ALBERT

Description: The mild compressional structures of Cenozoic age on the passive margins bordering Norway, the UK, the Faroes and Ireland have been the subject of much discussion in the literature. Nevertheless, their origin remains enigmatic. Candidate mechanisms must be able to explain the generation of sufficient stress to cause deformation, the episodic nature of the structures and why they developed where they did. We examine these mechanisms and conclude that multiple causes are probable, while favouring body force as potentially the most important agent. The geometry and setting of the structures are incompatible with gravitational sliding and toe-thrusting, probably the commonest compressive' structuring around the Atlantic margins. A passive mode of origin featuring drape or flank sedimentary loading probably emphasized some of the structures, but cannot be invoked as a primary mechanism. Likewise, reactivation of basement structure probably focused deformation but did not initiate it. Far-field orogenic stress from Alpine orogenic phases and from the West Spitsbergen-Eurekan folding and thrusting is also examined. This mechanism is attractive because of its potential to explain episodicity of the compressional structures. However, difficulties exist with stress transmission pathways from these fold belts, and the passive margin structures developed for much of their existence in the absence of any nearby contemporaneous orogeny. Breakup and plate spreading forces such as divergent asthenosheric flow have potential to explain early post-breakup compressional structuring, for example on the UK-Faroes margin, but are unlikely to account for later (Neogene) deformation. Ridge push, generally thought to be the dominant body force acting on passive margins, can in some circumstances generate enough stress to cause mild deformation, but appears to have low potential to explain episodicity. It is proposed here that the primary agent generating the body force was development of the Iceland Insular Margin, the significant bathymetric-topographic high around Iceland. Circumstantially, in Miocene times, this development may also have coincided with the acme of the compressional structures. We show that, dependent on the degree of lithosphere-asthenosphere coupling, the Iceland Plateau may have generated enough horizontal stress to deform adjacent margins, and may explain the arcuate distribution of the compressional structures around Iceland. Assuming transmission of stress through the basement we argue that, through time, the structures will have developed preferentially where the basement is hotter, weaker and therefore more prone to shearing at the relatively low stress levels. This situation is most likely at the stretched and most thermally-blanketed crust under the thickest parts of the young (Cretaceous-Cenozoic) basins. Although several elements of this model remain to be tested, it has the potential to provide a general explanation for passive margin compression at comparatively low stress levels and in the absence of nearby orogeny or gravitational sliding.

Description: Transform-margin development around the Arctic Ocean is a predictable geometric outcome of multi-stage spreading of a small, confined ocean under radically changing plate vectors. Recognition of several transform-margin stages in the development of the Arctic Ocean enables predictions to be made regarding tectonic styles and petroleum systems. The De Geer margin, connecting the Eurasia Basin (the younger Arctic Ocean) and the NE Atlantic during the Cenozoic, is the best known example. It is dextral, multi-component, features transtension and transpression, is implicated in microcontinent release, and thus bears close comparison with the Equatorial Shear Zone. In the older Arctic Ocean, the Amerasia Basin, Early Cretaceous counterclockwise rotation around a pole in the Canadian Mackenzie Delta was accommodated by a terminal transform. We argue on geometric grounds that this dislocation may have occurred at the Canada Basin margin rather than along the more distal Lomonosov Ridge, and review evidence that elements of the old transform margin were detached by the Makarov–Podvodnikov opening and accommodated within the Alpha–Mendeleev Ridge. More controversial is the proposal of transform along the Laptev–East Siberian margin. We regard an element of transform motion as the best solution to accommodating Eurasia and Makarov–Podvodnikov Basin opening, and have incorporated it into a three-stage plate kinematic model for Cretaceous–Cenozoic Arctic Ocean opening, involving the Canada Basin rotational opening at 125–80 Ma, the Makarov–Povodnikov Basin opening at 80–60 Ma normal to the previous motion and a Eurasia Basin stage from 55 Ma to present. We suggest that all three opening phases were accompanied by transform motion, with the right-lateral sense being dominant. The limited data along the Laptev–East Siberian margin are consistent with transform-margin geometry and kinematic indicators, and these ideas will be tested as more data become available over less explored parts of the Arctic, such as the Laptev–East Siberia–Chukchi margin.

Description: The Norwegian Atlantic margin, although frequently described as passive, has seen several significant and highly variable deformation events prior to and after early Cenozoic break-up. This chronology is strongly exemplified in the northern Vøring Basin, where deformation resulted in significant vertical motions, including deep erosion and sediment reworking. Post-break-up compressional deformation is well documented in the NE Atlantic margins, and is represented in the north Vøring Basin by the Vema and Naglfar domes. A prominent Maastrichtian–Paleocene pre-break-up phase of compression inverted the northern prolongation of the latest Turonian Vigrid Syncline. This syncline was the fairway for the approximately 1 km-thick Santonian–Campanian Nise Formation sandstone, shed from NE Greenland and/or the western Barents Sea margin. The inversion focused on the Vigrid Syncline axis, forming an anticline here referred to as the Vema–Nyk Anticline. The anticline may have been a major trap but was breached by erosion prior to collapse due to Late Paleocene extension. The remnant eastern half of the anticline is the Nyk High. The associated flanking syncline, the Någrind Syncline, also remains preserved. The collapsed side of the anticline is the Hel Graben, which itself was inverted in the Middle Miocene time forming the Naglfar and Vema domes. More speculatively, the development of the Vigrid Syncline and its bounding structural highs, the Gjallar Ridge and Utgard High, may also represent folds, marking the onset of compressional buckling in the mid-Norwegian–NE Greenland rift system. The repeated compressional deformation, as well as the extensional collapse, was focused on the area subjected to Early Cretaceous hyperextension. Compressional buckling under relatively low stress levels is proposed to have been due to significant lithosphere weakening caused by the hyperextension, whereby both high attenuation of the crystalline crust and serpentinization of the upper mantle contribute to the weakening. The Late Cenozoic compression post-dated the hyperextension by approximately 110 Ma, which suggests that the weakening is long-lived and that lithosphere has not been strengthened significantly through time.

Description: 〈p〉The Atlantic Ocean margins formed the basis for the seminal Wilson cycle concept, which suggests that oceans close, form fold belts, and later reopen in a concertina-like fashion. However, we observe that continental break-up of the North Atlantic–Arctic region only weakly reflects Wilson's concept. Rather than utilizing fold belts, transforms have been the dominant weaknesses that guided break-up, primarily because less force is required to break a plate via strike-slip related shearing than via rifting. Some transforms were inherited features, whereas others formed as part of the continental break-up process. Regardless of cause, once a transform has formed, the plate is broken and further rifting is not required before seafloor spreading can start. This is particularly well expressed in the NE Atlantic, where the line of Early Eocene break-up is very sharp, with minor or no preceding Paleocene rifting. Other examples include the De Geer, Ungava and Lomonosov transforms. We propose that the transform break-up mechanism is an important adjunct to the Wilson cycle theory and that it provides an explanation for ‘non-Wilson’ oceans, where old collision zones are not reactivated.〈/p〉

Description: The study focuses on the role of wrenching-involved continental break-up in microcontinent release, drawing from a review of examples. It indicates that the main groups of release mechanisms in this setting are associated with ‘competing wrench faults’, ‘competing horsetail structure elements’, ‘competing rift zones’ and ‘multiple consecutive tectonic events’ controlled by different stress regimes capable of release. Competing-wrench-fault-related blocks are small, up to a maximum 220 km in length. They are more-or-less parallel to oceanic transforms. The competing horsetail-structure-element-related blocks are larger (up to 610 km in length) and are located at an acute angle to the transform. Competing-rift-zone-related blocks are large (up to 815 km) and are either parallel or perpendicular to the transform. The multiple-consecutive-tectonic-event-related blocks have variable size and are generally very elongate, ranging up to 1100 km in length. The role of strike-slip faults in release of continental blocks resides in: linking the extensional zones, where the blocks are already isolated, by their propagation through the remaining continental bridges and subsequent displacement; facilitating rapid crustal thinning across a narrow zone of strike-slip-dominated faults; and slicing the margin into potentially detachable fault blocks.

Description: The Atlantic Ocean margins formed the basis for the seminal Wilson cycle concept, which suggests that oceans close, form fold belts, and later reopen in a concertina-like fashion. However, we observe that continental break-up of the North Atlantic–Arctic region only weakly reflects Wilson's concept. Rather than utilizing fold belts, transforms have been the dominant weaknesses that guided break-up, primarily because less force is required to break a plate via strike-slip related shearing than via rifting. Some transforms were inherited features, whereas others formed as part of the continental break-up process. Regardless of cause, once a transform has formed, the plate is broken and further rifting is not required before seafloor spreading can start. This is particularly well expressed in the NE Atlantic, where the line of Early Eocene break-up is very sharp, with minor or no preceding Paleocene rifting. Other examples include the De Geer, Ungava and Lomonosov transforms. We propose that the transform break-up mechanism is an important adjunct to the Wilson cycle theory and that it provides an explanation for ‘non-Wilson’ oceans, where old collision zones are not reactivated.

Description: The distribution of fitness effects of mutations is a factor of fundamental importance in evolutionary biology. We determined the distribution of fitness effects of 510 mutants that each carried between 1 and 10 mutations (synonymous and nonsynonymous) in the hisA gene, encoding an essential enzyme in the l -histidine biosynthesis pathway of Salmonella enterica . For the full set of mutants, the distribution was bimodal with many apparently neutral mutations and many lethal mutations. For a subset of 81 single, nonsynonymous mutants most mutations appeared neutral at high expression levels, whereas at low expression levels only a few mutations were neutral. Furthermore, we examined how the magnitude of the observed fitness effects was correlated to several measures of biophysical properties and phylogenetic conservation.We conclude that for HisA: (i) The effect of mutations can be masked by high expression levels, such that mutations that are deleterious to the function of the protein can still be neutral with regard to organism fitness if the protein is expressed at a sufficiently high level; (ii) the shape of the fitness distribution is dependent on the extent to which the protein is rate-limiting for growth; (iii) negative epistatic interactions, on an average, amplified the combined effect of nonsynonymous mutations; and (iv) no single sequence-based predictor could confidently predict the fitness effects of mutations in HisA, but a combination of multiple predictors could predict the effect with a SD of 0.04 resulting in 80% of the mutations predicted within 12% of their observed selection coefficients.

Description: We present a new approach for synthesis of GaN nanowires and microwires by metal organic chemical vapor deposition via a thin titanium film evaporated onto sapphire substrate prior to growth. Titanium etches a two-dimensional GaN layer deposited at the initial stage and GaN nanowires subsequently emerge at the boundaries of the etched grains. These wires grow at an exceptional elongation rate of 18 μ m/min and extend radially at a rate of 0.14 μ m/min. The GaN layer between the wires grows at a rate of 0.1 μ m/min. High material quality of these structures is confirmed by micro-photoluminescence spectroscopy. We investigate the initial nucleation stage, the time evolution of the wire length and diameter, the length and diameter distributions and speculate about a mechanism that yields the observed growth behavior.