Ridge subduction at an erosive margin - the collision zone of the Nazca Ridge in southern Peru

The 1.5-km-high, obliquely subducting Nazca Ridge and its collision zone with the Peruvian margin have been imaged by wide-angle and reflection seismic profiles, swath bathymetry, and gravity surveying. These data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust. The lowermost layer contributes 10–12 km to the total crustal thickness of the ridge. The sedimentary cover is 300–400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges. At the collision zone of ridge and margin, the following observations indicate intense tectonic erosion related to the passage of the ridge. The thin sediment layer on the ridge is completely subducted. The lower continental slope is steep, dipping at ∼9°, and the continental wedge has a high taper of 18°. Tentative correlation of model layers with stratigraphy derived from Ocean Drilling Program Leg 112 cores suggests the presence of Eocene shelf deposits near the trench. Continental basement is located <15 km landward of the trench. Normal faults on the upper slope and shelf indicate extension. A comparison with the Peruvian and northern Chilean forearc systems, currently not affected by ridge subduction, suggests that the passage of the Nazca Ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long-term erosive regime

Schallabsorption und Schallgeschwindigkeit in Salzschmelzen. Ueberpruefung eines Mikrogravitationseffektes Schlussbericht

The following report gives a summary of values of sound absorption and sound velocity in homogeneous alkali nitrate / silver nitrate molten salts measured by ultrasonics. Furthermore the molten salts were analysed by DTA (Differential-Thermo Analysing) in order to get their phase diagrams. Determining the sound absorption and sound velocity we use two different methods: the pulse-transmission method and the sound-lattice method also called DEBYE-SEARS method. On earth the sound-lattice method is disturbed by natural convection so that better values under microgravity are expected. Therefore an experiment according to the DEBYE-SEARS method was built up using in a sounding rocket called TEXUS (Technologisches Experiment Unter Schwerelosigkeit). Now the aim was to get values of ultrasound absorption under microgravity. In May 1990 TEXUS 25 started with the system potassium nitrate/silver nitrate and its sound absorption was measured. (orig.)SIGLEAvailable from TIB Hannover: F95B1116+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Einfluss von Mikrogravitation auf Schallabsorption und Schallgeschwindigkeit binaerer Salzschmelzen Schlussbericht

The following report gives a summary of sound absorption and sound velocity in binary nitrate molten salts measured by ultrasonics. Furthermore the molten salts were analysed by Differential-Thermo-Analysing (DTA) in order to get their phase diagrams. Determining the sound absorption and sound velocity the pulse-transmission method is used in the laboratory. The influence of microgravity should be investigated of these ultrasonic measurements. Therefore an experiment was built using in a sounding rocket called Mini-TEXUS (Technologisches Experiment Unter Schwerelosigkeit). It works with the same technique to give values of ultrasound absorption under microgravity. In November 1993 Mini-TEXUS 1 and in April 1995 Mini-TEXUS 4 started with the system rubidium nitrate/silver nitrate to measure its sound absorption. For further analysis the same experiment was repeated in the drop tower Bremer in November 1996. (orig.)SIGLEAvailable from TIB Hannover: F97B1737+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore–offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ∼5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s−1, from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s−1. The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s−1, from the top to the bottom, shows a gradual seaward thinning of ∼15 km over an ∼35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ∼2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW–SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica–Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin