Ministry of National infrastructures

Energy and Water Resources

Geological Survey of Israel

The subsurface structure of the Jericho Fault and the

associated deformation: geophysical observations and

mechanical model

Amir Sagy1, Nadav Wetzler,1,2 * Yael Sagy,1,2,3 Yoav Nahmias,1 Vladimir Lyakhovsky1

1The Geological Survey of Israel

2The Department of Geophysics, Atmospheric, and Planetary Sciences, Tel Aviv University

3The Geophysical Institute of Israel

Report GSI/23/2014 Jerusalem, September 2014

Abstract

The evolution of an asymmetric basin at the edge of a pull-apart structure is studied

using geophysical observations and mechanical modeling. A new seismic analysis of

the northern edge of the Dead Sea basin indicates recent subsidence, folding and

oblique faulting along the Jericho strike-slip fault, a main segment of the Dead Sea

Transform. The sub-vertical fault trace crosses the entire sedimentary sequence

typically branching in the shallow subsurface. The seismic analysis reveals the three-

dimensional structure of the area as an asymmetrical basin bordered on the east and

on the west by long monoclinic folds. The thickness of the sedimentary fill varies

from ~ 1.4 km near the present lake shores to a few hundreds of meters about 10 km

northward. The subsidence is partitioned between a vertical component of slip along

the Jericho Fault, and the simultaneously active monoclines bordering the basin. We

present a model and simulations that successfully reconstructs the present basin

structure and demonstrates the generation of flexures along the basin margins without

horizontal shortening. We show that the basin asymmetry and the association of folds

and fault can be explained as a long-term ductile response of the sediments to local

subsidence. The asymmetry along the basin is a consequence of a later vertical

displacement component on a pre-existing strike-slip fault. Our analysis points to

ongoing northward propagation of the Dead Sea basin and provides an alternative

explanation for the structural asymmetry of the basin margins.

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1. Introduction

The development of elongated deep basins along strike-slip faults is usually

associated with segmentation or bending of the fault (Mann et al., 1983; Quennell,

1959; Sylvester, 1988). The evolution of such basins as a function of time and space

combines horizontal extension with subsidence (Cunningham and Mann, 2007;

Garfunkel, 1981), and their three-dimensional structure is rarely simple, particularly

in mature and large basins (Armijo et al., 2002; Ben Avraham and Schubert, 2006).

The basic pull-apart model (e.g., Burchfiel and Stewart, 1966; Freund, 1971) assumes

the development of a rhomb-shaped graben while the horizontal and the vertical

displacements are absorbed by the bordering faults; yet analogue models demonstrate

that a much more complicated structure evolve even after a relatively small amount of

displacement (McClay and Dooley, 1995; Rahe et al., 1998).

Here we investigate the subsurface structure of the northern edge of the Dead

Sea basin, a relatively shallow zone compared to the deeper basin (Kashai and Croker,

1987; Ten Brink et al., 1993), and whose structural relationship to the main pull-apart

structure is unclear. The central structural element in this area is the Jericho Fault

that is interpreted as the southern tip of the Jordan Valley strike-slip segment

(Quennell, 1958). The basin in this area is also associated with flexures along both of

its margins (Al-Zoubi et al., 2007; Rotstein et al., 1991). Folding of rocks is generally

a process which related to compression and shortening. Therefore the main focus of

the present paper is to explain the development of meso-scale folds within a large

transtensional pull-apart regime.

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Following is a brief review of the areal and the local tectonic setting (Sec. 2).

We then analyze seismic reflection lines to reveal the subsurface structure of the

Jericho fault and the related deformation zone (Sec. 3). The controlling mechanism

that is associated with the asymmetrical elongated folds with a respect to the Jericho

Fault and the basin evolution is analyzed in section 4 using numerical modeling and

simulations. The applications of the structural architecture interpreted and the

mechanical model suggested are then discussed (Sec. 5).

2. Tectonic Setting

2.1 The Dead Sea basin

The Dead Sea basin is located along an active continental plate boundary

transform (Fig. 1a) which diverged along the suture of the Gulf-of-Aden, Red Sea

during the Early Miocene and separates the Arabian plate from the Sinai sub-plate

(Quennell, 1958; Freund, 1965; Wilson, 1965; Garfunkel, 1981). The southern section

of this transform consists of a depressed valley with up to a few hundreds of meters of

uplifted shoulders (Garfunkel, 2014). Since Mid-Miocene this area has

accommodated about 105 km of left-lateral slip and consists of several strike-slip fault

segments arranged in en-echelon geometry with transtensional basins along

overlapping zones (Eyal et al., 1981; Freund et al., 1970; Joffe and Garfunkel, 1987).

The Dead Sea basin is an elongated deep structure whose overall length is ~

150 km, and width is 15–20 km, which developed along a left step between the Jordan

Valley Fault and the Arava Fault (Fig. 1b). The basin consists of sediment filling

more than 10 km thick (Ginzburg and Ben Avraham, 1997; Ten Brink et al., 1993),

including continental sediments and thick marine evaporites which indicate temporary

penetrations of the sea into the basin (Garfunkel, 1997; Zak, 1967). The uppermost

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exposed sequences are mostly of Late Pleistocene to Holocene lake sediments (Bartov

et al. 2002).

The internal structure of the basin has been only partly revealed by

geophysical surveys. The southern transverse margin consists of a series of normal

faults, generated sub-basins and graben and horst structures (Al-Zoubi et al., 2002;

Ben Avraham and Ten Brink, 1989; Larsen et al., 2002; Neev and Hall, 1979). The

deepest part of the basin is located between the Amazyahu and Boqeq faults (Fig. 1b),

with an evaluated relative vertical throw of more than 8 km. This deep part is narrow

along the east-west direction, is 7-10 km wide and is interpreted as a full graben

structure (Ben Avraham & Schubert 2006). It is bordered on the west by the Sedom

diapir and on the north by an uplifted zone that includes the thick salt unit of the Lisan

salt diapir (Garfunkel and Ben-Avraham, 1996; Neev and Hall, 1979). The eastern

margins of the basin are dominated by dip-slip and strike-slip faults (Fig. 1b)

separating the basin from the elevated Arabian plate (Garfunkel et al., 1981; Neev and

Hall, 1979). The western boundary consists of a belt of oblique-normal faults and

eastward tilted blocks (Sagy et al,. 2003). The vertical throw measured in boreholes

such as En-Gedi-1, En-Gedi-2, and Emunah-1 located in the western part of this belt

is as much as 2.3 km. Seismic cross sections showed that the cumulative throw along

these marginal normal faults is probably much larger (Gardosh et al. 1997). On the

other hand, field observations (Bartov and Sagy, 2004) and seismologic data

(Hofstetter et al., 2012; Hofstetter et al., 2007; Shamir et al., 2005; Wetzler et al.,

2014) suggested dominant strike-slip deformation within the basin. These

observations might reflect slip partitioning between faults, which carry the horizontal

displacement in the inner basin and those that carry the vertical components along the

margins (Lubberts and Ben Avraham, 2002; Sagy et al., 2003).

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North of the Lisan diapir the basin is shallower (Choi et al., 2011; Ten Brink

et al., 1993) with the basement depth ranging from ~8 km near the Lisan diapir to ~ 6

km close to the present north shore of the lake (Ginzburg and Ben Avraham, 1997).

However, the stratigraphic information there is limited because only very few

borehole data exist (Kashai and Croker., 1987). A deeper sub-basin is suggested to be

located under the Arnon bathymetric depression (Neev and Hall, 1979), but its

structural expression in depth is unclear. It was also suggested that additional shallow

evaporite bodies exist in this area (Ginzburg and Ben Avraham, 1997) and that

transverse faults might be located near the present northern border of the lake and

even further north (Lazar et al. 2006). The northern part of the Dead Sea basin is a

seismically active zone where the two largest areal earthquakes occurred in the last

100 years, the 1927 6.25 ML magnitude earthquake (Shapira et al., 1993) and the 2004

5.1 magnitude earthquake (Hofstetter, 2008). The instrumental seismic catalog

suggests that the area south to the northern border of the lake is one of the most active

parts in the basin (Wetzler et al., 2014), but at the same time the Jericho valley (e.g.,

the ~15 km zone north of the lake) is seismically silent. This observation together

with the historical seismologic record might indicate that at present the Jericho fault is

seismically locked (Wetzler et al., 2014).

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Figure 1. Areal and local location maps. a) The Dead Sea Transform and the areal

tectonic plate configuration. b) Main tectonic elements in the Dead Sea basin: NB –

north basin; SB – south basin; JF-Jericho Fault; AF - Arava Fault; EBF - eastern

bordering fault; WMF- western margin faults; AMF – Amaziahu Fault; BF Boqeq

Fault; LD - Lisan diapir; SD – Sedom Diapir. c) A map of the Jericho valley (squered

by dotted lines in b) with Locations of fault traces and seismic lines that have been

used for the present work.

c

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2.2 The Jericho valley

The interior part of the Jordan River valley north to the Dead Sea is covered

by Pleistocene and Holocene lacustrine and fluvial sediments which are the

uppermost part of the sedimentary fill (Dead Sea Group). The margins mostly consist

of pre-basin Upper Cretaceous to Eocene sedimentary rocks. The present analysis

focuses on the area that is limited by the lake in the south, the town of Jericho in the

north, the upper Mesozoic outcrops in west and the Jordan River in the east (Fig. 1c).

The recent sediments exposed further eastward are bordered by the Dead Sea eastern

fault which turns northeastward in the southeast margin of the valley (Bender 1968),

while north of it Mesozoic rocks dip westward (Al-Zoubi et al., 2006).

The main structural features exposed west of the Jordan River are the marginal

normal faults of the Dead Sea basin (Fig. 1c), with a vertical displacement that

decreases northward from a few hundreds of meters in the northern part of the lake to

~100 m in the Jericho area and the Jericho fault (Fig. 1c). The fault, however, is only

partly exposed (Begin, 1973) while most of it is covered by recent sediments as a

result of a few Holocene transgressive episodes of the Dead Sea (Bartov et al., 2002;

Ken-Tor et al., 2001). The fault’s near-surface deformed zone was located in a few

paleoseismological trenches (Gardosh et al. 1990; Lazar et al. 2010; Nahmias & Sagy

2013; Reches & Hoexter 1981) and in a field outcrop which was exposed recently on

the shore of the retreated Dead Sea (Nahmias & Sagy 2013). A few large historical

and prehistorical events which ruptured the surface were identified and dated in these

locations (Lazar et al., 2010; Reches and Hoexter, 1981). At least two of them

occurred within the last 2000 years (Reches and Hoexter, 1981). The field

observations also suggest that the displacement along the fault vary locally, as

indicated by sub-horizontal striations on steep fault surfaces, local flexures, joints and

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normal fault branches (Gardosh et al., 1990; Nahmias and Sagy., 2013; Reches and

Hoexter, 1981). In the south of the lake the fault is expressed as a submarine cliff

(Lazar and Ben Avraham, 2002; Neev and Hall, 1979), while north of the Jericho

valley, the fault trace crosses the Jordan River and is revealed as a scarp in the Late-

Pleistocene-Holocene sediments and by segmented pressure ridges (Ferry et al., 2007).

The subsurface of the Jericho valley, or Jericho–Shuna basin, (Al-Zoubi et al.,

2007) was extensively studied by gravity (Ten Brink et al., 1993), seismology (Inbal

2010; Rotstein et al. 1991; Shamir et al. 2005) and seismic surveys (Al-Tarazi et al.,

2006; Al-Zoubi et al., 2007; Kashai and Croker, 1987; Lazar et al., 2006; Rotstein et

al., 1991; Shamir et al., 2005). Rotstein et al., (1991) analyzed a dozen reflection lines

across and along the valley and interpreted the Jericho fault as a plate boundary

deformation zone, associated with a compressional fold from the west (e.g., Kalia

fold). Shamir et al., (2005) added more deep seismic lines and suggested that the

Jericho fault was not active as a plate boundary since the Pliocene. Several northeast

oriented faults interpreted to be present in the western part of the valley (Shamir et al.,

2005). Al-Zoubi et al., (2006, 2007), on the other hand, analyzed seismic lines east of

the Jordan River and concluded that there are no evidence for a transform fault zone

located east to the Jordan River in the Jericho valley. They also revealed the existence

of an elongated young fold, the Al-Kharrar monocline, along the eastern part of the

valley oriented sub-parallel to the Jericho fault. Seismic lines were correlated using

deep borehole data and the structure was interpreted as an asymmetric basin crossed

by the Jericho fault with monoclinic folds on both sides of the basin (Al-Zoubi et al.,

2007). Below we further investigate this asymmetric structure and its relationship to

the transform kinematics and the pull-apart evolution.

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3. The subsurface structure of the Jericho Valley

3.1 The Jericho fault

We analyzed seismic reflection lines to disclose the structure of the Jericho

fault shallow subsurface and its related deformation zone. A dense grid of seismic

reflection profiles has been collected since 1980 in the Jordan Valley. The surveys

were collected for oil exploration and for various research projects. Therefore,

acquisition parameters and source energy in particular differ in each survey.

Interpretation of 18 seismic reflection lines was carried out (Fig. 1c). Five new high-

resolution seismic lines recorded with a sample rate of 0.5 msec (see App. A) were

acquired during 2008 - 2013 and in addition four seismic lines were reprocessed, two

of them of high resolution and the other two of semi-high resolution (recorded with a

sample rate of 2 msec). The results from the processing included post or pre-stack

time migrated sections and their corresponding (RMS) velocity models (Medvedev

and Ladell, 2011; Rochlin, 2013; Sagy, 2008). Acquisition parameters and processing

details of the newly acquired and reprocessed seismic lines that cross the fault are

summarized in Appendix A.

The upper part of the Jericho fault was identified on ten seismic reflection

lines that cross it, with a perfect match to the fault trace located in outcrops (Begin,

1973; Reches and Hoexter, 1981). Here we focus on the interpretation of the shallow

structure in three locations along the fault trace. The first is about 900 m north of the

lake border (Fig. 1c) where the fault is not exposed by an outcrop. Two partly

congruent lines cross the Jericho fault at this location: the GP-5009 semi-high

resolution line (Shamir et al., 2005) and the SV-130 high-resolution line which

focuses on the upper ~0.5 seconds (Fig. 1c). The two lines were reprocessed at the

Geophysical Institute of Israel for the present research (Rochlin, 2013).

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Line GP-5009 is 8 km long and crosses the Jericho valley in an E-W direction

(Fig. 1c). Figure 2 presents the Jericho fault in this location, down to 1.6 seconds

(about 1.8 km depth), with a large vertical exaggeration. The fault zone is clearly

identified as a sharp sub-vertical discontinuity between packages of reflectors. A

monoclinic folding is observed in the western part of the image with reflectors that

dip sharply towards the fault. An eastward thickening of reflectors is observed above

the black line in Figure 2 and indicates syn-tectonic deposition and simultaneous

activity of the faulting and the fold. At a depth greater than about one second, the fault

appears as a narrow zone with incoherent reflectors, while upward, the fault zone

consists of disconnected blocks which are bordered by branches of the fault (Figs. 2-

3). The correlation between the two sides of the fault below ~ 0.5 second is not trivial,

possibly because a significant amount of lateral motion occurred along the fault. East

of the fault the reflectors become horizontal while the additional variations of the dip

observed at the eastern most tip of the line might reflect change in the line orientation

(Fig. 1c).

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Figure 2. Semi-High resolution time migrated section of line GI-5009 (upper) and the

interpreted section (bottom). The Jericho Fault (JF) appear in the middle of the cross

section branched in its upper part. A monocline is observed west to the fault dipping

toward it with a normal fault in the hinge. Reflectors thicken and display on-lap

pattern above the horizon (green) which is interpreted as the base of the Dead Sea

Group. Variations of dip are observed in the eastern tip of the line and might reflect

the change in the line direction from E-W toward SE (Fig. 1). See location in Fig. 1.

Velocity model appear in appendix A.

Line SV-130 (Fig. 3) focuses on the uppermost part of the fault. It displays a

depressed zone ~ 250 m wide, separated by at least two branches of the fault. The

eastern block is downthrown and the correlation of reflectors from both sides of the

fault indicates ongoing thickening and subsidence of the eastern block.

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Figure 3. High resolution time migrated section of line SV-130 (upper) and the

interpreted section (bottom). The figure reveals the shallow subsurface structure of

Jericho Fault in the exact same location of Figure 2. Close to the upper surface the

branched fault generates a depressed zone. The correlation (marked by the blue

horizons) suggests that the sedimentary sequence is thickening eastward.

The shallow structure of the Jericho fault is presented in two additional

locations (Fig. 4a-b). The fault zone in these lines appears as a non-reflective zone

separating two packages of coherent reflectors. In both lines presented in figure 4, the

eastern sequence is thicker and downthrown with respect to the western one. The

seismic velocity (as obtained from the processing) of the non-reflective zone is similar

to that of its surroundings. Therefore the non-reflectivity interpreted as fragmentation

and incoherency of the sediments in the fault zone rather than lithological variation.

14

The subsurface of an intense fault zone is displayed in Figure 4b. A trench that

crosses the fault at the same location displays hundreds of fault branches, tilted blocks

and joints that are exposed along a 200 m wide zone (Fig. 5). The overall vertical

separation along this fault zone is 8 m since ~ 15 ky, which yields a minimal vertical

throw of ~0.5 mm/year (Nahmias and Sagy, 2013). Other high resolution lines

demonstrate that the fault is associated with small-scale sedimentary swelling or with

normal faults which indicate local compressional and extensional zones along it.

Figure 4. The shallow subsurface structure of Jericho Fault (JF) in two high resolution

time migrated sections: (a) line GP-320_11 and (b) line GP-319_11. The

interpretations of the two lines are in the bottom pictures. The blue horizons are

thickening eastward. The green horizon is interpreted as the base of the Dead Sea

Group based on a deeper seismic reflection line (GP-0320_11) in this area. See

locations in Figure 1.

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In summary, the shallow part of the Jericho fault was identified in our seismic

lines as a sub-vertical discontinuity between packages of reflectors. In all lines

examined, the upper part of the fault is defined by a steep 100-300 m wide

deformation zone. The eastern block is downthrown relative to the western block and

bundles of reflectors bend and thicken westward (Figs. 2-4). The field observations

above line GP-319_11 display tilted blocks and fault branches spread along a belt of ~

150 m across the fault (Nahmias and Sagy, 2013). Our observations therefore support

the assumption that the Jericho fault is a coherent active fault with a vertical

component of displacement. The vertical trace of the fault zone (Fig. 2) and the

asymmetry between reflectors from both sides of it support the geological and

tectonic evidence for significant lateral motion along the Jericho fault (Garfunkel

1981; Joffe & Garfunkel 1987; Reches & Hoexter 1981).

Figure 5. Fault branch and tilted block in Late-Pleistocene lacustrine sediments (The

upper member of Lisan Fm.). This segment is one of dozens of fault segments that are

exposed along ~ 150 m belt in a trench that crosses the Jericho Fault exactly above the

location of the fault in Line GP-319_11 (see Fig. 1 for location and Fig. 4b for

interpretation).

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3.2 The three-dimensional structure of the Jericho valley

Correlation of seismic reflectors across the transform fault is very speculative,

mostly because of the large lateral displacement and the absence of deep boreholes

that sample the Dead Sea Group west to the Jordan River (Rotstein et al., 1991). The

only deep borehole that exists west to the Jordan River is the Jericho 1, which directly

penetrated Cretaceous rocks. Nevertheless, we used the dense seismic data in the

Jericho valley for identifying the base of the Dead Sea Group and the top of the

Cretaceous Judea Group which is a prominent reflector in the areal subsurface

(Fleischer, 2003). Our interpretation of these markers, west of the Jericho fault,

primarily relies on outcrops of Upper Cretaceous to Paleogene rocks, which are

crossed by the western tip of some seismic lines (Fig. 1c). We also use data from

water wells drilled in the valley. Most of them are shallow and south of the city of

Jericho; they do not cross the base of the Dead Sea Group. A 380 m deep water well

drilled in the city of Jericho (Golani 1972) crossed 60 m of late Pleistocene sediments

and more than 300 m of Upper Mesozoic rocks (Mount Scopus Group). The seismic

lines in this area show that this borehole penetrated a small graben and therefore the

large thickness of the Mount Scopus Group measured indicates that this graben was

active during the Late Mesozoic.

The correlation of the base of the Dead Sea Group from both sides of the fault

was based on identification of unconformities and on thickness variations. For

example, a prominent unconformity appears in Figure. 2 (blue line), west of the

Jericho fault, and the layers above it thicken toward the fault, suggesting syn-tectonic

deposition. In other lines a sequence of thickening layers can be identified and

correlated from both sides of the fault zone (Fig. 3). Below these sequences the layers

cannot be correlated and thus at least a minimal thickness for the Dead Sea Group can

17

be assumed. For example, thickness changes are identified in Figure 4 above the

green horizon, which is interpreted as the base of the Dead Sea Group. We also used

Al Zoubi et al., (2007) correlation for identifying units east to the fault.

Figure 6 shows our interpretation for the two horizons, base of the Dead Sea

Group and top Cretaceous Judea Group along three east-west cross-sections

(composite section). The association of the Jericho fault with a monoclinic fold is

observed in all three cases. Our interpretation suggests that both the vertical throw

along the fault and the amplitude of the fold decrease northward. Normal faults were

also identified in the monoclinic fold hinges (see also (Rotstein et al. 1991)), yet their

vertical displacement south of Jericho city is limited. The strata in the area between

the Jericho fault and the Jordan River is relatively undisturbed and we did not locate

any other fault. The basin fill thickness, based on the velocity models (Rochlin 2013)

varies from about 1.3 km near the present Dead Sea shores to ~ 200 m in the Allenby

bridge area (the northern part of the study area). However the thickness of the basin in

the Allenby bridge area might be slightly larger eastward (Al-Zoubi et al., 2007).

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Figure 6. Interpretations of the Dead Sea Group base (green horizon) and the top of

the Judea Group (blue horizon) in three different E-W cross sections along the Jericho

valley (see Fig. 1c for locations): a) The eastern section of line DS-3047 in the north

of the area. b) Combinations of lines SI-7102 and GP-5038_02 in the central part of

the area. c) Line GP-5009 in the southern part of the area.

The structure of the research area is revealed in the structural map (in two way

travel time) of the base of the Dead Sea Group (Fig. 7) which shows that the basin

becomes shallower northward. Our interpretation also suggests that the monoclinic

fold is continuous along the area while the fold amplitude decreases northward. The

present results together with the data presented for the eastern side of the valley (Al-

Zoubi et al. 2007) show an asymmetrical elongated basin, limited in the east by a

monocline and in the west by a monocline and a vertical fault (see Fig. A2 in

Appendix 1).

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Figure 7. Structural map in two ways travel time of the base of the Dead Sea Group in

the Jericho valley. The contours and the colors mark the depth in seconds. Datum

level is in -400 m below MSL.

Monoclinic folds are usually associated with reverse faulting and compression

(Reches, 1978; Rotstein et al., 1991) or, less commonly, with normal faulting and

extension (Withjack and Callaway, 2000). However, the three-dimensional picture of

the research area (Fig. 7) implies that the monoclinic folding from both sides of the

basin (Fig. B1) might primarily be an effect of its subsidence. This possibility is tested

using mechanical modeling of the basin evolution.

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4. Mechanical model for the evolution of asymmetric folding in a

subsided basin

4.1 Model setup

Folding of a layered volume usually involves ductile deformation with

compressional buckling and shortening that forms billow-like deformations in parallel

layers (Johnson, 1977; Van Hise, 1896). General analytical solutions for folding of

viscoelastic rocks were developed by Biot (1957) and Ramberg (1959). The numerical

approach for viscous flow of rocks was introduced later (Dieterich, 1970; Parrish,

1973), including simulations of sedimentary basin evolution using models adopting

ductile rheology, e.g., (Cloetingh and Burov, 2011).

In this study, the assumption that the monoclinic folding from both sides of the

basin (Fig. B1) might primarily be an effect of its floor subsidence and that the

observed asymmetry is related to the faulting along the Jericho fault is tested. We

model a long-term ductile response of the ~5 km layered sedimentary sequence to a

subsided zone using a two-dimensional model of highly viscous Newtonian flow (Fig.

8). The model reproduces the flow dynamics and the geometrical response of the

rocks to the basin floor subsidence. The model also includes the horizontal transport

of the material that is controlled by the pressure gradient and the effective viscosity.

Based on the field and the subsurface observations described in Sec. 3, we assume

instantaneous infill of the subsided regions with lower density sediments (the blue

zone in Fig. 9).

The geometry and the boundary conditions of the model are illustrated in

Figure 8, which simplify an east-west cross-section prior to subsidence, with 5 km

thick pre-Miocene sediments (Ginzburg and Ben Avraham, 1997) above a rigid

basement (model base). With the onset of subsidence, the sediments are pulled down

21

at a constant rate from the central part of the bottom boundary (Fig. 8). This motion is

transferred through the whole model, leading to the formation of a negative

topography that is filled by lower density sediments.

Figure 8. The model is a rectangle 40 by 5 km, east-to-west cross-section of the north

Dead Sea basin. The boundary conditions of the model are set as follows: floor model

is set to wall (indicates the transition to the basement), except of a discrete region of 4

km that is set to velocity out (0.8 mm/y), sides of the model are set to wall with a free-

slip condition (indicated by the rollers), the top of the model is set to open channel

with an instantaneous soft sediments infill, and a low viscosity zone (shown by the

dashed lines) that is applied with the 2nd case of the simulations. Arrows at the top of

the model indicated the strike-slip motion along the JF.

The modeling was done using the FLUENT commercial software

(http://www.ansys.com) that numerically solves the Navier-Stokes equations by the

finite volume method. The 40 km wide and 5 km deep 2-D model is represented by

23,100 triangular elements of variable size. The nodal distance increases from 100 m

at the top of the model to 250 m at its bottom. The density used for the basin

sedimentary infill is 2,300 kg/m3, and the density used for the pre-basin sediments is

set to 2,600 kg/m3 (Ten Brink and Flores, 2012). The “open-channel” boundary

22

condition is applied at the top of the model, reproducing instantaneous sedimentation

and preserving a flat topography throughout the model run. Left and right side

boundaries with free-slip conditions are placed far enough away to reduce their

impact on the internal part of the model. A fixed condition is applied along the bottom

boundary, except for a 4 km width in the central part representing the subsiding part

of the basement. A constant vertical velocity of 0.8 mm/y is applied to this part of the

basement.

We ran the model for two different cases. In the first case, a uniform viscosity

of 5X1021

Pas is applied to the entire model. In the second case, the narrow vertical

zone with reduced viscosity up to 1X1021

Pas represents material weakening along the

vertical extension of the Jericho fault (Fig. 8). The simulated flow pattern and basin

formation in the first case is expected to be symmetric relatively to the centerline of

the basin. This case enables doing a feasibility test of the main model hypothesis and

settings (Fig. 8). The second case tests the possibility that the observed asymmetry of

the basin (Fig. A2) is broken due to the existence of a weak zone (the Jericho fault),

which is modeled here by a vertical low viscosity zone (Fig. 9).

4.2 Simulation results

Case #1: The simulated density distribution and flow pattern after 3 My is

shown in Figure 9. The subsided area is filled with young sediments (blue in Fig. 9)

reaching its maximum depth (~1.4 km) at the center of the basin, and extends up to ~6

km away from the centerline. Since the material properties are homogeneous, and

boundary conditions are symmetric with respect to the centerline, the simulation

predicts a symmetrical basin geometry and flow pattern (Fig. 9). The flow pattern

shown emphasizes that motion is not limited to vertical direction (piston-like motion),

23

but also includes a horizontal component responsible for transport of sediments

toward the central part of the basin. The set of green lines in Figure 9 represents the

accumulated vertical displacement of the pre-basin sediments along several horizons

at depths from 1 to 4 km. The shape of these lines manifests the deviation from the

piston-like flow and the decrease in the vertical velocity values at the upper part of the

model associated with the horizontal transport. The vertical velocity component along

the centerline of the model decreases from 0.8 mm/y at the bottom to 0.5 mm/y at the

top. Accordingly, the sediments at the bottom of the model are pulled down by 2.5 km,

and the maximal thickness of the young sediments is about half of the total

displacement. The flow pattern presented (Fig. 9) is almost identical for the different

viscosity values within the range of 1020

to 1025

Pas. Decreasing the viscosity of the

sediments to 5X1019

Pas decreases the subsidence rate at the top of the model to 0.26

mm/y and increases the width of the basin (blue area in Fig. 9). This reflects the

domination of the velocity boundary condition along the bottom boundary associated

with the prescribed subsidence.

Figure 9. Density profile of model-Case#1 at simulation time: 3 My. The blue

material (2300 km/m3) is associated with the basin young sediment infill, and the red

material (2600 km/m3) associates with the pre-basin sediment layers. Profiles of the

accumulated vertical displacement along horizons at depths of: 1, 2, 3, and 4 km are

plotted by in green lines. Arrows point to the flow direction and scaled by magnitude.

The accumulated subsidence at the bottom-center of the basin is illustrated by the 2.5

km heighted extended box. We note a symmetrical geometry of the basin with a

respect to the centerline.

24

The interface between pre-basin (red area) and young sediments (blue area) in

Figure 9 highlights the basin geometry and the bending of its boundaries. The

obtained geometry mimics the dimensions and the shape of the basin as described by

Al-Zoubi et al. (2007). Similarly to their interpretation, the young sediments mostly

accumulated within the range of about 5 km from the centerline, and vanish at about

10 km away (Fig. 9). However, the seismic lines at the western side of the basin show

a shorter deformation scale, dislocated at the intersection with the Jericho fault, and

positioned almost vertically, about 2 km west of the centerline. The effect of the weak

zone associated with the Jericho fault is discussed in the next section.

Case #2: The low viscosity zone is added to the model in this case (Fig. 10) to

account for the vertical weak zone ~500 m wide associated with the Jericho fault. The

simulated symmetrical flow pattern discussed above is broken in the model by the low

viscosity zone. Throughout all simulation times the model predicts an asymmetric

basin shape. Figure 10 presents three snapshots with the basin geometry obtained for

simulation times 0.9, 2.3, and 3.4 My. The basin shape in the eastern part of the model

is similar to that of the previous case with only a minor effect of the low viscosity

zone, whereas the basin shape in the west is strongly affected by this zone. The depth

of the basin is abruptly changed across the weak zone and extends westward to a

significantly shorter distance in comparison to the eastern part. The extent of the basin

along the surface is marked by vertical arrows in each snapshot (Fig. 10). The basin

extends with time to ~7 km away from the center on the east, but only ~5 km on the

west (Fig. 10c). The bottom of the basin (interface between red and blue zones in Fig.

10) is flat within a ~2 km interval between the low viscosity zone and the center of

the model. The shallow basin on the west and the flat bottom shape on the east of the

Jericho fault fit the geometry of the interpreted reflector of the base of the Dead Sea

25

Group. The interpreted horizon, taken from seismic lines DS-3047, GP-5038, and GI-

5009, are shown in snapshots (a), (b) and (c) of Figure 10, respectively. Since all the

seismic data are interpreted in the time domain, the time-to-depth conversion for this

comparison is done using Vp=2000 m/s (Fig. A1).

Figure 10. Density profile of model-Case #2 at simulation times: a) 0.9 My, b) 2.3

My and c) 3.4 My, along interpreted seismic horizons of three seismic lines: a) DS-

3047, b) GP-5038 and c) GI-5009. The blue material (2300 km/m3) is associated with

the basin young sediment infill, and the red material (2600 km/m3) associates with the

pre-basin sediment layers. The horizon of the Dead Sea group base is shown by the

green line. The extent of the basin along the surface is marked by vertical arrows in

each snapshot. The low viscosity zone is shown by the vertical grey line; associated

with the damage zone of the Jericho fault.

The sub-parallel E-W seismic lines DS-3047, GP-5038, and GI-5009 (Fig. 6)

reflect a variation of the basin structure in the N-S direction. The deeper basin (Fig.

10c) was obtained by the longest simulation time (3.4 My) and matches the southern

seismic line (GI-5009 Fig. 2, Fig. 6c), and the shallow basin structure (Fig. 10a) with

a simulation time of 0.9 My matches the northern seismic line (DS-3047 in Fig. 6a).

Assuming a constant subsidence rate and agreement between depths of the seismic

interfaces and the simulated basin depth for different ages, we suggest a scenario in

which the basin expands northward.

26

4.3 Model applications and limitations

The model successfully reconstructs the evolution of the Jericho basin with

respect to the internal structure (Fig. 10). It also points to the expansion of the basin

northward. The adopted fluid flow model enables reproducing large-scale and long-

term evolving structures in the frame of a ductile flow approach. This approach is not

supposed to reproduce any specific local architecture nor strong localized deformation

associated with faulting. Additionally, the suggested approach disregards several

physical properties that probably affect the basin formation, among them, the

formation of a topographic relief across the basin and the compaction of young

sediments. The rate of sedimentation and erosion should be taken into consideration

in a more detailed modeling. The compaction of young sediments and depth-

dependent density increase could affect the flow pattern, though for the size, depth

and resolution considered here, this feature could be neglected. Other properties of

the viscosity field, such as non-linear behavior, viscoelasticity, or power-law rheology

of the sediments are not included here. We also use a single viscosity value for both

pre-basin (red) and young sediment (blue) materials in Figure 10. But, as previously

discussed, increasing the viscosity value by several orders of magnitude shows no

significant influence on the flow pattern.

The evolution of the basin in the Jericho valley is successfully simulated by a

viscous-flow model that accounts for the material weakening around the Jericho fault.

The model demonstrates that a long-term flow mechanism driven by subsidence of the

basin can generate flexures without horizontal shortening (Fig. 10) and the breaking

of the basin symmetry could be generated by a preexisting weak zone.

27

5. Discussion

One of the main questions arising from this work regards to the architecture of

the northern border of the Dead Sea basin, or specifically: Is the research area an

internal part of the pull-apart structure and how is the main basin bordered from the

north? Our observations are consistent with previous suggestions that the main plate

boundary fault north of the present lake shore is in the western part of the valley (e.g.,

the Jericho fault), and that no large eastern strike-slip segment crosses the

Phanerozoic sediments (Al-Zoubi et al. 2007). At the same time, the seismic lines

demonstrate that the area is an active basin with sediment accommodation from both

sides of the Jericho fault. Such a pattern might be developed in a half graben

environment (Ben Avraham et al. 1990), but east of the Jericho fault the layers

become sub-horizontal and have no significant lateral thickness variations as typically

observed in half graben structures. Accordingly, the reconstruction of the observed

geometry required a horizontal subsidence of a narrow zone east of the Jericho Fault,

as shown in our model. This subsidence might point to an existence of an additional

buried normal fault in the basement, along the eastern side of the basin, underlying the

bended sediments. Yet, the present data (Al-Zoubi et al., 2007) is limited to the

sedimentary cover and further investigations of the basement are required for better

interpretation of the crustal structure of this area. Nevertheless, our model suggests

that the Jericho valley might reflect an ongoing northward development of the main

Dead Sea basin. If the present subsidence rate measured in the northern part of the

area is representative of the entire area and of a longer term, the basin age near the

Dead Sea lake shore is a few million years old, whereas near the Allenby Bridge in

the northern part of the area it is less than a million years old. Alternatively, if the

subsidence rates in the southern part of the research area are (or were) higher than

28

these in the northern part, then the basin depth represents a north-south bending of the

basin flour (Ten Brink et al., 1993).

In addition, the model presented provides a direct prediction that subsidence is

associated with basinward transport and bending of sediments. This prediction can

now be tested using observations along the main Dead Sea basin. Cross-sections in

the central Dead Sea basin demonstrated significant structural differences between the

two longitudinal margins of the basin (Garfunkel, 1997). The western margin is

covered by a thick sedimentary sequence and is structurally controlled by basinward

dipping blocks, flexures and normal faults (Gardosh et al., 1997; Sagy et al., 2003).

The sedimentary cover in the eastern margins is thin or absent, and a sharper

separation between the basin and its margins is revealed (Garfunkel 1997).

Generalization of our model results leads us to suggest that the basinward bending of

sediments in the western margin of the basin reflects the long term response of the

sediments to the basin subsidence, similarly to the picture described for the Jericho

valley. The large vertical separation in the main basin, driven by the marginal faults,

eventually leads to brittle deformation in the sediments above (Sagy et al., 2003). In

contrast, basinward bending is rare in the eastern margins where the upper rock

sequence consists mostly of less-viscose basement rocks.

29

6. Conclusion

In this study we analyzed the subsurface structure of the Jericho valley as an

example of an edge zone of a large active pull-apart basin. We developed a new

model to explain the observed structural architecture and discussed its implications.

We found that:

1. The Jericho fault is an active fault along the entire study area combining

horizontal and vertical displacement components. The uppermost part of the Jericho

fault includes a zone with fault branches and deformed sediments about 100-200 m

thick (Figs. 2-4). Both compressional and extensional deformed zones were identified

in the high resolution seismic lines. The fault is sub-vertical in the upper 2-3 km (Fig.

2) and the eastern block is consistently downthrown with a vertical separation of 0.5

second (~ 500-800 m) close to the present lake shore and about 0.1 second (evaluated

to a few dozen meters) near the north end of the research area (Fig. 6).

2. The subsidence in the Jericho valley involves bending of the sedimentary

sequence in the eastern flank of the basin and both bending and faulting in the western

flank. This structure suggests a moderate northern closure of the basin compared to a

simple rhomb-shaped graben model.

3. The numerical simulations demonstrate that a long-term flow of sediments

driven by subsidence of the basement can generate symmetrical flexures without

horizontal shortening. Shown also is that the symmetrical geometry of the evolving

basin is broken by introducing a localized lower viscosity zone to the model. This

zone is associated with the strike-slip faulting preceding the vertical motion.

Comparison between the simulated basin evolution with time and the seismic lines

across different parts of the area supports ongoing propagation of the Dead Sea basin

northward.

30

4. The basinward bending of the sedimentary sequence in the western margins

of the Dead Sea Transform (Gardosh et al., 1997; Sagy et al., 2003) and the structural

asymmetry between the western and the eastern margins (Garfunkel, 1997) are now

understood by the differences in the long term rheological response to the basin

subsidence. This large-scale application of our model suggests that folding by viscous

flow should be considered as an important mechanism of deformation of sedimentary

basins, even without horizontal shortening.

Acknowledgments

We thank Uri Frieslander, Eldad Levi, Benny Medvedev and Itay Rochlin for

their help with planning and processing of surveys. We thank the Israel Ministry of

Energy and Water Resources and the U.S. – Israel Binational Science Foundation for

allowing us to reprocess and interpret their data.

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Appendix A: Seismic data

The project consist seismic data that was collected in the Jericho valley. Data

Processing was performed using the Paradigm Geophysical software and included

mainly: amplitude compensation, filtering (band pass and deconvolution), several

iterations of velocity analysis, residual statics and post or pre-stack time migration.

Elevation statics were applied to all lines with final datum set to -400 m below MSL

(mean sea level).

We used data from 18 seismic reflection lines. Specification of the new

acquired reflection seismic lines and the reprocessed lines are presented in table A1.

35

Table A1: Specification of the new acquired (NA) and the reprocessed (RP)

reflection seismic lines.

More details are presented in three technical reports which have been

published as part of this project (Medvedev & Ladell 2011; Rochlin 2013; Sagy

2008). Example for one velocity model is presented in Fig. A1.

Figure A1. Stacking velocity for Line GP-5009 (Rochlin 2013), for the present

project).

Our model is also based on interpretation of reflection lines east to the Jordan

River that have been published in previous work (El Zoubi et al., 2007). This work

present the asymmetry of the basin in this area (Fig. A1).

36

Figure A2. Interpretation of combined W-E seismic lines across the intermediate part

of the Jericho–Shuna basin (from (Al-Zoubi et al. 2007)). The two marked horizons

demonstrate the basin asymmetry and the appearance of two flexures, east and west to

the Jericho Fault.

תקציר

סימטרי -בעבודה זו נחקרו מבנה העתק יריחו, מקטעו הדרומי של העתק עמק הירדן, והתפתחות אגן א

הקרקע בבקעת -לאורכו בעזרת ניתוח תצפיות גיאופיסיות ופיתוח מודל מכאני. האנליזה הסייסמית של תת

והה המכוונת חתכי רפלקציה שונים, חלקם בהפרדה גב 11 -מבוססת על מידע מ ,מצפון לים המלחיריחו,

מערב את מיקומו המשוער -להדמיה צפופה של החתך הרדוד. מספר רב מהחתכים חוצים בכיוון כללי מזרח

של העתק יריחו ומאפשרים מעקב אחר המעוות המקומי והמבנים השונים לאורכו. ניתוח המידע הסייסמי

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רציפות -פני השטח, מאזור החוף הצפוני של ים המלח ועד למעבר הגבול אלנבי. ההעתק מזוהה כאזור של אי

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ל החתכים באזור המחקר מזוהה הבלוק מצדו המעידים על אזורי מתיחה ולחיצה מקומיים לאורכו. בכ

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הקרקע של בקעת יריחו מפוענח כאן כהמשכו הצפוני של אגן ים המלח, אולם בניגוד לאגן העיקרי -תת

ים על כך שבאזור המחקר מפותח אגן לאורך העתק אחד בלבד. עובי הקרקע מצביע-שמדרום, נתוני תת

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הוא תוצר התקצרות ולחיצה, ולכן הופעת קמטים גדולים יחסית באזור מקובל לחשוב שקימוט סלעים

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