Abstract. We showed before that both the Aden and Red Sea plate boundaries are currently rifting and propagating along two distinct paths into Afar through the opening of a series of disconnected, propagating rifts. Here we use new geochronological, tectonic, and paleomagnetic data that we acquired mostly in the southeastern part of Afar to examine the geometry, kinematics, and time-space evolution of faulting related to strain transfer processes. It appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected. This mechanism implies the rotation about a vertical axis of small rigid blocks along rift-parallel faults that are shown to slip with a left-lateral component, which is as important as their normal component of slip (rates of-•2-3 mm/yr). By contrast, where rifts do not overlap, either a classic transform fault (Maskali) or an oblique transfer zone (Mak'arrasou) kinematically connects them. The length of the Aden-Red Sea overlap has increased in the last -•0.9 Myr, as the Aden plate boundary propagated northward into Afar.As a consequence, the first-order blocks that we identify within the overlap did not all rotate during the same time-span nor by the same amounts. Similarly, the major faults that bound them did not necessarily initiate and grow as their neighboring faults did. Despite these variations in strain distribution and kinematics, the overlap kept accommodating a constant amount of strain (7 to 15% of the extension amount imposed by plate driving forces), which remained distributed on a limited number (seven or eight) of major faults, each one having slipped at constant rates (-•3 and 2 mm/yr for vertical and lateral rates, respectively). The fault propagation rates and the block rotation rates that we either measure or deduce are so fast (30-130 mm/yr and 15-38ø/Myr, respectively) that they imply that strain transfer processes are transient, as has been shown to be the case for the processes of tearing, rift propagation, and strain jumps in Afar.

[1] This paper presents a compilation of intensity data covering the past 10 millennia (ArcheoInt). This compilation, which upgrades the one of Korte et al. (2005), contains 3648 data and incorporates additional intensity and directional data sets. A large majority of these data ($87%) were acquired on archeological artifacts, and the remaining $13% correspond to data obtained from volcanic products. The present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria. We show that $50% of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects (potsherds), the stability of the magnetization, and the data dispersion. The temporal and geographical distributions of this sub-data set are similar to those of the main data set, with $72% of the data dated from the past three millennia and $76% obtained from western Eurasia. Approximately half of the selected intensity data are associated with at least an inclination value. To constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western Eurasian data. We introduce a first-order regional weighting scheme based on the definition of eight widely distributed regions of 30°width within which the selected data are numerous enough. The regional curves of virtual axial dipole moments (VADM) and of mixed VADM-virtual dipole moments (VDM) averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components. The computation of global VADM and mixed VADM/VDM variation curves, assuming an equal weight for each region, yields a dipole evolution marked by a distinct minimum around 0 B.C./A.D. followed by a maximum around the third-fourth century A.D. A second minimum is present around the eighth century A.D. This variation pattern is compatible with the one deduced from earlier, more sophisticated analysis based on the inversion of both intensity and directional data. In particular, there is a good agreement among all VADMs and dipole moment estimates over the historical period, which further strengthens the validity of our weighting scheme.

Aims. Cosmogenic isotopes provide the only quantitative proxy for analyzing the long-term solar variability over a centennial timescale. While essential progress has been achieved in both measurements and modeling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. Here we aim at improving the reconstruction of solar activity over the past nine millennia using a multi-proxy approach. Methods. We used records of the 14 C and 10 Be cosmogenic isotopes, current numerical models of the isotope production and transport in Earth's atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo-and paleointensity database. The obtained series were analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. Results. A new reconstruction of the geomagnetic dipole field moment, referred to as GMAG.9k, is built for the last nine millennia. New reconstructions of solar activity covering the last nine millennia, quantified in terms of sunspot numbers, are presented and analyzed. A conservative list of grand minima and maxima is also provided. Conclusions. The primary components of the reconstructed solar activity, as determined using the SSA method, are different for the series that are based on 14 C and 10 Be. This shows that these primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. These components have therefore been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ≈2400-year quasi-periodicity, the so-called Hallstatt cycle, in both the 14 C and 10 Be based series. This Hallstatt cycle thus appears to be related to solar activity. Finally, we show that the grand minima and maxima occurred intermittently over the studied period, with clustering near lows and highs of the Hallstatt cycle, respectively.

[1] We studied 14 groups of French pottery fragments dated between the 4th and 16th centuries. The potsherds were analyzed using the Thellier and Thellier [1959] method, revised by Coe [1967]. Intensity values were corrected for thermoremanent magnetization (TRM) anisotropy and cooling rate dependence of TRM acquisition. We first analyzed modern ceramics produced following ancient techniques and fired in a wood-burning kiln inside of which field intensity was measured. The recovered mean intensity is within $3% of the expected value, which proves the reliability of our experimental procedure. Thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies. Our data indicate that large variations in intensity occurred in France over the last 2000 years. Two relative maxima in intensity are observed, one between the 8th and 10th centuries and the second between the 14th and 15th centuries. Similarities are observed between the archeointensity data from France and Ukraine, yielding some evidence for eastward drift of geomagnetic sources between western and eastern Europe from A.D. 800 to A.D. 1700. We also show that the dipole moment evolution proposed by McElhinny and Senanayake [1982] and Yang et al. [2000] for the last two millennia is likely biased toward higher values, mainly because of the absence of correction for the cooling rate dependence of TRM acquisition in most published archeointensity studies. We finally underline a possible relationship, valid at least in western Europe, between changes in direction and intensity of the geomagnetic field.

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Twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from Syria. These artifacts, archeologically well dated from ∼6000 B.C. to approximately A.D. 1200, have been analyzed using the Thellier and Thellier [1959] method as modified by Coe [1967]. Intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate. Our results indicate that field intensities were moderate in Syria from ∼6000 B.C. to ∼3500 B.C., with values of ∼30–40 μT. There was a significant increase in intensity by a factor of 2 from ∼3500 B.C. to ∼700 B.C., which was interrupted by a moderate decrease between ∼2550 B.C. and ∼1750 B.C. During more recent periods, our results show an intensity minimum approximately A.D. 200 and a maximum around the tenth century. Comparison with different data sets from the eastern Mediterranean and central Asia shows that geomagnetic field intensity variations were consistent at this large regional scale, at least over the last 5 millennia.