4. SITE 373: TYRRHENIAN BASIN Shipboard Scientific Party1 SITE DATA in drilling on the Mid-Atlantic Ridge. Basalt volcanism undoubtedly played a role in the genesis of the Tyrrhe- Position: 39°43.68N, 12°59.56E nian Basin. Water Depth (sea level): 3517 corrected meters, echo sounding. BACKGROUND AND OBJECTIVES Bottom Felt at: 3507 meters, drill pipe. Penetration: 457.5 meters. Background Number of Holes: 1 The understanding of the origin and evolution of the Number of Cores: 12 so-called small ocean basins and marginal seas is of Total Core Recovered: 27.5 meters critical importance in furthering our knowledge of Percentage Core Recovery: 24.1% ocean-continent relationships (Figure 2). Various hy- potheses have been put forward as to the origin of the Oldest Sediment Cored: Tyrrhenian Sea. Among them are: (a) the Tyrrhenian Depth subbottom: 270 meters Nature: Marl Basin is a relic of a once wider oceanic basin, perhaps Age: Early Pliocene as old as the Permian (Glangeaud, 1962); (b) the Basement: Tyrrhenian Basin is a marginal back-arc basin created Depth subbottom: 270-457 meters as a consequence of lithospheric subduction at a plate Nature: Basalt breccias, and flows. margin beneath the Ionian Basin; and (c) the Tyrrhe- Principal Results: Site 373 was located on the flank of a nian Basin was created in the late Tertiary by rapid seamount in the central Tyrrhenian Abyssal Plain (Figure subsidence of a continental craton. Recovery of base- 1), and its prime objective was to sample the basement. ment samples beneath the Tyrrhenian Abyssal Plain is The original hole was positioned too high on the flank and the bottom-hole assembly could not be stabilized. an essential first step in solving the problem. If the Hole 373A was an 800-meter offset to the west, where southern Tyrrhenian Sea is a relic of an ancient ocean acoustic basement was encountered at 270 meters, and the basin, it should be underlain by typical oceanic "layer hole was terminated in basalt at 457.5 meters. A Plio- 2" type material (abyssal tholeiitic basalt) of rather Quaternary sequence of nannofossil marls, zeolite marls, and volcanic ashes and sands overlies a basaltic basement old age and be characterized by low heat-flow values complex of calcareously cemented basalt breccias and typical of old ocean basins. If it is a recently formed, flow basalts. The limestone matrix of the basaltic breccias back-arc marginal basin, young oceanic-type crust contained foraminifers dated as not older than middle should underlie it. If, on the other hand, granite or Miocene and is probably early Miocene. The flow basalts metamorphic rocks were sampled at depth within the were extensively altered despite a penetration almost 200 meters beyond the top of basement. This basement central abyssal plain province, then it would provide a complex bears a general resemblance to that encountered strong confirmation of the hypothesis of oceanic sub- sidence of cratonic basement. However, the Tyrrhenian Abyssal Plain is commonly underlain by a sedimentary 1 sequence including evaporite deposits. Except locally Kenneth J. Hsü (Co-chief scientist), Eidg. Technisches Hoch- schule, Geologisches Institut, Zurich, Switzerland; Lucien Montadert on basement ridges, a relatively deep penetration (Co-chief scientist), Division Geologie, Institut Francais du Petrole, through evaporites is required before the basement can Rueil Malmaison, France; Daniel Bernoulli, Geologisch-palaontolo- be sampled. gisches Institut der Universitat Basel, Basel, Switzerland; Germaine Two types of basement highs have been recognized Bizon, Bureau d'Etudes Industrielles et de Cooperation de 1'Institut Francais du Petrole, Rueil Malmaison, France; Maria Cita, Instituto by geophysical surveys. On the edge of the Tyrrhenian di Geologia, Universita degli Studi di Milano, Milano, Italy; Al Abyssal Plain are a number of "highs" not associated Erickson, Department of Geology, University of Georgia, Athens, with magnetic anomalies. Drilling elsewhere in the Georgia; Frank Fabricius, Institut fur Geologie Techn. Universitat, western Mediterranean during Leg 13 has shown that Munich, Germany; Robert E. Garrison, University of California, Santa Cruz, California; Robert B. Kidd, Institute of Oceanographic this type of "basement high" represents subsided sialic Sciences, Wormley, United Kingdom; Frederic Mélières, Laboratoire crust bounding the central abyssal provinces (e.g., Sites de Geologie Dynamique, University of Paris, Paris, France; Carla 121, 134). A second type of "basement high," most Müller, Geologisch-Paleontologisches Instutut der Johann Wolfgang typical of the central abyssal provinces, is characterized Geothe-Universitat, Frankfurt, Germany (Present address: Bureau by highly positive magnetic anomalies. These are most d'Etudes Industrielles et de Cooperation de L'Institut Francais du Petrole, Rueil Malmaison, France); Ramil C. Wright, Beloit College, probably seamounts or submarine volcanoes. Dredging Department of Geology, Beloit, Wisconsin (Present address: Depart- on the Tyrrhenian basement highs has proven the ment of Geology, The Florida State University, Tallahassee, Florida. existence of both types of basement. SITE 373: TYRRHENIAN BASIN 373A NAPLES ITALY 100- 40° N ε 2oo- 300- 400- T. D. = 457.5 m 12°E 14°E 500J (b) Figure 1. (a) Site location map (depth contours in meters); and (b) generalized hole summary. Objectives the course was changed to 115° to follow the OGS profile MS-1 (Figures 3 and 4). The basins and There was considerable discussion during the meet- seamounts on the reference line were easily recognized. ings of the Mediterranean Advisory Panel concerning At 0546 LCT the vessel modified its course to 100°. the priority of drilling in the Tyrrhenian. Selli and The selected target on the flank of a seamount was associates presented arguments in favor of drilling on a reached at 0610 LCT and a 16-kHz beacon was nonmagnetic basement high (Site M-6 Alternate, dropped. The crew began to assemble the drill string 39°23N, 11°42E). Gasparini, Cita, Ryan, and Bonatti, immediately, and the string reached the sea bed at on the other hand, argued strongly for drilling on a 1400 LCT, 27 April. It was very difficult to determine Tyrrhenian seamount (Site M-6, 39°44N, 13°00E). It precisely the depth of the bottom from the PDR seemed a foregone conclusion that we would encounter because of the numerous side echoes from the flanks of sialic basement if we drilled on marginal nonmagnetic the seamount. The bottom was encountered by the drill highs, and that we would encounter volcanic basement string at 3461 meters some 30 meters above that if we drilled on seamounts in the central abyssal plain estimated on the basis of the PDR record. province. The Mediterranean Panel finally decided to Only a few meters were penetrated in Hole 373 give first priority to the proposed M-6 drillsite over the (39°43.86N, 13°00.10E). The thickness of sediments M-6 alternate drillsite on the basis that a sialic base- was insufficient to bury the bottom-hole assembly. It ment sample from a marginal basement ridge would was decided immediately to pull out and to make an not provide more information than that a basin-margin 800-meter offset towards the west where sediments can subside (a foregone conclusion), but that good, were expected to be thicker. The ship was positioned fresh basalt samples from a seamount might yield over Hole 373A (39°43.68N, 12°59.56E) by a side- important clues on basin genesis. Since such fresh ways movement using the thrusters. samples cannot be obtained by dredging, drilling is the only applicable method. The primary objective of Site Drilling Program 373 was, therefore, not so much to prove that basalt At 1530 LCT, 27 April, the crew began to lower the underlies this seamount, but to obtain fresh basalt drill string to drill Hole 373A. The sea bed was samples for petrological and geochemical studies, encountered at 1607 LCT, 3507 meters below the rig which might lead indirectly to an interpretation of the floor (as opposed to 3517 m indicated by the PDR basin genesis. record). Drilling operations were begun by washing down 96.5 meters. Core 1 was taken when the core OPERATIONS barrel was raised for the first heat-flow measurement at 106 meters subbottom. (Details of all heat-flow mea- Site Approach surements and results appear in Erickson and von On the 27 April Glomar Challenger approached the Herzen, this volume.) After washing down another 19 proposed site from the west southwest. At 0436 LCT meters, the second heat-flow measurement was success- 152 1 Tertiary Basin ^ I Position of DSDP Drilling [ Stable Platform Main Fault [ Alpine Belt I Main Thrust, front of nappe M Φ.. * I Salt and limite of Solt Basin I Front of Gravity Slide Escarpment , Fault I Limit of Post-Tectonic Bosin 1 \v| Volcanism | Deltas O North Dalmat Basin RiSβ Thick recent deposits Q Emile Beaudot Escarpment £5 O North- Balearic Accident 0 Shoal of A>boran Figure 2. Structural sketch map of the western Mediterranean from Biju-Duval et al. (1974). SITE 373: TYRRHENIAN BASIN 1 1 1 1 I i • i i ! 1 1 1 0430 LCT ——O—-4;27•7S 6 LCT - - 0528 LCTtX SNF 03282 ><s^ 0545 LCTOO^ Cln Proposed site 0 F 0546 LCT^~~Xiü — 0346Z ) — c SITE 373A 39°43.68•N 12° 59.56'E SITE 373 BEACON _ - 1210 LCT Or site SN 1010Z - - 1 1 1 1 I i 1 1 1 1 1 1 30°40'N Figure 3. Site approach, Site 373. fully run at 125 meters. As the reflector was estimated correct as the next barrel was pumped into place. At to be within the range 190 to 400 meters subbottom, it 1810 LCT Core 10 was raised and was found to was decided to drill ahead. At 2245 LCT and after contain several pieces of fresh basalt.