Tesfaye Kidane

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

[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

Remagnetization in lava flows recording pretransitional directions

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° for central Afar and 13 ± 4° for eastern Afar, consistent with the model of Manighetti et al. [2001a]. It appears that in the last

Paleomagnetic and geochronological identification of the Reunion subchron in Ethiopian Afar

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.

Chronologie K–Ar et TL du volcanisme aux extrémités sud du propagateur mer Rouge en Afar depuis 300 ka

Basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of La Palma are characterized by overlapping components with very high unblocking temperatures. A medium high-temperature (MHT) component with normal polarity is resistant up to 500–550°C, while in most cases the high-temperature (HT) primary reverse component cannot be isolated below 520°C. The same situation (with opposite polarities) is observed also in a lava flow associated with the onset of the upper Reunion reversal in the Gamarri sequence in Ethiopia. The presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling. The direction of MHT is either close or similar to the direction of the HT component of the overlying flows, which indicates that remagnetization occurred shortly after emplacement. Rock magnetic studies, microscopic observations, microprobe analyses and remagnetization experiments have been carried out to investigate the origin of MHT. The rock magnetic parameters do not exhibit specific anomalies. The unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling. The most plausible scenario is that baking by the overlying flows was accompanied by low-temperature oxidation of titanomagnetite to cation-deficient titanomagnetite. This description also includes the presence of cracks, fractures and other heterogeneities of the lava which induce variability between parallel profiles. Similarities with characteristics observed at Steens Mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution.

Palaeomagnetic constraints on continental break-up processes: observations from the Main Ethiopian Rift

This paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault-related scarp of Pliocene age in the central part of the Afar depression (Ethiopia). Paleomagnetic work uncovers a clear characteristic remanent magnetization carried by (titano-) magnetites, with high unblocking temperatures, close to the Curie temperature of pure magnetite. The characteristic direction, based on 27 non transitional flows, is D=8.1 o, I= 10.1 o (• 5=4.1 o). This is consistent with earlier determinations of the clockwise tectonic rotation, due to rift propagation and overlap, of the block to which the section belongs. Magnetic stratigraphy consists of a succession from bottom to top of 16 reversed, 7 normal, and 4 reversed flows. One flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field (AF) demagnetization, which is due to remagnetization by the overlying flow. K/Ar dating of five samples provides consistent determinations, averaging 2.07 + 0.05 Ma. All ages agree with this average, given their rather large individual uncertainties (from 0.04 to 0.08 Ma at the lc • level). Comparison with recent reference geomagnetic polarity timescales (GPTS) indicates that the normal subchron recorded in the Gamarri section must be linked with (one of) the R6union event(s). The normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double R6union event. Although uncertainties in ages are too large to allow unequivocal inferences, our age determination for the R6union event is in better agreement with earlier determinations of McDougall and Watkins [ 1973] than with a more recent value proposed by Baksi et al. [1993] and used in the GPTS.

Magmatic rifting and active volcanism

K-Ar and TL volcanism chronology of the southern ends of the Red Sea spreading in Afar since 300 ka. Continental rift segments linked to the propagation of the Red Sea plate boundary in Afar are dated using thermoluminescence and potassium-argon dating techniques. These new results constrain the mechanism of the two moderate extensional structures located at the southern ends of the propagator: the Manda Hararo and the Dadar graben. Ages obtained show that their internal floor are about 30 and 100 kyr old, respectively, and that the deduced vertical rate of fault scarps display values lower than those linked to the Gulf of Aden propagation. The lower deformation accommodated by the Red Sea structures, their youthfulness and the greater distance to the mature oceanic ridges could justify this contrast of evolution.  2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS K-Ar dating / thermoluminescence / Afar / rifting / Red Sea / Ethiopia

Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism

Abstract We report the first palaeomagnetic results from the Main Ethiopian Rift (MER), the northernmost sector of the East African rift system. This part of the MER shows an along-axis tectono-magmatic segmentation pattern similar to that of slow-spreading mid-ocean ridges, which developed during the past 1.9 Ma. The aims of our palaeomagnetic, structural and geochronological studies are to test plate kinematic models for the right-stepping, en echelon 60–80 km-long magmatic segments. Twenty palaeomagnetic sites were sampled on either basalt or ignimbrite outcropping in the region adjacent to, and within, the <1.9 Ma-old tectono-magmatic segments of Gademsa-Koka, Boset and Fentale-Dofan. Five K-Ar age determinations were made to bracket the age of units studied in the palaeomagnetic analyses. The natural remanent magnetization intensity possibly exhibited a unimodal distribution with a value of 6.6 A/m (σ = 5.6 A/m) for the basalts and a bimodal distribution with magnetization intensity of 0.69 A/m (σ = 0.55 A/m) and 0.03 A/m (σ = 0.02 A/m), statistically similar to values from previous studies in the Afar triple junction zone (e.g. Kidane et al. 1999, 2002). Progressive heating, alternating field analysis, and susceptibility vs. temperature measurements indicated unblocking temperature ranging between 300 °C−600 °C for basalts and between 500 °C−660 °C for ignimbrites, suggesting the magnetic mineralogy to be titanomagnetite and magnetite for the former and magnetite and titanohematite for the latter. Palaeomagnetic measurements using both TH and AF technique revealed quasi-single component of magnetization with viscous remanent magnetization (VRM) on a few samples. Principal component analysis and statistical averaging resulted in an overall mean palaeomagnetic direction of (Ds = 2.3°, Is = 7.8°, α95 = 7, K = 26.9, N = 17) which is statistically identical to the expected direction (D = 1.9°, I = 13.5°, α95 = 2.5, K = 105.6, N = 32) from the Apparent Polar Wander Path reference curve for Africa at 1.5 Ma (Besse & Courtillot 2003). The angular difference between the observed and expected directions above with their uncertainty is calculated to be 0.4° ± 7.5°. These results indicate that the Late Pliocene-Pleistocene rocks of the MER in the studied region do not suffer vertical axis rotation, arguing against transtensional and seafloor-spreading-transform kinematic models. We suggest that magma intrusion, rather than large offset faults, produce the right-stepping, en echelon magmatic segments of the MER, which is at the transition from continental to oceanic extension.

Magmatic rifting and active volcanism: introduction

A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a ten-day period, c. 2.5 km3 of magma were intruded along a 60 km-long dyke separating the Arabian and Nubian plates. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. This activity led to a renewed international focus on the role of magmatism in rifting, with major international collaborative projects working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This work has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.

Detection of geothermal anomalies using Landsat 8 TIRS data in Tulu Moye geothermal prospect, Main Ethiopian Rift

Abstract Palaeomagnetic investigations were carried out on tilted Jessoma sandstones in the Aïsha block. The average natural remanent magnetization intensity was 0.026A m−1 with good behaviour on demagnetization both by thermal and alternating fields. The normalized magnetization intensity decay curves indicated that fine-grained, Ti-poor titanomagnetites were the carriers of magnetization. A well-defined single-component magnetization with a poorly defined secondary component was found. The latter was removed at temperatures below 300°C or at 10 mT. After these steps, a straight-line segment directed towards the origin was identified, which was interpreted as the characteristic remanent magnetization. Site mean direction coordinates of Decg=227.8°, Incg=–33.9° (n=7, α95=2.8°, K=478) (in situ) and Decs=225.6°, Incs=0.4° (n=7, α95=2.8°, K=478) (tilt-corrected) were obtained. Corresponding mean virtual geomagnetic pole coordinates of Long=117.5° E, Latg=43.3° N (A95=3.2°, n=7) (in situ) and Lons=143.5° E, Lats=43.3° N (A95=2.8°, n=7) (tilt-corrected) were obtained. The corrected in situ mean pole position was compared with the African apparent polar wander path curve and a deposition age for the sandstone of 60±5 Ma was determined, which is consistent with previous reports. A vertical axis rotation (R±ΔR) of the Aïsha block of 29.1±4.06° was obtained, which is equivalent to an inferred 30° counterclockwise rotation of the Danakil block, in agreement with a ‘saloon-door’ mechanism for the opening of the Afar Depression.

Fault growth at a nascent slow-spreading ridge: 2005 Dabbahu rifting episode, Afar

Abstract A major rifting episode began in the Afar region of northern Ethiopia in September 2005. Over a 10-day period, c. 2.5 km3 of magma were intruded into the upper crust along a 60 km-long dyke separating the Arabian and Nubian plates. There was an intense seismic swarm and a small rhyolitic eruption; extension of up to 10 m occurred across the rift segment. Over the next five years, a further 13 dyke intrusions caused continued extension, eruptions and seismicity. The activity in Afar led to a renewed international focus on the role of magmatism in rifting, with major collaborative projects involving researchers from Ethiopia, the UK, the USA, France, Italy and New Zealand working in Afar and Ethiopia to study the ongoing activity and to place it in a broader context. This book brings together articles that explore the role of magmatism in rifting, from the initiation of continental break-up through to full seafloor spreading. We also explore the hazards related to rifting and the associated volcanism. This renewed focus on magmatism and its role in rifting has implications for our understanding of how continents break-up and the associated distribution of resources in rift basins and continental margins.