Metamorphic Changes Research Articles

Two lithospheric profiles across southern California derived from gravity and seismic data

Abstract We present two detailed 2-D density transects for the crust and uppermost mantle across southern California using a linear gravity inversion technique. This technique parameterizes the crust and upper mantle as a set of blocks that are based on published geologic and seismic models. Each block can have a range of densities that are constrained where possible by borehole measurements, seismic velocities, and petrologic data. To further constrain the models, it is assumed that the lithosphere is close to isostatic equilibrium at both ends of the profiles, in the deep ocean and east of the Mojave Desert. We calculate the lithostatic pressure variations field for the whole cross section to rule out the geophysically insignificant solutions. In the linear equation, ρ = a + bV ( V , seismic P-wave velocity; ρ , density), which approximates the mantle density–velocity ( ρ – V ) relationship, different coefficients for b were evaluated. Lower coefficients ( b b > 0.3) imply that other effects, such as composition and/or metamorphic changes, play an important role in the mantle. Density models were constructed with the coefficient b ranging from 0 to 0.6. The results indicate that a high b value in the mantle ρ – V relationship is associated with less dense crust in the Mojave block and more dense crust in the Catalina schist block. In the less dense Mojave block, the average density of the whole crust is ∼2.75 g/cm 3 , while that of the lower crust is ∼2.72 g/cm 3 . These densities imply a high silica content in the crust, and a minor fraction of basic rock in the lower crust, or perhaps the absence of a basaltic layer altogether. By comparison, the average density of a typical continental stable platform is ∼2.85 g/cm 3 . Models with higher b coefficients (0.5–0.6) are characterized by a large isostatic imbalance. On the other hand, lower b values (0–0.2) require a consolidated whole crust density in the Mojave Desert of ∼2.78 g/cm 3 , and a lower crust density of ∼2.89 g/cm 3 with mostly basaltic composition. This contradicts the observed, lower V p / V s -ratio in the Mojave Desert associated with mostly felsic and low-density crust. Models with lower b coefficients (0.1–0.2) are characterized by an absence of local Airy compensation beneath the San Gabriel Mountains at the LARSE-1 profile. These, and other non-gravity arguments, suggest optimal solutions to the mantle ρ – V relation of b ∼ 0.2–0.4. This, in turn, means that both thermal and petrological effects occur inside the downwelling of the uppermost mantle high velocity body located beneath the Transverse Ranges. During the development of this mantle downwelling, the basaltic layer of the Mojave block was likely eroded and pulled down into the high velocity body. Those basaltic fragments may have been transformed into eclogites, and this metamorphic change implies a higher b -coefficient density–velocity relationship than would be expected for a purely thermal process.

Interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the Pannonian Basin

SUMMARY The geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P-wave interval velocities to Meissner et al.’s activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes.

Feedback of the metamorphic changes on the subducting processes

Feedback of the metamorphic changes on the subducting processes

Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England

AbstractThe Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.

Electron transport chain activity during insect metamorphosis

The midgut of the caterpillar, Manduca sexta, is an obligatorily aerobic tissue that is fated to be destroyed at larval-pupal metamorphosis. Top-down metabolic control analysis has revealed that the substrate oxidation system (metabolite transporters, citric acid cycle, electron transport chain) confers most of the control over mitochondrial oxygen consumption and the activity of this system is greatly depressed during the early stages of metamorphosis (Am. J. Physiol. 287:R314–R321). In order to determine if there are metamorphic changes in the electron transport chain, the maximal activities of Complexes I, II, III and IV (cytochrome c oxidase) were determined in mitochondria isolated from midguts of feeding fifth instar larvae (day 2) and larvae committed to pupation (day 5 “wandering larvae”). The activity of Complex IV was determined polargraphically, whereas the activities of the other complexes were measured spectrophotometrically. There were no significant changes in the activities of Complexes I, II, and III during development. In contrast, cytochrome c oxidase activity was 38% higher in mitochondria isolated from pre-commitment larvae compared to that of wandering larvae. If, however, these activities are normalized to cytochrome aa3 content, there is no difference in cytochome c oxidase activity. These results indicate that the loss of cytochrome aa3 from the mitochondria is responsible for the decline in cytochrome c oxidase activity and this loss may play a part in the inhibition of the substrate oxidation system that occurs early in metamorphosis. This research was funded by the National Science Foundation (IBN-0131523).

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km 2 in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.

Thyroid hormone-induced expression of a bZip-containing transcription factor activates epithelial cell proliferation during Xenopus larval-to-adult intestinal remodeling

In the intestine during amphibian metamorphosis, stem cells appear, actively proliferate, and differentiate into an adult epithelium analogous to the mammalian counterpart. To clarify the molecular mechanisms regulating this process, we focused on a bZip-containing transcription factor (TH/bZip). We previously isolated TH/bZip from the Xenopus intestine as one of the candidate genes involved in adult epithelial development. Northern blot and in situ hybridization analyses showed that the transient and region-dependent expression of TH/bZip mRNA correlates well with the growth of adult epithelial primordia originating from the stem cells throughout the Xenopus intestine. To investigate its role in the adult epithelial development, we established an in vitro gene transfer system by using electroporation and organ culture techniques, and we overexpressed TH/bZip in the epithelium of Xenopus tadpole intestines. In the presence of thyroid hormone (TH) where the adult epithelial primordia appeared after 3 days of cultivation, overexpression of TH/bZip significantly increased their proliferating activity. On the other hand, in the absence of TH where the epithelium remained as larval-type without any metamorphic changes, ectopic expression of TH/bZip significantly increased the proliferating activity of the larval epithelium but had no effects on its differentiated state. These results indicate that TH/bZip functions as a growth activator during amphibian intestinal remodeling, although TH/bZip expression in the epithelium alone is not sufficient for inducing the stem cells.

Urochordate βγ-Crystallin and the Evolutionary Origin of the Vertebrate Eye Lens

A refracting lens is a key component of our image-forming camera eye; however, its evolutionary origin is unknown because precursor structures appear absent in nonvertebrates. The vertebrate betagamma-crystallin genes encode abundant structural proteins critical for the function of the lens. We show that the urochordate Ciona intestinalis, which split from the vertebrate lineage before the evolution of the lens, has a single gene coding for a single domain monomeric betagamma-crystallin. The crystal structure of Ciona betagamma-crystallin is very similar to that of a vertebrate betagamma-crystallin domain, except for paired, occupied calcium binding sites. The Ciona betagamma-crystallin is only expressed in the palps and in the otolith, the pigmented sister cell of the light-sensing ocellus. The Ciona betagamma-crystallin promoter region targeted expression to the visual system, including lens, in transgenic Xenopus tadpoles. We conclude that the vertebrate betagamma-crystallins evolved from a single domain protein already expressed in the neuroectoderm of the prevertebrate ancestor. The conservation of the regulatory hierarchy controlling betagamma-crystallin expression between organisms with and without a lens shows that the evolutionary origin of the lens was based on co-option of pre-existing regulatory circuits controlling the expression of a key structural gene in a primitive light-sensing system.

Olfactory metamorphosis in the Coastal Giant Salamander (Dicamptodon tenebrosus)

This study examined the gross morphology and ultrastructure of the olfactory organ of larvae, neotenic adults, and terrestrial adults of the Coastal Giant Salamander (Dicamptodon tenebrosus). The olfactory organ of all aquatic animals (larvae and neotenes) is similar in structure, forming a tube extending from the external naris to the choana. A nonsensory vestibule leads into the main olfactory cavity. The epithelium of the main olfactory cavity is thrown into a series of transverse valleys and ridges, with at least six dorsal and nine ventral valleys lined with olfactory epithelium, and separated by ridges of respiratory epithelium. The ridges enlarge with growth, forming large flaps extending into the lumen in neotenes. The vomeronasal organ is a diverticulum off the ventrolateral side of the main olfactory cavity. In terrestrial animals, by contrast, the vestibule has been lost. The main olfactory cavity has become much broader and dorsoventrally compressed. The prominent transverse ridges are lost, although small diagonal ridges of respiratory epithelium are found in the lateral region of the ventral olfactory epithelium. The posterior and posteromedial wall of the main olfactory cavity is composed of respiratory epithelium, in contrast to the olfactory epithelium found here in aquatic forms. The vomeronasal organ remains similar to that in large larvae, but is now connected to the mouth by a groove that extends back through the choana onto the palate. Bowman's glands are present in the main olfactory cavity at all stages, but are most abundant and best developed in terrestrial adults. They are lacking in the lateral olfactory epithelium of the main olfactory cavity. At the ultrastructural level, in aquatic animals receptor cells of the main olfactory cavity can have cilia, short microvilli, a mix of the two, or long microvilli. Supporting cells are of two types: secretory supporting cells with small, electron-dense secretory granules, and ciliated supporting cells. Receptor cells of the vomeronasal organ are exclusively microvillar, but supporting cells are secretory or ciliated, as in the main olfactory cavity. After metamorphosis two distinct types of sensory epithelium occur in the main olfactory cavity. The predominant epithelium, covering most of the roof and the medial part of the floor, is characterized by supporting cells with large, electron-lucent vesicles. The epithelium on the lateral floor of the main olfactory cavity, by contrast, resembles that of aquatic animals. Both types have both microvillar and ciliated receptor cells. No important changes are noted in cell types of the vomeronasal organ after metamorphosis. A literature survey suggests that some features of the metamorphic changes described here are characteristic of all salamanders, while others appear unique to D. tenebrosus.

A Role for Adenosine Deaminase in Drosophila Larval Development

Adenosine deaminase (ADA) is an enzyme present in all organisms that catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine. Both adenosine and deoxyadenosine are biologically active purines that can have a deep impact on cellular physiology; notably, ADA deficiency in humans causes severe combined immunodeficiency. We have established a Drosophila model to study the effects of altered adenosine levels in vivo by genetic elimination of adenosine deaminase-related growth factor-A (ADGF-A), which has ADA activity and is expressed in the gut and hematopoietic organ. Here we show that the hemocytes (blood cells) are the main regulator of adenosine in the Drosophila larva, as was speculated previously for mammals. The elevated level of adenosine in the hemolymph due to lack of ADGF-A leads to apparently inconsistent phenotypic effects: precocious metamorphic changes including differentiation of macrophage-like cells and fat body disintegration on one hand, and delay of development with block of pupariation on the other. The block of pupariation appears to involve signaling through the adenosine receptor (AdoR), but fat body disintegration, which is promoted by action of the hemocytes, seems to be independent of the AdoR. The existence of such an independent mechanism has also been suggested in mammals.

Genetic analysis of metamorphic and premetamorphic Xenopus ciliary marginal zone

A major event affecting the eye during amphibian metamorphosis is an asymmetrical growth of the ventrotemporal portion of the retina compared with its dorsonasal counterpart. This event is due to an increased proliferation of the precursors of the ventral ciliary marginal zone (CMZ). Here, we analyze the expression patterns of several key homeobox genes implicated in eye development (Xrx1, Xvax2, Xsix3, Xpax6, Xchx10, Xotx2) to understand whether they are active at the time in which the metamorphic changes of the retina occur. We also analyze their expression patterns in the ventral and dorsal CMZ and compare them with bromodeoxyuridine incorporation in the CMZ. Our results suggest that the metamorphic CMZ maintains the functional subdivisions described during embryonic development. Moreover, we find that genes involved in proliferation and cell type determination of the embryonic retina are actively transcribed in the proliferating CMZ, thus indicating a potential regulatory role for these genes in the metamorphic retina.

Thermo-mechanical model of the mantle wedge in Central Mexican subduction zone and a blob tracing approach for the magma transport

The origin of the Central Mexican Volcanic Belt (CMVB) and the influence of the subducting Cocos plate on the CMVB volcanism are still controversial. In this study, the temperature and mantle wedge flow models for the Mexican subduction zone are developed using the finite element method to investigate the thermal structure below CMVB. The numerical scheme solves a system of 2D Stokes equations and 2D steady-state heat transfer equation. Two models are considered for the mantle wedge: the first one with an isoviscous mantle wedge and the second one with strong temperature-dependent viscosity. The first model reveals a maximum temperature of ∼830 °C in the mantle wedge, which is not sufficient for melting of wet peridotite. Also, the geotherm of the subducting plate upper surface does not intersect the dehydration-melting solidus for mafic minerals. The second model predicts temperatures of more than 1200 °C beneath the CMVB for a wide range of rheological parameters (reference viscosity and activation energy). Up to 0.6 wt.% H 2O can be released down to 60 km depth through metamorphic changes in the oceanic crust of the subducting slab. The melting of this oceanic crust apparently occurs in a narrow depth range of 50–60 km and also melting of the mantle wedge hydrated peridotite is now expected to take place beneath CMVB. Considering that the melting processes on and in the vicinity of the subducting plate surface generate the most of the volcanic material, a dynamic model for the blob tracers is developed using Stokes flow at infinite Prandtl number. The blobs of 0.2–10.0 km in diameter migrate along very different trajectories only at low wrapping viscosities ( η w = 10 14–5 × 10 17 Pa s). The modeling results show that the “fast” trajectories terminate at the same focus location at the base of the continental crust, while the arrival points of “slow” trajectories, which are common for the blobs of smaller size (∼0.4–0.5 km), are scattered away from the average focus location. This observation may give us a hint on a possible mechanism of strato and mono volcanoes genesis. The rise time, which the blob detached from the subducted plate, needs to reach the bottom of the continental crust, is from 0.001 up to 14 million years depending on the blob diameter and surrounding viscosity.

Swinburne

Swinburne Margot K. Louis (bio) For many years impassioned Swinburneans have tried to make the scholarly world take Swinburne seriously, and have made some progress. Recently, however, at diverse cultural levels informed by, yet largely dissociated from Academe, something quite unexpected and astounding has happened: in rock, in jazz, in poetry, in fiction from magic realism to the crime novel, Swinburne is emerging as a living force in our own culture. In short (and as a scholar I shudder as I key in the words), Swinburne is cool. That it should come to this! Let us begin soberly, with two splendidly uncool items illuminating the poet's social and intellectual context. Those interested in Swinburne's family history can now visit The Swinburn(e) Family History Site at http://www.geocities.com/Athens/Atlantis/8805/index.html, but will pay special [End Page 401] attention to the site "Sir John Swinburne of Capheaton, Northumberland, England" at http://www.geocities.com/Athens/Atlantis/8805/sirjohn.html; this site provides basic information on the Swinburne family (beginning with a John Swinburne born c. 1612), along with photographs of Capheaton Hall and other relevant locations. At the entry on the poet, there is a link to a slightly fuller outline of Swinburne's career. An exploration of these sites deepens one's sense of the degree to which this poet was embedded in the English gentry. At the same time, of course, Swinburne was deeply involved with French culture, and this relationship deserves far greater attention than it has ever received. While various studies have explored his response to Sade, Hugo, Baudelaire, and Mallarmé, much more remains to be done in this area. Anyone working on Swinburne's connections with France would do well to consult Michael Kelly's "Comparing French and British Intellectuals: Towards a Cross-Channel Perspective" (French Cultural Studies 14, no. 3 [October 2003]: 336-348). Kelly has little to say of Swinburne directly, although he begins by discussing Jill Forbes, an academic who published on Swinburne in the 1970s and wrote a D.Phil. thesis on the poet's influence in France. The value of Kelly's article lies in his clarification of the practical and theoretical challenges in comparative discussions of French and British thinkers, and also in his outline of a wider "conceptual project around which a comparative study could be organized," focusing on "the forms of association, communication and action," and analysis of intellectuals' "relationship to the state, and to the institutions of political and social power" (p. 345). Significant criticism on Swinburne himself has appeared this year. Jerome McGann, whose Swinburne: An Experiment in Criticism (1972) may be said to have initiated modern Swinburne criticism, has happily returned to the field with "Swinburne's Radical Artifice; or, The Comedian as A.C." (Modernism/Modernity 11, no. 2 [2004]: 205-218). This richly celebratory article presents Swinburne himself as "a wonder, a terror, and finally a catalyst for great social and cultural change," and his aesthetic program as "a great, neglected cultural and scholarly obligation" (pp. 207, 206). McGann's rapid, sweeping, but deeply informed review of the scholarly tradition points out how responses to Swinburne by T. S. Eliot, Henry Adams, and William Empson were inflected by the critics' own cultural agendas, and points out that the fundamental issue at stake was and remains Swinburne's opposition to the replacement of "individual imaginative life" by institutional religion "at the center of human social and interpersonal exchange" (p. 212). By emphasizing poetry's self-reflexivity and its "Ovidian capacities for infinite metamorphic change and transformation," Swinburne reaffirms the individual imaginative life (pp. 212-213). [End Page 402] Brief but provocative close analysis of "Anactoria" and "The Garden of Proserpine" illustrates McGann's argument. For a generation, against the repudiation of Eliot expressed by such critics as Rikky Rooksby (and, I must add, myself), deconstructionist critics have celebrated Eliot's charge (that Swinburne provides only the hallucination of meaning) as a demonstration of Swinburne's sophistication and proto-Derridean genius. McGann absorbs but also transcends this line of argument: "Searching poems for their meanings, we often forget that in poetry, language is not a vehicle of reference...

Metamorphosis and developmental evolution in Ophionereis (Echinodermata: Ophiuroidea)

The brittle star Ophionereis schayeri has abbreviated non-feeding development through a reduced ophiopluteus and a vitellaria larva. The metamorphic changes involved with development from a bilateral larva to a radial juvenile were examined in detail. The reduced ophiopluteus has a continuous ciliated band that breaks up into discontinuous ciliary ridges at the vitellaria stage. As the vitellaria develops, the juvenile rudiment forms in the mid-ventral region. The rudiment then undergoes a morphogenetic movement to the left of the larval anterior/posterior axis. This results in a dramatic transformation from bilateral to radial symmetry and is accompanied by development of juvenile structures and settlement, 6–7 days after fertilisation. Ophiuroid development through one larval stage, the ophiopluteus, is termed Type I; whereas development through two larval stages, the ophiopluteus and vitellaria, is termed Type II. We examined the evolutionary changes in the expression of Type II development in Ophionereis by comparing the ontogeny of six species with a range of larval forms. O. fasciata has the ancestral-like planktotrophic ophiopluteus. Vestigial pluteal structures in the larvae of O. schayeri, provide a link between ophiopluteal and vitellarial forms during evolution of non-feeding development. The diversity of larval forms in Ophionereis indicates that evolution of non-feeding development through a vitellaria (Type II) may have involved an increase in egg size, reduction of pluteal structures and shortening of the time to metamorphosis. Assessment of the phylogenetic significance of Types I and II development awaits additional comparative data on the metamorphic stages of other ophiuroid genera.

Continental plate collision: Unstable vs. stable slab dynamics

Research Article| January 01, 2004 Continental plate collision: Unstable vs. stable slab dynamics Gisèle Toussaint; Gisèle Toussaint 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Evgene Burov; Evgene Burov 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Laurent Jolivet Laurent Jolivet 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar Author and Article Information Gisèle Toussaint 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Evgene Burov 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Laurent Jolivet 1Lab Tectonique UMR 7072, University of Pierre and Marie Curie, 4 place Jussieu, 75252 Paris, France Publisher: Geological Society of America Received: 02 Jul 2003 Revision Received: 23 Sep 2003 Accepted: 25 Sep 2003 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2004) 32 (1): 33–36. https://doi.org/10.1130/G19883.1 Article history Received: 02 Jul 2003 Revision Received: 23 Sep 2003 Accepted: 25 Sep 2003 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gisèle Toussaint, Evgene Burov, Laurent Jolivet; Continental plate collision: Unstable vs. stable slab dynamics. Geology 2004;; 32 (1): 33–36. doi: https://doi.org/10.1130/G19883.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We study possible scenarios for the evolution of continental collision zones by using a dynamic thermomechanical model that includes brittle-elastic-ductile rheology, surface erosion, and explicit metamorphic changes. This paper focuses primarily on the influence of four key parameters: (1) geotherm or thermotectonic age (which controls the rheological profile), (2) lower-crustal composition (weak or strong rheology), (3) convergence rate, and (4) metamorphic changes in the downgoing crust. The experiments suggest that, depending on these parameters, plate convergence is accommodated by four distinct mechanisms: stable subduction, shortening by pure-shear thickening or folding, and Rayleigh-Taylor instabilities. It appears that stable, oceanic-type subduction can only occur in the case of cold lithospheres (Moho temperature, TMoho < 550 °C), and basically needs high convergence rates (>4–5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the whole (upper and lower) crust or only the lower crust can be involved in subduction. It appears that in the case of weak metamorphic rheologies, phase changes only slightly improve chances for stable subduction. Lithospheric shortening becomes a dominant mechanism when TMoho > 550 °C or convergence rates are <4–5 cm/yr. Pure-shear thickening becomes important in all cases of hot lithospheres (TMoho > 650 °C). Large-scale folding is favored in the case of TMoho = 500–650 °C and is more effective in the case of mechanical coupling between crust and mantle (e.g., strong lower crust). Gravitational (Rayleigh-Taylor) instabilities overcome other mechanisms for very high values of TMoho (>800 °C) and may lead to development of subvertical cold spots. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Thyroid hormone controls the development of connections between the spinal cord and limbs during Xenopus laevis metamorphosis.

During premetamorphic stages, Xenopus laevis tadpoles expressing either a dominant-negative thyroid hormone (TH) receptor or a type-III iodothyronine deiodinase transgene in the nervous system have reduced TH-induced proliferation in the spinal cord and produce fewer hindlimb-innervating motorneurons. During prometamorphic stages, innervation of the hindlimbs is reduced, and few functional neuromuscular connections are formed. By metamorphic climax, limb movement is impaired, ranging from uncoordinated leg swimming to complete quadriplegia. This phenotype is due to transgene action in the tadpole spinal cord. The requirement of TH for neurogenesis during premetamorphosis is the earliest TH-regulated process reported to date in the sequence of metamorphic changes in anurans. The muscle formed during limb growth was previously shown to be a direct target of TH control. Here, we show that the same is true of the development of spinal cord cells that innervate the limbs.

Expression of type II iodothyronine deiodinase marks the time that a tissue responds to thyroid hormone-induced metamorphosis in Xenopus laevis

The thyroid gland synthesizes thyroxine (T4), which passes through the larval tadpole's circulatory system. The enzyme type II iodothyronine deiodinase (D2) converts thyroxine (T4) to the active hormone 3,5,3′-triiodothyronine (T3) in peripheral tissues. An early response to thyroid hormone (TH) in the Xenopus laevis tadpole is the stimulation of cell division in cells that line the brain ventricles, the lumen of the spinal cord, and the limb buds. These cells express constitutively high levels of D2 mRNA. Exogenous T4 induces early DNA synthesis in brain, spinal cord, and limb buds as efficiently as T3. The deiodinase inhibitor iopanoic acid blocks T4- but not T3-induced cell division. At metamorphic climax, both TH-induced cell division and D2 expression decrease in the brain. Then D2 expression appears in late-responding tissues including the anterior pituitary, the intestine, and the tail where cell division is reduced or absent. Therefore, constitutive expression of D2 occurs in the earliest target tissues of TH that will grow and differentiate, while TH-induced expression of D2 takes place in late-responding tissues that will remodel or die. This pattern of constitutive and induced D2 expression contributes to the timing of metamorphic changes in these tissues.

Teratogenic and anti-metamorphic effects of bisphenol A on embryonic and larval Xenopus laevis

Effects of bisphenol A (BPA) on embryonic and larval development were investigated. In Xenopus laevis blastulae treated with 2.5–3.0 × 10 −5 M BPA or with 10 −5 M 17β estradiol (E 2), malformation of the head region, scoliosis (curved vertebrate), and suppression of organogenesis were observed. In addition, 10 −5–10 −4 M BPA blocked tri-iodothyronine (T 3)-inducible resorption of the tail segments from premetamorphic (stage 52–54) larvae in vitro. When stage 52 tadpoles were immersed in 1.0–2.5 × 10 −5 M BPA, deceleration of both spontaneous and thyroxin (T 4)-induced metamorphic changes occurred. Furthermore, BPA suppressed thyroid hormone receptor (TR) β gene expression both in vivo and in vitro. Thus, we concluded that BPA at the concentrations examined affects both embryonic development and larval metamorphosis.

Effects of dexamethasone treatment on iodothyronine deiodinase activities and on metamorphosis-related morphological changes in the axolotl ( Ambystoma mexicanum)

In amphibians, there is a close interaction between the interrenal and the thyroidal axes. Hypothalamic corticotropin-releasing hormone or related peptides stimulate thyroidal activity by increasing thyrotropin synthesis and release, while corticosterone accelerates both spontaneous and thyroid hormone-induced metamorphosis. One of the mechanisms that is thought to contribute to this acceleration is a corticosterone-induced change in peripheral deiodinating activity. The present experiments were designed to investigate further the effects of glucocorticoid treatment on amphibian deiodinase activities and to explore the possible role of these effects in metamorphosis. Neotenic axolotls ( Ambystoma mexicanum) were treated either acutely or chronically with dexamethasone (DEX) and changes in type II and type III iodothyronine deiodinase (D2 and D3) activities were studied in liver, kidney, and brain. In addition, gill length, tail height, and body weight were measured at regular intervals in the chronically treated animals in search of metamorphosis-related changes. A single injection of 50 μ g DEX decreased hepatic D3 activity (6–48 h) while it increased D2 activity in brain (6–48 h) and to a lesser extent in kidney (24 h). These changes were accompanied by an increase in plasma T 3 levels (48 h). Samples taken during chronic treatment with 20 or 100 μ g DEX showed that both hepatic D2 and D3 activities were decreased on day 26, while renal D3 activity was decreased but only in the 20 μ g dose group. All other deiodinase activities were not different from those in control animals. At 25 days, all DEX-treated axolotls showed a clear reduction in gill length, tail height, and body weight, changes typical of metamorphosis. Prolongation of the treatment up to 48 days resulted in complete gill resorption by days 44–60. Although probably several mechanisms contribute to these DEX-induced metamorphic changes, the interaction with thyroid function via a sustained downregulation of hepatic D3 may be one of them.

Peculiarities of gas content and metamorphic changes in coals in the southwest coal region of the Lvov-Volyn Basin

Adequate ventilation is important because it protects both our health and home. However, the ventilation and energy requirements necessary to achieve good indoor air quality (IAQ) may interfere with one another. This paper presents the energy-saving potential of a ventilation system with an air-cleaning unit and demand control in a multi-residential building. This strategy is based on the demand to save energy in ventilation by reducing the supply of outdoor air using air cleaning while maintaining the desired air quality. Hence, the operation mode of the ventilation system in this study senses the indoor CO2 and HCHO concentrations according to the IAQ code for homes in Korea. The ventilation and air-cleaning modes are then operated independently or simultaneously to optimize the energy usage for ventilation.The simulation results indicate that, in maintaining the required IAQ level, the proposed ventilation system reduces the operation rate of the induction of outdoor air for slightly less than 50% of the operating time and decreases the energy use by approximately 20% with its air-cleaning and demand control mode compared to the conventional systems that employ a continuous ventilation mode.