The Geodynamic Evolution of Iran

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.

A revised age-model for the Eocene deep-marine siliciclastic systems, Aínsa Basin, Spanish Pyrenees

Using new palaeomagnetic and biostratigraphic data, we revise the age-model for the middle Eocene, deep-marine Aínsa Basin (Spanish Pyrenees), a tectonically active basin formed at a convergent-plate margin. This new age model provides a framework for evaluating the depositional history and sediment accumulation rates. New integrated magneto- and biostratigraphy data identify two normal and two reverse chrons of the geomagnetic polarity timescale (C21r, C21n, C20r, C20n) and place these Upper Hecho Group deposits in the middle Eocene (Lutetian). Nannofossil analysis identifies a biostratigraphic range from Subzone NP14b in the Gerbe System to Subzone NP15b at the top of the Aínsa System using key, age-diagnostic marker species such as Blackites inflatus, Blackites piriformis and Coccolithus gigas. We also present new nannofossil biostratigraphy from the Lower Hecho Group. This new Aínsa Basin chronostratigraphy enables inter-basinal correlations between the proximal fluvio-deltaic Tremp-Graus Basin and the more distal Jaca Basin, thereby providing a better understanding of the basin evolution.Supplementary materials: Field photographs of the sampled sections, magnetostratigraphic results, biostratigraphy results, alternative age model scenario and discussion on previous age model are available at https://doi.org/10.6084/m9.figshare.c.5083076

Forearc strike-slip displacement as an alternative to subduction erosion, with examples from Mexico and California (sinistral Nacimiento fault)

Slip on the Nacimiento fault of the central Coast Ranges of California has been variably interpreted as dextral, sinistral, or reverse. The currently prevailing interpretation is that the Nacimiento fault represents subduction erosion, by which the central to eastern part of the Cretaceous California batholith was thrust over the western part of the batholith and forearc basin, resulting in juxtaposition of the Salinian batholithic block against the Franciscan Complex, concurrently with Laramide flat-slab subduction (75–55 Ma) and underplating of the Pelona-Orocopia-Rand schist. No modern convergent plate margin includes such overthrusting. The closest modern analog to the likely configuration of the Salinian continental margin near the end of the Laramide deformation is southern Mexico, where arc plutons are exposed near the trench. Although commonly considered an example of subduction erosion, this margin is “missing” parts of the plutonic arc and forearc because they have been displaced to the southeast by sinistral slip. By analogy, the Nacimiento forearc was modified as a trench-trench-transform triple junction migrated southeastward along the continental margin during flat-slab subduction. This model makes testable predictions involving northwest-to-southeast younging of deep-marine deposits on batholithic crust underlain by contemporaneous schist. Correct restoration of later Cenozoic primarily dextral slip and Maastrichtian – Early Eocene primarily sinistral slip must result in realignment of north–south-trending belts of the Sierra Nevada – Salinia – Peninsular Ranges batholith, Great Valley forearc, and Franciscan Complex. These modern and ancient examples suggest that several “erosional” subduction zones are more plausibly explained by strike-slip truncation of forearcs.

Differentiating continental and oceanic arc systems and retro-arc basins in the Jiangnan orogenic belt, South China

The Neoproterozoic Jiangnan orogenic belt records the accretion and collision between the Yangtze and Cathaysia blocks in South China. The orogen is divisible into three units: a northeastern domain (also referred to as the Huaiyu or Shuangxiwu domain), a central domain (Jiuling domain) and an undifferentiated southwestern domain. Detrital zircons from the oldest sequences (Shuangqiaoshan, Lengjiaxi, Fanjingshan and Sibao groups) in the central and southwest domains yield similar age spectra with major age populations at c. 875–820 Ma, along with minor Palaeo- to Mesoproterozoic and Archaean ages. The dominance of detrital ages close to the deposition ages of the units, along with juvenile zircon Hf isotopic compositions and arc-like whole-rock compositional data, indicate the sedimentary units accumulated adjacent to a convergent plate margin magmatic arc. The presence of Mesoproterozoic and older zircons, both as detritus in the units and as xenocrysts within igneous rocks displaying a subduction-related signature, along with the compositional data, place the magmatic arc along a continental margin. In the northeastern domain, the oldest coeval sequence (Shuangxiwu and Qigong groups) and arc igneous suites are dated at c. 970–850 Ma, and lack older detritus and xenocrysts, indicating they represent an accreted oceanic arc system.

Provenance analysis of the Dezadeash Formation (Jurassic–Cretaceous), Yukon, Canada: implications regarding a linkage between the Wrangellia composite terrane and the western margin of Laurasia

The Mesozoic convergence of the allochthonous Wrangellia composite terrane (WCT) with the western margin of Laurasia coincided with the construction of the Chitina magmatic arc on the WCT, and the dispersal of volcanic flows and sediment gravity flows into an adjacent flysch basin. The basin, preserved as the Gravina–Nutzotin belt, includes the Dezadeash Formation in southwest Yukon, the Nutzotin Mountains sequence in southern Alaska, and the Gravina belt in southeastern Alaska. The Dezadeash Formation is a submarine fan system comprising stacked channel-lobe transition and lobe deposits interposed with overbank deposits. Conglomerate pebble-counts, sandstone point-counts, detrital zircon ages, and major element, trace element, rare earth element, and Sm–Nd isotopic geochemistry of sandstone, mudstone, and hemipelagite beds suggests that the deposits consist mainly of first-cycle volcanogenic detritus shed from the undissected Chitina arc, in addition to material eroded from the WCT. The arc was constructed of undifferentiated magma sourced from the depleted mantel, as well as older crustal material attributed to the WCT proxying for continental crust. The compositional provenance results, together with published paleocurrent data for the Dezadeash Formation and compositional and directional provenance indicators from the Nutzotin Mountains sequence and Gravina belt, does not require a sediment source from Laurasia. The provenance record is compatible with deposition of the Gravina–Nutzotin belt in a convergent plate margin setting.

The Tasmanides: Phanerozoic Tectonic Evolution of Eastern Australia

The Tasmanides occupy the eastern third of Australia and provide an extensive record of the evolution of the eastern Gondwanan convergent plate boundary from the Cambrian to the Triassic. This article presents a summary of the primary building blocks (igneous provinces and sedimentary basins) within the Tasmanides, followed by a discussion of the timing and extent of deformation events. Relatively short episodes of contractional deformation alternated with longer periods of crustal extension accompanied by voluminous magmatism. This behavior was likely controlled by plate boundary migration (trench retreat and advance) that was also responsible for bending and segmentation of the convergent plate margin. As a result, the Tasmanides were subjected to at least three major phases of oroclinal bending, in the Silurian, Devonian, and Permian. The most significant segmentation likely occurred at ∼420–400 Ma along a lithospheric-scale boundary that separated the northern and southern parts of the Tasmanides.