Late Miocene Time Research Articles

The Cibarco anticline of the northernmost San Jacinto fold belt: Possible influence of mud diapirism on the Neogene to the recent evolution of the structure where the first oil and gas wells were drilled in Colombia

The Cibarco anticline, located in the northwestern South American convergent margin, is a long (approximately 40 km) and narrow (2.5 km wide), south-southwest–north-northeast-trending structure that separates two much wider depocenters, the Tubará-Juan de Acosta depocenter (approximately 20 km wide) to the west, and the Sabanalarga syncline (approximately 11 km wide) to the east. Detailed geologic surface mapping and subsurface seismic interpretation suggest that the formation and evolution of the Cibarco anticline have been strongly influenced by mud diapirism, which has been focused along a west-southwest–east-northeast-trending, inverted pre-Oligocene extensional fault. Seismic interpretation and outcrop analyses also confirmed that the source of mobile shales is the Upper Oligocene marlstones of the Lower Ciénaga de Oro Formation. We suggest that the main mechanism that caused the overpressures that triggered and controlled shale mobilization was disequilibrium compaction related to the sedimentary loading of impermeable seals and the shortening and inversion of pre-Oligocene extensional faults. Growth strata indicate that the Upper Oligocene mud started moving in Early Miocene times during the deposition of the thick Porquero mudstones, and it stopped migrating in Latest Miocene times. After an uplift and erosion event, mud diapirism remained inactive in the Early Miocene, and it was then reactivated in Late Pliocene times. Slight mud migration focused on the inverted fault continued throughout the Pleistocene and to the present. Today, the slight mud migration to the surface is observed in volcanoes located in the northern strand of the Cibarco anticline, although the rest of the anticline has become welded. Our new interpretations, carried out more than a century after the first hydrocarbon exploration activities began in Colombia, will hopefully allow us to increase our understanding of shale tectonics, petroleum systems, and plays in this portion of the convergent margin.

Plio-Pleistocene hydrothermal events of the Baza Basin (Betic Cordillera, SE Spain) and their paleoecological implications

AbstractThe Lower Pliocene to upper Middle Pleistocene continental sedimentary infillings of the Baza Basin (Guadix-Baza Depression, SE Spain) are worldwide unique by their high stratigraphic completeness and exceptional preservation of their fossil record of terrestrial vertebrates. These sediments were deposited in fluvio-lacustrine environments and preserve huge assemblages of large mammals, including the oldest evidence of hominin presence in Western Europe at the late Lower Pleistocene sites of Barranco León and Fuente Nueva-3, dated to ∼1.4 Ma. Since latest Miocene times, the basin was subject to intense tectonic activity, showing sedimentary deposits originated from ancient hot springs. These hot springs were developed in a spatio-temporal network related to tectonic fractures, showing a higher concentration in the Orce sub-basin during Late Pliocene to Holocene times. Compositional analyses of cherts and other facies associated with these hot springs show geochemical and mineralogical markers linked to hydrothermal scenarios. The contribution of hot springs in the Baza Basin resulted in rich and productive biotopes, which supported a high diversity and biomass of terrestrial mammals. Similar geological, hydrochemical and ecological environments are found along hotspot lines in East Africa, the Levantine Corridor and the circum-Mediterranean realm. We hypothesize here that the presence of these hot springs corridors favored the dispersal of hominins and other large mammals during the Plio-Pleistocene, which led to the first hominin arrival in Europe.

A new early water frog (Telmatobius) from the Miocene of the Bolivian Altiplano

AbstractWe describe the new frog Telmatobius achachila sp. nov. from the late Middle to earliest Late Miocene of Achiri, based on a partial skeleton found at 3960 m above sealevel in the Bolivian Altiplano. This skeleton, attributed to a male adult, constitutes the first documented fossil record of the speciose living genus Telmatobius, endemic to the Andean Cordillera and the Altiplano. Phylogenetic analysis confirms the new species as being part of the crown group, and diverging both later than the T. verrucosus group and earlier than the T. bolivianus, T. marmoratus and T. macrostomus groups. Coupled with its accurate stratigraphic provenance and age, this phylogenetic position provides a relevant calibration point for timing the evolutionary history of these highland, mostly aquatic frogs. The skeleton of T. achachila indicates that several of the osteological peculiarities of extant Telmatobius were already acquired at c. 12 Ma, including some that might be linked to their aquatic lifestyle. Together with mixed montane–rainforest pollen vegetation uncovered in the same level, this fossil specimen further provides key data enabling a more accurate reconstruction of ancestral habitats and elevation ranges of Telmatobius, agreeing with the previously postulated conditions in which these water frogs might have first evolved. Ultimately, this discovery adds to the sparse evidence of a humid tropical Bolivian Altiplano just prior to: (1) the Late Miocene uplift pulse of the Central Altiplano; and (2) the drastic climate deterioration that occurred from Late Miocene time onward, leading to the harsh highland‐steppe environments reigning there today.

Geosites in the Gole della Breggia Geopark, Ticino, Southern Switzerland

This work is aimed at illustrating one of the most engaging geoparks in Europe, the Gole della Breggia Geopark. By following its geotrail, the visitors can observe a great deal of features that encompass a wide variety of sedimentary, stratigraphic, tectonic and paleogeographic processes. Its contents are popularized by a comprehensive geotrail that encompasses 23 geostops, enriched with explanatory panels, which enable viewers to gain an overall picture of the complex processes that led to the present-day configuration of the outcrops encountered in the geopark. We have selected five significative outcrops, which we have regarded as fully-fledged geosites, and which encompass the whole 170-million-year interval, from Early Jurassic to Late Miocene times, during which the rocks of the geopark were deposited and deformed. We have performed an assessment of the five geosites, using the most common criteria that can be applied for geosite evaluation. The results of the assessment show that the most valuable geosites are the Lombardian Maiolica, and the Limestone with Pelagic Bivalves. All five geosites, thanks to a QR code added to each figure along the text, can be viewed as virtual geosites (VGs), integrated by 360° videos.

Petroleum system modeling in complex tectonic setting; a study case of the Huallaga-Marañon retroforeland basin system, Peru

The present study explores the oil and gas potential of the Huallaga–Marañon Andean retroforeland basins in Perú, considering a regional transect which extends from the Eastern Cordillera in the west trough the Huallaga fold-and-thrust belt at the Andean deformation front to the Marañon Basin in the east. The sequential restoration of main tectonic events was derived from previous stratigraphic and structural interpretations revisited with reprocessed seismic lines, and new geochemical, petrographical, geochronological and thermochronological data. The resulting framework was tested in a 2-D Basin and Petroleum System Modeling (BPSM) to check the relationship between burial history, thermal maturity evolution and timing of hydrocarbon generation, expulsion and trapping at regional scale. BPSM suggests that during the Late Cretaceous (ca. 80 Ma) the petroleum generated in the Upper Triassic–Lower Jurassic source rock known as the Aramachay Formation migrated through the Middle–Upper Jurassic Sarayaquillo Formation basal sandstones up to 250 km and accumulated in the southeast of the Marañon Basin. From the Late Miocene, the reactivation of vertical faults favored the migration and accumulation of petroleum in Cretaceous siliciclastic reservoirs which is consistent with the hydrocarbon fields existence in this sector of the basin.In addition, results of modeling show that in the southwest of the Marañon Basin the development of structures during Late Miocene times postdated the generation and expulsion of hydrocarbons calculated for the Aramachay Formation which introduces a risk of synchronization between oil migration and charge. However, part of the generated hydrocarbons during the Late Cretaceous and Cenozoic are partially preserved in Jurassic reservoirs due to the high seal capacity of mudstones of the Sarayaquillo Formation. Such hydrocarbon fluids are redistributed and migrated into structural traps during Miocene times. In addition, the Miocene gas generation and expulsion also will reduce the risk of synchronization with the development of structures. In the Eastern Cordillera, organic-rich levels of the Aramachay Formation are in the late immature to early oil window show the occurrence of organic pores adding hydrocarbon storage capacity, suggesting this unit to be considered as a potential unconventional target in future exploration activities.

Internal architecture and structural evolution of a horst relay zone from the northern Gulf of Suez rift, Egypt: Implications for syn-rift sedimentation

A remarkable outcrop of an extensional NW-oriented horst block takes place between conjugate normal fault segments at the Akheider-El Ramliya fault block (ARFB), along the northwestern termination of the Gulf of Suez rift. Detailed field geological mapping is accomplished to identify the architecture, linkage style as well as evolution of the encountered normal fault system at the ARFB. The resultant structural model revealed several geometries of normal faults interaction that form soft- and hard-linkage relay zones affecting pre- and syn-rift rock units. Such relay zones are well developed between the boundary faults of the ARFB. Additionally, three E-W oriented fault belts are mapped at the northern, central and southern parts of the ARFB enclosing left-stepping en echelon normal faults, where several relay zones are formed between the overlapped fault segments. Structural analysis and evolution of the mapped area revealed that the overall architecture of the ARFB representing a broad horst relay zone between the conjugate divergent boundary normal faults. Also, two sub-basins have been recognized at the downthrows of the border faults comprising syn-rift Oligocene and Miocene sediments. The results show that these faults are rejuvenated by a continued NE-SW extension through Late Eocene, Oligocene and Miocene times. Additionally, the formed relay zones via growth and propagation of such faults have main controls on syn-sedimentation at the northern Gulf of Suez rift. The results of the present study offer constraints on horst relay zones including, internal structural architecture, geometries and interaction of boundary faults as well as role of inherited structures. Also, it provides an outcrop model for architecture and evolution of similar structural styles (e.g., the Gulf of Suez rift) as well as for subsurface mapping and exploration in extensional relay zones.

Rhinocerotoid fossils of the Linxia Basin in northwestern China as late Cenozoic biostratigraphic markers

This study reports on rhinocerotoid fossils from the Oligocene-Pleistocene strata of the Linxia Basin of Gansu Province, northwestern China. Rhinoceroses have been important components of mammalian faunas of these beds, and include the Late Oligocene giant rhinos (paraceratheres) and the Late Miocene hornless rhinos (aceratheres). As a result of their high diversity, wide geographical distribution and rapid evolution, rhinoceroses are good markers for stratigraphic division and may be used for intercontinental correlations. The Late Oligocene fauna of the Linxia Basin occurs in the lower part of the Jiaozigou Formation. Among 12 species of large mammal fossils, nine are of the superfamily Rhinocerotoidea, and giant rhinos are dominant. Rhinoceroses of the Middle Miocene fauna reflect a forest environment. Comparison with contemporaneous European rhinoceroses suggests a dispersal route for populations along the north side of the Tibetan Plateau. Rhinoceroses were most abundant in the Linxia Basin in the Late Miocene times. The most primitive species of the genus Chilotherium and the huge elasmotheres are key indicators for the base of the Upper Miocene (Tortonian). From the Late Miocene to Early Pliocene, the assemblages of the three-toed horse Hipparion comprise five horizons, each of which has different rhinocerotid fossils as markers of division and correlation. Because of the strong competition by ruminants, rhinoceroses declined greatly in the Early Pleistocene fauna. At that time, only a species of the woolly rhino persisted, but as an intermediate form, Coelodonta nihowanensis of the Linxia Basin can still connect the Pliocene Zanda Basin of the Tibetan Plateau and the Early Pleistocene Nihewan Basin of the North China Plain.

Environmental shifts in and around Lake Pannon during the Tortonian Thermal Maximum based on a multi-proxy record from the Vienna Basin (Austria, Late Miocene, Tortonian)

The Neogene Lake Pannon was the largest lake that ever existed in Europe. It attained its greatest extent during the Tortonian Thermal Maximum. For the first time, results from a detailed lake record documenting about 85 kyr of Late Miocene time in a continuously recovered, 60-m-long, clay-rich core of Lake Pannon are reported. This record includes the transition from the lake's maximum transgression into its highstand at around 10.4 Ma. The environmental development of Lake Pannon during its maximum extent is interpreted based on integrated paleontological, sedimentological, mineralogical and geochemical data. The maximum extent coincided with stable sedimentation of clay, little influx from the hinterland, low surface productivity and severe bottom-water anoxia. The clay mineralogy of the lower part of the core points to prevailing chemical weathering based on the illite/smectite ratios. Distinct Fe, Mn and Ba enrichments are interpreted to have formed close to sulfate-methane transition zones during the maximum flooding. The highstand phase was marked by rapid environmental shifts with frequent phases of well‑oxygenated bottom waters. These phases are reflected by rich benthic communities including stenohaline tunicates. The increased input of detritic kaolinite suggests a shift towards physical weathering and higher precipitation coinciding with a shift in the provenance of clay minerals.Increasing amounts of nutrients stimulated surface water productivity and nannoplankton blooms. Despite the offshore position of the core at ∼8 km from the mountainous ranges of the Alps, strong fluvial input is reflected from 32.5 to 30.3 m by coarser sediment and the occurrence of terrestrial and freshwater molluscs. The Rhenodanubian Flysch Unit was the main source of the siliciclastics of the core and was drained by the Paleo-Wien river. Drainage from the Calcareous Alps was limited to an exceptionally strong fluvial event and related deposits, which documents the presence of a second river in the southwest, which might represent the Paleo-Liesing.The dominance of smectite throughout the core suggests a temperate climate with distinct seasonality during the Tortonian Thermal Maximum. In view of the autochthonous ascidian sclerites in three samples, and assuming generally similar ecological requirements for both Pannonian and modern tunicates, we conclude a polyhaline salinity for Lake Pannon around 10.4 Ma.

A history of the latest and Neogene unconformities, offshore Palawan and the southern South China Sea

The age and character of the main unconformities offshore Palawan is resolved, and their history reviewed. Some very deep and very shallow unconformities have insufficient well data, but from the Oligocene to Late Miocene times a series of unconformities can be dated, characterised and correlated regionally to help develop a new tectono-stratigraphic framework. This framework does not support the widely cited idea of Early Miocene subduction of a Proto-South China Sea plate followed by collision over a plate suture.The absence or very weak expression of a Base Miocene Unconformity (c. 24 Ma), so strongly expressed across northern Borneo, also suggests a major revision of tectonic models is required. In spite of many cited reports, there is no evidence for an unconformity in the later Early Miocene (c. 17 to 15 Ma), which was a period of gradually increasing compression and uplift. The main mid-Neogene seismic unconformity is the Red Unconformity dated at about 12–13 Ma (equivalent to the DRU of Sabah). This was when the uplift of a foreland over-thrust system paused, and a locally erosional surface was rapidly transgressed.This review describes the neglect of analytical sciences that remain a crucial part of basic geological studies.

Late Miocene NE-SW strike-slip fault system of the Mexico basin, central Trans-Mexican Volcanic Belt: Origin, deformation, and reactivation

A NE-SW fault system, observed in the northeast portion of the eastern sector of the Trans-Mexican Volcanic Belt, has been active since the Pliocene-Holocene time with dip-slip kinematics. These NE-SW faults belong to the Tenochtitlan fault system that extends from the southwest coast of Mexico to the NE zone of the Mexico basin. The length of this fault system suggests a complex deformation history, which is unknown before the Pliocene. In addition, the kinematics of the NE-SW faults and the tectonic conditions are unknown before their current activity. In this study, we investigate the geometry and kinematics of the NE-SW faults in the Miocene rocks in the central part of the Mexico basin (Guadalupe Range) for to know the kinematics of these faults during the Late Miocene time. Our results suggest that the Miocene rocks record two deformation events, one event is related to crustal shortening which produced a strike-slip activity in the NE-SW faults during the Late Miocene. The second deformation event is associated with crustal extension and the activity of the NE-SW faults with dip-slip kinematics. This extensional event was active during the Pliocene-Holocene. The re-activation analysis and our observations in the Guadalupe Range suggests that the NE-SW normal faults in the Tizayuca-Chignahuapan region are the product of the re-activation of previous NE-SW strike-slip faults. The change in the kinematics of the NE-SW faults explains the complex geometry of the damage zone and the high amount of the geological fractures in the Miocene rocks of the Mexico Basin. The activity type of the NE-SW faults is probably related to the dynamic of the subduction process in the southwest of Mexico, associated with the change in the inclination (decrease) and the convergence velocity of the Cocos Plate.

A New Physeteroid from the Lower Miocene of Japan

We describe a new specimen of physeteroid from the lower Miocene (Burdigalian) of Japan. This specimen was recovered from the Toyohama Formation, Chita County, Aichi Prefecture, Japan in 1984 and includes a finely preserved cranium with detached teeth and ear bones (periotic, tympanic bulla, and malleus). Here we refer this specimen to a new genus and species of the Physeteroidea, Miophyseter chitaensis gen. et sp. nov. Our phylogenetic analysis suggests that Miophyseter is a physeteroid more closely related to the crown Physeteroidea (Physeteridae and Kogiidae) than the macroraptorial physeteroids that flourished in middle and late Miocene times. A deep and large excavation on the ventral surface of the palatine and pterygoid in Miophyster suggests an adaptation for deep dives and/or the development of robust pterygoid muscles for active biting.

The Nazca Drift System – palaeoceanographic significance of a giant sleeping on the SE Pacific Ocean floor

AbstractThe evolution and resulting morphology of a contourite drift system in the SE Pacific oceanic basin is investigated in detail using seismic imaging and an age-calibrated borehole section. The Nazca Drift System covers an area of 204 500 km2 and stands above the abyssal basins of Peru and Chile. The drift is spread along the Nazca Ridge in water depths between 2090 and 5330 m. The Nazca Drift System was drilled at Ocean Drilling Program Site 1237. This deep-water drift overlies faulted oceanic crust and onlaps associated volcanic highs. Its thickness ranges from 104 to 375 m. The seismic sheet facies observed are associated with bottom current processes. The main lithologies are pelagic carbonates reflecting the distal position relative to South America and water depth above the carbonate compensation depth during Oligocene time. The Nazca Drift System developed under the influence of bottom currents sourced from the Circumpolar Deep Water and Pacific Central Water, and is the largest yet identified abyssal drift system of the Pacific Ocean, ranking third in all abyssal contourite drift systems globally. Subduction since late Miocene time and the excess of sediments and water associated with the Nazca Drift System may have contributed to the Andean orogeny and associated metallogenesis. The Nazca Drift System records the evolution in interactions between deep-sea currents and the eastward motion of the Nazca Plate through erosive surfaces and sediment remobilization.

Sediment provenance, routing and tectonic linkages in the Nankai forearc region, Japan

AbstractUnderstanding sedimentary routing systems at continental margins can be challenging in tectonically active regions such as forearc basins. Here, we combine sand modal composition and detrital zircon U‐Pb data from the Kumano Basin and surrounding Nankai Trough region of Japan to explore sedimentation in an active forearc and its linkages to the regional tectonic history. Our analyses are on Miocene‐to‐Pleistocene samples collected by the Integrated Ocean Drilling Program (IODP) at inner and outer forearc basin sites C0009 and C0002, river samples from onshore Honshu, and published results from other Nankai IODP sites. Results show the sediments of many IODP samples are similar to that of the modern rivers. Data from C0009 demonstrate that the early accretionary prism already contained slope basins fed by sediments similar to those in the modern Kumano River by 7.1 Ma. The more distal C0002 section was predominantly fed by a system similar to the modern Yodo River, located to the west‐southwest. We conclude that the section was therefore tectonically transported, in agreement with the ‘mobile model’ of oblique tectonic convergence across SW Japan during late Miocene time. Deposition resumed after a ca. 5.6–3.8 Ma hiatus, with sediments primarily fed from the paleo‐Kumano River, with some input from the paleo‐Tenryu and Kiso/Nagara rivers located to the northeast. Input from the paleo‐Tenryu and Kiso/Nagara rivers was likely re‐routed towards the trench by ca. 1.3 Ma. Sediments similar to those of the distal Yodo River also make up part of the Kumano basin fill and are best explained by erosion of uplifted older accretionary prism sediments along the Megasplay Fault that bounds the forearc basin. Despite the temporal variability in sedimentation routed to the basin, basin‐fill dynamics appear to have been governed primarily by the critical‐wedge style dynamics driven by variations in frontal accretion and in‐ or out‐of‐sequence thrusting response.

Significance of Northern Andes Terrane Extrusion and Genesis of the Interandean Valley: Paleomagnetic Evidence From the “Ecuadorian Orocline”

AbstractGPS data suggest that the NW South America corner forms a semi‐rigid block drifting NE‐ward along the regional dextral strike‐slip faults that bound an oceanic terrane accreted in Late Cretaceous times to W Ecuador and Colombia. However, the relevance of both strike‐slip versus thrust tectonics during Cenozoic times and their relation with oceanic terrane accretion are unclear. Here we report on the paleomagnetism of 31 mid‐upper Eocene to upper Miocene mainly volcanic sites from the Cordilleras Occidental and Real of southern Ecuador. Eleven sites show that the western Cordillera Occidental underwent a 24° ± 10° clockwise (CW) rotation with respect to South America after late Miocene times, while no rotation occurred further east. We relate the regional CW rotation to the emplacement of the Cordillera Occidental nappe onto the continental sediments of the Interandean Valley, blanketing the Cordillera's eastern margin. As rotation and continental sedimentation onset ages are similar, we interpret such tectonic depression as a narrow flexural basin formed ahead of the advancing nappe front. The 20°–30° CW Neogene rotation of the Cordillera Occidental is indistinguishable from the post‐Cretaceous rotation of the Coastal forearc oceanic blocks, implying that the whole W Ecuador Andean chain was detached and rotated over a mid‐crustal detachment during the last 10 Ma. Eocene‐Miocene paleomagnetic inclination values are systematically consistent with those expected for South America, thus excluding latitudinal terrane drift. We suggest that the Andes of Ecuador and Peru form the “Ecuadorian Orocline”, formed by opposing orogenic rotations around the Amazonian craton indenter.

Extensive vertebrate tracksite from the Upper Red Formation (middle–late Miocene), west Zanjan, northwestern Iran

Abstract. An extensive vertebrate tracksite from the middle–late Miocene Upper Red Formation in western Zanjan Province, northwestern Iran, provides new records of paleobiogeographical significance. These are records of common footprints of felids referred to as Felipeda lynxi Panin and Avram, canids referred to as Canipeda longigriffa Panin and Avram and less common bird footprints referred to as Iranipeda abeli Lambrecht. The Canipeda record establishes the late Miocene presence of canids on the Iranian Plateau as part of the Eurasia-wide “Eucyon event”. The Felipeda footprints are consistent with body fossil and footprint records elsewhere in Eurasia that indicate a widespread distribution of felids by late Miocene time. An unusual trace associated with the footprints is a large, shallow grazing or locomotion trace similar to Megaplanolites in some features but distinctive in various features and its occurrence in nonmarine facies, and it will be the subject of further study.

No Yangtze River Prior to the Late Miocene: Evidence From Detrital Muscovite and K‐Feldspar40Ar/39Ar Geochronology

AbstractThe Yangtze River is the largest river in Asia and its age and evolution has been the subject of debate for more than one century. Here, we applied a combination of detrital muscovite and K‐feldspar40Ar/39Ar dating on a set of samples from late Cenozoic Nanjing fluvial gravel sediments (overlain by basalt with ages of ∼22.9 and ∼10.3 Ma) distributed along the lower Yangtze River to trace the late Cenozoic Yangtze drainage. It is found that the characteristic Cenozoic muscovite and K‐feldspar age signals of the upper Yangtze are absent in the gravel sediments, indicating that the upper Yangtze did not supply sediment to the lower Yangtze before at least late Miocene time. These results do not support a prelate Miocene birth of the Yangtze River, but favor an alternative model highlighting later integration of the upper Yangtze into the mid‐lower reaches to form the current Yangtze drainage system.

Styles of mesoscale brittle deformations associated with the Miocene folding of the Taishu Group, Tsushima, between the Japan Sea and East China Sea backarc basins

AbstractThe Taishu Group is a folded, Eocene–Lower Miocene, thick sedimentary package exposed widely on Tsushima Island between the Japan Sea and East China Sea. This location makes the strata important to understand tectonics and paleo‐environments in the Far East, but the timing of the folding is controversial. We studied the styles of brittle deformations of the strata. It was found that flextural‐slip folds were dominant. Mesoscale faults were classified into two groups: NE–SW trending reverse faults and NW–SE trending strike‐slip faults. Members of both the groups showed movements largely perpendicular to the fold axes. The latter group consisted of sinistral and dextral faults. Accordingly, we interpreted that they were transfer faults activated during the folding. Consequently, mesoscale faults and flexural‐slip faults evidence the map‐scale plane strain of the Taishu Group in the plane perpendicular to the NE‐trending fold axes. There were few transpressional deformations in the group. This is inconsistent with the transpression hypothesis for explaining the simultaneous folding and Japan Sea opening. Another hypothesis in which the folds in Tsushima are regarded as an onshore part of the Taiwan‐Shinji fold belt is inconsistent with the timing of folding suggested by mining geologists to be consistent with and contemporaneous with this deformation. On the other hand, we found that dolerite dikes and sills were involved in the folding. Therefore, we conclude that the folding began during the late Early Miocene time and climaxed during the ore mineralization at around 15 Ma. We suggest that the folding in Tsushima was the easternmost manifestation of the compressional regime around the Yellow Sea and East China Sea in the Early to early Middle Miocene, and that the compression was brought about by the arrival of the Philippine Sea plate to initiate buoyant subduction under Kyushu.

Oligocene-Early Miocene exhumation and shortening along the Anninghe fault in the southeastern Tibetan Plateau: insights from zircon and apatite (U-Th)/He thermochronology

ABSTRACT The growth mechanisms of the southeastern Tibetan Plateau are debated between models of Oligocene shortening and lateral extrusion of coherent crustal blocks versus Late Miocene flow of low-viscosity lower crust. To test these models, we examine brittle deformation of the north-striking Anninghe fault, which has been acting as a left-lateral strike-slip fault since Late Miocene time, using structural and bedrock low-temperature thermochronologic methods. Different from previous interpretations of Late Miocene strike-slip motion, our structural observations indicate westward contractional dip-slip along the southern segment of the fault. New zircon and apatite (U-Th)/He thermochronologic data along a vertical profile of Precambrian volcanoclastic cover both sides of the Anninghe fault. The age-elevation relationships and inverse thermal modelling suggest a phase of Oligocene-Early Miocene (~24-18 Ma) differential rock exhumation and cooling across the fault, with a mean erosion rate of ~0.30 ± 0.12 km/m.y. and ~0.007 ± 0.007 km/m.y. in the hanging wall and footwall, respectively. Consistent with the structural observations, the higher rate of rock exhumation in the hanging wall indicates an Oligocene-Early Miocene phase of west-verging reverse faulting. Combined with previously published data from adjacent sites, we suggest that the Anninghe fault has experienced a tectonic transition from Oligocene-Early Miocene east-west contraction to Late Miocene strike-slip motion. The early to Mid-Cenozoic crustal shortening contributed to the uplift of the southeastern Tibetan Plateau, supporting the crustal shortening and extrusion model.

Late Miocene Deformation Kinematics Along the NW Zagros Fold‐Thrust Belt, Kurdistan Region of Iraq: Constraints From Apatite (U‐Th)/He Thermochronometry and Balanced Cross Sections

AbstractApatite (U‐Th)/He (AHe) thermochronometric results are integrated with geologic cross sections, structural relationships, and stratigraphic data to reconstruct the growth of the NW Zagros orogenic belt in the Kurdistan region of Iraq. Prolonged exhumation is documented across the belt with deformation advances and retreats from ~14 Ma onward. After in‐sequence propagation of deformation during middle to late Miocene times, preserved growth strata and AHe data show a deformation retreat by latest Miocene time (~5 Ma). In the NW Zagros, the Phanerozoic succession contains two principal décollements in Lower Triassic and middle Miocene units. The Triassic strata are interpreted as the main décollement for a thin‐skinned system that was dominant during most of the Cenozoic. By ~8–5 Ma, the fold‐thrust belt shifted to basement‐involved deformation in association with growth of the mountain front flexure and reactivation of frontal structures. The shift from thin‐skinned to a hybrid thin‐ and thick‐skinned mode of shortening may reflect variations in the mechanical behavior of the upper crust and the presence of inherited basement discontinuities. On the basis of two NE‐SW balanced cross sections spanning the NW Zagros, the estimated total minimum horizontal shortening is ~18.2 km (6%) in the central and ~16 km (7%) in the southern sectors of the Kurdistan region of Iraq. These findings suggest that the evolution of the NW Zagros orogenic belt was likely driven by the mechanical stratigraphy of the sedimentary cover, inherited basement discontinuities, and the dynamic and thermomechanical effects of potential slab breakoff and lithospheric mantle delamination events.

The myth of the Messinian Dardanelles: Late Miocene stratigraphy and palaeogeography of the ancient Aegean-Black Sea gateway

The Dardanelles region has formed a key gateway connecting the Eastern Paratethys and the Aegean/Mediterranean since the late Miocene. Its sedimentary sequences contain crucial information about connectivity and tectonics but so far lack unambiguous age constraints. Only a few Miocene marine episodes have been documented and fossil assemblages are predominantly composed of Paratethyan fauna (mollusks and ostracods). Here, we apply an integrated stratigraphic approach and use the recently established chronostratigraphy for the Eastern Paratethys to re-evaluate the faunal assemblages and palaeoenvironments of the Seddülbahir and İntepe sections that allegedly played a crucial role in the geodynamic evolution of the Dardanelles during the Messinian-Zanclean. The Paratethyan ostracods and mollusks, however, clearly indicate that these sections correspond to the middle Tortonian (~9 Ma; Bessarabian–Khersonian in Eastern Paratethys terminology). Nannofossil assemblages are dominated by a mixing of reworked taxa from the late Eocene and Oligocene and no age-diagnostic taxa have been observed. Dinoflagellate analyses are also hampered by reworking and mainly reveal non-marine (fresh to oligohaline) aquatic conditions. Fossil mammal remains in the Seddülbahir section and other localities in the region confirm the late Miocene age. Strontium (87Sr/86Sr) isotope ratios of the anomalohaline (brackish) ostracods are significantly below open ocean values and similar to values obtained from Khersonian ostracods of Bulgaria. Fresh water assemblages reveal much higher 87Sr/86Sr values, which are interpreted to reflect the composition of local rivers. We conclude that in late Miocene times the Dardanelles region was a fresh to anomalohaline embayment, ephemerally connected to the Eastern Paratethys. We found no evidence for a major Messinian erosional surface nor for marine Mediterranean fossils indicative of the early Zanclean. Our results furthermore indicate that the proposed Messinian age for the propagation of the North Anatolian Fault into the Dardanelles region must be revised.