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Short term development of intracontinental rifts, with reference to the late Quaternary of the Rukwa Rift (East African Rift System)
Marine and Petroleum Geology 18 (2001) 307±317
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Short term development of intracontinental rifts, with reference to the late Quaternary of the Rukwa Rift (East African Rift System) T. Kjennerud a,*, S.J. Lippard b, P. Vanhauwaert c a
b
SINTEF Petroleum Research, N-7465 Trondheim, Norway Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway c Renard Centre of Marine Geology, Universiteit Gent, Gent, Krijgslaan 281-S8, B-9000 Gent, Belgium Received 7 August 2000; received in revised form 8 December 2000; accepted 15 December 2000
Abstract Relatively low-resolution seismic data and high contemporaneous rift topography normally limit quantitative analysis of normal faults in rifts. The availability of a recently collected high-resolution re¯ection seismic survey in the SE part of the presently active Rukwa Rift (East African Rift System) coupled with high sedimentation rates in the submerged part of the rift makes detailed quantitative analysis possible. High-resolution (down to about 1 m) seismic data penetrate ca. 300 m (representing about 150,000 years) of the uppermost sediments and show ®ne details of normal faulting and related structures. Displacements on the faults, which occur in the hanging wall of the major rift boundary fault, range from a few metres up to 100 m. They show increasing displacements with depth and characteristic rollover folding of the hanging walls. Fault propagation occurred upwards through the rapidly accumulating lake sediments and was preceded by ¯exuring and folding of the sediments. The geometry of the seismic sequences with alternating wedge-shaped and tabular units re¯ects pulsed activity on the faults. Fault displacement rates of up to 1.6 mm yr 21 have been estimated. Each extensional pulse has a duration of 1000s to 10,000s of years. The quiescent stages have a similar duration. Up to 2.5% extension has occurred during the recent evolution. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Normal faulting; Rukwa Rift; Late Quaternary
1. Introduction Detailed quantitative analysis of fault movements in rift basins is often hampered by the fact that fault displacement rates are generally high relative to sedimentation rates. This creates marked syn-rift relief with attendant footwall uplift and erosion and hanging-wall subsidence and sediment starvation (Bertram & Milton, 1989; Doglioni, D'Agostino, & Mariotti, 1998; RavnaÊs & Steel, 1998). As an example, the sedimentation rates on the Visund Fault block half-graben, northern North Sea have been reported as 40 mm ka 21 in the syn-rift late Jurassic (Fñrseth, Sjùblom, Steel, Liljedahl, Sauar, & Tjelland, 1995). These relatively low sedimentation rates partly explain the high relief at the time (e.g. Kjennerud et al., 2001). In such a setting it is dif®cult to calculate fault displacement rates. However, if suf®cient data are available in areas where the sedimentation rate equals or exceeds the fault displacement rate, then the displacement history can be established quantitatively * Corresponding author. Tel.: 147-7359-1257; fax: 147-7359-1102. E-mail address: [email protected] (T. Kjennerud).
over relatively short time periods. Doglioni et al. (1998) pointed out that if the sedimentation rate exceeds the subsidence rate, then the faults only control the geometry of the syn-tectonic beds but not the facies. This appears to be true of the south-eastern part of the Lake Rukwa Rift basin where the Neogene (Plio-Pleistocene) to Recent Lake Beds are uniform muds deposited at average rates ranging from 0.5 to 2 mm yr 21 (Ceramicola, Vanhauwaert, Kilembe, & De Batist, 1997; Talbot & Livingstone, 1989; Wescott, Krebs, Englehardt, & Cunningham, 1991). The Rukwa Rift is thus an area where it is possible to quantify rift development in a detailed manner. The present-day development in the Rukwa Rift can be used as an analogue to ancient rift basin formation. The Rukwa Rift (Fig. 1), situated between the Tanganyika and Malawi rifts, is a Neogene rift superimposed on a late Permian (Karroo) rift (Kilembe & Rosendahl, 1992; Morley, Cunningham, Harper & Wescott, 1992; Wescott et al., 1991). The NW±SE-trending rift is approximately 300±350 km long and 30±50 km wide and the shallow Lake Rukwa (165 km long and up to 50 km wide, but less than 16 m deep) covers a large part of the basin ¯oor. The
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(01)00007-1
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Fig. 1. Regional structural map of the southern part of the Rukwa Rift.
north-eastern ¯ank of the rift is continuously bounded by the remarkably straight Lupa Fault, with a maximum displacement of 10±12 km on the top basement. The south-western side is a segmented margin (U®pa fault escarpment). Kilembe and Rosendahl (1992) and Morley et al. (1992) documented the deep structure of the rift, based largely on conventional seismic data. The uppermost 200 m of sediments, probably dating back to some 150,000 yr BP, in the south-eastern part of the lake have been imaged by high-resolution re¯ection seismic data (Ceramicola et al., 1997). Morley, Vanhauwaert and DeBatist (2000) used these data to investigate highfrequency cyclic fault activity related to the Lupa Fault. The aim of the present paper is to quantify the recent structural history based on the high-resolution seismic data set. Particular emphasis is put on calculating fault displacement rates and extension by the restoration of two pro®les. 2. Geological setting The western branch of the East African Rift System
comprises the Kivu-Albert, Tanganyika, Rukwa and Malawi rift zones (Ebinger, 1989). The Tanganyika and Malawi rifts are characterised by large deep rift lakes. The Rukwa Rift lies east of Lake Tanganyika and north of Lake Malawi, and has a NW±SE trend that is oblique to the N±S trend of these two rifts. Two studies, based largely on the same conventional industrial seismic data, proposed contrasting origins for the Rukwa rift: (1) as an extensional to oblique-slip rift basin (Morley et al., 1992) and (2) as a strike-slip to oblique-slip pull-apart basin (Kilembe & Rosendahl, 1992). Evidence for strike-slip (¯ower structures, en-echelon fault patterns) was cited by Kilembe and Rosendahl (1992), while Morley et al. (1992) recognised that many of the faults within the basin are listric and dip-slip in character. Delvaux, Kervyn, Vittori, Kajara, and Kilembe (1998) have documented a possible recent change from dip-slip to dextral strike-slip during the late Quaternary. They also describe a general NE-ward tilting of the entire basin. However, modern earthquake data shows NE±SW extension (Foster & Jackson, 1998).
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conventional seismic data by the authors has recon®rmed the predominance of the NNW±SSE fault trend (mean N308W) within the southern part of the rift (Fig. 2). 3. Shallow seismic data set
Fig. 2. Rose diagram (half-circles at 5% intervals) of fault trends in the Rukwa rift. This diagram is a frequency versus azimuth plot of top basement faults (19 data points) within the rift.
A high-resolution seismic data set was collected in the southern part of Lake Rukwa (Fig. 3) in 1994 using a sparker source (Ceramicola et al., 1997). The data have a vertical resolution of approximately 1 m and penetrate down to
The deep seismic data image faults from the surface down to the top basement re¯ections. Some of the faults, particularly in the northern part of the basin, show evidence for larger displacements at top basement than at top Karroo (Morley et al., 1992), indicating that movements took place in the late Palaeozoic. In the central and southern parts of the rift the top basement and top Karroo re¯ectors are generally more or less equally displaced indicating dominantly Tertiary movements. However, the Karroo sequence clearly thickens towards the Lupa Fault, showing that this fault was active during both late Palaeozoic and late Tertiary rifting (Morley et al., 1992). Displacements of the Neogene Red Sandstones (see discussion in Morley et al., 2000) and the Pliocene±Recent Lake Beds can be seen to decrease upwards towards the lake ¯oor. This trend is con®rmed on the shallow seismic data (see below). The faults shown on Fig. 1 are mapped at the top Red Sandstone/base Lake Beds level. As noted by Kilembe and Rosendahl (1992) and Morley et al. (1992), the faults within the Rukwa rift show NNW±SSE trends that are oblique to the NW±SE Lupa Fault trend (N508W). More recent mapping of the
Fig. 3. Data base map showing the location of the seismic grid, the R96-1 core and the two reconstructed pro®les.
Fig. 4. (a) Seismostratigraphic units used in the present study and their dating. Note that only U10, U9 and the upper part of U8 have been cored. The remainder of the datings have been extrapolated. (b) Shallow core R96-1 tied in with line 22.
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about 200 m. The re¯ections show aggrading depositional geometries, with high re¯ection continuity and regular layering. The seismic sections show layers thickening towards the Lupa Fault in the north-east. Nine seismic units have been interpreted in the data set (Fig. 4a) (Morley et al., 2000). In the present study, the youngest unit U10 has been divided into U10a and U10b. The seismic data show few of the common
indicators of high-energy environments, such as onlapping, downlapping or mounding features. The mainly parallel to sub-parallel re¯ections suggest a relatively low-energy hemipelagic environment (Morley et al., 2000), an interpretation which is supported by the available core data (e.g. Talbot & Livingstone, 1989; Thevenon, Williamson, & Taieb, 2001). The seismic units have been interpreted on the
Fig. 5. Seismic example of the structures observed in the high-resolution data. The section has been taken from line 17. White bands on seismic are acquisition artefacts.
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basis of whether they display growth towards the faults or not. Generally the units showing internally parallel re¯ections separate units that show divergent re¯ections internally and thicken towards the faults. Morley et al. (2000) reported four periods with marked thickening towards the Lupa Fault and three periods of little or reduced thickening. This led them to propose cyclic fault activity within the Rukwa Rift. 3.1. Dating of the seismic units Two shallow cores have been collected in the southern part of Lake Rukwa. The ®rst was collected in 1960 (Haberyan, 1987; Talbot & Livingstone, 1989), and the second in 1996 (Barker, Telford, Francoise, & Thevenon, 2001; Thevenon et al., 2001). The second core (R96-1) has been tied to seismic line 22 (Fig. 4b) (Morley et al., 2000). This core penetrated units U10, U9 and the upper part of U8, giving radiocarbon dates of 8400 yr BP for the base of U10 and 14,500 yr BP for the base of U9 (Barker et al., submitted). The ages of the remaining units are uncertain, and extrapolation of sedimentation rates back in time is the only possibility. The core is from a fault block crest. It is 12 m long and covers ca. 15,000 yr. The sedimentation rate was relatively constant at the well position at ca. 1 mm yr 21 (Morley et al., 2000). However, the sedimentation rates are very different spatially for the two units. U10 is thickening towards both the intrabasinal faults and the Lupa Fault. Morley et al. (2000) reported maximum sedimentation rates for U10 up to 6 mm yr 21. U9 is a
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tabular unit and has a relatively constant sedimentation rate of ca. 1 mm yr 21. The exact position of the 1960 core is not known, due to navigation problems at the time, but it is 23.1 m long and covers 12,710 yr (Talbot & Livingstone, 1989), i.e. more or less the same time interval as the second core. Since the 1960 core contains twice as much sediment and covers slightly less time than the R96-1 core, it is likely that the older core was taken from the hanging wall of a fault, rather than a footwall crest. The sedimentation rates in the 1960 core vary between 0.64 and 8 mm yr 21, based on the dating reported by Talbot and Livingstone (1989). The maximum sedimentation rate (8 mm yr 21) occurred between 3300 and 4390 yr BP. This increase in sedimentation rate is not recorded in the R96-1 core. It is thus likely that this maximum sedimentation rate corresponds to an increase in accommodation space creation/fault displacement rates. Unfortunately, there is at present no additional seismostratigraphical resolution within U10 that is able to resolve this. The ages shown in Fig. 4a were obtained by using a crestal sedimentation rate of 1 mm yr 21 as suggested by Morley et al. (2000) and extrapolating downwards from the well position. Based on the available core data, the extrapolated age estimates are likely to be towards the maximum. There are also no indications in the high-resolution seismic data of any major break in sedimentation. It is worth noting that the seismic interpretation is most detailed for the younger part of the section. Compared to the younger units, units U2 and U3 cover about half of the total age span. It is important to note that, in addition to the tectonic in¯uence
Fig. 6. Structural map based on the shallow seismic data. Contours indicate the two-way time thickness (ms) of U10. The rose diagram (half-circles at 5% intervals) represents fault trends from the shallow seismic data on a length versus azimuth plot (39 data points).
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small (less than 0.5 m high) escarpments on the lake ¯oor. All the faults show increasing displacement with increasing depth. 3.3. Folding Folding and ¯exuring can be seen in the hanging walls of most of the faults. Deeper horizons show rollover into the fault plane, shallower horizons may, however, show a hanging-wall syncline and normal drag (arrow on Fig. 5). Folding can also be seen above upward-propagating faults A and B. The folds are due to syn-tectonic deformation and not due to post-tectonic modi®cation. 3.4. Structural map
Fig. 7. Fault displacement plot for the faults on lines 17 and 23. The displacements were measured from the reconstructed sections by plotting the displacement on the oldest horizon (top U2) against age. Three of the faults are blind and the curves do not reach the 0 age axis.
on sediment deposition within the rift, climatic variations have played a major role because of the shallow nature of the lake (e.g. Delvaux et al., 1998). 3.2. Fault expression The shallow seismic data clearly image the upper part of the Lake Beds, the faults that cut the sediments and the associated folding (Fig. 5). Recognisable displacements range from about 1 m up to about 100 m. Most faults appear to reach close to the lake ¯oor, but several are blind and appear to die out within the sediments (faults A and B on Fig. 5). Some faults even form
In the area of the shallow seismic survey most of the faults are synthetic to the Lupa Fault with NW±NNW trends and downthrow to the SW (Fig. 6) (mean trend is 3408). This agrees with the mapping of the faults cutting the top Red Sandstone/base Lake Beds on the deep seismic data (Morley, Wescott, Harper, & Cunningham, 1999). The open nature of the shallow seismic grid (line spacing 1±5 km) means that it is dif®cult to be con®dent in tying faults from line to line. As the faults have displacements of less than 100 m, it is unlikely that they have lengths much greater than 5 km (e.g. Dawers & Anders, 1995). In addition to the predominant NW±NNW trends, some faults strike N±NNE. Notable is a NNE-trending, 1±1.5 km wide, approximately 5 km long, graben seen on the NWtrending lines 26 and 12. Four to ®ve inward facing faults form nested inner and outer grabens. The NE-trending line 25 runs along the axis of the graben, but it is dif®cult to tie the faults to the crossing lines in order get a clearer picture of the structures. The origin of the graben is unclear, but it is clearly transverse to the main structures in the Rukwa Rift. The graben occurs where there is an increase in tilt of the
Fig. 8. Fault displacement versus time for the individual faults on line 17.
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Fig. 9. Fault displacement versus time for the individual faults on line 23.
beds towards the SE and the main depocenter adjacent to the Lupa Fault. It may thus be a relatively super®cial structure caused by gravity sliding of the sediments and not related to the NW-trending rift structures. 4. Fault displacement rates The seismic lines 17 and 23 (Fig. 3) were chosen for further analysis (fault displacement rates and structural restoration). The sections were depth converted using a velocity of 1450 m s 21 for the water column and 1600 m s 21 for the sediments. The restoration was carried out using the restoration/balancing software Locace (Beicip Franlab, 1999). A uniform lithology consisting of 90% shale and 10% silt was assigned to all the layers for use in the decompaction, using the exponential curves of Sclater and Christie (1980). A plot of fault displacement versus time is shown for the faults on lines 17 and 23 (Fig. 7). The displacements were measured from the reconstructed sections, by plotting the displacement on the oldest horizon (top U2) through time. Displacement rates range up to 1.6 mm yr 21, with the average displacement rate for the largest fault (103 m total displacement) over the whole period of ca. 150,000 yr being 0.7 mm yr 21. The individual faults on both lines are shown in Figs. 8 and 9. The general picture from the displacement plots is one of pulsating tectonic activity. The high resolution of the seismic data is crucial for this observation, as such ®ne extensional phases would not be observed in conventional seismic data. Distinct periods with marked displacement on most faults, represented by wedging units in the sections, are separated by periods of no or little displacement, represented by tabular units in the sections. Five periods of increased displacement and four more quiet periods are indicated in the plots. Most of the displacement occurred
between 152,200 and 109,900 BP (U2), 82,200 and 70,300 BP (U4), 55,000 and 38,800 BP (U6), 23,500 and 14,500 BP (U8) and during the last 8400 yr (U10). 5. Structural evolution The restorations of lines 17 and 23 (see Fig. 3 for location) were performed in two runs. The aim of the ®rst run was to balance the interpretation. The exact location of some of the fault planes was hard to de®ne at depth. Vertical simple shear (Gibbs, 1983; Verrall, 1981) was used as the deformation mechanism, which gave the best geometrical ®t. When assuming vertical shear, the extension is the same as that measured when summing the fault heaves. The second run was carried out in order to restore the two sections based on the corrected geometries. Shallow seismic line 17 (Fig. 10a) is located in the NW of the study area and is 8 km long. A symmetrical graben is located between 5 and 6 km, where also the maximum thickness of the interpreted seismic units was observed. The shallow seismic line 23 (Fig. 10b) is located in the centre of the study area and is 12.5 km long. The re¯ections are all generally dipping towards the Lupa Fault (northeastwards). The north-easternmost fault at 9.5 km shows the largest displacement. For every time step, the relief was restored ¯at. This was justi®ed because there are no indications of high relief and little indication of erosion observed in the seismic data set, and also because of the very high sedimentation rates. The evolution is described here in terms of each major pulse of extension. Most faults were active during the deposition of U2. The top U2 re¯ector was only interpreted on line 23, while the top U1 re¯ector was only interpreted on line 17. Line 17 displays the most uneven thickness distribution and the main sediment accumulation occurred in the
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Fig. 10. (a) Structural reconstruction of line 17. (b) Structural reconstruction of line 23.
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graben between 5 and 6 km. U3 was characterised by lower activity as seen on line 23, and ended the ®rst extensional pulse. There was renewed activity during U4, although the same faults were active as in the previous extensional stage. U5 is evenly distributed on line 17, while a slight thickness increase occurred most notably towards the fault at 9.5 km on line 23. U5 was characterised by little extension, and ended the second extensional pulse. Increased extension occurred during the deposition of U6. Some of the faults previously active on line 17 were no longer active. This is an indication that the extension had a longer wavelength in the NW. All faults were active on line 23. The sediments are generally thickening towards the Lupa Fault; however, the maximum sediment accumulation was in the half-graben at 9.5 km. U7 was deposited during a tectonically more quiet period. The unit shows a slight thickening towards the Lupa Fault on line 17, while the thickness is almost constant on line 23. Here the western boundary fault was active. U7 ended the third tectonic pulse. The U8 period was characterised by tectonic reactivation. The unit displays growth towards the intrabasinal faults and, most notably on line 23, also towards the Lupa Fault. The tabular unit U9 draped U8, thus ending the fourth tectonic pulse. Growth towards some of the faults occurred on line 23. Considerable extension and rotation towards the NE occurred during U10 (top U10a is only interpreted on line 23). At the same time as the NE part of the basin received sediment, the SW part near the shoreline was uplifted and eroded. The erosion was probably part tectonic and part climatic in origin. The erosion has been estimated to be maximum 6.3 m on line 17 and
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maximum 10.3 m on line 23. This was accommodated by a tilt of 0.128 on line 17 and mostly footwall uplift on line 23. U10b on line 23 displays more growth towards the Lupa Fault. Thus, most of the present extension is taken up on the Lupa Fault; however, thickening above some of the intrabasinal faults is seen in U10b. This indicates that the faults are propagating upwards at present. 5.1. Extension versus time The reconstruction indicates ca. 2.5% extension over 150,000 yr along line 17 and ca. 1% over 80,000 yr along line 23 (Fig. 11). This is equivalent to strain rates of ca. 5 £ 10215 s21 : Over shorter time intervals the rates were much higher, for example, most of the extension (1.5%) on line 17 appears to have occurred between 80,000 and 70,000 yr BP. The jumps in extension rate correlate with the jumps in fault displacement described in Section 4. The second largest increase in extension has occurred during the last 8400 yr. This event has accounted for 0.3% extension on line 17. Nicol, Walsh, Watterson, and Underhill (1997) presented strain rates calculated over 1±40 Myr for the North Sea 2 £ 10216 s21 ; Kenya Rift 2:5 £ 10216 s21 ; Basin and Range 4 £ 10216 s21 and Aegean 4 £ 10215 s21 : The average strain rates in Rukwa may be lower, if measured on a similar time scale for the Neogene rift phase. 18% extension (based on adding the extension of the Red Sandstone and Lake Bed units in Kilembe & Rosendahl, 1992) over 11 Myr (earliest late Miocene) gives a strain rate of 5 £ 10216 s21 : Out of the four examples cited above, only the North Sea and the Kenya Rift are directly comparable to the Rukwa Rift as intracontinental rifts. The recent strain rate in Rukwa is an order of magnitude larger than these two examples. 6. Discussion and conclusions
Fig. 11. Extension versus time for lines 17 and 23. Extension was computed from the reconstructed pro®les by taking (reconstructed pro®le length/ original length) £ 100.
In general most studies in palaeo-rift basins are based on conventional seismic data; these data usually have a vertical resolution around 25 m. With such data the features seen on the Rukwa high-resolution data set (ca. 1 m resolution) would barely be visible. The most uncertain part of the present analysis is related to the dating of the undrilled units. We have applied an extrapolation of sedimentation rates, where it is assumed that sedimentation rates stayed relatively constant as indicated by core R96-1. We are fully aware of the danger in doing this; however, the approximate datings give an indication of the rates of recent tectonic processes in the Rukwa Rift. It is hoped that, in the near future, new information on the dating of the sediments in the Rukwa Rift will become available. The high-resolution seismic data and the high sedimentation rates in Lake Rukwa have allowed the quanti®cation of movements on intrabasinal normal faults in a detailed
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manner. The fault displacement analysis indicates ®ve periods of increased extension, separated by periods of relative tectonic quiescence. These ®ve extensional pulses are also evident from the extension versus time plots (Fig. 11). The duration of each extensional pulse is between ca. 8000 and 40,000 yr, whilst the duration of the time intervals displaying fault quiescence is between ca. 6000 and 28,000 yr. This corresponds with the smallest fourth-order stage in the tectonic and stratigraphic hierarchy proposed by Fñrseth and RavnaÊs (1998) for the late Jurassic evolution of the Oseberg Fault Block in the North Sea Rift. Their fourthorder stages have durations of 1000s to 10,000s of years. By contrast, their third-order stages have durations of 1±2 Myr. It should be noted that their fourth-order stages were inferred rather than observed. We suggest that, added to the scheme of Fñrseth and RavnaÊs (1998), the fourthorder faulting stages should be separated by quiescence stages of the same order of magnitude duration as the faulting stages. Based on the restored sections, ®ve phases of extension have been quanti®ed in the Rukwa Rift during the last 150 ka. This is represented by a total of 2.5% extension on line 17 and 1% on line 23. 60% of the extension on line 17 appears to have taken place during just 10,000 yr during the deposition of U4. Although the dating for this interval is uncertain, this suggests that large amounts of extension may occur during very short time intervals in rift basins. The total amount of Neogene extension in the southern part of the Rukwa Rift probably amounts to 18% (Kilembe & Rosendahl, 1992). From the evidence presented in this paper, it seems that the Rukwa Rift is presently in the middle of a tectonically active stage. If the 2.5% extension is extrapolated over 11 Myr, it would represent more than 180% extension. It is thus likely that extension occurs in pulses of various orders of magnitude (e.g. Fñrseth & RavnaÊs, 1998; Morley et al., 2000). The structural restoration indicates that the most recent part of the development has been characterised by a tilt towards the NE, associated subsidence and sedimentation in the NE and uplift and erosion in the SW. Recent tilt towards the NE, has also been noted by Delvaux et al. (1998). The basin was tilted 0.128 on line 17. This event has occurred during the last 8400 yr. There are, however, also variations within this time span. As mentioned above, the ®rst shallow core suggested a sedimentation maximum between 3300 and 4390 yr BP, when 9.5 m of the total 23.1 m of the core was deposited. This leads us to suggest that the tectonic pulses recognised in the present work probably consist of even smaller scale faulting pulses and fault quiescence stages internally. Acknowledgements Tomas Kjennerud was ®nanced in the ®nal phase of the work by grant 138620/431 from the PetroForsk programme
(The Research Council of Norway). Pieter Vanhauwaert was ®nanced by a grant from IWT. Marc De Batist is thanked for initiating the Rukwa work at RCMG. Vedastus J. Seda is thanked for helping with the fault interpretation shown in Figs. 1 and 2. Cindy Ebinger, Roy H. Gabrielsen, Martin Hamborg, Chris K. Morley, Mike Talbot and Walther Wheeler are acknowledged for commenting on earlier versions of this paper. References Barker, P., Telford, R., Francoise, G., & Thevenon, F. (2001). Submitted for publication. Beicip Franlab (1999). Locace 2.4 user's guide (variously paginated). Bertram, G. T., & Milton, N. J. (1989). Reconstructing basin evolution from sedimentary thicknesses: the importance of palaeobathymetric control, with reference to the North Sea. Basin Research, 1, 247±257. Ceramicola, S., Vanhauwaert, P., Kilembe, E., & De Batist, M. (1997). High-resolution re¯ection seismic investigation of sediments of Lake Rukwa (Tanzania). International Project on Paleolimnology and Late Cenozoic Climate Newsletter, 10, 113±122. Dawers, N. H., & Anders, M. H. (1995). Displacement-length scaling and fault linkage. Journal of Structural Geology, 5, 607±614. Delvaux, D., Kervyn, F., Vittori, E., Kajara, R. S. A., & Kilembe, E. (1998). Late Quaternary tectonic activity and lake level change in the Rukwa Rift Basin. Journal of African Earth Sciences, 26, 397±421. Doglioni, C., D'Agostino, N., & Mariotti, G. (1998). Normal faulting versus regional subsidence and sedimentation rate. Marine and Petroleum Geology, 15, 737±750. Ebinger, C. J. (1989). Tectonic development of the western branch of the East African Rift System. Geological Society of America Bulletin, 101, 885±903. Fñrseth, R. B., & RavnaÊs, R. (1998). Evolution of the Oseberg Fault-Block in context of the northern North Sea structural framework. Marine and Petroleum Geology, 15, 467±490. Fñrseth, R. B., Sjùblom, T. S., Steel, R. J., Liljedahl, T., Sauar, B. E., & Tjelland, T. (1995). Tectonic controls on Bathonian±Volgian syn-rift succesions on the Visund fault block, northern North Sea. Sequence stratigraphy on the northwest European margin. R. J. Steel, V. L. Felt, E. P. Johannesen & C. Mathieu. Norwegian Petroleum Society Special Publication, 5, 325±346. Foster, A. N., & Jackson, J. A. (1998). Source parameters of large African earthquakes: implications for crustal rheology and regional kinematics. Geophysical Journal International, 134, 422±448. Gibbs, A. D. (1983). Balanced cross-section construction from sections in areas of extensional tectonics. Journal of Structural Geology, 5, 153± 160. Haberyan, K. A. (1987). Fossil diatoms and the palaeolimnology of Lake Rukwa, Tanzania. Freshwater Biology, 17, 429±436. Kilembe, E. A., & Rosendahl, B. R. (1992). Structure and stratigraphy of the Rukwa Rift, East Africa. Tectonophysics, 209, 143±158. Kjennerud, T., Faleide, J. I., Gabrielsen, R. H., Gillmore, G. K., Kyrkjebù, R., Lippard, S. J., & Lùseth, H. (2001). Structural restoration of Cretaceous±Cenozoic (post-rift) palaoebathymetry in the northern North Sea. Sedimentary environments offshore Norway Ð Palaeozoic to Recent. O. J. Marthinsen & T. Dreyer. Norwegian Petroleum Society Special Publication, X (in press). Morley, C. K., Cunningham, S. M., Harper, R. M., & Wescott, W. A. (1992). The geology and geophysics of the Rukwa Rift, East Africa. Tectonophysics, 11, 69±81. Morley, C. K., Vanhauaert, P., & De Batist, M. (2000). Evidence for high frequency cyclic fault activity from high resolution seismic re¯ection survey, Rukwa Rift, Tanzania. Journal of the Geological Society, London, 157, 983±993.
T. Kjennerud et al. / Marine and Petroleum Geology 18 (2001) 307±317 Morley, C. K., Wescott, W. A., Harper, R. M., & Cunningham, S. M. (1999). Geology and geophysics of the Rukwa Rift. Geoscience of rift systems Ð evolution of East Africa. C. K. Morley. American Association of Petroleum Geologists Studies in Geology, 44, 91±110. Nicol, A., Walsh, J. J., Watterson, J., & Underhill, J. R. (1997). Displacement rates of normal faults. Nature, 390, 157±159. RavnaÊs, R., & Steel, R. J. (1998). Architecture of marine rift±basin successions. American Association of Petroleum Geologists Bulletin, 82, 110± 146. Sclater, J. G., & Christie, P. A. F. (1980). Continental stretching; an explanation of the post-mid-Cretaceous subsidence of the central North Sea basin. Journal of Geophysical Research, 85, 3711±3739.
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Talbot, M. R., & Livingstone, D. A. (1989). Hydrogen index and carbon isotopes of lacustrine matter as lake level indicators. Palaeogeography Palaeoclimatology Palaeoecology, 70, 121±137. Thevenon, F., Williamson, D., & Taieb, M. (2001). Submitted for publication. Verrall, P. (1981). Structural interpretation with application to North Sea problems. Joint Association for Petroleum Exploration Courses, 3. Wescott, W. A., Krebs, W. N., Englehardt, D. W., & Cunningham, S. (1991). New biostratigraphic age dates from the Lake Rukwa basin in western Tanzania. American Association of Petroleum Geologists Bulletin, 75, 1255±1263.
www.elsevier.com/locate/marpetgeo
Short term development of intracontinental rifts, with reference to the late Quaternary of the Rukwa Rift (East African Rift System) T. Kjennerud a,*, S.J. Lippard b, P. Vanhauwaert c a
b
SINTEF Petroleum Research, N-7465 Trondheim, Norway Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway c Renard Centre of Marine Geology, Universiteit Gent, Gent, Krijgslaan 281-S8, B-9000 Gent, Belgium Received 7 August 2000; received in revised form 8 December 2000; accepted 15 December 2000
Abstract Relatively low-resolution seismic data and high contemporaneous rift topography normally limit quantitative analysis of normal faults in rifts. The availability of a recently collected high-resolution re¯ection seismic survey in the SE part of the presently active Rukwa Rift (East African Rift System) coupled with high sedimentation rates in the submerged part of the rift makes detailed quantitative analysis possible. High-resolution (down to about 1 m) seismic data penetrate ca. 300 m (representing about 150,000 years) of the uppermost sediments and show ®ne details of normal faulting and related structures. Displacements on the faults, which occur in the hanging wall of the major rift boundary fault, range from a few metres up to 100 m. They show increasing displacements with depth and characteristic rollover folding of the hanging walls. Fault propagation occurred upwards through the rapidly accumulating lake sediments and was preceded by ¯exuring and folding of the sediments. The geometry of the seismic sequences with alternating wedge-shaped and tabular units re¯ects pulsed activity on the faults. Fault displacement rates of up to 1.6 mm yr 21 have been estimated. Each extensional pulse has a duration of 1000s to 10,000s of years. The quiescent stages have a similar duration. Up to 2.5% extension has occurred during the recent evolution. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Normal faulting; Rukwa Rift; Late Quaternary
1. Introduction Detailed quantitative analysis of fault movements in rift basins is often hampered by the fact that fault displacement rates are generally high relative to sedimentation rates. This creates marked syn-rift relief with attendant footwall uplift and erosion and hanging-wall subsidence and sediment starvation (Bertram & Milton, 1989; Doglioni, D'Agostino, & Mariotti, 1998; RavnaÊs & Steel, 1998). As an example, the sedimentation rates on the Visund Fault block half-graben, northern North Sea have been reported as 40 mm ka 21 in the syn-rift late Jurassic (Fñrseth, Sjùblom, Steel, Liljedahl, Sauar, & Tjelland, 1995). These relatively low sedimentation rates partly explain the high relief at the time (e.g. Kjennerud et al., 2001). In such a setting it is dif®cult to calculate fault displacement rates. However, if suf®cient data are available in areas where the sedimentation rate equals or exceeds the fault displacement rate, then the displacement history can be established quantitatively * Corresponding author. Tel.: 147-7359-1257; fax: 147-7359-1102. E-mail address: [email protected] (T. Kjennerud).
over relatively short time periods. Doglioni et al. (1998) pointed out that if the sedimentation rate exceeds the subsidence rate, then the faults only control the geometry of the syn-tectonic beds but not the facies. This appears to be true of the south-eastern part of the Lake Rukwa Rift basin where the Neogene (Plio-Pleistocene) to Recent Lake Beds are uniform muds deposited at average rates ranging from 0.5 to 2 mm yr 21 (Ceramicola, Vanhauwaert, Kilembe, & De Batist, 1997; Talbot & Livingstone, 1989; Wescott, Krebs, Englehardt, & Cunningham, 1991). The Rukwa Rift is thus an area where it is possible to quantify rift development in a detailed manner. The present-day development in the Rukwa Rift can be used as an analogue to ancient rift basin formation. The Rukwa Rift (Fig. 1), situated between the Tanganyika and Malawi rifts, is a Neogene rift superimposed on a late Permian (Karroo) rift (Kilembe & Rosendahl, 1992; Morley, Cunningham, Harper & Wescott, 1992; Wescott et al., 1991). The NW±SE-trending rift is approximately 300±350 km long and 30±50 km wide and the shallow Lake Rukwa (165 km long and up to 50 km wide, but less than 16 m deep) covers a large part of the basin ¯oor. The
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(01)00007-1
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Fig. 1. Regional structural map of the southern part of the Rukwa Rift.
north-eastern ¯ank of the rift is continuously bounded by the remarkably straight Lupa Fault, with a maximum displacement of 10±12 km on the top basement. The south-western side is a segmented margin (U®pa fault escarpment). Kilembe and Rosendahl (1992) and Morley et al. (1992) documented the deep structure of the rift, based largely on conventional seismic data. The uppermost 200 m of sediments, probably dating back to some 150,000 yr BP, in the south-eastern part of the lake have been imaged by high-resolution re¯ection seismic data (Ceramicola et al., 1997). Morley, Vanhauwaert and DeBatist (2000) used these data to investigate highfrequency cyclic fault activity related to the Lupa Fault. The aim of the present paper is to quantify the recent structural history based on the high-resolution seismic data set. Particular emphasis is put on calculating fault displacement rates and extension by the restoration of two pro®les. 2. Geological setting The western branch of the East African Rift System
comprises the Kivu-Albert, Tanganyika, Rukwa and Malawi rift zones (Ebinger, 1989). The Tanganyika and Malawi rifts are characterised by large deep rift lakes. The Rukwa Rift lies east of Lake Tanganyika and north of Lake Malawi, and has a NW±SE trend that is oblique to the N±S trend of these two rifts. Two studies, based largely on the same conventional industrial seismic data, proposed contrasting origins for the Rukwa rift: (1) as an extensional to oblique-slip rift basin (Morley et al., 1992) and (2) as a strike-slip to oblique-slip pull-apart basin (Kilembe & Rosendahl, 1992). Evidence for strike-slip (¯ower structures, en-echelon fault patterns) was cited by Kilembe and Rosendahl (1992), while Morley et al. (1992) recognised that many of the faults within the basin are listric and dip-slip in character. Delvaux, Kervyn, Vittori, Kajara, and Kilembe (1998) have documented a possible recent change from dip-slip to dextral strike-slip during the late Quaternary. They also describe a general NE-ward tilting of the entire basin. However, modern earthquake data shows NE±SW extension (Foster & Jackson, 1998).
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conventional seismic data by the authors has recon®rmed the predominance of the NNW±SSE fault trend (mean N308W) within the southern part of the rift (Fig. 2). 3. Shallow seismic data set
Fig. 2. Rose diagram (half-circles at 5% intervals) of fault trends in the Rukwa rift. This diagram is a frequency versus azimuth plot of top basement faults (19 data points) within the rift.
A high-resolution seismic data set was collected in the southern part of Lake Rukwa (Fig. 3) in 1994 using a sparker source (Ceramicola et al., 1997). The data have a vertical resolution of approximately 1 m and penetrate down to
The deep seismic data image faults from the surface down to the top basement re¯ections. Some of the faults, particularly in the northern part of the basin, show evidence for larger displacements at top basement than at top Karroo (Morley et al., 1992), indicating that movements took place in the late Palaeozoic. In the central and southern parts of the rift the top basement and top Karroo re¯ectors are generally more or less equally displaced indicating dominantly Tertiary movements. However, the Karroo sequence clearly thickens towards the Lupa Fault, showing that this fault was active during both late Palaeozoic and late Tertiary rifting (Morley et al., 1992). Displacements of the Neogene Red Sandstones (see discussion in Morley et al., 2000) and the Pliocene±Recent Lake Beds can be seen to decrease upwards towards the lake ¯oor. This trend is con®rmed on the shallow seismic data (see below). The faults shown on Fig. 1 are mapped at the top Red Sandstone/base Lake Beds level. As noted by Kilembe and Rosendahl (1992) and Morley et al. (1992), the faults within the Rukwa rift show NNW±SSE trends that are oblique to the NW±SE Lupa Fault trend (N508W). More recent mapping of the
Fig. 3. Data base map showing the location of the seismic grid, the R96-1 core and the two reconstructed pro®les.
Fig. 4. (a) Seismostratigraphic units used in the present study and their dating. Note that only U10, U9 and the upper part of U8 have been cored. The remainder of the datings have been extrapolated. (b) Shallow core R96-1 tied in with line 22.
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about 200 m. The re¯ections show aggrading depositional geometries, with high re¯ection continuity and regular layering. The seismic sections show layers thickening towards the Lupa Fault in the north-east. Nine seismic units have been interpreted in the data set (Fig. 4a) (Morley et al., 2000). In the present study, the youngest unit U10 has been divided into U10a and U10b. The seismic data show few of the common
indicators of high-energy environments, such as onlapping, downlapping or mounding features. The mainly parallel to sub-parallel re¯ections suggest a relatively low-energy hemipelagic environment (Morley et al., 2000), an interpretation which is supported by the available core data (e.g. Talbot & Livingstone, 1989; Thevenon, Williamson, & Taieb, 2001). The seismic units have been interpreted on the
Fig. 5. Seismic example of the structures observed in the high-resolution data. The section has been taken from line 17. White bands on seismic are acquisition artefacts.
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basis of whether they display growth towards the faults or not. Generally the units showing internally parallel re¯ections separate units that show divergent re¯ections internally and thicken towards the faults. Morley et al. (2000) reported four periods with marked thickening towards the Lupa Fault and three periods of little or reduced thickening. This led them to propose cyclic fault activity within the Rukwa Rift. 3.1. Dating of the seismic units Two shallow cores have been collected in the southern part of Lake Rukwa. The ®rst was collected in 1960 (Haberyan, 1987; Talbot & Livingstone, 1989), and the second in 1996 (Barker, Telford, Francoise, & Thevenon, 2001; Thevenon et al., 2001). The second core (R96-1) has been tied to seismic line 22 (Fig. 4b) (Morley et al., 2000). This core penetrated units U10, U9 and the upper part of U8, giving radiocarbon dates of 8400 yr BP for the base of U10 and 14,500 yr BP for the base of U9 (Barker et al., submitted). The ages of the remaining units are uncertain, and extrapolation of sedimentation rates back in time is the only possibility. The core is from a fault block crest. It is 12 m long and covers ca. 15,000 yr. The sedimentation rate was relatively constant at the well position at ca. 1 mm yr 21 (Morley et al., 2000). However, the sedimentation rates are very different spatially for the two units. U10 is thickening towards both the intrabasinal faults and the Lupa Fault. Morley et al. (2000) reported maximum sedimentation rates for U10 up to 6 mm yr 21. U9 is a
311
tabular unit and has a relatively constant sedimentation rate of ca. 1 mm yr 21. The exact position of the 1960 core is not known, due to navigation problems at the time, but it is 23.1 m long and covers 12,710 yr (Talbot & Livingstone, 1989), i.e. more or less the same time interval as the second core. Since the 1960 core contains twice as much sediment and covers slightly less time than the R96-1 core, it is likely that the older core was taken from the hanging wall of a fault, rather than a footwall crest. The sedimentation rates in the 1960 core vary between 0.64 and 8 mm yr 21, based on the dating reported by Talbot and Livingstone (1989). The maximum sedimentation rate (8 mm yr 21) occurred between 3300 and 4390 yr BP. This increase in sedimentation rate is not recorded in the R96-1 core. It is thus likely that this maximum sedimentation rate corresponds to an increase in accommodation space creation/fault displacement rates. Unfortunately, there is at present no additional seismostratigraphical resolution within U10 that is able to resolve this. The ages shown in Fig. 4a were obtained by using a crestal sedimentation rate of 1 mm yr 21 as suggested by Morley et al. (2000) and extrapolating downwards from the well position. Based on the available core data, the extrapolated age estimates are likely to be towards the maximum. There are also no indications in the high-resolution seismic data of any major break in sedimentation. It is worth noting that the seismic interpretation is most detailed for the younger part of the section. Compared to the younger units, units U2 and U3 cover about half of the total age span. It is important to note that, in addition to the tectonic in¯uence
Fig. 6. Structural map based on the shallow seismic data. Contours indicate the two-way time thickness (ms) of U10. The rose diagram (half-circles at 5% intervals) represents fault trends from the shallow seismic data on a length versus azimuth plot (39 data points).
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small (less than 0.5 m high) escarpments on the lake ¯oor. All the faults show increasing displacement with increasing depth. 3.3. Folding Folding and ¯exuring can be seen in the hanging walls of most of the faults. Deeper horizons show rollover into the fault plane, shallower horizons may, however, show a hanging-wall syncline and normal drag (arrow on Fig. 5). Folding can also be seen above upward-propagating faults A and B. The folds are due to syn-tectonic deformation and not due to post-tectonic modi®cation. 3.4. Structural map
Fig. 7. Fault displacement plot for the faults on lines 17 and 23. The displacements were measured from the reconstructed sections by plotting the displacement on the oldest horizon (top U2) against age. Three of the faults are blind and the curves do not reach the 0 age axis.
on sediment deposition within the rift, climatic variations have played a major role because of the shallow nature of the lake (e.g. Delvaux et al., 1998). 3.2. Fault expression The shallow seismic data clearly image the upper part of the Lake Beds, the faults that cut the sediments and the associated folding (Fig. 5). Recognisable displacements range from about 1 m up to about 100 m. Most faults appear to reach close to the lake ¯oor, but several are blind and appear to die out within the sediments (faults A and B on Fig. 5). Some faults even form
In the area of the shallow seismic survey most of the faults are synthetic to the Lupa Fault with NW±NNW trends and downthrow to the SW (Fig. 6) (mean trend is 3408). This agrees with the mapping of the faults cutting the top Red Sandstone/base Lake Beds on the deep seismic data (Morley, Wescott, Harper, & Cunningham, 1999). The open nature of the shallow seismic grid (line spacing 1±5 km) means that it is dif®cult to be con®dent in tying faults from line to line. As the faults have displacements of less than 100 m, it is unlikely that they have lengths much greater than 5 km (e.g. Dawers & Anders, 1995). In addition to the predominant NW±NNW trends, some faults strike N±NNE. Notable is a NNE-trending, 1±1.5 km wide, approximately 5 km long, graben seen on the NWtrending lines 26 and 12. Four to ®ve inward facing faults form nested inner and outer grabens. The NE-trending line 25 runs along the axis of the graben, but it is dif®cult to tie the faults to the crossing lines in order get a clearer picture of the structures. The origin of the graben is unclear, but it is clearly transverse to the main structures in the Rukwa Rift. The graben occurs where there is an increase in tilt of the
Fig. 8. Fault displacement versus time for the individual faults on line 17.
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Fig. 9. Fault displacement versus time for the individual faults on line 23.
beds towards the SE and the main depocenter adjacent to the Lupa Fault. It may thus be a relatively super®cial structure caused by gravity sliding of the sediments and not related to the NW-trending rift structures. 4. Fault displacement rates The seismic lines 17 and 23 (Fig. 3) were chosen for further analysis (fault displacement rates and structural restoration). The sections were depth converted using a velocity of 1450 m s 21 for the water column and 1600 m s 21 for the sediments. The restoration was carried out using the restoration/balancing software Locace (Beicip Franlab, 1999). A uniform lithology consisting of 90% shale and 10% silt was assigned to all the layers for use in the decompaction, using the exponential curves of Sclater and Christie (1980). A plot of fault displacement versus time is shown for the faults on lines 17 and 23 (Fig. 7). The displacements were measured from the reconstructed sections, by plotting the displacement on the oldest horizon (top U2) through time. Displacement rates range up to 1.6 mm yr 21, with the average displacement rate for the largest fault (103 m total displacement) over the whole period of ca. 150,000 yr being 0.7 mm yr 21. The individual faults on both lines are shown in Figs. 8 and 9. The general picture from the displacement plots is one of pulsating tectonic activity. The high resolution of the seismic data is crucial for this observation, as such ®ne extensional phases would not be observed in conventional seismic data. Distinct periods with marked displacement on most faults, represented by wedging units in the sections, are separated by periods of no or little displacement, represented by tabular units in the sections. Five periods of increased displacement and four more quiet periods are indicated in the plots. Most of the displacement occurred
between 152,200 and 109,900 BP (U2), 82,200 and 70,300 BP (U4), 55,000 and 38,800 BP (U6), 23,500 and 14,500 BP (U8) and during the last 8400 yr (U10). 5. Structural evolution The restorations of lines 17 and 23 (see Fig. 3 for location) were performed in two runs. The aim of the ®rst run was to balance the interpretation. The exact location of some of the fault planes was hard to de®ne at depth. Vertical simple shear (Gibbs, 1983; Verrall, 1981) was used as the deformation mechanism, which gave the best geometrical ®t. When assuming vertical shear, the extension is the same as that measured when summing the fault heaves. The second run was carried out in order to restore the two sections based on the corrected geometries. Shallow seismic line 17 (Fig. 10a) is located in the NW of the study area and is 8 km long. A symmetrical graben is located between 5 and 6 km, where also the maximum thickness of the interpreted seismic units was observed. The shallow seismic line 23 (Fig. 10b) is located in the centre of the study area and is 12.5 km long. The re¯ections are all generally dipping towards the Lupa Fault (northeastwards). The north-easternmost fault at 9.5 km shows the largest displacement. For every time step, the relief was restored ¯at. This was justi®ed because there are no indications of high relief and little indication of erosion observed in the seismic data set, and also because of the very high sedimentation rates. The evolution is described here in terms of each major pulse of extension. Most faults were active during the deposition of U2. The top U2 re¯ector was only interpreted on line 23, while the top U1 re¯ector was only interpreted on line 17. Line 17 displays the most uneven thickness distribution and the main sediment accumulation occurred in the
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Fig. 10. (a) Structural reconstruction of line 17. (b) Structural reconstruction of line 23.
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graben between 5 and 6 km. U3 was characterised by lower activity as seen on line 23, and ended the ®rst extensional pulse. There was renewed activity during U4, although the same faults were active as in the previous extensional stage. U5 is evenly distributed on line 17, while a slight thickness increase occurred most notably towards the fault at 9.5 km on line 23. U5 was characterised by little extension, and ended the second extensional pulse. Increased extension occurred during the deposition of U6. Some of the faults previously active on line 17 were no longer active. This is an indication that the extension had a longer wavelength in the NW. All faults were active on line 23. The sediments are generally thickening towards the Lupa Fault; however, the maximum sediment accumulation was in the half-graben at 9.5 km. U7 was deposited during a tectonically more quiet period. The unit shows a slight thickening towards the Lupa Fault on line 17, while the thickness is almost constant on line 23. Here the western boundary fault was active. U7 ended the third tectonic pulse. The U8 period was characterised by tectonic reactivation. The unit displays growth towards the intrabasinal faults and, most notably on line 23, also towards the Lupa Fault. The tabular unit U9 draped U8, thus ending the fourth tectonic pulse. Growth towards some of the faults occurred on line 23. Considerable extension and rotation towards the NE occurred during U10 (top U10a is only interpreted on line 23). At the same time as the NE part of the basin received sediment, the SW part near the shoreline was uplifted and eroded. The erosion was probably part tectonic and part climatic in origin. The erosion has been estimated to be maximum 6.3 m on line 17 and
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maximum 10.3 m on line 23. This was accommodated by a tilt of 0.128 on line 17 and mostly footwall uplift on line 23. U10b on line 23 displays more growth towards the Lupa Fault. Thus, most of the present extension is taken up on the Lupa Fault; however, thickening above some of the intrabasinal faults is seen in U10b. This indicates that the faults are propagating upwards at present. 5.1. Extension versus time The reconstruction indicates ca. 2.5% extension over 150,000 yr along line 17 and ca. 1% over 80,000 yr along line 23 (Fig. 11). This is equivalent to strain rates of ca. 5 £ 10215 s21 : Over shorter time intervals the rates were much higher, for example, most of the extension (1.5%) on line 17 appears to have occurred between 80,000 and 70,000 yr BP. The jumps in extension rate correlate with the jumps in fault displacement described in Section 4. The second largest increase in extension has occurred during the last 8400 yr. This event has accounted for 0.3% extension on line 17. Nicol, Walsh, Watterson, and Underhill (1997) presented strain rates calculated over 1±40 Myr for the North Sea 2 £ 10216 s21 ; Kenya Rift 2:5 £ 10216 s21 ; Basin and Range 4 £ 10216 s21 and Aegean 4 £ 10215 s21 : The average strain rates in Rukwa may be lower, if measured on a similar time scale for the Neogene rift phase. 18% extension (based on adding the extension of the Red Sandstone and Lake Bed units in Kilembe & Rosendahl, 1992) over 11 Myr (earliest late Miocene) gives a strain rate of 5 £ 10216 s21 : Out of the four examples cited above, only the North Sea and the Kenya Rift are directly comparable to the Rukwa Rift as intracontinental rifts. The recent strain rate in Rukwa is an order of magnitude larger than these two examples. 6. Discussion and conclusions
Fig. 11. Extension versus time for lines 17 and 23. Extension was computed from the reconstructed pro®les by taking (reconstructed pro®le length/ original length) £ 100.
In general most studies in palaeo-rift basins are based on conventional seismic data; these data usually have a vertical resolution around 25 m. With such data the features seen on the Rukwa high-resolution data set (ca. 1 m resolution) would barely be visible. The most uncertain part of the present analysis is related to the dating of the undrilled units. We have applied an extrapolation of sedimentation rates, where it is assumed that sedimentation rates stayed relatively constant as indicated by core R96-1. We are fully aware of the danger in doing this; however, the approximate datings give an indication of the rates of recent tectonic processes in the Rukwa Rift. It is hoped that, in the near future, new information on the dating of the sediments in the Rukwa Rift will become available. The high-resolution seismic data and the high sedimentation rates in Lake Rukwa have allowed the quanti®cation of movements on intrabasinal normal faults in a detailed
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manner. The fault displacement analysis indicates ®ve periods of increased extension, separated by periods of relative tectonic quiescence. These ®ve extensional pulses are also evident from the extension versus time plots (Fig. 11). The duration of each extensional pulse is between ca. 8000 and 40,000 yr, whilst the duration of the time intervals displaying fault quiescence is between ca. 6000 and 28,000 yr. This corresponds with the smallest fourth-order stage in the tectonic and stratigraphic hierarchy proposed by Fñrseth and RavnaÊs (1998) for the late Jurassic evolution of the Oseberg Fault Block in the North Sea Rift. Their fourthorder stages have durations of 1000s to 10,000s of years. By contrast, their third-order stages have durations of 1±2 Myr. It should be noted that their fourth-order stages were inferred rather than observed. We suggest that, added to the scheme of Fñrseth and RavnaÊs (1998), the fourthorder faulting stages should be separated by quiescence stages of the same order of magnitude duration as the faulting stages. Based on the restored sections, ®ve phases of extension have been quanti®ed in the Rukwa Rift during the last 150 ka. This is represented by a total of 2.5% extension on line 17 and 1% on line 23. 60% of the extension on line 17 appears to have taken place during just 10,000 yr during the deposition of U4. Although the dating for this interval is uncertain, this suggests that large amounts of extension may occur during very short time intervals in rift basins. The total amount of Neogene extension in the southern part of the Rukwa Rift probably amounts to 18% (Kilembe & Rosendahl, 1992). From the evidence presented in this paper, it seems that the Rukwa Rift is presently in the middle of a tectonically active stage. If the 2.5% extension is extrapolated over 11 Myr, it would represent more than 180% extension. It is thus likely that extension occurs in pulses of various orders of magnitude (e.g. Fñrseth & RavnaÊs, 1998; Morley et al., 2000). The structural restoration indicates that the most recent part of the development has been characterised by a tilt towards the NE, associated subsidence and sedimentation in the NE and uplift and erosion in the SW. Recent tilt towards the NE, has also been noted by Delvaux et al. (1998). The basin was tilted 0.128 on line 17. This event has occurred during the last 8400 yr. There are, however, also variations within this time span. As mentioned above, the ®rst shallow core suggested a sedimentation maximum between 3300 and 4390 yr BP, when 9.5 m of the total 23.1 m of the core was deposited. This leads us to suggest that the tectonic pulses recognised in the present work probably consist of even smaller scale faulting pulses and fault quiescence stages internally. Acknowledgements Tomas Kjennerud was ®nanced in the ®nal phase of the work by grant 138620/431 from the PetroForsk programme
(The Research Council of Norway). Pieter Vanhauwaert was ®nanced by a grant from IWT. Marc De Batist is thanked for initiating the Rukwa work at RCMG. Vedastus J. Seda is thanked for helping with the fault interpretation shown in Figs. 1 and 2. Cindy Ebinger, Roy H. Gabrielsen, Martin Hamborg, Chris K. Morley, Mike Talbot and Walther Wheeler are acknowledged for commenting on earlier versions of this paper. References Barker, P., Telford, R., Francoise, G., & Thevenon, F. (2001). Submitted for publication. Beicip Franlab (1999). Locace 2.4 user's guide (variously paginated). Bertram, G. T., & Milton, N. J. (1989). Reconstructing basin evolution from sedimentary thicknesses: the importance of palaeobathymetric control, with reference to the North Sea. Basin Research, 1, 247±257. Ceramicola, S., Vanhauwaert, P., Kilembe, E., & De Batist, M. (1997). High-resolution re¯ection seismic investigation of sediments of Lake Rukwa (Tanzania). International Project on Paleolimnology and Late Cenozoic Climate Newsletter, 10, 113±122. Dawers, N. H., & Anders, M. H. (1995). Displacement-length scaling and fault linkage. Journal of Structural Geology, 5, 607±614. Delvaux, D., Kervyn, F., Vittori, E., Kajara, R. S. A., & Kilembe, E. (1998). Late Quaternary tectonic activity and lake level change in the Rukwa Rift Basin. Journal of African Earth Sciences, 26, 397±421. Doglioni, C., D'Agostino, N., & Mariotti, G. (1998). Normal faulting versus regional subsidence and sedimentation rate. Marine and Petroleum Geology, 15, 737±750. Ebinger, C. J. (1989). Tectonic development of the western branch of the East African Rift System. Geological Society of America Bulletin, 101, 885±903. Fñrseth, R. B., & RavnaÊs, R. (1998). Evolution of the Oseberg Fault-Block in context of the northern North Sea structural framework. Marine and Petroleum Geology, 15, 467±490. Fñrseth, R. B., Sjùblom, T. S., Steel, R. J., Liljedahl, T., Sauar, B. E., & Tjelland, T. (1995). Tectonic controls on Bathonian±Volgian syn-rift succesions on the Visund fault block, northern North Sea. Sequence stratigraphy on the northwest European margin. R. J. Steel, V. L. Felt, E. P. Johannesen & C. Mathieu. Norwegian Petroleum Society Special Publication, 5, 325±346. Foster, A. N., & Jackson, J. A. (1998). Source parameters of large African earthquakes: implications for crustal rheology and regional kinematics. Geophysical Journal International, 134, 422±448. Gibbs, A. D. (1983). Balanced cross-section construction from sections in areas of extensional tectonics. Journal of Structural Geology, 5, 153± 160. Haberyan, K. A. (1987). Fossil diatoms and the palaeolimnology of Lake Rukwa, Tanzania. Freshwater Biology, 17, 429±436. Kilembe, E. A., & Rosendahl, B. R. (1992). Structure and stratigraphy of the Rukwa Rift, East Africa. Tectonophysics, 209, 143±158. Kjennerud, T., Faleide, J. I., Gabrielsen, R. H., Gillmore, G. K., Kyrkjebù, R., Lippard, S. J., & Lùseth, H. (2001). Structural restoration of Cretaceous±Cenozoic (post-rift) palaoebathymetry in the northern North Sea. Sedimentary environments offshore Norway Ð Palaeozoic to Recent. O. J. Marthinsen & T. Dreyer. Norwegian Petroleum Society Special Publication, X (in press). Morley, C. K., Cunningham, S. M., Harper, R. M., & Wescott, W. A. (1992). The geology and geophysics of the Rukwa Rift, East Africa. Tectonophysics, 11, 69±81. Morley, C. K., Vanhauaert, P., & De Batist, M. (2000). Evidence for high frequency cyclic fault activity from high resolution seismic re¯ection survey, Rukwa Rift, Tanzania. Journal of the Geological Society, London, 157, 983±993.
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