Neogene to Recent Displacement and Contact of Sardinian and Tunisian Margins, Central Mediterranean

MAURICE G. GENNESSEAUX and DANIEL JEAN STANLEY

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Neogene to Recent Displacement and Contact of Sardinian and Tunisian Margins, Central Mediterranean

Maurice G Gennesseaux and Daniel Jean Stanley

SMITHSONIAN INSTITUTION PRESS City of Washington 1983 ABSTRACT Gennesseaux, Maurice G., and Daniel Jean Stanley. Neogene to Recent Displacement and Contact of Sardinian and Tunisian Margins, Central Mediterranean. Smithsonian Contributions to the Marine Sciences, number 23, 21 pages, 9 figures, 1983.—The seafloor between Sardinia, Tunisia, and Sicily occupies a key sector essential for understanding the geological evolution of the central Mediterranean. Although plate motion is generally considered as an explanation, this structurally complex region remains poorly defined. To interpret better the Neogene evolution, we prepared a detailed bathymetric chart and a map showing structural provinces and post-Miocene sediment patterns, which are constructed on the basis of seismic data (primarily a dense network of 30 KJ Sparker and 3.5 kHz profiles). The data suggest that the present-day configuration of the Tunisian and Sardinian margins results, in large part, from the contact of the southern part of the Corsican-Sardinian microplate with North Africa. Several dominant structural-stratigraphic trends are recognized in this study area: (1) NNW- SSE and NW-SE trends in the northwestern part of the study area are most likely related to the formation of the Algero-Balearic Basin since the late Oligocene. (2) Pronounced NNE-SSE trending structural axes (largely normal faults) are related to the near-parallel (N-S) tilted fault blocks in the Tyrrhenian Sea east of Sardinia. One of these tectonic structures on the margin east of Sardinia may possibly extend southward (190°-200°) onto, and across, the Tunisian margin. The largest, most obvious physiographic features south of Sardinia, including sea- mounts, ridges, and canyons, are associated with these trends. These features, for the most part of middle to upper Miocene age, are believed closely related to the opening and subsidence of the Tyrrhenian Sea. (3) Morphological, structural, and stratigraphic-sedimentary trends, particularly off Tunisia, suggest Pliocene-Quaternary compression (E-W trending tectonics and depositional axes), resulting from the northward movement of Africa. (4) Important NW-SE structural-depositional trends (many extensional, some strike-slip) of Miocene to Quaternary age dominate the Strait of Sicily area east of Tunisia and south of Sicily. These may be related to displacement along the Calabrian-Sicilian Arc and to a collisional regime between the arc, the Corsican-Sardinian block, and African margin. We believe that the present configuration of the two margins resulted from plate contact and welding during several major Miocene events and also from subsidence, first, of the Algero- Balearic Basin and, then, of the Tyrrhenian Sea. In theory, the Tunisian margin and adjacent land have been subjected to compression as a result of seafloor spreading and collision. The physiographic trends and subsurface structural-stratigraphic configuration we map, however, reveal a predominance of Neogene to Recent structures, primarily of extensional origin.

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Library of Congress Cataloging in Publication Data Gennesseaux, Maurice G. Neogene to recent displacement and contact of Sardinian and Tunisian margins, central Mediterranean. (Smithsonian contributions to the marine sciences ; no. 23) Bibliography: p. 1. Sea-floor spreading. 2. Geology—Mediterranean Sea. I. Stanley, Daniel J. II. Title. III. Series. QE511.7.G46 1983 551.8'7 83-16647 Contents

Page Introduction 1 Abbreviations 3 Acknowledgments 3 Geological Background 3 Seafloor Configuration of Study Area 5 Structural Trends 8 Stratigraphic-Sedimentary Patterns as Related to Tectonics 12 Tunisian Margin 15 Sardinian Margin 17 Teulada-Sardinia Valley Complex 17 Conclusions 17 Literature Cited 20

in

Neogene to Recent Displacement and Contact of Sardinian and Tunisian Margins, Central Mediterranean

Maurice G Gennesseaux and Daniel Jean Stanley

Introduction In this study, the configuration of the seafloor, major structural features, and Miocene to Recent The seafloor between Sardinia, Tunisia, and sediment series are detailed in the area between Sicily (Figure 1, inset) is physiographically one of 8°30'E and 11°15'E longitude and 37°15'N and the most complex regions in the central Mediter­ 39°00'N latitude. This is based on a close-grid 3.5 ranean. Most regional syntheses indicate that this kHz and 30,000 Joules Sparker survey by the sector occupies an area of convergence between USNS Kane in September 1975. About 2000 km Europe, including the Corsican-Sardinian micro- of continuous seismic profiles were collected along plate, and the northern part of the African plate 17 parallel transects, for the most part oriented that includes Tunisia, the Pelagian Sea to the NW-SE, between northern Tunisia and southern east, and southern Sicily. It should be pointed Sardinia (Figure 1). Navigational control was out, however, that the area between Tunisia and accomplished by satellite positioning, Loran C, Sardinia remains poorly defined geologically. On and radar in nearshore sectors; transects were the basis of earlier work on land and at sea, we made at a ship speed of about 10 knots. The would expect this Sardinian-Tunisian region to study also incorporates earlier soundings and record the effects of the welding of two or more nautical charts, and available published (Au­ plate boundaries and of their subsequent tectonic zende, 1969, 1971) and several unpublished seis­ deformation in the Neogene and Quaternary. In mic transects. A revised detailed chart of this theory, it would be expected that structural trends region, based on these data, is presented. and sedimentary patterns, as well as physiogra­ The purpose of this study is to focus on and phy, should display evidence of such plate contact interpret the major physiographic, structural, and and geologically recent (some Oligocene, and depositional traits mapped on the seafloor and in primarily Miocene to Quaternary) displacement the subbottom. Moreover, this information may (Stanley, 1977). help unravel the series of events that led, in the Neogene, to the formation of the Algero-Balearic Maurice G. Gennesseaux, Groupe d'Etude de la Marge Continentale, Basin to the west, the Tyrrhenian Sea immedi­ ERA 605, Department de Geologie Dynamique, Universite Pierre et ately to the northeast, and the Strait of Sicily- Marie Curie, 75230 Paris, France. Daniel Jean Stanley, Division of Pelagian Sea to the east and southeast of the Sedimentology, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560. study area. SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES

FIGURE 1.—Chart showing location of 3.5 kHZ and 30 KJ Sparker transects (SI to SI7) in the region between Sardinia, Tunisia, and Sicily (inset) obtained on the September 1975 cruise of the USNS Kane. Selected portions of Sparker transects are shown in Figures 5 and 6. Teulada Valley (TV), Sardinia Valley (SAV), and Sentinelle Valley (SV) are shown as reference points. In inset: ABB = Algero-Balearic Basin; LS = Ligurian Sea; TYS = Tyrrhenian Sea; IO = Ionian Sea. Depth in meters. NUMBER 23

ABBREVIATIONS.—The following abbreviations Prof. C. Morelli made available bathymetric plot- are used in the text and illustrations: ting-sheet notations. The manuscript was re­ ABB Algero-Balearic Basin viewed by A. Fabbri, I. Finetti, and other col­ BA Balearic Islands leagues who provided useful critique. This study CA Calabrian Arc was funded by the Mediterranean Basin (MEDIBA) CC Carbonara Canyon Project (Smithsonian Scholarly Studies grants CG Campidano Graben 1233S201 and 305), and by the Groups d'Etude CO Corsica cv Carbonara Valley de la Marge Continentale, Universite Pierre et EB Estafette Bank Marie Curie, Paris, France. This is Contribution GB Galite Bank and Island Number 194 of the French Centre National de la GC Gulf of Cagliari Recherche Scientifique, ERA 605. GK Grande Kabylie IO Ionian Sea IS Ichnusa Seamount Geological Background LM Lower Miocene LS Ligurian Sea The transformation from the Tethys Ocean to M Miocene the much restricted Mediterranean Sea as we MS Messinian know it today has resulted primarily in response N Sedimentary nappe series to the convergence of Africa and Europe. Inter­ P Pliocene pretations of this geological transformation have PA probable Paleozoic basement PG Pantelleria Graben been summarized in numerous studies. Biju-Du­ PK Petite Kabylie val et al. (1977), for example, have shown, by PQ Pliocene-Quaternary sediment series means of a sequence of time-lapse reconstructions, Q Quaternary the initiation and evolution of basins underlain RB Reagui Bank by oceanic crust (Algero-Provencal Basin, Ligur­ SA Sardinia ian Sea, and Tyrrhenian Sea) and the marked SAB Sardinia Basin SAV Sardinia Valley changes with time in the configuration of the SB Sentinelle Bank contiguous land mass. Creation of oceanic crust SI Sicily related to the displacement of microplates has SKB Skerki Bank occurred since the Oligocene (Le Pichon et al., SS Sardinia Shelf 1971) and, in certain areas, such as the Tyrrhen­ SSL Sardinian Slope ian Sea and Strait of Sicily, structural activity STS Strait of Sicily sv Sentinelle Valley (including crustal motion and volcanism) has TP Tunisian Plateau continued to the present. Moreover, regional geo­ TR probable salt diapir logical surveys indicate that our study area in the TS Tunisian Shelf central Mediterranean has experienced major Tunisian Slope TSL change since the Mesozoic. TU Tunisia TV Teulada Valley The highly simplified structural map of the TYS Tyrrhenian Sea western and central Mediterranean (Figure 2), a UM Upper Miocene summary of many investigations, shows the inter­ UO Upper Oligocene relationship among rift-opening of basins, for­ ACKNOWLEDGMENTS—We thank the U.S. Navy mation of oceanic crust, and development of ma­ (NAVOCEANO) for generously providing USNS jor subduction-collision zones. As noted on the Kane September 1975 cruise seismic data, includ­ chart, basins probably did not form contempor­ ing 3.5 kHz and Sparker profiles; we also express aneously. Rather, opening and deepening of the our appreciation to the Compagnie Francaise des basins have occurred during several stages, largely Petroles and ETAP-Tunis for releasing selected after the Oligocene. The major initial rifting seismic transects (including Figure 8, this study). phases of the Liguro-Provencal sector may be of SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES

FIGURE 2.—Geological map, a synthesis from many published studies, showing in simplified fashion the evolution of the western and central Mediterranean since the Oligocene. BA = Balearic Islands; CA = Calabrian Arc; CO = Corsica; GK = Grande Kabylie (GK and PK probably once formed part of an island arc); LM = lower Miocene; M = Miocene; M-Q = Miocene to Quaternary; P = Pliocene; PG = Pantelleria Graben; PK = Petite Kabylie; Q = Quaternary; SA = Sardina; SI = Sicily; UM = upper Miocene; UO = upper Oligocene. Dashed-line pattern indicates area underlain by oceanic crust; lines with triangles indicate normal faults and axes of compression. upper Oligocene to lower Miocene age (UO- cifically discuss the study area between Sardinia LM) as indicated by Auzende et al. (1974) and and Tunisia are provided by Auzende et al. Rehault (1981); this involved the counter-clock­ (1974), and Boccaletti and Manetti (1978). These wise rotation of the Corsican-Sardinian block (for studies, coupled with detailed mapping in north­ an updated summary, see Rehault, 1981). Rifting ern Tunisia and its margin (Auzende, 1971; of the Algero-Balearic Basin to the southwest is Caire, 1973; Rouvier, 1977), Sardinia and its probably somewhat younger, i.e., the major rift­ margin (Cocozza et al., 1974; Fanucci et al., ing phase ended in the middle-upper Miocene 1976), and Sicily and its margin (Wezel, 1974; (shown as M in Figure 2). The Tyrrhenian Sea is Grandjacquet and Mascle, 1978) have revealed the youngest basin in the western Mediterranean, the close relationship between structural trends that is, upper Miocene to Pliocene (UM-P; cf. on land and those immediately offshore. More­ Malinverno et al., 1981); a major subsidence over, the complexity and diversity of structural phase is documented during the Pliocene (cf. Selli trends in offshore sectors more distal from Tuni­ and Fabbri, 1971). Attention has been called to sia, Sardinia, and Sicily land masses also have the more recent development, largely by exten­ been shown. We show in Figure 2 that major sion (Pliocene and Quaternary, P-Q), in the structural features include N-S, NE-SW, and Strait of Sicily region (cf. Finetti and Morelli, NW-SE trends, probably of different ages, and 1972). that these axes appear to converge in the study Generalized structural syntheses that more spe­ area. NUMBER 23

To interpet these features better, the following on the outer plateau (Figure 5); it widens and three sections will focus on physiography, struc­ becomes U-shaped northward, where its axis ex­ tural trends, and dominant stratigraphic-sedi- tends to depths in excess of 2500 m on the south­ mentary patterns in this zone between the Tyr­ west Tyrrhenian Sea margin. Numerous small rhenian Sea, Algero-Balearic Basin, and Strait of depressions of subrounded, elongate or irregular Sicily-Pelagian Sea. shape (from 1 to over 2 km in length, and with a relief of less than 100 m) are mapped on this plateau; these depressions (in black) and possibly Seafloor Configuration of Study Area related paleo-drainage systems are shown in Fig­ A revised and more detailed chart (Figure 3) ure 4. has been compiled on the basis of 3.5 kHz profiles 3. The Tunisian Slope (TSL) forms the north­ taken by the USNS Kane and older bathymetric ern limit of the Tunisian borderland. The slope data (C. Morelli, unpublished sounding data (ranging from about 2° to 7°) dips northwest­ sheets, University of Trieste), the French research wardly, toward a large median valley (see 4 be­ vessel Jean Charcot (cruises 1970-1972, 1978), Ital­ low) whose axis is deeper than 2000 m. The ian CNR Bannock (cruise 1975), data in Gennes­ predominant strike of this slope is NE-SW. The seaux and Vanney (1979), and soundings on pub­ slope is markedly offset at about 10°E longitude, lished French, Italian (Morelli, 1970), and U.S. i.e., where the Tunisian Slope is incised by the (Carter et al., 1972) nautical charts of the Tuni­ large Sentinelle Valley. sian and Sardinian regions. The chart, prepared 4. The most distinctive negative topographic at a scale of 1:250,000, is contoured at 100 meter relief feature in the study area is the deep, arcuate intervals (corrected depth values after Mathews, median valley that separates the Tunisian from 1939). The major trends shown on our detailed the Sardinian physiographic provinces. Its flat chart also appear on the map recently published bed is about 9 km wide at its narrowest and by the UNESCO Intergovernmental Oceano­ shallowest point (about 1950 m). This median graphic Commission (1981, sheets 2, 7, and 8). valley is formed by two submarine canyons. The A series of physiographic features and morpho­ western canyon is called the Teulada Valley logical provinces are recognized from south to (TV); the northeastern one is the Sardinia Valley north and these are identified on the simplified (SAV) (cf. Morelli et al., 1975). The Teulada chart (Figure 4). Many of the geographic names Valley is generally V-shaped (Figure 6) and, from adopted here are those shown on the chart pub­ its shallowest point, trends westward toward the lished by Morelli et al. (1975, pi. 18). floor of the Algero-Balearic Basin (2600 m) where 1. The E-W trending Tunisian Shelf, north of it merges into a low, gentle deep-sea fan and the Cap Blanc and off Bizerte, dips gently seaward contiguous basin plain. From its shallowest point for a distance of about 40 km from shore and to (at about 9°20'E longitude), the Sardinia Valley a depth of approximately 200 m (Figure 3). continues towards the NE and NNE (also to a 2. The Tunisian Shelf is bordered seaward by depth of about 2500 m). On the lower Tyrrhenian a broad ENE-WSW borderland (to 100 km in margin, the valley merges on the rise without width) that consists of a low relief surface ranging very pronounced fan development. The NE- in depth from about 200 to 500 m. This prov­ trending Sardinia Valley branch is generally ince, termed Tunisian Plateau, comprises Galite more U-shaped than the Teulada Valley. More­ Island and bank (GB), Skerki Bank (SKB), Sen­ over the axial trend of the former is highly irreg­ tinelle Bank (SB), and Reagui Bank (RB); the ular and displays several sharp bends, presumably plateau is also cut by a large SSW to NNE the result of offset by recent faulting. The Sardi­ trending submarine valley (herein named Senti­ nia Valley is separated from the Sentinelle Valley nelle Valley, SV). This distinct valley is V-shaped in the study area proper (Figure 4); to the north­ and deeply incised (relief to as much as 600 m) east, however, the two valleys appear to merge SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES

FIGURE 3.—Physiographic chart of study area, based on 3.5 kHZ data collected by the USNS Kane (see track lines SI to SI7 in Figure 1) and earlier soundings (p. 5), contoured at 100 m intervals. Specific morphological features and physiographic provinces are identified in Figure 4. beyond the base of the slope in the SW Tyrrhen­ valleys. West of 9°30'E longitude, these are ori­ ian Sea basin. ented NNW-SSE. A very large valley, the Car­ 5. The Sardinia Slope (SSL) (2° to 4°) dips bonara Canyon (CC), extends from the Gulf of southward to the median valley described above. Cagliari (GC) to the SE (to a depth of about 1000 This province is about 50 km wide. The Ichnusa m), then veers to the SW, paralleling (and Seamount (IS), a large NE-SW trending high bounded by) the Ichnusa Seamount, and then feature on the slope, is about 75 km long, 18 km bends south where it merges with the Teulada wide, and has a relief of about 1800 m. The Valley near its shallowest point, at about 2000 m. Sardinian Slope, in contrast to the Tunisian 6. The Sardinia Shelf (SS) is wide (about 25 Slope, is highly dissected by numerous submarine km) south of Cape Teulada, but narrows mark- NUMBER 23

50

~S?f\f\ >.o- FIGURE 4.—Simplified chart (500 m intervals; modified from Figure 3) showing physiographic features discussed in text, including small depressions (in black), possible associated paleo- drainage pattern (dashed lines and open arrows); and major valley axes (solid arrows). CC = Carbonara Canyon; CG = Campidano Graben; EB = Estafette Bank; GB = Galite Bank and Island; GC = Gulf of Cagliari; IS = Ichnusa Seamount; RB = Reagui Bank; SA = Sardinia SAB = Sardinia Basin; SAV = Sardinia Valley; SB = Sentinelle Bank; SKB = Skerki Bank SS = Sardinia Shelf; SSL = Sardinian Slope; STS = Strait of Sicily; SV = Sentinelle Valley TP = Tunisian Plateau; TS = Tunisian Shelf; TSL = Tunisian Slope; TU = Tunisia; TV = Teulada Valley; TYS = Tyrrhenian Sea. edly in the eastern Gulf of Cagliari, near Cape of the shelf is clearly related to the wide Campi­ Carbonara. The sharp, angular (E —» NE —> SE dano (Graben) Valley (GG) in southern Sardinia —» NNE) configuration of the shelf edge parallels (shown on geological map in Cocozza et al., the southeastern Sardinian coastline. The shape 1974), and also to the Ichnusa Seamount. 8 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES

In summary, the new detailed chart highlights revealing faults are shown in Figures 5 to 8. In the physiographic complexity of both Tunisian Figure 9 we identify and interpret the following and Sardinian margins that are in juxtaposition. structures, proceeding from older to younger The sharply defined (angular, youthful) nature trends. of major submarine features is revealed by the 1. Probably the oldest structure constitutes a contours, and many of these features are directly series of N-S trends (some NNW-SSE) on the related with land forms on Sardinia and Tunisia. slope south of Sardinia and extends onto the Moreover, seismic profiles reveal that the youth­ Sardinia Shelf (observed in air-gun profiles col­ ful appearance of the seafloor surface is a function lected on the Bannock 1974 cruise (I. Finetti and of the plexus of geologically recent structures C. Morelli, pers. comm., 1981) and onto Sardinia mapped in this region. itself (Cherchi and Montadert, 1982). These structures, resulting from the rifting phase, could be as old as Upper Oligocene. The local NNW- Structural Trends SSE and NW-SE orientation of canyon axes may The configuration of the. region between Sar­ possibly be the result of subsequent displacement dinia and Tunisia, as highlighted in the previous of the margin and associated sedimentary pro­ section, could suggest a zone of contact resulting cesses. To the east, the much larger reentry form­ from the collision of plate boundaries. Geophysi­ ing the Gulf of Cagliari is directly associated cal studies indicate the presence of continental with the NW-SE Campidano Graben, the dom­ crust in this region, which now separates the inant structure in southern Sardinia. The Gulf of Algero-Balearic Basin from the Tyrrhenian Sea Cagliari and Campidano structure are super­ (Figure 2); to date, there is no evidence indicating posed on faults that are originally of Oligocene the presence of oceanic crust. Preliminary inves­ age, but along which vertical displacement has tigations also have attempted to reconstruct drift continued, particularly during the Pliocene and direction and define the boundary between mi- Quaternary (Coccoza and Jacobacci, 1975; Fan- croplates (cf. Auzende, 1971; Auzende et al. ucci et al., 1976). This land-to-offshore structural 1974). In these latter syntheses the predominant complex is associated with the major initial rifting drift direction was shown to extend, in gently phase that preceded and was associated with the arcuate fashion, along a NE-SW (N40°E) trend opening of the Ligurian-Provencal Basin in the for about 270-300 km between the eastern Alge­ northern and eastern parts of the western Medi­ rian margin (near Cap Rose) and the SE part of terranean. Studies in Sardinia (Cocozza et al., the Sardinian margin (off Cape Carbonara). Sub­ 1974) and of our offshore seismic transects reveal sequently, geophysical (gravimetric, magnetic) no clear evidence after this rifting phase of com- surveys (Morelli et al., 1975) have shown this pressional movement (in the Miocene and Re­ NE-SW trend is not, as previously postulated, cent) related to the counter-clockwise rotation of characterized by an alignment of extrusive vol­ the Corsican-Sardinian Block and its supposed canic bodies. Moreover, dredging indicates that contact with the African plate (Boccaletti and the Ichnusa Seamount comprises Paleozoic lith- Guazzone, 1974a). We recall, however, that in­ ologies comparable to the Sardinian basement dication of compressive motion is difficult at best (cf. Colantoni et al., 1981) locally covered by to demonstrate, even on multichannel records, evaporitic limestone (Wezel et al., 1977), and is and thus we cannot exclude some thrust motion. not a high relief feature formed by Tertiary ex­ 2. The broad, NE-SW trending Tunisian Pla­ trusive volcanism (Morelli, 1970). teau comprises a series of parallel and subparallel Our data tend to support these latter conclu­ structural axes, possibly including reversed faults, sions and brings to light a complex network of which likely are an extension of similarly oriented diverse tectonic trends, some superposed, which NE-SW structures in northern Tunisia and Al­ have evolved in space and time. Selected profiles geria. A large portion of the Plateau comprises a NUMBER 23

series of dislocated, imbricated series (Auzende, 3. The gently arcuate trend of the major me­ 1971): on land these are covered by the Numidian dian depression (E-W Teulada Valley, and Nappe that includes sediments of Oligocene to N50°E oriented Sardinia Valley) is well defined lower Miocene age (Rouvier, 1977). This inter­ on physiographic charts (Figures 3, 4). This fea­ pretation is based largely on surface morphology ture (illustrated in seismic profiles, Figures 5, 6) and shallow penetration seismic records. Clearly, appears to have originated primarily as a result better quality seismic coverage is required to of extension rather than erosion. Deposits inter­ accurately define deep structures and identify the preted as Messinian evaporites in the upper presence of nappes. Well-cemented series, partic­ reaches of Sardinia Valley, and non-evaporite ularly sandstones and limestones, form positive sediment series of possible upper Miocene age relief features prominent on the Plateau (Senti­ exposed along both valleys, suggest that this me­ nelle, Skerki, and other banks; cf. Dangeard, dian depression formed during the upper Mio­ 1928). This region does not comprise extrusive cene. This tensional phase, which occurred at volcanics or igneous basement series, except those about or shortly after the end of the collision- mapped on Galite Island. The Tunisian Slope compressive events to the south, on the Tunisian forming the external limit of this geological region margin, is probably related to large-scale subsid­ is oriented N50°E, and is markedly offset at about ence of both Tyrrhenian and Algero-Balearic ba­ 9°E and also at 10°E longitude. This NE-SW sins in the Miocene and Pliocene. The steep valley trending tectonic province, forming a large part walls (Figures 5, 6) are interpreted as step-fault of the Tunisian Plateau, is believed to have de­ surfaces, along which movement has continued veloped at the end of the major compressional during the Pliocene and Quaternary, primarily phase when the northern African plate came into by extension. Shallow, high-resolution 3.5 kHz contact with a European island arc. This island records suggest that motion has continued to the arc, separated from Europe, was formed by crys­ present. talline basement which extended northeastward 4. Structural trends oriented NNE-SSW on from the Petite (PK) and Grande Kabylie (GK) the Sardinian margin control large features, such (Figure 2). It is conceivable that the granitic as the Carbonara Valley (oriented about N30°E) substrate of the Galite Bank on, and the slope at and the Ichnusa Seamount that borders it; the outer edge of, the western part of the Tunisian roughly parallel to these is the Sentinelle Valley Plateau may represent another remnant of this on the Tunisian Plateau (N20°E). The trend of arc. As a result of convergence, the Numidian both valleys extends to the north by the N-S Nappe was dislocated and moved in a southward strike of slopes off SE Sardinia (one slope lies at direction. The end of the major collision phase about 1000 m, and the other, parallel, at about has been dated as Tortonian by Rouvier (1977). 2000 m). These eastern Sardinia slopes, recording We note that during lower Miocene time, sed­ major faults related to the subsidence of the iments deposited in an elongate N-S trending Tyrrhenian Sea, were very active in Plio-Quater­ basin on the slope off SE Sardinia (Sardinia nary time (Bacini Sedimentari, 1977). By anal­ Basin) were deformed by gentle, short-term ogy, we attribute a comparable age to the major compression before Tyrrhenian distensive phases formation of Carbonara (Fanucci et al., 1976) as shown by seismic reflection profiles (unpub­ and Sentinelle valleys, and relate these features lished, Compagnie Francaise des Petroles, Paris). to vertical movements along the Tyrrhenian mar­ This is the only evidence, albeit indirect, of post- gin. The opening of the Sentinelle Valley and, Oligocene compression recorded on the Sardinian more particularly, the northward change in di­ margin. This convergence-compressional trend rection of its axis (from NNE to NE) may be off Sardinia is poorly defined. It is almost cer­ related to the contemporaneous evolution of the tainly older than the above-cited major compres­ North Sicilian margin, also of Plio-Quaternary sional phase on the plateau off Tunisia. age. The dominant controlling factor is the sub- 10 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES

FIGURE 5 (left) .—Selected Sparker profiles, ori­ ented NW-SE (position shown in the inset; see also Figures 1 and 3). Sentinelle Valley (SV) and Sardinia Valley (SAV) are aligned. Tran­ sects (see Figure 4) cross Ichnusa Seamount, Carbonara Valley, and Sardinia Slope. Arrow a = Messinian evaporites underlying recent sed­ iments; arrow b = crystalline basement. Note thick sediment fill at head of Sentinelle Valley 500J (profiles 11 and 7). Depths of Sentinelle Valley Wat. axis in meters; Sed. = sediment thickness; Wat. = water column. "A

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FIGURE 6 (right).—Selected Sparker profiles (12- 17) across Teulada Valley (TV), oriented NW- SE (shown in the inset; see also Figures 1 and 3). Transects cross the Tunisian Plateau, Tuni­ sian Shelf, and Sardinian Slope (see Figure 4). The incised part of the thick sediments in the Teulada Valley are aligned in the different profiles. Arrow shows crystalline basement out­ cropping on valley slope. Depths of Teulada Valley axis in meters; Sed. = sediment thick­ ness; Wat. = water column. NUMBER 23 11

NW SE

10 ZO Km

500m Sed. 500 Wat 12 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES sidence on the Tyrrhenian Basin (cf. Selli and on land, where it is dated primarily as lower Fabbri, 1971; Bacini Sedimentari, 1977). Quaternary (Rouvier, 1977). This compression, 5. Large NW-SE trending features in the recording the continued motion of Africa north­ study area include the Gulf of Cagliari and Cam­ ward (Auzende et al., 1972; Purcaru and Berck- pidano Valley south of Sardinia. These are of hemer, 1982), is superposed on the major Miocene extensional origin, probably the direct result of deformation cited above. Displacement of the movement during the Miocene to Recent (accel­ type leading to compression of structures in north­ erated activity during the Plio-Quaternary), with ern Tunisia and the adjacent margin is probably subsidence taking place along reactivated Oligo­ comparable to that presently affecting Algeria; cene fractures on the Sardinia margin and adja­ evidence for the latter includes powerful earth­ cent Sardinian land mass. This fault-controlled quakes, such as the 1954 Orleansville and the topography is particularly apparent along the 1980 El Asnam events (Lepvrier, 1981; King, southwestern flank off Cape Carbonara. This 1981). trend is blocked to the SE by the Ichnusa Sea­ The E-W axes are somewhat oblique to those mount. produced by the NE-SW Miocene trend and To the south, in the Strait of Sicily, similarly overthrust phases cited earlier (see section 2 oriented NW-SE trends are mapped, but these above). , Compressive movement along both are of different origin and form topographically trends during the Quarternary may have pro­ less obvious features than off Sardina. The NE- duced most of the small depressions mapped on SW (Numidian Nappe) structure off Tunisia the Tunisian Plateau (Figure 4). It is noted, how­ (pattern 2, Figure 9) is interrupted by a series of ever, that the Tunisian Plateau most likely sub­ NW-SE normal faults, which are a northwest sided as a result of post-Miocene lowering of the extension of the Pantelleria Graben and associ­ Tyrrhenian Basin and extensional events in the ated tensional features (normal faults, volcanoes). Strait of Sicily. These extensional events also are This normal faulting of upper Miocene Plio-Qua­ related to the displacement of the Sicily-Cala- ternary age in the Strait of Sicily (Maldonado brian Arc (Dubois, 1976). It is also possible, how­ and Stanley, 1976) and Pelagian Sea (Blanpied, ever, that the small depressions on the Tunisian 1978) is related to geologically recent movement Plateau are related to fluvial systems, or karst in the Calabrian-Sicilian Arc (Dewey et al. 1973; development, that prevailed on this surface dur­ Boccaletti and Guazzone, 1974b; Dubois, 1976). ing Messinian dessication events. Thus, we view A NW-SE (and NNW-SSE) fault system (often surface relief on the Tunisian borderland as the dextral) is also well developed on the northern consequence of a complex structural evolution, Sicilian margin to about 11°E; this system may modified by the effects of major sea level oscilla­ dissect part of the Maghrebian chain in the study tions. area (A. Fabbri, 1983, personal communication). We note that the northwestern geographic limit Stratigraphic-Sedimentary Patterns as Related of this NW-SE trend is not at all obvious on the to Tectonics chart; subtle surface expression on the seafloor is, in part, due to masking by the sedimentary The previous sections attempt to correlate the cover. present seafloor configuration with the sequence 6. The E-W and NE-SW structural trends on of tectonic events that have modified this region. the Tunisian Shelf and inner Tunisian Plateau, The effects of structural deformation, so clearly apparent in seismic profiles as folds (synclines and recorded by the relief features and sedimentary anticlines), have resulted from the northerly mo­ sequences, are not all of the same age. For ex­ tion of Africa with respect to Europe, particularly ample, the physiography suggests that vertical important in the Miocene (Cohen et al., 1980). displacement during the Quaternary has been This same compressional trend has been mapped more active on the Sardinian (Campidano sector) NUMBER 23 13 than on the Tunisian margin; this is due to the and Shelf (Figure 9, pattern 6). This consists of proximity of the former to fault movement asso­ allochthonous sedimentary rock series of Tertiary ciated with the origin of the southwestern Tyr­ age (including Oligocene to lower Miocene for­ rhenian margin. Evidence of such neotectonic mations) emplaced and offset during the Miocene events is recorded by high-resolution Sparker (Figure 8A,B); these series have been displaced (Figures 5, 6) and 3.5 kHz profiles, and also by toward the southeast. Moreover, the physiogra­ deeper penetration acoustic records (Figures 7, 8). phy and examination of unpublished proprietary At least two types of acoustic basement are seismic records (CFP, ETAP, and others) indicate identified on the basis of sampling on land and that the amount of deformation affecting these dredging on the seafloor, and from available seis­ series decreases toward the northeast (i.e., on the mic profiles. The first consists of Paleozoic crys­ Plateau north of Skerki Bank). talline rocks (igneous and metamorphic, Figure The autochthonous sedimentary cover, consist­ 7) of the type that crops out in southern Sardinia; ing of near-parallel acoustic reflectors on seismic these rocks underlie large areas of the Sardinian profiles, is highly variable in thickness throughout margin. A substantial part of the Ichnusa Sea­ the study area (Figures 5-8). The high vertical mount, for example, is believed to be formed by exaggeration (to X 40) of Sparker profiles (Figures this type of Paleozoic basement (Colantoni et al., 5, 6) distorts acoustic reflectors. The limited qual­ 1981). Crystalline basement also underlies much ity of some records is such that it is not possible of the Tunisian Slope and parts of the Tunisian to precisely map the unconsolidated (post-nappe Plateau, particularly in the western part of the emplacement) sedimentary thicknesses and ac­ study area (example: Galite Bank). These lithol- curately plot isopach charts. On the basis of ogical series may correlate with rock formations seismic profiling data, however, we can identify cropping out in the Kabylie coastal range of areas covered by about or greater than 300 m of Algeria (Figure 2). Other areas where crystalline sediment and those covered by less than 300 m basement probably occur are shown in Figure 9. (Figure 9). Locally, such as off northeast Tunisia A second type of acoustic basement underlies, (Cap Blanc), this sedimentary cover exceeds 1500 and locally crops out on, the Tunisian Plateau m in some tectonic depressions (Figure 8). The

FIGURE 7.—Deeper-penetration Sparker profile: oriented NW-SE, and interpreted record, crossing Sardinian Shelf (SS) and slope (SSL), Carbonara Valley (CV), and Ichnusa Seamount (IS). PA = probable Paleozoic basement; M = Miocene (possibly upper Miocene) sediment series; MS = Messinian surface; PQ = Pliocene-Quaternary sediment series. SAV = Sardinia Valley; SA = Sardinia; SI = Sicily. Horizontal scale in km; vertical scale in seconds. 14 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES NUMBER 23 15 relative thicknesses of these deposits, which pre­ In other parts of the Tunisian Plateau and on sumably range in age from Miocene to Quater­ the slope (both affected by post-Miocene subsid­ nary, are not correlatable with seafloor depth, ence), the Plio-Quaternary cover generally ex­ that is, thicker sediment sequences do not neces­ ceeds 100 m. Generally, there is a reduced depo­ sarily occur in deeper or low-lying areas. In many sitional cover on high relief features (Figure 6: parts of the study area, however, there is a mod­ profiles 13-15) and on the slope, where the failure erate to good correlation between sediment thick­ and downslope redeposition of sediment exposes ness and the dominant morphostructural trends. older rock units. The borderland has undergone Furthermore, there is good evidence of an early somewhat less movement than the Tunisian Shelf Quaternary to Recent structural overprint on and inner plateau, presumably affected by depositional thickness and deformation of the compression. Indirect evidence of the minor sub­ upper sedimentary series throughout the study sidence affecting the plateau is indicated by the area (Figure 5: profiles 7 and 11, Figure 8A). small depressions (Figure 4), some of which also may have resulted from gentle synclinal defor­ mation and fault-displacement of the substrate, TUNISIAN MARGIN and/or dissolution of underlying Triassic salt On the Tunisian Shelf and adjacent inner pla­ piercement features. The thin sediment fill in teau, a series of E-W and NE-SW depositional these depressions is due partly to the structural trends are apparent. As noted on some NW-SE high of this region and to some of the depressions oriented profiles in the Gulf of Tunis (Figure 8B), that are of geologically recent tectonic origin (see thick wedges (>3 seconds, or to about 4000 m) of also p. 9). upper Miocene to recent sediment has filled sub­ The main supply of sediments on the sub­ siding depressions which, locally are presently merged Tunisian margin is land-derived, primar­ undergoing compression. These are comparable ily from rivers (oueds such as the Majardah) in to thick basin-filled synclines mapped by Rouvier northern Tunisia. These rivers transported much (1977) on the adjacent Tunisian land mass. more material seaward during wetter climatic In the vicinity of Skerki Bank (east of 11°E phases and eustatic lowstands in the Quaternary long; 38°N lat.; Figure 5: profiles 7 and 11), the than at present. These fluvial sediments, and sector west of Sicily, and in the Strait of Sicily those derived from coastal and sea-floor erosion, proper, there is also a thick (locally >500 m) have been displaced and redeposited by the In­ accumulation of Plio-Quaternary deposits. The termediate Water mass, which is presently flow­ depocenters appear structurally controlled by the ing northwestwardly through the Strait of Sicily network of presently still-active NW-SE trending and then diverges westward over the Plateau and normal faults. northward (toward the Tyrrhenian Sea) in the study area (Wiist, 1961; Miller, 1972). Moreover, FIGURE 8.—Selected deep-penetration air-gun profiles on the the less saline surface Atlantic Water mass, flow­ Tunisian margin east of Cap Blanc, A, NE-SW oriented ing generally eastwardly (Lacombe and Tcher- transect across the Tunisian Shelf north of Cap Bon, showing nia, 1972), also transports sediment into this re­ post-Miocene tectonic displacement of the basement (sedi­ mentary nappe series, N) which crops out at surface; reflec­ gion. The southern (headward) sector of the Sen­ tors interpreted as Messinian (MS?) deposits and overlying tinelle Valley has trapped large volumes of sedi­ prograding and deformed Pliocene-Quaternary (PQ) sedi­ ment (Figure 5: profiles 11, 7, 8), in part from ments also are identified. TR? = probable salt diapir. B, water masses flowing across the inner Tunisian NW-SE transect in the Gulf of Tunis region showing a thick Plateau. Sediments in this region have been (>4000 m) accumulation of Neogene sediments. Movement trapped in structurally deformed depressions; this of evaporite diapir (TR) appears to have stopped by the end of the Pleistocene. Abbreviations as in A; horizontal scale in tectonic effect has precluded the transport of km; vertical scale in seconds. sediment along the Sentinelle Valley axes further 16 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES -%r^-~

FIGURE 9.—Major structural trends and sedimentary patterns of study area. 1 = Contours of present seafloor surface, in meters. 2 = Overthrust symbols depicting recent direction of movement of Africa toward the north, resulting in compression off Tunisia. 3 = Low surface relief features, indicating structural axes, probably result from lower Miocene compression. 4 = Effect of extension as recorded by NW-SE grabens in and near the Strait of Sicily, by normal faults, oriented NNE-SSW, on the Sardinian margin and also forming the Sentinelle Valley (SV), by normal faults (step-faults) on the Tunisian and Sardinian slopes, and by the Campidano Graben. 5 = The trend of the Teulada and Sardinia Valley axes, denoting the major boundary at the present seafloor surface between the Tunisian and Sardinian margins. 6 = Acoustic basement, which includes Paleozoic crystalline rocks sampled on the island of Sardinia and, locally, on the two margins, and allochthonous (Nappe) units. 7 = Pliocene- Quaternary sediment series are shown as thin (<300 m; in white) or thick (>300 m; stippling pattern) accumulations. Thick, well-layered acoustic series consisting largely of mud turbidites, prevail in both the Tyrrhenian and Algero-Balearic basins. NUMBER 23 17

to the north across the Plateau (Figure 5: profile and Sardinian margins. Reflectors on the high- 5). resolution 3.5 kHz profiles suggest that a large portion of these deposits are of gravitative (tur­ SARDINIAN MARGIN bidite, slump) origin. We recall that in the Sar­ dinia Valley, sediments as old as Messinian, and In contrast to the Tunisian Plateau, the Sar­ some perhaps older, are observed (Figure 5: re­ dinian margin is characterized by a generally spectively arrow a in profile 6 and arrows b in thicker (>500 m) series of sediment that covers a profiles 6 and 8). substantially larger surface area (Figure 9). The It is of note that the sediment fill in the Teulada most extensive amount of sediment is mapped on Valley (Figure 6) is incised. This V-shaped cut is the slope south of Sardinia and is particularly at least 1 km wide, and has a relief of at least 100 important in the Gulf of Cagliari-Carbonara to 300 m (Figure 6). We suggest here that the Canyon complex (Fanucci et al., 1976). This thick origin of this feature is in part due to bottom series is related to the presence of the Ichnusa scour by gravity transport, such as turbidity cur­ Seamount, which serves as a tectonic dam behind rents (cf. Wezel et al., 1979). which sediments have been trapped. Tectonic It is also possible that the incision in the Teu­ damming resulting in thick sediment accumula­ lada Valley axis records the combined effects of tion also occurs in the Sardinia Basin southeast synsedimentary tectonic displacement (normal of Sardinia (Bacini Sedimentari, 1977); moreover, faulting related to a recent tensional phase) and this phenomenon of sediment entrapment in slope erosion by bottom currents. The currents may basins has been recorded along many sectors of result from a flow of Deep Mediterranean Water the Tyrrhenian margin (Selli, 1974). by way of the median valley, from the Tyrrhenian The generally thick sedimentary cover on the Basin westward toward the Algero-Balearic Ba­ Sardinian margin records the direct dispersal sin. It is also possible that scour was accelerated from the southern Sardinia land mass onto and during lowered eustatic sea-level stands in the across the smaller surface area of the contiguous late Pleistocene. The nature of this flow remains Sardinia margin. Furthermore, the north-to- poorly defined (Lacombe and Tchernia, 1972). south flow of the Intermediate Water mass along These latter hypotheses need to be tested. the eastern margin of Sardinia, and then to the A similar V-shaped incision is observed in the west (out of the Tyrrhenian Sea and off southern Carbonara Valley. This submarine canyon has Sardinia as indicated by Allen et al., 1972, and served as a major by-way for the long-term down- Miller, 1972) probably has also played an impor­ slope transport of sediment of Sardinian origin tant role in sediment deposition on the highly from the Campidano Graben Valley on land dissected margin. Sediment series are generally seaward to the Teulada Valley. Seismic profiles much thinner and locally absent on high relief that cross the axis of the Sardinia Valley do not features, such as the Ichnusa Seamount (Figure show a comparable incision (Figure 5: profiles 5, 7), as a result of seafloor erosion by flowing water 6). masses and of spill-over and downslope redeposi- tional processes (cf. Maldonado and Stanley, Conclusions 1976). The configuration of the present Mediterra­ nean is largely a result of convergence between TEULADA-SARDINIA VALLEY COMPLEX Africa and Europe, one which has involved a Sediment series are generally thick (in excess of progressively greater amount of closure as one 500 m) in the lower reaches of the Teulada- proceeds toward the east in the Mediterranean Sardinia Valley complex that serves as the major (Biju-Duval et al., 1977). In theory, the Sardi­ physiographic boundary between the Tunisian nian-Tunisian study area occupies a setting that 18 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES primarily has undergone compression during and Sardinia valleys which so obviously defines much of the period since the Upper Jurassic. In the present physiographic boundary between the more recent time we envision that the study area Tunisian and Sardinian margins. Caution, how­ occupies a key position involving the merging of ever, is needed in this respect, since the super­ the Tunisian and Sardinian margins. This likely posed major structural trends and associated sed­ resulted from the counter-clockwise rotation of imentary patterns, in themselves, do not prove the Corsican-Sardinian block in the Oligocene that this median depression necessarily closely and Miocene, and the continued northward conforms to and overlies directly above a deep- movement of Africa relative to the European lying crustal plate boundary. We do recognize plate; this latter also has induced displacement that the two valleys result primarily from ten- along the Calabrian Arc. Seismic profiles we ex­ sional motion related to geologically more recent amined tend to give some additional precision on subsidence of the Tyrrhenian Sea and Algero- this geologically recent evolution. Balearic Basin. Moreover, extensional motion in The effects of Quaternary (and probably con­ the southeastern part of this area, recorded by the tinuing) compression in northern Tunisia is be­ dominant NW-SE trend of normal faults, is re­ lieved to extend seaward on the contiguous Tu­ lated to horst and graben formation in the Strait nisian Shelf and inner plateau. Evidence for con­ of Sicily. This extensional trend, in turn, is prob­ tinuing lithospheric plate motion is provided by ably associated with the eastward displacement the E-W seismicity trend, including the destruc­ along the Calabrian-Sicilian Arc and subduction tive Orleansville and El Asnam earthquakes in in the Ionian Sea. Algeria in 1954 and 1980. Further evidence for a The dominant NNE-SSW orientation of tec­ northward-directed compression is indicated by tonic axes in the Sardinian-Tunisian region is seismic reflection profiles on the Algerian margin shown by seafloor relief, structural trends, and (Auzende et al., 1974). This E-W compressive depositional patterns (Figure 9). This orientation axis appears interrupted in the southeastern part (about N20°E to N30°E) clearly projects to the of the study area and adjacent Strait of Sicily, but reappears further to the east, in Sicily, as south the major structural trends mapped east of overthrusts that have continued to be displaced Sardinia in the Tyrrhenian Sea (Bacini Sedimen- as recently as the Quaternary. Some workers tari, 1977; Fabbri et al., 1981). Among the more envision that the Maghrebian chain is continuous obvious features are the Ichnusa Seamount and from North Africa to Sicily and that the sector Carbonara Canyon on the south Sardinia margin, between these two regions is dissected and dis­ and the Sentinelle Valley on the Tunisian bor­ placed by the NW-SE (and NNW-SSE) dextral derland. Some workers interpret the NNE-SSW fault systems (A. Fabbri, personal communica­ trending Sentinelle Valley as a reactivation, dur­ tion, 1983). ing the Plio-Quaternary, of an older fault (prob­ Although the Sardinian-Tunisian seafloor is ably transcurrent) on the African margin (A. believed to represent a welded plate boundary Fabbri, personal communication, 1983). In our and occupies a sector that has undergone view, however, the Sentinelle Valley, which cuts compression during much of the Neogene, our across most of the Tunisian Plateau, provides investigation indicates a predominance of struc­ evidence of the southward extent of Tyrrhenian- tural features of extensional origin. We propose related margin features, which have been active here that many of these features are related to the from the upper Miocene to the present. On the subsidence of the Tyrrhenian Sea involving much North African margin, this NNE-SSW trend is vertical movement from the Miocene to the Re­ superposed on the older Miocene NE-SW com- cent. It is tempting, for example, to place the pressional trend previously delineated by Au­ geological limit between the original Sardinian zende (1969, 1971). microplate and North Africa along the Teulada The E-W normal fault axes on the North Sicil- NUMBER 23 19 ian margin, in the Tyrrhenian Sea (Figure 2), do along the Calabrian Arc, and the NW-SE struc­ not appear to extend westward into and across tural trend related to the formation of the Strait the study area. The extent to which these still- of Sicily, are somewhat less obvious. It is possible active Sicilian East-West trending extensional that the convergence of tectonic-stratigraphic structures have played a role in the Plio-Quater­ trends in the study area indicates a triple junc­ nary evolution of this region is not determined tion. This aspect of plate motion remains to be here with available data. proven. Our assessment of the available data, In summary, the central Mediterranean region however, indicates that there is a correlation be­ highlights the southern extension (toward Tuni­ tween the complex configuration of this region sia) of N-S trending Tyrrhenian margin exten­ and the geologically recent evolution of the ad­ sional structures. The effects of compression due jacent deep (Tyrrhenian, Algero-Balearic, and to the northern movement of Africa and motion Ionian) Mediterranean basins. Literature Cited

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20 NUMBER 23 21

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