Late Oligocene Early Miocene Research Articles

Petrological relationships between plutonic and volcanic rocks in a continental volcanic arc system: Evidence from plutonic xenoliths in the Sierra de Pachuca, Mexico

Relatively abundant plutonic xenoliths found in some lava domes, lava flows, and scoria cone deposits were employed to study the features and processes between the magmatic plumbing systems and volcanic rocks in the Sierra de Pachuca, central-eastern Mexico. We present petrographic, geochemical, isotopic data and five U-Pb ages for some plutonic xenoliths and the Sierra de Pachuca host rocks to provide evidence of magmatic processes existing under this volcanic range. The phaneritic plutonic xenoliths were grouped into six lithological types based on petrography, chemical data, and isotopic ages. Four types (A to D) are hosted by the El Ventoso lava dome. Their compositions span from calc-alkaline gabbro to granite, with U-Pb zircon ages ranging from 23.1 ± 1.4 to 21.6 ± 0.6 Ma. These ages are contemporaneous with the emplacement of rhyolitic ignimbrites found at the base of the Sierra de Pachuca (22.8 ± 0.5 Ma U-Pb zircon age). The trace element patterns for these Late Oligocene-Early Miocene plutonic xenoliths display enrichment of LILE relative to HFSE and enrichment of LREE respecting to HREE typical of subduction-related rocks. Negative P, Ti, and Eu anomalies in these patterns also suggest fractional crystallization processes in magmas that produced the A to D types. Two more types of plutonic xenoliths (E and F) display gabbro and syenite compositions, respectively, and are hosted by the San Miguel-Santa María Regla columnar jointed basalts (gabbro samples) and by the El Prieto scoria cone (syenite samples). Based on one U-Pb zircon age (2.4 ± 0.2 Ma) and geological correlations, these plutonic xenoliths and their host rocks were emplaced in the Quaternary. They exhibit alkaline compositions, and their trace element patterns indicate an enriched mantle source with positive Ba and Pb anomalies, not typical of subduction-related magmas. The isotopic values (87Sr/86Sr = 0.70390 to 0.70502 and εNd = −2.99 to +0.76) for the Late Oligocene-Early Miocene A to D types and host rocks suggest the interaction of magmas with continental crust components. In contrast, the Quaternary xenoliths of E and F types and their host rocks display isotopic data (87Sr/86Sr = 0.70396 to 0.70602 and εNd = +0.13 to +3.66) that evidence of less interaction with continental crust components. In all studied xenoliths, the Pb isotopic ratios (206Pb/204Pb = 18.60–18.87; 207Pb/204Pb = 15.57–15.63 and 208Pb/204Pb = 38.39–38.68) lie on a mixing line between a mantle-like member and a radiogenic component such as the Paleozoic Acatlán complex that probably underlie the Sierra de Pachuca area. Based on the results obtained, it is proposed that the Late Oligocene-Early Miocene A to D type xenoliths represent the plumbing systems from which the volcanic rocks of the Sierra de Pachuca were extruded. On the other hand, the E and F types also represent the roots or magmatic reservoirs from which the Quaternary lava flows and scoria cones, located in the rear-arc part of the active front of the Trans-Mexican Volcanic Belt, were formed. Additionally, these two types of xenoliths and their Quaternary host rocks have hybrid geochemical characteristics of alkaline magmas influenced by crustal components produced during a slab rollback stage.

Episodic cooling of the Hannan-Micangshan Dome at the northern margin of the Yangtze Block, Central China: Response to progressive convergence in the eastern Tethys since the Mesozoic

The Hannan-Micangshan Dome (HMD) is a Meso-Cenozoic intra-continental compressional tectonic belt that provides an excellent natural laboratory to study intra-continental deformation processes and geodynamic mechanisms in Central China. Although many low-temperature thermochronological studies have been conducted in the HMD, the timing of initial cooling/exhumation, rapid exhumation phases and their relationship to progressive convergence in the eastern Tethys are still poorly constrained. In this study, we present new multiple thermochronological analyses of apatite (AFT) and zircon fission track (ZFT), hornblende and muscovite Ar-Ar, and zircon U-Pb dating of granitoid and sedimentary rocks from the HMD. Granitoids from the HMD yield Proterozoic zircon U-Pb ages and slightly younger hornblende and muscovite Ar-Ar ages, indicating rapid cooling after emplacement. ZFT dating of granitoids yielded apparent ages of ∼208–140 Ma. AFT dating of Triassic-Cretaceous sandstones and Proterozoic granitoids yielded apparent ages between ∼117 and ∼58 Ma. Thermal modeling results suggest that the HMD has experienced the onset of cooling and exhumation since the Early Jurassic followed by three phases of rapid cooling/exhumation during the Late Jurassic-Early Cretaceous (∼160–100 Ma), Middle Eocene (∼40 Ma) and since the Late Oligocene-Early Miocene (∼23–15 Ma). The initial exhumation in the HMD by the Early Jurassic may be related to the closure of the Paleo-Tethys Ocean between the Kunlun and Qiangtang terranes. The Late Jurassic-Early Cretaceous rapid exhumation in the HMD can be related to the basement-involved thrusting triggered by the far-field effects of the ongoing collision between the Qiangtang and Lhasa terranes following the closure of the Meso-Tethys since the Late Jurassic. Two phases of rapid Cenozoic exhumation by the Middle Eocene and since the Late Oligocene-Early Miocene in the HMD were attributed to the far-field stress effects of the collision between the India and Asian plates and the lateral growth of the Tibetan Plateau. Therefore, we proposed that the episodic cooling since the Early Jurassic of the HMD at the northern margin of the Yangtze Block can be interpreted as being related to the progressive convergence in the eastern Tethys since the Mesozoic.

Strain Partitioning in a Flattening Shear Zone: Re-Evaluation of a Cycladic Style Detachment.

The Almyropotamos tectonic window on southern Evia island in the NW Aegean Sea divides two high pressure-low temperature metamorphic units, representing distinct Hellenic thrust sheets. Ductile thinning along the major low-angle Evia Shear Zone has closely juxtaposed the lower (Basal Unit) marble-flysch sequence structurally below Styra marbles (Cycladic Blueschist Unit). The partially attenuated flysch comprises a matrix dominated by pelitic schist, with dispersed cm- to hm-scale blocks of marble, carbonate schist, quartzite, and metabasite. Structural investigation of the different lithotypes in the flysch reveals tectonic fabrics related to general flattening strain are developed most strongly in the pelitic matrix, whereas the compositionally diverse blocks exhibit differential preservation of older structures. Quartz c-axis distributions from quartz veins in the schists reflect an early, moderate temperature plane strain deformation. Colder deformation is evident in some pelitic schists, capturing Z-centered girdles consistent with the oblate finite strain ellipsoid inferred from macroscopic structures. New in situ 40Ar/39Ar and 87Rb/87Sr geochronology delineate the timing of the two deformation events. Geochronological data reaffirm the first-order observations of strain partitioning behavior at the scale of the shear zone, and confirm that the structure records two resolvable tectonometamorphic events: an early Oligocene HP-LT event, and a late Oligocene-early Miocene greenschist facies overprint coinciding with ductile thinning. The diffuse and discontinuous style of deformation recorded within the shear zone is unusual for major structures facilitating exhumation in the Aegean Sea, and may represent an analogue to mélange-hosted shear zones that accommodate progressive strain during subduction.

Elektroformica azquil gen. et sp. nov., a new formicine ant from Oligo-Miocene Mexican amber

A novel ant genus and species, Elektroformica azquil gen. et sp. nov. is described from Late Oligocene-Early Miocene Mexican amber. It is placed within the subfamily Formicinae based on the presence of an acidopore. It can be distinguished by its diminutive size, elongated scapes that extend beyond the head’s vertex by approximately double the length of the head, eyes located about 2/3 the distance between the mandible’s base and the head’s vertex. Additional diagnostic features include a distinctive keel-shaped clypeus, a prominently marked promesonotal groove, a discontinuous mesonotum with dorsal and declivitous edges, a curved propodeum lacking differentiated anterior and posterior margins, and a markedly reduced node on a thick petiole. Elektroformica azquil gen. et sp. nov. is in syninclusion with deciduous petals and filaments of a flower identified as cf. Hymenaea mexicana, along with elements closely associated with leaf litter and the soil environment. As an extinct taxon endemic to Oligo-Miocene deposits, Elektroformica azquil gen. et sp. nov. enriches the diversity within the subfamily Formicinae and offers valuable insights into the evolutionary history of formicine ants.

The last piece of the puzzle, the DP4 of Cephalomys arcidens (Rodentia, Caviomorpha) from the late Oligocene of Patagonia (Argentina)

ABSTRACT Deciduous dentition of extinct mammals has traditionally received little attention based on its poor representation in the fossil record. However, several aspects related to this tooth locus could have important evolutionary significance (time of replacement, retention throughout life, and morphology). In this work, we described a new specimen from late Oligocene levels of Cabeza Blanca (Chubut Province, Argentina). This fossil corresponds to a maxillary fragment with a deciduous premolar (DP4) assigned herein to Cephalomys arcidens based on morphological characteristics, size, and comparisons with other late Oligocene-Early Miocene caviomorphs. It can be identified as a DP4 due to its mesiodistally longer than wide diameter, shallow flexi, complex tooth morphology, and its position in the maxillary fragment. This is the first description of the DP4 of a Cephalomyidae and the last unknown dental locus of Cephalomys arcidens. Now, C. arcidens is one of the better-known late Oligocene caviomorphs since it is recognised through the complete dentition, crania, and mandibles. In addition, this new information would be used to evaluate the importance of deciduous dentition in resolving phylogenetic relationships in the enigmatic cephalomyids in particular and in Caviomorpha in general.

Biostratigraphy of the Late Oligocene-Early Miocene Anah and Bajwan Formations at Selected Sections of Northern Iraq

The Late Oligocene-Early Miocene including Anah and Bajwan formations have been studied in selected areas of Sulamaniyah (Aj-Dagh and Basara sections) and Kirkuk governorate within Bai Hassan Oilfield (BH-3 and BH-4), which is located within the Western Zagros Fold Thrust Belt, Northern, and Northeastern Iraq. The palaeontological investigations mainly depended on benthonic foraminifera of the studied sections of Aj-Dagh and Basara, as well as the studied wells of Bai Hassan OilField (BH-3, and BH-4) of Anah and Bajwan formations, twenty-four species belonging to fourteen genera recognized of benthonic foraminifera, with five index fossils determining the age of the formations as Late Oligocene-Early Miocene, which have been recognized through this study, especially benthonic foraminiferal, indicating four zones as Praerhapydionina delicata range zone, Austrotrillina howchini-Peneroplis evolutus zone, Concurrent range, and Miogypsinoides complanate-Lepidocyclina (Nephrolepidina) sp. Concurrent zone.

Revised timing of rapid exhumation in the West Qinling: Implications for geodynamics of Oligocene-Miocene Tibetan plateau outward expansion

Two contrasting age models for initial mountain building in the northeastern (NE) Tibetan Plateau (Paleocene-early Eocene versus late Oligocene-early Miocene) have led to the debate on how the deformed continental lithosphere absorbs plate convergence in general. The initial compressional deformation in the West Qinling (WQL) of the NE Tibetan Plateau figures prominently in this ongoing debate. Here, apatite (U-Th)/He (AHe) thermochronology combined with geomorphological analysis are used to refine the onset of compressional deformation in the WQL. New AHe ages from two vertical transects and an updated reconstruction of an obliquely-tilted erosion surface document the accelerated exhumation in the northern WQL at 23-22 Ma, interpreted as the onset of north-vergent thrusting. The AHe results, together with sedimentary records in the intermontane and foreland basins, suggest that the entire WQL began experiencing compressional deformation in the late Oligocene-early Miocene. When integrated with previous studies, our findings show that the northern plateau boundary has not remained stationary since the collision, but has instead experienced ∼750 km of outward expansion during the late Oligocene to middle Miocene. This phase of rapid plateau growth is coeval with the ∼30–50 % reduction of the India-Eurasia convergence rate, which suggests that the increased gravitational potential energy of orogenic belts played a key role in plate motion changes.

Configuration of the early-mid Cenozoic extensional arc volcanism of the North Patagonian and the Southern Central Andes (33–44°S)

The late Eocene to earliest Miocene volcanic arc of the Southern Central Andes (33–40°S) and North Patagonian Andes (40–44°S) exhibits distinctive features that shed light on the dynamics of subduction-related volcanic activity in extensional tectonic settings. This period is particularly significant in the Andes, as it records the opening of forearc, intra-arc, and retroarc extensional basins, accompanied by high-volume volcanism, preceding the middle-late Miocene final major uplift phase of the Andes. This extensional event occurred in two phases, associated with changes in the geometry and dynamics of the subduction system: from oblique convergence at low rates during the late Eocene-early Oligocene to an orthogonal convergence at high rates during the late Oligocene-early Miocene. This study presents a compilation of field, petrographic, geochemical, and isotopic data from volcanic sequences in the North Patagonian Andes and the Southern Central Andes, aiming to analyze petrogenetic processes at different stages of arc evolution.During the late Eocene to early Oligocene, subduction-related volcanism occurred in a wide area from 100 to 300 km from the trench. In this period, magmatic arc rocks exhibit intermediate composition and a relatively homogeneous geochemical signature, transitional between tholeiitic and calc-alkaline series, with a marked subduction-related signature. These magmas originated from depleted mantle sources with high slab-derived fluid and sediment contributions and limited crustal interactions. In contrast, the late Oligocene to earliest Miocene volcanic activity, at the peak of the extensional regime, displays significant geochemical differences along the Andes. In the Southern Central Andes (33–40°S), volcanism displays compositions similar to those of the previous stage with varying influence from the subducting slab. In some sectors, magmas register the climax of the extensional conditions, whereas the youngest volcanic rocks towards the north (33–34°S) present evidence of the onset of contractional tectonics, demonstrated by the increased interaction of the magmas with a progressively thickened crust. In the North Patagonian Andes (40–44°S), subaerial and subaqueous arc-related lava flows are interbedded with marine deposits, indicating a rapid subsidence regime. These rocks exhibit geochemical features transitional from arc to E-MORB and even OIB compositions, which suggest a genesis primarily dominated by decompression melting of a heterogeneous mantle source. Above all, the contrasting nature of late Eocene-early Miocene arc products in the Southern Central and Patagonian Andes highlights that no single process can explain the changes in composition, distribution, and evolution of arc magmatism; rather, it is a combination of factors, including variations in subduction zone configuration and mantle dynamics and composition.

Magmatic (Cr-Ni-PGE) and secondary/hydrothermal (emerald-peridot-rodingite-nephrite jade) mineralization associated with mafic-ultramafic rock complexes of Pakistan

Mafic magmatism has repeatedly taken place in Pakistan throughout its geologic history from the earliest Proterozoic to Recent. Much of it, however, does not have associated metallic mineralization. This paper concerns mineralization associated with mafic-ultramafic plutonic rock occurrences in the western- and northern suture zones of Pakistan, which demarcate the collision boundary between the northwestern Indian plate and Eurasian blocks. We discuss the petrology of the ophiolites and basaltic magma-related chromite deposits, showings of PGE and Ni minerals, and secondary mineral deposits (emerald and peridot gems, rodingite, and nephrite jade) associated with mafic-ultramafic complexes of the two suture zones. Chromite with high Cr#s (68 to 85) occurs in fair quantity and is mostly considered to be genetically related to basaltic magma of subduction-related environments. PGE and nickeliferous minerals are insignificant in quantity; PGE's are formed by a higher degree of partial melting and melt-rock reaction, whereas Ni-sulphides are the product of serpentinization of the ultramafic rocks. Emerald occurs in several localities in the ISZ, whereas the peridot has so far been reported from only one locality. Emerald is associated with Mg-carbonate ± quartz ± fuchsite ± talc ± serpentine, and peridot (Fo 89-92 ) is associated with chrysotile ± magnesite ± talc ± magnetite ± ludwigite. Both are formed via hydrothermal alteration of ultramafic rocks through the introduction of hydrous fluids incorporating CO 2 , Be, B, K, Al, and possibly Na. In view of the undeformed nature of the emerald and peridot and the 23 Ma age of emerald formation, the mineralization appears to post-date deformation and may be of Late Oligocene-Early Miocene age. The metasomatising fluids were either derived from the subducting Indian plate crust underneath the Kohistan arc or were related to 23-26 Ma silicic magmatism in this area of the NW Indian plate. The nephrite jade in the ophiolite mélange is hosted in serpentinites and has disseminated grains of chrome spinel with elevated Fe/(Mg + Fe) ratios (0.10–0.18), implying low-grade metamorphism of ultramafic rocks (protolith of serpentinite) with loss of Al and introduction of SiO 2 during accompanying metasomatism. The rodingite in Dargai ophiolite could have formed by the action of seawater channeled through fractures and cracks in rocks. However, their mineral assemblages indicate that they formed as a result of the influx of Ca, Al, and Na-bearing hydrothermal fluids related to the intrusion of gabbros/sheeted dykes in the serpentinized ultramafic rocks, resulting in Ca-metasomatism.

New insights into the structural and stratigraphic evolution of the Malay Basin using 3D seismic data: Implications for regional carbon capture and storage potential

AbstractThe Malay Basin is a mature hydrocarbon province currently being re‐assessed for CO2 storage. Selecting an appropriate storage site requires a comprehensive understanding of the structural and stratigraphic history of the basin. However, previous studies have been limited to observations from either regional 2D seismic lines or individual 3D seismic volumes. In this study, we access and utilise a basin‐wide (ca. 36,000 km2) 3D seismic and well database to describe the structural and stratigraphic features of the basin, particularly those within the uppermost ca. 4 km (Oligocene to Recent) and gain new insights into the basin's evolution. E–W transtensional rift basins first developed due to sinistral shear across an NW‐SE strike‐slip zone. The NW‐SE basin morphology seen today was generated during the late Oligocene–early Miocene, during which time dextral motion across marginal hinge zones created en‐echelon antithetic, extensional faults and pull‐apart basins, especially well preserved along the western margin of the basin. Collisional forces to the southeast during the early to middle Miocene resulted in the shallowing of the basin, intermittent connection to the South China Sea and a cyclic depositional pattern. Around 8 Ma (late Miocene), a significant uplift of the basin resulted in a major unconformity with up to 4.2 km of erosion and exhumation in the southeast. In the centre and northwest of the basin, the inversion of deeper E–W rifts resulted in the folding of Miocene sequences and the formation of large anticlines parallel to the rift‐bounding faults. The Pliocene to Pleistocene history is more tectonically quiescent, but some extensional faulting continued to affect the northwest part of the basin. Larger glacio‐eustatic sea‐level fluctuations during this time resulted in major changes in sedimentation and erosion on the Sunda Shelf, including the formation of a middle‐Pliocene unconformity. These structural events have created a variety of hydrocarbon traps across the basin of different ages, including transpressional anticlines, rollover anticlines and tilted fault blocks. Each of these has discrete and distinct trap elements with important implications for their CO2 storage potential.

Cechy mineralogiczno-petrograficzne i właściwości fizyko-mechaniczne piaskowca Macigno w rejonie Vellano (Pistoia, Toskania, Włochy)

The sandstones belonging to the terrigenous deposits of the Macigno Formation (Late Oligocene-Early Miocene) were widely used as building stones in Tuscany (Italy) for the wide distribution of their outcrops and the good qualities of the extracted stone. This research reports the petrographic and mineralogical data, and the physical and mechanical test values collected to evaluate the main technical properties of the Macigno sandstones from the Vellano area, with the purpose of comparing the quality of the stones extracted in this area with those from other quarries in north-western Tuscany. The results, obtained analysing 21 samples from the Vellano quarry and its surroundings, show that sandstones cropping out here are characterized by medium to medium-fine sand-sized grains made up of quartz, K-feldspar, plagioclases, phyllosilicates, lithic fragments, and accessory. Clayey materials and calcite are present as matrix and cement, respectively. The clay fraction is made up of mica-like minerals, chlorite, chlorite/smectite interlayers and, in some samples, corrensite and kaolinite. From the physical and mechanical point of view the analysed samples show low porosity and high flexural and compressive strengths. Compared to the other Macigno sandstone samples from north-western Tuscany, the best samples of Vellano stone show rather comparable mechanical resistance than those quarried at Matraia in Lucca province, which today is another active quarry of sure interest for the good quality of the extracted material.

Late Oligocene to Pleistocene thermo-tectonic evolution of the Karakoram Fault Zone: New insights from basement and detrital apatite fission track thermochronology

The Karakoram Fault Zone displays distinctive topographic and tectonic characteristics, with intricate river drainage systems. Although there are extensive thermochronological datasets for the western Tibetan Plateau, limited investigations have been carried out to uncover the low-temperature thermal history of the Karakoram Fault Zone, which connects the Lhasa terrane with the Pamir. In this contribution, apatite fission track (AFT) thermochronology was conducted on 13 basement and 4 conglomerate samples collected from the western Gangdese batholith and the Ayi Mountain Range, on either side of the Karakoram fault. Inverse thermal history modeling results reveal that the western Gangdese batholith experienced two stages of rapid basement cooling at 27–23 Ma (late Oligocene-early Miocene) and 16–4 Ma (middle Miocene-Pliocene). The adjacent Ayi Mountain Range experienced cooling in the late Miocene (∼8 Ma) and during the Pliocene to Pleistocene (4–1 Ma). We suggest that tectonic forces associated with rollback and lateral break-off of the Indian continental lithosphere, and the dextral strike-slip activity of the Karakoram fault were responsible for the first cooling phase. The second accelerated cooling phase was due to the deep fragmentation of the Indian slab, the activity of the Karakoram normal fault, and associated ∼E-W extension. Additionally, since the late Oligocene, the Indus River incision has likely also contributed to the exhumation of the western Tibetan Plateau. Subsequently, the western Gangdese batholith reached high elevations, resulting in the reversal of the Yarlung River. The dextral-normal motion of the Karakoram fault, which is still active today, was the main cause of the Pliocene to Pleistocene accelerated cooling detected in the Ayi Mountain Range.

Late Cenozoic Cooling History of the Xigaze Fore-Arc Basin along the Yarlung–Zangbo Suture Zone (Southern Tibet): New Insights from Low-Temperature Thermochronology

Abstract The Tibetan Plateau is currently the widest and highest elevation orogenic plateau on Earth. It formed as a response to the Cenozoic and is still ongoing collision between the Indian and Eurasian plates. The Xigaze fore-arc basin distributed along the Indus–Yarlung suture zone in southern Tibet preserves important information related to the late Cenozoic tectonic and topographic evolution of the plateau. In this study, apatite fission track (AFT) thermochronology was carried out on twelve sandstone samples from the middle segment of the Xigaze basin and additionally on four sedimentary rocks from the neighboring Dazhuka (Kailas) and Liuqu Formations. Inverse thermal history modeling results reveal that the fore-arc basin rocks experienced episodic late Oligocene to Miocene enhanced cooling. Taking into account regional geological data, it is suggested that the late Oligocene-early Miocene (~27–18 Ma) cooling recognized in the northern part of the basin was promoted by fault activity along the Great Counter thrust, while mid-to-late Miocene-accelerated exhumation was facilitated by strong incision of the Yarlung and Buqu rivers, which probably resulted from enhanced East Asian summer monsoon precipitation. Sandstone and conglomerate samples from the Dazhuka and Liuqu Formations yielded comparable Miocene AFT apparent ages to those of the Xigaze basin sediments, indicative of (mid-to-late Miocene) exhumation soon after their early Miocene burial (> ~3–4 km). Additionally, our new and published low-temperature thermochronological data indicate that enhanced basement cooling during the Miocene prevailed in vast areas of central southern Tibet when regional exhumation was triggered by both tectonic and climatic contributing factors. This recent and widespread regional exhumation also led to the formation of the high-relief topography of the external drainage area in southern Tibet, including the Xigaze fore-arc basin.

Tectonic and climatic significance of Oligocene-Miocene eolian sandstones in the Andean foreland basin of Argentina

Late Oligocene-early Miocene aridification in the retroarc foreland basin adjacent to the southern central Andes was recorded by widespread eolian conditions that coincided with accelerated subsidence driven by the main phase of Andean tectonic loading. An extensive eolian dune system, which is rare in most foreland basins, indicates specific conditions in terms of climate (aridity), atmospheric circulation, orography, sediment sources, dispersal patterns, and accommodation space. Detrital zircon U-Pb geochronological results for eolian sandstones spanning 20 foreland localities in northwestern to west-central Argentina, from 22°S to 36°S, reveal relatively localized Andean sources rather than regional cratonic provenance. Clastic detritus was largely derived from Phanerozoic igneous and sedimentary rocks of the Andean magmatic arc and retroarc fold-thrust belt during initial shortening-induced uplift of hinterland regions such as the Western Cordillera, Principal Cordillera, and Frontal Cordillera. This provenance record provides the earliest evidence for major westerly winds, transporting Andean detritus broadly eastward to the foreland basin (with common axial northward deflections) during a climate shift marked by the latest Oligocene-earliest Miocene inception of arid conditions. In addition to the climatic implications, independent stratigraphic data provide the basis for a compilation of sediment accumulation histories showing that eolian deposition was synchronous with accelerated subsidence attributable to enhanced Andean shortening. We propose that accelerated Andean uplift in the latest Oligocene-early Miocene generated rain-shadow conditions contemporaneous with increased sediment accommodation in the foreland basin, underscoring the important influence of tectonic processes on regional climate.

No significant provenance changes in the Hoh Xil Basin, central-northern Tibetan Plateau, from the Late Cretaceous to the Early Miocene

The construction and uplift of the Tibetan Plateau has played a critical role in Cenozoic global climatic cooling due to extensive chemical weathering and establishment of the Asian monsoons. However, the models, mechanisms, and timing of Tibet's uplift remain rigorously debated. Important outstanding questions include what were surface elevations of the Tibetan Plateau prior to the Paleogene India-Asia collision and what mechanisms were responsible for the Cenozoic uplift of the central Tibetan Plateau. In this study, we integrated new detrital zircon data from the Tuotuohe Basin, a sub-basin of the Hoh Xil Basin in the central Tibetan Plateau, with paleocurrent directions, petrological textures, sedimentary facies, and eastern and western Hoh Xil Basin isotopes to investigate the source-to-sink relationship between the Tanggula Range and the Hoh Xil Basin. Any notable provenance shifts would have implications for the topographic evolution of the central and northern Tibet and proposed plateau uplift mechanisms. Our detrital zircon data from the Tuotuohe sub-basin revealed no provenance changes between ca. 37 and ca. 20 Ma. Detrital zircon data from the Tuotuohe section and the eastern and western Hoh Xil Basins, paleocurrent directions, petrologic analysis, strata characteristics, and isotopic evidence suggest that there was a single expansive Hoh Xil Basin from the Late Cretaceous to the Early Miocene. Furthermore, the Tanggula Range has been the primary source region for the Hoh Xil Basin since the Late Cretaceous. However, the Fenghuo Range may have provided a significant secondary source of detritus starting in the Late Oligocene-Early Miocene. Our results imply that there was established topographic relief in central-northern Tibet between mountainous highlands in the south and the Hoh Xil Basin in the north since the Late Cretaceous. The Tanggula Range-Hoh Xil Basin experienced surface deformation and uplift from the Late Cretaceous to ca. 40 Ma, induced by the Lhasa-Qiangtang and India-Asia collisions, after which the deformation intensity ceased by ca. 27–24 Ma. Therefore, the additional ∼1.5 km surface uplift of the Hoh Xil Basin since the Middle Miocene cannot be the result of crustal shortening. Instead, surface uplift may have been caused by the convective removal of the mantle lithosphere or magmatic inflation, which explains the sub-horizontal attitudes of the Middle Miocene Wudaoliang Formation in the region.

Debris-slides, olistoliths and turbidites: keys to understanding the tectonostratigraphic affinities of a terrane block in a young orogenic belt, Timor-Leste

Timor lies at the centre of a rapidly evolving orogenic belt. The Matebian Range, one of its largest mountains, was previously mapped as Lower Miocene neritic Cablac Limestone and regarded as allochthonous (namely, Banda Terrane). New analyses have demonstrated a disjunct stratigraphy, extending from the Lower Jurassic to lowest Miocene and encompassing neritic to abyssal strata. Positioning each stratigraphic unit (six new) on a time v. bathymetry plot and identifying clast types in conglomerates and turbidites allows reconstruction of changes in provenance to depocentres through time. Terrane progression from the northern margin of Gondwana to the southern Sundaland margin (Asia) and then back to the NW margin of the Australian continent is indicated, involving progressive amalgamation and rifting and substantial episodes of uplift, particularly during the Middle Eocene and the Late Oligocene. The youngest unit (Late Oligocene–earliest Miocene) was deposited adjacent to a rapidly rising hinterland (southern Sundaland) very different from that on the coeval Australian margin. The study provides a model for the tectonostratigraphic reconstruction of limestone-dominated mountains in young orogenic belts and demonstrates the importance of using sedimentary clasts in mass-flow deposits to interpret ages and depositional environments of reworked material and to stitch together tectonostratigraphic pathways through time. Supplementary material: A description of stratigraphic units, including supplementary figures, a table and Appendices A and B, is available at https://doi.org/10.6084/m9.figshare.c.6852387

Hydrothermal lead-zinc-copper mineralizations in sedimentary rocks of Northern Apennines (Italy)

Lead and zinc hydrothermal mineralizations associated with late Oligocene-early Miocene sedimentary rocks (sandstones and marls) were found and investigated for the first time in the Northern Apennines (Northern Italy, province of Reggio Emilia). The mineralizations are located in a structurally complex area involved in the tectonic events forming the Northern Apennines chain. They are stratabound mineralizations mainly in the form of sub-vertical veins, suggesting a strong structural control. The ore minerals are sphalerite and galena (rare chalcopyrite), the gangue minerals are carbonates, barite and quartz, in different proportions in the three investigated sites (Busana, Casalino, Casenove). On the basis of microthermometric data on fluid inclusions in ore and gangue minerals, the ore-forming fluids belonged to an aqueous system containing Na-, Mg-, Ca-chlorides and/or carbonate and had a relatively high salinity, mainly in the range 11–17 wt% NaCl equivalent. The trapping temperatures were relatively high (142–231 °C range), suggesting basins characterized by anomalous geothermal gradients. The sphalerite composition reveals different FeS contents and different Zn/Cd ratios in the three sites. The δ34S values of the ore minerals vary from −7.98/−10.98 ‰ in Busana to 4.38/6.85 ‰ in the other two sites. The barite in Casalino has values of δ34S = 25.56 ‰ and δ18O = 13.81 ‰. The sphalerite composition and the isotopic data suggest complex interactions of the basin fluids involving crossed sediments, evaporite rocks, organic matter, as well as mixing between fluids and Jurassic marine waters. On the basis of their geological, petrographic-mineralogical and geochemical features, these mineralizations can be considered Alpine- or Irish-type MVT mineralizations.

The Age and the Accumulation Conditions of the Upper Paleogene–Lower Neogene Kurshskaya Formation Stratotype (Kaliningrad Oblast) by Palynological Data

The results of the palinologycal study of the Kurshskaya Formation stratotype (Primorsky quarry, Kaliningrad Oblast) are presented. In the lower part of the Kurshskaya Formation (Member of “chocolate” clays), a complex of dinoflagellate cysts with Areosphaeridium diktyoplokum, Glaphyrocysta semitecta, and Cordosphaeridium funiculatum of latest Eocene age was found. The formation is characterized by four spore-pollen assemblages: (1) Pinuspollenites–Inaperturopollenites–Sciadopityspollenites assemblage of latest Eocene age in the “chocolate” clays and in the lower part of brown sands; (2) Sequoiapollenites–Betulaepollenites betuloides assemblage of early Oligocene age in the lower part of the brown sands member; (3) Boehlensipollis hohli–Carpinipites carpinoides assemblage of early Oligocene age in the middle part of brown sands member; (4) Alnipollenites–Corylopollis assemblage of late Oligocene–early Miocene age in the upper part of the brown sands of the Kurshskaya Formation. The general age of the Kurshskaya Formation is terminal Eocene–early Miocene. In the lower parts of the Zamland Formation, which overlies the Kurshskaya Formation, the Pinuspollenites–Tricolporopollenites pseudocingulum–T. euphorii assemblage presumably of middle Miocene age was found. Based on the obtained palynological data, the conditions of sedimentation at the end of the Eocene–Oligocene–early Miocene in the South Baltic region are reconstructed. The regression of the sea basin began as early as the end of the Priabonian, the climate was still quite warm and humid, close to subtropical. Mesophytic mixed coniferous-broad-leaved forests grew along the shores of the strait, low-lying areas of land were occupied by marsh vegetation. Cooling at the Eocene/Oligocene boundary resulted in the appearance of hemlock in plant communities and an increase in the proportion of catkins (alder, birch, hornbeam). Presumably, in the late Oligocene–early Miocene, the proportion of small-leaved trees, especially alder and hazel, sharply increased in mesophytic forests, while the number of pine trees decreased. Wetter and warmer climatic conditions are assumed for the early Miocene: this time is characterized by an increase in the number of walnut, cypress, and Cyrillaceae. In the middle Miocene, the climate was still quite warm, but drier, and such moisture-loving species as podocarpus, spruce, glyptostrobus, and swamp cypress disappeared from plant communities.

Environmental and vegetation dynamics through the Oligocene to Early Miocene of North Africa (Egypt)

This study documents palynomorph assemblages from the Oligocene through the Early Miocene, across the significant Warm-house to Cool-house transition, using a subsurface section from the AG-5 well, north Western Desert of Egypt. Although the Paleogene and Neogene strata occupied a large area of the land, little about vegetation and ecosystem processes that shaped the climate and biodiversity during this time window is known. Here, based on a comprehensive palynological analysis of the Oligocene to Early Miocene Dabaa and Moghra formations encountered in the AG-5 well, we show that the studied area was characterized by a humid tropical to subtropical climate during the Oligocene-Miocene time interval. The obtained results provide clear evidence of the poorly known Paleogene-Neogene ecosystem in North Africa. In terms of palynostratigraphical framework, three palynozones covering the late Oligocene-Early Miocene interval are recognized. Moreover, the recovered palynomorphs inferred a deposition of the Dabaa Formation under a normal marine, inner shelf to littoral environment. However, for the basal part of the Moghra Formation, a deposition in an offshore marine environment, outer shelf (high sea level, 100–200 m depth) is assumed, whereas a deposition in shallow coastal lagoons, estuary, or delta environment (low sea level) is presumed for its uppermost part.

Late Oligocene–Early Miocene incision of the Three Gorges and the initial establishment of an east-flowing Yangtze River

The evolutionary history of the Yangtze River plays a key role for understanding the Cenozoic landscape evolution of East Asia. The Three Gorges currently separate the Sichuan Basin from the Jianghan Basin to the east, and their formation is considered to establish an initial connection between the Upper and Middle reaches of the modern Yangtze River. However, despite their importance in the Yangtze River evolution, the age of the initial incision of the Three Gorges remains an ongoing debate. In this study, we use apatite-fission track and (UTh)/He data to model the thermal history of the crust during the gorge formation. Based on the modeled time–temperature paths of samples on vertical profiles and the 3D thermos-kinematic modeling of the gorge region, we suggest that the gorges were initially carved during the Late Oligocene–Early Miocene. This event connected the drainages to the west and east of the gorge regions, leading to the formation of the modern east-flowing Yangtze River. This is also consistent with the sediment provenance of the Yangtze Gravels in the Lower Yangtze reported in previous studies. The gorge formation could be a response to the base level drop in the Jianghan and Subei Basins, which led to an upstream propagation of high erosion rates in the Three Gorges areas.