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Tephra layers are a unique tool for stratigraphy. Their geologically instantaneous deposition across wide areas makes them a powerful instrument for dating and correlating distant localities. In this paper, we apply tephra fingerprinting for highresolution stratigraphic purposes in the upper Miocene portion of the Pisco Formation, which crops out along the southern Peruvian coast. The Pisco Formation diatomaceous strata host an important marine vertebrate Fossil-Lagerstätte, whose palaeontological relevance has entailed the necessity of reconstructing a detailed chronostratigraphic framework using 39Ar-40Ar dating and tephra correlations. Distal ashes from the Central Andes volcanoes are frequent in the Pisco Formation, but their similar glass composition and mineral assemblage, together with the shallow marine environment limiting tephra preservation, could be unpromising for tephra-based correlations. In this study, tephra layers from measured stratigraphic sections were fingerprinted with a combined petrographic and microanalytical approach, including glass shard morphology and granulometric analyses. Based on the obtained results, we correlated sections several kilometres apart, greatly increasing the chronostratigraphic detail. Major element composition of biotite proved a valuable tool to discriminate the Central Andes metaluminous to peraluminous tephra deposited in the Peruvian forearc basins. This study highlights the applicability of tephra fingerprinting in unfavourable shallow marine environments. [ABSTRACT FROM AUTHOR]

Cenozoic convergence between the Eurasian and African plates and concurrent slab roll-back processes have produced a progressive extension in back-arc areas, such as the Aegean region and western Anatolia. There is still a long-standing controversy as to whether this was a continuous or stepwise process. To shed light on this controversy and on the driving mechanism of regional extension, we present palaeomagnetic and geochronological results from the Söke Basin located at the southeastern rim of the İzmir-Balıkesir Transfer Zone. Our improved geochronology shows that volcanic activity in the region occurred between 11.66 and 12.85 Ma. Middle to late Miocene palaeomagnetic data for the Söke Basin show a c. 23° clockwise rotation, whereas early Miocene data show a c. 28° counterclockwise rotation. The primary nature of the magnetization is indicated by a positive tilt test. The resulting c. 51° counterclockwise rotations during the middle Miocene signify a major tectonic reorganization, during a period when an interruption of exhumation of metamorphic massifs has been reported. We suggest that the İzmir-Balıkesir Transfer Zone is the main driver of the reorganization in the region. The regional fingerprint of this tectonic reorganization coincides with the acceleration of trench retreat and illustrates the surface impact of tearing of the Hellenic slab. [ABSTRACT FROM AUTHOR]

Volcanism across the North Tanzanian Divergence Zone (NTD), part of the East African Rift System, occurred episodically from the late Miocene to Recent. Here, we present a summary of previously published K-Ar and 40Ar/39Ar ages, new 40Ar/39Ar ages, and geochemical and Sr-Nd-Pb isotopic analyses on samples collected from several volcanoes distributed across the NTD: Burko, Monduli, Tarosero, Ketumbeine, Gelai, Kerimasi and Meru. The locus of volcanism over this period progressed from the southwestern portion of the NTD to the north and east, with a main pulse occurring at about 2.3 Ma, possibly marking the inception of a main rifting event. We model the source of the NTD volcanic rocks as a metasomatized subcontinental lithospheric mantle that includes minor and variable amounts of garnet and amphibole. REE data indicate variations in residual garnet content, consistent with varying depth of melt separation. Radiogenic isotopic data show systematic variations requiring the involvement of up to three components. Two alternative but not exclusive tectonic scenarios are proposed: one requiring the involvement of contributions from recent plume-related fluids, and one explaining the observed geochemical variations by melting of a lithosphere layered by multiple metasomatic events. [ABSTRACT FROM AUTHOR]

The large palaeosinkholes located in the NW of Gozo (central Mediterranean Sea, Malta) offer excellent exposures that provide information on the geometry and kinematics of large karst-related collapse structures. Detailed geological analysis of these peculiar palaeosinkholes indicates that deep-seated evaporite dissolution is the most feasible hypothesis to explain their formation, according to the following evidence. (1) Several structures have been formed by progressive foundering of cylindrical blocks with limited internal deformation as revealed by the synsedimentary subsidence recorded by their Miocene sedimentary fill. This subsidence mechanism is more compatible with interstratal dissolution of evaporites than karstification and cave development in limestone formations. (2) The dimensions and deformation style of the palaeosinkholes are similar to those of other collapse structures related to deep-seated dissolution of salt-bearing evaporites. (3) The arcuate monocline associated with some of these collapse structures is also a characteristic feature of subsidence related to dissolution of evaporites. However, no major evaporite formations have been documented so far in the subsurface of the Malta Platform. [ABSTRACT FROM AUTHOR]

Surficial silicifications have been long considered to be indicative of warm and dry climates. Here we describe various forms of supergene silicification in a Miocene lacustrine sequence in the Missour Basin near Jbel Ghassoul (Morocco) in a landscape with accentuated relief. The silicification is almost exclusively limited to a 10-40 m wide zone from the edges of scarp and mesa landforms. This distribution is interpreted to record the locations where groundwaters which produced the silicification discharged from a higher level palaeolandscape. The main component of the silica was imported late and significantly post-dates the deposition of the sediments. This implies that significant volumes of silica-bearing solutions flowed through these formations in response to a hydraulic gradient generated by relief. Silicification thus occurred only after uplift and incision of the sedimentary fill of the Missour Basin. The zones of silicification of the Jbel Ghassoul sequence can be linked geomorphologically to remnants of high-level pediments that have been dated in the literature to the early to middle Pleistocene and interpreted to have been formed during cold climates. Low temperatures in outcrops near the discharge zones during cold periods are considered to be a key factor in silica precipitation from groundwaters. [ABSTRACT FROM AUTHOR]

The only place where Neogene-Quaternary rocks crop out for the entire Tuscan Archipelago in the Northern Tyrrhenian Sea is the island of Pianosa. In particular, the Miocene deposits record the depositional and tectonic evolution of the Northern Tyrrhenian region during this time period. These deposits are subdivided into two successions separated by a low-angle unconformity. The older, middle Burdigalian succession represents a calciturbidite shallow marine system, whereas the younger late Tortonian-early Messinian succession comprises a continental alluvial system that evolves upwards into a lagoonal-marginal marine environment. Here we present sedimentological, palaeontological and petrographical data that support a new stratigraphic and palaeogeographical framework for reconstructing the opening of the Northern Tyrrhenian back-arc basin. The early Miocene succession records a pre-rift marine depositional phase followed by a late Burdigalian-Langhian erosional phase. This was followed by a period of synrift continental-marginal deposition, as recorded by the late Miocene succession, terminated by an important phase of uplift, probably induced by the start of magmatic activity in the Tuscan Archipelago area. [ABSTRACT FROM AUTHOR]

The Messinian Laga basin is the largest foreland basin within the Central Apennines fold and thrust belt (Italy). This area, actively investigated in the 1980s and 1990s for hydrocarbon resources, is considered a valuable analogue for clastic reservoirs developed in confined structural settings. Furthermore, it represents a key area for understanding the evolution of the Apennines, as it links the internal, structurally uplifted Early Miocene fold and thrust belt of the western Central Apennines with the more external and recent belt to the east. Despite several papers published on this area, the only reconstruction of the substratum structure is an internal and classified industry report. During the present study, we had access to a seismic database comprising 200 km of seismic profiles that were collected between 1983 and 1990. These data allowed us to reconstruct the structural setting of the Laga basin substratum, define the lateral continuity of the main compressional structures within the basin, construct a balanced cross-section, and define the shortening values. [ABSTRACT FROM AUTHOR]

Sillimanite- and staurolite-grade metamorphic rocks exhumed along the Pangong fault, the NE branch of the right-lateral Karakoram strike-slip fault in northern Ladakh, NW India, show multiple episodes of metamorphism and fabric development. Debate has centred on whether these metamorphic rocks were formed as a result of shear heating during strike-slip faulting, or whether they are exhumed earlier metamorphic rocks unrelated to movement on the Karakoram fault. Here we constrain the burial and exhumation history of the Pangong Metamorphic Complex combining the pressure-temperature evolution with accessory phase geochronology. Sillimanite-grade metamorphism in graphitic pelites was superseded by the preserved P-T conditions of a Bt + Ms + St + Grt + Qtz + Fsp assemblage at 585-605 °C and 6.05-7.25 kbar, equivalent to c. 20-25 km of burial. Laser ablation monazite U-Pb geochronology reveals that sillimanite-grade metamorphism occurred at 108.0 ± 0.6 Ma in rocks immediately adjacent to the Pangong strand of the Karakoram fault, implying that most metamorphic rocks along the Karakoram fault were not formed by shear heating during Miocene strike-slip faulting. This age correlates closely with the ages of the Hunza granite-granodiorite batholith, and the K2 orthogneiss in northern Pakistan, and confirms that some high-grade metamorphism occurred before collision and accretion of the Kohistan arc and the Indian plate to Asia; protracted high-grade metamorphism, and accompanying crustal thickening lasted at least 100 Ma along the South Asian plate margin. Our P-T and geochronology results also demonstrate the continuity of Cretaceous metamorphism across the Karakoram fault. [ABSTRACT FROM AUTHOR]

Throughout the Himalaya, the Tethyan sedimentary sequence forms the detached carapace, or superstructure, to exhumed mid-crustal rocks. In central Nepal, low metamorphic-grade Cambrian to Jurassic rocks of this carapace preserve five phases of deformation, structurally dominated by second phase (D2) north-verging back folds that oppose the vergence of the orogen. The folds are overprinted and cross-cut by the c. 22 Ma South Tibetan detachment system and the Manaslu leucogranite. New structural mapping in Hidden valley provides further constraints on the character of D2: megascopically, large asymmetric north-verging folds define D2; microscopically, it is characterized by an axial-planar cleavage. Balanced cross-sections and bed-length restoration of F2 folds indicate a minimum of 32-38% shortening and 180% thickening during D2. These data indicate that north-verging folds played a significant role in pre-Miocene crustal thickening of the Himalayan superstructure. The formation of these folds is compatible with wedge extrusion or channel flow of the underlying mid-crustal rocks, whereby their geometry reflects early coupling between the upper and middle crust. [ABSTRACT FROM AUTHOR]

An integrated analysis of facies and palaeontological content of the middle to late Miocene (c. 12.4-9.1 Ma) Angostura Formation led to the identification of facies associations indicative of shoreface and inner-shelf settings. The Angostura Formation comprises eight high-frequency sequences that are stacked to form a tectonically driven lowstand sequence set. The most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision: (1) a basal suite of deepening-upward shoreface sediments (transgressive systems tract), including a base-of-cycle shell concentration; (2) a mid-cycle shell bed, consisting of molluscan shells dispersed in a matrix of inner-shelf muddy fine-grained sandstones; (3) a shallowing-upward unit of inner-shelf to lower shoreface sediments almost barren of mollusc fossils (highstand systems tract). Biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late Miocene high-frequency glacio-eustatic changes derived from recent δ18O studies. This correlation has far-reaching implications and leads to the following conclusions: (1) glacio-eustasy in tune with oxygen isotope changes at fourth-order frequency (200 ka-1 Ma duration) may have been the principal factor regulating stratigraphic packaging in the Angostura Formation; (2) these sequences provide an excellent shallow-marine outcrop record of late middle to late Miocene Antarctic glaciations. [ABSTRACT FROM AUTHOR]

New K-Ar and 40Ar/39Ar data obtained for 29 samples of supergene alunite-, jarosite- and hollandite-group minerals from the Central Depression of Northern Chile document almost continuous supergene oxidation from 33 to 9 Ma. The downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m Ma-1. These calculated rates are sensitive to pre-existing topography and show that the present relief observed in the Central Depression represents a modified Miocene landscape. The results presented here are in good agreement with previously published geochronological data for the Atacama Desert. The combination of all published data, including the results presented here, defines a continuous skewed distribution, with a maximum between 21 and 14 Ma. The relative paucity of ages greater than 30 Ma reflects the removal of supergene profiles by erosion during and immediately after the Incaic compressive deformation. From 29 to 9 Ma, supergene oxidation profiles were generated under semi-arid conditions throughout the Atacama region. Ages between 9 and 5 Ma on the northern and southern margins of the desert mark the end of supergene oxidation processes, consistent with a Pliocene onset of present-day hyperaridity. [ABSTRACT FROM AUTHOR]

Focuses on the Cretan detachment and the subjacent subduction thrust that bounded an extruding wedge above the Miocene Hellenic subduction zone. Sketch of an extrusion wedge in a subduction setting; Discussion of the geometric reconstruction for the Early Miocene Cretan detachment; Presentation of a tectonic map of the Aegean.

Comments on the interpretation of G. Seyitoglu et al concerning the broad folds exposed in the Miocene sediments in western Turkey. Tectonic significance of the Sart Formation; Relationships between the normal faults; Description of the rollover anticline.

Focuses on a Miocene collisional belt in north Borneo. Principles of whole-crust gravity modelling; Continental crust beneath Sabah, Malaysia; Densities of selected Sabah rocks.

Presents information on a study which investigated the Miocene collisional mountain belt of the Western Cordillera of north Borneo. Geomorphology of the uplift history of Sabah; Apatite and zircon fission-track dating; Comparison of fision-track thermochronology studies with other collisional belts.

Provides information on a study which examined the stratabound sulphur deposits of northern Iraq which occurred in folded Middle Miocene evaporites overlying Lower Miocene hydrocarbon reservoirs. Regional geology of Iraq; Methodology of the study; Discussion on the events involved in sulphur formation.

Investigates the possibility that the planktonic foraminifera Globegerinoides trilobus and Orbulina universa share a common ancestor in the early Miocene as indicated by well-preserved fossil material from Limalok Guyot in the tropical Pacific. Increase in the morphological variance of the ancestral lineage; Habitat partitioning.

Discusses evidence for a short phase of north-south compression during the late Miocene to early Pliocene in western Turkey. Arguments about the published work on the Selendi and Usak-Gure basins; Stratigraphy of the basins.

The Mt Carmel structure is one of the most prominent topographical elements along the Levant continental margin and is the surface expression of a deep tectonic structure. The use of 3D seismic data allows us to set new constraints on the uplift of the Carmel structure and define the Or Akiva fault as its southern boundary. The Or Akiva fault was created during a Late Miocene extensional phase that had also created the Carmel fault at the northern edges of Mt Carmel. The different topographic expression across these two boundary faults reflects a change in the post-Miocene activity, where activity along the Or Akiva fault ceased and its southern side began subsiding, whereas its northern side bordered by the Carmel fault continued to rise. We conclude that the Late Miocene north-south extension, previously recognized only along branches of the Dead Sea Transform in northern Israel and Lebanon, affected larger areas than previously thought and can be related to regional kinematic changes during the breakup of Arabia from Africa and its collision with Eurasia. [ABSTRACT FROM AUTHOR]

A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7 Ma ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung-Tsangpo suture zone in southern Tibet. Garnet-clinopyroxene-plagioclase-quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130-1330 °C and pressures between 22 and 26 kbar indicating equilibration in the high-pressure and ultrahigh-temperature granulite field and defining a geotherm of c. 16 °C km-1. Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920-1130 °C and 17-24 kbar, whereas felsic granulites equilibrated at 870-900 °C at an inferred pressure of 17 kbar. In situ U-(Th)-Pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206Pb/238U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean 208Pb/232Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of c. 80 km by 17-14 Ma at the latest and has remained unchanged until the present day. [ABSTRACT FROM AUTHOR]

Research from biological and geological sources has highlighted the role of volcanoes in the outgassing of P, and thermal fixation and subsequent atmospheric oxidation of NOx in volcanic environments. The impact of these nutrient fluxes on biological systems has been demonstrated on present-day Hawai'i, and here we consider the impact on the plant communities within a large igneous province (LIP). The Miocene Columbia River Basalt Province, the youngest LIP on Earth, contains many sedimentary interbeds between the flows of the major extrusive phase, and these interbeds preserve variable but often diverse palynofloras. By integration of palynofloral analysis with analysis of macronutrient levels in the interbeds it can be suggested that there may have been significantly elevated levels of P within the lava field proximal to the source vents, a distribution mirrored by Ca and Mg. Evidence for potential volcanogenic eutrophication is restricted to >10³ year duration interbeds, contemporary with eruptive activity elsewhere in the LIP. The geochemistry and palynology of other interbeds demonstrate nutrient deficiency, with a potential route to nutrient sufficiency available from long-term symbiotic N-fixation. Elsewhere within the Columbia River Basalt Province, this process is short-circuited by the imput of felsic ash from the nearby Cascades Range of volcanoes. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) seismic reflection data reveal two silica diagenetic boundaries located in the North Sakhalin Basin, offshore Sakhalin Island. These boundaries form distinctive high-amplitude seismic reflections that predominantly cross-cut Miocene stratigraphy. The boundaries represent the opal-A to opal-CT (cristobalite and tridymite) transition and the subsequent deeper opal-CT to quartz transition and cover an area of c. 107 000 km². The reflections represent diagenetic reaction fronts that advance through silica-rich host sediments during burial. The oldest strata within which the diagenetic fronts have been identified are of Miocene age and the fronts commonly show a parallel relationship to a Late Miocene unconformity. These diagenetic fronts develop a variety of morphological forms. In the North Sakhalin Basin these front geometries are probably the result of the interference of the diagenetic fronts with inclined and folded stratigraphy together with lateral changes in the composition and, therefore, depth of transformation of the sediment. We define criteria for the recognition of these 2D cross-sectional morphological elements and by so doing provide the basis for the objective description and categorization of diagenetic front geometry. [ABSTRACT FROM AUTHOR]

In northern Calabria (Italy), the metasedimentary succession of the Lungro Verbicaro tectonic unit preserves mineral assemblages suggesting underthrusting to depths in excess of 40 km. Internal deformation of these rocks occurred continuously during the following decompression. Index mineral composition associated with progressively younger tectonic fabrics indicates that a substantial part of the structural evolution took place within the blueschist-facies P T field. Despite their tectonic and metamorphic history, the rocks of the Lungro Verbicaro Unit preserve significant sedimentary and palaeontological features allowing correlations with successions included in adjacent thrust sheets and the reconstruction of the Mesozoic continental margin architecture. The subduction exhumation cycle recorded by the Lungro Verbicaro Unit is entirely of Miocene age. This portion of the Apulia continental palaeomargin was involved in convergence-related deformation not earlier than the Aquitanian. The integration of our results with available constraints on the tectonic evolution of the Apennine Calabrian Arc system suggests that subduction and most of the subsequent exhumation of the Lungro Verbicaro Unit occurred, up to Langhian time, at maximum vertical rates in excess of 15 mm a ¹. The exhumation process was then completed, at much slower rates (<2 mm a ¹) in Late Miocene time, as indicated by both apatite fission-track data and stratigraphic information. [ABSTRACT FROM AUTHOR]