Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are ...either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ∼46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ∼31-30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ∼26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24-23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ∼21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea - Gulf of Suez rifting supports the interpretation that plate-boundary forces likely drove overall separation of Arabia from Africa.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Eurasia has largely grown to its present enormous size through episodic addition of crustal blocks by recurring birth and demise of oceans such as Paleotethys and Neotethys. Excluding the Kopet Dagh ...Mountains in the northeast, crystalline basement rocks of various dimensions are exposed in all continental tectonic zones of Iran. These rocks have traditionally been viewed as continental fragments with Gondwanan affinity and summarily been assigned Precambrian or younger ages, despite the fact that evidence from isotopic dating has largely been lacking. This study presents new ion microprobe and thermal-ionization zircon U-Pb geochronological data from granitoids and orthogneisses from several locations in central Iran and the Sanandaj–Sirjan structural zones to determine crystallization ages and investigate the origin and continental affinity of these various crustal fragments. The resulting U-Pb crystallization ages for the granites and orthogneisses range from late Neoproterozoic to Early Cambrian, matching the mostly juvenile Arabian–Nubian shield and Peri-Gondwanan terranes constructed after the main phase of Pan-African orogenesis. TIMS analyses of zircons with inherited cores from western Iran suggest that the Neoproterozoic crust of Iran might not be entirely juvenile, pointing to the potential presence of inherited older Proterozoic components as is common in the eastern Arabian shield. More importantly, the new zircon U-Pb crystallization ages unequivocally demonstrate that crystalline basement underlying the Sanandaj–Sirjan zone, central Iran, and the Alborz Mountains is composed of continental fragments with Gondwanan affiliation, characterized by wide spread late Neoproterozoic subduction-related magmatism. The exposure of these late Neoproterozoic–Early Cambrian basement rocks in the Iranian regions north of the Zagros is structurally controlled and linked to both large-scale crustal extension and exhumation during Mesozoic and Tertiary time as well as Tertiary collisional tectonics associated with the closure of Neotethys.
Variations in continental volcanic arc emissions have the potential to control atmospheric carbon dioxide (CO₂) levels and climate change on multimillion-year time scales. Here we present a ...compilation of ~120,000 detrital zircon uranium-lead (U-Pb) ages from global sedimentary deposits as a proxy to track the spatial distribution of continental magmatic arc systems from the Cryogenian period to the present. These data demonstrate a direct relationship between global arc activity and major climate shifts: Widespread continental arcs correspond with prominent early Paleozoic and Mesozoic greenhouse climates, whereas reduced continental arc activity corresponds with icehouse climates of the Cryogenian, Late Ordovician, late Paleozoic, and Cenozoic. This persistent coupled behavior provides evidence that continental volcanic outgassing drove long-term shifts in atmospheric CO₂ levels over the past ~720 million years.
The interplay between structural and metamorphic processes operating along the deep plate interface in subduction zones remains elusive as much of the geologic record is recycled into the mantle. In ...some cases, metamorphosed subducted rocks are underplated and exhumed to the surface, providing critical constraints on structural processes and the rheological evolution of subduction interfaces at convergent margins. One such exhumed high‐pressure/low‐temperature subduction complex is the Cenozoic Nevado‐Filábride Complex (NFC) in Southern Spain. This study presents new data from the NFC that elucidate the syn‐metamorphic deformation, stacking, and underplating of continental slivers along the subduction interface. The structurally lowest NFC dominantly comprises lithologically monotonous Paleozoic metamorphic basement rocks recorded by apatite U‐Pb ages and shows no evidence for large‐scale internal duplications suggesting it behaved as a coherent basement succession during subduction. In contrast, structurally higher levels of the NFC are characterized by the stacking of older‐on younger coherent slices and distinctly different metamorphic ages. These relationships document syn‐subduction structural repetitions and tectonic stacking of imbricate thin slivers (∼100s m) during subduction underplating. Structurally higher levels of the NFC exhibit both Eocene and Miocene metamorphic zircon rims and apatite ages, along with microstructures indicative of relatively higher temperature metamorphism. Large‐scale underplating and antiformal stacking of slivers in the subduction channel can provide buoyancy forces to underplate and assist exhumation. We demonstrate that the presubduction stratigraphic architecture is a key control on the style and timing of deformation and metamorphism, facilitating coherent subduction underplating.
Plain Language Summary
Subduction zones are tectonic boundaries where one rigid lithospheric plate sinks underneath another. At the interface between the two plates, rocks experience intense temperature, pressure, and stress conditions during metamorphism, causing deformation. The geologic record of these processes is often not accessible unless these rocks return to the surface. Our study targets one such exposure in Southern Spain, the Nevado‐Filábride Complex, which records deformation and structural mixing from subduction and subsequent transfer to the overriding plate. We perform geochronologic analyses to determine the age of zircon overgrowths that reveal the timing of metamorphism. Additionally, we date apatite minerals and examine deformation relationships at the microscopic scale to approximate the temperature conditions that these rocks experienced. We synthesize these new results with previously established geochronology of zircon grains from the same region that collectively show evidence for the large‐scale structural stacking of coherent rock slivers during deformation. The stacking pattern is observed only in the weak upper stratigraphic successions, while the deeper unit remained internally intact and experienced a lesser degree of metamorphism. Our results argue that the style and distribution of deformation during subduction are strongly influenced by the original stratigraphic architecture and properties prior to subduction.
Key Points
Two main Cenozoic metamorphic events were recorded in the Betic subduction zone in Eocene and Miocene by zircon and apatite
Imbrication and thrust faults coeval with underplating developed along the subduction interface
Subduction of thick coherent metamorphic basement enhances buoyancy forces and contributes to exhumation
Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are ...either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ∼46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ∼31–30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ∼26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24–23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ∼21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea – Gulf of Suez rifting supports the interpretation that plate–boundary forces likely drove overall separation of Arabia from Africa.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This study presents new down-hole zircon (U–Th)/He (ZrHe) ages, laboratory He diffusion measurements, and numerical thermal modeling of ZrHe ages from the Continental Deep Drilling Project (KTB) in ...Germany to investigate He diffusion kinetics in zircon in nature over geologic timescales and to test to applicability of laboratory-derived He diffusion kinetics. Single-grain laser (U–Th)/He ages, calculated using standard alpha-ejection correction procedures assuming homogenous parent-nuclide distribution, display a systematic decrease in ZrHe ages from ∼112 to <1Ma with increasing depth. Down-hole ZrHe results display consistent ages of ∼85±15Ma down to ∼4.7km, in agreement with rapid Cretaceous cooling documented by previous thermochronometric studies from the KTB drill hole. Below ∼5km, ZrHe ages systematically decrease in age and are completely reset (<1Ma) below ∼7.2km. The temperature range (∼130–200°C) in which ZrHe ages systematically decrease defines a well-behaved zircon helium partial retention zone (HePRZ). In addition, this study presents new, cycled step-heating experiments on zircon samples from the KTB drill hole. Results from these new KTB zircon diffusion experiments indicate an activation energy (Ea) of 160kJ/mol and a frequency factory (Do) of 0.03cm2s−1 with an estimated closure temperature (Tc) of 181°C, which are in excellent agreement with published He diffusion kinetics for zircon. To compare the ZrHe results and bulk diffusion kinetics, we modeled diffusion parameters using the well-established thermal history of the KTB drill hole. The computed zircon HePRZ for the KTB drill hole is consistent with observed down-hole ZrHe ages and published and KTB-specific laboratory-derived He diffusion kinetics. Our results from ZrHe analysis from the KTB drill hole suggest that He diffusion of zircon in nature may not be controlled by anisotropic diffusion behavior, but rather behaves in accordance with laboratory-derived diffusion kinetics. The observed ZrHe ages from the KTB drill hole are in excellent agreement with predicted ZrHe age data and underscore the validity and applicability of ZrHe dating as a reliable thermochronometer.
Cenozoic exhumation patterns in the internal and external Zagros reveal a long‐term deformation record associated with geodynamic restructuring of Arabia‐Eurasia collisional zone from continental ...subduction to plate suturing, which can be evaluated from thermochronometric, provenance, and subsidence analyses. Thermal modeling of zircon and apatite (U‐Th)/He ages and apatite fission track data from the Sanandaj‐Sirjan Zone (SSZ) indicates exhumation and inferred uplift along the leading edge of Eurasia starting in the Late Eocene (~35 Ma), coeval with initial foreland flexural subsidence of Arabia. Together with deceleration in Arabia‐Eurasia convergence and diminished subduction‐related magmatism, these events signal the final Neotethys closure and onset of long‐term (15–20 Myr) Arabian continental subduction beneath Eurasia, facilitated by the attenuated architecture of the precollisional Arabian margin. From 35 to 20 Ma, crustal shortening was relatively subdued and restricted to areas along the Arabia‐Eurasia plate boundary and diffuse inversion structures within continental interiors. Acceleration in SSZ cooling/exhumation rates from 19 to 16 Ma was synchronous with rapid basin subsidence and clastic progradation in the Zagros foreland. These events were contemporaneous with 20‐ to 16‐Ma surge in calc‐alkaline magmatism in central Iran and may have been linked to reorganization/deflection of Arabian plate vectors during the main phase of Red Sea rifting at 19–18 Ma. Transition from continental subduction to Arabia‐Eurasia suturing by ~12 Ma forced a transfer of strain from the subduction zone to intraplate deformational structures. This was marked by rapid outward expansion of the Zagros orogen, involving a shift in exhumation from the SSZ to Zagros fold‐thrust belt and Iranian plate interior.
Key Points
Collisional exhumation in the Sanandaj‐Sirjan Zone initiated by the Late Eocene (~35 Ma) based on modeling of multithermochronometric data
Accelerated pulse of Zagros exhumation and magmatism recorded at 20‐16 Ma, linked to changes in external forces during rapid Red Sea rifting
Transition from continental subduction to plate suturing by 12 Ma triggered outward expansion of Zagros deformation into continent interiors
In the Aegean region, the Cycladic Blueschist Unit (CBU) and the Ambelakia unit (Mt. Ossa, Thessaly, Greece) represent early Cenozoic subduction‐related HP‐LT metamorphic complexes exhumed in the ...back‐arc of the Hellenic subduction zone. The Ambelakia unit has been linked to the classic CBU in the Cyclades; however, the tectonic affinity, structural position, nature of the protolith, or timing of metamorphism for these rocks in eastern Thessaly remains largely unresolved. This study provides detailed new insights into both the provenance, protolith age, and tectonic affinity of the Pelagonian, Ambelakia, and Olympos‐Ossa tectonic units by integrating U‐Pb detrital zircon (DZ) and detrital apatite (DA) data in a structural context. DZ results suggest the existence of distinct metasedimentary units, spanning in depositional age from Carboniferous to Late Cretaceous, with provenance signatures that support a correlation with the classic CBU and strengthen the argument for a similar pre‐subduction tectonic relationship. Depth‐profiling analysis reveals metamorphic rims that record HP‐LT metamorphism in the Ossa Ambelakia during the Paleocene‐Eocene. Apatite U‐Pb data from the metasedimentary units preserve a detrital signature similar to the DZ signatures, indicating that apatites were not reset (<450 C) nor recrystallized during subduction metamorphism. These new data suggest that the Ambelakia unit is a lateral equivalent of the CBU subduction complex; however, this unit experienced distinctly lower pressure and temperature conditions during metamorphism than the along‐strike classic CBU.
Plain Language Summary
We investigated a potential correlation between two similar rock units in Greece. Both units were subducted to blueschist‐facies metamorphic conditions; however, such rocks at Mt. Ossa are very poorly understood in comparison to the rocks of the Cycladic Blueschist Unit (CBU) in the Cyclades. We measured radioactive and stable elements within zircon and apatite grains to calculate geologic ages from the rocks at Mt. Ossa, then compared these ages to a compiled data set of ages from the CBU to demonstrate a reasonable correlation between the two units. We found that the rocks at Mt. Ossa are still in stratigraphic order from oldest to youngest, despite undergoing subduction. We also observed that few zircons have younger ages, which we interpret as a record of the subduction event, but no apatite grains record this young subduction event. The Ambelakia rocks were not subducted as deep as some rocks from the Cycladic islands in the southern Aegean.
Key Points
The Ambelakia unit represents a single, upright, stratigraphically coherent structural sliver consisting of Permian‐Late Cretaceous strata
Zircon provenance of the Pelagonian domain, Ambelakia unit, and the Ossa Flysch support a direct correlation between Cycladic Blueschist Unit and Ambelakia
The Ambelakia unit was metamorphosed at blueschist‐facies conditions during the Paleocene‐Eocene
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