Radiometric dates of uplifted marine fauna in Greece:Implications for the interpretation of recent earthquake and tectonic histories using lithophagid dates

n AD 365 a great (Mw N 8) earthquake lifted up western Crete, exposing a shoreline encrusted by marine organisms, and up to 10 m of marine substrate beneath it. Radiocarbon ages determined for corals and bryozoans exposed between the paleo-shoreline and present sea level are consistent, within measurement error, with each other and with the date of the earthquake. But radiocarbon ages determined for the boring bivalve Lithophaga lithophaga found on the same substrate are at least 350 years, and up to 2000 years, older than the date of the earthquake that lifted them above sea level. These observations reveal two important effects that limit the use of radiocarbon lithophagid ages in tectonic and paleoseismological studies. The first is that the exceptional preservation potential of lithophagids allows them to remain intact and in situ long after natural death, while the substrate continues to be colonised until eventual uplift. The second, which we confirm with radiocarbon analysis of museum specimens of known age, is the incorporation of old (14C-free) carbon into lithophagid shells from the limestone host rock into which the lithophagids bored. The two effects are both significant in Crete and central Greece, and can cause the radiocarbon lithophagid ages to be up to 2000 years older than the uplift event which exposed them. Understanding these effects is important because lithophagids are far more abundantly preserved, and used to date uplift, than most other marine organisms. This study shows that they can rarely be used to distinguish uplift events, or date them to better than 1000 years, or even to distinguish whether observed uplift occurred in a single or in multiple events. After taking account of these uncertainties, the ages of the lithophagids are, however, consistent with the hypothesis that the highest prominent marine notches and exposed lithophagid holes within a few metres of sea level in Greece formed when sea level became relatively stable ~ 6000 years ago, following rapid rise after the last glacial maximum

Formation of Corrugated n = 1 2D Tin Iodide Perovskites and Their Use as Lead-Free Solar Absorbers

Major strides have been made in the development of materials and devices based around low-dimensional hybrid group 14 metal halide perovskites. Thus far, this work has mostly focused upon compounds containing highly toxic Pb, with the analogous less toxic Sn materials being comparatively poorly evolved. In response, the study herein aims to (i) provide insight into the impact of templating cation upon the structure of n = 1 2D tin iodide perovskites (where n refers to the number of contiguous two-dimensional (2D) inorganic layers, i.e., not separated by organic cations), and (ii) examine their potential as light absorbers for photovoltaic (PV) cells. It was discovered through systematic tuning of organic dications, that imidazolium rings are able to induce formation of (110)-oriented materials, including the examples of “3 × 3” corrugated Sn-I perovskites. This structural outcome is a consequence of a combination of supramolecular interactions of the two endocyclic N-atoms in the imidazolium functionalities with the Sn-I framework and the higher tendency of Sn2+ ions to stereochemically express their 5s2 lone pairs relative to the 6s2 electrons of Pb2+. More importantly, the resulting materials feature very short separations between their 2D inorganic layers with iodide–iodide (I···I) contacts as small as 4.174 Å, which is amongst the shortest ever recorded for 2D tin iodide perovskites. The proximate inorganic distances, combined with the polarizable nature of the imidazolium moiety, eases the separation of photogenerated charge within the materials. This is evident from the excitonic activation energies as low as 83(10) meV, measured for ImEA[SnI4]. When combined with superior light absorption capabilities relative to their lead congeners, this allowed fabrication of lead-free solar cells with incident photon-to-current and power conversion efficiencies of up to 70 % and 2.26 %, respectively, which are amongst the highest values reported for pure n = 1 2D group 14 metal halide perovskites. In fact, these values are superior to the corresponding lead iodide material, which demonstrates that 2D Sn-based materials have significant potential as less toxic alternatives to their Pb counterparts

A physical model for the motion of the Sierra Block relative to North America.

We explain the motion of the Sierra Block with respect to North America by treating it as a rigid inclusion within a thin sheet of non-Newtonian fluid, representing the continental lithosphere of North America. Deformation within North America is driven by a constant velocity applied to its western boundary by the Pacific plate. The deforming lithosphere applies forces to the edges of the block, and the requirement that these forces sum to zero leads to an approximate analytical solution in which the speed of the block, normalized by the rate of plate relative motion, depends on the dimensions of the block, its distance from the boundary, and on the power-law index, n, in the constitutive relation for the non-Newtonian fluid. Numerical solutions to the thin viscous sheet equation confirm the validity of this approximation. The speed of the Sierra Block with respect to North America is consistent with that of a rigid body, of the same shape and position, embedded within a thin viscous sheet of non-Newtonian fluid having a power-law index, n, between 2 and 6. A tighter constraint on n is provided by profiles of velocity, taken in the direction perpendicular to plate relative motion; comparison between observed profiles and those from a thin sheet model support a value of n 3 for the index

Constraints from GPS measurements on the dynamics of the zone of convergence between Arabia and Eurasia

We investigate the dynamics of deformation in the zone of convergence between the Arabian and Eurasian plates using a physical model that treats the lithosphere as a thin fluid sheet deforming in response to lateral variations in gravitational potential energy (GPE). This model has two free material parameters, the power-law exponent, n, of the vertically-averaged rheology of the lithosphere, and the Argand number, Ar, which expresses the relative importance of GPE and stresses required to deform the lithosphere. With boundary conditions described by three free parameters, the model fits the observed deformation, as measured by 367 GPS velocities, with a root-mean-square residual of <2.4 mm/yr. We find negligible improvement when variations in material properties are introduced that represent increases or decreases in the lithospheric strength of the Central Iranian and Turkish-Iranian Plateaux and the Zagros Mountains. Effective viscosity of the lithosphere ranges from 5 × 1022 Pa s at 10 nanostrain/yr to 1022 Pa s at 100 nanostrain/yr. As well as matching the decadal-timescale, predominantly interseismic, strain rates determined from GPS, the computed distribution of strain rates is also consistent with the distribution and types of earthquake focal mechanism. Significant historical earthquake activity is seen in regions with strain rates lower than 20 nanostrain/yr, implying that it is prudent to base assessments of seismic hazard on regional strain rates

A physical model for the motion of the Sierra Block relative to North America

We explain the motion of the Sierra Block with respect to North America by treating it as a rigid inclusion within a thin sheet of non-Newtonian fluid, representing the continental lithosphere of North America. Deformation within North America is driven by a constant velocity applied to its western boundary by the Pacific plate. The deforming lithosphere applies forces to the edges of the block, and the requirement that these forces sum to zero leads to an approximate analytical solution in which the speed of the block, normalized by the rate of plate relative motion, depends on the dimensions of the block, its distance from the boundary, and on the power-law index, n, in the constitutive relation for the non-Newtonian fluid. Numerical solutions to the thin viscous sheet equation confirm the validity of this approximation. The speed of the Sierra Block with respect to North America is consistent with that of a rigid body, of the same shape and position, embedded within a thin viscous sheet of non-Newtonian fluid having a power-law index, n, between 2 and 6. A tighter constraint on n is provided by profiles of velocity, taken in the direction perpendicular to plate relative motion; comparison between observed profiles and those from a thin sheet model support a value of n 3 for the index

Archimedes and the Tauern eclogites: the role of buoyancy in the preservation of exotic eclogite blocks

The eclogite fragments of the Tauern Window formed at pressures around 20 kbar and temperatures in the region 600-650°C; these pressures are higher by 10-12 kbar than those experienced by the units now surrounding the eclogite-bearing zone. The eclogites probably formed in a subduction zone prior to the main Austroalpine collision: downward shearing forces dominated over buoyancy forces in the subduction zone mélange, permitting the subduction of relatively light material to great depth. At the cessation of subduction this buoyant material returned to the surface, carrying eclogite blocks that were too small to sink rapidly through it. A similar mechanism could account for the emplacement of certain Alpine-type garnet peridotites