Turkana, Kenya): Implications for Local and Regional Stresses

Research Paper

GEOSPHERE Early syn-rift igneous dike patterns, northern Kenya Rift (Turkana, Kenya): Implications for local and regional stresses,

GEOSPHERE, v. 16, no. 3 tectonics, and magma-structure interactions https://doi.org/10.1130/GES02107.1 C.K. Morley PTT Exploration and Production, Enco, Soi 11, Vibhavadi-Rangsit Road, 10400, Thailand 25 figures; 2 tables; 1 set of supplemental files

CORRESPONDENCE: [email protected] ABSTRACT basins elsewhere in the eastern branch of the East African Rift, which is an active rift, several studies African Rift. (Muirhead et al., 2015; Robertson et al., 2015; Wadge CITATION: Morley, C.K., 2020, Early syn-rift igneous dike patterns, northern Kenya Rift (Turkana, Kenya): Four areas (Loriu, Lojamei, Muranachok-Muru- et al., 2016) have explored interactions between Implications for local and regional stresses, tectonics, angapoi, Kamutile Hills) of well-developed structure and magmatism in the upper crust by and magma-structure interactions: Geosphere, v. 16, Miocene-age dikes in the northern Kenya Rift (Tur- ■■ INTRODUCTION investigating stress orientations inferred from no. 3, p. 890–918, https://doi.org/10.1130/GES02107.1. kana, Kenya) have been identified from fieldwork cone lineaments and caldera ellipticity (dikes were

Science Editor: David E. Fastovsky and satellite images; in total, >3500 dikes were The geometries of shallow igneous intrusive sys- insufficiently well exposed). Muirhead et al. (2015) Associate Editor: Eric H. Christiansen mapped. Three areas display NNW-SSE– to N-S– tems in rifts are highly varied, and range from those suggested that variable lineaments were the result oriented dike swarms, with straight, radial, and dominated by dikes and pipes, to those where sills of interplay between the regional stress field, local Received 18 December 2018 concentric patterns in zones <15 km long, and (fed by dikes and/or transgressive sills) dominate magma-induced stress fields, and stress rotations Revision received 29 November 2019 indicate NNW-SSE to N-S regional maximum hor- the shallow systems, particularly in syn-rift sedi- caused by interaction between rift segments. Pre- Accepted 20 February 2020 izontal principal stress (SHmax) directions in the early mentary basins (Galerne et al., 2011; see reviews in existing structures may influence the storage and

Published online 2 April 2020 to middle Miocene. Individual dikes are typically Magee et al. [2016] and Galland et al. [2018]). The orientation of deeper magma reservoirs, while shal- <2 m wide and tens to hundreds of meters long nature of these highly variable shallow intrusion low magmatism and intra-rift faulting are affected and have accommodated <2% extension. In places systems provides information about differences in by the local stress regime (Robertson et al., 2015), (Loriu, Lojamei, Lokhone high), dikes trend at a volcanic plumbing systems (e.g., Bell and Butcher, which can be influenced by the loading effects of high angle to the rift trend, suggesting some local 2002; Smallwood and Maresh, 2002; Planke et al., large topographic features such as rift flanks and influence (e.g., overpressured magma chamber, 2005; Cartwright and Hansen, 2006; Schofield et al., major volcanic edifices (Maccaferri et al., 2014; cracked lid–style dike intrusions over a sill or lacco- 2018; Magee et al., 2017), magma-fault interactions Wadge et al., 2016). lith, preexisting fabric in basement) on orientation, (e.g., Rateau et al., 2014; Schofield et al., 2016; Muir- There are few studies in the East African Rift in addition to the influence from regional stresses. head et al., 2016; Dumont et al., 2017; Morley, 2018), (Fig. 1) that assess the relationships between Only a minor influence by basement fabrics is seen upper crustal stress variations in rifts (Muirhead the large-boundary-fault stage of rifting and dike on dike orientation. The early- to middle-Miocene et al., 2015; Robertson et al., 2015; Wadge et al., emplacement in the upper crust in parts of the rift dikes and extrusive activity ended a long phase (up 2016), interaction between magmatic activity and system where igneous activity is less dominant to 25 m.y.) of amagmatic half-graben development petroleum systems (Schutter, 2003; Senger et al., (e.g., Muirhead et al., 2015). The reasons for this in central Kenya and southern Turkana, which lay 2017; Spacapan et al., 2018), and the processes con- paucity of studies partly lie in the difficulty of find- on the southern edge of the early (Eocene–Oligo- trolling extension in rifts (e.g., Swain, 1992; Ebinger ing a region of good exposure where a diversity of cene) plume activity. The Miocene dike sets and and Casey, 2001; Buck, 2004, 2006; Bialas et al., rift-basin settings can be found coupled with the extension on major border faults in Turkana con- 2010; Karakas and Dufek, 2015; Ebinger et al., 2017). right erosion levels to reveal dikes; in this respect, trast with larger, more extensive arrays of dikes in Interactions between syn-rift basins, rift structure, the Turkana area (Figs. 1, 2) of northern Kenya is evolved systems in the Main Ethiopian Rift that and igneous processes vary greatly between rifts, an exception. In Turkana, there are a number of are critical for accommodating crustal extension. and first-order differences can largely be explained Cenozoic rift basins that were initially filled by base- By the Pliocene–Holocene, magmatism and intru- in terms of passive versus active rifts (see review ment-derived alluvial, fluvio-deltaic, and lacustrine sion along dikes had become more important in van Wyk de Vries and van Wyk de Vries [2018]). deposits, and were then filled by extrusive igneous This paper is published under the terms of the for accommodating extension, and the tectonic There are also important temporal and spatial dif- deposits and clastics with a strong volcanic source CC‑BY-NC license. characteristics began to resemble those of rift ferences within passive and active rifts. For the East component (Morley et al., 1992; 1999a; Vétel et al.,

GEOSPHERE | Volume 16 | Number 3 Morley | Igneous dikes, Turkana, Kenya Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/3/890/5022267/890.pdf 890 by guest on 29 September 2021 Research Paper

2004; Muia, 2015), while contemporaneous basins further north have been filled predominantly by volcanic and volcaniclastic rocks (Boschetto et al., Gulf of 1992). In some of the Turkana basins, erosion has Aden exposed well-developed arrays of dikes (Fig. 2). 10° Afar The dikes occur in areas that have only been infre- Sudan Rift Ethiopian quently visited by geologists, although one of the Fig. 1B Rift areas described, in the Lokichar Basin (Fig. 2), has become a production area for hydrocarbons, which Fig. 5 5° has considerably opened up the outcrops around that basin. This paper discusses the occurrences Turkana Depression and morphology of well-exposed dike swarms in Albert Rift Anza four areas in Turkana: Muranachok-Muruangapoi, Fig. 2 Graben Kamutile Hills, Lojamei (South Lokichar Basin), 0° Lake Loriu (Fig. 2). This is a preliminary study based Kenya Baringo largely on analysis of satellite images, whereby Rift Western >3500 dikes have been mapped. To a limited Tanzania Branch Tanganyika Divergence extent, this analysis is supplemented by fieldwork Rift 5° Eastern that the author conducted in the area at various Figure 1. (A) Topographic map (Aster Global Branch Rukwa Digital Elevation Map, https://asterweb.jpl times between 1987 and 2013. However, that work Indian Rift .nasa.gov/gdem.asp) of the East African Rift, was focused on the petroleum system of the area Ocean 4 showing the locations of the study area and (Morley et al., 1992, 1999a; Wescott et al., 1993, key features discussed in the text. (B) Topo- 1999; Talbot et al., 2004; Tiercelin et al., 2004), not 3 graphic map (Aster Global Digital Elevation 10° the igneous activity, although some basic infor- Map) for East Africa showing the distribution of Mesozoic–Paleogene rift basins in the Turkana 2 mation about the intrusions was gathered. This Malawi area, Kenya, compiled from Morley et al. (1999a, information is supplemented by work from other Rift 1999b), Wescott et al. (1999), and Ebinger and Elevation (km)Elevation 1 studies in the region, notably Vetel (2005), Vétel et Ibrahim (1994). Crustal thickness contours are from Sippel et al. (2017). SB—Segen Basin; RR— al. (2004), Vetel and Le Gall (2006), Tiercelin et al. 30° 35° 40° 0 Ririba Rift; CBR—Chew Bahir Rift; KSFB—Kino Sogo fault belt. (2012a), and Muia (2015). The primary aims behind 4 Southern this study are: (1) to describe the relationships of Ethiopian RIft dike sets to structural location at a time when conti- Gofa Sudan Province 3 nental extension by faulting, particularly during the Rift formation of half grabens, in the brittle crust was Basin 2 Elevation (km)Elevation the primary mode; (2) to use the dike orientations Gatomi Basin Proto-Turkana basin to infer the stress orientations at particular times 40 30 SB 40 during the Cenozoic development of the Turkana 1 part of the eastern branch of the East African Rift; CBR RR (3) to assess whether relationships between dikes, 0 Lotikipi 16 rift structures, and preexisting fabrics can be iden- Basin 46 tified; and (4) to assess differences and similarities KSFB between dikes developed during the half-graben Lapur Range phase of development in Turkana and those in Lake 40 Turkana 28 Ethiopia, Afar, and southern Kenya to understand N 40 Anza Graben 24 whether the dikes are highly active in accommodat- 20 ing crustal extension, or are secondary features to 100 km 36 faulting (e.g., Hayward and Ebinger, 1996; Keranen Normal faults Mid Cretaceous-Cenozoic rift Crustal thickness (km) et al., 2004; Keir et al., 2006; Bastow et al., 2010;

Ebinger et al., 2010; Belachew et al., 2013; Beutel et Chew al., 2010; Weinstein et al., 2017; Ebinger et al., 2017; Bahir Rift Dumont et al., 2017; Rooney et al., 2018). Lapur Range

Lotikipi Lokitaung Gorge Plains Gatome Basin ■■ GEOLOGICAL BACKGROUND S. Lapur 4°N Outside of the initial preliminary studies of the Kataboi area (e.g., Walsh and Dodson, 1969; Hackman et fault zone Jibisa al., 1990), historically, a wide range of geological ring studies in the Turkana region have arisen largely as Lake complex Turkana a consequence of research into hominids (e.g., Cer- Allia Bay Kino Sogo ling and Brown, 1982; Feibel et al., 1989; Boschetto fault belt Muranachok- Jarigole et al., 1992; McDougall and Brown, 2009; Feibel, Muruangapoi 2011; Brown and Jicha, 2016) or hydrocarbon exploration (Morley et al., 1992, 1999a; Dunkelman et al., 1988, 1989; Karson and Curtis, 1994; Talbot et Moiti al., 2004; Tiercelin et al., 2004, 2012a, 2012b; Vétel Fig. 20

et al., 2004; Le Gall et al., 2005a; Vetel and Le Gall, Lokichar fault Napadet 2006). The focus of both types of study has been Hills 3°N the sedimentary section, although the extensive Kajong Turkana Kamutile tracts of volcanic and pyroclastic units are useful North Basin Hills Fig. 16 for reconstructing the tectonic history of the area, Lokichar Kathigithigiria Basin Mount understanding the paleo-geomorphology and sed- Hills Loriu Porr iment provenance, and providing radiometric ages Fig. 17 Fig. 9 of the basin fill. Mount Mount Kulal Moroto Fig. 10 Seismic reflection data have demonstrated that Lokhone there are a number of half-graben rift basins in west- FWU Fig. 19 Kerio Turkwell escarpment Lokichar Basin ern Turkana (e.g., Turkana, North Lokichar, Lokichar, Basin Fig. 4 Fig. 4 and Kerio Basins, Fig. 2; Morley et al., 1992, 1999a; Fig. 15 The Barrier Turkwell fault Vetel, 2005; Vetel and Le Gall, 2006), Lake Turkana Fig. 13 (Dunkelman et al., 1988, 1989), and the Omo Basin (Alemu, 2017). The age of rifting is diachronous and spans the time period between the Eocene and Lokhone- 2°N Loperot the present day; there is a general easterly and Lojamei Suguta Valley 35°E 36°E southerly younging in both rift-basin and volca-

nic activity (Fig. 3; Morley et al., 1992, 1999a; Vetel, NNW-SSE- to N-S- trending NNE-SSW- to NE-SW- trending foliations NNW-SSE- to 2005; Vetel and Le Gall, 2006; Boone et al., 2018b). foliations N-S- trending foliations Some basins or stages in basin development are Dike swarms discussed in Trend of Precambrian foliations filled primarily by basement-derived coarse clastics this study N Oval/circular features in volcanics, and lacustrine shales, while other basins or stages Boundary of Precambrian eroded volcanoes, and/or forced folds foliation trend province in basins are dominated by lavas and pyroclastic 50 km and volcaniclastic deposits (Boschetto et al., 1992; Normal fault Morley et al., 1999a; Vetel and Le Gall, 2006; Fig. 3). The eastward migration of extensional activity has Figure 2. Topographic map (digital elevation model, slope-shader image Aster Global Digital Elevation Map) of the Turkana region, Kenya, showing key rift features and the location of the study area. Rift structure is from Morley et al. (1999a), caused the Turkana region to be the widest part with some modification from satellite image interpretation (this study) and from Vétel et al. (2005) and Vetel and Le Gall of the East African Rift, forming an ~250-km-wide (2006). FWU—footwall uplift.

diffuse zone of faulting that is three to five times North South the width of the East African Rift in other places Lapur NW Lake Suguta Ma Range Turkana Muranachok Lothidok North Lokichar Loriu Kerio Lokichar Valley S Kerio (outside of the Afar Triangle and North Tanzania 0 Divergence) (Figs. 1, 4). Today, fault systems west Gombe Group 22 Turkwell Basalt (13,c) 1 8 Beds 12 of Lake Turkana are largely abandoned, and Plio- 10 12 Loperi Basalts 19 cene–Holocene fault activity has been focused II 15 2 Lothidok Fm III IV around Lake Turkana and east of the lake (Morley I 16 20 16 11 Kalakol et al., 1999a; Vétel et al., 2004; Vetel and Le Gall, b 14 Basalts 12 2006; Corti et al., 2019). The main Pliocene–Holo- 3 17 30 Lk? cene volcanic activity is also focused along Lake 2,4, Ld? 12 18 5 ? Lo? Turkana (e.g., South, Central, and North Islands) 6 40 20 ? and on the eastern side of the lake (Mount Kulal, ? 21 Mount Marsabit) (Hackman et al., 1990; Curtis, 1991; 6 9 Karson and Curtis, 1994). 50 Muranachok 4 Grits A north-south seismic wide-angle reflection-re- a ? ? ? ? ? ? 10 fraction profile was acquired on the western side 60 of Lake Turkana as part of the KRISP 90 program (e.g., Keller et al., 1994). This line shows that the 70 crust west of the lake is as thin as 20 km (Keller ? ? ? ? ? 7 et al., 1994; Khan et al., 2000). Gravity modeling 80 indicates that this zone of crustal thinning trends Miocene-recent clastics; Predominantly basement- north-south and runs along the northern half of Period of Dominantly extrusive mixed uvial-alluvial- derived clastics; typically Unconformity extension volcanics lacustrine, mixed volcanic coarse grained Lake Turkana (Fig. 1B). Estimates of upper crustal basement sources Timing Timing extension, related to superimposed Cretaceous, Volcaniclastics and uncertain uncertain Presence of rift section Lacustrine minor ows Paleogene, and Neogene rifting events, from ? possible but highly shales balanced cross-sections and flexural models are uncertain to hypothetical ~35–40 km (Morley et al., 1992; Hendrie et al., 1994). Figure 3. Correlation of extensional episodes and stratigraphy in Turkana and central Kenya. Based on Morley et al. (1999a) and In southern Ethiopia and northern Turkana, the Brown and McDougall (2011), modified and/or supported by the following studies: 1—Boone et al. (2018a); 2—Ragon et al. (2018); presence of 45–35 Ma tholeiitic basalts and associ- 3—Brown and Jicha (2016); 4—Wescott et al. (1999); 5—Zanettin et al. (1983); 6—Tiercelin et al. (2012a); 7—O’Connor et al. (2011); ated silicic extrusives (Amaro-Gamo Unit, Chibelet 8—Abdelfettah et al. (2016); 9—Ebinger and Ibrahim (1994); 10—Owusu Agyemang et al. (2019); 11—Boschetto et al. (1992); 12— Morley et al. (1999a); 13—Haileab et al. (2004); 14—Boone et al. (2018b); 15—Wescott et al. (1993); 16—Muia (2015); 17—Ducrocq et Volcanics, Balesa-Koromto Unit, Turkana Volcanics; al. (2010); 18—Boone et al. (2019); 19—Chapman et al. (1978); 20—Hautot et al. (2000); 21—Morley et al. (1992); 22—Bosworth and see review in Rooney [2017]) indicates that anom- Maurin (1993). Notes: a—Timing of extension is uncertain; basin underlies an Eocene–Oligocene volcanic series imaged on seismic alously hot mantle related to a mantle plume had data and gravity data (Wescott et al., 1999). Extension during Cretaceous-Eocene was likely episodic. Lapur Sandstones have no obvious fault control. b—Based on Figure 5 (and related references). c—Equivalent to the Orange horizon of Morley et al. (1999a). arrived beneath northeastern Africa by 40–45 Ma Study areas: I—Muranachok-Muruangapoi, II—Napadet-Kamutile-Kathigithigiria Hills; III—Loriu; IV—Lojamei. Other abbreviations: (Davidson and Rex, 1980; Ebinger, et al., 1993, 2000; Fm—Formation; Ld?—basin fill seen on seismic reflection data, but has not been drilled, probably equivalent stratigraphy to that George et al., 1998; George and Rogers, 2002; Pik et in Lothidok Range; Lk? Lo?—basin fill seen on seismic reflection data, but has not been drilled, probably equivalent stratigraphy to that in Lokichar Basin or the Loriu area. al., 2006). In northwestern Turkana, the oldest lavas are of late Eocene age (Fig. 5; e.g., Zanettin et al., 1983; Bellieni et al., 1987; Rooney, 2017). The lavas extend to the eastern side of Lake Turkana in the volcanism covers a region today that extends from anomalously high mantle temperatures, probably Kajong area (Fig. 2), where a sample was 40Ar/39Ar northwestern Turkana, where the crust is relatively associated with a mantle plume, were required to dated at 39.2 ± 04 Ma by Furman et al. (2006). thick, to the central area of Lake Turkana, which cause magmatic activity in the Turkana area, par- The key evolution in volcanism in Turkana is a today is the main zone of thinning. The igneous ticularly the very early Eocene–Oligocene lavas shift to the southeast from the Eocene–Oligocene activity is associated with very shallow partial melt- present in northwestern Turkana. However, the volcanism to the Miocene volcanism, and a shift to ing of the mantle (50–65 km; Furman et al., 2006). greater depths of melting at 20 Ma (Fig. 4; Furman the east from the Miocene to the Pliocene–Holocene Structural data (e.g., Hendrie et al., 1994) and geo- et al., 2006) suggest that the initial Paleogene ther- (Fig. 5; Morley et al., 1999a). The Eocene–Oligocene chemical data (Furman et al., 2006) indicate that mal anomaly had dissipated or at least diminished,

Thin lower crustal layer (Mechie et al., 1997; Sippel et al., 2017) ca 45(?) Ma-7 Ma ca 7 Ma- present “Mixed active and passive rift” pattern “Active rift” pattern (Fig. 22) Figure 4. Semi-schematic crustal-scale W Napadet-Kamutile- Loriu area Anza Graben E Kathigithigiria Hills area Anza Graben cross-section through the Turkana region, showing the relationship between up- Mount Moroto Turkwell Basin Lokichar Basin Kerio Basin Suguta- Mt. Kulal Mount Marsabit S. Lake Turkana per-crustal rift basins (Morley et al., 1999a), 0 the estimated depth range of zones where Upper partial melting has produced surface vol- Crust canism (Furman et al., 2006), and Moho Lower morphology (Sippel et al., 2017). See Fig- Crust Moho ure 2 for locations. The deformation style is based in part on information from Mechie 50 et al. (1997) and the current model for

Depth (km) 40-30 ma 5-0 Ma southern Kenya (Ebinger et al., 2017; Plas- Mantle man et al., 2017; Roecker et al., 2017; Tiberi et al., 2019). Depth estimates for the zones Partial melting 20-10 Ma of partial melting that produced magmas 100 possibly from detached for certain time periods are as follows (Fur- lithospheric mantle “drip” 50 km (Furman et al., 2006) man et al., 2006): 39–35 Ma basanite lavas, 15–20 kbar, 50–65 km; 20 Ma mafic lavas, 85–100 km; Pliocene–Holocene lavas east of Cenozoic rift basin ll (sediments and volcanics) Cretaceous-Paleogene rift basin ll (Anza Graben) Lake Turkana, 20–30 kbar, 65–90 km; South Regions within which partial melting of mantle may have occurred at Island, Lake Turkana, 15–20 kbar, 50–65 km. a particular age (as indicated); depth based on Furman et al. (2006), extent based Ductile extensional shears zones in on surface radiometric ages of igneous rocks (as reviewed in this study) lower crust (?)

29-10 Ma intrusions - magma chamber, 7 Ma- present, intrusions; magma chamber, sills, laccoliths, dikes sills, laccoliths, dikes

and probably the onset of a new mantle thermal ■■ METHODOLOGY were overlain on the Aster DEM data set, which anomaly, plus the effects of lithospheric stretching, has a resolution of 1 arc-second (~30 m in Kenya). both had an effect on Miocene volcanism. An alter- Dikes and other geological features were inter- Hence the DEM data are good for providing gen- native explanation to plume-related melting for the preted within Global Mapper software (https://www. eral topographic information but lack the resolution 16–23 Ma volcanism in Turkana is partial melting bluemarblegeo.com/products/global-mapper.php), to aid in interpretation of dikes. Interpretation of related to detachment of a plume-metasomatized using the U.S. National Aeronautics and Space features was cross-checked with that of previous “drip” of lithosphere that descended into the asthe- Administration (NASA) Aster Global Digital Eleva- publications (in particular: Morley et al., 1999a; nosphere (Furman et al., 2016). tion Map (DEM) V2 data sets (https://asterweb.jpl. Vetel, 2005; Tiercelin et al., 2012a; Muia, 2015) and In the depth of melting and distribution of the nasa.gov/gdem.asp). World Imagery ESRI server unpublished material from previous expeditions I igneous activity, it can be seen that for the Miocene of Community maps program, SPOT satellite was involved with. Turkana is arid to semiarid and and Pliocene–Holocene phases, some arguments images, https://www.arcgis.com/home/item.htm- has little tree cover. Young alluvial, fluvial, lacus- can be made for crustal thinning exerting an influ- l?id=10df2279f9684e4a9f6a7f08febac2a9) provides trine, and aeolian deposits commonly mask the ence on the location and depth of melting. However, variable-resolution data that typically is 1 m or bet- bedrock. But there are also extensive tracts where the great activity of Pliocene–Holocene volcanism ter, but can be of lower resolution in some areas. bedrock is exposed almost 100%. Geological maps passing from the Suguta Valley (Fig. 2) to the east- The imagery used for the study areas (Fig. 2) dates of the areas used in this study are from Morley et ern side of the lake (Morley et al., 1999a; Vetel and from 2011 and has a resolution of 0.5 m and an al. (1999a) and Vetel (2005). The Loriu area geology Le Gall, 2006), where the crust is showing increas- accuracy of 10.2 m. Dikes as small as ~1–2 m in was remapped based on previous unpublished field ing thickness (Figs. 1B, 4), indicates that partial width (and probably smaller) are imaged, and can notes of the author and satellite images. melting within the mantle is more strongly influ- be mapped on this data set. Typically dikes that run Dikes have a variety of characteristics on sat- enced by mantle temperature anomalies than by for hundreds of meters to kilometers in length in ellite data. Most typically they are composed of lithospheric stretching. Turkana are ~1–10 m wide. The World Imagery data dark gray to black basalt, and so form thin, linear,

erosionally resistant features that are darker than the adjacent regions. Dikes are commonly so thin (<1 m) that they are not imaged clearly. The wider ETHIOPIA SOUTH SUDAN dikes (~5–10 m), particularly those in the Lojamei 26.0 area, display internal features; in particular, in some 25.9 cases chilled margins tend to be more resistant to weathering, and so stand higher than the center of the dike (Fig. 6). In other examples, an apparently KENYA wide dike is actually the product of two or three Todenyang separate intrusive events. The effects of chilled Lokwanamur margins or multiple intrusion events can be con- Kalin fusing in some areas on satellite images, because Mogila 31.2 Lapur 35.7 such dikes can resemble a straight dirt road. This 32.3 26.4 Lokitaung is particularly an issue in the Lojamei area, where 31.7 O 36.2 N 30 25.6 trend in Gatome Basin 35.7 a grid of linear, bulldozed, seismic reflection lines Songot Lotikipi Plain volcanics 27.2 are present. Another area where the appearance of 4° N dikes is atypical is the Napadet-Kamutile-Kathigithi- 32.5 giria (NKK) Hills (Fig. 7). The presence of the white KFZ

material seen in Figure 7 is related primarily to Moruerith 27.9 16.6 wind-blown sediment and to sediment (commonly Kaeris 15.7 diatomaceous lacustrine deposits and pyroclastic Depression 36.0 17.5 deposits) interbedded within the volcanic sequence. 31.0 Pelekech The dikes form subtle topographic ridges that trap UGANDA 27.0, 27.1 Lake Turkana the sediment. It is a problem that some straight Radiometric ages (in Ma) Kalakol bedding trends, fractures, and faults are also asso- 22.9 Zanettin et al. (1983) Muranachok 18.5 Lothidok 17.9 3° N ciated with linear tracks of white sediment, hence 32.5 Brown and Jicha (2016) 17.7 17.6 the presence of dikes had to be verified when map- 13.8 McDougall and Brown (2009) 24.2 27.5 ping. Conversely, this probably meant that some Normal fault 16.3 Trend of bedding/layering 16.8 white tracks associated with sub-satellite-resolution Fig. 20 16.5 13.8 16.6 Trend of foliation in Precambrian gneisses 12.2 dikes remained unmapped, and there is probably Pliocene sedimentary rocks 25.5 11.6 overinterpretation of dikes as well. Eocene-Miocene volcanic and sedimentary rocks Muruangapoi 22.9 In some areas, particularly where dikes are wide Cretaceous-Paleogene N Lodwar arkosic, coarse clastics 35° E 36° E (>5 m), the relative timing of different dike sets can Metamorphic basement 10 km Eroded volcano or crater be established. This is particularly useful in the Loja- mei area where a few dikes of different orientations Figure 5. Geological map of northwestern Turkana, modified from Brown and Jicha (2016) and Vetel (2005). Radiometric ages Trend of foliation in Precambrian gneisses have been dated (Morley et al., 1999a; Tiercelin et al., (in Ma) are from the three sources compiled by Brown and Jicha (2016), with error ranges omitted. See Figure 1 for location. KFZ—Kataboi fault zone. Trend of bedding, layering or foliation 2004, see Lojamei Dikes section), and this information is complemented by the relative timing of the

Dike dikes of different orientations from their cross-cutting relationships seen in the field and from satellite the trend of the older dike (Fig. 8C); and (2) a local widths, and timing in the study area is provided in Normal fault data (Fig. 8). In outcrop, the cross-cutting relation- change in direction (jog) where the younger dike Table 1. A summary of the published dating of dikes ships of dikes are readily observed, but commonly runs parallel to the older dike for a short distance in the study areas in western Turkana is provided such relationships are not clearly imaged on sat- before resuming its original orientation (Figs. 8A, in Table 2. Shapefiles (ESRI) of the mapped dike 1 Supplemental Files. ESRI Shapefiles of the mapped ellite data, particularly for narrow dikes. Other 8D). Rarely, instead of cross-cutting an older dike, a patterns, faults, and other features for this study dike patterns, faults, and other features for this effects commonly accompany the cross-cutting younger dike may simply be deflected into the ori- are available in the Supplemental Files1. study. Please visit https://doi.org/10.1130/GEOS.S .12054126 or access the full-text article on www.gsa- relationships and so aid in identification of rela- entation of the older dike (Figs. 8B, 8E). A summary Dikes were initially mapped in Global Mapper pubs.org to view the Supplemental Files. tive timing. These effects are: (1) a small offset in of the potential problems of mapping dike traces, without sorting into different types. Subsequently,

the dikes identified in the NKK Hills and Loriu E W were sorted into groups simply on basic patterns: which dikes looked compatible with a radial dike pattern, or which ones were compatible with ring dike trends. In the Lojamei area, such patterns were not apparent, and whether the different dike trends tended to have consistent relative timing (Fig. 8) was considered instead. Rose diagrams of linear feature orientations represent the frequency of occurrence of a particular azimuthal trend (commonly divided into 10° or Dike 20° increments). Dike orientations are given as the average orientation for a particular unit length, e.g., 100 m. Hence, all dikes up to 100 m in length would

Sandstone beds with be represented by one orientation each, while a gentle dip 5-km-long dike counts as 50 orientations. This has the advantage of giving a more accurate weighting Road-like appearance when dike lengths are highly variable, and reduces to wide dike with the problems of representing dikes that exhibit vari- prominent chilled margins, ca10 m wide able azimuths.

Figure 6. Aerial view of dikes in the Lojamei area (see Fig. 2 for general location of the Lojamei area). ■■ DIKE PATTERNS AND TIMING WEST OF LAKE TURKANA

Loriu Area

Introduction

The coarse arkosic clastics of the Loriu area (formerly Lariu) are known as the Loriu Sandstone (Muia, 2015). The sandstones are of uncertain age and directly overlie Precambrian gneisses, and in turn are unconformably overlain by early Mio- cene volcanic rocks with interbedded pyroclastic, volcaniclastic, and lacustrine deposits (Wescott et 2°45’28” N al., 1993; Muia, 2015). The sandstones are present 35°58’69” N in a 3.4 km long × 2.6 km wide NE-SW–trending zone (Fig. 9). In places, the sandstones onlap the gneisses; elsewhere, they are in normal fault con- N tact (Fig. 9). The fault geometry changes in a NE-SW direction and gives rise to two main transfer zones, one with a colinear geometry, the other with a relay 500 m ramp (Fig. 9). The Precambrian gneisses and Loriu Sandstone were deformed by a phase of normal

Figure 7. Satellite image of dikes in the Napadet-Kamutile-Kathigithigiria Hills area acting as traps for faulting that occurred prior to extrusion of the vol- light-colored sediment, which considerably enhances the visibility of dike sets. canic rocks. Subsequently, renewed normal faulting

Deection into trend of during the Miocene–Pliocene tilted the volcanic older E-W dike series. The dips within the Loriu Sandstone are commonly higher and more variable (>30° dip) 2 o 13’ 45.11” N 35 o 59’ 36.17” E than would typically be expected for simple normal o 2 12’ 27.95” N faulting, suggesting that volcanic intrusions, and 35 o 58’ 12.97” E possibly inversion, have also deformed the area

Cross-cut N Younger (Morley et al., 1999a; Le Gall et al., 2005a). Outcrops of the Loriu Sandstone, volcanic rocks, Youngest Cross-cut and and to a lesser degree the Precambrian gneisses jog along side 100 m Older of older dike are extensively intruded by dikes (Figs. 9, 10, 11); N possibly thick underlying sills or laccoliths have Oldest caused broad monoclinal folding (with bedding Younger 50 m dips ~35°–40°NE) along the southwestern margin of the Loriu Sandstone (Fig. 11). Wescott et al. (1993) used whole-rock K-Ar dating to obtain an age of Cross-cut and Older Cross-cut and small o set in o short jog 15.7 ± 0.7 Ma for a basalt sample from the base of 2 13’ 52.24” N trend of older o along side of 35 59’ 27.78” E Younger the volcanic sequence, and an age of 14.7 ± 0.17 Ma dike older dike for a dike intruding the sediments (Table 2; see Fig. 9 for locations). The igneous outcrops overlie Precambrian gneisses and Loriu Sandstone, and 2° 14’ 15.53” N include lavas interbedded with pyroclastic and Older 35° 39’ 33.68” E N N volcaniclastic deposits and lake beds. Intrusions include basaltic dikes with augite phenocrysts, and 50 m 50 m Younger agglomerates in vent complexes. Muia (2015) sam- LOJ 20 N 015o <12.9 Ma pled a N110°-trending vertical basalt dike intruding (12.9 ±1.5 Ma) o LOJ 01 o N 25 15.9 ±1.2 Ma 40 39 o N 170 o the Loriu Sandstone (Fig. 9). Ar- Ar dating of the Dike trends and ages based on (16.5 ± 0.4 Ma) N140 N 50 radiometric dating and on LOJ 98-25 dike did not result in definition of a plateau age; cross-cutting relationships (15.7 ± 0.8 Ma) o a pseudo–plateau age for increments between N 090 16.5-15.7 Ma (11.9 ± 0.8 Ma) o 10% and 65% of Ar release suggests an age of ca. N 080 16.5 Ma 18.5 Ma (Muia, 2015; Table 2). E

Dike 3 2°14’ 01” Intrusions in the Loriu Area Dike 2 35° 57 ‘37” Dike trends in the Loriu area are separated into Dike 3 Dike 2 two types, comprising radial and non-radial pat- Dike 1 terns (blue and red dikes, respectively, in Fig. 10). Dike 1 N The radial dikes are centered around an eroded volcanic complex (area A in Fig. 10). The longest

50 m intrusions are ~2.1 km long, and dikes are commonly in the range of hundreds of meters to 1 km long. On the eastern side of the radial dikes is area Figure 8. Satellite images of dikes from the Lojamei area showing criteria for determining the relative timing of dikes. Coordinates of white square are given as locations for the images. (A) Youngest dike jogs along the trace of the intermediate-age dike for a short B, which comprises a high density of predominantly distance and cross-cuts the oldest dike. (B) Younger dike deflects into the trace of the older dike. (C) Cross-cutting relationship NW-SE–trending intrusions. The NW-SE–trending with small visible offset of older dike. (D) Short deflection of cross-cutting younger dike along older dike trace. (E) Dikes 2 and 3 dikes reach maximum lengths of ~700 m, and more deflected into the trend of older Dike 1. (F) Suggested scheme for the ages of dikes (color coded to match those shown in A–D) based on limited radiometric dating (Morley et al., 1999a; Tiercelin et al., 2004) and the types of relationships shown in A–D. LOJ typically dikes are tens of meters to a few hundred are sample numbers in Tiercelin et al. (2004); see Table 2. meters long. The presence of other trends gives

TABE 1. POTENTIA PITFAS FOR INTERPRETATION OF DIKES FROM SATEITE DATA Dike trace hite sediment that may mask dike, highlight dike, or represent a linear feature unrelated to a dike. Introduces an element of over- and underinterpretation of dike features in the Napadet-Kamutile-Kathigithigiria Hills. Interruption of dike trace by deposition of recent deposits or younger volcanics results in underestimation of dike trace length. Dike resembles other linear features (e.g., bulldoed dirt tracks), particularly a problem in the oamei area. Dike features in Precambrian basement may be much older than the Cenooic. Dikes that eroded ith a consistent dark apron of material strongly resemble those that cut through the Cenooic section and are very likely to be Cenooic in age. inear features that lack this apron are harder to distinguish and may be older dikes that eather in a similar ay to gneissic host rock. Sometimes the same trend of dikes can be seen in adacent Cenooic rocks, hich increases the likelihood that basement-involved dikes of similar orientation are also of Cenooic age. Dike idth Satellite resolution issues. In part mitigated by outcrop field observations. Multiple parallel intrusions mistaken for a single intrusion. Dike timing Dike segmentation and different depths of eposure due to the presence of a slope, hich may create the false perception of cross-cutting relationships. Satellite resolution insufficient to determine timing relationships for most dikes. Only the idest dikes (5 m) may ehibit observable relationships. The most reliable relative timing is established for ide dikes in a flat rock pavement (this is the case for parts of the oamei area).

TABE 2. SMMARY OF RADIOMETRIC AGES OF IGNEOS INTRSIONS AND REATED AA FOS IN TRKANA, KENYA ocation Composition, Comments Age and method Reference sample number

apur Range 015′01″N, 358′23″E Te, KER 65/05, OK A5 Dike, 10 m ide, N60 strike 27.03 0.57 Ma, 0Ar-39Ar Tiercelin et al. (2012a) 010′02″N, 357′32″E R, NATH 08/0, 09/0 Sill and dikes; deep eathering affects dating 28.73 0.6 Ma, 0Ar-39Ar Tiercelin et al. (2012a) 27.0 0.66 Ma, 0Ar-39Ar oamei 0212′06.2″N, 3557′36.63″E D, O 20 Dike, 10 m ide, N170 strike 12.09 1.5 Ma, K-Ar hole rock Tiercelin et al. (200) 0211′30.02″N, 3557′57.16″E AB, O 98-25 Dike, 5 m ide, N50 strike 15.70 0.8 Ma, K-Ar hole rock Tiercelin et al. (200) 0209′3.65″N, 3558′3.32″E AB, O 01 Dike, 2 m ide, N10 strike 16.5 0. Ma, K-Ar hole rock Tiercelin et al. (200) 0213′2.58″N, 3559′05.66″E B Dike, N025 strike 15.9 1.2 Ma, K-Ar hole rock Morley et al. (1992) 0212′20.22″N, 3557′13.2″E B Dike, N050 strike; location E, Figure 13 11.9 1.9 Ma, K-Ar hole rock Morley et al. (1992) Muranachok-Muruangapoi 032′58″N, 3520′12″E R, 87-3182 ava flo 36.0 0. Ma, K-Ar anorthoclase McDougall and Bron (2009) 0319′51″N, 3535′27″E B, 87-3155 ava flo 16.3 0.2 Ma, K-Ar hole rock McDougall and Bron (2009) 0316′57″N, 3527′1″E B, 87-3191 ava flo 16.5 0.2 Ma, K-Ar hole rock McDougall and Bron (2009) 0333′1″N, 3521′12″E B ava over Muranachok Grits 18.5 0.7 Ma, K-Ar hole rock anettin et al. (1983) 0313′20″N, 3529′15″E B ava over Muranachok Grits 25.5 1.2 Ma, K-Ar hole rock anettin et al. (1983) NKK Hills 0256′18″N, 3551′1″E B, 92-15 ava flo 12.8 0.01 Ma, K-Ar hole rock McDougall and Bron (2009) 0229′01″N, 3556′27″E B, 87-1 ava flo 13.9 0.02 Ma, K-Ar hole rock McDougall and Bron (2009) 0229′01″N, 3556′27″E B87-311 ava flo 15.0 0.02 Ma, K-Ar hole rock McDougall and Bron (2009) 0256′59″N, 3553′12″E B ava flo 1.9 1.5 Ma, K-Ar hole rock Morley et al. (1992) 0236′7″N, 3501′27″E B ava flo 13.2 1.5 Ma, K-Ar hole rock Morley et al. (1992) oriu 023′6″N, 3625′13″E B Base of lava flo overlying oriu Sandstone dike 15.70 0.7 Ma, K-Ar hole rock escott et al. (1993) 022′26″N, 3625′29″E B ava flo 1.70 0.17 Ma, K-Ar hole rock escott et al. (1993) 022′50″N, 3625′7″E B Dike, 1 m ide, 500 m long, N135 strike Estimate 18.5 Ma, 0Ar-39Ar Muia (2015) pseudo-plateau age only Notes: NKKNapadet-Kamutile-Kathigithigiria; ABalkaline basalt; Bbasalt; Ddifferentiated lava; Rrhyolite; Tetephrite. ocations estimated from position given on map in publication. Some sample numbers are not present here or in the paper.

Wescott et al. (1993) probably of late Eocene age (Boone et al., 2019) lava 15.7 ± 0.7 and comprises basement-derived arkosic fluvial Colinear transfer zone concealed grits and lacustrine shales (Loperot Group; Morley 18° by later volcanic unit et al., 1999a; Fig. 3). In the late early Miocene to

10° Muia, (2015) middle Miocene section, volcanic flows and vol- dike 18.5 Ma 2° 43’ N caniclastic deposits (Auwerwer Basalts, Auwerwer 30° 17° Sandstone Formation) become important features of the basin fill (Morley et al., 1999a; Tiercelin et Wescott et al. (1983) Y al., 2004; Fig. 3). Detailed descriptions of the basin dike 14.7 ± 0.17 have been given by Morley et al. (1992, 1999a), 10° 76° Tiercelin et al. (2004), and Talbot et al. (2004). 44° More recently, hydrocarbons were discovered in the basin by Tullow Oil, which has made it a

2° 42’ N significant center for exploration. The older section is exposed in numerous low-relief outcrops N 28° Lake around the flexural margin of the basin marked by the Lokhone footwall uplift area (Fig. 2). With 40° 35° Turkana variable degrees of intensity, these outcrops are Synthetic transfer intruded by early–middle Miocene basic and inter- zone (relay ramp) 1 km mediate dikes (Morley et al., 1999a, Tiercelin et al., 35° 2004). The intrusions in the southern part of the basin form a particularly dense network and are

View direction 40° discussed in the following section. Fig. 12 33°

Lojamei Dikes 36° 24’ E 36° 25’ E 36° 26’ E Fig. 11 The dikes are exposed in the lowest volcanic Early Miocene volcanic rocks, with some thin interbedded Loriu Sandstone, probable Late Cretaceous layers and the Oligo-Miocene sediments (predomi- sedimentary rocks and/or Paleogene nantly Lokhone Sandstone Formation) that underlie Plio-Pleistocene Precambrian gneisses Slumped Loriu Sandstone lacustrine deposits the middle Miocene Auwerwer Basalts and Auwer- wer Sandstone Formation in the Lokichar Basin. In 17° Dip and strike of bedding in Early Miocene Normal fault Loriu Sandstone basic dikes the basin center, the volcanic section is ~1–1.5 km thick (Morley et al., 1999a). The uplifted region that Figure 9. Geological map of the Loriu area based on aerial photo and satellite image mapping together with limited field exposes the dikes is on the flexural margin of the checking. Considerably updated from Wescott et al. (1993). See Figure 2 for location. basin, and in this direction the basin fill thickness rapidly decreases, hence the likely maximum depth of emplacement of the dikes seen at the surface rise to a network of dikes that makes for impressive South Lokichar Basin–Lojamei Area today is <1 km. viewing from the air (Fig. 12). Both the orientation The Lojamei area (Figs. 2, 13) is intensively and spacing of the intrusions in area B are very dif- The Lokichar Basin is the largest and most intruded by dikes typically 1–10 m wide. Morley et ferent from those of the adjacent radial dike swarm. accessible sedimentary basin in Turkana, and al. (1999a) and Tiercelin et al. (2004) reported on The third trend of dikes in the area lies northeast forms a classic half graben, with the Kerio base- dikes from the area that were dated using 40K-40Ar of area B and comprises a NE-SW–trending dike ment high marking the flexural margin to the whole-rock fractions; the ages range between 16.5 set (area C, Fig. 10) that lies approximately parallel east, and the basin thickening westward into the and 12 Ma (Table 2) for dikes with a variety of ori- to the trend of Precambrian basement foliations. east-dipping Lokichar fault (Morley et al., 1999a; entations. The 16.5–15 Ma basalts coincide with a Typical dike lengths are between 50 m and 300 m. Figs. 2, 4). The oldest part of the basin fill is major phase of volcanic activity that affected the

northern and central regions of the Kenya rift (Sam- buru Basalts, 20–16 Ma; Turkana Basalts, 23–14 Ma; 11.2 km N Baker and Wohlenberg, 1971; Baker et al., 1972; 0 2 km Joubert, 1966; Walsh and Dodson, 1969; Tatsumi and Kimura 1991; Muia, 2015). 30% 20 10 10 20 30 The younger-age dikes coincide with the Auw- erwer Basalts (12.5–10.7 Ma) in the Lokichar Basin (Morley et al., 1999a). There is a considerable vari- C ety to the dikes in hand specimen: while many are C simply dark gray to black, fine-grained basalts, other dikes are porphyritic (commonly augite 2° 42’ N phenocrysts), contain amygdales filled by white zeolites, or are intermediate in composition and rich in mica. Some mica-rich dikes show a cleavage developed where mica is concentrated and aligned along the chilled margins (e.g., Fig. 13, location A). Lake Volcanic center Turkana Dike Trends

The relative timing relationships of some dikes B identified from satellite images (Fig. 8) can be 2° 40’ N matched with the radiometric ages to suggest 45.2 km some preferred trends for dikes of different age:

pre–16.5 Ma, N070°–N090° dikes; ca. 16.5 Ma, N130°– 31.7 km 20% 10 10 20% N150° dikes; ca. 15–16 Ma, N25°–N60° dikes; ca. 12.9–11.9 Ma, N180°–N160° dikes; and post–11.9 Ma, ~N005°–N020° dikes (but also one N050°-trending 20% 10 10 20% dike dated to this age). The N130°–N150° trend is B the strongest in the area and gives rise to a number of long dikes (e.g., Fig. 14, dike 1). Small volca- A nic cones (e.g., Fig. 14, location X) tend to lie on A dikes of this trend, or form NNW-SSE alignments 36° 23’ E 36° 25’ E of cones. While some relationship between timing

and orientation can be defined, it is unlikely that all Figure 10. Map of dikes in the Loriu area, overlain on a shaded relief map (Aster Global Digital Elevation Map). Labeled dikes of a particular age follow a single trend. For areas: A—radial dike swarm focused on volcanic center, with dikes relating to this pattern shown in blue, and dikes not example, at small volcanic centers, dikes exhibit compatible with the radial trend shown in red; B—predominantly NW-SE dike trend; C—area of NE-SW–trending dikes, compatible with radial dike swarm, but possibly also influenced by NE-SW–trending basement fabrics. Rose diagrams a radial geometry (Fig. 14, location Y). The radial showing dike orientations in the three areas; total length of dikes measured is given in kilometers (km). geometry could just represent intersection of dikes of different age and orientation, but it is also common for dikes to form simultaneously with a radial geometry at volcanic centers (see review in Tibaldi the Lokhone Sandstone (Oligocene–early Miocene), Lokichar Basin, the approximately east-west dike [2015]). An indication that dikes striking N70°–N120° and the Lokhone Shale (early Miocene). Two dike orientations are found. These dikes feed a number reflect a particular time period of intrusions comes trends are present, approximately east-west and of sills intruded into the shales. More radiometric from the Lokhone area to the north (Fig. 15). This ~NNW-SSE. For these dikes, there is no indication dating, geochemistry, and detailed fieldwork are area, which lies in the central part of the Lokichar that they are part of a radial dike swarm, and in required to determine how the different pulses of Basin, displays intrusions in Precambrian gneisses, a number of places around the margins of the dike intrusion have developed and interacted.

Figure 11. Schematic cross-sec- SSW NNE NNW-SSE trending dikes continuing the trend of tion for the Loriu area, showing Igneous intrusion a possible explanation for the the fault zone to the south. Loriu Sandstone NW-SE–trending dike swarm, and The Lojamei area was potentially affected by the NW-SE–trending boundary to Precambrian basement complex stress patterns during the syn-rift stage, the Precambrian basement and Laccolith related to stress deflections around the major splay- the Loriu Sandstone (see Fig. 9 for 1 km location). Inflation by the laccolith ing fault termination (local stress rotations related under the region of outcropping to fault morphology have been observed in other Precambrian basement, and deflation of the laccolith accompanying dike intrusion in the Loriu Sandstone, contributed to rift systems; e.g., Tingay et al., 2010). The absence the subsidence of the area of outcropping Loriu Sandstones relative to the top of the Precambrian basement. of clear regional stress control on dike orientations may reflect the development of a shallow magma Interpretation The structural position of the Lojamei area is chamber or chambers, or pipes that dominated the near the flexural margin of the Lokichar Basin. local stress conditions, plus stress rotations related The dikes in the South Lokichar Basin cover the The region lies south of the plunging nose of the to structure. full range of possible orientations from east-west Lokhone basement outcrop. To the east, the Pre- through to north-south, with the attendant implica- cambrian gneiss exposures are limited by the tion that the local principal stress directions have boundary fault of the Kerio Basin. But passing to Napadet-Kamutile-Kathigithigiria (NKK) Hills rotated spatially and/or with time. The maximum the south, this NNE-SSW–trending fault abruptly dike lengths are on the order of kilometers: 3.6 km turns to the southeast and dies out (Morley et al., The western side of the Napadet Hills cor- (ENE-WSW trend), 2.7 km (north-south trend), 1999a). However, some splays from the fault trend responds with the flexural margin of the late 4.4 km (NNW-SSE trend), 5.4 km (NNE-SSW trend), more north-south and reach the northern part of Miocene–Pliocene North Lokichar Basin, and field and 2.0 km (east-west) trend. The period from ca. the Lojamei area (Fig. 13) and die out, with some observations indicate that the volcanic layers dip 17 Ma to 10 Ma coincides with extension in the Lokichar Basin. The main faults in the basin trend Precambrian Miocene Grits Precambrian north-south to NNW-SSE, with some local NW-SE gneisses Grits volcanics gneisses and NE-SW trends (Morley et al., 1999a). The basin Grits appears to have behaved as a simple extensional Precambrian gneisses half graben (Morley et al., 1999a; Vetel, 2005). Faults in outcrop exhibit more variety of orientation than those mapped from seismic data (Figs. 2, 13), which may reflect a resolution issue, where smaller-displacement faults exhibit more variety of orientations than the faults apparent on seismic reflection data. This may in turn indicate that some low-displacement faults observed in outcrop responded to local strains and volume Networks of dikes in grits changes caused by igneous intrusion emplacement and deflation, rather than the regional stresses affecting the larger-displacement faults. Eight examples of normal faults intruded by dikes were observed during fieldwork (in 1988); they exhibit strikes between N160° and N030° and dips between 55° and 70°, dominantly to the west (Figs. 13, 14). However, the majority of the dikes in the Lojamei area are subvertical, very linear features inde- NE-SW–trending dike pendent of fault control. Bedding dips are also NW-SE–trending dikes (part of radial trend)

generally low (<15°; Fig. 13) and so do not exert a Figure 12. Aerial view toward the northeast of the Loriu area, with dikes of area B (Fig. 10) in the foreground. control on dike orientation. See Figure 9 for view direction. From foreground to the coast is ~5 km.

westward ~8°–10° as a result of rotation in the Lokhone Shales TVK 112 hanging wall of the Lokichar fault to the west. The eastern margin of the hill is the footwall of two east-dipping boundary faults, the Turkana fault and L Splay of Kerio Basin the Lothagam-Lokhone fault zone (Fig. 16). A few 10% K bounding fault dikes are visible on satellite images on the eastern J 10° margin of the Napadet Hills. In outcrop where a 115 km I 20° Fault Dike H orientations orientations large gorge along the Lokichar lugga (lugga is sea- G sonal watercourse) is present (Fig. 16, location Y), 15° 16° extensive deposits related to volcanic centers are exposed in cliffs. Dip amount and direction in the volcanic strata are highly variable, ranging from F subhorizontal to locally >70°. While there is structural tilting of the volcanic series related to normal faulting, the high dip values are related to depo- 02°15’N sition and slumping on unstable, steep volcanic slopes. Slumped units comprising mud flows and volcaniclastics with volcanic blocks and soft-sediment folds are mixed with agglomerates, lavas P Q R (basalt, trachyte, phonolite), lava bombs, agglom- Fig. 14 N O M erates, tuffs, lapilli tuffs, and intrusions. The strike 8° TVK 111 directions of subvertical intrusions in outcrop are predominantly north-south and NE-SW, with a few oriented east-west. While some dikes are composed of basalts and trachytes, a number of dikes are filled by pyroclastic rocks. One 2-m-wide E pyroclastic dike that strikes N045° and dips 64°N was observed to be intruded along a normal fault.

LOJ 20 On satellite images, these pyroclastic dikes form distinctive narrow (<5 m), linear, light gray to white features against the dark-colored volcanic LOJ 98-25 rocks. They are spaced tens to hundreds of meters Volcano 02°11’N Volcano apart. Intrusions in the Napadet Hills are generally more widely spaced and absent of the cross-cut- TVK 113 D N ting networks compared with the Kamutile Hills C 2 km (Fig. 16). No dike ages have been published, but 5° five K-Ar whole-rock ages from related lavas from B the NKK Hills have been published (Morley et al., TVK 115 A 2 km 1992; McDougall and Brown, 2009), and indicate LOJ 01 36°00’E that volcanism lasted from 15 to 13 Ma (Table 2). Dip, strike, The well-developed dike swarm of the Kamutile Strike of dip amount Dike Normal fault Volcanic cone sedimentary of Loperot Hills (Fig. 17) lies in the large-scale relay ramp bedding Sandstone between the Kerio Basin bounding fault to the Figure 13. Early–middle Miocene dike patterns in the Lojamei area, overlain on satellite image with southeast (Lothidok-Lokhone fault zone) and 80% opacity over a shaded-relief map. See Figure 2 for location. Rose diagram shows dike orientations the Turkana fault to the northwest (Fig. 16). The measured for 100 m long segments, for 115 km total length of dikes. Lower hemisphere stereonet for 14 fault orientations is also shown. “TVK” numbers are seismic line numbers (see Morley et al., 1999a). dikes exhibit the following patterns (Figs. 17, 18): A–R are field localities described in detail in the Supplemental Files (text footnote 1). (1) a strong overall north-south trend over a 13 km

distance, with the north-south trend as the long axis of a radial dike pattern; (2) a concentric dike pattern, particularly on the western side of the trend; (3) a WNW-ESE trend, particularly in the northern half of the dike field; and (4) a NNE-SSW trend in two en echelon zones. Notes from one field stop (from 1987) at the northern edge of the dikes (Fig. 17, location X) describe outcrops along a lugga which expose purple, white, and light green pyroclastic rocks (tuffs, lapilli tuff, welded tuffs) interbedded with 2° 13’ N basic and intermediate lava flows. The sequence locally dips north, but is also locally rotated at intrusions, including intrusions along faults. Dikes vary in composition from black basalts to gray to bluish-gray trachytes and pyroclastic dikes. The N dikes are commonly porphyritic and include various combinations of feldspar, biotite, amphibole, and clinopyroxene phenocrysts. Several cross-cut- 36° 00’ E 500 m ting generations of dikes are present. Dikes occur at a density of eight to 15 dikes per 100 m, range in thickness from 0.5 m to 4 m, and occupy ~20% of the section. Dikes vary widely in orientation,

7° but trends of N110°–N135° and N350°–N010° are 64° 66° 1 8° the most common. This field locality appears to TVK 111 60° 66° be broadly representative of the zone affected Z by dikes. At the center of the concentric and radial dike pattern is a region of white sediment and pyro- Y clastic rocks, ~2 km in diameter, that is punctuated by numerous volcanic cones (~200 m in diameter) 1 (Fig. 17). This region is a minor depression that has trapped sediment that overlies and masks the dikes. Possibly the depression represents minor subsidence associated with the concentric dikes, caused X by deflation of the underlying magma chamber. North of this area is the highest density of dikes in 1 the trend (Figs. 17, 18); dike orientations on satel-

500 m lite images are highly varied, but north-south and NW-SE trends dominate (Fig. 17). Dike Volcanic cone Fault Bedding in sandstones

Figure 14. Satellite image showing a typical area of well-developed early–middle Miocene dikes in the Lojamei area (see Fig. 13 for Southern Extension of the Turkana Fault and location). (A) Uninterpreted image. (B) Interpreted image. X—volcanic cone overlying a NW-SE–trending dike (1), which is probably the plumbing system for the cone; Y—eroded volcanic cone showing short radial dikes; Z—area of Oligocene–early Miocene clastics the Lokhone Basement High (Footwall Uplift) (north-south–trending bedding). Blue line labeled “TVK 111” is a seismic line (see Morley et al., 1999a). The area of dikes in Figure 17 is a relatively low-relief region, in contrast to higher-relief areas

Lokhone Shale Lokhone Sandstone Precambrian Gneisses localized nature of the Miocene intrusive activity.

012°, 14°W Figure 19B shows a rare example for the Lokhone high, where an almost completely eroded Miocene basalt outlier on basement reveals a central pipe 004°, 10°W associated with short (<200 m long) NW-SE– and C NNE-SSW–trending dikes. The main indications of any basement influence on intrusions are shown in Figure 19C, where a few short dikes (<200 m) strike NNE-SSW, parallel to basement foliation. Two longer (>1 km) east-west–trending dikes cross- cut foliation. The southern east-west dike does not B appear to be associated with any offset or deflec- 20° A 25° tions in foliation across the dike, but the northern 25° dike does show such features (Fig. 19D), indicating that it is following a preexisting fault. TVK-12 200°, 78°E N Muranachok-Muruangapoi Area

In the Muranachok-Muruangapoi area, there are 0 500 m 1000 m three main mappable units: Precambrian gneisses, Base Lokhone Sandstone/unconformity Basalt dike Strike of foliation Normal fault Top Lokhone Sandstone with Precambrian basement the Muranachok Sandstones (of probably Late Cre- taceous or Late Cretaceous–Paleogene age), and Figure 15. Satellite image of the western Lokhone high area, showing predominantly east-west– and NNW-SSE–trending Miocene Oligo-Miocene volcanic rocks (Fig. 5). The Mura- dikes affecting the flexural margin of the Lokichar Basin. See Figure 2 for location. Field descriptions of localities A–C are given in the Supplemental Files (text footnote 1). Line labeled “TVK-12” is a seismic line (see Morley et al., 1999a). nachok Sandstones have yet to be reliably dated. However, their possible equivalents to the north in the Lapur Range are now interpreted to represent to the west and the south that exhibit many small topographic profiles across the central part of the episodic deposition from the Late Cretaceous to (typically 1–2 km diameter) volcanic cones. The cen- Kathigithigiria Hills showing the nature of the vol- the Eocene, because in the lower part they contain tral part of the Kamutile and Kathigithigiria Hills canic edifice that lies as much as ~400 m above the dinosaur bones, while the upper part of the section is marked by a linear NNW-SSE–trending valley unconformity level with basement. The presence displays a transitional relationship with the Turkana that strikes northward into the major normal fault of dikes in the synthetic transfer zone between the Volcanics whose base is radiometrically dated at (Turkana fault) on the eastern side of the Napa- Turkana fault and the Lothidok-Lokhone fault zone, ca. 37–38 Ma (O’Connor et al., 2011; Tiercelin et al., det Hills (Fig. 16). This valley is interpreted here and the volcanic high centered along strike of the 2012a). Detrital zircons confirm a Paleogene age for as representing the extension of the fault to the Turkana fault, strongly suggest that syn-rift fault-re- the upper part of the section (Owusu Agyemang et south; a similar interpretation is made by McDou- lated pathways were important for the migration al., 2019). The Turkana Volcanics have previously gall and Brown (2009). The fault is not visible within of magma in the NKK Hills area. been dated by Zanettin et al. (1983) and McDougall the Precambrian basement exposures of the Lok- The NKK Hills north-south dike trend (Figs. 16, and Brown (2009) as part of a belt that ranges in age hone footwall uplift to the south, suggesting that 17) and the NNE-SSE–striking Turkana fault (Fig. 16) between ca. 36 and 16.3 Ma (Table 2). Unfortunately, it dies out below the volcanic sequence (Fig. 19). converge toward the south and end at a large it is not possible to be certain of the age of the dikes The volcanic rocks of the NKK hills unconformably eroded volcanic cone ~2 km in diameter with a in the Muranachok-Muruangapoi area within this overlie the Precambrian basement on a sub-hor- present-day relief above the basement-volcanic broad age range. But given that the closest ages izontal surface that lies at ~580–600 m elevation unconformity of >250 m (location U, Fig. 19C). to the study area are early Miocene, this seems to (Fig. 19C). This relationship indicates that the uplift South of the volcano, there is no indication of be the most likely age for the dikes. that exposed the gneisses in the footwall of the related intrusions in basement exposures. Despite Within the gneisses, dikes are spaced variably, Kerio Basin bounding fault was largely established the close proximity to the Kathigithigiria Hills, intru- between ~1.2 km and 150 m apart, and trend pre- by the middle Miocene. Figures 16B and 16C are sions in the basement are few, demonstrating the dominantly NNW-SSE (Fig. 20). Locally, at eroded

volcanic centers at the north end of the basement A TF 36°00’ E outcrops, local radial dike patterns are developed. LoF In the belt of volcanic rocks to the west, NNW-SSE– 3°00’ N trending faults are present on satellite images. The Lothagam eastern margin of the gneisses is interpreted to Hill Turkana be bounded by the northern continuation of the Basin Lokichar fault (Figs. 2, 20). Overall NNW-SSE– trending faults and dikes are superimposed on North Napadet NNE-SSW–trending fabrics in basement (Fig. 2). Lokichar Hills These igneous features and normal faults are inter- Basin preted as being indicative of a NNW-SSE–trending Y maximum horizontal principal stress (S ) direc- Pyroclastic dike Hmax N tion, probably during the early Miocene. Fault continues North of the dikes, Muranachok Sandstones, along topographic and the Precambrian gneisses are outcrops of Oli- low in the NKK Hills go-Miocene volcanic rocks, which in places have low-relief, well-developed circular to oval patterns 200 m (Fig. 20). These volcanic rocks are a mixture of B lavas, pyroclastic rocks, volcaniclastic rocks, and Kamutile Lokichar Lugga minor intrusions (sills, small dikes, and some vent Hills agglomerates). Bedding dips gently (5°–10°) away from the center of the circular patterns, indicating an eroded dome pattern. Some of the features are perfectly circular, while others are asymmetric Kathigithigiria (Figs. 20, 21) and both are ~1–4 km in diameter. Hills The patterns suggest eroded volcanoes, yet on satellite images there are no clear intrusive complexes C (dikes, pipes) in the centers of many of the features Lokichar (Fig. 21). Hence, at least some of the domes are N Basin forced folds associated with underlying sill com- LokF plexes or laccoliths (e.g., Le Bas, 1971; Pollard and 10 km Johnson, 1973; Acocella et al., 2001; Magee et al., 2013, 2017; van Wyk de Vries et al., 2014). Kerio Basin 2°30’ N Lokhone footwall Edge of Muranachok-Lapur Range and the Kataboi Precambrian uplift E basement Fault Zone

B TF C The Muranachok-Muruangapoi area displays a 750 m W TF E W E 1000 m very straightforward pattern of dikes and faults that 500 m

Elevation (m) Elevation 750 m indicate a NNW-SSE SHmax direction. The complexity 0 20 km Region of dike (m) Elevation in this area lies in the presence of very different intrusion 500 m 0 29 km trends of faults and dikes in the adjacent area to the northeast (Fig. 2). Vetel (2005) identified a corridor Figure 16. (A) Shaded-relief map (Aster Global Digital Elevation Map) of the Napadet-Kamutile- Kathigithigiria of NE-SW–trending faults that runs from the south- (NKK) Hills region, showing the location of the middle Miocene dike swarm and its relationship to major Cenozoic rift faults. (B, C) Topographic profiles along the sections marked in A. See Figure 2 for location. ern Lapur Range to the northern Lothidok Basin, Locality Y—Lokichar Lugga; TF—Turkana fault; LoF—Lothagam fault; LokF—Lokhone fault. which he called the Kataboi fault zone. According to

Vetel (2005), the Kataboi fault zone is ~20 km wide N = 206 km 35° 58’ E and comprises a network of ~N50° faults. Synkine- matic volcanic rocks related to small basins with X the fault zone have been radiometrically dated to 20% 10 10 20% between ca. 28 Ma and 25 Ma (Ragon et al., 2018). For the Kataboi fault zone region, Ragon et al. (2018) interpreted three stages of development. The first extensional pulse is marked by 28–25 Ma; they attributed the non-optimal structural orientations to be due to reactivation of preexisting structures. Fig. 18 High intensity of intrusions A period of relative tectonic quiescence is marked 2° 45’ N by 25–14 Ma, accompanied by development of a Central region of significant weathering profile. At 14 Ma, there was white lacustrine the development of a large north-south–oriented sediment and half graben called the North Lokichar Basin accom- small volcanic cones panied by north-south–trending dikes (Boone et al., 2018a). This represents the main phase of rifting, with the basin undergoing changes in morphology between 4.2 Ma and 0.7 Ma (Boone et al., 2018a; Ragon et al., 2018). Further north, ~2 km south of Lokitaung Gorge, there is another NE-SW fault trend (Fig. 2). This trend is accompanied by NE-SW– and NNE-SSW–trending dikes and a later set of cross-cutting north-south dikes, all lying north of the fault trend. Tiercelin et al. (2012a) dated several dikes in this area to between 29 and 27 Ma (Table 2).

■■ DISCUSSION

Origin and Types of Dike Geometries in the Turkana Area

In this study, the areas of well-developed dikes coincide with the tips of major faults or transfer zones between faults (Figs. 2, 13, 16, 19). In the Muranachok-Muruangapoi and Napadet Hills areas, the NNW-SSE– to north-south–trending dikes and 2° 41’ N N faults ignore the NNE-SSW– to NE-SW–trending basement fabric; this suggests that the trends fol-

lowed the regional SHmax direction at their time of 2 km formation. In the Lokhone high area, the intensity of intrusions is low, but most of the intrusions that are present ignore any basement fabrics (Figs. 15, 19). However, in Figure 19, a few examples are shown Figure 17. Dike pattern in the Kamutile Hills area (see Fig. 2 for location). The dike pattern is overlain on a light- ened satellite image of the area. The rose diagram shows the orientations of 100 m long dike segments, with where short dikes follow the NNE-SSW trend of the total length of dikes measured (N) = 206 km. basement foliations, and one east-west dike follows

N = 22.6 km Radial and circumferential patterns tend to arise as a consequence of the interplay between stresses arising from overpressure in the magma N 20% 10 10 20% reservoir, magma chamber shape (Gudmundsson, 2006; Tibaldi, 2015), surface loading by volcanic 0 400 m edifices (Pinel and Jaupart, 2003), and regional stresses (e.g., Nelson et al., 1992; Pollard and Aydin, 1984; Olson and Pollard, 1991; Tentler, 2003; Muir-

35° 58’ E 35° 59’ E head et al., 2015), as well as by magma chamber inflation and collapse (Harker, 1904; Bailey et al., 1924; Anderson, 1942; Phillips, 1974; Bistacchi et al., 2012). For an individual volcanic center, the transition from numerous small volcanic cones to large volcanic edifices with relief >1 km affects dike development due to the changing topographic 2° 45’ N load with time (Pinel and Jaupart, 2003). Hence the roles played by magma reservoir size, geometry, and overpressure and by topographic load in influencing dike patterns evolves with time (Pinel and Jaupart, 2003). The more regional effects of topographic loads, including rift flank loads, on intrusion distribution are described by Maccaferri et al. (2014). The volcanic fields in the NKK Hills, Lojamei, and Loriu comprise numerous small 2° 46’ N cones, most commonly <1 km diameter, and even at the center of the radial dike complexes, those in the concentrated area of small cones are <2 km diameter. The highest demonstrable edifice is the Kathigithigiria Hills, where the extrusive activity of

Figure 18. Detail of the most densely intruded region of the Kamutile Hills area. The rose small volcanic cones has built a high as much as diagram for the local area shows little difference from the dike distributions for the whole ~400 m thick above basement. But even this relief area (Fig. 17). The rose diagram shows the orientations of 100 m long dike segments, with falls short of the 1 km relief considered as exert- the total length of dikes measured (N) = 22.6 km. See Figure 17 for location. ing a significant load by Pinel and Jaupart (2003). The footwall of the Lokichar fault, which today is of very low relief, shows low amounts of exhumation an older fault (Fig. 19D). In general, the dikes in [footnote 1]; Wilkinson, 1988). These represent that occurred over a long time period (cooling of the study area appear to be more influenced by the simplest arrangement expected for syn-rift ~100 °C gradually over a 40 m.y. time period; Boone local and regional stress directions than by base- faults and igneous features. In two areas, the NKK et al., 2018b, 2019), suggesting that topographic ment fabric. Hills and Loriu, well-developed dike swarms are relief was never very high (i.e., <1 km), an inference In the Muranachok-Muruangapoi area, NNW- elongate in a north-south direction that is parallel supported by the relatively limited development of

SSE–trending dike swarms lie subparallel to to the expected regional SHmax direction. In both boundary-fault alluvial fan deposits adjacent to the extensional faults and the alignment of eroded of these examples, the dike swarms are modified Lokichar fault in the Lokichar Basin (e.g., Morley circular to oval volcanic features (eroded volca- from simple subparallel orientations. In the Loriu et al., 1999a; McClymont, 2018). Consequently, for noes and/or forced folds above sills and laccoliths). area, a well-developed radial pattern about a volca- the stages outlined above, the dikes in the Kathig- This is also the case for north-south– to NNW- nic center is present, while in the NKK Hills region, ithigiria Hills and Loriu are interpreted to have SSW–trending early–middle Miocene dikes in the concentric and weak radial patterns are developed, developed in an area of relatively low topographic Moiti-Jarigole area (Fig. 2; Supplemental Files particularly on the western side of the swarm. relief, where the effects of volcanic edifice building

Figure 19. Satellite images of the northern Lokhone high (footwall uplift) area. (A) Regional map showing the trend of dikes and faults in the Napadet-Kamutile-Kathigithigiria Hills area, Strike of and how they do not continue into the Precambrian basement Turkana gneissic foliation exposures (see Fig. 2 for location). (B) Detailed image of Miocene fault volcanic outliers (dark brown circular features) over NNE–trend- Boundary ing Precambrian gneisses; some very minor Miocene intrusions fault to are present (red lines are Miocene dikes). See C for location. Kerio (C) Northern Lokhone high area, showing the presence of a Pipe basin few scattered Miocene dikes. Location U—large eroded volcano. (D) Detail of dike intruded along a fault zone, as indicated by offset and bending of foliations. See C for location.

2° 37’N

and rift flank topography were limited. In turn, this N suggests that the circumferential and radial dikes N U N were influenced by regional stresses and magma reservoir size, geometry, and overpressure, rather C Lokhone high than topographic load. 5 km 36° 00’E 100 m In the case of the Loriu-area high-density dike zone (area B in Fig. 10), there is a pronounced Miocene volcanics 36° 00’E Dike NW-SE trend, which is oblique to other dikes and structural trends in the area, including NNE-SSW– Unconformity between trending Precambrian basement fabrics (Fig. 2). Miocene volcanics U and Precambrian The short length (average 0.3 km) and narrow width gneisses (<1 m) of the dikes suggest that they are sourced 100 m from a shallow magma reservoir. In area B in the 2° 32’N Loriu area, dike spacing is 21.5 dikes per kilometer, and dike length is between 25 and 660 m. Dike thickness is generally <1 m, but even using 1 m dike thickness, extension is low, <2%, or a total of 64.5 m extension over the 3-km-wide area in D which they are present. One interpretation that Precambrian N gneiss accounts for the shallow depth source for the dikes, the trend of the dikes, and the anomalous WNW- ESE strike and NNE dip to the boundary between 1 km B the sandstones and the Precambrian basement, is a sill or laccolith-like body within the area of Precambrian gneisses outcrop that passes to the northeast to a region where the laccolith is less Fault tip zone inflated (or has been deflated) in the vicinity of the NW-SE–trending dikes (Fig. 11). Some of the NE-SW–trending faults related to the radial fault O set of Bending of trend are superimposed on the NW-SE trend. The foliation into N dark marker proposed formation of dikes of variable orienta- zone fault 200 m tion above a sill or laccolith is analogous to the Trend of foliation cracked-lid model, where dike networks are fed by Dike along fault zone Fault zone without dike in Precambrian gneiss a sill and form above the sill, and the dike patterns

are related to sill emplacement processes, not far- 35°20’ N 35°30’ N field stresses (e.g., Muirhead et al., 2014; Coetzee and Kisters, 2017). The South Lokichar Basin exhibits networks of highly variably oriented dikes, which could fit a Circular- oval features cracked-lid model (Muirhead et al., 2014; Coetzee in volcanic rocks and Kisters, 2017). However, the dike network was built up over an ~6 m.y. time span, and the large aspect ratios (3–5 km long, 5–10 m wide) of some dikes suggest a relatively deep magma source. Hence the cracked-lid model seems less appli- 3°30’ N cable for the Lojamei area than the Loriu. Many Figure 20. Interpreted satellite im- more dikes need to be dated to determine whether age of the Muranachok area. See Figure 2 for location. there are systematic changes in dike orientation with time, or whether multiple trends are associated with each period. Of the four study areas, Local more radial Lokichar dike pattern around Fault Lojamei exhibits the widest dikes (~10 m). Average small volcanic centre dike spacing in the areas most heavily intruded

NNW-SSE trending is about eight dikes per kilometer, which would dikes Boundary to indicate a maximum of 80 m extension per kilo- Precambrian gneisses meter if all dikes were 10 m thick (~9% extension). Normal fault Boundary to Muranachok Thicknesses cannot be measured accurately from Grits Igneous dike N satellite images, but the proportion of narrow dikes (<2 m) to wide dikes (5–10 m) is ~8:1, suggesting Arcuate feature in volcanic layering 5 km 3°20’ N that 2 m is a more appropriate average than 10 m, which gives 16 m/km extension (~1.6% extension).

o 3 47’ 05” N 3 o 35’ 34” N o 35 29’ 50” E 35 o 25’ 35”E

500 m

500 m N

Figure 21. Satellite images of the Muranachok area (area of Fig. 20), showing examples of oval to circular features in the Eocene–Miocene volcanic sequences.

Comparison of Dike Development in Turkana even at an early stage in the rift history (Roecker earthquakes are present down to the base of the versus Southern Kenya and Ethiopia et al., 2017; Plasman et al., 2017; Tiberi et al., 2019). crust (~42 km depth in southern Lake Tanganyika), For the Magadi area of the southern Kenya Rift, rapid stressing of faults by active magmatic intru- The southern Kenya and northern Tanzania rift Muirhead et al. (2016) estimated that the border sions or related volatile-rich fluids may cause brittle system represents a young (<7 Ma), well docu- faults accommodated regional extension at rates deformation in crust that is ductile at lower strain mented, low-extension active rift system, and it is between 1.23 and 1.78 mm/yr for the first 3 m.y. By rates (Fagereng, 2013; Hodgson et al., 2017; Lava- an example where mantle processes control the 7 m.y., the smaller intra-rift faults had taken over yssière et al., 2019). extension of thick, initially cold lithosphere (e.g., much of the extension (1.34–1.60 mm/yr), along The Main Ethiopian Rift is a more mature Muirhead et al., 2016; Roecker et al., 2017; Plasman with magmatic volatile release occurring in the magma-dominated extensional system than the et al., 2017; Tiberi et al., 2019). The region exhibits rift center and intrusion of dike swarms at depth southern Kenya Rift, where 80% of the exten- continental crust with an upper-crustal volcanic (Muirhead et al., 2016). This evolution demon- sion across the Main Ethiopian Rift is confined to plumbing system that is interpreted to be domi- strates the importance of flow of magmatic fluids ~20-km-wide magmatic segments dominated by

nated by a volatile fluid rich in CO2 (5–15 km depth) within fault systems, and how they weaken the dike injection and related faulting (Hayward and accumulated at the top of crystal-rich magma mush– lithosphere and focus upper-crustal strain early in Ebinger, 1996; Ebinger and Casey, 2001; Keir et al., filled chambers, while a network of molten rock lies the rift history, even prior to magmatic segments 2006). Swarms of low-magnitude earthquakes are in the lower crust and upper mantle (Fig. 22; e.g., being established (Muirhead et al., 2016). A link concentrated at 8–14 km depth, and coincide with Roecker et al., 2017; Ebinger et al., 2017). Mid-crust- between overpressures related to magma (or mag- the top of a 20–30-km-wide zone of mafic intrusions al-level and lower-crustal magma storage areas ma-derived fluids) and modern faulting activity is (Keir et al., 2006). In the brittle seismogenic layer, are interpreted to be dominant, with major dike also suggested by recent GPS (Jones et al., 2019) both faulting and dike intrusions are ongoing, with systems being mostly fed from lower-crustal res- and earthquake (Baer et al., 2008; Oliva et al., 2019) dikes aligned perpendicular to the minimum stress ervoirs and accommodating significant extension studies. In particular, where boundary fault–related (Keir et al., 2006). In the Main Ethiopian Rift, diking, not extension on bounding faults, is the dominant mechanism for extension, which occurred at a rate Border fault Axial zone of 4.0 ± 0.9 mm/yr between 1992 and 2003 CE (Ben- 0 dick et al., 2006). Within the dike swarms, individual Upper Crust dike widths are typically ~0.5–1.5 m, but dike activ-

10 CO2 –rich volatiles ity tends to be cyclic, and the first dikes in a cycle + magma tend to be wider (~5–6 m; Wright et al., 2006). 20 Crystalline mush The rift system in Turkana contains elements Depth (km) Lower Crust of active rift development as determined for the 30 X southern Kenya and Main Ethiopian Rifts outlined High-velocity above, but also diverges in important ways. The lower crust 40 tectonic evolution of the Turkana–southern Ethio- Mantle magmatic pian Rift from the onset of magmatism is shown in Metasomatism of mantle and underplating Predominantly volcanic rocks, Figure 23. The first phase of magmatism (40–23 Ma) volcaniclastic rocks, and igneous- lower crust was focused on southern Ethiopia and northwest- rock–sourced sedimentary rocks Magma bodies Seismicity ern Turkana (Figs. 3, 23D) and, as discussed in the Figure 22. Schematic cross-section illustrating the early stage of continental extension in an active rift (e.g., southern Geological Background section, is plume related Kenya Rift). Uplift of the asthenospheric mantle (due to development of a thermal anomaly and/or extensional thin- and affected by minor crustal extension (Hendrie et ning of the lithosphere) has progressed to a stage where there is a large supply of magma to the central part of the rift, and so extension by dike intrusion is in the process of replacing shear failure along large faults as the dominant al., 1994). This early plume-related magmatism did mode of extension. X—deep seismicity associated with brittle displacement along boundary faults (e.g., Ebinger et not, however, result in Turkana progressing rapidly al., 2017). Upper crust shows faulting, infiltration of CO -rich fluids from the lower crust, small magma chambers, and 2 to a magma-dominated system like the southern numerous small intrusions; lower crust demonstrates some faulting, large crystalline mush–filled chambers, extension mostly accommodated on axial dike complexes, and magmatic underplating in basal part of crust. The classic upper Kenya Rift. The area covered by late Paleogene (brittle)–lower (ductile) crust boundary is blurred by the occurrence of deep earthquakes (e.g., those indicated by X) in volcanic rocks in northern Kenya is substantial old Precambrian crust that was cold at the start of rifting. Deep earthquakes may be related to lower-crust composition (~42,000 km2; Morley, 1994). However, to the south and/or overpressuring by fluids related to magmatism. Conversely, heat flow in the crust is elevated by crustal thinning, igneous intrusions, and convecting hydrothermal fluids. Based on data from Ebinger et al. (2017), Plasman et al. (2017), and southeast, there is a large region of late Paleo- Roecker et al. (2017), and Tiberi et al. (2019). gene–early Miocene extensional basins filled by

6-0 Ma 15-6 Ma 6º N 6º N ~12 Ma Figure 23. Tectonic development of the Tur- kana–southern Ethiopian Rift area during the late Eocene–recent. Compiled from in- ~8 Ma formation in Morley et al. (1999a), Ebinger et al. (2000), Vetel (2005), Vetel and Le Gall (2006), Balestrieri et al. (2016), Emishaw et al. (2017), McDougall and Brown (2009), 5º N 5º N Brown and Jicha (2016), and Rooney (2017). OB The Gatome Basin (panel D) is an older ba- CB sin of probable Late Cretaceous–Paleogene age that underlies the late Eocene–Oligo- cene volcanic rocks. CB—Chew Bahir Basin; DV TB J—Jibisa; LB—Lokichar Basin; OB—Omo Ba- sin; TB—Turkana Basin; TuB—Turkwell Basin. 4º N 4º N 6 KSFB—Kino Sogo fault belt; SV—Suguta KSFB Valley. Pliocene–Holocene volcanic centers (panel A) mapped from satellite images 6 in this study: AV—Asie volcanic centers; DV—Dukana volcanic centers; HV—Hurri HV Hills volcanic centers; KV—Kulal volcanic 3º N 3º N hills; MV—Marsabit volcanic centers. Exten- AV sion direction estimates: 1—Kataboi fault 5 zone area, activity ca. 28–25 Ma (Ragon et KV al., 2018). 2—Lojamei area; inferred from 6 dip-slip normal faults to be approximately MV 2 east-west extension; dike orientations are variable and suggest local perturbation of 2º N 2º N the stress field by magma chambers ca. SV 17–15 Ma, and more north-south–oriented dikes and approximately east-west extension direction during the 12–10 Ma period 36ºE 37º E 50 km 38º E 36ºE 37º E 38º E of dike emplacement (panels B and C; see the South Lokichar Basin–Lojamei Area 23-15 Ma 40-23 Ma section in text). 3—Loriu area; from dike 6º N 6º N orientations suggesting east-west extension direction (minimum horizontal stress), with strong local stress field perturbation by magma chambers (panel C; see the Loriu Area section in text). 4—Muranachok- Muru- angapoi; from fault and dike trends (panel C; see Muranachok-Muruangapoi Area section 5º N 5º N in text). 5—Napadet-Kamutile-Kathigithi- giria Hills; from north-south dike trends, plus local stress field perturbation by magma chambers (panel B; see Napa-

Gatome Basin det-Kamutile-Kathigithigiria Hills section J in text). 6—Minimum horizontal stress 1 direction orthogonal to strong NE-SW to 4º N 4º N NNE-SSW trend of small volcanic cones and craters (see Discussion section) of AV, 4 DV, HV, and MV for the Pleistocene (panel A; Strecker and Bosworth, 1991). Recent ex- 1,2 tensional activity (red arrows) as indicated by seismicity and deformed Holocene lake shorelines appears to be focused along the 3º N 3º N trend of Suguta Valley and Lake Turkana 3 Anza Graben (panel A; Pointing et al., 1985; Melnick et al., 2012). Details of Cenozoic activity within the Anza Graben are not shown, but in general, 2 LB Paleogene activity (panel D) is more import- TuB ant in the central and southeastern part of the graben, while Neogene activity (panels 2º N 2º N B and C) was significant in the northwestern part (Morley et al., 1999b). When passively subsiding, it was still an important depres- 36ºE 37º E 38º E 36ºE 37º E 38º E sion and exerted an influence on drainage; during sea-level highstands, it acted as a Probable extent of volcanic Alluvium or older (than time of map) Cenozoic Plio-Pleistocene marine gulf, apparently enabling an un- Sedimentary basin volcanic or sedimentary rocks volcanic centers rocks for the time interval fortunate whale to be preserved amongst covering basement Sedimentary basin freshwater fossils in the Loperot area of Approximate outcropping area inter ngering with Precambrian Predominantly volcanic basin ll the Lokichar Basin, Turkana (Wichura et of volcanic rocks for the volcanic rocks basement mapped time interval al., 2015). Active normal Modern extension Large volcanoes Probably active Inferred extension direction faults faults

continental, basement-derived sediments, where than one basin lateral to the Lokichar Basin (e.g., with similar length:thickness ratios being observed, igneous activity was absent until the early Miocene Turkwell and/or Kerio Basins) was also active (but but where the longest dikes in Turkana are about an (Fig. 3; Morley et al., 1999a). The Lokichar Basin with lower heaves), possibly rates could have been order of magnitude shorter than the longest dikes and possibly the Turkana Basin (Fig. 2) represent ~1 mm/yr. Such rates are in line with modern geo- in Ethiopia. the northern limit of these syn-rift basins, which detic rates across the whole East African Rift, which The 6 Ma–recent period in Turkana is marked by extend 300 km south to the Tugen Hills–Elgayo indicate predominantly east-west extension with inner rift–style faulting, which trends NNE-SSW from escarpment area (Kamego Formation; see review an angular velocity of 1.99 mm/yr (Stamps et al., Suguta Valley in the south, through southern Lake in Muia [2015]). Sanidine grains from a phonolite 2018) and are of similar magnitude, but lower than, Turkana, to the Kino Sogo fault belt–Chew Bahir Rift interfingering with the top of the Kamego Forma- estimates of border-fault displacement from the in northern Kenya and southern Ethiopia (Fig. 23A). tion in the Tugen Hills yielded 39Ar/40Ar ages (19.9 ± southern Kenya Rift (Muirhead et al., 2016). How- The volcanism is narrowly focused in Suguta Valley 0.1 Ma, 19.8 ± 0.1 Ma), that place the end of domi- ever, unlike in the southern Kenya Rift, there is no and expands toward the north to cover a large area nantly basement-derived syn-rift sedimentation as evidence for an inner fault zone or a segmented from Lake Turkana in the west to the Hurri Hills and <20 Ma (Muia, 2015). The 300-km-long (north-south) magmatic axial belt developing at this time, and Mount Marsabit area in the east (Fig. 23A). This vol- belt of rift basins, with minor to no volcanic input magmatism played a very minor role at least in canism is part of the regional trend in the Kenya Rift until the early Miocene, indicates that rift tectonics the upper crust in southern Turkana until the early where the erupted volume increases from a range of progressed differently over an ~25–30 m.y. period, Miocene (23–15 Ma; Fig. 23C). 2800–1000 km3/m.y. during the Miocene to 7300 km3/ compared with the last 7 m.y. for the southern The areas of well-developed dikes described in m.y. during the Pliocene–Holocene (Guth, 2015). In Kenya Rift (as summarized in Fig. 22). The onset this study are of early–middle Miocene age, and are the Hurri Hills and Mount Marsabit areas, the trends of volcanism at ca. 18–20 Ma (Tatsumi and Kimura, representative of the first time during the syn-rift of the volcanic edifices and their volcanic cones and

1991; Morley et al., 1999a; Muia, 2015) above basins stage when significant quantities of magma encoun- craters indicate NNE-SSW– to NE-SW–trending SHmax filled by basement-derived sediments is a signifi- tered well-developed systems of extensional faults directions or NW-SE to WNW-ESE extension direc- cant feature of the 300-km-long belt. On the eastern in the upper crust. In the Moruerith and Lothidok tions during the Pleistocene (Fig. 23; Strecker and side of Lake Turkana are notable early–middle Mio- Hills areas (Fig. 5), there is overlap between the Bosworth, 1991). However, present-day extension in cene volcanic rocks around Moiti-Jarigole and the regions with late Eocene–Oligocene volcanism and the region is WNW-ESE to east-west (Stamps et al., Jibsia ring complex, which include basic lavas those with Miocene volcanism (Fig. 23B), but over 2018), with Holocene and modern extension rates and syenite intrusions (e.g., Wilkinson, 1988; Key, much of the area, the first entry of volcanism into across the southern Lake Turkana–Suguta Valley 1989; Furman et al., 2006). The early extension in the rift occurred ca. 18–20 Ma and volcanism con- area estimated in the range of 3.2–6.7 mm/yr (Mel- Turkana overlaps with minor periods of late Paleo- tinued until ca. 10 Ma (Figs. 3, 23). More extensive nick et al., 2012) and 4.3–5.1 mm/yr (Stamps et al., gene extension in the Sudan and Anza rifts to the dating of dikes is required to determine in detail 2008; Knappe et al., 2020). Mantle xenoliths from the northwest and east respectively (e.g., Schull, 1988; what patterns in magmatic behavior may exist in the Mount Marsabit volcanics record different stages Morley et al., 1999b; Mohamed et al., 2016; Yas- region. The short lateral extent of the area of intense in mantle evolution, with evidence for non-rifted sin et al., 2017). The distribution of these basins dike intrusions (<15 km), the minor, highly local con- sub-continental lithospheric mantle, a cooling and over an area ~1800 km in a NW-SE direction and tribution to upper-crustal extension (<2%), and the decompression stage related to Anza Graben rifting, >400 km wide, and oblique to the more north-south overall area affected by volcanism (~13,150 km2) and a subsequent stage of magmatism and metaso- trend of the Eocene–Oligocene volcanic rocks in indicate that igneous activity was relatively localized matism related to Quaternary volcanism, including

southern Ethiopia and northern Turkana (Fig. 1B), and of low volume. The dike systems described for evidence for infiltration of 2H O- and CO2-rich silicic suggest that far-field stresses, not just those arising Turkana are noticeably shorter, thinner and more melts (Kaeser et al., 2006). from active rifting, could have played a role at least closely spaced and occupy a smaller area than those The development of a Plio-Pleistocene igneous during the early stages of rift formation in central described for the Oligocene of the Main Ethiopian trend east of the region of maximum crustal thinning Kenya and Turkana. Rift (Fig. 24). The presence of pyroclastic dikes in (Fig. 4) suggests that by the Pliocene, mantle pro- Extension and deposition in the predominantly the NKK Hills indicates a close association of the cesses, akin to those affecting the southern Kenya Oligocene–middle Miocene–age Lokichar Basin dikes with the shallow plumbing systems of vol- Rift (see Fig. 22), had come to dominate extension (Morley et al., 1999a) is dominated by the east-dip- canoes (e.g., Torres-Hernández et al., 2006). The in Turkana. Although some short-lived extensional ping boundary fault, which exhibits as much as graph plots of length-thickness variations exhib- trends have affected the area east of Lake Turkana 14 km of heave (Morley et al., 1999a), indicating ited by the Turkana dikes match with results of a in the Plio-Pleistocene (Kino Sogo fault belt, Ririba time-averaged extension rates of ~0.56 mm/yr for previous comparison of dike lengths that included Rift; Fig. 23), the area of consistent extension during the 25 m.y. duration of rifting. If at times more examples from Ethiopia (Fig. 25; Cruden et al., 2018), this time has been Lake Turkana (Morley et al., 1999a;

S. Lapur N 10s m-2-3 km long 4° N spacing 370 m Kataboi Location at fault zone tip of Lokichar 13° N fault Lake Turkana

Muranachok- Muruangapoi 2 to 55 km long spacing – 4.5 km 2 to 28 km long Kajong Fig. 20 spacing – 4.1 km Turkana Kamutile Hills Napadet Basin 10s m-2-3 km long spacing 18-100 m Hills 3° N Lokichar fault Kathigithigiria Lake Hills Tana North 12° N Lokichar Loriu 10s m-1.5 km long Basin Fig. 16 Fig. 9 Fig. 10 spacing 50 m Location in Kerio major relay Fig. 15 Basin zone between two synthetic The Barrier boundary faults Fig. 13 Location at N Lokichar fault tip. 3 to 20 km long Volcanic center Basin Lojamei Lokichar fault 2° N also dies out average spacing – 1.6 km 10s m-5 km long 37° E to south 36° E spacing 130 m Suguta Valley

Dike swarms discussed in Oval/circular features in volcanics; 50 km Normal fault this study eroded volcanoes and/or forced folds

Figure 24. Comparison of the size of dikes from Turkana (this study) (A) and those associated with Oligocene flood basalts in the northern Ethiopian Rift (redrawn from Rooney et al., 2018) (B). Dike colors in B are: red—ENE-WSW trend; green—NW-SE trend; black—radial and concentric dikes around volcanic center.

100 Shiprock (New Mexico, USA) dikes Ethiopia dikes Sudan dikes Longest dikes in Lojamei area (Turkana) 10 Field occupied by most dikes in Turkana Figure 25. Log thickness versus log length for igneous dikes, from Cruden et al. (2018) with Turkana data added. Thickness (m) 1

0.1 0.1 1 10 100 1000 10,000 100,000 Length (m)

Melnick et al., 2012; Corti et al., 2019; Knappe et al., of rifting contrasts both with the development of Balestrieri, M.L., Bonini, M., Corti, G., Sani, F., and Phillipon, M., 2016, A refinement of the chronology of rift-related faulting 2020). It would appear that the attempts to propa- other parts of the Eastern Rift Branch and with in the Broadly Rifted Zone, southern Ethiopia, through apa- gate across the Anza Graben from the south (Suguta the Pliocene–Holocene style of rifting around and tite fission-track analysis: Tectonophysics, v. 671, p. 42–55, Valley–Kino Sogo fault belt) and from the north (Main east of Lake Turkana, where magmatism and dikes https://doi.org/10.1016/j.tecto.2016.01.012. Ethiopian Rift–Ririba Rift) failed, and extension was have become more important for accommodating Bastow, I.D., Pilidou, S., Kendall, J.-M., and Stuart, G.W., 2010, Melt-induced seismic anisotropy and magma assisted rift- more easily focused west of the Anza Graben along extension, and smaller fault swarms have tended ing in Ethiopia: Evidence from surface waves: Geochemistry Lake Turkana. This still may have involved reactiva- to develop (although some larger boundary faults Geophysics Geosystems, v. 9, Q0AB05, https://doi.org/10 tion of an older Cretaceous rift basin (Ebinger and in Lake Turkana continued to be active). .1029/2010GC003036. Belachew, M., Ebinger, C., and Coté, D., 2013, Source mecha- Ibrahim, 1994), but this basin is probably north-south nisms of dike-induced earthquakes in the Dabbahu-Manda trending to match the crustal thinning trend, unlike Hararo rift segment in Afar, Ethiopia: Implications for fault- the NW-SE trending Anza Graben (Fig. 1B). ACKNOWLEDGMENTS ing above dikes: Geophysical Journal International, v. 192, In reviewing data from this manuscript, I was frequently reminded p. 907–917, https://doi.org/10.1093/gji/ggs076. of the contribution of Jean-Jacques Tiercelin to the region. I am Bell, B., and Butcher, H., 2002, On the emplacement of sill grateful that he kept me involved in work in the area. I enjoyed his complexes: Evidence from the Faroe-Shetland Basin, in ■■ CONCLUSIONS company, as well as his expertise; his passing in 2017 was far too Jolley, D.W., and Bell, B.R., eds., The North Atlantic Igneous premature, and so I would like to dedicate this paper to his mem- Province: Stratigraphy, Tectonic, Volcanic and Magmatic Processes: Geological Society of London Special Publication Well-exposed dikes in western Turkana are ory. Amoco, Elf Aquitaine, and Tullow Oil are thanked for sending me or enabling me to do fieldwork in Turkana episodically from 197, p. 307–329, https://doi .org /10 .1144 /GSL .SP .2002 .197 .01 .12. described in detail for the first time in this study. The 1987 to 2011. Cindy Ebinger, Bernard Le Gall, and anonymous Bellieni, G., Visentin, E.J., Piccirillo, E.M., and Zanettin, B., 1987, early–middle Miocene dikes and extrusive activity reviewers are thanked for very useful and constructive comments Volcanic cycles and magmatic evolution in northern Turkana that considerably helped to improve this manuscript. (Kenya): Tectonophysics, v. 143, p. 161–168, https://doi .org ended a long phase (as long as 25 m.y.) of amag- /10.1016/0040-1951(87)90085-0. matic half-graben development in central Kenya and Bendick, R., McClusky, S., Bilham, R., Asfaw, L., and Klemperer, southern Turkana, which lay on the southern edge S., 2006, Distributed Nubia-Somalia relative motion and REFERENCES CITED dike intrusion in the Main Ethiopian Rift: Geophysical Jour- of the early (Eocene–Oligocene) plume activity. A Abdelfettah, Y., Tiercelin, J.-J., Tarits, P., Hautot, S., Maia, M., and nal International, v. 165, p. 303–310, https://doi .org /10 .1111/j wide range of dike patterns (parallel, circumferential, Thuo, P., 2016, Subsurface structure and stratigraphy of the .1365-246X.2006.02904.x. radial, cross-cutting networks, superimposed trends) northwest end of the Turkana Basin, Northern Kenya Rift, Beutel, E., van Wijk, J., Ebinger, C., Keir, D., and Agostini, A., 2010, are present. In places (Loriu, Lojamei, Lokhone high), as revealed by magnetotellurics and gravity joint inversion: Formation and stability of magmatic segments in the Main Journal of African Earth Sciences, v. 119, p. 120–138, https:// Ethiopian and Afar rifts: Earth and Planetary Science Letters, dikes trend a high angle to the rift trend, suggesting doi.org/10.1016/j.jafrearsci.2016.03.008. v. 293, p. 225–235, https://doi .org /10 .1016 /j .epsl .2010 .02 .006. some local influence (e.g., overpressured magma Acocella, V., Cifelli, F., and Funiciello, R., 2001, The control of Bialas, R.W., Buck, W.R., and Qin, R., 2010, How much magma is chamber, cracked lid–style dike intrusions over a overburden thickness on resurgent domes: Insights from required to rift a continent?: Earth and Planetary Science Let- ters, v. 292, p. 68–78, https://doi .org /10 .1016 /j .epsl .2010 .01 .021. sill or laccolith, preexisting fabric in basement) on analogue models: Journal of Volcanology and Geothermal Research, v. 111, p. 137–153, https://doi.org/10.1016/S0377 Bistacchi, A., Tibaldi, A., Pasquarè, F.A., and Rust, D., 2012, The orientation as well as the influence from regional -0273(01)00224-4. association of cone-sheets and radial dikes: Data from the stresses. Despite diverse dike orientations being Alemu, T., 2017, Seismic reflection studies of South Omo Basin, Isle of Skye (UK), numerical modelling, and implications for shallow magma chambers: Earth and Planetary Science present, the overall north-south to NNW-SSE dike South Western Ethiopia [M.S. thesis]: Addis Ababa, Ethiopia, Addis Ababa University, 74 p. Letters, v. 339–340, p. 46–56, https://doi.org/10.1016/j.epsl trends are supportive of approximately east-west Anderson, E.M., 1942, The Dynamics of Faulting and Dyke For- .2012.05.020. extension during the early–middle Miocene. mation with Application to Britain: Edinburgh, Oliver and Boone, S.C., Seiler, C., Kohn, B.P., Gleadow, A.J.W., Foster, D.A., The early to middle Miocene–age dikes occur in Boyde, 191 p. and Chung, L., 2018a, Influence of rift superposition on litho- Baer, G., Hamiel, Y., Shamier, G., and Nof, R., 2008, Evolution of spheric response to East African Rift System extension: localized areas as much as ~15 km long and 5 km a magma-driven earthquake swarm and triggering of the Lapur Range, Turkana, Kenya: Tectonics, v. 37, p. 182–207, wide, are closely spaced (tens of dikes per kilome- nearby Oldoinyo Lengai eruption, as resolved by InSAR https://doi.org/10.1002/2017TC004575. ter), relatively short (<5 km), and numerous (>3500 ground observations and elastic modeling, East African Rift, Boone, S.C., Kohn, B.P., Gleadow, A.J.W., Morley, C.K., Seiler, 2007: Earth and Planetary Science Letters, v. 272, p. 339–352, C., Foster, D.A., and Chung, L., 2018b, Tectono-thermal dikes have been mapped), and have only accom- https://doi.org/10.1016/j.epsl.2008.04.052. evolution of a long-lived segment of the East African Rift modated low amounts of upper-crustal extension Bailey, E.B., Clough, C.T., Wright, W.B., Richey, J.E., and Wil- System: Thermochronological insights from the North Loki (<2%). This limited development of dikes differs son, G.V., 1924, Tertiary and post-Tertiary geology of Mull, char Basin, Turkana, Kenya: Tectonophysics, v. 744, p. 23–46, from the larger, more extensive arrays of dikes in Loch Aline, and Oban: Memoir of the Geological Survey https://doi.org/10.1016/j.tecto.2018.06.010. of Scotland, 445 p. Boone, S.C., Kohn, B.P., Gleadow, A.J.W., Morley, C.K., Seiler, evolved systems in the Main Ethiopian Rift that Baker, B.H., and Wohlenberg, J., 1971, Structure and evolution C., and Foster, D.A., 2019, Birth of the East African Rift Sys- are critical for accommodating crustal extension. of the Kenya Rift Valley: Nature, v. 229, p. 538–542, https:// tem: Eocene nucleation of magmatism and strain in the Instead, in the Oligocene–Miocene Turkana Rift doi.org/10.1038/229538a0. Turkana Depression: Geology, v. 47, p. 886–890, https://doi Baker, B.H., Mohr, P.A., and Williams, L.A.J., 1972, Geology of the .org/10.1130/G46468.1. basins, extension was primarily accommodated Eastern Rift System of Africa: Geological Society of America Boschetto, H.B., Brown, F.H., and McDougall, I., 1992, Stratigra- along the very large border faults. The early style Special Paper 136, 67 p., https://doi.org/10.1130/SPE136-p1. phy of the Lothidok Range, northern Kenya, and K/Ar ages

of its Miocene primates: Journal of Human Evolution, v. 22, Ducrocq, S., Boisserie, J.-R., Tiercelin, J.-J., Delmer, C., Garcia, Feibel, C.S., Brown, F.H., and McDougall, I., 1989, Stratigraphic p. 47–71, https://doi.org/10.1016/0047-2484(92)90029-9. G., Kyalo, M.F., Leakey, M.G., Marivaux, L., Otero, O., Pei- context of fossil hominids from the Omo group deposits: Bosworth, W., and Maurin, A., 1993, Structure, geochronology gné, S., Tassy, P., and Lihoreau, F., 2010, New Oligocene Northern Turkana Basin, Kenya and Ethiopia: American and tectonic significance of the northern Suguta Valley vertebrate localities from northern Kenya (Turkana basin): Journal of Physical Anthropology, v. 78, p. 595–622, https:// (Gregory Rift), Kenya: Journal of the Geological Society, Journal of Vertebrate Paleontology, v. 30, p. 293–299, https:// doi.org/10.1002/ajpa.1330780412. v. 150, p. 751–762, https://doi.org/10.1144/gsjgs.150.4.0751. doi.org/10.1080/02724630903413065. Furman, T., Kaleta, K.M., Bryce, J.G., and Hanan, B.B., 2006, Brown, F.H., and Jicha, B.R., 2016, New high-precision 40Ar/39Ar Dumont, S., Klinger, Y., Socquet, A., Doubre, C., and Jacques, Tertiary mafic lavas of Turkana, Kenya: Constraints on East ages on Oligocene volcanic rocks of northwestern Kenya: E., 2017, Magma influence on propagation of normal faults: African plume structure and the occurrence of high-μ vol- Journal of African Earth Sciences, v. 114, p. 63–66, https:// Evidence from cumulative slip profiles along Dabbahu-Man- canism in Africa: Journal of Petrology, v. 47, p. 1221–1244, doi.org/10.1016/j.jafrearsci.2015.11.012. da-Hararo rift segment (Afar, Ethiopia): Journal of Structural https://doi.org/10.1093/petrology/egl009. Brown, F.H., and McDougall, I., 2011, Geochronology of the Tur- Geology, v. 95, p. 48–59, https://doi.org/10.1016/j.jsg.2016 Furman, T., Nelson, W.R., and Elkins-Tanton, L.T., 2016, Evolution kana Depression of northern Kenya and southern Ethiopia: .12.008. of the East African rift: Drip magmatism, lithospheric thin- Evolutionary Anthropology, v. 20, p. 217–227, https://doi .org Dunkelman, T.J., Karson, J.A., and Rosendahl, B.R., 1988, ning and mafic volcanism: Geochimica et Cosmochimica /10.1002/evan.20318. Structural style of the Turkana Rift, Kenya: Geology, v. 16, Acta, v. 185, p. 418–434, https://doi.org/10.1016/j.gca.2016 Buck, W.R., 2004, Consequences of asthenospheric variability p. 258–261, https://doi .org /10 .1130 /0091 -7613 (1988)016 <0258: .03.024. on continental rifting, in Karner, G.D., Taylor, B., Driscoll, SSOTTR>2 .3 .CO;2. Galerne, C.Y., Galland, O., Neumann, E.-R., and Planke, S., 2011, N.W., and Kohlstedt, D.L., eds., Rheology and Deforma- Dunkelman, T.J., Rosendahl, B.R., and Karson, J.A., 1989, 3D relationships between sills and their feeders: Evidence tion of the Lithosphere at Continental Margins: New York, Structure and stratigraphy of the Turkana rift from seis- from the Golden Valley Sill Complex (Karoo Basin) and Columbia University Press, p. 1–30, https://doi.org/10.7312 mic reflection data: Journal of African Earth Sciences, v. 8, experimental modeling: Journal of Volcanology and Geo- /karn12738-002. p. 489–510, https://doi .org /10 .1016 /S0899 -5362 (89)80041 -7. thermal Research, v. 202, p. 189–199, https://doi.org /10 .1016 Buck, W.R., 2006, The role of magma in the development of Ebinger, C., Ayele, A., Keir, D., Rowland, J., Yirgu, G., Wright, T., /j.jvolgeores.2011.02.006. the Afro-Arabian Rift System, in Yirgu, G., Ebinger, C.J., Belachew, M., and Hamling, I., 2010, Length and timescales Galland, O., Holohan, E., van Wyk de Vries, B., and Burchardt, S., and Maguire, P.K.H., eds., The Afar Volcanic Province within of rift faulting and magma intrusion: The Afar rifting cycle 2018, Laboratory modelling of volcano plumbing systems: the East African Rift System: Geological Society of London from 2005 to present: Annual Review of Earth and Planetary A review, in Breitkreuz, C., and Rocchi, S., eds., Physical Special Publication 259, p. 43–54, https://doi.org/10.1144 Sciences, v. 38, p. 439–466, https://doi.org/10.1146/annurev Geology of Shallow Magmatic Systems: Dykes, Sills and /GSL.SP.2006.259.01.05. -earth-040809-152333. Laccoliths: Cham, Switzerland, Springer, p. 147–214, https:// Cartwright, J., and Hansen, D.M., 2006, Magma transport Ebinger, C.J., and Casey, M., 2001, Continental breakup in mag- doi.org/10.1007/11157_2015_9. through the crust via interconnected sill complexes: Geol- matic provinces: An Ethiopian example: Geology, v. 29, George, R., and Rogers, N., 2002, Plume dynamics beneath the ogy, v. 34, p. 929–932, https://doi.org/10.1130/G22758A.1. p. 527–530, https://doi .org /10 .1130 /0091 -7613 (2001)029 <0527: African plate inferred from the geochemistry of the Tertiary Cerling, T.E., and Brown, F.H., 1982, Tuffaceous marker horizons CBIMPA>2 .0 .CO;2. basalts of southern Ethiopia: Contributions to Mineralogy in the Koobi Fora region and the Lower Omo Valley: Nature, Ebinger, C.J., and Ibrahim, A., 1994, Multiple episodes of rifting and Petrology, v. 144, p. 286–304, https://doi.org/10.1007 v. 299, p. 216–221, https://doi.org/10.1038/299216a0. in Central and East Africa: A re-evaluation of gravity data: /s00410-002-0396-z. Chapman, G.R., Lippard, S.J., and Martyn, J.E., 1978, Stratigra- Geologische Rundschau, v. 83, p. 689–702, https://doi .org George, R., Rogers, N., and Kelley, S., 1998, Earliest magmatism phy and structure of the Kamasia Range, Kenya Rift Valley: /10.1007/BF00251068. in Ethiopia: Evidence for two mantle plumes in one flood Journal of the Geological Society, v. 135, p. 265–281, https:// Ebinger, C.J., Yemane, T., WoldeGabriel, G., Aronson, J.L., and basalt province: Geology, v. 26, p. 923–926, https://doi .org /10 doi.org/10.1144/gsjgs.135.3.0265. Walter, R.C., 1993, Late Eocene–Recent volcanism and .1130/0091-7613(1998)026<0923:EMIEEF>2.3.CO;2. Coetzee, A., and Kisters, A.F.M., 2017, Dyke-sill relationships in faulting in the southern main Ethiopian rift: Journal of the Gudmundsson, A., 2006, How local stresses control mag- Karoo dolerites as indicators of propagation and emplace- Geological Society, v. 150, p. 99–108, https://doi.org /10 .1144 ma-chamber ruptures, dyke injections, and eruptions in ment processes of mafic magmas in the shallow crust: /gsjgs.150.1.0099. composite volcanoes: Earth-Science Reviews, v. 79, p. 1–31, Journal of Structural Geology, v. 97, p. 172–188, https://doi Ebinger, C.J., Yemane, T., Harding, D.J., Tesfaye, S., Kelley, S., and https://doi.org/10.1016/j.earscirev.2006.06.006. .org/10.1016/j.jsg.2017.03.002. Rex, D.C., 2000, Rift deflection, migration, and propagation: Guth, A.L., 2015, Volcanic volumes associated with the Kenya Corti, G., Cioni, R., Franceschini, Z., Sani, F., Scaillet, S., Molin, P., Linkage of the Ethiopian and Eastern rifts, Africa: Geolog- Rift: Recognition and correction of preservation biases, in Isola, I., Mazzarini, F., Brune, S., Keir, D., Erbello, A., Muluneh, ical Society of America Bulletin, v. 112, p. 163–176, https:// Wright, T.J., Ayele, A., Ferguson, D.J., Kidane, T., and Vye- A., Illsley-Kemp, F., and Glerum, A., 2019, Aborted propaga- doi.org /10 .1130 /0016 -7606 (2000)112 <163: RDMAPL>2 .0 .CO;2. Brown, C., eds., Magmatic Rifting and Active Volcanism: tion of the Ethiopian rift caused by linkage with the Kenya Ebinger, C.J., Keir, D., Bastow, I.D., Whaler, K., Hammond, J.O.S., Geological Society of London Special Publication 420, rift: Nature Communications, v. 10, 1309, https://doi.org Ayele, A., Miller, M.S., Tiberi, C., and Hautot, S., 2017, Crustal p. 31–42, https://doi.org/10.1144/SP420.3. /10.1038/s41467-019-09335-2. structure of active deformation zones in Africa: Implications Hackman, B.D., Charsley, T.J., Key, R.M., and Wilkinson, A.F., Cruden, A.R., McCaffrey, K.J.W., and Bunger, A.P., 2018, Geo- for global crustal processes: Tectonics, v. 36, p. 3298–3332, 1990, The development of the East African Rift System in metric scaling of tabular igneous intrusions: Implications https://doi.org/10.1002/2017TC004526. north-central Kenya: Tectonophysics, v. 184, p. 189–211, for emplacement and growth, in Breitkreuz, C., and Rocchi, Emishaw, L., Laó-Davila, D.A., Abdelsalam, M.G., Atekwana, https://doi.org/10.1016/0040-1951(90)90053-B. S., eds., Physical Geology of Shallow Magmatic Systems: E.A., and Gao, S.S., 2017, Evolution of the broadly rifted Haileab, B., Brown, F.H., McDougall, I., and Gathogo, P.N., 2004, Dykes, Sills and Laccoliths: Cham, Switzerland, Springer, zone in southern Ethiopia through gravitational collapse Gombe Group basalts and initiation of Pliocene deposition p. 11–38, https://doi.org/10.1007/11157_2017_1000. and extension of dynamic topography: Tectonophysics, in the Turkana depression, northern Kenya and southern Curtis, P.C., 1991, Competing magmatic processes at a continen- v. 699, p. 213–226, https://doi .org /10 .1016 /j .tecto .2016 .12 .009. Ethiopia: Geological Magazine, v. 141, p. 41–53, https://doi tal rift: Petrology and geochemistry of Quaternary volcanoes Fagereng, Å., 2013, Fault segmentation, deep rift earthquakes and .org/10.1017/S001675680300815X. in the Turkana Rift Zone, East Africa [Ph.D. thesis]: Chapel crustal rheology: Insights from the 2009 Karonga sequence Harker, A., 1904, The Tertiary Igneous Rocks of Skye: Memoir Hill, North Carolina, University of North Carolina at Chapel and seismicity in the Rukwa-Malawi rift zone: Tectonophysics, of the Geological Survey of the United Kingdom, 481 p. Hill, 210 p. v. 601, p. 216–225, https://doi .org /10 .1016 /j .tecto .2013 .05 .012. Hautot, S., Tarits, P., Whaler, K., Le Gall, B., Tiercelin, J.-J., Davidson, A., and Rex, D.C., 1980, Age of volcanism and rifting Feibel, C.S., 2011, A geological history of the Turkana Basin: and Le Turdu, C., 2000, Deep structure of the Baringo Rift in southern Ethiopia: Nature, v. 283, p. 657–658, https://doi Evolutionary Anthropology, v. 20, p. 206–216, https://doi Basin (central Kenya) from three-dimensional magneto- .org/10.1038/283657a0. .org/10.1002/evan.20331. telluric imaging: Implications for rift evolution: Journal of

Geophysical Research, v. 105, p. 23,493–23,518, https://doi B., Deresse, B., Musila, M., Kanoti, J., and Perry, M., 2020, influence of syn-rift structure on igneous intrusion archi- .org/10.1029/2000JB900213. Accommodation of East African Rifting across the Turkana tecture: Geosphere, v. 14, p. 2533–2584, https://doi.org/10 Hayward, N.J., and Ebinger, C.J., 1996, Variations in along- Depression: Journal of Geophysical Research. Solid Earth, .1130/GES01645.1. axis segmentation of the Afar Rift system: Tectonics, v. 15, https://doi.org/10.1029/2019JB018469. Morley, C.K., Wescott, W.A., Stone, D.M., Harper, R.M., Wig- p. 244–257, https://doi.org/10.1029/95TC02292. Lavayssière, A., Drooff, C., Ebinger, C., Gallacher, R., Illsley-Kemp, ger, S.T., and Karanja, F.M., 1992, Tectonic evolution of the Hendrie, D.B., Kusznir, N.J., Morley, C.K., and Ebinger, C.J., 1994, F., Oliva, S.J., and Keir, D., 2019, Depth extent and kinematics northern Kenya Rift: Journal of the Geological Society, v. 149, Cenozoic extension in northern Kenya: A quantitative model of of faulting in the Southern Tanganyika Rift, Africa: Tecton- p. 333–348, https://doi.org/10.1144/gsjgs.149.3.0333. rift basin development in the Turkana region: Tectonophysics, ics, v. 38, p. 842–862, https://doi .org /10 .1029 /2018TC005379. Morley, C.K., Wescott, W.A., Stone, D.M., Harper, R.M., Wigger, v. 236, p. 409–438, https://doi .org /10 .1016 /0040 -1951 (94) 90187-2. Le Bas, M.J., 1971, Cone-sheets as a mechanism of uplift: Geo- S.T., Day, R.A., and Karanja, F.M., 1999a, Geology and geo- Hodgson, I., Illsley-Kemp, F., Gallacher, R.J., Keir, D., Ebinger, logical Magazine, v. 108, p. 373–376, https://doi .org/10.1017 physics of the western Turkana basins, Kenya, in Morley, C.J., and Mtelela, K., 2017, Crustal structure at a young con- /S0016756800056399. C.K., ed., Geoscience of Rift Systems—Evolution of East tinental rift: A receiver function study from the Tanganyika Le Gall, B., Vétel, W., and Morley, C.K., 2005a, Inversion tectonics Africa: American Association of Petroleum Geologists Stud- Rift: Tectonics, v. 36, p. 2806–2822, https://doi.org/10.1002 during continental rifting: The Turkana Cenozoic rifted zone, ies in Geology 44, p. 19–54. /2017TC004477. northern Kenya: Tectonics, v. 24, TC2002, https://doi .org/10 Morley, C.K., Bosworth, W., Day, R.A., Lauck, R., Bosher, R., Jones, J.R., Stamps, D.S., Wauthier, C., Saria, E., and Biggs, .1029/2004TC001637. Stone, D.M., Wigger, S.T., Wescott, W.A., Haun, D., and J., 2019, Evidence for slip on a border fault triggered by Maccaferri, F., Rivalta, E., Keir, D., and Acocella, V., 2014, Off-rift Bassett, N., 1999b, Geology and geophysics of the Anza magmatic processes in an immature continental rift: Geo- volcanism in rift zones determined by crustal unloading: Graben, in Morley, C.K., ed., Geoscience of Rift Systems— chemistry Geophysics Geosystems, v. 20, p. 2515–2530, Nature Geoscience, v. 7, p. 297–300, https://doi .org/10.1038 Evolution of East Africa: AAPG Studies in Geology, v. 44, https://doi.org/10.1029/2018GC008165. /ngeo2110. p. 67–90, https://doi.org/10.1306/St44623C4. Joubert, P., 1966, Geology of the Loperot area: Degree sheet 18, Magee, C., Briggs, F., and Jackson, C.A.-L., 2013, Lithological Muia, G., 2015, The “Turkana Grits”: Potential hydrocarbon res- S.E. quarter: Geological Survey of Kenya Report 74, 52 p. controls on igneous intrusion-induced ground deformation: ervoirs of the northern and central Kenya Rifts [Ph.D. thesis]: Kaeser, B., Kalt, A., and Pettke, T., 2006, Evolution of the litho- Journal of the Geological Society, v. 170, p. 853–856, https:// Rennes, France, Université de Rennes, 207 p. spheric mantle beneath the Marsabit volcanic field (northern doi.org/10.1144/jgs2013-029. Muirhead, J.D., Airoldi, G., Whitec, J.D.L., and Rowland, J.V., Kenya): Constraints from textural, P-T and geochemical Magee, C., Muirhead, J.D., Karvelas, A., Holford, S.P., Jackson, 2014, Cracking the lid: Sill-fed dikes are the likely feeders of studies on xenoliths: Journal of Petrology, v. 47, p. 2149– C.A.L., Bastow, I.D., Schofield, N., Stevenson, C.T.E., McLean, flood basalt eruptions: Earth and Planetary Science Letters, 2184, https://doi.org/10.1093/petrology/egl040. C., McCarthy, W., and Shtukert, O., 2016, Lateral magma v. 406, p. 187–197, https://doi.org/10.1016/j.epsl.2014.08.036. Karakas, O., and Dufek, J., 2015, Melt evolution and residence in flow in mafic sill complexes: Geosphere, v. 12, p. 809–841, Muirhead, J.D., Kattenhorn, S.A., and Le Corvec, N., 2015, Vary- extending crust: Thermal modelling of the crust and crustal https://doi.org/10.1130/GES01256.1. ing styles of magmatic strain accommodation across the magmas: Earth and Planetary Science Letters, v. 425, p. 131– Magee, C., Bastow, I.D., van Wyk de Vries, B., Jackson, C.A.-L., East African Rift: Geochemistry Geophysics Geosystems, 144, https://doi.org/10.1016/j.epsl.2015.06.001. Hetherington, R., Hagos, M., and Hoggett, M., 2017, Struc- v. 16, p. 2775–2795, https://doi.org/10.1002/2015GC005918. Karson, J.A., and Curtis, P.C., 1994, Quaternary volcanic centers ture and dynamics of surface uplift induced by incremental Muirhead, J.D., Kattenhorn, S.A., Lee, H., Mana, S., Turrin, B.D., of the Turkana Rift, Kenya: Journal of African Earth Sciences, sill emplacement: Geology, v. 45, p. 431–434, https://doi .org Fischer, T.P., Kinji, G., Dindi, F., and Stamps, D.S., 2016, Evo- v. 18, p. 15–35, https://doi .org /10 .1016 /0899 -5362 (94)90051 -5. /10.1130/G38839.1. lution of upper crustal faulting assisted by magmatic volatile Keir, D., Ebinger, C.J., Stuart, G.W., Daly, E., and Ayele, A., 2006, McClymont, D., 2018, Identification of lateral facies trends using release during early-stage continental rift development in Strain accommodation by magmatism and faulting as rifting seismic and well velocity data, South Lokichar Basin Kenya: the East African Rift: Geosphere, v. 12, p. 1670–1700, https:// proceeds to breakup: Seismicity of the northern Ethiopian Abstract presented at American Association of Petroleum doi.org/10.1130/GES01375.1. rift: Journal of Geophysical Research, v. 111, B05314, https:// Geologists International Conference and Exhibition, Cape Nelson, R.A., Patton, T.L., and Morley, C.K., 1992, Rift-segment doi.org/10.1029/2005JB003748. Town, South Africa, 4–11 November. interaction and its relation to hydrocarbon exploration in Keller, G.R., Prodehl, C., Mechie, J., Fuchs, K., Khan, M.A., Magu- McDougall, I., and Brown, F.H., 2009, Timing of volcanism and continental rift systems: American Association of Petroleum ire, P.K.H., Mooney, W.D., Achauer, U., Davis, P.M., Meyer, evolution of the northern Kenya Rift: Geological Magazine, Geologists Bulletin, v. 76, p. 1153–1169, https://doi.org/10 R.P., Braile, L.W., Nyambok, I.O., and Thompson, G.A., 1994, v. 146, p. 34–47, https://doi.org/10.1017/S0016756808005347. .1306/BDFF898E-1718-11D7-8645000102C1865D. The East African rift system in the light of KRISP 90: Tec- Mechie, J., Keller, G.R., Prodehl, C., Khan, M.A., and Gaciri, S.J., O’Connor, P.M., Sertich, J.J.W., and Manthi, F.K., 2011, A ptero- tonophysics, v. 236, p. 465–483, https://doi .org /10 .1016 /0040 1997, A model for the structure, composition and evolution dactyloid pterosaur from the Upper Cretaceous Lapurr -1951(94)90190-2. of the Kenya rift: Tectonophysics, v. 278, p. 95–119, https:// sandstone, West Turkana, Kenya: Anais da Academia Bra- Keranen, K., Klemperer, S.L., Gloaguen, R., and EAGLE Work- doi.org/10.1016/S0040-1951(97)00097-8. sileira de Ciências, v. 83, p. 309–315, https://doi .org /10 .1590 ing Group, 2004, Three-dimensional seismic imaging of a Melnick, D., Garcin, Y., Quinteros, J., Strecker, M.R., Olago, D., /S0001-37652011000100019. protoridge axis in the Main Ethiopian rift: Geology, v. 32, and Tiercelin, J.-J., 2012, Steady rifting in northern Kenya Oliva, S.J., Ebinger, C.J., Wauthier, C., Muirhead, J.D., Roecker, p. 949–952, https://doi.org/10.1130/G20737.1. inferred from deformed Holocene lake shorelines of the S.W., Rivalta, E., and Heimann, S., 2019, Insights into fault- Key, R.M., 1989, A note on the Jibisa Ring Complex of northern Suguta and Turkana basins: Earth and Planetary Science magma interactions in an early-stage continental rift from Kenya: Journal of African Earth Sciences, v. 8, p. 113–125, Letters, v. 331–332, p. 335–346, https://doi .org /10 .1016 /j .epsl source mechanisms and correlated volcano-tectonic earth- https://doi.org/10.1016/S0899-5362(89)80016-8. .2012.03.007. quakes: Geophysical Research Letters, v. 46, p. 2065–2074, Khan, M.A., Mechie, J., Birt, C., Byrne, G., Gaciri, S., Jacob, B., Mohamed, A.Y., Whiteman, A.J., Archer, S.G., and Bowden, S.A., https://doi.org/10.1029/2018GL080866. Keller, G.R., Maguire, P.K.H., Novak, O., Nyambok, I.O., Patel, 2016, Thermal modelling of the Melut basin Sudan and Olson, J.E., and Pollard, D.D., 1991, The initiation and growth J.P., Prodehl, C., Riaroh, D., Simiyu, S., and Thybo, H., 2000, South Sudan: Implications for hydrocarbon generation and of en echelon veins: Journal of Structural Geology, v. 13, The lithospheric structure of the Kenya Rift as revealed by migration: Marine and Petroleum Geology, v. 77, p. 746–762, p. 595–608, https://doi.org/10.1016/0191-8141(91)90046-L. wide-angle seismic measurements, in Mac Niocaill, C., and https://doi.org/10.1016/j.marpetgeo.2016.07.007. Owusu Agyemang, P.C., Roberts, E.M., Downie, B., and Ser- Ryan, P.D., eds., Continental Tectonics: Geological Society Morley, C.K., 1994, Interaction of deep and shallow processes tich, J.J.W., 2019, Sedimentary provenance and maximum of London Special Publication 164, p. 257–269, https://doi in the evolution of the Kenya Rift: Tectonophysics, v. 236, depositional age analysis of the Cretaceous? Lapur and .org/10.1144/GSL.SP.1999.164.01.13. p. 81–91, https://doi.org/10.1016/0040-1951(94)90170-8. Muruanachok sandstones (Turkana Grits), Turkana Basin, Knappe, E., Bendick, R., Ebinger, C., Birhanu, Y., Lewi, E., Floyd, Morley, C.K., 2018, 3-D seismic imaging of the plumbing sys- Kenya: Geological Magazine, v. 156, p. 1334–1356, https:// M., King, R., Kianji, G., Marlita, T., Temtime, B., Waktola, tem of the Kora Volcano, Taranaki Basin, New Zealand: The doi.org/10.1017/S0016756818000663.

Phillips, W.J., 1974, The dynamic emplacement of cone sheets: Rooney, T.O., 2017, The Cenozoic magmatism of East-Africa: Part Strecker, M., and Bosworth, W., 1991, Quaternary stress-field Tectonophysics, v. 24, p. 69–84, https://doi .org /10 .1016 /0040 1—Flood basalts and pulsed magmatism: Lithos, v. 286–287, change in the Gregory Rift, Kenya: Eos (Transactions, Amer- -1951(74)90130-9. p. 264–301, https://doi.org/10.1016/j.lithos.2017.05.014. ican Geophysical Union), v. 72, p. 17, 21–22, https://doi.org Pik, R., Marty, B., and Hilton, D.R., 2006, How many mantle Rooney, T.O., Krans, S.R., Mège, D., Arnaud, N., Korme, T., /10.1029/90EO00017. plumes in Africa? The geochemical point of view: Chem- Kappelman, J., and Yirgu, G., 2018, Constraining the mag- Swain, C.J., 1992, The Kenya rift axial gravity high: A re-inter- ical Geology, v. 226, p. 100–114, https://doi.org/10.1016/j matic plumbing system in a zoned continental flood basalt pretation: Tectonophysics, v. 204, p. 59–70, https://doi .org .chemgeo.2005 .09 .016. province: Geochemistry Geophysics Geosystems, v. 19, /10.1016/0040-1951(92)90269-C. Pinel, V., and Jaupart, C., 2003, Magma chamber behavior p. 3917–3944, https://doi .org /10 .1029 /2018GC007724. Talbot, M.R., Morley, C.K., Tiercelin, J.-J., Le Hérissé, A., Pot- beneath a volcanic edifice: Journal of Geophysical Research, Schofield, N., Holford, S., Millett, J., Brown, D., Jolley, D., Passey, devin, J.-L., and Le Gall, B., 2004, Hydrocarbon potential of v. 108, 2072, https://doi.org/10.1029/2002JB001751. S.R., Muirhead, D., Grove, C., Magee, C., Murray, J., Hole, M., the Meso-Cenozoic Turkana Depression, northern Kenya. II. Planke, S., Ramussen, T., Rey, S.S., and Myklebust, R., 2005, Jackson, C.A.-L., and Stevenson, C., 2016, Regional magma Source rocks: Quality, maturation, depositional environments Seismic characteristics and distribution of volcanic intru- plumbing and emplacement mechanisms of the Faroe-Shet- and structural control: Marine and Petroleum Geology, v. 21, sions and hydrothermal vent complexes in the Vøring and land Sill Complex: Implications for magma transport and p. 63–78, https://doi .org /10 .1016 /j .marpetgeo .2003 .11.008. Møre basins, in Doré, A.G., and Vining, B.A., eds., Petroleum petroleum systems within sedimentary basins: Basin Tatsumi, Y., and Kimura, N., 1991, Secular variation in basalt Geology: North-West Europe and Global Perspectives— Research, v. 29, p. 41–63, https://doi.org/10.1111/bre.12164. chemistry in the Kenya Rift: Evidence for the pulsing of Proceedings of the 6th Petroleum Geology Conference: Schofield, N., Jarram, D.A., Holford, S., Archer, S., Mark, N., asthenospheric upwelling: Earth and Planetary Science Geological Society of London Petroleum Geology Confer- Hartley, A., Howell, J., Muirhead, D., Green, P., Hutton, D., Letters, v. 104, p. 99–113, https://doi.org/10.1016/0012-821X ence Series 6, p. 833–844, https://doi.org/10.1144/0060833. and Stevenson, C., 2018, Sills in sedimentary basins and (91)90241-9. Plasman, M., Tiberi, C., Ebinger, C., Gautier, S., Albaric, J., Peyrat, petroleum systems, in Breitkreuz, C. and Rocchi, S., eds., Tentler, T., 2003, Analogue modelling of overlapping spreading S., Déverchère, J., Le Gall, B., Tarits, P., Roecker, S., Wam- Physical Geology of Shallow Magmatic Systems: Dykes, centers: Insights into their propagation and coalescence: bura, F., Muzuka, A., Mulibo, G., Mtelela, K., Msabi, M., Kianji, Sills and Laccoliths: Cham, Switzerland, Springer, p. 273– Tectonophysics, v. 376, p. 99–115, https://doi.org/10.1016/j G., Hautot, S., Perrot, J., and Gama, R., 2017, Lithospheric 294, https://doi.org/10.1007/11157_2015_17. .tecto.2003.08.011. low-velocity zones associated with a magmatic segment of Schull, T.J., 1988, Rift basins of interior Sudan: Petroleum explo- Tibaldi, A., 2015, Structure of volcano plumbing systems: A the Tanzanian Rift, East Africa: Geophysical Journal Interna- ration and discovery: American Association of Petroleum review of multi-parametric effects: Journal of Volcanology tional, v. 210, p. 465–481, https://doi.org/10.1093/gji/ggx177. Geologists Bulletin, v. 72, p. 1128–1142. and Geothermal Research, v. 298, p. 85–135, https://doi.org Pointing, A.J., Maguire, P.K.H., Khan, M.A., Francis, D.J., Swain, Schutter, S.R., 2003, Hydrocarbon occurrence and exploration /10.1016/j.jvolgeores.2015.03.023. C.J., Shah, E.R., and Griffiths, D.H., 1985, Seismicity of the in and around igneous rocks, in Petford, N., and McCaffrey, Tiberi, C., Gautier, S., Ebinger, C., Roecker, S., Plasman, M., northern part of the Kenya Rift Valley: Journal of Geodynam- K.J.W., eds., Hydrocarbons in Crystalline Rocks: Geological Albaric, J., Déverchère, J., Peyrat, S., Perrot, J., Ferdinand ics, v. 3, p. 23–37, https://doi .org /10 .1016 /0264 -3707 (85) 90020-1. Society of London Special Publication 214, p. 7–33, https:// Wambura, R., Msabi, M., Muzuka, A., Mulibo, G., and Kianji, Pollard, D.D., and Aydin, A., 1984, Propagation and linkage of doi.org/10.1144/GSL.SP.2003.214.01.02. G., 2019, Lithospheric modification by extension and mag- ocean ridge segments: Journal of Geophysical Research, v. 89, Senger, K., Millett, J., Planke, S., Ogata, K., Eide, C.H., Festøy, matism at the craton-orogenic boundary: North Tanzania p. 10,017–10,028, https://doi .org /10 .1029 /JB089iB12p10017. M., Galland, O., and Jerram, D.A., 2017, Effects of igneous Divergence, East Africa: Geophysical Journal International, Pollard, D.D., and Johnson, A.M., 1973, Mechanics of growth of intrusions on the petroleum system: A review: First Break, v. 216, p. 1693–1710, https://doi.org/10.1093/gji/ggy521. some laccolithic intrusions in the Henry mountains, Utah, II: v. 35, no. 6, p. 47–56. Tiercelin, J.-J., Potdevin, J.-L., Morley, C.K., Talbot, M.R., Bellon, Bending and failure of overburden layers and sill formation: Sippel, J., Meeßen, C., Cacace, M., Mechie, J., Fishwick, S., H., Rio, A., Le Gall, B., and Vétel, W., 2004, Hydrocarbon Tectonophysics, v. 18, p. 311–354, https://doi.org/10.1016 Heine, C., Scheck-Wenderoth, M., and Strecker, M.R., 2017, potential of the Meso-Cenozoic Turkana Depression, /0040-1951(73)90051-6. The Kenya rift revisited: Insights into lithospheric strength northern Kenya. I. Reservoirs: Depositional environments, Ragon, T., Nutz, A., Schuster, M., Ghienne, J.-F., Ruffet, G., and through data-driven 3-D gravity and thermal modelling: diagenetic characteristics, and source rock–reservoir rela- Rubino, J.-L., 2018, Evolution of the northern Turkana Solid Earth, v. 8, p. 45–81, https://doi.org/10.5194/se-8-45 tionships: Marine and Petroleum Geology, v. 21, p. 41–62, Depression (East African Rift System, Kenya) during the -2017. https://doi.org/10.1016/j.marpetgeo.2003.11.007. Cenozoic rifting: New insights from the Ekitale Basin (28– Smallwood, J.R., and Maresh, J., 2002, The properties, morphol- Tiercelin, J.-J., Potdevin, J.-L., Thuo, P.K., Abdelfettah, Y., Schus- 25.5 Ma): Geological Journal, v. 54, p. 3468–3488, https:// ogy and distribution of igneous sills: Modeling, borehole ter, M., Bourquin, S., Bellon, H., Clément, J.-P., Guillou, H., doi.org/10.1002/gj.3339. data and 3D seismic from the Faroe-Shetland area, in Jol- Nalpas, T., and Ruffet, G., 2012a, Stratigraphy, sedimen- Rateau, R., Schofield, N., and Smith, M., 2014, The potential role ley, D.W., and Bell, B.R., eds., The North Atlantic Igneous tology and diagenetic evolution of the Lapur Sandstone of igneous intrusions on hydrocarbon migration, West of Province: Stratigraphy, Tectonic, Volcanic and Magmatic in northern Kenya: Implications for oil exploration of the Shetland: Petroleum Geoscience, v. 19, p. 259–272, https:// Processes: Geological Society of London Special Publication Meso-Cenozoic Turkana depression: Journal of African doi.org/10.1144/petgeo2012-035. 197, p. 271–306, https://doi .org /10 .1144 /GSL .SP .2002 .197 .01 .11. Earth Sciences, v. 71–72, p. 43–79, https://doi.org/10.1016 Robertson, E.A.M., Biggs, J., Cashman, K.V., Floyd, M.A., and Spacapan, J.B., Palma, J.O., Galland, O., Manceda, R., Rocha, /j.jafrearsci.2012.06.007. Vye-Brown, C., 2015, Influence of regional tectonics and E., D’Odorico, A., and Leanza, H.A., 2018, Thermal impact Tiercelin, J.-J., Nalpas, T., Thuo, P., and Potdevin, J.-L., 2012b, pre-existing structures on the formation of elliptical calderas of igneous sill-complexes on organic-rich formations and Hydrocarbon prospectivity in Mesozoic and early–middle in the Kenyan Rift, in Wright, T.J., Ayele, A., Ferguson, D.J., implications for petroleum systems: A case study in the Cenozoic rift basins of central and northern Kenya, Eastern Kidane, T., and Vye-Brown, C., eds., Magmatic Rifting and northern Neuquén Basin, Argentina: Marine and Petro- Africa, in Gao, D., ed., Tectonics and Sedimentation: Impli- Active Volcanism: Geological Society of London Special leum Geology, v. 91, p. 519–531, https://doi.org/10.1016/j cations for Petroleum Systems: American Association of Publication 420, p. 43–67, https://doi.org/10.1144/SP420.12. .marpetgeo.2018.01.018. Petroleum Geologists Memoir 100, p. 179–207, https://doi Roecker, S., Ebinger, C., Tiberi, C., Mulibo, G., Ferdinand-Wamb- Stamps, D.S., Calais, E., Saria, E., Hartnady, C., Nocquet, J.-M., .org/10.1306/13351553M1001742. rua, R., Mtelela, K., Kianji, G., Muzuka, A., Gautier, S., Albaric, Ebinger, C.J., and Fernandes, R.M., 2008, A kinematic model Tingay, M.R.P., Morley, C.K., Hillis, R.R., and Meyer, J., 2010, Pres- J., and Peyrat, S., 2017, Subsurface images of the Eastern for the East African Rift: Geophysical Research Letters, v. 35, ent-day stress orientation in Thailand’s basins: Journal of Rift, Africa, from the joint inversion of body waves, sur- L05304, https://doi.org/10.1029/2007GL032781. Structural Geology, v. 32, p. 235–248, https://doi.org /10 .1016 face waves and gravity: Investigating the role of fluids in Stamps, D.S., Saria, E., and Kreemer, C., 2018, A geodetic strain /j.jsg.2009.11.008. early-stage continental rifting: Geophysical Journal Interna- rate model for the East African Rift System: Scientific Reports, Torres-Hernández, J.R., Labarthe-Hernández, G., Aguillón-Ro- tional, v. 210, p. 931–950, https://doi .org /10 .1093 /gji /ggx220. v. 8, 732, https://doi .org /10 .1038 /s41598 -017 -19097 -w. bles, A., Gómez-Anguiano, M., and Mata-Segura, J.L., 2006,

The pyroclastic dikes of the Tertiary San Luis Potosi vol- analyses: Basin Research, v. 16, p. 165–181, https://doi .org C.K., ed., Geoscience of Rift Systems—Evolution of East canic field: Implications on the emplacement of Panalillo /10.1046/j.1365-2117.2003.00227.x. Africa: American Association of Petroleum Geologists ignimbrite: Geofísica Internacional, v. 45, p. 243–253. Vétel, W., Le Gall, B., and Walsh, J.J., 2005, Geometry and Studies in Geology 44, p. 55–66, https://doi.org/10.1306 van Wyk de Vries, B., and van Wyk de Vries, M., 2018, Tectonics growth of an inner rift fault pattern: The Kino Sogo Fault Belt, /St44623C3. and volcanic igneous plumbing systems, in Burchardt, S., Turkana Rift (North Kenya): Journal of Structural Geology, Wichura, H., Jacobs, L.L., Lin, A., Polcyn, M.J., Manthi, F.K., Win- ed., Volcanic and Igneous Plumbing Systems: Understand- v. 27, p. 2204–2222, https://doi.org/10.1016/j.jsg.2005.07.003. kler, D.A., Strecker, M.R., and Clemens, M., 2015, A 17-My-old ing Magma Transport, Storage, and Evolution in the Earth’s Wadge, G., Biggs, J., Lloyd, R., and Kendall, J.-M., 2016, Histori- whale constrains onset of uplift and climate change in east Crust: Amsterdam, Elsevier, p. 167–189, https://doi .org /10 cal volcanism and the state of stress in the East African Rift Africa: Proceedings of the National Academy of Sciences of .1016/B978-0-12-809749-6.00007-8. System: Frontiers of Earth Science, v. 4, 86, https://doi .org the United States of America, v. 112, p. 3910–3915, https:// van Wyk de Vries, B., Márquez, A., Herrera, R., Granja Bruña, /10.3389/feart.2016.00086. doi.org /10 .1073 /pnas .1421502112. J.L., Llanes, P., and Delcamp, A., 2014, Craters of elevation Walsh, J., and Dodson, R.G., 1969, Geology of northern Turkana: Wilkinson, A.F., 1988, Geology of the Allia Bay area: Degree sheet revisited: Forced-folds, bulging and uplift of volcanoes: Geological Survey of Kenya Report 82, 42 p. 11, with coloured 1:250 000 geological map: Kenya Mines Bulletin of Volcanology, v. 76, 875, https://doi.org/10.1007 Weinstein, A., Oliva, S.J., Ebinger, C.J., Roecker, S., Tiberi, C., and Geology Department Report 109, 54 p. /s00445-014-0875-x. Aman, M., Lambert, C., Witkin, E., Albaric, J., Gautier, S., Wright, T.J., Ebinger, C., Biggs, J., Ayele, A., Yirgu, G., Keir, D., Vetel, W., 2005, Dynamic of intra-continental extension in Peyrat, S., Muirhead, J.D., Muzuka, A.N.N., Mulibo, G., and Stork, A., 2006, Magma-maintained rift segmentation at magmatic rift context: Turkana Rift (Northern Kenya) from Kianji, G., Ferdinand-Wambura, R., Msabi, M., Rodzianko, continental rupture in the 2005 Afar dyking episode: Nature, Eocene to Present [Ph.D. thesis]: Brest, France, Université A., Hadfield, R., Illsley-Kemp, F., and Fischer, T.P., 2017, v. 442, p. 291–294, https://doi.org/10.1038/nature04978. de Bretagne Occidentale, 213 p. Fault-magma interactions during early continental rifting: Yassin, M.A., Hairi, M.M., Abdullatif, O.M., Korvin, G., and Mak- Vetel, W., and Le Gall, B., 2006, Dynamics of prolonged conti- Seismicity of the Magadi-Natron-Manyara basins, Africa: kawi, M., 2017, Evolution history of transtensional pull-apart, nental extension in magmatic rifts: The Turkana Rift case Geochemistry Geophysics Geosystems, v. 18, p. 3662–3686, oblique rift basin and its implication on hydrocarbon explo- study (North Kenya), in Yirgu, G., Ebinger, C.J., and Magu- https://doi.org/10.1002/2017GC007027. ration: A case study from Sufyan Sub-basin, Muglad Basin, ire, P.K.H., eds., The Afar Volcanic Province within the East Wescott, W.A., Morley, C.K., and Karanja, F.M., 1993, Geology of Sudan: Marine and Petroleum Geology, v. 79, p. 282–299, African Rift System: Geological Society of London Special the “Turkana Grits” in the Lariu range and Mt. Porr areas, https://doi.org/10.1016/j.marpetgeo.2016.10.016. Publication 259, p. 209–233, https://doi.org/10.1144/GSL.SP southern Lake Turkana, Northwestern Kenya: Journal of Zanettin, B., Justin Visentin, E., Bellieni, G., Piccirillo, E.M., and .2006.259.01.17. African Earth Sciences, v. 16, p. 425–435, https://doi.org/10 Rita, F., 1983, Le volcanisme du bassin du Nord-Turkana Vétel, W., Le Gall, B., and Johnson, T.C., 2004, Recent tectonics in .1016/0899-5362(93)90101-U. (Kenya): Age, succession et évolution structurale: Bulletin the Turkana Rift (North Kenya): An integrated approach from Wescott, W.A., Wigger, S.T., Stone, D.M., and Morley, C.K., 1999, des Centres de Recherches Exploration-Production Elf-Aqui- drainage network, satellite imagery and reflection seismic Geology and geophysics of the Lotikipi Plain, in Morley, taine, v. 7, p. 249–255.