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A morphotectonic study of an extensional fault zone in a magma-rich rift: the Baringo Trachyte Fault System, central Kenya Rift

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The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N–S-trending linked extensional fault network. This fault system bounds to the west the 8km deep Baringo Basin which itself lies within the axial valley of
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  Tectonophysics 320 (2000) 87–106www.elsevier.com /  locate /  tecto A morphotectonic study of an extensional fault zone in amagma-rich rift: the Baringo Trachyte Fault System,central Kenya Rift B. Le Gall  a, *, J.-J. Tiercelin  a , J.-P. Richert  b , P. Gente  a , N.C. Sturchio  c ,D. Stead  d , C. Le Turdu  e a  UMR CNRS 6538 ‘‘Domaines Oce´aniques’’, Institut Universitaire Europe´en de la Mer, Universite´ de Bretagne Occidentale,Place Nicolas Copernic, 29280 Plouzane´ , France b  3 rue des Ajoncs, 64160 Morlaas, France c  Environmental Research, ER-203, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA d  Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK  e  Elf Petroleum Norge AS, Dusavik, P.O. Box 168N, 4001 Stavanger, Norway Received 2 July 1998; accepted for publication 31 January 2000 Abstract The Baringo Trachyte Fault System is located within the central Kenya Rift and forms part of a N–S-trendinglinked extensional fault network. This fault system bounds to the west the 8 km deep Baringo Basin which itself lieswithin the axial valley of the central Kenya Rift. It mainly a ff  ects a middle Pleistocene trachytic dome (510 ka), theso-called Baringo Trachyte (BT). A morphotectonic study of the 10 km long BT master fault and associateddownthrow geometries provides constraints on the evolution of a magma-type rift fault system from an initial stageof crack opening through to propagation. A model of radial fault growth is proposed in order to account for thelongitudinal segmentation of the main fault escarpment from the median part to the tips. The small-scale half-grabengeometry developed in the median high-strain zone is progressively accommodated laterally by both flexure andrelated narrow compensation grabens. The resulting crack swarms are well-developed at the free southern tip zone.Both the spatial distribution of rock-breaking products and their relations to the immediate hangingwall providefurther evidence for this hypothesis. Well-developed screes and other gravity-driven structures (slumps) preferentiallyoccur along the median part of the Baringo Trachyte Fault Escarpment, probably as earthquake-induced features.The hangingwall fault zone shows an asymmetrical triangular-shape with a maximum width of about half the lengthof the main scarp. This zone of maximum deformation and subsidence appears to be laterally controlled by twomajor, conjugate, transverse basement discontinuities lying with a conjugate geometry. Its internal architecture isdominated by antithetic westerly-dipping normal faults bounding discrete half-grabens, locally infilled by syn-tectonicvolcaniclastics. Chronological data on hydrothermal silica filling open cracks on the BT footwall suggest that themaster fault evolution occurred from 345 to 198 ka, as the possible result of at least four major normal faultingearthquakes. © 2000 Elsevier Science B.V. All rights reserved. Keywords:  Baringo Trachyte; basement control; central Kenya Rift; earthquake; extensional faulting; morphostructure* Corresponding author. Tel.: + 33-298-498756; fax: + 33-298-498760. E-mail address:  blegall@sdt.univ-brest.fr (B. Le Gall)0040-1951 /  00 /  $ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0040-1951(00)00069-X  88  B. Le Gall et al.  /   Tectonophysics 320 (2000) 87–106  1. Introduction  half-graben-like structure bounded to the west bythe Elgeyo Border Fault, and to the east by theLaikipia Border Fault system. In the axial part of Part of the Eastern Branch of the East AfricanRift system, the Kenya Rift is a linear graben this half-graben, the uplifted Tugen Hills separatesthe Kerio Basin from the tectonically activestructure which extends for 900 km from theTurkana Depression at ca. 5 °  latitude N to the Baringo Basin, presently occupied by the fresh-water Lake Baringo (Fig. 1b and c).Tanzanian Plateau at ca. 3 °  latitude S (Fig. 1a).North of the equator up to 2 ° N, the central As suggested by recent geophysical investiga-tions in the Kerio and Baringo basins (Mugishasegment of the Kenya Rift extends as a 100 kmwide and 100 km long N–S-trending asymmetrical et al., 1997; Hautot et al., 2000), this part of the Fig. 1. (a) Simplified tectonic map of the East African Rift system showing the location of the central Kenya Rift. (b) Simplifiedstructural map of the central Kenya Rift between the equator and 1 ° N, showing the main rift border faults (Elgeyo, Saimo andLaikipia border faults), and the present-day active inner trough, presently occupied by the Bogoria and Baringo lake basins. (c)Detailed structural interpretation from SPOT imagery of the Baringo–Bogoria inner trough, showing the main N–S and NW–SEstructural features and the location of the volcanic and sedimentary formations: Baringo Trachyte (BT); Chemakilani Basalts (CB);Loyamarok Phonolite (LP); Kapthurin Formation (KF); and Baringo Basalts (BB). (d) Lithostratigraphic log of the volcano-sedimentary syn-rift sequence infilling the Baringo Basin [modified after Hautot et al. (2000)].  89 B. Le Gall et al.  /   Tectonophysics 320 (2000) 87–106  Kenya Rift started to develop as early as Paleogene deep basement-controlled features (Wohlenbergand Bhatt, 1972; Smith and Mosley, 1993;times contemporaneous with the development of the Lokichar–Loperot basins in the Turkana area Grimaud et al., 1994; Le Turdu et al., 1999).Analysis of fault populations and associated stria-(Morley, 1988) (Fig. 1a). Gravity, magnetotelluricand seismic data indicate 6 to 8 km thick volcanic tions along the POKTZ in the PorumbonyanzaRiver further indicate dominantly strike or obliqueand sedimentary strata filling these basins (Morley,1988; Mugisha et al., 1997; Hautot et al., 2000), fault movements (Le Turdu, 1998) (Fig. 1c). Atthe map scale, the existence of sigmoid fault-and overlying basement rocks that belong to theMozambique Belt of Panafrican age (650 to bounded horsts observed on SPOT imagery alongthe inferred northwest extension of POKTZ north450 Ma) (Hackman, 1988). Voluminous fissuraland multi-centre volcanic eruptions accompanied of Nakuratabem (Fig. 1c) is also consistent withdominant lateral faulting (Le Turdu et al., 1995).the tectonic evolution of the central Kenya Riftsince Lower Miocene times, giving rise to thick The majority of work on the central Kenya Rifthas focused on large-scale rift-related structural,( > 1000 m) extensive flood volcanics interbeddedwith tens to hundreds of metres thick fluvio- volcanic and sedimentary patterns (Hill et al.,1985; Williams and Chapman, 1986; Tiercelin andlacustrine syn-rift deposits (Chapman et al., 1978;Williams and Chapman, 1986; Renaut et al., 1999) Vincens, 1987; Smith and Mosley, 1993; Grimaudet al., 1994; Hetzel and Strecker, 1994; Mugisha(Fig. 1d). Estimates of cumulate crustal extensionfor the central Kenya Rift were calculated by et al., 1997). Few studies to date have discussedthe mechanisms and tectonic evolution of local-Chapman et al. (1978) using the tilted-blockmodel. They indicated crustal extension of ca. scale rift extensional fault systems. Such a struc-tural analysis has been carried out by the authors1 km across the Elgeyo and Tugen Hills structures,and up to 2.5 km for the whole rift at this latitude on the western flank of the Baringo–Bogoria innertrough north of latitude 0 ° 45 ∞ N, where a complex(i.e. 2 % of its total width). It should be noted thathigher extension estimates based upon geological N–S trending fault system — the Baringo TrachyteFault System (BTFS) — cuts the 510 ka-oldand geophysical evidence indicate a 5–30 kmcrustal extension for the Kenya Rift since the early Baringo Trachyte dome (BT) (Martyn, 1969;Truckle, 1977; Le Turdu, 1998) (Fig. 1c). Thisphases of rifting (McKenzie et al., 1970; Searle,1970; Baker and Wohlenberg, 1971; Baker et al., system provides on its eastern border an exception-ally well exposed example of a N–S-trending1988). In contrast, studies of palaeostress fields inthe central Kenya Rift suggest that extension was normal fault escarpment named the BaringoTrachyte Fault Escarpment (BTFE) (Fig. 2). Thisoriented E–W prior to 400 to 200 ka (Le Turduet al., 1995; Bosworth and Strecker, 1997). fault scarp displays accurate geometries that helpto better constrain the evolutionary developmentDuring the last 2 Ma, rift deformation in thecentral Kenya Rift is known to have migrated of synrift extensional faulting in relation to pre-existing heterogeneities. In order to investigate theeastward up to the present-day active axis whichis centred on the 60 km long by 30 km wide overall fault pattern at varying scales, detailedmapping of BTFS has been undertaken based onBaringo–Bogoria inner trough (Lippard, 1972;Chapman et al., 1978; Hill et al., 1986; Richert SPOT imagery, aerial photographs and field mor-phostructural evidence (Fig. 3).et al., 1987; Le Turdu et al., 1995) (Fig. 1b andc). At this location, E–W extensional tectonics areexpressed by a dense network of anastomosing,globally N–S-trending normal faults coeval with  2. The Baringo Trachyte Fault System three NW–SE transverse lineaments — thePorumbonyanza–Ol Kokwe (POKTZ), Wasages–   2.1. General geometry of the faulted volume Marmanet (WMTZ) and Bahati (BTZ)Transverse Zones. These lineaments have been The western side of the Lake Baringo Basin isformed by a N–S-trending linked grid-fault systeminterpreted from gravity and aeromagnetic data as  90  B. Le Gall et al.  /   Tectonophysics 320 (2000) 87–106  Fig. 2. Oblique aerial view from the south of the 10 km long, N–S-trending BTFE showing the well-marked zigzag pattern formedby a succession of kilometre-long fault segments oriented N–S, NW–SE or NE–SW. Lake Baringo lies to the east of the faultescarpment (at the top of the photo). developed during the important post-500 ka phase old Baringo Trachyte (BT) dome and volcaniclas-tic sediments; and (2) in the north the 894 ka-oldof faulting in the central Kenya Rift (Dunkleyet al., 1993; Le Turdu et al., 1995). This extensional Loyamarok Phonolite (Cle´ment et al., 2000) anda small outcrop of Chemakilani Basalts [datedfault pattern is dominated by two major structuresknown as the Kapthurin Fault to the south 1.8 Ma; Truckle (1977)] (Figs. 1c and 3). Due tomarked contrasts in the overall petrostructural(Tiercelin and Vincens, 1987), and a 20 km longcomposite structure known as the BTFS (Fig. 1c). character of these three adjoining lava complexes,major di ff  erences are observed in the morphologyAccording to recent geophysical investigations,this intricate fault network is assumed to bound of the resulting normal fault system, with a moresubdued expression along the Chemakilani– to the west a 8 km thick basin of Palaeocene toRecent age (Hautot et al., 2000) (Fig. 1c). Loyamarok segment. As a consequence, this faultsegment is here considered as a specific structuralThe BTFS involves: (1) in the south the 510 ka- Fig. 3. (a) Three-dimensional SPOT view of the BTFS (Le Turdu et al., 1995). Black dot indicates the Kampi-ya-Samaki village(KyS) onshore Lake Baringo. 1, Footwall of the BT master fault; 2, its immediate hangingwall; and 3, the more distal part of thefaulted zone. Black arrow (rg) shows the sigmoid fault-bounded horsts that relate lateral movement along the northern extension of POKTZ (Fig. 1c). (b) and (c) Aerial photograph (Royal Air Force, January 1969) and morphotectonic interpretation of the BTFS.The dipping attitude of the lava flow in the di ff  erent fault blocks is shown. Bold dotted line, lateral oblique limits of the asymmetrictriangle-shaped faulted zone. The northeastern boundary is formed by the POKTZ. The structural meaning of the N40–50 ° E-trendingsoutheast limit is less clear, but it can be considered as a conjugate trend with regard to the POKTZ, both fault systems beinggenerated during an E–W extension mechanism. Lithostratigraphy of the BT master fault footwall and hangingwall: 1: BaringoTrachyte (footwall); 2: Chemakilani Basalts and Loyamarok Phonolite; 3: Recent alluvium; 4: Marigat alluvial fan; 5: Baringo Basalts;6: Kampi-ya-Samaki Beds; 7: Baringo Trachyte (hangingwall).  91 B. Le Gall et al.  /   Tectonophysics 320 (2000) 87–106 
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