Search Results (429)

Powell Basin is an ocean basin formed as a result of the Scotia Sea evolution. The existing tectonic models propose a variety of starting and ending ages for the spreading of the basin based on seafloor magnetic anomalies. Here, we use recent magnetic field data obtained from eight magnetic profiles in Powell Basin to provide insights into the oceanic spreading evolution. The differences found between the number of anomalies on both sides of the axis and the asymmetry in the spreading rates suggest different opening models for different parts of the basin. We propose a spreading model starting in the late Eocene (38.08 Ma) and ending in the early Miocene (21.8 Ma) for the northern part of Powell Basin. For the southern part, the opening started in the late Eocene (38.08 Ma) and ended in the middle Paleogene (25.2 Ma). The magnetic data have been combined with gravity and sediment thickness data to better constrain the age models. The gravity and sediment thickness information allow us to more accurately locate the position of the extinct spreading axis. Geothermal heat flow measurements are used to understand the relationship between the low amplitudes of the magnetic anomalies and the heat beneath them. Our proposed oceanic spreading models suggest that the initial incursions of the Pacific mantle outflow into the Powell Basin occurred in the Oligocene, and the initial incursions of oceanic currents from the Weddell Sea occurred in the Eocene. Full article

The Zheduo–Gongga Mountain, an enormous tower located at the boundary of the eastern Tibetan Plateau, is an ideal place to study the contribution of the climate and/or tectonics to the mountain building. Here, we report new zircon U–Pb ages, biotite ⁴⁰Ar–³⁹Ar, and apatite fission track (AFT) ages of granites along the Zhonggu transect in the northern part of the Zheduo–Gongga massif to investigate the detailed exhumation history and mechanism. The results show zircon U-Pb ages of 14.3 ± 0.3 and 11.3 ± 0.2 Ma, Biotite ⁴⁰Ar–³⁹Ar ages of 4.39 ± 0.07 and 3.62 ± 0.05 Ma, and AFT ages of ~2.6–0.9 Ma. Combining previous structural and geochronological studies, we argue that the growth and exhumation of the Zheduo–Gongga Mountain experienced the following stages. Late Oligocene–early Miocene crust shortening and magmatism marked the initiation of the crustal thickening and surface uplift during ~32–11 Ma, forming a migmatite–granitic belt along the Xianhuihe fault, in response to the northward advancing of the Indian plate into the Eurasian plates. Subsequently, the massif experienced episodic phases of exhumation with variable rates. The exhumation occurred at a rate of ~1–1.5 km/Ma with a cooling rate of 70 ± 20 °C/m.y. during ~11–5 Ma coinciding with the coeval intensification of the Asian monsoon and clockwise rotation of the Chuandian block, south of the Xianshuihe fault. During ~5–2 Ma, a phase of accelerated exhumation (~2–5 km/Ma) started, followed by a possible phase of decelerated exhumation (~1–1.5 km/Ma, corresponding to a cooling rate of 120 ± 20 °C/m.y.) since ~2 Ma, when alpine glaciations initiated due to global cooling. This study highlights the importance of tectonic deformation during ~11–5 Ma in controlling the early growth and exhumation of high mountains in the eastern Tibetan Plateau. The climate may account for the later exhumation of the Zheduo–Gongga mountain since ~5 Ma. Full article

A notable feature of volcanic activity in the Halmahera arc since the Middle Miocene is that eruptions have occurred episodically from south to north, rather than simultaneously. However, the dynamic cause of the volcanic age disparity between the northern and southern regions remains unresolved. To investigate this, thermal simulation methods were employed to calculate the temperature structures of the subducted slab and mantle wedge under varying movement rates of subduction and overriding plates. Simulation results indicate that arc magmatism is favored at lower subduction rates or higher overriding plate movement rates. During the Middle Miocene, the northward drift of the Australian plate propelled the Obi microplate southward via left-lateral strike-slip faults, accelerating its movement rate. In contrast, the movement rate of the Halmahera microplate in the north remained lower, unaffected by the Australian plate. The accelerated movement of the Obi microplate resulted in the rapid generation of a higher temperature zone, facilitating arc volcanism on Obi Island in the late Middle Miocene. Meanwhile, the lower movement rate of the Halmahera microplate delayed the formation of a favorable high-temperature zone for arc volcanism, leading to the emergence of volcanism on Halmahera Island in the late Pliocene. Full article

Zircon U-Pb dating, rock geochemistry, Sr-Nd-Pb, and zircon Hf isotope analyses were conducted on the ultrabasic and basic rocks of ophiolites in the Sabah area (Borneo, SE Asia). The zircon U-Pb ages of ultrabasic and basic rocks range from 248 to 244 Ma, indicating that the ophiolites already existed in the early Triassic. The rare earth elements of basic rocks in Central Sabah show N-MORB-type characteristics and E-MORB-type characteristics in the northwest and southeast. The ε_Nd(t) values of basic rocks range from 3.66 to 8.73, and the ε_Hf(t) values of zircon in ultrabasic rocks are between −10.2 and −6.1. Trace element analysis shows that the magmatic source was influenced by melts and fluids from the subducting plate of the Paleo-Tethys Ocean. The tectonic evolution of the Sabah area can be traced back to the Early Triassic. At that time, the fast subduction of the Paleo-Tethys Ocean plate and the retreating of the Paleo-Pacific plate resulted in the upwelling of mantle material in relatively small extensional settings, leading to the formation of the ophiolites. From the Jurassic to the Early Cretaceous, the Paleo-Pacific plate was intensely subducted, and the ophiolite intrusion in the Sabah area moved to the continental crust of South China or the Sundaland margin as fore-arc ophiolites. From the Late Cretaceous to the Miocene, with the expansion of the Proto-South China Sea and South China Sea oceanic crust, the ophiolites in the Sabah area drifted southward with microplate fragments and sutured with East Borneo. Full article

The lower Messinian Calcare di Rosignano Formation (Tuscany, Italy, 43° N) preserves one of the youngest and northernmost examples of coral reefs in the Mediterranean. The outcropping succession of the Acquabona quarry consists of four main facies, namely, in ascending stratigraphic order: (1) coral boundstone, (2) coralline algal rudstone, (3) serpulid floatstone to packstone, and (4) peloidal packstone to grainstone. The succession displays a trend toward increasingly more shallow conditions and progressively more restricted water circulation. The coral reef displays a limited coral biodiversity and a remarkable abundance of heterotrophs, similar to modern coral reefs developed at the edges of the ecological niche of symbiont-bearing colonial corals. The widespread presence of coral colonies pervasively encrusted by coralline algae and benthic foraminifera suggests that short-term environmental perturbations caused temporary shutdowns of the coral-dominated carbonate factory. Moving upwards, there are fewer corals and more highly adaptable carbonate producers like coralline algae and serpulids. This suggests that the decline of corals had been caused by the conditions in the basin becoming more stressful, up to the collapse of the coral community. The overall succession indicates that coral-dominated ecosystems located at the edges of the coral zone are very sensitive; they can be affected even by minor perturbations and easily collapse if negative conditions persist. Full article

It is hypothesized that the present tectonic setting of the Anatolian, Aegean and Balkan regions has been deeply influenced by the different deformation styles of the inner and outer belts which constituted the Oligocene Tethyan system. Stressed by the Arabian indenter, this buoyant structure has undergone a westward escape and strong bending. The available evidence suggests that in the Plio–Pleistocene time frame, the inner metamorphic core mainly deformed without undergoing major fragmentations, whereas the orogenic belts which flanked that core (Pontides, Balkanides, Dinarides and Hellenides) behaved as mainly brittle structures, undergoing marked fractures and fragmentations. This view can plausibly explain the formation of the Eastern (Crete–Rhodes) and Western (Peloponnesus) Hellenic Arcs, the peculiar time-space features of the Cretan basins, the development of the Cyprus Arc, the North Aegean strike-slip fault system, the southward escapes of the Antalya and Peloponnesus wedges and the complex tectonic setting in the Balkan zone. These tectonic processes have mostly developed since the late Late Miocene, in response to the collision of the Tethyan belt with the Adriatic continental domain, which accelerated the southward bending of the Anatolian and Aegean sectors, at the expense of the Levantine and Ionian oceanic domains. The proposed interpretation may help us to understand the connection between the ongoing tectonic processes and the spatio-temporal distribution of major earthquakes, increasing the chances of estimating the long-term seismic hazard in the study area. In particular, it is suggested that seismic activity in the Serbo–Macedonian zone may be favored by the post-seismic relaxation that develops after seismic crises in the Epirus thrust front and inhibited/delayed by the activations of the North Anatolian fault system. Full article

The Cenozoic glaciation of Antarctica proceeded through two distinct steps around 35 and 15 million years ago. The first icing was attributed to thermal isolation due to the opening of the Drake/Tasman passages and the development of the Antarctic circumpolar current. I also subscribe to this “thermal isolation” but posit that, although the snowline was lowered below the Antarctic plateau for it to be iced over, the glacial line remains above sea level to confine the ice sheet to the plateau, a “partial” glaciation that would be sustained over time. The origin of the second icing remains unknown, but based on the sedimentary evidence, I posit that it was triggered when the isostatic rebound of West Antarctica caused by heightened erosion rose above the glacial line to be iced over by the expanding plateau ice, and the ensuing cooling lowered the glacial line to sea level to cause the “full” glaciation of Antarctica. To test these hypotheses, I formulate a minimal box model, which is nonetheless subjected to thermodynamic closure that allows a prognosis of the Miocene climate. Applying representative parameter values, the model reproduces the observed two-step icing followed by the stabilized temperature level, in support of the model physics. Full article

We present a molecular phylogeny for the subtribe Ecliptinae (Asteraceae, Heliantheae) based on three plastid (matK, psbA-trnH, and trnQ-rps16) and two nuclear (nrITS and nrETS) markers. The results of the phylogenetic reconstruction were utilised as a topological constraint for a subsequent divergence dating analysis and ancestral range reconstructions. We sampled 41 species and 40 genera (72%) of Ecliptinae and two species of Montanoa (as outgroups) to elucidate the generic relationships between the genera of this subtribe. The Bayesian inference (BI) and Maximum Likelihood (ML) analyses were performed for the combined molecular dataset. The divergence dating analysis was performed using a relaxed, uncorrelated molecular clock with BEAST v1.8.4 and calibrated using a single secondary calibration point from a recently published chronogram for the family. The ancestral range reconstructions focusing on continents (i.e., South America, North America, Africa, Asia, and Oceania) and biomes (Dry forests, Altitudinal grasslands, Savannas, and Rainforests) were performed on BioGeoBEARS. Our phylogenetic results indicate that the genera of Ecliptinae are grouped into five clades, informally named the Monactis, Oblivia, Blainvillea, Wedelia, and Melanthera clades. The most recent, common ancestor of Ecliptinae was widespread in the North and South American dry forests at 8.16 Ma and mainly radiated in these regions up to the Pleistocene. At least eight dispersal events to South America and four dispersal events from North America to Africa, Asia, and Oceania took place during this period in all five informal clades of Ecliptinae. At least 13 biome shifts from dry forests to rainforests were evidenced, in addition to ten biome shifts from dry forests to altitudinal grasslands and savannas. These results corroborate the mid-late Miocene to early Pleistocene radiation of Ecliptinae in tropical dry forests. Future studies should aim to sample the remaining 14 unsampled genera of Ecliptinae to position them in one of the five informal clades proposed in this study. Full article

Iran is a country characterized by high biodiversity and complex biogeographic patterns. Its diverse landscape and steep climatic gradients have resulted in significant faunal diversity and high level of endemism. To better understand these patterns, we investigated the historical environmental drivers that have shaped Iran’s current geological and climatological conditions, and, consequently, have shaped the current zoological distribution patterns. Furthermore, we provide an overview of the country’s zoological diversity and zoogeography by reviewing published studies on its fauna. We analyzed nearly all available catalogs, updated checklists, and relevant publications, and synthesized them to present a comprehensive overview of Iran’s biodiversity. Our review reports approximately 37,500 animal species for Iran. We also demonstrated that the country serves as a biogeographic transition zone among three zoogeographical realms: the Palearctic, Oriental, and Saharo-Arabian, where distinct faunal elements intersect. This biogeographic complexity has made it challenging to delineate clear zoogeographical zones, leading to varying classifications depending on the taxon. The uplift of mountain ranges, in particular, has played a crucial role in shaping faunal diversity by serving as barriers, corridors, and glacial refugia. These mountains are largely the result of orogeny and plate collisions during the Mesozoic and Cenozoic eras, coupled with the development of the Tethyan Sea and the uplift of several ranges during the Miocene. Despite these insights, our understanding of biodiversity distribution in Iran remains incomplete, even for some well-studied taxa, such as certain vertebrate families and arthropods. We highlight the existing gaps in knowledge regarding zoogeographical patterns and propose approaches to address these gaps, particularly concerning less-studied species and the highly diverse group of insects. Full article

Upper Miocene volcanic and plutonic rocks on Kos island preserve a record of magmatic and tectonic events in the transition zone between the Aegean and Anatolian microplates. Their field setting, syn-intrusion deformation, mineralogy, and geochemistry were investigated. Volcanic rocks, including trachyandesite flows and trachyandesite to rhyolite domes, were extruded on a central E–W horst and directly overlie Alpine basement. Thick successions of trachytic flow tuffs are interbedded with fluvial and lacustrine basinal sediments to the south of this horst. Volcanism was synchronous with the emplacement of the Dikeos monzonite pluton, which is geochemically similar to some lithic clasts in the thick flow tuffs and is cut by mafic dykes including lamprophyres. Two main types of mafic magma were present: a K-rich lamprophyric magma that evolved to trachyandesite and more calc-alkaline magma similar to mafic enclaves in the monzonite. Syn-intrusion structures in the monzonite indicate emplacement during E–W sinistral strike-slip faulting that created local transtensional deformation, providing accommodation for a Dikeos magma reservoir. A change in the style of deformation in the Late Miocene led to NW-striking extension in the footwall, occupied by mafic dykes and mineralized veins, and extensional detachment of the hanging wall, resulting in unroofing of the monzonite. Full article

The distribution of species in relation to latitude and altitude is of fundamental interest to ecologists and is expected to attain increasing importance as the Earth’s climate continues to change. Species diversity is commonly greater at lower than higher latitudes on a global scale, and the similarity of communities frequently decreases with distance. Nevertheless, reasons for such patterns are not well understood. We investigated species richness and changes in community composition of mayflies (Ephemeroptera) over 13 degrees of latitude at 81 locations throughout New Zealand by light-trapping and the benthic sampling of streams. Mayflies were also sampled along an altitudinal gradient on a prominent inactive volcano in the east of North Island. Sampled streams were predominantly in the native forest, at a wide range of altitudes from sea level to c. 1000 m a. s. l. A total of 47 of the 59 described New Zealand mayflies were recorded during the study, along with five undescribed morphospecies. Species richness declined and the degree of dissimilarity (beta diversity) of mayfly communities increased significantly from north to south but less strongly with increasing altitude. Our results suggest that the southward decline in species richness has historical origins with the north of the country having acted as a major refuge and region of speciation during the Pleistocene. The increasing dissimilarity of the northern and southern communities may reflect an increasingly harsh climate, variable amounts of subsequent southward dispersal of northern species and, in the South Island, the presence of species which may have evolved in the newly uplifted mountains during the Miocene–Pliocene. Full article

Petrographic studies have been carried out on the Early Miocene Darnó Conglomerate Formation, which consists only of debris of ophiolitic mélange and is found today on Darnó Hill in SW Bükk, NE Hungary. The studied sediments are bounded by the Darnó line from Darnó Hill. The aim of this work was to show if it is possible to reconstruct the petrographic composition of the source area only from its debris. The rock types were determined in thin sections using a polarizing microscope, and a quantitative analysis of the different rock types was carried out using the grain counting method, the results of which were interpreted as volume ratios. The main rock types observed in the studied samples (textural varieties of basalt, dolerite/microgabbro, claystone, siltstone, and radiolarite) are similar to the rock types of the mélange assemblage of Darnó Hill. Based on the volume calculations of basaltic detrital grains with different textures characteristic for pillow basalts, it could be established that pillow basalts dominated the igneous rocks in the source area of the Darnó Conglomerate on Darnó Hill already in the Miocene. Thus, this work shows that the lithological composition of a source area can be precisely outlined by a detailed petrographic analysis of the debris eroded from the immediate vicinity. Full article

New data obtained from core samples of two boreholes and dredged samples from the Alba Guyot in the Magellan Seamount Trail (MST), Western Pacific, including the ⁴⁰Ar/³⁹Ar age determinations of basanite, and the mineralogy of basanite, tuff, tuffite, mantle-derived inclusions in basanite and tuff (lherzolite xenolith and Ol, Cpx, and Opx xenocrysts), and calcareous nannofossil biostratigraphy, have implications for the guyot′s development and history. Volcanic units in the upper part of the Alba Guyot main edifice and its Oma Vlinder satellite, at sea depths between 3600 and 2200 m, were deposited during the Cretaceous 112 to 86 Ma interval. In the following ~60 myr, the Alba Guyot became partly submerged and denuded with the formation of a flat summit platform while the respective fragment of the Pacific Plate was moving to the Northern Hemisphere. Volcanic activity in the northeastern part of the guyot summit platform was rejuvenated in the Miocene (24–15 Ma) and produced onshore basanitic volcanoes and layers of tuff in subaerial and tuffite in shallow-water near-shore conditions. In the Middle-Late Miocene (10–6 Ma), after the guyot had submerged, carbonates containing calcareous nannofossils were deposited on the porous surfaces of tuff and tuffite. Precipitation of the Fe-Mn crust (Unit III) recommenced during the Pliocene–Pleistocene (<1.8 Ma) when the guyot summit reached favorable sea depths. The location of the MST guyots in the northwestern segment of the Pacific Plate near the Mariana Trench, along with the Miocene age and alkali-basaltic signatures of basanite, provide first evidence for petit-spot volcanism on the Alba Guyot. This inference agrees with the geochemistry of Cenozoic petit-spot basaltic rocks from the Pacific and Miocene basanite on the Alba Guyot. Petit-spot volcanics presumably originated from alkali-basaltic melts produced by decompression partial melting of carbonatized peridotite in the metasomatized oceanic lithosphere at the Lithosphere–Asthenosphere Boundary level. The numerous volcanic cones with elevations of up to 750 m high and 5.1 km in basal diameter, discovered on the Alba summit platform, provide the first evidence of voluminous Miocene petit-spot basanitic volcanism upon the Cretaceous guyots and seamounts of the Pacific. Full article

The surface and subsurface karst features of the Eocene limestone plateaus along the Middle Nile Valley in Egypt were formerly believed to be epigene in origin and to have developed during post-Eocene pluvial periods. However, the morphology of the caves and their restriction to particular stratigraphic intervals suggests that they are hypogene. The geochemistry and mineralogy of the soft, thick-bedded, brown/black cave infills shows that these sediments originated from hydrothermal processes, as evidenced by their Fe, Mn, Co, Ni, and Cu concentrations. Thus, the karst features are hypogene and probably formed during the opening of the Red Sea Rift at the end of the Oligocene and early Miocene. At this time, there was abundant volcanic activity, as shown by basalt lavas ~70 km northwest of Assiut; this triggered the release of large amounts of CO₂ that made the hydrothermal waters acidic and dissolved the caves. Full article

The trend in partial pressure of atmospheric CO₂, P(CO₂), across the 66 MYr of the Cenozoic requires elucidation and explanation. The Null Hypothesis sets sea surface temperature (SST) as the baseline driver for Cenozoic P(CO₂). The crystallization and cooling of flood basalt magmas is proposed to have heated the ocean, producing the Paleocene–Eocene Thermal Maximum (PETM). Heat of fusion and heat capacity were used to calculate flood basalt magmatic Joule heating of the ocean. Each 1 million km³ of oceanic flood basaltic magma liberates ~5.4 × 10²⁴ J, able to heat the global ocean by ~0.97 °C. Henry’s Law for CO₂ plus seawater (H_S) was calculated using δ¹⁸O proxy-estimated Cenozoic SSTs. H_S closely parallels Cenozoic SST and predicts the gas solute partition across the sea surface. The fractional change of Henry’s Law constants, Hn−HiHn−H0 is proportional to ΔP(CO₂)_i, and Hn−HiHn−H0×∆P(CO2)+P(CO2)min, where ΔP(CO₂) = P(CO₂)_max − P(CO₂)_min, closely reconstructs the proxy estimate of Cenozoic P(CO₂) and is most consistent with a 35 °C PETM ocean. Disparities are assigned to carbonate drawdown and organic carbon sedimentation. The Null Hypothesis recovers the glacial/interglacial P(CO₂) over the VOSTOK 420 ka ice core record, including the rise to the Holocene. The success of the Null Hypothesis implies that P(CO₂) has been a molecular spectator of the Cenozoic climate. A generalizing conclusion is that the notion of atmospheric CO₂ as the predominant driver of Cenozoic global surface temperature should be set aside. Full article