Lowest Structural Levels Research Articles

Is the Inverted Field Gradient in the Catalina Schist Terrane Primary or Constructional?

AbstractNew geothermometry using laser‐Raman data on carbonaceous material from low and intermediate grade rocks on Santa Catalina Island, California, together with existing thermobarometric data, show that there is a quasi‐continuous increase in peak metamorphic temperature from 327 ± 8°C in lawsonite blueschist facies rocks at the lowest structural levels, through ∼433°C in overlying epidote blueschists, 546 ± 20°C in albite‐epidote amphibolite facies rocks, to 650–730°C in amphibolite facies rocks at the top of the sequence. Rocks of different metamorphic grade are separated from one another by tectonic contacts across which temperature increases by ∼100°C in each case. Previously published geochronological data indicate that peak metamorphism in the highest grade rocks at 115 Ma preceded deposition of blueschist facies metasediments by ∼15 million years, so that the present inverted grade sequence does not represent an original inverted temperature gradient. The present structure results from progressive underplating of oceanic rocks in a cooling subduction zone following a high‐T metamorphic event at 115 Ma. An inverted temperature gradient of ≥100°C/km across the subduction channel likely existed during the high‐T event, decreased during underplating, and reached zero by ∼90 Ma.

Desain Taman Sekolah untuk Anak Berkebutuhan Khusus di Sekolah Alam Bogor

The condition of a school landscape is important because it affects the learning process which can increase participation among students. Nature school is one form of an educational institution that takes account the importance of a landscape and how the students can appreciate the environment. One nature school is in Bogor, more specifically at Tanah Baru. This nature school has an inclusive education program for children with special needs, in which they can interact with other children of the same age. The aim of this research is to create a concept and design of the school landscape and playgrounds that can function as an educational and healing space through programs and activities that stimulate growth and development of children with environmental qualities and esthetics. The method that was used for this research was analyzing qualitative and quantitative aspects through three approaches that are biophysics, social, and education of special needs children. This method was done through four stages which are: (1) preparation stage, (2) research and analyzing the existing conditions (physical and biophysical) and educational process for special needs children through direct observation and literature review, (3) data processing stage, and (4) designing stage. The main concept of this school landscape is to create an educational and interactive landscape which can be used for therapeutic activities by combining the existing therapeutic activities. The main concept of this landscape comes from the concept “building blocks for learning” which consists of skills obtained from therapeutic activities. The design concept, “building blocks of life”, is a development from the main concept which can be interpreted as the compositional structure of all living things, in which cells are the lowest structural level of living things, as well as elementary school in the educational system. The output of this research is limited to the landscape design phase, with the final product in the form of a site plan and other supporting detail drawings.

Auxins and Cytokinins-The Role of Subcellular Organization on Homeostasis.

Plant hormones are master regulators of plant growth and development. Better knowledge of their spatial signaling and homeostasis (transport and metabolism) on the lowest structural levels (cellular and subcellular) is therefore crucial to a better understanding of developmental processes in plants. Recent progress in phytohormone analysis at the cellular and subcellular levels has greatly improved the effectiveness of isolation protocols and the sensitivity of analytical methods. This review is mainly focused on homeostasis of two plant hormone groups, auxins and cytokinins. It will summarize and discuss their tissue- and cell-type specific distributions at the cellular and subcellular levels.

Deformation temperatures, vorticity of flow and strain symmetry in the Loch Eriboll mylonites, NW Scotland: implications for the kinematic and structural evolution of the northernmost Moine Thrust zone

Abstract The Moine Thrust zone (MTZ) marks the Caledonian foreland-to-hinterland transition zone at the base of the Scandian ( c . 430 Ma) orogenic wedge. In the Loch Eriboll region, the upper ductile part of the MTZ is composed in ascending order of two regionally extensive thrust sheets (Upper Arnaboll-Creag na Faoilin and Creagan) and is overlain by the Moine Nappe. Quartz crystal fabrics, kinematic vorticity (W m ), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. At the lowest structural levels, quartzite and gneiss in the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust sheet yield rigid grain-based arithmetic mean minimum (W m min) and mean maximum (W m max) vorticity estimates of 0.57 and 0.67, respectively (60–53% pure shear). Creagan thrust sheet mylonites yield W m min and W m max estimates of 0.59 and 0.72 (59–48% pure shear). At the highest structural levels, Moine Nappe mylonites yield W m min and W m max estimates of 0.59 and 0.71 (59–49% pure shear). Quartz c - and a -axis fabrics qualitatively indicate an increase in non-coaxial deformation (top-to-the-west) traced towards structurally higher levels, which is accompanied by increases in deformation temperature ( c . 370 °C to c . 550 °C). Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal shortening has occurred as nappe stacking progressed.

Proterozoic ophiolites and mafic–ultramafic complexes marginal to the İstanbul Block: An exotic terrane of Avalonian affinity in NW Turkey

Among the Proterozoic inliers in the İstanbul Block, the lowest structural levels are exposed in the Sünnice Massif, north of Bolu. Amphibolite-facies mafic and subordinate ultramafic rocks of the Çele meta-ophiolite underlie the greenschist-facies Ediacaran calc-alkaline Yellice metavolcanics, which are intruded by the ∼ 565–576 Ma Dirgine granitoids. Hornblende gneisses of the Çele meta-ophiolite comprise island arc meta-tholeiites and transitional to calc-alkaline metabasalts which, together with minor serpentinite are disposed in a broadly antiformal structure. The meta-ophiolitic rocks are therefore the oldest ophiolites in NW Turkey, and are themselves thrust on to a putative pre-existing continental margin, now represented by the metasedimentary migmatites of the Demirci gneisses, which may thus be the oldest rocks of the complex. The İstanbul Block is an exotic terrane. Unlike other western Turkish terranes, it lacks Variscan metamorphism: its different provenance, indicated by its geological record, faunal affinities, and inherited mid-Proterozoic isotopic dates, suggests a former link with Avalonian basement in England, NW Europe and the Maritime Provinces of Canada. Hence, together with other terranes now situated further east than the Avalonian terranes of NW Europe, the İstanbul Block may represent the easternmost extremities of Avalonia, which were detached during its end-Ordovician collision with the Bruno-Silesian Promontory on the SW margin of Baltica. Subsequent migration of the İstanbul Block to its present location occurred by eastward displacement by sinistral transpression along the southern margin of Baltica to a point east of the Dobrogea and south of the Scythian Platform, followed by collision with the Sakarya Block in the Mesozoic and Late Cretaceous southward displacement with the opening of the Black Sea basin.

Extreme tectonic rotations within an eastern Mediterranean ophiolite (Baër–Bassit, Syria)

Palaeomagnetic results from 27 sites at five localities within the dismembered Baër–Bassit ophiolite of northern Syria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of the Arabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four locality mean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within one of these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions of remanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotations on a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressive increase in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thrust stack record the highest rotation (exceeding 200°). Although tectonic rotation during imbricate thrusting has been reported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differential thrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodos ophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of a coherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partially rotated Baër–Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the Late Tertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slip fault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed in the study are therefore of composite origin, and reflect the complex development of structural fabrics within the ophiolite.

Nd isotopic and geochemical signature of the Paleoproterozoic Trans-Hudson Orogen, southern Baffin Island, Canada: implications for the evolution of eastern Laurentia

The southern Baffin Island segment of the Trans-Hudson Orogen comprises a three-part crustal architecture. The lowest structural level (level 1) includes ca. 2.88 Ga tonalite-diorite-granite orthogneisses of the parautochthonous Superior Province. Level 2 comprises monzogranite-tonalite-diorite orthogneisses of the 1.86–1.82 Ga Narsajuaq arc. At the highest structural level (level 3), ca. 1.95 Ga tonalite-monzogranite orthogneiss, ca. 1.93 Ga shelf metasediments of the Lake Harbour Group, and dominantly psammitic metasediments of the Blandford Bay assemblage are tectonically imbricated and cross-cut by 1.86–1.85 Ga monzogranitic to tonalitic rocks of the Cumberland batholith. Level 2 Narsajuaq arc felsic orthogneisses display strongly fractionated rare earth elements, with La n /Yb n >100, ε Nd (1.85 Ga) values ranging from −12 to −19, and T DM ages from 2.7 to 3.6 Ga, consistent with derivation by underplating and melting of strongly fractionated crust bearing components as old as Paleoarchean, such as parts of the Nain Province or the northernmost Superior Province. The Narsajuaq arc rocks on southern Baffin Island show more evolved Nd isotopic compositions and strongly fractionated REE profiles relative to correlated rocks on the Ungava Peninsula in northern Quebec. The north-south change in Nd isotopic and REE geochemical signatures is consistent with northward subduction beneath an Archean cratonic nucleus. At structural level 3, the peraluminous Cumberland batholith shows La n /Yb n mostly<50, negative Eu anomalies, weakly fractionated HREE, ε Nd (1.85 Ga) values from −2.7 to −7.4, and T DM ages from 2.35 to 3.08 Ga. The data are consistent with pervasive assimilation of Lake Harbour Group metasediments ( ε Nd (1.85 Ga)=−4.5 to −7.3) and lesser amounts of Ramsay River orthogneiss ( ε Nd (1.85 Ga)=−5.2 to −16.5). Lake Harbour Group Nd isotopic and REE compositions are similar to the Tasiuyak metasedimentary gneiss of Labrador, and support correlation of the two units. The Blandford Bay metasediments show Nd isotopic evidence for contribution of Mesoarchean detritus ( ε Nd (1.85 Ga)=−10.9 to −13.4), and may be derived from Archean Nain Province. The distinctive lithologies, Nd isotopic signatures, REE chemistry and crystallization ages of structural levels 2 and 3 metaplutonic rocks suggest that they represent two separate Paleoproterozoic terranes showing distinctive crustal evolution histories involving separate and distinctive Archean crustal nuclei. Tectonic models for the eastern Trans-Hudson Orogen require two spatially-distinct 1.86–1.82 Ga subduction systems reworking distinct Archean cratonic margins in order to account for the differences between Paleoproterozoic orthogneisses on southern Baffin Island. One subduction system would have produced the Narsajuaq arc of level 2, by pervasive assimilation of crustal rocks with ε Nd (1.85 Ga) lower than −11.6 and La n /Yb n >100. The differences between the less evolved Nd isotopic compositions of the Narsajuaq arc of Ungava Peninsula and the highly evolved Nd isotopic compositions of the Narsajuaq arc rocks on Baffin Island may explained by: (i) subduction beneath a northward-thickening Archean cratonic block; or (ii) by a relict north–south age boundary in the Archean cratonic margin. A second subduction zone would have produced the Cumberland batholith by melting and assimilation of a cratonic margin less fractionated REE and more elevated ε Nd values. Data from structural levels 2 and 3 are consistent with correlation of these rocks across the eastern segments of the Trans-Hudson Orogen.

Contrasting P-T Paths in the Eastern Himalaya, Nepal: Inverted Isograds in a Paired Metamorphic Mountain Belt

Petrology and phase equilibria of rocks from two profiles in Eastern Nepal from the Lesser Himalayan Sequences, across the Main Central Thrust Zone and into the Greater Himalayan Sequences reveal a Paired Metamorphic Mountain Belt (PMMB) composed of two thrust-bound metamorphic terranes of contrasting metamorphic style. At the higher structural level, the Greater Himalayan Sequences experienced high-T/moderate-P metamorphism, with an anticlockwise P–T path. Low-P inclusion assemblages of quartz + hercynitic spinel + sillimanite have been overgrown by peak metamorphic garnet + cordierite + sillimanite assemblages that equilibrated at 837 ± 59°C and 6·7 ± 1·0 kbar. Matrix minerals are overprinted by numerous metamorphic reaction textures that document isobaric cooling and re-equilibrated samples preserve evidence of cooling to 600 ± 45°C at 5·7 ±1·1 kbar. Below the Main Central Thrust, the Lesser Himalayan Sequences are a continuous (though inverted) Barrovian sequence of high-P/moderate-T metamorphic rocks. Metamorphic zones upwards from the lowest structural levels in the south are: Zone A: albite + chlorite + muscovite ± biotite; Zone B: albite + chlorite + muscovite + biotite + garnet; Zone C: albite + muscovite + biotite + garnet ± chlorite; Zone D: oligoclase + muscovite + biotite + garnet ± kyanite; Zone E: oligoclase + muscovite + biotite + garnet + staurolite + kyanite; Zone F: bytownite + biotite + garnet + K-feldspar + kyanite ± muscovite; Zone G: bytownite + biotite + garnet + K-feldspar + sillimanite + melt ± kyanite. The Lesser Himalayan Sequences show evidence for a clockwise P–T path. Peak-P conditions from mineral cores average 10·0 ± 1·2 kbar and 557 ± 39°C, and peak-metamorphic conditions from rims average 8·8 ± 1·1 kbar and 609 ± 42°C in Zones D–F. Matrix assemblages are overprinted by decompression reaction textures, and in Zones F and G progress into the sillimanite field. The two terranes were brought into juxtaposition during formation of sillimanite–biotite ± gedrite foliation seams (S3) formed at conditions of 674 ± 33°C and 5·7 ± 1·1 kbar. The contrasting average geothermal gradients and P–T paths of these two metamorphic terranes suggest they make up a PMMB. The upper-plate position of the Greater Himalayan Sequences produced an anticlockwise P–T path, with the high average geothermal gradient being possibly due to high radiogenic element content in this terrane. In contrast, the lower-plate Lesser Himalayan Sequences were deeply buried, metamorphosed in a clockwise P–T path and display inverted isograds as a result of progressive ductile overthrusting of the hot Greater Himalayan Sequences during prograde metamorphism.

Upper and lower plate juxtaposition, deformation and metamorphism during crustal convergence, Trans-Hudson Orogen (Quebec–Baffin segment), Canada

The lower plate of the Quebec–Baffin segment of Paleoproterozoic Trans-Hudson Orogen comprises parautochthonous Archean crystalline basement of the Superior Province (3.22–2.74 Ga) and imbricated Paleoproterozoic rift-margin strata (2.04–1.92 Ga). The collisional upper plate contains (1) allochthonous ophiolitic units (2.00 Ga), (2) a continental arc terrane (Narsajuaq arc; 1.86–1.82 Ga), (3) shelf sequence strata (ca. <1.93 Ga and >1.86 Ga) and basement orthogneisses (ca. 1.95 Ga), (4) foreland basin deposits (<1.93 Ga and >1.86 Ga), and (5) a batholith which intrudes the shelf and foreland basin rocks (Cumberland batholith; 1.86–1.85 Ga). Lower and upper plate assemblages were juxtaposed during a 1.82–1.79 Ga accretion–collision event, and deformed into regional-scale folds during two post-collisional folding episodes (ca. 1.76–1.74 Ga). As a result of the structural relief generated by the post-collisional folding and subsequent exhumation, all orogenic elements can be directly examined along a 400 km, across-strike transect. In the external zone (northern Quebec), lower plate rift-margin units were internally imbricated (Cape Smith Belt) and translated southward across the Superior Province basement. Paleoproterozoic amphibolite facies mineral growth is syn to post-thrusting. Tectonic imbrication has resulted in a structural inversion of the accreted (i.e. upper plate) ophiolite stratigraphy. North of the Cape Smith Belt, mid-crustal plutonic units of the Narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt's basal décollement. Within Narsajuaq arc, syn-magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion–collision faults. In the internal zone (southern Baffin Island), a tripartite orogen-scale tectonostratigraphy is exposed on the flanks of the post-collisional fold structures. At the lowest structural level, lower plate Superior Province orthogneisses are imbricated with Paleoproterozoic rift-margin units, and mineral growth is syn-thrusting. At intermediate structural level, upper plate orthogneisses are correlated with the Narsajuaq arc in northern Quebec. At the highest structural level, a north-facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence. Dominantly-monzogranite plutons of the Cumberland batholith are restricted to the upper structural level and indicate pre-emplacement imbrication of tectonostratigraphic units. Within both upper and lower plates, granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1.82–1.79 Ga accretion–collision structures. Exposures of Superior Province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units, from a narrow shear zone in the external zone to km-scale imbrication and more pervasive shear deformation in the internal zone. A minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units. Basement involvement during post-collision shortening is manifest as thick-skinned folding, indicating coupling across the collisional basal décollement.

Structural evolution of the High Himalayan Gneiss sequence, Langtang Valley, Nepal

Abstract Within the High Himalayan Crystalline Sequence (HHCS) of the Langtang Valley of central Nepal, four different lithotectonic units have been identified in the field. The structurally lowest unit, immediately in the hanging wall of the Main Central Thrust (MCT) records an inverted metamorphic sequence with kyanite-grade rocks overlain by rocks containing sillimanite. Pressure-temperature-deformation histories of the different units since Himalayan collision 50 Ma ago, in combination with geochemical data, indicates that the ‘apparent’ metamorphic inversion is a result of the tectonic juxtaposition along high-temperature shear zones that accommodated top-to-southwest thrusting. It is also argued that the contacts between different sillimanite-grade units are also tectonic in origin and that the HHCS in the Langtang Valley does not represent a structurally undisturbed section through the crust. Internally the different units contain minor extensional and thrust faults which at the same structural level developed at similar metamorphic conditions. Deformation along the high temperature shear zones of the HHCS are coeval with the earliest of these minor fault zones at the lowest structural levels. With time, this deformation migrated upwards and minor faulting within the upper levels of the HHCS took place at chlorite-grade. These minor fault zones are interpreted to be the result of dynamic spreading of the developing orogenic wedge associated with changes in the geometry of this wedge due to underplating and accretion of material to the base of the metamorphic pile.

The structure and petrology of ultrabasic rocks in the southern part of the Cuillin Igneous Complex, Isle of Skye

AbstractThe ultrabasic rocks of the southern portion of the Early Tertiary Cuillin Igneous Complex, Isle of Skye, are recognised as forming a Peridotite Series s.l. and have been separated into six distinct structural–lithological units. These units range from almost pure dunite (Unit 1, at the lowest structural level), through to feldspathic peridotites and allivalites (Units 5 and 6, at the highest structural levels). Detailed field and mineralogical studies indicate that both cumulus and postcumulus processes involving ultrabasic (picritic) magmas may be identified, and that the latter processes have significantly modified many of the primary features of these rocks.Layering, both modal and phase, is present within all six units, although it is more prominent within the higher units, especially Units 5 and 6. Differing orientations of fabrics defined by cumulus spinel and intercumulus plagioclase layers within Unit 3 indicate the important role of compaction and intercumulus melt migration. Unit 4 is extremely heterogeneous, involving material ranging in composition from peridotite to allivalite, and provides clear evidence for postcumulus melt movement, magma-mixing, disruption and brecciation. Units 5 and 6 developed with a more porous cumulus framework, giving rise to dendritic growths involving cumulus olivine and poikilitic plagioclase.It is concluded that postcumulus melt movement, injection and magma-mixing, involving ultrabasic magmas, were significant processes in the formation of the ultrabasic rocks of the Cuillin Igneous Complex.

Yukon-Tanana terrane: A partial acquittal

Research Article| April 01, 1990 Yukon-Tanana terrane: A partial acquittal Vicki L. Hansen Vicki L. Hansen 1Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 Search for other works by this author on: GSW Google Scholar Author and Article Information Vicki L. Hansen 1Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (4): 365–369. https://doi.org/10.1130/0091-7613(1990)018<0365:YTTAPA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Vicki L. Hansen; Yukon-Tanana terrane: A partial acquittal. Geology 1990;; 18 (4): 365–369. doi: https://doi.org/10.1130/0091-7613(1990)018<0365:YTTAPA>2.3.CO;2 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 The Yukon-Tanana composite terrane, northern Canada and eastern Alaska, is divisible into two distinct terranes on the basis of metamorphic cooling ages and inferred structural level. The Nisling terrane records Early Cretaceous cooling ages, whereas the Teslin-Taylor Mountain terrane records pre-Late Jurassic cooling. The Nisling terrane, present at the lowest structural levels within the Yukon-Tanana terrane, represents North American continental-margin crust that was partly subducted beneath the accretionary prism of a southwest-dipping (present-day coordinates) early Mesozoic subduction complex, represented by the Teslin-Taylor Mountain and Slide Mountain terranes. The Nisling is exposed as a result of mid-Cretaceous crustal extension subsequent to Jurassic crustal thickening related to Teslin-Taylor Mountain-Slide Mountain emplacement. Although the Nisling and Teslin-Taylor Mountain terranes have many petrologic and tectonic differences, their mutual relations provide a unified view of early to middle Mesozoic tectonics of the western North American margin, a view that dispels the notion that large tracts of the northern Cordillera are "suspect." This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Franciscan melanges compared with olistostromes of Taiwan and Italy

Details of origin of Franciscan melanges are unknown, although subduction is accepted as the controlling process. Some melanges near plate boundaries in Taiwan and Italy are evidently olistostromes. How do Franciscan melanges compare with these? The Lichi melange and units of “argille scagliose” type in the Northern Apennines rest upon normal marine sediments. The time of accumulation was brief, as shown by limiting time brackets. These key types of evidence for olistostrome origin are rare or absent in the Franciscan, but the pervasive shearing would probably have obliterated such evidence. Similarities between the above-cited olistostromes and Franciscan melanges include the following: argillaceous matrix; large and small blocks of sedimentary rocks and ophiolites; phacoidal and joint-block shapes; soft-sediment deformation in some sandstone; rotation of blocks; extreme dispersal of distinctive rocks; reappearance of older rocks at younger levels. Collectively, these similarities suggest that Franciscan melanges were originally assembled by olistostrome accumulation. Differences between presumed olistostromes and the Franciscan include the following, in addition to stratigraphic relations mentioned above. The Lichi melange shows faint original gross layering where shearing is minimal. Franciscan melanges show various compositional units, but shearing allows tectonic explanations. Blueschist metamorphism is rare or absent in olistostromes of Taiwan and the Northern Apennines. It occurs in the Franciscan not only in random blocks, but also as extensive units of schist and phyllite near the structural top of the complex, toward the “hanging wall” (Great Valley sequence). In the structurally lowest levels, only zeolite facies metamorphism is prevalent. Similar generalities apply to ages of rocks at highest and lowest structural levels. The age distribution would be just the opposite if the entire Franciscan were simply an east-dipping pile of olistostromes. It is concluded that neither subduction alone nor olistostromes alone could have produced all the features of Franciscan melanges, but both played an important role. Critical original features of olistostromes have been modified or destroyed by recurrent underthrusting.

The Ballybofey Anticline: A solution of the general structure of parts of Donegal and Tyrone

AbstractA tight early fold, the Ballybofey anticline, trends south‐eastwards across central Donegal. South‐west of its axial trace the Dalradian strata are inverted so that the rocks high in the Series occur at the lowest structural level, next to, and facing downwards, the supposedly Moinian psammites in the core of the Lough Derg antiform. Rapid facies changes occur in the Dalradian rocks across the Caledonian strike.

Metamorphic and Structural History of “Colville Batholith” Gneisses, North-Central Washington

The Tonasket Gneiss crops out along and east of the Okanogan Valley in north-central Washington. The predominant rocks are medium- and high-grade banded migmatitic and feldspathic gneisses, basic gneisses, subordinate isochemically metamorphosed amphibolites and calc-silicate rocks, and mylonitized derivatives of these rocks. These gneisses were considered by earlier workers to be the protoclastic border zone of a large batholith. The findings of this study indicate that the gneisses were formed mainly during regional metamorphism, which was followed by periods of mylonitization, gentle folding, and faulting. The earliest metamorphism of these rocks, as recorded in the amphibolites and basic gneisses, was probably in the granulite facies. The second metamorphism, which was of the amphibolite facies, is responsible for nearly all the current mineral assemblage and was accompanied by sodium, silica, and subordinate potassium metasomatism, producing feldspathized amphibolites and quartzo-feldspathic gneisses. For the most part both of these metamorphic facies resulted in synkinematic crystallization, which produced small-scale isoclinal folds, boudinage, and aligned mineral grains or streaks, all of which consistently trend west-north-west. These synkinematic rocks are well preserved in the lowest structural levels of the gneisses, which are in the eastern part of the mapped area. Higher structural levels are characterized by more complete feldspathization of isochemical rocks, more potassium activity, and more post-crystal-line deformation than are in the lower levels. However, the metamorphic grades are similar. Recrystallization following renewed deformation remained incomplete, but some porphyroclasts, especially of K feldspar, continued to grow after this deformation. The upper-medium or lower-high-grade assemblages that had formed during this stage did not undergo subsequent retrogression. The upper levels of the gneiss mass, now exposed along the southern, western, and northern boundaries, were subsequently sheared and mylonitized along subhorizontal s planes under low temperatures and in a dry environment. This mylonite zone probably represents a distributed flat thrust near the top of the gneiss body. Lineation in this zone parallels other linear features in the lower levels of the gneiss. The mass underwent broad gentle folding and arching after the mylonitization. Still later, high-angle faulting occurred along the western and southern borders of the area.