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Science Article Vol. 5, No. 11 November 1995 INSIDE • Science in National Parks, p. 216 • North-Central Section Meeting, GSA TODAY p. 225 A Publication of the Geological Society of America • Southeastern Section Meeting, p. 227 Evidence of Collisional Processes Associated with Ophiolite Obduction in the Eastern Mediterranean: Results from Ocean Drilling Program Leg 160 Scientific Participants of Leg 160: A. H. F. Robertson* and K.-C. Emeis (Co-Chief Scientists), C. Richter (Staff Scientist), M.-M. Blanc-Valleron, I. Bouloubassi, H.-J. Brumsack, A. Cramp, G. J. De Lange, E. Di Stefano, R. Flecker, E. Frankel, M. W. Howell, T. R. Janecek, M-J. Jurado-Rodriguez, A. E. S. Kemp, I. Koizumi, A. Kopf, C. O. Major, Y. Mart, D. F. C. Pribnow, A. Rabaute, A. P. Roberts, J. H. Rüllkotter, T. Sakamoto, S. Spezzaferri, T. S. Staerker, J. S. Stoner, B. M. Whiting, J. M. Woodside Figure 1. JOIDES Resolution at Marseilles, France, prior to departing on Leg 160. ABSTRACT driving force was tectonic loading by longer active (e.g., the Upper Creta- sional settings on land to the east to Recent drilling in the eastern the Eurasian plate. In this area the ceous Oman ophiolite). However, the areas of incipient collision on the Mediterranean Sea south of Cyprus leading edge is represented by the Troodos is an example of an ophiolite Mediterranean Ridge in the west (Cita has revealed important insights into Troodos ophiolite. The breakup and that is undergoing active emplacement and Camerlenghi, l990). One such area fundamental tectonic processes asso- subsidence history of the Eratos- today, in an area that is easily accessi- of incipient collision includes the ciated with continental collision and thenes Seamount, as revealed by ble and geologically well known. Much Eratosthenes Seamount, south of ophiolite emplacement. A transect drilling during Leg 160, is clearly of the evidence for this emplacement is Cyprus. Previous geophysical studies of four sites was drilled across the linked with the emplacement of the recorded beneath the sea to the south (Woodside, l977, l991; Krasheninnikov Africa-Eurasia plate boundary. The Troodos ophiolite. This underwent of Cyprus, along the boundary between et al., l994) suggest that the Eratos- results show that the African plate, strong surface uplift during active the African and Eurasian plates. This thenes Seamount is a fragment of con- represented by the Eratosthenes tectonic emplacement, associated area was a target of drilling during Leg tinental (or transitional-type) crust that Seamount, underwent rapid with collision of the seamount with 160 in March and April l995 (Figs. 1 is undergoing subsidence and breakup subsidence and related faulting the Cyprus margin. and 2). in a zone of incipient collision between in Pliocene-Pleistocene time. The The eastern Mediterranean Sea the African and Eurasian plates (Robert- INTRODUCTION (Fig. 3) is a remnant of the Mesozoic son, l990; Kempler, l993; Limonov et Tethys ocean (Dercourt et al., l993). al., l994; Robertson et al., l994, l995). *Corresponding author—Alastair Robertson, Many ophiolites were emplaced Different areas record various stages of Department of Geology and Geophysics, Uni- long ago by processes that are no versity of Edinburgh, Edinburgh, EH9 3JW, UK. Tethys closure, ranging from fully colli- Obduction continued on p. 219 ABCD E Figure 2. Core photographs illustrating stages in the tectonic-sedimentary evolution of the Eratosthenes Seamount and the adjacent Cyprus margin. A: Oncolites (algal balls) that accumulated on a shallow-water carbonate platform in Miocene time (Site 966, Core 17R, Section 1, 45–65 cm). B: Limestone (Miocene?) composed of various types of clasts, reflecting current reworking in a shal- low-water shelf setting (Site 966, Core 13R, Section 1, 1–21 cm). C: Debris flow composed of clasts of altered shallow-water limestone in a matrix of muddy nannofossil ooze of early Pliocene age (Site 966, Core 8H, Section 1, 1-23 cm). D: Clast-rich nannofossil ooze of Pliocene age showing slumping and color banding. The slumping is related to collapse of the Eratosthenes Seamount (Site 965, Core 3H, Section 3, 35–58 cm). E: Calcareous silty mud with detrital gypsum of Messinian(?) age, possibly deposited in a saline lake located at the foot of the Cyprus slope (Site 968, Core 25X, Section 2, 108–132 cm). Obduction continued from p. 213 This hypothesis could be tested by drilling a transect of relatively shallow holes (most <300 m) across the Africa- Eurasia plate boundary. One hole was Figure 3. Outline map of drilled on the northern part of the the Mediterranean showing seamount plateau (Site 966), one on the main tectonic features the upper northern slopes (Site 965), and the locations of the sites one on a small high at the base of the drilled during Leg 160. This article deals only with the northern slope of the seamount (Site transect of sites drilled 967), and one on the lower slope of the between Cyprus and the Cyprus margin to the north (Site 968; Eratosthenes Seamount Fig. 4, inset). (Sites 965, 966, 967, and In this article we summarize the 968). The line with teeth new information gained by drilling, marks the deformation front and we demonstrate how this can be between the African and used to test a model of collisional pro- Eurasian plates. cesses and ophiolite obduction that is of general importance. The details of the recovery at the individual sites are recorded elsewhere (Emeis, Robertson, Richter, et al., 1995). SUMMARY OF DRILLING RESULTS the Eratosthenes Seamount gave way to erosion and/or local accumulation of The nature of the basement of the gypsum and ferruginous muds in small Eratosthenes Seamount is unknown, marginal lagoons and/or lakes, as but it might represent transitional crust recorded at Sites 965 and 967 (Fig. 5). of a rifted passive margin. The presence By contrast, a much thicker (~150 m) of mafic igneous intrusions at depth is succession of inferred Messinian age suggested by the existence of a strong was deposited at Site 968 on the lower magnetic anomaly beneath the Cyprus slope. This was probably seamount and adjacent seafloor areas deposited in a large lake or inland sea, (Woodside, l977, l991). The oldest well below eustatic sea level. The exis- sediments recovered during Leg 160 are tence of Messinian lakes was previously undated shallow-water carbonates that inferred in the western Mediterranean underlie Upper Cretaceous pelagic Tyrrhenian Sea (Kastens, Mascle, et al., carbonates at Site 967 (Fig. 4). In the l990). pre–Upper Cretaceous, the Eratosthenes Distinctive matrix-supported brec- Seamount formed part of a carbonate cias occur between underlying shallow- platform, as widely exposed around the water carbonates and overlying nanno- eastern Mediterranean (e.g., in south- fossil oozes of early Pliocene age at Site ern Turkey and the Levant). Overlying 966. They formed by mainly mass-flow pelagic carbonates, of Upper Creta- processes during the early Pliocene ceous and middle Eocene age at Site (pre–4.5 Ma). The source of many of 967 and of exclusively middle Eocene the clasts was a shallow-water lime- age at Site 966, indicate accumulation stone, similar to the underlying succes- in a quiet deep-water (bathyal) setting. sion. Pliocene accumulation took place The absence of gravity input (e.g., tur- in a relatively deep-marine setting bidites) within the Upper Cretaceous (more than several hundred meters), and middle Eocene pelagic sediments based on microfossils in the matrix. is consistent with deposition on a Erosion of the limestones that are submerged platform or promontory, redeposited as clasts in the lower isolated from terrigenous input. The Pliocene might have been subaerial, Figure 4. Summary of the successions drilled on the Eratosthenes Seamount and lower slope of Eratosthenes Seamount area was proba- subaqueous, or both, but there is little the Cyprus margin. Lower left: Simplified bathymetry of the transect area. bly then still located well to the south evidence of clast rounding via sedimen- of areas of Tethys that experienced tary transport. A tectonic fabric present ophiolite emplacement and active mar- within several clasts could suggest gin deformation in the latest Creta- derivation from a faulted rock (e.g., ceous (Campanian-Maastrichtian; an erosional fault scarp). The presence ¸Sengör and Yilmaz, l981). of nannofossil mud clasts also indicates Middle Eocene pelagic carbonate that deep-marine sediments were accumulation was followed by shallow- reworked in an unstable slope setting. water carbonate deposition. The car- The Pliocene-Pleistocene succes- bonates accumulated in a near-reef, sions at each site are composed of nan- platform setting and are dated as early nofossil oozes interbedded with cal- Miocene at Site 967, on the basis of careous muds, numerous sapropels, benthic foraminifera (I. Premoli-Silva, and minor volcanic ash. The Eratos- 1995, personal commun.). Water thenes Seamount sites (Sites 965, 966, depths must have decreased by more and 967) are now at water depths rang- than several hundred meters between ing from 700 to 2900 m. These differ- the middle Eocene and the Miocene, ences in water depth are the result of which implies that surface tectonic differential subsidence of the seamount uplift of the Eratosthenes Seamount area. Much of this subsidence took must have taken place, in addition to place relatively rapidly. The early the effects of any eustatic sea-level Pliocene sediments accumulated in change. This uplift is not likely to have deep water, without evidence of a grad- been collision-induced, because the ual upward transition from shallow- subsequent Miocene shallow-water water conditions. Benthic foraminifers deposition took place under relatively indicate a further deepening after the tectonically stable conditions, but it late Pliocene, at least at Site 967. could instead reflect tectonic move- ments related to an earlier history of TECTONIC MODEL OF subduction.
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