Extensional Collapse Research Articles

Geochemistry and geochronology of Paleozoic intrusions in the Nalati (Narati) area in western Tianshan, Xinjiang, China: Implications for Paleozoic tectonic evolution

The Nalati (Narati) area in the Chinese western Tianshan is characterized by abundant Paleozoic intrusions, including granites, diorites and gabbros. They are not only indicators of the interaction between crust and mantle, but also useful clues for tracing the tectonic history of the Tianshan Orogen. Most Early Paleozoic granitoids (biotite monzonitic granites and muscovite granites) of this study are from the Yili Block. The biotite monzonitic granites (mixed-source-derived I-type granites) have a zircon U–Pb age of 497.0±5.9Ma, indicating the time of the subduction of the Terskey Oceanic crust in the Late Cambrian. The 427.2±5.7Ma Zircon U–Pb age of the S-type muscovite granites let us interpret that these granites may have been formed during the crust thickening process after the collision between the Yili Block and the Nalati Block. In western Tianshan the Late Paleozoic biotite granites, muscovite granites, quartz diorites, biotite monzonitic granites, granodiorites and alkali granites have respectively a LA-ICPMS zircon U–Pb age of 371.8±6Ma, 357.2±7.5Ma, 313.9±2.5Ma and 296.9±2.4Ma. The biotite granites display I-type geochemical features and are considered to have been derived from a lower continental crust source. The muscovite granites have a pronounced S-type affinity and are considered to have been formed by the partial melting of thickened continental crust after the collision between the Central Tianshan Belt and the South Tianshan Belt. The quartz-diorites are adakite-like and have an I-type affinity, which are considered to have been formed by partial melting of a delaminated lower crust in a post-orogenic extension setting. The granodiorites also show some typical geochemical features of adakite. Their formation is considered to be related to the lower crustal delamination and the ascending of asthenosphere after the collision event. The alkali granites show an A-type granite affinity. They may have been formed in a within-plate tectonic setting and possibly related to rifting as a consequence of extension.The gabbros can be divided into two groups based on their geochemical characteristics and ages. The older gabbros (415–429Ma) with lower TiO2 contents are formed in an arc-related setting. The younger ones (340–318Ma) with higher TiO2 content are formed in an intracontinental setting and influenced by crustal contamination.Magmatic activities recorded by Paleozoic intrusions can constrain the tectonic history of the Tianshan region from subduction of oceanic crust to intraplate extensional collapse and to rifting, and can also corroborate that the final closure of the South Tianshan Ocean occurred in the Late Devonian–Early Carboniferous time.

Late thrusting extensional collapse at the mountain front of the northern Apennines (Italy)

Thrust‐related anticlines exposed at the mountain front of the Cenozoic Appenninic thrust‐and‐fold belt share the presence of hinterlandward dipping extensional fault zones running parallel to the hosting anticlines. These fault zones downthrow the crests and the backlimbs with displacements lower than, but comparable to, the uplift of the hosting anticline. Contrasting information feeds a debate about the relative timing between thrust‐related folding and beginning of extensional faulting, since several extensional episodes, spanning from early Jurassic to Quaternary, are documented in the central and northern Apennines. Mesostructural data were collected in the frontal anticline of the Sibillini thrust sheet, the mountain front in the Umbria‐Marche sector of the northern Apennines, with the aim of fully constraining the stress history recorded in the deformed multilayer. Compressional structures developed during thrust propagation and fold growth, mostly locating in the fold limbs. Extensional elements striking about perpendicular to the shortening direction developed during two distinct episodes: before fold growth, when the area deformed by outer‐arc extension in the peripheral bulge, and during a late to post thrusting stage. Most of the the extensional deformation occurred during the second stage, when the syn‐thrusting erosional exhumation of the structures caused the development of pervasive longitudinal extensional fracturing in the crestal sector of the growing anticline, which anticipated the subsequent widespread Quaternary extensional tectonics.

Geochemistry, geochronology, and Sr–Nd isotopes of the Late Neoproterozoic Wadi Kid volcano-sedimentary rocks, Southern Sinai, Egypt: Implications for tectonic setting and crustal evolution

The Kid Group is one of the few exposures of Neoproterozoic metavolcano-sedimentary rocks in the basement of southern Sinai in the northernmost Arabian–Nubian Shield. It is divided into the mostly metamorphosed volcaniclastic Melhaq and siliciclastic Um Zariq formations in the north and the mostly volcanic Heib and Tarr formations in the south. The Heib, Tarr, and Melhaq formations reflect an intense episode of igneous activity and immature clastic deposition associated with core-complex formation during Ediacaran time, but Um Zariq metasediments are relicts of an older (Cryogenian) sedimentary sequence. The latter yielded detrital zircons with concordant ages as young as 647±12Ma, which may indicate that the protolith of Um Zariq schist was deposited after ~647Ma but 19 concordant zircons gave a 206Pb/238U weighted mean age of 813±6Ma, which may represent the maximum depositional age of this unit. In contrast, a cluster of 11 concordant detrital zircons from the Melhaq Formation yield a weighted mean 206Pb/238U age of 615±6Ma. Zircons from Heib Formation rhyolite clast define a 206Pb/238U weighted mean age of 609±5Ma, which is taken to approximate the age of Heib and Tarr formation volcanism. Intrusive syenogranite sample from Wadi Kid yields a 206Pb/238U weighted mean age of 604±5Ma. These constraints indicate that shallow-dipping mylonites formed between 615±6Ma and 604±5Ma. Geochemical data for volcanic samples from the Melhaq and Heib formations and the granites show continuous major and trace element variations corresponding to those expected from fractional crystallization. The rocks are enriched in large ion lithophile and light rare earth elements, with negative Nb anomalies. These reflect magmas generated by melting of subduction-modified lithospheric mantle, an inference that is further supported by εNd(t)=+2.1 to +5.5. This mantle source obtained its trace element characteristics by interaction with fluids and melts from subducting oceanic crust during the Late Cryogenian time, prior to terminal collision between fragments of East and West Gondwana at ~630Ma. Positive εNd(t) values and the absence of pre-Ediacaran zircons in all but Um Zariq metasediments indicate minor interaction with Cryogenian and older crust. A model of extensional collapse following continental collision, controlled mainly by lithospheric delamination and slab break-off is suggested for the origin of the post-collision volcanics and granites at Wadi Kid. No evidence of pre-Neoproterozoic sources was found. Kid Group Ediacaran volcanic rocks are compositionally and chronologically similar to the Dokhan Volcanics of NE Egypt, which may be stratigraphic equivalents.

Performance of pillar design in underground stone mines that include discontinuities

This paper describes a pillar design methodology that was developed from a study of pillar performance in operating stone mines. A number of methodologies have been developed to calculate full-scale mine pillar strengths based on laboratory scale strength obtained from specimens. Data were collected on rock mass quality, pillar conditions, mining dimensions and intact rock strength. Results showed that current mining practices have resulted in generally stable pillar layouts, without recent cases of extensive pillar collapses; however, failure of the pillars was found to be related to spalling of hard brittle rock, shearing along pre-existing angular discontinuities or progressive extrusion of soft infill material on bedding planes. A numerical model (FLAC3D) for the entire rock mass has been developed in order to analyze the stability of the entire underground opening. A preliminary monitoring phase has been realized, aimed at controlling abandoned rock structure movement at the most significant discontinuities. Some measurements of the vertical stress in the pillars and in the walls have also been performed and were used for model calibration. Once the model has been calibrated, analysis of the actual stress and deformation conditions can then be evaluated, the stability condition of the entire structure can be computed and a forecast analysis of what intervention could be realized to guarantee underground public access can be performed. The developed strength equation can be used to design stable pillar layouts, keeping in mind that the safety factor is greater than 1.5 and the width:height ratio of the pillars is between 0.6 and 0.8. It is concluded that, by applying the developed equation and selecting appropriate input parameters, it should then be possible to calculate the factor of safety.

Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides

Research Article| June 01, 2012 Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides Vegard V. Vetti; Vegard V. Vetti Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Search for other works by this author on: GSW Google Scholar Haakon Fossen Haakon Fossen Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Search for other works by this author on: GSW Google Scholar Author and Article Information Vegard V. Vetti Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Haakon Fossen Department of Earth Science, University of Bergen, Postboks 7803, 5020 Bergen, Norway Publisher: Geological Society of America Received: 30 May 2011 Revision Received: 13 Jan 2012 Accepted: 25 Jan 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2012) 40 (6): 571–574. https://doi.org/10.1130/G32512.1 Article history Received: 30 May 2011 Revision Received: 13 Jan 2012 Accepted: 25 Jan 2012 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 Vegard V. Vetti, Haakon Fossen; Origin of contrasting Devonian supradetachment basin types in the Scandinavian Caledonides. Geology 2012;; 40 (6): 571–574. doi: https://doi.org/10.1130/G32512.1 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 Supradetachment basins that formed during the Devonian extensional collapse of the Scandinavian Caledonides have been explained as hanging-wall basins formed along the listric breakaway zone of a major detachment fault system. This model involves significant rotation of bedding as the detachment flattens to an approximately horizontal orientation, and explains the large stratigraphic thicknesses of east-dipping layers overlying in tectonic contact the top-to-the-west Nordfjord-Sogn detachment zone (NSDZ). However, it fails to explain one of the basins, the Håsteinen basin, where east-dipping Devonian strata rest unconformably on a metamorphic substrate that forms part of the upper plate of the detachment system. Based on detailed field mapping, we present a model where the Håsteinen forms as a ramp basin that develops on the upper plate above a major west-facing ramp in the NSDZ. We test both of these models by forward modeling using 2DMove (two-dimensional kinematic modeling software). The results show how two different types of supradetachment basins can coexist in a subhorizontal detachment zone in an area that underwent many tens of kilometers of lateral crustal extension. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Exhumation of Triassic HP–LT rocks by upright extrusional domes and overlying detachment faults, Ishigaki-jima, Ryukyu islands

The Tomuru Formation of Ishigaki-jima in the southernmost part of the Ryukyu arc, comprises blueschist facies subduction complex rocks metamorphosed in Triassic time. D1 structures related to subduction, blueschist facies mineral growth, and possibly early stages of exhumation, are deformed by D2 structures that appear to reflect the last stage of exhumation. D2 structures define several anticlines with parasitic overturned folds verging away from anticlinal axes. The shortening recorded by this deformation appears to reflect upward extrusion relative to flanking material. The anticlines are flanked by detachment faults with normal sense-of-shear parallel to D2 vergence. Hanging wall rocks that include the Fusaki Formation, an accretionary prism with early Cretaceous metamorphic ages, and late Eocene limestone, conglomerate, and andesitic volcanics. The Eocene strata contain metamorphic detritus derived from the Tomuru and Fusaki Formations indicating pre-late Eocene surface exposure of these units. Ultramafic rocks and gabbro blocks of the Tomuru Formation were incorporated by sedimentary sliding into the trench prior to subduction and high-pressure metamorphism rather than being emplaced as diapirs along a post-metamorphic fault as previously proposed. Geochronologic, metamorphic, and thermal considerations suggest exhumation of the Tomuru Formation to relatively shallow crustal depths prior to or concurrent with early Cretaceous metamorphism of the Fusaki Formation. Arcward-vergent thrusting may have placed the younger, and formerly structurally lower, subduction complex (Fusaki Formation) over the older one (Tomuru Formation). D2 extrusional doming began after the emplacement of the Fusaki Formation at high structural levels. The D2 transport directions are subparallel to the strike of the orogen suggesting that the upright extrusion may have occurred along a forearc strike-slip fault system. This final stage of exhumation concluded in the late Eocene with extensional collapse and the development of detachment faults. The progression from initial exposure of the Tomuru and Fusaki Formations, deposition of late Eocene strata, extrusional doming and late detachment faulting may have been associated with migrating step-overs rather than changes in regional tectonics.

Improved Design of Thin-Walled Square Tube for Energy Absorption

Thin-walled tubes are the main structures to absorb the energy in front impact at the desired controlled collision. A thin-walled square tube is designed by use of a double deck square tube with clapboards which has a stable structure with less Euler deformation in front impact. More deformations of extensional collapse in the double deck square tube with clapboards improve the performance of structures for energy absorption. Induced slots are employed in the square tube to decrease the peak acceleration in front impact. Collision process can be controlled by the slots for the stable deformation and reduced initial peak acceleration

Basin-scale salt tectonic processes of the Laurentian Basin, Eastern Canada: insights from integrated regional 2D seismic interpretation and 4D physical experiments

Abstract This study integrates seismic interpretation and 3D analogue experiments monitored by digital image correlation techniques to investigate the evolution of the salt structures and the related depositional systems in the Laurentian Basin offshore Atlantic Canada. During the late Triassic, a layer of more than 3 km thick salt was deposited locally in a set of interconnected rift half-grabens forming a 50–70 km wide evaporite basin in the northern part of the Scotian Basin salt provinces. High sediment input in the Jurassic and early Cretaceous mobilized the salt into complex salt tectonic features, which suggest four kinematic domains with: (1) salt welds and pillows; (2) extensional diapirs and canopies; (3) contractional diapirs and folds; and (4) allochthonous salt nappe. The landward grabens trapped most of the Early Jurassic sediments by passive downbuilding into the salt with local extension. The expelled salt has been evacuated basinwards into a large contractional salt massif. The rapid advance of the allochthonous nappe was coeval with the Late Jurassic extensional collapse of the inflated salt massif due to seaward sediment progradation. Late Cretaceous and Tertiary progradation over the salt nappe caused extensional deformation with growth faulting and formation of minibasins on the secondary salt detachment level.

Extensional Flow during Gravitational Collapse: A Tool for Setting Plate Convergence (Padrón Migmatitic Dome, Variscan Belt, NW Iberia)

AbstractPlate convergence analysis in collisional orogens is usually based on the study of major contractional structures and strike-slip shear zones. Here we show how the structural analysis of extensional structures may report the regional or far stress field during relatively local, gravity-driven extensional collapse of a thickened continental crust and how this information may be used to constrain the broad vectors of plate convergence at that time. The Padron migmatitic dome is a synconvergent extensional system developed in the axial zone of the Variscan belt exposed in the NW part of the Iberian Massif of Spain. This system affected the allochthonous and autochthonous sequences involved in Pangaea’s assembly in Southern Europe. It includes three major extensional shear zones, which have been analyzed in detail to provide a wide ground data set for the discussion of the proposed model. The tectonic flow in the Padron migmatitic dome and in other coeval structures is characterized by vectors ranging...

Discussion: A unifying model for the Torridon Group (early Neoproterozoic), NW Scotland: Product of post-Grenvillian extensional collapse, by GE Williams and J Foden

The Torridonian succession in NW Scotland is a well-exposed sequence of sedimentary rocks that record the early Neoproterozoic geological evolution of Laurentia. An excellent review by Stewart (2002) summarised the observations on the three main 'Torridonian' units: the Stoer, Sleat and Torridon groups. The Torridon Group, deposited between 1000 – 950 Ma, was initially interpreted as a rift basin ( [Stewart, 1982] and [Williams, 2001]), but a number of studies ( [Rainbird et al., 2001], [Kinnaird et al., 2007] and [Krabbendam et al., 2008]) have now concluded that it represents a foreland basin positioned in front of the Grenville Orogen (1200 – 960 Ma). In a recent paper in Earth Science Reviews, Williams and Foden (2011) propose, instead, that the Torridon Group was deposited in an ‘extensional collapse basin’. We question this conclusion and take this opportunity to highlight aspects of their new interpretation that we find equivocal.

Late Ottawan extension in the eastern Adirondack Highlands: Evidence from structural studies and zircon and monazite geochronology

Research Article| May 01, 2012 Late Ottawan extension in the eastern Adirondack Highlands: Evidence from structural studies and zircon and monazite geochronology Martin S. Wong; Martin S. Wong † 1Department of Geology, Colgate University, Hamilton, New York 13346, USA †E-mail: mswong@colgate.edu Search for other works by this author on: GSW Google Scholar Michael L. Williams; Michael L. Williams 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar James M. McLelland; James M. McLelland 1Department of Geology, Colgate University, Hamilton, New York 13346, USA Search for other works by this author on: GSW Google Scholar Michael J. Jercinovik; Michael J. Jercinovik 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar Justin Kowalkoski Justin Kowalkoski 1Department of Geology, Colgate University, Hamilton, New York 13346, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Martin S. Wong † 1Department of Geology, Colgate University, Hamilton, New York 13346, USA Michael L. Williams 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA James M. McLelland 1Department of Geology, Colgate University, Hamilton, New York 13346, USA Michael J. Jercinovik 2Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Justin Kowalkoski 1Department of Geology, Colgate University, Hamilton, New York 13346, USA †E-mail: mswong@colgate.edu Publisher: Geological Society of America Received: 06 Jan 2011 Revision Received: 15 Jul 2011 Accepted: 20 Jul 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (5-6): 857–869. https://doi.org/10.1130/B30481.1 Article history Received: 06 Jan 2011 Revision Received: 15 Jul 2011 Accepted: 20 Jul 2011 First Online: 08 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 Martin S. Wong, Michael L. Williams, James M. McLelland, Michael J. Jercinovik, Justin Kowalkoski; Late Ottawan extension in the eastern Adirondack Highlands: Evidence from structural studies and zircon and monazite geochronology. GSA Bulletin 2012;; 124 (5-6): 857–869. doi: https://doi.org/10.1130/B30481.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Syn- to postorogenic collapse is increasingly being recognized as a fundamental process in most orogens, although the mechanism of collapse at mid- to lower-crustal levels is not well understood. The Adirondack Highlands of the Grenville Province in northern New York are an exhumed granulite terrane and represent an ideal setting in which to investigate mid- to lower-crustal orogenic processes. Although the highlands experienced high-grade metamorphism and crustal thickening during the Ottawan orogeny, few extensional shear zones have been recognized in this region. We document a new shear zone in the eastern Adirondack Highlands that accommodated postorogenic extensional collapse during the late Ottawan. The shear zone accommodated extension via noncoaxial top-to-the-southeast shear, although we cannot rule out a significant coaxial vertical thinning component. Some earlier thrust motion may also have occurred in the zone. U-Pb sensitive high-resolution ion microprobe zircon and in situ electron probe U-Th-Pb monazite ages demonstrate that extension occurred from ca. 1050 to 1025 Ma, following peak metamorphism during the Ottawan orogeny. Our results suggest that late Ottawan extension in the Adirondack Highlands was more widespread than previously recognized and was likely driven by gravitational collapse. Extension along this eastern Adirondack Highlands shear zone may connect with the Tawachiche shear zone to the north in Quebec, indicating that tectonic exhumation during the Ottawan occurred on a system of interconnected shear zones. Top-to-the-northeast extension in the eastern Adirondack Highlands was synchronous with top-to-the-northwest extension on the Carthage Colton shear zone in the western Adirondack Highlands, suggesting that the highlands granulite terrane may have been partially exhumed as a large-scale symmetrical gneiss dome. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Scott model of linear logic is the extensional collapse of its relational model

We show that the extensional collapse of the relational model of linear logic is the model of prime-algebraic complete lattices, a natural extension to linear logic of the well known Scott semantics of the lambda-calculus.

Isotopic Evolution of the Idaho Batholith and Challis Intrusive Province, Northern US Cordillera

The Idaho batholith and spatially overlapping Challis intrusive province in the North American Cordillera have a history of magmatism spanning some 55 Myr. New isotopic data from the 98Ma to 54Ma Idaho batholith and 51Ma to 43Ma Challis intrusions, coupled with recent geochronological work, provide insights into the evolution of magmatism in the Idaho segment of the Cordillera. Nd and Hf isotopes show clear shifts towards more evolved compositions through the batholith’s history and Pb isotopes define distinct fields correlative with the different age and compositionally defined suites of the batholith, whereas the Sr isotopic compositions of the various suites largely overlap.The subsequent Challis magmatism shows the full range of isotopic compositions seen in the batholith.These data suggest that the early suites of metaluminous magmatism (98^87 Ma) represent crust^mantle hybrids. Subsequent voluminous Atlanta peraluminous suite magmatism (83^67 Ma) results primarily from melting of different crustal components. This can be attributed to crustal thickening, resulting from either subduction processes or an outboard terrane collision. A later, smaller crustal melting episode, in the northern Idaho batholith, resulted in the Bitterroot peraluminous suite (66^54 Ma) and tapped different crustal sources. Subsequent Challis magmatism was derived from both crust and mantle sources and corresponds to extensional collapse of the over-thickened crust.

Endothelial Cells Promote Pancreatic Stem Cell Activation During Islet Regeneration in Mice

Objectives Diabetes is the clinical consequence of the loss of the majority of the β-cell population and failure to regenerate new pancreatic β cells. The current therapies based on β-cell replacement have failed to achieve β-cell renewal and thus, long-term insulin freedom. We have hypothesized that early rejection of endothelial elements within the islet grafts may seriously hamper islet regeneration in both native and islet grafts. Methods In the present study, we analyzed the role of endothelial cells to activate pancreatic stem cells during islet regeneration. Mice were pretreated with or without endothelial pharmacological ablation of endothelial cells, followed by an acute β-cell injury using a single intraperitoneal injection of streptozotocin. We performed comparative morphometric analyses of recovered pancreata on days 3, 7, 10, and 30 after streptozotocin injury, staining with bromodeoxyuridine (BrdU) for representative cell types, β cells, endothelial elements, and stem cells. Blood glucose levels were measured continuously after the injury to monitor the capacity for metabolic control. Results Morphometric analyses revealed an increasing number of cells over time to be stained with a stem cell and BrdU markers among animals only injured with streptozotocin but not with endothelial ablation. Notably, on day 10, stem cell markers were dramatically decrease nearly to basal levels, with appearance of numerous insulin-positive cells. Intact vessels with cobblestone-shaped endothelial elements were observed in direct proportion to the better outcomes, both by morphometric and by metabolic parameters. In contrast, fewer insulin-positive cells were observed in pancreata that had been ablated of endothelial cells showing extensive collapse of endocrine functions. Conclusions We observed that endothelial elements promoted stem cell proliferation and islet regeneration after a β-cell insult. We believe that preservation of endothelial cells positively affects the process of pancreatic regeneration.

Finite Element and Reliability Analyses for Slope Stability of Subansiri Lower Hydroelectric Project: A Case Study

The partly constructed and excavated power house slopes of Subansiri Lower Hydroelectric Project experienced extensive collapses through complex mode of failure. A detailed study is attempted in this paper to understand the reasons for the failure and assess the stability of the existing constructed slopes using limit equilibrium and FEM solutions and also to propose modified design for rebuilding the slopes. To take into account the uncertainty associated with the rockmass and soil properties, probability and reliability analyses have also been carried out. Based on the field observations and stability analyses of the natural and cut slopes, suitable support systems such as slope flattening with various angles, weldmesh, shotcrete, rockbolts and drainage holes have been considered to meet the stability requirements. In this study, it is demonstrated that the probabilistic approach when used in conjunction with deterministic approach helps in providing a rational solution for quantification of stability in the estimation of risk associated with the power house slope construction.

Megacrystic Gore Mountain–type garnets in the Adirondack Highlands: Age, origin, and tectonic implications

Spectacular exposures of the world's largest megacrystic garnets (to 35 cm diameter) occur in a coarse-grained amphibolite at the Barton Garnet Mine in the Adirondack Highlands (Gore Mountain, New York State, USA). Over the years, numerous geologists have concluded that the large size of the garnets resulted from an influx of fluids during ca. 1050 Ma upper amphibolite facies metamorphism of a ca. 1155 Ma olivine metagabbro. The presence of fluids under such mid-crustal pressure-temperature conditions is anomalous and warrants explanation. Evidence indicates that the fluids were introduced along, and close to, a steep border fault that juxtaposes charnockite against the garnet ore at the southern margin of the mine. Granitic pegmatites and quartz veins are present in the border zone and locally intrude the garnet ore. Geochronology has played a critical role in resolving the genesis of the Gore Mountain garnets. Over the past 20 yr Sm-Nd and Lu-Hf techniques have been used to date the crystallization of the garnets as 1049 ± 5 Ma, an age that coincides with the termination of the contractional phase of the Ottawan orogeny, the onset of extensional orogen collapse, and the emplacement of the Lyon Mountain Granite. New U-Pb zircon age determinations of 1045 ± 7.5 (Barton Garnet Mine) and 1055 ± 7.4 (New York State Route 3 near Cranberry Lake) for Lyon Mountain granite pegmatites directly associated with megacrystic garnet amphibolites corroborate the synchronicity of emplacement of Lyon Mountain magmas and the growth of the garnet megacrysts. It is argued that during the ca. 1050 Ma extensional collapse of the Ottawan orogen, fluids gained access to extensional fault networks and interacted with country rocks. We further suggest that increasing temperature due to advected heat from Lyon Mountain Granite carried the Gore Mountain megagarnet amphibolite into granulite facies conditions that resulted in reactions between hornblende and garnet that produced orthopyroxene and calcic plagioclase intergrowths, both as symplectites and coarsely textured pods developed in pressure shadows. Geothermal modeling of garnet zoning in metapelites ([Storm and Spear, 2005][1]) and oxygen isotope zoning in titanite ([Bonamici et al., 2011][2]) require a short period of rapid cooling ca. 1050 Ma, which we interpret to be related to the extensional collapse of the Ottawan orogen at that time ([Rivers, 2008][3]; [McLelland et al., 2010a][4], [2010b][5]). Reconnaissance of the southern and central Adirondacks reveals that a number of megacrystic garnet occurrences similar to those at Gore Mountain are present in areas that contain both metagabbros and megacrystic garnet amphibolites, and we propose that all of these formed during orogen collapse, intrusion of Lyon Mountain Granite, and fluid-related alteration at high temperature. [1]: #ref-51 [2]: #ref-5 [3]: #ref-45 [4]: #ref-37 [5]: #ref-38

Pleural effusion complicates monitoring of respiratory mechanics*

Although pleural effusion reduces respiratory system compliance by restricting the lungs, the effusion volume is partially accommodated by chest wall expansion. The implications for these opposing volume changes on airway pressure monitoring in ventilated patients with pleural effusion are unreported. We investigated the interactions among pleural effusion, positive end-expiratory pressure, and indices of respiratory mechanics in a swine model. Interventional animal model. Hospital animal research facility. Nine deeply anesthetized swine. The preparation included tracheostomy, arterial/venous catheter placement, and chest tube insertion. Animals were ventilated throughout the study at 9 mL/kg, and frequency was adjusted to maintain normocapnia (inspiratory:expiratory=1:2, FIO2=0.5) and positive end-expiratory pressure of 1 cm H2O and 10 cm H2O. Fluid was instilled into the right pleural space to simulate effusions of 13 mL/kg (pleural effusion 1) and 26 mL/kg (pleural effusion 2). Quantitative computerized tomography studies (in five animals) and functional residual capacity volumes (wash-in/wash-out technique) were obtained for each pleural effusion/positive end-expiratory pressure combination. Mean functional residual capacity compared to baseline at positive end-expiratory pressure of 1 cm H2O was decreased by pleural effusion 1 and pleural effusion 2 (-42%, -64%) and restored by positive end-expiratory pressure of 10 cm H2O (moderate) to +23% of baseline for pleural effusion 1 and +1% for pleural effusion 2. Plateau pressure increased and compliance decreased in response to pleural effusion 1 and pleural effusion 2. Moderate positive end-expiratory pressure applied during both pleural effusion quantities restored plateau pressure and tidal compliance to prepleural effusion values. Computed tomography studies revealed lung compression and tidal derecruitment cycles occurring with pleural effusion at positive end-expiratory pressure of 1 cm H2O, whereas a moderate positive end-expiratory pressure restored prepleural effusion functional residual capacity and prevented lung and intratidal derecruitment. When pleural effusion is present, respiratory mechanics must be interpreted cautiously and sufficient positive end-expiratory pressure should be applied to prevent extensive collapse and intratidal cycles of recruitment/derecruitment.

The exposed base of a collapsing wedge: The Nestos Shear Zone (Rhodope Metamorphic Province, Greece)

The Nestos Shear Zone (NSZ) in the Rhodope Metamorphic Province is a major high‐strain zone between two metamorphic terranes. Microdiamond‐bearing ultrahigh‐pressure (UHP) rocks occur in the NSZ which was therefore interpreted as a suture zone where subduction and exhumation of these rocks and terrain accretion occurred during the Mesozoic. Our petrological study of samples from the lower part of the NSZ, together with monazite dating of a microdiamond‐bearing garnet schist, structural observations, already published results from the upper part, and other published timing constraints, results in a fundamentally different picture: the NSZ is the base of an Eocene age southwestward thrust wedge which included not only the structurally higher parts of the Rhodope Metamorphic Province but also the entire Internal Hellenides. The UHP rocks, for the peak pressure of which we derive an age of ∼200 Ma by monazite dating, are unrelated to the tectonic processes in the NSZ and probably represent slivers of a higher tectonic unit captured by thrusting along the NSZ. Pressure decrease in the footwall and regional extension and basin formation in the hanging wall during the activity of the NSZ show that the overlying thrust wedge was collapsing in late Eocene times. We hypothesize that the transition from subduction to continental collision may start with a pronounced accretion event when the first detachment horizon forms in the middle crust of the downgoing plate. Such an event could trigger slab retreat, an extensional collapse of the internal wedge, and subsequent magmatism.

Tectonic evolution of a continental subduction‐exhumation channel: Variscan structure of the basal allochthonous units in NW Spain

A regional study starting from detailed geological mapping has been carried out in the Malpica‐Tui Complex of Galicia in NW Spain. The complex is formed by two units representing pieces of the external edge of Gondwana, subducted and exhumed during the Variscan collision. The study shows that synsubduction and early synexhumation structures in continental subduction channels tends to be obscured and even erased once exhumation is complete. Detailed structural analysis, matched with the knowledge of the history, and available data for other Galician basal units have elucidated the major structures developed during the subduction‐exhumation process. The results include evidence of the plate convergence causing early Variscan continental subduction of the Gondwana margin. Subduction was followed by exhumation driven by ductile thrusting within the subduction channel, which, in turn, provoked crustal duplication in the subducted slab and modified the initial tectonometamorphic architecture of the subduction wedge. The next step was accretion to the adjacent continental domains, placing the subduction wedge on top of unsubducted parts of the Gondwana margin via ductile thrusting. Thrusting was preceded by progressive propagation of a train of recumbent folds toward the foreland that affected the previous structural stack. Subsequent transference of oceanic (Rheic) and peri‐Gondwanan terranes to the Gondwana margin took place by out‐of‐sequence thrusting followed by crustal extensional collapse and strike‐slip tectonics.

Mesozoic contraction deformation in the Yanshan and northern Taihang mountains and its implications to the destruction of the North China Craton

Mesozoic contraction deformation in the Yanshan and Taihang mountains is characterized by basement-involved thrust tectonics, basement-cored buckling anticlines and ductile thrust and nappe tectonics. Most of these deformations are orientated west-east, west-northwest and northeast to north-northeast. The contraction deformations began in the Permian, continued through the Triassic and Jurassic and terminated in the Early Cretaceous, and constitute an important part of the destruction of the North China Craton. It is estimated, from balanced cross-section reconstructions, that the north-south shortening of the central part of the Yanshan belt before 135 Ma was around 38%. The initial crust thickness, pre-dating the major contraction deformation in late Paleozoic and early Mesozoic, was estimated to be around 35 km based on paleogeographic characteristics. Assuming that the inferred depth of ductile thrusting deformation, 20-25 km, was the crust thickness involved in the contraction deformation, and also assuming that the N-S contraction deformation was accommodated by vertical crust thickening, the thickness of the crust after the contraction deformation was expected to be around 47-50 km. This was the approximate crust thickness required for the eclogitization of the lower crust for delamination. The gravity potential accumulated by the isostatic uplift of the thickened crust, together with the decrease in crustal strength caused by the coeval magmatisms associated with the contraction deformation, led to the subsequent extensional collapse of the middle and upper crust although the regional stress regime associated with the plate interactions remained constant. It is inferred that the Mesozoic contraction deformations in the Yanshan and Taihang mountains were not only a significant tectonic process contributing to the destruction of the craton in middle and upper crust but also stimulated delamination at a deep level and the extension of the shallow crust. In other words, both the suspected delamination of the lower crust and upper mantle and the well constrained extension deformations of the shallow crust in the eastern North China Craton during the late Mesozoic are a consequence of crust thickening due to previous contractions. Extensional deformations could be expected to occur independently in the shallow crust, and are not necessarily associated with or responding to de-lamination at a deep level.