Obduction processes in ancient, modern and future ophiolites

Development of concepts concerning the genesis and emplacement of Tethyan ophiolites in the Eastern Mediterranean and Oman regions

The results of recent geochemical investigations of several island arc - marginal basin systems in the Scotia Sea area and in the western Pacific are outlined. Marginal basins in different stages of evolution are represented, from those in the initial stages of formation to those with an extensive and multiple history of back-arc spreading. Some are completely intraoceanic, others have developed at continental margins. Basalts erupted at back-arc spreading centres seem to be as geochemically varied as those from normal mid-ocean ridges, and record evidence for similar processes of partial melting, fractional crystallization and magma mixing in their genesis. They appear to have been derived from mantle sources with incompatible trace element characteristics ranging from ‘depleted’ to ‘enriched’, but with the ‘enriched’ mantle sources being sampled during the earlier stages of back-arc spreading. Submarine back-arc basalts are more vesicular than their normal ocean ridge equivalents, and their corresponding glasses have higher water contents. This, together with other geochemical features such as the higher ratios of lithophile to high field strength elements in some back-arc basalts, suggests that a component from the subducted slab may be involved in their petrogenesis. The chemistry of the corresponding arc volcanics is described in relation to the subduction and extensions history of marginal basin development. In intraoceanic arcs the early stages of arc magmatism are dominated by the eruption of large volumes of island arc tholeiites and subsidiary high-Mg andesites. In the Mariana region, after the initial volcanic arc is split and separated by back-arc spreading, the later frontal arc volcanics have calc-alkaline characteristics. Basalts erupted during the early stages of back-arc spreading more commonly have arc-like geochemical features when the marginal basin has developed through splitting of a calc-alkaline volcanic arc. The secular variation in the geochemistry of the arc volcanics may be related to the progressive development of a lithophile element enriched mantle source beneath the arc. This source contributes to the basalts produced during the early stages of arc rifting and back-arc spreading. Ophiolite complexes which represent marginal basin floor may well carry these arc-like geochemical features.

Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum‐Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific‐type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward‐dipping subduction zone(s). The 650‐500 Ma arc‐related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490‐470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present‐day western Pacific. At 480‐470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460‐430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc‐continent collision(s). From 435‐400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430‐400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro‐Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390‐370 Ma collision of Gondwana‐derived Aquitaine‐Cantabrian blocks with Eastern Avalonia‐Baltica and removed the part of the Iapetus south of Baltica.Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present‐day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid‐Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a < 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably < 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present‐day Mediterranean.

Plate kinematics, origin and tectonic emplacement of supra-subduction ophiolites in SE Asia

Summary The Semail Ophiolite in Oman represents one of the well-preserved ophiolite complexes globally and provides a unique window into the processes of obduction. The emplacement of Semail ophiolite onto the Arabian lithosphere is a result of intra-oceanic subduction, was strongly influenced by inheritance features preserved from pre-obduction tectonic processes. Therefore, a detailed characterization of the crustal architecture and rheological properties of the lithosphere are essential for improving our understanding of obduction processes. In this study, we investigated the crustal structure and Moho depth beneath the central Oman Mountains through analysis of P-wave receiver functions (PRFs) and Bouguer gravity anomalies. We utilize broadband seismic data recorded at 12 seismic stations spanning the ophiolite belt and surrounding regions (Ghaba basin and Saih Hatat Dome). PRFs analysis reveals noticeable lateral variations in Moho depths ranging from ∼39 km beneath the sedimentary basin to ∼46 km beneath the ophiolite belt, and decreasing to ∼30 km underneath Saih Hatat Dome (SHD). 2D forward modeling of Bouguer gravity anomalies (−36 to 91 mGal) constraints with seismological results shows flexural bending of the Moho topography and thin crust (∼ 30 km) beneath the SHD. The 2D forward flexural modelling analysis suggests that lithospheric flexure is due to the emplacement of the ∼5 km thick Semail Ophiolite. The presence of a thin crust beneath the SHD is caused by Permian rifting and thinning of the continental lithosphere. The observed high value of Vp/Vs (1.75 – 1.87) also provides support for Permian mafic intrusions to the lower crust. The Arabian lithosphere exhibits lower mechanical strength in the southern region (Te = 25 km) relative to the northern area, a characteristic likely inherited from pre-obduction magmatic processes. These results provide new geophysical constraints on the crustal architecture of the southern Oman Mountains and emphasize the role of surface loading in shaping lithospheric structure during ophiolite emplacement.

Ever since their recognition, ophiolites have been a source of controversy, although all workers have agreed upon their importance. A number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers. An expanded version of the Penrose ophiolite definition is proposed, herein called the ophiolite association, taking into account rocks which bear upon the origin and significance of ophiolite complexes. An ideal ophiolite association includes the following units from bottom to top: (1) a crystalline basement and shallow water sedimentary sequence, (2) a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and/or melange, (3) a metamorphic unit, as much as a few hundred meters thick, generally with higher‐grade rocks over lower‐grade ones, (4) ultramafic tectonite unit composed dominantly of multiply deformed peridotite, dunite, and minor chromitite, (5) a cumulate complex, ultramafic at the base grading to mafic or intermediate at the top, (6) a noncumulus unit of varitextured gabbro and minor trondhjemite (7) a sheeted dike complex, (8) an extrusive section of massive and pillowed flows and intercalated sediments, (9) an abyssal or bathyal sediment sequence which may include radiolarian chert, red pelagic limestone, metalliferous sediments, breccias, and/or pyroclastic deposits, and (10) postemplacement deposits of laterite, reef limestone, or shallow marine or subaerial sediments. This expanded ophiolite association allows greater precision in inferring original origin and emplacement. Some ophiolite complexes, particularly in the Tethys (herein designated Tethyan type), exhibit units 1, 2, 3, and 4, whereas others, particularly in the Cordilleran belt (herein termed Cordilleran type), lack units 1, 2, 3, and, in places, 4. The emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone. Emplacement of the Cordilleran type generally is not clear‐cut. The nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5–9 than by chemistry or mineralogy. Some ophiolite complexes lack complete sections and may have formed along transform faults. Others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc. Several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone. Such conversion may result from normal plate evolution or from major plate reorganization. Major times of ophiolite emplacements appear to correspond with global plate reorganizations.

Precambrian evolution of the Sirwa Window, Anti-Atlas Orogen, Morocco

For reference to paper by H. H. Wilson under discussion, see this Bibliography Vol. 33, No. 5, 01 E69-08579

A combination of geochemical and field investigations of the Ballantrae Complex allows some control on the relative timing of events in its tectono-magmatic history. In particular, the presence of boninitic lava and hyaloclastite confirms an oceanic, supra-subduction zone origin for this early Ordovician ophiolite assemblage. Boninite eruption was preceded by a substantial phase of late Tremadoc island-arc volcanicity and the boninitic units themselves are interbedded with early Arenig primitive island-arc tholeiites. Various supra-subduction zone extensional processes have been invoked to explain boninite occurrences in recently active oceanic island arcs; interarc rifting as a precursor to marginal basin formation is preferred as a model for the Ballantrae Complex. Evidence to support this preference is drawn from the early to mid-Arenig part of the Complex where sandstones derived from mature volcanic arc lavas are conformably overlain by tholeiitic lavas erupted in a back-arc basin; a possible analogue is seen in the active Bransfield Strait marginal basin. Thus, an evolutionary sequence can be envisaged for the Ballantrae Complex which begins with the establishment of an oceanic island arc where supra-subduction zone extension led to the genesis of boninitic lavas. Next, as extension continued, a marginal basin developed wherein sandstones derived from the arc lavas were deposited and tholeiites of ocean island type were erupted. Imbrication of the back arc sequence and accretion of the ophiolite terrane was initiated in the late Arenig by arc-continent collision.

Detailed airborne magnetic studies conducted over the region of the S.W. Pacific marginal basins extending from the Solomon Islands to New Zealand suggest that three major phases of basin formation and island arc development have occurred in this region. Development of the Tasman Sea took place during the Late Cretaceous‐Paleocene. Development of the basins to the east of the Tasman Sea occurred predominantly during the Oligocene as well as during the Upper Miocene to Recent. The South Fiji Basin, consisting of the Kupe and Minerva Abyssal Plains, is marked by the presence of possibly two RRR triple junction spreading centers that were active between the times of anomalies 13 to 7 (36–25.5 m.y.). The Kupe Abyssal Plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center. The western limb appears to have been subducted beneath the present site of the Three Kings Rise. This seafloor spreading phase (calculated half‐spreading rate of 35 mm/yr) was coincident with the overthrusting phase of the New Caledonia ultramafic rocks. During that period, active volcanism along the then continuous Solomons‐New Hebrides‐Fiji‐Lau Island arc was taking place. Magnetic anomalies from 1 to 4 (0–8 m.y. B.P.) are seen to extend along a clearly defined lineation pattern over the North Fiji Basin. Analysis of the magnetic anomalies suggests that during the Upper Miocene, at about the time of magnetic anomaly 4 (8 m.y.), the Solomons–Lau Island arc commenced breaking up through the development of an active spreading center (calculated half‐spreading rate of 35 mm/yr) located between the New Hebrides and Fiji portions of the island arc. Portions of the northeastern sector of the Minerva Abyssal Plain were subducted beneath the North Fiji Basin during this period between 0–8 m.y. B.P. At the time of about anomaly 3 to 2′ (4.5–3.5 m.y.) the development of the North Fiji Basin extended into the Lau Basin and Havre Trough, thus separating the Tonga and Lau Ridges. As a result, a distinct series of active spreading centers developed southward along the axis of the Havre Trough–Lau Basin. The Havre Trough is marked by present seismic and submarine volcanic activity. Residual magnetic anomalies 1–2 (0–1.8 m.y. B.P.) form a symmetrical pattern around the axis of the trough extending south from the Lau Basin to the active Taupo‐White Island Volcanic Zone of New Zealand. Phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of Pacific Plate motion.

ABSTRACTThe Tibetan–Tethys zone of the Zanskar Himalaya shows a complete Mesozoic shelf carbonate sequence overlying metamorphic basement of the Central crystalline complex and Palaeozoic sedimentary rocks. Continental rifting in the Permian produced the alkaline and basaltic Panjal volcanic rocks and by Triassic time a small ocean basin was developed in the Indus-Tsangpo zone. Stable sedimentation continued until the Middle-Late Cretaceous when a thick sequence of tholeiitic to andesitic island arc lavas (Dras arc) were erupted in the basin above a N-dipping subduction zone. The Spontang ophiolite was emplaced southwards onto the Zanskar shelf edge during latest Cretaceous or earliest Tertiary times.Following emplacement of the Spontang ophiolite, deep-sea sedimentation ended abruptly with initial collision between the Indian plate and the Dras island arc. Emplacement of the massive Ladakh (Trans-Himalayan) batholith along the southern margin of Tibet in late Cretaceous-Eocene time occurred by crustal melting as a result of northward subduction of Mesozoic oceanic crust along the Indus subduction zone. Southward-directed thrusting in both Zanskar and Indus zones accompanied ocean closure during the late Cretaceous–Eocene. Late Tertiary compression caused intense folding, overturning and a phase of northward-directed thrusting along the Indus suture zone and the northern margin of the Tibetan–Tethys zone, resulting in a large amount of crustal shortening.

The upwelling mantle within the Banda Sea is modeled using computer visualization techniques. The Banda Sea is considered a triple junction of the Pacific, Australian, and Eurasian Plates within the plate tectonic hypothesis. Evaluation of mantle depths from gravity and seismic studies indicates upwelling of mantle from approximately 30-40 km under the continental shelf of Australia to 21 km in the Banda Arc. From here the mantle rises to 14 km within the Weber Deep and finally reaches a depth of 7 km in the North Banda Sea. Seismic epicenter data delineate spatial boundaries of flow regimes and define magmatic migration routes. Epicenter magnitudes are visualized in 3 dimensions by color-coding. Topographic and bathymetric data from in-house sources define geographic position and geomorphology of the model domain, while altimetry data delineate the gravity field associated with a component of mantle dynamics using contour lines or interpolated color fields. Conceptual animation portrays upwelling and divergence of mantle flow structures (geostreams) underlying the tectonic trends of the region and the resulting counterflow within the volcanic arcs based on the surge tectonic hypothesis. This animation uses a series of color-coded arrows and particle systems to represent these flow structures in motion. Induced micro-gravity oscillations in the triple junction may be caused by planetary gravity waves. This phenomena is explored to determine the coupling effects with the atmospheric pressure flux of the Southern Oscillation, which modulates El Nino.

High heat flow and shallow crustal depths have been observed in many marginal basins landward of the trench-island arc system in the western Pacific. All the basins in the north-western Pacific where adequate heat-flow data exist, except the deep west Philippine basin, show a high mean heat flow. This is in marked contrast to the southwestern Pacific, where only the Fiji plateau and Lau basin have high heat flow. All active midocean ridges show a uniform relationship between heat flow and elevation: high and shallow where the ridge is young, and uniform and deep where the oceanic crust is old. In the absence of magnetic data in the marginal basins, this relationship has been used to determine whether or not these basins have the same thermal and dynamic structure as the midocean ridges. The heat flow and elevation data from the southwestern Pacific basins are compatible with the ridge data, whereas heat flow and elevation data from the Japan abyssal plain, Parece Vela basin, and the Shikoku basin in the northwestern Pacific have too high a heat flow for their mean depth. Convincing evidence has been presented that the marginal basins of the northwestern Pacific owe their origin and development to large-scale crustal extension landward of the island arc system. However, the intrusion of oceanic crust associated with this extension cannot alone adequately account for the heat flow and elevation data. Three other factors are discussed that, in addition to the intrusion process, may account for the observed distribution of heat flow and crustal depth: (1) continuous incipient intrusion, which raises the ‘effective’ thermal conductivity of the crust and hence the heat flow in these basins; (2) development of a thinner lithospheric plate landward of the island arc system of the northwestern Pacific; and (3) dynamic forces acting on the upper mantle landward of the downgoing slab, decreasing the observed elevation of the marginal basins. There are problems with all three of these factors, but a model using only the second is preferred, because it is simple to develop and is compatible with rigid plate tectonics.

The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet+ diopside amphibolites, enstatite+cordierite+sillimanite+spinel±sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet+diopside+scapolite+plagioclase±wollastonite. P–T conditions of granulite facies metamorphism are in the range 800–860°C and 10.5±1.1 kbar to 14.7±2.8 kbar. Garnet+clinopyroxene+hornblende+plagioclase amphibolites from the metamorphic sole record peak P–T conditions of 840±70°C and 11.6±1.6 kbar (THERMOCALC average P–T mode) and 840–870°C and 13.9–11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57–46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. 40Ar39Ar hornblende ages from the amphibolites range from 95–93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian–Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic–Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100–1500°C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.

Suniniary High heat flow in marginal basins and island arc volcanism are explained by numerical modelling of flow induced by the downgoing slab. The hypothesis that frictional heating above the slab causes thermal diapirs to rise is mechanically unfeasible since the region above the slab must be fluid enough to convect thermally, yet so viscous that the motion of the slab produces over 2 kilobars of stress. The calculation of induced flow is two-dimensional, time dependent, with a variable viscosity dependent on temperature and pressure. A yield stress was used to approximate fault dislocations. The calculation is very sensitive to the material properties and to localized flow near the base of the interface between the slab and the landward plate. The highest stress occurred at the base of the slab interface. This area ablated creating a narrow channel necessary for separating melt from mostly crystalline mush to establish a source of volcanic magmas. In the marginal basin area, tensile yielding occurred in the plate causing it to thin and raise the heat flow significantly. The yielding continued until a complete rift occurred. Induced flow is, then, a likely explanation for island arc volcanics and high heat flow in marginal basins.