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Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implications
No full textWe use teleseismic waveform inversion, along with depth phase analysis, to constrain the centroid depths and source parameters of large African earthquakes. The majority of seismic activity is concentrated along the East African Rift System, with additional active regions along stretches of the continental margins in north and east Africa, and in the Congo Basin. We examine variations in the seismogenic thickness across Africa, based on a total of 227 well-determined earthquake depths, 112 of which are new to this study. Seismogenic thickness varies in correspondence with lithospheric thickness, as determined from surface wave tomography, with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km. In regions of thin lithosphere, the seismogenic thickness is typically limited to ≤20 km. Larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of ≥1500 Ma, where the East African Rift passes around the Archean cratons of Africa, through the older Proterozoic mobile belts. These correlations are likely to be related to the production, affected by method and age of basement formation, and preservation, affected by lithospheric thickness, of a strong, anhydrous lower crust. The Congo Basin contains the only compressional earthquakes in the continental interior. Simple modelling of the forces induced by convective support of the African plate, based on long-wavelength free-air gravity anomalies, indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in Africa. Seismicity along the margins of Africa reflects a mixture between oceanic and continental seismogenic characteristics, with earthquakes in places extending to 40 km depth
The Fine Structure of the Subducted Investigator Fracture Zone in Western Sumatra as Seen by Local Seismicity
No full textThe Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004 Mw = 9.1, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity
Control of the symmetry of plume-ridge interaction by spreading ridge geometry
No full textThe Iceland, Gal´apagos and Azores plumes have previously
been identified as interacting asymmetrically with adjacent spreading centres.
We present evidence that the flow fields in these plume heads are radially
symmetric, but the geometry of the mid-ocean ridge systems imparts
an asymmetric compositional structure on outflowing plume material. First,
we quantify the degree of symmetry in geophysical and geochemical observables
as a function of plume centre location. For each plume, we find that
bathymetry and crustal thickness observations can be explained using a single
centre of symmetry, with these calculated centres coinciding with independently
inferred plume centre locations. The existence of these centres of
symmetry suggests that the flow fields and temperature structure of the three
plume heads are radially symmetric. However, no centres of symmetry can
be found for the incompatible trace element and isotopic observations. To
explain this, we develop a simple kinematic model to predict the effect of midocean
ridge geometry on the chemical composition of outflowing plume material.
The model assumes radially symmetric outflow from a compositionally
heterogeneous plume source, consisting of a depleted mantle component
and enriched blebs. These blebs progressively melt out during flow through
the melting regions under spreading centres. Asymmetry in trace element
and isotopic profiles develops when ridges either side of the plume centre receive
material that has been variably depleted according to the length of flow
path under the ridge. This model can successfully explain compositional asymmetry around Iceland and Gal´apagos in terms of an axisymmetric plume interacting
with an asymmetric ridge system
The Fine Structure of the Subducted Investigator Fracture Zone in Western Sumatra as Seen by Local Seismicity
No full textThe Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004 Mw = 9.1, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity
Late Cenozoic volcanism and rates of active faulting in eastern Iran
No full textWe present new 40Ar/39Ar ages of samples of volcanic rock exposed along the remote margins of the Dasht-e Lut desert in eastern Iran. Close spatial relationships between the volcanic rocks and the trace of active strike-slip faults allow us to determine the slip rates of two major faults, averaged since eruption of the volcanics. Our study shows that the Nayband fault at the western margin of the Dasht-e Lut has a slip rate of 223C1.4 o?= 0.5 mm yr22121 averaged over 2.25 Ma. The East Neh fault, one of several active strike-slip faults within the Sistan Suture Zone at the eastern margin of the Dasht-e Lut, has a minimum slip rate of 223C1.2 mm yr22121 averaged over 223C1.7 Ma. The rates of slip on major active faults in eastern Iran are largely unknown, and the slip rates our data provide, though limited, are a significant increase on what is known of the faulting within this remote and relatively inaccessible desert region. We also present analyses of the major and trace element concentrations within the volcanic rocks. The chemistry of the volcanic rocks is typical of intracontinental melts with an overall signature similar to that of ocean island basalts. Inversion of rare earth element distributions suggests some melting has occurred at depths of 223C80 km, indicating the presence of a relatively thin lithosphere beneath eastern Iran, in agreement with recently published maps of lithospheric thickness derived from shear wave velocities
Influence of the Iceland mantle plume on oceanic crust generation in the North Atlantic
Get PDFWhen a mantle plume with elevated temperature underlies an oceanic spreading centre it
affects the generation of oceanic crust by creating thicker crust. We map the variation in
crustal thickness and seismic velocity along three long-offset seismic profiles acquired over
oceanic crust generated shortly after continental breakup in the North Atlantic: a 212-kmlong
flowline from the Faroes rifted continental margin across crust of 51–42 Ma age, where
oceanic spreading developed close to the inferred centre of the Iceland mantle plume; a 256 km
flowline extending from the Hatton rifted continental margin across crust of 52–40 Ma age,
about 800 km south of the presumed centre of the mantle plume; and a 99 km strike line over
oceanic crust formed at 43 Ma in the Iceland Basin off the Hatton continental margin. The
crustal velocity structure along each profile is constrained by multichannel seismic reflection
data, which is used primarily to map the sediments, and by densely spaced ocean-bottom
seismometers, which recorded wide-angle reflections and refractions to offsets of more than
100 km. Over 56 000 crustal diving wave and Moho wide-angle reflection arrivals were used in
joint crustal refraction and reflection tomographic inversions. Quantitative error analysis shows
that the seismic velocity of the crust is mostly constrained to within 0.1 km s−1 and the depth
of the Moho to within ±250 m. We interpret the crustal thickness and velocity changes along
the profiles as caused primarily by changes in the mantle temperature at the time of crustal
formation. If all the oceanic crustal thickness variations are ascribed to mantle temperature
changes, we infer that as mature seafloor spreading developed following continental breakup,
the mantle cooled by ca. 75 ◦C over a 10 Myr period, although it still remained hotter than
the global average of normal oceanic crust. The crust formed close to Iceland is at all times
thicker than that formed further away, which we interpret as reflecting higher temperatures
close to the centre of the thermal anomaly created by the mantle plume. Currently at the
Reykjanes Ridge, south of Iceland, we interpret thicker than normal oceanic crust as being
caused by the presence of hotter mantle, modulated by thickness variations of 1.5–2.0 km
which are attributed to temporal variations in the mantle plume temperature of about 25 ◦C
on a 3–6 Myr timescale. A 1.5 km increase in thickness of oceanic crust generated between
48 and 45 Ma on the Faroes line is similar in magnitude and duration to those occurring on
the present day Reykjanes Ridge, which we suggest is due to a temperature pulse of ∼25 ◦C.
Gravity lineations in the northern North Atlantic suggest that the oceanic crust has exhibited
small thickness fluctuations of similar size throughout its history, interpreted as due to small
fluctuations in the temperature of the Iceland mantle plume
Quaternary adakite—Nb-enriched basalt association in the western Trans-Mexican Volcanic Belt: is there any slab melt evidence?
No full textAbstract A spatial and temporal association between
adakitic rocks and Nb-enriched basalts (NEB) is recognised
for the first time in the western sector of the Trans-Mexican
Volcanic Belt in the San Pedro–Cerro Grande Volcanic
Complex (SCVC). The SCVC is composed of subalkalic
intermediate to felsic rocks, spanning in composition from
high-silica andesites to rhyolites, and by the young transitional hawaiite and mugearite lavas of Amado Nervo shield volcano. Intermediate to felsic rocks of the SCVC show many geochemical characteristics of typical adakites, such as high Sr/Y ratios (up to 180) and low Y (\18 ppm) and Yb contents. Mafic Amado Nervo rocks have high TiO2 (1.5–2.3 wt%), Nb (14–27 ppm), Nb/La (0.5–0.9) and high
absolute abundances of HFSE similar to those shown by
NEB. However, the Sr and Nd isotopic signature of SCVC rocks is different from that shown by typical adakites and
NEB. Although the adakites–NEB association has been traditionally considered as a strong evidence of slab-melting,we suggest that other processes can lead to its generation.
Here, we show that parental magmas of adakitic rocks of the
SCVC derive their adakitic characteristic from high-pressure
crystal fractionation processes of garnet, amphibole and
pyroxene of a normal arc basalt. On the other hand, Amado
Nervo Na-alkaline parental magmas have been generated by sediment melting plus MORB-fluid flux melting of a heterogeneous mantle wedge, consisting of a mixture of depleted and an enriched mantle sources (90DM + 10EM). We cannot exclude a contribution to the subduction component of
slab melts, because the component signature is dominated by
sediment melt, but we argue that caution is needed in interpreting the adakites–NEB association in a genetic sense
Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control
No full textIn rivers draining the Himalaya-Tibetan-Plateau region, the 26Mg/24Mg ratio has a range of 2‰ and the 44Ca/42Ca ratio has a range of 0.6‰. The average δ26Mgδ26Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ26Mgδ26Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ26Mgδ26Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their View the MathML sourceδ4442Ca values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ26Mgδ26Mg (−1.31‰) and View the MathML sourceδ4442Ca (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated View the MathML sourceδ4442Ca values in rivers and rock reported so far demonstrating that both riverine and bedrock View the MathML sourceδ4442Ca values may show greater variability than previously thought.
Although rivers draining TSS limestone have the lowest View the MathML sourceδ26Mgandδ4442Ca values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ26Mgδ26Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ26Mgδ26Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in 26Mg/24Mg ratios between silicate rock and solute Mg reflects the 26Mg/24Mg isotopic fractionation factor (View the MathML sourceαsilicate–dissolvedMg) between silicate and dissolved Mg during incongruent silicate weathering. The value of View the MathML sourceαsilicate–dissolvedMg of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average View the MathML sourceδ4442Ca value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ26Mgδ26Mg value of −0.46‰ and an average View the MathML sourceδ4442Ca value of 0.5‰, distinct from riverine values for δ26δ26Mg but similar to riverine View the MathML sourceδ4442Ca values. Although rivers draining each major rock type have View the MathML sourceδ4442Ca and δ26Mgδ26Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments.
The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle
Martensitic transformation B2–R in Ni–Ti–Fe: experimental determination of the Landau potential and quantum saturation of the order parameter
No full textThe Landau potential of the martensitic phase transformation in
Ni46.8Ti50Fe3.2 was determined using high resolution x-ray diffraction
to measure the spontaneous strain and calorimetric measurements to
determine the excess specific heat of the R phase. The spontaneous
strain is proportional to the square of the order parameter which is
tested by the relation of the excess entropy and the order parameter.
The parameters of the Landau free energy were determined by fitting
the temperature evolution of the order parameter and using the scaling
between the excess entropy and the order parameter. The double well
potential at absolute zero temperature was calculated and the
interface energy and domain wall thickness were estimated
Quartz-bearing C-O-H fluid inclusions diamond: Retracing the pressure-temperature path in the mantle using calibrated high temperature IR spectroscopy
No full textInfrared spectra of C-O-H micro-inclusions were collected from a micro-inclusion bearing diamond during step-heating and freezing experiments to examine fluid speciation as a function of pressure and temperature. The inclusions contain H2O, CO2, carbonate, apatite, quartz and mica, which together represent the oxidising remnant mantle fluid composition after diamond crystallisation. The internal pressure of the inclusions, measured from calibrated shifts of the quartz peaks, increases from 1.3 GPa at ambient temperature, to approximately 4-5 GPa at 737 °C, close to the conditions of crystallisation of the host diamond in the mantle