bluuebunny/crossref_metadata_2025_split huggingface dataset:
Twenty years of deep seismic reflection profiling in Germany—a contribution to our knowledge of the nature of the lower Variscan crust
Authors: Meissner R. 
Published: April 2008

Summary

Near-vertical reflection studies in Germany have concentrated on the following objectives:

(1)
The nature of crustal reflectors

(2)
A detailed velocity structure

(3)
Zones of increased tectonic activity, e.g., faults.

Approximately 20 years ago, the identity between reflectors in the near-vertical and the wide-angle range was established, wide-angle events being strongly influenced by velocity gradients. A pronounced, but limited low-velocity body below the Urach geothermal anomaly was detected by using an especially long spread length. An alternating polarity of deep crustal reflectors could be demonstrated. A deep fault zone, cutting the whole crust at steep angles, and shallow-angle overthrust structures were detected in the Variscan area. There is generally a strong reflectivity in the Variscan lower crust between the Conrad level at 6 s two-way travel time, i.e., 18 km, and the Moho at 9–10 s, i.e., 27–30 km depth. The lower crust seems to consist of thin lenses or lamellae with alternating velocity. They were probably generated by an extensive syn- and post-orogenic melting process leaving behind mafic and ultramafic ‘splinters’ in the lower crust and a compact ultramafic residue in the uppermost mantle, the Moho being formed as a new and uniform boundary. The poorly reflective upper crust is probably dominated by vertically oriented plutons in a rigid environment.
A physical model of the lower crust from North America based on seismic reflection data
Authors: Smithson S. B. 
Published: April 2008

Summary
In speculations about crustal composition, structure, and genesis, the nature of the deep crust is always the major unknown. Seismic reflection data indicate a complex, finely (about 30–100 m) layered lower crust in some areas and an apparently transparent lower crust in other areas. Lower crustal layering is folded to sub-horizontal. The number of seismic events may be highly influenced by recording conditions. Variations in lower crustal structure appear extensive. Best lower crustal reflections are found in areas of young extension in the western US, ancient gneiss in Minnesota, and in offshore Long Island. The lower crust may be affected by a wide range of processes after its formation. Variability and complexity of the lower crust is consistent with postulated exposed deep crustal sections.
The physical properties of layered rocks in deep continental crust
Authors: Hall J. 
Published: April 2008

Summary

Normal incidence seismic reflection profiles often show strong reflectivity—indicating sub-horizontal layering— in the deep continental crust, below poorly reflective upper crust. An example from the WINCH profile across the Caledonides of NW Britain shows necking of the lower crustal reflective layer near Islay. Combining the seismic data with gravity modelling constrains the lower crustal density to be about 3.1 Mg m
−3
and the P-wave velocity to be about 7.3 km s
−1
. This layer can be correlated with the deepest crustal layer of the parallel LISPB refraction profile where the layer has a similar P-wave velocity, Poisson’s ratio of 0.25 and electrical conductivity of more than 0.003 S m
−1
. These data suggest that a) the lower crust is heterogeneous, b) its mean composition is like that of very basic igneous rocks, and c) hydrated minerals and possibly free water are significant components. Aqueous fluids are shown to permeate deep crustal rocks even more effectively than they do sedimentary rocks and a mechanism of water capture in the lower crust is presented. Layering may result from interleaving of mantle and crust, fabric re-orientation due to shearing or, most likely, by intrusive underplating. Reflectivity may be enhanced by layer-variable water content.
Diversity in the lower continental crust
Authors: Dewey J. F. 
Published: April 2008

Summary

Arguments based upon tectonic style and process suggest that the lower continental crust is laterally exceedingly inhomogeneous on the scale of tens and hundreds of kilometres. At least twenty basic variations exist apart from unmodified Archaean nuclei. Some 80% of the continental crust was generated by 2.5 × 10
9
years and most of this has been modified and/or redistributed by tectonic and sedimentary processes. Two general secular trends have occurred to increase homogeneity in the continental crust; addition of mafic igneous rocks to the lower crust, and differentiation, in both extensional and shortening environments, into a more mafic, refractory lower crust and an upper, wetter, more silicic, crust enriched in lithophile and radiogenic elements.
Continental lithosphere strength: the critical role of lower crustal deformation
Authors: Kusznir N. J. , Park R. G. 
Published: April 2008

Summary
A thermo-rheological model of lithosphere deformation, incorporating the elastic, ductile and brittle behaviour of lithosphere material, has been used to examine intraplate continental lithosphere strength, brittle-ductile transition depth and flexural rigidity. These parameters are critically dependent on crust and mantle rheology and consequently on geothermal gradient, crustal thickness and lower crustal composition.

For lithosphere subjected to a lateral tectonic force, creep in the lower crust and mantle leads to stress release, and the subsequent stress redistribution generates stresses in the upper lithosphere sufficient to cause brittle fracture. The extent of creep in the lower crust and mantle and the degree of upper lithosphere stress amplification (which together determine bulk lithosphere strength) increase with geothermal gradient. For significant lithosphere extension, under maximum likely levels of available tectonic stress, a lithosphere surface heat flow of 60 mW m
−2
or greater is required, while for compressive lithosphere deformation, heat flow must exceed 75 mW m
−2
. Similarly flexural rigidity increases with increase in the thermal age of the lithosphere at the time of loading.

The depth of the brittle-ductile transition decreases with increase in geothermal gradient. For a limited range of gradients (expressed by heat flow
q
= 50–55 mW m
−2
) multiple brittle-ductile transitions may exist in the middle and lower crust and upper mantle, with important tectonic implications (e.g. for intra-crustal detachments and crust-mantle decoupling).

Lithosphere strength in extension and compression, and flexural rigidity, are both controlled by the quartzo-feldspathic rheology of the crust for thermally young lithosphere and by the olivine rheology of the mantle for older lithosphere. Lithosphere strength is therefore critically influenced by the thickness of the crust (decreasing with increase in crustal thickness) and by the composition of the lower crust, particularly for lithosphere with intermediate heat flows.
Crustal structure and evolution of the central Australian basins
Authors: Lambeck K. 
Published: April 2008

Summary
The basins of central Australia formed over a period of 600 Ma or more from late Proterozoic to Carboniferous time. These basins appear to overlie crust of normal thickness whereas the crust of the intervening arches has been uplifted and reduced in thickness. The Moho, in consequence, varies in depth by up to ± 10 km about its mean depth, over horizontal distances of 50 km or less. The stresses associated with this lower crustal structure are of the order of a few kbars and extend throughout the crust. These stresses have existed since at least late Proterozoic time, and because the Moho undulations appear to follow the near surface deformations, this rules out the existence of a mechanically weak lower crust. The region is out of isostatic equilibrium and this state must be maintained by either a mechanically strong lithosphere of more than 150 km thickness or by horizontal compressive stresses, of the order of a few kbars. The former possibility is ruled out by the available mantle flow laws and geotherms proposed for the central Australian shield. In the latter case the horizontal compressive forces have also played the key role in shaping the basins and crustal structure. The horizontal compression must act down to at least the Moho and must characterize the dominant state of stress since late Proterozoic time.
The chemical composition of the Archaean crust
Authors: Taylor S. R. , McLennan S.M. 
Published: April 2008

Summary

Three separate observations, from sedimentary rock compositions, subduction zone igneous rocks and heat flow constraints combine to suggest that the composition of the Archaean crust was distinct from that of the present-day continental crust. The sedimentary REE patterns are consistent with derivation of the upper crust from a 1:1 mixture of the Archaean bimodal basic-felsic suite. The andesite model, while adequate to account for present-day continental growth does not provide enough Cr and Ni for the Archaean crust. Average crustal component heat flow values of 14 mW m
−2
for Archaean terrains are significantly lower than the post-Archaean average of 23 mW m
−2
for crustally produced heat.

Correlations from the sedimentary trace element data provide Archaean upper crustal values of 1.5% K, 5.7 ppm Th and 1.5 ppm U. Heat flow constraints restrict such compositions to the upper layers. The total crustal values are consistent with a 2:1 basic/felsic composition, and give values for the total crust of 0.75% K, 2.9 ppm Th and 0.75 ppm U, which provide crustal heat flow components of 14 mW m
−2
for a 30 km thick crust and 19 mW m
−2
for a 40 km crust.
The structural evolution of the lower crust of orogenic belts, present and past
Authors: Windley B. F. , Tarney J. 
Published: April 2008

Summary
The lower crust of orogenic belts may be exposed by post-growth uplift in magmatic arcs or by tectonic uplift on post-collisional crustal-scale thrusts. Each tectonic zone of a belt can be expected to have a different type of lower crust, and there is a similar tectonic zonation in collisional belts ranging from the early Proterozoic to the Tertiary. Tonalitic gneisses make up the bulk of the lower crust of Andean-type arcs. Deep sections of sutures may expose nepheline gneisses, blueschists and granulites. Exposed basic dyke swarms represent the feeders to eroded lavas in aulacogens in continental forelands. Post-collisional thrusts uplift earlier lower crust of amphibolite facies gneisses or granulites, of either igneous or sedimentary parentage. The result of the many factors controlling variable uplift in the different tectonic zones of an orogenic belt is that a deep-seated cross-section can only be constructed by compilation of type sections from different belts.
Crustal generation in the Archaean was dominantly magmatic and broadly similar to that seen in the deeper parts of Andean-type arcs, but on a much more extensive scale. Magmatic underplating, associated with strong horizontal thrusting, produced a strongly foliated lower crust with interlayering of mafic and silicic components. Archaean crust probably had a primary vertical zonation with respect to the proportion of these components and of heat-producing elements.
Evolution of the late Archaean lower continental crust in southern West Greenland
Authors: Robertson S. 
Published: April 2008

Summary
Accretion of new continental crust onto the base of pre-existing sialic crust is indicated by field, geochemical and isotopic data from two suites of Archaean gneisses from the Inner Godthåbsfjord region of southern West Greenland. Crustal accretion combined with horizontal tectonic interleaving of existing rock suites thickened the crust resulting in high-grade metamorphism. Enrichment in LIL elements resulted from the depletion in these elements of granulite-facies gneisses below the present level of erosion. Related fluid movements were significant in leading to the anatexis of rocks enriched in LIL elements. The present-day distribution of rock suites is at least in part a function of the juxtaposition of different levels of the late Archaean crust.
Granulitic xenoliths from the French Massif Central—petrology, Sr and Nd isotope systematics and model age estimates
Authors: Downes H. , Leyreloup A. 
Published: April 2008

Summary

The protoliths of granulitic xenoliths found in Tertiary/Quaternary alkali volcanics from the French Massif Central are shown to have been principally immature sediments and calc-alkaline intrusives. The association may indicate formation of the crust in an island-arc or continental margin environment. Pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite-facies metamorphism (9–15 kbar) followed by uplift to lower pressures (4–9 kbar). Rb-Sr and Sm-Nd isotopic data and REE abundances are presented for both meta-igneous and meta-sedimentary suites. ɛSr present-day values for the granulitic xenoliths range from +2.7 to +325, with ɛNd values between +0.2 and −12.0. There are no isochron relationships within the Sr and Nd data, but possible mixing lines in the Rb-Sr system may indicate sedimentation at 700–1000 Myr. T
Nd
DM
model ages indicate a Proterozoic (800–2100 Myr) origin for the protoliths of the meta-igneous samples and a similar provenance age for the meta-sediments.
Xenoliths from southern Africa: a perspective on the lower crust
Authors: van Calsteren P. W. C. , Harris N. B. W. , Hawkesworth C. J. , Menzies M. A. , Rogers N. W. 
Published: April 2008

Summary
Granulite-facies xenoliths from southern African kimberlites yield temperatures from 780–900°C and pressures from 10–15 kb. They have Nd model ages of 1.4–3.7 Ga and retain Rb-Sr mica and Sm-Nd garnet ages from 0.14–1 Ga, significantly older than kimberlite emplacement ages. It is argued that low abundances of U, K and Rb in lower crustal rocks can be either primary features resulting from plagioclase-dominated cumulate processes, or secondary effects caused by the granulite-facies breakdown of mica in wet intermediate-acid rocks. Combined trace element and isotope arguments suggest that contamination with lower crustal material does not significantly affect the geochemistry of flood basalts from the Karoo, the Parana and the Deccan.
The lower crust in eastern Australia: xenolith evidence
Authors: Griffin W. L. , O’Reilly S. Y. 
Published: April 2008

Summary

Hundreds of Mesozoic to Recent xenolith-bearing basalt flows, diatremes, cinder cones and maars have erupted through the low-grade Palaeozoic metamorphic rocks of the Tasman Fold Belt, which makes up the eastern third of the Australian continent. Granulite-facies xenoliths, interpreted as lower-crustal material, have been found at more than 40 of these localities. A palaeogeotherm, based on garnet pyroxenite xenoliths from Victoria, also fits available data from New South Wales and Queensland. A crustal thickness of 25—30 km, and lower-crustal temperatures of 700–850°C, have been derived by referring T estimates for spinel lherzolite xenoliths to this geotherm. The lower-crustal xenolith suites are dominated by basic pyroxene granulites and garnet granulites; more silicic xenoliths are very rarely reported. The granulites cover a range in composition typical of intraplate basaltic magmas and cumulates. Granoblastic microstructures are typical, but common relict igneous features suggest that all the mafic granulites originated as igneous rocks. The lower crust in this area probably formed through multiple intrusion of basic magmas near the crust/mantle interface over a long time span. The strongly layered ‘lower crust’ seen on seismic reflection studies in eastern Australia probably represents this thick crust-mantle transition zone. The seismic ‘Moho’ reflects the depth at which the proportion of basic to ultrabasic rocks drops below a critical value, and probably lies well below the
petrographically
defined base of the crust.
Geophysical controls on sedimentation within the African Rift Systems
Authors: Fairhead J. D. 
Published: April 2008

Summary
Gravity and seismological data are used, together with unpublished oil company data, to investigate the lithospheric structures associated with the East, and the West and Central African Rift Systems. Although these two rift systems are the result of extensional tectonics, they appear to be tectonically very different, with uplift and volcanism in East Africa, and subsidence and sedimentation in West and Central Africa. These differences can be explained in terms of the amounts of crustal extension (and thinning) that these rift systems have undergone and the isostatic response of these crustal structures during, and subsequent to, the extensional phase. In West and Central Africa, gravity modelling indicates crust extensions in the order of 70 km, while in East Africa extension across the Kenya Rift could be as little as 10 km. The implications of these lithospheric models for the formation of sedimentary basins are discussed, as well as how the rift structures relate to the crustal structure of continental margins.
Structural expressions of rifting: lessons from Lake Tanganyika, Africa
Authors: Rosendahl B. R. , Reynolds D. J. , Lorber P. M. , Burgess C. F. , McGill J. , Scott D. , Lambiase J. J. , Derksen S. J. 
Published: April 2008

Summary
The fundamental building blocks of Lake Tanganyika are half-graben that tend to be arcuate or crescent-shaped in plan view. When combined with the asymmetric subsidence of the half-graben, this geometry creates horizontal components of motion which are expressed as shearing and rotation along the ends of the units. The key to understanding rift-basin morphology is recognizing the various ways in which these fundamental units can be linked together in time and space. Although many modes of linking occur, the basic theme is one of alternating half-graben polarities along the strike of the rift. Where adjacent half-graben face in opposite directions and do not overlap with each other, they are usually separated by interbasinal ridges trending oblique to the rift axis. In cases where facing half-graben overlap, the apparent morphology can be that of a full-graben with a central high or a half-graben abutting a structural platform, depending upon the orientation of the observation line relative to the overlap geometry. Mechanically, such full-graben should be viewed as opposing half-graben which hinge away from the central high. Examination of the linking models in Tanganyika has enabled us to derive an empirical model of rifting. The model is extremely useful in understanding the stratigraphy of Tanganyika, and it seems to explain many of the styles and patterns of deposition which are peculiar to rift basins. If the model has general applicability to continental rifts, it can also be useful to those who deal with the mechanics of rifting and the deep structure of passive margins.
Tectonics and volcanism of the southern Kenya Rift Valley and its influence on rift sedimentation
Authors: Baker B. H. 
Published: April 2008

Summary
Volcanism in the northern half of the Kenya Rift Valley began at 30 Ma, and in the southern half it began at 15 Ma. In the southern rift tectonic development started with gentle downwarping followed by repeated flooding of the depression with lavas. A half-graben formed at 7 Ma by faulting on the W, succeeded by further eruptions in the rift floor. By 4 Ma a graben had formed with faulting of the flexed eastern margin of the depression. Succeeding periods of faulting and volcanism migrated inwards, creating step-fault platforms and a narrow inner graben, but in the Naivasha-Nakuru sector voluminous trachytic volcanism completely filled the rift depression at intervals between 6 and 2 Ma. The inner graben was cut by dense swarms of minor faults from 2 Ma; and in the last 0.5 Ma a series of trachytic caldera volcanoes was built axially in the inner graben. Large sedimentary basins are found in broad half-graben in the northern and southern splay-fault sectors, and are subdivided into local basins where volcanic damming has occurred. Minor basins formed on back-tilted step-fault platforms, and numerous smaller lakes existed in graben of the rift floor. In the central sector lake basins spread broadly over pyroclastic deposits between volcanic piles. Ramp and step-ramp structures locally divert drainage from the high rift flanks into rift basins. Hypersaline lakes with trona evaporites in the Natron-Magadi basin were caused by the arid climate, eruption of sodium carbonate-rich ash, and recirculation of alkaline groundwater by hot springs into closed structural basins.
Slope processes, sediment derivation and landform evolution in a rift valley basin, northern Kenya
Authors: Reid I. , Frostick L. E. 
Published: April 2008

Summary

The Plio-Pleistocene sediments of the Lake Turkana basin are the product of episodic deposition interspersed with long periods of reworking. Their stratigraphy is complex and includes a wide range of lithotypes from fine lake clays to extensive coarse gravel sheets. The current phase of landscape evolution is dominated by fluvial incision and slope retreat under semi-arid conditions. Rates of sediment derivation are high, with measured values of up to 1600 t km
−2
a
−1
, and a catalogue of evidence of surface processes (rainsplash pillars, mobile soil mantles, etc.) illustrates the erosional dynamism of an area that currently receives a rainfall of only 300 mm a
−1
. Detailed process studies reveal the ten-fold decline in infiltration capacity of silty regoliths that results from impaction of the surface by the first fall of a rain season. They also explain the rapid onset of overland flow which only accounts for between 2 and 7% of stormwater but which ravages the soil, causing erosion that ranges from 0.2 to 0.5 kg m
−2
a
−1
. Shallow gradient slopes are shown to export fine material (D
50
= 0.2 mm) and to concentrate coarse fragments > 4 mm as a surface lag. But studies on steeper gradient slopes where the regolith is typically a talluvium, and where processes are gravity-dominated, sliding or rolling movements indicate a direct relationship between particle size and downslope movement towards the stream system. Despite the variable importance of different processes on different type-slopes, a clearcut relationship emerges between slope gradient and erosion rate that allows us to model slope retreat and landscape evolution. A volumetric analysis gives an age to the present basin landform of 0.11 Ma. Analysis of sequential links in the sediment transport train permits us to match clastic materials deposited in the accumulation zones of rift valley basins (riverine floodplain, lake delta, beach) with those of the erosional source areas.