A. Nicolas

Structures of ophiolites and dynamics of oceanic lithosphere

I - Introduction and Analytical Methods.- 1. Introduction.- 1.1. Historical development of the ophiolite concept.- 1.2. Interest of ophiolite studies.- 1.2.1. Ophiolites as key for the study of oceanic lithosphere and asthenosphere.- 1.2.2. Ophiolites as markers of past plate tectonics.- 1.3. Scope and structure of the book.- 2. Analytical methods in ophiolites.- 2.1. Introduction.- 2.2. The oceanic reference frame.- 2.2.1. The ridge referential.- 2.2.2. Ridge side of origin of a given ophiolite.- 2.3. Structural studies in the hypovolcanic and volcanic sequences.- 2.4. Structural studies in the plutonic sequence.- 2.4.1. Principal structures.- 2.4.2. Viscous/plastic deformation.- 2.4.3. Importance of viscous flow.- 2.5. Structural studies in the ultramafic section.- 2.5.1. Homogeneity of mantle structures.- 2.5.2. Principal structures.- 2.5.3. Melt products: evidence for segregation/impregnation.- 2.5.4. Microstructures in peridotites and kinematic analysis.- 2.5.5. Microstructural imprint of asthenospheric and lithospheric flow.- 2.5.6. Serpentinization and low temperature deformations.- 2.6. Expected asthenospheric flow patterns.- II - Typical Ophiolite Complexes.- 3. Oman ophiolite: the harzburgite ophiolite type.- 3.1. Introduction.- 3.2. Geological setting.- 3.2.1. Geodynamic setting.- 3.2.2. History of the Hawasina basin.- 3.3. General description of the ophiolite.- 3.3.1. Introduction.- 3.3.2. Mafic section.- 3.3.3. Ultramafic section.- 3.3.4. Metamorphic aureoles.- 3.3.5. High pressure metamorphism.- 3.4. Structure of the Oman ophiolite.- 3.4.1. Introduction-main structural events.- 3.4.2. Structures related to accretion at the spreading center.- 3.4.3. Structures related to oceanic thrusting and obduction.- 3.5. General interpretation of the Oman ophiolite.- 3.5.1. Introduction.- 3.5.2. Spreading rate estimation.- 3.5.3. Paleo-environment of origin and obduction history.- 4. Xigaze and Trinity ophiolites-Plagioclase lherzolite massifs: the lherzolite ophiolite type.- 4.1. Introduction.- 4.2. Xigaze ophiolite.- 4.2.1. Introduction.- 4.2.2. Geological setting.- 4.2.3. Description.- 4.2.4. Structural analysis.- 4.2.5. Geochemistry.- 4.2.6. Discussion.- 4.3. Trinity ophiolite.- 4.3.1. Introduction.- 4.3.2. Geological setting.- 4.3.3. Description.- 4.3.4. Structural analysis.- 4.3.5. Melt extraction and melt reaction.- 4.3.6. Petrology and geochemistry.- 4.3.7. Discussion.- 4.4. The western Alps ophiolites.- 4.5. The spinel-plagioclase lherzolite massifs.- 4.5.1. Penological zonation.- 4.5.2. Structural zonation.- 4.5.3. Structure and geodynamic environment.- 4.5.4. Contact metamorphism and nature of metamorphosed formations.- 5. Bogota Peninsula and N.E. districts of New Caledonia - Wadi Tayin in Oman - Coastal Complex of Newfoundland: possible origin in transform faults.- 5.1. Introduction.- 5.2. Bogota Peninsula and N.E. ophiolitic districts of New-Caledonia.- 5.2.1. Introduction.- 5.2.2. Geological setting.- 5.2.3. Description of the Bogota Peninsula shear zone.- 5.2.4. Description of the Tiebaghi-Poum-Belep shear zone.- 5.2.5. Discussion.- 5.3. Coastal Complex of Newfoundland.- 5.3.1. Introduction.- 5.3.2. Geological setting.- 5.3.3. Description.- 5.3.4. Petrology and geochemistry.- 5.3.5. Interpretation.- 5.4. Wadi Tayin massif in Oman.- 5.4.1. Introduction.- 5.4.2. Structural description.- 5.4.3. Discussion.- 5.5. Conclusion.- 5.5.1. The diversity of ophiolitic transforms.- 5.5.2. Dike orientation in transform zones.- 6. Canyon Mountain ophiolite: possible origin in an island arc.- 6.1. Introduction.- 6.2. Geological setting.- 6.3. Description.- 6.4. Structural analysis.- 6.5. Petrology and geochemistry.- 6.6. Discussion.- 6.6.1. Specific characteristics of the Canyon Mountain ophiolite.- 6.6.2. Structural models.- 6.6.3. Geodynamic environment of origin.- III - Activity of Oceanic Spreading Centers and the Origin of Ophiolites.- 7. Melt generation and extraction in mantle diapers.- 7.1. Introduction.- 7.2. Melt extraction from the asthenosphere.- 7.2.1. Conditions of adiabatic melting.- 7.2.2. Asthenospheric path and the meeting with lithospheric conditions.- 7.2.3. Depth of first melting.- 7.2.4. Maximum depth of melt extraction.- 7.3. Physical mechanisms of melt extraction.- 7.3.1. Fraction of stable melt in a peridotite.- 7.3.2. Melt extraction.- 7.4. A model of melt extraction by hydrofracturing.- 7.4.1. The model.- 7.4.2. Melt velocity within dikes, episodicity and duration of episodes of melt extraction.- 7.4.3. Geochemical implications.- 7.5. Melt extraction by solid compaction and melt percolation in transition zones of ophiolites.- 7.6. Focusing of melt extraction below oceanic ridges.- 8. The various ophiolites and their oceanic environments of origin.- 8.1. Introduction.- 8.2. Harzburgite and lherzolite types of ophiolites - Role of spreading rate.- 8.2.1. Distinctive characteristics.- 8.2.2. Harzburgite and lherzolite types of ophiolites and mantle partial melting.- 8.2.3. Harzburgite and lherzolite types of ophiolites and oceanic environments.- 8.3. Island-arc, back-arc or mid-ocean ophiolites.- 8.3.1. Geochemical characteristics.- 8.3.2. Other criteria.- 9. Mantle flow, tithospheric accretion and segmentation of oceanic ridges.- 9.1. Introduction.- 9.2. Mantle flow in the Oman ophiolite.- 9.2.1. Introduction.- 9.2.2. Homogeneous mantle flow away from the ridge-Relation with seismic anisotropy.- 9.2.3. Channeling of mantle flow along the ridge axis.- 9.2.4. Mantle flow in transform faults.- 9.2.5. Mantle flow in diapers.- 9.2.6. Mantle flow patterns beneath the Oman paleo-ridge.- 9.3. Mantle flow in the Trinity ophiolite and lherzolite massifs.- 9.4. Mantle diapirism and ridge segmentation.- 9.4.1. Introduction.- 9.4.2. Models of mantle diapers.- 9.4.3. Return flow and thickness of the buoyant layer.- 9.4.4. Spacing of mantle diapirs and ridge segmentation.- 9.4.5. Stability of mantle diapers.- 10. Magmatic processes in the uppermost mantle at oceanic spreading centers.- 10.1. Introduction.- 10.2. Principal characteristics of transition zones.- 10.3. Origin of the wehrlitic intrusions.- 10.4. Origin of dunites.- 10.4.1. Introduction.- 10.4.2. Field occurrences.- 10.4.3. Residual/magmatic origin.- 10.4.4. Mechanism of formation of residual dunites.- 10.4.5. Geochemical reequilibration.- 10.4.6. Conclusion as to the origin of dunites.- 10.5. Structure and origin of the chromite deposits.- 10.5.1. Introduction.- 10.5.2. Setting of chromite deposits.- 10.5.3. Structure of chromite deposits.- 10.5.4. Composition of chromite deposits.- 10.5.5. Origin of chromite deposits.- 11 - Generation of oceanic crust.- 11.1. Introduction.- 11.2. Lithology of ophiolites and seismic structure of the oceanic crust.- 11.3. Serpentinite sea-floor in slow spreading environments and LOT.- 11.3.1.Abyssal and ophiolitic peridotites.- 11.3.2. Serpentinized peridotites as sea-floor.- 11.3.3. Nature of the Moho.- 11.4. The plutonic section and the problem of magma chambers.- 11.4.1. Introduction.- 11.4.2. Origin of the layering in the plutonic gabbro sequence.- 11.4.3. Magma chamber models.- 11.4.4. Conclusions about magma chamber models.- 11.4.5. Plating of gabbros and diking at the roof of magma chambers.- 11.4.6. Initiation of a new magma chamber.- 11.5. Sheeted dikes and volcanic units.- 11.5.1. Introduction.- 11.5.2. Generation at rifts and ridges.- 11.5.3. Structural evolution of the volcanic-hypovolcanic units.- 11.6. Crustal discontinuities in lherzolite type of ophiolite and episodic oceanic spreading.- 11.6.1. Variable basalt delivery along ridge-strike.- 11.6.2. Episodic basalt delivery in time.- 11.7. Early metamorphism in ophiolites and hydrothermal activity at oceanic ridges.- 11.7.1. Introduction.- 11.7.2. Metamorphic-zonation in ophiolites.- 11.7.3. Relationship with the sequence of hydrothermal alteration in oceanic crust.- IV - Emplacement of Ophiolites Trough Space and Time.- 12 - Ophiolites emplacement.- 12.1. Introduction.- 12.2. Ophiolite belts.- 12.2.1. Passive margins of continents.- 12.2.2. Active margins of continents.- 12.2.3. Collision belts.- 12.3. Emplacement-related features in ophiolites.- 12.3.1. Introduction.- 12.3.2. Ophiolite nappes and high temperature aureoles.- 12.3.3. Ophiolitic melanges and high pressure metamorphism.- 12.4. Mechanisms of ophiolite emplacement.- 12.4.1. Introduction.- 12.4.2. Thrusting on passive continental margins.- 12.4.3. Upheaval in the accretionary prism of active margins.- 12.5. Summary and concluding remarks.- 13 - Ophiolite belts through time.- 13.1. Introduction: a reappraisal of ophiolites and their oceanic environments.- 13.2. Ophiolites generation and emplacement through time.- 13.3. Ophiolites as witness of pangean cycles.

Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge

Abstract High-temperature N-S to NW-SE trending shear zones have been discovered in the peridotites and gabbros of the Oman ophiolite. They reach their maximum development (1–2 km across and 50 km long) in the northern massifs of the ophiolite belt. The major shear zones run parallel to the paleo-ridge as defined by the orientation of the diabase dike swarm. The shear zones are in structural and kinematic continuity with the basal thrusts in the peridotites and underlying metamorphic aureoles. They are marked by an important magmatism, synchronous with the shearing. The source of this magmatism is probably in the dying activity of the ridge and in the partial melting of the metamorphic aureole. The shearing and the associated southward thrusting occurred at the ridge and along strike when compressional tectonics superseded extensional tectonics 95–100 m.y. ago. This first motion, probably controlled by the ridge geometry, was rapidly followed by a WSW-thrusting.

Magma chambers in the Oman ophiolite: fed from the top and the bottom

Recent models of magma chambers at fast-spreading ridges are based on the idea that the entire gabbro section of the oceanic crust crystallizes from a thin melt lens located just below the sheeted dike complex. The shape of the lens has been deduced from seismic reflection data at fast-spreading ridges. On the basis of structural studies in the Oman ophiolite, we suggest that the accretion of the lower crust may not proceed entirely in this way. We emphasize the contrast between: (1) upper level gabbros characterized by a magmatic foliation which, from a flat attitude at depth, rapidly steepen upward and tend to become oriented parallel to the sheeted dikes; and (2) lower gabbros, flat-lying, magmatically deformed, and more or less strongly layered. Wehrlite layers and lenses which contribute to the layering of these gabbros have previously been interpreted as sills. We suggest here that the modally graded bedding, which is an important feature of the lower layered gabbros, may have similarly originated as sills. This is deduced from the fact that, above mantle diapirs, the several hundred metre thickness of the transition zone contains sills of layered gabbros, commonly organized in modally graded sequences. These sills, which are interlayered with dunite or harzburgite, contain gabbros which are shown here to be structurally similar to those in the layered gabbro unit at all scales. If this interpretation is correct, the gabbro section of the oceanic crust in Oman is built up by crystallization, both along the walls and the floor of the perched magma lens, followed by subsidence, and also in sills intruded either in the subsiding foliated gabbros or in the mantle dunites of the Moho transition zone. Supply from the perched melt lens generates the upper foliated gabbros, and supply by sills emplaced near Moho level gives rise to the basal layered gabbros and the gabbro-troctolite lenses of the transition zone. Feeding of the perched melt lens by vertical dikes and feeding of the Moho horizon by sills may correspond to successive stages of a basaltic melt injection episode.

Harzburgite and lherzolite subtypes in ophiolitic and oceanic environments

Abstract In most ophiolites the ultramafic section is harzburgitic. It is rarely composed of residual lherzolites, except for the local occurrence of impregnation lherzolites within harzburgitic massifs. Several characters (environmental formations, thickness and composition of the crust, geometry of the asthenospheric flow, serpentinization) validate the distinction between an harzburgite and a lherzolite ophiolitic subtype. The harzburgite subtype can be derived from any oceanic spreading center, provided the rate is larger than 1 cm/yr. The lherzolite subtype would correspond to situations (vicinity of transform fault, very slow spreading rates) where the lithospheric front penetrates at 20–30 km into the mantle below the spreading center. These lherzolitic massifs are best explained as being derived from slow-spreading rifts. Finally the particular asthenospheric flow discovered in these massifs is discussed.

Textures, structures and fabrics due to solid state flow in some European lherzolites

Abstract Structures, textures and fabrics caused by solid state flow in lherzolites are described with special reference to the Lanzo Massif (Italian Alps). Extension and shearing movements have produced a foliation, which, for the case of inhomogeneous deformation is developed in the axial surface of shear folds. High simple shear angles have been estimated. The “b” kinematic direction is evidenced in the field by an enstatite lineation. The flow mechanism responsible for the development of the main subfabric is considered to be translation gliding in olivine and enstatite. Syntectonic and annealing recrystallizations are also present; they commonly result in a distinct subfabric.

A new magma chamber model based on structural studies in the Oman ophiolite

Abstract Structural mapping in the mafic-ultramafic transition zone and in the overlying crustal section of the Oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros, and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites. The crustal section near the spreading axis, and in particular the magma chamber are fed by two independent melts: a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone. Layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow. Whatever its origin, the layering orientation in the newly created crust does not necessarily reflect its original attitude. The final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen. A new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges.

Development of shape and lattice preferred orientations: application to the seismic anisotropy of the lower crust

We review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock-forming silicates of the lower crust (feldspar, quartz, pyroxene and amphibole). The orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized. The fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters: shear strain, aspect ratio and interference between crystals. The same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow. In order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic P-wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks. The calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16%, compared with 5 and 8% for typical rock compositions. The strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6% compared to the 14% suggested by model studies of the observed seismic laminations. We suggest that other effects, such as compositional layering and/or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy.

Mantle flow patterns at an oceanic spreading centre: The Oman peridotites record

Abstract The mantle section of the Oman ophiolite is the largest piece of the uppermost oceanic mantle exposed at the Earths surface. Extensive structural mapping of these rocks has been conducted throughout the Oman range in order to unravel mantle processes associated with the generation of the oceanic lithosphere. The mantle peridotites of Oman have recorded two successive plastic deformations: the first one related to the accretion of the lithosphere (the “asthenospheric” shear flow), and the second one imprinted during the first step of the emplacement of the peridotites (intraoceanic thrusting). These two events have been distinguished on the basis of microstructural criteria. Four well-contrasted asthenospheric flow patterns have been documented. The diapir pattern, particularly relevant to the mantle process beneath spreading ridges, features vertical flow lines and elliptic flow plane trajectories in a pipe, the extension of which along the ridge axis is in the order of 10 km. These structures rotate to the horizontal and diverge in every direction in a narrow transition zone a few hundred metres thick below the Moho discontinuity. Such a diapiric pattern has been recognized in a few places along the Oman palaeo-ridge. A large amount of magma has circulated through these mantle diapirs, which are probably the main feeding zones of the overlying magma chamber. The second flow pattern features very intense plastic flow channelled along the ridge axis, away from a diapir. One ridge segment fed in such a way by one diapir is several times longer than the diapir section. The pattern which is by far the most common ( ~ 70% of the Oman peridotites) features very regular structures over several tens of kilometres along the strike of the palaeo-ridge; the flow plane weakly dips away from the ridge axis, and the flow line is parallel to the spreading direction. This flow pattern is frozen during the gradual accretion of the lithospheric mantle away from the ridge in a steady-state spreading regime. A shear-sense inversion at shallow depth below the Moho is frequently observed, pointing to a forced asthenospheric flow. Forced flow on the ridge flank is consistent with the existence upstream of mantle diapirs making space for themselves below the ridge axis. The structural homogeneity of the oceanic lithosphere, revealed by seismic anisotropy studies, is acquired farther from the ridge when this forced flow induced by the partially molten diapirs is superseded by the more regular flow induced by the drift of the overlying plate. This occurs at a distance from the ridge which, in the Oman case, can be roughly estimated to be a few tens of kilometres. The last flow pattern has been observed in only one area and it corresponds to a 20 km thick asthenospheric shear zone that strikes at a right angle to the ridge axis. This zone could represent a broad diffuse transform zone as described along present-day fast spreading ridges.

Mountain building: strike-parallel motion and mantle anisotropy

Abstract Geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction. Several hundred to a few thousand kilometers of displacement may have occurred this way, resulting in a pervasive ductile deformation of the lithosphere. Consequently, both crustal and mantle fabrics may reflect these movements, and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building. On the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement, a variety of tectonic models have been proposed which differ in: 1. (1) the timing of strike-parallel motions, (early—before or during continental accretion—or late—after accretion was completed); 2. (2) the geometric pattern of strike-parallel displacement zones (low-angle or high-angle); and 3. (3) the location of orogen-parallel movement zones (in outboard terranes on the subducting plate, along the suture zone or within the continental hangingwall). Large-scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle. Although few studies have been carried out from this view point, thermal, structural and petrological data support the conclusion that lithospheric faults, involving the upper mantle, may be discriminated from crustal faults rooted in some crustal decoupling zone. Where available, data on mantle flow beneath orogenic belts are in agreement with mountain-parallel displacements inferred from crustal studies. This has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts. The fast velocity direction, which is parallel to the flow direction, should therefore be recorded parallel to the strike of the belt.

Mantle—crust transition zone and origin of wehrlitic magmas: Evidence from the Oman ophiolite

Abstract Abundant exposure and well-preserved outcrops in the Oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment. The mantle-crust transition zone shows large lateral variations in thickness and composition, but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations, and gabbro lenses. Compositionally, the transition zone marks a gradational passage from mantle to crust. The Moho may be placed at the base of the continuous gabbroic crustal section. Structures both within the transition zone and near the base of the crust are nearly parallel to the Moho, however, the dominant flow mechanisms differ from the upper mantle to the lower crust. Plastic flow dominates in the upper mantle, while viscous flow, resulting in strong magmatic fabrics, dominates at crustal levels. Wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted. The parent magma of the wehrlitic instrusions is in fact a crystal-melt mixture composed of olivine and chromite, probably including a component of mantle xenocrysts and xenoliths, and of reequilibrated basaltic melt.