Hajime Shiobara

Crustal structure of the northern part of the Vøring Basin, mid-Norway margin, from wide-angle seismic and gravity data

Abstract Three regional Ocean Bottom Seismograph (OBS) profiles with a total of fifteen recovered OBSs were acquired in the northern part of the Voring Basin, mid-Norway margin, in 1996. The data have been modelled by inversion and forward modelling of the OBS vertical components (P-waves). The velocity is found to increase downwards within the sedimentary layers, due to increasing depth of burial (confining pressure). Within the two deepest sedimentary layers there seems to be an increase of 0.5–1 km/s in velocity northwestwards. This increase is most likely caused by high-velocity sill-intrusions in the sedimentary rocks, emplaced during the rift episode leading to Early Eocene opening of the NE Atlantic. The presence of significant amounts of sills seems to terminate close to the Nyk High along profile 5 (dip-profile), and close to a dome northwest of the shelf edge along profile 6 (dip-profile). The velocity in the top of this dome is anomalously high (3.4 km/s at only 1 km depth beneath the seafloor), suggesting that the high consists of volcanic rocks, or heavily intruded sedimentary rocks. The relatively low velocities derived from within the upper crystalline crust (5.9–6.2 km/s) confirms that the crust is of continental origin. Within the lower crust there seems to be a clear increase in velocity northwestwards (from about 7.0 km/s to about 7.4 km/s), suggesting that the amount of high-velocity intrusions (underplating) in the lower crust decreases landwards. The modelling of the strike-profile (profile 7) suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile. The model for profile 7 ties closely southwestwards to the models derived from OBS data acquired in 1992, and northeastwards to Lofoten (OBS data acquired in 1988). The models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey, and that the transition zone is related to the Surt Lineament. The distribution of magmatic rocks thus seems to correlate strongly with pre-breakup structures. In addition to the P-wave modelling, gravity modelling has been performed along all profiles. The gravity modelling confirmed all main aspects of the OBS models, and provided constraints on the crustal structures towards the ends of the profiles.

Crustal structure of the central part of the Vøring Basin, mid-Norway margin, from ocean bottom seismographs

Abstract Regional Ocean Bottom Seismograph (OBS) data acquired in the central and northern part of the Voring Basin, mid-Norway margin, have been modelled by use of two-dimensional (2-D) ray-tracing. The regional dataset comprises thirty OBSs deployed along seven 100–170 km long profiles. The deeper part of the Voring Basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels (2–5 km below sea-floor), but the modelling of the OBS-data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea-floor to the upper mantle. The shallow and intermediate sediments (to 5 km below sea-floor) are characterized by a vertical increase in velocity due to increased confining pressure. There is also considerable lateral variation in the velocities within sedimentary layers at all levels. The OBS-data confirm that intrusions of sills are important at intermediate and deep sedimentary levels (2–10 km below sea-floor) in most parts of the area. The sills seem to vary in lateral extent from about 20 km to more than 100 km, and their thickness is generally inferred to be about 200 m. The low velocity in the upper crystalline crust (6.2 km/s) confirms that the crust in the Voring Basin is of continental origin. In most parts of the area the velocity of the lower crust is as high as 7.3–7.6 km/s. This high-velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness. The base of the 7.3 km/s layer is interpreted as the Moho, and the upper mantle velocity is estimated to 8.2 km/s.

Crustal structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland

Results from the Reykjanes-Iceland Seismic Experiment (RISE) show that the thickness of zero-age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off-axis at 63°N, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9–11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide-angle reflections from this body suggests that it corresponds to a zone of high-velocity (≥7.5 km s−1), high-magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age.

Crustal structure of the outer Vøring Plateau, offshore Norway, from ocean bottom seismic and gravity data

Four ocean bottom seismograph (OBS) profiles acquired across the continent-ocean transition zone of the outer Voring Plateau have been modeled by inversion and forward modeling of the OBS vertical component traces (P waves). The thickness of the sedimentary layers deposited since the early Eocene continental breakup varies from up to 2.2 km above the basaltic inner flows (IF) to a minimum of 0.2 km about 20 km seaward of the Voring Escarpment (VE), with a velocity of 1.6–2.55 km/s. The thickness of the uppermost layer of basalt varies from 0.5 to 4.5 km, and its velocity ranges from 3.4 to 5 km/s. The modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend ∼40 km seaward of the VE. Farther seaward, another layer of extrusives/intrusives is found beneath the uppermost layer of basalt. The lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent/ocean transition extends over a 30–50 km wide zone in between (strike) profile 3 (oceanic; 6.6–7.35 km/s) and (strike) profile 4 (continental; 6.1–7.0 km/s). An up to 7 km thick lower crustal high-velocity body (7.1–7.4 km/s) is interpreted in terms of intrusions in the lower crust beneath and landward of the continent/ocean transition. The total thickness of the normal oceanic crust is estimated to be 7–9 km, the anomalously thick oceanic crust reaches a maximum thickness of about 25 km, and the crustal thickness is estimated to be 17–22 km beneath the IF. Gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low.

A detailed subduction structure in the Kuril trench deduced from ocean bottom seismographic refraction studies

Abstract In 1983, we conducted an ocean bottom seismograph (OBS) experiment in the southernmost part of the Kuril trench, beneath which the Pacific Plate is subducting under Hokkaido Island, Japan. The aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling, using explosives and airguns. The total length of the profiles was 560 km, along which ten OBSs were deployed. The observed data were of good quality, which enabled us to obtain a detailed velocity structure of the active margin down to 20–30 km. We constructed a velocity structure model by ray-tracing and amplitude modeling. In the oceanic basin, the crust has a typical oceanic structure characterized by three layers with P-wave velocities of 1.8, 3.8–6.5 and 6.5–7.0 km/s. The velocity gradient in layer 3 increases downward from 0.075 to 0.10 s −1 . The Moho discontinuity was well constrained by clearly observed P n and P m P phases. The total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin. The P-wave velocity is 7.9 km/s beneath the Moho discontinuity, which increases downward with a rather small velocity gradient, 0.015–0.03 s −1 . Beneath the continental slope, we found relatively low velocity (2.5 to 5.5–5.8 km/s) material. The oceanic Moho discontinuity associated with the subducting plate was traced down to a depth of 25 km. Our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure. This result is in remarkable contrast with the velocity structure in the active margin of the Ryukyu trench area (about 1000 km south of the present experimental area), where we found a 12 km-thick, prominent low-velocity wedge is situated 50–150 km landward from the trench axis. On the seaward side of the wedge, the surface of the igneous basement undulates severely. The wedge was probably formed by materials of oceanic origin. Such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern Pacific.

Oceanic crust in the Japan Basin of the Japan Sea by the 1990 Japan‐USSR Expedition

In September of 1990, a seismic refraction and reflection survey was conducted in the Japan Basin, in the northeastern part of the Japan Sea, as a part of the Japan-USSR joint expedition. Twenty-six ocean bottom seismometers (OBSs) were deployed on two 200-km long lines. Explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines. Airguns were also used as the source for the multi-channel reflection profiles. n nThe crustal structure deduced is that of a typical oceanic basin: the crustal thickness is about 8.5 km including 2 km of sediment. We obtained a more detailed crustal structure than that obtained previously. From the dense airgun shooting data, the crustal structure is well resolved to show layer 1A, layer 1B, layer 2A, 2B, 2C, layer 3, and the mantle. The crust basically consists of laterally homogeneous layers but the Moho deepens slightly westward.

Crustal lineaments, distribution of lower crustal intrusives and structural evolution of the Vøring Margin, NE Atlantic; new insight from wide-angle seismic models

Abstract Five lineaments on the volcanic Voring Margin, NE Atlantic, have been identified in crustal scale models derived from Ocean Bottom Seismograph (OBS) data. It is suggested that the Voring Basin can be divided in four compartments bounded by the Jan Mayen Fracture Zone/Lineament, a new lineament defined from this study, the Gleipne Lineament, the Surt Lineament and the Bivrost Lineament. The NW–SE trending Jan Mayen-, Gleipne- and Bivrost lineaments probably represent old zones of weakness controlling the onset of the early Eocene seafloor spreading, whereas the Surt- and New lineaments, rotated ca. 30° symmetrically from the azimuth of the Gleipne Lineament, may represent adjustment features related to the early Cretaceous/early Tertiary rifting. The longest landward extent of a lower crustal high-velocity body, assumed to represent intrusions related to the last phase of rifting, is found between the New Lineament and the Gleipne Lineament, where the body extends across the Helland Hansen Arch. Northeastwards in the Voring Basin, the landward limit of the body steps gradually seawards, closely related to the interpreted lineaments. Northeast of the Gleipne Lineament, the body terminates close to the Fles Fault Complex, north of the Surt Lineament, it extends across the Nyk High, and northeast of the Bivrost Lineament the intrusions terminate around the Voring Escarpment. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Icelandic plume, and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement. The increased thickness of the continental crust on the seaward side of the Voring Escarpment, the upwarping of Moho and thinning of the lower crustal high-velocity layer in the western part of the Voring Basin, as well as a strong shallowing of the Moho observed in parts of the area between the Jan Mayen Fracture Zone/Lineament and the New Lineament, can be explained by lithospheric delamination models.

Vp/Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Voring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The Vp/Vs ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The Vp/Vs ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Voring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Voring Basin, as well as on the southern Lofoten Margin further north. The Vp/Vs ratio in the extrusive rocks on the Voring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the Vp/Vs ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The Vp/Vs ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Voring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the Vp/Vs ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).

Deep crustal structure of the eastern Nankai Trough and Zenisu Ridge by dense airgun–OBS seismic profiling

Abstract An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough.

A regional shear-wave velocity model in the central Vøring Basin, N. Norway, using three-component Ocean Bottom Seismographs

Abstract A regional Ocean Bottom Seismometer (OBS) dataset was acquired in the central and northern part of the Voring Basin, N. Norway, in 1992. Thirty three-component OBSs were deployed along seven regional profiles, and the shear waves recorded on the horizontal components have been modelled by use of 2-D ray-tracing utilizing an established P-wave model. In the shallow sediments (1–4 km depth) mean Vp/Vs ratios of 2.1–2.2 and 2.3–2.5 are inferred along the NW–SE- and NE–SW-trending profiles, respectively. The relatively high mean Vp/Vs ratios in this interval are probably related to unconsolidated sediments in the uppermost layers. The azimuthal variation in the Vp/Vs ratios with the fast direction NW–SE, indicates a shear-wave anisotropy of 5–10% in this interval, consistent with observations further north in the Lofoten area. In the 5–10 km depth interval high-velocity sill intrusions in the sediments are mapped by the P-wave modelling. These high-velocity intrusions act as efficient conversion interfaces, confirmed by the shear-wave modelling. A regional decrease in the Vp/Vs ratios to 1.7–1.8 are observed at 4–6 km depth, possibly related to sand-rich sediments in the middle Cretaceous. The upper crust is clearly mapped by the shear waves, and the inferred Vp/Vs ratio of 1.75 confirms that the crust in the Voring Basin is of continental origin. An increase of the Vp/Vs ratio (1.8–1.85) in the lower crust indicates a change in its composition, possibly explained by a `mixing of continental crust and magmatic underplating.