J.E. Meulenkamp

Tertiary palaeogeography and tectonostratigraphic evolution of the Northern and Southern Peri-Tethys platforms and the intermediate domains of the African–Eurasian convergent plate boundary zone

Abstract The increasing effects of African–Eurasian convergence during the Tertiary resulted in the uplift and emergence of the Northern and Southern Peri-Tethys platforms. Palaeogeographic maps covering six selected time slices, including the Middle Eocene, late Early Oligocene, late Early Miocene, early Middle Miocene, early Late Miocene and Middle to Late Pliocene illustrate that environmental and depositional differentiations on the northern platform and along the bordering domains of the convergence zone were more pronounced than on the southern platform and its adjacent areas. The tectonic evolution of the northern platform included an overall eastward-directed trend in the onset of basin uplift and emergence, which started at about the Eocene–Oligocene transition, at 34 Ma. Tectonostratigraphic analyses indicate a striking contemporaneity of the events which defined the temporal and spatial development of both the domains of the convergent plate boundary zone and the bordering platforms. Five episodes of major regional change in palaeogeographic and tectonic setting are distinguished. They occurred in the Late Eocene (37–34 Ma), early Late Oligocene (30–27 Ma), latest Early to earliest Middle Miocene (17–15 Ma), early Late Miocene (9–8 Ma) and late Early to early Middle Pliocene (4–3 Ma). These episodes encompassed changes which were most probably induced by geodynamic events primarily related to the relative motions of the African/Arabian and Eurasian plates. In turn, the plate motions are assumed to have ‘triggered’ discrete steps in the regional kinematics and geodynamics that governed the palaeogeographic evolution of the Peri-Tethys platforms and the intermediate domains of the African–Eurasian plate boundary zone.

On the Hellenic subduction zone and the geodynamic evolution of Crete since the late Middle Miocene

Abstract In recent syntheses of the geodynamic evolution of the Aegean area the Hellenic subduction zone and in particular its initiation, plays a central role. Two basic aspects of the recent comprehensive model put forward by Le Pichon, Angelier and co-workers, are: (a) initiation of subduction along the Hellenic arc is inferred to have taken place approximately 13 Ma ago. whereas several others have suggested approximately 5 Ma, and (b) migration of the trench system to the south-southwest, giving rise to an extensional regime in the back-arc region, which led to general subsidence and formation of the present-day Aegean Sea. New data on the structure of the Upper Mantle underneath the Aegean area indicate that the former aspect requires modification. The seismic velocity structure indicating a northward dipping slab down to a depth of at least 600 km and the depth distribution of earthquake hypocenters can not be reconciled with straightforward initiation of subduction approximately 13 Ma ago. Estimates of about 5 Ma for the time of initiation of the present subduction zone are even more difficult to reconcile. Assuming 400 km as an upper bound for the effect of stretching of the Aegean Sea and of other processes contributing to the separation of Crete from the Eurasian mainland we arrive at an age for the Hellenic subduction zone of at least 26 Ma. As to aspect (b): geological data of Crete lend strong support to the idea that (on Crete) compressional tectonics is of much greater importance than current models imply. Combining geophysical and geological data we propose a modification of the tectonic evolution for the Cretan segment of the Hellenic arc. Whereas the basic process of extension caused by a migrating trench system (roll-back) in a land-locked basin remains a very useful and sound concept, the tectonics of the margins of a region subject to stretching (in this case Crete) may be more complicated than hitherto thought. In our model the documented fragmentation of Crete into several basins which started about 12 Ma ago is not attributed to initiation of subduction but to inception of the roll-back process. Compressional tectonics on Crete is taken to be associated with deformation of the crust resulting from the overall tensional stress regime.

Nappe stacking resulting from subduction of oceanic and continental lithosphere below Greece

We quantitatively investigate the relation between nappe stacking and subduction in the Aegean region. If nappe stacking is the result of the decoupling of upper-crustal parts (5-10 km thick) from subducting lithosphere, then the amount of convergence estimated from balancing the nappe stack provides a lower limit to the amount of convergence accommodated by subduction. The balanced nappe stack combined with the estimated amount of completely subducted lithosphere indicates 700 km of Jurassic and 2400 km of post-Jurassic convergence. From seismic tomographic images of the underlying mantle, we estimate 2100-2500 km of post-Jurassic convergence. We conclude that (1) the imaged slab represents the subducted lithosphere that originally underlay the nappes, (2) since the Early Cretaceous, subduction in the Aegean has occurred in one single subduction zone, and (3) the composition of the original basement of the nappes indicates that at least 900 km of sub-upper-crust continental lithosphere subducted in the Aegean.

On late miocene to recent vertical motions in the Cretan segment of the Hellenic arc

Abstract High-resolution foraminiferal and calcareous nannoplankton biostratigraphy and reliable paleobathymetry estimates based on plankton-benthos ratios in foraminifera make it possible to reconstruct in detail Late Neogene vertical motions along the Central Cretan segment of the Hellenic arc. These motions are considered to express the surficial effect of the roll-back process of the Hellenic subduction zone, which started about 12 Ma ago. In contrast to earlier views there is no sustained uplift since the late Middle Miocene. Successive paleotopographic profiles for central Crete show a predominance of subsidence, coupled with an increase of differential reliefs, from the latest Serravallian until the Messinian. Subsidence was most pronounced between the Tortonian-Messinian boundary interval and the early part of the Early Pliocene, locally up to a magnitude of more than 1000 m. A two-phased uplift history, coupled with tilting to the north or northeast, can be inferred from the late Early Pliocene-Recent record separated by a short, early Late Pliocene episode of subsidence. Rates of uplift were highest in the Early Pliocene up to about 125 cm/ka. The Central Cretan Late Neogene to Recent paleotopographic profiles are compared with those reconstructed from seismic interpretations and drilling data from the Cretan Sea. The combined results show that paleotopographic configurations between the Cyclades and Crete were fairly similar to those on central Crete until Messinian time. Foundering of the Cretan Basin started in the course of the Early Pliocene, but subsidence rates were less than the contemporaneous uplift rates of Crete. The timing and magnitude of the vertical motions along the Cretan Sea-Central Cretan transect put detailed geological constraints on tectonophysical modelling of the surficial effects of the roll-back process. These vertical motions are discussed in view of the model of southward translation of a large supracrustal slab, resulting in the origin of the Cretan Sea and in thrusting and uplift in the frontal parts of the slab, where it thrusted over the northern limb of the subducted Ionian Plate.

ON LATE OLIGOCENE TO PLIOCENE DEPOCENTRE MIGRATIONS AND THE EVOLUTION OF THE CARPATHIAN-PANNONIAN SYSTEM

Abstract The Late Oligocene to Pliocene evolution of foredeep basins of the Eastern Alps-Carpathians fold and thrust belt is marked by pronounced internal to external and lateral depocentre shifts. The latter shifts, covering a present along-arc distance of 1700 km, portray accelerating rates of foredeep depocentre migration, particularly so in the late Early and in the Middle Miocene, from about 7 to about 45 cm/yr. Lateral depocentre migration came to a close at the beginning of the Late Miocene; the subsequent Late Miocene to Pliocene foredeep infill history was characterized by exponentially increasing accumulation rates in the intersection area of the East European (Ukrainian) and Moesian platforms. Successive steps in the evolution of the foredeep basins had pronounced counterparts in the intra-Carpathian area. The beginning of acceleration of foredeep depocentre migration in latest Early Miocene times was coeval with the inception of intra-Carpathian extensional tectonics. The extremely rapid Middle Miocene depocentre shift, coupled with a change in direction of foredeep depocentre migration, corresponded with maximum extension in the intra-Carpathian area. The ensuing end of foredeep depocentre migration around the Middle-Late Miocene transition was coeval with the end of extension of the intra-Carpathian area, which, in turn, was followed by the inception of overall, thermal subsidence at about 11.5 Ma. The timing of and spatial relationships between discrete, coeval events in arc and intra-arc evolution put unambiguous geological constraints on geodynamic modelling of the evolution of the Carpathian-Pannonian system. It is speculated that the geological observations may best be understood in terms of the surficial effects of lateral migration of slab detachment. Such effects would mirror the dominant role of a concentrating slab pull, taken to result in, e.g., a time-progressive acceleration of foredeep depocentre migration.

Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record

Continental deposits from the early Pliocene lacustrine Ptolemais basin in NW Greece display rhythmical alternations of lignite and marl beds. Three parallel sections from this area are studied using magnetostratigraphy and cyclostratigraphy. The presence of the greater part of the Gilbert Chron enables the recognition of astronomical periodicities in the succession. Especially the precessional influence is evident, as it determines the lithological cycles. The continental Ptolemais composite section is correlated to the most recent astronomical time scale — and thus to the marine reference section: the Rossello composite from Sicily [C.G. Langereis, F.J. Hilgen, The Rossello composite: a Mediterranean and global reference section for the Early to early Late Pliocene, Earth Planet. Sci. Lett. 104 (1991) 211‐225] — on a bed-to-bed scale. It is concluded that lignite corresponds to an insolation minimum (beige layer in the Rossello composite), and marl to an insolation maximum (grey layer in the Rossello composite). This implies a precipitation increase during insolation maxima in early Pliocene continental Greece.

Lateral shifts of Apenninic foredeep depocentres reflecting detachment of subducted lithosphere

Abstract The hypothesis of lateral migration of slab detachment was formulated on the basis of seismic tomography results on the 3-D structure of subduction zones in the Mediterranean region [M.J.R. Wortel and W. Spakman, Structure and dynamics of subducted lithosphere in the Mediterranenan region, Proc. K. Ned. Akad. Wet. 95 (1992) 325–347]. The redistribution of slab pull forces associated with the tearing process is taken to affect foredeep development by adding a lateral component to internal–external depocentre migrations. In the case of the Apenninic Arc the direction of detachment and associated depocentre migration is expected to be southeast. This prediction has been confronted with the tectonostratigraphy of Oligocene to Recent Apenninic foredeeps. Results show there are two episodes marked by important lateral shifts of foredeep depocentres. During the early Late Miocene (Tortonian) the depocentre of the Northern Apennines foredeep shifted towards the southeast. This is evidenced by the relative positions of the depocenters of the Burdigalian–Serravallian Marnoso Arenacea and the Tortonian Umbrian–Marchean and Lazian “Minor Basins”. A further, stepwise southeastward migration can be inferred from the Plio-Pleistocene records of the Central and Southern Apennines. In a more general sense, our results indicate that slab detachment may play a significant role in the evolution of foredeeps along convergent plate boundaries.

Aspects of the Late Cenozoic Evolution of the Aegean Region

The late Neogene to early Pleistocene history of the Aegean region was mainly defined by tectonic events, which led to large-scale, roughly contemporaneous changes in paleogeographic configurations and sedimentation conditions. The most important of these events occurred in the late Serravallian/early Tortonian, during the Tortonian-Messinian boundary interval, within the Messinian, and during the course of the Pliocene.

Sedimentary cycles and volcanic ash beds in the Lower Pliocene lacustrine succession of Ptolemais (NW Greece): discrepancy between 40 Ar= 39 Ar and astronomical ages

A high-resolution cyclostratigraphy for the rhythmically bedded lignite‐marl sequences of the Lower Pliocene Ptolemais Formation is combined with 40 Ar= 39 Ar dating results of intercalated volcanic ash beds. Detailed field reconnaissance in three open-pit lignite mines reveals three end-member sediment types: lignites, composed primarily of organic material; grey marls, a mixture of carbonate and organic material; and beige marls, almost exclusively composed of carbonate. These lithologies are arranged in two basic types of sedimentary cycles: lignite‐grey marl and lignite‐beige marl cycles. A cyclostratigraphic composite section comprising 56 lignite‐marl cycles is constructed which combines the consistent cycle patterns from three parallel sections. The concordant positions of 20 volcanic ash beds in these sections confirm the cyclostratigraphic correlations and indicate that the lignite‐marl cycles result from regional, basin-wide forcing rather than lateral facies migrations. 40 Ar= 39 Ar ages on sanidine and biotite separates from nine volcanic ash beds were obtained by multiple total fusion and incremental-heating experiments. The 40 Ar= 39 Ar ages range between 5:00 0:05 and 4:04 0:04 Ma and are, in general, consistent with the stratigraphic order. A least-square linear regression using the measured 40 Ar= 39 Ar ages gives an average duration of 21:8 0:8 kyr per lignite‐marl cycle. Evidently, the lignite‐marl cycles in the Ptolemais Formation are linked to the precessional variation in the Earth’s orbit through its influence on Mediterranean climate. For the first time, 40 Ar= 39 Ar dating results, totally independent from any other dating and or tuning technique, confirm the astronomical theory of climate change. The 40 Ar= 39 Ar ages of the volcanic ash beds show a constant200 kyr (4.5%) age discrepancy with the astronomical ages of the same ash beds. This inconsistency remains difficult to explain. The discrepancy is unlikely to have resulted from erroneous astronomical ages, through incorrectness in the astronomical tuning, inaccuracies of the magnetostratigraphic data or the orbital time-series used, and=or errors in the APTS. The 40 Ar= 39 Ar dating results neither give clear indications for a possible source of error. From the excellent data set it is evident that neither loss of radiogenic 40 Ar, nor an underestimation of the contribution of Ca- and K-derived Ar isotopes could have caused the discrepancy. Moreover, the discrepancy is also beyond the errors in the systematic variables, like the decay constants of 40 K or the ages for the neutron-fluence monitors.

PRESENT STATUS OF THE ASTRONOMICAL (POLARITY) TIME-SCALE FOR THE MEDITERRANEAN LATE NEOGENE

Sedimentary cycles may reflect orbitally induced climate oscillations and can then be used to construct astronomical time–scales. Following the initial tuning of the Late Pleistocene, the ‘anchored’ astronomical time–scale was extended to the base of the Pliocene, using palaeoclimatic records from Ocean Drilling Project (ODP) sites in the eastern equatorial Pacific and North Atlantic and sedimentary cycle patterns in marine successions exposed onland in the Mediterranean. In this paper we present a review of the progress subsequently made in establishing a Late Neogene astronomical (polarity) time–scale (A(P)TS) in the Mediterranean region. Major steps forward are (1) the evaluation of the initial time–scale, using high–resolution climatic proxy records, different astronomical solutions and the additional influence of obliquity on sedimentary cycle patterns, (2) the extension of the A(P)TS into the Middle Miocene, i.e. back to about 12 3Ma, (3) the closure of the Messinian gap in the A(P)TS, (4) the incorporation of the continental record, and (5) the intercalibration of astronomical and radioisotopic time.