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The timing of magmatism, uplift and crustal extension: preliminary
observations from Yemen
M.
A.
M E N Z I E S 1, J.
A. HURFORD
B A K E R 1, D .
2, M. A L ' K A D A S I
B O S E N C E 1, C.
D A R T 1, I.
D A V I S O N 1,
1, K. M c C L A Y x, G . N I C H O L S 1, A . A L ' S U B B A R Y
& A. YELLAND 2
1
lDepartment of Geology, Royal Holloway and Bedford New College, University of
London, Egham, Surrey TW20 OEX, UK
ZDepartment of Geology, University College, Gower Street, London WC1E 6BT, UK
Al~traet: The Red Sea and the Gulf of Aden form young, oceanic rift basins, situated between the diverging African and Arabian plates and bordered by highly elevated, volcanic
margins. Yemen in the southeastern Red Sea, was once centred over the Afar plume/
triple-junction (c. 30 Ma) forming part of the Arabian 'passive' margin. The present high
elevation of the Afro-Arabian rift-flanks (up to 3.6 km as in Yemen), is the combined result
of a number of endogenic rift processes which served to generate both the initial crustal uplift and also preserve the elevated topography. A further isostatic response generating uplift
is likely to have been driven by differential erosion of the rift-flanks. However, the sedimentary record of the pre-Jurassic to early Tertiary period provides little evidence for major
changes in relief or elevation. Furthermore, structural and volcanological observations indicate that most of the crustal extension occurred during mid-late Tertiary. The voluminous
Oligo-Miocene basalt-rhyolite magmatism of Yemen was not apparently associated with
pre-volcanic (> 30 Ma) uplift despite the commonly held belief that the Afar plume existed
beneath the region 30 Ma ago. Geological data point to an episode of uplift that occurred
after the initiation of magmatism. Fission track data indicate that uplift related exhumation
postdates magmatism by some 10-15 Ma, perhaps the amount of time needed to change the
thermal character of the Pan-African lithosphere "above the Afar plume. A sequence of
magmatism followed by synchronous crustal extension and uplift for Yemen does not fit
with the traditional categories of active (uplift-magmatism-rifting) and passive (riftinguplift-magmatism) rifting. Clearly such end-member models do not simply apply to the Red
Sea or the Great Basin of the western USA where a period of tectonic quiescence, followed
by post-volcanic extension and uplift (1 km), post-dated the Oligo-Miocene ignimbrite
flare-up.
The relative timing of surface uplift, magmatism
and extension is thought to be pivotal to understanding whether the mantle was a passive or
active participant in rift formation (Seng6r &
Burke 1978). The passive model requires that
extension predates any uplift and magmatism,
while in the active model uplift predates magmatism and extension. However, observations
of rift margin sequences, particularly in the Red
Sea, show that rift formation is in practice more
complex. It has long been recognized that the
triple-junction structure of active rifts is strongly
associated with domal surface uplift and volcanism (Cloos 1939). Receiit theoretical considerations (McKenzie & Bickle 1988; White &
McKenzie 1989; Houseman 1990; Farnetani &
Richards 1991) indicate that mantle, plumes or
hot spots are inextricably linked to the rapid
effusion of continental flood basalts. The generation of large volumes (c. 2 x 106 km 3) of magma
involved in flood volcanism requires superposition of rifting on anomalously hot mantle (i.e.
plumes, > 1380°C). According to these models,
based on swells in oceanic lithosphere, considerable amounts of uplift of the order of 1-2 km at
the plume centre are expected to pre-date magmatism and rifting, and because of the lateral
dimensions of plumes, uplift is expected to have
some effect up to 1500-2000 km radius from the
plume centre. Houseman (1990) further suggests that, at triple-junctions where extension is
followed by the separation of a failed arm and
two-armed passive margin (e.g. Ethiopia and
southern Red Sea/Gulf of Aden), the point of
inflection between the two arms of the passive
margin (e.g. Yemen) ' . . . should be associated
FromSTOREY,B. C., ALABASTER,T. & PANKHURST,R. J. (eds), 1992, Magmatismand the Causes
of ContinentalBreak-up, Geological Society Special Publication No. 68, pp. 293-304.
293
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294
M.A. MENZIES E T A L .
with the most intense pre-break-up uplift and
volcanism and the earliest initiation of rifting'.
Such theoretical considerations have important
implications for the relative timing of surface uplift, magmatism and extension in flood basalt
provinces as they imply that plume involvement
will trigger significant surface uplift prior to magmatism and finally extension.
This paper summarizes recent geological observations from Yemen and utilizes these data to
constrain the timing and amount of uplift, magmatism and crustal extension. Preliminary fission track data from Yemen are compared with
the fission track data from the Sinai Peninsula,
eastern Egypt and Saudi Arabia in an attempt to
evaluate the appropriateness of active and passive models of tiffing.
20,
30'
Yemen
Excellent exposure exists along the Yemen riftflank of the southern Red Sea (Fig. 1) due to 3.5
km of relief. This offers a unique opportunity to
study the detailed geological relationships between magmatism, sedimentation and tectonics
within the framework of uplift and subsidence of
the Arabian rift margin. The exposed lithologies
bracket a considerable period of time from PanAfrican (c. 500-900 Ma) basement through
Mesozoic to Tertiary sediments and > 2 km
thickness of Tertiary to Recent flood volcanism.
The inter-relationships of these lithologies record distinct phases of tectonics, sedimentation
and volcanicity which may be used to elucidate
the particular nature of the rift process. In conjunction with an on-going programme of K-Ar
40.
So °
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EURASIAN
PLATE
TURKISH
40"
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:::::::Mediterrar
•~::::
Sea
20"
30'
ARABIAN
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/
20"
20"
YEMEN
AFRICAN
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r
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::iiiiiiiiii!!iiiiiiii!:iiii:iiiililili!iiiiiiii!iiii:ii:iiiiii:i
50 °
Fig. 1. Tectonic setting of the Red Sea. Note the northward movement of the African and Arabian plates and the
possible 'sphere of influence' of the Afar plume (White & McKenzie 1989). The Yemen and Ethiopian flood
volcanics are located above the postulated plume.
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MAGMATISM, UPLIFT AND EXTENSION
age determinations in the Yemen volcanics (e.g.
Menzies et al. 1990; Huchon et al, 1992), the
existence of pre-, syn-, and post-volcanic
structure can help constrain the temporal relation between tiffing and volcanism.
Sedimentologieal studies (Al'Subbary &
NiehoUs 1991; Bosenee et al. 1992) indicate that
the Kholan Formation (of unknown age), which
immediately overlies the Pan-African basement,
is up to 200 m thick. These sediments represent
a transition from continental sedimentation to a
shallow-marine environment and pass upwards
into a succession of carbonates that reach a
thickness of 400 m (Amran Formation of CaUovian to Kimmeridgian age). The Amran carbonates
eventually became emergent and the siliciclastic
Tawilah Formation, of Cretaceous to Palaeocene age, was deposited on an eroded Amran
surface. This is thought to be a major sequence
boundary where shallow marine sediments onlap a shoreline of cemented Amran limestones.
Within the Tawilah sandstones there is evidence
for shallow-marine sedimentation but the bulk
of the formation is a sequence of braided fluvial
channel deposits interbedded with palaeosols
developed on overbank deposits. The thickness
of the formation appears to be relatively constant at around 400 m across the traverse. Eventually a transition occurs between the Tawilah
Formation and the overlying volcaniclastic
units. In this transition a variety of lithologies
occur including terrestrial or shallow-marine
sandstones, lateritic palaeosols, shallow-marine
sediments and volcanic rocks. Here gastropodrich horizons, presumed to be of shallow marine
origin, are exposed at different altitudes ranging
from 900-2400 m, implying 1.0-2.5 km of uplift
after the onset of the overlying magmatism (30
Ma). Lateritic disconformities also occur at the
sediment-basalt contact and are widespread in
Yemen as they are in Saudi Arabia (Camp &
Roobol 1989) and elsewhere. The presence of
marine sediments above these laterites and the
lack of evidence for erosion indicates that at
this time (30 Ma) uplift was minimal. Also,
the lack of angular unconformities between the
sediments and overlying basalts can be used as
evidence that crustal extension and possibly uplift did not significantly pre-date magmatism.
However, it should be remembered that if uplift
is distributed over a broad area (1000-2000 km)
around the plume head without concomitant extension it will not necessarily be associated with
significant breaks in the sedimentary succession.
In contrast, if uplift had occurred synchronously
with magmatism and extension then clastic
sediments shed from any uplifted region should
appear in the sedimentary record and palaeocur-
295
rent directions may be expected to radiate from
the updomed region. Theoretically, the unroofing sequence preserved in the sediments
should record the inverse of the present stratigraphy. This was not observed anywhere within
the traverse and only one basaltic pebble (Tertiary?) was found within a conglomeratic horizon in the Tawilah Formation. Consequently
there is no evidence in the Jurassic to Tertiary
sedimentary record for pre-volcanic uplift with
associated erosional unroofing of older rocks
and their involvement in sedimentary processes.
Any elevation change within these sedimentary
units can be measured a t most in a few tens of
metres, and not in kilometres as would be required with significant pre-volcanic uplift and
extension above a plume.
It is vital to determine the timing of crustal extension and to ascertain whether it predates, is
synchronous with, or postdates the volcanic
rocks. While structural investigations (McClay
et al. 1991) in the plaform stratigraphy indicate
little or no evidence for widespread pre-volcanic
structure ( > 30 Ma) there is evidence for minor
amounts of crustal extension, in the form of
block faulting, within the upper part of the volcanic sequence (25-20 Ma) and considerable
amounts of crustal extension that post-dates the
eruption of the volcanic rock units ( < 20 Ma).
Pre-volcanic extensional structures (> 30 Ma)
as a result of rifting or uplift would be apparent
as angular unconformities between basal volcanic units and underlying lithologies, basal conglomerates or breccias, and control of basement
fault blocks on the distribution of volcanic units.
No marked angular conformities were observed
at the sediment-volcanic contact throughout the
study area in Yemen (see also Menzies et al.
1990) and no evidence was found, in the underlying lithologies, for faulting that dies out upwards. All of the pre-volcanic sediments have
been rotated by the same amount indicating that
they were deposited before the main episode of
extensional faulting. Syn-volcanic crustal extension (30-20 Ma) would result in angular unconformities between volcanic units, fanning dips,
sedimentary deposits within the volcanic pile,
significant lateral variations in the thickness of
ash flows within and between adjacent fault
blocks and limitations on the lateral extent of ash
flows due to topographic highs. Possible synvolcanic extension was observed within the
Yemen volcanics at one locality where an angular unconformity may occur within the uppermost volcanic units ( 4 25 Ma). Here, the upper
volcanic units may have been rotated 20 degrees
less than the underlying sediments suggesting
that the upper volcanics were erupted during ex-
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296
M.A. MENZIES E T A L .
tensional faulting. However, this is difficult to
evaluate since the section studied is in close
proximity to a granitic intrusion which may have
disrupted the adjacent volcanic rocks. If late
syn-volcanic extension did occur it may be related to the reported change in the extensional
stress field from E - W ( > 22 Ma) to N - S ( < 22
Ma) (Huchon etal. 1992). It is important to point
out that Huchon et al. (1992) dated dyke intrusion which does not necessarily date the episode
of crustal extension particularly if it occurred by
block faulting. Post-volcanic crustal extension
(< 20 Ma) generates structures that are the most
widespread and best developed throughout the
region. Rotated fault blocks contain hundreds of
metres of volcanic rocks resting on platform sediments which in turn rest on Pan-African basement. Assessment of the presence or absence Of
pre-, syn- and post-volcanic structure indicates
no pre-volcanic uplift ( > 30 Ma). Uplift appears
to have occurred during or immediately after
most of the Tertiary volcanism. Not only is this
conclusion consistent with what was deduced
from the nature of the pre-volcanic sediments in
Yemen but it is also consistent with the lack of
pre-voleanic structure to the north in Saudi
Arabia (Bohannon et al. 1989).
Volcanological studies indicate that present
exposures of sub-aerial volcanic rocks are some
2500 m thick. However, this may not constitute
the true thickness of the volcanic rocks because
contemporaneous Tertiary granites ( < 24 Ma)
intruding the volcanic rocks are now exposed,
unroofed, at 3015 m altitude. Age data indicate
that minor volcanism may have begun around 45
Ma but reached a peak at 30-19 Ma (Civetta et
al. 1978; Chiesa et al. 1983, 1989; Capaldi et al.
1987; Menzies et al. 1990; Huchon et al. 1992).
Since the 30-19 Ma range is determined on the
erosional remnants of the volcanic pile, erosional
unroofing may have removed as much as 1-2 km
of the volcanics. For example, peak basaltrhyolite volcanism may have lasted in total for
longer than 11 million years (30-19 Ma) with a
significant amount of the volcanic pile having
been removed by erosion. Age determinations
on dyke rocks (Huchon et al. 1992) indicate a
possible major structural change around 22 Ma
when a dominant E - W extensional stress regime
was replaced by a N - S extensional system. This
structural change appears to be synchronous
with the major period of granite emplacement
and also marks the onset of syn-volcanic
extension.
Four important observations can be made by
considering the sedimentological, structural and
volcanological evolution of the region. Firstly,
the palaeoenvironmental record within the
Mesozoic to Tertiary sediments indicates n o
marked (> 100 m) n0n-eustatic sea-level change
as one might expect with significant pre-volcanic
uplift. Secondly, the sedimentary rocks record
no marked erosional periods that would be
caused by erosional unroofing during doming or
uplift. A disconformity occurs between the
Tawilah Formation and the flood volcanics.
Thirdly, the lack of pre-volcanic crustal extension and the presence of late syn-volcanic and
post-volcanic structure indicates that most
crustal extension postdated the onset of magmatism at 30 Ma. Fourthly, erosional unroofing after
volcanism may have removed a significant
amount of the flood volcanics.
One can deduce from these geological observations that significant uplift did not occur prior
to 30 Ma and extension began to affect the
region at ,~ 22 Ma, some 8 million years after
the onset of peak magmatism and at a time of
major structural change. Alternatively, if uplift
did happen synchronously with the onset of
volcanism at 30 Ma it had little or no affect
on the geological record. If one accepts that the
Pan-African lithosphere beneath Yemen had a
pre-rift thickness of c. 180 km (McGuire &
Bohannon 1989) then it follows that > 10 Ma
may be required, from the time of plume impingement, before uplift is registered in such thick
lithosphere (Spohn & Schubert 1983). This
would require that the plume had been under the
region for several million years prior to the onset
of uplift.
Preliminary fission track data
Fission track (VI') research in Yemen is facilitated by three major advantages: (a) the geodynamic position of the rift margin, close to the
centre of the Afar thermal anomaly and triplejunction (Fig. 1), should ensure a maximum
crustal response to the thermo-tectonic processes of rifting; (b) excellent exposure and completeness of section, encompassing Proterozoic
basement (c. 900 Ma) to Quaternary extrusives,
allow detailed stratigraphic relationships to be
resolved; and (c) a rigorous geological framework which can be integrated with quantitative
FT estimates of exhumation and shallow crustal
cooling. Since FT dating of apatites indicates the
age at which the rock cooled below 120-125°C
care was taken to sample basement rocks at
some distance from dyke swarms and other intrusives which may have reset the apatite FT
ages. Dixon et al. (1989)pointed out that FT
ages may record the age of local magmatic pulses
rather than the beginning of exhumation. In a
2080
1710
1380
970
Elevation
m
apatite
20
apatite
6
apatite
9
apatite
9
0.550
(202)
0.171
(18)
0.154
(81)
2.736
(360)
Spontaneous
~
(Ns)
8.610
(3160)
2.556
(269)
2.216
(1163)
1.733
(228)
Induced
pi
(Ni)
11%
75%
7%
30%
PX2
1.309
(9068)
1.309
(9068)
1.309
(9068)
1.309
(9068)
Dosimeter
pd
(Nd)
379+_41
17+_2
16_+4
16+1
FT
Central
age
Ma (_+lor)
Track densities (p) are as measured and are (x 106 tr cm-2); numbers of tracks counted (N) shown in brackets.
Analyses by external detector method using 0.5 for the 4~2¢r geometry correction factor. See Hurford & Carter (1991).
Ages calculated using dosimeter glass CN-5 for apatite with ~c~5 = 374+9.
PX2 is probability for obtaining X2 value for v degrees of freedom, where v = no. crystals- 1.
Yem969
F4
Yem 970
F12
Yem971
F13
Yem973
F28
Sample No.
Field No.
Mineral
and no.
crystals
12.83_+0.12
(68)
--
13.85+0.34
(36)
D
Apatite
mean track
length
0zm)
1.00
m
2.01
Length
standard
deviation
(/tin)
Table 1. Fission track ages and length data for apatites from Pan-A~can basement rocks of Yemen. Samples 969, 970 and 971 are amphibolites and 973 a gneiss
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298
M.A. MENZIES E T A L .
rift environment this cooling can be brought
about by (a) surface uplift and subsequent
erosional exhumation, (b) exhumation without
surface uplift due to the competing effects of isostatic compensation and erosion, or (c) crustal
thinning and unroofing due to extensional block
faulting. FF dates were determined on samples
of Pan-African basement from Yemen (Table
1). Apatite FT ages of c. 16 Ma with long mean
track lengths indicate rapid crustal cooling and
exhumation of the proto-Red Sea rift-flanks.
This period of cooling and exhumation occurred
approximately 14 million years after the onset of
significant flood volcanism, presumed to have
begun around 30 Ma (Fig. 2). In Yemen the
large thickness (> 3 km) of erupted volcanics
(30-20 Ma) and the associated high geothermal
gradient will have annealed all apatites in the
Pan-African basement such that any pre-volcanic uplift will not have been recorded in the FT
ages. Approximately 3-4 km of erosion at < 20
Ma can be demonstrated using FT data. This
may have important implications for the possible
removal of younger volcanics. The presence of a
sample with a partially reset, apparent apatite
FT age of c. 380 Ma, outside the extended area,
indicates slower exhumation from shallower
crustal levels. There is a general increase of
sample FT age with elevation, and the base of
the uplifted partial annealing zone (once at c.
3000-4000 m depth) in north Yemen is currently
believed to be located at 900-1700 m elevation.
It is possible that Fir' ages in Yemen may record
the cessation of volcanism and subsequent erosion coupled with the development of late synvolcanic and post-volcanic extensional structures rather than post-volcanic surface uplift.
However, geological evidence points to surface
uplift initiating exhumation. Marine sediments
near the base of the flood volcanics are found
at a regional elevation of 2400 m which attests
to this amount of surface uplift at some point
during the past 28 Ma.
Theoretically, eruption of 3-4 km of flood volcanics in Yemen would produce an increase in
elevation of approximately 600 m (assuming
Airy isostasy), which is much less than the average 2000 m elevation seen today in the Yemen
plateau where there is no observable extension.
In fact a thickness of 11 km of magmatic underplating would be required to produce this
observed average surface elevation. This is
theoretically possible (McKenzie & Bickle 1988)
if we assume that the potential temperature in
the mantle beneath this extended region (for
fl = 2) is elevated relative t o normal asthenosphere. Such a scenario would generate 10-15
km of melt of which c. 5 km was erupted. Studies
of xenoliths from Saudi Arabia (McGuire 1988)
point to relatively hot shallow mantle (1020°C at
36 km). Garnet pyroxenite xenoliths which crystallized over the depth range 40-50 km at 9001000°(2 (McGuire & Bohannon 1989) may have
formed as a result of magmatic underplating
near the crust-mantle boundary. However recent gravity modelling indicates that the crust
underneath the Yemen rift mountains is 35 km
thick or less (Makris et al. 1991). The gravity
data therefore suggests that magmatic underplating (with crustal densities) is not the reason
for the present uplift and that it maybe a transient phenomenon due to thermal expansion
above the plume. This is supported by the
presence of numerous hot springs and young
volcanic cones throughout the Yemen highlands. Moreover evidence exists in Saudi Arabia
for anomalously high temperatures at the base of
the crust (McGuire & Bohannon 1989) almost
twice as hot as would be expected from surface
heat flow data. The role of underplating and/or
thermal expansion close to the Afar plume need
to be resolved if the cause of uplift is to be fully
understood.
Sinai, eastern Egypt and Saudi Arabia
One of the most contentious issues in the Red
Sea is the relative timing of uplift, extension and
magmatism. Gass (1970a, b) drew attention to
the temporal and spatial coincidence of volcanism and surface uplift associated with the formation of the Afro-Arabian dome. He proposed
that the causal mechanism was localized thermal
disturbances in the mantle (i.e plumes), an idea
that has recently gained wide acceptance (e.g.
White & McKenize 1989; Fig. 1). For some time
it has been accepted that uplift and formation of
broad domal structures predated magmatism
(Gass 1970a, b; Kohn & Eyal 1981) but more
recently this has been questioned (Almond
1986; Bohannon et al. 1989).
The presence of distinct domes and resultant
differential uplift is supported by fission track
studies for the Sinai Peninsula and the southeastern desert of Egypt (Kohn & Eyal 1981; Omar et
al. 1987; Garfunkel 1988). It is apparent that uplift at c. 27 Ma predates rifting and the 20 Ma
peak of magmatism (23-17 Ma) in the Sinai Peninsula (Baldridge et al. 1991; Fig. 2). Kohn &
Eyal (1981) estimate up to 3 km of erosion to
have occurred since c. 9 Ma and Omar et al.
(1987) proposed that distinct domes existed in
the southeastern desert of Egypt, and that these
produced variable uplift induced erosion that occurred some 5 million years prior to magmatism.
A detailed study of the basement of eastern
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MAGMATISM, UPLIFT AND EXTENSION
Egypt (Omar e t al. 1989) concluded, however,
that the length distributions of fission tracks
were vital in understanding the uplift age. Of the
three distinct groupings only one group characterized by unimodal, narrow negatively skewed
track length distributions and long mean lengths
gave the best 'cooling ages'. These authors
299
concluded that rift-flank uplift began around
21-23 Ma and that is was contemporaneous with
extension and subsidence.
In an investigation of the Pan-African basement in Saudi Arabia, Bohannon e t al. (1989)
concluded that doming cannot have occurred at
any time between the late Cretaceous to early
AGE OF INITIAL EXTENSION, IGNEOUS
ACTIVITY AND EXHUMATION AROUND THE RED SEA
NORTH CENTRAL
Sinai
so
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28
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TIME
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I
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30
20
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50
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I
I
40
30
20
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TIME (Ma)
TIME (Ma)
LEGEND
APPROXIMATE
BEGINNING OF
EXTENSION
/
I
AGEOF
RIFT-RELATED
ITITITITI
IGNEOUS ACTIVITY
BEGINNING
OF
UPLIFT?/EXHUMATION
/
Fig. 2. Relative timing of initial extension, igneous activity and exhumation from north to south along the margins
of the Red Sea (after Dixon et al. 1989). Note that volcanism in Yemen and Saudi Arabia predates extension and
exhumation. Data are taken from several sources (Sinai, Omar et al. 1989 and Baldridge et al. 1991; Saudi
Arabia, Bohannon et al. 1989; Yemen, Civetta et al. 1978 and Chiesa et al. 1989; Ethiopia, Hart et al. 1989 and
Mohr & Zanettin 1988; eastern Egypt, Omar et al. 1987 and Ressetar et al. 1981).
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on May 11, 2016
300
M.A. MENZIES E T A L .
Oligocene in the central and northern Red Sea
because of complete sections of Upper Cretaceous-Eocene (c. 73-45 Ma) marine rocks in
Egypt and Saudi Arabia (Fig. 1). Bohannon et
al. (1989) thought that the axial part of a dome
cannot have been uplifted at any time between
late Cretaceous to early Oligocene times in the
northern and central Red Sea. Similarly, the
existence of thin continuous marine sediments,
coastal zone non-marine rocks and thick lateritic
soils of late Cretaceous to mid-Oligocene age
over a large part of Arabia argue strongly against
late Cretaceous to early Tertiary domal structures. This is because these sediments indicate
that the entire Afro-Arabian continent was at a
low elevation or below sea level for 45 million
years prior to Red Sea rifting. It is important
to note that all the surrounding marine rocks
are fine grained so presumably there was little
difference in elevation between the areas of marine deposition and of soil development. Consequently there is little evidence for Cretaceous
doming in the northern and central Red Sea but
late Oligocene to early Miocene uplift appears to
have been active in the northern Red Sea (Egypt
and Sinai).
Geological observations and a detailed fission
track investigation of the Pan-African basement
in Saudi Arabia (Almond 1986; Bohannon 1986;
Bohannon et al. 1989; Camp & Roobol 1989;
Camp et al. 1991) have provided evidence that
exhumation presumably related to doming or
uplift, postdates magmatism. In contrast, Almond (1986) provided evidence that early extension was related to subsidence not uplift and
doming and that the uplift which eventually produced the Afro-Arabian dome occurred around
10 Ma. However, it should be noted that all of
the flood volcanics studied in Yemen are subaerial with no known submarine eruptives. This
would argue against significant subsidence during their formation (30-20 Ma). Most of the
faulting that formed the Red Sea rift occurred
during late Oligocene (Bohannon 1986) with a
peak of crustal extension around 25 Ma. In a
later study Bohannon et al. (1989) stressed the
lack of evidence for pre-volcanic rifting or crustal
extension. This is similar to observations in
Yemen. Early Oligocene volcanic rocks in Saudi
Arabia conformably overlie sedimentary rocks
deposited in marine and coastal zone environments and the oldest angular unconformity is
beneath 15-18 Ma old flows. These geological
observations and FT results (Bohannon et al.
1989) indicate 2.5-4 km of uplift in early to
middle Miocene times (Fig. 2). The geographical
distribution of FT ages across the western Saudi
Arabian escarpment (Bohannon et al. 1989)
show trends in common with other rift-flanks
throughout the world. The youngest FT ages
generally occur at the lowest elevations along
the base of the escarpment and the older ages
occur along, and to the east, of the escarpment
crest. Overall the FT data for Saudi Arabia
tentatively suggest exhumation marginal to
the central Red Sea, beginning at 20 Ma and
accelerating at < 14 Ma. A significant phase of
erosion postdates rifting and magmatism by 1015 million years. On the basis of these data
Bohannon et al. (1989) invoked a passive rifting
model for the Red Sea, contrasting with the
active, doming (early uplift) models proposed
elsewhere (Gass 1970a, b).
Several aspects of the geological and FT data
have important implications for the timing of
uplift, magmatism and extension. Firstly, uplift
cannot significantly predate magmatism (c. 30
Ma) due to the lack of any evidence in the
sedimentary record for major changes in sea
level. From geological observations one can constrain the beginning of uplift to be around early to
mid-Miocene in Sinai, eastern Egypt and Saudi
Arabia. The lack of pre-volcanic structure cannot be used as an indicator of the lack of uplift as
uplift may occur without significant extension.
Secondly, some 2.5-4.0 km of exhumation
marginal to the central Red Sea postdated rifting
in Saudi Arabia and Egypt by 5-10 Ma (Fig. 1).
In Sinai recent data indicate that uplift, rifting
and magmatism were broadly synchronous (Fig.
2). Thirdly, plume impingement on a moving
plate may result in a systematic increase in the
age of volcanism, and possibly uplift, away from
the plume head such that regions in the northern
Red Sea may have been uplifted earlier than
those in the southern Red Sea as the latter are
within the present-day 'sphere of influence' of
the Afar plume. In Sinai, eastern Egypt and the
northern Red Sea, uplift is believed to have
started around 25-20 Ma whereas in Saudi
Arabia and Yemen, in the southern Red Sea,
exhumation appears to have started around 2015 Ma (Fig. 2). With regard to systematic
changes in the age of the volcanic rocks Dixon et
al. (1989) reported temporal changes that are
the opposite of that produced by a stationary
plume and a 'northward'-moving plate. However, a recent evaluation of all available age data
does not support any systematic regional variations in volcanism around the margins of the Red
Sea (Menzies et al. 1990). A detailed investigation of the timing of initiation of volcanism and
the temporal and spatial evolution of the flood
volcanism in Yemen is underway at present,
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MAGMATISM, UPLIFT AND EXTENSION
Discussion
In an overview of the Red Sea, Dixon et al.
(1989) stated that the timing of magmatism, uplift and extension support neither a purely active
or passive rifting model but that the early volcanism implied a causal association between upwelling mantle and rift initiation. In contrast,
White & McKenzie (1989) suggested that the
Afar plume must have existed under the southern Red Sea at 30 Ma coincident with the initiation of a period of major flood volcanism.
Implicit in this model is c. 1-2 km of surface
uplift synchronous with, or just prior to, magmatism. White & McKenzie (1989) stress that
such uplift is spread over an area of 1000-2000
km, with a maximum directly above the plume
head. Although the dynamic uplift associated
with initial plume impingement may effectively
cause instantaneous uplift, several tens of millions of years may be required before the conductive thermal-uplift effects are registered in
150 km thick lithosphere (Spohn & Schubert
1983). Following the work of Houseman (1990),
the proximity of Yemen towards the centre of
the proposed plume, suggests that we might expect significant amounts of dynamic uplift prior
to magmatism. While the White & McKenzie
(1989) model requires pre-volcanic uplift, most
authors report no widespread pre-volcanic uplift
or structure in Yemen or Saudi Arabia with only
one report of pre-volcanic faulting (Hempton
1987). Most of the lowermost volcanic units in
Saudi Arabia, Yemen and Ethiopia are erupted
disconformably onto palaeosol horizons or
fluviatile sediments without an intervening angular unconformity. Some of the lower volcanic
units conformably overlie gastropod-rich horizons or fluviatile sediments.
Recently, passive and active models for the
evolution of the Red Sea and elsewhere have
come under scrutiny. Pallister (1987) and Dixon
et al. (1989) invoked active rifting models mainly
because magmatism predated extension and uplift. In contrast, Bohannon et al. (1989) invoked
a passive rifting model for the Red Sea on the
basis of FT data and Menzies et al. (1990) supported such a model primarily on the presence of
synsedimentary structures in the Tawilah sandstones underlying the Yemen volcanics. This
was interpreted as evidence for pre-volcanic
crustal extension. A recent investigation of these
features revealed that they are deeply weathered
igneous dykes and consequently there now
appears to be no unequivocal evidence f o r prevolcanic structure in the sediments underlying
the volcanic rocks in Yemen. Moreover crustal
extension in Yemen has primarily been accommodated by domino-style block faulting rather
301
than dilation due to dyke intrusion. As such, passive and active models do not adequately explain
the Yemen data and Red Sea rifting.
The Great Basin of the western USA offers an
interesting comparison with Yemen. The Great
Basin is also a region of Oligo-Miocene basaltrhyolite magmatism in a region of crustal extension and uplift. In the Great Basin, Taylor et al.
(1989) demonstrated the existence of pre- (> 32
Ma), syn- (30-27 Ma) and post-volcanic (16-14
Ma; < 5.3 Ma) structure but stressed that only
period of faulting was synchronous with magmatism. Therefore magmatism and faulting
need not be closely related in space and time. Although support can be found for a passive rifting
model (i.e. extension began prior to volcanism)
in the Great Basin, the genetic relationship between volcanism and extension is not simple and
direct. This is similar to the southern Red Sea
where most of the structure tends to be late synvolcanic or post-volcanic and the eentres of volcanism do not always coincide with extended
areas. In both the Red Sea and the western USA
evidence exists for only local extension prior
to the main episode of volcanism. Little or no
evidence exists for significant regional extension
during the peak of volcanism when the greatest
volume of magma was erupted. Extension after
peak volcanism is apparent both in the western
USA (Best & Christensen 1991) and the Red Sea
where faults cut the entire volcanic sequence and
older rocks. Best & Christensen (1991) coneluded that regional extension did not occur in
the Basin and Range and that it was episodic.
Basal angular unconformities are not widespread in the Great Basin (Best & Christensen
1991) or the Red Sea and faulted angular discordances are limited in distribution. Bohannon
et al. (1989) reported angular unconformities
under 18 Ma flows in Saudi Arabia indicating
that crustal extension had begun by that time.
Angular unconformities have only been reported in Yemen at Jabal an Nar and Jabal
Khariz. At Jabal an Nar, late Miocene (10 Ma)
basalts unconformably rest on early Miocene
silicified rhyolites (18 Ma) (Capaldi et al. 1987;
Huchon et al. 1992). This constrains extension to
have occurred between 20 and 10 Ma. A similar
angular unconformity is apparently located at
Jabal Khariz on the southern coast of Yemen west
of Aden. Here late Miocene volcanics (9.6 Ma)
rest unconformably on block faulted Yemen Volcanics of inferred early Miocene age (Cox et al.
1969). It is interesting to note that the FI" data
indicate that the initiation of exhumation (17
Ma) coincided with the age of the angular unconformities in Yemen (20-10 Ma) and Saudi
Arabia (18 Ma). This may point to a similar
evolutionary history for both regions. Other
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302
M.A. MENZIES E T A L .
evidence for syn-volcanic crustal extension in
the form of intervolcanic sedimentary deposits
are volumetrically rather limited. In the case of
the western USA they amount to 1% of the cumulative thickness of Tertiary. sediments (Best &
Christensen 1991). In the case of Yemen no accurate assessment of the amount of sedimentary
material has been undertaken but preliminary
results of traverse work indicate that in parts of
northern Yemen there is an east (10%) to west
( < 1%) variation in the amount of sediments.
Best & Christensen (1991) concluded that
simple models of active and passive rifting (Sengor & Burke 1978) are inappropriate when
applied to complexly evolving terrains like the
Great Basin of the western USA. This equally
well applies to Yemen and Saudi Arabia and
elsewhere (Brown et al. 1991) where magmatism
frequently pre-dates crustal extension and uplift/
exhumation, a sequence that is neither passive
nor active. Perhaps the reason no uplift occurred
before or during early magmatism is that magmas were efficiently transported to the surface
via narrow conduits in which magma velocity
greatly exceeded conductive heat transfer. The
crust would take ten million or more years to respond to the heat perturbation caused by the arrival of the Afar plume under the lithospheric
plate (Spohn & Schubert 1983). This is consistent with a gradual change in the lithological
character of the Yemen Volcanics. The early
volcanism was predominantly mafic perhaps the
result of efficient magma transfer with magma
velocities exceeding heat transfer into the lithosphere. In contrast, later volcanism was more
silicic (granites and rhyolitic ignimbrites) indicating storage and differentiation of mafic magmas in crustal magma chambers thus enhancing
conductive heat transfer into the lithosphere.
Summary
The relationship between magmatism, crustal
extension and uplift in continental rifts can only
be properly evaluated with an integration of
geological field observations, age determinations
and fission track analysis. In Yemen, adjacent to
the southern Red Sea, geological and preliminary fission track data indicates that the onset of
flood volcanism (c. 3-4 km) predates significant
crustal extension and uplift/exhumation. This is
an area that is frequently cited as a classic example of the opposite phenomenon where plumedriven uplift precedes magmatism. It is becoming increasingly apparent in the southern
Red Sea, the Basin and Range of the western
USA, southeastern Africa and elsewhere that
the development of volcanic and non-volcanic
margins cannot be adequately explained by
traditional active and passive models. We
suggest that a plume was responsible for flood
volcanism, but the earliest expression of continental break-up was magmatism rather than
domal uplift.
This work was funded by an expedition grant from the
Royal Society which made fieldwork possible. Additional support from British Petroleum and the Industrial Association of the department of Geology
RHBNC is gratefully acknowledged. The British
Council is thanked for supporting postgraduate field
studies in Yemen (J.B., M.A. and A.A.). The University of Sana'a is thanked for its continued hospitality
and for provision of vehicular support. S. Muir is
thanked for the diagrams. R. G. Bohannon and N.
Harris are thanked for their comments on an earlier
version of this manuscript.
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