Axial segmentation at a fossil oceanic spreading centre in the Haylayn block (Semail nappe, Oman): off-axis mantle diapir and advancing ridge tip

J. Geodynamics Vol. 13, Nos 2-4, pp. 253-278, 1991 Printed in Great Britain. All rights reserved

0264-37117/91 $3.00+1k00 Copyright © 1991 Pergamon Press plc

A X I A L S E G M E N T A T I O N AT A FOSSIL O C E A N I C S P R E A D I N G C E N T R E IN T H E H A Y L A Y N B L O C K ( S E M A I L N A P P E , OMAN): O F F - A X I S M A N T L E D I A P I R A N D A D V A N C I N G R I D G E TIP

1. R E U B E R , P. N E H L I G and T. J U T E A U

GDR GEDO, UniversitO de Bretagne Occidentale, 6, av Le Gorgeu, 29287 Brest Cedex, France

ABSTRACT Reuber, i., Nehlig, P. and Juteau, T., 1991. Axial segmentation at a fossil oceanic spreading centre in the Haylayn block (Semail nappe, Oman): off-axis mantle diapir and advancing ridge tip. Journal of Geodynamics, 13: 253-278. The Haylayn block, located in the central part of the Oman ophiolite, is characterized by a number of special features, in its south-eastern end, that previously led to various interpretations. These characteristics are: - - a n important change in the strike of the primary dykes at the level of the sheeted dyke complex, crosscut by a dense pattern of ridge-parallel secondary dykes; - - a steepening and rotation of the petrological Moho and the cumulate layering in places cut by numerous ridge-parallel diabase dykes; --intrusive planar laminated gabbronorites, associated with important magmatic breccias; --large bodies of intrusive wehrlites related to a dome of mantle harzburgites outcropping within the plutonic sequence. The dome of harzburgites is interpreted as an off-axis diapir placed approximately 5 km south of the spreading axis, generating the magma of the huge wehrlite bodies. Some of the gabbronorites (Haymiliyah area) might result from the ultimate differentiation, in a closed fractionation system, of the old rift system, whereas the structural relationships of the others (SE of the studied area) require the existence of a new propagating rift.

INTRODUCTION

Recent studies along the EPR combining high resolution "swath"-mapping systems, multi-channel seismic experiments and detailed geochemical investigations on dredged samples have demonstrated the occurrence of various types of discontinuities along the spreading axis of the ridge. These discontinuities partition the axis into segments which may migrate, lengthen or shorten with time (Macdonald et al., 1988). These discontinuities occur at a range of scales. The first-order segmentation is tectonically defined by major transform faults and propagating rifts: these are long-living boundaries, partitioning the ridge into distinctive tectonic units which persist for millions of years. They occur at intervals of 300-500 km and are 253

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characterized by both large axial depth anomalies (500-3000 m) and pronounced geochemical features (i.e. highly Fe-Ti fractionated basalts). The second-order segmentation, at intervals of 50-300 kin, is defined by smaller, non-rigid offsets of the spreading centre: these include large offset overlapping spreading centres (OSCs), and, eventually, small offset ( < 20 km) transform faults. They are characterized by axial depth anomalies of several hundred metres. Small-offset (0.5 to 3-5 km) OSCs, with axial depth anomalies of several tens of metres, and very small lateral offsets ( < 0 . 5 kin) of the axial summit graben (SNOOs or Small Non-Overlapping Offsets, Batiza et al., 1986), are usually inferred geochemically, or by small changes in strike (1-5 ° ) of the axis (DEVALs or Deviations of Axial Linearity, Langmuir et al., 1986) and rarely have a clear tectonic or bathymetric signature. The segmentated character of ridges has been explained by the distribution of mantle sources of partial melting at discrete locations beneath the ridge axis (Macdonald et al., 1986; Crane, 1985; Kastens et al., 1986: Schouten and Klitgord, 1982; Whitehead and Schouten, 1984; Macdonald et al., 1988). In this model, partial melts segregate and ascend, feeding axial reservoirs at discrete places beneath the ridge axis. These melts locally swell the crustal magma reservoir and create a local high in the axial depth profile of the ridge. Local eruptions are concomitant with along-strike migration of magma away from the source of upwelling. In this magmatic model, ridge-axis discontinuities occur at the distal ends of magmatic pulses, usually at deep points along the axial depth profile, and define the ends of ridge segments. Basalts dredged at these discontinuities are frequently highly fractionated (Langmuir et al., 1986; Sinton et al., 1983). If the morphotectonic features and basalt compositions of oceanic ridge segmentation are now reasonably well documented, we still have a poor idea of its expression at depth, at the level of Layers 2 and 3 of the oceanic crust. Ophiolites may provide useful information, but most of the ophiolite complexes do not offer sufficient length of observation along the strike of the fossil spreading axis, indicated by the attitude of the sheeted dyke complex. The Semail Ophiolite in Oman is a remarkable exception: this piece of Cretaceous oceanic lithosphere extends over more than 400 km, roughly parallel to the mean attitude of the sheeted dyke complex. Most authors agree on the point that this fossil lithosphere was created at an intermediate-to-fast spreading centre (Pallister and Hopson, 1981; Nicolas et al., 1988a,b), and favour a comparison with the EPR or more complex areas in the Pacific, or, alternately, with rather fast spreading centers in marginal basins (Lippard et al., 1986; Beurrier, 1987), rather than with slow spreading ridges such as the Mid Atlantic Ridge. Figure 1 shows the size of the Semail Ophiolite compared to a significant segment of the EPR. It is clear that, along the 400 km of the Semail Nappe, various axial discontinuities should be recorded in the fossil oceanic crust, especially in the well-exposed plutonic sequence as suggested by previous work: (i) Major transverse structures evoking fossil fracture zones have not been observed along the Semail Ophiolite. Hence, axial discontinuities, if recorded in

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Fig. 1. Location of the ophiolite blocks of the Semail Nappe, with indication of possible areas of axial segmentation (A = Rajmi, B = W u q b a h , C = H a w q u a i n , D = I b r a , E = Ghuzayn). The rectangle indicates the area studied in this paper. A s e g m e n t of the East Pacific Rise is given at the same scale for comparison.

the Semail Nappe, should be of the second order, and the Semail Nappe as a whole was probably detached between two first order fracture zones (namely the Dibba zone in the north and the Owen fracture zone in the south). (ii) Several diapiric structures have been mapped in detail in the mantle sequence (Ceuleneer, 1986; Ceuleneer et al., 1988; Nicolas et al., 1988a), which could correspond to the location of the extraction and upwelling of partial melts, in the central parts of the fossil accretion segments. Four occurrences of such diapirs have been mapped in detail with their corresponding mantle structures (Ceuleneer et al., 1988), and about nine of them have been deduced from variations in the petrology of the crust and mantle (abundance of wehrlites and transitional dunites, etc: Nicolas et al., 1988). This gives a minimum length of about 30 km for the elementary accretion segments. (iii) The extremities of such segments are less documented: they may be sought in areas where the strike of the dyke complex is modified, or where two generations of sheeted dykes exist. Such areas are, from north to south (Fig. 1, areas A to D respectively): the northern Fizh Block, interpreted as a leaky transform

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fault (Smewing, 1980), or as a possible OSC (Reuber, 1988); the Wuqbah area (Rothery, 1983); the Haylayn Block in its SE part (studied here, Browning, 1982); and the Ibra area, recently interpreted as a leaky transform fault (Nicolas et al., 1988a). Based on the drastic decrease of the cumulate thickness, the NW end of the Haylayn block was also proposed as the tip of a spreading segment by Dahl (1984, E on Fig. 1). In this paper, we present cartographic and structural evidences of axial segmentation in one specific area of the Semail Nappe crustal sequence, which provides valuable insights about the deep seated structures of these singular spots, inaccessible in the present day oceans.

GEOLOGICAL CONTEXT OF THE HAYLAYN BLOCK

The Haylayn Block is located in the central part of the Oman Mountains (Fig. 1), where it outcrops on the northern flank of the Jabal Akhdar NW-SE anticline and the Hawasina window. It exhibits the classical lithological units defined in the ophiolitic section of the Semail Nappe (Browning, 1982; Boudier et al., 1983; Dahl et al., 1983; Dahl, 1984; Ceuleneer, 1986; Beurrier, 1987; Juteau et al., 1988a; Nehlig, 1989). Harzburgitic tectonites make up the south-western half of the Haylayn Block. They overlie the northern flank of the Jabal Akhdar, and their basal contact with the Hawasina volcano-sedimentary nappes is marked by discontinuous outcrops of metamorphic rocks (amphibolites and metasediments) (Searle and Malpas, 1980: Ghent and Stout, 1981; Boudier et al., 1985; Ceuleneer, 1986; Rabu, 1987). The north-western part of the block exhibits a classic succession of lithological units (harzburgites, layered ultramafics, layered gabbros, isotropic gabbros, sheeted dyke complex and extrusives) from SW to NE, that is, from bottom to top, (Juteau et al., 1988a,b). The south-eastern part, however, is marked by several specific features (Fig. 2): (1) the reappearance of mantle tectonites within the plutonic sequence, in the middle of an antiform structure oriented NW-SE, crosscut by the Wadi al Hawquain; (2) the spectacular development of non layered, planar laminated, opx-rich gabbronorites and norites; (3) a marked curvature of the planar structures of the gabbroic sequence (layering and lamination planes), in the Wadi Haymiliyah area; (4) the abundance of magmatic breccias developed at the top of these gabboic units; (5) numerous and huge wehrlitic intrusions intruding the whole gabbroic sequence, especially at the base of the crust; (6) a dense net of dykes crosscutting the layered cumulate sequence, and the existence of two sets of sheeted dykes in the south-eastern extremity of the Haylayn Block.

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The first descriptions of the Haylayn Block (Browning, 1982; Dahl et al., 1983) pointed out the exceptional thickness of the plutonic sequence in the Wadi Haymiliyah area, described the main units in the plutonic sequence, and noticed the great n u m b e r of wehrlitic and dunitic intrusions in the gabbros. Subsequently, Dahl (1984) introduced the concept of a "sandwich horizon" composed of laminated noritic gabbros associated with magmatic breccias, interbedded between an upper layered cumulate sequence crystallized at the roof of the magma chamber, and a lower one crystallized at the floor of the magma chamber. He interpreted the thick Haymiliyah plutonic sequence as developed over an active feeding zone of the magma chamber. The reappearance of the mantle tectonites in the plutonic sequence, the complex attitude of the layerings and laminations in the south-eastern part of the Block, as well as the exceptional thickness of the Wadi Haymiliyah plutonic sequence, were ascribed by Boudier et al. (1983) and Beurrier (1987) to postaccretional folding during an early compressive phase, related to the intraoceanic detachment of the ophiolitic slab, or to post-obductional tectonic events, respectively. More recently, the cartographic curvature of the layering and lamination of the gabbros of Wadi Haymiliyah has been interpreted as representing a primary, magmatic feature, contouring the outline of a magma chamber tip (Juteau et al., 1988a). Detailed mapping at 1/20,000 scale of the south-eastern end of the block and new analyses of the interactions between the magmatic and tectonic processes are now presented: these data strongly suggest that we are dealing here with a case of axial segmentation, as suggested by Juteau et al. (1988a).

NEW GEOLOGICAL

A N D S T R U C T U R A L D A T A IN T H E S O U T H - E A S T E R N P A R T O F

THE HAYLAYN BLOCK

A gentle NE-ward dip of the whole nappe results in a general upward succession of the section from SW to NE (or down-wadi). This structure is well observed in the northern and central part of the Haylayn block, notably north of Wadi Haymiliyah. In the south-eastern part presented in Fig. 2 (from Wadi Haymiliyah to the south-eastern end of the block), the structures are more complicated. Three domains can be distinguished in this area, which are in gradual contact but have their own dynamic significance: (1) The dome of harzburgites and associated transitional dunites and wehrlites, interpreted as a mantle diapir; (2) the area NW of Wadi Hawquain, interpreted as the south-eastern end of a starving magma chamber; (3) the crustal sequence SE of Wadi Hawquain, interpreted as the northwestern tip of a new magma chamber, intrusive and propagating into an older crust.

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The dome of mantle harzburgites and surrounding transitional ultramafics The harzburgites. South H a w q u a i n , foliated harzburgites occur over about 13 × 6 km and are s u r r o u n d e d by transitional dunites along their southern, eastern and north-western margins, and by ultramafic and mafic cumulates to the northeast and to the west. The quite h o m o g e n o u s harzburgites ( o p x = 25%, no cpx), with rare pyroxenitic or dunitic banding are affected by a rather weak asthenospheric deformation. The g e o m e t r y of this d e f o r m a t i o n is quite complex (Fig. 3): - - a p r e d o m i n a n t l y linear fabric with a near to vertical lineation associated with variable ( W N W - E S E or NE-SW) striking foliation planes are observed in three areas on both sides of Wadi H a w q u a i n , alligned NW-SE; - - s t e e p foliations associated with subhorizontal lineations suggesting horizontal flow and possible strike-slip m o v e m e n t s separate the areas with .near to vertical lines;

260

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--flat foliations are c o m m o n in the southern part of the harzburgite dome, where they are parallel to the flat overlying gabbroic cumulates towards the SE. The associated lineations show a nearly radial pattern around the southeastern occurrence of the steep structures. - - s h e a r senses indicating a flow away from the central part of the vertical structures are observed, a dextral m o v e m e n t on WNW-ESE striking high angle foliations, and a less convincing conjugate sinistral one in north-southerly directions. These observations suggest that these small structures of a few km in diameter represent small superficial domes developed on the summit of a mantle diapir. The pattern of reported structures is similar to those observed in other mantle diapirs (Ceuleneer et al., 1988). Furthermore, two chromite pods of remarkably similarity with those of the Maqsad diapir (Ceuleneer and Nicolas, 1985) were discovered recently at the summit of the northwestermost diapir (G. Ceuleneer, pers. comm.). Transitional ultramafics surrounding the harzburgitic diapir. The mantle harzburgites pass southwards and north-westwards into dunites and wehrlites. Deformational structures are less visible in the dunites, and clinopyroxene and plagioclase are interstitial and undeformed. These facies are considered to represent the top of the mantle impregnated by magma and consequently called "transition zone" (Benn et al., 1988). In the large area of transitional dunites, stretching from Hajir to upper Wadi Falah (Fig. 2), lenses of layered gabbro appear intrusive or 'in situ" sills within the transition zone. At the eastern margin of the dome, these dunites grade to pegmatitic or weakly layered wehrlites, which, in turn, are intrusive into layered gabbros, and are crosscut by diabase dykes. A n o t h e r huge body of dunites and wehrlites is rooted in the NW margin of the harzburgite dome, and rises to a level near to the sheeted dykes between Huwayl and Gharwah (Fig. 2). The wehrlites are commonly isotropic and contain centimetric poikilitic diopside. In some parts of the huge mass a weak layering can be observed. The layered wehrlites of Gharwah, showing a gently northwestward dip, represent an upper, slightly differentiated part of this magmatic body, which continues further north-west with isolated patches of wehrlite that intrude even the magmatic breccia of Wadi Haymiliyah. Towards the SE, these wehrlites grade to transitional dunites, which are commonly impregnated, and then to the harzburgites. A b u n d a n t diabase and plagiogranite dykes intrude the wehrlites and transitional dunites here. As in other parts of the Semail Nappe (Benn et al., 1988; Juteau et al., 1988a,b), the wehrlites of this area are interpreted as representing the crystallization product of partial melts of picritic composition rooted in the residual dunitic transition zones. The map of Fig. 2 shows that important volumes of wehrlites were produced by the al Hawquain harzburgitic dome, and were intruded NWward and SE-ward, grossly parallel to the fossil ridge.

A X I A L S E G M E N T A T I O N A T A FOSSIL O C E A N I C S P R E A D I N G C E N T R E

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Diorite and granite plugs. Small (1 to 50 m wide) dioritic and granitic dykes and lenses outcrop in the vicinity of Hajir (Fig. 2). They have a mean orientation of N120°E and crosscut the topmost part of the tectonites and the base of the plutonic sequence. They have a monzonitic texture and consist of quartz, plagioclase, potassic feldspar and biotite. Structural, geochemical and geochronological evidence indicates that they were intruded well after accretion during obduction by partial melting of either continental crust (Lippard et al., 1986), or of the volcanosedimentary sole during earlier oceanic detachment (Boudier et al., 1988).

The Wadi Haymiliyah area: the south-eastern tip of a starving magma chamber This area grades into the regular ophiolitic sequence towards the NW, but exhibits a number of distinctive peculiarities in the area considered. Harzburgites representing the depleted upper mantle constitute the south-western limit of the studied area. Here, they show a high temperature foliation of medium intensity and steep dip, trending east-west in the northern part (Haylayn to Musayfiyah), and grading to NNW-SSE in the southern part (Wadi Haymiliyah). The associated lineation is near to horizontal to the North, and has a steep pitch to the South (Fig. 4). The ultramafic rocks. From Wadi Haylayn to the Wadi north of Haymiliyah, layered gabbros or wehrlites/pyroxenites rest directly over the mantle harzburgites. Near Musayfiyah, dunites and pyroxene-plagioclase-impregnated dunites, less than 300 m thick, represent a small transition zone. Two 300 m-thick laccoliths of dunite-wehrlite are clearly intrusive into the lower layered gabbros of Wadi Haymiliyah: they represent magmatic rocks and not transitional depleted facies here, though they are gradually rooted in the transitional dunites of Hajir. Other non-intrusive and often well-layered ultramafics are commonly itercalated throughout the studied area close to the petrological Moho. They are made up of wehrlites and pyroxenites with minor interstitial plagioclase and occasional gabbro layers. This well-layered facies of medium grain size is normally interlayered within the cumulate gabbros between Musayfiyah and north of Wadi Haymiliyah (Fig. 2). The layering is generally parallel to the petrological Moho and to the foliation in the harzburgites (Figs 2 and 4). Southeast of Hajir, ultramafic rocks are the predominant plutonic facies. Their layering is generally weak, and pegmatitic grain sizes (1-2 cm) are abundant, commonly with interstitial plagioclase in the cores of the pegmatitic zones. The transition between pegmatitic and layered facies is gradual, indicating magma percolation for the pegmatitic crystallization. SE of Haymiliyah, the layering attitude changes gradually from Moho-parallel to Moho-perpendicular, with a north and north-west to westward steep dip (Figs 2 and 4). These layers are perpendicular to the cartographic margin of the Hawquain harzburgitic dome (Fig.

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2): at the contact, these gabbro layers take the aspect of veins rooted in the harzburgites, whose foliation is parallel to them, and thus also perpendicular to the general outline of the contact. In the ultramafic facies, magmatic lineations are rare and dispersed. Layered gabbros. Layered gabbros alternate with the ultramafic cumulates described above and succeed them towards the NE (up-section). They make up 1 to 2 km in thickness between Wadi Haylayn and Wadi Hajir. Their layering is parallel to the layering of the ultramafic cumulates, and describes the same cartographic curvature (Fig. 2). Magmatic lineations are quite regular, near to horizontal or weakly eastwards dipping in the northern part, but steepen and may be dip-parallel in the Wadi Haymiliyah area. In this basal part of the crustal section, magmatic lineations thus appear to follow the attitude of the plastic lineations in the mantle, as suggested by Nicolas et al. (1988b). A n o t h e r series of NW-SE trending layered gabbros, including minor ultramafic cumulates, occurs west of Hawquain, to the north of the huge wehrlite body and harzburgitic dome. Steep dips are c o m m o n around Hawquain, but more gentle ones towards the NW (Fig. 4). Lineations are weak and quite variable even on a small scale. Laminated gabbronorites and associated magmatic breccias. Laminated gabbronorites make up the core of the curvated cumulates in the Haymiliyah area, and are well exposed between Haymiliyah and Gharwah. Their lamination is due to the orientation of pyroxene and feldspar tablets d e f n i n g a lamination plane. The orthopyroxene is an abundant cumulus phase--in place of olivine, which disappears completely--and develops as prismatic, pleochroic crystals of bronzite to hypersthene compositions (En 80-60), often more abundant than clinopyroxene crystals (Juteau et al., 1988a). The transition between layered cumulates and the first fine-grained laminated gabbros is sharp. They develop over a thickness of about 1 km and exhibit numerous reccurrences of coarse grained and layered cumulates. These layers are concordant with the lamination of the fine-grained massive gabbronorites. Fig. 4. (opposite) Simplified geological map of the same area as Fig. 2 showing the orientations of the principal planar and linear structures of the SE-Haylayn Block, showing the poles of the sheeted dyke complex, the magmatic layering and lamination planes in the cumulates, and the foliation and lineation planes in the mantle peridotites (equal area projection--lower hemisphere). - - S h e e t e d dyke complex: A = Wadi Haylayn/Mabrah = 28; 1, 5, 10, 15% ; B = Wadi Wadiyah (N = 20: 1, 5, 10, 15%); C = W a d i Haymiliyah ( N = 2 7 ; 1, 5, 10, 15%); D = E - H a w q u a i n ( N = 6 9 ; 1, 2, 5, 10%): E = D a r i s (N - 3 3 : 3, 6, 9, 12%). Note the progressive rotation of the primary dykes from west to east (Fig. 4A to 4E). - - F = Dykes in gabbros Wadi Falah (N = 60: 1, 5, 10, 15%). - - M a g m a t i c layering and lamination: O = Wadi Haylayn/Mabrah (N = 36; 1, 5, 10, 15%): N = Wadi Wadihay ( N = 2 5 : 1, 5, 10%); M = H a y m i l i y a h , Layered gabbros ( N = 106; 1~ 5, 10, 15%): K = W a d i Haymiliyah, differentiated wehrlitic intrusion (N =48; 1, 5, 10, 20); J = Wadi Hajir (N = 19; 1, 10, 15%): I = SE Haymiliyah ( N = 2 1 ; 4, 8%); H = W-Hawquain ( N = 5 , 10, 20%); G = E-Hawquain ( N = 57; 1, 3, 5%). --Foliation in peridotites: P = between Wadi Haymiliyah and Wadi Haylayn ( N = 13; 5, 10%): R = Mantle dome, South of Hawquain (N = 130; 1, 2, 3, 4%). - - L i n e a t i o n in peridotites: Q = between Wadi Haymiliyah and Wadi Haylayn (N = 11: 5, 10, 20%): S = Mantle dome, South of Hawquain (N = 120; 1, 2, 3%).

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The sharp contact between both gabbros, and occasional oblique sills of laminated gabbro indicate however, that the laminated gabbros are intrusive. A r o u n d Gharwah, in the Wadi Haymiliyah area, an important rotation of the strike and dip is observed; the layering and planar lamination rotate gradually from N 160°E to EW and then to NS with a steep dip toward the north-east and then to the west (Figs 2 and 4). These laminated gabbronorites represent differentiated facies from a tholeiitic magma evolved in a closed fractionation system, as indicated by their high Fe-Ti content and the the co-precipitation of magmatic sulphides (Juteau et al., 1989). Between Haymiliyah and Gharwah oases, an horizon of spectacular magmatic breccias develops, more than 500 m thick. They are rooted in the underlying planar laminated gabbronorites, which are more and more brecciated by dioritic, tonalitic to plagiogranitic veins and dykes going upsection. The elements of the breccia are both planar laminated gabbros and layered cumulates. The matrix is composed mainly of leuco-diorites to plagiogranites. The shape of the blocks indicates that brecciation was induced by hydraulic fracturing processes. In the upper part of the breccias, near the Gharwah oasis, a small isotropic wehrlite body intrudes the magmatic breccia, indicating that at least some of the wehrlitic intrusions postdate the development of these breccias. The upper part of the crustal section. Isotropic "high level" gabbros are poorly represented in the area: a thin strip follows the base of the dykes between Wadi al Hawquain and Wadi Haymiliyah, and between Wadi Wadiyah and Haylayn (Fig. 2), becoming more developed NW of Mabrah. These amphibole-rich gabbros are characterized by a variable grain size (often pegmatoid) and ophitic to intergranular textures. The sheeted dyke complex is exposed along nearly continuous outcrops, and appears as a set of subparallel, fine- to medium-grained diabase dykes overlying the plutonic sequence. The transition between the high level gabbros and the overlying sheeted dyke complex is usually marked by small plugs, vein networks and well defined ten to few hundred metres wide intrusive bodies of plagiogranite. The transition between 90% gabbros and 90% dykes occurs usually within less than 100 m. Intrusive relationships are complex: as usual, isotropic gabbros, doleritic dykes and plagiogranites are mutually intrusive. In the area considered here, northwest of Wadi al Hawquain, the sheeted dyke complex has an almost constant orientation of N 155°E, vertical, grading towards N 120 ° E, vertical, around Mabrah and Haylayn (Figs 2 and 4). The volcanic rocks overlying the sheeted dyke complex are only poorly exposed and are usually masked by the quaternary gravels and sands of the Batinah plain or the tectonically emplaced formations of the Hawasina nappes. Three volcanic units have been distinguished in this area by Beurrier (1987). The lower volcanic unit (SV1) appears at the top of the sheeted dyke complex, as decametric to hectometric outcrops, in the Mabrah area. It consists of slightly

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vesicular, reddish-brown elongated basaltic pillows. The upper volcanic unit (SV2) is widely exposed from Daris to Wadi Mabrah and is generally in direct cartographic contact with the sheeted dyke complex. It includes spilitic basalts, andesites and dacites and contains numerous umber lenses. A third volcanic unit (SV3) was distinguished by Beurrier (1987). It corresponds to the last volcanic unit, is composed of alkali basalts and outcrops only in the Mabrah area. This key-area of Haymiliyah was already interpreted as representing the southeastern tip of an axial magma chamber (Juteau et al., 1988a). The main arguments in favour of this hypothesis are: (a) the curvature of the magmatic structures in the gabbroic sequence: the absence of any plastic deformation in this sequence, and the subvertical axis of the curvature, totally discordant with the NW-SE regional fold axes, indicate that this curvature is contemporaneous with the accretionary stage. The intrusion of the al Hawquain peridotites may have accentuated the curvature of the Haymiliyah gabbroic sequence, when the gabbros were not yet fully consolidated. (b) the closed-fractionation system developed at the top of the gabbroic sequence (huge development of Fe-Ti-rich gabbronorites) indicates an at least partial disconnection of the Haymiliyah end of the magma chamber from its main part towards the NW. This is in agreement with the existence of an important necking of the plutonic sequence, just north of the Haymiliyah area (Juteau et al., 1988a, Fig. 17). (c) the mapped contacts between the gabbronorites and the layered gabbros (Fig. 2) indicate an intrusion of the gabbronorites from the North-West: these differentiated magmas may have migrated along-strike over some distance and have been trapped in the Haymiliyah chamber, at the extremity of the spreading cell, while the Haymiliyah axial segment was probably starving and retracting north-westward.

The crustal sequence east o f Hawquain: a westward propagating magma chamber

The crustal sequence here comprises the same main units described above, with different structural relationships. At the base of the crustal sequence, the cumulate gabbros and interbedded wehrlites/troctolites are in discordant contact with transitional dunites or harzburgites, and are intruded by wehrlites (Figs 2 and 4). Their layering trends NE-SW, and is moderately NW-ward dipping (Fig. 4G). These cumulates are intruded by a dense series of diabase dykes making up to 50% of the unit, which have the "normal" NW-SE strike, and are subvertical (Fig. 4F). The most striking feature in this area is an intrusive, subvertical body of finegrained, laminated gabbronorites, south-west of Daris, forming a N 125°E trending strip from Wadi Falah to al Hawquain. This body is narrowing northwestward (Fig. 2). In contrast to the adjacent cumulates, they are quite devoid of

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intrusive diabase dykes. The lamination (NW-SE) is highly discordant with respect to the layering of the adjacent cumulates. They are different from the laminated gabbronorites of Haymiliyah by their low Ti contents (Cotten, pers. comm.). Rare magmatic breccias were mapped in Wadi Falah in relation with these laminated gabbros (Fig. 2). The high level gabbros (to the North of the gabbronorites) present their usual thickness in this area (several hundreds of m). Two larger bodies of plagiogranite are observed in Wadi Falah and east of Wadi al Hawquain (Fig. 2). This area is also characterized by cross-cutting sheeted dykes represented by two main sets (Figs 2 and 4D,E): - - N 10 to 20 ° E, dipping 70 to 80 ° E; - - N 120 to 145 ° E, dipping 70 to 80 ° SW. The NNE trending dykes are always crosscut by the N 120-145 ° trending ones; both are crosscut by late subhorizontal sills (Beurrier, 1987). Both generations of dykes have almost the same volumetric importance and appear in the field as decametric packs of dykes in one direction, then in the other (Fig. 5). Both families of dykes are highly epidotized, suggesting vigourous hydrothermal circulation during their emplacement (Nehlig and Juteau, 1988). Geochemical data (Beurrier, 1987) show for the first generation dykes (N10-20 °) the same flat R E E patterns as for the majority of the dykes (N 155°) north of Wadi al Hawquain. The second generation dykes (N 120-145°), and those crosscutting the Daris/Wadi al Hawquain gabbroic sequence, exhibit similar patterns of generally lower R E E contents and a weak negative Eu anomaly. These dykes appear coeval with the gabbronorite (Beurrier, 1987). The upper extrusives crop out east of Wadi al Hawquain, mostly in direct contact above the dyke complex, except for a small outcrop, which can be ascribed to the lower extrusives. Drill holes (made by the BRGM) in the Daris area across a sulphide prospect showed the SV1 lava unit underlying the SV2 unit, suggesting the existence of graben structures, contemporaneous with the volcanic activity (Beurrier, 1987).

R E L A T I V E C H R O N O L O G Y OF M A G M A T I C A N D T E C T O N I C EVENTS IN T H E S T U D I E D A R E A

The following chronological scenario can be deduced from the exposed data, which are slightly different for the parts NW and SE of Hawquain (Fig. 6). The steps described here certainly represent a continuous evolution, and are probably overlapping in time from one step to the next: (i) Ultramafic and mafic cumulates, crystallized from wehrlitic and gabbroic liquids, constitute in some places coexisting but immiscible liquids, as they may be generated alternately and injected into the same magmatic chamber (Juteau et al., 1988b; Reuber 1988). This 'normal' primary crust extends from Wadi Haylayn to beyond Hajir, where it becomes increasingly ultramafic. Cumulates of the same type east and west of Hawquain and in upper Wadi Falah, probably

AXIAL SEGMENTATION AT A FOSSIL OCEANIC SPREADING CENTRE

267

Fig. 5. Outcrop showing a pack of primary dykes (left) crosscut by a pack of secondary dykes (right), East of Hawquain.

also represent the original oceanic crust. This crust included the isotropic gabbros, the first generation of sheeted dykes, and the V1 lavas. The first set of sheeted dykes was injected in NW-SE direction in the NW, turning to northsouth and NE-SW further SE around Hawquain. (ii) The laminated, opx-rich gabbronorites of the Haymiliyah area are intrusive into the layered gabbro cumulates: they are concordant with the layered gabbros and intrude them from the NW. They were brecciated at their top either by their own residual liquids (Juteau et al., 1988a), or by the residual liquids of the underlying layered gabbros (Regba et al., in press). (iii) The wehrlite intruding the breccias (Wadi Haymiliyah) is geometrically related to the Hawquain mantle dome. Thus all the wehrlitic bodies, intrusive into the cumulates and rooted in the dunitic transition zone, appear to belong to the same event and to be related to the diapir. They represent ultramafic partial melts extracted from this diapir, and injected along-strike towards the NW and SE (Fig. 2). The intrusion of the diapir itself is quite contemporaneous with the production of the wehrlites. It has probably deformed the not yet consolidated gabbros, and accentuated their primary curvature in the Haymiliyah area. (iv) The laminated, opx-rich gabbronorites of the upper Wadi Falah are intrusive into the layered gabbroic cumulates and the wehrlites, which they post-date here: they constitute a kind of vertical blade injected from the south-east between the layered cumulates and the high level gabbros. They are parallel to, and seem to be contemporaneous with, the second generation of dykes in the dyke complex and the dense swarms of diabase dykes which crosscut the layered sequence.

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DISCUSSION

We shall first give a possible mechanistic model to explain the observed features, and then discuss the magmato-tectonic evolution of this piece of oceanic crust. The deviation of the primary dykes from their regional NE-SW trend by 75 ° can result either from a rotation post-dating their emplacement as ridge-parallel dykes, or from an initially different orientation of dyking related to a change of the stress field at the ridge axis. As the rotation of the dykes appears incompatible with the bending of the cumulates, we prefer the second interpretation, based on the existence of an off-axis diapir as represented by the harzburgites south of Hawquain.

Stress or velocity field of a diapir The deviation of the primary dykes can be explained by the specific location of the observed crust with respect to a mantle diapir. The resulting stress field can tentatively be constructed by simple vector addition of the h o m o g e n e o u s extensive field, created by the tension of the plates, and a radial stress field generated

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by the diapir. The parametres are (x along-strike; y perpendicular to the ridge), while dyke injection is always perpendicular to the extensional stress (Fig. 7): the homogenous extensional field: h = (0,H) a radial extensional field at the periphery of an elliptical diapir: r = (a cos +, b sin +) The models of Fig. 7 have been constructed by simple vector addition of the thus defined velocities along the surface of the diapir, for a few simple cases (Fig. 7). The surface shape of the diapirs considered, and the corresponding stress field are: (1) a circle (sin +, cos +) with a radial field (sin +, cos +), (2) an ellipse with an axial ratio of 2/1 (2sin +, cos +) and an elliptic-radial field of equal strength around the periphery (sin +, cos +), (3) the same ellipse (2 sin +, cos +) with a velocity field of proportional elliptical strength (2 sin +, cos +). These diapiric fields can be superposed with the homogenous field, admitting different positions of the diapir, and different strength relationships between both stress fields (Fig. 7): (A) A diapir centered on the ridge axis basically adds to the general stress field (Fig. 7A). However, a deviation of this ridge perpendicular field of 30-60 ° can be observed at the longstrike extremities of the diapir. (B) An off-axis diapir with a field half as strong as the h o m o g e n e o u s field (Fig. 7B) shows again the maximum deviation of the velocity field at the longstrike extremities of the diapir, which reaches 30-45 °. (C) An off-axis diapir with a field as strong as the homogeneous one (Fig. 7C) results in a deviation of the stress field of up to 90 ° at the ridge-sided central part of the diapir, suggesting a strong deviation of the dyke orientation at the ridge itself, when this diapir is emplaced close to the ridge. (D) An off-axis diapir with a velocity field greater than the homogeneous would imply a jump of the ridge itself, and lead to an overlap.

Comparison with the field data Coming back to our field data, the statistical orientations of the sheeted dyke complex in the studied area (Fig. 4) show two remarkable features (i) the first generation of dykes (Fig. 4A to E) shows a fan-like disposition and a gradual rotation from west to east, from N140°E in the west (Fig. 4A), to N20 ° to the east (Fig. 4E, end of the dyke complex in the massif). (ii) the maximum deviation of the primary dykes is observed just to the NE of the central part of the al Hawquain mantle dome, evoking configuration (C) in Fig. 7. Any structural interpretation of the studied area should take into account these important data. We propose that the emplacement of the Hawquain mantle dome, slightly off-axis as in the sketch of Fig. 7C, is responsible for the deviation of the primary dykes of the sheeted dyke complex described in Fig. 4. For

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geometrical reasons, and to satisfy the observed dyke deviation and other structures, this diapir had to be located southwest of the ridge segment where the primary cumulates crystallized. Consequently the whole area would represent the south-western flanc of the spreading axis that created the rest of the Haylayn block. The bend in the cumulates should be due to their interaction with the diapir, before being completely consolidated. This interaction is a combination of the upward intrusion of the peridotites and the sideward m o v e m e n t of the crust. The diapir would have been located at the edge of the initial magma chamber that shows viscous deformation and magma percolation and interaction, thus at a maximum of a few km off the ridge. At the end of its activity the peridotite dome is obviously incorporated into the crust, whereas, in the beginning the different motion of the two created the special structures. The life-time of the diapir may be in the same order as the time the crustal sequence created at the ridge takes to attain the diapir, i.e. 100,000 years (for 5 km), or even less. This viscous deformation appears to affect the gabbroic sequence only. In the upper part of the crustal section, more brittle deformation results in the magmatic breccia observed, thus transmitting the stress to deviate the direction of dyking. The dimensions of the complete diapiric structure would be close to those of the harzburgitic outcrop, that is, 6 km across and 13 km along-strike, which is comparable to the size of mantle diapirs described elsewhere in the Semail Nappe (Ceuleneer et al., 1988). Off-axis seamounts at short distance from the ridge are c o m m o n in actual oceans along fast spreading ridges (Monti et al., 1987), and having their independant magma supply should correspond to small mantle diapirs. Interaction o f the oceanic crust created at the ridge with an off-axis diapir Accepting the existence of a mantle diapir located just off-axis, we examine now its effect on a crust created at the axis of an oceanic ridge. We first test a simple model with a continuous spreading axis, and then add the existence of an axial discontinuity to better explain the observations. Off-axis diapir at a continuous ridge (Fig. 8A). The evolution of a piece of oceanic crust created at the ridge axis, at a place influenced by the mantle diapir, and passing subsequently over this diapir, may be depicted in four steps, within a continuous evolution (Fig. 8A). The centre of the diapir is proposed to be located at few km south of the ridge, approximately at the lateral limit of the magma chamber along a normal fast spreading ridge segment, where the cumulates are not yet completely consolidated (Nicolas et al., 1988b). During this time (less than 100,000 years) the diapir is considered fixed, but in the end of its activity it becomes incorporated into the moving lithosphere. (i) Accretion along the normal ridge segment is modified by the proximity of the off-axis diapir, which modifies the local stress pattern (Fig. 7C): the direction

272

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of extension is greatly modified and dykes related to primary accretion may intrude nearly perpendicular to the ridge axis (Fig. 8A, 1st step). Sideways from the diapir, the dykes are oblique and 'en echelon', indicating a dextral wrench component NW of the diapir, and a sinistral one SE of it, as expected from the diapiric field. (ii) As the crust migrates off-axis, the still soft cumulates are deformed approaching the diapir representing a structural dome (Fig. 8A, 2nd step). Magma generated by the diapir percolates the not yet fully crystallized cumulates and results in pegmatitic recrystallizations. Under the assumption of a continous spreading axis, this magma, differentiated during percolation across the cumulates would be be at the origin of the laminated gabbronorites. The deformation is viscous in the lower part of the plutonic sequence, and consists in magma induced hydrofracturing in its higher levels. (iii) As the considered part of the crust arrives above the central part of the diapir (Fig. 8A, 3rd step), diabase dykes are injected into the overriding crustal sequence, now again parallel to the ridge axis. The percolating liquid would assemble in the higher levels of the cumulates, and form local magma chambers, that evolve into gabbronorite, due to the reduced size and closed fractionation system. Here, the gabbronorites of Wadi Haymiliyah and of Wadi Falah would have the same origin, which is not supported by either structural or petrological evidence. This asymmetry cannot fully be explained by the diapir alone. (iv) As the crust observed leaves the off-axis diapir, whose activity is about to end, the picritic liquid or crystal mush extracted from the well developed transition zone on top of the diapir injects the observed part of oceanic crust (Fig. 8A, 4th step). The extraction may take place by the "rolling mill" effect (Rabinowicz et al., 1984, 1987; Nicolas et al., 1988b) on the off-ridge side of the diapir, as only here the flow-lines in the mantle bend definitively to a horizontal attitude. Even more, the lateral velocity is specially high in this area (Fig. 7C), allowing an efficient magma extraction by filtre-press. These wehrlites intrude in a generally ridge-parallel to slightly oblique trend (cf. Fig. 2, 8A). Evidence for an advancing ridge tip in the SE (Fig. 8B). The vertical blade of laminated gabbros widening toward the SE, and the striking abundance of secondary diabase dykes in the same area, strongly suggest that these features were propagating north-westward. For this reason, a ridge advancing from the SE is proposed, which interferes with the main structures described above. A similar geometry of north-westward propagating dykes has been suggested for the Wuqbah massif by Rothery (1983). The initial ridge of Wadi Haymiliyah was starving and retracting towards the NW (Fig. 9). This way the gabbronorites would represent ultimate differentiates of the cumulate series in the Haymiliyah area (Fig. 8B), whereas the gabbronorites of Wadi Falah belong to the new advancing ridge. These assumptions are in better concordance with the observed structural and compositional differences between the two gabbronorite bodies. (i) The primary cumulates are formed at a ridge that is retracting towards the NW, maintaining a smaller magmatic chamber in the area studied at the diapir.

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The orientation of the primary dykes is deviated by the mantle diapir for the same reasons as above. (ii) As the crust advances over the diapir, the NW segment is further retracting. The diapir has less tectonic influence on the southeastern part, where the cumulates are already consolidated, than on the north-western part, where bending and the isolation of a small magma chamber occurs. In this relict magma chamber, the magma evolves in a closed system to crystallize the norites and some magmatic sulphides (Juteau et al., submitted). The breccias form at the transition between the viscously deforming plutonics and the brittle upper levels of the crust. Secondary ridge parallel dykes are at this stage related to the diapir. (iii) Except for these few diabase dykes and percolating magma difficult to evidence, the magma related to the diapir is represented essentially by the transitional and intrusive ultramafic bodies, extracted the same way as described above. Continuous dyking may already be related to the new ridge advancing NW-ward from SE of the diapir that is about to end its activity. (iv) The dykes related to the new ridge are more abundant and earlier in the SE than in the NW, demonstrating the direction in which this ridge advances. Our

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small off-axis diapir cedes activity, possibly in relation with the new advancing ridge tip, and migrates SW-wards together with the crust. The new ridge increases in importance and injects the body of laminated gabbronorite, as precursor of a cumulate magmatic chamber. It is probably fed along strike from a magma chamber already existing further SE (western part of the adjacent Rustaq-Nakhl Block, Fig. 9). Comparison with oceanic structures

Coming back to a comparison with the present day oceanic ridges, the size of the structures described here should be kept in mind: the offset between the two ridge segments is less than 10 km. This corresponds to the size of a very small OSC only. The proposed disposition could correspond to an en-echelon arrangement of three successive ridge segments, each of them fed by a mantle diapir (Fig. 9): In the NW, the diapir centered below the huge dunitic transition zone of Wadi Bidit, active at the time tl is ceasing its activity. The central Hawquain diapir, described in this paper, apparently created smaller amounts of magma only during time t2, and could not fully develop, because it was rapidly 'overridden" by a magma chamber tip propagating from the Rustaq diapir, starting its activity later, time t3. Its location is well d o c u m e n t e d by the huge dunitic transition zones existing in the western part of Rustaq-Nakhl massif and circular mantle structures (Fig. 9). The small Hawquain diapir studied here in detail could find its oceanic equivalent in off-axis seamounts, that also show an evolution different from the ones of a spreading axis.

CONCLUSION

The very peculiar structures described in the south-eastern part of the Haylayn block may be summarized as follows: - - a thick pile of ultramafic and gabbroic cumulates bends from Moho-para!lel to near to vertical to it; --a dome of mantle harzburgites outcropping in the middle of the crustal sequence is related with important amounts of intrusive wehrlites; --laminated gabbronorites constitute concordant and discordant intrusive bodies and are related to two different events; - - t h e primary dykes are progressively deviated from parallel to the general trend of the ridge axis to nearly perpendicular to it; --a second generation of diabase dykes crosscutting the plutonic sequence, and the primary dykes, are again parallel to the general ridge orientation. We propose a model in which the d o m e of mantle harzburgites corresponds to a mantle diapir located a few km off-axis, and having a stress field at its periphery nearly equal to the h o m o g e n e o u s tensional field developed by the plate motion. Interference of this diapir-related stress field with the general one explains the

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deviation of the first dykes as a primary phenomenon. The cumulates are still soft while passing over this dome and are consequently steepened and bent. Concordant laminated gabbronorites in the N W are either the products of magmas generated by this diapir, differentiated during percolation, or more probably the ultimate differentiates of a starving ridge segment in the Haymiliyah area. The intrusive and transitional wehrlites are clearly related to the diapir, whereas the second generation of diabase dykes may be related to the advancing ridge in the SE. The discordant body of laminated gabbronorites in the SE represents a first injection of differentiated magma of an advancing ridge in the SE.

ACKNOWLEDGEMENTS

This work has benefitted from numerous discussions and collective field work together with M. Beurrier, F. Boudier, G. Ceuleneer, A. Nicolas, H. Whitechurch and M. Cannat. J. L. Bouchez and S. Crambert kindly supplied additional structural data from the peridotites. J. L. Travers effectuated the drawings. We thank these individuals and also the Oman Ministery of Petroleum and Minerals represented by M. H. Kassim and H. A1 Azri for their constant support. This study was financially supported by the French CNRS/INSU and I F R E M E R ( P N E H O and D B T Programs).

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