Changes In Crustal Thickness Research Articles

Gravity constraints on lithosphere flexure and the structure of the late Paleozoic Ouachita orogen in Arkansas and Oklahoma, south central North America

Spectral analysis of Bouguer gravity anomalies in western central Arkansas and eastern Oklahoma indicates that the thickness of the crust in the Ouachita fold and thrust belt increases from 38 km in the western Ouachitas to 44 km in the eastern Ouachitas. The change in crustal thickness occurs near the western end of the Broken Bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1.8×1024 N m in the western Ouachitas to 5.0×1023 N m in the eastern Ouachitas. The flexural rigidity in the western Ouachitas is similar to values determined in the Appalachian fold and thrust belt and coincides with the depth of the 450°C isotherm predicted by conductive cooling models for the thermal evolution of the early Paleozoic southern Laurentian rifted continental margin. The thick crust in the eastern Ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age. The thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the Paleozoic continental margin. A layered density model derived from the gravity data shows that strata interpreted to be deformed Ouachita facies rocks are thickest in the eastern Ouachitas and are consistent with a greater amount of shortening in the central thrust belt in Arkansas as compared to Oklahoma. The opposite relationship is observed in the frontal Ouachita province, where shortening appears greater in Oklahoma. The cross‐strike changes in the locus of shortening, crustal thickness, flexural rigidity, and the inferred transition from rift to transform segments of the early Paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the Benton uplift. Flexural modeling indicates that the load required to produce the observed Bouguer gravity low in the Arkoma foreland basin trends parallel to the Benton and Broken Bow uplifts but is located 114 to 276 km farther south. In the western Ouachitas, the position of the load coincides with the northern edge of the Sabine uplift and is interpreted to mark the southern extent of Ouachita facies rocks that were emplaced on the Laurentian continental margin and/or attached remnant oceanic crust.

Seismic exploration of continental strike-slip zones

Seismic exploration of both Palaeozoic and active strike-slip zones shows strike-slip faults that penetrate all or most of the crust. Offsets on the Moho are evident, particularly at young and active zones with a component of compression, such as the Alpine Fault of New Zealand where a change in crustal thickness of about 20 km is observed. Moho offsets for the old Palaeozoic strike-slip zones are usually much less prominent. Careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets, the Moho structure consists of a localized keel-type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide. The large Moho offsets of young strike-slip zones may in some cases partially decay with time. Active strike-slip zones are becoming an important focus of study, but seismic exploration is hampered by complex near-surface geology, 3D structure and the difficulty of imaging steeply dipping structure in the subsurface. In order to meet this challenge, a wide range of seismic techniques is now being deployed. These include wide-angle seismic reflection, refraction, P-wave delays and the study of guided S-waves. Results from California give geophysical images of vertical strike-slip faults that penetrate to the Moho. In contrast, the Alpine Fault of New Zealand appears to be a surface manifestation of an inclined (∼40°) ramp, extending down to the lower crust and along which uplift and exhumation of the continental crust, and possibly strike-slip motion, is taking place.

Seismic structure and indicators of magma budget along the Southern East Pacific Rise

In this paper we re‐examine the relationship between seismically constrained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment‐scale variations in axial depth, morphology, basalt geochemistry, and hydrothermal activity that have often been attributed to along‐axis differences in the supply of magma to the mid‐ocean ridge. Along >800 km of the fast spreading SEPR, good correlations exist between axial depth, ridge cross‐sectional area, mantle Bouguer anomaly, and the MgO weight percent of basalts recovered from the rise axis. These correlations indicate along‐axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of ∼100,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of seismic layer 2A, and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross‐sectional area. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (seismic layer 2A) thickness. We hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (∼10–100 years) than the longer‐term variations in magma supply reflected in along‐axis changes in the shape and depth of the ridge axis.

A model for the structure, composition and evolution of the Kenya rift

The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle P n velocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle P n velocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and P n velocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal P n velocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.

The Southeast Indian Ridge between 88°E and 118°E: Gravity anomalies and crustal accretion at intermediate spreading rates

Although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical survey of the intermediate spreading rate Southeast Indian Ridge between 88°E and 118°E, a 2300‐km‐long section of the ridge located between the Amsterdam hot spot and the Australian‐Antarctic Discordance where satellite gravity data suggest that the Southeast Indian Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103°30′E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well‐developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifted high that is lower and pervasively faulted, sometimes with significant (>50 m throw) faults within a kilometer of the axis. A shallow axial valley (<700 m deep) is observed from 104°E to 114°E with a sudden change to a deep (>1200 m deep) valley across a transform at 114°E. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near‐axis ridge flank depth from 2800 m near 88°E to 3300 m near 114°E and by a 50 mGal increase in the regional level of mantle Bouguer gravity anomalies over the same distance. The regional changes in depth and mantle Bouguer anomaly (MBA) gravity can be both explained by a 1.7–2.4 km change in crustal thickness or by a mantle temperature change of 50°C–90°C. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneously and these estimates serve as upper bounds. The along‐axis MBA gradient is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103°30′E–104°E and at 114°E–116°E and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them. The change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly. The presence or absence of a quasi‐steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have an MBA low located at a depth minimum near the center of the segment. At EPR‐like segments, the MBA low is about 10 mGal with along‐axis gradients of 0.15–0.25 mGal/km, similar to those observed at the EPR. Rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply. Segments with a shallow axial valley have very flat along‐axis MBA profiles with little correspondence between axial depth and axial MBA gravity.

Tomography of the Andean crust and mantle at 20°S: first results of the Lithoscope experiment

In 1994, a temporary network of 41 short-period seismic stations operated for 6 months along an east-west profile in Northern Chile and Bolivia. The profile crossed the entire Andean chain at a latitude of 20°S. We present the results of the teleseismic tomography of the crust and mantle along an east-west vertical cross-section beneath the profile. The most striking feature of the tomographic image is the identification of the subducted Nazca plate. It appears as a zone of high velocities which is continuous over the aseismic part of the slab down to the 660 km discontinuity. In the continental lithosphere, strong lateral P-wave velocity variations are found in the crust whereas the upper mantle shows only small velocity variations. Major lateral changes in crustal velocity coincide with the limits of the main structural units of the Andean range. The distribution of velocity anomalies in the crustal layer matches the lateral changes in crustal thickness and average velocity deduced from refraction surveys. A comparison with a previous tomographic experiment in Northern Bolivia confirms that the northern and southern segments of the Bolivian orocline have different lithospheric structures.

Slant stacks of refraction data from maine: Effects of lateral variations in velocity structure

Abstract We discuss some of the advantages and pitfalls associated with τ( p ) analysis of wide-angle profiles recorded over terranes characterized by significant lateral heterogeneity. We construct composite slant stacks for a pair of reversed shots recorded along the axis of the Merrimack Synclinorium in southwest Maine and invert picks for bounds on averaged one-dimensional slowness-depth structure. Extremal bounds on total crustal thickness derived from shot point 12 are 40 to 43 km. Average crustal velocities for individual slowness models corresponding to the extremal depths range from 6.5 to 6.6 km/sec. Predicted normal-incidence two-way travel times for the Moho lie between 12.2 and 13.2 sec. The utility of one-dimensional inversion of τ( p ) data is largely in estimating the maximum resolving power of the data set; this is information that forward modeling approaches do not provide. Comparisons of slant stacks of reversed shot gathers with intercept times predicted for a two-dimensional model derived by ray tracing highlight some of the pitfalls associated with interpretation of τ( p ) data in terms of one-dimensional structure. In some cases, the principal arrival may become multi-valued, resulting in misidentification of phases and compounding biases in estimates of average crustal parameters. Slant stacks can be highly useful, however, in refining two-dimensional models derived from travel times because of the sensitivity of ray parameters to dip and lateral velocity gradients. We use slant stacks in conjunction with the original shot gathers to refine our estimate of the configuration of the Moho beneath southwest Maine. Our final model shows a step in the Moho that is consistent with variations in normal two-way travel times inferred from analysis of postcritical fan shots. This abrupt change in crustal thickness may mark the northeast extent of significant underthrusting of Avalon crust beneath Medial New England basement during the Acadian Orogeny. The preservation of offsets in Moho that were formed during Paleozoic compression is consistent with the limited amount of rifting experienced by this portion of the orogen during the opening of the Atlantic.

Relationship between axial morphology, crustal thickness, and mantle temperature along the Juan de Fuca and Gorda Ridges

We analyze gravity data over the Juan de Fuca and northern Gorda Ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate (28 mm/yr half rate). The Cleft Segment, at the south end of the Juan de Fuca Ridge, has an axial high morphology while the northern segment of the Gorda Ridge has a rift valley. Residual mantle Bouguer anomalies (RMBA) on the northern Gorda Ridge are elevated relative to the Cleft Segment by 10–20 mGal, indicating thinner crust and/or a colder mantle. The minimum value (−50 mGal) of the RMBA along the Juan de Fuca Ridge is over Axial Seamount and gradually increases south toward the Blanco Transform. The observed RMBA are interpreted to result from along axis variations in crustal thickness and mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. We estimate that mantle temperatures are elevated by 30°–40°C beneath Axial Seamount, resulting in an excess crustal thickness of ∼1.7 km. The Cleft Segment is associated with crust that is estimated to be only 300–700 m thicker, and mantle temperatures are only 10°–15°C higher than beneath the northern Gorda Ridge. However, even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology. These results are consistent with the predicted sensitivity of recent thermo‐mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates. We attribute the systematic differences in axial morphology, crustal thickness, mantle temperature, and lithospheric strength along the Juan de Fuca/Gorda ridge system to the presence of the Cobb thermal anomaly at Axial Seamount. The hotter mantle beneath the Juan de Fuca Ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner, weaker lithosphere than along the Gorda Ridge.

Approximations for surface-wave propagation in laterally varying media

A number of techniques which exploit the waveforms of seismic surface waves depend on simple approximations for the character of the propagation process from source to receiver based on the representation for a stratified medium. Commonly the propagation path is assumed to lie along a great circle and to be representable by a path-averaged structure. The influence of structure near the source and near the receiver is included by using local modal formulations. However, the terms that depend on source depth and receiver depth in the stratified medium results are not purely local in character, and so care has to be taken to ensure a simple mapping between the modal shapes for the different structures. For frequencies less than 0.03 Hz, different crustal structures can be used at the source, near the receiver, and along the propagation path, provided that the change in crustal thickness is not more than 10 km between contiguous structures. Furthermore, for frequencies up to 0.035 Hz, it should be possible to use a single modal set in non-linear waveform inversions for perturbations of up to 5 per cent in lithospheric velocities along the propagation path. For propagation paths of length from 1000 to 4000 km, typical of a continental scale, the path-averaged structure approximation should be suitable for waveform fitting for frequencies in the range 0.01-0.03 Hz. The lower limit depends on the use of asymptotic approximations and the upper on the influence of heterogeneity on the modal content of the seismograms. Where surface waves cross a major structural boundary such as the continent-ocean transition, some aspects of the wavefield can still be represented using the path-averaged approximation.

Geophysical evidence for crustal thickness variations between the Denali and Tintina Fault Systems in west‐central Yukon

Analyses of receiver functions recorded at two broadband seismic stations in the northern Cordillera indicate that the crust is thinner beneath Dawson (∼ 35 km) than Whitehorse (∼ 39 km). A simple two‐dimensional gravity model, constrained by the seismic results, shows that this change in crustal thickness occurs at about 63°N, in a zone approximately 35 km wide, where the Moho dips at ∼8° to the south. The Bouguer anomaly high associated with the thinner crust can be traced westward into east‐central Alaska where crustal thinning and extension are well documented; therefore we propose that west‐central Yukon (north of 63°N) has been extended also. On the basis of geological and other geophysical data we examine three likely time windows, in the Cretaceous, early Tertiary, and Present. We discuss data which indicate that early Tertiary and younger extension may be related to the transfer of motion from the Denali Fault System inboard to the Tintina Fault System. Earlier Cretaceous extension has been variously attributed to collision related back arc extension, syncollisional processes, or gravitational collapse of an overthickened crustal section. The relative importance of extension in these periods remains nebulus; however, the correspondence between the transition in crustal thickness determined in this study and a mapped boundary between “lower” plate and "upper" plate rocks suggests mid‐Cretaceous extension exerted a significant influence on current Moho depths.

Crustal and upper mantle contribution to the axial gravity anomaly at the southern East Pacific Rise

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14°S and 17°S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8–10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6–7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid‐to‐lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature‐related mantle density variations predicted by a simple, plate‐driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MBA within the experimental uncertainty (±1 mGal). However, the retention of a small amount of melt (≤1–2% at 14°S; ≤4% at 17°S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (∼10 km wide) partial melt conduit that extends to depths of 50–70 km with melt concentrations up to 2% higher than the surrounding mantle. Along‐axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties in crustal thermal models. The good correlation between along‐axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along‐axis variation. Instead, this variability can be adequately explained by a combination of along‐axis changes in crustal thermal structure and/or along‐axis crustal thickness changes of a few hundred meters.

The transition between the Erris and the Rockall basins: new evidence from wide-angle seismic data

The Rockall Trough is a deep-water basin west of Britain and Ireland. The origin of the basin and the nature of the crust beneath it is controversial. A series of seismic wide-angle experiments carried out during 1988 and 1990, help to clarify the crustal and upper mantle structure in the region. Results for the crustal structure from a profile which straddles the shelf break between the basin and the Irish Shelf are discussed here. These data together with the available geological information indicate that the basin probably formed in the late Palaeozoic to early Mesozoic as part of a regionally linked basin assemblage which includes the Hatton Basin and the shallow sea basins surrounding Ireland and Britain. Good data quality has allowed the transition between the relatively unstretched crust of the Irish and British mainland, defined by previous onshore seismic refraction experiments, to be well resolved. The Erris Trough, one of the small late Palaeozoic to early Mesozoic basins which fringe the mainland shelf region north of the Porcupine Basin is a half-graben. Its major bounding fault is located on its western margin and the basin is divided from the Rockall Trough by a narrow horst, the Erris Ridge. The shelf trajectory along the western flank of the horst deepens smoothly towards the trough centre, where the crust thins to 5 km near the trough margin below a sedimentary and water column 8 km thick. Surprisingly, the crustal thickness is slightly greater over a 150 km broad zone at the trough centre (i.e., ca. 6 km). This change in crustal thickness may be due to lateral strain migration to the warmer basin margins as its centre cooled during the deformation. The crustal structure beneath the sediment pile at the trough centre is two layered, as opposed to the three-layered seismic refraction structure found onshore. However, the basic character of the lower crust present in onshore Ireland, in particular the presence of a gradient zone defining the crust/mantle transition, is still preserved in the Trough. This similarity in structure precludes the presence of magmatic underplating. The crustal structure observed in the Rockall Trough can be formed by differential stretching of the lithosphere. In this model the lower ductile crust and mantle lithosphere are stretched over a wide region byβ 2 = 2−3. Strain focusing into a much narrower region of brittle upper crust generates severe amounts of crustal thinning (β 1 = 8−10), and is responsible for the fusing of the upper and mid-crustal seismic refraction layers found beneath onshore Ireland and Britain. Mediating detachment surfaces, sited at the brittle/ductile transition at any time, served to relay the strain from the lower lithosphere into the upper crust. Syntectonic heat loss plays an important role in controlling the deformation pattern.

Calculation of synthetic seismograms in a laterally varying medium by the boundary element-discrete wavenumber method

Abstract We investigate the propagation of Lg waves in laterally varying crustal structures by numerical simulation. The method of calculation is formulated in terms of boundary integral equations where the Green9s functions are evaluated by wavenumber summation. The approach is well suited to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others. We investigate the effect of a crustal fault with vertical offset and study the case of a lateral change in crustal thickness. The results show that the Lg wave amplitude is only slightly affected by the presence of these heterogeneities. They confirm the robustness of Lg wave propagation in presence of lateral heterogeneities observed in other numerical simulations. They show that large-scale geometric features of the crust cannot account alone for the strong attenuation of Lg waves observed in many regions. The results also suggest a possible relation between the level of Lg wave coda and the degree of roughness of the Moho. They further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal heterogeneities.

Comparison of the tectonics and geophysics of the major structural belts between the northern and southern continental margins of the South China Sea

A comparison of the tectonics and geophysics of the major structural belts of the northern and the southern continental margins of South China Sea has been made, on the basis of measured geophysical data obtained by ourselves over a period of 8 years (1984–1991). This confirmed that the northern margin is a divergent one and the southern margin is characterized by clearly convergent features. The main extensional structures of the northern margin are, from north to south: 1. (1) The Littoral Fault Belt, a tectonic boundary between the continental crust and a transitional zone, along the coast of the provinces of Guangdong and Fujian in South China. It is characterised by earthquake activities, high magnetic anomalies and a rapid change in crustal thickness. 2. (2) The Northern and Southern Depression zones (i.e., the Pearl River Mouth Basin), this strikes NE-ENE and is a very large Cenozoic depression which extends from offshore Shantou westwards to Hainan Island. 3. (3) The Central Uplift Zone. This includes the Dongsha Uplift, Shenhu Uplift and may be linked with the Penghu uplift and Taiwan shoals to the east, forming a large NE-striking uplift zone along the northern continental slope. It is characterized by high magnetic anomalies. 4. (4) Southern Boundary Fault Belt of the transitional crust. This has positive gravity anomalies on the land side and negative ones on the sea side. 5. (5) The Magnetic Quiet Zone. This is located south of the southern Boundary Fault Belt and between the continental margin and the Central Basin of the South China Sea. Magnetic anomalies in this belt are of small amplitude and low gradient. We consider the Magnetic Quiet Zone to be a very important tectonic zone. The major structures of southern continental margin southwards are: 1. (1) The Northern Fault Belt of the Nansha Block. This extends along the continental slope north of the Liyue shoal (Reed Bank) and Zhongye reef, and is a tectonic boundary between oceanic crust and the Nansha Block continental crust. 2. (2) The Nansha Block Uplift Zone. Due to the development of reefs and shoals, there are many channels and valleys. Our long-distance multichannel seismic profiles indicated that there are thick Paleogene sediments and thin Neogene sediments all over the central part of the block. 3. (3) The Nansha Trough, a nappe structure formed by the southeastward drifting of Nansha Block and northwestward overthrusting of Palawan-northwest Borneo. 4. (4) Zengmu Shoal Basin, southwest of the Nansha Block; the maximum thickness of Cenozoic strata is over 9 km in this important petroliferous basin.

Fine‐scale magnetic anomaly field over the southern Juan de Fuca Ridge: Axial magnetization low and implications for crustal structure

Sea surface, deep tow, and submersible magnetic surveys show that the spreading axis of the Cleft segment of the southern Juan de Fuca Ridge (SJDF) is overlain by a distinctive magnetic anomaly low in contrast to the more typical magnetic anomaly high found over the adjacent Vance segment and most mid‐ocean ridge spreading centers. Forward iterative thickness modeling, based on the near‐bottom magnetic data, shows that this axial magnetic low can be produced by a thin magnetic source layer beneath the Cleft axial valley. Thick crust is required to produce the magnetic anomaly high over the Vance axial valley. This magnetic modeling demonstrates that variations in the base of the magnetic source layer, which is assumed to be equivalent to the base of the extrusive lavas, can produce a significant proportion of the observed short‐wavelength magnetic anomaly signal. These magnetic layer thickness models also correlate quite well with nearby seismic results which show that seismic layer 2A thins beneath the axial valley of the Cleft and is thicker beneath the Vance axial valley. Recent seismic results over other fast and medium spreading rifts also show rapid changes in the thickness of the uppermost crust near the axis of spreading, which could produce significant magnetic contrasts. The results of this study and the correlation with independent seismic measurements suggest that while magnetization may vary due to geomagnetic behavior or because of petrology or alteration, first‐order change in upper crustal thickness is one of the most important variables to consider in the production of fine‐scale magnetic anomalies in young ocean crust.

Heat flow, gravity and structure of the Abitibi belt, Superior Province, Canada: Implications for mantle heat flow

The results of new heat flow measurements are presented for eleven sites located in the Abitibi greenstone belt of the Archean Superior Province. These new and previously published heat flow data are used with gravity and seismic data to constrain the crustal structure and composition of the Abitibi subprovince, and to determine the mantle heat flow beneath the Canadian Shield. In order to analyze the long-wavelength variations in heat flow and Bouguer gravity anomaly, the crust was divided into three layers with distinctive composition and physical properties. Average density, heat production and thickness for each layer were varied within a range compatible with measurements and yielded crustal models consistent with gravity and heat flow data. The dominant contribution to crustal heat production is that of a tonalitic type layer which has the lowest density. The models require mantle heat flow to range between 10 and 14 mW m −2. The westward increase in heat flow requires the tonalitic layer to thicken from the Grenville Front toward the Kapuskasing uplift at the expense of the greenstone supracrustals. Long-wavelength gravity data are more affected by changes in crustal thickness than by composition, and the low Bouguer anomaly near the eastern Grenville Front is caused by 5–6 km of crustal thickening. In greenstone terranes, the long-wavelength variations in heat flow are more useful for determining average crustal composition and the thickness of supracrustals than the gravity data.

Deep seismic reflection data across the conjugate margins of the Labrador Sea

A deep seismic reflection transect of the conjugate margins of the Labrador Sea is described, which represents one of the few data sets of this kind. A characteristic reflectivity is ascribed to a 120 km wide ribbon of very thin crust that may be either thinned continental crust, which has perhaps been intruded, or oceanic crust, perhaps modified by the proximity of the continent. Most of the major changes in crustal thickness and in the subsidence and sedimentation patterns on the margins occur landward of these transitional zones, which are found on both margins. An interpretation of these regions as continental in origin is compatible with other seismic observations on the west Greenland margin, but does not match the magnetic anomaly interpretation, which requires the transitional crust to be oceanic in origin. Models that satisfy the gravity anomalies and the subsidence history have been used to assist in interpreting the seismic data. The subsidence models include the effects of decompression melting during lithospheric extension and rifting, and we predict the thickness of igneous crust produced. However, the gravity models suggest that a lower crustal layer may extend farther inland below the Labrador shelf than is predicted by magmatic underplating. The present seismic results, combined with the other geophysical data, are consistent with a pure shear model of lithospheric stretching, with faulting confined to the upper crust. Many of the problems raised by this data set are similar to those identified in comparing the nonvolcanic margin of Iberia with the conjugate Grand Banks margin in the North Atlantic. If the transition zone results from stretching the continental lithosphere, then a large component of the very thin crust there must consist of igneous material formed by melting. Under these conditions a sharp, vertical ocean–continent boundary would be unlikely.

A Preliminary Interpretation of Deep Seismic Reflection and other Geophysical Data from the Darling Fault Zone, Western Australia

Deep seismic data were recorded along a 75 km long traverse that covered some 50 km within the Yilgarn Craton and 25 km within the Perth Basin, down to 12 s two-way time (TWT). Gravity data were recorded at 500 m intervals and magnetic data at 100 m intervals along the same traverse. The data were processed to stack stage using a conventional land seismic processing sequence. Migration proved difficult because of the poor knowledge of the subsurface velocity distribution in the Yilgarn. Good reflections were obtained for the full 12 s TWT. The preferred interpretation of the seismic data suggests that the crust beneath the western Yilgarn Craton may be divided vertically into three structural zones: 1 (0 km to 7 km) a zone of thin-skinned compressional tectonism which probably occurred between 2650 Ma and 2500 Ma; 2 (7 km to 25 km) a zone that appears to contain several minor detachments (or shear zones) and seismic events characteristic of intruded lenses of magma; and 3 (25 km to 40 km) easterly dipping continuous seismic events. Reflection events between 11 s and 12 s TWT in the east of the profile may represent the top of the Moho reflection package, but previous work suggests that it may be slightly deeper than this. The seismic data also image well the fault plane of the Phanerozoic Darling Fault. Modelling of the gravity data suggests that the Darling Fault Zone may separate the crust (43 km thick) beneath the Yilgarn Craton from the crust (27 km thick) beneath the Darling Mobile Zone and Perth Basin. This change of crustal thickness is probably coincident with the Proto-Darling Fault, and is not related to the Phanerozoic Darling Fault that is evident on the seismic data. Modelling of the magnetic data indicates numerous intrusive bodies between the surface and approximately 20 km depth. Some of these bodies exhibit strong magnetic remanence.

Sn to Sg conversion and focusing along the Atlantic Margin, Morocco: Implications for earthquake hazard evaluation

Digital data from a telemetered, short‐period seismic network in Morocco provide a new perspective for understanding the cause of severe shaking and macroseismic reports in Morocco produced by large, offshore earthquakes located along the Azores‐Gibraltar seismic zone. Even though the earthquake epicenters are 500–1000 km away from the Moroccan coast, historical records show that such events are capable of producing considerable damage in inland areas. We analyze 15 earthquakes that occurred in this region. The records show multiple S phases with varying frequencies and amplitudes. The S phase with the largest amplitude, usually misinterpreted as Sn, has a phase velocity of 4.2–4.4 km s−1. We show that these S arrivals can best be explained as Sn to Sg converted phases. Calculated locations of the conversion points for these phases exhibit two distinct zones almost parallel to the Atlantic coastline: one is located along the passive continental margin and the other is located about 100 km inland from the coastline. We interpret these two zones to be regions where a sudden change in crustal thickness occurs. Such zones act to focus and magnify the amplitudes of seismic phases. This interpretation explains the unusually strong felt reports within Morocco from such distant offshore events, and it has a significant effect on earthquake hazard evaluation and mitigation studies.

Convective removal of thermal boundary layer of thickened continental lithosphere: A brief summary of causes and consequences with special reference to the Cenozoic tectonics of the Tibetan Plateau and surrounding regions

England, P., 1993. Convective removal of thermal boundary layer of thickened continental lithosphere: a brief summary of causes and consequences with special reference to the Cenozoic tectonics of the Tibetan Plateau and surrounding regions. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earth's Surface. Tectonophysics, 223: 67–73. The tectonics of Tibet and its surroundings underwent marked changes at about 8 Ma. Normal faulting of the plateau appears to have initiated at about this time. Widespread highly potassic volcanism, interpreted as the result of rapid heating of the mechanical boundary layer, began about 12-8 Ma. Such rapid heating could be the consequence of convective removal of the lower lithosphere and its replacement by asthenosphere. This process would have raised the surface height of the plateau and could have provided the driving force for extension. The diffuse deformation in the Indian Ocean probably began at about 8 Ma; analysis of the buckling of a plastic lithosphere implies that the horizontal force per unit length required to drive this deformation is consistent with uplift of the Tibetan plateau by 1–3 km. The Indian monsoon intensified at about 8 Ma. This intensification is consistent with the rapid passage of the surface height of the plateau across a threshold required to disrupt the pre-existing atmospheric circulation of the region. Taken together, these observations imply that the surface height of the Tibetan plateau rose by between 1 and 3 km in a few million years around 8 Ma. Such a rapid increase in surface height affecting an area the size of the Tibetan plateau could not be achieved by crustal thickness changes, but could have been caused by the convective removal of a substantial thickness of lower continental lithosphere of the region.