New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ∼2 Ma reference pole for stable Africa

Abstract

[1] Joint French-Ethiopian field trips in 1995-1996 yield new geochronologic and paleomagnetic data, which significantly expand our knowledge of the recent magmatic and tectonic history of the Afar depression. Twenty-four new K-Ar ages range from 0.6 to 3.3 Ma. There is quite good agreement between magnetic polarities and Geomagnetic Polarity Timescale (GPTS). Eight age determinations with uncertainty less than 50 kyr can be used in future reassessments of the GPTS (upper and lower Olduvai/Matuyama reversals and Reunion and Mammoth subchrons). Paleomagnetic analysis of 865 cores from 133 sites confirms that low-Ti magnetites are the main carrier of the Characteristic Remanent Magnetization (ChRM). A positive tilt test (based on two subgroups with 63 and 23 sites, respectively) confirms that this ChRM is likely the primary magnetization. The main paleomagnetic results can be summarized as follows. A $2 Ma reference pole for stable Africa is determined based on 26 sites located on either side of the northern termination of the East African rift. It is located at l = 87.2°N, f = 217.1°E (A 95 = 4°). A 4.6 ± 1.8°(2s) inclination shallowing is identified within a population of 231 stratoid lava flows, consistent with a global axial quadrupole of 6 ± 2% of the axial dipole. Combined with earlier data of Acton et al. [2000], our new data allow mean paleomagnetic field directions to be determined for five individual, fault-bounded blocks previously identified by tectonic analysis within central Afar. These all have suffered negligible rotations about vertical axes since emplacement of the lava. This contrasts with the significant rotations previously uncovered to the east in Djiboutian Afar for three major individual blocks. Taken altogether, the declination differences with respect to reference directions are 2 ± 4°f or central Afar and 13 ± 4°for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last $3 Ma the Afar depression was extensively floored by trap-like basalts, which were deformed by a single but complex physical (tectonic) process, combining diffuse extension, rift localization, propagation, jumps and overlap, and bookshelf faulting.

Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements

“…The distribution of deformation at a large scale over Afar recalls the Oligo-Miocene for the evolution of the extension over Afar by Stab et al (2016). Unlike Wolfenden et al (2005) who draw the evolution of the locus of extension as a migration of focused extension toward the current active axes, Stab et al (2016) show that the focusing of the extension in Afar is very recent and coeval with the emplacement of large magmatic centres corresponding to the Quaternary segments (∼1.1 Ma, Kidane et al 2003;Lahitte et al 2003). If the magma supply is insufficient within the magmato-tectocic segments, and especially those at the tip of the spreading axes (MI and MG), the extensive deviatoric stresses can potentially be released during tectonic events through the slip on numerous and intricate faults, such as the 1989 sequence Dobi (Sigmundsson 1992;Jacques et al 1999Jacques et al , 2011).…”

“…Whereas Acton et al (2000) propose three to four blocks, Manighetti et al (2001) propose seven to eight. The decrease of the mean block rotation from 7-11 • in Eastern Afar and to 3-4 • in Central Afar is interpreted as the result of the propagation of the spreading axes Acton et al 2000;Manighetti et al 2001;Kidane et al 2003;Audin et al 2004).…”

The time scales of continental rifting: Implications for global processes

“…The magmatic segmentation postdates the ca. 1-3 Ma Stratoid basalts, which covered much of the Afar depression (e.g., Barberi and Varet, 1977;Kidane et al, 2003;Lahitte et al, 2003) and may have initiated as recently as ca. 70 ka (Williams et al, 2009;Medynski et al, 2013).…”

Seismicity of the central Afar rift and implications for Tendaho dam hazards

“…This style of deformation initiated within the broader Oligo-Pliocene rift zone at around 2 Ma (e.g. Kidane et al 2003). The highly oblique c. E-W extension direction of the MER relative to the NE-SW extension directions of the Red Sea and obliquely-opening Gulf of Aden rifts is decoupled by the NW-striking TGD fault scarp, which passes beneath the region (Manighetti et al 1997;Figs 1, 4 & 5).…”