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24. Structure and Tectonic Stresses in Metamorphic Basement, Site 976, Alboran Sea1

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24. Structure and Tectonic Stresses in Metamorphic Basement, Site 976, Alboran Sea1 Zahn, R., Comas, M.C., and Klaus, A. (Eds.), 1999 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 161 24. STRUCTURE AND TECTONIC STRESSES IN METAMORPHIC BASEMENT, SITE 976, ALBORAN SEA1 François Dominique de Larouzière,2,3 Philippe A. Pezard,2,4 Maria C. Comas,5 Bernard Célérier,6 and Christophe Vergniault2 ABSTRACT A complete set of downhole measurements, including Formation MicroScanner (FMS) high-resolution electrical images and BoreHole TeleViewer (BHTV) acoustic images of the borehole wall were recorded for the metamorphic basement section penetrated in Hole 976B during Ocean Drilling Program Leg 161. Because of the poor core recovery in basement (under 20%), the data and images obtained in Hole 976B are essential to understand the structural and tectonic context wherein this basement hole was drilled. The downhole measurements and high-resolution images are analyzed here in terms of structure and dynamics of the penetrated section. Electrical resistivity and neutron porosity measurements show a generally fractured and consequently porous basement. The basement nature can be determined on the basis of recovered sections from the natural radioactivity and photoelectric fac- tor. Individual fractures are identified and mapped from FMS electrical images, providing both the geometry and distribution of plane features cut by the hole. The fracture density increases in sections interpreted as faulted intervals from standard logs and hole-size measurements. Such intensively fractured sections are more common in the upper 120 m of basement. While shallow gneissic foliations tend to dip to the west, steep fractures are mostly east dipping throughout the penetrated section. Hole ellipticity is rare and appears to be mostly drilling-related and associated with changes in hole trajectory in the upper basement schists. In the gneisses, a very slight short-axis ellipticity pointing steadily N035° is obtained. A few narrow intervals with sharp changes in hole geometry are also identified from mechanical calipers in lower basement. These sections are found to correspond to active faults from BHTV images. The analysis of an 80-m-long basement interval reveals the presence of 20 active faults. The mapping of each fault plane and drilling-induced slip vector leads to the identification of an extensional tec- ° tonic regime, with the minimum stress axis S3 pointing N080 . Assuming a near vertical maximum stress direction (S1; about 33 MPa at 800 mbsf), and that pore pressure (Pe) equals lithostatic pressure (Se), the magnitude of the three principal stresses can be determined as a function of the friction coefficient (µ). As Hole 976B is located in the western part of the Alboran Basin, this new result indicates a present-day N080° directed extensional or transtensional stress component. This result is compatible with the actual kinematics of Europe-Africa plate convergence in the westernmost Mediterranean. INTRODUCTION breccias and faults, the total core recovery was rather low, averaging 19% in this metamorphic interval (Fig. 1). The dip of the main folia- The Alboran Sea is considered as an extensional basin located in tion planes was determined on board ship, but the dip direction of a collisional setting. The basin is rimmed by orogenic cordilleras these planes is unknown from cores that were not oriented. (Betic Chain in southeastern Spain, Rif in Morocco). One of the ob- jectives for Ocean Drilling Program (ODP) Leg 161 was to obtain Core Analysis and Lithostratigraphy samples of basement lithology in the western Mediterranean, as well as to identify the tectonic characteristics of this basement. Site 976 is The basement lithology derived from core in Hole 976B may be located on a structural high within the southern Spanish margin, off divided in two main units (Fig. 1): (1) from 669.7 down to 794.0 Malaga, and close to Deep Sea Drilling Project (DSDP) Site 121. Af- mbsf, dark gray high-grade micaschists (quartz-biotite-sillimanite- ter drilling through a thick Pleistocene and Miocene sedimentary se- plagioclase), within which layers of calcite and dolomite marble are quence, the basement was perforated at Site 976 in Holes B and E, interbedded (Comas, Zahn, Klaus, et al., 1996); (2) below 794.0 and the first direct evidence of a metamorphic lithology in this area mbsf, the recovered samples consist of medium gray massive high- was obtained. grade metasedimentary gneissic rocks (quartz-biotite-sillimanite), This paper is focused on the study of basement structures pene- with local migmatitic facies. Leucogranitic material (quartz, feldspar, trated in Hole 976B, where a full set of downhole data (standard logs) plagioclase, and cordierite) was found episodically, probably occur- and images (Formation MicroScanner [FMS] and BoreHole Tele- ring as small bodies or dikes cross-cutting the metamorphic series. Viewer [BHTV]) was recorded. Basement was reached at 669.7 Numerous zones of thick breccias were encountered for example at meters below seafloor (mbsf), and the hole terminated at 928.7 mbsf. 677.0, 705.0, 750.0, 765.0, and 913.0 mbsf. In Hole 976E, the base- Because of the hard-rock lithology and the presence of numerous ment was reached at 652.1 mbsf and the hole ends at 736.3 mbsf. De- spite the fact that the two holes 976B and 976E are very close to one another (about 20 m apart), the lithologic units are quite different, ev- 1Zahn, R., Comas, M.C., and Klaus, A. (Eds.), 1999. Proc. ODP, Sci. Results, 161: idence of the presence of subvertical structures. College Station, TX (Ocean Drilling Program). 2 Laboratoire de Mesures en Forage, ODP-NEB, BP 72, F-13545 Aix-en-Provence cedex 20, France. Downhole Geophysical Measurements 3 GAÏA, 16 boulevard Notre-Dame, F-13006 Marseille, France. [email protected] 4 Pétrologie Magmatique, CEREGE (FU 17 du CNRS), BP 80, F-13545 Aix-en- Downhole measurements were recorded in both basement holes, Provence cedex 4, France. but operational difficulties prevented the recording of data and imag- 5 Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, Campus Fuen- tenueva, E-18002, Granada, Spain. es through the sediment/basement contact in Hole 976B, and bore- 6 Université de Montpellier II-CNRS (UMR 5569), cc 57, F-34095 Montpellier hole wall images in Hole 976E (Fig. 2). The electrical resistivity pro- cedex 5, France. file obtained in Hole 976E images a sharp, probably recently faulted 319 F.D. DE LAROUZIÈRE ET AL. Hole 976B Lithology 650 Re- Core covery Sediment Fault gouges and breccias 75R 76R 77R 78R 700 79R 80R 81R 82R 83R 84R 85R 750 86R 87R 88R 89R 90R 91R 92R 800 93R 94R 95R 96R Depth (mbsf) 97R 98R 850 99R 100R 101R 102R 103R 900 104R 105R 106R High-grade schist 950 0 20406080 Gneiss Dip of Foliation (°) Figure 1. Structural dip of foliation mapped from core Marble in the basement section of Hole 976B. Main lithotypes, Granite occurrences of fault gouges and breccias, and core recovery are indicated to the right. Breccia basement/sediment contact. The electrical resistivity measurement trodes (Ekstrom et al., 1987; Lüthi and Banavar, 1988). The slimhole magnitude in the sediment and basement however is comparable in configuration developed for ODP uses four pads, each with 16 but- the two holes. A full suite of logging tools from Schlumberger was tons. A single pass of the tool maps about 22 percent of the borehole run into the accessible basement section of Hole 976B, from 700.0 to surface in a 25.4-cm-diameter (10 in) borehole. Each electrode is ori- 900.0 mbsf. This suite included the “quad” combination of geophys- ented in space with 3-axes accelerometers and fluxgate magnetome- ical sensors, the FMS electrical imager, the geochemical tool (GLT), ters, making it possible to derive the strike and dip of geological and the BHTV acoustic imager. structures. Because of electrode geometry, the tool has a moderate Hole 976B conditions were found to be better in the lower part of depth of investigation (a few centimeters). While FMS data are re- this basement section where gneisses were sampled (Fig. 3). While corded every 2.5 mm as the tool moves up the borehole, the vertical the hole shape is close to the bit size (9.875 in) over this interval, it is resolution of individual features is on the order of 1.0 cm. generally much larger above 800.0 mbsf, in the schists. The hole de- The tool can, however, detect thinner features provided that they viation from vertical remains under 7° (Fig. 3), with sharp changes in have a sufficient resistivity contrast with the surrounding matrix. The front of large hole and high neutron-porosity intervals. These short images recorded with the FMS show electrical conductivity changes, intervals are often the site of natural radioactivity and photoelectric particularly those resulting from beds different in nature, as well as factor (Pef) decrease (e.g., 718, 783, 808, 828, and 833 mbsf; Fig. 3), the presence of fractures, either open or mineralized. Data processing and can be associated with the presence of fault gouges and breccias (using Logos software of Schlumberger) is required to convert the (Fig. 1). Other intervals with low radioactivity, high Th/U ratio, but raw data into a grayscale (or color scale) image representative of con- not necessarily high neutron-porosity or caliper-size changes (e.g., ductivity changes in the rock. Data processing includes conversion of 812 and 858 mbsf) point at probably older features, not necessarily the current intensity to variable-intensity gray or color images, then active at present. to an electrical conductivity map of the borehole surface. In the former, black corresponds to higher conductivity, and white corre- DOWNHOLE ELECTRICAL IMAGES sponds to lower values. FMS Sensor and Image Processing FMS Data and Image Analysis The FMS creates a picture of the borehole wall by mapping the The FMS images recorded in Hole 976B (Fig.
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