Neotectonic stress analysis of the Red Sea rift by Finite ...ir.lib.u-ryukyu.ac.jp/bitstream/20.500.12000/448/1/bull...Sunil Kumar Dwivedi and Daigoro Hayashi weakness in the lithosphere,

Title Neotectonic stress analysis of the Red Sea rift by FiniteElement Modeling

Author(s) Dwivedi, Sunil Kumer; Hayashi, Daigoro

Citation 琉球大学理学部紀要 = Bulletin of the College of Science.University of the Ryukyus(83): 3-28

Issue Date 2007-03

URL http://hdl.handle.net/20.500.12000/448

Rights

Bull. Fac. Sci., Univ. Ryukyus, No.83 : 3 - 28 (2007)

Neotectonic stress analysis of the Red Sea rift

by Finite Element Modeling

Sunil Kumar Dwivedi and Daigoro Hayashi

Simulation Tectonics Laboratory, Faculty of Science,

University of the Ryukyus, Okinawa, 903-0213, Japan.

Abstract

The Red Sea is a tectonic rift that was formed in the late Oligocene-early Miocene when

the originally connected African and Arabian land masses broke apart. At first it was a

continental rift, then, as Arabia drifted away, developed into an intercontinental system

that today separates the independent Arabian plate from the African plate. The Red Sea rift

is part of an extensive global system of faults running approximately north to south. In the

present study, numerical modeling on the Saudi Arabian seismic reflection profiles is carried

out to examine the neotectonic stress field in the south western Red Sea-Arabian plate

margin to reveal a kinetics of active fault system using two-dimensional elastic finite

element method (FEM) under plane strain condition. The Mohr-Coulomb failure criterion

has been adopted to analyze the relationship between stress distribution and fault formation.

A Saudi Arabian reflection profile (Mooney et al, 1985; Prodehl, 1985) is adopted for the

modeling and extensional displacement boundary condition is imposed along NE-SW

direction. Our result shows the extensional displacement and physical properties of rock

layer control the distribution, orientation, magnitude and intensity of the stress and fault

development. According to the calculated stress patterns of failure elements, normal faults

develop in the Red Sea and Arabian Plate margin. The results from our simulation are in

good agreement with those of the seismicity, focal mechanism solution of earthquakes and

active faulting in the Red Sea.

1. Introduction

Modern rifts offer an opportunity to better understand the processes that control

continental breakup by comparing direct observations and measurements with theoretical

models. Among the factors that contribute to the initiation and evolution of continental

rift zones, far-field stress in the lithosphere (horizontal traction), inherited zones of

Received: January 10, 2007


weakness in the lithosphere, and the rheological structure of the lithosphere are thought

to be dominant. The Red Sea rift, about 2000 km long NNW-SSE trending depression, is

a spectacular example of continental extension (Fig. 1). It forms the broad zone of active

deformation between Africa and Arabia. The Red Sea rift which was initiated in Oligocene

is accompanying with a high seismic activity and is well expressed as fractures and

normal faults (Fig. 2). The average spreading rate is thought to be 20 mm/yr (Chu and

Gordan, 1998). Seismicity shows that axial trough is an area of active spreading and

normal faulting associated with Afro-Arabian plate extension and northerly motion of the

Arabian Plate against the Eurasian Plate provided by left lateral motion along the Dead

Sea fault (Amri, 1994, 1998; Ghebreab, 1998; Huang and Solomon, 1987). Most of the

constraints for the dynamics of this actively deforming region have been evidenced by

various seismological and geological observations. This relative wealth of structural,

seismotectonic, and geophysical data makes the Red Sea rift a natural laboratory for the

study of continental extension.

There are various concepts and interpretations regarding the origin and evolution of

the Red Sea rift and which has produced it as debatable and open topic. Several rifting

models have been suggested - prolonged normal faulting (Lowell and Genik, 1972),

lithospheric thinning by faulting and dike injection (Berhe, 1986), diffuse extension

followed by brittle deformation (Cochran, 1983; Cochran and Martinez, 1988), lithospheric

simple shear (Wernicke, 1985; Voggenreiter et al., 1988) and combinations involving

detachment faults and prolonged magmatic expansion (Bohannon, 1986, 1989; Bohannon

and Eittreim, 1991), asymmetric rifting (Dixon et al., 1989) and pull-apart basins (Makris

30"

/^RICAN f

I Nubian

PLATE

30°NAP North Arololbn bull

EAF Eon AttUolloil fault Strike-slip fiult

Fig.l Cenozoic tectonic setting of the Red Sea and adjacent areas (after Johnson, 1998).

Neotectonic stress analysis of the Red Sea rift by Finite Element Modeling

36°

LEGEND

Phancrozoic features

j l:*p»Fi:ii ( re rjcciim. Ptflcogcuv. and Nccjgi-n

j sedinieinar. dcpusiR oflhc R«l Sea basin:

—i Tiilnij"'. iuL Nk-Mj'tTK ^u.iIt

10 prcscm,

* * * Krur^M,,,

H bfcdribo■■■■I indkiOmi

Precambrii

jnp s.,,.-,,,,,

Dlhd At»hkitiiil)(di!(Jih3t!

in features

|n«kl ■: btiMk-ducdb: Iran

ll liiinK- doede ahcJr /obi;

ilcil Itrrjiit MidiitlJ Jashi

rvj bcncnUs ripjiin-F-^i'R: c

,i.iii-iiJiiip"i«l"-*-*lpf-"'te

"^ Pivi-iimlifian Mtuirvs


and Rhim, 1991). Some authors argue for passive rifting (McGuire and Bohannon, 1989),

others for active rifting (White and McKenzie, 1989) and others have advocated neither

purely active nor purely passive rifting (Davison et al., 1994; Drury et al., 1994). Hamid

and Hayashi (2004) simulated the Red Sea rift and proposed the active rifting model.

The stresses responsible for rifting have been suggested to fall into a spectrum

between two active and passive end members (Sengor and Burke, 1978). In the active

model, deviatoric stresses responsible for rifting are imposed by upwelling mantle beneath

the rift whereas passive rifting is caused by plate driving forces outside the immediate

area of the rift. It has been suggested that the position of a particular rift in this

spectrum might be distinguished by the relative timing of uplift, volcanism and rifting.

Volcanism precedes normal faulting and rifting in active rifts whereas the reverse holds

for the passive rifts. In all cases, very large extensional stresses, up to 200 MPa are

apparently required to initiate rifts in typical continental crust (Bott, 1981). Therefore,

during rift development this stress must be developed within the upper crust either as an

intraplate stress related to plate boundary forces, or as a result of lateral density

contrasts within the plate. The gravitational body forces developed by uplift in oceanic

swells are never great enough to overcome the ridge-related compressive stresses, with the

result that oceanic swells never rift. However the gravitational body forces developed in

continental swells uplifted 1 km or more are sufficient to overcome a regional compressive

stress field, and rifting can occur (Crough, 1983). Unless the crust is perfectly

homogeneous, the locus of crustal failure for both active and passive types of rifts will be

influenced by preexisting weakness in the crust which may also control other aspects of

rift formation and evolution (Dixon et al., 1987).

The study of architecture of the earth with the form, symmetry, geometry of the

earth's materials, at present and at the time they were formed and deformed is known as

structural analysis. Such analysis is very useful to understand many problems in

structural geology such as the relationships between the present observed geometries of

structures, their initial configuration and the stress distribution under which they

developed etc. In this study an approach has been made to model the Red Sea rift zone by

considering elastic rheology under extensional boundary conditions according to available

geological and geophysical data. Although the vertical forces for the study cannot be

excluded but seem currently to be of several orders of magnitude lower than the

horizontal forces. For the study Red Sea rift has been chosen because of its rather simple

geometry (extension is mostly concentrated along axial trough expressed by normal

faults), it's purely extensional strain regime, relatively young age (~3.5 Myr), and the

large amount of geological, geophysical, and geodetic data available to constrain the

models.

In particular, the approach has been made to study the feasibility of passive

stretching of the lithosphere by a regional stress field and consider how a suitable


2000 m-j

50 +

mo-

"1

6 km/s contour I

Moho'

SP-4

8 km/s contour

Model A

NE

SP-3 SP-2 SP-I

T i T ,-2000 m

500 km

■Om

1001000 km

Fig.3 Simplified profile of Saudi Arabian seismic line data (after Prodehl, 1985), topographic profile

is vertically exaggerated.

Tihania Asir Khamis Mushayl

(Coastal plain) Rk| Se.isnclssPhanern/oic-Ncoprotcm/oic

IN-jV, O-t O. 6.2 ft.3 *~~~ (..I 65?6.*63 t 6.2 6.3 (U [Cruslal layer! I

I SPI NE

.'(tin "

-6r3. i Id

fi.5 I (o

67 I Li'(i 8

ol_T__i L-.™8.0

50 f Model B60 j M 8


2. Physiography, Geology and Tectonic setting

2.1 Arabian Shield

The Arabian Shield is a stable craton that comprises a crystalline basement,

predominantly metavolcanic, metasedimentary, and plutonic rocks, of Precambrian

continental crust about 40-45 km thick and mostly 870-550 million years old (Davison et

al., 1994; Genna et al., 2002). Some Precambrian rocks in this region date back to Archean

but most are Neoproterozoic (1000-540 Ma). They originated as volcanic islands or as

chains of volcanoes along spreading centers and subduction zones in a Neoproterozoic

ocean and against ancient continental margins, and were folded and uplifted towards the

end of the Precambrian as a large belt of mountains. The mountains existed between

about 680-540 Ma and were part of one of the largest mountain belts ever known to have

existed on earth. By the end of Precambrian, the mountains had been eroded and only

their roots are preserved, exposed in western Saudi Arabia in the Arabian Shield. The

Phanerozoic cover of the Arabian Shield comprises of younger sedimentary rocks that

range in age from Cambrian to Pleistocene and in thickness from zero to 10 km that crop

out as relatively flat lying beds of sandstone, siltstone, limestone and evaporates (salt

deposits), and volcanic rocks (Davison et al., 1994; Bosence et al., 1996). These rocks were

deposited unconformably on the underlying Precambrian basement, in river beds, in glacial

valleys and in shallow seas, or were extruded from subaerial volcanoes. The rocks north

and east of the Arabian shield are referred to as the Arabian Platform; those on the

shield are mainly harrat (fields of Cenozoic basalt); and those west of the shield are

Cenozoic rocks that occupy the Red Sea basin (Fig. 2). The youngest deposits in the

region include coral limestone and unconsolidated sand, silt, gravel, and sabkah, filled

dried up lake beds and wadis, and fringed the coastlines. Prior to the opening and uplift

of the rifted margins of the Red Sea and Gulf of Aden, Phanerozoic rocks covered and

concealed the basements rocks, but erosion and unroofing since then exposes them as the

Arabian Shield, in the west, and in minor outcrops elsewhere.

The Precambrian terranes converged and amalgamated during orogenic events between

750-650 Ma involved deformation, metamorphism, and uplift such as magmatic intrusion,

orogenic collapse, extension, exhumation, and strike-slip faulting (Davison et al., 1994;

Windley et al., 1996; Genna et al., 2002). The Phanerozoic rocks which are unconformable

over the Precambrian, are mostly little deformed, and affected by open folds and block

faulting in the Arabian Platform (east and north of the exposed shield), Red Sea and

Gulf of Aden basins. The Cenozoic sedimentary, evaporitic, and minor volcanic rocks that

fill the Red Sea basin were deposited in an initial intracontinental rift that evolved with

ongoing spreading into the narrow marine basin (Bosence et al., 1996). The separation of

Arabia and Africa, which began about 25 million years ago, entailed rifting and seafloor

spreading along the axes of the Red Sea and Gulf of Aden and the northward drift of the


Arabian Plate and eventual collision with Eurasia. During this period, in addition to the

formation of new oceanic crust and sedimentation in the Red Sea and Gulf of Aden basins,

western and southern margins of the Arabian Plate were uplifted and partly covered by

subaerial flood basalt, resulting in the creation of the Red Sea Escarpment and fields of

lava (harrat), and the northern and northeastern margins were sutured to rocks in Iran

and Turkey, causing crustal shortening and formation of the Zagros fold-and-thrust belt.

2.2 Red Sea

The Red Sea rift formed in the late Oligocene - early Miocene is a narrow oceanic

trough, and shows slightly sinuous basin shape some 2,000 km long that runs NW-SE

direction. It begins in the area of the Strait of Bab al Mandeb (Fig.l), which connects

Red Sea with the Indian Ocean via the Gulf of Aden. The geometry of this connection is

illustrated by compilations of seismicity (Ambraseys et al., 1994; Hofstetter and Beyth,

2003). The Red Sea extends from 13° to 28° N, and at its northern end it divides into the

two arms of the Gulf of Suez and the Gulf of Aqaba. Its coastline morphology is similar

on either side with a narrow, steep-sided axial trough having an irregular bottom (1.5 to

2.5 km depth). The sinuous shape of the axial trough parallels to that of the coastlines.

Wide, shallow (0-600 m) shelves flank the axial trough (Coleman, 1974; Cochran; 1983).

The shelf is 125 km wide at 17° N, offshore of the study area, and is associated with

many islands of the Farasan group. The width of north Red Sea is 180 km, while it

widens to 350 km to the south and then narrows down at the Strait of Bab el Mandeb to

30 km. The coastal plain rises gently landward to maximum elevations of 100 m along

most of the Arabian coast. The east edge of the coastal plain is marked locally by

foothills as high as 650 m, but in other areas the plain gives way eastward to pediment

and dissected terrain of slightly steeper slope. The foothills and pediment are 15 km wide,

and inland of them the land rises abruptly, in the deeply dissected mountainous terrain

known as the Asir, to the west facing Arabian escarpment. The mountainous terrain is 30

to 60 km wide at 17° N. The land surface slopes gently eastward in the tablelands east of

the escarpment.

The Red Sea rift was formed when the originally connected African and Arabian land

mass broke apart. At first it was a continental rift, then, as Arabia drifted away,

developed into an intercontinental system that today separates the independent Arabian

plate from the African plate (Martinez and Cochran, 1988). The Red Sea rift is part of an

extensive global system of faults running approximately north to south (Fig.l). At the

northern end of the Red Sea, this rift system again splits into the Gulf of Suez and the

Gulf of Aqaba, and both units have completely different structures. While former is a

tensional rift system, the latter is part of an extensive shear system that is followed to

the Dead Sea and Jordan, and from there to the Alpidic chains of Taurus in southern

Turkey. Within the entire meridional system, the Red Sea is the most advanced section,

10 Sunil Kumar Dwivedi and Daigoro Hayashi

as far as the breaking-apart is concerned. Geological and geophysical evidence strongly

support the idea of new oceanic crust being created along the axial trough (Cochran, 1983;

Girdler and Underwood, 1985). This process of active ocean spreading is responsible for

the ever-increasing drift of the Arabian plate. The Red Sea continues to develop from an

originally seismically and tectonically active rift rim to a passive continental margin. The

decreasing tectonic activity in the marginal area is an expression of the gradual

establishment of a state of equilibrium on the new edge of the continent.

2.2.1 Nature of the Red Sea crust

There is general agreement that young oceanic crust exists along parts of the length

of the Red Sea (Roeser, 1975, Girdler, 1969). The crust is exposed in the Arabian margin

and may be present to an unknown extent beneath the coastal plains to the west

(Bohannon, 1986; Davison et al., 1994). Oceanic crust is present in the axial trough of the

Red Sea, where large-amplitude, marine magnetic anomalies as old as 5 Ma are well

documented (Drake and Girdler, 1964; Girdler, 1969; Roeser, 1975). Oceanic crust probably

extends to at least 70 km from the spreading axis at 21° N. The Red Sea shelves and

coastal plains are underlain by sediment having thickness of 4 to 6 km documented by

well-log data, seismic-refraction studies and seismic reflection profiles (Drake and Girdler,

1964; Girdler, 1969; Coleman, 1974). The character of sub-sediment crust is uncertain.

Some researchers (Girdler and Underwood, 1985; Mooney et al., 1985) have argued about

an abrupt transition from continental to oceanic crust 30 km inland of the Arabian

shoreline between 18° 30' and 18° N. Recent seismic data under the eastern side of the Red

Sea in Saudi Arabia (Richter et al., 1991) and Yemen (Makris and Rhim, 1991) confirm

the stretched character of the continental crust, and discount the presence of oceanic crust

beneath the coastal plain close to the exposed Precambrian shield. Some authors

(LaBrecque and Zitellini, 1985; Girdler and Underwood, 1985) use magnetic models to infer

oceanic crust beneath the shelves and possibly the coastal plain. Cochran (1983), and

Coleman (1974) argue that block faults in extended continental crust account for the shelf

anomalies and limit oceanic crust to that proven in the axis. Rifting in this area began

in the Late Oligocene (24 Ma) and oceanic crust along the axial trough is estimated to be

5-6 Ma old (Izzeldin, 1987; Sultan et al., 1992).

2.2.2 Red Sea margin

The Saudi Arabian Red Sea coastal margin has a complex geomorphologic history

reflecting the interactions among structure, uplift, erosion and volcanism. The major

landscape features (Fig.2) comprise: 0-50 km wide coastal plain, an erosional escarpment,

a region of hills and mountains between the coastal plain and escarpment that are the

remnants of erosional retreat of the escarpment; and one or more erosional surfaces east

of the escarpment (broadly referred to as the Najd Pediplain) that include in places

Neotectonic stress analysis of the Red Sea rift by Finite Element Modeling 11

exhumed remnants the surface that developed on the Arabian shield at the end of the

Precambrian (Huchon et al., 1991; Davison et al., 1994). Structurally, the margin (Fig.2)

includes: the Cenozoic Red Sea basin, which underlies the Red Sea and, most of the coastal

plain, continental crust composed of Precambrian rocks of the Arabian Shield, small

basins inland from the Red Sea that contain Cenozoic sedimentary rocks and reflect local

extensions, fractures inland from and subparallel to the coast line that are intruded by

Cenozoic dikes; and large fields of Cenozoic to Recent basalt (harrats), which were

extruded unconformably on the shield and on sedimentary rocks of the Red Sea basin

(Huchon et al., 1991; Davison et al., 1994).

The contact between the Red Sea basin and Arabian shield is partly faulted and partly

depositional (Bohannon, 1986). It coincides with a zone of crustal attenuation and

modification in which continental crust of the Arabian Plate (effectively the crystalline

rocks of the shield) thins from 40-45 km thickness (inland) to 15-5 km (at the coast) and

is replaced beneath the Red Sea by juvenile oceanic crust. However, details of the structure

and evolution of the Red Sea basin are not well established.

2.2.3 Geophysical configuration of the Red Sea margin

Several interpretation of crustal thickness beneath the Arabian Shield and southern

Red Sea has been made from Saudi Arabian seismic refraction line data (Milkerieit and

Fluh, 1985; Mooney et al., 1985; Prodehl, 1985) (F.igs. 3 and 4). The Moho is located

subhorizontal 38-45 km beneath the Arabian Shield in these models. There are differences

in their interpretations west of the Asir where the Moho rises from its sub-shield depth

to 8 to 15 km beneath the Farasan Islands. Three contrasting interpretations are

illustrated by Bohannon (1986). The interpretations by Mooney et al. (1985) suggests that

the Moho rises abruptly from a continental 38 km beneath the Shield, slightly west of the

Asir, to a transitional 18 km beneath the eastern coastal plain (Fig.4). From the coastal

plain it gently rises to an "oceanic" 8 km under the Farasan Islands. The seismically

determined transition coincides at the surface of the Tihama Asir with a sharp boundary

between the Precambrian rocks and the Tertiary volcanic and intrusive rocks. A simple

three layer crust composed of sediments (4.2 km/s), an upper crust (about 6.2 km/s), and

a thin lower crust (6.8 km/s) characterizes the Red Sea shelf and rift. Prodehl (1985)

proposed an alternative interpretation in which the Moho rises gently from 43 km beneath

the Shield, slightly east of the Asir, to about 14 km beneath the Farasan Islands. The

transition from continental to oceanic crust is not obvious in his model. The interpretation

of Milkereit and Fluh (1985) is similar to that of Mooney et al. (1985). They interpreted

a flat Moho at about a 13 km beneath the shelf of the eastern Red Sea and proposed a

double Moho between the Asir and shelf; one, nearly level, at 13 to 15 km and a deeper

one with an east slope.


3. Cenozoic tectonic development of the Red Sea rift system

The Red Sea rift system is one of the world's largest active continental rift systems,

which comprises a variety of rifting stages starting from initial faulting, advancing

through several stages of continental rifting. The rifting of the Red Sea and associated

system was initiated in the Oligocene, with thermally driven uplift and domal arching of

the Arabian-African Shield. Two triple junction is characteristic feature of the Red Sea

rift system, the Afar to the south and the Sinai to the north. As a result of rifting, the

Arabian plate separated and moved NE to collide with the Eurasian plate. In general the

Gulf of Suez and the Gulf of Aqaba are considered to be the result of the divergence of

lithosphere plates, whereas the Gulf of Aqaba mainly results from strike-slip movements

with few extensional components. The formation of Gulf of Aden has resulted from

divergence of the Arabian, African and Somalian continental plates.

Prior to the formation of Red Sea the north eastern Afro-Arabian continent had low

relief and was largely below the sea level from the Late Cretaceous to the early Oligocene.

The events leading to the formation of the Red Sea followed the sequence (Bohannon et

al.,1989)-

i) Alkaline volcanism and rifting beginning about 30-32 Ma affecting narrow linear

zone in the continent.

ii) Rotational block faulting and detachment faulting, well underway by 25 Ma.

in) Gabbro and diorite magmatism, andesite to rhyolite volcanism, and nonmarine

sedimentation in the rift between 20 and 25 Ma.

iv) Marine sedimentation in the rift as the early shelves started to subside in the

middle Miocene, and

v) Uplift of the adjacent continents (about 3 km) and subsidence of the shelves (about

4 km) between 13.8 and 5 Ma.

The sequence volcanism and rifting followed by uplift leads a passive mantle model for

rift origin. The rift starts with mechanical extension in a narrow zone of lithosphere

between 25-32 Ma. The thinned lithosphere is replaced by upwelling asthenosphere and by

rocks from the adjacent deep continental lithosphere, which flow into the rift. Ductile flow

of the deep continental lithosphere is accelerated by partial melting as rocks flow upward

toward the rift axis. Once partially melted rocks join the upwelling asthenosphere, a rapid

erosion of the lithospheric mantle beneath the continent near the rift edge occurs resulting

uplift. The Red Sea began a consequence of changing plate geometries resulting from the

collision of India and Eurasia. After the collision, the segment of the Owens fracture zone

north of the Carlsberg Ridge became locked, forcing the northeast corner of Afro/Arabia

to rotate with the Indian plate away from the rest of Africa.

Red Sea rifting in the present stage is dominated by two processes, first - the

concentration of extensional deformation, which had been widely distributed across the


rift, and second - the segmentation of the rift. The northern Red Sea region is floored by

thinned and stretched continental crust, which is associated with diffuse basaltic

intrusions (Bonatti, 1985). The axial injections of oceanic crust and seafloor spreading

have not yet started in the northern region of the Red Sea area. The crust beneath the

main trough is continental crust and that was extended and modified by normal faulting

and dike injection during late Oligocene to early Miocene phase of continental rifting. The

Red Sea is a northward-propagating rift and this fact is favored by: thinned continental

crust that floors the northern Red Sea (Cochran et al., 1986), axial trough widest in the

south (Dixon et al., 1987), heat flow decreases northward, magmatic anomalies

symmetrical to the north and the depth of normal fault mechanisms increases northward

from the south (Huang and Solomon, 1987).

4. The Model

Elastic finite element method (Hayashi, 2002) has been applied to 2D geometry of the

models. The FEM has been used in order to solve the elastic equations and for that we

have to define the geometry of the models and the correct boundary conditions to apply

to it. The models are two-dimensional and plane strain conditions are assumed. This

means that no appreciable strain will occur outside the plane. Moreover, the out-of-plane

stress is always a principle stress and is perpendicular to the model. Therefore, the other

two principle stresses must lie in the plane of the model. By subsequently comparing the

out-of-plane principle stress with the two in-plane principle stresses, and by assuming

Andersonian fault criteria (Anderson, 1951) it is possible to determine the type of fault,

i.e., normal, strike-slip or thrust.

Table 1: Physical Properties of the rock layers.

Model

layer

Layer 1

Layer 2

Layer 3

Layer 4

Layer 5

Rock layer

Upper continental

crust

Lower continental

crust

Transitional crust

Oceanic crust

Syntectonic

deposits

Vp

(km/s)

6.4

7.3

6.8

6.4

4.2

P

Density

(kg/nf)

2700

2800

2900

3100

2300

Y

Young's

Modulus

(GPa)

73.7

98.1

89.9

84.6

27.0

0

Poisson's

ratio

0.25

0.25

0.25

0.25

0.25

c

cohesion

(GPa)

18

21

25

30

15

Friction

angle (°)

35

40

45

50

30


Primarily based on the seismic sections of Mooney et al. (1985) and Prodehl (1985)

(Fig's. 3 and 4), 2D elastic finite element models are constructed. The location of the

profile is shown in Fig.2. The dimension of the model is 360 km in length and 38 km in

depth that stretches from the Red Sea axis to the Arabian shield. Considering the

sufficient care of tectonostratigraphy of the area, all models are divided into five

individual layers; upper continental crust, lower continental crust, transitional crust,

oceanic crust and syntectonic deposits (Table 1). For the FEM calculation two

independent elastic constants Young's modulus (E) and Poisson ratio (u) are needed.

Since the density (p) of major rock unit are known and Poisson ratio is assumed 0.25 for

each layer, average P-wave velocity for each layer (Vp) obtained from the seismic section

makes possible to calculate Young's Modulus by using equation 1 (Timosenko and

Goodier, 1970) (Fig.5).

LEGEND

Upper cont. crust

Lower cont. crust

Transitional crust

Oceanic crust

Syntectonic deposits

2700

4.2 2300 27.0 15 30

Vp(km/s)

Density

(kg/m3)

Young's modulus

(GPa)

Poisson ratio Cohesion

(MPa)

Friction angle

(degree)

Fig.5 Rock layer properties for FEM calculation.

E=pV;(1 + v) (1 - 2 v)

(equation 1)(1 + v)

For calculation we use the static Young's modulus for each tectonic rock unit

assuming that the static Young's modulus is 80% of the dynamic Young's modulus. The

values of Young's modulus are consistent with the values for crustal rocks determined in

the laboratory (Sydney and Clark, 1966).

Elastic behavior of crust is assumed for the modeled section and this is justifiable

since rheology changes from elastic to viscous below Mono which exists at 38 km depth.


a i, 02, and a a as the maximum, intermediate and minimum principal stresses

respectively. As shown in Fig.7, the Mohr-Coulomb criterion is written as a linear

relationship between shear and normal stresses,

x = c + an t&n • (ii)

where, c and


4.2 Model Results

Modeling results will be presented first and discussed for two cases. All the models

are built up in order to evaluate the influence of both the litho-mechanical behavior of the

involved rock layers and the geometry of the model on neotectonic distribution of stress

and faulting in response to extensional displacement. Gravitational force is taken into

account in all models. Since the models consist of two types of geometries, the simulated

a)

Scale: H=V 100 MPa

50 m

I

360 km

b)

100 m

360 km

Fig.8 Stress distribution in the Red Sea for model A: a) at 50 m extension; b) at 100 m extension.

Each pair of perpendicular lines represents a i (long lines) a 3 (short lines) in the stress field,

and red bar shows the tensional stress field.

Model B

Scale: H=V 100 MPa

50 m

360 km

100 m

360 km

Fig.10 Stress distribution in the Red Sea for model B: a) at 50 m extension and b) at 100 m

extension. Each pair of perpendicular lines represents O\ (long lines) a a (short lines) in the

stress field, and red bar shows the tensional stress field.


stress regimes are explained with respect to both model geometries.

The obtained stress distribution for the two models A and B are shown in Figs. 8 and

10. In both models, the pattern of stress distribution shows close similarity with the depth

distribution of the earthquake hypocenters in the Red Sea (Huang and Solomon, 1987;

Amri, 1994 and 1998; Hoister et al., 2003). Confirming seismogenic studies the models

Model A

0 m

50 m

360 km

d)

e)

Scale: H=V100 MPa

I OOm

360 km

-»- 125m

360 km

150 m

360 km

*" 200 m

Fig.9 Failure elements in the Red Sea for model A with extension at: a) 0 m, b) 50 m, c) 100 m, d)

125 m, e) 150 in and c) 200 m.. Each pair of perpendicular lines represents a, (long lines)

a,< (short lines) in failure elements, and red bar shows the tensional stress field.


show normal faulting in the Red Sea axis to Arabian plate margin. In an extensional

regime, the maximum compressive stress (aO is vertical, whereas the minimum is

horizontal (a3) (Anderson, 1951) and Fig.8 and 10 show that throughout the entire

model, the orientation of a i is almost vertical, thus giving an indication that the applied

boundary condition is suitable for the simulation, and represents the natural condition of

Red Sea tectonics. Models also show the maximum values of stress are located at the

shallow depth 4-8 km in the Red Sea. The difference between the two models in the

distribution of stress is mainly localized in the upper part of the section. Comparing the

modeling results, we can see the role of both the geometry of Moho and the rock layer

property changes. Results show the Model B giving more realistic stress field and

faulting and is able to justify geological and geophysical observable in the Red Sea.

4.2.1 Model A

Stress-field

Model A is adopted from seismic section of Prodehl (1985). The stress field under 0

m horizontal extensional displacement is shown in Fig.9. Compressive nature of stress is

found in all layers. Lower magnitude of a i and a 3 is found in upper part of the model.

Magnitude of Oi and oVincreases with depth and the orientation of O\ and a3 is vertical

and horizontal respectively. With increasing horizontal extensional displacement,

extensional stress is predominantly observed in the upper part of models (Fig.8), which

corresponds to normal faulting in the extensional regime of the Red Sea.

Failure pattern

As described earlier, failure pattern is analyzed using Mohr-Coulomb criterion

(Melosh and Williams, 1989). During progressive extension of 0 m to 200 m, the region

where failure has taken place is analyzed to understand the mode of faulting with their

implication to the Red Sea rifting. The red bar denotes the principle stress during

tensions! failure while black bar for compressional failure (Fig.9). As extension is

progressively increased, the zone of failure in compression increases with depth. After

boundary displacement of 0 m to 100 m no significant failure elements are developed in

the models. However, at extensional displacement of 125 m few elements are failed in the

upper part of the model near coastal plain and Red Sea rift zone. With increasing the

extensional displacement progressively from 125 m to 200 m, failure elements are

progressively developed in the upper part of the model from Red Sea rift zone through

syntectonic deposits to Red Sea margin and Arabian Shield (Fig. 9). However, the

localization of failure elements are not in the right position and do not replicate the

natural situation of the Red Sea.


4.2.2 Model B

Stress-field

Model B is adopted from seismic section of Mooney et al. (1985) and shows not so

significant changes in the state of stress and magnitude of stress compared with Model A.

In the initial stage of extension, mostly compressive stress is observed throughout the

upper part of the model that cause normal faulting in extensional tectonic regime

(Fig. 10). Compressive nature of stress is found in all layers. Lower magnitude of CTi and

ct3 is found in upper part of the model. Magnitude of Oi and o3 increases with depth and

the orientation of O\ and a3 is vertical and horizontal respectively. With increasing

horizontal extensional displacement, extensional stress is predominantly observed in the

upper part of models (Fig. 10), which corresponds to the normal faulting in the

extensional regime of the Red Sea.

Failure pattern

Mode of faulting in Model B is similar to the previous Model A. However, the

localization of faulting corresponds to natural situation of the Red Sea. Under 0 m

extension no elements are failed in the model. But with increasing extensional

displacement progressively from 50 m to 200 m, elements are failed in the Red Sea rift

zone through Red Sea shelves and near and below the coastal plain, and are deeply rooted

because of the abrupt change in Moho morphology (Fig. 11) corresponding to normal

faulting in the Red Sea

5. Discussion

5.1 Model set up assumptions

Considering homogeneous and isotropic material for each layer, all models have been

simulated by means of 2D FEM under plane strain condition. However, in nature, the

behavior of rock layer is not homogeneous and isotropic. Since, physical properties of rock

layers were not found experimentally, series of calculations were performed to determine

the appropriate values of rock parameters. Finally, most suitable set of layer properties

was adopted for the modeling. During calculations attention has been paid to avoid wide

fluctuation of key parameters from their real values. Elastic rheology is assumed for the

modeled section of the crust although it is plastic-elastic in nature. Though our models

are simple, assumed data are consistent with available field data.

5.2 State of stress in the Red Sea

The predominant stress field is the more recent stress field for Red Sea rifting, of

which the primarily compressive stress acts more or less perpendicular to the spreading

axis of the Red Sea (Al Amri, 1994). Studies of continental stress fields have shown that


b)

c)

Model B

Scale: H=V 100 MPn

0 m

360 km

360 km

100 m

360 km

d)Scale: H=V

360 km

360 km

150 m

200 m

360 km

Fig.ll Failure elements in the Red Sea for model B with extension at: a) 0 m, b) 50 m, c) 100 m, d)

125 m, e) 150 m and f) 200 m.. Each pair of perpendicular lines represents a, (long lines)

a i (short lines) in failure elements, and red bar shows the tensional stress field.

principle stress axes can rotate through large angles over geologically short periods. This

phenomenon has been well documented in the Red Sea and East African Rift system

(Strecker et al., 1990; Streaker and Bosworth, 1991; Ring et al., 1992). Number of authors

made attempts to assess the state of stress in the Red Sea and adjacent areas (Strecker

et al., 1990; Strecker and Bosworth, 1991; Bosworth and Strecker; 1997). They deduced the


NE-SW direction of least horizontal stress direction (aHmin) for the Red Sea. They noted

that the direction of a Hmin have changed following the change in direction of relative

movement of Afro-Arabian plates. Those studies clearly indicate that regional direction of

aHmin is consistent with the relative movements of Afro-Arabian plates. Since, direction

of our model profile coincides with am™ direction, therefore results of modeling are

consistent with the stress state derived from those studies.

5.3 Seismicity and active faults in the Red Sea

Tectonic activities are manifested by the occurrence of earthquakes. The geology of

southwestern Arabia is largely affected and controlled by the geodynamic process acting

in the Red Sea region, characterized by its opening from the southeast (Ambraseys et al.,

1994). During the rifting process, a system of regional transform and normal faults has

been formed that run across and along the Red Sea. Some of these faults extend inland

over tens or hundreds of kilometers. Barzangi (1981) pointed out that such faults can be

classified as potentially active and movements along them are likely to cause damaging

earthquakes in the region. Merghelani and Gallanthine (1980) studied the microseismicity

of the Tihama Asir and Jeddah area and show a higher level of seismicity. Makris et al.

(1991) showed that microearthquakes with maximum hypocenter depths of 18 km are

concentrated along the margins of the axial trough and grabens with the eastern graben

margins more active than the western. In the central Red Sea, frequency of seismicity is

shallow and mainly associated with the deep axial trough zone (Al Amri, 1994; Hofster et

Fig.12 Seismicity map of the Red Sea (redrawn from Amri, 1994; 1998).


al., 2003). Huang and Solomon (1987) investigated the 1967 earthquake that occurred on

a surface with a strike of 309 and dip of 45 with largely dip-slip movement and fault

plane solutions showed the normal faulting in the axial trough. The scatter of some

epicenters in the shield area is expected due to the complexity of rift faulting, while the

low level of seismicity in the coastal plains is caused by the fact that some deep faults

exist without surface traces (Al Amri, 1994) (Fig. 12).

Studies on neotectonics of the region are relatively few. Some studies (Coleman et al.,

1979; Bohannon, 1986) indicated that the neotectonics of the region are dominated by an

Red Sea

axial trougb

0

Red Sea Shelf I CotnalPUh IDeformed Belj

SPS

Neogene sediment and sedimentary rocks 1 Large normal fault=—— *»■ - ■ * ■ ' * I-C

360 km

Fig.13 Field evidence of faulting near coastal plain (Tihama) of southwestrn Arabian plate margin

(redrawn from Bohannon, 1989).

45*

Oceanic cnnt (early)

Oceanic crass (toe)

Oceanic eras) in proto-pull

Fig.14 Map of active faults of Red Sea (redrawn from Makris and Rhim, 1991).

24 Sunil Kumar Dwivedi and Daigoro Hayaahi

overall SW-NE extension stress while others (Giraud et al., 1986) showed compressional

stresses in the same direction. Bohannon (1986) proposed a gently east dipping

hypothetical detachment zone within the Proterozoic crystalline rocks in the area (Fig. 13).

Gillman (1968) have revealed a large flexure-like fault zone with NW-SE strike and

westward dip as well as some northeast-trending faults, interpreted as transform faults in

the coastal area of the Tihama Asir. Surface Quaternary faults have been observed in

Farasan Islands near Jizan, and in the wadis near Jeddah (Baraznagi, 1981). Recent uplift

in the Red Sea is evident by raised coral terraces (Morris, 1975) that show the

neotectonics of the Tihama region. All of these seismological and neotectonic studies

indicate that the southwestern part of Arabia is an active tectonic region (Fig. 14).

In the present study, FEM simulation has reproduced the realistic stress field, failure

elements and displays realistic fault patterns, which are in close agreement with the

microseismicity and active faulting of the region. Active faults of both normal and

compressive types are predicted in the Red Sea axial trough, coastal plain (Tihama Asir)

and the Red Sea escarpment. We are successful in simulating several active faults at their

proper locations. All of the faults display the characteristics initiating at depth,

transmitting to the surface and finally propagating towards Red Sea-Arabian Shield with

increasing the extensional displacement, which is consistent with the field observation.

6. Conclusion

Elastic models have long been used to model earthquakes and displacement fields and,

as far as the present study is concerned, the application of an elastic FEM simulation

enabled us to improve our understanding of the available data obtained from the

geological and geophysical investigations in the Red Sea. In particular following

considerations are made from the present study:

1. The rock layer properties of the present models characterizing Red Sea and

Southwestern Arabia are defined by means of elastic parameters. The study has

enabled us to evaluate the lithological behavior of the rock layers involved in the

extensional deformation. The performed simulations show that the rock layers play an

important role not only on the growth of faults but also on the distribution of the

stresses in the area.

2. The stress distribution pattern shows the existence of both tensional and compressive

state of stress in the Red Sea. The magnitude of principal stresses depends upon layer

properties and applied extensional displacement. With increasing displacement, the

magnitude of Oi decreases and its axis rotates towards vertical resulting in normal

fault. The fault pattern obtained by simulation suggests the direct correlation with

the present day active faults development in the Red Sea.


3. Our modeling shows faults are confined to the Red Sea-Arabian Shield transition

area. The reason is due to the strong lithological variation and sharp boundary

between the Precambrian rocks and the Tertiary volcanic intrusive rocks.

4. Comparison between the two models enables us to distinguish between the influences

of the rock layer properties and those induced by the model geometry.

5. The comparison between the Model A (Prodehl, 1985) and Model B (Mooney et al.,

1985) show neotectonic stress state and failure elements of Model B is more likely to

represent present situation of Red Sea tectonics, i.e. Model B satisfies in a regional

sense the constraints of the available geologic, seismologic data.

Acknowledgements

S.K. Dwivedi is obliged to the Ministry of Education, Science, Sports and Culture,

Japan (Monbukagakusho) for the scholarship to carry out the research.

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Documents

Neotectonic stress analysis of the Red Sea rift by Finite ...ir.lib.u-ryukyu.ac.jp/bitstream/20.500.12000/448/1/bull...Sunil Kumar Dwivedi and Daigoro Hayashi weakness in the lithosphere,