Two-stage rifting in the Kenya rift_ implications for half-graben models

ELSEVIER Tectonophysics 278 (1997) 63-81 TECTONOPHYSICS Two-stage rifting in the Kenya rift: implications for half-graben models E Mug isha a,b, C.J. Eb inger a'*, M . S t recker c, D. Pope a,1 a Department of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK b Petroleum Exploration and Production Department, P.O. Box 9, Entebbe, Uganda c lnstitutfiir Geowissenschaften, Universitiit Potsdam, Postfach 601553, 14415 Potsdam, Germany Accepted 25 April 1997 Abstract The Kerio sub-basin in the northern Kenya rift is a transitional area between the southern Kenya rift, where crustal thickness is 30 kin, and the northern Kenya rift, where crustal thickness is 20 kin. The lack of data on the shallow crustal structure, geometry of rift-bounding faults, and rift evolution makes it difficult to determine if the crustal thickness variations are due to pre-rift structure, or along-axis variations in crustal stretching. We reprocessed reflection seismic data acquired for the National Oil Corporation of Kenya, and integrated results with field and gravity observations to (1) delineate the sub-surface geometry of the Kerio sub-basin, (2) correlate seismic stratigraphic sequences with dated strata exposed along the basin margins, and (3) use new and existing results to propose a two-stage rifting model for the central Kenya rift. Although a classic half-graben form previously had been inferred from the attitude of uppermost strata, new seismic data show a more complex form in the deeper basin: a narrow full-graben bounded by steep faults. We suggest that the complex basin form and the northwards increase in crustal thinning are caused by the superposition of two or more rifting events. The first rifting stage may have occurred during Palaeogene time contemporaneous with sedimentation and rifting in northwestern Kenya and southern Sudan. The distribution of seismic sequences suggests that a phase of regional thermal subsidence occurred prior to renewed faulting and subsidence at about 12 Ma after the eruption of flood phonolites throughout the central Kenya rift. A new border fault developed during the second rifting stage, effectively widening the basin. Gravity and seismic data indicate sedimentary and volcanic strata filling the basin are 6 km thick, with up to 4 km deposited during the first rifting stage. Keywords: rift basin; East Africa; seismic stratigraphy; gravity model 1. Introduction The seismically active Kenya (Gregory) rift, East Africa, encompasses several sedimentary basins, and it has been the site of voluminous basaltic and acidic * Corresponding author. Fax: +44-0113-233-5259; E-mail: cindy @ earth.leeds.ac.uk J Present address: Amoco Exploration, Amoco House, West Gate, London, UK. volcanism since the Miocene (Fig. 1). The crust be- neath the N-S-trending Kerio-Baringo sub-basins thins to the north along the rift axis, as well as to the east and west beneath the rift flanks (KRISR 1991; Keller et al., 1994). Models proposed for the along variations in crustal structure differ in the sub-surface geometry of the main border faults (e.g., Chapman, 1971; Bosworth, 1985; Bosworth et al., 1986; Mor- ley et al., 1992), pre-rift crustal heterogeneities (e.g., Swain et al., 1981; Smith and Mosley, 1993; Maguire 0040-1951/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0040-1951(97)00095-4 64 F. Mugisha et el./Tectonophysics 278 (1997) 63-81 4ON 2°N 0 ° 34°E 36 ° I ~ /%- . . ' . " . ' . ' . ' , / I SUDA.N ~. 'o" ' • ",." ".. / /? : L. ' . I / '~ l / / / I / , I , \ / i UGANDA\ Mt. ,, Elgon k ~/ - ys, t" t / - ' - -~ , Kerio 38°E I --TH IOP IA i ~ Neogene-Recent ~, sedimentary & volcanic • . rocks : : :~' ~ Mesozoic & Palaeogene sedimentary rocks - - - -1 Proterozoic Precambrian Basement Archaean Basement / Border fault / " sin __Sub- [ / / \ \ ~'~' / / " " " -~" / -i Mr. Kenya- L~ e ~ ~ ~ . ~ "~ . . . . . Vi o r i a ~ / ~ " \ ~ ~ / , • I I . / \ " # j U ~ ~ / " - \ - t , <)|l l\Nairobi TANZANIA ' ' ".,. . - - "~. / ~ . . /~ / / 0 km 100 Fig. I. Summary of regional geology and location of the Kerio sub-basin within the Neogene-Recent Kenya rift systems, East Africa (after Maguire et el., 1994). N.B.: Palaeogene and Mesozoic rift basins underlie Neogene-Recent volcanic and sedimentary strata. et el., 1994), and modifications to the lower crust by magmatic underplating (Hay et el., 1995). Key in many of these interpretations is the Kerio sub-basin, which is cited as a classic, west-tilted half-graben based on the attitude of strata exposed within the rift. Seismic refraction data and teleseismic models (e.g., Green et el., 1991; Maguire et el., 1994) provide few constraints on the sub-surface geometry of the basins and thickness of sedimentary and volcanic strata contained therein. Interpretations of the seismic ve- locities and short-wavelength gravity lows observed across the rift basins differ, in part due to treatment of the high-amplitude regional field: Morley (1988) proposes an 8-9-km-thick sedimentary pile; Swain et el. (1981) suggest a 4-km-thick sedimentary fill underlain by anomalously low-density upper crust; Hay et al. (1995) propose a 3-km-thick succession of volcanic and sedimentary rocks and a lens of higher- density lower crust. Thus the extensional history and sub-surface geometry of these well-studied central Kenya rift basins remain largely unresolved. In 1989 reflection seismic data were acquired for the National Oil Corporation of Kenya in the Kerio sub-basin, which is characterised by a narrow Bouguer anomaly low superposed on a broader low (Fig. 2). In this paper we re-process and enhance these seismic reflection profiles, and integrate results with field and gravity observations to (1) delineate the sub-surface geometry of the Kerio sub-basin, (2) correlate seismic stratigraphic sequences with F. Mugisha et al . / Tectonophysics 278 (1997) 63-81 65 35 E 36 E 37 E 38 E 2 1 A' l 35 E 36 E ;3'7 E 38 E Fig. 2. Bouguer gravity anomalies in central and northern Kenya. Kerio Valley study area enclosed by box. A-A ~ indicates the location of the gravity profile shown in Fig. 8. Bold lines are seismic reflection profiles KVI, KV2, KV3 (Figs. 5-7). Note the narrow gravity low corresponding to the Kerio sub-basin south of the Palaeogene Lokichar basins in NW Kenya. Dashed bold lines indicate the approximate axis of the Neogene-Recent Kenya rift. 66 F. Mugisha et al . / Tectonophysics 278 (1997) 63-81 dated strata exposed along the basin margins, and (3) use new and existing results to propose a two-stage rifting model for the central Kenya rift. 2. Tectonic and geological setting The Kenya rift system has developed along the eastern margin of the 1300-km-wide East African plateau that is cored by the Archaean Tanzania craton and surrounded by Proterozoic mobile belts (e.g., Fig. 1). The uplifted plateau is characterized by a broad, strongly negative Bouguer gravity anomaly that rises to the east along the Indian Ocean passive continental margin (Figs. 1 and 2). Gravity data and seismic tomography models suggest that the Kenya rift formed above a mantle plume, although the spatial and temporal extent of the asthenospheric upwelling are weakly constrained by existing data (e.g., Ebinger et al., 1989; Nyblade et al., 1990; Green et al., 1991; Achauer, 1992). Seismic refraction and tomographic studies of the crustal structure beneath the southern Kerio and Baringo sub-basins show that 'normal' crust of a thickness of ~36 km outside the rift thins to 30 km beneath the rift valley (e.g., Maguire et al., 1994). An intracrustal reflector is interpreted as the base of the granitic upper crust, and it rises from depths of 15 km beneath the flanks to 10 km beneath the Baringo basin, the present locus of seismic and volcanic activity (e.g., Tongue et al., 1992; Smith, 1994). The approximately 20-kin-wide, 100-km-long Kerio sub-basin is bounded to the west by the 1500-m-high Elgeyo escarpment, a fault-line scarp (Fig. 3). The uplifted footwall of the Kamasia fault, which separates the Kerio sub-basin from the tecton- ically active Baringo sub-basin, exposes Neogene- Recent strata (Fig. 3). Seismic refraction data indi- cate that the western border fault maintains a dip >45 ° to depths of at least 9 km into the sub-sur- face (Maguire et al., 1994). The elongate, -60 mGal Bouguer gravity anomaly observed over the central Kerio basin decreases to the north and south as does the topographic expression of the Elgeyo es- carpment. To the northwest of the Elgeyo fault lies the Eocene-Oligocene Lokichar rift system whose southern extent is poorly constrained (e.g., Hendrie et al., 1994; Ebinger and Ibrahim, 1994; Fig. 1). The Elgeyo escarpment exposes Late Proterozoic (Mozambique belt) gneisses covered by Miocene sed- imentary and volcanic rocks (Lippard, 1972; Fig. 3). The gneisses are characterised by steep E-dipping foliations generated during orogen-parallel shearing during the Pan-African orogeny (e.g., Smith and Mosley, 1993; Hetzel and Strecker, 1994). The ex- posed basement of the Elgeyo escarpment shows a se- ries of foliation domains juxtaposed to NW-trending brittle shear zones that were active into the Late Pro- terozoic (Hetzel and Strecker, 1994). The Elgeyo to- pographic escarpment and subsidiary Cenozoic faults generally follow these basement foliation trends and change orientation when intersecting the NW-trend- ing shear zones (e.g., Smith and Mosley, 1993; Hetzel and Strecker, 1994; Smith, 1994). Along the southern part of the Elgeyo escarp- ment outside the area of seismic coverage, the oldest known strata overlying basement rocks are sandstone and conglomerate (Kimwarer sediments) overlain by Mid-Miocene basanites (Elgeyo basalts) (Figs. 3 and 4). Adjacent to the seismic survey the oldest rocks overlying basement are sedimentary rocks (Tambach sediments, Fig. 4). They can be divided into a 200- m-thick fluvial unit of conglomerates, clays, and siltstones, overlain by 200 m of finely laminated la- custrine clays containing fish fossils (Lippard, 1972). While the lower conglomerates contain basement- derived clasts, the upper conglomeratic horizons also contain nephelinite and phonolite clasts. Lippard (1972) linked the phonolite and nephelinite clasts in the uppermost strata to the Mid-Miocene phono- litic centres to the southwest. Clast imbrications also suggest transport from south to north. The Tambach sediments are overlain by Mid- Miocene flood phonolites (Uasin Gishu and Tiim phonolites), which reach a thickness in excess of 500 m (Lippard, 1972). These ca. 14-m.y.-old flows pro- vide a minimum age for the Tambach sediments, consistent with the age implied by the volcanic clasts. The youngest flow unit is dated at 12 Ma and has a relatively constant thickness of 50 m (Bishop and Chapman, 1970; Lippard, 1972). Subse- quent flows in the region accumulated in the Kerio and Baringo sub-basins east of the Elgeyo escarp- ment, but did not overstep the western escarpment. Farther west on the escarpment shoulder the Mid- Miocene phonolite lavas lie directly on the basement rocks and dip <5 ° to the west. Thickness variations F. Mugisha et al./Tectonophysics 278 (1997} 63 81 67 UPPER MIOCENE SEDIMENTS MID-MIOCENE FLOOD PHONOLITE MIOCENE FLUVIO-LACUSTRINE SEDIMENTS MID-MIOCENE BASANITE & PHONOLITE CRYSTALLINE BASEMENT j QUATERNARY SEDIMENTS PLEISTOCENE TRACHYTE MIO-PLIOCENE BASALTS UPPER MIOCENE PHONOLITES & TRACHYTES FAULT Fig. 3. Generalized geological map and stratigraphic section of the Kerio Valley and adjacent regions. Broken lines are seismic reflection profiles KV1, KV2, and KV3 shown in Figs. 5-7. Fault with broken lines corresponds to the buried Elgeyo fault. KF, Kerio fault; BF, is Barwesa fault; TF, transfer fault interpreted from seismic reflection profiles (Figs. 4-6). Modified after Chapman et al. (1978), Hackman et al. (1990), Strecker (t991), Smith and Mosley (1993), and Hetzel and Strecker (1994). The Miocene Ewalel phonolites are marked by 'e'. and flow directions suggest that the source areas for the phonolite lavas were located along the margins of the present-day rift near the Kamasia and Elgeyo faults (Lippard, 1972; Fig. 3). Sedimentary and volcanic strata are exposed along the footwall of the Kamasia fault, which we define as the eastern topographic margin of the Kerio sub-basin (Fig. 3). Similar to the situation along the southern Elgeyo escarpment, the oldest unit above basement gneisses is a quartzite-rich sandstone and siltstone (Martyn, 1969; Fig. 4), overlain by Mid-Miocene phonolites (Sidekh phonolites, 16-14.4 Ma) alter- nating with ftuvio-lacustrine sandstones and shales (Chepkuno and Muruyur Beds) and reaching a cumu- 68 F. Mugisha et aL /Tectonophysics 278 (1997) 63-81 Seismic stratigraphic units Stratigraphic chart " .v . ' . ' . ' , ' . ' . ' . ' . ' . ' . ' . ' . ' . ' . " ".','.'.'.'.'.V.V.',V,'.V ".'.'.'.'.'.','.'.'.'.V.'.'.'." ".'.'.'.'.'.'.'.'.'.'.'.v.','." o OO °0o o o ~0 0 O0 o 0 o 00o 0 O0 000 000 0 o iiiii iiiiiNiiii iiiiii ,..~ . . . . ,U',2- " " - ~ 1 1 ' " . x l - -~ l l ~ . Quaternary Sediments Quaternary Sediments (Kerio Fm) Upper Miocene Upper Miocene Phonolites & 7-9 Ma Volcanic Flows Trachytes (Ewalel & Kaption Phono- lite, Kabarnet Trachyte, Mpesida Beds) Upper Miocene Upper Miocene Sediments 9-11 Ma Sedimentary Strata (Ngorora Fm) Middle Miocene Mid-Miocene Flood Phonolite Volcanic Flows (Uasin Gishu, Sidekh & Tiim Phono- 12-14 Ma lites, Chepkuno & Muruyur Beds) Middle Miocene Miocene Fluvio-Lacustrine Sediments Sediments (Tambach Fm) >14 Ma Paleogene(?) Sediments Seismic Basement Kimwarer & Kamego Sediments Crystalline Basement Direct ties Indirect ties ~ Unconformity Fig. 4. Seismic stratigraphic units and correlative stratigraphic units of the Elgeyo escarpment and Kamasia Range. Modified after Lippard (1972), Chapman et al. (1973, 1978), Deino et al. (1990), and Strecker (1991). lative thickness of ca. 700 m. These deposits are over- lain by the up to 1100-m-thick Tiim phonolites (14.3- 11 Ma; Lippard, 1972; Deino et al., 1990) that are tentatively correlated with the Mid-Miocene Uasin- Gishu phonolites of the Elgeyo escarpment (e.g., Chapman et al., 1978). Mid-Miocene phonolites across the Kerio and Baringo sub-basins are conformably overlain by Up- per-Miocene sedimentary rocks (Ngorora Formation, 11-9 Ma), which reach a thickness of up to 400 m along the Kamasia fault (Chapman et al., 1978; Deino et al., 1990). This unit consists of conglomer- ates, tuffaceous sandstones, pyroclastic deposits, and fossiliferous lacustrine mudstones (Martyn, 1969; Chapman et al., 1978). Up to 120-m-thick con- glomeratic channel-fill deposits within the flood phonolites along the Elgeyo escarpment may be chronostratigraphically equivalent with the Ngorora Fm. The width of the erosional cuts is up 200 m and their orientation is roughly E-W (Hetzel, 1992). The Ngorora Formation is unconformably overlain by Upper-Miocene lava flows (Ewalel phonolites, 9-7.5 Ma) which range in thickness from 180 to 600 m (Martyn, 1969; Lippard, 1972; Chapman and Brook, 1978). An angular unconformity and weathering surface separates the phonolites from the widespread, 350-m-thick Upper-Miocene trachytes (Kabarnet trachytes, 7.2-6.8 Ma). As in the older volcanic sequences, there are also lacustrine interca- lations within the Kabarnet trachytes (Mpesida Beds, F. Mugisha et al./Tectonophysics 278 (1997) 63 81 69 Martyn, 1969; Hill et al., 1986). Miocene-Pliocene basalts (Kaparaina Formation) are up to 350-m-thick but of limited areal extent in the study area and occur along the eastern front of the range (Fig. 3). Within the Kerio sub-basin, only the Quaternary Kerio Formation onlaps the westward-tilted Kabar- net trachytes of the Kamasia block. The Kerio For- mation consists of broad, coarse, alluvial fan facies and massive landslide deposits (Hetzel, 1992) at the foot of the escarpment and fluvio-lacustrine facies in the vicinity of the Kerio River. High-resolution Landsat imagery and field observations show no faults that cut the Recent strata, which may indicate that extension has migrated eastward to the volcani- cally and seismically active Baringo sub-basin where young faults are common (e.g., Smith, 1994). In summary, the available stratigraphic informa- tion and field observations suggest that the morphol- ogy of the Kenya rift, as we see it today, was pro- duced by faulting and tilting after the deposition of the widespread Mid-Miocene phonolite flows. These flows are exposed in deeply incised canyons along the western rift flanks and along the Elgeyo and Ka- masia escarpments (Chapman, 1971; Lippard, 1972). Renewed faulting along the Elgeyo fault and basin subsidence occurred by about 11 Ma, as indicated by the distribution and attitude of younger volcanic and sedimentary units (Chapman, 1971; Lippard, 1972; Chapman et al., 1978; Deino et al., 1990). 3. Kerio sub-basin structure Three seismic reflection lines were acquired within the Kerio Valley by Halliburton Geophysical in a project supported by the National Oil Corporation of Kenya (Fig. 3). Gravity data were collected ev- ery ten shotpoints along the two E -W profiles (KV1, KV2) and the along-axis tie line, KV3 (Figs. 2 and 3). The Kerio River was unusually high in 1989, pre- venting data acquisition in ca. 3-km-wide gaps along both KV 1 and KV2 (Figs. 5-7). These gaps prevented migration of the two dip lines (see Appendix A). The cross-rift seismic reflection profiles (KV1, KV2) do not show the simple westward-tilted half- graben expected from extrapolation of surface struc- tures (e.g., Chapman et al., 1978; Morley et al., 1992). Instead, strata below a shallow unconformity dip eastward or are flat-lying along the western side of the basin, and there are several other uncon- formities and faults (Figs. 5 and 6). Unfortunately, penetration was insufficient to image the top of crystalline basement beneath the deepest sequences, as discussed below. The plane of the Elgeyo fault bounding the western side of the Kerio sub-basin is poorly imaged on both KV1 and KV2, but its presence is indicated by offsets of reflectors, roll- over or drag structures, and a lateral discontinuity in interval velocity, particularly along KV1 (Fig. 5). The surface projection of the fault lies about 500 m east of the erosional Elgeyo escarpment, whereas the surface projection of the Kerio fault, the major fault bounding the deep, inner basin lies approximately 2 km to the east of the present escarpment (e.g., CDP190W; Fig. 5). No offsets of Quaternary alluvial fan deposits are visible at the surface projections of either of the two faults. Several faults mark the eastern margin of the deep Kerio basin. The Barwesa fault is a buried, west- dipping fault bounding the deep basin (CDP320; Fig. 6). Along KV2 it is a steep reverse fault with small throw, as revealed in the stratigraphic analyses below. Reflectors below 1.0 s dip to the east and abut the fault. Its continuation to the south along KV1 is difficult to ascertain due to poor penetration (Fig. 5). Instead, at the end of KV1 the seismic signature changes to convex reflectors defining a body with a diapiric shape, against which lower reflectors appear to be truncated (D, Fig. 5). This intrusive body may be one of several Upper-Miocene (Kaption) phono- lite laccoliths and sills that form a N-S-trending