The Heterogeneous Tethyan Oceanic Lithosphere of the Alpine Ophiolites

Elements ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 23-28 ◽  
Author(s):  
Elisabetta Rampone ◽  
Alessio Sanfilippo
Keyword(s):  
Oceanic Lithosphere ◽  
Passive Margin ◽  
Crustal Material ◽  
Crustal Accretion ◽  
Mantle Dynamics ◽  
Western Tethys ◽  
Mantle Peridotites ◽  
Slow Spreading ◽  
Tethys Ocean ◽  
Basaltic Crust

The Alpine–Apennine ophiolites are lithospheric remnants of the Jurassic Alpine Tethys Ocean. They predominantly consist of exhumed mantle peridotites with lesser gabbroic and basaltic crust and are locally associated with continental crustal material, indicating formation in an environment transitional from an ultra-slow-spreading seafloor to a hyperextended passive margin. These ophiolites represent a unique window into mantle dynamics and crustal accretion in an ultra-slow-spreading extensional environment. Old, pre-Alpine, lithosphere is locally preserved within the mantle sequences: these have been largely modified by reaction with migrating asthenospheric melts. These reactions were active in both the mantle and the crust and have played a key role in creating the heterogeneous oceanic lithosphere in this branch of the Mesozoic Western Tethys.

The ophiolites of the Tibetan Geotraverses, Lhasa to Golmud (1985) and Lhasa to Kathmandu (1986)

1988 ◽  
Vol 327 (1594) ◽  
pp. 215-238 ◽  
Keyword(s):  
Subduction Zone ◽  
Oceanic Lithosphere ◽  
Island Arc ◽  
Passive Margin ◽  
Jinsha River ◽  
Geochemical Composition ◽  
The North ◽  
Kunlun Mountains ◽  
Slow Spreading ◽  
Back Arc

Ophiolite belts are found in Tibet along the Zangbo, Banggong and Jinsha River Sutures and in the Anyemaqen mountains, the eastern extension of the Kunlun mountains. Where studied, the Zangbo Suture ophiolites are characterized by: apparently thin crustal sequences (3-3.5 k m ); an abundance of sills and dykes throughout the crustal and uppermost mantle sequences; common intraoceanic melanges and unconformities; and an N-MORB petrological and geochemical composition. The ophiolites probably formed within the main neo-Tethyan ocean and the unusual features may be due to proximity to ridge-transform intersections, rather than to genesis at very slow -spreading ridges as the current consensus suggests. The Banggong Suture ophiolites have a supra-subduction zone petrological and geochemical composition — although at least one locality in the Ado Massif shows MORB characteristics. However, it is also apparent that the dykes and lavas show a regional chemical zonation, from boninites and primitive island arc tholeiites in the south of the ophiolite belt, through normal island arc tholeiites in the central belt to island arc tholeiites transitional to N-MORB in the north. The ophiolites could represent fragments of a fore-arc, island arc, back-arc complex developed above a Jurassic, northward-dipping subduction zone and emplaced in several stages during convergence of the Lhasa and Qiangtang terranes. The ophiolites of the Jinsha River Suture have a N-MORB composition where analysed, but more information is needed for a proper characterization. The Anyemaqen ophiolites, where studied, have a within-plate tholeiite composition and may have originated at a passive margin: it is not, however, certain whether true oceanic lithosphere, as opposed to strongly attenuated continental lithosphere, existed in this region.

Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

2020 ◽  
Author(s):  
Yanhui Suo ◽  
Sanzhong Li ◽  
Xianzhi Cao
Keyword(s):  
Large Scale ◽  
Shear Velocity ◽  
Oceanic Lithosphere ◽  
Small Scale ◽  
Mantle Plumes ◽  
Crustal Accretion ◽  
Southeast Indian Ridge ◽  
Indian Ridge ◽  
Tethys Ocean ◽  
Mantle Temperature

Hot mantle plumes and ancient cold slabs have been observed beneath modern mid-ocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°−120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of >2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of 0.75−2.5 km are observed on the northern flank of the westernmost (70°−80°E) Southeast Indian Ridge and the southern flank of the eastern (120°−140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20−53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo-Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago.

The interplay between recycled and primordial heterogeneities: predicting Earth's mantle dynamics via numerical modeling

2021 ◽  
Author(s):  
Matteo Desiderio ◽  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Large Scale ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Mantle Dynamics ◽  
Density Anomaly ◽  
Mantle Mineral ◽  
Intrinsic Strength

<p>According to geochemical and geophysical observations, Earth's lower mantle appears to be strikingly heterogeneous in composition. An accurate interpretation of these findings is critical to constrain Earth's bulk composition and long-term evolution. To this end, two main models have gained traction, each reflecting a different style of chemical heterogeneity preservation: the 'marble cake' and 'plum pudding' mantle. In the former, heterogeneity is preserved in the form of narrow streaks of recycled oceanic lithosphere, stretched and stirred throughout the mantle by convection. In the latter, domains of intrinsically strong, primordial material (enriched in the lower-mantle mineral bridgmanite) may resist convective entrainment and survive as coherent blobs in the mid mantle. Microscopic scale processes certainly affect macroscopic properties of mantle materials and thus reverberate on large-scale mantle dynamics. A cross-disciplinary effort is therefore needed to constrain present-day Earth structure, yet countless variables remain to be explored. Among previous geodynamic studies, for instance, only few have attempted to address how the viscosity and density of recycled and primordial materials affect their mutual mixing and interaction in the mantle.</p><p>Here, we apply the finite-volume code <strong>STAGYY</strong> to model thermochemical convection of the mantle in a 2D spherical-annulus geometry. All models are initialized with a lower, primordial layer and an upper, pyrolitic layer (i.e., a mechanical mixture of basalt and harzburgite), as is motivated by magma-ocean solidification studies. We explore the effects of material properties on the style of mantle convection and heterogeneity preservation. These parameters include (i) the intrinsic strength of basalt (viscosity), (ii) the intrinsic density of basalt, and (iii) the intrinsic strength of the primordial material.</p><p>Our preliminary models predict a range of different mantle mixing styles. A 'marble cake'-like regime is observed for low-viscosity primordial material (~30 times weaker than the ambient mantle), with recycled oceanic lithosphere preserved as streaks and thermochemical piles accumulating near the core-mantle boundary. Conversely, 'plum pudding' primordial blobs are also preserved when the primordial material is relatively strong, in addition to the 'marble cake' heterogeneities mentioned above. Most notably, however, the rheology and the density anomaly of basalt affect the appearance of both recycled and primordial heterogeneities. In particular, they control the stability, size and geometry of thermochemical piles, the enhancement of basaltic streaks in the mantle transition zone, and they influence the style of primordial material preservation. These results indicate the important control that the physical properties of mantle constituents exert on the style of mantle convection and mixing over geologic time. Our numerical models offer fresh insights into these processes and may advance our understanding of the composition and structure of Earth's lower mantle.</p>

The formation, preservation and seismic signatures of chemical heterogeneities in the lower mantle

2020 ◽  
Author(s):  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer ◽  
Paul J. Tackley ◽  
Paula Koelemeijer
Keyword(s):  
Physical Properties ◽  
Lower Mantle ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Global Scale ◽  
Seismic Velocities ◽  
Mantle Dynamics ◽  
Wide Range ◽  
Scientific Challenge

<p>Despite stirring by vigorous convection over billions of years, the Earth’s lower mantle appears to be chemically heterogeneous on various length scales. Constraining this heterogeneity is key for assessing Earth’s bulk composition and thermochemical evolution, but remains a scientific challenge that requires cross-disciplinary efforts. On scales below ~1 km, the concept of a “marble cake” mantle has gained wide acceptance, emphasising that recycled oceanic lithosphere, deformed into streaks of depleted and enriched compositions, makes up much of the mantle. On larger scales (10s-100s of km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or intrinsically-strong materials. Intrinsically dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous domains (e.g., enhanced in the strong mineral bridgmanite) may survive as “blobs” in the mid-mantle for large timescales, such as plums in the mantle “plum pudding”<sup>1,2</sup>. While many studies have explored the formation and preservation of either intrinsically-dense (recycled) or intrinsically-strong (primordial) heterogeneity, only few if any have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.<span> </span></p><p>To address this objective, we use state-of-the-art 2D numerical models of global-scale mantle convection in a spherical-annulus geometry. We explore the effects of the <em>(i)</em> physical properties of primordial material (density, viscosity), <em>(ii)</em> temperature/pressure dependency of viscosity, <em>(iii)</em> lithospheric yielding strength, and <em>(iv)</em> Rayleigh number on mantle dynamics and mixing. Models predict that primordial heterogeneity is preserved in the lower mantle over >4.5 Gyr as discrete blobs for high intrinsic viscosity contrast (>30x) and otherwise for a wide range of parameters. In turn, recycled oceanic crust is preserved in the lower mantle as “marble cake” streaks or piles, particularly in models with a relatively cold and stiff mantle. Importantly, these recycled crustal heterogeneities can co-exist with primordial blobs, with piles often tending to accumulate beneath the primordial domains. This suggests that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles.<span> </span></p><p>Finally, we put our model predictions in context with recent discoveries from seismology. We calculate synthetic seismic velocities from predicted temperatures and compositions, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Convection models including preserved bridgmanite-enriched domains along with recycled piles have the potential of reconciling recent seismic observations of lower-mantle heterogeneity<sup>3</sup> with the geochemical record from ocean-island basalts<sup>4,5</sup>, and are therefore relevant for assessing Earth’s bulk composition and long-term evolution.<span> </span></p><p><sup>1</sup> Ballmer et al. (2017), <em>Nat. Geosci</em>., 10.1038/ngeo2898<br><sup>2</sup> Gülcher et al. (in review), <em>EPSL</em>: Variable dynamic styles of primordial heterogeneity preservation in Earth’s lower mantle <br><sup>3</sup> Waszek et al. (2018), <em>Nat. Comm., </em>10.1038/s41467-017-02709-4 <br><sup>4</sup> Hofmann (1997), <em>Nature, </em>10.1038/385219a0; <br><sup>5</sup> Mundl et al. (2017), <em>Science, </em>10.1126/science.aal4179</p>

scholarly journals Small-sized <i>Trochammina</i> assemblages in deep-water Eocene flysch deposits (Outer Carpathians, Poland) and their palaeoecological implications

2015 ◽  
Vol 34 (1) ◽  
pp. 1-19 ◽  
Author(s):  
Anna Waśkowska
Keyword(s):  
Deep Water ◽  
Global Climate ◽  
Early Eocene ◽  
Northern Margin ◽  
Western Tethys ◽  
Thermal Maximum ◽  
Energy Environment ◽  
Outer Carpathians ◽  
Tethys Ocean ◽  
C 50

Abstract. The global climate change in the early Eocene contributed significantly to the turnover of benthic foraminifera. A major extinction within agglutinated and calcareous forms and the occurrence of opportunistic assemblages resulted. The Trochammina material described here belongs to these post-crisis assemblages. Foraminiferal assemblages with numerous Trochammina species are identified within deep-water Eocene deposits of the Polish part of the Outer Carpathians. Trochammina reach up to 80% of the assemblages, the remainder consists of cosmopolitan agglutinated foraminifera, mainly Bathysiphon, Recurvoides, Paratrochamminoides and Trochamminoides. The low biodiversity (average number of species 24, of genera 15) and the presence of dwarf forms are the main characteristics of the assemblages. These assemblages occur predominantly in shales with numerous organic traces (lower Hieroglyphic beds), deposited in the Silesian Basin (Outer Carpathians) which was on the northern margin of the western Tethys Ocean during the early Eocene (Ypresian, c. 50 Ma). The Trochammina biofacies developed in the Silesian Basin after the Paleocene–Eocene Thermal Maximum crisis, and is dominated by opportunistic forms, mainly represented by mobile epifauna and shallow-water infauna, interpreted as a recolonizing assemblage in a low energy environment.

Where are the passive margins of the western Tethys Ocean?

Geology ◽  
1978 ◽  
Vol 6 (10) ◽  
pp. 597 ◽  
Author(s):  
Xavier Le Pichon ◽  
René Blanchet
Keyword(s):  
Western Tethys ◽  
Passive Margins ◽  
Tethys Ocean

scholarly journals Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Nature ◽  
2008 ◽  
Vol 455 (7214) ◽  
pp. 790-794 ◽  
Author(s):  
J. Escartín ◽  
D. K. Smith ◽  
J. Cann ◽  
H. Schouten ◽  
C. H. Langmuir ◽  
...  
Keyword(s):  
Oceanic Lithosphere ◽  
Detachment Faults ◽  
Slow Spreading

scholarly journals Boninitic blueschists record subduction initiation and subsequent accretion of an arc–forearc in the northeast Proto-Tethys Ocean

Geology ◽  
2021 ◽  
Author(s):  
Dong Fu ◽  
Bo Huang ◽  
Tim E. Johnson ◽  
Simon A. Wilde ◽  
Fred Jourdan ◽  
...  
Keyword(s):  
Subduction Zones ◽  
Plate Tectonics ◽  
Oceanic Lithosphere ◽  
Island Arc ◽  
Early Paleozoic ◽  
Subduction Initiation ◽  
North Qilian Orogenic Belt ◽  
Mariana Arc ◽  
Tethys Ocean ◽  
North Qilian

Subduction of oceanic lithosphere is a diagnostic characteristic of plate tectonics. However, the geodynamic processes from initiation to termination of subduction zones remain enigmatic mainly due to the scarcity of appropriate rock records. We report the first discovery of early Paleozoic boninitic blueschists and associated greenschists from the eastern Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which have geochemical affinities that are typical of forearc boninites and island arc basalts, respectively. The boninitic protoliths of the blueschists record intra-oceanic subduction initiation at ca. 492–488 Ma in the eastern North Qilian arc/forearc–backarc system, whereas peak blueschist facies metamorphism reflects subsequent subduction of the arc/forearc complex to high pressure at ca. 455 Ma. These relations therefore record the life circle of an intra-oceanic subduction zone within the northeastern Proto-Tethys Ocean. The geodynamic evolution provides an early Paleozoic analogue of the early development of the Izu–Bonin–Mariana arc and its later subduction beneath the extant Japanese arc margin. This finding highlights the important role of subduction of former upper plate island arc/forearcs in reducing the likelihood of preservation of initial subduction-related rock records in ancient orogenic belts.

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