Françoise Boudier

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The shearing and the associated southward thrusting occurred at the ridge and along strike when compressional tectonics superseded extensional tectonics 95–100 m.y. ago.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.

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.

Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge

Abstract Structures induced in peridotites by high-temperature plastic flow at the oceanic spreading centre source have been mapped in the Oman ophiolite. They can be reset to the geometry of the spreading system by rotating the boundary between these peridotites and the layered gabbros (the Moho) to the horizontal and by equating the trend of the sheeted dyke complex with that of the ridge. A number of typical asthenospheric flow patterns thus emerge which are more specifically described in a companion paper ∗ . The most spectacular is a structure characterized by divergence of flow planes and flow lines from the vertical to the horizontal above and around mantle diapirs 10 × 15 km in diameter. Such diapirs are also recognized by traces of an intense magmatic activity (magmatic impregnation, and an abundance of gabbro dykes and intrusions and chromite pods). The general structure of the 500 km long Oman palaeo-ridge is dominated by these diapirs which are spaced approximately every 50 km along strike. In the preliminary picture which is revealed by this study, no other mantle structure typical of a ridge segmentation is documented with the possible exception of a transform structure within the Wadi Tayin massif. Evidence converges to indicate that Oman ophiolites correspond to a fast-spreading ridge.

Kinematics of oceanic thrusting in the Oman ophiolite: model of plate convergence

Abstract Kinematics of early thrusting of Oman ophiolites is deduced using analysis of homogeneous deformation in their metamorphic soles, and is constrained by new K Ar ages. A model of decollement from a Tethyan ridge system is proposed for the 100-95 Ma period of time. The local geometry of the ridge system controls the thrust direction; a NE toward SW shortening direction of the lithosphere is deduced from the bulk vectorial composition of the senses of shear recorded in the high-temperature thrust soles. A uniform NE toward SW transport direction is also recorded in the lower-grade rocks of the thrust soles, and in the high-pressure/low-temperature metamorphic rocks of the Saih Hatat, for the 90-80 Ma period of time. This last convergence direction is that of Africa with respect to Eurasia deduced from magnetic anomalies study in the Atlantic Ocean.

Root zone of the sheeted dike complex in the Oman ophiolite

The root zone of the sheeted dike complex representing a thin zone (hundred meters thick) of extreme thermal gradient (∼5°C/m) is regarded as a thermal boundary between the convective magma chamber system below, and the main convective hydrothermal circuit which closes above, at the base of this root zone. The root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit, where the foliation steepens, and where the first diabase dikes appears. It is a complex zone characterized by mutual intrusions of microgabbros dikes (that we call protodikes) with brownish microgranular contacts against the gabbro matrix. Upward, viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex. Protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis. Later diabase dikes cross-cut the section. These relationships are obscured by the hydrothermal circulation which has generated, in particular, isotropic amphibole gabbro veins. These veins tend to propagate horizontally; they may be interpreted as the downward closure of the main hydrothermal convective circuit.

Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust

From 32 detailed cross sections through the gabbro unit of the Oman ophiolite, it is concluded that the thickness of this unit is on average 3.6 km. The lower layered gabbros represent two thirds, and the upper homogeneous foliated gabbros represent one third the gabbro unit. Assuming that the overlying basaltic lid (sheeted dikes and extrusives) is 1.5–2 km thick, the average crustal section in the Oman ophiolite is 0.5–1 km thinner than the standard 6-km-thick oceanic crust usually considered to be produced at fast spreading ridges, a point which is discussed. Variations in gabbro thickness between 5.4 km and 1.5 km are recorded. There is a general correlation throughout the ophiolite belt, particularly in the southeastern massifs, such that the thinnest gabbro units (2.2–2.5 km thick) overlay the thickest (300–700 m) transition zones which separate them from the mantle harzburgites and the thickest gabbro units (3.6–3.9 km thick) overlay the thinnest (5–100 m) transition zones. The combination of thinner gabbro units and thicker transition zones is observed above mantle diapirs or in domains which, following our structural models, were accreted above diapirs and have drifted in the spreading direction. If it is assumed that the extrusive basalt and the sheeted dike complex units have a constant thickness, such large variations indicate similar variations in the Moho level below the ridge of origin; in particular, the Moho above mantle diapirs should be some 1–1.5 km shallower than away from diapirs. As the Oman ophiolite is considered to derive from a fast spreading paleoridge, this doming should be detected in actual fast spreading ridges, as suggested by Barth and Mutter [this issue] and Wang et al. [this issue].

Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)

The Mirdita ophiolite in Albania occupies a N-S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be excluded. Classically, the western massifs have been considered “lherzolitic” and the eastern massifs “harzburgitic”. Detailed structural mapping reveals that the deep mantle section was harzburgitic and that the major differences between the two are restricted to the uppermost mantle and lower crustal section. These are typically “ophiolitic” in the eastern massifs, being composed of a thick dunitic transition zone rich in basaltic impregnations and chromite deposits overlain by a lower crust of layered gabbros. In contrast, in the western massifs the uppermost mantle is composed of highly strained to mylonitic lherzolites which originate from more depleted harzburgites by impregnation and tectonic dispersion of melt during deformation occurring at 1000°–800°C. Layered gabbros are locally absent, and the crust can be reduced to diabase dikes or sills and extrusives. The diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites. The contrast between the eastern and western massifs is ascribed to successive episodes of magmatic and amagmatic spreading in a slow spreading environment. The low-T, high strain deformation of the western massifs is localized in the dome-shaped envelopes of these massifs. This structure, and even the present-day topography of the western massifs, evoke the “turtleback” domes described along the Mid Atlantic Ridge (MAR) and explained by mantle denudation [Tucholke et al., 1998].