ALBERT

Notes: Abstract In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.

Notes: Abstract Results are presented from a deep seismic sounding experiment with the research vessel POLARSTERN in the Scoresby Sund area, East Greenland. For this continental margin study 9 seismic recording landstations were placed in Scoresby Sund and at the southeast end of Kong Oscars Fjord, and ocean bottom seismographs (OBS) were deployed at 26 positions in and out of Scoresby Sund offshore East Greenland between 70° and 72° N and on the west flank of the Kolbeinsey Ridge. The landstations were established using helicopters from RV POLARSTERN. Explosives, a 321 airgun and 81 airguns were used as seismic sources in the open sea. Gravity data were recorded in addition to the seismic measurements. A free-air gravity map is presented. The sea operations — shooting and OBS recording — were strongly influenced by varying ice conditions. Crustal structure 2-D models have been calculated from the deep seismic sounding results. Free-air gravity anomalies have been calculated from these models and compared to the observed gravity. In the inner Scoresby Sund — the Caledonian fold belt region — the crustal thickness is about 35 km, and thins seaward to 10 km. Sediments more than 10 km thick on Jameson Land are of mainly Mesozoic age. In the outer shelf region and deep sea a ‘Moho’ cannot clearly be identified by our data. There are only weak indications for the existence of a ‘Moho’ west of the Kolbeinsey Ridge. Inside and offshore Scoresby Sund there is clear evidence for a lower crust refractor characterised byp-velocities of 6.8–7.3 km s−1 at depths between 6 and 10 km. We believe these velocities are related to magmatic processes of rifting and first drifting controlled by different scale mantle updoming during Paleocene to Eocene and Late Oligocene to Miocene times: the separation of Greenland/Norway and the separation of the Jan Mayen Ridge/Greenland, respectively. A thin igneous upper crust, interpreted to be of oceanic origin, begins about 50 km seaward of the Liverpool Land Escarpment and thickens oceanward. In the escarpment zone the crustal composition is not clear. Probably it is stretched and attenuated continental crust interspersed with basaltic intrusions. The great depth of the basement (about 5000 m) points to a high subsidence rate of about 0.25 mm yr−1 due to sediment loading and cooling of the crust and upper mantle, mainly since Miocene time. The igneous upper crust thickens eastward under the Kolbeinsey Ridge to about 2.5 km; the thickening is likely caused by higher production of extrusives. The basementp-velocity of 5.8–6.0 km s−1 is rather high. Such velocities are associated with young basalts and may also be caused by a higher percentage of dykes. Tertiary to recent sediments, about 5000 m thick, form most of the shelf east of Scoresby Sund, Liverpool Land and Kong Oscars Fjord. This points to a high sedimentation rate mainly since the Miocene. The deeper sediments have a rather high meanp-velocity of 4.5 km s−1, perhaps due to pre-Cambrian to Caledonian deposits of continental origin. The upper sediments offshore Scoresby Sund are thick and have a rather low velocity. They are interpreted as eroded material transported from inside the Sund into the shelf region. Offshore Kong Oscars Fjord the upper sediments, likely Jurassic to Devonian deposits, are thin in the shelf region but thicken to more than 3000 m in the slope area. The crust and upper mantle structure in the ocean-continent transition zone is interpreted to be the result of the superposition of the activities of three rifting phases related to mantle plumes of different dimensions: 1. the ‘Greenland/Norway separation phase’ of high volcanic activity, 2. the ‘Jan Mayen Ridge/Greenland separation phase’ and 3. the ‘Kolbeinsey Ridge phase’ of ‘normal’ volcanic activity related to a more or less normal mantle temperature. During period 2 and 3 only a few masses of extrusives were produced, but large volumes of intrusives were emplaced. So the margin between Scoresby Sund and Jan Mayen Fracture Zone is interpreted to be a stretched margin with low volcanic activity.