Changes In Crustal Structure Research Articles

Crustal structure beneath Yinggehai basin and adjacent Hainan Island, and its tectonic implications

Two NE-SW trending wide-angle seismic profiles were surveyed across the Chinese side of the Yinggehai (莺歌海) basin (YGHB) with ocean bottom hydrophones (OBHs) and piggyback recorded by onshore stations located on the Hainan (海南) Island. Detailed velocity-depth models were obtained through traveltime modeling and partially constrained by amplitude calculations. More than 15 km Tertiary sedimentary infill within the YGHB can be divided in to three layers with distinct velocity-depth distribution. Overall, the upper layer has a high velocity gradient with 3.8–4.1 km/s at its bottom, consistent with progressive compaction and diagenesis. Its thickness increases gradually towards the basin center, reaching 4.5 km along the southern profile. The middle layer is characterized in its most part as a pronounced low velocity zone (LVZ) with average velocity as low as 3.0 km/s. Its thickness increases from 3.0 to over 4.5 km from NW towards SE. The primary causes of the velocity inversion are high accumulation rate and subsequent under-compaction of sediments. The velocity at the top of the lower layer is estimated at about 4.5 km/s. Despite strong energy source used (4 × 12L airgun array), no reflections can be observed from deeper levels within the basin. Towards NE the basin is bounded sharply by a clear and deep basement fault (Fault No. 1), which seems to cut through the entire crust. A typical continental crust with low-velocity middle crust is found beneath the coast of the western Hainan Island. Its thickness is determined to be 28 km and shows no sign of crustal thinning towards the basin. The sharp change in crustal structure across Fault No. 1 indicates that the fault is a strike-slip fault. The crustal structure obtained in this study clearly favors the hypothesis that the YGHB is a narrow pull-apart basin formed by strike-slip faulting of the Red River fault zone.

Heat flow and lithospheric thermal regime in the Northeast German Basin

New values of surface heat flow are reported for 13 deep borehole locations in the Northeast German Basin (NEGB) ranging from 68 to 91 mW m − 2 with a mean of 77 ± 3 mW m − 2 . The values are derived from continuous temperature logs, measured thermal conductivity, and log-derived radiogenic heat production. The heat-flow values are supposed free of effects from surface palaeoclimatic temperature variations, from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow. Two-D numerical lithospheric thermal models are developed for a 500 km section along the DEKORP-BASIN 9601 deep seismic line across the basin with a north-eastward extension across the Tornquist Zone. A detailed conceptual model of crustal structure and composition, thermal conductivity, and heat production distribution is developed. Different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow. The best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the NEGB. This estimate is corroborated by seismological studies and somewhat less than typical for stabilized Phanerozoic lithosphere. Modelled Moho temperatures in the basin are about 800 °C; heat flow from the mantle is about 35 to 40 mW m − 2 . In the southernmost part of the section, beneath the Harz Mountains, higher Moho temperatures up to 900 to 1000 °C are shown. While the relatively high level of surface heat flow in the NEGB obviously is of longer wave length and related to lithosphere thickness, changes in crustal structure and composition are responsible for short-wave-length anomalies.

Abrupt changes in crustal structure beneath the Coast Ranges of northern California - developing new techniques in receiver function analysis

SUMMARY New approaches to receiver function analyses at a suite of broad-band stations in the Coast Ranges of northern California identify rapid changes in crustal thickness accompanied by melt accumulation in the lower crust. These processes are associated with the northward migration of the Mendocino triple junction through the area over the past ∼8 Myr. By simultaneously solving for crustal thickness, Poisson's ratio and Moho strike and dip, we facilitate the application of receiver function analyses in areas of crustal complexity where data sets are sparse. We also introduce a straightforward approach to calculating the Vp/Vs velocity ratio of the seismogenic zone, which is used here to aid in the interpretation of results from the receiver function analysis. Results are consistent with triple junction related crustal modifications occurring in the plate motion direction in a narrow swath of crust between the Maacama and Bartlett Springs Faults. Crustal thicknesses decrease from over 35 km thick in Round Valley, south of the current triple junction location, to 20 km thick just 100 km further south, near Clear Lake. Major crustal thinning is localized to a narrow area of the crust around Redwood Valley, where the Moho dips approximately 17° to the northeast. This thinning may also be restricted to the lower crust rather than being evenly distributed throughout the crustal column. Coincident with these changes, melts are inferred in the lower crust from low velocity zones and elevated values of Poisson's ratio (σ≥ 0.32) at two seismic stations. These melts occur in the same area as melt bodies suggested in other seismological studies. Together these results provide the first detailed description of the crustal modifications—and the spatial extent of that deformation—associated with the passage of the Mendocino triple junction.

Gravity anomalies of the Cyprus Arc and their tectonic implications

Continental crust forms from structurally thickened remnants of oceanic crust and overlying sediments, which are then invaded by arc magmatism. Understanding this process is a first order problem of lithospheric dynamics. The transition in young mountain belts, from ocean crust through the agglomeration of arc systems with long histories of oceanic closures, to a continental hinterland is well exemplified by the plate margin in the eastern Mediterranean. The boundary between the African plate and the Aegean/Anatolian microplate is in the process of transition from subduction to collision along the Cyprus Arc. In the west, north of the oceanic Herodotus Basin, subduction may be continuing; in the east, microcontinental blocks such as the Eratosthenes Seamount are already colliding with Cyprus to the north of the suture. The changes in crustal structure along and across this convergent zone are not known except by inference from bathymetry, and from a couple of deep-penetrating wide-angle seismic transects on the African plate margin. In this contribution we model four Bouguer gravity profiles across the Cyprus Arc. The profiles are similar in showing a gravity low over the plate boundary suture attributed to thick sediments, with Bouguer highs over oceanic or transitional crust to the south, and over ophiolites and thinner sediments, on thin continental crust to the north. Bouguer lows at the northern ends of the profiles are characteristic of elevated continental crust. Well-sedimented oceanic or transitional crust on the African plate margin is replaced north of the suture by variably sedimented basement, in which continental crustal thickness increases from 10 km to 30 km over the forearc region. The models involve assumptions about characteristic densities and trade-offs between sediment and crustal thicknesses, but they do provide well-defined targets for future wide-angle seismic tests.

A viscoelastic model of interseismic strain concentration in Niigata-Kobe Tectonic Zone of central Japan

The high strain rates of the Niigata-Kobe Tectonic Zone (NKTZ) of central Japan have been simulated using a viscoelastic finite element model of great interplate earthquake cycles that includes the subducting Pacific plate and lateral changes in crustal structure beneath NKTZ. As a result of long-term loading at trenches, the elastic crust over a viscoelastic mantle can transfer stress or strain rate not only as an average of geological time scales, but also during an interplate earthquake cycle. Thus, if the earthquake cycle is several times longer than the relaxation time of the viscoelastic system, deformation of an inland area far from trenches responds to the cycles of interplate earthquakes, and the crust deforms like an elastic sheet during the later stage of an earthquake cycle, allowing efficient transmission of stress during this period. Therefore, in the case of long interplate earthquake cycle, the NKTZ crustal structure with small thickness and rigidity enhances local interseismic strain concentration there. The “plate coupling ratio” (0.4∼0.5) between Pacific and Amurian plates and local crustal heterogeneity beneath the high strain rate zone explain the large strain rate belt if the earthquake cycle offshore Kanto is enough long.

Contrasts in regional seismic wave propagation to station WMQ in central Asia

SUMMARY The varying character of regional seismic phase propagation in central Asia is well illustrated by the character of the seismograms at the Urumqi station (WMQ) in Xinjiang, China. Three different classes of behaviour are seen for propagation across the relatively stable platforms to the north, along the Tien Shan chain and across the Tarim basin from Tibet. These paths demonstrate the influences of the variation in crustal structure on the regional phases particularly the Tibetan plateau and its north and south boundaries, but also the influence of the Pamirs on propagation along the Tien Shan. As an interference phase, Lg is sensitive to changes in crustal structure and can be used to study crustal heterogeneity. Lg can propagate along the Tien Shan chain trapped by the gradients in structure on either side, but with a progressive loss in amplitude and frequency. However, once the northern boundary of Tibet is crossed the high frequencies in the Lg train are lost and there is little recognizable arrival. The amplitude ratios of the different crustal phases Pn, Sn and Lg can be used to characterize the behaviour and the different classes of propagation can be further separated when account is made of the frequency content in the Lg window. The influence of the transitions in crustal structure at the northern edge of the Tibetan plateau on propagation to WMQ is investigated using numerical modelling with the pseudospectral method for models of crustal variations, this demonstrates how the crustal pinch at the northern boundary operates to modify the fairly efficient propagation inside Tibet with substantial transfer of energy into the mantle.

Quaternary convergent margin tectonics of Costa Rica, segmentation of the Cocos Plate, and Central American volcanism

Along Costa Rica, new geophysical data indicate considerable control of Quaternary convergent margin tectonics by the subducting lower plate. Three types of ocean crust enter the subduction zone: (1) Cocos Ridge with its underlying thick crust stands 2 km high, (2) on its north flank is normal crust covered 40% by seamounts, and (3) along the adjacent Nicoya margin the underthrust crust has a smooth sea floor. A 3‐ to 10‐km‐wide base of slope frontal prism varies little opposite different subducting crusts except where subducting seamounts eroded it. Once the breaching seamount has passed the prism it is quickly restored. The effect of oceanic crust on continental margin structure is most evident in the middle and upper slope. Where Cocos Ridge and its flanking seamounts subduct, erosion is pronounced relative to the stable slope where smooth lower plate subducts. Aligned upper plate features above lower plate segment boundaries extend more than 120 km landward of the trench axis and correspond in varying degrees with volcanic arc segmentation. The offset of volcanoes across the Costa Rica/Nicaragua border corresponds with a change in crustal structure and depth of the lava source. Subducted sediment shows little correlation with the slab signal in volcanic arc lavas but the magnitude of faulting associated with ocean plate flexure adjacent to the trench axis parallels it well. Thus fluids in ocean crust fractures and bound water in serpentinite may have a recognizable geochemical effect in arc lavas.

An integrated study of the NE German Basin

The NE German Basin is part of the Southern Permian Basin south of the TransEuropean Suture Zone (TESZ). Here we report an attempt to integrate a variety of geological and geophysical data in order to reveal the present day deep crustal structure of the NE German Basin. Special focus is taken on detailed geological information, available reflection seismic data, wide angle refraction seismic data and gravity data.Based on this integrative approach, a concise crustal model is developed which can be subject to further evaluation. Furthermore, it is shown that the NE German Basin is an outstanding feature quite different from classical basin types. As the Moho is flat below the basin, neither a simple-shear-, nor a pure-shear-extension model can be applied. Instead, a thick high-velocity lower crustal layer is present below the basin centre. This high-velocity lower crust is also observed in the western part of the basin and can be taken as characteristic for East Avalonia. Results of gravity modelling indicate the presence of a high-density lower crust, thus supporting the wide-angle data. Furthermore, changes in crustal structure can be derived from changes in reflectivity pattern and from the gravimetric signature along the Elbe Fault System, indicating the presence of different crustal domains north and south of the fault system.An elastic plate model, where the forces applied are the sediment load, a vertical load at the southern margin accounting for the Harz Moutains and a NNE–SSW-directed compression, indicates a buckling of the crust during Late Cretaceous to Tertiary inversion.However, the correlation between the observed crustal structure and the tectonic events that have affected the area remains a subject of discussion. A Permian thermal event affected the upper mantle and the lower crust and, therefore, may have modified the ‘crustal memory’ with regard to Caledonian and Variscan events. Additional tectonic events affected the area during the Mesozoic. In conclusion, it remains an open question whether the high-velocity and high density lower crustal structure represents a remnant of East Avalonia, whether it is related to the formation of the Permo-Triassic basin, or is the cumulative result of a series of events during the Palaeozoic and Mesozoic.

The crustal structure of the Canary Basin: Accretion processes at slow spreading centers

Multichannel seismic reflection and gravity data define the structure of Mesozoic ocean crust of the Canary Basin, formed at slow spreading rates. Single and multichannel seismics show a transition from smooth to rough basement topography from Jurassic to Cretaceous crust and a coeval change in crustal structure. Internal reflectivity of the rough basement area comprises upper, upper middle or whole crust cutting discrete dipping reflections. Lower‐crustal reflectivity is almost absent and reflections from the crust‐mantle transition are short and discontinuous or absent for several kilometers. In contrast, crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust‐mantle boundary. The crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust. In the smooth basement area, fracture zones exhibit neither a clear topographic expression nor crustal thinning. We interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from Jurassic to Cretaceous times. In addition, the crust imaged across the path of the Cape Verde Hot Spot in the Canary Basin exhibits a widespread lower crustal reflectivity, very smooth topography and apparently thick crust. Our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the Mesozoic Mid‐Atlantic Ridge.

Evidence of crustal thinning beneath the Limpopo Belt and Lebombo monocline of southern Africa based on regional gravity studies and implications for the reconstruction of Gondwana

The 600-km-long by 250-km-wide Limpopo Belt forms a wedge-shaped structure between the Zimbabwe and Kaapvaal cratons in southern Africa. The results of a gravity survey which included the part of the Limpopo Belt within Zimbabwe are incorporated with other data and presented as a regional gravity map. Three gravity profiles crossing the Limpopo Belt and one within Mozambique have been interpreted in terms of lateral changes in crustal structure. These interpretations are controlled by density measurements and seismic refraction data across the northern and central parts of the Belt. The Zimbabwe craton appears from gravity modelling to be characterised by a horizontal Moho at 34 km depth. The Limpopo Belt is parallel to the ENE-trending gravity contours and in the east has a southwest-northeast line of gravity highs coincident with the Nuanetsi-Sabi volcanic rocks. This study has shown that the regional gravity high associated with these features is consistent with crustal thinning probably caused by the breakup of Gondwana. The crust thins in a south-southeast direction to about 30 km in the Central Limpopo Belt with a dip on the Moho which varies from about 4° to 7°. The short-wavelength positive gravity anomalies occurring on the Nuanetsi-Sabi regional gravity high are due to late Karoo igneous intrusions and volcanics. A north-south similar line of gravity highs exists along the Lebombo monocline on the Mozambique/South Africa border. Beyond the Lebombo monocline to the east the gravity anomaly suggests a thickness of 6 km or more of sediments that could represent a paleo-deltaic environment prograding over the continental margin. The Lebombo-Nuanetsi-Sabi gravity anomaly that dominates the gravity response in the eastern part of the study area is likely to be a failed-arm triple junction and could imply a much tighter fit of Gondwana than has been previously suggested. As such the geometry of such a margin may provide a major constraint on the geometric fit of East Antarctica with Africa.

Magsat magnetic anomaly contrast across Labrador Sea passive margins

Many passive margins not complicated by nearby anomalous crustal structure have satellite elevation crustal magnetic anomaly contrasts across them that are recognizable in reduced‐to‐pole versions of the Magsat and POGO data. In the Labrador Sea region this contrast is particularly well developed with strong positive anomalies overlying the continental crust of Greenland and eastern Canada and prominent negative anomalies situated over the Labrador Sea and Baffin Bay. We use forward modeling of the large‐scale crustal bodies in this region (continental, oceanic, passive margin, several anomalous structures) to show that the Magsat anomaly contrast is due simply to the change in crustal susceptibility and thickness at the continental/oceanic crustal transition. Because the thickness varies more than the average susceptibility from continental to oceanic crust, the strong anomaly contrast is essentially an edge effect due mostly to the change in crustal structure.

Crustal structure of the sheared southwestern Barents Sea continental margin

Eighteen expanding spread profiles from the southwestern Barents Sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data. The velocity-depth profiles from the slope-intercept, Herglotz-Wiechert and τ-sum methods were compared and contrasted to the velocity-depth curve calculated from the iterative forward modelling of the amplitudes. The velocity-depth profiles derived from the τ-sum and forward modelling of the amplitudes were used to produce synthetics in both the X- T and τ-P domains to choose the technique that most accurately represented the data. The results showed the oceanic crust at the Senja Fracture Zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5.7–6.4 km s −1 and slightly smaller crystalline crustal thickness. The continent-ocean transition at crustal depths was shown to be narrow, in the range of 10–20 km. Our results were consistent with two different crustal provinces east of the Senja Fracture Zone, the Tertiary marginal basin and the Cretaceous basin region. The velocity-depth curves enabled us, in a gross sense, to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles. Depths down to 20 km were inferred for the base of the sedimentary section in the southwestern Barents Sea. The crust thinned gently towards the continent-ocean boundary at the Senja Fracture Zone, whereas the crustal thickness changed abruptly below the Hornsund Fault Zone at the margin farther north. We associated the change in crustal structure with Cenozoic in the thick crust beneath the Svalbard Platform in the north. The depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area.

Lithospheric structure in southern Sweden—results from FENNOLORA

The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Småland-Värmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0–150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a transition zone of limited lateral extent, less than a few tens of kilometers. We suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the Småland-Värmland Granite Belt and the Svecofennides.

Lg waves and structural boundaries

Abstract The propagation of Lg waves in complex media can be described either by means of a modal superposition scheme with numerical integration through the heterogeneity or by using ray diagrams. Rays are set off at equal horizontal intervals in a stratified zone adjacent to the heterogeneity, with phase velocities appropriate to particular modes. The constructive interference pattern in the stratified medium is modified in the horizontally varying region to give a graphic illustration of the propagation effects. The ray method agrees well with the modal calculations but may be conveniently applied in more general circumstances, e.g., to include surface topography or permanent changes in crustal structure. Structural boundaries which involve sudden thinning of the crustal wave guide are particularly disruptive to the Lg train, as at a pinch in crustal thickness or the continent-ocean transition. The effects of localized thickening are more subtle. The relatively sharp cutoff for Lg waves in some structures, e.g. in the Tibetan Plateau, can be explained by the source being no longer able to couple into the crustal wave guide at the receiver.

Attenuation of intensity for the 1887 northern Sonora, Mexico earthquake

Abstract Contouring the intensity data for the northern Sonora earthquake of 1887 yields an area of perceptibility defined by the MM III isoseismal that is greatly elongated to the southeast of the epicenter along the Sierra Madre Occidental. North of the epicenter, the isoseismals are compressed due to the change in crustal structure between the Colorado Plateau and the southern Basin and Range province. The higher intensity isoseismals are more symmetric, although local structure strongly influences individual intensity values. To evaluate the attenuation of intensity with distance, three regression curves are calculated as below all data: I = 12.0 + 3.2 − 0.0015 R − 1.5 In ( R ) 25 − 500 km: I = 12.0 + 0.69 − 0.0069 R − 0.82 In ( R ) Isoseismal: I = 12.0 + 8.1 − 0.0031 R − 2.3 In ( R ) where R is the epicentral distance in kilometers. The “All data” curve is derived from a regression computed using all 171 intensity values. The “25-500 km” curve eliminates the near and far points, and the “Isoseismal” curve is a regression on the average radii of the area enclosed by each isoseismal contour. The latter two curves are similar to curves for the Western United States as a whole (Anderson, 1978) or the Cordilleran curve of Howell and Schultz (1975) for intensities less than or equal to MM X. The curve for “All data” indicates less attenuation at lower isoseismals than the other two 1887 regressions and falls between the Cordilleran and Eastern United States models of Howell and Schultz.

Determining source parameters of moderate-size earthquakes from regional waveforms

Waveform modeling of the teleseismic long-period body-wave phases of shallow earthquakes has proven quite effective in determining source parameters for events of magnitude larger than six. Unfortunately, these teleseismic phases become too weak for smaller events, and regional data must be used which are generally much more complicated. This is because the crust-mantle system acts as a leaky waveguide and a large number of rays are required to model the observations. In this report we review a procedure for the systematic determination of source parameters from regional body waves. A least-squares inversion technique is used which is based on a cross-correlation of the data and a synthetic seismogram. The synthetic waveforms are a linear combination of those for the three fundamental types of fault. A set of profiles of the synthetic waveforms is presented. Although the synthetic waveforms are generated for a model developed for the western United States, this model seems to be adequate for most continental regions. The inversion is parameterized in terms of fault strike, dip and rake; these parameters are relatively insensitive to small changes in crustal structure. Several examples are presented.

A seismic refraction and reflexion study of the continent-ocean transition beneath the north Biscay margin

The structure of the northern margin of the Bay of Biscay consists of a series of tilted and rotated blocks bounded by prominent listric faults whose polarity is consistently down toward the continent-ocean boundary. These blocks formed by rifting in late Jurassic - early Cretaceous time and are now thinly covered by post-rift sediments of Aptian to Recent age. Seismic refraction profiles were occupied on the shelf, on either side of and across the continent-ocean transition to the shelf, using P ubs and O bs with explosives and a 4 x 1000 in 3 (4 x 16400 cm 3 ) airgun array. Two-ship expanding spread multichannel (48-trace) seismic reflexion profiles and 30 km fixed offset reflexion profiles were located along the seismic refraction profiles on either side of the transition. A two-ship 30 km fixed offset multichannel profile was located across the transition as well as a 5 km fixed offset multichannel profile extending from the ocean crust to the shelf. Conventional 48-trace single ship multichannel profiles were located along all the refraction and two-ship reflexion lines. Interpretation of the refraction profiles has been made by using ray tracing as well as synthetic seismograms. Conventional seismic processing techniques have been used to prepare the two-ship multichannel seismic data for interpretation. The survey is believed to be the first attempt to apply two-ship multichannel seismic data to the study of the change in crustal structure of a rifted passive margin from the shelf to the ocean crust. The results from the experiment led to the identification of a zone of transition between continental and oceanic crust about 8 km wide. The seismic refraction data show progressive thinning of the continental crust from 33 km to about 5 km close to the transition zone. However, extension values calculated in the upper crust from the rotation of fault blocks are much less (1.1—1.4) and suggest that the majority of the thinning is achieved by extensive attenuation of the lower crust.

New geophysical evidence for sea-floor spreading in central Baffin Bay

Geophysical data collected during a detailed survey in Baffin Bay have shown that lineated magnetic anomalies trending north-northwest occupy the deep central region. These anomalies exhibit maximum amplitudes of about 300 nT and can be modelled by a 1-km thick magnetic source layer divided into blocks of normal and reversed polarity. The magnetizations required are comparable with those of oceanic basalts. A striking feature of the gravity field is a 20 mGal gravity low, about 20 km wide, which runs through the centre of the bay with approximately the same trend as the magnetic lineations. The gravity low is associated with a change in crustal structure measured from seismic refraction data and sometimes with a deepening of the sediment-basement interface, reminiscent of a median valley. These results suggest that the magnetic anomalies were produced by sea-floor spreading and that the gravity low marks an extinct spreading centre in Baffin Bay. Comparisons of the magnetic anomaly profiles with a model profile computed for magnetic anomalies 13–24 (38 to 60 Ma), show good correlation between the observed and computed anomalies in the time period represented by anomalies 13–21, with slow spreading rates of about 0.3–0.4 cm yr−1 perpendicular to the spreading axis. These results are in reasonable agreement with magnetic anomaly identifications and spreading rates deduced from geophysical data in the Labrador Sea. The direction of plate motion in Baffin Bay is not well defined from the data, but the Labrador Sea data require plate motions at a highly oblique angle to the spreading centre in the bay. Peculiarities of the postulated spreading centre, including the change in crustal structure beneath the gravity low along its strike from south to north, and the decrease in coherence and amplitude of the magnetic anomalies immediately north of the survey area, may be the result of these very low spreading rates, oblique spreading and changes in spreading direction, or the proximity of this area to the junction with a possible major transform fault through the Nares Strait.

Tidal loading: crustal structure of Nova Scotia and the M2 tide in the northwest Atlantic from tilt and gravity observations

Summary. Earth-tide tilt and gravity observations from five locations in Nova Scotia, Canada are used to study both the structure of the Earth beneath Nova Scotia and the structure of the M2 ocean tide around Nova Scotia. The observed Earth tides are functions of the spatial distribution of the ocean tides and the elastic properties of the Earth. It is therefore possible, in principle, to invert tidal loading observations for both the distribution of the ocean tides, and the elastic properties of the Earth. In practice our observations from Nova Scotia are imperfectly suited to this task. They contain both observational errors and are poorly distributed. However, it is possible to separate the loading due to tides in waters adjacent to Nova Scotia from that due to the remaining ocean, thereby removing errors that result from uncertainties in the global tide distribution. This is done by considering the spatial derivative of the tilt, since the tilt due to distant loads is virtually the same at all stations within a small geographic area. Differencing pairs of stations nearly eliminates the load tilt caused by distant oceans. Using differenced tilt data the problem is reduced to the estimation of the elastic properties of the Earth beneath Nova Scotia because the tidal load in adjacent waters is reasonably well known. The crustal model inferred from a comparison of observed and theoretical difference tilts is in agreement with the results of seismic refraction experiments conducted along the Atlantic coast of Nova Scotia. Both tilt and seismic data suggest that the crust is 35 km thick, consists of two major layers and is underlain by normal mantle. Having determined an approximate crustal structure, the tilt and gravity observations (as opposed to difference observations) are used to demonstrate that the M2 tidal distribution in the northwest Atlantic is intermediate between the co-tidal charts of Tiron et al. and Zahel and is in fact closely described by Dietrich's empirical chart. A finite element model of loading on the continental margin is used to demonstrate that the lateral changes in crustal structure across the continental margin in no way influence the conclusions. The results also indicate that variations in crustal structure will modify the gravity load tide by at most 10 per cent. Consequently, the inverse problem of inferring the global tide distribution from gravity observations may be considered to be linear to a first approximation.

Propagation of Lg and lateral variations in crustal structure in Asia

Lg propagates efficiently with predominant periods of about 1 to 1 1/2 s across the stable regions of Asia: the Indian shield, the Eurasian platform, the Tarim basin, and (less efficiently) eastern China. This suggests that the structure is relatively homogeneous and Q must be high in the crust in these regions. For paths across the Tien Shan, where the crust is abnormally thick, Lg is observed with somewhat less sharp arrivals, but nevertheless, frequencies are high, and amplitudes large. For paths along the Tien Shan, signals are weaker, and arrivals are still less sharp. Lg was not observed for paths crossing the Tibetan Plateau. We infer that the change in crustal structure on the margins of the Tibetan Plateau and at the Tien Shan may disrupt the wave guide for Lg and may scatter energy out of the relatively homogeneous wave guide of the more stable crust. Alternatively or in addition, Lg may not propagate across the Tibetan Plateau because of an unusual velocity structure or because of high attenuation in the crust.