Continental Shield Research Articles

Degree 12 model of shear velocity heterogeneity in the mantle

We obtain a three‐dimensional (3D) model of shear wave velocity heterogeneity of the Earth's mantle by inverting a large set of seismic data consisting of 27,000 long‐period seismograms and 14,000 travel time observations. About 60% of the data has been collected through the efforts of several research groups and used in earlier studies. The new data, which come from stations of different seismic networks including the Chinese Digital Seismographic Network (CDSN) and Geoscope, are extracted to provide sampling of mantle heterogeneity as uniform as possible. Because of the improved data coverage, we expand our model to degree 12 in spherical harmonics to describe horizontal variations, and to order of 13 in Chebyshev polynomials to describe radial variations. The resulting model shows a clear pattern of slower‐than‐average shear velocities at shallow depths underlying the major segments of the world‐wide ridge system. These anomalies extend to depths greater than 300 km and in some cases appear to continue into the lower mantle. There is also a good correlation between the major continental shields and fast‐velocity perturbations at depths extending to 300–400 km. Some of the continental “roots” extend to depths greater than in other studies. The pattern of heterogeneity is more complex in the midmantle, where the power spectrum is almost flat and has a relatively low amplitude; therefore the results in this depth range should be interpreted with caution. The pattern of the heterogeneity indicates a rapid change at a depth of about 1700 km. At this depth, the power spectrum of the model shifts from one which is almost flat in the midmantle to that dominated by degrees 2 and 3; this pattern then continues to the core‐mantle boundary (CMB). The model is dominated by a few megastructures of velocity heterogeneity below the depth of 2000 km, in agreement with previous studies. Among these megastructures are the “Pangea Trough,” “Great African Plume,” and “Equatorial Pacific Plume Group.” The model predicts well the large‐scale pattern of observed S, SS absolute travel times, and SS‐S, ScS‐S differential travel times. It also predicts well the waveforms of mantle wave and body wave. We compare our model with several other recently published models. There is generally a good agreement in the long‐wavelength pattern of the models, especially at shallow depths and near the CMB. However, the amplitude as well as the pattern of shorter‐wavelength features are in some cases quite different.

Interpretation of SKS-waves using samples from the subcontinental lithosphere

The seismic properties of five South African kimberlite nodules have been calculated from petrofabric measurements. These garnet lherzolite and harzburgite composition nodules (56–80% olivine) are known from previous studies to have originated at depths of 120–170 km in the subcontinental lithosphere. Their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature (900–1050°C) and pressure (3.0–3.5 GPa) conditions. The average of the five samples has a maximum S-wave anisotropy ( δV s) of 3.7% which is sufficiently high to explain previously reported teleseismic SKS delay times ( δt) in continental shield areas of up to 1.7 s. The biggest delay time is observed when the foliation is vertical, the lineation horizontal, and the fastest S-wave polarized parallel to the lineation, i.e. an orientation for transcurrent motion. Delay times of over 1 s can only be generated by this orientation. A strong mantle-crust mechanical coupling is suggested for such situations. The potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine, the most abundant phase (70%) in the upper mantle. The V p and V s seismic anisotropy increases with fabric strength and finite strain. Recrystallization tends to reduce fabric strength and seismic anisotropy, resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50–60% strain or 8–9% δV s for pure olivine aggregates. A survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum S-wave anisotropy for the olivine component of 9%, or about 8% for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account. The ophiolite samples are twice as anisotropic as the kimberlite nodules. If an average anisotropy value is representative of the lithosphere, then the SKS delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal. The magnitude of S-wave anisotropy is less sensitive than V p to variations in olivine volume fraction in range 50–100%, and to deformation or fabric intensity, further suggesting that S-wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness. For smaller delay times there is some trade off between structural orientation and thickness.

Upper mantle xenoliths from alkali basalts of the Vogelsberg, Germany: implications for mantle upwelling and metasomatism

Alkali basalt-hosted mantle xenoliths of type I (Frey & Prinz, 1978) from the Vogelsberg area (W Germany) cover a compositional range from mildly depleted lherzolites (19 wt.% clinopyroxene) to harzburgites (3 wt.% clinopyroxene), whereas dunites and pyroxenites seem to be extremely rare. Due to partial melting and magma extraction, major and moderately incompatible trace-element compositions of bulk rocks and minerals correlate with the variable modal clinopyroxene abundance, indicating a degree of ∼ 30 % partial melting for the most depleting harzburgites. Even though these peridotites represent residues of partial melting, some harzburgites show light rare-earth-element enrichment and a Sr-Nd isotopic signature similar to that of the Vogelsberg basanites. The xenoliths equilibrated in narrow temperature intervals typical for each locality (1125, 1025, and 925 °C), with exception of xenoliths from one outcrop, which reveal a bimodal temperature distribution (1050 and 900 °C). The elevation of the thermal condition above that expected for a stable continental shield appears to be the result of mantle uplift and heating due to magma intrusion. The presence of «spinel-pyroxene-clusters» interpreted as former garnet suggests the rise of asthenospheric mantle into the lithosphere. The peridotite/magma interaction is deduced from Fe-Ti-metasomatism observed in high-temperature xenoliths (1020-1070 °C) considered as parts of thermal aureoles adjacent to local magma conduits

Dynamic surface topography: A new interpretation based upon mantle flow models derived from seismic tomography

The very long wavelength (i.e. in the degree range ℓ = 1–8) dynamic topography at the Earth's surface, estimated by isostatic reduction of the observed topography, is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle. This flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large‐scale seismic shear velocity heterogeneity in the mantle. Our estimate (and prediction) of the long wavelength dynamic surface topography reveals significant depressions (≈ 2–3 km) of the continental shields relative to their hydrostatic reference positions.

Tomographic inversions for mantle P wave velocity structure based on the minimization of l2 and l1 norms of International Seismological Centre Travel Time Residuals

We use International Seismological Centre (ISC) P arrival data (1964–1987) and iterative algorithms which minimize the l1 and l2 residual norms to solve simultaneously for three‐dimensional P velocity variations in Earth's mantle, source mislocations, and station corrections. We find that the maximum velocity perturbations produced by the l1 minimization (approximately ±4% in our final model) are relatively insensitive to smoothing and damping constraints. Therefore, using an l1 norm criterion allows us to keep the bias introduced to the inversion to a minimum. Among the well‐resolved features contained in both the l1 and the l2 velocity models are a fast anomaly in the lower mantle beneath the Tonga‐New Hebrides subduction zone to a depth of 1670 km and another fast anomaly beneath the Japanese island arc and eastern Asia. Continuity between these anomalies and shallower fast anomalies is not clear. A fast anomaly extending from 670 km to 2070 km depth appears beneath eastern North America, the Caribbean, and north central South America. A broad, fast anomaly appears beneath eastern Asia just above the core‐mantle boundary as well as several slow anomalies under the Pacific basin of comparable size. Both models contain a circum‐Pacific ring of 2% lower velocities in the depth range 0–200 km, associated with back arc basins. High velocities (over 2%) associated with the continental shields tend to disappear below 400 km, though a significant region of high velocity remains beneath the Siberian platform in the 400 to 670 km depth interval.

Magnetotelluric soundings, structure, and fluids in the southern Canadian Cordillera

The Cordillera of western Canada lies in a region of oceanic and island-arc lithosphere accreted to North America during subductions of the last 200 Ma. Magnetometer arrays have shown the crust of the region to be highly conductive. Magnetotelluric (MT) soundings across the Intermontane and Omineca tectonic belts between 50°N and 54°N reveal structure in terms of electrical resistivity. Pseudosections of phase and apparent resistivity and preliminary resistivity–depth sections are shown for three transects. The resistivity range is from less than one ohm metre to several thousands of ohm metres. In old continental shields, crustal resistivities cover a similar four-decade range transposed up two decades, i.e., 102–106 Ω∙m. We show that the observed resistivities can be produced by water with NaCl and (or) CO2 in solution, at the high temperatures of the Cordilleran crust, in fractured rock of effective porosity 4–5%. The resistivity variations may represent varying fracture densities. By following structures from outcrops we infer that the more resistive rocks are probably granitoid plutons, with low fracture densities. The highly conductive basalts probably have higher fracture densities. Sections and phase maps indicate that granitoid plutons continue from the Coast Plutonic Complex, under a thin layer of basalt, across the southwestern half of the Intermontane Belt. Near the centre of the Intermontane Belt, in line with the Fraser fault system, highly conductive rock continues from the surface at least to midcrustal depths. Resistivities as low as 1 Ω∙m in the uppermost crust under the Cariboo Mountains, in the Omineca Belt, are ascribed to intense fracturing or mineralization. For the southernmost transect, between 50°N and 51°N, a phase pseudosection shows informative resemblances to the sections farther north. Resistivity–depth inversions at seven sites from six-decade MT data give penetration into the upper mantle, but some of these sites may be affected by static shift. All results fit the mantle upflow hypothesis advanced earlier by Gough.

Some thermomechanical aspects of the subduction of continental lithosphere

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.

Coda attenuation in the Lützow-Holm Bay region, East Antarctica

The frequency-dependent coda Qc−1 was estimated for the Lutzow-Holm Bay region, East Antarctica, on the basis of the single-scattering model, and discussed together with Qc−1 data of Kyoto, Japan, as an example of an active region. Coda waves from six shallow earthquakes, observed with a local telemetry seismic network installed along the Soya Coast, were analyzed with narrow band-pass filters. Qc−1 for a lapse time of 20–40 s was estimated to be 0.00495 ƒ−0.776 for a frequency of 4–16 Hz, and Qc−1 for 150–210 s was 0.00335 ƒ−0.926 for 1–24 Hz. In comparison with Qc−1 data for Kyoto and other regions with various tectonic conditions, Qc−1 in the Lutzow-Holm Bay region was characterized by large values at low frequency (around 1 Hz) and smaller values at higher frequency. From the single-scattering model, Qc−1 suggests a strong frequency dependence of intrinsic absorption for S waves. Assuming that the observed Qc−1 reflects scattering loss of energy, and taking account of the geological conditions and the extremely low seismic activity in the East Antarctic continental shield, it is suggested that the frequency-dependent Qc−1 is attributable to large scattering loss in the lower-frequency range caused by large-scale heterogeneities as a result of velocity and/or density perturbations.

Crustal structures from MT soundings in the Canadian Cordillera

Magnetotelluric soundings have been made across the Intermontane and Omineca tectonic belts of the Canadian Cordillera between latitudes 51.5 and 53.5°N. The frequency range, 0.016 to 130 Hz, gives penetration into the middle crust. In this part of the Cordillera the upper crust has very low resistivities, ranging from 3 to 300 ohm m, compared with continental shields and stable platforms. The most resistive rocks (100–300 ohm m) rise to the surface as the Coast Plutonic Complex is approached, and we identify them with confidence as granodiorites and similar plutonic rocks (hereafter “plutonics”). Phase pseudosections and resistivity-depth sections are used to infer that these plutonics continue northeastward from the Coast Plutonic Complex, across more than half of the width of the Intermontane Belt, with a sharp edge well located in the phase pseudosections. The Miocene basalts have extremely low resistivities (3–30 ohm m) and form a sheet 0–2 km thick covering the plutonics. The very low resistivities in all rocks are probably caused by saline hot water in connected spaces, with low fracture density giving relatively high resistivities in the plutonic rocks, and much greater fracture densities giving extremely low resistivities in the volcanics. This is consistent with the lower mechanical strength of basalt as against granodiorite; the volcanics may have accommodated most of the post-Miocene extension of the upper crust. Off the edge of the plutonic rocks beneath the basalts, very low resistivities extend at least into the middle crust; this deep extension of the highly conductive rock may mark the feeder channel of the Miocene basalt to the surface. Three resistivity-depth sections show a fall of resistivity with depth, to values of 10 ohm m or even less, at a depth of only 8 km. Heat flow is high in the region, and the temperature at 8 km may be as high as 350°C. The increase in conductivity may be due in part to the temperature effect on NaCl solutions, and in part to release of water from hydrated minerals. All crustal features disclosed in this work support the hypothesis advanced earlier, that the interior Canadian Cordillera lie above an elongated upflow in the mantle inland from the currently active subduction.

Solidus of carbonated fertile peridotite under fluid-saturated conditions

Research Article| March 01, 1990 Solidus of carbonated fertile peridotite under fluid-saturated conditions Trevor J. Falloon; Trevor J. Falloon 1Geology Department, University of Tasmania, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar David H. Green David H. Green 1Geology Department, University of Tasmania, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Trevor J. Falloon 1Geology Department, University of Tasmania, Hobart, Tasmania 7001, Australia David H. Green 1Geology Department, University of Tasmania, Hobart, Tasmania 7001, Australia Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (3): 195–199. https://doi.org/10.1130/0091-7613(1990)018<0195:SOCFPU>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Trevor J. Falloon, David H. Green; Solidus of carbonated fertile peridotite under fluid-saturated conditions. Geology 1990;; 18 (3): 195–199. doi: https://doi.org/10.1130/0091-7613(1990)018<0195:SOCFPU>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The solidus for a fertile peridotite composition ("Hawaiian pyrolite") in the presence of a CO2-H2O fluid phase has been determined from 10 to 35 kbar. The intersection of the decarbonation reaction (olivine + diopside + CO2 ←→ orthopyroxene + dolomite) with the pyrolite solidus defines the point Q′, located at 22 kbar and 940 °C. At pressures less than Q′, the solidus passes through a temperature maximum at 14 kbar, 1060 °C. The solidus is coincident with amphibole breakdown at pressures less than 16 kbar. At pressures above Q′, the solidus is defined by the dissolution of crystalline carbonate into a sodic, dolomitic carbonatite melt. The solidus is at a temperature of 925 °C at ∼28 kbar. The solidus temperature above the point Q′ is similar to the solidus determined for Hawaiian pyrolite-H2O-CO2 for small contents of H2O (<0.3 wt%) and CO2 (<5 wt%), thus indicating that the primary sodic dolomitic carbonatite melt at both solidi has a very low and limited H2O solubility. The new data clarify the roles of carbonatite melt, carbonated silicate melt, and H2O-rich fluid in mantle conditions that are relatively oxidized (fO2 ∼ MW to FMQ). In particular, a carbonatite melt + garnet lherzolite region is intersected by continental shield geothermal gradients, but such geotherms only intersect regions with carbonated silicate melt if perturbed to higher temperatures ("kinked geotherm"). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-frequency seismic observations and models of chemical explosions: Implications for the discrimination of ripple-fired mining blasts

Discrimination of large chemical explosions (e.g., mining blasts) from possible clandestine nuclear tests (e.g., decoupled explosions) is a significant issue for seismic verification. Unless discrimination is possible, the numerous mining blasts within the USSR would give ample opportunity for concealing a clandestine test. Evernden et al. (1986) advocate high frequencies as a means to detect these clandestine tests; however, mining blasts must still give a unique signature for their discrimination. Specific conditions are also necessary for high-frequency verification: low attenuation and quiet seismic stations are among the most important. In this study we address these problems under conditions representing a best case scenerio. During the summer of 1985, Lawrence Livermore National Laboratory (LLNL) deployed a regional array and high frequency seismic station near RSON (Red Lake, Ontario, Canada) to study regional seismic signals at a low-noise site with excellent propagation characteristics, the continental shield. One objective was to evaluate the performance of regional seismic arrays and high-frequency stations in this geological environment. Using high-frequency data from mining blasts within the Mesabi Iron Range in northern Minnesota (distance of 380 km), we also observed the high-frequency Pn phase of ripple-fired blasts, modeled their source properties, and evaluated a possible discriminant for chemical explosions. This study suggests the existence of a distinctive signature for large chemical explosions: strong, high-frequency spectral peaks in the P spectra introduced by delay shooting or ripple firing. Its application depends, however, on a frequency band extending to at least 35 Hz at regional distances, reasonably uniform delay times in the blast pattern, predictably quiet seismic sites, and minimal path attenuation for high frequencies. At 380 km, we observe good propagation to 30 Hz; at 50 Hz the evidence suggests severe attenuation or scattering which is consistent with a total Q between 1500 and 2500. Attenuation estimates assuming a simple source behavior (e.g., f −2 or f −3) at high frequencies for mining explosions would give incorrect values of Q unless corrected for the blast pattern. These attenuations will limit the value of high frequencies for the detection of small decoupled explosions at regional distances.

High-frequency seismic observations and models of chemical explosions: Implications for the discrimination of ripple-fired mining blasts

AbstractDiscrimination of large chemical explosions (e.g., mining blasts) from possible clandestine nuclear tests (e.g., decoupled explosions) is a significant issue for seismic verification. Unless discrimination is possible, the numerous mining blasts within the USSR would give ample opportunity for concealing a clandestine test. Evernden et al. (1986) advocate high frequencies as a means to detect these clandestine tests; however, mining blasts must still give a unique signature for their discrimination. Specific conditions are also necessary for high-frequency verification: low attenuation and quiet seismic stations are among the most important. In this study we address these problems under conditions representing a best case scenerio.During the summer of 1985, Lawrence Livermore National Laboratory (LLNL) deployed a regional array and high frequency seismic station near RSON (Red Lake, Ontario, Canada) to study regional seismic signals at a low-noise site with excellent propagation characteristics, the continental shield. One objective was to evaluate the performance of regional seismic arrays and high-frequency stations in this geological environment. Using high-frequency data from mining blasts within the Mesabi Iron Range in northern Minnesota (distance of 380 km), we also observed the high-frequency Pn phase of ripple-fired blasts, modeled their source properties, and evaluated a possible discriminant for chemical explosions.This study suggests the existence of a distinctive signature for large chemical explosions: strong, high-frequency spectral peaks in the P spectra introduced by delay shooting or ripple firing. Its application depends, however, on a frequency band extending to at least 35 Hz at regional distances, reasonably uniform delay times in the blast pattern, predictably quiet seismic sites, and minimal path attenuation for high frequencies. At 380 km, we observe good propagation to 30 Hz; at 50 Hz the evidence suggests severe attenuation or scattering which is consistent with a total Q between 1500 and 2500. Attenuation estimates assuming a simple source behavior (e.g., f−2 or f−3) at high frequencies for mining explosions would give incorrect values of Q unless corrected for the blast pattern. These attenuations will limit the value of high frequencies for the detection of small decoupled explosions at regional distances.

Seismic modelling of the Earth’s large-scale three-dimensional structure

Several different kinds of seismological data, spanning more than three orders of magnitude in frequency, have been employed in the study of the Earth’s large-scale three-dimensional structure. These yield different but overlapping information, which is leading to a coherent picture of the Earth’s internal heterogeneity. In this article we describe several methods of seismic inversion and intercom pare the resulting models. Models of upper-mantle shear velocity based upon mantle waveforms (Woodhouse &amp; Dziewonski ( J. geophys. Res . 89 , 5953-5986 (1984))) ( f ≲ 7 mHz) and long-period body waveforms ( f ≲ 20 mHz; Woodhouse &amp; Dziewonski ( Eos, Wash . 67 , 307 (1986))) show the mid-oceanic ridges to be the major low-velocity anomalies in the uppermost mantle, together with regions in the western Pacific, characterized by back-arc volcanism. High velocities are associated with the continents, and in particular with the continental shields, extending to depths in excess of 300 km. By assuming a given ratio between density and wave velocity variations, and a given mantle viscosity structure, such models have been successful in explaining some aspects of observed plate motion in terms of thermal convection in the mantle (Forte &amp; Peltier ( J. geophys. Res . 92 , 3645-3679 (1987))). An im portant qualitative conclusion from such analysis is that the magnitude of the observed seismic anomalies is of the order expected in a convecting system having the viscosity, tem perature derivatives and flow rates which characterize the mantle. Models of the lower mantle based upon P-wave arrival times ( f ≈ 1 Hz; Dziewonski ( J. geophys. Res . 89 , 5929-5952 (1984)); Morelli &amp; Dziewonski ( Eos, Wash . 67 , 311 (1986))) SH waveforms ( f ≈ mHz; Woodhouse &amp; Dziewonski (1986)) and free oscillations (Giardini et al . ( Nature, Lond . 325 , 405-411 (1987); J. geophys. Res. 93 , 13716—13742 (1988))) ( f ≈ 0.5-5 mHz) show a very long wavelength pattern, largely contained in spherical harmonics of degree 2, which is present over a large range of depths (1000-2700 km). This anomaly has been detected in both compressional and shear wave velocities, and yields a ratio of relative perturbations in v s and v P in the lower mantle in the range 2-2.5. Such values, which are much larger than has sometimes been assumed, roughly correspond to the case that perturbations in shear modulus dominate those in bulk modulus. It is this anomaly that is mainly responsible for the observed low-degree geoid undulations (Hager et al. Nature, Lond . 313 , 541-545 (1985))). In the upper part of the lower mantle this pattern consists of a high-velocity feature skirting the subduction zones of the Pacific and extending from Indonesia to the Mediterranean, with low velocities elsewhere; thus it appears to be associated with plate convergence and subduction. The pattern of wave speeds in the lowermost mantle is such that approximately 80% of hot spots are in regions of lower than average velocities in the D" region. The topography of the core-mantle boundary, determined from the arrival times of reflected and transmitted waves (Morelli &amp; Dziewonski ( Nature, Lond . 325 , 678-683 (1987))), exhibits a pattern of depressions encircling the Pacific, having an amplitude of approximately ± 5 km, which has been shown to be consistent with the stresses induced by density anomalies inferred from tom ographic models of the lower mantle (Forte &amp; Peltier ( Tectonphysics (In the press.) (1989))). By using both free oscillations (Woodhouse et al . ( Geophys. Res. Lett . 13 , 1549-1552 (1986))) and travel-time data (Morelli et al . ( Geophys. Res. Lett . 13 , 1545—1548 (1986))), the inner core has been found to be anisotropic, exhibiting high velocities for waves propagating parallel to the Earth ’s rotation axis and low velocities in the equatorial plane. Tomographic models represent an instantaneous, low-resolution image of a convecting system. They require for their detailed interpretation knowledge of mineral and rock properties that are, as yet, poorly known but that laboratory experiments can potentially determ ine. The fact that the present distribution of seismic anomalies must represent the current configuration of therm al and compositional heterogeneity advected by m antle flow, imposes a complex set of constraints on the possible modes of convection in the m antle of which the implications have not yet been worked out; this will require num erical modelling of convection in three dimensions, which only recently has become feasible. Thus the interpretation of the ‘geographical’ information from seismology in terms of geodynamical processes is a matter of considerable complexity, and we may expect that a number of the conclusions to be drawn from the seismological results lie in the future.

The P-wave structure of the lithosphere-asthenosphere in the Western Pacific: a comparison of ocean versus continent

In this paper, we present the lithosphere-asthenosphere P-wave velocity structures in the Western Pacific as determined by ocean-bottom seismometer array observations to illustrate the contrasts and similarities between continental and oceanic regions. These structures are compared with representative velocity structures beneath continental shield regions and the tectonically active regions. The subcrustal lithosphere in the western Northwest Pacific Basin possesses a layered P-wave structure (apparent surface velocities are 8.0, 8.4 and 8.6 km/s) and extends to a depth of about 149 km. The lithosphere is underlain by a low-velocity layer which appears to be the major transition zone from the lithosphere to asthenosphere. This velocity structure is similar to that of the continental shield regions. In the East Mariana Basin, the uppermost mantle layer below the Moho is about 30 km thick with a thin high-velocity lid (8.3 km/s) at its base. These layers form the subcrustal lithosphere, below which several alternating low- and high-velocity layers together form a thick asthenosphere. The upper mantle structure beneath the East Mariana Basin seems to show an intermediate feature between continental shield regions and tectonically active regions. The differences between the upper mantle structure of the continent and ocean could be regarded as aspects of an evolution of upper mantle. The western Northwest Pacific Basin could be regarded as being almost matured, while the East Mariana Basin could be regarded as being rejuvenated.

The old continental shields stability related to mantle convection

Two dimensional thermal convection of a fluid layer overlayed by a conductive lid is studied. Lateral thermal conductivity variation is assumed within the rigid lid, in order to see how an increase of the thermal conductivity within the lithosphere can stabilize a downwelling plume beneath it. Calculations are performed for a constant viscosity fluid with Rayleigh numbers (Ra) up to 4 × 105. Different lid thicknesses (δ) and thermal conductivity (k) values are considered. The numerical results reveal a coupling between the zone of high thermal conductivity and the downwelling plumes. The adjustment time of the convective circulation decreases when the values of parameters (Ra, δ, k) increase. The lowest value for the adjustment time (500 My), is obtained for Ra = 4 × 105. The calculations suggest that an increase of the effective thermal conductivity causes the anchoring of downwelling mantle jets below primitive continental nuclei. The presence of such sinking plumes attached to the cratonic lithosphere may protect them against disruption by mantle flow. The increase in the effective thermal conductivity of the cratonic lithosphere is attribued to thermal conductivity anisotropy of olivine crystal. Such a mechanism can explain the stability of old continental shields.

Correlation between the asthenosphere and the structure of the Earth's crust in active margins of the Pacific Ocean

This paper deals with geological aspects of the correlation between deep and surface structures of the island arcs, marginal seas and folded structures of the active margins of the Pacific. The asthenosphere in these regions is most clearly identified with the areas of partial melting where magma is generated. A thicker and more complete asthenosphere is confined to tectonically active regions. Within the interarc basins of island arcs, which are characterized by the outflows of present-day tholeiitic basalts, the anomalous mantle reaches up to the crust. In deep-water basins the asthenosphere lies at a depth ranging from 30 km in the Parece Vela basin to 50–80 km in other marginal seas. Accordingly, the eruptive tholeiites are of different ages ranging from Miocene to Paleogene and even Late Cretaceous. Marginal seas are characterized by a crustal structure similar to the oceanic one. The base of the deep basins comprises rift structures associated with upwelling of the asthenosphere causing extension stresses in the lithosphere and magmatic activity. Endogenic regimes can be distinguished by the distribution of the heat flow, which is high in tectonically active regions such as island arcs, marginal seas or young fold mountain belts and is low in the abyssal basins of the Pacific and over the territory of continental shields. The processes in the upper mantle, in particular in the asthenosphere, give rise to endogenic phenomena in the transition zones. The state of the asthenosphere is determined by partial melting, which also means a lowering of its density. A lower density of the substance in the asthenosphere leads to an increase in volume and the consequent rise and extension of the overlying lithosphere. The deep basins of the marginal seas and island arcs are formed. Eruptions of tholeiite basalts may take place along the rifts. The processes occurring within the zones where the asthenosphere has risen are accompanied by intense mineralization. The tectonics in the back-arc and interarc basins is similar to processes taking place in the mid-oceanic ridges where intensive hydrothermal activity is associated with the deposition of zinc, copper and ferrous Sulfides. Consequently, the ancient paleorift structures of transition zones could also be a source of useful minerals.

Radioactivity, Heat Flow, and Rifting of the Indian Continental Crust

The relationship between heat flow (Q) and heat production (A) in India is quite different for the northern and southern parts of the peninsula. In the north, locations in a variety of tectonic settings (including Gondwana rifts) show a linear relationship between Q and A with a common reduced heat flow (Q*) about $40 mWm^{-2}$. In southern India, the heat production of surface rocks is greater than is expected for the measured heat flow. Relative to other continental shields, g* is high in northern India and about average in the south. Apparently the Indian crust evolved with a content of radioactive elements higher than in most shields, and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle. This depletion may coincide with a regional granulite event about 2,500 m.y. ago. The high heat flow and heat production in northern India may explain several unique characteristics of the Indian peninsula. One feature is the continued reactivation of rif...

Ray path interpretation of the crustal structure beneath Saudi Arabia

Interpretation of a long-range seismic refraction line in Saudi Arabia has shown that beneath the Arabian Shield velocity generally increases with depth, from about 6 km s −1 at the surface to about 7 km s −1 at the top of the crust-mantle transition zone. The base of this transition zone (Moho) occurs at 37–44 km in depth. Intracrustal discontinuities can also be recognized, the most important being in the 10–20 km-depth range and separating the upper from the lower crust. Laterally, the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions. Beneath the Red Sea shelf and coastal plain the crust, including 4 km of sediments, is only 15–17.5 km thick. With the aid of both seismic and gravity data an abrupt, steeply dipping transition from the crust of the Red Sea shelf and coastal plain to that of the Arabian Shield has been derived. With a jump of more than 20 km in Moho depth, this appears to be the major discontinuity between the Red Sea depression and the Arabian continental shield.

Crustal block of the western Ganga Basin: A fragment of oceanic affinity?

Abstract Three-component, long-period New Delhi records of two earthquakes occurring near Multan, central Pakistan, are used to determine group velocities of multimode surface waves in the period range of 5 to 57 sec. Inversion of these data has provided insight regarding the crustal shear velocity structure in the western Ganga Basin, in the northwestern wedge of the Indian subcontinent. The shear velocity is less than 3.10 km/sec within the topmost 10 km of its 40-km-thick crust. The estimated velocity in the 10- to 20-km depth range is 3.80 ± 0.09 km/sec. In the lower half crust, the velocity is as high as 3.98 ± 0.08 km/sec. Unlike the continental shield crust in the New Delhi-Shillong region, the crustal structure in this western part of the Ganga Basin is strongly reminiscent of certain oceanic plateaus. This suggests that the crustal block underlying the sediments is not part of the stable core, or the craton of the Indian Shield. The available data do not allow the determination of fine velocity structure in the upper mantle. The average velocity in the uppermost 80 km of the mantle is 4.52 km/sec. This value appears to be characteristic of the entire northern Indian subcontinent and may be indicative of active tectonism in the mantle south of the Himalayas.

Impact Craters: Their Importance in Geologic Record and Implications for Natural Resource Development: ABSTRACT

Impacting bodies of sufficient size traveling at hypervelocities carry tremendous potential energy. This relatively infrequent process results in the instantaneous formation of unique structures that are characterized by extensive fracturing and brecciation of the target material. Impacts onto continental shield areas can create rich ore deposits, such as the Sudbury mining district in Canada. Impacts into the sedimentary column can instantaneously create hydrocarbon reservoirs out of initially nonporous rocks, such as at Red Wing Creek and Viewfield in the Williston basin. Associated reservoirs are usually limited to a highly deformed central uplift in larger craters, or to the fractured rim facies in smaller craters. The presence of reservoirs and trapping mechanisms is largely dependent, however, upon the preservation state of the crater in the subsurface. A catastrophic extraterrestrial event (a large asteroid impact) has also been suggested as the cause for the extinction of the dinosaurs, but the latest theory proposes a companion star with a 26 m.y. periodicity as the cause for numerous lifeform extinctions over a similar time interval. Regardless of their magnitude and distribution over the earth, it is clear that catastrophic extraterrestrial events have been responsible for altering the geologic column locally, regionally, and quite possiblymore » on a global scale.« less