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Post-Miocene rifting and diapirism in the northern Red Sea
Marine Geology, 74 (1987) 173-190
173
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
POST-MIOCENE RIFTING AND DIAPIRISM IN THE NORTHERN RED SEA Y O S S I M A R T a n d D A V I D A. ROSS National Oceanographic Institute, Tel-Shikmona, Haifa 31080 (Israel) Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.)
(Received March 31, 1986; revised and accepted July 21, 1986)
Abstract Mart, Y. and Ross, D.A., 1987. Post-Miocene rifting and diapirism in the northern Red Sea. Mar. Geol., 74:173 190. Rifts and diapirs are the dominant structures in the bathyal zone of the northern Red Sea, where commonly the diapirs ascend along normal faults associated with the rifts. In this manner, the diapirs form elongated features sometimes exceeding lengths of 30 km and widths of 4 km. They also penetrate a sedimentary sequence about 1000 m thickness or less, thicknesses generally considered to be barely adequate for the gravity conversion required to initiate diapiric motion. We suggest that development of diapirs under these minimal overburden conditions was made possible by the high thermal gradients in the area, resulting from the underlying attenuated continental crust and ascending mantle, that prevail in the rift system of the northern Red Sea. Diapiric evolution was also dependent on the extensional tectonic regime, with its numerous normal faults forming weaknesses along which the diapirs could move upwards.
Introduction T h e n o r t h e r n Red S e a is a u n i q u e e x a m p l e of a pre-sea-floor rift s y s t e m s e p a r a t i n g t h e relat i v e l y u n d e f o r m e d A r a b i a n a n d A f r i c a n plates. T h e r e g i o n is c h a r a c t e r i z e d by t h e r e m a r k a b l e j i g s a w fit of t h e N W - S E t r e n d i n g o p p o s i t e coastlines, w h i c h led W e g e n e r (1929) to p r e s e n t t h e Red S e a as a n e x a m p l e of t h e e a r l y s t a g e s of c o n t i n e n t a l drift. T h e m o u n t a i n o u s t e r r a i n s t h a t b o r d e r t h e Red Sea on b o t h t h e A f r i c a n a n d t h e A r a b i a n c o a s t l i n e s s h o w steep s e a w a r d slopes f l a n k e d by n a r r o w c o a s t a l plains a n d c o n t i n e n t a l shelves (Fig.l). W h e r e a s t h e c o a s t a l p l a i n s a r e b u i l t of N e o g e n e a n d Q u a t e r n a r y sediments, t h e a d j a c e n t m o u n t a i n s consist of e x t e n s i v e o u t c r o p s of P r e c a m b r i a n , igneous, a n d m e t a m o r p h i c suites (Coleman, 1974). C o n t i n e n t a l slopes a r e steep, descend0025-3227/87/$03.50
ing to a m a i n t r o u g h a t d e p t h s of 1200-1500 m. T h e deep a x i a l t r o u g h , w h i c h is a d o m i n a n t m o r p h o l o g i c a l f e a t u r e in t h e c e n t r a l Red Sea, is c o n s p i c u o u s l y a b s e n t in the n o r t h ( M a r t a n d Hall, 1984; U c h u p i a n d Ross, 1986). T h e m o r p h o l o g y of t h e m a i n t r o u g h of t h e nort h e m Red S e a is c o m m o n l y r u g g e d (Fig.2). F a u l t s , rifts a n d d i a p i r s a r e a b u n d a n t , a n d t h e d o m i n a n t t e c t o n i c r e g i m e is one of c r u s t a l e x t e n s i o n ( C o c h r a n , 1983; M a r t a n d Hall, 1984). As a rule, t h e s t r u c t u r a l a s s o c i a t i o n of rifts a n d diapirs is a c o m m o n o c c u r r e n c e , b u t t h e n o r t h e r n Red S e a a s s o c i a t i o n is unique, a n d differs f r o m m o s t o c c u r r e n c e s in s e v e r a l aspects: (1) T h e diapirs in t h e n o r t h e r n Red S e a a r e c o v e r e d by a 1 k m o r less t h i c k o v e r b u r d e n , w h e r e a s in m o s t r e g i o n s d i a p i r s r e s u l t f r o m a
© 1987 Elsevier Science Publishers B.V.
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Fig.1. (a) A generalized morphological map of the Red Sea and adjacent terrains. Deeps are marked by asterisks, and contours are in meters. (Prom Choubert, 1968, and Coleman, 1974). (b) Ship tracks of the R/V "Atlantis II" and the R/V "Shikmona" in the northern Red Sea and locations of profiles shown in
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Fig.2. Bathymetric map of the bathyal zone of the northern Red Sea (contours in meters). Note that the deeper parts of the northern Red Sea are located in its western section. The arcuate configuration of the deeper parts of the bathyal zone changes its trend from NNW SSE to NNE SSW, so that a gradual orientational shift from the Red Sea trend to the Dead Sea trend can be discerned. See Fig.lb for the ship tracks.
salt l a y e r b u r i e d u n d e r a n o v e r b u r d e n seq u e n c e a t t a i n i n g t h i c k n e s s e s of 3-6 km (Gussow, 1968; H u m p h r i s , 1979). (2) In the n o r t h e r n Red Sea the diapirs are typically elongated, a t t a i n i n g lengths exceeding 30 km and widths of a b o u t 4 km (Fig.3), whereas in m a n y o t h e r places diapirs commonly form s t r u c t u r a l domes t h a t are r o u n d or oblate. (3) In the n o r t h e r n Red Sea the diapirs c o m m o n l y a s c e n d e i t h e r a l o n g the b o u n d a r y faults of the rift or into the rift floor.
Geological setting T h e e v o l u t i o n of sea-floor s p r e a d i n g axes proceeds from an initial stage of c o n t i n e n t a l uplift and i g n e o u s activity, t h r o u g h an inter-
mediate stage of rifting and a t t e n u a t i o n of c o n t i n e n t a l crust, to the m a t u r e stage of a c c r e t i o n of o c e a n i c c r u s t and sea-floor spreading. These stages are well r e p r e s e n t e d in the Red Sea region. T h e s e p a r a t i o n b e t w e e n the African and A r a b i a n plates s t a r t e d d u r i n g the late O l i g o c e n e - e a r l y M i o c e n e with a N W - S E t r e n d i n g r e g i o n a l uplift and extensive intrusive and e x t r u s i v e v o l c a n i s m (Coleman, 1974). T h e e a r l y Red Sea uplift p r o b a b l y exceeded 3 km, as i n d i c a t e d by the widespread outcropping of the A r a b i a n - N u b i a n P r e c a m b r i a n massif on b o t h flanks of the Red Sea (Choubert, 1968). The 22-20 M a old alkali-basalt dikes associated with this t e c t o n i c phase in the n o r t h e r n Red Sea show a d o m i n a n t northw e s t w a r d t r e n d ( B a r t o v et al., 1980).
176
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Fig.3. Distribution of rifts and diapirs in the northern Red Sea. The arcuate configuration of the bathymetric features (Fig.2) can be closely correlated to the structural elements, showing the gradual change of tectonic trends. The association between the rifts and the diapirs can be seen in most places. F o l l o w i n g the r e g i o n a l uplift and volcanism, rifting o c c u r r e d a l o n g the axis of the e l o n g a t e d dome and e v e n t u a l l y a m a r i n e basis developed. Rifting of the Red Sea o c c u r r e d d u r i n g t h e e a r l y to middle M i o c e n e a l o n g t h e same n o r t h w e s t e r n trend. It is of i n t e r e s t to n o t e t h a t w h e r e a s the t e c t o n i c d e v e l o p m e n t of the Red Sea and the G u l f of A d e n is similar and a l m o s t c o n t e m p o r a n e o u s , and b o t h are extensions from the n o r t h e r n s e g m e n t of the N W I n d i a n O c e a n t e c t o n i c s p r e a d i n g system, the Red Sea was c o n n e c t e d d u r i n g this time to the M e d i t e r r a n e a n Sea (Stoffers and Ross, 1974; C o c h r a n , 1981, 1983). T h e Red S e a - M e d i t e r r a n e a n c o n n e c t i o n is significant, b e c a u s e w h e n
the M e d i t e r r a n e a n was affected by its late M i o c e n e salinity crisis (Ryan et al., 1973), the Red Sea was affected as well, and a s e q u e n c e of e v a p o r i t e s a t t a i n i n g t h i c k n e s s e s of a b o u t 3 km was also deposited t h e r e (Ahmed, 1972). A c c r e t i o n of o c e a n i c basalts in the Red Sea in the c e n t e r of the m a i n t r o u g h n e a r l a t i t u d e 18°N s t a r t e d 5 M a ago (Roeser, 1975; Le P i c h o n and F r a n c h e t e a u , 1978), and the a c c r e t i o n zone propagated northwestwards and southe a s t w a r d s at an a v e r a g e r a t e of 120 mm y r - 1 ( M a r t and Hall, 1984). An essentially continuous zone of sea-floor spreading now extends from l a t i t u d e s 16 ° to 21°N. F r o m 21 ° to 24°N is a t r a n s i t i o n a l zone w h e r e o c e a n i c basaltic accre-
177
tion is sporadic and occurs in basins separated by attenuated continental crust (Pautot et al., 1984). Farther north, between 24 ° and 28°N, is a rifted zone underlain by attenuated continental crust (Cochran, 1983; Uchupi and Ross, 1986). The basis structural trends in the northern Red Sea region were set during the middle-late Miocene. These include the NW-SE-oriented regional rifts that led to the evolution of the coastal plains and the continental margins on the Arabian and the African flanks. This structural framework extended from the southern Red Sea into the Suez rift, and was associated with the N E - S W extension into the Red Sea. The late Miocene-early Pliocene was a critical period in the development of the Red Sea. Concurrently with the beginning of seafloor spreading in the central part of the sea, the axis of the continental separation between Arabia and Africa in the northern region changed its trend from N W - S E to N-S. This change caused the termination of large-scale tectonic activity in the Suez rift and initiated the evolution of the Dead Sea rift (Mart and Hall, 1984). As a result of these tectonic changes, the Red S e a - M e d i t e r r a n e a n connection was disrupted and the Red Sea-Indian Ocean connection was opened through the Straits of Bab-al-Mandeb at the southern end of the Red Sea. The Indian Ocean connection ended the evaporitic deposition in the Red Sea. The continuation of the continental divergent motion in the Red Sea region during the Plio-Pleistocene led to the evolution of a double trough system in the central Red Sea. This consists of the main trough, formed in the Miocene and underlain by the late Miocene evaporites resting on attenuated continental crust (Drake and Girdler, 1964), and an axial trough, where accretion of oceanic crust occurred, dissecting the evaporitic sequence. At the same time crustal attenuation and tectonic distension continued in the Red Sea, leading to the formation of a series of rifts within the main trough. Concurrent with this distension, diapirs started to develop.
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Fig.5 (continued). (b) Interpretation of (a). Note the sequence of alternating concordant bedding and unconformities (heavy lines), suggesting a sedimentatiorL regime of intermittent activity. See Fig.lb for location.
Mechanical c o n s t r a i n t s o f shallow diapirs The diapirs in the northern Red Sea occur in a region where their sediment overburden is relatively thin. In most of the well-known diapiric regions, such as the Gulf of Mexico or northern Germany, diapirs pierce upward through a thick overburden, exceeding 5 km in places (e.g. Kupfer, 1968; Murray, 1968). These thick overburdens and the subsequent burial depths of the r o c k - s a l t sequences cause high pressures and high temperatures. The high pressure leads to consolidation of the overburden and to the increase in its density from about 2.0g cm -3 to values of 2.6g cm -3 or more, whereas halite maintains its 2.0-2.2 g cm-3 density. The high temperature enhances the ductility of the rock salt (Carter and Hansen, 1983). The density contrast between the evaporites and the overburden results in a metastable gravity configuration t h a t leads to diapirism when enhanced by increased salt ductility and affected by a perturbation (Parker and McDowell, 1955; Gussow, 1968). The perturbation is considered to be associated with faulting, which both triggers a seismic
shock and provides a favorable emplacement zone for the ascending diapir along the fault plane. The thickness of the Plio-Pleistocene sedimentary sequence, atop the late Miocene evaporites in the northern Red Sea, is approximately 500-1000m. This approximation is based on a 0.5-1.0 s average reflection time from the sea floor to reflector S (Phillips and Ross, 1970), which marks the top of the evaporite sequence, and on interval seismic velocity of about 2.0km s -~ (Drake and Girdler, 1964). Evidence t h a t the northern Red Sea diapirism was indeed triggered under thin overburden is suggested by the density conversion encountered in DSDP borehole 225. At this site in the central Red Sea, the density of the sediments at 5 m was 1.7 g cm -3, at 100m the same type of detrital sediments had a density of 2.2g cm -3, and dolomite beds encountered at 150 m depth had a density of 2.8 g cm-3. In comparison, halite found in the same borehole at depths greater than 180 m had a density of 2.2 g cm- 1, which is its normal density. Heat flow measurements in the central Red Sea showed anomalously high values of 153-276 mW m - i (Langseth and Taylor, 1967; Girdler, Ericson et al., 1974). These high values indicate favorable thermal conditions required for initiation of salt flow (Carter and Hansen, 1983). The measured thermal gradient of 117 K km-~ in the Red Sea (Whitmarsh et al., 1974) further supports this presumption, because laboratory tests have shown that lithostatic confining pressure is a critical parameter to salt ductility at pressures below 20 MPa. However, under conditions of higher pressures, temperature becomes the determining parameter (Carter and Hansen, 1983). The analysis of the origin of the northern Red Sea diapirs is based on the rough calculation of possible pressure and temperature at a depth of 1 km, which suggests a confining lithostatic pressure of about 30 MPa and a temperature of approximately 130°C. These temperatures and pressures are barely sufficient to trigger diapirism (Carter and Hansen,
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Fig.6. A seismic reflection profile showing a diapir piercing through a sequence of Plio-Pleistocene sediments. The sediment thickness is at least 0.75 s reflection time, which is approximately 750 m. The diapir is topped by a collapse structure due to dissolution of its upper part when it reaches the sea floor. See Fig.lb for location.
181
1983). However, the tectonic regime of extensional movement and normal faults in the region also can contribute to the development of the diapirs. The fault planes can be preferred zones for salt flow since they represent zones of weakness as a result of lithological discontinuity and structural gaps. Most fault types are favorable zones for diapiric ascent (Bishop, 1978), but normal faults seem to be the most likely to enhance diapirism under otherwise marginal conditions. Thus the burial depth of the Red Sea salt, the subsurface temperature, the overburden density, and the faulting regime all seem to enhance the development of diapiric structures provided that adequate perturbation occurs. The perturbation that triggers the diapirism is probably associated with the faulting and rifting in the northern Red Sea, but other types of triggering mechanisms should not be ruled out. A structural analysis of the salt domes of east Texas suggests that these diapirs were triggered by differential loading (Jackson and Seni, 1983). Reconstruction of the sequence of events in east Texas indicates that gravity gliding and prograding deltaic deposits initiated a flow of the underlying middle Jurassic Louann Salt. Deposition of the salt was probably associated with rifting, but the latter developments preceded the diapirism. Basin analysis of the east Texas salt domes enabled Jackson and Seni (1983) to suggest that there are critical thicknesses of the salt as well as the overburden below which the diapirs are not likely to develop. They suggested that the critical salt thickness for the initiation of the flow is approximately 600 m, and a similar thickness of the overburden is required as well. Although the triggering mechanism of the east Texas salt domes seems to differ from that of the northern Red Sea, the basic mechanical characteristics are very similar.
Rifts, diapirs and tectonic regime Rifts are very prominent in the northern Red Sea: a N W - S E trending system occurs at the
continental margins, and a N N W - S S E to N - S trending one is present in the bathyal zone. The outer system is associated with at least two levels of seaward reduction of topographic elevation - - the first drops from the elevated plateaus of Arabia and Africa to the coastal plains of the Red Sea, and the second drops from the coastal plains, across the continental margins, to the bathyal zone. The combined drop in elevation, including the calculated effect of erosion, is at least 5 km (Choubert, 1968; Coleman, 1974). The inner or bathyal rifting system, on the other hand, is associated with vertical offsets that rarely exceed 500 m, and the combination of grabens and horsts yields only a small change in bathymetry. The difference in the orientation between the two rifting systems is significant. Whereas the outer rifts extend from the northern Red Sea into the Gulf of Suez, the bathyal rifts extend from the Red Sea into the Dead Sea rift (Figs.1 and 3). The dimension of the rift-lows in the bathyal rift system can reach lengths exceeding 50 km and widths of up to 10km (Fig.3). Their morphology and internal structure suggest that they are active, as indicated by the concave stratification of the fill and normal boundary faults (Fig.4). The boundary faults are longer than the morphological depressions associated with them. However, structural evidence shows that these faults are not presently active along their entire lengths. In places of regular bathymetry, seismic reflection profiles show that the subsurface layers have the typical concave, inter-rift structure except for their upper sections, where the stratification shows little displacement between the rift and the adjacent terrain. These rifts are buried and do not affect the sea floor (Fig.5), but their correlation with active rifts and half-rifts supports the interpretation of structural continuity (Mart and Hall, 1984). The spatial association of the rifts of the northern Red Sea and diapiric structures suggests generic correlation between them. Although the diapirs have not been sampled in ~:he northern Red Sea, they probably are built
182 of late Miocene evaporitic rocks comparable to the late Miocene evaporites that crop out nearby and have been sampled in wells near the coast. Drilling results from the marginal and bathyal zones of the Red Sea (Ahmed, 1972; Whitmarsh et al., 1974) suggest that gypsum was deposited on the margins and rock salt in the bathyal zone during the late Miocene. Further evidence of the composition of the evaporites is provided by several diapirs topped by collapse structures, resulting from dissolution of the evaporites by sea water (Fig.6). In some places the diapirs pierce the overlying sediments and affect the sea floor forming elongated hills reaching lengths exceeding 30 km (Fig.3). These diapirs which protrude through the sea floor are commonly covered by an envelope of non-evaporitic sediments so that they are not affected by sea water dissolution (Fig.7). Piercement structures are also evident in the seismic profiles from places where the diapirs do not affect the sea-floor morphology. In these instances it is suggested that the diapirs either ceased to ascend or, alternatively, have not yet reached the sea floor (Fig.8). Where the diapirs barely reach the sea floor, they cause the uppermost sediments to be uplifted, folded and faulted by the ascending diapir (Fig.9). Diapirs that cause intensive upward folding in the overlying sediments without piercing them are common. This type of diapir is also abundant in other diapiric provinces (Bishop, 1978), but is difficult to identify positively from the seismic profiles due to its similarity to nondiapiric fold structures. The proximity and the structural association between t h e piercing and the non-piercing diapirs, as well as the abundance of anticlines compared with the extreme rareness of synclines in the northern Red Sea, support the presumption that most of the anticlinal structures in the northern Red Sea are probably fold-forming, non-piercing diapirs. Most of the diapirs observed in the northern Red Sea are associated with faulting and rifting. Many of the diapirs pierced their way upward along boundary faults of rifts (Figs.7
and 10); others just forced their way into rift floors (Fig.ll). The displacements between the diapirs and the most recent sediments suggest that in many places diapirism is active at present (Fig.9). Red Sea diapirs are smaller than the associated rifts, and it seems the rifting was instrumental in initiating the diapiric ascent. Our findings are supported by experimental and theoretical models, which indicate that rifts can be the product of structural extension and collapse that resulted from diapiric piercing and uplift motion (Withjack, 1979; Withjack and Scheiner, 1982). Therefore the evidence indicates that the faults of the northern Red Sea bathyal rift formed planes of weakness in the overburden which facilitated the diapiric activity under marginal constraints.
The origin o f the diapirs and the rifts Data from the northern Red Sea show two systems of rifts: regional and local. The regional rift system is the Dead Sea-Red S e a - E a s t African Rift system which is a product of mantle diapirism that led, in the central Red Sea, to accretion of new oceanic crust (Ross and Schlee, 1973). The local system is that of rifts in the bathyal zone. The regional system was initiated in the late Oligoceneearly Miocene, whereas the local system is of Plio-Pleistocene age. However, both systems are presently active. The evolution of the regional system started with uplift and subsequent development of a rift along the apex of the uplifted zone. This type of structural evolution was described by experimental and numeric models (Withjack, 1979; Withjack and Scheiner, 1982). Analysis of the local diapir system suggests that the evolution of the Red Sea bathyal-zone structures was dependent primarily on the deposition of the thick evaporitic series during the late Miocene. Then in the Plio-Pleistocene continued crustal attenuation contributed to the structural evolution of the northern Red Sea by steepening the thermal gradient and thus increasing ductility of the late Miocene
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184
evaporites. Furthermore, the extensional tectonic regime led to the development of a series of rifts in the bathyal zone of the northern Red Sea. The rifts developed in the Plio-Pleistocene, and the late Miocene evaporites were not deposited in them (Mart and Hall, 1984). Sedimentation in the northern Red Sea also played a critical role in the evolution of the diapirs. The thickness of the Plio-Pleistocene sedimentary sequence, which seems to be barely adequate for the densities required for gravity inversion, was an essential step in the process. The average rate of sediment deposi-
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tion during the last 5 Ma is 0.2 m 1000 yrs-1, which is low compared with the calculated rate of 1.0 m 1000 y r s - 1 in the Gulf of Elat (Reiss et al., 1980). However, sediment densities higher than 2.8g cm -3 found at depths of 150m indicate t h a t the lithostatic confining pressure was sufficient to trigger diapirism under the prevailing geothermal and tectonic regimes. The depicted sequence of events in the structural evolution of the northern Red Sea indicates that the diapiric structures of the region stemmed from late Miocene evaporites. Subsequent to burial under at least 600 m of
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Fig.8. (a) A seismic reflection profile showing a series of anticlines. The flanks of the anticlines dip steeper with the increase in depth, and it is suggested t h a t they represent incipient diapirs. The steeply dipping seismic reflections should not be confused with diffraction effects. (b) A seismic reflection profile showing two adjacent diapirs. The n o r t h e r n one has already pierced its way t h r o u g h the sedimentary cover, but the southern one is still in its incipient stage, upfolding the overlying strata. See Fig.lb for location.
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FigA0. A seismic reflection profile showing multiple diapiric structures in a faulted zone. See Fig.lb for location.
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Fig.ll. (a) A seismic reflection profile showing a diapiric piercement into a rift floor. (b) A seismic reflection profile showing a series of diapirs that penetrated a rift floor. Note the morphological similarities between these diapirs and the incipient diaplrs presented in Fig.8a. See Fig.lb for location.
P l i o - P l e i s t o c e n e sediments, d i a p i r i c g r o w t h w a s t r i g g e r e d by h i g h heat-flow a n d c o n t r o l l e d b y rifting. T h e r e is g r o u n d to p r e s u m e t h a t d i a p i r i c t e r r a i n s a l o n g rifted p a s s i v e c o n t i n e n tal margins started their evolution under c o n s t r a i n t s s i m i l a r to t h o s e of t h e n o r t h e r n Red Sea.
P r o g r a m . D.A.R. w a s funded by t h e N a t i o n a l S c i e n c e F o u n d a t i o n , G r a n t 76-10417, a n d t h e D e p a r t m e n t of C o m m e r c e , N O A A , N a t i o n a l Sea G r a n t College P r o g r a m , u n d e r G r a n t No. NA84AA-D-00033 (M/0-1). T h i s p a p e r is W o o d s H o l e O c e a n o g r a p h i c I n s t i t u t i o n Contrib u t i o n N u m b e r 6189.
Acknowledgements
References
W e wish to t h a n k N.L. C a r t e r , R. R e z a k a n d E. U c h u p i for t h e i r s u g g e s t i o n s d u r i n g t h e p r e p a r a t i o n of this paper. Y.M. w a s p a r t l y s u p p o r t e d by t h e I n t e r n a t i o n a l O c e a n D r i l l i n g
Ahmed, S.S., 1972. Geology and petroleum prospects in eastern Red Sea. Bull., Am. Assoc. Pet. Geol., 56: 707 710. Bartov, Y., Steinitz, G., Eyal, M. and Eyal, Y., 1980, Sinistral movement along the Gulf of Aqaba - - its age
190 and relation to the opening of the Red Sea. Nature (London), 285: 220-222. Bishop, R.S., 1978. Mechanism for emplacement of piercement diapirs. Bull., Am. Assoc. Pet. Geol., 62: 1561-1583. Carter, N.L. and Hansen, F.D., 1983. Creep of rocksalt. Tectonophysics, 92: 275-333. Choubert, G., 1968. International Geological Map of Africa. 1:5,000,000. UNESCO, Paris, 9 sheets. Cochran, J.R., 1981. The Gulf of Aden: structure and evolution of a young ocean basin and continental margin. J. Geophys. Res., 86:263 288. Cochran, J.R., 1983. A model for the development of the Red Sea. Bull. Am. Assoc. Pet. Geol., 67: 40-69. Coleman, R.G., 1974. Geological background of the Red Sea. In: R.B. Whitmarsh, O.A. Weser, D.A. Ross et al., Initial Reports of the Deep Sea Drilling Project, Vol.23. U.S. Govt. Printing Office, Washington, D.C., pp.813-820. Drake, C.L. and Girdler, R.W., 1964. A geophysical study of the Red Sea. Geophys. J. R. Astron. Soc., 8: 473-495. Girdler, R.W., Ericson, A.J. and Von Herzen, R.P., 1974. Downhole temperature and shipboard thermal conductivity measurements aboard R/V Glomar Challenger in the Red Sea. In: R.B. Whitmarsh, O.A. Weser et al., Initial Reports of the Deep Sea Drilling Project, Vol.23. U.S. Govt. Printing Office, Washington, D.C., pp.879-886. Gussow, W.C., 1968. Salt diapirism: importance of temperature and energy sources of emplacement. Mem., Am. Assoc. Pet. Geol., 8: 16-53. Humphris, C.C., 1979. Salt movement on continental slope, northern Gulf of Mexico. Bull., Am. Assoc. Pet. Geol., 63: 782-798. Jackson, M.P.A. and Seni, S.J., 1983. Geometry and evolution of salt structures in the marginal rift basin of the Gulf of Mexico, East Texas. Geology, 11: 131-135. Kupfer, D.H., 1968. Relationship of internal to external structure of salt domes. Mere., Am. Assoc. Petrol. Geol., 8: 79-89. Langseth, M.G. and Taylor, P.T., 1967. Recent heat flow measurements in the Indian Ocean. J. Geophys. Res., 72: 6249-6260. Le Pichon, X. and Francheteau, J., 1978. A plate tectonic model of the Red Sea - - Gulf of Aden area. Tectonophysics, 46: 369-406.
Mart, Y. and Hall, J.K., 1984. Structural trends in the northern Red Sea. J. Geophys. Res., 89: 352-364. Murray, G.E., 1968. Salt structures of the Gulf of Mexico basin. Mem. Am. Assoc. Pet. Geol., 8: 99-121, Parker, T.J. and McDowell, A.N., 1955. Model studies of salt-dome tectonics. Bull. Am. Assoc. Pet. Geol., 39: 2384 2470. Pautot, G. et al., 1984. Decouverte d'une nouvelle fosse a saumures, sediments metalliferes et appareil vocanique central en Mer Rouge. Soc. Geol. Fr., 10e Reun. Ann. (Abstr.), 10: 435-436. Phillips, J.D. and Ross, D.A., 1970. Continuous seismic reflection profiles in the Red Sea. Philos. Trans. R. Soc. London, Ser. A, 267: 143-152. Reiss, Z. et al., 1980. Late Quaternary paleoceanography of the Gulf of Aqaba (Elat), Red Sea. Quat. Res., 14: 294-308. Rezak, R. and Bright, T.J., 1981. Sea floor instability at East Flower Garden Bank, northwest Gulf of Mexico. Geo-Mar. Lett., 1: 97-103. Roeser, H.A., 1975. A detailed magnetic survey of the southern Red Sea. Geol. Jahrb., Reihe D, 13: 131-153. Ross, D.A. and Schlee, J., 1973. Shallow structure and geologic development of the southern Red Sea. Geol. Soc. Am. Bull., 84: 3827-3848. Ryan, W.B.F. et al., 1973. Initial Reports of the Deep Sea Drilling Project, Vol.13. U.S. Govt. Printing .Office, Washington, D.C., 1447 pp. Stoffers, P. and Ross, D.A., 1974. Sedimentary history of the Red Sea. In: R.B. Whitmarsh et al,, Initial Reports of the Deep Sea Drilling Project, Vol.23. U.S. Govt. Printing Office, Washington, D.C., pp.849-865. Uchupi, E. and Ross, D.A., 1986. The northern Red Sea: Continental i'ifting and salt tectonics. Geo-Mar. Lett., 5: 203-209. Wegener, A., 1929. Die Entstehung der Kontinente und Ozeane. Vieweg, Braunschweig. Whitmarsh, R.B. et al., 1974. Initial Reports of the Deep Sea Drilling Project, Vol.23. U.S. Govt. Printing Office, Washington, D.C., pp.849-865. Withjack, M., 1979. An analytical model of continental rift fault patterns. Tectonophysics, 59: 59-81. Withjack, M. and Scheiner, C., 1982. Fault patterns associated with domes - - an experimental and analytical study. Bull. Am. Assoc. Pet. Geol., 66: 302-316.
173
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
POST-MIOCENE RIFTING AND DIAPIRISM IN THE NORTHERN RED SEA Y O S S I M A R T a n d D A V I D A. ROSS National Oceanographic Institute, Tel-Shikmona, Haifa 31080 (Israel) Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.)
(Received March 31, 1986; revised and accepted July 21, 1986)
Abstract Mart, Y. and Ross, D.A., 1987. Post-Miocene rifting and diapirism in the northern Red Sea. Mar. Geol., 74:173 190. Rifts and diapirs are the dominant structures in the bathyal zone of the northern Red Sea, where commonly the diapirs ascend along normal faults associated with the rifts. In this manner, the diapirs form elongated features sometimes exceeding lengths of 30 km and widths of 4 km. They also penetrate a sedimentary sequence about 1000 m thickness or less, thicknesses generally considered to be barely adequate for the gravity conversion required to initiate diapiric motion. We suggest that development of diapirs under these minimal overburden conditions was made possible by the high thermal gradients in the area, resulting from the underlying attenuated continental crust and ascending mantle, that prevail in the rift system of the northern Red Sea. Diapiric evolution was also dependent on the extensional tectonic regime, with its numerous normal faults forming weaknesses along which the diapirs could move upwards.
Introduction T h e n o r t h e r n Red S e a is a u n i q u e e x a m p l e of a pre-sea-floor rift s y s t e m s e p a r a t i n g t h e relat i v e l y u n d e f o r m e d A r a b i a n a n d A f r i c a n plates. T h e r e g i o n is c h a r a c t e r i z e d by t h e r e m a r k a b l e j i g s a w fit of t h e N W - S E t r e n d i n g o p p o s i t e coastlines, w h i c h led W e g e n e r (1929) to p r e s e n t t h e Red S e a as a n e x a m p l e of t h e e a r l y s t a g e s of c o n t i n e n t a l drift. T h e m o u n t a i n o u s t e r r a i n s t h a t b o r d e r t h e Red Sea on b o t h t h e A f r i c a n a n d t h e A r a b i a n c o a s t l i n e s s h o w steep s e a w a r d slopes f l a n k e d by n a r r o w c o a s t a l plains a n d c o n t i n e n t a l shelves (Fig.l). W h e r e a s t h e c o a s t a l p l a i n s a r e b u i l t of N e o g e n e a n d Q u a t e r n a r y sediments, t h e a d j a c e n t m o u n t a i n s consist of e x t e n s i v e o u t c r o p s of P r e c a m b r i a n , igneous, a n d m e t a m o r p h i c suites (Coleman, 1974). C o n t i n e n t a l slopes a r e steep, descend0025-3227/87/$03.50
ing to a m a i n t r o u g h a t d e p t h s of 1200-1500 m. T h e deep a x i a l t r o u g h , w h i c h is a d o m i n a n t m o r p h o l o g i c a l f e a t u r e in t h e c e n t r a l Red Sea, is c o n s p i c u o u s l y a b s e n t in the n o r t h ( M a r t a n d Hall, 1984; U c h u p i a n d Ross, 1986). T h e m o r p h o l o g y of t h e m a i n t r o u g h of t h e nort h e m Red S e a is c o m m o n l y r u g g e d (Fig.2). F a u l t s , rifts a n d d i a p i r s a r e a b u n d a n t , a n d t h e d o m i n a n t t e c t o n i c r e g i m e is one of c r u s t a l e x t e n s i o n ( C o c h r a n , 1983; M a r t a n d Hall, 1984). As a rule, t h e s t r u c t u r a l a s s o c i a t i o n of rifts a n d diapirs is a c o m m o n o c c u r r e n c e , b u t t h e n o r t h e r n Red S e a a s s o c i a t i o n is unique, a n d differs f r o m m o s t o c c u r r e n c e s in s e v e r a l aspects: (1) T h e diapirs in t h e n o r t h e r n Red S e a a r e c o v e r e d by a 1 k m o r less t h i c k o v e r b u r d e n , w h e r e a s in m o s t r e g i o n s d i a p i r s r e s u l t f r o m a
© 1987 Elsevier Science Publishers B.V.
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Fig.1. (a) A generalized morphological map of the Red Sea and adjacent terrains. Deeps are marked by asterisks, and contours are in meters. (Prom Choubert, 1968, and Coleman, 1974). (b) Ship tracks of the R/V "Atlantis II" and the R/V "Shikmona" in the northern Red Sea and locations of profiles shown in
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Fig.2. Bathymetric map of the bathyal zone of the northern Red Sea (contours in meters). Note that the deeper parts of the northern Red Sea are located in its western section. The arcuate configuration of the deeper parts of the bathyal zone changes its trend from NNW SSE to NNE SSW, so that a gradual orientational shift from the Red Sea trend to the Dead Sea trend can be discerned. See Fig.lb for the ship tracks.
salt l a y e r b u r i e d u n d e r a n o v e r b u r d e n seq u e n c e a t t a i n i n g t h i c k n e s s e s of 3-6 km (Gussow, 1968; H u m p h r i s , 1979). (2) In the n o r t h e r n Red Sea the diapirs are typically elongated, a t t a i n i n g lengths exceeding 30 km and widths of a b o u t 4 km (Fig.3), whereas in m a n y o t h e r places diapirs commonly form s t r u c t u r a l domes t h a t are r o u n d or oblate. (3) In the n o r t h e r n Red Sea the diapirs c o m m o n l y a s c e n d e i t h e r a l o n g the b o u n d a r y faults of the rift or into the rift floor.
Geological setting T h e e v o l u t i o n of sea-floor s p r e a d i n g axes proceeds from an initial stage of c o n t i n e n t a l uplift and i g n e o u s activity, t h r o u g h an inter-
mediate stage of rifting and a t t e n u a t i o n of c o n t i n e n t a l crust, to the m a t u r e stage of a c c r e t i o n of o c e a n i c c r u s t and sea-floor spreading. These stages are well r e p r e s e n t e d in the Red Sea region. T h e s e p a r a t i o n b e t w e e n the African and A r a b i a n plates s t a r t e d d u r i n g the late O l i g o c e n e - e a r l y M i o c e n e with a N W - S E t r e n d i n g r e g i o n a l uplift and extensive intrusive and e x t r u s i v e v o l c a n i s m (Coleman, 1974). T h e e a r l y Red Sea uplift p r o b a b l y exceeded 3 km, as i n d i c a t e d by the widespread outcropping of the A r a b i a n - N u b i a n P r e c a m b r i a n massif on b o t h flanks of the Red Sea (Choubert, 1968). The 22-20 M a old alkali-basalt dikes associated with this t e c t o n i c phase in the n o r t h e r n Red Sea show a d o m i n a n t northw e s t w a r d t r e n d ( B a r t o v et al., 1980).
176
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Fig.3. Distribution of rifts and diapirs in the northern Red Sea. The arcuate configuration of the bathymetric features (Fig.2) can be closely correlated to the structural elements, showing the gradual change of tectonic trends. The association between the rifts and the diapirs can be seen in most places. F o l l o w i n g the r e g i o n a l uplift and volcanism, rifting o c c u r r e d a l o n g the axis of the e l o n g a t e d dome and e v e n t u a l l y a m a r i n e basis developed. Rifting of the Red Sea o c c u r r e d d u r i n g t h e e a r l y to middle M i o c e n e a l o n g t h e same n o r t h w e s t e r n trend. It is of i n t e r e s t to n o t e t h a t w h e r e a s the t e c t o n i c d e v e l o p m e n t of the Red Sea and the G u l f of A d e n is similar and a l m o s t c o n t e m p o r a n e o u s , and b o t h are extensions from the n o r t h e r n s e g m e n t of the N W I n d i a n O c e a n t e c t o n i c s p r e a d i n g system, the Red Sea was c o n n e c t e d d u r i n g this time to the M e d i t e r r a n e a n Sea (Stoffers and Ross, 1974; C o c h r a n , 1981, 1983). T h e Red S e a - M e d i t e r r a n e a n c o n n e c t i o n is significant, b e c a u s e w h e n
the M e d i t e r r a n e a n was affected by its late M i o c e n e salinity crisis (Ryan et al., 1973), the Red Sea was affected as well, and a s e q u e n c e of e v a p o r i t e s a t t a i n i n g t h i c k n e s s e s of a b o u t 3 km was also deposited t h e r e (Ahmed, 1972). A c c r e t i o n of o c e a n i c basalts in the Red Sea in the c e n t e r of the m a i n t r o u g h n e a r l a t i t u d e 18°N s t a r t e d 5 M a ago (Roeser, 1975; Le P i c h o n and F r a n c h e t e a u , 1978), and the a c c r e t i o n zone propagated northwestwards and southe a s t w a r d s at an a v e r a g e r a t e of 120 mm y r - 1 ( M a r t and Hall, 1984). An essentially continuous zone of sea-floor spreading now extends from l a t i t u d e s 16 ° to 21°N. F r o m 21 ° to 24°N is a t r a n s i t i o n a l zone w h e r e o c e a n i c basaltic accre-
177
tion is sporadic and occurs in basins separated by attenuated continental crust (Pautot et al., 1984). Farther north, between 24 ° and 28°N, is a rifted zone underlain by attenuated continental crust (Cochran, 1983; Uchupi and Ross, 1986). The basis structural trends in the northern Red Sea region were set during the middle-late Miocene. These include the NW-SE-oriented regional rifts that led to the evolution of the coastal plains and the continental margins on the Arabian and the African flanks. This structural framework extended from the southern Red Sea into the Suez rift, and was associated with the N E - S W extension into the Red Sea. The late Miocene-early Pliocene was a critical period in the development of the Red Sea. Concurrently with the beginning of seafloor spreading in the central part of the sea, the axis of the continental separation between Arabia and Africa in the northern region changed its trend from N W - S E to N-S. This change caused the termination of large-scale tectonic activity in the Suez rift and initiated the evolution of the Dead Sea rift (Mart and Hall, 1984). As a result of these tectonic changes, the Red S e a - M e d i t e r r a n e a n connection was disrupted and the Red Sea-Indian Ocean connection was opened through the Straits of Bab-al-Mandeb at the southern end of the Red Sea. The Indian Ocean connection ended the evaporitic deposition in the Red Sea. The continuation of the continental divergent motion in the Red Sea region during the Plio-Pleistocene led to the evolution of a double trough system in the central Red Sea. This consists of the main trough, formed in the Miocene and underlain by the late Miocene evaporites resting on attenuated continental crust (Drake and Girdler, 1964), and an axial trough, where accretion of oceanic crust occurred, dissecting the evaporitic sequence. At the same time crustal attenuation and tectonic distension continued in the Red Sea, leading to the formation of a series of rifts within the main trough. Concurrent with this distension, diapirs started to develop.
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179 NE
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Fig.5 (continued). (b) Interpretation of (a). Note the sequence of alternating concordant bedding and unconformities (heavy lines), suggesting a sedimentatiorL regime of intermittent activity. See Fig.lb for location.
Mechanical c o n s t r a i n t s o f shallow diapirs The diapirs in the northern Red Sea occur in a region where their sediment overburden is relatively thin. In most of the well-known diapiric regions, such as the Gulf of Mexico or northern Germany, diapirs pierce upward through a thick overburden, exceeding 5 km in places (e.g. Kupfer, 1968; Murray, 1968). These thick overburdens and the subsequent burial depths of the r o c k - s a l t sequences cause high pressures and high temperatures. The high pressure leads to consolidation of the overburden and to the increase in its density from about 2.0g cm -3 to values of 2.6g cm -3 or more, whereas halite maintains its 2.0-2.2 g cm-3 density. The high temperature enhances the ductility of the rock salt (Carter and Hansen, 1983). The density contrast between the evaporites and the overburden results in a metastable gravity configuration t h a t leads to diapirism when enhanced by increased salt ductility and affected by a perturbation (Parker and McDowell, 1955; Gussow, 1968). The perturbation is considered to be associated with faulting, which both triggers a seismic
shock and provides a favorable emplacement zone for the ascending diapir along the fault plane. The thickness of the Plio-Pleistocene sedimentary sequence, atop the late Miocene evaporites in the northern Red Sea, is approximately 500-1000m. This approximation is based on a 0.5-1.0 s average reflection time from the sea floor to reflector S (Phillips and Ross, 1970), which marks the top of the evaporite sequence, and on interval seismic velocity of about 2.0km s -~ (Drake and Girdler, 1964). Evidence t h a t the northern Red Sea diapirism was indeed triggered under thin overburden is suggested by the density conversion encountered in DSDP borehole 225. At this site in the central Red Sea, the density of the sediments at 5 m was 1.7 g cm -3, at 100m the same type of detrital sediments had a density of 2.2g cm -3, and dolomite beds encountered at 150 m depth had a density of 2.8 g cm-3. In comparison, halite found in the same borehole at depths greater than 180 m had a density of 2.2 g cm- 1, which is its normal density. Heat flow measurements in the central Red Sea showed anomalously high values of 153-276 mW m - i (Langseth and Taylor, 1967; Girdler, Ericson et al., 1974). These high values indicate favorable thermal conditions required for initiation of salt flow (Carter and Hansen, 1983). The measured thermal gradient of 117 K km-~ in the Red Sea (Whitmarsh et al., 1974) further supports this presumption, because laboratory tests have shown that lithostatic confining pressure is a critical parameter to salt ductility at pressures below 20 MPa. However, under conditions of higher pressures, temperature becomes the determining parameter (Carter and Hansen, 1983). The analysis of the origin of the northern Red Sea diapirs is based on the rough calculation of possible pressure and temperature at a depth of 1 km, which suggests a confining lithostatic pressure of about 30 MPa and a temperature of approximately 130°C. These temperatures and pressures are barely sufficient to trigger diapirism (Carter and Hansen,
180
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Fig.6. A seismic reflection profile showing a diapir piercing through a sequence of Plio-Pleistocene sediments. The sediment thickness is at least 0.75 s reflection time, which is approximately 750 m. The diapir is topped by a collapse structure due to dissolution of its upper part when it reaches the sea floor. See Fig.lb for location.
181
1983). However, the tectonic regime of extensional movement and normal faults in the region also can contribute to the development of the diapirs. The fault planes can be preferred zones for salt flow since they represent zones of weakness as a result of lithological discontinuity and structural gaps. Most fault types are favorable zones for diapiric ascent (Bishop, 1978), but normal faults seem to be the most likely to enhance diapirism under otherwise marginal conditions. Thus the burial depth of the Red Sea salt, the subsurface temperature, the overburden density, and the faulting regime all seem to enhance the development of diapiric structures provided that adequate perturbation occurs. The perturbation that triggers the diapirism is probably associated with the faulting and rifting in the northern Red Sea, but other types of triggering mechanisms should not be ruled out. A structural analysis of the salt domes of east Texas suggests that these diapirs were triggered by differential loading (Jackson and Seni, 1983). Reconstruction of the sequence of events in east Texas indicates that gravity gliding and prograding deltaic deposits initiated a flow of the underlying middle Jurassic Louann Salt. Deposition of the salt was probably associated with rifting, but the latter developments preceded the diapirism. Basin analysis of the east Texas salt domes enabled Jackson and Seni (1983) to suggest that there are critical thicknesses of the salt as well as the overburden below which the diapirs are not likely to develop. They suggested that the critical salt thickness for the initiation of the flow is approximately 600 m, and a similar thickness of the overburden is required as well. Although the triggering mechanism of the east Texas salt domes seems to differ from that of the northern Red Sea, the basic mechanical characteristics are very similar.
Rifts, diapirs and tectonic regime Rifts are very prominent in the northern Red Sea: a N W - S E trending system occurs at the
continental margins, and a N N W - S S E to N - S trending one is present in the bathyal zone. The outer system is associated with at least two levels of seaward reduction of topographic elevation - - the first drops from the elevated plateaus of Arabia and Africa to the coastal plains of the Red Sea, and the second drops from the coastal plains, across the continental margins, to the bathyal zone. The combined drop in elevation, including the calculated effect of erosion, is at least 5 km (Choubert, 1968; Coleman, 1974). The inner or bathyal rifting system, on the other hand, is associated with vertical offsets that rarely exceed 500 m, and the combination of grabens and horsts yields only a small change in bathymetry. The difference in the orientation between the two rifting systems is significant. Whereas the outer rifts extend from the northern Red Sea into the Gulf of Suez, the bathyal rifts extend from the Red Sea into the Dead Sea rift (Figs.1 and 3). The dimension of the rift-lows in the bathyal rift system can reach lengths exceeding 50 km and widths of up to 10km (Fig.3). Their morphology and internal structure suggest that they are active, as indicated by the concave stratification of the fill and normal boundary faults (Fig.4). The boundary faults are longer than the morphological depressions associated with them. However, structural evidence shows that these faults are not presently active along their entire lengths. In places of regular bathymetry, seismic reflection profiles show that the subsurface layers have the typical concave, inter-rift structure except for their upper sections, where the stratification shows little displacement between the rift and the adjacent terrain. These rifts are buried and do not affect the sea floor (Fig.5), but their correlation with active rifts and half-rifts supports the interpretation of structural continuity (Mart and Hall, 1984). The spatial association of the rifts of the northern Red Sea and diapiric structures suggests generic correlation between them. Although the diapirs have not been sampled in ~:he northern Red Sea, they probably are built
182 of late Miocene evaporitic rocks comparable to the late Miocene evaporites that crop out nearby and have been sampled in wells near the coast. Drilling results from the marginal and bathyal zones of the Red Sea (Ahmed, 1972; Whitmarsh et al., 1974) suggest that gypsum was deposited on the margins and rock salt in the bathyal zone during the late Miocene. Further evidence of the composition of the evaporites is provided by several diapirs topped by collapse structures, resulting from dissolution of the evaporites by sea water (Fig.6). In some places the diapirs pierce the overlying sediments and affect the sea floor forming elongated hills reaching lengths exceeding 30 km (Fig.3). These diapirs which protrude through the sea floor are commonly covered by an envelope of non-evaporitic sediments so that they are not affected by sea water dissolution (Fig.7). Piercement structures are also evident in the seismic profiles from places where the diapirs do not affect the sea-floor morphology. In these instances it is suggested that the diapirs either ceased to ascend or, alternatively, have not yet reached the sea floor (Fig.8). Where the diapirs barely reach the sea floor, they cause the uppermost sediments to be uplifted, folded and faulted by the ascending diapir (Fig.9). Diapirs that cause intensive upward folding in the overlying sediments without piercing them are common. This type of diapir is also abundant in other diapiric provinces (Bishop, 1978), but is difficult to identify positively from the seismic profiles due to its similarity to nondiapiric fold structures. The proximity and the structural association between t h e piercing and the non-piercing diapirs, as well as the abundance of anticlines compared with the extreme rareness of synclines in the northern Red Sea, support the presumption that most of the anticlinal structures in the northern Red Sea are probably fold-forming, non-piercing diapirs. Most of the diapirs observed in the northern Red Sea are associated with faulting and rifting. Many of the diapirs pierced their way upward along boundary faults of rifts (Figs.7
and 10); others just forced their way into rift floors (Fig.ll). The displacements between the diapirs and the most recent sediments suggest that in many places diapirism is active at present (Fig.9). Red Sea diapirs are smaller than the associated rifts, and it seems the rifting was instrumental in initiating the diapiric ascent. Our findings are supported by experimental and theoretical models, which indicate that rifts can be the product of structural extension and collapse that resulted from diapiric piercing and uplift motion (Withjack, 1979; Withjack and Scheiner, 1982). Therefore the evidence indicates that the faults of the northern Red Sea bathyal rift formed planes of weakness in the overburden which facilitated the diapiric activity under marginal constraints.
The origin o f the diapirs and the rifts Data from the northern Red Sea show two systems of rifts: regional and local. The regional rift system is the Dead Sea-Red S e a - E a s t African Rift system which is a product of mantle diapirism that led, in the central Red Sea, to accretion of new oceanic crust (Ross and Schlee, 1973). The local system is that of rifts in the bathyal zone. The regional system was initiated in the late Oligoceneearly Miocene, whereas the local system is of Plio-Pleistocene age. However, both systems are presently active. The evolution of the regional system started with uplift and subsequent development of a rift along the apex of the uplifted zone. This type of structural evolution was described by experimental and numeric models (Withjack, 1979; Withjack and Scheiner, 1982). Analysis of the local diapir system suggests that the evolution of the Red Sea bathyal-zone structures was dependent primarily on the deposition of the thick evaporitic series during the late Miocene. Then in the Plio-Pleistocene continued crustal attenuation contributed to the structural evolution of the northern Red Sea by steepening the thermal gradient and thus increasing ductility of the late Miocene
183
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184
evaporites. Furthermore, the extensional tectonic regime led to the development of a series of rifts in the bathyal zone of the northern Red Sea. The rifts developed in the Plio-Pleistocene, and the late Miocene evaporites were not deposited in them (Mart and Hall, 1984). Sedimentation in the northern Red Sea also played a critical role in the evolution of the diapirs. The thickness of the Plio-Pleistocene sedimentary sequence, which seems to be barely adequate for the densities required for gravity inversion, was an essential step in the process. The average rate of sediment deposi-
W
tion during the last 5 Ma is 0.2 m 1000 yrs-1, which is low compared with the calculated rate of 1.0 m 1000 y r s - 1 in the Gulf of Elat (Reiss et al., 1980). However, sediment densities higher than 2.8g cm -3 found at depths of 150m indicate t h a t the lithostatic confining pressure was sufficient to trigger diapirism under the prevailing geothermal and tectonic regimes. The depicted sequence of events in the structural evolution of the northern Red Sea indicates that the diapiric structures of the region stemmed from late Miocene evaporites. Subsequent to burial under at least 600 m of
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Fig.8. (a) A seismic reflection profile showing a series of anticlines. The flanks of the anticlines dip steeper with the increase in depth, and it is suggested t h a t they represent incipient diapirs. The steeply dipping seismic reflections should not be confused with diffraction effects. (b) A seismic reflection profile showing two adjacent diapirs. The n o r t h e r n one has already pierced its way t h r o u g h the sedimentary cover, but the southern one is still in its incipient stage, upfolding the overlying strata. See Fig.lb for location.
186
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Fig.9. A seismic reflection profile of an ascending diapir that uplifts and breaks the overlying sediments. See Fig.lb for location.
187
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FigA0. A seismic reflection profile showing multiple diapiric structures in a faulted zone. See Fig.lb for location.
188
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Fig.ll. (a) A seismic reflection profile showing a diapiric piercement into a rift floor. (b) A seismic reflection profile showing a series of diapirs that penetrated a rift floor. Note the morphological similarities between these diapirs and the incipient diaplrs presented in Fig.8a. See Fig.lb for location.
P l i o - P l e i s t o c e n e sediments, d i a p i r i c g r o w t h w a s t r i g g e r e d by h i g h heat-flow a n d c o n t r o l l e d b y rifting. T h e r e is g r o u n d to p r e s u m e t h a t d i a p i r i c t e r r a i n s a l o n g rifted p a s s i v e c o n t i n e n tal margins started their evolution under c o n s t r a i n t s s i m i l a r to t h o s e of t h e n o r t h e r n Red Sea.
P r o g r a m . D.A.R. w a s funded by t h e N a t i o n a l S c i e n c e F o u n d a t i o n , G r a n t 76-10417, a n d t h e D e p a r t m e n t of C o m m e r c e , N O A A , N a t i o n a l Sea G r a n t College P r o g r a m , u n d e r G r a n t No. NA84AA-D-00033 (M/0-1). T h i s p a p e r is W o o d s H o l e O c e a n o g r a p h i c I n s t i t u t i o n Contrib u t i o n N u m b e r 6189.
Acknowledgements
References
W e wish to t h a n k N.L. C a r t e r , R. R e z a k a n d E. U c h u p i for t h e i r s u g g e s t i o n s d u r i n g t h e p r e p a r a t i o n of this paper. Y.M. w a s p a r t l y s u p p o r t e d by t h e I n t e r n a t i o n a l O c e a n D r i l l i n g
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