No abstract

The Miocene basin evolution of southeastern Spain and eastern Morocco is linked to a "shear zone" elongated from SW across the Alboran Sea. In Spain the magmatism is mostly calc-alkaline (or K-rich calc-alkaline). Most of the products are locatred on strike-slip faults (Almeria-Cabo de Gata). Lavas of dacitic compositions are interpreted as products of crustal anatexis. During Messinian time, lamproites are erupted over an extended area. Later (Plio-Quaternary), alkali basalts are located near Cartagena. In Morocco, calc-alkaline magmatism is not as developed as in Spain; late Tortonian-Messinian volcanoes (Gourougou, Guilliz) have erupted of shoshonitic lavas. Alkali basalts are abundant and appear from the end of Messinian to Quaternary all over northwestern Africa. In the studied area, there are no chronological nor geochemical polarity of the magmatism according to the existence of a Miocene subduction. The association of the magmatism with tectonics and basin evolution shows that it is linked with their aperture. The structure of the lithosphere, as it appears from the geophysical data, shows the existence of two different crusts, separated by the western part of the "shear zone". Trans-Alboran calc-alkaline magmatism is clearly correlated with the activity of this "shear zone", from Miocene to present time.

A complete set of downhole measurements along with Formation MicroScanner (FMS) electrical images of the borehole surface was recorded in the basement of Hole 896A during Ocean Drilling Program Leg 148. This complete geophysical data set offers, largely because of the quality of the recorded data, the first opportunity to integrate core results, meter-scale logs, and centimeter-scale borehole wall images in a structural sense for oceanic basement. At an even larger scale, extremely low resistivity values and consistently high acoustic velocity were measured in the hole. These values indicate that the basement has been altered to a large extent at this topographic high and heat-flow maximum, but also that cracks and fractures once conveying fluid to alter fresh basalts are now sealed.Nearly 7700 features were mapped over 225 m of images (34.3 per meter on average) and described in terms of geometry and aperture. The penetrated basement section consequently appears to be intensively fractured. In the laboratory, Archie's formula (1942) is found to provide a reasonable first-order estimate of porosity for this severely fractured basement section. Once applied to downhole measurements at meter-scale, this relationship yields a continuous porosity profile through the penetrated section. The combination of meter-scale (with the Dual LateroLog) and centimeter-scale (with FMS images) investigations of the rock structure outlines the presence of four narrow and subvertical fracture zones in the analyzed interval.At 356 mbsf, one of these narrow intervals appears to separate two different fracture sets, as well as stress regimes in terms of borehole shape and rupture mode. This might indicate the presence of a presently active fault zone at this depth, which divides the upper basement and may explain the surprisingly high permeability values inferred deeper in the hole from packer experiments. Borehole elongations are very small throughout, but coherent in azimuth and distribution with those obtained only 1 km away at Site 504. In the near future, a detailed description of upper basement structures both in terms of geometry and volume should arise from the combined analysis of standard downhole measurements and fracture sets obtained from FMS image analysis.

A compaction model was developed and applied to five sites drilled as part as Ocean Drilling Program Leg 161, during which overpressured sediments were cored. The long-term compaction coefficient for the porosity variation is very high (1.3-3.4 × 10-7 Pa-1). Assuming that the fluid overpressures result from disequilibrium compaction, the fluid overpressures can be estimated from the difference between the hydrostatic porosity (i.e., the porosity distribution that would have resulted at equilibrium compaction with hydrostatic pore-fluid pressures) and the porosity deduced from downhole measurement analysis. Fluid overpressuring starts at very shallow depths (120-150 m below seafloor) and, in some cases, very quickly reaches the reduced lithostatic pressure. The reduced lithostatic pressure corresponds to the upper limit of pore-fluid overpressure before which natural fracturing occurs in unconsolidated sediments. Fluid overpressure is commonly correlated with the presence of gas (mostly methane). The ingredients for capillary sealing, two fluid phases in a layered sequence of fine and coarse sediments, exist in all the sedimentary sections described in this paper, and layers filled with free gas are clearly revealed as spikes in the porosity derived from the density log. Capillary sealing is shown to be quantitatively capable of retaining the overpressures observed.

Downhole electrical images obtained in the bottom 200 m of Hole 504B during ODP Leg 148 are analyzed here in terms of geometry, with a precise description of fracturing given. Because core recovery was poor, these images are essential to the understanding of the structural context within which this deep hole is drilled. Nearly 4500 traces of fractures were mapped over 167 m of images (26.9 per m on average). The steep to nearly vertical structures that dominate throughout this interval are oriented mainly in a 015°N strike direction, which is identical to the maximum horizontal stress (SH max ) determined higher up in Hole 504B from BHTV images.Two zones with intense horizontal fracturing are described (1920-1950 and 1975-2005 mbsf) from FMS images. Although the top one is also detected at a meter scale by deep-reaching electrical resistivity measurements, the bottom one is not. The top interval is also the site of a change in hole deviation (at 1950 mbsf), possibly in relation to a change in dike orientation or in structure of the stress field near the bottom of the hole.The large number of nearly vertical fractures reported from FMS images in this bottom section were probably induced in response to drilling. Particularly concentrated over a 70-m-long interval (1975-2045 mbsf), these steep features coincide in azimuthal direction with those of borehole enlargements. This might arise from tension-generated failures under a strike-slip and strongly deviatoric environment, as suggested also by modes of failure obtained in cores. In such a case, borehole enlargements developed in the direction of tensile failure and pointing 015°N would not correspond to the traditional "breakout" terminology, but would confirm the previous determination of SH max orientation in this hole.From 1980 to 2003 mbsf, steep features are also detected at depth into the rock from electrical measurements. Located at the bottom of the section drilled during Leg 140, this interval is that in which cooling from fluid circulation was applied at the beginning of Leg 148 in order to protect the core bit from high temperatures. Therefore, it is likely that borehole enlargements and nearly vertical fracturing detected from 1975 to 2045 mbsf originate in drilling, as the propagation of failures deep into the diabase is particularly efficient at depths where cooling was specifically performed.