ALBERT

Notes: [Auszug] Two papers in Science, report the first observations of martian magnetic anomalies, made with magnetometers on board the Mars Global Surveyor spacecraft. This discovery was completely unexpected. Some of the anomalies are large (about 1,000 nanotesla) and well resolved, and they are mainly in the ...

Notes: We investigate a particular potential cause of deformation within the subducting Tonga slab: that associated with material that moves over a template while remaining in contact with it. In such a situation both the location and the style of deformation within the material depend, in a predictable way, on the shape of the template, and in particular on its Gaussian curvature. We look for such an association in the Tonga slab, using earthquake locations to define the slab shape and their focal mechanisms to indicate the style of deformation.Only in one place, at 25°S and 500-600 km depth, does the style of the faulting in the earthquakes demonstrably correspond with that required by the Gaussian curvature if the slab were moving over a template. Although the Gaussian curvature in other parts of the slab, particularly near the ‘hook’ at its northern end, would also require deformation if the slab were moving over a template, the pattern of earthquake mechanisms were in those places is not clear enough to confirm the association.Although we are limited by our ability to resolve only the coarsest features (≳ 300 km) of the slab shape, we reach the important conclusion that deformation in response to motion over a template is not the main cause of the intermediate and deep seismicity in the Tonga slab. Most of the earthquakes, and all the biggest ones, occur where, even if the slab were moving over a template, it would not need to deform to do so. Some other explanation is required for these earthquakes.

Notes: Deep seismic-reflection profiles have shown that the lower part of the continental crust often contains many strong, sub-horizontal reflectors. Various suggestions have been made about the origin of such reflections. One explanation, which does not require the invocation of any complex tectonic process, is layering of mafic/ultramafic intrusions of partial melts derived from the upper mantle. We show that the internal layering within large mafic and ultramafic igneous intrusions can form the necessary impedance contrasts with appropriate length scales to produce seismograms similar to those observed from the lower crust, and suggest that such intrusions could be responsible for the character of the observed lower crustal reflections. Such layering could be formed at any stage of crustal formation. This suggestion is consistent with earlier geochemical proposals and does not impose any new constraints on the origin and structure of the continental crust.

Notes: In this paper we examine the connection between the westward motion of Turkey relative to Europe and the extension in and around the Aegean Sea. The principal new data available since the last attempt to synthesize the tectonics of this region by McKenzie (1978) are much improved focal mechanisms of earthquakes, constrained by P and SH body wave modelling as well as by first motions. These mechanisms show that the faulting in the western part of the Aegean region is mostly extensional in nature, on normal faults with a NW to WNW strike and with slip vectors directed NNW to NNE. There is evidence from palaeomagnetism that this western region rotates clockwise relative to stable Europe. In the central and eastern Aegean, and in NW Turkey, distributed right-lateral strike–slip is more prevalent, on faults trending NE to ENE, and with slip vectors directed NE. Palaeomagnetic data in this eastern region is more ambiguous, but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations, relative to Europe, in the south. The strike–slip faulting that enters the central Aegean from the east appears to end abruptly in the SW against the NW-trending normal faults of Greece.The kinematics of the deformation is controlled by three factors: the westward motion of Turkey relative to Europe; the continental collision between NW Greece–Albania and the Apulia–Adriatic platform in the west; and the presence of the Hellenic subduction zone to the south. As the right-lateral slip on the North Anatolian Fault enters the Aegean region it splays out, becoming distributed on several parallel faults. The continental shortening in NW Greece and Albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed E–W right-lateral shear, and thus leads to E–W shortening in the northern Aegean, which is compensated by N–S extension as the southern Aegean margin can move easily over the Hellenic subduction zone. The dynamics of the system, once initiated, is self-sustaining, being driven by the high topography in eastern Turkey and by the roll-back of the subducted slab beneath the southern Aegean.The geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate. In spite of its extreme simplicity, a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the Aegean region, including: the slip vectors and nature of the faulting in the eastern and western parts; the senses and approximate rates of rotation; the overall extensional velocity across the Aegean; and the distribution of strain rates, as seen in the seismicity and topography or bathymetry, and using geodetic measurements.As part of this study, we re-examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 Gulf of Corinth earthquakes, and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting. An important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves.

Notes: [Auszug] In the ocean basins magma is generated in two different environments. Plate separation at ridges leads to adiabatic upwelling and melting. This process produces most of the oceanic crust. The other type of feature which produces magma in the ocean basins is intra-plate volcanism such as that which ...

Notes: [Auszug] The surface of Mars is cut by long linear faults with displacements of metres to kilometres, most of which are thought to have been formed by extension. The surface has also been modified by enormous floods, probably of water, which often flowed out of valleys formed by the largest of these ...

Notes: [Auszug] The correlation between variations of the bathymetry and of the geoid in the Pacific Ocean suggests that both are the surface expression of mantle convection. The planform of the convection which is required to generate the observed anomalies does not consist of rolls but is three dimensional, with ...

Notes: [Auszug] Plate tectonics offers a framework in which aspects of the evolution and dispersal of organisms can be ...