bluuebunny/crossref_metadata_2025_split huggingface dataset:
The intracratonic Irumide Belt of Zambia and its bearing on collision orogeny during the Proterozoic of Africa
Authors: Daly M. C. 
Published: April 2008

Summary
The Irumide Belt of Zambia is often quoted as being typical of African intracratonic mobile belts which involve no crustal shortening or major displacements. This belt is shown here to include the NW-facing foreland fold and thrust zone of the Southern Moçambique Belt and to have involved considerable crustal shortening. A crustal scale ‘pop-up’ structure separates this foreland from the internal higher-grade granulite-facies rocks of Malawi and Moçambique where major SE-facing nappe structures of Irumide age have been described. Several possible suture zones are identified and the Irumide-Moçambique Belt is interpreted as resulting from collisional processes during the Mid-Proterozoic. The adjacent Ubendian Belt represents a continental transform fault zone associated with crustal shortening across the Irumide Belt. The implications of these interpretations regarding the onset of plate tectonics in the Proterozoic and Archaean are briefly discussed.
Precambrian collision tectonics in Africa
Authors: Shackleton R. M. 
Published: April 2008

Summary
Stages of the collisional process are separated. Criteria are proposed by which A- and B-subduction may be distinguished. Late Precambrian structures in NE Africa are attributed to successive gentle collisions of island arcs; the sutures dip E or SE and relative plate motion was NW-SE. In the polycyclic Proterozoic Mozambique Belt of E Africa there are probably three sutures (Sekerr, Baragoi, Moyale-Shakiso) with ophiolites; they appear to dip E. It is argued that despite the lack of geochronological evidence, Archaean basement probably underlies the metasedimentary sequences, right across the Mozambique Belt of E Africa, from the Archaean Tanzania Craton on the W to the Archaean Craton, represented by Madagascar and S India, on the E. The belt is therefore interpreted as having been built by collision of continental plates. Widespread recumbent structures, complex and intense deformation and high-grade metamorphism imply crustal thickening and a multistage collisional history. Relative NW-SE motion was probably followed by N-S post-collisional ductile shear. The Archaean Limpopo Belt in southern Africa shows evidence of drastic crustal thickening and intense deformation in contrast to the much weaker deformation and little crustal thickening in the Kaapvaal and Rhodesian Cratons to the S and N. Collision is implied but in the absence of convincing ophiolites, subduction-type calc-alkaline magmatism and abyssal sediments, B-subduction remains unproven.
Post-collision isostatic readjustment in the Southern Banda Arc
Authors: Milsom J. , Audley-Charles M. G. 
Published: April 2008

Summary
The collision between the Australian continent and island arcs marginal to SE Asia is currently taking place in the Banda Arc region, with some of the most intense deformation on, and around, the island of Timor. From Late Miocene to Mid-Pliocene times the area was dominated by a compressional tectonic regime which resulted in the emplacement from the N of large thrust sheets on the deformed Australian continental margin. During the last 3 Ma, however, compression seems to have been unimportant in the islands and vertical movements, both positive and negative; have been commonplace and rapid.
Considerable departures from isostatic equilibrium are suggested by gravity surveys in the region and in some cases the sign of the isostatic anomalies accords well with the observed sense of vertical movement. In other areas, such as Northern Timor and the inner (volcanic) arc, uplift is occurring where the the gravity data suggest that there should be subsidence. A possible explanation is that a contribution to the high gravity values is made by a cold, and therefore dense, subducted slab which is now sinking independently after rupture near the continental margin. Because of the rupture, the sinking slab no longer exerts a downward pull on the overlying lithosphere which has thus been freed to rebound isostatically.
Collision tectonics in the Cordillera of western N America: examples from Alaska
Authors: Jones D. L. , Silberling N. J. , Coney P. J. 
Published: April 2008

Summary
The western edge of N America grew during the Mesozoic and Early Cenozoic by the addition of numerous accreted terranes consisting of fragments of island arcs, scraps of oceanic basins, seamounts, continental rise deposits and pieces of continents. The result of the accretionary process was production of new continental crust of extraordinary heterogeneity and structural complexity. This complexity is best portrayed in Alaska, where virtually 95% of the state was assembled during the Cretaceous, with attendant folding, large-scale thrust faulting, penetrative deformation and regional metamorphism. In this brief report, we discuss evidence for large-scale displacement of Wrangellia and for its time of incorporation into the Alaskan tectonic collage. The structures produced during accretion of this large coherent body are contrasted with those produced during coeval collapse of interval flysch basins that lay N of Wrangellia and with those associated with emplacement of enormous thrust sheets of oceanic rocks on top of continental platforms in Central and Northern Alaska. Despite compelling evidence for thousands of km of relative displacement between accreted terranes and between the terranes and the N American Craton, the resulting collage carries few readily discernible signals that allow application of currently popular simple plate tectonic models.
A critique of graptolite classification, and a revision of the suborders Diplograptina and Monograptina
Authors: Rigby J. 
Published: April 2008

Summary
Three broad styles of modern graptolite classifications are identified, each being based on a framework laid down in the nineteenth century. Each style is critically reviewed with regard to its potential for the expression of presently know phyletic relationships. Following the style of classification adopted by Bulman (1970), family and subfamily taxa within the suborders Diplograptina and Monograptina are revised.
Heterochrony, punctuated equilibrium, and graptolite zonal boundaries
Authors: Finney Stanley C. 
Published: April 2008

Summary
Many graptolite taxa are important biostratigraphically because their first appearances in stratigraphic sequences represent unique events that are recognizable and correlative worldwide. However, because the direct ancestry of many of these taxa are cryptic, the biostratigraphical value of their first appearances may be questioned.

Astogenetic and ontogenetic studies of well-preserved, isolated specimens of
Nemagraptus gracilis, Dicellograptus
(four species),
Leptograptus trentonenesis, Glossograptus ciliatus
and
Glyptograptus euglyphus
from the middle Ordovician Athens Shale of Alabama show that considerable graptolite evolution occurs through heterochony. And, what are here considered to be small heterochronic changes, often produce large morphological discontinuities between ancestors and descendants. In turn the presence of these morphological discontinuities in graptolite sequences suggest that graptolite evolution follows the model of punctuated equilibrium, rather that that of phyletic gradualism. Whether nor not this is the case is here considered to be a matter of semantics. More importantly, heterochronic evolution is here considered to explain the cryptic ancestry of many graptolite taxa, and with this mode of evolution in mind, the graptolite biostratigrapher may find it easier to determine ancestor-descendent relations and thus recognize those first appearances that are valid for biostratigraphical correlation.
Morphometric studies of
Climacograptus
(Hall) and the phylogenetic significance of astogeny
Authors: Mitchell Charles E. 
Published: April 2008

Summary

Climacograptus typicals
and its varieties, along with the closely related
C. pygmaeus
, are abundant at many horizons in North American Caradoc strata. Morphometric analysis of colony form in
C. typicalis
and
C. pygmaeus
, from large collections of isolated, three-dimensionally preserved material, reveals low levels of variation for a colony. The analysis of astrogeny and its relationship to colony form suggests that many aspects of overall form are highly correlated with astogeny and that differences in the astogenetic pattern of
C. typicalis
and
C. pygmaeus
are correlated with some of the differences in colony form that distinguish the two species.

A theoretical framework is needed for the incorporation of astogenetic data into systematic and phylogenetic practice. The graptoloid colony was an integrated, functional entity and a unit of selection. Astogeny was a colonial character that displayed a morphometric and evolutionary pattern of change that suggested an analogy with von Baer’s Law and its evolutionary interpretation: morphogenesis during astogeny proceeds from generalized states to specific stages and shared astogenetic pattern is indicative of shared descent. This is proposed as a working hypothesis in the formulation of systematic relationships and phylogenies.
The lower Silurian graptolites of Oued In Djerane: a study of populations at the Ordovician-Silurian boundary
Authors: Legrand P. 
Published: April 2008

Summary

Three zones can be distinguished in the Oued in Djerane region, Algerian Sahara, from the bottom to the top:
Climacograptus (Climacograptus)
tilokensis sp. nov. Zone;
Diplograptus(?) kiliani
Zone;
Glyptograptus (Glyptograptus) tariti
Zone. An example of a detailed study of the different forms of the
Climacograptus (Climacograptus) normalis
group is given. In addition the problem of the Ordovician-Silurian boundary and its application to the Algerian Sahara sequence is discussed.
Silurian graptolites from N Greenland
Authors: Bjerreskov Merete 
Published: April 2008

SummaryRecent collections of Silurian graptolites from N Greenland have yielded much new information on the biostratigraphy of the area. The graptolites occur mainly in slope and trough sediments, some of which are now incorporated in the N Greenland foldbelt. The full sequence of graptolite zones established in Britain cannot be recognized. The oldest Silurian graptolites indicate a lower LlandoveryatavusorcyphusZone age and above this an intermittent succession of graptolite faunas are found throughout the Llandovery, Wenlock, Ludlow and into the Pridoli (transgrediensZone only). There are differences between the zones recognized in the western and eastern parts of N Greenland. ThespiralisZone appears to be the most extensive, occurring all over N Greenland. The graptolite sequences in N Greenland correspond most closely with those reported from the nearby Canadian Arctic Archipelago, but are easily correlated with the graptolite faunas from the Cordillera region, especially from the northern part. Differences of faunal assemblages are tentatively correlated with changes in sedimentary environments, possibly indicating a difference in the spatial distribution of species in a living graptolite community.
A mathematical study of the
Cyrtograptus sakmaricus
lineage with discussions of the evolutionary trends in this lineage
Authors: Huo Shi-cheng , Shu De-gan , Li-pu Fu 
Published: April 2008

Summary

On the basis of a mathematical study of the
Cyrtograptus sakmaricus
lineage, we have found that all members of this lineage have the shape of a logarithmic spiral, as seen in plan view, which may be expressed in terms of logarithmic equation
ρ
= Ae
aθ
. Tightness of coiling and the shape of the volution are determined by the value of
a
. The principle and methods concerned are presented. This value which controls the form and function of cyrtograptids, increases in stratigraphically higher members and may be used as a basis for dividing the genus
Cyrtograptus
into various species. The cyrtograptids of the
Cyrtograptus sakmaricus
lineage from the Silurian deposits in Ziyang, Shaanxi, China show trends toward increase in number of thecae in number of volutions in tightness of coiling.
Unconformities in the Cenozoic of the North-East Atlantic
Authors: Pearson I. , Jenkins D. Graham 
Published: April 2008

Abstract
Unconformities in the North-East Atlantic and Norwegian Greenland Sea have been examined from 60 DSDP boreholes, and correlated to a single biostratigraphic time-scale of the Cenozoic. The unconformities can be divided into two types: (a) widespread events with equivalent episodes outside this study area, and (b) locally restricted events.
Unconformities equivalent to those recognized by Vail & Hardenbol (1979) and Keller & Barron (1983) are present in the late Pliocene, Miocene, mid-Oligocene and late to mid-Eocene. Hiatuses corresponding to other unconformities noted by Vail & Hardenbol are difficult to recognize, being encompassed within large local unconformities.
Hiatuses have been formed either by (a) a change in bottom water activity or (b) tectonic events, diapirism and slope instability. Recognition of these types through a series of filters allows greater accuracy in defining circulatory events.
Sedimentation on mid-ocean sediment drifts
Authors: Kidd Robert B. , Hill Philip R. 
Published: April 2008

Summary

The paper reports results from DSDP Leg 94, where holes were drilled in the Feni and Gardar sediment drifts, with particular emphasis placed on studying sediment waves that ornament the surfaces of the drifts. The Feni Drift is a 1500–1700 m thick accumulation of sediment constructed on an early Eocene to late Oligocene seismic reflector. The Gardar Drift is 1300–1600 m thick, and is probably built directly on late Eocene oceanic basement. At Site 610 on Feni Drift, the deepest hole penetrated to upper Miocene sediments at 720 m, and at Gardar Drift, to upper Miocene sediment at 500 m. Pre-site surveys showed that the sediment wave fields on the drift surfaces were more irregular than Gloria sidescan records suggest, consisting of interfingered
en echelon
mounds. The waves did not show any well-defined sense of migration. The sediments recovered at Sites 610 and 611 show that the drifts consist of pelagic ooze and calcareous mud, with very few primary sedimentary structures preserved. No significant differences were observed between lithologies on sediment wave crests and those in through locations. On the other hand, systematic differences in sedimentation rates between crest and trough holes at Gardar Drift support the hypothesis that the waves migrated during the Pliocene but were evenly draped during the Pleistocene.
Turbidite deposition and the origin of the Madeira Abyssal Plain
Authors: Weaver P. P. E. , Searle R. C. , Kuijpers A. 
Published: April 2008

Summary
Extensive investigations in the Madeira Abyssal Plain have revealed thick sequences of turbidites. Individual turbidites can be correlated over an area of 2° × 2°, and their emplacement can be shown to coincide with glacial onsets and terminations. The turbidites originate on the NW African margin and contain up to 2% organic carbon. They are virtually ungraded massive silts and clays, some with coarser basal layers.
Seismic records through the abyssal plain show distinct changes in sediment character at a few hundred metres depth. Rates of accumulation are calculated from piston cores to be about 7.5 cm/1000 years, suggesting that the turbidites are a recent phenomenon. If the link with glacial onsets and terminations has held throughout, the first major turbidites may have been emplaced at 2.4 Ma, at the onset of Northern Hemisphere glaciation. Some turbidites may also have been emplaced during the supposed late Miocene glaciations. The Madeira Abyssal Plain is therefore a recent feature, and before its initiation the area would have had a ridge-flank type topography.
North-east Atlantic Neogene benthic foraminiferal faunas: tracers of deep-water palaeoceanography
Authors: Schnitker Detmar 
Published: April 2008

Summary
The qualitative and quantitative composition of deep-sea benthic foraminiferal faunas from DSDP Sites 400 (4399 m), 552 (2301 m), 553 (2329 m), 554 (2576 m), and 555 (1659 m) in the north-eastern North Atlantic have undergone strong changes during the past 27 Ma, very probably reflecting changes in the deep oceanic environment. Throughout this time interval the faunas of the deepest site (400) were distinctly different from the faunas of the shallower sites, indicating a persistent depth stratification of the deep water masses. The strongest qualitative faunal change occurred during the middle Miocene, about 15–13 Ma ago, when at all depths several species that had persisted since the Oligocene became extinct, several new species appeared for the first time, and the quantitative faunal composition of all sites changed drastically. This change is thought to be indicative of the (re-)initiation of North Atlantic Deep Water formation, and occurred at the same time as the rapid growth of the Antarctic ice sheet and a strong change in the carbon and oxygen isotopic composition of deep sea foraminifers worldwide. At the shallower and intermediate sites, the composition of benthic faunas changed again about 6.5–7 Ma ago, a change that also affected several key species at the abyssal site as well. This change was probably caused by the closure of the Mediterranean, the Messinian Crisis. The introduction of a new ‘shallow water’ species into the bathyal and upper abyssal sites, as well as a change in dominance in the deep abyss at 2.5 Ma, signifies the onset of modern glacial conditions.
Early to Middle Miocene benthic foraminiferal faunas from DSDP Sites 608 and 610, North Atlantic
Authors: Thomas Ellen 
Published: April 2008

Summary

Benthic foraminifera were studied in lower and middle Miocene cores from DSDP Site 608 (3534 m, 42°50′N., 23°05′W.) and Site 610 (2427 m, 53°13′N., 18°53′W.), north-eastern North Atlantic Ocean. There were extremely high relative abundances of
Bolivina spp.
between about 19.5 and 17 Ma at both sites. These high abundances were probably caused by oxygen deficiency in the bottom waters, which is suggestive of sluggish circulation.

Some changes in the faunal composition occurred at the beginning of the period of sluggish circulation (20–19 Ma), and between about 15 and 14 Ma, at which time there was a global increase in oxygen isotopic ratios in benthic foraminiferal tests. The changes in faunal composition involved less than 20% of the total fauna. Most first and last appearances of benthic species were not coeval at the two sites. The first appearance of
Cibicidoides wuellerstorfi
is an exception and occurred at the same time as the middle Miocene increase in oxygen isotopic ratios.
Miocene to Recent bottom water masses of the north-east Atlantic: an analysis of benthic foraminifera
Authors: Murray J. W. , Haddon C. A. , Powell A. D. J. , Weston J. F. 
Published: April 2008

Summary
The modern benthic foraminifera of the north-east Atlantic Ocean show a distribution pattern related to bottom water masses. Varimax factor analysis of data from depths > 2000 m gives factors which correlate with North-East Atlantic Deep Water (NEADW), North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW); a fourth factor corresponds with the basal part of the Mediterranean Water.
Miocene to Pleistocene benthic foraminiferal assemblages have been compared with the modern assemblages. On the assumption that species have not changed their environmental preferences through time, the fossil assemblages can be used to infer the past existence of water masses similar to those of today. The results presented here show that in the late Miocene, AABW penetrated to the southern flank of Rockall Plateau at the time of build-up of the East Antarctic ice sheet. During the early Pliocene the rate of production of NADW increased, perhaps as a result of a decrease in size of the Antarctic ice sheet, which in turn led to the southward retreat of the limit of penetration of AABW. Throughout much of the Pleistocene, as now, AABW has not penetrated north of the Azores Ridge. NEADW was the most extensive bottom water mass in the late Pliocene and Pleistocene.
Palaeoclimatic and palaeoceanographic development in the Pliocene North Atlantic:
Discoaster
accumulation and coarse fraction data
Authors: Backman Jan , Hermelin Otto , Pestiaux Pierre , Zimmerman Herman 
Published: April 2008

Summary

Accumulation variations of the calcareous nannofossil genus
Discoaster
have been determined from a high latitude North Atlantic sediment sequence (DSDP Hole 552A) of late Pliocene age. Spectral analysis for the preglacial Pliocene reveals a dominant quasiperiodicity associated with obliquity-induced temperature variations in surface water. Spectral peaks corresponding to the 100 Kyr eccentricity and the 21 Kyr precession periodicities are also detected.

Coarse fraction analysis of core V26-145 (Blake Plateau) illustrates the Pliocene development of an increasingly intense subtropical circulation in the North Atlantic. A trend towards increased current velocity occurs through the early Pliocene and probably reflects the progressive emergence of the Panamanian Isthmus, and concomitant intensification of surface water circulation of the North Atlantic subtropical gyre system. In the Rockall Plateau area (Hole 552A), however, no evidence is seen for a related surface water warming.
Discoaster
accumulation data indicate that the preglacial Pliocene development of surface water temperature in the high latitude North Atlantic reflects a progressive climatic deterioration.
Palaeoceanographic setting of the Callovian North Atlantic
Authors: Robertson A. H. F. , Ogg J. G. 
Published: April 2008

Summary
DSDP/IPOD Site 534 located in the Blake Bahama Basin adjacent to the Florida coast off eastern USA exemplifies the early opening stages of the North Atlantic in Callovian (mid-Jurassic) time.

The earliest sediments accumulated on a rugged ocean floor located near the edge of a WNW-ESE-trending fracture zone, not far from the spreading axis. At that time the ocean was
c.
500 km wide with a borderland experiencing a tropical climate. Input of both inorganic and organic material was mostly of marine origin, but also included quartz, illite, chlorite and large quantities of fine plant material. Carbonate banks flourished during a time of raised sea-level, shedding large quantities of sediment, mostly peri-platform ooze, which was redeposited by turbidity currents to Site 534 after a marginal ridge (Blake Spur ridge) was finally breached during later Callovian time. The site subsided
c.
300 m during the Callovian and for a time lay below the CCD which was still shallower than 3000 m. Distinctive hummocky seismic reflections and sedimentary structures in the cores suggest that sluggish bottom water circulation existed, giving rise to sediment-drifts (‘muddy contourites’). Hydrothermal precipitates drifted westwards from the ridge. Inferred trade wind patterns favoured extensive coastal upwelling along the African borderland, the closest margin to Site 534 during the Callovian. The favoured origin of numerous, but volumetrically minor, black shale intervals is that marine organic matter was concentrated where the oxygen-minimum-zone intersected the shelf-continental rise and that this material was subsequently redeposited by currents and/or gravity flows to form layers which readily became black and de-oxygenated during diagenesis.