Dense Oceanic Lithosphere Research Articles

Bilateral Heterogeneity in an Upwelling Mantle via Double Subduction of Oceanic Lithosphere

Vietnam is a major field of Cenozoic volcanism in Southeast (SE) Asia. Two contrasting models have been proposed to explain the mantle upwelling and volcanism in this region; collision of the Indian and Eurasian continents or subduction of the Pacific or Indo-Australian oceanic lithosphere. To place constraints on the origin of the intraplate volcanism in SE Asia, new geochronological and geochemical data for Cenozoic basalts in Vietnam are presented. Based largely on Sr-Nd-Pb isotope systematics, it was found that the sources of basalts from Central and Southern Vietnam are chemically distinct forming a sharp boundary at 13.5°N. The basalts north of the boundary show isotopic features similar to Enriched Mantle type 2 (EM2) ocean island basalts. Whereas the basalts south of the boundary show isotopic features similar to Enriched Mantle type 1 (EM1) ocean island basalts. The EM1 and EM2 basalts display positive Sr anomalies and elevated Pb/Ce and Th/La ratios, respectively. Such features suggest the origins of the sources through the recycling of deeply-subducted crustal lithologies. Furthermore, subduction of dense oceanic lithosphere can induce a convecting cell in the upper mantle. Therefore, we suggest that the chemically different basalts from Central and Southern Vietnam represent the surface expression of melting in two different convecting cells, one is driven by subduction of the Pacific plate and the other by subduction of the Indo-Australian plate.

Chapter 5 Tectonic evolution of the Oman Mountains

Abstract The tectonic evolution of the Oman Mountains as of the Neoproterozoic begins with a major extensional event, the Neoproterozoic Abu Mahara rifting. It was followed by the compressional Nabitah event, still during the Neoproterozoic, in Oman but possibly not in the study area. During the earliest Cambrian, the Jabal Akhdar area was affected by the Cadomian Orogeny, marked by NE--SW shortening. It is unclear, whether the Saih Hatat area was exposed to the Cadomian deformation, too. Still during the lower Cambrian, the Angudan Orogeny followed, characterized by NW--SE shortening. An episode of rifting affected the Saih Hatat area during the mid-Ordovician. During the mid-Carboniferous, both dome areas were deformed by tilting and large-scale open folding in the course of the ‘Hercynian’ event. As a consequence, a major unconformity formed. As another Late Paleozoic event, the Permian break-up of Pangaea and subsequent formation of the Hawasina ocean basin, are recorded in the Southeastern Oman Mountains. As a result, a passive margin formed which existed until the mid-Cretaceous, characterized by deposition of mostly shelfal carbonates. This interval of general tectonic quiescence was interrupted during the early Jurassic by uplift and tilting of the Arabian Platform. The platform collapsed during the late Cretaceous, related to the arrival of the obducted allochthonous nappes including the Semail Ophiolite, transforming the passive margin to an active margin. The Semail Ophiolite formed most likely above a subduction zone within the Neo-Tethys Ocean during the Cenomanian while parts of the Arabian Plate were subducted to the NE. Formation of oceanic lithosphere and SW-thrusting was broadly coeval, resulting in ophiolite obduction onto the Hawasina Basin. The Semail Ophiolite and the Hawasina rocks combined were thrust further onto the Arabian Plate. Their load created a foreland basin and forebulge within the Arabian Platform. Once the continental lithosphere of the Arabian Platform was forced into the subduction zone, a tear between the dense oceanic lithosphere and the buoyant continental lithosphere developed. This led to rapid uplift and exhumation of subducted continental lithosphere of the Saih Hatat area, while obduction was still going on, causing in multiple and intense folding/thrusting within the eastern Saih Hatat Dome. Exhumation of the Saih Hatat Dome was massive. The emplacement of the ophiolite was completed during the Campanian/Maastrichtian. For completeness, we also present alternative models for the developmental history of the Semail Ophiolite. Immediately after emplacement, the Arabian lithosphere underwent intense top-to-the-NE extensional shearing. Most of the Saih Hatat Dome was exhumed during the latest Cretaceous to Early Eocene, associated with major extensional shearing at its flanks. Further convergence during the late Eocene to Miocene resulted in exhumation of the Jabal Akhdar Dome and some gentle exhumation of the Saih Hatat Dome, shaping the present-day Southeastern Oman Mountains. In the coastal area, east and SE of the Saih Hatat Dome, some late Cretaceous to present-day uplift is evident by, e.g., uplifted marine terraces. The entire Oman Mountains are uplifting today, which is evident by the massive wadi incision into various rock units, including wadi deposits which may form overhangs.

Evolution of seaward-dipping reflectors at the onset of oceanic crust formation at volcanic passive margins: Insights from the South Atlantic

Seaward-dipping reflectors (SDRs) have long been recognized as a ubiquitous feature of volcanic passive margins, yet their evolution is much debated, and even the subject of the nature of the underlying crust is contentious. This uncertainty significantly restricts our understanding of continental breakup and ocean basin–forming processes. Using high-fidelity reflection data from offshore Argentina, we observe that the crust containing the SDRs has similarities to oceanic crust, albeit with a larger proportion of extrusive volcanics, variably interbedded with sediments. Densities derived from gravity modeling are compatible with the presence of magmatic crust beneath the outer SDRs. When these SDR packages are restored to synemplacement geometry we observe that they thicken into the basin axis with a nonfaulted, diffuse termination, which we associate with dikes intruding into initially horizontal volcanics. Our model for SDR formation invokes progressive rotation of these horizontal volcanics by subsidence driven by isostasy in the center of the evolving SDR depocenter as continental lithosphere is replaced by more dense oceanic lithosphere. The entire system records the migration of >10-km-thick new magmatic crust away from a rapidly subsiding but subaerial incipient spreading center at rates typical of slow oceanic spreading processes. Our model for new magmatic crust can explain SDR formation on magma-rich margins globally, but the estimated crustal thickness requires elevated mantle temperatures for their formation.

A Study on the Dominant Driving Force of Plate Movement presented in the High School Earth Science Textbooks

In the early model of plate tectonics, the plate was depicted as a passive raft floating on the convecting mantle and carried away by the mantle flow. At the same time, ridge push at spreading boundaries and drag force exerted by the mantle on the base of lithosphere were described as the dominant driving forces of plate movements. However, in recent studies of plate tectonics, it is generally accepted that the primary force driving plate motion is slab pull beneath subduction zones rather than other forces driven by mantle convection. The current view asserts that the density contrast between dense oceanic lithosphere and underlying asthenosphere is the substance of slab pull. The greater density of oceanic slab allows it to sink deeper into mantle at trenches by gravitational pull, which provides a dominant driving force for plate motion. Based on this plate tectonics development, this study investigated the contents of plate tectonics in high school Earth Science textbooks and how they have been depicted for the last few decades. Results showed that the early explanation of plate movement driven by mantle convection has been consistently highlighted in almost all high school textbooks since the 5th curriculum, whereas most introductory college textbooks rectified the early theory of plate movement and introduced a newly accepted theory in revised edition. Therefore, we suggest that the latest theory of plate tectonics be included in high school textbooks so that students get updated with recent understanding of it in a timely manner.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

AbstractThe Phanerozoic subduction system of the Korean peninsula is considered to have been activated by at least Middle Permian time. The geochemically arc-like Andong ultramafic complex (AUC) occurring along the border between the Precambrian Yeongnam massif and the Cretaceous Gyeongsang back-arc basin provides a rare opportunity for direct study of the pre-Cretaceous mantle wedge lying above the subduction zone. The tightly constrained SHRIMP U–Pb age of zircons extracted from orthopyroxenite specimens (222.1±1.0 Ma) is indistinguishable from the Ar/Ar age of coexisting phlogopite (220±6 Ma). These ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment. The nearly coeval occurrence of a syenite-gabbro-monzonite suite in the SW Yeongnam massif also suggests an extensional tectonic setting along the continental margin side during Late Triassic time. The relatively enriched ɛHf range of dated zircons (+6.2 to −0.6 at 222 Ma) is in contrast to previously reported primitive Sr–Nd–Hf isotopic features of Cenozoic mantle xenoliths from Korea and eastern China. This enrichment is not ascribed to contamination by the hypothetical Palaeozoic crust beneath SE Korea, but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo-Pacific plate. Most AUC zircons show a restricted core-to-rim spread of ɛHf values, but some grains testify to the operation of open-system processes during magmatic differentiation.

Segmentation of the Hellenides recorded by Pliocene initiation of clockwise block rotation in Central Greece

New paleomagnetic data from Early Miocene to Pliocene terrestrial sedimentary and volcanic rocks in Central Greece constrain the history of vertical-axis rotation along the central part of the western limb of the Aegean arc. The present-day pattern of rapid block rotation within a broad zone of distributed deformation linking the right-lateral North Anatolian and Kephalonia continental transform faults initiated after Early Pliocene time, resulting in a uniform clockwise rotation of 24.3±6.5° over a region >250km long and >150km wide encompassing Central Greece and the western Cycladic archipelago. Because the published paleomagnetic dataset requires clockwise rotations of >50° in Western Greece after ∼17Ma, while our measurements resolve no vertical-axis rotation of Central Greece between ∼15Ma and post-Early Pliocene time, a large part of the clockwise rotation of Western Greece must have occurred during the main period of contraction within the external thrust belt of the Ionian Zone between ∼17 and ∼15Ma. Pliocene initiation of rapid clockwise rotation in Central and Western Greece reflects the development of the North Anatolia–Kephalonia Fault system within the previously extended Aegean Sea region, possibly in response to entry of dense oceanic lithosphere of the Ionian Sea into the Hellenic subduction zone and consequent accelerated slab rollback. The development of the Aegean geometric arc therefore occurred in two short-duration pulses characterized by rapid rotation and strong regional deformation.

Surface deformation and slab–mantle interaction during Banda arc subduction rollback

The reason for the spectacular curvature of the Banda subduction zone is debated. Tomographic images and plate reconstructions reveal subduction of a single slab. The ancient geometry of the Australian plate, as well as the interaction between the slab and the mantle, caused the deformation of the slab. The spectacularly curved Banda arc comprises young oceanic crust1,2 enclosed by a volcanic inner arc, outer arc islands and a trough parallel to the Australian continental margin3,4,5. Strong seismic activity in the upper mantle defines a folded surface6,7, for which there are two contrasting explanations: deformation of a single slab5,8 or two separate slabs subducting from the north and south6,9. Here we combine seismic tomography with the plate tectonic evolution of the region to infer that the Banda arc results from subduction of a single slab. Our palaeogeographic reconstruction shows that a Jurassic embayment, which consisted of dense oceanic lithosphere enclosed by continental crust, once existed within the Australian plate. Banda subduction began about 15 million years ago when active Java subduction tore eastwards into the embayment. The present morphology of the subducting slab is only partially controlled by the shape of the embayment. As the Australian plate moved northward at a high speed of about 7 cm yr−1, the Banda oceanic slab rolled back towards the south–southeast accompanied by active delamination separating the crust from the denser mantle. Increasing resistance of the mantle to plate motion progressively folded the slab and caused strong deformation of the crust. The Banda arc represents an outstanding example of large-scale deformation of the Earth’s crust in response to coupling between the crust, slab and surrounding mantle.

The anatomy and ontogeny of modern intra-oceanic arc systems

Abstract Intra-oceanic arc systems (IOASs) represent the oceanic endmember of arc–trench systems and have been the most important sites of juvenile continental crust formation for as long as plate tectonics has operated. IOASs' crustal profiles are wedge-shaped, with crust up to 20–35 km thick; a more useful definition is that IOASs occur as chains of small islands, generally just the tops of the largest volcanoes. A very small fraction of IOASs lie above sea level, but advancing marine technologies allow their most important features to be defined. Modern IOASs subduct old, dense oceanic lithosphere and so tend to be under extension. They consist of four parallel components: trench, forearc, volcanic–magmatic arc, and back-arc, occupying a ≥200 km zone along the leading edge of the overriding plate. These components form as a result of hydrous melting of the mantle and reflect the strongly asymmetric nature of subduction processes. Forearcs preserve infant arc lithosphere whereas magmatism in mature IOASs is concentrated along the volcanic–magmatic front. Mature IOASs often have minor rear-arc volcanism and, because most IOASs are strongly extensional, sea-floor spreading often forms back-arc basins. Sub-IOAS mantle is also asymmetric, with serpentinized harzburgite beneath the forearc, pyroxene-rich low- V p mantle beneath the magmatic front, and lherzolite–harzburgite beneath back-arc basins. Because most IOASs are far removed from continents, they subduct oceanic lithosphere with thin sediments and have naked forearcs subject to tectonic erosion. IOASs evolve from broad zones of very high degrees of melting and sea-floor spreading during their first 5–10 Ma, with the volcanic–magmatic front retreating to its ultimate position c. 200 km from the trench.

Conditions for the onset of plate tectonics on terrestrial planets and moons

Plate tectonics on Earth is driven by the subduction and stirring of dense oceanic lithosphere into the underlying mantle. For such a regime to exist on any planet, stresses associated with mantle convection must exceed the strength of the lithosphere. This condition is sufficiently restrictive that plate tectonics currently operates only on Earth, and mantle convection in most terrestrial planets and moons is probably in a stagnant lid regime. Convective stresses on the lithosphere depend on the viscosity and velocity of underlying cold downwellings. The lithospheric yield stress is controlled by its friction coefficient and elastic thickness (the depth to the brittle–ductile transition or BDT). Both convective stresses and the plate's yield strength depend critically on the size, thermal state and cooling history of a planet. Accordingly, here we use numerical simulations and scaling theory to identify conditions in which mantle convection leads to lithospheric failure for a range of conditions relevant to the terrestrial planets. Whereas Earth is expected to be in a plate-tectonic regime over its full thermal evolution, the Moon and Mercury are expected to have always remained in a stagnant lid regime. Venus, Io and Europa currently fall on the transition between the two regimes, which is consistent with an episodic style of mantle convection for Venus, a tectonic component to deformation on Io, and the resurfacing history and lithospheric evolution of Europa. Our results suggest that Venus may have been in a plate-tectonic regime in the past. While stagnant now, it is plausible that Mars may have also been in an active-lid regime, depending on whether there was liquid water on the surface.

Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene

Abstract: We present a tectonic synthesis and an animation of the tectonics of the western Mediterranean since the Oligocene. This work is based on data derived from different geological datasets, such as structural geology, the distribution of metamorphic rocks, magmatic activity, sedimentary patterns, palaeomagnetic data and geophysics. Reconstruction was performed using an interactive software package (PLATYPLUS), which enabled us to apply rotational motions to numerous microplates and continental terranes involved in the evolution of the western Mediterranean basins. Boundary conditions are provided by the relative motions of Africa and Iberia with respect to Europe, and the Adriatic plate is considered here as an African promontory. The reconstruction shows that during Alpine orogenesis, a very wide zone in the interface between Africa and Europe underwent extension. Extensional tectonics was governed by rollback of subduction zones triggered by gravitational instability of old and dense oceanic lithosphere. Back-arc extension occurred in the overriding plates as a result of slow convergence rates combined with rapid subduction rollback. This mechanism can account for the evolution of the majority of the post-Oligocene extensional systems in the western Mediterranean. Moreover, extension led to drifting and rotations of continental terranes towards the retreating slabs in excess of 100-800 km. These terranes - Corsica, Sardinia, the Balearic Islands, the Kabylies blocks, Calabria and the Rif-Betic - drifted as long as subduction rollback took place, and were eventually accreted to the adjacent continents. We conclude that large-scale horizontal motions associated with subduction rollback, back-arc extension and accretion of allochthonous terranes played a fundamental role during Alpine orogenesis.

Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens

We present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision (slab breakoff), allowing an explanation of syn- to post-collisional magmatism and metamorphism. Continental collisions are preceded by subduction of dense oceanic lithosphere, and followed by attempted subduction of buoyant continental lithosphere. This situation of opposing buoyancy forces leads to extensional deformation in the subducted slab. A narrow rifting mode of deformation will result if there is strain localization. Slab breakoff results. We have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere, and have compared it with the change in buoyancy force during continental sbduction. Whether breakoff will occur, and the depth at which it will occur, is a strong function of temperature and hence the subduction velocity. For a subduction velocity of 1 cm/yr breakoff could occur at depths of between 50 and 120 km, while at higher velocities it is still likely to occur, but at deeper depths. As a result of the rifting during breakoff, the asthenosphere upwells into the narrow rift, and following breakoff it impinges on the mechanical lithosphere of the overriding plate. The resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere, producing basaltic magmatism that leads to granitic magmatism in the crust. Dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca. 50 km. Breakoff removes the force at the downdip side of the continental crust, while the enhanced heating leads to a reduction of the strength of the underlying crust. Both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface, leading to the rapid exhumation of eclogite facies continental crust. The cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen. We have tested the variety of predicted expressions of slab breakoff in the geological record of the Alps, the Aegean Islands, and the Dabie Shan. A comparison of the various belts highlights (1) that the magmatism and metamorphism are found near the suture and the centre of the orogen, demonstrating the general steepening of suture during collision, (2) that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn-orogenic magmatism, and (3) that the melts can be emplaced in both compressive and extensional environments. We suggest that slab breakoff is an important step in the evolution of many orogenic belts, and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra-high-pressure metamorphics.

On The initiation of subduction

Estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation. Three environments proposed as preferential sites of incipient subduction are investigated: passive continental margins, transform faults/fracture zones, and extinct ridges. None of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses. Subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins. The attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates. This is the only tectonic force sufficient to trigger the nucleation of a new subduction zone. The ubiquitous distribution of transform faults and fracture zones, combined with the common proximity of these features to mature subduction complexes, suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere.

Delamination in collisional orogens

During continental collision, the weight of the downgoing slab may lead to extensional shearing in the descending plate, permitting separation of the downdip, dense oceanic lithosphere from the adjacent lighter continental lithospher. Separation may occur by means of a lithosphere-penetrating normal-sense shear zone, by means of pure shear necking of the lithosphere, or by some combination of these mechanisms. During separation, the continental crust attached to the lower plate may undergo an episode of extensional tectonics. The proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism. If the continents continue to converge following delamination, a compressive tectonic regime will be reestablished in the crust. The subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat Moho. The likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime, as well as underplating associated with syntectonic plutonism, may account for the seismic complexities observed at Moho depths beneath some mountain belts.

Subduction of young oceanic lithosphere and extensional orogeny in southwestern North America during Mid‐Tertiary time

An “extensional orogeny” deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the “ignimbrite flareup,” massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1–2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid‐Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low‐K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high‐K rocks rest on an autochthonous sialic basement. Within the high‐K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K‐h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid‐Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab‐pull is likely to approach zero. Even prior to ridge‐trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge‐trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province fractured into fault blocks.