127,300 research outputs found
The continental record and the generation of continental crust
No full textContinental crust is the archive of Earth history. The spatial and temporal distribution of Earth's record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and seawater and atmospheric proxies are distributed about a series of peaks and troughs. This distribution reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. The physio-chemical resilience of zircons and their derivation largely from felsic igneous rocks means that they are important indicators of the crustal record. Furthermore, detrital zircons, which sample a range of source rocks, provide a more representative record than direct analysis of grains in igneous rocks. Analysis of detrital zircons suggests that at least ∼60%–70% of the present volume of the continental crust had been generated by 3 Ga. Such estimates seek to take account of the extent to which the old crustal material is underrepresented in the sedimentary record, and they imply that there were greater volumes of continental crust in the Archean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at ca. 3 Ga, which may have been linked to the onset of significant crustal recycling, probably through subduction at convergent plate margins. The Hadean and Early Archean continental record is poorly preserved and characterized by a bimodal TTG (tonalites, trondhjemites, and granodiorites) and greenstone association that differs from the younger record that can be more directly related to a plate-tectonic regime. The paucity of this early record has led to competing and equivocal models invoking plate-tectonic– and mantle-plume–dominated processes. The 60%–70% of the present volume of the continental crust estimated to have been present at 3 Ga contrasts markedly with the <10% of crust of that age apparently still preserved and requires ongoing destruction (recycling) of crust and subcontinental mantle lithosphere back into the mantle through processes such as subduction and delamination.Peer reviewe
Soil crusting in Western Samoa. Part II - Experimental investigation of factors influencing crust formation
No full textThe Alafua Penetrometer was used to measure relative differences in soil crust strength. Crust strength and thickness were shown to increase with increases in rainfall amount, drying time, droplet size, kinetic energy and soil clay and silt content. The investigations were designed to illustrate some of the factors influencing crust formation to a diploma level soil conservation class
A seismic study of the mid- and lower-crust beneath the sea of Bothnia: BABEL line 1
No full textIn the Autumn of 1989, Durham University took part in the BABEL Project, a collaboration of scientists from five nations recording wide angle and normal incidence seismic data in the Baltic Shield. Recording stations were set up along the Swedish coast of the Sea of Bothnia to record marine airgun shots at wide angle. Similar stations were operated by Finnish teams on the eastern coast, and by a German team on Aland. The data recorded are of high quality and high resolution in comparison to previous wide angle surveys in the region, with a shot spacing of 75 m. A large proportion (around fifty percent) of the project involved developing software for processing this data. The in-line data from Line 1, and also those recorded at two off-line stations, have been interpreted using Cerveny's Gaussian Beam forward modelling package BEAM87, the in-line model being further constrained by 2D gravity modelling. The resulting models are compared and contrasted with normal incidence data from the same line, other models derived from BABEL data in the Sea of Bothnia and older refraction lines in the vicinity. The models show a highly complex crust whose thickness varies between 50 and 60 km along the profile. The seismic velocity is high, increasing from 5.85 km s(^-l) near the surface to 7.4 km s(^-1) at the base of the crust. Lateral velocity variations are seen in the mid-upper crust while discontinuous reflectors and diffracting bodies are seen at 30 km depth. In the central/northern part of the line the crust thickens and there is a change in seismic velocity. Using other geophysical information from the region, two hypotheses are put forward for interpreting this part of the seismic model. The first is the presence of a large igneous intrusion, and the second is the existence of a shear zone or tectonic boundary cutting Line 1. Further work will be required to confirm either or both of these hypotheses
Towards using seismic anisotropy to interpret ductile deformation in mafic lower crust
No full textThe lower crust forms an important geodynamic control in continental tectonics and the communication and coupling of kinematics between surface and deep-Earth processes. An understanding of the relationship between seismic properties, finite strain and fabric orientation thus provides a useful tool in the remote sensing and
interpretation of deformation in the lower crust.
This thesis outlines a work-flow model by which the seismic properties of a single and representative lower crustal lithology can be calculated and calibrated against
finite strain from petrofabric development across a strain gradient. The work-flow model constitutes a multi-disciplinary approach, incorporating field mapping and
sample collection, experimental petrofabric determination, and seismic modelling.
A review of compositional estimates of the deep crust, including xenoliths, exposed sections and estimates from wide-angle seismic profiles, indicates the importance
of mafic lithologies.
The Laxfordian-age high-grade shear zone at Upper Badcall, NW Scotland, exhibits a strain gradient in a deformed doleritic Scourie dyke (Lewisian complex) that intersects the zone at a high angle. From an analysis of field data from detailed mapping, the shear zone is shown to be characterised by generally simple shear, but where the tectonic movement direction varies transversely across the shear zone. Calculation of the strain profile across the deformation zone gives shear strains, y up to 57, but with y < 15 being perhaps more realistic. Cumulative displacements
total ~1000m left-laterally, and ~600m vertical displacement, north-side up. Nine samples were collected across the shear zone in the mafic dyke, representing a
strain gradient from undeformed protolith to the highest recorded stains.
The sample suite is characterised as a hornblende-plagioclase-quartz aggregate that develops macroscopic planar and linear fabrics with strain, from an essentially
isotropic protolith. Quantification of the aggregate lattice preferred orientation (LPO) using electron backscatter diffraction (EBSD) showed the dominance of
fabric development in the hornblende phase, with (100) poles clustering forming normal to the foliation plane and [001] axes parallel to the tectonic X direction.
Plagioclase and quartz retained random fabrics from the wall-rock protolith with increasing finite strain. The hornblende LPO fabric, described by the texture index, J, shows a positive logarithmic relationship with strain, where LPO intensity saturated by y ~10.
The strain-calibrated quantitative petrofabric description of each sample is used to calculate their aggregate elasticity tensors (Cij) via a Voigt-Reuss-Bill average,
and from which seismic properties are derived using Christoffel's equation. Hence, a framework of petrofabric- and strain-calibrated seismic properties is described
for a strain gradient in a representative high-grade mafic lithology. P-wave anisotropies up to ~10% are-recorded in the most deformed samples with Vsmax typically between 6.42-6.63kms/-1. S-wave anisotropies record up to 7.23% AV,
in the most deformed samples, with Vpmax ranging between 3.62-3.75kms-1 for all samples. The relationship between petrofabric-derived seismic anisotropy and finite strain across the sample suite show a positive relationship, approximated by a logarithmic function, whereby P- and S-wave anisotropy exhibit a steep positive gradient with strain up to y~10.
The sample-wise framework of petrofabric- and strain-calibrated seismic properties is interpolated to estimate the continuum relationship between seismic properties,
finite strain and petrofabric orientation. In a move to illustrate the application of results in seismic and structural modelling, case study models of crustal deformation are presented for the eastern Basin and Range province, the North Sea rift, and Tibet. Models are promising in their ability to differentiate between regions of lower crust characterised by a uniform mafic composition but different finite strain state and/or petrofabric geometry, although multiple seismic survey methods may
be needed to fully interpret results in terms of strain and fabric orientation.
In summary, a multidisciplinary approach combining field mapping and sampling, petrofabric characterisation with EBSD, and seismic modelling provides an efficient and reproducible work-flow for the determination of petrofabric-derived strain-calibrated seismic properties of lower crustal materials
Design, construction and testing of an ascending micropenetrometer to measure soil crust resistance
No full textThe increasing world population is putting pressure on global food production.
Agriculture must meet these growing demands by increasing crop yields. One phenomenon
which prevents seedling emergence and damages crop yield is soil crusting. Understanding
of soil crusting and the factors which influence it is fundamental to ensuring good crop
production. An instrument which will test soil crust strength in a novel way, mimicking
seedling growth, may lead to pre-emptive agricultural soil management which could
increase crop production. This work details the process of design, construction and testing
of an ascending penetrometer to measure soil crust strength. The full design process is
discussed from concept generation and evaluation, using experimental methods and a
multi-criteria decision making tool, through to final design configuration, specification,
manufacture and testing.
Traditionally, soil penetrometers measure soil strength by forcing a probe from the surface
of the soil into the bulk soil below. To more accurately measure the direct impedance a
seedling would experience a device should measure impedance from the bulk soil upwards
and into the soil crust, mimicking what a growing seedling would experience. Results
prove that the manufactured ascending penetrometer with a force resolution of 0.01N and
displacement resolution of 0.0004mm is capable of detecting differences in soil crusts. At
these resolutions and accuracy to 0.1N and 0.1mm excellent repeatability was achieved.
The machine is therefore a useful and realistic tool for quantitatively comparing soil crusts
in soil. It is hoped that being able to compare soil crust strength will lead to improved soil
management techniques
Heterogeneous crust and upper mantle across southern Kenya and the relationship to surface deformation as inferred from magnetoteluric imaging.
No full textWe have used magnetotelluric data imaging to determine the resistivity structure across southern Kenya and our results suggest the presence of a buckled blocky or segmented lithosphere across the region. Prominent steep conductive zones at the Oloololo (OLO) escarpment and eastern rift margin allow us to subdivide the region into three crustal domains. West of OLO, a bow-shaped conductor underlies a 10 km thick resistive upper crustal unit spatially correlating with an exposed Archaean greenstone belt. Between OLO and the eastern rift margin are found steeply dipping alternating conductive and resistive zones that appear buckled. East of this belt are found prominent, 5 to 20 km deep, subhorizontal conductors atop steep resistive blocks with flanking conductors. The main steep features in the crust appear to extend below the seismic Moho and thus suggest the presence of anomalously thick crust across the region. A 50 km-wide and 4–8 km deep w-shaped (double half-graben) structure is suggested at the position of the Kenyan rift. We show that our inferred lateral zoning is consistent with collocated gravity and seismic measurements. We propose a link between the deep resistivity heterogeneity and surface deformation pattern in the area
Magma mobilization by downward-propagating decompression of the Eyjafjallajökull volcanic plumbing system
No full textDetailed observations of the 2010 Eyjafjallajökull eruptions in Iceland show seismic activity propagating vertically through the entire crust during a ten-week period of volcanic unrest comprising multiple eruption episodes. Systematic changes in magma chemistry suggest a complex magmatic plumbing system, tapping several accumulation zones at different depths containing magma of differing ages and compositions. During the eruption, a systematic downward propagation of seismicity through the crust and into the upper mantle to ~30 km depth occurred in a series of steps, each of which preceded an explosive surge in eruption rate. Here we show that the sequence of seismicity and eruptive activity may be explained by the downward propagation of a decompression wave that triggers magma release from progressively deeper sills in the crust. Comparing observations of the downward-propagating seismicity with the decompression of a series of model elastic sills suggests that each sill was 1-10 km3 in size
Crustal flow beneath Iceland
No full textTheoretical and experimental studies indicate that when oceanic crust is hotter than about 800°C, variations in crustal thickness drive lower crustal flow. We investigate the nature of crustal flow beneath Iceland, where zero-age crust varies in thickness from under 20 km to over 40 km over a distance of 100 km and temperatures exceed 1000°C below depths of a few kilometers. We model the regional characteristics of crustal flow using the two-dimensional channel flow equation with depth-dependent viscosity. The model predicts the observed decay in crustal thickness contrasts between zero-age and off-axis crust on shore Iceland and development of a sharp Moho step as the plate moves away from the rift axis. These features become locked in place as plate cooling increases the viscosity. Observations are best matched by model predictions when the solidus viscosity is of order 1018 Pa s, in agreement with viscosity estimates from deformation rates associated with deglaciation and plate boundary processes. Crustal flow acts to erase the crustal thickness memory of ridge-plume interaction, so that maps of Icelandic crustal thickness can be used neither to derive a detailed plume flux history nor to test in detail whether the Iceland Plume center is fixed relative to other hot spots. Crustal flow beneath Iceland is an unusually clear example of the kind of flow postulated to explain continental phenomena including exhumation of metamorphic core complexes and associated development of low-angle detachments within extensional terrains and sharp-edged plateau topography in major collisional belts
Seismic data reveal eastern Black Sea Basin structure
No full textRifted continental margins are formed by progressive extension of the lithosphere. The development of these margins plays an integral role in the plate tectonic cycle, and an understanding of the extensional process underpins much hydrocarbon exploration. A key issue is whether the lithosphere extends uniformly, or whether extension varies\ud
with depth. Crustal extension may be determined using seismic techniques. Lithospheric extension may be inferred from the waterloaded subsidence history, determined from\ud
the pattern of sedimentation during and after rifting. Unfortunately, however, many rifted margins are sediment-starved, so the subsidence history is poorly known.\ud
To test whether extension varies between the crust and the mantle, a major seismic experiment was conducted in February–March 2005 in the eastern Black Sea Basin (Figure 1), a deep basin where the subsidence history is recorded\ud
by a thick, post-rift sedimentary sequence. The seismic data from the experiment indicate the presence of a thick, low-velocity zone, possibly representing overpressured sediments. They also indicate that the basement and\ud
Moho in the center of the basin are both several kilometers shallower than previously inferred. These initial observations may have considerable impact on thermal models of the petroleum system in the basin. Understanding\ud
the thermal history of potential source rocks is key to reducing hydrocarbon exploration risk. The experiment, which involved collaboration between university groups in the United Kingdom, Ireland, and Turkey, and BP and\ud
Turkish Petroleum (TPAO), formed part of a larger project that also is using deep seismic reflection and other geophysical data held by the industry partners to determine the subsidence history and hence the strain evolution of\ud
the basin
A high-resolution wide-angle seismic study of the crust beneath the Northumberland trough
No full textIn June 1987, during the BIRPS MOBIL normal-incidence seismic profiling programme, off the East coast of England, the University of Durham recorded simultaneously at several land based seismic stations in Northern England. The resulting wide-angle data, particularly from Line 1, have excellent resolution in both space and time due to the airgun source and 50 m shot spacing. The interpretation of the Line 1 wide-angle data at Durham used BEAM87, Cerveny's Gaussian beam modelling package. The main arrivals interpreted include the upper crustal refraction (Pg), the Moho wide-angle reflection (PmP), the upper mantle refraction (Pn), and a very high amplitude arrival (D) which merges into PmP. Modelling gave a crust about 30 km thick with a change in velocity gradient and a slight velocity contrast at about 20 km depth. There are several wide-angle reflections from interfaces at mid-crustal depths, between 10 and 20 km depth, and the bottom 2 km of the crust has a high velocity of about 7 kms(^-1). Two interesting results are that a lateral velocity change about 40 km offshore is required to fit the Pg travel times; also that arrival D is modelled best as the remnant of a step on the Moho at the same location. These appear to be borne out by the normal incidence data for line 1, which show a lateral decrease in the mid-crustal reflectivity above a set of strong, westerly-dipping reflections at Moho depths. These results suggest the presence of a major crustal fault about 40 km offshore. It is suggested that this fault may be the northward continuation of the Dowsing Fault Zone
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