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., © 2020 The Authors 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 A 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 B 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; 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 891 by guest on 29 September 2021 Research Paper 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).