Seismic Refraction Lines Research Articles

Shallow seismic investigations of the accretionary complex offshore Central Chile

Thrust ridges are accretionary structures often associated with local uplift along splay faults and cold seep activity. We study the influence of a NS-trending thrust ridge system on the transition between the accretionary prism and the continental framework (shelf break) offshore the Maule Region (central Chile at 35°–36°S) by examining its 2-D and 3-D seismic velocity structure. The experiment comprises five densely spaced seismic refraction lines running subparallel to the trench and recorded at nine OBH/S (ocean bottom hydrophone/seismometers) deployed along the central line. Results show a narrow margin-parallel volume (approximately 6 × 50 × 5 km3) whose velocity distribution is consistent with sedimentary rocks. The shallow sedimentary unit is characterized by the presence of very low velocity hydrate-bearing sediments (<1.7 km/s), which are interpreted as highly porous sedimentary rocks (>50% porosity) within the Gas Hydrate Stability Zone (GHSZ) suggesting low hydrate content. These zones spatially correlate with fluid activity in the vicinity of the NS trending thrust ridges based on local high heat flow values (>40 mWm‐−2) and seepage mapping. On the other hand, the splay faults that crop out on the flanks of the thrust ridge structures might be responsible for tectonically induced vertical fluid migration.

Rift structures and its related unconformities on and adjacent the Dongsha Rise: insights into the nature of the high-velocity layer in the northern South China Sea

It is curious that the typical feature of high-velocity layer (HVL) underneath the crust for the magma-rich margin was found in the magma-poor margin of the South China Sea. In order to better understand the nature and age of the HVL in the South China Sea, here we provide seismic imaging on the Cenozoic structure and sedimentation using a regional seismic reflection profile combined with existing drilling data and seismic refraction lines in the northeastern South China Sea. An important finding is that the extent of erosion since Neogene is only on the Dongsha Rise and is much smaller than that of the HVL observed across the northern margin of the South China Sea, which suggests the occurrence of the HVL has no influence on the erosion on Dongsha Rise through Neogene. On the regional seismic profiles, the different structures between the deepwater rifted margins and the continental shelf were also observed. The faults in the Zhuyi Sub-basins on the continental shelf are imaged to be high-angle and bound a series of grabens and horsts. In contrast, the deepwater region is characterized by tilted fault blocks and an array of listric normal faults, which extremely thinned the crust to be ~ 5–10 km. After middle Miocene, active magmatism occurred in the deepwater region at the time of post-spreading of oceanic crust. These listric faults on the hyper-extended crust is interpreted to be conduits for magma migration to the shallower levels. This typical structure in the deepwater region changed the geophysical nature of the HVL underneath the extreme thinned crust, which should be formed before the event of hyper-extension and is likely to be associated with the subduction of the paleo-Pacific toward the Asia in the late Cretaceous. Our study will answer the question why the South China Sea with the HVL underneath the crust is a magma-poor passive margin.

Estimating the water holding capacity of the critical zone using near‐surface geophysics

AbstractIn high‐mountain watersheds, the critical zone holds crucial life‐sustaining water stores in the form of shallow groundwater aquifers. To better understand the role that the critical zone plays in moderating hydrologic response to fluxes at the surface and in the subsurface, the hydrologic properties must be characterized over large scales (i.e., that of the watershed). In this study, we estimate porosity from geophysical measurements across a 58‐ha area to depths of ~80 m. Our observations include velocities from seismic refraction, downhole nuclear magnetic resonance logs, downhole sonic logs, and samples acquired by push coring. We use a petrophysical approach by combining two rock physics models, a porous medium for the saprolite and a differential effective medium for the fractured rock, into a Bayesian inversion. The inverted geophysical porosities show a positive correlation with measured values (R2 = 0.93). We extrapolate the porosity estimates from 30 individual seismic refraction lines to a 3D volume below our study area using ordinary kriging to quantify the water holding capacity of our study area. Our results reveal that the critical zone in our study area holds ~2.9 × 106 ± 9.6 × 105 m3 of water, where 34% of this storage is in the saprolite, 55% is in the fractured rock, and the remaining 11% is in the bedrock.

Invariant TE and TM magnetotelluric impedances: application to the BC87 dataset

Invariants under rotation of the magnetotelluric impedance tensor relax the directional requirements when fitting data using two-dimensional (2D) models. This happens, for instance, when using the familiar impedance derived from the determinant of the tensor. In this work, we use two particular invariants that reduce in 2D to the traditional transverse electric (TE) and transverse magnetic (TM) impedances. They are obtained as the missing first link of an infinite chain of invariants that are computed iteratively without any reference to a strike direction. We challenge the current view that the TE and TM impedances are not invariant. We argue in favor of the invariance. We apply our approach to the well-known and difficult BC87 dataset. These data present (1) electro-galvanic distortions, (2) static shifts and (3) inconsistent strikes. To remove the electro-galvanic distortions, we use recent developments that combine the phase tensor with the quadratic equation. To handle statics, we use a mimic of the electromagnetic array profiling (EMAP) method; the property of the TE mode of acting as a natural filter for static effects, as reported recently in the literature. Finally, the multiple strikes are handled by the invariant character of the TE and TM impedances. The processed TE responses are interpreted as one-dimensional models to construct a 2D resistivity image of the subsurface, which is accompanied by the complementary image of the multiple strikes. Our model correlates very well with an independent EMAP survey over the Nelson Batholith and with seismic reflection and refraction lines.

Crustal Models Assessment in Western Part of Romania Employing Active Seismic and Seismologic Methods

In the years 1999 - 2000 two regional seismic refraction lines were performed within a close cooperation with German partners from University of Karlsruhe. One of these lines is Vrancea 2001, with 420 km in length, almost half of them recorded in Transylvanian Basin. The structure of the crust along the seismic line revealed a very complicated crustal structure beginning with Eastern Carpathians and continuing in the Transylvanian Basin until Medias. As a result of the development of the National Seismic Network in the last ten years, more than 100 permanent broadband stations are now continuously operating in Romania. Complementary to this national dataset, maintained and developed in the National Institute for Earth Physics, new data emerged from the temporary seismologic networks established during the joint projects with European partners in the last decades. The data gathered so far is valuable both for seismology purposes and crustal structure studies, especially for the western part of the country, where this kind of data were sparse until now. Between 2009 and 2011, a new reference model for the Earth’s crust and mantle of the European Plate was defined through the NERIES project from existing data and models. The database gathered from different kind of measurements in Transylvanian Basin and eastern Pannonian Basin were included in this NERIES model and an improved and upgraded model of the Earth crust emerged for western part of Romania. Although the dataset has its origins in several periods over the last 50 years, the results are homogeneous and they improve and strengthen our image about the depth of the principal boundaries in the crust. In the last chapter two maps regarding these boundaries are constructed, one for mid-crustal boundary and one for Moho. They were build considering all the punctual information available from different sources in active seismic and seismology which are introduced in the general maps from the NERIES project for Romania. The depths maps in the study region are presented with all their regional peculiarities as they appear, projected on the local tectonic structure for the area under examination.

High lateral resolution exploration using surface waves from noise records

Determination of the shear-wave velocity structure at shallow depths is a constant necessity in engineering or environmental projects. Given the sensitivity of Rayleigh waves to shear-wave velocity, subsoil structure exploration using surface waves is frequently used. Methods such as the spectral analysis of surface waves (SASW) or multi-channel analysis of surface waves (MASW) determine phase velocity dispersion from surface waves generated by an active source recorded on a line of geophones. Using MASW, it is important that the receiver array be as long as possible to increase the precision at low frequencies. However, this implies that possible lateral variations are discarded. Hayashi and Suzuki (2004) proposed a different way of stacking shot gathers to increase lateral resolution. They combined strategies used in MASW with the common mid-point (CMP) summation currently used in reflection seismology. In their common mid-point with cross-correlation method (CMPCC), they cross-correlate traces sharing CMP locations before determining phase velocity dispersion. Another recent approach to subsoil structure exploration is based on seismic interferometry. It has been shown that cross-correlation of a diffuse field, such as seismic noise, allows the estimation of the Green’s Function between two receivers. Thus, a virtual-source seismic section may be constructed from the cross-correlation of seismic noise records obtained in a line of receivers.In this paper, we use the seismic interferometry method to process seismic noise records obtained in seismic refraction lines of 24 geophones, and analyse the results using CMPCC to increase the lateral resolution of the results. Cross-correlation of the noise records allows reconstructing seismic sections with virtual sources at each receiver location. The Rayleigh wave component of the Green’s Functions is obtained with a high signal-to-noise ratio. Using CMPCC analysis of the virtual-source seismic lines, we are able to identify lateral variations of phase velocity inside the seismic line, and increase the lateral resolution compared with results of conventional analysis.

Three-Dimensional Geologic Model of Glacial Outwash in Mclean County, Illinois, Based on Seismic Refraction Studies

Seven two-dimensional (2-D) seismic refraction lines were used to determine the thickness and geometry of a valley train outwash deposit of the Quaternary Henry Formation near Heyworth in southern McLean County, Illinois. These refraction data were collected and processed in 2-D, then imported into a Petrel, a three-dimensional (3-D) geological modeling software package. The 3-D geologic model was built using the velocity attribute of the seismic refraction data. The 3-D velocity model was then verified manually by moving a cross-section through the velocity model at 20 m increments. These selected data points were used to create 3-D horizons, surfaces, and contacts constraining the target Henry Formation from the overlying alluvium of the Cahokia Formation and the underlying Delavan Till. Results of the 3-D model show the Henry Formation outwash trends about S10°E, which is oblique to S55°W-trending modern Kickapoo Creek valley. The Henry Formation outwash is confined to the Kickapoo valley, and consists of well-stratified sand and gravel at that is as much as 25 m in thickness in the channel. The thickness of the Henry Formation in the terrace is 8–10 m. The Cahokia Formation is everywhere about 2 m in thickness. The Henry Formation here is interpreted to be deposited in a subglacial tunnel valley that was deposited about 20,000 years ago as the Laurentide ice sheet retreated from its maximum southerly extent.

Crustal structure off Kong Oscar Fjord, East Greenland: Evidence for focused melt supply along the Jan Mayen Fracture Zone

The complex structure of the North-East Greenland margin is the consequence of two rifting events: the initial separation of Greenland and Scandinavia around 56Ma, and the breaking off of the Jan Mayen microcontinent from Greenland around 33Ma likely driven by the arrival of the Iceland Plume beneath the east coast of Greenland. The boundary between these two rifting events is the Jan Mayen Fracture Zone. While seismic lines north of this pronounced topographic structure document the presence of a high velocity lower crust, existing data south of the fracture zone show no indications that the lower crust consists of a large, high velocity body. This gap is closed by a 500km long seismic transect, which starts in Kong Oscar Fjord and extends out beyond the Kolbeinsey Ridge. It provides a detailed view of the crustal structure just south of the Jan Mayen Fracture Zone. The transect includes parts of the Caledonian crust of Greenland, crosses the present-day shelf and the oceanic crust formed since the separation of the Jan Mayen microcontinent. The seismic refraction line shows a constant ~9km thick oceanic crust between the extended continental margin and the mid-ocean ridge. Evidence for an only 3km thick high-velocity lower crustal body is observed within the continent-ocean transition zone. While such high velocity crust is ubiquitous beneath the shelf to the north of the fracture zone, to the south it only occurs beneath Mesozoic basins that have been attributed to extensional collapse of the Caledonian orogeny. Inline with earlier interpretations of high velocities in the lower crust of extended continental margins, we suggest that Kong Oscar Fjord is underlain by the products of excess magma production that were focused along the Jan Mayen Fracture Zone during the breakup of the Jan Mayen microcontinent from Greenland.

The oceanic crustal structure at the extinct, slow to ultraslow Labrador Sea spreading center

AbstractTwo seismic refraction lines were acquired along and across the extinct Labrador Sea spreading center during the Seismic Investigations off Greenland, Newfoundland and Labrador 2009 cruise. We derived two P wave velocity models using both forward modeling (RAYINVR) and traveltime tomography inversion (Tomo2D) with good ray coverage down to the mantle. Slow‐spreading Paleocene oceanic crust has a thickness of 5 km, while the Eocene crust created by ultraslow spreading is as thin as 3.5 km. The upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading, with a velocity drop of 0.5 to 1 km/s when compared to Paleocene upper crustal velocities (5.2–6.0 km/s). The overall crustal structure is similar to active ultraslow‐spreading centers like the Mohns Ridge or the South West Indian Ridge with lower crustal velocities of 6.0–7.0 km/s. An oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites (7.3–7.9 km/s) found 1.5 km below the basement. The second, ridge‐parallel line also shows extremely thin crust in the extinct axial valley, where 8 km/s mantle velocity is imaged just 1.5 km below the basement. This thin crust is interpreted as crust formed by ultraslow spreading, which was thinned by tectonic extension.

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Abstract. High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

Geophysical characterization of the upper crust in the transitional zone between the Pampean flat slab and the normal subduction segment to the south (32–34°S): Andes of the Frontal Cordillera to the Sierras Pampeanas

Abstract The Nazca Plate subducting beneath the South American Plate has strongly influenced Cenozoic mountain growth in western Argentina and Chile sectors (32–34°S; 70–66°W). At these latitudes, the Pampean flat slab has induced the development of prominent mountain systems such as the Frontal Cordillera, the Precordillera, and the associated Sierras Pampeanas in the eastwards foreland region. Through a gravity study from the Frontal Cordillera to the Sierras Pampeanas region between 32 and 34°S, we delimit a series of geological structures that are accommodating shortening in the upper crust and others of regional and subsurface development, without any clearly defined mechanics of deformation. Additionally, through an isostatic residual anomaly map based on the Airy-Heiskanen local compensation model, we obtain a decompensative gravity anomaly map that highlights anomalous gravity sources emplaced in the upper crust, related to known geological structures. In particular, by applying the Tilt method which enhances the gravity anomalies, the NW-trending Tunuyan Lineament is depicted south of 33.4°S following previous proposals. Using the decompensative gravity anomaly, two profiles were modelled through the northern sector of the study area using deep seismic refraction lines, borehole data and geological information as constraints. These density models of the upper crust of this structurally complex area accurately represent basin geometries and basement topography and constitute a framework for future geological analysis.

Crustal structures of the Boreas Basin and the Knipovich Ridge, North Atlantic

The Boreas Basin is located in the Norwegian-Greenland Sea between Northeast Greenland and Svalbard. Towards the east, it is bounded by the ultraslow mid-ocean Knipovich Ridge. Here, we present a 340-km-long seismic refraction line acquired during the expedition ARK-XXIV/3 of research vessel Polarstern in 2009, using 18 ocean bottom seismometers. It crosses the central Boreas Basin from the Knipovich Ridge to the Northeast Greenland margin. Thus, the line provides the first reliable crustal structure information of this basin. In addition, the gravity data acquired parallel to the seismic refraction line are used to calculate a 2.5-D gravity model. The P-wave velocity model shows an unusual ∼3-km-thin oceanic crust with seismic velocities less than 6.3 km s−1, indicating the absence of a significant oceanic layer 3. Mantle velocities vary between 7.5 kms−1 in the uppermost mantle and 8.0 km s−1 at approximately 15 km depth. The low velocities within the upper mantle may be explained by 13 per cent serpentinisation, which is negligible at about 15 km depth. Furthermore, the S-wave velocity model shows low Vp/Vs ratios in the mantle, indicating a highly serpentinised mantle at shallow depths. The gravity model has crustal densities between 2.3 and 2.9 g cm−3, which also point towards the absence of a significant thick oceanic layer 3. The results of our seismic refraction line and other geophysical data indicate that the entire Boreas Basin opened at ultraslow spreading rates since at least ∼28 Ma. No evidence for an extinct spreading ridge in the centre of the Boreas Basin was found.

The crustal structure of southern Baffin Bay: implications from a seismic refraction experiment

Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.8–4.0 km s-1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 to 6.4 km s-1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.4–7.2 km s-1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle and lower crust with P-wave velocities from 5.3 to 7.0 km s-1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transition zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay.

Crustal structure and evolution beneath the Colorado Plateau and the southern Basin and Range Province: Results from receiver function and gravity studies

Over the past several decades, contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long-term stability of the Colorado Plateau (CP) and crustal thinning beneath the Basin and Range Province (BRP) in the southwestern United States. Here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15,500 P-to-S receiver functions recorded at 72 USArray and other broadband seismic stations on the southwestern CP and the southern BRP. Our results reveal that the BRP is characterized by a thin crust (28.2 ± 0.5 km), a mean Vp/Vs of 1.761 ± 0.014 and a mean amplitude (R) of P-to-S converted wave (relative to that of the direct P wave) of 0.181 ± 0.014 that are similar to a typical continental crust, consistent with the model that the thin crust was the consequence of lithospheric stretching during the Cenozoic. The CP is characterized by the thickest crust (42.3 ± 0.8 km), largest Vp/Vs (1.825 ± 0.009) and smallest R (0.105 ± 0.007) values in the study area. In addition, many stations on the CP exhibit a clear arrival before the P-to-S converted phase from the Moho, corresponding to a lower crustal layer of about 12 km thick with a mafic composition. We hypothesize that the lower crustal layer, which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines, contributed to the long-term stability and preuplift low elevation of the Colorado Plateau.

Chapter 44 Geology and petroleum potential of the Lincoln Sea Basin, offshore North Greenland

Abstract A seismic refraction line crossing the Lincoln Sea was acquired in 2006. It proves the existence of a deep sedimentary basin underlying the Lincoln Sea. This basin appears to be comparable in width and depth to the Sverdrup Basin of the Canadian Arctic Islands. The stratigraphy of the Lincoln Sea Basin is modelled in analogy to the Sverdrup Basin and the Central Spitsbergen Basin, two basins between which the Lincoln Sea intervened before the onset of seafloor spreading in the Eurasian Basin. The refraction data indicates that the Lincoln Sea Basin is capped by a kilometre-thick, low-velocity layer, which is taken to indicate an uplift history similar to, or even more favourable than, the fairway part of the Sverdrup Basin. Tectonic activity in the Palaeogene is likely to constitute the major basin scale risk. We conclude that the Lincoln Sea Basin is likely to be petroliferous and contains risked resources on the order of 1×10 9 barrels of oil, to which comes an equivalent amount of (associated and nonassociated) gas.

Genetic algorithm inversion of the average 1D crustal structure using local and regional earthquakes

Knowing the best 1D model of the crustal and upper mantle structure is useful not only for routine hypocenter determination, but also for linearized joint inversions of hypocenters and 3D crustal structure, where a good choice of the initial model can be very important. Here, we tested the combination of a simple GA inversion with the widely used HYPO71 program to find the best three-layer model (upper crust, lower crust, and upper mantle) by minimizing the overall P- and S-arrival residuals, using local and regional earthquakes in two areas of the Brazilian shield. Results from the Tocantins Province (Central Brazil) and the southern border of the São Francisco craton (SE Brazil) indicated an average crustal thickness of 38 and 43 km, respectively, consistent with previous estimates from receiver functions and seismic refraction lines. The GA+HYPO71 inversion produced correct Vp/Vs ratios (1.73 and 1.71, respectively), as expected from Wadati diagrams. Tests with synthetic data showed that the method is robust for the crustal thickness, Pn velocity, and Vp/Vs ratio when using events with distance up to about 400 km, despite the small number of events available (7 and 22, respectively). The velocities of the upper and lower crusts, however, are less well constrained. Interestingly, in the Tocantins Province, the GA+HYPO71 inversion showed a secondary solution (local minimum) for the average crustal thickness, besides the global minimum solution, which was caused by the existence of two distinct domains in the Central Brazil with very different crustal thicknesses.

Cavola experiment site: geophysical investigations and deployment of a dense seismic array on a landslide

Geophysical site investigations have been performed in association with deployment of a dense array of 95 3-component seismometers on the Cavola landslide in the Northern Apennines. The aim of the array is to study propagation of seismic waves in the heterogeneous medium through comparison of observation and modelling. The small-aperture array (130 m×56 m) operated continuously for three months in 2004. Cavola landslide consists of a clay body sliding over mudstone-shale basement, and has a record of historical activity, including destruction of a small village in 1960. The site investigations include down-hole logging of P- and S-wave travel times at a new borehole drilled within the array, two seismic refraction lines with both P-wave profiling and surface-wave analyses, geo-electrical profiles and seismic noise measurements. From the different approaches a consistent picture of the depths and seismic velocities for the landslide has emerged. Their estimates agree with resonance frequencies of seismic noise, and also with the logged depths to basement of 25 m at a new borehole and of 44 m at a pre-existing borehole. Velocities for S waves increase with depth, from 230 m/s at the surface to 625 m/s in basement immediately below the landslide.

New insight into the deep structure of Antarctic Peninsula continental margin by methods of 2D gravity/magnetic modelling and 3D seismic tomography

The Antarctic Peninsula, one of several terrains of Western Antarctica, is a Mesozoic magmatic arc at the southeastern Pacific margin. In order to investigate the structure of the crust and uppermost mantle of Antarctic Peninsula continental margin we have developed joint geophysical models by 2D gravity and magnetic modelling along two most representative and lengthy seismic refraction lines, acquired by Polish Academy of th Sciences in 1980-1990 . Resulting model along line I-I (along the DSS line 12), crosssing the Antarctic Peninsula margin near the Anvers Island, shows the features of passive continental margin of convergent type. The joint model on line II-II, passing from the Drake Passage through the South-Shetland Trench/Islands system and Bransfield Strait to Antarctic Peninsula, indicates continental margin structure of active style, caused by recent subduction and on-going continental rifting in the Bransfield Strait. This subduction activity is responsible to the stripes of gravity and magnetic anomalies along the Antarctic Peninsula shelf, caused by many batholits and plutons of mafic rocks (gabbro and diorites), intruded into the crust due to partial melting of the upper mantle caused by south-eastward progradation of subduction front. On-going tectonic processes within the South Shetland Islands – Bransfield Strait block relate to mobilization of the upper mantle substance characterized by the lowest density (3.18 g/cm ) against the uppermost mantle densities of 3.21 and 3.30 g/cm revealed beneath the oceanic crust and Antarctic Peninsula continental crust respectively. To study the lithosphere structure of the region of Antarctic Peninsula and adjoining sea-water areas we implemented the seismic tomography method which is based on Backhus-Gilbert approximation and uses the data on earthquake foci and time arrival of P-waves recorded by network of seismic stations. On the first stage only 27 events from the International Seismology Center (ISC) for the period 1992-2005, recorded by five permanent seismic stations, were used. Preliminary computation of P-wave velocity field results in construction of a velocity section at the depth of 100 km. It shows a low-velocity zone, which covers the major part of the Drake Passage, South-Shetland Islands and western part of the Scotia Sea.

Crustal thickness variations along the Southeast Indian Ridge (100°–116°E) from 2‐D body wave tomography

Axial morphology along the Southeast Indian Ridge (SEIR) systematically changes from an axial high to a deep rift valley at a nearly uniform intermediate spreading rate between 100°–116°E, west of the Australian‐Antarctic Discordance (AAD). Basalt geochemistry has a consistent Indian–mid‐ocean ridge basalt (MORB) type isotopic signature, so changes in axial topography are attributed to variations in both mantle temperature and melt supply. Wide‐angle seismic refraction lines were shot to four ocean bottom hydrophones within SEIR segments P1, P2, S1, and T, where each segment is characterized by a different morphology. We constructed 2‐D crustal velocity models by jointly inverting hand‐picked P wave refraction (Pg) and Moho reflection (PmP) traveltime data using a top‐down, minimum‐structure methodology. The results show a 1.5 km eastward decrease in crustal thickness across the study area, with segment averages ranging from 6.1 km at P1 to 4.6 km at T. Melt generation models require a ∼30°C decrease in mantle temperature toward the AAD to account for the crustal thickness trend. Significant changes in axial morphology accompany small‐scale variations in crustal thickness, consistent with models of crustal accretion where ridge topography is determined by a balance between mantle temperature, melt supply, and cooling from hydrothermal circulation. Layer 3 thins by 3.0 km as layer 2 thickens by 1.4 km between segments P1 and T, reflecting the eastward decrease in melt supply and increase in melt lens depth. The trade‐off in seismic layers may be explained by models relating the increase in overburden pressure on a deepening melt lens to the volume of magma erupted into the upper crust rather than cooling at depth to form new lower crustal material.

Initial results from wide-angle seismic refraction lines in the southern Cape

One of the projects within the framework of Inkaba yeAfrica, an earth system science initiative between German and South African research communities, is the Agulhas-Karoo transect. This 800 km north-south off-onshore transect runs from the offshore Agulhas Plateau onto the South African coast, across the Cape Fold Belt, Beattie Magnetic Anomaly, the Karoo Basin, the Great Escarpment and into the Kaapvaal Craton. Among the number of geophysical measurements taken along the transect are two wide-angle on-shore seismic lines collected in April and May 2005. The lines run roughly parallel to each other approximately 200 km apart, starting at Mossel Bay and St. Francis, and running about 200 km north to Fraserburg and Graaf Reinet, respectively. At each line 48 seismic receivers were used to record data from 13 shots. The profiles cross a wide variety of geological terrains, such as the siliciclastic sequences of the Paleozoic – Mesozoic Karoo and Oudtshoorn basins, the lower Paleozoic Cape Fold Belt, and the Eocambrian Kango and Kaaimans inliers. They also cross the Beattie Magnetic Anomaly, a large east-west orientated crustal feature within the upper crust, and more than 1000 km long, first reported almost a century ago, but still not fully understood. The overall quality of seismic data is very good. First (P-wave) arrivals were manually picked on the available traces, and tomographic inversion was done using these travel times. The ray coverage made it possible to create the P-wave velocity model to depths of up to 25 km. We find excellent correlation of the shallow features with surface rock type. Deeper down we can identify both stratigraphic and tectonic contacts between geological groups. These include an inferred possible blind Paleozoic thrust fault, and the unconformity between the Cape Supergroup and the Namaqua-Natal Metamorphic Complex. The normal listric geometry of the Kango and Gamtoos Faults is clearly seen to a minimum depth of 15 km. We also observe a high velocity anomaly within the NNMC at ~10 km depth that we relate to the source of the Beattie Magnetic Anomaly.