bluuebunny/crossref_metadata_2025_split huggingface dataset:
Thermal gradients in the continental crust
Authors: Chapman D. S. 
Published: April 2008

Summary
A set of preferred geotherms for the continental crust is calculated using assumptions of steady state conductive heat transfer. The calculations use observed heat flow as the principal constraint, temperature and pressure dependent thermal conductivity functions, and a radiogenic heat generation profile that is consistent with a generalized petrological model of the lithosphere. Properties of these geotherms include: (1) a nearly constant gradient through the upper crust because the decrease in thermal conductivity at higher temperatures for felsic rocks counteracts the decreasing heat flow caused by crustal radioactivity; (2) divergence of the geotherms amounting to temperature differences of more than 500 K between rifts and shields at Moho depths, and (3) convergence of geotherms below 250 km in the asthenosphere as convective heat transfer dominates.
Emphasis is also placed on understanding the sensitivity of calculated temperatures to measureable properties. Computed geotherms are overall most sensitive to surface heat flow or reduced heat flow, and uncertainties in this quantity will ultimately limit the accuracy of temperature estimations for the deeper crust and lithosphere. Otherwise, the sensitivity to parameters such as heat production, thermal conductivity and its temperature dependence throughout the lithosphere and crustal thickness depends on depth and whether the region is characterized by high or low heat flow. Upper mantle heat production, pressure coefficients of thermal conductivity, and the position of the upper/lower crust boundary are relatively insensitive parameters for lower crustal temperature calculations.
Dynamic events accompanying crust forming processes lead to significant perturbations of the steady state static temperature field described above. Pressure-temperature pairs from granulite terrains plot near high temperature limits for those steady state geotherms and imply either a transient P-T-t path or a magmatic heat input from the mantle accompanying crustal thickening.
Geochronology and related isotope geochemistry of high-grade metamorphic rocks from the lower continental crust
Authors: Moorbath S. , Taylor P.N. 
Published: April 2008

Summary
We discuss the distribution of Rb, Sr, Sm, Nd, U, Th and Pb in medium-to-high pressure granulites, which are considered representative of the material of the lower crust. The depletion of large-ion-lithophile, heat-producing elements in such granulites is emphasized, together with its consequencs for radiogenic isotope evolution. Geochronological studies of granulites show that sometimes high-grade metamorphism closely follows original crust formation, whilst in other cases a long time interval separates these major events. Isotopic criteria for the recognition of lower crustal involvement in magma genesis are also discussed.
Sedimentary carbonates as isotopic marker horizons at Lake Turkana, Kenya
Authors: Abell P. I. , McClory J. P. 
Published: April 2008

Summary

The Plio-Pleistocene sediments of the Koobi Fora region of Lake Turkana contain a variety of carbonate horizons: molluscs, stromatolites, indurated calcitic sediments and concretions. The stable oxygen isotope ratios of these carbonates are largely unchanged since deposition, and constitute a record of environmental conditions at the time of deposition. Two factors influence these isotope ratios: (1) temperature, which produces more negative δ
18
O values with increasing temperature; and (2) evaporative concentration of the heavier oxygen isotope in the rainfall and body of the lake, which makes for more positive values of δ
18
O under arid conditions. The major change in climate which took place at about the time of deposition of the KBS Tuff is isotopically plain in most sections, but stratigraphic control is not sufficient to correlate other significant changes. Nevertheless, the general trends in environmental change during the deposition of any particular section of sediments can be clearly perceived provided there are enough carbonate horizons. Gastropods provide the least equivocal evidence for environmental conditions, and a survey of the entire continent of Africa for stable isotope ratios in contemporary gastropods has been conducted to provide a database for comparison of environmental conditions in the fossil gastropods.
Sedimentation by termites in the Malawi Rift Valley
Authors: Crossley R. 
Published: April 2008

Summary

Homogeneous red clayey sand sheets, averaging 5 m thick, cover a discontinuous area totalling 8800 km
2
in the Karonga section of the Malawi Rift Valley floor. The sheets accumulated in a terrestrial environment, sedimentation was slow and gradual, and fossil termite mounds occur at the base of the sheet in one area. Topographic considerations rule out deposition by overland flow, and texture is incompatible with aeolian sedimentation. Modern termites (
Macrotermes falciger
) are transporting sediment from beneath the sand sheets onto the ground surface. The textures of fossil and modern
Macrotermes
mound deposits are very similar to the textures of the sand sheets. It is concluded that the sand sheets have accumulated through vertical transfer of sediment by
Macrotermes
. Scrutiny of the literature suggests that similar deposits may be widespread in African savanna and rainforest areas, both within the African Rift System and in epeirogenic basins. Equivalent sediments must occur in ancient tropical sequences since fossil termites are first recorded in the mid-Cretaceous.
The role of hydrocarbon source rocks in petroleum exploration
Authors: Brooks J. , Cornford C. , Archer R. 
Published: April 2008

Summary
Petroleum is generated from organic-rich sediments (source rocks) containing organic matter originating from biological materials. During burial of sediments, the increase in temperature results in a series of geochemical reactions which leads from biopolymers to geopolymers, often collectively called kerogen, which are precursors of petroleum.
The amount, type and composition of petroleum generated is dependent upon the nature and geological history of the source rock. The most important parameters are the nature of the organic matter in the source rock and its maturity governed by its time/temperature history.
Petroleum exploration is now using modern understandings of hydrocarbon generation, migration and accumulation to help improve its success in predicting petroleum discoveries. The initial stages of an exploration programme can regionally evaluate the source rocks, together with the hydrocarbon potential of each source rock horizon and integrate this information with the geological development of the basin to enhance predictions of where and when hydrocarbon generation and accumulation have occurred.
The formation of organic-rich deposits in two deep-water marine environments
Authors: Morris R. J. 
Published: April 2008

Summary
Organic-rich sediments have been collected from deep-water core sites in the Eastern Mediterranean (6500–9000 years BP) and the Guinea Basin (50,000–250,000 years BP estimated). The results of geochemical analyses suggest that similar processes may have been involved in their formation. The same processes are known to give rise to present day organic-rich oozes on the continental shelves off Peru and Namibia. The deep-water organic deposits are believed to have been a direct result of periods of greatly increased productivity in the overlying water column. Their formation is not thought to have involved the development of widespread stagnant conditions in the associated body of water, conditions which are often taken to be essential for the preservation of organic matter in deep water.
The magmatic evolution of the Scourian complex at Gruinard Bay
Authors: Rollinson H. R. , Fowler M. B. 
Published: April 2008

Summary

Scourian (Archaean) gneisses at Gruinard Bay form a bimodal suite of trondhjemites and amphibolites in which the trondhjemites form about 75% of the complex. The gneisses form a large-scale agmatite complex in which amphibolite, ultramafic rocks and tonalitic gneiss are enclosed in trondhjemitic and associated granitic gneiss. All lithologies have been metamorphosed to hornblende-granulite facies and subsequently retrogressed. Amphibolites are the earliest recognizable component of the agmatite complex and have a varied geochemistry, which in part reflects the heterogeneous nature of their source. It is postulated that they are fragments of Archaean ocean floor. As such they are potential source rocks for the quartzofeldspathic gneisses, and calculations show that the tonalite and trondhjemite can be derived by 30% and 20–30% partial melting respectively, of a basaltic source, similar in chemistry to the amphibolites, leaving a residue of clinopyroxene, garnet and hornblende. Granitic gneisses associated with the trondhjemite require a slightly lower degree of partial melting (
c.
10%) and an eclogitic residue. Comparisons with experimentally determined phase equilibria indicate that the partial melting took place in the pressure range 10–20 kb, significantly shallower than the depth of partial melting in modern subduction zones.
The geochemistry of Lewisian marbles
Authors: Rock N. M. S. , Davis A. E. , Hutchison D. , Joseph M. , Smith T. K. 
Published: April 2008

Summary

To rectify the dearth of published petrographical and analytical data for Lewisian marbles, 307 samples, from nearly all outcrops known, have been examined in thin section, and 114 samples analysed for 35 elements. Lewisian marbles can be divided texturally into: 1)
macrocrystalline
(generally white, pure to silicate-rich)
eg
from Glenelg; 2)
microcrystalline
(often coloured, ornamental, pure to silicate-rich),
eg
from Iona; 3)
microbrecciated
(grey, crushed, fine-grained, silicate-opaque-rich),
eg
from the Loch Maree Group (LMG). Lewisian marbles are geochemically typical of ancient carbonates, tending to have lower contents of trace elements (notably Sr) than younger (
eg
Scottish Dalradian) marbles. Whereas all analyzed marbles from the South Harris and Coll foreland outcrops and from the Glenelg and Kintail inliers are
dolostones
(%MgO 15–20, %SiO
2
5–15), those of the eastern inliers (Glen Strathfarrar, Shinness) are pure
limestones
(%MgO < 10, %SiO
2
< 10). Elsewhere, limestones and dolostones are intimately associated or interbanded (
eg
Scardroy inlier). Microbrecciated LMG marbles (Flowerdale belt of Gairloch; Furnace belt of Loch Maree) have notably higher chalcophile and siderophile element contents than macrocrystalline marbles in adjacent belts (Cloiche of Gairloch; Gleann Tulacha of Loch Maree). Aside from such local differences, however, limestones and dolostones of the inliers, LMG and remaining foreland reveal few non-trivial differences in composition. Moreover, Lewisian marbles collectively show strong Pearson and Spearman correlations between elements concentrated in accessory phases (
eg
Y-La-Ce, K-Rb-Al, Fe-Co-V). Given established secular variations in the chemistry of global carbonate rocks, these relationships between both marble groups and chemical variables are difficult to explain if some Lewisian marbles (
eg
S. Harris) represent late Scourian, and others (
eg
LMG) early Proterozoic sediments. They may provide a tentative hint that most Lewisian marbles were deposited contemporaneously.
The role of mid-crustal shear zones in the Early Proterozoic evolution of the Lewisian
Authors: Coward M. P. , Park R. G. 
Published: April 2008

Summary
The major shear zones in the Lewisian complex are either steep NW-SE zones, with a generally dextral strike-slip component, or were subhorizontal before subsequent deformation. They appear to share the same NW-SE movement direction. The Outer Hebrides lie mostly within a major mid-crustal ‘flat’ which on the mainland, at a higher structural level, is seen only north of Loch Laxford and south of Loch Torridon. Inclined NW-SE-striking shear zones at Diabaig, Carnmore and Loch Laxford are interpreted as ramps by which this zone descends below the central region.

Movements on these major zones record a long period of probably intermittent activity from
c.
2600 Ma to
c.
1400 Ma. Early movements (Inverian) were widespread on the mainland, and indicate a generally overthrust regime with a small dextral component. A major change in kinematic regime occurred during dyke emplacement and in the early Laxfordian (D
1
–D
2
) where relative movements appear to have been dominantly strike-slip and extensional (transtensional). In the later Laxfordian (D
3
) major upright folds and steep dextral shear zones indicate a return to a dextral transpressional regime.

This sequence, together with evidence from Greenland, can be interpreted in the context of a reconstructed N. Atlantic, as the result of relative movements between two Archaean ‘plates’ to N and S of the combined Nagssugtoqidian-Lewisian belt.

The Inverian-Nag I structure indicates a dominantly N-S convergence. A change to a NW-SE convergence direction in the Early Laxfordian explains a dextral strike-slip regime in the Lewisian and convergence in W Greenland. A further change to NNW-SSE convergence in the later Laxfordian would explain the D
3
transpressional regime in Scotland.
Sedimentary models for extensional tilt-block/half-graben basins
Authors: Leeder M. R. , Gawthorpe R. L. 
Published: April 2008

Summary
Extensional tectonism produces characteristic half-graben/tilt-block systems whose facies mosaics are influenced by tectonically induced slopes resulting from hanging wall downtilting and footwall uplift. The characteristic asymmetrical subsidence vectors that therefore develop across the graben also exert a fundamental control upon facies distributions. A number of predictive tectono-sedimentary facies models are presented in which these various influences are explored. Alluvial fans and cones react to tilting by becoming segmented, those in the hanging wall showing down-dip hanging wall off-lap and those sourced in the footwall showing progradation from the apex. Lake and coastal waters react instantly to tilting, causing transgression and seiche-induced erosion. Axial through-flowing river channels and delta lobes tend to migrate or avulse towards the axis of maximum subsidence but may be constrained by the toes of footwall-sourced fans. Peat accumulation or soil development are accentuated up the hanging wall dip slope away from the locus of maximum deposition. In coastal areas, fan deltas sourced in the footwall pass offshore into small submarine fans whilst axial fans issue from delta fronts where individual fan lobes may migrate under fault control. In carbonate provinces the footwall scarp may become a bypass margin whilst the hanging wall dip slope may undergo a ramp-to-rimmed shelf evolution with time.
Crustal extension in the Basin and Range Province, southwestern United States
Authors: Hamilton W. 
Published: April 2008

Summary
Cenozoic extension of areally varying ages and amounts has on average doubled the width of the Basin and Range Province. Extensional structures that formed at all depths down to 20 km, and which range in age from Oligocene to Holocene, are widely exposed and are here interpreted in terms of a model of depth-varying deformation. The middle crust is extended by discontinuous ductile shear as internally underformed lenses slide apart along gently dipping zones of mylonite. The tops of these lenses are undulating detachment faults, the composite area of which increases with time as deep lenses slide out from underneath shallower ones. Brittle blocks of upper-crust bedrock above the detachments respond first by rotating between range-front faults, the same direction of rotation being maintained across a series of lenses, and then by pulling completely apart, while basinal strata fill the gaps and are dragged directly on detachment faults. Some faults rise gently from the main detachment zones and surface as range-front faults. Most tilted-block ranges are isolated atop detachments.
Detachment faults cut out crust. Beneath them are mid-crustal rocks of any age and type and above them are mostly upper-crustal rocks, including extensive syndeformational basin sediments rotated to steep or moderate dips. As attenuation proceeds and components rise, detachment faults evolve from ductile to brittle, develop splays, and are themselves broken by steep brittle structures related to new, deeper detachments. Parts of detachment faults remain active even after exposure at the surface, but slip on them is then limited to the down-dip direction. It is inferred from seismic reflection profiles and rock-mechanic considerations that the unexposed lower crust is extended by more pervasive ductile flattening.
Rupture characteristics of normal faults: an example from the Wasatch fault zone, Utah
Authors: Bruhn Ronald L. , Gibler Pamela R. , Parry William T. 
Published: April 2008

Summary

The Salt Lake fault segment is one of several independent rupture segments in the Wasatch normal fault zone, Utah. The segment is about 35 km long and consists of several, approximately linear fault sections that intersect in geometrical barriers defined by bends and branching of the fault trace. The dips of these fault sections vary based on a preliminary model of the fault zone, with estimated dips ranging from 45 to 90°. The palaeo-stress tensor was characterized by sub-vertical maximum principal compressive stress, a minimum principal stress axis trending about 050° and
φ
≈ 0.2−0.5. The average slip direction across the fault segment was ≈240°. Three large, non-conservative barriers occur within the fault segment—at the northern and southern ends of the segment, and a branch point in the centre of the segment. Bifurcation of the rupture zone into two primary fault systems has presumably increased the fracture toughness of the northern part of the segment due to the overlap of secondary faulting adjacent to the primary fault planes. Two large historic earthquakes in the eastern Basin and Range and Rocky Mountain transition region have ruptured bifurcated fault zones similar in structure to the Salt Lake segment; the Hebgen Lake, Montana earthquake (
M
= 7.5) in 1959 and the Borah Peak, Idaho earthquake (
M
= 7.3) in 1983. The earthquake foci were located at depths of 12–18 km and the ruptures propagated towards the regions of fault-zone bifurcation in both cases. A characteristic earthquake (
M
≈ 7−7.5) may initiate at either the centre or the southern end of the Salt Lake segment. In the first case, the rupture would propagate bilaterally, but in the latter case the rupture would propagate unilaterally to the N. A unilateral propagation direction is preferred based on inferences drawn from the structural geology of the segment and direct comparison with the rupture traces of the Borah Peak and Hebgen Lake earthquakes.
Extensional tectonics and crustal structure: deep seismic reflection data from the northern North Sea Viking graben
Authors: Beach Alastair , Bird Tim , Gibbs Alan 
Published: April 2008

Summary
A preliminary interpretation of a deep seismic reflection profile across the northern North Sea provides an insight into the crustal structure beneath the major extensional rift, the Viking graben. The Viking graben was initiated during the early Mesozoic and evolved during the Jurassic-Cretaceous into an asymmetrical half-graben, fault-bounded on its western side. Deep seismic reflections enable an interpretation of the position of the Moho to be made, and an estimate of the amount of crustal extension, within the plane of the section, of 50% has been calculated. The pre-Mesozoic crust has been thinned to a minimum of 14 km, but generally varies from 15–20 km beneath the rift. Interpretation of extensional faults within the Jurassic generally gives less than 50% extension, and much of the early Mesozoic faulting is probably not imaged. Calculation of Mesozoic subsidence in the graben leads to an estimated stretching factor of 3 or greater. A larger scale of linked tectonics involving major strike-slip movements along the length of the graben is invoked to explain the discrepancy between subsidence and crustal extension. Fault-related subsidence occurred in three distinct phases of finite duration—during the Triassic (40 My), the late Jurassic (30 My), and the early Tertiary (30 My). During the late stages of pervasive crustal thinning (late Jurassic-Early Cretaceous) a series of major fault and shear zones developed, dipping E and passing through the crust, offsetting the Moho. The change in tectonic style corresponds to a change from a dominantly extensional to a dominantly strike-slip mode of rift evolution. The seismic profile also images the Magnus basin related to the Great Glen fault system, and the Horda Platform near the coast of Norway.
Structures associated with extensional tectonics in the Suez rift
Authors: Chénet P. Y. , Colletta B. , Letouzey J. , Desforges G. , Ousset E. , Zaghloul E. A. 
Published: April 2008

Summary
The opening of the Suez rift, which was initiated at the beginning of the Miocene (23 Ma), was the result of a regional extensional stress more or less perpendicular to the rift axis. A detailed study of selected areas within the onshore part of the rift (Abu Durba, Gebel Ekma and Wadi Dib) shows that the fault geometries visible in the Precambrian basement and the overlying sedimentary rocks are distinct; brittle deformation in one, ductile deformation in the other. In the basement (metamorphic and granitic rocks), faults display a rectilinear pattern. Two sets of faults are predominant; (i) N140-150E faults, parallel to the Gulf trend, with pure dip-slip displacement, (ii) cross-faults (transfer faults) trending N00 to N30E with strike-slip component. These faults delineate diamond-shaped tilted blocks. In the overlying sedimentary rocks (Cretaceous-Eocene sands, shales and limestones) faults are spoon-shaped. Spoon faults are generally superimposed over concave wedges of the basement blocks but are decoupled from basement fault sets. The decoupling is accommodated by ductile deformation and detachment in the Cretaceous marks.