161 Preliminary Report


Early in scientific drilling, the Mediterranean attracted the interest of the earth science community. In 1970, the first expedition of Glomar Challenger (Leg 13; Ryan, Hsü, et al., 1973) to the Mediterranean investigated the "Messinian salinity crisis," which led to the deposition of the famed evaporitic sequences. Leg 42 (1975; Hsü, Montadert, et al., 1978) and Leg 107 (1985; Kastens, Mascle, Auroux, et al., 1987) provided important information on the pre-Messinian environmental history and tectonic evolution of the Mediterranean.

Leg 161 in the western Mediterranean was the second in a two-leg ODP program to address both tectonic and paleoceanographic objectives in the Mediterranean Sea. During Leg 161, JOIDES Resolution drilled a transect of five sites across the western Mediterranean (Fig. 1 ) from the Tyrrhenian Sea to the Alboran Sea, immediately east of the Strait of Gibraltar. Sites 974 and 975 in the Tyrrhenian Sea and on the Menorca Rise, respectively, were dedicated to paleoceanographic studies. Sites 976, 977, 978, and 979 in the Alboran Sea focused on tectonic goals, but also involved paleoceanographic objectives.

The ODP Leg 160 paleoceanographic investigation in the eastern Mediterranean was devoted to obtaining Pliocene–Pleistocene records containing a detailed record of sapropel deposition. Leg 161 was designed to retrieve time-equivalent sedimentary sequences that would allow documentation of the Miocene-through-Pleistocene paleoceanography of the western Mediterranean and a determination of the Mediterranean-wide circulation patterns at times of sapropel formation in the east. Site 974, in the Tyrrhenian Sea, was chosen, as it represents the westernmost documented occurrence of sapropels in the Mediterranean. Site 975, in the southern Balearic Sea, was selected to document the hydrography of western Mediterranean surface and deep waters. The paleoceanographic goals at the Alboran Sea sites included study of the late Cenozoic history of Atlantic-Mediterranean water exchange and the development of biological productivity patterns. The paleoceanographic and tectonic objectives in the Alboran Sea are related in that the paleo-geographic evolution of the western Mediterranean gateway is a central theme for understanding the Messinian desiccation and circulation in the western Mediterranean Sea.

For tectonic studies in the western Mediterranean, the Alboran Sea was chosen as the optimum area to study the origin of Neogene extensional basins in collisional settings. Among the Mediterranean convergent boundaries, the collision between the Eurasian and the African plates at the western-most Mediterranean Sea has resulted in a broad region of distributed deformation rather than a discrete plate boundary. This broad region comprises the Betic, Rif, and Tell cordilleras, which are linked across the Gibraltar Arc, and includes the extensional basins that form the Alboran and South Balearic seas (Fig. 2 ). The apparent paradox of extensional basin formation and crustal stretching during the collision of the Eurasian and Africa plates has been a long-standing problem in Mediterranean tectonics.

The Alboran Basin was formed during the early to middle Miocene by extension at the site of a former collisional orogen. The basin, which is floored by extended continental crust and surrounded by a thrust belt that was tectonically active during basin extension, closely resembles the northern Tyrrhenian Sea or the Panonnian Basin in that there is no geological or geophysical evidence that oceanic lithosphere subduction was associated with basin extension. The extension directions in the basin, and those of the coeval thrusting in the surrounding orogenic arc, are not clearly related to the Eurasian-African relative motion.

The prime tectonic objective of the Alboran Sea drill sites was to determine the response of the crust to compressional and extensional forces, and to better understand the kinematics and deformation of the Mediterranean continental lithosphere. Furthermore, the continental rift system that led to the development of the Alboran Basin provided an opportunity to examine the nature of brittle and ductile deformation of the crust, the role of magmatism in rifting processes, and the role of upper mantle in crustal modification and lithosphere evolution. The young, and tectonically active Alboran Sea is an ideal natural laboratory where these active tectonic processes can be investigated. The Alboran Sea drilling results are expected to have immediate applications in establishing geodynamic models for the origin and evolution of Mediterranean-type backarc extensional basins.

In summary, the primary paleoceanographic objectives of Leg 161 were as follows.

(1) To investigate the timing of sapropel formation in the Tyrrhenian Sea. At the time Leg 161 was being planned, this was the westernmost documented occurrence of sapropels in the Mediterranean.

(2)To investigate the circulation pattern in the western Mediterranean during periods of sapropel formation in the east. A determination of the hydrographic patterns across the entire Mediterranean was needed to better define the various factors that may have contributed to the formation of sapropels (i.e., basinwide anoxia vs. biological productivity).

(3)To understand environmental conditions during the onset of evaporitic conditions and to investigate the reestablishment of open-ocean conditions during the earliest and latest Miocene.

(4) To investigate Atlantic-Mediterranean water exchange and its influence on the Mediterranean's hydrography during the onset of Northern Hemisphere glaciation, about 3.0 m.y. ago. Monitoring hydrographic paleo-gradients across the Mediterranean,as well as vertical paleo-gradients between surface- and deep-water hydrographic proxies, was an important objective of Leg 161.

The primary tectonic objectives of Leg 161 were as follows.

(1) To investigate the dynamics, kinematics, and deformation of the continental lithosphere margins, including the development of extensional basins on collisional orogens, the dynamics of the collapse of collisional ridges, which result in extensional basins surrounded by orogenic belts, and collisional processes affecting continental margins.

(2) To determine the nature of the Mediterranean crust to develop a lithosphere model for the Alboran Sea rifting; to establish models for Miocene rifting on the basis of the nature of the basement and the geometry of rifting; to examine the magnitude and timing of extensional faulting; to examine syn-rift vs. post-rift subsidence and the pattern of total tectonic subsidence; and to determine the timing and role of magmatism during extension.

(3)To investigate post-rift deformation, in particular the late Miocene to Holocene contractive reorganization, recent strike-slip tectonics, the role of volcanism, and the recent collapse of the Alboran Basin.

To address these topics, six sites were selected for drilling in the western Mediterranean (Figs. 1 , 2 , 3 , and 4 ).

Site 974 (proposed site MedSap-5) is located in the Tyrrhenian Sea and reoccupies ODP Leg 107 Site 652, which recovered eight sapropels and several volcaniclastic deposits (Kastens, Mascle, Auroux, et al., 1987). The primary objective of Site 974 was to obtain a continuous Pliocene–Pleistocene record of organic-rich sedimentary events and a comprehensive record of volcaniclastic sedimentation that could be tied into the paleoceanographic and tephrochronologic concepts of the Mediterranean.

Site 975 (proposed site MedSap-6A), on the South Balearic Margin, was chosen, as it is located in a key position to monitor the history of inflowing Atlantic waters on their passage east, and of outflowing Mediterranean waters on their passage west to the Alboran Sea. Site 975 is a central tiepoint along the trans-Mediterranean drilling transect, which was drilled during Legs 160 and 161.

Site 976 (proposed site Alb-2A) is located in the western Alboran Sea on a structural high at the southern Spanish margin. The site's primary objective was to penetrate through the Pleistocene and Miocene sediment cover and to recover hard rock samples at least 200 m into basement to yield information on the origin and evolution of the Alboran Sea as a typical "Mediterranean backarc" basin. A primary paleoceanographic objective was to monitor the Atlantic-Mediterranean water exchange during the late Cenozoic.

Sites 977 and 978 (proposed sites Alb-4A and Alb-4, respectively) are located in the eastern Alboran Sea and lie in small sub-basins south and north of the Al-Mansour Seamount. At Sites 977 and 978, we aimed to drill through a sequence of tilted and hummocky seismic reflectors, likely representing syn-rift sedimentary structures, that may yield information on the subsidence history and tectonic evolution of the eastern Alboran Basin. Paleoceanographic objectives included an investigation of the Miocene-through-Pleistocene history of the Atlantic-Mediterranean water exchange, and of productivity patterns in the eastern Alboran Sea in relation to climatically driven circulation changes.

Site 979 (proposed site Alb-3A), in the southern Alboran Sea, is located on the southern flank of the Alboran Ridge, about 45 km north of Cabo Tres Forcas. The main objective was to penetrate through a zone of syn-sedimentary deformation on the flank of the ridge, which is depicted in seismic reflection profiles across the site, to yield the age and nature of folding, faulting, and associated unconformities. Ultimately, information obtained from this site will provide the database needed to establish the history of subsidence and/or uplift of the southern Alboran Basin and of the later contractive reorganization of the Alboran Basin as a whole.

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