Evolution of the Southwest Indian Ridge from 55°45′E to 62°E : changes in plate-boundary geometry since 26 Ma

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q06022, doi:10.1029/2006GC001559.From 55°45′E to 58°45′E and from 60°30′E to 62°00′E, the ultraslow-spreading Southwest Indian Ridge (SWIR) consists of magmatic spreading segments separated by oblique amagmatic spreading segments, transform faults, and nontransform discontinuities. Off-axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the SWIR. Individual magmatic segments show varying magnitudes and directions of asymmetric spreading, which requires that the shape of the plate boundary has changed significantly over time. In particular, since 26 Ma the Atlantis II transform fault grew by 90 km to reach 199 km, while a 45-km-long transform fault at 56°30′E shrank to become an 11 km offset nontransform discontinuity. Conversely, an oblique amagmatic segment at the center of a first-order spreading segment shows little change in orientation with time. These changes are consistent with the clockwise rotation of two ~450-km-wide first-order spreading segments between the Gallieni and Melville transform faults (52–60°E) to become more orthogonal to spreading. We suggest that suborthogonal first-order spreading segments reflect a stable configuration for mid-ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge-axis that can more easily accommodate seafloor spreading.Funding for this work came from a JOI-Schlanger Fellowship to Baines and NSF grant 0352054 to Cheadle and John

The Iceland Microcontinent and a continental Greenland-Iceland-Faroe Ridge

The breakup of Laurasia to form the Northeast Atlantic Realm was the culmination of a long period of tectonic unrest extending back to the Late Palaeozoic. Breakup was prolonged and complex and disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed, which are blanketed by lavas and underlain variously by magma-inflated, extended continental crust and mafic high-velocity lower crust of ambiguous and probably partly continental provenance. New rifts formed by diachronous propagation along old zones of weakness. North of the Greenland-Iceland-Faroe Ridge the newly forming rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis displaced ~ 150 km to the west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a transverse barrier. Thereafter, the ~ 400-km-wide latitudinal zone between the stalled rift tips extended in a distributed, unstable manner along multiple axes of extension that frequently migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day. It is the surface expression of underlying magma-assisted stretching of ductile mid- and lower continental crust which comprises the Icelandic-type lower crust that underlies the Greenland-Iceland-Faroe Ridge. This, and probably also one or more full-crustal-thickness microcontinents incorporated in the Ridge, are capped by surface lavas. The Greenland-Iceland-Faroe Ridge thus has a similar structure to some zones of seaward-dipping reflectors. The contemporaneous melt layer corresponds to the 3–10 km thick Icelandic-type upper crust plus magma emplaced in the ~ 10–30-km-thick Icelandic-type lower crust. This model can account for seismic and gravity data that are inconsistent with a gabbroic composition for Icelandic-type lower crust, and petrological data that show no reasonable temperature or source composition could generate the full ~ 40-km thickness of Icelandic-type crust observed. Numerical modeling confirms that extension of the continental crust can continue for many tens of Myr by lower-crustal flow from beneath the adjacent continents. Petrological estimates of the maximum potential temperature of the source of Icelandic lavas are up to 1450 °C, no more than ~ 100 °C hotter than MORB source. The geochemistry is compatible with a source comprising hydrous peridotite/pyroxenite with a component of continental mid- and lower crust. The fusible petrology, high source volatile contents, and frequent formation of new rifts can account for the true ~ 15–20 km melt thickness at the moderate temperatures observed. A continuous swathe of magma-inflated continental material beneath the 1200-km-wide Greenland-Iceland-Faroe Ridge implies that full continental breakup has not yet occurred at this latitude. Ongoing tectonic instability on the Ridge is manifest in long-term tectonic disequilibrium on the adjacent rifted margins and on the Reykjanes Ridge, where southerly migrating propagators that initiate at Iceland are associated with diachronous swathes of unusually thick oceanic crust. Magmatic volumes in the NE Atlantic Realm have likely been overestimated and the concept of a monogenetic North Atlantic Igneous Province needs to be reappraised. A model of complex, piecemeal breakup controlled by pre-existing structures that produces anomalous volcanism at barriers to rift propagation and distributes continental material in the growing oceans fits other oceanic regions including the Davis Strait and the South Atlantic and West Indian oceans

Topological photonic crystals in the visible: design and angle-resolved characterization of the bulk and edge states

We fabricated and characterized photonic crystals with non-trivial topological bulk states and pseudo-time-reversal-symmetry protected helical edge states in the visible regime. With a 30-keV electron beam, we excite coherent cathodoluminescence in the nanostructured material and derive photonic band structures

Transformational leadership and moral reasoning.

Terms such as moral and ethical leadership are used widely in theory, yet little systematic research has related a sociomoral dimension to leadership in organizations. This study investigated whether managers’ moral reasoning (n 132) was associated with the transformational and transactional leadership behaviors they exhibited as perceived by their subordinates (n 407). Managers completed the Defining Issues Test (J. R. Rest, 1990), whereas their subordinates completed the Multifactor Leadership Ques-tionnaire (B. M. Bass & B. J. Avolio, 1995). Analysis of covariance indicated that managers scoring in the highest group of the moral-reasoning distribution exhibited more transformational leadership behav-iors than leaders scoring in the lowest group. As expected, there was no relationship between moral-reasoning group and transactional leadership behaviors. Implications for leadership development are discussed. There has been growing interest in the development and pro-motion of moral or ethical leadership in organizations. Recent attention to this somewhat ethereal notion has created inspiring profiles (e.g., Coles, 2000; H. E. Gardner, 1996; J. W. Gardner, 1990) of leaders celebrated for their actions in commerce and history; however, few systematic attempts have been made to operationalize this dimension in relation to everyday leadership in organizations. To date, organizational researchers interested in the moral potential of leadership (e.g., Bass & Steidlmeier, 1999

Along-strike trace element and isotopic variation in Aleutian Island arc basalt: subduction melts sediments and dehydrates serpentine

Trace element and Sr-Nd-Pb isotope compositions of basaltic lavas from 11 volcanoes spanning 1300 km of the Aleutian Island arc provide new constraints on the recycling of elements in melts and fluids derived from subducted oceanic crust and sediment. Despite a nearly twofold variation in the flux of sediment subducted along the Aleutians, proxies indicating the presence of sediment melt in the magma source, including Th/La and Th/Nd, do not vary systematically along strike. In contrast, ratios including B/La, B/Nb, B/Be, Cs/La, Pb/Ce, and Li/Y suggest that the quantity or composition of fluid transferred from the slab into the mantle wedge varies an order of magnitude along strike and is apparently correlated with sediment flux. However, the most distinctive fluid addition corresponds spatially with subduction of the Amlia Fracture Zone (AFZ), a likely repository for H2O-rich serpentinite. Sr, Nd, and Pb isotope ratios, together with Th/Nd and B/La ratios, show that the majority of these basalts reflect a common baseline metasomatism of the mantle that accumulated, perhaps over millions of years, via small additions of both slab fluids and partially melted sediment. The paradox of requiring slab surface temperatures high enough to melt a layer of sediment, while lower-temperature dehydration reactions that supply water occur sufficiently deep to flux melting \u3e80 km beneath the volcanoes is reconciled in a four-stage model: (1) as sediment and altered ocean crust is carried to ∼60 km depth and temperatures increase to ∼650°C, metamorphic dehydration reactions release most of the fluid and B to the shallow mantle wedge beneath the fore arc, but some of this mantle is metasomatized and flows downward; (2) the uppermost layer of sediment begins to melt at ∼750°C and \u3e60 km depth; this small volume of melt physically mingles with the overlying metasomatized mantle wedge as it flows further downdip; (3) below the sediment veneer, the uppermost 1 km of ocean crust reaches 650°C at ∼90 km depth where antigorite breaks down, releasing B-rich H2O; and (4) this fluid infiltrates the layer of residual unmelted metasediment leaching the remaining inventory of fluid mobile elements and ascends into the modified mantle lowering its solidus and inducing partial melting. Where antigorite is likely abundant at the surface of the Pacific plate along the AFZ, deep H2O flux to the mantle is enhanced. This is reflected in higher B and lower incompatible element contents in the magmas, a relationship that links the amount of serpentine subducted to the extent of partial melting and the major element composition of arc basalt

Pathways Disrupted in Human ALS Motor Neurons Identified through Genetic Correction of Mutant SOD1

Direct electrical recording and stimulation of neural activity using micro-fabricated silicon and metal micro-wire probes have contributed extensively to basic neuroscience and therapeutic applications; however, the dimensional and mechanical mismatch of these probes with the brain tissue limits their stability in chronic implants and decreases the neuron–device contact. Here, we demonstrate the realization of a three-dimensional macroporous nanoelectronic brain probe that combines ultra-flexibility and subcellular feature sizes to overcome these limitations. Built-in strains controlling the local geometry of the macroporous devices are designed to optimize the neuron/probe interface and to promote integration with the brain tissue while introducing minimal mechanical perturbation. The ultra-flexible probes were implanted frozen into rodent brains and used to record multiplexed local field potentials and single-unit action potentials from the somatosensory cortex. Significantly, histology analysis revealed filling-in of neural tissue through the macroporous network and attractive neuron–probe interactions, consistent with long-term biocompatibility of the device.Stem Cell and Regenerative Biolog