Ingrid Reuber

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.

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.

Duality of magmatism in the plutonic sequence of the Sumail Nappe, Oman

Abstract Detailed field mapping of the crustal sequence in several areas of the Sumail Ophiolite Nappe, mainly in the Fizh, the Salahi and the Haylayn blocks, and comparisons with other well-studied areas, have shown that the crustal sequence was constructed by two distinct, although nearly contemporaneous, magmatic series: The first, a gabbroic series, was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit, the high-level isotropic gabbro unit and their by-products, the sheeted dyke complex and the lower extrusives. Fine-grained homogeneous, planar-laminated gabbros commonly occur at the top of the layered gabbro sequence. In some specific areas (Wadi Haymiliyah and Wadi Rajmi), these are strongly differentiated Fe-Ti-rich gabbronorites and norites, which coincide with large-scale discontinuities within the plutonic sequence, interpreted as axial discontinuities of the spreading magma chambers. The second series, an ultramafic wehrlitic series, is locally rooted in the upper mantle (i.e., the impregnated dunitic transition zone). This has intruded the whole crustal sequence up to the pillow lavas. Countless discordant and concordant wehrlitic bodies intruded the plutonic sequence, making up 20–40% of it in volume. We describe the various settings and forms of these intrusions, which were largely underestimated in previous studies. We also describe their mineralogy and textures, and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems, independently from each other. We infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion. At the base of the crust however, indications of immiscibility between gabbros and wehrlites are common, suggesting that both magmatic series were quite contemporaneous. The wehrlites are commonly isotropic and may exhibit an internal layering of the dunite-wehrlite-clinopyroxenite type, sometimes appearing at the upper or lower margins of the intrusions. In this case, the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros. These plagioclase-free ultramafic layered cumulates, therefore, are not interbedded within the gabbro cumulates, but actually belong to the wehrlitic intrusive series. Picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives, which are for the main part cogenetic with the wehrlites. The time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre. We believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust.

Precambrian accretionary tectonics in the Bou Azzer-El Graara region, Anti-Atlas, Morocco

Ophiolites of the Anti-Atlas of Morocco occur as highly dismembered slivers of mafic and ultramafic rocks sandwiched between a Precambrian craton and a magmatic arc. Tectonic slices of accretionary melange, ophiolites, and forearc basins have been juxtaposed by oblique subduction and transpression in the forearc region of a Late Proterozoic subduction zone. Synkinematic to postkinematic calc-alkalic magmatism in these terranes indicates migration of the magmatic front over the deformed forearc terranes.

Structure and emplacement of the Alpine-type peridotites from Beni Bousera, Rif, Morocco: A polyphase tectonic interpretation

Abstract The Beni Bousera peridotite antiform, in the southern branch of the Gibraltar orocline exhibits a lherzolitic core, containing thin pyroxenite layers surrounded by harzburgites, then by dunites and garnet-bearing dunite. In the peripheral areas, the amount of pyroxenite greatly increases by the development of garnet-pyroxenite veins. These data indicate a gradient of partial-melting increasing upward. We conclude that a reversed geothermal gradient was temporarily established in the upper part of the peridotite, just under its migmatitic roof (kinzigite aureole, then sillimanite-gneiss). Study of ductile, penetrative structures and discussion of the later “cold” structures show that the foliation of the tectonites was originally flat, with a NW-SE-trending stretching lineation. Orthopyroxene crystals are used as tectonic markers. “Isostrain zones” indicate that simple shear and finite strain increase upward. We conclude that a ductile shear zone acted between peridotite and gneiss. Strain heating along this shear zone probably has been responsible for partial melting of the upper part of the peridotite. In our model, the peridotite emplacement into the mid-continental crust began during crustal extension. Then, as comparison with the Ronda massif suggests, an intracrustal thrusting stage was followed by high-temperature and relatively low-pressure metamorphism and by intrusion of acidic dikes. The uplift of the mantle slab continued by compressive upthrusting and isostatic doming. This complex, polyphase evolution probably extended beyond the Alpine orogeny, as suggested by Kornprobst.

Complexity of the crustal sequence in the northern Oman ophiolite (Fizh and southern Aswad blocks): the effect of early slicing?

Abstract On the basis of detailed mapping of the Fizh and southern Aswad blocks in northern Oman, the complexity of the crustal sequence is shown. Lateral facies variations are at least as important as vertical ones, and no section can be reconstructed as a “stratigraphic” section. Sharp and transitional Moho boundaries alternate over a distance of 10 km. The important residual mantle-crust transition zones are root zones for intrusive wehrlites, and cause doming in the Moho. Fractures, facilitating the penetration of seawater in the upper crustal parts (dikes and lavas) show intense hydrothermal alteration and can be related to shear zones in the lower parts of the crustal sequence and occasionally be followed into the peridotites. Intrusive wehrlites reach as high into the crust as the base of the dike complex and occasionally even the lavas. They follow fault zones or N-S to NW-SE-directions. Recrystallization of isotropic gabbro occurs in the vicinity of the wehrlites. The geometry observed is not compatible with an oceanic transform zone; it may indicate the location of an overlapping spreading centre, although too little is known of the deeper structures to confirm this. There was certainly an important reaction to the early compression at the oceanic spreading ridge prior to final consolidation, confirming an earlier model of the setting.

The Dras arc: two successive volcanic events on eroded oceanic crust

Abstract The Dras arc is recognized as a volcanic arc system in the western part of the Indus suture zone and it constitutes the link between the Ladakh batholith and the Kohistan arc. This study is based on detailed mapping of the area between Dras, Kargil and Sanku which revealed the following: 1. (1) The ultramafics of Dras and Thasgam can be followed across the Suru Dras ridge and are not intrusive into the arc volcanics, but instead constitute the most probably oceanic substratum of these volcanics. 2. (2) Successive volcanic events are distinguished: 2.1. (a) Dras I is a variable volcaniclastic series rich in slates and carbonates, which can probably be assigned to the Albo-Cenomanian, as dated by orbitolines. This series is intruded by gabbro, diorite and granite and is deformed, essentially in the northern part. It is unconformably overlain by 2.2. (b) the Dras II pyroclastics which grade southward into volcanic breccia and thus enable the location of the centres of volcanic activity during this younger period.

Obduction-related ophiolitic polymict breccias covering the ophiolites of Antalya (southwestern Turkey)

The ophiolites of the Antalya nappes, cropping out in the area of Kemer (southwestern Turkey), are capped by a spectacular detrital sequence composed of various ophiolitic breccias whose total thickness reaches 1000 m. Four main formations have been distinguished within the breccias. The basal formations are in depositional contact over the previously tectonized and eroded deepest part of the ophiolites. The upper breccias are dated as middle Maastrichtian, which is the age of the deposition of synchronous serpentinitic olistostromes onto the continental margin. These data suggest that the breccias formed as debris slides from bathymetric highs on the ocean crust as a result of renewed tectonic activity accompanying ob-duction onto the continental margin.