Antony Morris

Alternative tectonic models for the Late Palaeozoic-Early Tertiary development of Tethys in the Eastern Mediterranean region

Abstract A summary and discussion is given of alternative models of the tectonic evolution of the Tethyan orogenic belt in the Eastern Mediterranean region, based on recent information. Model 1 (Robertson & Dixon 1984). A single Tethyan ocean continuously existed in the Eastern Mediterranean region, at least from Late Palaeozoic onwards. The dominant influences were episodic northward subduction of Tethyan oceanic crust beneath Eurasia, and the northward drift of continental fragments, from Gondwana towards Eurasia. During the Mesozoic, the south Tethyan area was interspersed with Gondwana-derived microcontinents and small ocean basins. Ophiolites formed mainly by spreading above subduction zones in both northerly (internal) and southerly (external) oceanic basins during times of regional plate convergence, and were mainly emplaced as a result of trench-passive margin collisions. In a related model, Stampfli et al. (1991) argued for spreading along the North African margin in the Late Permian. Model 2A (Dercourt et al. 1986). Only one evolving Tethys existed. Triassic-Jurassic oceanic crust (Neotethys) formed in a single Tethyan ocean basin located north of Gondwana-related units. Spreading later formed a small ocean basin in the present Eastern Mediterranean Sea area during the Cretaceous. Jurassic and Cretaceous ophiolites formed at spreading ridges and record times of regional plate divergence. In an update version, Model 2B (Dercourt et al. 1993), spreading extended along the northern margin of Gondwana, with an arm extending through the south Aegean, splitting off a large microcontinent. Further spreading in the Cretaceous then opened the Eastern Mediterranean basin and fragmented pre-existing carbonate platforms. The Mesozoic ophiolites were seen as being mainly far-travelled from northerly (i.e. internal) orogenic areas. Model 3 (Şengör et al. 1984). Subduction in the Late Palaeozoic was dominantly southwards, beneath the northern margin of Gondwana in the Eastern Mediterranean. This subduction led to opening of Triassic backarc basins; and a rifted Gondwana fragment (Cimmeria) drifted across a pre-existing Tethys (Palaeo-Tethys) to collide with a passive Eurasian margin. In their model, a backarc basin (Karakaya Basin) rifted and then closed prior to collision of a Cimmerian microcontinent in the Mid Jurassic, and this was followed by renewed rifting of a small ocean basin in the Early Jurassic. Mesozoic ophiolites mainly formed above subduction zones; they were variously seen as far-travelled (in the ‘Greek area’), or more locally rooted (in the ‘Turkish area’). Recent evidence shows that difficulties exist in detail with all three models. However, four key elements are met in Model 1: dominantly northward subduction in the north; multiple ocean basins from Triassic onwards in the south; supra-subduction spreading of the major ophiolites; and emplacement from both northerly and southerly Mesozoic oceanic basins. Palaeomagnetism has played an important role, in setting the large-scale Africa-Eurasia relative motion framework and in providing tests for the tectonic affinities of smaller units, but such smaller-scale studies have often been compromised by the geological complexity and by the remagnetisation of tectonically thickened units.

First palaeomagnetic results from the Cycladic Massif, Greece, and their implications for Miocene extension directions and tectonic models in the Aegean

Abstract The Aegean region is characterised by major fault-bound blocks with markedly contrasting structural trends. Recent models account for these variations either through differential block rotation or through a process of radial extension. The Cycladic Massif in the back-arc region to the Hellenic trench represents a critical area where apparently divergent extension directions are observed. This contribution provides the first palaeomagnetic results from this area. Magnetic remanence and AMS data are presented for Middle Miocene granodiorites on the islands of Mykonos and Naxos. These rocks are exposed in the footwalls of major extensional detachments within the metamorphic core complexes that characterise the Cycladic Massif. A clear correlation between the orientation of an L-S fabric developed within the intrusions and the magnetic fabric data is observed, demonstrating the presence of a ‘normal’ magnetic fabric of extensional origin. The mean extension directions inferred from the orientations of the maximum axes of the magnetic susceptibility ellipsoids are markedly divergent at 064° (Mykonos) and 351° (Naxos). Northeasterly in situ magnetic remanences are observed on Mykonos, whereas those on Naxos are directed to the NNW. Neither direction corresponds to the appropriate palaeomagnetic reference direction for the Miocene. Standard palaeomagnetic tilt corrections cannot be applied at these sites because of the lack of palaeohorizontal markers. Deviations of in situ remanences away from the reference direction are potentially due to rotation around any one of an infinite number of net tectonic rotation axes. Each of these may be arbitrarily decomposed into components of tilting and vertical axis rotation. Geologically realistic solutions which restore the in situ magnetisations back to the reference direction may be found by consideration of regional structural trends, assuming that tilting is controlled by the major block-bounding faults. These solutions indicate significant components of differential vertical axis rotations across the Cyclades. Mykonos is bound by NW-SE trending regional faults and is shown to have rotated clockwise by ca. 22°. In contrast, Naxos is bound by ENE-WSW trending structures and has rotated anticlockwise by ca. 33°. The apparent divergence of extension directions inferred from both the magnetic anisotropy and structural data is resolved by correcting the mean AMS data for subsequent rotation using the rotation solutions derived from the magnetic remanence data. The data imply that the primary Middle Miocene extension direction in the Cycladic Massif was nearly uniform along a NNE-SSW azimuth. We consider that the uniform extension direction and opposing senses of block rotation determined for this portion of the Cycladic massif are most consistent with a recent model describing deformation within the Aegean in terms of a system of broken, fault-bounded slats. Within this scenario, Mykonos occupies a position on a clockwise rotating slat which continues northwest, at least as far as Evvia. A major ENE-WSW trending fault between the islands, previously identified on shallow seismic sections, separates this domain from an anticlockwise rotating slat which includes Naxos.

Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid‐Atlantic Ridge 30°N

Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Miocene remagnetisation of carbonate platform and Antalya Complex units within the Isparta angle, SW Turkey

Abstract A palaeomagnetic study has been carried out within the Mesozoic and Tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep-sea volcanics and sediments of the Antalya Complex, exposed around the Isparta angle, SW Turkey. The Antalya Complex is interpreted as a mosaic of carbonate platforms, basinal sediments, volcanic and ophiolitic rocks which formed within a southerly strand of the Neotethyan ocean, adjacent to Gondwana. The results indicate a widespread remagnetisation event. Negative fold tests show that the remanence at most sites is of secondary origin (e.g., within the cirali lavas). The magnetisation is carried by magnetite of presumed authigenic origin. The remagnetisation event is believed to have occurred in the Early-Middle Miocene (Burdigalian-Langhian). It was possibly triggered by the migration of orogenic fluids ahead of the advancing Lycian nappes during their emplacement onto the carbonate platforms. Subsequent to remagnetisation, a large segment of the Isparta angle underwent an anticlockwise rotation of 30°. This rotation is attributed to the overall convergence and bending of the Hellenic arc and the final stages of emplacement of the Lycian Nappes during the Late Miocene, in agreement with previous studies. Previously, southerly palaeolatitudes were inferred from Late Triassic extrusives of the Godene Zone (Calbali Dag unit). The post-folding magnetisation identified here within the Cirali lavas of the Godene Zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections. The available data cannot be used to substantiate an origin for the Antalya units south of the equator in the early Mesozoic. Instead, a position close to the northern margin of Gondwana is indicated.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Quantitative constraint on footwall rotations at the 15°45'N oceanic core complex, Mid-Atlantic Ridge: Implications for oceanic detachment fault processes

The subsurface geometry of detachment faults at slow spreading mid-ocean ridges is debated: are they planar features that form and slip at low angles, as often inferred for their continental equivalents, or do they initiate at steep angles and then flatten in response to flexural unloading as displacement proceeds, as predicted in “rolling hinge” conceptual models? An essential difference is that significant rotation of the footwall should occur in the rolling hinge but not the planar fault model. This can be tested using paleomagnetism. Previous attempts to address this question have relied upon data from azimuthally unoriented drill cores. Although results are consistent with large rotations having occurred, these interpretations are very nonunique, and other solutions that require minimal rotations are equally permissible. We here present a rigorous analysis of paleomagnetic and structural data from a unique set of azimuthally oriented cores, collected using a seabed rock drill, from the 15°45′N oceanic core complex on the Mid-Atlantic Ridge. By considering the full paleomagnetic remanence vector in combination with kinematic data from the detachment fault shear zone we are able to quantitatively constrain the geometrically permissible axes and magnitudes of rotation of the detachment fault footwall, for the first time without having to make a priori assumptions about the orientation of the axis. We show that significant rotations (64° ± 16°) have indeed occurred, about a gently plunging, near-ridge-parallel axis, robustly supporting the rolling hinge models. We further discuss the geological implications of this result for oceanic detachment fault processes.

Palaeomagnetic evidence for clockwise rotations related to dextral shear along the Southern Troodos Transform Fault, Cyprus

Abstract The Southern Troodos (Arakapas) fault represents a Late Cretaceous oceanic fracture zone which was locally disrupted during Late Cretaceous-Eocene palaeorotation of the Troodos microplate. Opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation. Palaeomagnetic studies of Turonian zeolite facies lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the Arakapas fault belt, the western margin of the Limassol Forest Complex and the eastern flank of the Troodos ophiolite. At seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes. One fault block at the western end of the Arakapas fault belt has experienced a net anticlockwise rotation, while at six other sites only simple tilting about sub-horizontal axes is indicated. The overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right-lateral slip along the transform. Cross-cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post-spreading disruption of the fracture zone. Sites located further south, in the Eastern Limassol Forest Complex, show no relative rotation with respect to the main Troodos ophiolite to the north and have experienced only simple tilting. However, if our results are considered in conjunction with existing palaeomagnetic data from the area, it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform. Some areas were rotated by over 100° about steeply inclined axes, whereas others experienced only simple tilting. The whole area was later subjected to a bulk 90° anticlockwise rotation along with the Troodos microplate. Data obtained from the umbers and radiolarites (Perapedhi Fm.) exposed on the eastern flank of the Limassol Forest block indicate that at least 30°, and possibly up to 45°, of this 90° rotation took place over a maximum of 15 Ma.

Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes

Serpentinization of ultramafic rocks during hydrothermal alteration at mid-ocean ridges profoundly changes the physical, chemical, rheological, and magnetic properties of the oceanic lithosphere. There is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans. Unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults, which eventually results in structures known as oceanic core complexes (OCCs). However, we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of OCCs. Since magnetite is a direct product of serpentinization, the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere. Here we present new results from an integrated, rock magnetic, paleomagnetic, and petrological study of variably serpentinized peridotites from the first fossil OCC recognized in an ophiolite. Integration with existing data from mid-ocean ridge-related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range (0–100%) of degrees of serpentinization. Variations in a range of magnetic parameters with serpentinization, and associated paleomagnetic data, provide: (i) key constraints on the mechanism(s) of serpentinization at mid-ocean ridges; (ii) insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies; and (iii) evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at OCCs.

Proxy-climate and geomagnetic palaeointensity records extending back to Ca. 75,000 BP derived from sediments cored from Lago Grande di Monticchio, Southern Italy

As part of a multi-disciplinary study of maar lake sediments, magnetic properties of some 3000 samples from two long cores extracted from the bottom sediments of Lago Grande di Monticchio located in southern Italy have been measured and the results connected with lithological, palynological and geochemical data obtained from the same cores. A time-scale, based on microlaminations, radiocarbon and ArAr age determinations (see companion papers in this special issue of Quaternary Science Reviews) dates the base of the longer 51 m core at about 76 ka before the present. The strengths of anhysteretic remanence, saturation isothermal remanence and susceptibility are highly sensitive to the contrast between stadial and interstadial conditions, showing almost flip-flop transitions at Oxygen Isotope Stage 5a/4 and Stage 2/1 boundaries as previously observed for Lac du Bouchet, another maar lake site located in the Haute Loire Province of France. On the other hand, these laboratory installed magnetizations vary smoothly through sediment deposited during stadials. A prolonged period of heavy (modelled) precipitation starting at ∼25 ka BP coincides with a marked decrease in measured magnetic intensities possibly associated with the consequent increase in lake level and lake water chemistry. The natural remanent magnetization is dominated by climatic and environmental influences: nevertheless a weak signal attributed to variations in the palaeointensity of the geomagnetic field has been isolated, and it shows good broad-scale agreement with that previously reported for Lac du Bouchet.

Multiple tectonic rotations and transform tectonism in an intraoceanic suture zone, SW Cyprus

Abstract New palaeomagnetic data from the Late Cretaceous, Neotethyan Troodos ophiolite and related units in SW Cyprus provide compelling evidence that transform tectonism is recorded outside the main outcrop of the fossil Southern Troodos Transform Fault Zone (STTFZ). High-level intrusive and extrusive sequences were sampled along the SW margin of the Troodos massif and in ophiolitic slivers preserved along arcuate fault lineaments further to the south and west. These units show cross-cutting relationships with corresponding differences in remanent magnetisation directions, characteristic of syn-magmatic rotation in an active transform setting. Net tectonic rotation analyses allow decomposition of the total rotation at these sites into early and late components. Early transform-related rotations are consistently clockwise, in agreement with all other studies of rotations associated with the STTFZ further to the east. Late rotations and the net rotations derived at localities where cross-cutting relationships were not present are regarded as composite in origin. The composite rotation axes closely relate to the orientation of observed regional structures. In these circumstances, additional constraints are used to attempt geologically viable interpretations. The overall distribution of transform-related rotations (steeply and shallowly plunging rotation axes) and spreading axis-related rotations (sub-horizontal rotation axes) is comparable with that along the STTFZ and adjacent parts of the Troodos massif. This is most readily explained by simple along-strike extension of the primary spreading axis and transform fabrics into SW Cyprus, with minor late disruption by high-angle dextral shear zones and neotectonic graben systems.