Steady increase in geomagnetic reversal frequency after the Cretaceous Normal Superchron

The Earth's core is constantly and efficiently cooled by mantle convection. The heat flux transferred from the core to the mantle through the core-mantle boundary (CMB) is critical for understanding the dynamics of solid Earth. Although it is difficult to estimate the CMB heat flux, its history could be reconstructed from geomagnetic reversal frequency. However, overlooked short geomagnetic reversals may exist in the geomagnetic polarity time scale (GPTS), which affects the estimation of the heat flux history. Here, we report four new high-precision 40Ar/39Ar ages of the Oligocene Ethiopian traps. The traps may contain undiscovered reversals in marine magnetic anomaly. Based on the ages, we identified new reversals in Chron C12n, which was not found in marine magnetic anomalies. Our non-parametric analysis of GPTS suggests four potential periods of missing geomagnetic reversals, which correspond to long polarity intervals in GPTS. We found that C12n correspond to one of the periods. This indicates that several undetected reversals may exist within or near the edge of long polarity intervals after the Cretaceous Normal Superchron (prolonged stable polarity period). Considering the undetected reversals, we conclude that the CMB heat flux increased more slowly and monotonically after the Superchron than that ever estimated.

Obliquity of the Eastern Mediterranean Sea rifted margins: Reconciling structures and kinematic models.

<p>Orthogonal, oblique and transform rifted margins are defined by the comparison of the structural trend of the margin versus the orientation of the oceanic spreading ridge marked by marine magnetic anomalies. However, when neither transform fault nor marine magnetic anomalies can be identified in the oceanic domain, the determination of the obliquity of extension is delicate and deduced from the architecture of the rifted margins. This setting is illustrated by the Eastern Mediterranean Sea, which is a relic of an oceanic domain, now partly subducted northward underneath Anatolian, Aegean and Calabrian domains. Although the Southern and Eastern margins, from Malta to Lebanon, escaped compressional reactivation during Late Cretaceous and Cenozoic, their potential orthogonal, oblique or transform components have been the subject of extensive debates. Multiple geodynamic scenarios implying different ages and directions of oceanic opening have been proposed suggesting that either the southern or the eastern margins had a transform motion (or highly oblique).</p><p>In this contribution, we investigate the architecture of the different margin segments using 2D and 3D seismic data combined with available stratigraphic records and potential field maps. Based on these observations, we identified and mapped the different rift domains of the Eastern Mediterranean margins, adapting the terminology developed for hyper-extended rifted margins. The Eastern Mediterranean rifted margins are characterized by Mesozoic thick post-rift carbonate platforms developed over moderately thinned continental crust. Distal domains are dominated by thick sedimentary basins (>10 km) where the top basement is barely visible on reflection seismic data. Between the carbonate platform and the distal basin, the transition is always sharp (<30km in width) and marked by large normal faults. The resulting rift domain map highlights different structural trends, which are not coherent with a simple pair orthogonal-transform margins. Moreover, we reconstructed the extensional evolution of the former Northern and Western conjugate margins, which are now integrated in the Alps, Balkanides, Hellenides and Taurides by compiling boreholes and onshore geological data. These fossil margins recorded evidence for different tectonic extensional phases from Permian to Cretaceous.  </p><p>Our preliminary conclusion suggests that poly-phased and poly-directional extension led to distinct breakup ages in the Herodotus and northern Levant Basins. It results in the superposition of extensional structures of different orientations and ages, which inhibit the clear determination of orthogonal, oblique or transform margins. We tentatively explain this architectural complexity by the close position of the East Mediterranean Sea to the migrating rotation pole between Africa and Eurasia during the Mesozoic in relation with the Central Atlantic spreading to the West and the multiple subduction systems of the Neo-Tethys to the North.</p>

Geomagnetic Field Paleointensity during the Cretaceous Normal Superchron from Marine Magnetic Anomalies

<p>The knowledge of the geomagnetic field intensity during the Cretaceous Normal Superchron, a long term of forty million years without polarity reversals, may have a large impact on our understanding of the dynamo process occurring in Earth’s outer core. How, it is difficult to get the geomagnetic field behavior during the Cretaceous Normal Superchron resulting from the inadequate sampling or data of variable qualities from igneous rocks and sedimentary. Here we examine 20 magnetic anomaly profiles across the Cretaceous magnetic quiet zone of the Central Atlantic Ocean in the African flank extracted from the EMAG2v3, and calculate a synthetical magnetization profile based on the forward modeling method. We suggest that this profile records the high strength of geomagnetic field at the beginning of ~30 million years and low signal during the late period, which could be correlated with the low-resolution relative paleointensity record from the sediment samples at the Falkland Plateau, and which also could be found the VDMs/VADMs averaged by a 7-Ma sliding window from the absolute intensity records mostly from the MagIC database. Our results support the hypothesis that the distribution of heat flow along the core-mantle boundary is positively correlative to the intensity of the dipole field.</p>