bluuebunny/crossref_metadata_2025 huggingface dataset:
Oceanic fracture zones
Authors: WHITE R. S. , WILLIAMS C. A.
Published: December 2007

Report of the William Smith meeting of the Geological Society, London held at Burlington House on 1-2 May 1985, and introduction to published papers and abstracts.

Oceanic fracture zones are one of the most conspicuous features of the ocean basins. They are formed where offsets occur in the oceanic spreading centres, and they leave scars that stretch thousands of kilometres across entire ocean basins. Through the 1960s and 7Os, oceanic fracture zones were mapped extensively by magnetic and bathymetric surveys and were used frequently in global reconstructions Only in the past 5 years or so have they been studied intensively in their own right using more and more sophisticated geophysical and geological techniques.

This conference brought together 25 speakers from around the world to present some of the huge amount of recent work on oceanic fracture zones. Perhaps the most striking feature of the meeting was that with modem high resolution geophysical tools it is at last becoming possible to narrow the gap between observations from remote measurements and those of field geologists. That does not necessarily mean that interpretations are becoming easier;rather the converse, because the closer you look the more complicated things become. But there is hope at last that soon we may be able to make geological maps of the ocean floor just as we can on land.

A great deal can be learnt about the tectonics of young oceanic features simply by looking at the bathymetry and the ‘grain’ of the seafloor. During the meeting a
Structure and seismotectonics of the Vema Fracture Zone, Atlantic Ocean
Authors: LOUDENR K. E. , WHITE R. S. , POTTS C. G. , FORSYTH D. W.
Published: December 2007

The structure and seismo-tectonics of the Vema Fracture Zone and the adjacent regions of its transverse ridge to the south and the Mid-Atlantic Ridge spreading centre to the north is discussed, based on an analysis of seismic refraction profiles, micro-earthquake activity and gravity anomalies. Crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers. Micro-earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge-transform intersection.

Immature crust, similar to that reported by
Fowler (1976)
from the FAMOUS area, lies beneath the spreading centre. The lower crust thins and normal mantle appears as the fracture zone is approached. The thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection. This is also the locus of current microearthquake activity at sub-bottom depths of 3–4 km, which a composite fault-plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley.

Within the central region of the fracture zone valley, the crust thickens but still retains an anomalous velocity structure. Seismic layer 3 is absent and crustal velocities are low. This is probably the consequence of attenuated crustal accretion, of faulting and cracking, and of hydrothermal alteration. The pattern of residual gravity anomalies, after correction for topography, for sediment thickness and for mantle temperature variations, is consistent with the crustal structure defined by the seismic experiments. The gravity measurements indicate that there is a broad thinning of the crust over 20–40 km in the vicinity of the fracture zone, on which are superimposed localized regions of low and high densities.

To the south of the fracture zone, the crust thickens and assumes a normal velocity structure including a mature seismic layer 3. The seismic Moho is warped upwards under the transverse ridge, suggesting a tectonic origin from buckling rather than uplift by serpentinization.
The structure of the Charlie–Gibbs Fracture Zone
Authors: CALVERT A. J. , WHITMARSH R. B.
Published: December 2007

This short paper summarizes the results of a major combined seismic refraction and gravity survey carried out over the Charlie–Gibbs Fracture Zone. Crust 3–4.5 km thick, and possessing an anomalously low seismic velocity structure, was found beneath the fracture zone and gradual crustal thinning was detected over some 40 km to the south of the transform. The most plausible explanation for the ridge to the north is that it consists of uplifted normal crust.
A 3-dimensional analysis of magnetic anomalies over fracture zones in the Cretaceous Magnetic Quiet Zone (Madeira Abyssal Plain)
Authors: VERHOEF J. H. , DUIN E. J. TH.
Published: December 2007

Magnetic anomalies over fracture zones in a Magnetic Quiet Zone are not complicated by reversals in the earth's magnetic field and thus are easier to interpret, showing only the effect of fracture zones as such. Observations on the Kane Fracture Zone (

Twigt
et al.
1983

), Tyro Fracture Zone (
Slootweg & Collette 1985
) and other fracture zones in the Cretaceous Magnetic Quiet Zone all point to an enhanced magnetization over fracture zones, mainly situated on the younger side of the fracture zone axis. A model which accounts for this, relates the asymmetric magnetization to the asymmetric development of topography at the intersection of fracture zone and transform fault.

The three dimensional analysis of Hayes, Charis and Cruiser Fracture Zones, presented in this study, leads to a distribution of the magnetization of the oceanic crust which may be interpreted in terms of a varying magnetization intensity or a varying thickness of the magnetic layer. On the younger side of the fracture zone axis the magnetization is systematically higher than average (up to 6 A/m) in the intensity model. Also, there are indications that there exists a reduced magnetization on the older side in several places. In the study area a few large changes in seafloor spreading direction occur. These are accompanied by anomalies which do not conform to the general model. Evidently, the proposed mechanism of emplacement of magnetic bodies may not be effective under these extreme circumstances. For the remainder, the general validity of the magnetic fracture zone model is confirmed.
The Fifteen Twenty Fracture Zone and the North American–South American plate boundary
Authors: ROEST W. R. , COLLETTE B. J.
Published: December 2007

The Fifteen Twenty Fracture Zone offsets the Mid-Altantic Ridge over 195 km in a sinistral sense. It was surveyed in 1982 between 30
°
W and 60
°
, i.e. from anomaly 34 on the African plate to anomaly 34 on the American plate where it disappears as the Barracuda Ridge into the Caribbean subduction zone. The fossil limbs of the Fifteen Twenty Fracture Zone show a remarkable symmetry to both sides of the Mid-Atlantic Ridge. Barracuda Ridge appears to have undergone a later deformation. The sections of the Fifteen Twenty FZ between 49
°
W and 54
°
W to the west of the Mid-Atlantic Ridge and between 37.5
°
W and 42.5
°
W to the east do not exhibit the normal characteristics of a fracture zone but have elongate and extremely narrow transverse ridges (dykes) along the fracture zone axis. The present transform domain shows signs of leaking. This is interpreted as a result of a fairly recent (about 7 Ma
bp
) change in spreading direction of the area.

This change in spreading direction seems to have affected only the area south of 17
°
50′N. It may have resulted from a northward migration of the plate boundary between the North American and South American plates and the coming into existence of a triple point near 16
°
N. From this reconstruction, the location of the differential pole between the North American and South American plates is 16
°
N/53.5
°
W. This location leads to increasing compression from 53.5
°
to 60
°
W, affecting the Barracuda Ridge and Trough with a N–S shortening of about 16 km in 7 Ma. From 53.5
°
to 45
°
W, N–S extension occurred (about 19 km near the Mid-Atlantic Ridge), which led to the origin of the volcano–tectonic complex of Researcher Ridge, Researcher Trough and Royal Trough to the west of the Mid-Atlantic Ridge.
The palaeoenvironment and deformation of
Beaconites
-like burrows in the Old Red Sandstone at Gortnabinna, SW Ireland
Authors: O'SULLIVAN M. J. , COOPER M. A. , MACCARTHY I. A. J. , FORBES W. H.
Published: December 2007

The Gortnabinna area contains a number of well-preserved, deformed burrows within the fluviatile Old Red Sandstone of S Ireland. Detailed logging of the local succession enables subdivision into four facies; Facies 1 represents sheet-flood deposits, Facies 2 ephemeral stream channels, Facies 3 marginal sheet-flood deposits and Facies 4 low-energy settling of fines. The burrows occur in Facies 1 and 4. The sediments are now folded into a large, upright, westward plunging anticline. The fold has a well-developed axial planar cleavage and is extended along strike by listric normal faults. The burrows in Facies 1 were originally circular and orthogonal to bedding and reveal a N–S layer parallel shortening strain of 2:1 followed by a shear strain of 1.48 during folding. These burrows have a bimodal distribution of diameters with peaks in the ranges 50–80 and 110–170 mm. The burrows of Facies 4 are sub-parallel to the bedding plane and are generally curved, with a maximum width of 180 mm and lengths of up to 2.3 m. These burrows display passive infill following abandonment. The burrows are morphologically similar though distinct from
Beaconites barratti,
which has been described from elsewhere in the Old Red Sandstone, and are tentatively interpreted as lungfish burrows.
Transport and accumulation of river-derived sediment on the Washington continental shelf, USA
Authors: STERNBERG R. W.
Published: December 2007

The combined results of many physical oceanographic, geochemical, and geological investigations provide a coherent picture of the sedimentology of the Washington continental shelf. The major source of modern sediment is the Columbia River. Sediment on the shelf is transported by bottom currents resulting from surface wind stress and gravity waves which occur as individual storm or wave events lasting one to several days, primarily during the winter months.

Field measurements of the sediment response to wind- and wave-generated bottom currents suggest that the modern sediment is transported as suspended load and disperses primarily northward parallel to the isobaths and is associated with a well-developed mid-shelf silt deposit. Smaller quantities of sediment are transported seaward over the shelf edge and into the numerous submarine canyons that incise the shelf. Analysis of available wave data suggests that thresholds of sediment motion were exceeded on the order of 22%, 16%, and 1.5% of the time annually on the inner, central, and outer shelf respectively. Bottom current measurements suggest that threshold conditions were exceeded 13% and 6% of the time annually on the inner and central shelf respectively.

Measurements of accumulation rates using
210
Pb geochronology show that about 67% of the annual sediment discharge from the Columbia River is being deposited in the mid-shelf silt deposit; 6% is being deposited over the shelf edge on the open slope, and 11% is accumulating in the four major submarine canyons.
Magnetic mapping of the Butterton Dyke: an example of detailed geophysical surveying
Authors: SOWERBUTTS W. T. C.
Published: December 2007

A detailed vertical gradient magnetic survey of part of a small intrusion known as the Butterton Dyke has been made with the aid of a microcomputer-based data gathering system. The results of over 16 500 magnetic measurements made by one person in less than a day have given results which reveal a detailed pattern of magnetic anomalies. From these it is possible to trace the course of two dykes for a distance of over 300 m, and identify places where they change direction and show small offsets. The advantage of making vertical gradient rather than total field magnetic measurements include a faster surveying speed and better resolution of near-surface anomalies.
The history of Quaternary sedimentation in the UK sector of the North Sea Basin
Authors: CAMERON T. D. J. , STOKER M. S. , LONG D.
Published: December 2007

Reconnaissance seismic and sampling surveys have shown that up to 500 m of Lower and early Middle Pleistocene deltaic sediments are preserved in the UK sector of the North Sea. The distributions of delta-related facies record the progressive expansion of peripheral delta systems northwards across the southern North Sea during the early Pleistocene. A late Middle Pleistocene unconformity is present throughout the UK sector, and sedimentation above the unconformity was dominated by glacial erosional and depositional processes. A complex system of deeply incised valleys was eroded during each of three regional Middle and Upper Pleistocene glaciations and the valleys have been filled or partially filled by glaciolacustrine or glaciomarine and interglacial marine sediments. The valleys of the earliest (Elsterian) glaciation are up to 400 m deep in the southern North Sea. The Saalian and Weichselian valleys are commonly more than 100 m deep, they are eroded through their contemporary tills nearshore but occur well beyond the limits of these tills offshore. The origins of these valleys are attributed to a combination of fluvial, subglacial and periglacial processes
Tectono-sedimentary evolution of the Bowland Basin, N England, during the Dinantian
Authors: Gawthorpe R. L.
Published: December 2007

The Bowland Basin is a NE–SW-trending sedimentary basin, lying between the Askrigg–Bowland High to the N and NW and the Central Pennine High to the SE. The basin was initiated in ? late Devonian–Tournaisian times. Sediment thickness and facies variations within the Dinantian succession show marked asymmetry on both a basin-wide and intra-basin scale, as a result of approximate NW–SE to N–S extension across normal faults. Thickening of the sedimentary sequence from NE to SW over much of the basin resulted from fault-controlled extension along the SE margin of the basin with downthrow to the NW, giving the basin a half-graben character. Extension occurred in two distinct episodes, marked by basin-wide debris flow and sedimentary slide/slump deposits. During the late Chadian/early Arundian the simple half-graben became modified by a series of NE–SW-trending antithetic and NW–SE-trending transfer faults associated with local unconformities and marked lateral thickness and facies variations. This interval marks the transition from carbonate ramp to slope sedimentation and a temporary halt in carbonate supply to the basin. The second episode of extension, the late Asbian/early Brigantian, marks the break up of the northern basin margin, ending major carbonate deposition within the basin. Reactivation of these structures at the end of the Carboniferous created the Ribblesdale Fold Belt as a result of NW–SE to N–S compression/transpression. Application of this research to areas surrounding the Bowland Basin has led to the recognition of additional shelf basins, the orientation of which suggests control by normal and transfer fault systems. Discussion and
Geochemical and geological constraints on the formation of unconformity-related vein baryte deposits of Central Europe
Authors: DILL H. , NIELSEN H.
Published: December 2007

Baryte vein deposits confined to the unconformity between Palaeozoic or Precambrian basement and the Permian–Triassic cover are of widespread occurrence in Central Europe. Age dating carried out on pitchblende predating and postdating baryte, indicates that baryte mineralization took place between Lower Permian and Lower Jurassic times (290 to 200 Ma).
87
Sr/
86
Sr ratios exclude any derivation of ore-bearing fluids from a mantle source and point to a crustal origin. Sulphur was derived from sulphate and sulphide contained in the host rocks folded during the Variscan orogeny (Sudetian phase) and from the Permian and Mesozoic sedimentary cover. Strontium isotopes bear witness to a mixture of granite-derived strontium and strontium from the enclosing wall rocks. The unconformity is considered to be a first-order geohydraulic surface which favoured fluid migration
via
convective cells. The thermal convection may be related to Variscan granites which might have provided additional heat by the decay of U, Th and K.
The iron ores and associated sediments of the Chichali formation (Oxfordian to Valanginian) of the Trans-Indus Salt Range, Pakistan
Authors: HALLAM A. , MAYNARD J. B.
Published: December 2007

A distinctive and widespread belemnite-bearing shale of the Tethyan region is well exposed in the Trans-Indus Salt Range as the Chichali Formation, where it contains two workable ironstone seams. Facies analysis indicates a pronounced deepening in the Upper Oxfordian and Kimmeridgian followed by two regressive cycles marked by a transition from glauconitic–phosphatic shales to glauconitic silts and sands and finally to winnowed and oxidized glauconitic sandy ironstones. The first cycle ends in Berriasian belemnite-rich beds; the second cycle, which has few belemnites, extends into the Valanginian, and is followed by deltaic sediments of the Lumshiwal Formation.

Comparison with other areas indicates that the source area for the clastic components was the Indian Shield to the east and south. The increased clastic supply in the Upper Jurassic–Lower Cretaceous is thought to result from tectonic uplift in this source area rather than from climatic change.

The ironstone beds are distinctive in being pelletal glauconites compared with the oolitic chamosite and goethite so typical of the Jurassic elsewhere. Enrichment of the glauconite to commercial grade was by winnowing and oxidation on the seafloor. The absence of oolitic chamosite and goethite could be related to generally deeper water and an overall Eh in the environment too high to allow iron to migrate in appreciable quantities as Fe
2+
.
Sediment exchanges across the coastal margins of NW Europe
Authors: KIRBY R.
Published: December 2007

This contribution serves as an introduction to the ‘Coastal Margins as Sediment Traps’ thematic set.

One of the most basic questions which faces those concerned with sediment transport in the marine environment is whether the coastal margin is a net source or sink for sediment. The coastal margin is taken to mean the coast itself with its cliffs, bays and estuaries, and the immediate coastal zone stretching a few hundred metres to a few kilometres off shore. It is not clear to what extent the coasts of NW Europe have recovered from the last period of depressed sea level. Uncertainty exists as to whether sediment lost from the coastal margin is deposited in offshore sinks, whether sediment originally laid down on the continental shelves is returning to depositional sites located principally in bays and estuaries, or more particularly whether all coastal margins are subject to the same balance of processes.

The reason for this lack of knowledge is clear. The calculation of the sediment budget of an estuary is difficult. The calculation of the budget for a whole coastal sector, including the continental shelf fronting it, is even more problematical. For entire seas our capacity for reliable quantification at the present time is negligible.

A perspective on this issue can be obtained by considering the situation in a parallel discipline. Oceanographers have been studying the circulation of our shallow seas for 70–80 years, latterly with co-operative experiments using tens of simultaneously deployed, self-recording, current meters and the mathematical models
Transport of clay minerals and anthropogenic compounds into the German Bight and the provenance of fine-grained sediments SE of Helgoland
Authors: IRION G. , WUNDERLICH F. , SCHWEDHELM E.
Published: December 2007

A large mud body, 500 km
2
and up to 21 m thick, SE of Helgoland has been investigated using geochemical, mineralogical, and sedimentological methods. The analysis of 27 sediment cores from this area revealed that, overlying older fine-grained sediments, there is a 'cake' of muddy, polluted sediments, up to 3 m thick, which has accumulated over the last 100 years.

It is assumed that human activities like dredging and dumping are responsible for this rapid rate of deposition. Generally it has been estimated that all of this mud has been supplied by rivers entering this part of the North Sea. But analyses of sediments from the Ems, Weser, and the Elbe rivers show that, under normal conditions, the river sediments do not enter the open sea but are trapped in the estuaries. It may, therefore be concluded that much of the underlying unpolluted sediments of the mud area were derived from elsewhere or were laid down during an earlier period. One possibility is that the lower sediments were deposited when the German Bight was a sheltered area, protected by a barrier of morainic sediments in the north between Helgoland and Schleswig Holstein. This barrier must have been breached more than 2000 years ago.
Sediment exchanges along the coastal margin of the Moray Firth, Eastern Scotland
Authors: REID G. , McMANUS J.
Published: December 2007

The Moray Firth is the largest of only three major coastal embayments on the East Coast of Scotland. The relationships between the offshore, coastal margin and fluvial sediments are examined in terms of processes and deposits. These suggest that the large post-glacial accretions of sediment infilling the inner firths are in part attributable to transport of material from the offshore zone. Since the fluvial input to the area appears to be relatively small and much is retained on estuarine tidal flats it is likely that the offshore contribution is the most important constituent of the deposits of the coastal margin. The other major source is glacial and fluvioglacial material now being reworked by marine activity.
Delimitation of a cryptic Eocene tectono-thermal domain in the Eastern Cordillera of the Bolivian Andes through K–Ar dating and
40
Ar–
39
Ar step-heating
Authors: McBRIDE S. L. , CLARK A. H. , FARRAR E. , ARCHIBALD D. A.
Published: December 2007

New K–Ar ages and
40
Ar–
39
Ar age spectra for samples from granitoid rocks and meta-sediments of the Cordillera Real, Bolivia, reveal a complex mid-Tertiary tectono-thermal history for this segment of the Central Andean eastern cordillera. Previous radiometric dating of the Cordillera Real intrusions defined two, areally-distinct, plutonic domains: in the N, Upper Triassic–Lower Jurassic; in the S, Upper Oligocene–Lower Miocene. Our new K–Ar ages, many from deformed intrusions, fall between these two episodes. For the variably foliated Zongo massif, there is a marked discordance between biotite and muscovite K–Ar dates, each decreasing consistently from SW to NE across this 8 km wide plutonic centre. The age gradients are developed within the foliated and unfoliated facies of the Zongo intrusions, but do not continue NE into the Palaeozoic meta-sedimentary rocks, for which K–Ar dates are erratic.
40
Ar–
39
Ar step-heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux (210 and 39 Ma), whereas those with intermediate ages exhibit disturbed spectra. A plateau date for a biotite from the NE margin of the Zongo massif shows that final cooling through
c.
300 °C occurred in the Late Eocene. The new data reveal that a marked thermal discontinuity existed 39 Ma ago at the NE contact of the Zongo intrusion. Additional discordant dates for the Yani and Illampu intrusions, to the NW, indicate that the discontinuity was of regional scale. There is no direct relationship between this tectono-thermal event, which affected at least 2000 km
2
, and the Upper Palaeozoic foliation of several of the plutons involved, and thus it is cryptic on outcrop and microscopic scales. We propose that the thermal discontinuity resulted from Late Eocene crustal shortening, SW-verging thrust faulting, and attendant cordilleran uplift at the inner (eastern) boundary of the Andean orogen, coincident with the Incaic deformation defined in Peru.
The compaction of igneous and sedimentary rocks
Authors: McKENZIE D. P.
Published: December 2007

In many problems of interest to geology and soil mechanics a fluid moves through a solid matrix which is also being deformed. In most cases of interest the effective viscosity of the matrix is many orders of magnitude greater than that of the percolating fluid, and both form interconnected networks in three dimensions. The partial differential equations governing such behaviour can now be obtained and a variety of simple model problems have been investigated. Some of the more surprising solutions contain solitary waves and compaction fronts. Some of the less exotic solutions are presently of more geological interest. The mobility of volatile rich melts at melt fractions as small as 0.1% has important consequences for trace element geochemistry. The geometry of layered intrusives appears to result from differential compaction of the crystal mush, and the rates required can be used to estimate a viscosity of 3 x 10
18
Pa s for an olivine matrix. This value agrees excellently with laboratory experiments when proper account is taken of the grain size dependence. Compaction in sedimentary rocks results in overpressure when the porosity becomes sufficiently small, and can also lead to the development of secondary porosity.
Some aspects of fluid motion during metamorphism
Authors: THOMPSON A. B.
Published: December 2007

Most of the mineralogical, chemical and textural features that can be observed in metamorphic rocks are produced during devolatilization reactions (Fyfe
et al.
1978; Thompson 1983). In many cases only the last such event is recorded. Between devolatilization pulses, fluid-rock interaction will be minimal and microstructural evidence of even the last event is often modified by recrystallization (Ridley & Thompson 1986). The mechanisms by which the fluids escape varies depending upon whether a connected porosity, or fracture network, is available and the magnitudes of temperature and fluid-pressure gradients (e.g. Norton & Knight 1977; Etheridge
et al.
1984; Yardley 1986). These factors will change during the metamorphic process. Fluids released deeper in the metamorphic pile do not necessarily flow pervasively through the overlying rocks but rather appear to be focused into high permeability channelways through a variety of processes (Fyfe
et al.
1978; Wood & Walther 1986). The patchiness of retrogressive alteration points also to the focused flow of fluid after a metamorphic peak and perhaps under different rheological conditions. Metamorphism in the absence of fluid results in incomplete mineral reactions and other disequilibrium features in metamorphic rocks (Thompson 1983; Ridley & Dixon 1984).

Many papers have appeared in recent years dealing with aspects of fluid motion during metamorphism. They have variously demonstrated the transient nature of fluid flow, indicated some controls on different modes of fluid behaviour, and suggested that mechanical responses will be different depending upon whether a rock is undergoing devolatilization or not. Attention is focused here on
Fluid flow and mineral reactions at high temperatures and pressures
Authors: HELGESON H. C. , LICHTNER P. C.
Published: December 2007

Although fluid flows in response to differences in hydraulic head, chemical reactions occur in response to changes in fluid pressure. Depending on the flow direction, cross-sectional area, and hydraulic conductivity of the system, flow of fluids through porous media may lead to increases and/or decreases in fluid pressure along the flow path. At high temperatures corresponding to those in the supercritical region of the system H
2
O, mass transfer among minerals and aqueous fluids in hydrothermal systems is sensitive to changes in fluid pressure as well as temperature. As a consequence, mineral solubilities may both increase and decrease in the direction of fluid flow, causing development of isolated channelways and a heterogeneous distribution of secondary porosity and alteration products. For example, vertical downward flow is accompanied by increasing fluid pressure if the Darcy velocity is less than the hydraulic conductivity of the medium. Hence, for a constant flow rate an abrupt increase in cross-sectional area may lead to dissolution of quartz, but precipitation of sulphides.
The tectonic history of Kohistan and its implications for Himalayan structure
Authors: COWARD M. P. , BUTLER R. W. H. , KHAN M. ASIF , KNIPE R. J.
Published: December 2007

The tectonic history of Kohistan, northern Pakistan, involves two collisional events. Cleavage and folding developed at 90-100 Ma along the northern suture between the Kohistan island arc and the Asian plate. At the same time there was major folding and shearing of the lower part of the Kohistan arc, approximately 100 km south of the suture. This deformation was followed by ocean subduction south of the Kohistan arc, generating the Kohistan calc-alkaline batholith, with subsequent ocean closure during the Eocene and obduction of the Kohistan arc, together with the adjacent part of the Asian plate, over the Indian continental crust. The construction of balanced cross-sections through the imbricated upper part of the Indian continental crust, in the footwall to this southern suture indicates a minimum displacement of 470 km, requiring the western Himalayan hinterland to be underlain by a large wedge of Indian middle to lower crust. There is some shortening of the overriding Kohistan and Asian plates by thrusts and shear zones, but it is insufficient to satisfy the palaeomagnetic data; there must be major crustal shortening, involving thrusts, in the Hindu Kush and Pamirs north of Kohistan.

The post-Eocene thrust direction, which for most of Pakistan is towards 160°, is almost perpendicular to that immediately to the east in the Himalayan belt, generating complex refolded thrust patterns in the Hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the Nanga Parbat syntaxis. These different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics, cannot always be related to plate movement vectors, but are modified by, or develop from, complex rotations during place collision or from the gravitational spreading of a thickened crust. A regional approach is required to recognize and correctly attribute the various components in thrust displacements.
U-Pb zircon geochronology from the Precambrian Annagh Division gneisses and the Termon Granite, NW County Mayo, Ireland
Authors: AFTALION M. , MAX M. D.
Published: December 2007

Two sets of U-Pb zircon data from widely spaced specimen sets in the oldest migmatites in the Annagh Division gneisses yield upper intercept ages of 1297
+164
-101
and 1224
+192
-64
Ma and lower intercept ages of 869
+69
–118
and 813
+166
–
269
Ma. Zircons from the metamorphosed alkaline Doolough Granite (gneiss) yielded an upper intercept age of 1093
+8
–7
Ma; thus the intrusion is chronologically related, as an immediate precursor, to the widespread granitization and migmatization previously dated by U-Pb at 1070 ±30 Ma to 1000 ±30 Ma and by Rb-Sr whole-rock at 1069 ±72 Ma to 985 ± 186 Ma. Much of the Annagh Division is composed of pre-Grenville (Elsonian?) or very early Grenville gneisses which were intensively reworked during the main Grenville orogeny. The lower intercept ages from the older gneisses may possibly define a separate post-Grenville and pre-Caledonian tectono-thermal event.

The Termon Granite at the southern tip of the Mullet Peninsula in NW Country Mayo was emplaced at 404
+2
-3
Ma but the U-Pb upper intercept age of 1956
+127
–121
Ma indicates that basement of Lewisian age may well underlie the exposed pre-Caledonian basement and its Caledonian (Dalradian) cover in the NW County Mayo metamorphic inlier.
Sedimentary and botanical factors influencing peat accumulation in the Mississippi Delta
Authors: KOSTERS E. C. , CHMURA G. L. , BAILEY A.
Published: December 2007

The Holocene Mississippi Delta System consists of six cyclic deltaic packages. Associated peat deposits are planar and eutrophic, and accumulate in areas abandoned by deltaic sedimentation. Peats that formed in large-scale interlobe basins are the subject of this paper. True peats (>75% organic matter) average 81.7% organic matter on a dry-weight basis. Controls on quality and lateral distribution of peat deposits are: botanical parent material, the balance between subsidence and accretion rates, detrital clastic influx and marine inundation. Botanical parent material, inferred from
13
C signatures and microtome sections, is always freshwater, originating either in forested swamps, or under herbaceous (floating) mats.
Sphagnum spp.
is essentially absent.

In order for peat to accumulate, rates of accretion and subsidence must balance. Long-term subsidence rates in the study area increase from about 0.10 to 0.30 cm a
-1
over a distance of 40 km from upper to lower delta plain. Subsurface accretion rates for true peats at a depth of 2-3 m, however, average about 0.05 cm a
-1
. Renewed coarse detrital clastic influx terminates peat accumulation. The largest quantities of true peat are found together with relatively large amounts of organic-poor clastic sediments (mostly clays).

Delta lobe abandonment and consequent coastal erosion causes salt water intrusion into freshwater environments. Resulting saline marsh sediments have maximum organic matter contents of 35%. In a transgressive setting, these may overlie older high-quality (low-ash) freshwater peats. Leaching experiments reveal the presence of water-soluble salts in both transgressed and fresh peats. Leaching reduces ash values by 1/5 to 1/3, suggesting that such losses may occur during early diagenesis. It is possible that some commercial coal seams originated in analogous interlobe environments, where leaching and possibly silica mobilization reduced the originally high ash contents to acceptable levels.
Palaeochannels in mine workings in the High Hazles Coal (Westphalian B), Nottinghamshire Coalfield, England
Authors: GUION P. D.
Published: December 2007

Palaeochannels have numerous deleterious effects on coal mining including the formation of ‘washouts’, and their detection and delineation in the subsurface is of paramount importance. A palaeochannel system in the roof of the High Hazles Coal (Westphalian B) at Gedling Colliery, Nottinghamshire, is described. The roof strata of this seam are interpreted as the deposits of a lacustrine delta complex formed on an upper delta plain, with the palaeochannels constituting minor deltaic distributaries. The lacustrine delta front deposits consist of a coarsening-upwards sequence of siltstones and sandstones, which form a sheet-like body of coalescing distributary mouth bars that were laid down by friction-dominated effluents. The distributary channels were filled by a variety of processes, giving rise to complex channel deposits. Bedload deposition of sand on bars, collapse and sloughing of channel banks, and deposition of silts and muds from suspension were important mechanisms. The nature of the deposits reflects the mode of channel abandonment, rather than equilibrium channel processes. Overbank flood deposits and prodelta/lacustrine mudstones are also described from the High Hazles roof. Based on this study, criteria are suggested that may enable the successful detection and delineation of palaeochannels in mine workings elsewhere.
Stages in the compaction of peat; examples from the Stephanian and Permian of the Massif Central, France
Authors: COUREL L.
Published: December 2007

Thick coal seams in the intramontane basins of the Massif Central allow a detailed study of peat/coal compaction. Three stages of compaction can be detected. Stage 1—occurred during the deposition of the peat. Coal beds which thicken and dip away from penecontemporaneous sandstones suggest rapid compaction. Stage 2—compaction of peat related to overburden pressure in some cases resulted in the formation of lakes over peat beds. The thickness of the lacustrine deposits may give an indication of the amount of compaction that took place. One possibility is that the early compaction was related to the progradation of siliciclastics over the peat. Stage 3—occurred after the organic sediment achieved some rigidity. Compaction can be calculated from deformed clastic dykes.
The Westphalian tetrapod fauna; some aspects of its geography and ecology
Authors: MILNER A. R.
Published: December 2007

Fossil tetrapods (amphibians and reptiles) have been found in Westphalian Series beds at over 40 localities, seven of which have produced assemblages with a diversity of six or more genera. All tetrapod-bearing assemblages derive from the southern lowlands of Euramerica which correspond to the Westphalian equatorial belt. The preservational environments and the eight major tetrapod groups of the Westphalian are briefly reviewed and apparent associations are identified. There appear to be four overlapping tetrapod associations which may correspond to four communities, namely: (i) terrestrial, (ii) lowland open water body (fresh water or brackish), (iii) lowland restricted water body (fresh water or brackish), and (iv) intermontane basin restricted water body (fresh water). Factors which may determine the nature of the preserved assemblages include marine influence, openness of water-body and seasonal variation.
The Rb-Sr geochronological evolution of the Ras Gharib segment of the northern Nubian Shield
Authors: ABDEL-RAHMAN ABDEL-FATTAH M. , DOIG R.
Published: December 2007

Rb-Sr whole-rock isochron ages have been determined for six igneous rock suites from the Ras Gharib. These represent all major crustal components of the northern Nubian Shield and are, from oldest to youngest: diorite-tonalite, extrusive rocks, granodiorite-adamellite and leucogranite, muscovite trondhjemite (orogenic Pan-African suites), dyke swarms and anorogenic peralkaline granites. The ages obtained are: 881 ± 58 Ma, 620 ± 16 Ma, 552 ± 7 Ma, 516 ± 7 Ma, 493 ± 7 Ma and 476 ± 2 Ma. The initial
87
Sr/
86
Sr ratios of the Pan-African suites and the dykes are in the range 0.7039 to 0.7047. The initial ratio of the anorogenic peralkaline granites is 0.7110 ±0.0012, higher than any previously reported for igneous rocks from the Egyptian belt.

The 493 ± 7 Ma age of the dyke swarms represents a younger limit for the Pan-African compression–accretion event and an upper (older) limit for an extensional tectonic regime. Linked with this is the anorogenic peralkaline granite magmatism (476 ±2 Ma), which is the last manifestation of igneous activity in this region.

The low initial ratios of the Pan-African suites provide no support for the existence of an older (Archaen) sialic continental crust in the region. The anorogenic peralkaline granite with the high initial ratio was produced by partial melting of earlier Pan-African suites. The age data are used to construct a tentative correlation scheme for shield rocks of Egypt, Sudan and Saudi Arabia.
The origin of the Upper Palaeozoic Chañaral mélange of N Chile
Authors: BELL C. M.
Published: December 2007

The fragmented and mixed strata of the Chañaral melange extend for some 220 km from N to S in the coastal ranges of N Chile (26°20′ to 28°20′S). The mélange was produced in an accretionary wedge resulting from the NE subduction of the ancestral Pacific oceanic plate beneath the Gondwana-land margin during Carboniferous times. The mélange consists essentially of blocks of sandstone in a pelitic matrix. These sediments were deposited as deep-sea basin-plain turbidites prior to their fragmentation. Other blocks in the mélange include mafic volcanic rocks, with the geochemical characteristics of oceanic island basalts, and rare shallow-marine limestones.

Structures in the mélange suggest an origin by two distinct deformation processes involving unlithified sediments. Initial boudinage and break-up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge. The second process produced cross-cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting.
The relationship between tectonic strain and the three-dimensional shape fabrics of pebbles in deformed conglomerates
Authors: FREEMAN B. , LISLE R. J.
Published: December 2007

The bias associated with the use of the shape of deformed inclusions to estimate bulk strain from ‘conglomerate’ systems is examined. Mechanical considerations based on Eshelby's theory for the deformation of ellipsoidal inclusions, supported by data from naturally deformed conglomerates with more than one type of inclusion, reveal a discrepancy between the degree of prolateness-oblateness shown by the inclusion and the bulk rock strain ellipsoid. This bias is most marked in situations where the difference of competence between the inclusion and matrix is high and the volume fraction of inclusions is low. In these situations, an inclusion which is more competent than the matrix will deform to a shape which is more prolate than the bulk strain ellipsoid. This has important implications for strain analysis of conglomeratic rocks and the interpretation of pebble shape fabrics in the context of regional tectonics.
The tectonic implications of high-pressure metamorphism in the western Alps
Authors: FRY N. , BARNICOAT A. C.
Published: December 2007

Five mineralogical stages are recognized in the metamorphism of Alpine ophiolites from one area in the Zermatt–Saas zone. Constraints on
P-T
values indicate an abrupt change at the metamorphic peak (550–600 °C, 17–20 kbar) from a cold positive slope of 25°C/kbar or less, to cooling initially without discernible change of pressure. Correspondence of the prograde path to the
P-T
trend of
T
(in °C) = 150 + 23
P
(in kbar) shown by a variety of Alpine units suggests that they all have
P-T
histories for which an England & Richardson (1977) relaxation model (also England & Thompson 1984) is inappropriate. Major tectonism at the peak pressure is indicated by association between minerals (kyanite, chloritoid, lawsonite in eclogite), vein growths, fluid action and thrust surfaces. The proposed metamorphic environment involves thrust repetition, subcreting ophiolite to the over-riding slab in a thermally mature subduction zone at a depth of approximately 70 km. Preservation is favoured when ocean closure leads to subduction of sialic crust, so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials.
The ammonium content of Caledonian granites
Authors: HALL A.
Published: December 2007

The Caledonian granites of the British Isles have NH
+
4
contents in the range of 0–167 parts per million. High contents (>60 ppm) are confined to muscovite-rich granites, occurring principally in a restricted area, namely SE Ireland and the Isle of Man. Their magmas are inferred to have formed by the partial melting of a sedimentary protolith rich in organic matter. There is no apparent correlation between
NH
+
4

content and initial
87
Sr/
86
Sr ratios.
Crystallographic fabrics and deformation histories
Authors: LAW R. D.
Published: December 2007

Report of a meeting of the Tectonic Studies Group held on 19 May 1986 at the Department of Earth Sciences, Leeds University. The meeting was organized by R. D. Law, R. J. Knipe, S. Bowler and G. E. Lloyd.

This meeting was arranged to reflect the recent resurgence of interest in the relationships between deformation histories and crystallographic fabric development in plastically deformed rocks. This resurgence of interest has been due largely to recent advances in both mathamatical modelling of fabric development and the development of analytical techniques for measuring crystallographic preferred orientation. The meeting was attended by some 40 participants, of whom 75% came from either mainland Europe or North America.

The meeting was opened by
S. M. Schmid
who presented a general introduction to: (1) the principles of crystallographic preferred orientation (fabric) development, 2) recent advances in analytical techniques for the measurement of crystallographic fabrics, and (3) calcite fabric development associated with experimental simple shear deformation. In this introductory paper the importance of integrating microstructural and crystallographic fabric studies was emphasized. Such integrated studies within quartz mylonite belts indicate significant departures from plane strain (simple shear) deformation. This theme was followed by
Behrmann
who reviewed recently published petrofabric evidence from the Moine thrust zone and Betic Cordilleras for partitioning of deformation within thrust sheets into domains of non-coaxial and essentially coaxial flow. Behrmann argued that this type of deformation, which incorporated sub-vertical shortening and movement parallel stretching, may be a reflection of gravity induced spreading of the
The geochemistry of early diagenetic dolostones from a low-salinity Jurassic lagoon
Authors: ANDREWS J. E. , HAMILTON P. J. , FALLICK A. E.
Published: December 2007

In the Middle Jurassic Great Estuarine Group of NW Scotland, argillaceous dolostones and dolomitic limestones occur interbedded with limestones and mudstones of low salinity, 'closed lagoon' palaeoenvironments. These microcrystalline dolostones are imperfectly ordered and calcium-rich, and have escaped pervasive dissolution-reprecipitation processes during burial diagenesis. Geochemical studies show that the dolostones retain a significant early diagenetic memory. δ
18
O
dol
values =≈ 0%o are the heaviest reported from the Great Estuarine Group and probably formed by evaporative fractionation from meteoric water. δ
13
C
dol
values ≈ −2%o may represent precipitation from a standing body of meteoric water partially equilibrated with atmospheric CO
2
. Calcite coexisting with the dolomite is lighter in both
13
C and
18
O by 0.8 and 3.7%o respectively. It is suggested that the minerals were not co-precipitates from one solution. Strontium is preferentially concentrated in the dolomite with
87
Sr/
86
Sr values between 0.7095 and 0.7099. These values, which are slightly more radiogenic than Jurassic seawater, may originate from the continental weathering of ancient limestones, mixed with seawater derived from the compaction of underlying sediments. Sm-Nd model ages are consistent with weathering of Dalradian rocks in the drainage hinterland. Dolomite formation proceeded by dolomitization of a CaCO
3
precursor in a warm seasonal climate. During dry, arid seasons the lagoon water evaporated concentrating
18
O, raising Mg/Ca and Sr/Ca ratios and thereby promoting dolomitization. During humid wet periods Sr-poor CaCO
3
precipitated with lighter δ
18
O and δ
13
C values.
The isotopic and structural age of the Aberdeen Granite
Authors: KNELLER B. C. , AFTALION M.
Published: December 2007

New uranium–lead isotope data from monazites yield an emplacement age of 470 ± 1 Ma for the 'S'-type Aberdeen granite. Emplacement of this intrusion was late-tectonic with respect to the Grampian (early Caledonian) orogeny, and followed closely on the upper amphibolite facies meta-morphic climax in the Dalradian country rocks. The field relations of the associated vein complexes suggest that this isotopic age also dates the local D3 deformation of the Dalradian supergroup. Previously published isotope data indicate that the granite originated largely by melting of metasediments with a minimum crustal residence time of
c.
1.2 to 1.8 Ga. Rocks with comparable isotopic characteristics form the migmatitic country-rock envelope to the granite, and we advocate extensive contemporaneous melting of such rocks at shallow depths below the level of emplacement (then 17–20 km).