Mid‐Pleistocene to Recent Crustal Extension in the Inner Graben of the Northern Kenya Rift

Abstract

Magmatic continental rifts often constitute nascent plate boundaries, yet long‐term extension rates and transient rate changes associated with these early stages of continental breakup remain difficult to determine. Here, we derive a time‐averaged minimum extension rate for the inner graben of the Northern Kenya Rift (NKR) of the East African Rift System for the last 0.5 m.y. We use the TanDEM‐X science digital elevation model to evaluate fault‐scarp geometries and determine fault throws across the volcano‐tectonic axis of the inner graben of the NKR. Along rift‐perpendicular profiles, amounts of cumulative extension are determined, and by integrating four new 40Ar/39Ar radiometric dates for the Silali volcano into the existing geochronology of the faulted volcanic units, time‐averaged extension rates are calculated. This study reveals that in the inner graben of the NKR, the long‐term extension rate based on mid‐Pleistocene to recent brittle deformation has minimum values of 1.0–1.6 mm yr−1, locally with values up to 2.0 mm yr−1. A comparison with the decadal, geodetically determined extension rate reveals that at least 65% of the extension must be accommodated within a narrow, 20‐km‐wide zone of the inner rift. In light of virtually inactive border faults of the NKR, we show that extension is focused in the region of the active volcano‐tectonic axis in the inner graben, thus highlighting the maturing of continental rifting in the NKR.

Magma‐Assisted Continental Rifting: The Broadly Rifted Zone in SW Ethiopia, East Africa

“…For regions farther south, in the southern Turkana Rift, Rooney et al (2022) suggested migration of deformation toward a narrow zone accompanied by spatially focused magmatism. Similarly, a recent structural and geochronological study in the nKR documented a significant influence of magmatism associated with Quaternary strain localization within the inner graben of the rift (Riedl et al, 2022), similar to observations made in the central and southern sectors of the Kenya Rift (Baker, 1986;Baker et al, 1988;Strecker et al, 1990).…”

“…With respect to the rift‐related magmatic evolution of the study area, it can be concluded that the volcanic rocks in the BRZ were emplaced in three major eruption periods during the Eo‐Oligocene, the Miocene, and the Pliocene‐Holocene. These distinct eruptive episodes were virtually coeval with phases of tectonic activity in the Red Sea Rift (e.g., Bosworth & Stockli, 2016), the eastern Tanzanian Craton (e.g., Rooney, 2020), and the nKR (Muirhead et al., 2022; Riedl et al., 2022).…”

“…Typically, large continental rift zones evolve from isolated tectonic basins during the early stages of extension into larger, throughgoing, and topographically linked depressions (e.g., Ebinger & Scholz, 2012; Ebinger et al., 1999; and references therein). Before this connectivity between individual basins is achieved, kinematic linking between these basins is accommodated within structurally complex transfer zones by the generation of closely spaced normal faults, the formation of local transfer faults, and pronounced magmatic activity accompanied by diking (Bosworth & Morley, 1994; Morley et al., 1992; Riedl et al., 2022).…”

“…Magmatic continental rifts are often associated with areally extensive volcanic rocks that were emplaced prior to the onset of widespread extensional deformation and tectonic subsidence (Aitken et al., 2013; Corti, 2008; Kendall & Bertelloni, 2016; McKenzie, 1978; White & McKenzie, 1989); examples include the Red Sea Rift (Bosworth & Stockli, 2016; Bosworth et al., 2005), the North and South Atlantic rifts (Courtillot & Renne, 2003; Courtillot et al., 1999; Saunders et al., 1997; Torsvik & Cocks, 2005), and the East African Rift (Kendall & Bertelloni, 2016; Wolfenden et al., 2004). Over time, both magmatism and extensional deformation migrate toward the interior of these tectonic depressions, and a narrow, volcanically and tectonically active rift develops during more advanced stages of extension (e.g., Corti, 2008; Ebinger et al., 1993; Morton et al., 1979; Richter et al., 2021; Riedl et al., 2022).…”

“…• Initial diffuse normal faulting across the Broadly Rifted Zone of southern Ethiopia occurred soon after the emplacement of flood basalts during the late Oligocene • Extensional deformation migrated toward a narrow zone along the Mali-Dancha and Bala-Kela basins in the Gofa Province during late-stage rifting • Thermal modeling results reveal a close temporal relationship between exhumation and regional pulses in magmatism, indicating magma-assisted continental rifting toward the interior of these tectonic depressions, and a narrow, volcanically and tectonically active rift develops during more advanced stages of extension (e.g., Corti, 2008;Ebinger et al, 1993;Morton et al, 1979;Richter et al, 2021;Riedl et al, 2022).…”