Subsidence in the gulf of suez: implications for rifting and plate kinematics

Teftonophysic~,

249

153 (1988) 249-270

Elsevier Science Publishers

B.V., Amsterdam

- Printed

’

in The Netherlands

Subsidence in the Gulf of Suez: implications for rifting and plate kinematics MICHAEL S. TTECKLER ‘, FRANCOIS

BERTHELOT

XAVIER ’ Lament-Doherty

Geological Observatory

LE PICHON of Colombia

2, NICOLAS LYBERIS ’ and 3

University, Palisades. NY 10964 (U.S.A.)

2 Universitt! de Paris VI, Paris (France) ’ Laboratoire de GPoiogie, Ecole Normale Sup&ewe, (Revised

version

accepted

24 rue Lhomond February

75.231. Paris [France)

1, 1988).

Abstract Steckler,

MS.,

rifting

Berthelot,

F., Lyberis,

and plate kinematics.

Tectonophysics,

phases of tectonic slowed

again.

quantitative

subsidence.

fourth,

(Rudeis

values

throughout

Stress directions

of the rift, the rate of subsidence

Uplift

extension.

opening

phase.

By the Middle

cross-section

for the large regional

uplift (-

of the rift decreases The total extension

km at the triple junction.

of the second calculated

together

for the kinematic

Sea, and both open at rifting

Correcting

Approximately

Slower

together

was very low. In the

Miocene,

of the Gulf

the subsidence of Suez allows

1100 m on the Egyptian the net tectonic

at the latitude l/3

extension

with th,: Gulf

Gulf of Suez shows three distinct

to l/2

continued

had better

side) is critical

subsidence

by ov.er one

of Gebel Zeit (2S” N) i:. - 30

of the extension

occurred

for the rest of the Miocene

during and

the Plio-Pleistocene.

Suez. These data, scenario

for

The Gulf of Suez and Red Sea Rifting.

and southern

in subsidence.

to a two-dimensional

by uniform

to 32-36

subsidence

increase

for the extension.

to that predicted

km, which corresponds the rapid

the initial stage of opening

the backstripping

of extension.

accurate

relative

During

of well data in the central

fm.) there is a rapid

Extending

estimates

for obtaining

in the Gulf of Suez: implications

(Editors).

rift which has evolved as one arm of the Sinai triple junction

and the Red Sea. Backstripping

late Burdigalian

X., 1988. Subsidence

and J.R. Cochran

153: 249-210.

The Gulf of Suez is a Neogene of Aqaba

N. and Le Pichon,

In: X. Le Pichon

from microstructures

with constraints evolution

- N30°.

with very low amounts

Subsequently,

the direction

slowed. The post-Miocene minor change is probably

formed

of extension

shift to oblique

phase

This geometry

directional

as a strike-slip opening

This change

phases

to the opening

of the Sinai triple junction,

in the Gulf continued

boundary.

clockwise

in the Gulf of Aqaba is below the current

of Suez represents through

the main

As the Arabia-Africa

in the Gulf of Suez rotated

towards

continuation an initial phase

into a

of the Red startup

of the

of Suez extension.

motion perpendicular

and the Dead Sea transform

resolution

of the Gulf of

can be combined

transferred extension

to the and

is the rest& of a

of the data for the Gulf of Suez, which

at < 1 mm/yr.

Introduction There is a general understanding of the plate motions involving the Neogene separation of Arabia from Africa. The northeastward movement of Arabia, relative to Africa, has created the young oceanic basins of the Gulf of Aden and Red Sea. 0040-1951/88/$03.50

several

At first, the Gulf of Suez is the northward

The first subsidence

in the pole of opening. opening

of the region.

of extension.

the Gulf of Aqaba

Gulf of Aqaba,

exhibit

from the other arms

0 1988 Elsevier Science Publishers

B.V.

At the northern end of the Red Sea, the kinematics become more complex. Here the Red Sea bifurcates into the Gulf of Suez rift and the Gulf of Aqaba-Dead Sea Transform zone (Fig. 1). The Dead Sea transform has undergone 105 km of strike-slip and transtensional movement and is currently the main continuation of the plate

250

Medi

Fig. 1. Map showing the location of the rifts bordering the Sinai peninsula (adapted from Garfunkel, 1981; Garfunkef et al., 1981).

boundary on the northwest side of Arabia. The Gulf of Suez rift appears to initially have been the northern extension of the Red Sea, but is now the least active of the three branches which meet south of the Sinai peninsula. In fact, whether or not the Gulf of Suez is still undergoing extension (Garfunhel and Bartov, 1977) or is inactive (Tamsett, 1984; Steckler and ten kinlc, 1986) is under debate. The details of the temporal history of each of the three rifts in this region is poorly constrained and the kinematic evolution of the plates around this Sinai triple junction is not well e&.abhshed. Several possible movement histories have aiready been described (Tamaett, 1984; Gird&r, 1985; Girdler and Southren 1937; Ste&ler and ten Brink, 1986; CourtiRot et al., 1987a;b; Joffe and GarfunkeI, WV). Asce&kAg how the rifts bordering the Sinai triple junction have

evolved can contribute to a general understanding of the factors which control whether rifts are abandoned or successfully develop into oceanic basins. Each of the three arms of the Sinai triple junction has undergone a very different rifting history. Furthermore, from each rift, through its particular sedimentary and structural record, only an incomplete evaluation of that rifting history has or can be obtained. These individual datasets contain information on different aspects of the evolution of the system. The combination of these datasets, therefore, holds the potential to place tighter constraints on the evolution of the whole region than any of the individual components (Joffe and Garfunkel, 1987). The northern Red Sea is now a deep water basin with great thicknesses of evaporites making

the acquisition

of sufficiently

detailed

subsidence

adjacent

and

data

Recent

marine

the plates around

structural

periments

difficult.

(Martinez

and Cochran,

et al., this vol.), however, our knowledge structures,

and crustal

Red Sea. Along

of

is

best

(Freund

there are limited

improving

in the northern

are unfossiliferous known

at

The Gulf of Suez rift, althou~

the evolution

the Sinai triple junction.

reconstruction

cern will be proposed

of

Finally,

of the region

of con-

and discussed.

nonmarine and direction

the

Geological setting

most of

Dead

et al., 19’70; Garfunkel,

data constraining

a kinematic

constrain

conditions,

Sea transform,

So, while the total amount

opening

Transform

thermal

thicknesses

the Dead

the early sediments deposits.

are greatly

of the current

ex-

1988; Gaulier

rifts to better

Sea 1981)

The crystalline posed African Stoeser

the least active,

and

is com-

rocks of Pan-

Ma-Stern

et al.,

1984;

1985). The metamorphics

by a NW-SE

(Garfunkel

in the region

and granitic

age (710-510 and Camp,

characterized

the rates.

basement

of metamorphic

Bartov,

direction

are

of foliation

1977; Ries et al., 1983).

has the best available data pertaining to the temporal evolution of the region. The Gulf of Suez rift

The crystalline massifs are also affected by numerous dikes of basic and acid rocks (mainl;/ dolerites.

has been a relatively

andesites

shallow-water,

marine

basin

for virtually its entire existence. Although large thicknesses of unfossiliferous evaporites mask the deeper section, great numbers of hydrocarbon exploration wells penetrate the entire rift sequence in much of the basin. As a result, the subsidence history, reflecting the rift evolution, is best known in the Gulf of Suez. Uplift related to the rifting has also produced excellent exposures of the synrift sediments and structures. The Gulf of Suez is therefore the best site to attempt a subsidence analysis to obtain the extension history. Even here, however, regional uplift, probably due to small-scale convection (Steckler, 1985), limits the precision of results. Directional data on the opening history of the Gulf of Suez rift can be obtained via slickenside surfaces on the rift faults. Stress directions consistent with simultaneous motion on fault surfaces of different o~entation can be derived (Angelier and Mechler, 1977; Angelier et al., 1982). These results provide additional data on the successive reorganizations of the rifts under study. The overall purpose of this paper is to develop a kinematic reconstruction of the rifts at the northern ter~nation of the Red Sea and to understand their evolution. Toward this end, this paper will assess the extension history of the Gulf of Suez rift with data obtained from field studies and backstripping analysis. This subsidence information on extension rates for the Gulf of Suez will be combined with microstructural determinations of paleostress directions and the development of the

and rhyolites).

as the later

rift

These

structures

intrusions

as well

preferentially

follow

three directions: NW-SE, known as the clysmic direction, ENE-WSW, and N-S to NNE-SSW. known as the Aqaba son, 1971).

direction

(Vail,

1970; Rob-

Immediately overlying the basement the “Nubian Sandstones”, a collection arily nonmarine

and eolian

sandstones

rocks are of primranging

in

age from Cambrian to Cretaceous. These are succeeded by shallow-water carbonates and shales of ad-Cretaceous to Eocene age. The Mesozoic and Tertiary Jurassic terranean

sediments continental (Garfunkel

are related to the margin of the Eastern and

Derin,

1985;

Lower MediSteckler

and ten Brink, 1986). The hinge zone of this margin runs ENE from near Cairo across the northern part of the Sinai Peninsula. These sediments, which thin from this hinge zone southwards to a feather edge, comprise the coastal plain wedge of this margin. Rocks of Oligocene age are missing south of the hinge zone, except for a few isolated outcrops and subsurface occurrences. This is most likely the result of the Oligocene sea-level fall which produced similar regressions at other passive continental margins. The earliest syn-rift sediments are those of the Nukhul formation composed of shallow-water elastics, carbonates, and evaporites. The basal age of the Nukhul formation is poorly determined and is probably diachronous. In the Midyan region of Saudi Arabia, the equivalent rocks are Chattian (Bayer et al., this vol.). The Nukhul formation

252

contains Foraminifera of the N4 and N5 zones (Fig. 2). Open marine conditions become widespread with the deposition of the Rudeis formation of Burdigalian age (Fig. 2). Globigerina marls dominate the formation, although sands are important locally. The mid-Clysmic event, marked by a reorganization of the active fault blocks (Garfunkel and Bartov, 1977) and an angular unconformity and erosion of highs (Beleity, 1982) separates the Upper and Lower Rudeis. Paleobathymetric interpretations of the faunal assemblages during the Rudeis formation indicate open marine conditions and deeper water depths than either earlier or later. Moretti (1987) argues for average water depths around 100 m while others propose greater values up to 1000 m (Evans, this vol.). The Rudeis formation is succeeded by the shallower-water limestones of the Kareem formation. In the subsequent Belayim formation, evaporitic conditions begin. The Belayim is composed of alternating limestones, shales and anhydrite or halite. The Kareem is Langhian in age and the Belayim extends to Foraminifera zone N13 in the upper Serravalian (Scott and Govean, 1985; Fig. 2). The Abu Alaqa formation is the elastic lateral equivalent of the Upper Rudeis to Belayim formations (Fig. 2; Garfunkel and Bartov, 1977; Webster and R&son, 1982). The main evaporite sequence comprises &he South Gharib and Zeit formation. The South Gharib includes massive halite deposits with interbeds of shales, sands and limestones. The Zeit is generally more anhydritic with frequent altemations of non-evaporite sediments. The boundary between the evaporitic formations is time-transgressive and towards the margins of the rift changes to elastic lateral equivalents. The post-Zeit sediments show a return to open marine conditions with the establishment of a seaway to the Indian Ocean. The age of the return to elastic deposition is not firmly established, but is generally regarded as occurring near the Miocene-Pliocene boundary. Major unconformities in the syn-rift sequence, in addition to the mid-Clysmic event, are the post-Nukhul, post-Kareem and post-Zeit (Beleity,

1

I

T

!i2 L

mm

FORMATIONS

rsl I

Lmtll

2 3

ZEIT

4 5 6 7 8 9 IO II

I2 I3 14 I5 I§ I7 18 (9 m et 22 aullANl 23 24 2s

cn4lTu

,a

Fig. 2. Geologic timde showing agea of symift sediment fon!natiolls in the Gulf of Suez.

1982; Webster and R.&son, 1982). As wiB be seen, these appear to correspond to changes in the extensional history of the region.

253

29”

29

32”

34”

33”

Fig. 3. Geologic and location map of the Gulf of Suez. The lighter shaded regions represent the location of pre-Neogene and the darker shading represents the location of basement

outcrop. The hatched bands indicate the position

sediments

of accommodation

zones. The circles show the locations of wells used in this study with the filled circles marking the wells shown in Fig. 4. The line locates the profile analyzed and the diamonds

locate the fission track samples utilized.

The Suez rift trends N30”-40° W, parallel to the major normal faults. An important aspect of the tilted fault blocks is their asymmetry. The dip direction of the tilted blocks and faults tend to remain constant across the rift, but alternate along strike (Moustafa, 1976). Two zones have been identified in which dip reversals occur (Moustafa, 1976; Thiebaud and Robson, 1979). In addition,

Sultan and Shtitz (1984) demonstrate major lateral offsets in the tilted blocks in the central Gulf of Suez between the October and Belayim oil fields. Their work, together with the geometry of the faults on the east, where two major border faults appear to curve and cross, suggest that this is another accommodation zone. Thus, the Gulf of Suez rift is composed of four roughly equal length

254

sets of tilted blocks with dip reversals occurring on the northern and southern accommodation zones (Fig. 3). The accommodation zones are not simple throughgoing faults, but complex zones. The southern, or Morgan zone, appears to cross the Gulf approximately perpendicularly and to be composed of numerous en echelon faults with an Aqaba trend (e.g. Webster and Ritson, 1982). The central zone, on its eastern side, is more like the accommodation zones in the East African rift with curved border faults crossing and replacing each other along strike. The northern, or Galala zone, is also oblique to the rift. It is a complex transition from sets of northeast dipping blocks to sets of southwest dipping blocks. Robson (1971) and Garfunkel and Bartov (1977) show the complex arrangement of block dips on land for the eastern side of the rift. We consider this change in style and obliquity of the accommodation zones along strike to be related to the increase in extension from north to south in the Gulf of Suez. Backstipping

In order to determine the tectonic subsidence in the Gulf of Suez through time, a dozen wells in the central and southern Gulf were backstripped. Because compaction corrections for shales of different composition vary widely, it is necessary to always use the local porosity data to determine compaction coefficients. Processed well logs were used to determine porosity and density versus depth for over 30 wells. Insufficient data on the detailed lithology of the sediments versus depth was available to fully determine the compositional percentages and compaction coefficients for all the lithologies present in each formation. Therefore, compaction coefficients were calculated for the average, non-evaporite, lithological composition of each formation. This is justified because the formations are defined on gross lithological grounds. Plots were made of porosity and density vs. depth for each formation (Berthelot, 1986). The differences between several of the formations were small and we grouped them into only four distinct lithologies. The pre-evaporitic sediments (Nukhul, Rudeis, Kareem), halite, gypsum/ anhydrite, and non-evaporitic younger (Relayim-

post-Zeit) sediments. The cause of the differences between the two non-evaporite, compacting lithologies was not determined, but may he due to several causes such as an increased elastic fraction due to eroding uplifts, different diagenesis in the presence of evaporitic deposits or environments, or varied composition of different water depth deposits. The anhydrite now found in the Gulf of Suez is believed to originally have been deposited as gypsym. The phase change from gypsum to anhydrite results in the expulsion of large amounts of water and a reduction of sediment volume. The depth of the gypsym-anhydrite phase change is primarily temperature controlled, but also pressure and compositionally dependent (Kern and Weisbrod, 1967). Relative to most exponentially compacting sediments, this change is abrupt. The depth of this transition in the Gulf of Suez varies from the surface to over 400 m. This phase change was included as occurring at 400 m depth and, like normal compaction, has the effect of decreasing the tectonic subsidence of the younger post-Zeit sediments and increasing tectonic subsidence of the evaporite bearing formations. The maximum possible change in tectonic subsidence due to including the gypsum-anhydrite transition is - 120 m. Figure 4 shows the results from the backstripping of several wells in the Gulf of Suez. There is great variability from well to well in the subsidence curves as is expected in an active rift zone. Still, comparison of curves from numerous locations throughout the Gulf of Suez show some consistent patterns. Although determining an exact rate of tectonic subsidence during the deposition of the Nukhul formation is difficult because the age of its lower boundary is uncertain, the subsidence rate is undoubtedly low. Subsidence rates increase abruptly during the deposition of the Rudeis formation. This transition is delineated by the post-Nnkhul unconformity (Beleity, 1982) and also corresponds to a major change in sediment lithology and of the depositional environment to open marine conditions (Garfunkel and Bartov, 1977; Beleity, 1982). The increase in subsidence rate at the beginning of the Rudeis formation did not occur universally

255

-mn-

-4Km

/

-25

-2s

-20

-2s

-20

-I5

-10

-5

0

Fig. 4. Backstripping

plots of six wells from the Gulf of Suez. The group

the heavy solid line is the tectonic

sediment

subsidence.

accumulation,

the dashed

See text for further

and the amount of increase is highly variable. This can be attributed to the varying positions of the wells on the tilting fault blocks. Wells (a) and (b) on Fig. 4 illustrate the differences in subsidence curves between wells at the high and low end of tilting fault blocks. By mid-Miocene

-10

-5

n

-1s

-10

-5

0

Age (Ma)

Age(Ma) plot, the light solid line is the observed

-IS

I

I

-20

d

time, the subsidence

rate de-

creased once again to modest levels. Commonly, there is a break in the subsidence rates at the mid-clysmic unconformity (Beleity, 1982) separating the upper and lower Rudeis formations. This may in part be due to non-deposition or erosion at exposed noses of fault blocks. Garfunkel and Bartov (1977) found a reorganization of fault

of wells are marked

by the filled circles on Fig. 3. In each

line is the compaction-corrected

basement

subsidence,

and

discussion.

blocks at this time suggesting that an actual tectonic event did occur at this time. The backstripping did not include corrections for paleowater depths. Paleobathymetric interpretations of the Rudeis formation indicate open marine conditions and deeper water depths than either earlier or later. These water depths would add directly onto the tectonic subsidence (Steckler and Watts, 1978). Thus, the rapid subsidence shown for the Early Miocene is a minimum estimate. Since shallower water depths are interpreted for the limestones of the Kareem and Belayim formations, the flattening of the subsidence during the Middle Miocene must have been even more

256

pronounced. If the larger paleobathymetric estimates are correct, then net tectonic uplift may have occurred during this period. Similarly, sealevel corrections were not included because of controversy over the amplitudes of these changes. However, even a fraction of the sea-level fall during the Serravallian shown by Haq et al. (1987) could counterbalance the extreme flattening observed in all of the tectonic subsidence curves at Belayim time (Fig. 4). Beginning with the Belayim formation, evaporites were deposited in the Gulf of Suez. This is very likely a response to the then developing uplifts bordering the Gulf of Suez restricting the access of the Gulf to the Mediterranean Sea (Steckler, 1985) together with a falling sea level. The evaporites were deposited in shallow water (Richardson and Arthur, in press). This is not surprising as evaporites can be deposited extremely rapidly (l-100 m/ky; Schreiber and Hsii, 1980) and, therefore rapidly fill any unloaded tectonic subsidence. However, if the Gulf of Suez and Red Sea was a dessicated basin similar to the Mediterranean during the Messinian, the deposition level of the evaporites may have been below the current sea level. Richardson and Arthur (in press) suggest that this was the case and that the surface of the Gulf of Suez was - 100 m below sea level. Examination of the limestones and shales which alternated with the evaporites during periods of flooding for their paleoenvironment may help to better resolve this uncertainty. The rate of tectonic subsidence during the Upper Miocene appears to increase slightly. In the southern part of the Gulf of Suez, salt tectonics and diapirism distort the tectonic subsidence curves (Fig. 4d). This results in apparent high rates of subsidence during the South Gharib formation at salt diapirs and low rates in the salt withdrawal region. Actual tectonic subsidence cannot accurately be determined from the point estimates provided by well data. The overall tectonic subsidence in many of the wells from mid-Miocene time, and all but one in the post-Miocene is very low, similar to that which would occur during the post-rift thermal cooling phase. As mentioned above, several of the wells which do show rapid Late Miocene subsicknce are

affected by salt flowage. Still, despite low subsidence rates, continued rifting cannot be excluded because any subsidence could be counterbalanced by the regional uplift. There is structural and fission track evidence of continued faulting and uplift during recent times (Garfunkel and Bartov. 1977; Kohn and Eyal, 1981). Whether this mostly vertical tectonics and isostatic adjustment or a continuing low rate of extension is unclear from the well data. Overall, three main phases can be clearly distinguished. The early phase is during the Nukhul when the extension rate and tectonic subsidence rates were low. At the beginning of the Rudeis, there was a rapid increase in subsidence as the Gulf of Suez entered its main phase of extension. During this time period, the uplift bordering the Gulf of Suez began (Steckler et al., 1987), affecting the net tectonic subsidence. From Middle Miocene onwards, the net tectonic subsidence rate was greatly decreased. The change in rate is great enough that the extension rate must have been lower than in the earlier phase, even when consideration of the uplift is taken into account. There is a large variation in subsidence from well to well due to differential movement of the tilted fault blocks during rifting. Gebel Zeit profile Backstripping of individual wells can give a qualitative picture of the major changes in extension history and the approximate timing of those changes. The variability of subsidence across the rift since the Middle Miocene, for example, precludes numerical estimates based on individual wells. In order to make quantitative estimates of the temporal history of extension, bar&&ripping of entire profiles and averaging of the subsidence histories of those profiles must be done. A profile across the Gulf of Suez through the northern end of Gebel Zeit (Fig. 3) was therefore analyzed in this manner. The cross-section was wmpiled using Abdine (1981) for the central portion of the Gulf, Webster and Ritson (1982) for the Sinai, C&&a et al. (1986) for Gebel Zeit, and a wmpilation of well data. This entire profile was then backstripped

251

Present

Top Zeit

b I

I

I

I

I

I

I

Top S. Gharib 5 E E. B

C

i

I 20

10

?

I 50

I 60

I 70

I

I

I

I

I

I

I

I

I

I

I 80

d I

Top Kareem

h

2 ‘i

I 40

Y : I

s

I 30

Top Belayim

h

$

-

Y2;--

L e

:4

I

I

a”

Top Rudeis

h

g

g

f

Top Nukhul UL

h

3

2 &

-

Y : 1

L

9

:4

a”

-

-

I

10

I

20

30

I

I 40

I

50

I

I 70

60

80

Distance (km) Fig. 5. Projected

profile

only on present-day sediments exaggeration.

from

across

profile

each

layer

Gulf of Suez (top). The cross-section

is located

are layers below heavy line. Below are successive in turn

and

decompacting

remaining

layers.

on Fig. 3. Pre-rift reconstructions Profile

is not

carbonates

and elastics

of the cross-section. paleospastically

illustrated

bacKstripping

restored.

No

the

vertical

258

Tectonic Subsidence

I

10

20

I

30

I

40

I

50

I

60

I

70

I

80

DisranGe(km) Fig. 6. a. Tectonic subsidence across the profile at each formation calculated by unloading the remaining sediments from each of the profiles in Fig. 5. b. Tectonic subsidence along the cross-section interpolated to 2 m.y. time intervals. Relative rates of subsidence through time can be more easily seen.

using the same assumptions and parameters as the indi~du~ wells. The unloading of sediments was done assuming Airy isostasy. Figures 5b-g show the reconstructed sediment thicknesses at the times of the various formation boundaries. Profiles of tectonic subsidence through time are plotted in Fig. 6. Several caveats must be noted before interpreting the subsidence history represented by this cross-section. The cross-section was not balanced nor was the extension along faults reconstructed. Thus, the gaps in the deposition of the Nukhul seen in Fig. 511;are an artifact of the method. They represent the horizontal displacements along the faults. The continuity of thickness between these gaps, in fact, suggests that the Nukhul deposition was a rather broad sag with little high relief block tilting. The fault induced artifacts are also present in the other profiles as apparent horst blocks at major faults. The profiles do, however, preserve the cross-sectional area of the subsidenee during each formation. Some salt flowage is apparent in the profiles at kilometers 43 and 52. The thickening and thinning of the South Gharib and Z&t formations do not reflect tectonic subsidence vari-

ations, but rather the salt motion. As long as the flow remains in the plane of the profde, the effect on the average tectonic subsidence will be minimal. The formation boundaries used in the backstripping are not all time-synchronous and there are major facies changes across the profile. The most significant is the change from evaporites of the South Gharib and Zeit formations in the central basin to elastics of the Abu Alaqa and Post-Zeit formations at the rift margins. Since the Miocene and post-Miocene elastics of the upper Abu Alaqa and Post-Zeit could not be separated at the rift margins, some of the Late Miocene tectonic subsidence. is incorrectly assigned to the post-Miocene. This, coupled with the generally diachrunous facies change between the South Gharib and Zeit formations and the poorly known ages associated with these formations suggest caution in interpreting any rate changes of subsidence from the Late Miocene onwards. Still, the cross-sections shown in Fig. 5 reiterate the conclusion based on the individual wells. The amount of extension which occurs during the Nukhul is smalI. There is a rapid phase of subsidence during the Rudeis followed by an inter-

259

-loot-

-1oow 5

2 c’

;=” -2000 -

-2000 -

Ew

5.

a”

B

-3000 -

-3OOw

Backstripping

-

1 -20

-4Oorl -2.5

I -IS

Geohistory

a

I -5

I -10

-4000 0

-2s

-15

-20

Age (Ma) Fig. 7. Backstripping dashed

plot calculated

line is average

heavy dashed

corrected

line shows the average

tectonic

-5

0

Age (Ma)

by integrating

compaction

-10

along profile.

Light solid line is the average

subsidence.

The heavy solid line is the average

subsidence

after correction

sediment

tectonic

thicknesses

subsidence

while the light

throu:$

time. The

for later extension.

mediate rate during deposition of the Miocene evaporites and post-Miocene. The rapid accumulation of the South Gharib salt observed in some

By integrating across the rift, it is possible to obtain the mean tectonic subsidence for the Gulf of Suez. Because extension continued throughout

of the wells can be seen to be a local phenomena related to salt movement. The subsidence pattern is further clarified by

the development of the rift, it is necessary to correct the average subsidence for subsequent thinning by extension. As a first order correction

the plots of tectonic

we use the extension

subsidence

illustrated

in Fig.

served

6. The upper plots show a reconstructed depth to backstripped basement at the time of each forma-

subsidence

needed

to produce

the ob-

with a one-dimensional

instan-

taneous extension model. This yields aterage values of p = 1.6. This is clearly inaccurate given the

tion. The compaction and backstripping corrections are most apparent in the increased thickness

effects

of Nukhul. A better understanding of the relative rates during the formation of the Gulf of Suez is

regional (Steckler,

given by Fig. 6b. In this cross-section, the tectonic subsidence has been interpolated to 2 m.y. time intervals. The relative thicknesses, therefore, re-

and uncorrected tectonic subsidence estimates are shown for the Gebel Zeit profile in Fig. 7 and Table 1. From these values, it can be dem-

flect actual tectonic subsidence rates and the points made earlier clearly stand out.

onstrated that extension definitely the Middle Miocene. The extension

TABLE

of vertical

and

lateral

heat

loss and

the

uplift due to small scale convection 1985; Buck, 1984, 1986). The corrected

continued past represented by

1

Extension Formation

of the Gulf of Suez from backstripping Depth

Age (Ma)

(m)

Depth corrected

a

Extension

b

Projected

’

(km)

(m)

Rate (km)

Nukhul

19.0

120.0

178.0

Rudeis

16.2

611.0

751.0

11.0-17.0

12.0-21.0

Kareem

15.2

690.0

826.0

12.0-19.0

13.0-23.0

0.14- 0.22

Belayim

11.3

756.0

885.0

(13.0-20.0)

(14.0-25.0)

S. Gharib Zeit

diachr. (5.1)

905.0 1038.0

1010.0 1112.0

(16.0-22.0)

(18.0-29.0)

(0.03- 0.07) _

(18.0-26.0)

(20.0-32.0)

(0.08-0.24)

Post-Zeit

0.0

1275.0

1275.0

32.0-36.0

(0.0660.27)

* Adjusted ’ Minimum recorded ’ Extension

for thinning assumes

4.0

29.0

2.0-

5.0

0.04- 0.09 0.33- 0.62

of layer by later extension.

instantaneous

in sediments.

3.0-

extension

Values in parentheses

at 27.5 o N triple junction

location.

and regional

uplift postdates

are considered

less reliable.

layer. Maximum

assumes

uplift proportional

to extension

260

Distance (km) Fig. 8. Profile across the Gulf of Suez and its rift shoulders showing tqxqraphy subsidence. The dotted lines represent the reconstruction

and sediment thickness. The thin line is the tectonic

of the tectonic uplift, corrected for erosion and the concommitant

&static

rebound.

the tectonic subsidence recorded in the Miocene elastics is only 12 km and is insufficient to account for the subsequent subsidence as due solely to post-rift cooling. In order to obtain more accurate values of the extension from the tectonic subsidence and take into account the effect of regional uplift, it is necessary to separate the components which comprise the vertical movements. These are uplift due to heating of the lithosphere and subsidence due to crustal thinning. The estimates made above linked these two amounts by assuming uniform instantaneous extension of the lithosphere. As pointed out above, the thermal component is affected by many additional, time-dependent factors. We therefore wish to estimate the extension by utilizing only the subsidence due to crustal thining. By using the regional shape of the rift flank uplift, it is possible to estimate the present heating at the rift independent of models, as was done in Steckler (1985). In this manner, a more accurate value for the crustal thinning and extension can be obtained. The uplift bordering the cross-section of the Gulf of Suez is shown in Fig. 8. This uplift in Sinai is due, not only to the Gulf of Suez rifting,

but also to the Gulf of Aqaba and Red Sea tectonics. Furthermore, the uplift is modified by dynamic effects such as small-scale convection and by conductive cooling, so that it does not directly reflect the total extension. We will therefore not try to use the uplift to estimate the thermal effect of the Gulf of Suez rifting. Instead, the uplift, after correcting for erosion, will be used as a baseline to determine the total subsidence resulting from crustal thinning. The extreme elevations of the mountains in Sinai are due to a regional response to erosion. As described by Garfunkel (1988), dissection by erosion lowers the average elevation of an area, but uneroded peaks undergo uplift caused by the regional isostatic response to the denudation. The maximum height of peaks is equal to the tectonic uplift plus the isostatic uplift. We assume the central high mountains of Sinai between the Gulf of Suez and Aqaba faults act as a block responding regionally to erosion. The average elevation of the dissected terrain along the profile is 1360 m. Fission track ages of 107 and 26.6 Ma at 720 m and 1250 m elevation close to the profile (Kahn and Eyal, 1981) suggest between 750 and 1100 m of now eroded basement, in addition to 650 m of

261

sediments

(Steen,

1982; Garfunkel,

1988)

overlay

gional

uplift,

the net subsidence

peak. Adopting

densities

of 2.4, 2.78,

therefore

reflects

and 3.33 for the sediments,

basement

and mantle,

extension.

Thus the additional

the highest

respectively, the tectonic uplift in the Sinai is - 1700 _t 100 m. This is in good agreement with

the regional

the

difficulty

pattern

of values

(1988). On the Eastern more difficult

estimated

Desert

is

This introduces

response

and ero-

an asymmetry

to the erosion.

not averaged,

duce an underestimate

the uplift

to

This profile

since that would

of the amount

pro-

of erosion

at

the border fault. Instead, a number of individual points were reconstructed using an Airy response to the erosion and a smooth curve fitted to them. The data points were taken from along the top of the uneroded Eocene section, the sediment-basement contact, and two nearby basement sites with fission track ages (Omar,

1985). The curve projects

to - 1100 m elevation at the border fault (Fig. 8). Preliminary results from recent fission track sampling Eastern Desert basement (Steckler et al., 1987; Omar et al., in prep.) support the estimates made here. Additional fission track work is also being done on the mountains of Sinai (B. Kohn and G. Omar, pers. commun., 1986). We estimate the average thermal uplift over the Gulf of Suez rift itself to lie between the uplift in Sinai and the average of the Sinai and Eastern Desert needed

uplifts. Calculating to account for

the crustal thinning subsidence from this

elevated surface to the backstripped subsidence in the Gulf of Suez, assuming

loss

uplift

basement an initial

about

3/4

of the

total

heat represented

conductive

is the uncertainty

A major

in the timing not

of the

uplift.

significant

relief in the Gulf of Suez until after the event (Garfunkel

If the regional

uplift

appear

contribution

the extension.

from

develop

the Gulf

only after

to be

and Bartov,

is due to induced

1977).

small-scale

(Buck, 1984, 1986; Steckler,

it will lag behind would

does

by the

additional

convection

There

cooling.

There

of Aqaba,

its initiation

1985) then is also a but

this

and prob-

ably not until it became transtensional We have therefore calculated the extension history of the Gulf of Suez using two assumptions (Table 1). The first is that the entire uplift postdates the formation of concern. the extension

This yields equal

a minimun

to that calculated

value lsing

for only

the observed subsidence. The second assumption is that the uplift is proportional to the extension recorded in the subsidence. This does not allow for any time lag in the uplift and is therefore an overestimate. The actual extension history is likely close to the minimum initially, and thereafter approaches the maximum estimate. Further theoretical work on small-scale convection and better estimates of the uplift/erosion history are needed to refine these estimates. The results, given in Table 1 and projected to the triple junction position, indicate a few kilometers of slow extension during the Nukhul, the most through the Rudeis, encompassing opening

of the Gulf

slightly

mate 32 + 4 km of extension at the junction. Geometric considerations suggest the higher end of the range is probably the more accurate. We must now reestimate the extension through

the kinematic history of rifting. the faults which control the Neogene basins are similar to basement structures. The Gulf Gulf of Aqaba are dominated ented NW-SE and N-S to

variable

rapid extension almost half the

of Suez, and a nel. slow and

crustal thickness of 40 km, yields an average of 40% thinning. This corresponds to an average p of 1.7 and 27-31 km of extension. A smaller initial crustal thickness would yield greater values for the extension. Projecting this value and the estimate by Steckler (1985) south to the triple junction location (Martinez and Cochran, 1988) we esti-

time represented by the subsidence, taking into account the effect of the uplift. Due to the re-

and

more than counterbalances

through

mid-Clysmic

to make. The uplift is fault-bounded

sion are greatest. was therefore

Garfunkel

side, the correction

on one side only, where both the isostatic

by

heat

only

was reduced

continued

extension

thereafter.

Direction of the extension In addition to the amount and timing of extension of the Gulf of Suez. determinaton of the direction of opening is critical to reconstructing The directions geometry of those seen in of Suez and by fractures NNE--SSW.

of the the the oriBy

1

a

b

Fig. 9. Maps showing the locations

of the microtectonic

been sorted into three distinct sets of directions

measurements

and the stress directions

found for each site. The results have

and ages.

analyzing the slickensides on these faults, the stress field responsible for the deformation of interest can be determined. For each population of faults measured at a site, we have applied the method of right dihedrals (Angelier and Mechler, 1977)

These clusters were then dated using the few sites where dating is possible. We note that the results obtained in this manner are temporally consistent. The Lower Miocene sediments of the Lower Rude& formation, at two sites in the region of

and/or the calculation

Hurghada and of Safaga, display synsedimentary tectonics in conglomerates. Faults having throws

of a tensor of constraints

(Angelier et al., 1982). Using these methods, we have been able to obtain constraints on the stress field active at the time of motion along the faults. Details on the observations, site locations and analysis are given in the accompanying paper by Lyberis (this vol.). Control on the timing of the fault movements is difficult to obtain in the Gulf of Suez. Few of the fault faces containing slickensides also yield sufficient constraints on the age of the fault activity. We presume that there is a consistent regional stress direction. Furthermore, we suggest that the clustering of the stress directions, obtained from the microstructures, about a few consistent directions reflects these regional stresses and not local perturbations. We have therefore sorted the measurement sites according to direction (Fig. 9).

of several meters which affect only the Abu Zenima formation were also observed. All of these faults have, for the most part, a N-S direction. The movements along the length of these faults are left-lateral with a normal component The analysis of populations of fractures showed that they were produced by a N15°-N400 extension direction. Outside of these three sites, where the faulting can be tied to the basal synrift formations, all the other faulting yielding this direction involves only the Mesozoic and Eocene formations. Nevertheless, these faults are clearly related to the formation of the Gulf of Suez. These solutions are often found on the bounding faults of the major subbasins of the rift. For example, the major and satellites faults bordering Gebel Zeit and the Abu

263

Rudeis plain, are produced by NNE-SSW extension. Figure 9a presents the sites where faults

border throw

yielded

attributed

this direction

of extension.

The N15 O-N40 o direction been found

in faults affecting

than the Lower Rudeis. sides related

has not

formations

In two locations,

to this phase of motion

by striae associated Miocene.

of extension

with motions

We thus attribute

Bartov,

to the Abu Alaqa group (Garfunkel 1977).

younger

show activity

slicken-

sinestral

were overlain

dated

of the Quaternary Abu Rudeis plain have a of several kilometers affecting sediments

as Middle

this phase an age prior

The

slickensides

in two phases

dip slip movement.

phase of motion extensional

of these

where

the first is a

The analysis

yields an NE-SW

and faults of this

to ENE-WSW

stress.

This activity sediments evaporitic

was available

to de-

the extension

event.

tures of the Gulf of Suez. This is why faults of the

However, comparing these results with the individual subsidence curves (Fig. 4), we find that the first phase of rapid subsidence in the Gulf of Suez correlates with this period. This NNE-SSW direction of extension is not

clysmic direction have a purely normal movement whereas the N-S faults have a left-lateral dip-slip movement. It should be noted that this second phase of motion is found only in the Gulf of Suez. We have not observed it on the Egyptian coast of

limited

the Red Sea south of Safaga.

(Fig. termine

event”

In the average

7) insufficient a data point

dating

(Garfunkel

at the “mid-Clysmic”

solely to the Gulf of Suez. Observations

on

formations.

least

the

Miocene

and

“mid-Clysmic 1977).

at

the Middle

curve

the

Bartov,

to

affects

subsidence

to

up

thus

uppermost

pre-

In the course of this phase,

is perpendicular

to the m.ijor struc-

As described

above,

the Egyptian coast of the Red Sea, from Safaga south to Ras Benas, have shown that this exten-

successive reactivation on the same fault planes confirms that this ENE-WSW phase of extension

sion direction is present throughout the entire region. This is the only direction found in this

corresponds

region of the Red Sea and is in agreement with the N30 o -40 o E estimated for the total opening direc-

subsidence during the Middle Miocene. At some places in the Gulf of Suez, we found

tion of the northern Red Sea (Co&ran, 1983; Joffe and Garfunkel, 1987). A second set of faults in the Gulf of Suez yield ENE-WSW extension directions (Fig. 9b). The

evidence of another stress field distinct described above (Fig. SC). We observed

crests of major fault blocks of the Gulf of Suez, such as Gebel Zeit and Gebel Esh el Mellaha, contain calcareous reefal deposits which cover slickensides on the faulted surface of the tilt block. The population

of faults

measured

in this setting

are pure normal faults which result from an extension direction averaging N60° E. The overlying

succeeds

the NNE-SSW

(Rouchy et al., 1983). However, the contacts between the basement and the carbonates of these border faults also show evidence of activity during

Kinematic reconstruction

carbonates

are

dated as Langhian (Rouchy et al., 1983). On the eastern coast of the Gulf of Suez, deformation by normal faults, some including the upper part of the Gharandal group, are sealed by the sediments attributed to the Ras Malaab (evaporite) group. Elsewhere, the N-S faults which form the eastern

of Suez

irom those .i family of

of extension, constitute the most recent deformation observed. This stress trajectory ha:; also been observed in the Gulf of Aqaba. The age of this phase is unknown

of the reef. These

of the Gulf

striations which cut the preceding and indicate nearly pure E-W extension. For example, the N-S section of the border fault of the Abu Rudeis plain exhibits, superimposed over the striae just described, a family of striations with a pitch of 90 O. These cases, which document an EI-W phase

reefs were formed on the already faulted slope of this range, prior to the deposition of the evaporites

the formation

phase. This second phase

to the slowing

We now investigation

but it appears

to be recent.

attempt to combine results of this of the Gulf of Suez with i:nformation

about the other two arms of the Sinai triple junction into a consistent history of motion. Joffe and Garfunkel (1987) have shown that simultaneous consideration of the several interrelated rifts provides much tighter constraints about the kinematics than does analysis of individual rifts. They

264

give an excellent summary of the limits on the acceptable poles of opening of the Arabian-African rift system. However, they only distinguished the total and recent (O-5 Ma) poles. Using the subsidence data from the Gulf of Suez and microtectonic data, we attempt to separate the earlier phases of motion. We begin by briefly summarizing what is known of the kinematics of each rift. The Red Sea is the major zone of separation between Africa and Arabia. Due to the lack of transforms, the Red Sea pole of opening is poorly known independent of the regional constraints. Still, most determinations of both total and recent opening poles (Laughton et al., 1970; Freund et al., 1970; Girdler and Darracott, 1972; Izzeldin, 1982; Joffe and Garfunkel, 1987) yield a location near the Cyrenaica platform of Libya. Poles farther north (e.g. McKenzie et al., 1970) are inconsistent with the regional geology (Joffe and Garfunkel, 1987). These observations indicate a relatively constant N30°-N35O opening direction for the northernmost Red Sea throughout its entire rifting history. This direction of opening coincides with the single extensional stress direction for the northern Red Sea found by microtectonic analysis. In the southern Red Sea, seafloor spreading, and associated magnetic anomalies, constrain the opening rate for the last 5 Ma (Laughton et al., 1970). Consideration of the Gulf of Aden magnetits further indicates that there have been no major changes in the opening rate or pole for lo-12 Ma (Co&ran, 1983; Joffe and Garfunkel, 1987). Thus, two-stage models of the Red Sea opening which incorporate quiescence during the Late Miocene evaporite deposition (Hempton, 1987; Voggenreiter and Altherr, 1987; Girdler and Southern, 1987) are untenable. The Gulf of Aqaba-Dead Sea transform has undergone 105 km of left-lateral motion since its inception between 19 Ma (Eyal et al., 1981) and 15.5-11.5 Ma (Steinitz et al., 1978). This motion has taken place in two stages (Freund et al., 1970), an early phase of pure transform motion and a later transtensional stage. The change, which created the pull-apart basins of the Dead Sea and Gulf of Aqaba occurs at the Miocene-Pliocene boundary or later. Garfunkel(l981) has calculated pole positions for these two stages, which shows

that there was only a slight change of direction. Joffe and Garfunkel (1987) also suggest that the change in relative motion was minor and add that the second phase only encompasses - 30 km of motion, less than previous estimates. Although the Gulf of Aqaba has been primarily a transverse boundary and the Red Sea primarily extensional, the opening directions during either phase could not have differed by more than 10’ from the Red Sea direction and be consistent with the amount of extension of the Gulf of Suez. The differences in opening direction for the two phases in the Gulf of Aqaba are thus below the resolution of microstructural analyses. These analyses yield abundant evidence along the Gulf of Aqaba of both strike-slip and pure extension due to NNE-SSW extensional stresses (Lyberis, this vol.). As in the Gulf of Suez, a young E-W extensional stress field was also detected. The Gulf of Suez shows the least offset of the three, probably about 32-36 km at the triple junction. The first phase of motion is represented by the slow subsidence during the Nukhul, prior to the initiation of the Dead Sea transform. Projected to 27.5” N, this is only 3-4 km of extension. This period is still transitional between the Syrian arc NW-SE compressional stress state and the developing NNE-SSW extension, and as such shows only minor opening (Garfunkel and Bartov, 1977). During this time the Gulf of Suez is the sole northward continuation of the Red Sea. The first set of Gulf of Suez stress determinations is consistent with this scenario, indicating colinear opening of the two rifts (Fig. 9). The Rudeis formation documents a rapid increase in the rate of extension. The rate during this time is almost as large as the current northern Red Sea opening rate (Joffe and Garfunkel, 1987). This and the observation of the N30 o E stress field in the Lower Rudeis sediments suggest the Gulf of Suez was still the primary continuation of the Red Sea. The Rudeis formation, following the postNukhul unconformity, marks the beginning of significant extension in the Red Sea system. At some point during this period, the Dead Sea transform probably began to form. The subsidence rate in the Gulf of Suez slowed by Kareem time, probably starting at the rnid-

265

Clysmic tion

event. The stress analyses

of the

Closure

opening

direction

of the velocity

tion requires had begun together

towards

up the motion

The large decrease

cates that the Gulf of Aqaba plate boundary.

sion continued, northward

N60” E.

between

component

on the transform

Arabia

in subsidence direction

had become

rate indi-

the main

The Gulf of Suez exten-

but at a much

slower rate as the

of separation

boundary

related

was taken

up

(Fig. 9).

During subsidence

the time of Belayim deposition, the rate in the Gulf of Suez becomes a

minimum.

The rate is low enough

that it could be

to the regional

parts of the Arabian The

at the triple junc-

with the change in opening

northern

a rota-

that by this time, the Gulf of Aqaba taking

and Africa.

triangle

indicate

post-Miocene

leaky oblique-slip the other hand, the rift system There

events

is poor

section

switch

clearly around

during

strike-slip

to on

a modification

of

the Sinai

triple junction.

on the post,.evaporite

of changes

in the Gulf of

this time are little better than

above.

However,

the Gulf of Aqaba

suggests

the Suez extension

rate. Courtillot

its direction.

the ex:ension

a further

found that the present motion must be less than 1 mm/yr, termine

in other

Sea transform,

indicates

age control

those discussed

from

on the Dead

and estimations

Suez opening

occurring

plate.

in

reduction

in

et al. :1987a, b)

of the Gulf of Suez but cou1.d not de-

Nevertheless,

they favored

accounted for solely by thermal subsidence. A pause or slowdown in the Red Sea rifting during

model with a component of right-lateral extension. This would be consistent with the E-W directed

the Serravallian would correlate in the Bitlis suture (Hempton,

(1987)

with uncertainties

with the collision 1987). However,

in ages and the regional

uplift

rate, and especially the size of the coeval sea level drop, this cannot be confirmed. Reconciling a stop in the Red Sea rifting with the constraints on rates and amount of opening for the branches would be

normal

faulting

observed.

on the other

hand,

terns at the northern end indicate left-lateral motion,

Joffe argue

and that

a

Sarfunkel fault

pat-

of the Gulf of Suez and determned poles

yielding NNE opening. This is in agreement with the magnitude 6.5, 1969 earthquake off Shadwan Island which indicated almost pure normal fault-

difficult. The subsidence rate also shows a slight acceleration during the main evaporite depositional period (Berthelot, 1986). One possibility is

ing at N30 o E (Huang and Solomon, 1987). Figure 10 summarizes the proposed scenario

that these changes, if real, may reflect minor adjustments in partitioning of the Red Sea extension

Sea-Gulf of Aqaba system. Schematic “snapshots” of the rifts at critical times are depicted in Fig. 11.

between

One

the Gulfs

of Suez and

Aqaba,

perhaps

for

the

evolution

possible

of

solution

the

Gulf

of

for directions,

Suez-Red

rates,

and

amounts of movement for each of the three arms of the system is given in Table 2. The direction of the Red Sea extension

has been

assumed

stable.

The results suggest that the Red Sea, after an initially slow start, rapidly increased its rate of opening which has been relatively constant since the Middle Miocene. The rate of Red Sea opening during the upper Burdigalian is slightly less than later on, but no less than half the current rate. The Gulf of Suez started as the northward continuation of the Red Sea, but soon after was mostly succeeded by the Gulf of Aqaba. This is similar to

N”kh”l

Fig.

Rudels

10. Cartoon

BeI

illustrating

tween rifting and the directions Velocity

triangles

evolution

of the rifts.

are shown

.ZWl

the geometric of opening

Port-ze,,

relationships

be-

in the Sinai region.

for the different

phases

in the

the evolution proposed by Steckler and ten Brink (1986) except that the subsidence data indicates that slow extension of the Gulf of Suez has been sustained. The Gulf of Aqaba-Dead Sea system originated during the Rudeis, possibly at the midclysmic event. However, it accommodated rela-

266

END. MlOCENE

1

Fig. 11. Schematicreconstructionof the rifts and plates in the region of investigationat several times throughouttheir development.

tively little motion (O-10 km) until after the Rudeis. Kareem time may be transitional between the two and the post-Kareem unconformity (Beleity, 1982) may mark the end of the readjustment. The lowered subsidence rates, as well as the develaping uplift (SteckIer, 1985), may have contributed to the restricted environments favoring

evaporite deposition for the rest of the Miocene. Since the Belayim, Aqaba has been the primary plate boundary, accommodating 4-8 times more of the African-Arabian separation than Suez. Garfunkel (1981) has shown that the postMiocene change in direction at the Gulf of Aqaba was relatively minor. Its main effect was to intro-

261

TABLE

2

Evolution

of motion

Formation

at Sinai triple junction Gulf of Suez

Time period

Northern

Red Sea

Gulf of Aqaba

distance

direction

rate

distance

direction

rate

distance

direction

rate

(km)

Co)

(cm/W

(km)

(“)

(cm/v)

(km)

to)

(cm/v)

kO.1

Nukhul

Aquit.-E.

4.0

32

0.07-0.1

4.0

32

0.07-0.1

Rudeis

L. Burdigalian

13.0

38

0.44

18.0

32

0.62

5.0

26

Kareem-Zeit

Mid.-Late

14.0

59

0.13

80.0

32

0.73

67.0

26

0.61

Post-Zeit

Plio.-Pleist.

32-90

0.08

37.0

32

0.73

35.0

29

0.69

139.0

32

107.0

27

Burd. Mio.

Total motion

4.0 34.0

50

-

duce a component of extension into the transform creating several basins along its length. The direc-

The post-Miocene is poorly constrained.

tion of motion along Aqaba cannot be parallel to the E-W directed stress tensors as suggested by

direction and amount. If the direction is easterly. then it must be small (I 3 km) and the Aqaba and

Mart and Rabinowitz (1986). E-W directed extensional stress has also been observed farther north

Red Sea rates are very close. If the current opening is subparallel to the other rifts, then the Mid-

in Israel (Eyal and Reches,

dle-Late

1983; Ron and Eyal,

Miocene

extension in the Gulf of Suez There is a tradeoff between

rate could

be maintained.

This

1985). It appears to be a local stress field associated with the transform. If the transform is now a

second possibility again leads to the conclusion that the young E-W extensional stress field does

zone of weakness, given the current extension across it, then the least principal stress would be expected to rotate perpendicular to the fault. Thus, the E-W normal faulting does not reflect the regional stress field, but a local perturbation in the vicinity of the major transform zone. Lacking any

not reflect the current motion unless recent change with negligible offset.

other evidence substantiating a different motion, we accept the kinematic and field evidence of leaky strike-slip movement (Garfunkel, 1981). If we accept this explanation for the direction of stress field near Aqaba, then we must consider whether

it also affects the other slickenside

results.

this is a very

Summary and conclusions Subsidence analysis of the Gulf of Suez provides important constraints of the opening history of this rift. Backstripping of individual wells, while yielding qualitative data on tectonic subsidence, displays a large amount of scatter. Constructing and backstripping cross-sections perpendicular to the rift produces a better overview of the tectonic

Could the ENE extensional stresses in Suez also reflect a local rotation perpendicular to the rift? We note that there are mechanical differences

movements in the rift. However, because structed cross-sections may not have the resolution as well data, analysis of individual

between extensional rifts incorporating movement on low-angle detachments, which would allow for

are still needed to obtain the timing of events. Integrating across these cross-sections i.; a method of obtaining quantitative estimates of extension.

stress continuity across rifts (Bell et al., 1988) and near-vertical strike-slip boundaries, which under extension may act more like a free surface. The microtectonic analyses of the earlier motion in the Gulf of Aqaba, prior to the initiation of pull-apart basins, do exhibit stress fields consistent with the direction of motion. We therefore conclude that the ENE-WSW solutions in the Gulf of Suez do reflect the regional stress field.

consame wells

In actively extending regions, such as the Gulf of Suez, it is necessary to correct for the thinning of the sedimentary layers by later extension. The substantial uplift at the Gulf of Suez rift decreases the net tectonic subsidence. In order to obtain accurate values of extension, it is necessary to correct for the additional uplift. Reconstructing the erosion of the rift flanks is important for

268

distinguishing between the current topography, the tectonic uplift, and the structural uplift. In the southern Gulf, the uplift in excess of that produced by uniform extension decreased by - l/4 the estimated extension. The two-dimensional backstripping results confirmed the rapid subsidence and extension rate during the Rudeis formation (late Burdigalian). One-third to one-half of the extension of the Gulf of Suez occurred during this relatively short period. A low average rate of extension was found for the Middle Miocene to the present, but facies changes and uncertainties about the uplift history limited the resolution of variations during this time period. There was slight evidence for a slowdown or pause during the Serravallian. The early opening during the Nukhul ( - Early Miocene) reflects an initial startup phase with only minor amounts of extension. Stress directions obtained from the analysis of slickensides show several stress trajectories. The sparse age control indicates consistency with the phases identified in the subsidence data. A rotation of the direction of extension in the Gulf of Suez from NNE to NE and perhaps to E is suggested. This and the changes in extension rate confirm a progressive shift in rifting from the Gulf of Suez to the Gulf of Aqaba. This shift was not a simple replacement as proposed by Tamsett (1984), rather it has continued for 13-17 m.y. and appears to adjust discontinuously. Once rifting started, the Gulf of Suez became a local zone of weakness and has persisted in accommodating some of the extension between Africa and Arabia, even though the Gulf of Aqaba may be the preferred location for the northward continuation of the Red Sea (Steckler and ten Brink, 1986).

Acknowbtgements

We would like to thank AMOCO, GUPCO and TOTAL CFP for their generous release of data. M. Steckler would like to thank the &c&e Normale Superieure for support in France and 3. Bott for assistance in processing the cross-section. This paper was improved by reviews from W.R. Buck and J.R. Co&ran. This research was supported by

National Science Foundation grants OC’E-83 09983 and OCE-86-10213. L-DGO contribution number 4332.

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