Results of magnetotelluric and gravimetric measurements in western Nicaragua, Central America

Magnetotelluric and gravity data have been collected within a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. 2-D interpretation of the magnetotelluric (MT) data, collected at 13 stations, indicates four regions of high electrical conductivity in addition to the conductive coastal region. Two of these are associated with conducting sediments and pyroclastics in the upper part of the crust. Two other conductive structures have been defined at depth around 20 km and the one best defined is located below the depression. From the distribution of seismic events, volcanic activity in the depression and the similarity in geophysical characteristics with areas such as the Rio Grande Rift, this conductor is interpreted as a melt layer or a complex of magma chambers. Models of the upper lithosphere, constrained by the MT model, vertical electrical sounding (VES) data, seismic data and densities, have been tested using gravity data. A model that passes this test shows a gradual thickening of the crust eastwards from the Pacific coast. An anomaly centred over the depression is interpreted to have its origin in a thinning of the crust. In this model the melt layer is situated on top of the bulge of the lower lithosphere. A change in the composition of the crust, from the Pacific Coastal Plain to the Highland, is indicated from the change in character of the MT response and from the density distribution in the gravity model. This may support the hypothesis that the Pacific region is an accreted terrane. MT and gravity data indicate a depth to a resistive and high-density basement in the depression of ca. 2 km. On the basis of this, the vertical setting in the depression is estimated to be of the order of 2.5 km.     

Magnetotelluric interpretation of crustal and mantle structure in the Grenville Province

Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.     

Deep seismic reflection evidence for the role of extension in the evolution of continental crust

Summary. Deep seismic reflection profiling, as well as geologic studies, indicate that extensional basins are a common feature of continental crust. The wide-spread occurrence of extensional basins, combined with published models on the effects of extension on lower crustal rocks, suggests that extensional processes play an important role in the evolution of continental crust. Extension is now recognized to have been the last major tectonic event to affect approximately 50% of the United States. This observation and (he preservation of extensional features in areas which have experienced subsequent episodes of compression, suggests that extensional processes may lead to a strengthening of continental crust. In areas of active extension such as the western United States, seismic and petrologic data, as well as theoretical modeling of heat flow data, suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present Mohorovicic discontinuity formed during extension. Although the interpretation of the various data is somewhat speculative, we suggest that the volume of continental material in some areas has doubled as a result of extension. Thus, extension may result in the addition of a significant amount of new material to the continents.     

Deep seismic reflection studies in Israel - an update

Summary. The results of three deep crustal reflection lines are presently available from Israel. A 90 km line from near the Dead Sea rift to the Mediterranean coast was carried out for deep study. Two other lines in the Mediterranean coastal area were derived by recorrelation of oil exploration lines. The data shows a division between continental inner Israel and the coastal plain. In the first area a reflective lower crust is apparent with transparent upper crust and almost transparent upper mantle. Near the coast, in an area which was previously suggested as underlain by an ancient fossil oceanic crust, strong reflections characterize the uppermost mantle. Comparison between the reflection pattern and previous deep refraction and MT data indicates some agreement away from the coast and lack of correlation in the area of possible fossil oceanic crust near the coast.     

COCORP deep seismic reflection traverses of the U.S. Cordillera

Summary. COCORP seismic reflection traverses of the U.S. Cordillera at 40°N and 48.5°N latitude reveal some fundamental similarities as well as significant differences in reflection patterns. On both traverses, autochthonous crust beneath thin-skinned thrust belts of the eastern part of the Cordillera is unreflective; immediately to the west the Cordilleran interior is very reflective above a flat, prominent reflection Moho. Mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns, in constrast to more easily deciphered patterns beneath areas of Cenozoic accretion. The prominent reflection Moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during Cenozoic extension. The main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the Cordilleran interior; these differences are probably related to the way Cenozoic extension was accommodated at depth. Laminated middle and lower crust above the reflection Moho in the western Basin and Range (40°N) may be related to magmatism, ductile pure shear and large-scale transposition during Cenozoic extension. By contrast, beneath the eastern Basin and Range (40°N), and the orogenic interior in the NW United States (48.5°N), Cenozoic extension was probably accommodated along dipping deformation zones throughout the crust.     

Physical properties and structure of the lower crust revealed by one- and two-dimensional modelling

Summary. Data from a refraction and a reflection seismic survey in the Black Forest, southwest Germany, are used for extensive one- and two-dimensional modelling. The data are available along approximately the same line, and therefore the same piece of crust is probed by two seismic methods. We utilize this favorable circumstance for detailed model calculations concerning both data sets. Lower crustal properties vary on the scale of a wavelength and thus full solutions of the elastic equations are required: the Reflectivity Method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response. Details of the geometry and physical properties of the lamination are derived. Vertical layering on a scale of 100 m is found; horizontal extent of reflecting elements is in the range of a few hundreds of meters; rocks with velocities between 5.6 and 7.2 km/s constitute the lower crust.     

Results of deep reflection seismic profiling in the Oberpfalz (Bavaria)

Summary. In order to investigate the target area of the Continental Deep Drilling (KTB) in the Oberpfalz a network of six seismic reflection lines was acquired in 1985 using the Vibroseis technique. The average length of these lines was 50 km. In addition, the 185 km long NW/SE striking line DEKORP 4 with its short appendix line 4-Q of 40 km length was acquired with the same technique. The results reveal a strongly structured upper crust. This is in contrast with previous surveys in the German Variscides which show a poorly reflective upper crust and a strongly reflective lower crust. Except for the S part of DEKORP 4 in the Oberpfalz area the Mono is only weakly reflective. In addition to the Vibroseis survey 96 shots along line DEKORP 4 were recorded by conventional reflection techniques and by portable reflection and refraction stations from university institutes and geological surveys in order to obtain wide-angle reflection and expanding spread data.     

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.     

COCORP: new perspectives on the deep crust

Summary. Relict sutures from colliding continents, regions characterized by a "young" Mono, layering and faulting throughout the crust, mid-crustal magma traps, and seismic "bright spots" which suggest deep crustal fluids are among recent COCORP findings. In addition, new studies of signal penetration, noise mitigation, recording geometry, and coherency filtering have yielded better understanding of, and substantial improvements in, data quality. Amplitude anomalies, or "bright spots", in the Basin and Range may be due to magma at mid-crustal levels; in one case, a normal fault appears to link the deep magma with young surface volcanics. Another bright spot. 15 km deep in southeastern Georgia, has a flat geometry that suggests a gas/liquid interface, perhaps within fluids underthrust along an Appalachian suture. The Mono continues to appear relatively undisturbed in many regions of past deformation, suggesting that its formation post-dates these major tectonic episodes. The diversity of reflection patterns from the U.S. Cordillera casts further doubt on the generality of the common model of a reflective, layered lower crust underlying a transparent upper crust.     

Crustal thickening under the palaeo-meso-Proterozoic Delhi Fold Belt in northwestern India: evidence from deep reflection profiling

The results of deep reflection profiling studies carried out across the palaeo-meso-Proterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling.
The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT. Another band of subhorizontal Moho reflections, at ≈ 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.     

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

We report results from the Seismic Wide-Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the information from a conventional deep multichannel reflection profile and a coincident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide-angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide-angle data, and an excellent match was obtained. It was possible to fine-tune velocities in less well-resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth-convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward-dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Ocean.     

The Agulhas Plateau: structure and evolution of a Large Igneous Province

Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s⁻¹. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 10⁶ km³ of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.     

Seismic reflection models of continental crust based on metamorphic terrains

Summary. Laboratory seismic velocity measurements on rock samples from metamorphic terrains, coupled with geologic cross sections, provide the basis for synthetic seismic reflection profiles for various types of continental crust. Results from greenstone belts, mylonite zones and partial cross sections of continental crust indicate that lithologic heterogeneity and geometrical factors control crustal reflection characteristics.     

Combined seismic reflection and refraction profiling in southwest Germany - detailed velocity mapping by the refraction survey

Summary. Within the framework of site survey studies of the Deep Drilling Program of the Fedral Republic of Germany (KTB), coincident deep-seismic reflection and refraction experiments in the Black Forest, southwest Germany, were carried out. The simultaneous interpretation of the reflection and the refraction data reveals in particular both a strong velocity reduction in the upper crust and a laterally varying laminated structure of the lower curst. Additional refraction lines result in a three-dimensional crustal model which shows two distinct crustal types of different seismic properties. These crustal types seem to correlate with the major geologic units of Southwest Germany. Variations of Poisson's ratio derived from clearly recorded shear wave data show a similar trend.     

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.     

Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

西南极菲尔德斯半岛长城站地区深部电性结构

本文分析了1992年12月长城站附近三个测点的大地电磁测深资料(1s到4096s的低频信号数据),得出菲尔德斯半岛风谷断裂电性主轴为北东110°,半岛地壳厚度为22.3km,壳内含四个主要电性层,厚度分别为:1.3km,6.7km,1.2km和13.3km,其中上地壳为9.2km,下地壳为13.1km。     

Reflectivity of the crystalline crust: hypotheses and tests

Summary. The nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop. Geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms, but most geological information still comes from speculative interpretations of seismic experiments run in different regimes. Pronounced lower-crustal reflectivity, detected worldwide, is ascribed in various geological hypotheses to primary lithologic layering, to ductile strain banding, or to trapped fluids. A BIRPS deep crustal profile across the Atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone. Study of worldwide crustal profiles shows that exposed high-pressure terranes are not as reflective as in situ lower crust at high pressure, suggesting either that these granulite terranes are not representative of the lower crust or that physical properties, possibly the presence of fluids or thermally controlled ductile strain banding, are more likely responsible for observed reflectivity than are simple lithologic boundaries. The argument for the importance of physical properties in causing observed lower-crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat-flow.     

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.     

Reflection mapping across the convergent margin of western Canada

Summary. Five marine multichannel seismic reflection profiles totalling 520 km were recorded across the western Canada convergent margin where the Juan de Fuca plate is subducting beneath North America. The data extend the results of LITHOPROBE on Vancouver Island. The primary objectives are definition of the offshore accretionary structures and clarification of the convergent interaction between the two plates. The main features of this preliminary interpretation are: (1) the subduction deformation front is complex with evidence of sediments being accreted and subducted; (2) the top of the oceanic crust and the Mono are imaged below the deep water sedimentary basin; (3) the top of the subducting plate is clearly imaged below the shelf; (4) beneath the inner shelf, one band of high reflectivity underlain by a zone of lesser reflectivity lies above the plate; (5) alternative interpretations place the present zone of decoupling at the base of the reflective band or the top of the plate; (6) the San Juan and Leech River faults that bound small accreted terranes are imaged as thrusts that merge at depth.