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Within the uncertainties of involved astronomical and biological parameters, the Drake Equation typically predicts that there should be many exoplanets in our galaxy hosting active, communicative civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing the importance of planetary tectonic style for biological evolution. We summarize growing evidence that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated emergence and evolution of complex species. We further suggest that both continents and oceans are required for ACCs because early evolution of simple life must happen in water but late evolution of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox (1) by adding two additional terms to the Drake Equation: foc (the fraction of habitable exoplanets with significant continents and oceans) and fpt (the fraction of habitable exoplanets with significant continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by demonstrating that the product of foc and fpt is very small (< 0.00003–0.002). We propose that the lack of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life. [ABSTRACT FROM AUTHOR]

Iran is a remarkable geoscientific laboratory where the full range of processes that form and modify the continental crust can be studied. Iran's crustal nucleus formed as a magmatic arc above an S-dipping subduction zone on the northern margin of Gondwana 600–500 Ma. This nucleus rifted and drifted north to be accreted to SW Eurasia ∼250 Ma. A new, N-dipping subduction zone formed ∼100 Ma along ∼3,000 km of the SW Eurasian margin, including Iran's southern flank; this is when most of Iran's many ophiolites formed. Iran evolved as an extensional continental arc in Paleogene time (66–23 Ma) and began colliding with Arabia ∼25 Ma. Today, Iran is an example of a convergent plate margin in the early stages of continent-continent collision, with a waning magmatic arc behind (north of) a large and growing accretionary prism, the Zagros Fold-and-Thrust Belt. Iran's crustal evolution resulted in both significant economic resources and earthquake hazards. Iran is a natural laboratory for studying how convergent plate margins form, evolve, and behave during the early stages of continental collision. Iran formed in the past 600 million years, originating on the northern flank of Gondwana, rifting away, and accreting to SW Eurasia. Iran is actively deforming as a result of collision with the Arabian plate, but earthquakes do not outline the position of the subducting slab. The Cenozoic evolution of Iran preserves the main elements of a convergent plate margin, including foredeep (trench), accretionary prism, and magmatic arc. [ABSTRACT FROM AUTHOR]

To understand how plate tectonics became Earth's dominant mode of convection, we need to address three related problems. (i) What was Earth's tectonic regime before the present episode of plate tectonics began? (ii) Given the preceding tectonic regime, how did plate tectonics become established? (iii) When did the present episode of plate tectonics begin? The tripartite nature of the problem complicates solving it, but, when we have all three answers, the requisite consilience will provide greater confidence than if we only focus on the long-standing question of when did plate tectonics begin? Earth probably experienced episodes of magma ocean, heat-pipe, and increasingly sluggish single lid magmatotectonism. In this effort we should consider all possible scenarios and lines of evidence. As we address these questions, we should acknowledge there were probably multiple episodes of plate tectonic and non-plate tectonic convective styles on Earth. Non-plate tectonic styles were probably dominated by 'single lid tectonics' and this evolved as Earth cooled and its lithosphere thickened. Evidence from the rock record indicates that the modern episode of plate tectonics began in Neoproterozoic time. A Neoproterozoic transition from single lid to plate tectonics also explains kimberlite ages, the Neoproterozoic climate crisis and the Neoproterozoic acceleration of evolution. [ABSTRACT FROM AUTHOR]

Abstract How new subduction zones form is an emerging field of scientific research with important implications for our understanding of lithospheric strength, the driving force of plate tectonics, and Earth's tectonic history. We are making good progress towards understanding how new subduction zones form by combining field studies to identify candidates and reconstruct their timing and magmatic evolution and undertaking numerical modeling (informed by rheological constraints) to test hypotheses. Here, we review the state of the art by combining and comparing results coming from natural observations and numerical models of SI. Two modes of subduction initiation (SI) can be identified in both nature and models, spontaneous and induced. Induced SI occurs when pre-existing plate convergence causes a new subduction zone to form whereas spontaneous SI occurs without pre-existing plate motion when large lateral density contrasts occur across profound lithospheric weaknesses of various origin. We have good natural examples of 3 modes of subduction initiation, one type by induced nucleation of a subduction zone (polarity reversal) and two types of spontaneous nucleation of a subduction zone (transform collapse and plumehead margin collapse). In contrast, two proposed types of subduction initiation are not well supported by natural observations: (induced) transference and (spontaneous) passive margin collapse. Further work is therefore needed to expand on and understand the implications of these observations. Our future advancing understanding of SI will come from better geologic insights, laboratory experiments, and numerical modeling, and with improving communications between these communities. Graphical abstract Image 1 Highlights • Progress in our understanding of how new subduction zones form is reviewed. • 3 ways that new subduction zones formed are presented: polarity reversal, transform collapse, and plume margin collapse. • Future advances in both 2D and 3D numerical modeling expected [ABSTRACT FROM AUTHOR]

When Earth's tectonic style transitioned from stagnant lid (single plate) to the modern episode of plate tectonics is important but unresolved, and all lines of evidence should be considered, including the climate record. The transition should have disturbed the oceans and atmosphere by redistributing continents, increasing explosive arc volcanism, stimulating mantle plumes and disrupting climate equilibrium established by the previous balance of silicate-weathering greenhouse gas feedbacks. Formation of subduction zones would redistribute mass sufficiently to cause true polar wander if the subducted slabs were added in the upper mantle at intermediate to high latitudes. The Neoproterozoic Snowball Earth climate crisis may reflect this transition. The transition to plate tectonics is compatible with nearly all proposed geodynamic and oceanographic triggers for Neoproterozoic Snowball Earth events, and could also have contributed to biological triggers. Only extraterrestrial triggers cannot be reconciled with the hypothesis that the Neoproterozoic climate crisis was caused by a prolonged (200-250 m.y.) transition to plate tectonics. [ABSTRACT FROM AUTHOR]

Abstract: Extension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench‐normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench‐parallel extension has split apart the Eocene‐Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano‐tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near‐trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid‐ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting. [ABSTRACT FROM AUTHOR]

Iran is a mosaic of continental terranes of Cadomian (520–600 Ma) age, stitched together along sutures decorated by Paleozoic and Mesozoic ophiolites. Here we present the current understanding of the Mesozoic (and rare Cenozoic) ophiolites of Iran for the international geoscientific audience. We summarize field, chemical and geochronological data from the literature and our own unpublished data. Mesozoic ophiolites of Iran are mostly Cretaceous in age and are related to the Neotethys and associated backarc basins on the S flank of Eurasia. These ophiolites can be subdivided into five belts: 1. Late Cretaceous Zagros outer belt ophiolites (ZOB) along the Main Zagros Thrust including Late Cretaceous–Early Paleocene Maku–Khoy–Salmas ophiolites in NW Iran as well as Kermanshah–Kurdistan, Neyriz and Esfandagheh (Haji Abad) ophiolites, also Late Cretaceous–Eocene ophiolites along the Iraq–Iran border; 2. Late Cretaceous Zagros inner belt ophiolites (ZIB) including Nain, Dehshir, Shahr-e-Babak and Balvard–Baft ophiolites along the southern periphery of the Central Iranian block and bending north into it; 3. Late Cretaceous–Early Paleocene Sabzevar–Torbat-e-Heydarieh ophiolites of NE Iran; 4. Early to Late Cretaceous Birjand–Nehbandan–Tchehel–Kureh ophiolites in eastern Iran between the Lut and Afghan blocks; and 5. Late Jurassic–Cretaceous Makran ophiolites of SE Iran including Kahnuj ophiolites. Most Mesozoic ophiolites of Iran show supra-subduction zone (SSZ) geochemical signatures, indicating that SW Asia was a site of plate convergence during Late Mesozoic time, but also include a significant proportion showing ocean-island basalt affinities, perhaps indicating the involvement of subcontinental lithospheric mantle. [ABSTRACT FROM AUTHOR]

Highlights: [•] Banded Iron Formation of the Arabian-Nubian Shield (ANS BIF) was deposited ∼750Ma. [•] Deposition was cyclic (alternating Fe-rich and Si-rich bands) and rapid (soft-sediment slumping). [•] ANS-BIF is associated with metavolcanic and volcaniclastic sedimentary rocks. [•] ANS BIF and other ∼750Ma BIF indicate major change in marine redox before Sturtian glaciation. [ABSTRACT FROM AUTHOR]

Abstract: The Late Cretaceous Gogher–Baft ophiolite is one of the best preserved remnants of Neo-Tethyan oceanic lithospheric within the inner Zagros ophiolite belt. The ophiolite comprises from bottom to top, harzburgites, pegmatite and isotropic gabbroic lenses within the mantle sequence, pillowed to massive basalts to dacites and pyroclastic rocks associated with blocks of pelagic limestone and radiolarite. Basaltic to dacitic sills crosscut the pyroclastic rocks. The ophiolite sequence is overlain by Turonian–Maastrichtian pelagic limestones (93.5–65.5Ma). Mineral compositions of harzburgites are similar to those of fore-arc peridotites and overlap with abyssal peridotites. Most Gogher–Baft ophiolite magmatic rocks show supra-subduction zone affinities, except for some E-MORB type lavas. The geochemical characteristics suggest that Gogher–Baft ophiolite magmatic rocks were generated during subduction initiation. These show progressive source depletion leading to the formation of MORB to boninitic magmas. Early E-MORB-type pillow lavas may have originated by melting mantle that was not affected by subduction components as the Tethyan oceanic plate began to sink beneath Eurasia as subduction began in the Late Cretaceous. Initial εNd (t) values range from +2.6 to +9 for Gogher–Baft magmatic rocks. Samples with radiogenic Nd overlap with least radiogenic MORBs and with Oman and other Late Cretaceous Tethyan ophiolitic rocks. The initial 87Sr/86Sr ratios range from 0.7048 to 0.7057, indicating modification due to seafloor alteration. Radiogenic 207Pb/204Pb isotopic compositions (systematically above the NHRL) and less radiogenic Nd isotopic compositions suggest the involvement of sediments in the mantle source in some magmatic rocks. Our results for Gogher–Baft ophiolite and the similarity of these to other Iranian Zagros ophiolites suggest a subduction initiation setting for the generation of these magmatic rocks. [Copyright &y& Elsevier]

The southern Mariana Arc-Trench system is rapidly deforming, resulting in unusual interactions between arc and back-arc basin (BAB) magmatic systems. New geochemical data for volcanoes in this region are presented and explored. Tracey Seamount, an extinct submarine volcano about 30 km northwest of Guam, is the southernmost stratovolcano of the Mariana Arc. Tracey is built about 125 km above the subducted slab and has erupted a bimodal suite of typical arc mafic and felsic lavas as recently as 0.527 ± 0.023 Ma (40Ar/39Ar age). An unusual cluster of small basaltic volcanoes, informally termed the Alphabet Seamount Volcanic Province (ASVP), is found about where the next arc volcano to the southwest of Tracey Seamount should have grown. Samples from six of these volcanoes were studied here. At least two ASVP volcanoes were recently active, as shown by hydrothermal activity. The lack of magmatic focusing to build a single stratovolcano where the ASVP is situated reflects strong extension in the BAB. Construction of northern ASVP volcanoes is controlled by east-west extension accompanying opening of the Mariana Trough. In contrast, southern ASVP volcanoes are affected by north-south extension due to rapid rollback of a narrow slab of Pacific seafloor that is subducted along the east-west trending Challenger Deep segment of the Mariana Trench to the south. ASVP lava compositions are distinct from Tracey Seamount and other Mariana Arc lavas, instead showing affinities with Mariana Trough BAB basalt (BABBs): they are mafic, tholeiitic, low-K2O and LREE-depleted, with low 87Sr/86Sr, but show a subtle gradient from somewhat more arc-like lavas closer to the trench to BABBs farther west. The unusual tectonic setting of ASVP provides a unique perspective on how different arc magma batches reflect melting of mantle with strong compositional gradients which are mixed together beneath long-lived arc volcanoes but here rise to form scattered small ASVP volcanoes. [ABSTRACT FROM AUTHOR]

Any basin with seafloor spreading that forms over an active subduction zone is a backarc basin (BAB). The Gulf of Mexico (GoM) opened behind the 232-150 Ma Nazas arc over an east-dipping subduction zone in Late Jurassic time, beginning ca. 165 Ma, and thus is a BAB. The hypothesis that the Gulf of Mexico formed as a backarc basin explains two enigmas: (1) Why was the GoM opening pole (79-84°W, 23-30°N) so different from that of the central Atlantic (15-18°W, 65-67°N)? and (2) Why was the GoM opening so short-lived (ca. 165-142 Ma), when there was no collision or other obvious reason for seafloor spreading to stop? The GoM BAB hypothesis also illuminates the relationship between the GoM and the Border rift system, which can be traced from the GoM near the mouth of the Rio Grande >2000 km along the U.S.-Mexico border into the Independence Dike Swarm of eastern California. Late Jurassic rifting in the Border rift system was succeeded by thermotectonic subsidence through Early Cretaceous time. In addition, the segmentation of the transitional crust beneath the northern GoM into a magmatically robust segment beneath the Texas coast and a stretched margin beneath Louisiana is also consistent with BAB behavior: igneous activity is most prolifi c nearest the arc and diminishes with distance from the trench. A possible objection to the GoM BAB hypothesis is that the spreading ridge was oriented at high angles to the Nazas arc trend, whereas modern oceanic BAB spreading ridges generally parallel the associated arc. Continental BABs like the GoM develop spreading ridge orientations that are often at high angles to the associated convergent margin; for example, spreading ridges associated with the Miocene Sea of Japan and Andaman Sea BABs trend perpendicular to the associated arc. Such geometries reflect the presence of extensional stresses that are not orthogonal to the subduction zone, a situation that also existed in the GoM region during Late Jurassic time. [ABSTRACT FROM AUTHOR]

Abstract: The Arabian Plate originated ∼25Ma ago by rifting of NE Africa to form the Gulf of Aden and Red Sea. It is one of the smaller and younger of the Earth''s lithospheric plates. The upper part of its crust consists of crystalline Precambrian basement, Phanerozoic sedimentary cover as much as 10km thick, and Cenozoic flood basalt (harrat). The distribution of these rocks and variations in elevation across the Plate cause a pronounced geologic and topographic asymmetry, with extensive basement exposures (the Arabian Shield) and elevations of as much as 3000m in the west, and a Phanerozoic succession (Arabian Platform) that thickens, and a surface that descends to sea level, eastward between the Shield and the northeastern margin of the Plate. This tilt in the Plate is partly the result of marginal uplift during rifting in the south and west, and loading during collision with, and subduction beneath, the Eurasian Plate in the northeast. But a variety of evidence suggests that the asymmetry also reflects a fundamental crustal and mantle heterogeneity in the Plate that dates from Neoproterozoic time when the crust formed. The bulk of the Plate''s upper crystalline crust is Neoproterozoic in age (1000–540Ma) reflecting, in the west, a 300-million year process of continental crustal growth between ∼850 and 550Ma represented by amalgamated juvenile magmatic arcs, post-amalgamation sedimentary and volcanic basins, and granitoid intrusions that make up as much as 50% of the Shield''s surface. Locally, Archean and Paleoproterozoic rocks are structurally intercalated with the juvenile Neoproterozoic rocks in the southern and eastern parts of the Shield. The geologic dataset for the age, composition, and origin of the upper crust of the Plate in the east is smaller than the database for the Shield, and conclusions made about the crust in the east are correspondingly less definitive. In the absence of exposures, furthermore, nothing is known by direct observation about the composition of the crust north of the Shield. Nonetheless, available data indicate a geologic history for eastern Arabian crust different to that in the west. The Neoproterozic crust (∼815–785Ma) is somewhat older than in the bulk of the Arabian Shield, and igneous and metamorphic activity was largely finished by 750Ma. Thereafter, the eastern part of the Plate became the site of virtually continuous sedimentation from 725Ma on and into the Phanerozoic. This implies that a relatively strong lithosphere was in place beneath eastern Arabia by 700Ma in contrast to a lithospheric instability that persisted to ∼550Ma in the west. Lithospheric differentiation is further indicated by the Phanerozoic depositional history with steady subsidence and accumulation of a sedimentary succession 5–14km thick in the east and a consistent high-stand and thin to no Phanerozoic accumulation over the Shield. Geophysical data likewise indicate east–west lithospheric differentiation. Overall, the crustal thickness of the Plate (depth to the Moho) is ∼40km, but there is a tendency for the crust to thicken eastward by as much as 10% from 35–40km beneath the Shield to 40–45km beneath eastern Arabia. The crust also becomes structurally more complex with as many as 5 seismically recognized layers in the east compared to 3 layers in the west. A coincident increase in velocity is noted in the upper-crust layers. Complementary changes are evidenced in some models of the Arabian Plate continental upper mantle, indicating eastward thickening of the lithospheric mantle from ∼80km beneath the Shield to ∼120km beneath the Platform, which corresponds to an overall lithospheric thickening (crust and upper mantle) from ∼120km to ∼160km eastward. The locus of these changes coincides with a prominent magnetic anomaly (Central Arabian Magnetic Anomaly, CAMA) in the extreme eastern part of the Arabian Shield that extends north across the north-central part of the Arabian Plate. The CAMA also coincides with a major structural boundary separating a region of northerly and northwesterly basement trends in the west from a region of northerly and northeasterly trends in the northeastern part of the Plate, and with the transition from high-stand buoyant Shield to subsided Platform. Its coincidence with geophysically indicated changes in the lower crust and mantle structure suggests that a fundamental lithospheric boundary is present in the central part of the Arabian Plate. The ages and isotopic characteristics of xenoliths brought to the surface in Cenozoic basalt eruptions indicate that the lower crust and upper mantle are largely juvenile Neoproterozoic additions, meaning that the lower crust and upper mantle formed about the same time as the upper crust. This implies that the lithospheric boundary in the central part of the Arabian Plate dates from Neoproterozoic time. We conclude that lithospheric differentiation across the Arabian Plate is long lived and has controlled much of the Phanerozoic sedimentary history of the Plate. [Copyright &y& Elsevier]

We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible for cross-chain geochemical and isotopic variations. [ABSTRACT FROM AUTHOR]

Earth is the only known planet with subduction zones and plate tectonics, and this fact demonstrates that special conditions are required for this mode of planetary heat loss. Sinking of cold, dense lithosphere in subduction zones is the principal plate-driving force, so plate tectonics could not have begun until Earth cooled sufficiently to allow lithosphere to collapse into the underlying asthenosphere. Direct geologic evidence for when the modern episode of subduction tectonics began focuses on the first appearance of ophiolitic graveyards, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Ophiolites manifest two modes of lithospheric motion expected from subduction tectonics: seafloor spreading and obduction. High-pressure, low-temperature metamorphic blueschists and ultrahigh-pressure terranes indicate subduction and exhumation of oceanic and continental crust, respectively. These lines of evidence indicate that the modern style of subduction tectonics began in Neoproterozoic time. This revolution in the functioning of the solid Earth may have driven wild fluctuations in Earth's climate, described under the "snowball Earth" hypothesis. These conclusions may be controversial, but suggest fruitful avenues for research in geodynamics and paleoclimate. [ABSTRACT FROM AUTHOR]

Subduction initiation is a key process in the operation of plate tectonics. Our understanding of melting processes and magmatic evolution during subduction initiation has largely been developed from studies of the Izu-Bonin-Mariana forearc. Many suprasubduction zone ophiolites are analogous to the Izu-Bonin-Mariana forearc sequence. However, whether there are differences between Izu-Bonin-Mariana subduction initiation sequences and suprasubduction zone ophiolites remains unclear. Here, we report field geological, geochemical, and geochronological data from mafic and felsic rocks in the Xigaze ophiolite (southern Tibet) mantle and crustal section; the same types of published data from both this ophiolite and the Izu-Bonin-Mariana forearc are compiled for comparison. The ophiolite section is intruded by various late-stage dikes, including gabbroic pegmatite, diabase, basalt, and plagiogranite. The compositions of clinopyroxene and amphibole suggest that gabbroic pegmatite formed from hydrous high-SiO2 depleted melts, while whole-rock compositions of basaltic and diabase dikes show negative Nb and Ta anomalies, suggesting flux melting of depleted mantle. Along with the mafic rocks, plagiogranite has a roughly constant content of La and Yb with increasing SiO2 contents, implying hydrous melting of mafic amphibolite. Early-stage pillow basalts exhibit geochemical affinities with Izu-Bonin-Mariana forearc basalts, but they are slightly enriched. Synthesized with the regional geological setting and compared with Izu-Bonin-Mariana forearc magmatism, we propose that the transition from mid-ocean ridge basalt-like lavas to subduction-related mafic and felsic dikes records an Early Cretaceous subduction initiation event on the southern flank of the Lhasa terrane. However, the mantle sources and the magmatic evolution in the Xigaze ophiolite are more variable than those for the Izu-Bonin-Mariana forearc. [ABSTRACT FROM AUTHOR]

The tectonic setting in which Jurassic igneous rocks of the Sanandaj-Sirjan Zone (SaSZ) of Iran formed is controversial. SaSZ igneous rocks are mainly intrusive granodiorite to gabbroic bodies, which intrude Early to Middle Jurassic metamorphic basement; Jurassic volcanic rocks are rare. Here, we report the age and petrology of volcanic rocks from the Panjeh basaltic-andesitic rocks complex in the northern SaSZ, southwest of Ghorveh city. The Panjeh magmatic complex consists of pillowed and massive basalts, andesites and microdioritic dykes and is associated with intrusive gabbros; the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex. Igneous rocks belong to a metaluminous sub-alkaline, medium-K to high-K calc-alkaline mafic suite characterized by moderate Al 2 O 3 (13.7–17.6 wt%) and variable Fe 2 O 3 (6.0–12.6 wt%) and MgO (0.9–11.1 wt%) contents. Zircon U-Pb ages (145–149 Ma) define a Late Jurassic (Tithonian) age for magma crystallization and emplacement. Whole rock compositions are enriched in Th, U and light rare earth elements (LREEs) and are slightly depleted in Nb, Ta and Ti. The initial ratios of 87 Sr/ 86 Sr (0.7039–0.7076) and εNd(t) values (−1.8 to +4.3) lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle. Immobile trace element (Ti, V, Zr, Y, Nb, Yb, Th and Co) behavior suggests that the mantle source was enriched by fluids released from a subducting slab (i.e. deep-crustal recycling) with some contribution from continental crust for andesitic rocks. Based the chemical composition of Panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the Ghorveh area we offer two interpretations for these (and other Jurassic igneous rocks of the SaSZ) as reflecting melts from a) subduction-modified OIB-type source above a Neo-Tethys subduction zone or b) plume or rift tectonics involving upwelling metasomatized mantle (mostly reflecting the ~550 Ma Cadomian crust-forming event). [ABSTRACT FROM AUTHOR]

South of the latitude of Guam, the Mariana Trough exhibits both trenchparallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deepand shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20-40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough. [ABSTRACT FROM AUTHOR]

In this study we use detrital zircons to probe the Early Paleozoic history of NE Iran and evaluate the link between sediment sources and Gondwanan pre-Cadomian, Cadomian and younger events. U–Pb zircon ages and Hf isotopic compositions are reported for detrital zircons from Ordovician and Early Devonian sedimentary rocks from NE Iran. These clastic rocks are dominated by zircons with major age populations at ~ 2.5 Ga, ~ 0.8–0.6 Ga, 0.5 Ga and ~ 0.5–0.4 Ga as well as a minor broad peak at ~ 1.0 Ga. The source of 2.5 Ga detrital zircons is enigmatic; they may have been supplied from the Saharan Metacraton (or West African Craton) to the southwest or Afghanistan–Tarim to the east. The detrital zircons with age populations at 0.8–0.6 Ga probably originated from Cryogenian–Ediacaran juvenile igneous rocks of the Arabian–Nubian Shield; this inference is supported by their juvenile Hf isotopes, although some negative εHf (t) values suggest that other sources (such as the West African Craton) were also involved. The age peak at ca 0.5 Ga correlates with Cadomian magmatism reported from Iranian basement and elsewhere in north Gondwana. The variable εHf (t) values of Cadomian detrital zircons, resembling the εHf (t) values of zircons in magmatic Cadomian rocks from Iran and Taurides (Turkey), suggest an Andean-type margin and the involvement of reworked older crust in the generation of the magmatic rocks. The youngest age population at 0.5–0.4 Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys, which probably started in Late Cambrian–Ordovician time. A combination of U–Pb dating and Hf-isotope data from Iran, Turkey and North Gondwana confirms that Iran and Turkey were parts of Gondwana at least until late Paleozoic time. [ABSTRACT FROM AUTHOR]

Carboniferous igneous rocks constitute volumetrically minor components of Iranian crust but preserve important information about the magmatic and tectonic history of SW Asia. Ghushchi granites and gabbronorites in NW Iran comprise a bimodal magmatic suite that intruded Ediacaran–Cambrian gneiss and are good representatives of carboniferous igneous activity. Precise SIMS U–Pb zircon ages indicate that the gabbronorites and granites were emplaced synchronously at ~ 320 Ma. Ghushchi granites show A-type magmatic affinities, with typical enrichments in alkalis, Ga, Zr, Nb and Y, depletion in Sr and P and fractionated REE patterns showing strong negative Eu anomalies. The gabbronorites are enriched in LREEs, Nb, Ta and other incompatible trace elements, and are similar in geochemistry to OIB-type rocks. Granites and gabbronorites have similar ε Nd (t) (+ 1.3 to + 3.4 and − 0.1 to + 4.4, respectively) and zircon ε Hf (t) (+ 1.7 to + 6.2 and + 0.94 to + 6.5, respectively). The similar variation in bulk rock ε Nd (t) and zircon ε Hf (t) values and radiometric ages for the granites and gabbronorites indicate a genetic relationship between mafic and felsic magmas, either a crystal fractionation or silicate liquid immiscibility process; further work is needed to resolve petrogenetic details. The compositional characteristics of the bimodal Ghushchi complex are most consistent with magmatic activity in an extensional tectonic environment. This extension may have occurred during rifting of Cadomian fragments away from northern Gondwana during early phases of Neotethys opening. [ABSTRACT FROM AUTHOR]