Lateral Variations in Lower Crustal Strength Control the Temporal Evolution of Mountain Ranges: Examples From South-East Tibet

Controversy surrounds the rheology of the continental lithosphere, and how this rheology controls the evolution and behavior of mountain ranges. In this study, we investigate the effect of lateral contrasts in the strength of the lower crust, such as those between cratonic continental interiors and weaker rocks in the adjacent deforming regions, on the evolution of topography. We combine numerical modeling with recently published results from stable-isotope palaeoaltimetry in south-east Tibet. Stable-isotope palaeoaltimetry in this region provides constraints on vertical motions, which are required to distinguish between competing models for lithosphere rheology and deformation. We use numerical modeling to investigate the effect of lateral strength contrasts on the shape and temporal evolution of mountain ranges. In combination with palaeoaltimetry results, our modeling suggests that lateral strength contrasts provide a first-order control on the evolution of topography in south-east Tibet. We find that the evolution of topography in the presence of such strength contrasts leads to laterally varying topographic gradients, and to key features of the GPS- and earthquake-derived strain-rate field, without the need for a low-viscosity, lower-crustal channel. We also find that palaeoaltimetric samples may have been transported laterally for hundreds of kilometers, an effect which should be accounted for in their interpretation. Our results are likely to be applicable to the evolution of mountain ranges in general and provide an explanation for the spatial correlation between cratonic lowland regions and steep mountain range-fronts

Subduction tractions and vertical axis rotations in the Zagros-Makran transition zone, SE Iran: The 2013 May 11 M<inf>w</inf> 6.1 Minab earthquake

The source parameters and slip distribution of the 2013 May 11 Mw 6.1 Minab earthquake are studied using seismology, geodesy and field observations. We observe left-lateral strike-slip motion on a fault striking ENE–WSW; approximately perpendicular to previously studied faults in the Minab–Zendan–Palami fault zone. The fault that ruptured in 2013 is one of a series of ∼E–W striking left-lateral faults visible in the geology and geomorphology. These accommodate a velocity field equivalent to right-lateral shear on ∼N–S striking planes by clockwise rotations about vertical axes. The presence of these faults can reconcile differences in estimates of fault slip rates in the western Makran from GPS and Quaternary dating. The longitudinal range of shear in the western Makran is likely to be controlled by the distance over which the underthrusting Arabian lithosphere deepens in the transition from continent–continent collision in the Zagros to oceanic subduction in the Makran.This work forms part of the NERC- and ESRC-funded project ’EarthquakesWithout Frontiers‘. CP is funded by a NERC PhD studentship.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/gji/ggv20

Unexpected earthquake hazard revealed by Holocene rupture on the Kenchreai Fault (central Greece): Implications for weak sub-fault shear zones

High-resolution elevation models, palaeoseismic trenching, and Quaternary dating demonstrate that the Kenchreai Fault in the eastern Gulf of Corinth (Greece) has ruptured in the Holocene. Along with the adjacent Pisia and Heraion Faults (which ruptured in 1981), our results indicate the presence of closely-spaced and parallel normal faults that are simultaneously active, but at different rates. Such a configuration allows us to address one of the major questions in understanding the earthquake cycle, specifically what controls the distribution of interseismic strain accumulation? Our results imply that the interseismic loading and subsequent earthquakes on these faults are governed by weak shear zones in the underlying ductile crust. In addition, the identification of significant earthquake slip on a fault that does not dominate the late Quaternary geomorphology or vertical coastal motions in the region provides an important lesson in earthquake hazard assessment.This work forms part of the NERCand ESRC-funded project ‘Earthquakes Without Frontiers’, and was partly funded by the NERC grant ‘Looking Inside the Continents from Space’

Changes in the Status and Distribution of the Yellow-faced Bumble Bee

Bombus vosnesenskii, the distinctively-patterned Yellow-faced Bumble Bee, has undergone a significant and rapid range extension in British Columbia. Known initially from a single record of a few specimens at Osoyoos in 1951, it was put forward in 1996 as a species that warranted a threatened or endangered status because of its severely restricted range in the province. However, since 2000, the species has expanded north in the Okanagan Valley, west to the Similkameen Valley and, especially, has become firmly established in south coastal regions of the province, including Vancouver Island. Population increases in B. vosnesenskii to the south of BC have also been reported. The reasons for the rapid expansion of B. vosnesenskii in BC are unclear. Particularly in lowland southwestern BC, the range expansion might have been enhanced through escapes from colonies kept as pollinators of agricultural crops. The spread of B. vosnesenskii has coincided with the decline of B. occidentalis, so the former may have been introduced or naturally expanded its range at the same time as a niche was becoming vacant

Towards a Microscopic Model of Magnetoelectric Interactions in Ni3V2O8

We develop a microscopic magnetoelectric coupling in Ni$_3$V$_2$O$_8$ (NVO) which gives rise to the trilinear phenomenological coupling used previously to explain the phase transition in which magnetic and ferroelectric order parameters appear simultaneously. Using combined neutron scattering measurements and first-principles calculations of the phonons in NVO, we determine eleven phonons which can induce the observed spontaneous polarization. Among these eleven phonons, we find that a few of them can actually induce a significant dipole moment. Using the calculated atomic charges, we find that the required distortion to induce the observed dipole moment is very small (~0.001 \AA) and therefore it would be very difficult to observe the distortion by neutron-powder diffraction. Finally, we identify the derivatives of the exchange tensor with respect to atomic displacements which are needed for a microscopic model of a spin-phonon coupling in NVO and which we hope will be obtained from a fundamental quantum calculation such as LDA+U. We also analyze two toy models to illustrate that the Dzyaloskinskii-Moriya interaction is very important for coexisting of magnetic and ferroelectric order but it is not the only mechanism when the local site symmetry of the system is low enough.Comment: 20 pages, 10 figure

Observations and dynamical implications of active normal faulting in South Peru

SUMMARY Orogenic plateaus can exist in a delicate balance in which the buoyancy forces due to gravity acting on the high topography and thick crust of the plateau interior are balanced by the compressional forces acting across their forelands. Any shortening or extension within a plateau can indicate a perturbation to this force balance. In this study, we present new observations of the kinematics, morphology and slip rates of active normal faults in the South Peruvian Altiplano obtained from field studies, high-resolution DEMs, Quaternary dating and remote sensing. We then investigate the implications of this faulting for the forces acting on the Andes. We find that the mountains are extending ∼NNE–SSW to ∼NE–SW along a normal fault system that cuts obliquely across the Altiplano plateau, which in many places reactivates Miocene-age reverse faults. Radiocarbon dating of offset late Quaternary moraines and alluvial fan surfaces indicates horizontal extension rates across the fault system of between 1 and 4 mm yr–1—equivalent to an extensional strain rate in the range of 0.5–2 × 10−8 1 yr–1 averaged across the plateau. We suggest the rate and pattern of extension implies there has been a change in the forces exerted between the foreland and the Andes mountains. A reduction in the average shear stresses on the sub-Andean foreland detachment of ≲4 MPa (20–25 per cent of the total force) can account for the rate of extension. These results show that, within a mountain belt, the pattern of faulting is sensitive to small spatial and temporal variations in the strength of faults along their margins.Denman Baynes Senior Studentship, Clare College Cambridge Arup Santander Mobility Grant (University of Cambridge

Neutron scattering and scaling behavior in URu2Zn20 and YbFe2Zn20

The dynamic susceptibility chi"(deltaE), measured by inelastic neutron scattering measurements, shows a broad peak centered at Emax = 16.5 meV for the cubic actinide compound URu2Zn20 and 7 meV at the (1/2, 1/2, 1/2) zone boundary for the rare earth counterpart compound YbFe2Zn20. For URu2Zn20, the low temperature susceptibility and magnetic specific heat coefficient gamma = Cmag/T take the values chi = 0.011 emu/mole and gamma = 190 mJ/mole-K2 at T = 2 K. These values are roughly three times smaller, and Emax is three times larger, than recently reported for the related compound UCo2Zn20, so that chi and gamma scale inversely with the characteristic energy for spin fluctuations, Tsf = Emax/kB. While chi(T), Cmag(T), and Emax of the 4f compound YbFe2Zn20 are very well described by the Kondo impurity model, we show that the model works poorly for URu2Zn20 and UCo2Zn20, suggesting that the scaling behavior of the actinide compounds arises from spin fluctuations of itinerant 5f electrons.Comment: 7 pages, 5 figure