Geometrical-statistical modelling of systems of fracture zones along oceanic ridges

A mechanical-statistical model is presented that aims to help to understand the history and geometry of the process of formation of fracture zones along oceanic ridges. It uses ideas of statistical fracture theory used in engineering, namely the Weibull fracture model. The approximate parallelism of the fracture zones along ridges makes it possible to use a one-dimensional point process model with points along the ridge axes, which represent the transform faults. The ratios of the lengths of the corresponding fracture zones to the ocean width are used to obtain a rough estimate of the Weibull modulus, which is an important material parameter in fracture theory. The theory is refined by introducing a hard-core point process model. The corresponding positive minimum distance between subsequent fracture zones results from stress relaxation in the vicinity of a given fracture zone.     

Crustal structure of Atlantic fracture zones–II. The Vema fracture zone and transverse ridge

Summary. The crustal structure beneath the Vema fracture zone and its flanking transverse ridge was determined from seismic refraction profiles along the fracture zone valley and across the ridge. Relatively normal oceanic crust, but with an upwarped seismic Moho, was found under the transverse ridge. We suggest that the transverse ridge represents a portion of tectonically uplifted crust without a major root or zone of serpentinite diapirism beneath it. A region of anomalous crust associated with the fracture zone itself extends about 20 km to either side of the central fault, gradually decreasing in thickness as the fracture zone is approached. There is evidence to suggest that the thinnest crust is found beneath the edges of the 20 km wide fracture zone valley. Under the fracture zone valley the crust is generally thinner than normal oceanic crust and is also highly anomalous in its velocity structure. Seismic layer 3 is absent, and the seismic velocities are lower than normal. The absence of layer 3 indicates that normal magmatic accretionary processes are considerably modified in the vicinity of the transform fault. The low velocities are probably caused by the accumulation of rubble and talus and by the extensive faulting and fracturing associated with the transform fault. This same fracturing allows water to penetrate through the crust, and the apparently somewhat thicker crust beneath the central part of the fracture zone valley may be explained by the resultant serpentinization having depressed the seismic Moho below its original depth.