrain the model at
the surface. Due to ambiguities in potential field data
interpretation a rather comprehensive knowledge of
the regional geology of the AV and the adjoining
region is necessary to constrain the model structures
and interpretation of the gravity data in particular. In
the following, a short introduction of the regional
geology of the AV is given.
The DST is a system of left-lateral strike-slip faults
that accommodate the relative motion between the
African and Arabian plates. Except for a mild com-
pressional deformation starting about 80 Ma ago, the
larger Dead Sea region has remained a stable platform
since the early Mesozoic. Approximately 17 Ma ago,
this tectonic stability was interrupted by the formation
of the DST, with a total left-lateral displacement of
105 km until today (Quennell 1958; Freund et al. 1970;
Garfunkel 1981). The AV forms a part of the large
Tertiary–Quaternary rift system which extends from
the Gulf of Aqaba in the south to Syria and Turkey in
the north (Fig. 1). The rift, like the rest of the East
African Rift System, has undergone a very complicated
geological evolution and tectonic history. The regional
geology and structural evolution of the AV System
have been extensively described and well documented
(Quennell 1958, 1959, 1983; Bender 1974; Jarrar et al.
1983; Atallah 1992).
The pre-Cambrian basement rocks in the Dead Sea
Rift System, except in the northern Aqaba/Elat gulf at
the extreme south of the AV, are mostly covered by an
(1) Early Cambrian volcanic sedimentary succession,
(2) Mesozoic sediments and (3) more recent Tertiary
volcanic rocks. The oldest sedimentary sequence, on
the other hand, is masked by sediments of Middle to
Upper Pleistocene age. In the rift valley thin deposits
of Pleistocene–Holocene age are common. Late
Paleozoic igneous rocks, early Cambrian volcano-
sedimentary successions, and minor Cenocoic mafic
volcanic rocks form the plateau adjoining the DST
(Jarrar 1985).
The main trend of the tectonic structure in the
Arava/Araba valley is the same as that of the AF,
which is dominantly of NNE–SSW direction (Atallah
and Mikbel 1983; Atallah 1992). Within the valley
floor, three major geotectonical provinces are recog-
nizable between latitudes 28? 00¢ and 33? 20¢: The Gulf
of Aqaba/Elat, the Wadi Araba and the Dead Sea-
northern region.
Recent geological and geochemical studies of the
AF by the DESERT team show that now exhumed
fault sections were deformed at temperatures between
150 and 300?C, indicating depths of up to 3 km.
Analysis of the fluids in the fault shows that expulsed
fluids were replaced by fluids from stratigraphically
higher reservoirs on very short time scales and that the
age of the AF is younger than 30 Ma (Janssen et al.
2004).
This brief summary shows that the Dead Sea region
provides an excellent site to study geodynamic pro-
cesses on different scales. This then facilitates an
understanding of how the interplay of structure and
dynamics controls the occurrence and rate of earth-
quakes. In this context an important aspect is that the
DST crosses a land area, which was a stable platform
throughout most of the Phanerozoic (Garfunkel 1981).
Therefore, late Cenozoic activities can be observed in
‘‘pure form’’, without being masked by earlier events.
Thus, the Dead Sea area is a unique natural laboratory.
It allows the study of the geometry of upper crustal
faulting, the mechanics of the lithosphere, and growth
and subsidence of pull-apart basins. This pull-apart
formation forms the most negative topographic feature
on Earth. The Dead Sea lies approximately –430 m
below normal sea level.
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Gravity data
As described in the introduction, the international
DESERT project was aimed at the investigation of the
lithosphere at different scales in time and space.
Therefore, it provides insight into differently scaled
processes, which act on the transform. Towards this
end, the gravity group worked on density models of the
area at both, a regional and a local scale. To test
the modelled gravity against the measured gravity field
the first task was to homogenise existing databases (e.g.
Hassouneh 2003). Gaps in the data coverage existed in
particular in the central part of the DESERT project,
where the seismic profile crosses the Arava fault and
on the plateau in the south east (see Fig. 1).
The existing measurements were reprocessed by
Hassouneh (2003), who calculated a topographic
correction for the entire region and identified and cor-
rected errors in the various databases. Unfortunately,
only a few metadata (information about field cam-
paigns, gravity meters, processing procedures) were
available for this task. In many cases, data had to be
eliminated, where no correction of the errors was pos-
sible (false station heights, geographic coordinates,
gravity values, misprints and many other error sources).
This modified data set was the basis of our stud-
ies—without further evaluation for consistency. In the
Arava valley, the DESERT gravity team completed the
database with additional measurements (refer to next
chapter), to enable a more local gravity interpretation.
DESERT gravity campaign
From March to May 2002 the gravity survey was per-
formed in the area of Sde-Boker and Zofar, with a
general gravity station spacing of about 500 m. De-
tailed gravity measurements, with a station spacing of
approx. 50 m, have been carried out in the Zofar area
along the seismic lines GP-2150 and GP-2152 (Frie-
slander 2000). Some 480 gravity stations were mea-
sured at an average production rate of 50 stations per
day. The gravity field data were collected using a
Scintrex CG-3M AutoGrav gravity meter no. 808426.
This instrument can measure variations in the Earth’s
gravity with a readout resolution of 0.01 · 10–5m/s2.
The local survey west of the DST was linked via the
Sde-Boker and E’n-Yahav base stations to Jordan’s
national gravity work (IGSN 72). Observations at these
base stations were twice a day, in the morning and
evening of each working day. During the field survey,
the daily drift was less than 0.07 · 10–5m/s2for the
Scintrex meter. Repeated observations were carried
outat all stations and resultedin anerror
of ± 0.02 · 10–5m/s2. For height determinations of
field stations, two Trimble GPS 5700 instruments were
used in a real-time kinematics mode. Elevations were
surveyed to the top of the instrument in real time with
an accuracy of a few centimetres. All data were pro-
cessed after the return from the field survey on the
same day by the in-house JAVA software. Therefore,
erroneous measurements could be detected instantly
and occasionally remeasured.
Bouguer anomaly and isostatic residual gravity field
The DESERT Bouguer anomaly compilation inte-
grates the following data sets:
•
Regional gravity data by the Natural Resources
Authority (NRA) of Jordan (Hassouneh, 2003).
Regional gravity data by the Geophysical Institute
of Israel (GII) (Rybakov, personal communica-
tion).
The local DESERT gravity surveys of Sde-Boker
and Zofar as described above.
•
•
In total we homogenised approximately 86,800 sta-
tions, covering the AV and the Dead Sea area. They are
fairly uniformly spaced with an average distance of
some 500 m. All stations are processed according to
standard procedures, using the 1967 Geodetic Refer-
ence System and the standard density of 2.67 Mg/m3.
The terrain corrections were calculated up to Hayford
zone O2(167 km), using a digital terrain model with a
25 m grid compiled by the Geological Survey of Israel
(J.K. Hall, personal communication). The overall
accuracy of the station complete Bouguer anomaly
values are estimated to be 0.1 · 10–5m/s2. Figure 2
shows the central area of the derived Bouguer anomaly
map with a constant contour interval of 5 · 10–5m/s2,
based on a grid with 500 m node spacing. All anomalies
range from 30 · 10–5m/s2
10–5m/s2(minimum). The highest Bouguer gravity
values are located west of the AV; here the anomalies
show a SW–NE trend. Due to the lack of meta data we
cannot calculate an overall error of the Bouguer
anomaly.
To enhance the local anomalies, which are caused by
near surface density inhomogeneities, an isostatic re-
gional gravity field has been calculated, based on the
assumption of regional compensation, i.e. a Vening–
Meinez model. The model parameters are:
(maximum) to –120 ·
Density of the topographical masses: 2.67 Mg/m3
Density of the upper mantle: 3.3 Mg/m3
Depth of the normal crust: 30 km
Flexural rigidity: 1023N m
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After subtracting this isostatic Vening Meinesz re-
gional field from the Bouguer gravity field, we derived
an ‘‘isostatic residual field’’, which is shown in Fig. 3.
For further enhancement of local features in the
isostatic residual gravity, we calculated the curvature
of the field by another in-house software, based on
algorithms published by Roberts (2001) (Fig. 4). Cur-
vature is an attribute of a curve or a surface in the 2-D
or 3D space, respectively, and describes, how much the
curve deviates from a straight line (planar surface) and
how bent a curve/surface is at a particular point.
Therefore, curvature is a surface-related attribute
which provides insight into particular aspects or prop-
erties of a surface, which are otherwise difficult or not
possible to observe. From the methodological point of
view it is related to the second derivative of the sur-
face. Curvature has previously been used in the anal-
ysis of 3D seismic surveys (Roberts 2001), but has also
been tested and applied on gravity data (Tas ˇa ´rova ´
2004). Here, we used ‘‘dip curvature’’ which is calcu-
lated in the direction of the largest dip of the Bouguer
gravity ‘‘surface’’. It enhances linear features (gravity
lineaments) in the residual field and may help to
identify general strike directions. Figure 4 shows the
mapped ‘‘dip curvature’’ of the isostatic residual field
shown in Fig. 3. An interpretation of the gravity field is
given in the following section.
Qualitative interpretation of resulting gravity fields
The Bouguer gravity anomalies (Fig. 2) do not show a
unique trend. In general the magnitude of Bouguer
Fig. 2 The measured (complete) Bouguer anomaly map in the
Dead Sea area (contour interval 5 · 10–5m/s2). The white lines
mark the positions of the vertical cross-sections shown in Fig. 5
(cross-section a corresponds with Fig. 5a, b with Fig. 5b and c
with Fig. 5c; the grey line indicates the vertical NS cross-section
in Fig. 6. The WRR profile is indicated by circles
Fig. 3 Isostatic residual field, based on a Vening–Meinesz model
(contour interval 5 · 10–5m/s2). The WRR profile is indicated by
circles
Fig. 4 Dip curvature of the isostatic residual field shown in
Fig. 3. Negative curvatures are blue/green colours, while positive
curvatures are shown in orange/red colours. Zero curvatures are
yellow. The thick black lines represent an interpretation of some
prominent curvature lineaments. G Gharandal basin, T Timna
basin. The WRR profile is indicated by circles
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