Subduction Zone Processes Research Papers (original) (raw)

Archean, Paleoproterozoic, and Mesoproterozoic rocks, assemblages, and structures differ greatly both from each other and from modern ones, and lack evidence for subduction and seafloor spreading such as is widespread in Phanerozoic terrains. Most specialists nevertheless apply non-actualistic plate-tectonic explanations to the ancient terrains and do not consider alternatives. This report evaluates popular concepts with multidisciplinary information, and proposes options. The key is fractionation by ca. 4.45 Ga of the hot young Earth into core, severely depleted mantle, and thick mafic protocrust, followed by still-continuing re-enrichment of upper mantle from the top. This is opposite to the popular assumption that silicate Earth is still slowly and unidirectionally fractionating. The protocrust contained most material from which all subsequent crust was derived, either directly, or indirectly after downward recycling. Tonalite, trondhjemite, and granodiorite (TTG), dominant components of Archean crust, were derived mostly by partial melting of protocrust. Dense restitic protocrust delaminated and sank into hot, weak dunite mantle, which, displaced upward, enabled further partial melting of protocrust. Sinkers enriched the upper mantle, in part maintaining coherence as distinct dense rocks, and in part yielding melts that metasomatized depleted-mantle dunite to more pyroxenic and garnetiferous rocks. Not until ca. 3.6 Ga was TTG crust cool enough to allow mafic and ultramafic lavas, from both protocrust and re-enriched mantle, to erupt to the surface, and then to sag as synclinal keels between rising diapiric batholiths; simultaneously upper crust deformed ductily, then brittly, above slowly flowing hot lower TTG crust. Paleoproterozoic and Mesoproterozoic orogens appear to be largely ensialic, developed from very thick basin-filling sedimentary and volcanic rocks on thinned Archean or Paleoproterozoic crust and remaining mafic protocrust, above moderately re-enriched mantle. Subduction, and perhaps the continent/ocean lithospheric dichotomy, began ca. 850 Ma – although fully modern plate- tectonic processes began only in Ordovician time – and continued to enrich the cooling mantle in excess of partial melts that contributed to new crust. “ Plumes ” from deep mantle do not operate in the modern Earth and did not operate in Precambrian time.

- by
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- Tectonics, Archean (Precambrian tectonics), Archean geology, Mantle Petrology
- by Chris Goldfinger
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- Tectonics, Subduction Zone Processes

The devastating 2004 Indian Ocean tsunami caught millions of coastal residents and the scientific community off-guard. Subsequent research in the Indian Ocean basin has identified prehistoric tsunamis, but the timing and recurrence intervals of such events are uncertain. Here we present an extraordinary 7,400 year stratigraphic sequence of prehistoric tsunami deposits from a coastal cave in Aceh, Indonesia. This record demonstrates that at least 11 prehistoric tsunamis struck the Aceh coast between 7,400 and 2,900 years ago. The average time period between tsunamis is about 450 years with intervals ranging from a long, dormant period of over 2,000 years, to multiple tsunamis within the span of a century. Although there is evidence that the likelihood of another tsunamigenic earthquake in Aceh province is high, these variable recurrence intervals suggest that long dormant periods may follow Sunda megathrust ruptures as large as that of the 2004 Indian Ocean tsunami.

- by Patrick Daly
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- Geology, Geoarchaeology, Seismic Hazard, Paleoenvironment

The Etna volcano is located in an apparently anomalous
position on the hinge zone of the Apennines subduction and
its Na-alkaline geochemistry does not favour a magma source
from the deep slab as indicated for the Aeolian K-alkaline
magmatism. The steeper dip of the regional foreland monocline
at the front of the Apennines in the Ionian Sea than in Sicily,
implies a larger rollback of the subduction hinge in the Ionian
Sea. Moreover, the lengthening of the Apennines arc needs
extension parallel to the arc. Therefore, the larger southeastward
subduction rollback of the Ionian lithosphere with respect
to the Hyblean plateau in Sicily, should kinematically produce
right-lateral transtension and a sort of vertical `slab window'
which might explain (i) the Plio-Pleistocene alkaline magmatism
of eastern Sicily (e.g. the Etna volcano) and (ii) the late Pliocene
to present right lateral transtensional tectonics and seismicity
of eastern Sicily. The area of transfer of different dip and
rollback occurs along the inherited Mesozoic passive continental
margin between Sicily and the oceanic Ionian Sea, i.e. the
Malta escarpment.

The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low (5C/km), lawsonite may carry water into great depths of 300 km. In the hot subduction zone where the geothermal gradient is high (>25C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of <80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hy-drous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of 60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultra-high-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.

- by Yong-Fei Zheng and +2
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- Metamorphic Petrology, Isotope Geochemistry, Subduction Zone Processes, Igneous petrogenesis

The comparison between Alps and Apennines
shows they are orogens with distinct characters both in terms of
geological and geophysical data. They have respectively 1)
convergence rate faster than the slab retreat vs. convergence rate
slower than the slab retreat; 2) double vs. single vergence; 3)
high vs. low morphological and structural elevation; 4) deep vs.
shallow rocks involved; 5) the occurrence of higher
metamorphic degree vs. lower metamorphic degree; 6) the basal
décollement involves the crust and the LID of both upper and
lower plates whereas only the shallow crust of the lower plate
contributes to the accretionary prism; 7) shallow vs. deep
foredeep; 8) low vs. high dip of the foreland monocline; 9)
thickened vs. thinned crust under the ridge; 10) the Alps have
both in the upper and in the lower plate a pre-subduction Moho,
whereas the Apennines have in the footwall plate a presubduction
Moho, but in the hangingwall they present a new
forming Moho; 11) thickened lithosphere vs. a shallow
asthenosphere in the hangingwall; 12) no vs. well developed
backarc basin and related alkaline-tholeiitic magmatism; 13)
scarce vs. larger abundance of subduction-related volcanism;
14) smooth vs. high amplitude gravity and heath flow
anomalies.
The two belts interfered since the southern prolongation of
the Alps has been incorporated into the internal Apennines, and
the Apennines slab retreat is subsiding much part of the Alps,
partly counteracting their uplift.
These differences mimic worldwide asymmetries as the
eastern vs. western Pacific (i.e., Andes vs. Marianas) or
Himalayas vs. Barbados subduction zones, and favor a global
scale explanation such as the “westward” drift of the lithosphere
rather than regional slab-pull related variations in the tectonic
style.

- by Carlo Doglioni
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- Tectonics, Geodynamics, Plate Tectonics, Subduction Zone Processes

The Tyrrhenian Sea is the easternmost basin of
the boudinated backarc lithosphere in the hangingwall of the
Late Oligocene to Present Apennines subduction, which
started in the Provençal and Valencia troughs and
progressively moved to the Algerian and Tyrrhenian basins. All
basins and in particular the Tyrrhenian Sea are asymmetric,
being more extended and magmatically intruded in the eastern
side, as testified also by the higher heat flow. The Apennines
slab retreated “eastward”, which kinematically requires an
eastward mantle flow either to compensate or push the slab
rollback.
Corsica and Sardinia represent the major lithospheric boudin
in the backarc basin and their crustal and lithospheric roots
have an eastward offset with respect to the superficial
topography, possibly related to the shear induced by the
underlying relative eastward mantle flow. It is interpreted that
the mantle that generated the oceanic crust of the Provençal
basin was depleted and consequently it became lighter; during
its eastward transit below Sardinia and Corsica the depleted
mantle could have generated the Miocene uplift of the
continental swell.
Fault spacing in the brittle upper crust has an average value of
4-5 km in the Northern Tyrrhenian and 4 km to 16-17 km in
the southern part. Internal sub-basins developed at different
bathymetries, due to variable stretching and sediment supply in
the different parts of the Tyrrhenian Sea. Northern and
southern Tyrrhenian basins present respectively as minimum
estimates 25 and 253 km of extension, according to the larger
subduction of the Ionian heavier lithosphere of the
Apennines foreland. The whole Tyrrhenian basin opened
obliquely to the pre-existing alpine orogen. Therefore the main
Tyrrhenian architecture and magmatism seem to have been
primarily controlled by the composition and thickness of the
downgoing subducting lithosphere beneath the Apennines,
i.e., continental in the Adriatic and oceanic in the Ionian, and
the westward motion of the lithosphere relative to the mantle.

- by Carlo Doglioni
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- Structural Geology, Tectonics, Geodynamics, Subduction Zone Processes

"We present a geodynamic reconstruction of the Central–Western Mediterranean and neighboring areas
during the last 50 Myr, including magmatological and tectonic observations. This area was interested by
different styles of evolution and polarity of subduction zones influenced by the fragmented Mesozoic and
Early Cenozoic paleogeography between Africa and Eurasia. Both oceanic and continental lithospheric plates
were diachronously consumed along plate boundaries. The hinge of subducting slabs converged toward
the upper plate in the double-vergent thick-skinned Alps–Betics and Dinarides, characterized by two
slowly-subsiding foredeeps. The hinge diverged from the upper plate in the single-vergent thin-skinned
Apennines–Maghrebides and Carpathians orogens, characterized by a single fast-subsiding foredeep. The
retreating lithosphere deficit was compensated by asthenosphere upwelling and by the opening of several
back-arc basins (the Ligurian–Provençal, Valencia Trough, Northern Algerian, Tyrrhenian and Pannonian basins).
In our reconstruction, the W-directed Apennines–Maghrebides and Carpathians subductions nucleated along the
retro-belt of the Alps and the Dinarides, respectively. The wide chemical composition of the igneous rocks
emplaced during this tectonic evolution confirms a strong heterogeneity of the Mediterranean upper mantle
and of the subducting plates. In the Apennine–Maghrebide and Carpathian systems the subduction-related
igneous activity (mostly medium- to high-K calcalkaline melts) is commonly followed in time by mildly sodic
alkaline and tholeiitic melts. The magmatic evolution of the Mediterranean area cannot be easily reconciled
with simple magmatological models proposed for the Pacific subductions. This is most probably due to synchronous
occurrence of several subduction zones that strongly perturbed the chemical composition of the upper
mantle in the Mediterranean region and, above all, to the presence of ancient modifications related to past
orogeneses. The classical approach of using the geochemical composition of igneous rocks to infer the coeval
tectonic setting characteristics cannot be used in geologically complex systems like the Mediterranean area."

- by Carlo Doglioni
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- Mediterranean Studies, Geodynamics, Plate Tectonics, Subduction Zone Processes
- by Alessandro Aiuppa
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- Earth Sciences, Geochemistry, Thermodynamics, Subduction Zone Processes

Alps and Apennines developed along opposite subductions, which inverted the tethyan passive continental margins located along the boundaries of Europe, Africa and the Adriatic plates. The Alps have higher morphological and structural elevation, two shallow, slow subsiding foreland basins. The Apennines have rather low morphological and structural elevation, one deep and fast subsiding foreland basin. While the Alps sandwiched the whole crust of both upper and lower plates, the Apennines rather developed by the accretion of the upper crust of the lower plate alone. Alpine relics are boudinated in the hangingwall of the Apennines, stretched by the Tyrrhenian backarc rifting. Relative to the upper plate, the subduction hinge moved toward it in the Alps from Cretaceous to Present, whereas it migrated away in the Apennines from late Eocene to Present, apart in Sicily where since Pleistocene(?) it reversed. The asymmetry appears primarily controlled by the slab polarity with respect to the westward drift of the lithosphere.

Chemical geodynamics is an integrated discipline that studies the geochemical structure and tectonic evolution of geospheres with the aim of linking tectonic processes to geochemical products in the Earth system. It was primarily focused on mantle geochemistry, with an emphasis on geochemical recycling in oceanic subduction zones. It has been extended to geochemical reworking and recycling under high-pressure (HP) to ultrahigh-pressure (UHP) conditions in all convergent plate margins. In particular, UHP terranes, along with UHP metamorphic minerals and rocks in continental subduction zones, represent natural laboratories for investigating geochemical transport and fluid action during subduction and exhumation of continental crust. As a result of this extension, the study of UHP terranes has significantly advanced our understanding of tectonic processes in collisional orogens. This understanding has principally benefited from the deciphering of petrological and geochemical records in deeply subducted crustal rocks that occur in different petrotec-tonic settings. This review focuses on the following issues in continental subduction zones: the time and duration of UHP metamorphism, the origin and action of metamorphic fluid/melt inside UHP slices, the element and isotope mobilities under HP to UHP conditions during continental collision, the origin of preme-tamorphic protoliths and its bearing on continental collision types, and the crustal detachment and crust– mantle interaction in subduction channels. The synthesis presented herein suggests that the nature of preme-tamorphic protoliths is a key to the type of collisional orogens and the size of UHP terranes. The source mixing in subduction channels is a basic mechanism responsible for the geochemical diversity of continental and oceanic basaltic rocks. Therefore, the geochemical study of HP to UHP metamorphic rocks and their derivatives has greatly facilitated our understanding of the geodynamic processes that drive the tectonic evolution of convergent plate margins from oceanic subduction to continental collision. Consequently, the study of chemical geodynamics has been developed from oceanic subduction zones to continental collision zones, and it has enabled important contributions to development of plate tectonic theory.

Despite the growing amount of data on surface horizontal displacement, the vertical movements of the lithosphere and exhumation processes at convergent plate boundaries are still poorly known. Petrological and geochronological data on High-Pressure to Ultra High Pressure Low-temperature metamorphic rocks provide invaluable constraints on the behaviour of convergent zone. On one hand, the development of in situ datings coupled with more and more precise and continuous pressure-temperature estimates allow the trajectory of subducted rocks to be followed in the 2D thermal-depth (T-Z) field. On the other hand, thermo-mechanical numerical model allow the trajectory of subducted rocks to be followed in the 4D X-Z- T-deformation space. The combination of worldwide natural data with numerical model emphasizes the following salient results: - Whatever their origin (continental or oceanic), the exhumation of HP to UHP rocks is related to convergent processes. - Exhumation of solid rocks requi...

- by Keiko Hattori
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- High Pressure, Subduction Zone Processes, Continental Tectonics, Subduction Zone

The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of super-continent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.

- by Dietmar Müller and +1
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- Plate Tectonics, Subduction Zone Processes, Tectonic Evolution of Rifted Continental Margins, Supercontinents
- by Carlo Doglioni
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- Mediterranean Studies, Tectonics, Geodynamics, Subduction Zone Processes

An integrated interpretation of the late Paleozoic structural and geochronological record of the Iberian Massif is presented and discussed under the perspective of a Gondwana-Laurussia collision giving way to the Variscan orogen. Compressional and extensional structures developed during the building of the Variscan orogenic crust of Iberia are linked together into major tectonic events operating at lithosphere scale. A review of the tectonometamorphic and magmatic evolution of the Iberian Massif reveals backs and forths in the overall convergence between Gondwana and Laurussia during the amalgamation of Pangea in late Paleozoic times. Stages dominated by lithosphere compression are characterized by subduction, both oceanic and continental, development of magmatic arcs, (over-and under-) thrusting of continental lithosphere, and folding. Variscan convergence resulted in the eventual transference of a large allochthonous set of peri-Gondwanan terranes, the Iberian Allochthon, onto the Gondwana mainland. The Iberian Allochthon bears the imprint of previous interaction between Gondwana and Laurussia, including their juxtaposition after the closure of the Rheic Ocean in Lower De-vonian times. Stages governed by lithosphere extension are featured by the opening of two short-lived oceanic basins that dissected previous Variscan orogenic crust, first in the Lower-Middle Devonian, following the closure of the Rheic Ocean, and then in the early Carboniferous, following the emplacement of the peri-Gondwanan allochthon. An additional, major intra-orogenic extensional event in the early-middle Carboniferous dismem-bered the Iberian Allochthon into individual thrust stacks separated by extensional faults and domes. Lateral tec-tonics played an important role through the Variscan orogenesis, especially during the creation of new tectonic blocks separated by intracontinental strike-slip shear zones in the late stages of continental convergence.

- by Ruben Diez Fernandez and +1
- •
- Geology, Structural Geology, Tectonics, Continental Subduction

Partial melting at continental lithosphere depths plays an important role in generating geochemical variations in igneous rocks. In particular, dehydration melting of ultrahigh-pressure (UHP) metamorphic rocks during continental collision provides a petrological link to intracrustal differentiation with respect to the compositional evolution of continental crust. While island arc magmatism represents one end-member of fluid-induced large-scale melting in the mantle wedge during subduction of the oceanic crust, the partial melting of UHP rocks can be viewed as the other end-member of fluid-induced small-scale anatexis during exhumation of the deeply subducted continental crust. This latter type of melting is also triggered by metamorphic dehydration in response to P–T changes during the continental collision. It results in local occurrences of hydrous melts (even supercritical fluids) as felsic veinlets between boundaries of and multiphase solid inclusions in UHP metamorphic minerals as well as local accumulation of veinlet-like felsic leucosomes in foliated UHP metamorphic rocks and metamorphically grown zircons in orogenic peridotites. Thus, very low-degree melts of UHP rocks provide a window into magmatic processes that operated in continental subduction zones. This article presents a review on available results from experimental petrology concerning the possibility of partial melting under conditions of continental subduction-zone metamorphism, and petrological evidence for the occurrence of dehydration-driven in-situ partial melting in natural UHP rocks during the continental collision. Although the deeply subducted continental crust is characterized by a relative lack of aqueous fluids, the partial melting in UHP rocks commonly takes place during decompression exhumation to result in local in-situ occurrences of felsic melts at small scales. This is caused by the local accumulation of aqueous fluids due to the breakdown of hydrous minerals and the exsolution of structural hydroxyl and molecular water from nominally anhydrous minerals in UHP rocks during the exhumation. The dehydration melting of UHP rocks would not only have bearing on the formation of supercritical fluids during subduction-zone metamorphism, but also contribute to element mobility and ultrapotassic magmatism in continental collision orogens. Therefore, the study of dehydration melting and its effects on element transport in UHP slabs, rocks and minerals is a key to chemical geodynamics of continental subduction zones.

- by Keiko Hattori
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- High Pressure, Subduction Zone Processes, Continental Tectonics, Subduction Zone

To better understand the establishment of subduction zones, considerable effort is required regarding various factors, such as the time of subduction initiation and the age of the slab’s lithosphere. In this study, adakitic dikes (ca. 17.3 Ma: εNd of 7.4–7.9 and 206Pb/204Pb of 18.27–18.42) were recognized to crystallize along with the normal and enriched mid-ocean ridge basalt (N-and E-MORB)-like oceanic crust (ca. 17.8–14.1 Ma: εNd of 8.8–13.3 and 206Pb/204Pb of 17.71–18.22) in the East Taiwan Ophiolite (ETO). Given that the outcropping in ultramafic sequence, the distinct Nd–Pb isotopes, and the slight precedence than corresponding arc volcanism (≤ca. 14.1 Ma), the origins of the ETO adakitic dikes are related to amphibolite-facies melting of the subducted slab beneath the Luzon forearc spreading center during subduction initiation. In this respect, we conclude that the sparsely exposed but isotopically heterogeneous E-MORB-type rocks (εNd of 8.8–10.2 and 206Pb/204Pb of 18.06–18.22) are Nb-enriched basalts and relate to the Nb-enriched slab melts derived from further melting of the hot eclogitic residuum of the young South China Sea subducted slab. In addition, the isotopic diversity of N-MORB-like samples can be attributed to the diking of adakitic and Nb-enriched slab melts beneath the Luzon forearc spreading center. Combined with the previously reported early Miocene uplift of the Zambales Ophiolite in the western Luzon, we infer an early Miocene induced-subduction initiation of the South China Sea and the delayed near-trench spreading of the Luzon forearc by <5 Myr. Compared to the absence of Nb-enriched slab melts and >20 Myr subducted slab in the genesis of the in-situIzu-Bonin-Mariana (IBM) forearc and the voluminous Nb-enriched rocks and ca. 0 Myr subducted slab of the Central Palawan–Amnay ophiolites, the sparse Nb-enriched rocks and <15 Myr subducted slab of the ETO sequence invoke the governing factor of slab’s lithosphere age during subduction initiation magmatism. We propose that the lack of Nb-enriched slab melts in the forearc chemostratigraphy of the Oman Ophiolite may indicate the subduction initiation of an old and cold oceanic plate.

The kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings. Two
reference frames are used here to describe the simple 2D kinematics of subduction zones. In the first, the upper plate is assumed
fixed, whereas in the second frame upper and lower plates move relative to the mantle.
Relative to a fixed point in the upper plate U, the transient subduction hinge H can converge, diverge, or be stationary. Similarly,
the lower plate L can converge, diverge or be stationary. The subduction rate VS is given by the velocity of the hinge H minus the
velocity of the lower plate L (VS=VH−VL). The subduction rate 1) increases when H diverges, and 2) decreases when H converges.
Combining the differentmovements, at least 14 kinematic settings can be distinguished along the subduction zones.Variable settings
can coexist even along a single subduction zone, as shown for the 5 different cases occurring along the Apennines subduction zone. Apart
from few exceptions, the subduction hinge converges toward the upper plate more frequently along E- or NE-directed subduction zone,
whereas it mainly diverges from the upper plate along W-directed subduction zones accompanying backarc extension.
Before collision, orogen growth occurs mostly at the expenses of the upper plate shortening along E–NE-directed subduction zones,
whereas the accretionary prism of W-directed subduction zones increases at the expenses of the shallow layers of the lower plate. The
convergence/shortening ratio is N1 along E- or NE-directed subduction zones, whereas it is b1 along accretionary prisms ofW-directed
subduction zones.
Backarc spreading forms in two settings: along the W-directed subduction zones it is determined by the hinge divergence
relative to the upper plate, minus the volume of the accretionary prism, or, in case of scarce or no accretion, minus the volume of
the asthenospheric intrusion at the subduction hinge. Since the volume of the accretionary prism is proportional to the depth of the
decollement plane, the backarc rifting is inversely proportional to the depth of the decollement. On the other hand, along E- or NEdirected
subduction zones, few backarc basins form (e.g., Aegean, Andaman) and can be explained by the velocity gradient within
the hangingwall lithosphere, separated into two plates.
When referring to the mantle, the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot
reference frames. The subduction hinge is mostly stationary being the slab anchored to the mantle along W-directed subduction
zones, whereas it moves W- or SW-ward along E- or NE-directed subduction zones. Surprisingly, along E- or NE-directed
subduction zones, the slab moves “out” of the mantle, i.e., the slab slips relative to the mantle opposite to the subduction direction.
Kinematically, this subduction occurs because the upper plate overrides the lower plate, pushing it down into the mantle. As an
example, the Hellenic slab moves out relative to the mantle, i.e., SW-ward, opposite to its subduction direction, both in the deep
and shallow hotspot reference frames. In the shallow hotspot reference frame, upper and lower plates move “westward” relative to
the mantle along all subduction zones.
This kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and
plate motions.

- by Carlo Doglioni and +1
- •
- Geodynamics, Plate Tectonics, Subduction Zone Processes, Geotectonics and Geodynamics
- by Ian Metcalfe
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- Geology, Geochemistry, Geophysics, Oceanography
- by Yildirim Dilek
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- Geology, Geochemistry, Geophysics, Subduction Zone Processes
- by Yildirim Dilek
- •
- Geology, Geochemistry, Geophysics, Subduction Zone Processes

Post-collisional volcanism in northwestern Iran is represented by the Saray high-K rocks including leucite-bearing under-saturated and leucite-free silica saturated rocks. We report Ar–Ar age data which constrain the age as ca. 11 Ma (late Miocene). Most of clinopyroxene phenocrysts from the volcanic rocks have complex oscillatory zoning, with high Ti and Al cores, low Ti and high Al mantled clinopyroxenes, grading into low Ti and Al outer rims. All the rocks are highly enriched in incompatible trace elements and have identical Sr–Nd–Pb isotopes. Enrichment in incompatible elements and other geochemical features for the Saray lavas suggest a metasomatized sub-continental lithospheric mantle (SCLM) as the magma source. The negative Nb–Ta–Ti anomalies for the Saray lavas compare with the features of subduction-related magmatism with negligible contamination with ancient crustal components. The highly radiogenic 87 Sr/ 86 Sr and 207 Pb/ 204 Pb isotopic values of the Saray lavas imply the involvement of slab terrigenous sediments and/or a continental lithosphere. Isotopically, the volcanic rocks define a binary trend, representing 5–8% mixing between the primary mantle and sediment melts. Our melting models suggest residual garnet in the source and are incompatible with partial melting of amphibole and/or phlogopite bearing lherzolites, although the complex geochemical features might indicate the result of mixing between melts produced by different sources or a homogenous melt passing through a compositionally-zoned mantle during multiple stages of partial melting and melt migration. The geochronological, geochemical and isotopic data for the Saray rocks suggest that these Late Miocene magmas were derived from a small degree of partial melting of subduction-metasomatized (subcontinental) lithospheric mantle source in a post-collisional setting.

- by DrMohamed Khedr
- •
- Volcanology, Iranian Studies, Mineral Chemistry, Isotopic tracers

The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic
processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most
enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By
taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD)
project in the Sulu orogen, a series of combined studieswere carried out for UHP metamorphic rocks from themain
hole (MH) at continuous depths of 100 to 5000m. The results provide newinsights into the chemical geodynamics
of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the
surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as
follows. (1) An O isotope profile of 100 to 5000mis established for the UHP metamorphicminerals, with finding of
18O depletion as deep as 3300m. Alongwith areal 18O depletion of over 30,000 km2 along the Dabie–Sulu orogenic
belt, three-dimensional 18O depletion of over 100,000 km3 occurs along the northern margin of the South China
Block. (2) Changes in mineralOisotope,Hisotope andwater content occur in eclogite-gneiss transitions, concordant
with petrographic changes. The contact between different lithologies is thus themost favorable place forfluid action;
fluid for retrogression of the eclogites away fromthe eclogite-gneiss boundarywas derived fromthe decompression
exsolution. For the eclogites adjacent to gneiss, in contrast, the retrogrademetamorphismwas principally caused by
aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance,
petrography and δ18O values of adjacent samples shows O isotope heterogeneities between the different and same
lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental
collision. (3) Studies of major and trace elements in the two continuous core segments indicate highmobility of LILE
and LREE but immobility ofHFSE andHREE. Someeclogites have andesitic compositionswith high SiO2, alkalis, LREE
and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses,
possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the
exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by
melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results
provide evidence for melt-induced elementmobility in the UHPmetamorphic rocks, and thus the possible presence
of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO2,
LREE and LILE occur at the contact between eclogite and gneiss. This indicates theirmobility between different slab
components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the
widespread retrogression, retrogradefluidwas internally buffered in stable isotope compositions, and the retrograde
fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and
molecularwater in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals
the episode of HP eclogite-facies recrystallization at 216±3Ma, with timescale of 1.9 to 9.3Myr or less. Collectively,
theDabie–SuluUHP terrenes underwent the protracted exhumation (2–3 mm/yr) in the HP-UHP regime. (6) Zircon
U–Pb ages and Hf isotopes indicate that mid-Neoproterozoic protoliths of bimodal UHPmetaigneous rocks formed
during supercontinental rifting along preexisting arc-continent collision orogen, corresponding to dual bimodal
magmatism in response to the attempted breakup of the supercontinent Rodinia at about 780Ma. The first type of
bimodalmagmatismwas formed by reworking of juvenile Late Mesoproterozoic crust, whereas the second type of
bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust. In
conclusion, the geochemical studies of CCSD-MHcore samples have placed important constraints on the nature and
scale of fluid action and element mobility during the continental subduction and UHP metamorphism.

- by Yong-Fei Zheng and +1
- •
- Stable Isotope Geochemistry, Continental Subduction, Subduction Zone Processes, Isotope Geochronology

A conceptual shift is overdue in geodynamics. Popular models that present plate tectonics as being driven by bottom-heated whole-mantle convection, with or without plumes, are based on obsolete assumptions, are contradicted by much evidence, and fail to account for observed plate interactions. Subduction-hinge rollback is the key to viable mechanisms. The Pacific spreads rapidly yet shrinks by rollback, whereas the subduction-free Atlantic widens by slow mid-ocean spreading. These and other fi rstorder features of global tectonics cannot be explained by conventional models. The behavior of arcs and the common presence of forearc basins on the uncrumpled thin leading edges of advancing arcs and continents are among features indicating that subduction provides the primary drive for both upper and lower plates. Subduction rights the density inversion that is produced when asthenosphere is cooled to oceanic lithosphere: plate tectonics is driven by top-down cooling but is enabled by heat. Slabs sink more steeply than they dip and, if old and dense, are plated down on the 660 km discontinuity. Broadside-sinking slabs push all sublithosphere oceanic upper mantle inward, forcing rapid spreading in shrinking oceans. Down-plated slabs are overpassed by advancing arcs and plates, and thus transferred to enlarging oceans and backarc basins. Plate motions make sense in terms of this subduction drive in a global framework in which the ridge-bounded Antarctic plate is fixed: most subduction hinges roll back in that frame, plates move toward subduction zones, and ridges migrate to tap fresh asthenosphere. This self-organizing kinematic system is driven from the top. Slabs probably do not subduct into, nor do plumes rise to the upper mantle from, the sluggish deep mantle.

- by Warren Hamilton
- •
- Geodynamics, Continental Subduction, Plate Tectonics, Subduction Zone Processes
- by vincenzo picotti
- •
- Geodesy, Coevolution, Tectonics, Sedimentary Basins
- by Lina Patiño
- •
- Geology, Geochemistry, Geophysics, Subduction Zone Processes

The Andaman-Nicobar Accretionary Ridge forms the eastern boundary of the Bay of Bengal and is presently being constructed by accretion and underplating of sediments offscraped from the obliquely colliding Bengal Fan. Net accretion is relatively low (~28%) with the rest subducted mostly into the upper mantle. Although subduction initiated along the margin at ~95 Ma, large-scale subduction accretion likely accelerated during the Early Miocene by which time wedge top basins had formed. Prior to that time sediment offscraped against the ophiolitic backstop was probably derived from the adjacent magmatic arc of Burma during the Eocene, as well as some erosion of continental sources, probably from the Sibumasu Block, which forms the western edge of Sundaland. The scale of this accretion was small and potentially interrupted by times of tectonic erosion during the Palaeogene. The influence of continental erosion increased into the Oligocene, potentially accompanied by modest flux from the Indus-Yarlung Suture via the Irrawaddy River. Drainage capture in eastern Tibet in the Early Miocene and opening of the Andaman Sea, probably in the Late Miocene, has removed these source areas to the Andaman Trench.

- by Massimo Verdoya
- •
- Rheology, Multidisciplinary, Subduction Zone Processes, Heat Flux

The deep structure of the southern Apennines
(SA) accretionary wedge is still debated since
industrial seismic reflection and well data provide
reliable constraints only to a depth of about 10 km. As
a consequence, two directly linked questions regard
(1) the shortening in the accretionary prism
(particularly within the buried Apulian thrust units)
and (2) the degree of involvement of the lower plate
basement (i.e., the Apulian crystalline basement). To
address these issues, we have constructed a regional
section along a recently released deep seismic
reflection profile (CROP-04) which intersects the
entire SA. The resulting cross section, adequately
constrained to a depth of about 15 km, has been
framed in a geodynamic scenario characterized by the
eastward roll-back of the westward subducting Apulo-
Adriatic lithosphere. On the basis of this section we
speculate on the deep structure, building both thin- and
thick-skinned thrust models. A cross-check of these
end-members models against documented tectonic,
geophysical, and geochemical features shows that the
thin-skinned model is generally more consistent with
the available data. The development of basement slices
with thicknesses of tens of kilometers is unlikely,
while it remains possible that the Apulian basement
could have been involved with its upper few
kilometers. In the thin-skinned model, the total
shortening of the allochthonous units (i.e., Apennine
and Apulian carbonate platforms and Lagonegro
basin) is estimated to be greater than 280–300 km.
Some 90 km of shortening can be attributed to the
Apulian thrust units.

- by Carlo Doglioni and +1
- •
- Continental Subduction, Plate Tectonics, Subduction Zone Processes, Subduction Zones

The Neyriz ophiolite along the northeast flank of the Zagros fold-thrust belt in southern Iran is an excellent example of a Late Cretaceous supra-subduction zone (SSZ)-related ophiolite on the north side of the Neotethys. The ophiolite comprises a mantle sequence including lherzolite, harzburgite, diabasic dikes, and cumulate to mylonitic gabbro lenses, and a crustal sequence comprising a sheeted dike complex and pillow lavas associated with pelagic limestone and radiolarite. Mantle harzburgites contain less CaO and Al 2 O 3 , are depleted in rare earth elements, and contain spinels that are more Cr-rich than lherzolites. Mineral compositions of peridotites are similar to those of both abyssal and SSZ-peridotites. Neyriz gabbroic rocks show boninitic (SSZ-related) affinities, while crustal rocks are similar to early arc tholeiites. Mineral compositions of gabbroic rocks resemble those of SSZ-related cumulates such as high forsterite olivine, anorthite-rich plagioclase, and high-Mg# clinopyroxene. Initial εNd(t) values range from +7.9 to +9.3 for the Neyriz magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic mid-ocean ridge basalts and with Semail and other Late Cretaceous Tethyan ophiolitic rocks. Initial 87 Sr/ 86 Sr ranges from 0.7033 to 0.7044, suggesting modification due to seafloor alteration. Most Neyriz magmatic rocks are characterized by less radiogenic 207 Pb/ 204 Pb (near the northern hemisphere reference line), suggesting less involvement of sediments in their mantle source. Our results for Neyriz ophiolite and the similarity to other Iranian Zagros ophiolites support a subduction initiation setting for its generation.

- by DrMohamed Khedr
- •
- Stable Isotope Analysis, Iranian Studies, History of Iran, Stable Isotopes

In the Southern Apennines and Calabria outcrop metamorphic slices of hercynian and/or alpine age. The reconstructions by Glauco Bonardi and co-authors of an alpine subduction-related origin for the emplacement of those rocks is still supported by a number of tectonic and geodynamic constraints. Alps and Apennines are two end members of opposite subduction zones (i.e., “E”-ward and “W”-ward), having different depth and evolution of the accretionary prism basal decollement. The PTt evolution of the metamorphic rocks of Calabria can be better reconciled with an earlier alpine-type evolution, where the continental and oceanic slices of crust have been subducted and re-exhumed by the deepening of the decollement planes into both the upper and lower plates and accreting the orogen. The early alpine emplacement of those metamorphic rocks is testified by their high structural elevation, the HP/LT assemblages and by stratigraphic constraints (e.g., Bonardi et alii, 2005). Therefore it can be interpreted that the earlier alpine subduction was gradually substituted by the opposite W-directed Apennines subduction, having in its hangingwall the boudinated relics of the alpine history.

- by Carlo Doglioni
- •
- Subduction Zone Processes

We re-evaluate the possibility that Earth's rotation contributes to plate tectonics on the basis of the following observations: 1) plates move along a westerly polarized flow that forms an angle relative to the equator close to the revolution plane of the Moon; 2) plate boundaries are asymmetric, being their geographic polarity the first order controlling parameter; unlike recent analysis, the slab dip is confirmed to be steeper along W-directed subduction zones; 3) the global seismicity depends on latitude and correlates with the decadal oscillations of the excess length of day (LOD); 4) the Earth's deceleration supplies energy to plate tectonics comparable to the computed budget dissipated by the deformation processes; 5) the Gutenberg–Richter law supports that the whole lithosphere is a self-organized system in critical state, i.e., a force is acting contemporaneously on all the plates and distributes the energy over the whole lithospheric shell, a condition that can be satisfied by a force acting at the astronomical scale. Assuming an ultra-low viscosity layer in the upper asthenosphere, the horizontal component of the tidal oscillation and torque would be able to slowly shift the lithosphere relative to the mantle.

- by Carlo Doglioni and +1
- •
- Geodynamics, Plate Tectonics, Subduction Zone Processes, Geotectonics and Geodynamics

Records of ancient intraoceanic arc activity, now preserved in continental suture zones, are commonly used to reconstruct paleogeography and plate motion, and to understand how continental crust is formed, recycled, and maintained through time. However, interpreting tectonic and sedimentary records from ancient terranes after arc–continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others. In this synthesis we examine what is lost, and what is preserved, in the translation from modern processes to the ancient record of intraoceanic arcs. Composition of accreted arc terranes differs as a function of arc–continent collision geometry. ‘Forward- facing’ collision can accrete an oceanic arc on to either a passive or an active continental margin, with the arc facing the continent and colliding trench- and forearc-side first. In a ‘backward-facing’ collision, involving two subduction zones with similar polarity, the arc collides backarc-first with an active continental margin. The preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case. Preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision, rather than tectonic erosion. The prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent. Arc systems are most likely to undergo tectonic accretion shortly before forward-facing collision with a continent, and thus most forearc and accretionary-prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity, when sediment flux to the trench was greatest and tectonic accretion prevailed. Collision geometry and tectonic erosion vs. accretion are important controls on the ultimate survival of material from the trench, forearc, arc massif, intra-arc basins, and backarc basins, and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts, oblique plate convergence, and arc rifting. Forward-facing collision involves substantial recycling, melting, and fractionation of continent-derived mate- rial during and after collision, and so produces melts rich in silica and incompatible trace elements. As a result, forward-facing collision can drive the composition of accreted arc crust toward that of average conti- nental crust.

Subduction zones appear primarily controlled by the polarity of their direction, i.e., W-directed or E- to NNE-directed,
probably due to the westward drift of the lithosphere relative to the asthenosphere. The decollement planes behave
differently in the two end-members. In the W-directed subduction zone, the decollement of the plate to the east is warped
and subducted, whereas in the E- to NNE-directed, it is ramping upward at the surface. There are W-directed subduction
zones that work also in absence of active convergence like the Carpathians or the Apennines. W-directed subduction zones
have shorter life 30–40 Ma. than E- or NE-directed subduction zones even longer than 100 Ma.. The different
decollements in the two end-members of subduction should control different PTt paths and, therefore, generate variable
metamorphic assemblages in the associated accretionary wedges and orogens. These asymmetries also determine different
topographic and structural evolutions that are marked by low topography and a fast ‘eastward’ migrating structural wave
along W-directed subduction zones, whereas the topography and the structure are rapidly growing upward and expanding
laterally along the opposite subduction zones. The magmatic pair calc-alkaline and alkaline–tholeiitic volcanic products of
the island arc and the back-arc basin characterise the W-directed subduction zones. Magmatic rocks associated with E- or
NE-directed subduction zones have higher abundances of incompatible elements, and mainly consist of calc-alkaline–
shoshonitic suites, with large volumes of batholithic intrusions and porphyry copper ore deposits. The subduction zones
surrounding the Adriatic plate in the central Mediterranean confirm the differences among subduction zones as primarily
controlled by the geographic polarity of the main direction of the slab. The western margin of the Adriatic plate
contemporaneously overridden and underthrust Europe toward the ‘west’ to generate, respectively, the Alps and the
Apennines, while the eastern margin subducted under the Dinarides–Hellenides. These belts confirm the characters of the
end-members of subduction zones as a function of their geographic polarity similarly to the Pacific subduction zones.
q1999 Elsevier Science B.V. All rights reserved.

- by Carlo Doglioni
- •
- Geology, Tectonics, Subduction Zone Processes, World Orogeny

Dike-like bodies of garnet (Py 77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of hornblende (pargasite to pargasitic hornblende), [depleted in LREE with flat HREE segments (normalized to CI-chondrite), and enriched in LILE (normalized to N-MORB)] and crystallized from sub-arc tholeiitic melt that derived from wet mantle wedge (615–600 Ma ago), at P = 23–27 kbar and T = 510–600 °C. Diapirism of hot mantle due to decompression caused partial dehydration melting (P = $12 kbar and T = 800–>1100 °C) of hornblende into pyrope-almandine garnet and diopside (Wo 49– 51 En 32–34 Fs 16–17.5). The clockwise P-T path continued by isothermal decomposition (10–11 kbar) of garnet into hercynite-rich spinel (XFe 2+ = 0.46–0.64) and quartz. By continuous cooling and probably by introduction of the Abu Ziran granite-related solutions, the hornblendites followed a retrograde greenschist facies hydration path, including transformation of diopside to tremolite (T = 280–420 °C and pressure > 5 kb) and garnet to chlorite (T = 190–345 °C). In addition, metasomatic minerals mainly of feld-spars and titanite, and minor ilmenite, rutile, topaz and calcite formed, as well as composition of the original amphibole changed having increases of FeO, TiO 2 and K 2 O and decreases of Al 2 O 3 , MgO and Na 2 O. Deep source of the Meatiq hornblendites implies for a probable local crustal thickening of Meatiq Core Complex by crustal shortening, while their exhumation was most probably accompanied the NW-SE extension and thinning of the previously thickened crust that occurred during oblique island arc convergence with the closure of the Mozambique ocean and the collision of East and West Gondwanaland.

- by Mohamed Hamdy and +1
- •
- Subduction Zone Processes, Mantle, dehydration melting, extension-related exhumation

The Quaternary geodynamic evolution and the tectonic
processes active along the Central and Northern Apennines
thrust fronts and in the adjacent Padane-Adriatic foredeep
domains are analysed and discussed.
A reinterpretation of the available geophysical and geological
data reveals that the south-eastward prolongation of
the Apennines thrust front in the Adriatic Sea is most likely located
along the north-eastern side of the Adriatic ridge, i.e., in
a more external position with respect to traditional interpretations.
Further south, the Apennine thrust front is segmented
in correspondence with the Tremiti lithospheric right-lateral
transfer zone.
This new interpretation of the Apennine thrust front bears
some relevant implications since it rejuvenates to Late Quaternary
the most recent contractional deformations in its
Adriatic portion. This is consistent with the Late Quaternary
activity of the buried thrust-related folds associated with the
Apennine front along the Marche coastal belt and in the Po
Plain documented by geomorphological analysis and by seismic
refl ection profi les. Moreover, active shortening associated
with the Apennines accretionary prism in the Po Plain and
in the central and northern Adriatic Sea is documented by GPS
data and by historical and instrumental seismicity.
The Quaternary evolution of one of the active thrust-related
folds recognised in the Po Plain subsurface (the Mirandola
anticline) has been investigated in detail by backstripping
high-resolution stratigraphic data. Our results show decreasing
relative uplift rates during the Quaternary. However, tectonic
relative uplift rate of about 0.16 mm/a can still be recognised
during the last 125 ka. Horizontal shortening faster than
1 mm/a should be expected in agreement with available GPS
data.
Furthermore, the SW-ward (or W-ward) increasing dip of
the foreland monocline in the Po Plain and in the centralnorthern
Adriatic and the asymmetric distribution of the Quaternary
to Recent subsidence indicate a still active fl exural retreat
of the subducting lithosphere in these domains.
The Quaternary to Recent fl exural retreat of the subducting
Adriatic lithosphere and the related frontal accretion of
the Apennines prism are framed in a coherent geodynamic
scenario characterized by a retreating west-directed subduction
zone, which is the natural evolution of the Neogene geodynamic
history.

The Calabrian Arc (CA) is a subduction system related to the slow convergence between Africa and Eurasia, well comparable in its geodynamic evolution to the adjacent Mediterranean Ridge against which it impinges in the eastern side of the Ionian Sea. The external part of the arc is represented by a well developed accretionary complex resting on the seaward dipping continental backstop and bordered by the Malta and Apulian escarpments. Although the regional architecture of the margin geometry has been described through the analysis of high penetration seismic data, location of major active faults and the fine texture of this tectonic structure is still poorly constrained. We reconstructed geometry and structural setting of the external CA through the interpretation of seismic data belonging to the Crop-Mare and -MS datasets. Three seismic sections have been selected and re-processed through the application of the pre-stack depth migration (PSDM). Seismic images show that at the toe o...

- by Luigi Torelli and +1
- •
- Geology, Geophysics, Seismic Hazard, Tectonics

Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically per- turbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting the Permian volcanic products have been interpreted as developed either in a late- collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between late Variscan orogenic evolution and litho- spheric thinning that, since Permian – Triassic time, announced the Tethys opening. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, accounted by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active exten- sion is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement poorly reactivated during Mesozoic–Tertiary convergence.

- by Gisella Rebay and +2
- •
- Numerical Modeling, Subduction Zone Processes, Variscan tectonic

When considering a migrating subduction hinge, the kinematics of convergent geodynamic settings shows that subduction zone rates can be faster or slower than convergent rates as a function of whether the subduction hinge migrates away or toward the upper plate. This opposite behaviour occurs in particular along W-directed and E- or NE-directed subduction zones respectively. Along W-directed slabs, the subduction rate is the convergence rate plus the slab retreat rate, which tends to equal the backarc extension rate. Along E- or NE-directed slabs, the subduction rate is decreased by the shortening in the upper plate. Relative to the mantle, the W-directed slab hinges are fixed, whereas they move west or southwest along E- or NE-directed subduction zones. Therefore, subduction zones appear as passive features controlled by the far field plate velocities and their motion relative to the underlying “eastward” mantle flow rather than by the negative buoyancy alone of the downgoing plate.
Several observations cast doubts about the efficiency of the slab pull alone in triggering plate motions. For example, kinematically, the slab is moving out of the mantle along some E- or NE-directed subduction zones, i.e., moving in the direction opposite to the one predicted by the pull of the slab. Mantle convection is also inadequate to explain the Earth’s surface kinematics. Plate motions driven by the Earth’s rotation seem to be the simplest explanation for the asymmetry along the subduction zones and the aforementioned incongruence.

- by Carlo Doglioni
- •
- Tectonics, Geodynamics, Plate Tectonics, Subduction Zone Processes

The Ossa-Morena zone in SW Iberia represents a section of the northern margin of West Gondwana that formed part of a Cordilleran-type orogenic system during the Neoproterozoic (Cadomian orogeny). The crustal section in this zone preserves the record of rifting that led to the opening of the Rheic Ocean in the early Paleozoic and the collision of Gondwana and Laurussia in the late Paleozoic (Variscan orogeny). We present U-Pb zircon data from three alkaline to peralkaline syenites that intruded Neoproterozoic and Cambrian strata and give crystallization ages ranging between ca. 490 Ma and 470 Ma. Lu/Hf isotopic data from these zircons give positive initial εHf values (0 ≤ εHf(t) ≤ +11.5) that approach the model values for the depleted mantle at the time of crystallization. This suggests that a significant proportion of the magma was derived from the mantle, with limited mixing/assimilation with crustal-derived melts. Alkaline/peralkaline magmatic suites of similar age and chemical composition intruded other sections of the northern margin of West Gondwana and along the boundaries of the continental blocks that today make up Iberia. These blocks are further characterized by the presence of high-pressure metamorphic belts that formed during accretion and subsequent collision of peri-Gondwanan domains against Laurussia during the Devonian and Carboniferous (Variscan orogeny). Our U-Pb and Lu-Hf data set indicates that during the Cambrian−Ordovician transition, lithosphere extension reached a stage of narrow intracontinental rifting, where deeply sourced magmas, probably coming from the lower crust and/or the upper mantle, intruded continental upper crust across various sections of previously stretched crust. We propose that necking of the Gondwana lithosphere into several continental microblocks with fertile mantle beneath them compartmentalized extension (multiblock model), which favored the onset of early Paleozoic peralkaline and alkaline magmas. The boundaries of microblocks represent zones of inherited crustal weakness that were later reactivated during the late Paleozoic as major accretionary faults related to the amalgamation of Pangea during the Variscan orogeny. Our dynamic model provides an explanation for the unusual spatial relationship between peralkaline and alkaline igneous provinces (usually shallow in the crust) and the occurrence of high-pressure rocks. Our observations suggest that Cordilleran-type orogens subjected to extension after long-lived subduction can develop wide continental platforms that feature multiple continental blocks. In addition, the formation of sequenced high-pressure belts in collisional orogens can be explained as the ultimate consequence of multiple necking events within continental lithosphere during previous collapse of a Cordilleran-type orogen.

"The Cenozoic geological evolution of the Italian area is characterized by the formation of two major mountain chains - the Alps to the north and the Apennines throughout the peninsula – plus the opening of two oceanic basins (the Ligurian-Provençal and the Tyrrhenian Sea). Associated with the formation of these two belts, a volumetrically important and chemically complex magmatic activity developed. The Alps and the Apennines show very different styles of evolution: the Alps display double-verging growth, with the involvement of large volumes of basement and the exhumation of metamorphic rocks (thick-skinned tectonics). On the other hand, the Apennines are a
single-verging belt, mostly characterized by thin-skinned tectonics and associated to a radial "eastward" translation (coupled to extensional tectonics in the Ligurian-Provençal, Tyrrhenian, and western Apennines areas). The Apennines generated an arc from the northern Apennines down to Sicily, possibly merging with the Maghrebides along the northern Africa coast. The different evolution of the Alpine and Apenninic belts is mirrored by the different
geometry of the respective foredeep or foreland basins (shallow in the Alps and deep in the Apennines), as recorded also by the dip of the foreland monocline (shallow in the Alps, 2-4°, and steeper in the Apennines, 6-15°). The paradox is evident: the higher the belt, the thinner the foreland basin. The Alps consist of rocks belonging to the continental margings of the European and Adriatic-African plates, as well as remnants of Mesozoic intervening ocean(s). On the other hand, the Apennines, with the exception of the Calabro-Peloritani arc and other scattered basement outcrops, are mainly made up of rocks of Adriatic origin (Mesozoic Laziale-Abruzzese and Apulian carbonate platforms plus basinal successions), with subordinate ophiolites. From a magmatological point of view, the Alpine magmatism is essentially concentrated in a relatively narrow area, the so-called Insubric Lineament and in a relatively short time (mostly ~32-24 Ma). On the other hand, the Apennines-related igneous activity spans a larger
time range (essentially from 22 Ma to Present), with several peaks in magma production. This magmatism took place over a much wider area, characterized by variable lithospheric thickness, Moho age and depth. On the basis of thermo-tectonic, magmatological, and plate-kinematics constraints, a geodynamic evolutionary model of the Italian area is proposed. We suggest that three subduction zones have been active and have consumed oceanic and, partially, continental lithosphere: the Alpine subduction zone, with the European plate under-thrusting the Adriatic microplate; the Apenninic subduction zone, with the ancient (Mesozoic?) Ionian/Mesogean oceanic
lithosphere and the Adriatic micro-plate under-thrusting westward the European plate; and the Dinaric subduction zone, with the Adriatic micropate under-thrusting northeastward the European plate. Such a geodynamic scenario is summarised in a movie, spanning the last 50 Ma."

By selecting a limited number of variables (westward vs. eastward subduction polarity; oceanic vs. continental origin
of downgoing and overriding plates), we identify eight end-member scenarios of plate convergence and orogeny. These
are characterized by five different types of composite orogenic prisms uplifted above subduction zones to become
sources of terrigenous sediments (Indo-Burman-type subduction complexes, Apennine-type thin-skinned orogens,
Oman-type obduction orogens, Andean-type cordilleras, and Alpine-type collision orogens). Each type of composite
orogen is envisaged here as the tectonic assembly of subparallel geological domains consisting of genetically associated
rock complexes. Five types of such elongated orogenic domains are identified as the primary building blocks of
composite orogens: magmatic arcs, obducted or accreted ophiolites, neometamorphic axial belts, accreted paleomargin
remnants, and accreted orogenic clastic wedges. Detailed provenance studies on modern convergent-margin settings
from the Mediterranean Sea to the Indian Ocean show that erosion of each single orogenic domain produces peculiar
detrital modes, heavy-mineral assemblages, and unroofing trends that can be predicted and modeled. Five corresponding
primary types of sediment provenances (magmatic arc, ophiolite, axial belt, continental block, and clastic wedge
provenances) are thus identified, which reproduce, redefine, or integrate provenance types and variants originally
recognized byW. R. Dickinson and C. A. Suczek in 1979. These five primary provenances may be variously recombined
in order to describe the full complexities of mixed detrital signatures produced by erosion of different types of
composite orogenic prisms. Our provenance model represents a flexible and valuable conceptual tool to predict the
evolutionary trends of detrital modes and heavy-mineral assemblages produced by uplift and progressive erosional
unroofing of various types of orogenic belts and to interpret petrofacies from arc-related, foreland-basin, foredeep, and
remnant-ocean clastic wedges.

- by Carlo Doglioni and +1
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- Geology, Tectonics, Sedimentary provenance, Subduction Zone Processes

The upper subcontinental lithospheric mantle below the French Massif Central is more oxidized than the average continental lithosphere, although the origin of this anomaly remains unknown. Using iron oxidation analysis in clinopyroxene, oxybarometry, and melt inclusions in mantle xenoliths, we show that widespread infiltration of volatile (HCSO)-rich silicic melts played a major role in this oxidation. We propose the first comprehensive model of magmatism and mantle oxidation at an intraplate setting. Two oxidizing events occurred: (1) a 365– 286 Ma old magmatic episode that produced alkaline vaugnerites, potassic lamprophyres, and K-rich calc-alkaline granitoids, related to the N–S Rhenohercynian subduction, and (2) b 30 Ma old magmatism related to W–E extension, producing carbonatites and hydrous potassic trachytes. These melts were capable of locally increasing the subcontinental lithospheric mantle fO 2 to FMQ + 2.4. Both events originate from the melting of a metasomatized lithosphere containing carbonate + phlogopite ± amphibole. The persistence of this volatile-rich lithospheric source implies the potential for new episodes of volatile-rich magmatism. Similarities with worldwide magmatism also show that the importance of volatiles and the oxidation of the mantle in intraplate regions is underestimated.

- by Audrey M Martin
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- Geology, Petrology, Volatiles In Magmas, Igneous petrology