Time-dependent rotational stability of dynamic planets with elastic lithospheres

True polar wander (TPW), a reorientation of the rotation axis relative to the solid body, is driven by mass redistribution on the surface or within the planet and is stabilized by two aspects of the planet's viscoelastic response: the delayed viscous readjustment of the rotational bulge and the elastic stresses in the lithosphere. The latter, following Willemann (1984), is known as remnant bulge stabilization. In the absence of a remnant bulge, the rotation of a terrestrial planet is said to be inherently unstable. Theoretical treatments have been developed to treat the final (equilibrium) state in this case and the time-dependent TPW toward this state, including nonlinear approaches that assume slow changes in the inertia tensor. Moreover, remnant bulge stabilization has been incorporated into both equilibrium and linearized, time-dependent treatments of rotational stability. We extend the work of Ricard et al. (1993) to derive a nonlinear, time-dependent theory of TPW that incorporates stabilization by both the remnant bulge and viscous readjustment of the rotational bulge. We illustrate the theory using idealized surface loading scenarios applied to models of both Earth and Mars. We demonstrate that the inclusion of remnant bulge stabilization reduces both the amplitude and timescale of TPW relative to calculations in which this stabilization is omitted. Furthermore, given current estimates of mantle viscosity for both planets, our calculations indicate that departures from the equilibrium orientation of the rotation axis in response to forcings with timescale of 1 Myr or greater are significant for Earth but negligible for Mars

Tectonic events, continental intraplate volcanism, and mantle plume activity in northern Arabia: constraints form geochemistry and Ar-Ar dating of Syrian lavas

New (40)Ar/(39)Ar ages combined with chemical and Sr, Nd, and Pb isotope data for volcanic rocks from Syria along with published data of Syrian and Arabian lavas constrain the spatiotemporal evolution of volcanism, melting regime, and magmatic sources contributing to the volcanic activity in northern Arabia. Several volcanic phases occurred in different parts of Syria in the last 20 Ma that partly correlate with different tectonic events like displacements along the Dead Sea Fault system or slab break-off beneath the Bitlis suture zone, although the large volume of magmas and their composition suggest that hot mantle material caused volcanism. Low Ce/Pb (<20), Nb/Th (<10), and Sr, Nd, and Pb isotope variations of Syrian lavas indicate the role of crustal contamination in magma genesis, and contamination of magmas with up to 30% of continental crustal material can explain their (87)Sr/(86)Sr. Fractionation-corrected major element compositions and REE ratios of uncontaminated lavas suggest a pressure-controlled melting regime in western Arabia that varies from shallow and high-degree melt formation in the south to increasingly deeper regions and lower extents of the beginning melting process northward. Temperature estimates of calculated primary, crustally uncontaminated Arabian lavas indicate their formation at elevated mantle temperatures (T(excess) similar to 100-200 degrees C) being characteristic for their generation in a plume mantle region. The Sr, Nd, and Pb isotope systematic of crustally uncontaminated Syrian lavas reveal a sublithospheric and a mantle plume source involvement in their formation, whereas a (hydrous) lithospheric origin of lavas can be excluded on the basis of negative correlations between Ba/La and K/La. The characteristically high (206)Pb/(204)Pb (similar to 19.5) of the mantle plume source can be explained by material entrainment associated with the Afar mantle plume. The Syrian volcanic rocks are generally younger than lavas from the southern Afro-Arabian region, indicating a northward progression of the commencing volcanism since the arrival of the Afar mantle plume beneath Ethiopia/Djibouti some 30 Ma ago. The distribution of crustally uncontaminated high (206)Pb/(204)Pb lavas in Arabia indicates a spatial influence of the Afar plume of similar to 2600 km in northward direction with an estimated flow velocity of plume material on the order of 22 cm/a

Identification of a putative transcription factor gene (WBSCR11) that is commonly deleted in Williams-Beuren syndrome

Williams-Beuren syndrome (WBS) is a complex developmental disorder involving the hemizygous deletion of genes on chromosome 7q11.23. The cardiovascular aspects of the disorder are known to be caused by haploinsufficiency for ELN, but the genes contributing to the other features of WBS are still undetermined. Fifteen genes have been shown to reside within the WBS deletion, and here we report the identification and cloning of an additional gene that is commonly deleted. WBSCR11, which was identified through genomic DNA sequence analysis and cDNA library screening, was positioned toward the telomeric end of the WBS deletion. The gene is expressed in all adult tissues analyzed, including many regions of the brain. The predicted protein displays hemology to another gene from the WBS deletion, GTF2I, which is known to be a transcription factor. We postulate that WBSCR11 is also a transcription factor and may contribute to the spectrum of developmental symptoms found in WBS.link_to_subscribed_fulltex

Media 1: Phase-gradient contrast in thick tissue with a scanning microscope

Originally published in Biomedical Optics Express on 01 February 2014 (boe-5-2-407

Continuous reorientation of synchronous terrestrial planets due to mantle convection

A large fraction of known rocky exoplanets are expected to have been spun-down to a state of synchronous rotation, including temperate ones. Studies about the atmospheric and surface processes occurring on such planets thus assume that the day/night sides are fixed with respect to the surface over geological timescales. Here we show that this should not be the case for many synchronous exoplanets. This is due to True Polar Wander (TPW), a well known process occurring on Earth and in the Solar System that can reorient a planet without changing the orientation of its rotational angular momentum with respect to an inertial reference frame. As on Earth, convection in the mantle of rocky exoplanets should continuously distort their inertia tensor, causing reorientation. Moreover, we show that this reorientation is made very efficient by the slower rotation rate of synchronous planets. This is due to the weakness of their combined rotational/tidal bulge---the main stabilizing factor limiting TPW. Stabilization by an elastic lithosphere is also shown to be inefficient. We thus expect the axes of smallest and largest moment of inertia to change continuously over time but to remain closely aligned with the star-planet and orbital axes, respectively

Crustal and Upper Mantle Structures Beneath the Arabian Shield and Red Sea

The Arabian Shield and Red Sea region is considered one of only a few places in the world undergoing active continental rifting and formation of new oceanic lithosphere. We determined the seismic velocity structure of the crust and upper mantle beneath this region using broadband seismic waveform data. We estimated teleseismic receiver functions from high-quality waveform data. The raw data for RF analysis consist of 3-component broadband velocity seismograms for earthquakes with magnitudes Mw > 5.8 and epicentral distances between 30° and 90°. We performed several state-of-the-art seismic analyses of the KACST and SGS data. Teleseismic P- and S-wave travel time tomography provides an image of upper mantle compressional and shear velocities related to thermal variations. We present a multi-step procedure for jointly fitting surface-wave group-velocity dispersion curves (from 7 to 100 s for Rayleigh and 20 to 70 s for Love waves) and teleseismic receiver functions for lithospheric velocity structure. The method relies on an initial grid search for a simple crustal structure, followed by a formal iterative inversion, an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve surface-wave dispersion discrepancy. Inversions of receiver functions have poor sensitivity to absolute velocities. To overcome this shortcoming we have applied the method of Julia et al. (Geophys J Int 143:99–112, 2000), which combines surface-wave group velocities with receiver functions in formal inversions for crustal and uppermost mantle velocities. The resulting velocity models provide new constraints on crustal and upper mantle structure in the Arabian Peninsula. While crustal thickness and average crustal velocities are consistent with many previous studies, the results for detailed mantle structure are completely new. Finally, teleseismic shear-wave splitting was measured to estimate upper mantle anisotropy. These analyses indicate that stations near the Gulf of Aqabah display fast orientations that are aligned parallel to the Dead Sea Transform Fault, most likely related to the strike-slip motion between Africa and Arabia. The remaining stations across Saudi Arabia yield statistically the same result, showing a consistent pattern of north-south oriented fast directions with delay times averaging about 1.4 s. The uniform anisotropic signature across Saudi Arabia is best explained by a combination of plate and density driven flow in the asthenosphere. By combining the northeast oriented flow associated with absolute plate motion with the northwest oriented flow associated with the channelized Afar plume along the Red Sea, we obtain a north-south oriented resultant that matches our splitting observations and supports models of the active rifting processes. This explains why the north-south orientation of the fast polarization direction is so pervasive across the vast Arabian Plate. Seafloor spreading in the Red Sea is non-uniform, ranging from nearly 0.8 cm/a in the north to about 2 cm/a in the south. The Moho and LAB are shallowest near the Red Sea and become deeper towards the Arabian interior. Near the coast, the Moho is at a depth of about 22–25 km. Crustal thickening continues until an average Moho depth of about 35–40 km is reached beneath the interior Arabian Shield. The LAB near the coast is at a depth of about 55 km; however, it also deepens beneath the Shield to attain a maximum depth of 100–110 km. At the Shield-Platform boundary, a step is observed in the lithospheric thickness where the LAB depth increases to about 160 km. This study supports multi plume model, which states that there are two separated plumes beneath the Arabian Shield, and that the lower velocity zones (higher temperature zones) are related to volcanic activities and topographic characteristics on the surface of the Arabian Shield. In addition, our results suggest a two-stage rifting history, where extension and erosion by flow in the underlying asthenosphere are responsible for variations in LAB depth. LAB topography guides asthenospheric flow beneath western Arabia and the Red Sea, demonstrating the important role lithospheric variations play in the thermal modification of tectonic environments