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Active compressional tectonics in the Jericho area, Dead Sea rift
239
Tecronophysics, 198 (1991) 239-259 Elsevier Science Publishers B.V., Amsterdam
Active compressional tectonics in the Jericho area, Dead Sea Rift Y. Rotstein a, Y. Bartov b and A. Hofstetter
a
u The Institutefor
Petroleum Research and Geophysics, POB 2286, Holon, 58117, Israel h Israel Geological Survey, 30 Malchei Yisrael, Jerusalem, 95501, Israel
(Received October 10, 1989; revised version accepted August 14, 1990)
ABSTRACT Rotstein, Y., Bartov, Y. and Hofstetter, A., 1991. Active compressional tectonics in the Jericho area, Dead Sea Rift. In: J. Makris, P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Oceanic Basin. Tectonophysics, 198: 129-259.
The results of several seismic reflection surveys in the Jericho area, north of the Dead Sea, are described and the data are used to analyze the structure of this part of the Dead Sea Rift. In particular, we describe the structure of the Dead Sea Transform, which in this area is a zone of intense deformation, rather than a distinct fault plane. The transform has a westerly dip, away from the rift, suggesting that it may have a reverse-faulting component. Associated with the transform are several folds, of which the best documented is the Kalia Monocline, immediately north of the Dead Sea. The Kalia Monocline lies along the transform and appears to be a young structure associated with it, indicating that this part of the plate boundary is characterized by local compression. The southerly extent of the Kalia Monocline is not clear presently, but the available data suggest that the compressional field also characterize the northwestern part of the Dead Sea Basin. The other folds appear north of the Kalia Monocline and seem to be similar to it. They are, most probably, also the result of recent compression along the transform. A number of mechanisms of small earthquakes in this area are computed and several of them show thrusting, suggesting that the local compression is still active
Introduction
The Dead Sea Rift is the transform plate boundary between the Arabian Plate and the Sinai Block of the African Plate (Fig. 1). The transform plate motion along the rift is taking up the opening of the Red Sea into the zone of continental convergence in southeastern Turkey. Motion along this transform is now thought to have started in the Middle Miocene, with the end of the main opening phase of the Gulf of Suez (e.g. Steckler et al., 1988; Shaliv, 1989). The instantaneous rate of motion along the transform is about 0.6 cm/yr, based on plate kinematics (Joffe and Garfunkel, 1987) and is about the same as the estimate of the average slip based on the 105 km total documented slip (Steinitz et al., 1978). The Dead Sea Transform is a left lateral system of en-echelon faults. Moving northward along the plate boundary, the faults are usually offset westward. In the overlap areas, a series of pronounced depressions appear, and these are frequently re-
ferred to as rhomb-shaped grabens or pull-apart basins. The deep depressions, particularly the Dead Sea Basin, have traditionally attracted most of the attention in the rift, and as they became targets for oil exploration, most of the geophysical work was concentrated in them. Nevertheless, some oil-related geophysical surveys, in particular seismic reflection lines, have been carried out in other parts of the rift. In this work we use the existing data from the Jericho area (Fig. 2), which recently became open to the public, to study the transform and the structures that are associated with it. We find that the strike-slip motion along the transform is accompanied by compression and we use first-motion data of recent small earthquakes to document that compression is still active in the area. Geological setting The study area is part of the Jordan Valley, a segment of the Dead Sea Rift. It is situated be-
0040-1951/91/%03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved
240
33”
32”
50
LEGEND Fault . .._..___.. Anticlinat Axis
. .
. . . .-
.
. .-t_
Volcano . . . , . Direction of plate motion m
Odaternary
MEDlTERRANEAN
SEA
Fig. 1. Generalized tectonic map of Israel and adjacent areas, showing major tectonic elements (after Bartov, 1990).
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
the Jordan River in the centre of the rift valley and the prominent normal fault which marks the western boundary of the rift north of the Dead tween
SEA
241
RIFT
Sea (Figs. 2, 3). This area was studied by Picard (1931) who described the Dead Sea as a rift valley bordered by “ two considerable border faults”,
200
JOROAN VALLEY 4
I
Fig. 2. Location map of present study, showing the location of the seismic reflection Iines. Heavy lines denote parts of seismic sections shown as fries in this work and numbers in brackets indicate @ure numbers. Also sbown are station numbers (every 100) along these lines and the two deep wells in the area. The thick line marks the location of the plate transform following Reches and Hoexter (1981).
242
Y
KOICTF-IY
1
16f
’
84 m. Turonian
+Tili2
JORDAN VALLI 276 m. Eo
153m.
E
T/2 +llOm. /
Katie 1
i
Kalia 5m.e Emme?
LEGEND Dead Sea
I
Fault
-
Anticlinal
axis
_
Synclinol
axis
-A
-L @
Flexure Drill hole Mazar
Fm. ouitcrops
Cretoceous ouitW0p.S
I:‘1 41
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
SEA RIFT
243
Fig. 3. (a) Simplified geological map of the study area. Pre-rift Cretaceous series is stippled and axes of suggested pre-rift Syrian Arc folds are shown (following Rot, 1970 and Begin, 1975a). Also shown are wells in which pre-rift Upper Mesozoic and Tertiary sediments were found; numbers indicate depth to pre-rift sediments and their ages appear when known. (b) LANDSAT 5 imagery showing the trace of the Dead Sea Transform in the Jericho area. The plate transform is recognized by the terrace which it forms in the white chalk of the Pleistocene Lisan Formation; it is marked by arrows. Location as (a).
and noted that the Western Boundary Fault is the oldest tectonic feature in the area which is related to this rift. He estimated the throw on the western border fault to be several hundred metres, with most of the motion on it being pre-Late Pleistocene Lisan Formation. Rot (1970) and Begin (1975a) studied parts of this area, but focused on the folds which are observed in the Mesozoic sequence west of the rift. In particular, they mapped the Mar Sava and Auja anticlines and the
Buqia Syncline (Fig. 3), relating facies and thickness changes across them to Late Turonian-Middle Eocene folding, which formed the Syrian Arc (Krankel, 1924), system of ~y~et~c folds throughout the Levant (Fig. 1). This system was active up to the Miocene, but no evidence for this late tectonism was documented in the study area. The Jericho area is located immediately north of the Dead Sea Basin, which is considered to be an active pull-apart basin (e.g., Freund et al.,
t
244
1970; Freund and Garfunkel, 1976; Garfunkel, 1981: Aydin and Nur, 1982). The plate transform that extends from the south along the eastern margin of the Dead Sea Rift, bends to the east and dies out to the northeast (Fig. 3). The next segment of the transform lies along the western side of the Dead Sea and extends obliquely across the rift in the study area (an-Men~em et al., 1976; Garfunkel et al., 1981; Reches and I-Ioexter, 1981: Fig. 21, and continues to the north-northeast. toward the Bet Shean-Kinneret Graben. We re-examined the available air photographs from the area and mapped it as a semi~nt~uous line on both sides of the Jordan River (Figs. 2, 3). The air photograph shows no record for a diagonal fault north of the Dead Sea as suggested by Kashai and Croker (1987) and ten Brink and Ben-Avraham (1989) Surface sediments in this area are mostly horizontal, consisting mainly of coarse elastics and chalks of the Pleistocene Sarnra and Lisan Formations. At a few locations, outcrops of marls prohably of Pliocene age and sandstones of the Mazar F~~rrnati~~n(lower part of Samra Formation; Begin. 1975b). dip to the east up to 5O (Fig. 3a). These beds are unconformably overlain by the Late Pleistocene Lisan Formation, known from the central Jordan Valley south of the Sea of Galilee, and from the northern Arava Valley, south of the Dead Sea. Many wells are found in the area but most are shallow water wells. Nevertheless, some of them are deep enough to penetrate prerift sediments, which appear to be quite shallow in this area (Fig. 3a). Jericho 1 is the only deep well in the area and is located on pre-rift Senonian chert. Jordan Valley 1, which is the only drill hole in the eastern side of the rift, penetrated 276 m of Neogene to Pleistocene elastic rift sediments overlying pre-rift Lower EZocene beds. Thus, as noted by Kashai and Croker (19871, a deep basin does not exist in the Jericho area and the transform is characterized by strike-slip motion. The Dead Sea Transform in the study area is known to be seismically active, with the last known strong earthquake in the area being the M = 6.25 Jericho event in 1927 (e.g. Ben-Menahem et al., 1976). Strong earthquakes in this area have a repeat time of several hundred years (Reches and
KOl\lI
14 t I 41
Hoexter. 1981; Rotstein, 1987) and the seismic activity in the area is presently quite low, possibly because littie strain has accumulated since the relatively recent strong earthquake (Rotstein and Arieh,
1986).
Seismic reflection data
Multichannel seismic reflection data coverage in the area is shown in Fig. 2. All lines were surveyed between 1977 and 1985, in four different exploration phases. Vibrators were used as the source in all the lines, but otherwise the different exploration phases varied in field parameters. Current practice in conventional seismic exploration for oil in onshore Israel calls for maximum source power, high multiplicity and source sweep which starts at the lowest possible frequencies. None of the lines in this area seem to have been carried out using optimal parameters, even though most yield useful data. Most of the lines were carried out using multiplicity of only 24 in contrast to the 60 which is generally presently used. The more recent lines use multiplicity of 60, but the source sweep of 16-70 Hz in these lines lacks the low end of the possible frequency range and emphasizes information from shallow depth. Maximum spread which was employed in this area was 4500 m, but in most of the lines the spread was 2650-3150 m. Since, in general, depth of investigation in seismic reflection work is estimated roughly as equivalent to the maximum spread length. the available seismic sections can be expected to yield information up to a depth of 25~-5~ m. Conventional commercial processing was ap plied to all the lines and several of the lmes were processed by more than one centre. Only one line was reprocessed as part of this work, yielding somewhat improved results, but not justifying reprocessing the entire data set. Most of the lines have migrated sections. l7hthquake
source mechanism
A countrywide earthquake monitoring system has been operated in Israel since 1982 by the Institute for Petroleum Research and Geophysics
ACTIVE
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SEA RIFT
Fig. 4. Focal mechanisms for individual earthquakes in the Jericho area. Solid circles denote compression and open circles denote dilatation. Stations are pro$cted on the lower hemisphere. The letters P and T refer to the compressional end the tensional axes, respectively.
TABLE 1 Fault parameters of the events that were used in this study No.
1 2 3 4 5 6 7 8 9 X0 11 12
Date
8404100342 8~80211~ 8501101407 850125~8 8501261303 8501271936 8605120653 8~7~~818 8612042238 87~230811 8805250146 8802221934
Mag.
2.1 3.1 2.5 4.7 1.6 2.0 2.4 2.1 2.0 1.0 2.1 1.3
No. obs.
P 6
18 6.5 13 36 a 11 22 25 23 9 23 13
10 14 65 35 30 20 24 0 3 25 5 24
T
Fault plane
Auxiliary plane
+
6
#
+J
8
x
Q,
8
x
315 I32 92 112 69 104 329 309 129 282 72 122
10 73 5 54 60 70 35 14 10 52 81 16
224 280 194 299 249 284 220 219 219 SO 202 220
0 35 125 25 340 15 10 355 355 55 170 170
75 60 55 80 75 65 45 80 85 30 40 85
0 80 -60 93 90 90 10 10 10 150 100 -30
9090 234 259 185 160 195 212 263 264 171 337 262
31 44 10 15 25 82 80 80 75 50 60
165 106 -125 70 90 90 134 169 174 63 81 -174
Mis.
Sta. dist.
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.11 0.09 0.0
0.77 0.67 0.64 0.75 0.82 0.76 0.86 0.76 0.72 0.68 0.63 0.80
Event 2 is a composite solution. Date is composed of year, month, day, hour, and minute. Following the notations of ski and Richards (1980, p, 105) we use dip, 8, strike (or azimuth for P and T stress axes), +, and rake, X. Event numbers are referred to in the text.
246
TWO 0 G
WAY TIME
(set) 9 N
ACTIVE
247
CO1 dPRES
u
m
a [I:
a
c:
248
TWO 8
WAY
TIME
(set)
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COMPRESSIONAL
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DEAD
(Seismological Bulletins IPRG, 1982-1988). For earthquakes within the rift, stations do not encircle the sources, as is usually required for a reliabie determination of source location and mechanism. However, additional azimuthal coverage was achieved through the use of published catalogs of the Jordanian network with stations east of the rift (JSO Bulletins, 19841988). During that period of time, I1 earthquakes were recorded with sufficient number of P onsets for reliable fault plane solutions (Fig. 4 and Table 1). One additional earthquake mechanism (No. 2 in Fig. 4 and Table 1) is a composite solution consisting of data from three earthquakes, with similar waveforms and closely spaced in time and location. The reliability of the fault plane solutions was tested using the misfit and station distribution statistical criteria (Reasenberg and Oppenheimer, 1985). The misfit parameter describes how well the fault plane solution fits the available data, and our results (misfit < 0.11) show a very good fit (Table 1). The station distribution parameter can
249
RIFI-
be viewed as a factor of robustness which is related to the radiation pattern of a given set of phase readings. It varies from 1, in a weIl constrained case of several stations close to the nodal planes, to 0 in a poorly constrained distribution. We define robustness if station distribution parameter is greater than 0.5 and note that all solutions passed this criterion and are, most likely, reliable. Results and discussion Main fault zone
Three of the seismic lines extended eastward into the Arabian Plate and show clearly the main plate boundary fault zone (Figs. 5-7). Line SI-7102 (Fig. 8) may also cross the main fault, but cannot be used for a detailed study of its structure. In all the lines where the plate boundary fault zone is observed, it appears to be about a 1 km wide zone of intense deformation, rather than a distinct main
Sl-7102
WESTERN
-w-
SEA
BOUNDARY FAULT 200
150
SYluTECTONlC SEQUENCE \
-E-
Ikm
Fig. 8. Migrated seismic section of line SI-7102. For location see Fig. 2. MFZ denotes the main fault zone which in this section is not observed clearly. Heavy line denotes the marker used for structural mapping. Syntectonic rift-related sequence is discussed in the
250
fault
plane
with
near-surface
splays.
deformation
fault zone extends
In
these
associated
sections.
with the main
over a wider area and in two of
them (Figs. 5 and 7) it is also significantly intense tions,
than
further
the main
at depth,
In these
fault zone, below
many coherent
indicating
the presence
of relatively
disturbed
slivers.
In contrast, and coherent
surface
by a 6-10
I s (ap-
reflections,
the upper
part
the plate
phone
the plate
sideswipes,
which
lines because
are not observed
in the other
these lines are not as oblique
In all the seismic
sections
which
to it.
cross it, the
fault zone dips to the west, away from the
rift valley. Only
line DS-3047
(Fig. 7) is approxi-
mately
perpendicular
to the fault and can be ex-
pected
to show
true
the
dip.
The
present
data
that the main
fault zone is steeper
in the
m terrace (Fig. 3) which is too
southern
part of the study area as compared
to the
by a single earthquake.
boundary,
but
that
it is not
and a very short geo-
array, may be required
to study the details
of the plate boundary. In the absence of sufficiently detailed data, we chose to describe the plate boundary as a wide zone of deformation rather than try and trace individual faults. In DS-3048 (Fig. 6) the main fault zone has significantly different characteristics than evident which
lies along
indicate
higher frequencies
in the lines
which
is
apparent with the present resolution of the seismic data. A high resolution survey using short group intervals,
structure
are rare.
Thus, it is likely that a distinct master fauh does exist within the zone of deformation which characterizes
steep
boundary (see next section). This structure, the Kalia monocline, may he the source of these
on the
high to have been formed
is evident
large
main
large and un-
reflections
We note that the plate boundary
more two sec-
about
prox. 1.5 km), includes
highly deformed
the
cross
it north
and
south
of
DS-3048 (Figs. 5 and 7). Here, the fault zone is narrower. in spite of the obliqueness of the line which makes it appear even wider than it really is. In addition, continuous reflectors at all depths inside the main fault zone indicate that deformation is not as large as in the areas crossed by the other two lines. Thus, as in the case of northern Israel (Rotstein and Bartov, 1989) the characteristics of the plate boundary show rapid changes along the strike. Two other phenomena are apparent in line DS-3048 and are not observed in the two other lines which cross the plate boundary. The first is the observation of two sets of reflectors, one almost horizontal and the other dipping to the SE. The second one is the appearance of distinct reflectors in this line beneath the plate boundary zone. We consider these two phenomena to be related and suggest that the dipping reflectors. including those in the main fault zone area, are sideswipes. This line crosses at a small angle a
northern part, further indicating rapid changes in the geometry of the plate boundary along its strike. C’orrelation of seismic reflectors across strikeslip faults is often difficult. since sedimentary sections appear that
on both sides were not de-
posited at the same place. In the present study area such a correlation was attempted by Kashai and Croker (1987), using Iine DS-3047. However, since none of the lines penetrated deeply into the Arabian consider In line
Plate, or is tied to wells on both sides, we this correlation to be rather speculative. DS-3048 (Fig. 6). which includes the
clearest section from the Arabian Plate, correlation across the main fault zone is not apparent. In general, this type of correlation is important in analyzing the sense of motion and the detailed development of the faults. However, in this case, even without such a correlation, some of the characteristics of the fault can be inferred. For example, the dip away from the rift, indicates that the main
fault
zone
is not
associated
with a rift-re-
lated normal faulting component, such as the Western Boundary Fault. Instead, in the study area, the Dead Sea Transform may have the characteristics of a reversed fault. The main component of motion across the transform is of a strikeslip nature, but some reversed motion may aIso be present, as shown by Gardosh (1987), who found a small compression structure in the Lisan Formation. This is in contrast with the possible southerly extension of this segment of the transform into the deep northern Dead Sea Basin, that was suggested to have distinct normal faulting characteristics (Neev and Hall, 1979). A reversed motion component on this segment of the transform indicates that the plate boundary
ACTIVE
251
CC3MPRES
P
$i
a
lL
.Y
I
a i
9
0
-
n
252
tco14it IV
Y
is associated
with local compression.
field is currently seismological
active,
ated
with
the
main
10). Other
in
two of the earthquakes
fault
zone,
to be associ-
yield
thrusting
(events Nos. 2 and 4 in Figs. 4
earthquakes
Dead Sea yield solutions with a component
be evident
area which appear
source mechanisms and
it should
data. Indeed,
from the Jericho
If this stress
in the northernmost
which indicate
of reverse motion,
ing active compression
monocline. transform
also suggest-
in the area.
is a distinct
which can serve as independent pression
across
monocline
this part
is apparent
evidence
the
Lisan
main
fault
zone
lated
the Jericho
extent
in both directions
The Kalia
Monocline
observed area
near
(Begin
active compres-
by an earthquake area
(No.
divergent
sedimentation
the
1975a; source
2 in
sections
Fig.
10).
clearly
dis-
reflectors,
indicating
(Figs. 7-9).
Their posi-
tion next to the main fault zone suggests that these are young rift sediments associated with the tectonism which formed the underlying monocline.
its full
is not yet known. is not apparent
near-surface
features
ap-
those not The entire
however,
compressional
this
this
struc-
The
1 deep well and various
by the seismic data;
from
is sug-
of this linear
Formation in
the
fault zone. 1t is also
several of the seismic
syntectonic
shallow wells (Fig. 3a). The monocline appears to be a narrow, elongated feature that lies along the transform, with some 15 km of its length presently documented
mild
with
of local com-
interpretation
et al., 1981). As noted,
for com-
is seen in Fig. 10, which shows a time map of a Cretaceous reflector, corre-
using
This
is also indicated
play
of the transform.
the
within
structure
in all the lines which
proach the plate boundary, including crossing the transform (Figs. 5-9). structure structural
by
mechanism
Monocline
as a result
ture with the plate boundary
Finally, Kalia
created
\I
Monocline
associated
gested by the close proximity
Garfunkel
Kalia Monocline
that the Kalia
structure
in this area.
supported
strike-slip
active and
pression
sion
The
We suggest
is a young,
t I
on the
The normal faulting seismic sections and,
observed in some of the of course, known from
surface mapping, does not preclude local compression. The prominent boundary faults of the rift, with vertical throws of several hundred metres, clearly
demonstrate
that
extension
was
an
im-
portant element in the region. However, temporal tectonic changes are known from other parts of the rift (e.g. Zak and Freund, 1981; Marcus and
surface. Its existence as a pre-rift structure was suggested by Begin (1975b) while studying
Slager, 1985) and we suggest that they also characterize this area. Namely, that the extension which
palaeocurrents first described
had been important in the past is no longer active. West of the Kalia Monocline, across the West-
the region
in the Samra Formation. It was in the course of oil exploration in
using
the same seismic
reflection
data
as in this work (Hardman, 1985). This author also suggested that the Kalia Monocline is delimited to the east by a large easterly dipping normal fault, located in the area where we map the plate boundary. We find some normal faulting in the Kalia Monocline but no evidence for a major normal fault downfaulting the area east of the
Fig. 10. Interpretation Monocline north
map
and earthquake
showing
contoured
source mechanisms
of the Kalia Monocline.
Surface
two-way
ern Boundary
the Buqia
Syncline
(Fig.
3)
was mapped by Rot (1970). This feature is related to the Syrian Arc and shows typical changes in thickness structure.
of Late Cretaceous sediments We observe a similar change
across the in section
DS-730 which displays thickening of uppermost Cretaceous sediments towards the top of the suggest that monocline (Fig. 5). These relations
time (in milliseconds)
to a Cretaceous
of Fig. 4. Also shown are the axis of the Jericho
extent of the main fault zone, indicated
main fault zone at the level of the marker
Fault,
marker
Anticline
from the seismic data,
which is mapped, are shown by heavy broken the seismic marker.
lines. Other
which
details
and of another, is stippled.
the Kalia smaller
fold
The traces of the
faults are shown at the level of
ACTIVE
COMPRESSlONAL
TECTONICS
IN THE
JERICHO
AREA,
DEAD
SEA
253
RIFT
1.0 - 1.9
*
2.0- 2.9
l
3.0 - 3.9 a
&ROAN
VALLEY 0
I
I
190
200
254
prior
to its deformation
form,
the Kalia
dipping,
eastern
strike-slip
by the Dead
Monocline
was part
structures
which
faults are quite common
restraining
bends
of such faults
1981; Christie-Blick what similar
and
monocline
(1978) and termed
fault block. Along
ever, smaller
accompany and appear
further at
was described
by Stearns
(1985) a contrac-
the Dead Sea Transform structure
is the regional
have been
How-
also described
and Bartov,
A restraining bend on the Dead Sea Transform in this area can be inferred in the structural intermap (Fig. 10). Moreover,
the transform
in this area has approximately a N-E trend, rather than the almost N-S trend of the rift and of the further
north.
If, on the other hand, the
long-term, regional direction of plate motion is approximated by the trend of the rift, then the transform is oblique to this direction. This geometry has the effect of a restraining bend and will be associated with compression, as shown elsewhere along the rift (Rotstein
and Bartov,
south.
are crossed give
In addition,
by one line
different
no
indication
tectonic
displayed
the three
appear
Monocline,
some
data exist, but are insufficient for reliable mapping. The N-S line DS-3048 (Fig. 11) indicates that the Kalia Monocline dies out in this direction, being one of a series of compressional structures in the region. The line displays a second, smaller anticline centred at SP 1040 and a third anticline centred at SP 900. We interpret this last anticline to be the extension of the Jericho Anticline (Fig. lo), tested by the Jericho 1 well. The two E-W seismic lines from the area north of the Kalia Monocline both show the sedimentary sections in the eastern parts of the lines to dip towards the plate boundary (Figs. 12 and 13). The plate boundary itself is not crossed by these lines but is expected to be only in short distance further
21
III
phases.
of being
with the plate mechanisms
boundary.
folds
adjacent
to
to be similar the
result
These observations of the Kalia
cline are both the result of compression
of sug-
Mono-
associated
The earthquake
source
(Figs. 4 and 10) are either pure thrusts
or have a large thrusting component. These data support active compression in this area and are consistent with our conclusions that all the anticlines in the Jericho area are young structures not related to the Syrian Arc folding phase. The observation of an apparent compressional dome, the Zahret el-Qurein dome (Garfunkel et al.. 1981; Fig. 3), in the northernmost part of the area, is also consistent with this interpretation. The surface trace of the active fault is associated with the plate boundary in this area and is displaced eastward (Fig. 3). This geometry is in agreement with local compression in the immediate vicinity of the jump. The observation of a continuous compressional Dead Sea is more likely
regime north of the to infer an overall ob-
of the transform in this area with the of plate motion. The angle between the
two directions of the Kalia
where and
gest that the two folds north
liqueness direction
1989).
Jericho Anticline To the north
structure
and
ches, 1987; Gardosh, 1987; Rotstein 1989; Goldberg and Beyth, 1991)
transform
2). The
1985). A some-
in various places along the rift (Garfunkel et al., 1981; Bayer, 1985; Heiman and Ron, 1987; Re-
pretation
(Fig.
each other (Fig. 11) they also appear
with the bend in Lebanon.
structures
to the east
ti
them is quite similar to the observed structure m the lines that cross the young Kalia Monocline
(e.g. Garfunkel,
Biddle,
by Harding
the largest compressional uplift associated
Sea Transof the NW
flank of the Buqia Syncline.
Compressional
tional
Kol4ltlh
L
is quite small, in light of the small
magnitude of the structures relative to that north of the Sea of Galilee (Rotstein and Bartov, 1989) and, in particular, to the Mount Hermon area (Garfunkel,
1981).
Dead Sea Basin To the south of the Kalia Monocline there is also no indication that the end of the compressional regime is approached. This area is of particular interest, since it lies along the northwestern boundary of the Dead Sea Basin, in a region which is traditionally considered to undergo extension and normal faulting (e.g. Neev and Hall, 1979; Garfunkel, 1981; Ten Brink and Ben-Avraham, 1989). The Kalia Monocline itself assumes its prominence near the southernmost extent of
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE
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RIFT
TWO WAY TIME (MC)
255
256
Y
the present southward
study
area (Fig.
until it is delimited
10) and may extend
the Lisan
by the rift boundary
a small amount
fault. South of it, in the Cretaceous the rift, Rot (1970) mapped ture
(Fig.
system
3a), but
a compressional
related
Cretaceous
sion along the northwestern Sea Basin servation
strata.
in relative
boundary
the
single
channel
suggest sistent
Arc thick-
even though
of recent
faulting
with significant
northwest
boundary
normal
showed
of the Dead
many
surface
expression
seismic
present
seismic
data
at some places
faulting
along
the
Neev and Hall (1979) part
seismological
of the
them as the near-
of salt diapirism and
We
Sea Basin, may
in the western
Dead Sea Basin and interpreted
with the ob-
\I
which are incon-
of the Dead
folds
I.1
is observed.
that these observations,
be due to local compression.
Active compres-
may, also, be consistent from
struc-
it to the Syrian
on the basis of changes
ness of Upper
series west of
Formation,
ROISII~IN
at depth. data
The
from
the
from this area (Neev and Hall, 1979). As noted by
region suggest that these folds may be yet another
Garfunkel
expression
steep
et al. (1981), these data
eastern
boundary
fault
show that the
of the
Dead
Sea
Basin is void of sediments, indicating a continuing activity; the presence of a 20-30 m thick sedimentary layer across the normal fault under the western part of the lake, indicates that for a few lo4 years, little normal faulting occurred on it. They also note that most of the activity on the large normal
faults west of the Dead Sea are older than
-W-
Fig. 12. Unmigrated
for the compressional
stress
seismic section of line DS-3046
showing
in
this part of the rift. The Dead Sea is commonly considered to be a pull-apart basin resulting from the stepwise jump of the main transform westward (e.g. Garfunkel, 1978). Using this model, the western master fault of the graben is inferred to have a normal faulting component. The present data suggest that the compressional field is not limited to the Jericho
DS - 3046
east. It is not clear if this is part of the Jericho Anticline,
field
a dip towards
-E-
the Dead Sea Transform
or of the smaller anticline
which is a short distance
to the south of it. For location
to the
see Fig. 2.
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE
JERICHO
AREA
DS -
DEAD
SEA
25-l
RIFT
3038 .EJERICHO
ANTICI
INE
2
I.0
:!
Fig;. 13. Unseats
seismic section of line ES-3038 showing the sou~e~terly flank of the Jericho Antidine. Note the reselmblance to the sections which cross tlte Kalia Anticline. For location see Fig. 2.
area, but rather extends into the Dead Sea itself. If so, the internal structure in the basin is significantly more complex than predicted by the pullapart model. The obviation that the eastern boundary fault of the graben is active, while the western bounds is less active, suggests that the deep northern Dead Sea Basin is the product of asymmetric graben formation, much in the way found to characterize the East African grabens (Rozendahl, 1987). On the other hand, a combination of extension along the eastern side of the Dead Sea and compression along its western part, is generally consistent with block rotation being an important mechanism in the formation of pull-apart basins as suggested by Eyal et al. (1986).
tween Arabia and Africa is an appro~mately 1 km wide zone of intense deformation, rather than a distinct fault plane. The main fault zone dips to the west, away from the rift, su~esting that although most of the motion is of a strike-slip nature, a reverse-motion ~mponent is also present. Support for compression associated with reversed faulting is indicated by the presence of the young Kalia Monroe along the main fault zone. Other folds appear north of the Kalia Monocline, su~esting that the compression associated with the plate boundary extends northward from the Dead Sea for at least 25 km. The limited number of earthquake source mechanisms from the study area include thrusting, demonstrating that this compression is still active.
Conclusions
Seismic reflection data show some of the main tectonic characteristics of the Dead Sea Rift in the Jericho area. The transform plate boundary be-
The authors with to thank the Earth Science Research Administration of the Ministry of En-
25x
ergy and Infrastructure, seismic
reflection
for permission
to use the
data. We also thank
and H. Ron who reviewed
B. Katz
who processed
of line DS-3048 and I. Chelinskya
part
who drafted
the
figures.
Ilarding.
T.I’., 19X5. Seismic
of negative positive
Theory
and Methods.
1980. Quantitative
Freeman,
Seismology:
San Francisco.
Calif..
A. and
Aydin,
A. and Nur, A.. 1982. Evolution
and their scale independence. Bartov.
Y., 1990. Geological
Countries. Bayer,
of pull-apart
Tectonics,
Photomap
basins
1: 91-105.
S. and
des Golfes
und Kompressionstektonik
von Aqaba.
Geol. Rundsch..
am
74: 599-
610.
central
The geology
of the Jericho
sheet.
Geol.
Begin, Z.B., 1975b. Paleocurrents Formation cation.
(Jericho
Sediment.
Ben-Menahem, seismicity
region, Geol.,
A.. Nur.
Samra impli-
14: 191-218.
Z.,
of an incoherent
plate.
Seismological
Kashai,
Bulletins,
and Adjacent
Earth
Planet.
basin
N. and
formation
Biddle.
along
Econ. Paleontol. 1986. The origin Eastern
K.T..
strike-slip
Mineral..
Eyal. Y., Eyal, M., Bartov,
Freund,
Observatory,
Sinai. Tectonics,
G. and Folkman,
Y.,
graben,
Philos. Trans.
of Geology.
Hebrew
Z., Zak, I.. Goldberg,
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Quaternary
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Univ.,
Garfunkel,
10th
Int.
(rift) in relation
physics,
Congr.
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synthesis
of sedimen-
Sedimentol..
Jerusalem,
kinematics.
Tectono-
R., 1981. Active faulting
transform.
Island-an
of the Red Sea rift and
In: J. Makris,
in
in the Judean
fault-plane
Desert. FPPLOT
for calculating
solutions.
USGS,
aspects
Z. and
basins
Sea Trans-
and
141: 75-88.
Hoexter,
activity
of pull-apart to the Dead
D.F.,
1981.
in the Dead
Holocene
seismic
and
Sea area. Tectonophysics.
80:
235-254. Qilt. Geological Rotstein,
Y.. 1987.
Gaussian
occurrence
Tectonophysics.
ment
prc-hability
in the Jordan
estimates
of large
Valley, Dead Sea Rift.
141: 95-105.
Y. and
Earth Planet. Rotstein,
1 : 50000. Wadi el
map of Israel.
Survey of Israel.
Arieh,
E., 1986.
Tectonic
implications
data from Israel and adjacent
of areas.
Sci. Lett.. 78: 237-244.
Y. and Bartov, transform:
of the Dead
Y.. 1989. Seismic reflection an example Sea Rift.
across
from a convergent
J. Geophys.
a
seg-
Res.. 94 (B3):
2902-2912.
80: l-26.
M. and Beyth, M., 1991. Tiran at the junction
in
investigations
programs
swells with applications
continental
80: El--108. Z.. Zak. I. and Freund,
9:
Sea Rift.
D.. 1985. FPFIT.
computer
earthquake
form. Tectonophysics,
Rotstein.
of the Dead Sea leaky
to plate
Mineral.,
in time and space. Isr. J. Earth
Researches
Z., 1987. Mechanical
earthquake
structure
141: 33-60.
Rept.. 85-739.
recent microearthquake
the Dead Sea Rift. Tectonophysics, Goldberg,
Thesis.
Part I, pp. 35-72.
Z., 1981. Internal
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M.Sc.
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77 pp.
Z., 1978. The Negev-regional basins.
Guidebook,
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Sea Basin.
and
Geol.. 23: 209-238.
Rot, I., 1970. The geological
Ser. A, 267: 107-130. Activity
geometry
Rift system as deduced
1979. Geophysical
Fortran
displaying
tectonic
M., Weissbrod,
and Tectonic
in the Dead
Univ., Jerusalem,
Garfunkel, tary
R. Sot. London,
in
Jordan
Jordan.
Bogen, Zentralbl.
P. and Oppenheimer.
Open-File
Reches.
M., 1987. Stratigraphy
Amman,
1 well (Jordan-Dead
L.. 1931. Geological
push-up
to Excursions
T. and Derin, B.. 1970. The shear along the Dead Sea Rift. Gardosh,
and Reches.
5: 267-277. Z., 1976. Guidebook
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J., 1985. The sedimentary-magmatic
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27 pp.
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Reasenberg,
rhomb-shaped
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Division,
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Sot.
Y.. Steinitz,
of the
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E. and
faults.
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Tectono-
kinematics
P.F., 1987. Structural
of the Dead Sea-Jordan
sequence
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37: l-34.
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Marcus,
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IS.
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Tectonophysics,
Seismological
E., 1924. Der Syrische
1985. Deformation
of the Bir Zreir
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Inter.. 12: I-50. Christie-Blick.
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E.L. and Croker.
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M., 1976. Tectonics.
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Israel) and its emplacement
A. and Vered,
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the breaking
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Bull..
5-22.
274-281;
Surv. Isr. Bull.. 67: l-35.
Dead
re-evaluation.
from new subsurface Z.B., 1975a.
and
Pet. Geol.
I ~-recommendation
H.. 1987. Young
Sea -a
Seismological
1985. Kollisions-
Am. Assoc.
(1983) Ltd., Rept. No. 85/l
Garfunkel,
Red
Jordan
Survey of Israel.
H.J.,
Ostsaum Begin.
Joffe.
JSO.
of Israel and Adjacent
and identification flower structures.
141: 117-124.
circum
PP.
Ron.
basin,
physics.
932
Kalia
Oil Exploration
pull-apart P.G.,
inversion.
I?.. 1985.
Heiman. Richards,
positive
63: 582-600. Sismica
Aki, K. and
characteristics
flower structures.
structural
Hardman.
References
()cea~~ Ihrvn.
f-il‘;torv of a NCW
19X:000-000.
I‘cctonophy\ich.
Z.B. Begin
the manuscript.
who edited it, Y. Ben Harush
Red Sea: Birth and flarl>
internal Dead
Sea
P. Mohr and R. Rihm (Editors),
Rozendahl. special
B.R.. 1987. Architecture reference
Sci.. IS: 445-503.
to East Africa.
of continental Annu.
rifts with
Rev. Earth
Planet.
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
Seismological Bulletins, 1982-1988. Earthquakes in and around Israel. Institute for Petroleum Research and Geophysics, Seismological Division, Bull. No. 1-7. Shahv, G., 1389. Stages in the Tectonic and Volcanic History of Neogene Continental Basins in Northern Israel. Ph.D. Thesis, Hebrew Univ., Jerusalem, 100 pp. (in Hebrew). Steams, D.W., 1978. Faulting and forced folding in the Rocky Mount~n foreland. Geol. Sot. Am. Mem., 151: l-37. Steckler, M.S., Bertbeiot, F., Lybris, N. and Le Pichon, X., 1988. Subsidence in the Gulf of Suez: impli~tions for rifting and plate kinematics. Tectonophysics, 153: 249-270.
SEA
RIFT
259
Stein&, G., Bartov, Y. and Hunziker, J.C., 1978. K-Ar age definition of some Maine-Pli~ne basalts in IsraeI: their significance to the tectonics of the rift valley. Geol. Msg., 115 (5): 329-340. Ten Brink, U.S. and Ben-Avraham, Z., 1989. The anatomy of a pull-apart basin: seismic reflection evidence from the Dead Sea Basin. Tectonics, 8 (2): 333-350. Zak, I. and Freund, R., 1981. Asy~et~ and basin ovation in the Dead Sea Rift. Tectonophysics, 80: 27-38.
Tecronophysics, 198 (1991) 239-259 Elsevier Science Publishers B.V., Amsterdam
Active compressional tectonics in the Jericho area, Dead Sea Rift Y. Rotstein a, Y. Bartov b and A. Hofstetter
a
u The Institutefor
Petroleum Research and Geophysics, POB 2286, Holon, 58117, Israel h Israel Geological Survey, 30 Malchei Yisrael, Jerusalem, 95501, Israel
(Received October 10, 1989; revised version accepted August 14, 1990)
ABSTRACT Rotstein, Y., Bartov, Y. and Hofstetter, A., 1991. Active compressional tectonics in the Jericho area, Dead Sea Rift. In: J. Makris, P. Mohr and R. Rihm (Editors), Red Sea: Birth and Early History of a New Oceanic Basin. Tectonophysics, 198: 129-259.
The results of several seismic reflection surveys in the Jericho area, north of the Dead Sea, are described and the data are used to analyze the structure of this part of the Dead Sea Rift. In particular, we describe the structure of the Dead Sea Transform, which in this area is a zone of intense deformation, rather than a distinct fault plane. The transform has a westerly dip, away from the rift, suggesting that it may have a reverse-faulting component. Associated with the transform are several folds, of which the best documented is the Kalia Monocline, immediately north of the Dead Sea. The Kalia Monocline lies along the transform and appears to be a young structure associated with it, indicating that this part of the plate boundary is characterized by local compression. The southerly extent of the Kalia Monocline is not clear presently, but the available data suggest that the compressional field also characterize the northwestern part of the Dead Sea Basin. The other folds appear north of the Kalia Monocline and seem to be similar to it. They are, most probably, also the result of recent compression along the transform. A number of mechanisms of small earthquakes in this area are computed and several of them show thrusting, suggesting that the local compression is still active
Introduction
The Dead Sea Rift is the transform plate boundary between the Arabian Plate and the Sinai Block of the African Plate (Fig. 1). The transform plate motion along the rift is taking up the opening of the Red Sea into the zone of continental convergence in southeastern Turkey. Motion along this transform is now thought to have started in the Middle Miocene, with the end of the main opening phase of the Gulf of Suez (e.g. Steckler et al., 1988; Shaliv, 1989). The instantaneous rate of motion along the transform is about 0.6 cm/yr, based on plate kinematics (Joffe and Garfunkel, 1987) and is about the same as the estimate of the average slip based on the 105 km total documented slip (Steinitz et al., 1978). The Dead Sea Transform is a left lateral system of en-echelon faults. Moving northward along the plate boundary, the faults are usually offset westward. In the overlap areas, a series of pronounced depressions appear, and these are frequently re-
ferred to as rhomb-shaped grabens or pull-apart basins. The deep depressions, particularly the Dead Sea Basin, have traditionally attracted most of the attention in the rift, and as they became targets for oil exploration, most of the geophysical work was concentrated in them. Nevertheless, some oil-related geophysical surveys, in particular seismic reflection lines, have been carried out in other parts of the rift. In this work we use the existing data from the Jericho area (Fig. 2), which recently became open to the public, to study the transform and the structures that are associated with it. We find that the strike-slip motion along the transform is accompanied by compression and we use first-motion data of recent small earthquakes to document that compression is still active in the area. Geological setting The study area is part of the Jordan Valley, a segment of the Dead Sea Rift. It is situated be-
0040-1951/91/%03.50 0 1991 - Elsevier Science Publishers B.V. All rights reserved
240
33”
32”
50
LEGEND Fault . .._..___.. Anticlinat Axis
. .
. . . .-
.
. .-t_
Volcano . . . , . Direction of plate motion m
Odaternary
MEDlTERRANEAN
SEA
Fig. 1. Generalized tectonic map of Israel and adjacent areas, showing major tectonic elements (after Bartov, 1990).
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
the Jordan River in the centre of the rift valley and the prominent normal fault which marks the western boundary of the rift north of the Dead tween
SEA
241
RIFT
Sea (Figs. 2, 3). This area was studied by Picard (1931) who described the Dead Sea as a rift valley bordered by “ two considerable border faults”,
200
JOROAN VALLEY 4
I
Fig. 2. Location map of present study, showing the location of the seismic reflection Iines. Heavy lines denote parts of seismic sections shown as fries in this work and numbers in brackets indicate @ure numbers. Also sbown are station numbers (every 100) along these lines and the two deep wells in the area. The thick line marks the location of the plate transform following Reches and Hoexter (1981).
242
Y
KOICTF-IY
1
16f
’
84 m. Turonian
+Tili2
JORDAN VALLI 276 m. Eo
153m.
E
T/2 +llOm. /
Katie 1
i
Kalia 5m.e Emme?
LEGEND Dead Sea
I
Fault
-
Anticlinal
axis
_
Synclinol
axis
-A
-L @
Flexure Drill hole Mazar
Fm. ouitcrops
Cretoceous ouitW0p.S
I:‘1 41
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
SEA RIFT
243
Fig. 3. (a) Simplified geological map of the study area. Pre-rift Cretaceous series is stippled and axes of suggested pre-rift Syrian Arc folds are shown (following Rot, 1970 and Begin, 1975a). Also shown are wells in which pre-rift Upper Mesozoic and Tertiary sediments were found; numbers indicate depth to pre-rift sediments and their ages appear when known. (b) LANDSAT 5 imagery showing the trace of the Dead Sea Transform in the Jericho area. The plate transform is recognized by the terrace which it forms in the white chalk of the Pleistocene Lisan Formation; it is marked by arrows. Location as (a).
and noted that the Western Boundary Fault is the oldest tectonic feature in the area which is related to this rift. He estimated the throw on the western border fault to be several hundred metres, with most of the motion on it being pre-Late Pleistocene Lisan Formation. Rot (1970) and Begin (1975a) studied parts of this area, but focused on the folds which are observed in the Mesozoic sequence west of the rift. In particular, they mapped the Mar Sava and Auja anticlines and the
Buqia Syncline (Fig. 3), relating facies and thickness changes across them to Late Turonian-Middle Eocene folding, which formed the Syrian Arc (Krankel, 1924), system of ~y~et~c folds throughout the Levant (Fig. 1). This system was active up to the Miocene, but no evidence for this late tectonism was documented in the study area. The Jericho area is located immediately north of the Dead Sea Basin, which is considered to be an active pull-apart basin (e.g., Freund et al.,
t
244
1970; Freund and Garfunkel, 1976; Garfunkel, 1981: Aydin and Nur, 1982). The plate transform that extends from the south along the eastern margin of the Dead Sea Rift, bends to the east and dies out to the northeast (Fig. 3). The next segment of the transform lies along the western side of the Dead Sea and extends obliquely across the rift in the study area (an-Men~em et al., 1976; Garfunkel et al., 1981; Reches and I-Ioexter, 1981: Fig. 21, and continues to the north-northeast. toward the Bet Shean-Kinneret Graben. We re-examined the available air photographs from the area and mapped it as a semi~nt~uous line on both sides of the Jordan River (Figs. 2, 3). The air photograph shows no record for a diagonal fault north of the Dead Sea as suggested by Kashai and Croker (1987) and ten Brink and Ben-Avraham (1989) Surface sediments in this area are mostly horizontal, consisting mainly of coarse elastics and chalks of the Pleistocene Sarnra and Lisan Formations. At a few locations, outcrops of marls prohably of Pliocene age and sandstones of the Mazar F~~rrnati~~n(lower part of Samra Formation; Begin. 1975b). dip to the east up to 5O (Fig. 3a). These beds are unconformably overlain by the Late Pleistocene Lisan Formation, known from the central Jordan Valley south of the Sea of Galilee, and from the northern Arava Valley, south of the Dead Sea. Many wells are found in the area but most are shallow water wells. Nevertheless, some of them are deep enough to penetrate prerift sediments, which appear to be quite shallow in this area (Fig. 3a). Jericho 1 is the only deep well in the area and is located on pre-rift Senonian chert. Jordan Valley 1, which is the only drill hole in the eastern side of the rift, penetrated 276 m of Neogene to Pleistocene elastic rift sediments overlying pre-rift Lower EZocene beds. Thus, as noted by Kashai and Croker (19871, a deep basin does not exist in the Jericho area and the transform is characterized by strike-slip motion. The Dead Sea Transform in the study area is known to be seismically active, with the last known strong earthquake in the area being the M = 6.25 Jericho event in 1927 (e.g. Ben-Menahem et al., 1976). Strong earthquakes in this area have a repeat time of several hundred years (Reches and
KOl\lI
14 t I 41
Hoexter. 1981; Rotstein, 1987) and the seismic activity in the area is presently quite low, possibly because littie strain has accumulated since the relatively recent strong earthquake (Rotstein and Arieh,
1986).
Seismic reflection data
Multichannel seismic reflection data coverage in the area is shown in Fig. 2. All lines were surveyed between 1977 and 1985, in four different exploration phases. Vibrators were used as the source in all the lines, but otherwise the different exploration phases varied in field parameters. Current practice in conventional seismic exploration for oil in onshore Israel calls for maximum source power, high multiplicity and source sweep which starts at the lowest possible frequencies. None of the lines in this area seem to have been carried out using optimal parameters, even though most yield useful data. Most of the lines were carried out using multiplicity of only 24 in contrast to the 60 which is generally presently used. The more recent lines use multiplicity of 60, but the source sweep of 16-70 Hz in these lines lacks the low end of the possible frequency range and emphasizes information from shallow depth. Maximum spread which was employed in this area was 4500 m, but in most of the lines the spread was 2650-3150 m. Since, in general, depth of investigation in seismic reflection work is estimated roughly as equivalent to the maximum spread length. the available seismic sections can be expected to yield information up to a depth of 25~-5~ m. Conventional commercial processing was ap plied to all the lines and several of the lmes were processed by more than one centre. Only one line was reprocessed as part of this work, yielding somewhat improved results, but not justifying reprocessing the entire data set. Most of the lines have migrated sections. l7hthquake
source mechanism
A countrywide earthquake monitoring system has been operated in Israel since 1982 by the Institute for Petroleum Research and Geophysics
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE JERICHO
AREA,
DEAD
SEA RIFT
Fig. 4. Focal mechanisms for individual earthquakes in the Jericho area. Solid circles denote compression and open circles denote dilatation. Stations are pro$cted on the lower hemisphere. The letters P and T refer to the compressional end the tensional axes, respectively.
TABLE 1 Fault parameters of the events that were used in this study No.
1 2 3 4 5 6 7 8 9 X0 11 12
Date
8404100342 8~80211~ 8501101407 850125~8 8501261303 8501271936 8605120653 8~7~~818 8612042238 87~230811 8805250146 8802221934
Mag.
2.1 3.1 2.5 4.7 1.6 2.0 2.4 2.1 2.0 1.0 2.1 1.3
No. obs.
P 6
18 6.5 13 36 a 11 22 25 23 9 23 13
10 14 65 35 30 20 24 0 3 25 5 24
T
Fault plane
Auxiliary plane
+
6
#
+J
8
x
Q,
8
x
315 I32 92 112 69 104 329 309 129 282 72 122
10 73 5 54 60 70 35 14 10 52 81 16
224 280 194 299 249 284 220 219 219 SO 202 220
0 35 125 25 340 15 10 355 355 55 170 170
75 60 55 80 75 65 45 80 85 30 40 85
0 80 -60 93 90 90 10 10 10 150 100 -30
9090 234 259 185 160 195 212 263 264 171 337 262
31 44 10 15 25 82 80 80 75 50 60
165 106 -125 70 90 90 134 169 174 63 81 -174
Mis.
Sta. dist.
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.11 0.09 0.0
0.77 0.67 0.64 0.75 0.82 0.76 0.86 0.76 0.72 0.68 0.63 0.80
Event 2 is a composite solution. Date is composed of year, month, day, hour, and minute. Following the notations of ski and Richards (1980, p, 105) we use dip, 8, strike (or azimuth for P and T stress axes), +, and rake, X. Event numbers are referred to in the text.
246
TWO 0 G
WAY TIME
(set) 9 N
ACTIVE
247
CO1 dPRES
u
m
a [I:
a
c:
248
TWO 8
WAY
TIME
(set)
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE
JERICHO
AREA.
DEAD
(Seismological Bulletins IPRG, 1982-1988). For earthquakes within the rift, stations do not encircle the sources, as is usually required for a reliabie determination of source location and mechanism. However, additional azimuthal coverage was achieved through the use of published catalogs of the Jordanian network with stations east of the rift (JSO Bulletins, 19841988). During that period of time, I1 earthquakes were recorded with sufficient number of P onsets for reliable fault plane solutions (Fig. 4 and Table 1). One additional earthquake mechanism (No. 2 in Fig. 4 and Table 1) is a composite solution consisting of data from three earthquakes, with similar waveforms and closely spaced in time and location. The reliability of the fault plane solutions was tested using the misfit and station distribution statistical criteria (Reasenberg and Oppenheimer, 1985). The misfit parameter describes how well the fault plane solution fits the available data, and our results (misfit < 0.11) show a very good fit (Table 1). The station distribution parameter can
249
RIFI-
be viewed as a factor of robustness which is related to the radiation pattern of a given set of phase readings. It varies from 1, in a weIl constrained case of several stations close to the nodal planes, to 0 in a poorly constrained distribution. We define robustness if station distribution parameter is greater than 0.5 and note that all solutions passed this criterion and are, most likely, reliable. Results and discussion Main fault zone
Three of the seismic lines extended eastward into the Arabian Plate and show clearly the main plate boundary fault zone (Figs. 5-7). Line SI-7102 (Fig. 8) may also cross the main fault, but cannot be used for a detailed study of its structure. In all the lines where the plate boundary fault zone is observed, it appears to be about a 1 km wide zone of intense deformation, rather than a distinct main
Sl-7102
WESTERN
-w-
SEA
BOUNDARY FAULT 200
150
SYluTECTONlC SEQUENCE \
-E-
Ikm
Fig. 8. Migrated seismic section of line SI-7102. For location see Fig. 2. MFZ denotes the main fault zone which in this section is not observed clearly. Heavy line denotes the marker used for structural mapping. Syntectonic rift-related sequence is discussed in the
250
fault
plane
with
near-surface
splays.
deformation
fault zone extends
In
these
associated
sections.
with the main
over a wider area and in two of
them (Figs. 5 and 7) it is also significantly intense tions,
than
further
the main
at depth,
In these
fault zone, below
many coherent
indicating
the presence
of relatively
disturbed
slivers.
In contrast, and coherent
surface
by a 6-10
I s (ap-
reflections,
the upper
part
the plate
phone
the plate
sideswipes,
which
lines because
are not observed
in the other
these lines are not as oblique
In all the seismic
sections
which
to it.
cross it, the
fault zone dips to the west, away from the
rift valley. Only
line DS-3047
(Fig. 7) is approxi-
mately
perpendicular
to the fault and can be ex-
pected
to show
true
the
dip.
The
present
data
that the main
fault zone is steeper
in the
m terrace (Fig. 3) which is too
southern
part of the study area as compared
to the
by a single earthquake.
boundary,
but
that
it is not
and a very short geo-
array, may be required
to study the details
of the plate boundary. In the absence of sufficiently detailed data, we chose to describe the plate boundary as a wide zone of deformation rather than try and trace individual faults. In DS-3048 (Fig. 6) the main fault zone has significantly different characteristics than evident which
lies along
indicate
higher frequencies
in the lines
which
is
apparent with the present resolution of the seismic data. A high resolution survey using short group intervals,
structure
are rare.
Thus, it is likely that a distinct master fauh does exist within the zone of deformation which characterizes
steep
boundary (see next section). This structure, the Kalia monocline, may he the source of these
on the
high to have been formed
is evident
large
main
large and un-
reflections
We note that the plate boundary
more two sec-
about
prox. 1.5 km), includes
highly deformed
the
cross
it north
and
south
of
DS-3048 (Figs. 5 and 7). Here, the fault zone is narrower. in spite of the obliqueness of the line which makes it appear even wider than it really is. In addition, continuous reflectors at all depths inside the main fault zone indicate that deformation is not as large as in the areas crossed by the other two lines. Thus, as in the case of northern Israel (Rotstein and Bartov, 1989) the characteristics of the plate boundary show rapid changes along the strike. Two other phenomena are apparent in line DS-3048 and are not observed in the two other lines which cross the plate boundary. The first is the observation of two sets of reflectors, one almost horizontal and the other dipping to the SE. The second one is the appearance of distinct reflectors in this line beneath the plate boundary zone. We consider these two phenomena to be related and suggest that the dipping reflectors. including those in the main fault zone area, are sideswipes. This line crosses at a small angle a
northern part, further indicating rapid changes in the geometry of the plate boundary along its strike. C’orrelation of seismic reflectors across strikeslip faults is often difficult. since sedimentary sections appear that
on both sides were not de-
posited at the same place. In the present study area such a correlation was attempted by Kashai and Croker (1987), using Iine DS-3047. However, since none of the lines penetrated deeply into the Arabian consider In line
Plate, or is tied to wells on both sides, we this correlation to be rather speculative. DS-3048 (Fig. 6). which includes the
clearest section from the Arabian Plate, correlation across the main fault zone is not apparent. In general, this type of correlation is important in analyzing the sense of motion and the detailed development of the faults. However, in this case, even without such a correlation, some of the characteristics of the fault can be inferred. For example, the dip away from the rift, indicates that the main
fault
zone
is not
associated
with a rift-re-
lated normal faulting component, such as the Western Boundary Fault. Instead, in the study area, the Dead Sea Transform may have the characteristics of a reversed fault. The main component of motion across the transform is of a strikeslip nature, but some reversed motion may aIso be present, as shown by Gardosh (1987), who found a small compression structure in the Lisan Formation. This is in contrast with the possible southerly extension of this segment of the transform into the deep northern Dead Sea Basin, that was suggested to have distinct normal faulting characteristics (Neev and Hall, 1979). A reversed motion component on this segment of the transform indicates that the plate boundary
ACTIVE
251
CC3MPRES
P
$i
a
lL
.Y
I
a i
9
0
-
n
252
tco14it IV
Y
is associated
with local compression.
field is currently seismological
active,
ated
with
the
main
10). Other
in
two of the earthquakes
fault
zone,
to be associ-
yield
thrusting
(events Nos. 2 and 4 in Figs. 4
earthquakes
Dead Sea yield solutions with a component
be evident
area which appear
source mechanisms and
it should
data. Indeed,
from the Jericho
If this stress
in the northernmost
which indicate
of reverse motion,
ing active compression
monocline. transform
also suggest-
in the area.
is a distinct
which can serve as independent pression
across
monocline
this part
is apparent
evidence
the
Lisan
main
fault
zone
lated
the Jericho
extent
in both directions
The Kalia
Monocline
observed area
near
(Begin
active compres-
by an earthquake area
(No.
divergent
sedimentation
the
1975a; source
2 in
sections
Fig.
10).
clearly
dis-
reflectors,
indicating
(Figs. 7-9).
Their posi-
tion next to the main fault zone suggests that these are young rift sediments associated with the tectonism which formed the underlying monocline.
its full
is not yet known. is not apparent
near-surface
features
ap-
those not The entire
however,
compressional
this
this
struc-
The
1 deep well and various
by the seismic data;
from
is sug-
of this linear
Formation in
the
fault zone. 1t is also
several of the seismic
syntectonic
shallow wells (Fig. 3a). The monocline appears to be a narrow, elongated feature that lies along the transform, with some 15 km of its length presently documented
mild
with
of local com-
interpretation
et al., 1981). As noted,
for com-
is seen in Fig. 10, which shows a time map of a Cretaceous reflector, corre-
using
This
is also indicated
play
of the transform.
the
within
structure
in all the lines which
proach the plate boundary, including crossing the transform (Figs. 5-9). structure structural
by
mechanism
Monocline
as a result
ture with the plate boundary
Finally, Kalia
created
\I
Monocline
associated
gested by the close proximity
Garfunkel
Kalia Monocline
that the Kalia
structure
in this area.
supported
strike-slip
active and
pression
sion
The
We suggest
is a young,
t I
on the
The normal faulting seismic sections and,
observed in some of the of course, known from
surface mapping, does not preclude local compression. The prominent boundary faults of the rift, with vertical throws of several hundred metres, clearly
demonstrate
that
extension
was
an
im-
portant element in the region. However, temporal tectonic changes are known from other parts of the rift (e.g. Zak and Freund, 1981; Marcus and
surface. Its existence as a pre-rift structure was suggested by Begin (1975b) while studying
Slager, 1985) and we suggest that they also characterize this area. Namely, that the extension which
palaeocurrents first described
had been important in the past is no longer active. West of the Kalia Monocline, across the West-
the region
in the Samra Formation. It was in the course of oil exploration in
using
the same seismic
reflection
data
as in this work (Hardman, 1985). This author also suggested that the Kalia Monocline is delimited to the east by a large easterly dipping normal fault, located in the area where we map the plate boundary. We find some normal faulting in the Kalia Monocline but no evidence for a major normal fault downfaulting the area east of the
Fig. 10. Interpretation Monocline north
map
and earthquake
showing
contoured
source mechanisms
of the Kalia Monocline.
Surface
two-way
ern Boundary
the Buqia
Syncline
(Fig.
3)
was mapped by Rot (1970). This feature is related to the Syrian Arc and shows typical changes in thickness structure.
of Late Cretaceous sediments We observe a similar change
across the in section
DS-730 which displays thickening of uppermost Cretaceous sediments towards the top of the suggest that monocline (Fig. 5). These relations
time (in milliseconds)
to a Cretaceous
of Fig. 4. Also shown are the axis of the Jericho
extent of the main fault zone, indicated
main fault zone at the level of the marker
Fault,
marker
Anticline
from the seismic data,
which is mapped, are shown by heavy broken the seismic marker.
lines. Other
which
details
and of another, is stippled.
the Kalia smaller
fold
The traces of the
faults are shown at the level of
ACTIVE
COMPRESSlONAL
TECTONICS
IN THE
JERICHO
AREA,
DEAD
SEA
253
RIFT
1.0 - 1.9
*
2.0- 2.9
l
3.0 - 3.9 a
&ROAN
VALLEY 0
I
I
190
200
254
prior
to its deformation
form,
the Kalia
dipping,
eastern
strike-slip
by the Dead
Monocline
was part
structures
which
faults are quite common
restraining
bends
of such faults
1981; Christie-Blick what similar
and
monocline
(1978) and termed
fault block. Along
ever, smaller
accompany and appear
further at
was described
by Stearns
(1985) a contrac-
the Dead Sea Transform structure
is the regional
have been
How-
also described
and Bartov,
A restraining bend on the Dead Sea Transform in this area can be inferred in the structural intermap (Fig. 10). Moreover,
the transform
in this area has approximately a N-E trend, rather than the almost N-S trend of the rift and of the further
north.
If, on the other hand, the
long-term, regional direction of plate motion is approximated by the trend of the rift, then the transform is oblique to this direction. This geometry has the effect of a restraining bend and will be associated with compression, as shown elsewhere along the rift (Rotstein
and Bartov,
south.
are crossed give
In addition,
by one line
different
no
indication
tectonic
displayed
the three
appear
Monocline,
some
data exist, but are insufficient for reliable mapping. The N-S line DS-3048 (Fig. 11) indicates that the Kalia Monocline dies out in this direction, being one of a series of compressional structures in the region. The line displays a second, smaller anticline centred at SP 1040 and a third anticline centred at SP 900. We interpret this last anticline to be the extension of the Jericho Anticline (Fig. lo), tested by the Jericho 1 well. The two E-W seismic lines from the area north of the Kalia Monocline both show the sedimentary sections in the eastern parts of the lines to dip towards the plate boundary (Figs. 12 and 13). The plate boundary itself is not crossed by these lines but is expected to be only in short distance further
21
III
phases.
of being
with the plate mechanisms
boundary.
folds
adjacent
to
to be similar the
result
These observations of the Kalia
cline are both the result of compression
of sug-
Mono-
associated
The earthquake
source
(Figs. 4 and 10) are either pure thrusts
or have a large thrusting component. These data support active compression in this area and are consistent with our conclusions that all the anticlines in the Jericho area are young structures not related to the Syrian Arc folding phase. The observation of an apparent compressional dome, the Zahret el-Qurein dome (Garfunkel et al.. 1981; Fig. 3), in the northernmost part of the area, is also consistent with this interpretation. The surface trace of the active fault is associated with the plate boundary in this area and is displaced eastward (Fig. 3). This geometry is in agreement with local compression in the immediate vicinity of the jump. The observation of a continuous compressional Dead Sea is more likely
regime north of the to infer an overall ob-
of the transform in this area with the of plate motion. The angle between the
two directions of the Kalia
where and
gest that the two folds north
liqueness direction
1989).
Jericho Anticline To the north
structure
and
ches, 1987; Gardosh, 1987; Rotstein 1989; Goldberg and Beyth, 1991)
transform
2). The
1985). A some-
in various places along the rift (Garfunkel et al., 1981; Bayer, 1985; Heiman and Ron, 1987; Re-
pretation
(Fig.
each other (Fig. 11) they also appear
with the bend in Lebanon.
structures
to the east
ti
them is quite similar to the observed structure m the lines that cross the young Kalia Monocline
(e.g. Garfunkel,
Biddle,
by Harding
the largest compressional uplift associated
Sea Transof the NW
flank of the Buqia Syncline.
Compressional
tional
Kol4ltlh
L
is quite small, in light of the small
magnitude of the structures relative to that north of the Sea of Galilee (Rotstein and Bartov, 1989) and, in particular, to the Mount Hermon area (Garfunkel,
1981).
Dead Sea Basin To the south of the Kalia Monocline there is also no indication that the end of the compressional regime is approached. This area is of particular interest, since it lies along the northwestern boundary of the Dead Sea Basin, in a region which is traditionally considered to undergo extension and normal faulting (e.g. Neev and Hall, 1979; Garfunkel, 1981; Ten Brink and Ben-Avraham, 1989). The Kalia Monocline itself assumes its prominence near the southernmost extent of
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE
JERICHO
AREA,
DEAD
SEA
RIFT
TWO WAY TIME (MC)
255
256
Y
the present southward
study
area (Fig.
until it is delimited
10) and may extend
the Lisan
by the rift boundary
a small amount
fault. South of it, in the Cretaceous the rift, Rot (1970) mapped ture
(Fig.
system
3a), but
a compressional
related
Cretaceous
sion along the northwestern Sea Basin servation
strata.
in relative
boundary
the
single
channel
suggest sistent
Arc thick-
even though
of recent
faulting
with significant
northwest
boundary
normal
showed
of the Dead
many
surface
expression
seismic
present
seismic
data
at some places
faulting
along
the
Neev and Hall (1979) part
seismological
of the
them as the near-
of salt diapirism and
We
Sea Basin, may
in the western
Dead Sea Basin and interpreted
with the ob-
\I
which are incon-
of the Dead
folds
I.1
is observed.
that these observations,
be due to local compression.
Active compres-
may, also, be consistent from
struc-
it to the Syrian
on the basis of changes
ness of Upper
series west of
Formation,
ROISII~IN
at depth. data
The
from
the
from this area (Neev and Hall, 1979). As noted by
region suggest that these folds may be yet another
Garfunkel
expression
steep
et al. (1981), these data
eastern
boundary
fault
show that the
of the
Dead
Sea
Basin is void of sediments, indicating a continuing activity; the presence of a 20-30 m thick sedimentary layer across the normal fault under the western part of the lake, indicates that for a few lo4 years, little normal faulting occurred on it. They also note that most of the activity on the large normal
faults west of the Dead Sea are older than
-W-
Fig. 12. Unmigrated
for the compressional
stress
seismic section of line DS-3046
showing
in
this part of the rift. The Dead Sea is commonly considered to be a pull-apart basin resulting from the stepwise jump of the main transform westward (e.g. Garfunkel, 1978). Using this model, the western master fault of the graben is inferred to have a normal faulting component. The present data suggest that the compressional field is not limited to the Jericho
DS - 3046
east. It is not clear if this is part of the Jericho Anticline,
field
a dip towards
-E-
the Dead Sea Transform
or of the smaller anticline
which is a short distance
to the south of it. For location
to the
see Fig. 2.
ACTIVE
COMPRESSIONAL
TECTONICS
IN THE
JERICHO
AREA
DS -
DEAD
SEA
25-l
RIFT
3038 .EJERICHO
ANTICI
INE
2
I.0
:!
Fig;. 13. Unseats
seismic section of line ES-3038 showing the sou~e~terly flank of the Jericho Antidine. Note the reselmblance to the sections which cross tlte Kalia Anticline. For location see Fig. 2.
area, but rather extends into the Dead Sea itself. If so, the internal structure in the basin is significantly more complex than predicted by the pullapart model. The obviation that the eastern boundary fault of the graben is active, while the western bounds is less active, suggests that the deep northern Dead Sea Basin is the product of asymmetric graben formation, much in the way found to characterize the East African grabens (Rozendahl, 1987). On the other hand, a combination of extension along the eastern side of the Dead Sea and compression along its western part, is generally consistent with block rotation being an important mechanism in the formation of pull-apart basins as suggested by Eyal et al. (1986).
tween Arabia and Africa is an appro~mately 1 km wide zone of intense deformation, rather than a distinct fault plane. The main fault zone dips to the west, away from the rift, su~esting that although most of the motion is of a strike-slip nature, a reverse-motion ~mponent is also present. Support for compression associated with reversed faulting is indicated by the presence of the young Kalia Monroe along the main fault zone. Other folds appear north of the Kalia Monocline, su~esting that the compression associated with the plate boundary extends northward from the Dead Sea for at least 25 km. The limited number of earthquake source mechanisms from the study area include thrusting, demonstrating that this compression is still active.
Conclusions
Seismic reflection data show some of the main tectonic characteristics of the Dead Sea Rift in the Jericho area. The transform plate boundary be-
The authors with to thank the Earth Science Research Administration of the Ministry of En-
25x
ergy and Infrastructure, seismic
reflection
for permission
to use the
data. We also thank
and H. Ron who reviewed
B. Katz
who processed
of line DS-3048 and I. Chelinskya
part
who drafted
the
figures.
Ilarding.
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