Early Miocene Period Research Articles

Genetic resources in Trifolium and their utilization in plant breeding

Clovers (Trifoliumspecies) are a large and widespread genus of legumes. Recent work supports the Mediterranean origin of the genus in the Early Miocene period, and centres of diversity for clovers occur in the Eastern Mediterranean, East Africa and South America. A number of clovers are of agricultural importance as forage species in grassland agriculture around the world, particularly in temperate areas. White clover (Trifolium repensL.) is the most important legume of grazed pastures, and red clover (T. pratense) is widely cut and conserved as a winter feed. We consider the current state of collected resources in clovers, which have been collated in the most important databases. These are European Internet Search Catalogue (EURISCO), germplasm resources information network and system-wide information network for genetic resources together with the collection held at AgResearch, New Zealand. In total, collections have been made of 204 species with 48 species having more than 100 accessions in these databases. As expected, the majority of accessions are of the agriculturally important species. The geographical origin of collected material is outlined and, for the major species, accessions are broken down according to their status, e.g. wild population, breeders' lines. We then describe some of the ways in which genetic resources of white and red clovers and their relatives have been used in the breeding of these species. These include introgression of stress tolerance traits, targetting improvements in resource use efficiency and increasing seed yield.

Orientations of palaeotectonic features as a key to understanding the neotectonic block rotation of the Kocaeli peninsula, NW Turkey

Due to northward subduction of Neotethys, the İstanbul zone collided with the Sakarya zone in northwest Turkey during the early Eocene. Subsequently, this region was subjected to compressional forces during the late Eocene–early Miocene period. Folds, thrusts and reverse faults developed approximately parallel to long axes of the İstanbul zone. NNW–SSE oriented conjugate strike‐slip faults developed with continued contraction. In addition to the orientations of palaeotectonic features, the morphotectonic, stratigraphic and seismic characteristics expose differences between the northeastern Marmara peneplain and the southern Black Sea highland. This study reports causes of this diversity reflecting the neotectonic evolution of the İstanbul zone. The diversity is related to the clockwise rotation of the Kocaeli peninsula between two dextral zone‐bounding faults and two sinistral block‐bounding faults. The principle factors of this process were the development of the North Anatolian fault zone (NAFZ) and the related evolution of the Adapazarı–Karasu fault zone (AKFZ), the Bosphorus fault zone (BFZ) and the Northern Boundary fault (NBF).

Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin

Existing evidence shows that an Oligocene erosion event occurred on the northern continental margin of the South China Sea, and the Tainan Basin area might be at the center of this event, followed by a rapid tectonic subsidence in the late Oligocene and early Miocene period. The rapid tectonic subsidence is mainly thermal‐controlled, and the effect of the Yichu Fault on the Tainan Basin is limited to the basin's eastern part. We develop a 2‐D thermal‐mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the Tainan Basin. Our modeling indicates that the Oligocene uplift, erosion, and subsequent rapid subsidence could be caused by a thermal event, and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults. However, it also suggests that other mechanisms might be needed to jointly account for the observed erosions.

Mangrove (<i>Kandelia</i> sp.) pollen in Middle Miocene sediments from the Oguni area, Yamagata Prefecture, Northeast Japan

Molluscan and pollen fossils in the mangrove swamp are known from the upper Lower-lower Middle Miocene sediments in Southwest Japan. Pollen analysis was performed for the purpose of finding a fossil mangroves in Northeast Japan. Sixteen samples were collected from two sites of the Middle Miocene Myozawa- bashi Formation in the Oguni area, Yamagata Prefecture, Northeast Japan. As a result, evergreen Quercus occurred dominantly in the pollen composition of both sites, which is characteristic for the NP-2 zone of the Neogene pollen zones. In particular, mangrove pollen (Kandelia sp.) fossils were found in an oyster fossil bed. This is the northernmost record of fossil mangroves in Japan. The living mangrove Kandelia candel (L.) has a northern limit at South Kyushu (Kiire, Kagoshima Prefecture), and so the average winter (January) temperature of Oguni town in the early Middle Miocene period is estimated to have been at least 9°C higher than today. Since a mangrove swamp-dwelling bivalve Geloina was recently found in northern Iwate Prefecture, the mangrove pollen may be found in more northern places than the Oguni area.

Initial uplift of the Tibetan Plateau and environmental implications

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.142

Phylogenetic relationships and divergence times among dormice (Rodentia, Gliridae) based on three nuclear genes

We examined phylogenetic relationships among six species representing three subfamilies, Glirinae, Graphiurinae and Leithiinae with sequences from three nuclear protein‐coding genes (apolipoprotein B, APOB; interphotoreceptor retinoid‐binding protein, IRBP; recombination‐activating gene 1, RAG1). Phylogenetic trees reconstructed from maximum‐parsimony (MP), maximum‐likelihood (ML) and Bayesian‐inference (BI) analyses showed the monophyly of Glirinae (GlisandGlirulus) and Leithiinae (Dryomys,EliomysandMuscardinus) with strong support, although the branch length maintaining this relationship was very short, implying rapid diversification among the three subfamilies. Divergence time estimates were calculated from ML (local clock model) and Bayesian‐dating method using a calibration point of 25 Myr (million years) ago for the divergence betweenGlisandGlirulus, and 55 Myr ago for the split between lineages of Gliridae and Sciuridae on the basis of fossil records. The results showed that each lineage ofGraphiurus,Glis,GlirulusandMuscardinusdates from the Late Oligocene to the Early Miocene period, which is mostly in agreement with fossil records. Taking into account that warm climate harbouring a glirid‐favoured forest dominated from Europe to Asia during this period, it is considered that this warm environment triggered the prosperity of the glirid species through the rapid diversification.Glirulus japonicusis suggested to be a relict of this ancient diversification during the warm period.

Oligocene to Recent tectonic history of the Central Solomon intra-arc basin as determined from marine seismic reflection data and compilation of onland geology

Systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the Solomon Islands constrains the late Tertiary sedimentary and tectonic history of the Solomon Island arc and its convergent interaction with the Cretaceous Ontong Java oceanic plateau (OJP). The OJP, the largest oceanic plateau on Earth, subducted beneath the northern edge of the Solomon arc in the late Neogene, but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained. The Central Solomon intra-arc basin (CSB), which developed in Oligocene to Recent time above the Solomon arc, provides a valuable record of the tectonic environment prior to and accompanying the OJP convergent event and the subsequent arc polarity reversal. Recognition of regionally extensive stratigraphic sequences—whose ages can be inferred from marine sedimentary sections exposed onland in the Solomon Islands—indicate four distinct tectonic phases affecting the Solomon Island arc. Phase 1: Late Oligocene–Late Miocene rifting of the northeast-facing Solomon Island arc produced basal, normal-fault-controlled, asymmetrical sequences of the CSB; the proto-North Solomon trench was probably much closer to the CSB and is inferred to coincide with the trace of the present-day Kia-Kaipito-Korigole (KKK) fault zone; this protracted period of intra-arc extension shows no evidence for interruption by an early Miocene period of convergent “soft docking” of the Ontong Java Plateau as proposed by previous workers. Phase 2: Late Miocene–Pliocene oblique convergence of the Ontong Java Plateau at the proto-North Solomon trench (KKK fault zone) and folding of the CSB and formation of the Malaita accretionary prism (MAP); the highly oblique and diachronous convergence between the Ontong Java plateau and the Solomon arc terminates intra-arc extension first in the southeast (Russell subbasin of the CSB) during the Late Miocene and later during the Pliocene in the northwest (Shortland subbasin of the CSB); folds in the CSB form by inversion of normal faults formed during Phase 1; Phinney et al. [Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)] show a coeval pattern of southeast to northwest younging in folding and faulting of the MAP. Phase 3: Late Pliocene–early Pleistocene arc polarity reversal and subduction initiation at the San Cristobal trench. Effects of this event in the CSB include the formation of a chain of volcanoes above the subducting Australia plate at the San Cristobal trench, the formation of the broad synclinal structure of the CSB with evidence for truncation at the uplifted flanks, and widespread occurrence of slides and “seismites” (deposits formed by seismic shaking). Phase 4: Pleistocene to Recent continued shortening and synclinal subsidence of the CSB. Continued Australia-Pacific oblique plate convergence has led to deepening of the submarine, elongate basin axis of the synclinal CSB and uplift of the dual chain of the islands on its flanks.

MAGNETIC FABRICS OF THE TERTIARY VRONDOU PLUTONIC COMPLEX, NORTHERN GREECE.

The emplacement and tectonic evolution of the Tertiary Vrondou granitoid in the Greek Rhodope has been studied by a combination of field tectonic, microtextural and anisotropy of magnetic susceptibility (AMS) methods. The Vrondou pluton is composed of at least two intrusions, a Mid-Oligocene one at its eastern and an Early Miocene at its western parts. Room for the emplacement in both cases was provided by the opening of an extensional ramp space within a several km thick, mid-crustal subhorizontal shear zone, with a top to the SW sense, active during the Mid-Oligocene to Early Miocene period. The older eastern part of the pluton is much less deformed than the younger western part. Magmatic textures are well preserved in the older intrusion whereas they are scarcely present in the younger one. They indicate a NW-SE direction of magmatic flow in both cases. Sub-solidus plastic deformation affected mostly the younger western part of the pluton and only locally the older eastern one. In both cases mineral lineations trending NE-SW were developed. An increase in the shear rate by the Early Miocene or a localization of shear mainly in the west, in combination with the more silica saturated rock types present there, are probably the reasons for the different deformational behaviour of the two parts of the pluton.

Age and style of deformation and stratal thinning at the transition, Wasatch Plateau to Great Basin, central Utah

The Salina area of central Utah contains an unusually complete Tertiary stratigraphic record, affording unique opportunities to study details of the Tertiary geologic history. Stratigraphic and structural studies reveal Neogene ages for tectonically significant parts of all major structures such as the 80-km-long Sanpete-Sevier Valley anticline (SSVA) and its flanking structures, the Wasatch monocline and Sevier Valley syncline. Newly identified critical components of the Neogene history in the SSVA include: (1) a protracted Eocene through early Miocene period of quasi-continuous sedimentation and relative structural quiescence that buried an early phase of the anticline; (2) east-trending folds in rocks of the anticlinal core; (3) widespread plutons in the southern part of the core; (4) strike-slip faults; (5) major thinning of strata (vertical structural convergence) between uplifted Jurassic rocks of the anticlinal core and downdropped superjacent Tertiary rocks; and (6) a widespread zone of altered rock at the contact between core rocks and overlying Tertiary rocks. We interpret the thinning and widespread alteration (5 and 6) to be linked through protracted processes of extensional faulting (possibly including detachment faulting), fluid flow, massive dissolution, and dissolution collapse at the contact between the Jurassic core rocks and overlying Tertiary rocks. We suggest that the SSVA, one of several diapir-like elongate uplifts in central Utah, is somewhat analogous to flank ridges that develop at landslide margins by outward and upward flow of mechanically weak clay-rich rock or gouge. By analogy, the SSVA results from outward and upward movement of the relatively incompetent Arapien Shale from beneath the adjacent Sevier Valley. If the structures formed during these newly recognized components of Neogene history are palinspastically restored, much of the evidence for contractional deformation at shallow structural levels is eliminated; this affects resource assessments based on models of that deformation. Also, the association of extensional faulting with non-tectonic dissolution collapse and with lateral motion of mechanically weak rocks implies shallow penetration and non-seismogenic to weak seismogenic behavior of some normal fault systems in central Utah.

Geology of the northern side of the Gulf of Edremit and its tectonic significance for the development of the Aegean grabens

This paper describes the Neogene evolution of northwestern Anatolia based on geological data collected in the course of a new mapping program. The geological history of the region, as recorded by the Neogene sedimentary and magmatic rocks that overlie the Paleozoic–Triassic basement, began after a lake invasion during the Early Miocene period with the deposition of shale-dominated successions. They were accompanied by calc-alkaline intermediate lavas and pyroclastic rocks ejected through NNE trending fractures and faults. The Lower–Middle Miocene successions were deformed under a compressional regime at the end of the Middle Miocene. The deposition of the overlying Upper Miocene–Lower Pliocene successions was restricted to within NE–SW trending graben basins. The graben bounding faults are oblique with a major strike-slip displacement, formed under approximately the N–S extension. The morphological irregularities formed during the Miocene graben formations were obliterated during a severe erosional phase to the end of the deposition of this lacustrine succession. The present E–W graben system as exemplified from the well-developed Edremit graben, postdates the erosional phase, which has formed during the Plio-Quaternary period.

The Kozak Pluton and its emplacement

The Kozak pluton is located in central western Anatolia, where it crops out in the core of an antiformal culmination. The pluton was emplaced into regionally deformed and metamorphosed basement rocks, during the Late Oligocene–Early Miocene period. The pluton consists mainly of a homogeneous granodiorite with fine-grained margins. A suite of hypabyssal intrusives of the same composition formed a fringe around the pluton, during the late phases of emplacement. The hypabyssal rocks are intimately mixed with cogenetic volcanic rocks of Early Miocene age. Field evidence suggests that the pluton is a very high level intrusion, although it has features typical of more deep-seated granites. The pluton exhibits different effects on the host rocks along different contacts, which suggests that during its long history of ascent a variety of emplacement mechanisms operated at different depths. These emplacement mechanisms range from diapiric intrusion at deeper levels through cauldron subsidence to passive emplacement. During the latter phase, faults and fractures allowed the pluton to be finally emplaced at sub-volcanic levels. Copyright © 1999 John Wiley & Sons, Ltd.

Thermal and mechanical models for the structural and metamorphic evolution of the Zanskar High Himalaya

Abstract The regional Barrovian facies metamorphic rocks of the High Himalayan Slab in Zanskar are bounded along the base by the southwest vergent Main Central Thrust (MCT) with its characteristic zone of inverted isograds, and along the top of the slab by the northeast dipping Zanskar Shear Zone (ZSZ), part of the South Tibetan Detachment (STD) System of normal faults. Summarized here are the results of systematic mapping combined with detailed P-T-t data from the Zanskar Himalaya, and models for the thermal and mechanical evolution of the middle and deep crustal rocks of the Himalaya are discussed. Temperatures and pressures increase dramatically up-structural section across the MCT Zone from biotite through garnet, staurolite and kyanite grade to sillimanite + muscovite, and decrease along the top of the stab beneath the ZSZ normal faults. In Zanskar, peak P-T conditions of the M1 kyanite-grade rocks are 550–680°C and 9.5–10.5kbar, and M2 sillimanite-grade gneisses were formed at 650–770°C and 4.5–7 kbar. The core of the High Himalayan Zone is a 30 km wide zone of sillimanite + K-feldspar-grade gneisses, migmatites and anatectic leucogranites with a right way-up isograd sequence above and an inverted sequence below. Thermal models of thrusting a hot slab over a cold slab and frictional heating along the MCT are not supported by the structural and thermobarometric data. Mechanical models of post-metamorphic structural disruption by folding and thrusting of a pre-existing, right way-up metamorphic sequence are compatible with structural and P-T data. Ductile shearing along the MCT Zone has structurally condensed the isograds, and final motion along the ZSZ postdated leucogranite crystallization (21.5-19.5 Ma; U-Pb monazite ages) and emplacement in the footwall. Crustal shortening and thickening, resulting in prograde metamorphism, lasted from the time of India-Asia collision at 54-50Ma until at least 30-25 Ma. Crustal melting is constrained at 20.8-19.5 Ma along the core of the stab. Late stage out-of-sequence thrusting within the High Himalayan Slab effectively maintained the crustal thickening process enabling P-T conditions to remain high for 10-5 Ma. Rapid exhumation between 21-18.5 Ma was accompanied by removal of 18–25 km of overburden by erosion at exhumation rates of 6–10 mm yr −1 . This early Miocene period of high exhumation rates, rapid erosion and exhumation of rocks buried at τ25 km depth is interpreted to indicate the uplift of high mountains and erosion of deep valleys, probably with high precipitation during that time.

Evolution of the Bayramiç magmatic complex, northwestern Anatolia

During the Oligocene–Middle Miocene period widespread magmatic activity developed in Western Anatolia, following the continental collision between the Sakarya continent and the Tauride–Anatolide platform. This produced both intrusive and extrusive rocks, which appear to be associated in space and time, as exemplified from the Bayramiç area. In the Bayramiç area, the magmatic activity started with the intrusion of the Evciler granite, and the coeval lower volcanic association. This was followed by the development of the upper volcanic association. These rock groups form collectively the Bayramiç magmatic complex, which was generated under an on-going north–south compressional regime. The Bayramiç magmatic complex has a subalkaline composition, displaying a calcalkaline trend. Trace elements and REE contents resemble to island-arc and collision-related magmas. According to the isotope values the Bayramiç magmatic complex was derived from the magmas of lithospheric mantle origin, which were later contaminated, while passing through the thick continental crust, in a post-collisional tectonic setting, during the Oligocene–Early Miocene period. The latest product of the magmatism is the Late Miocene–Pliocene basalt lavas. Their geochemical properties are clearly different from the Oligocene–Early Miocene magmatic rocks. The basalts were generated when the north–south compression gave way to the north–south extensional regime.

Geology and Evaluation of the Nap Region (the Metamorphic Massifs) of the Southeast Anatolian Orpgenic Belt

One of the important tectonic units of the Southeast anatolian orogenic belt is the nappes. They form of a nappe package consistin of the metamorphic massifs at the top, and ophiolitic assciations lying below the massifs. the metamorphic massifs are composed of a high grade metamorphic core and a lower grade matemorphic envelope. The carbonate rocks dominate the envelope in which a succession varying from Permo-Carbonifereous to Upper Cretaceous have been identified. The metamorphic massifs which underwnt metamorphizm during the late Crateceous belonged orijinally to the Taurus tectonic belt. Following the matemorphism they begun to move southward as a nappe. Initially they were emplaced on to the ophiolites and, collectively formed a nappe package. During their transport, the nappes were internally imbricated, fagmented and then were overlain by different cover successions during the early Eocene-early Miocene period. During this soutward transport, initially an oceanic environment were totally destroyed by the middle Eocene, Later the remnant seas were filled by the late early Miocene units. At the latest stage the nappes were emplaced on the Arabian platform in the early-middle Miocene.

Pillow metabasalts in a mid-Tertiary extensional basin adjacent to the Liquiñe-Ofqui fault zone: the Isla Magdalena area, Aysén, Chile

Pillow metabasalts and interbedded slates adjacent to the Liquiñe-Ofqui fault zone (LOFZ) of the southern Chilean Andes have been studied using electron-microprobe mineral analysis, major and trace element whole-rock geochemistry and RbSr geochronology. The rocks show previously unrecognized mineralogical evidence of two metamorphic events in a low-pressure environment: an early greenschist-facies metamorphism, possibly during emplacement on the sea-floor, and a patchy amphibolite-facies overprint, which could represent either continuing sea-floor metamorphism or contact metamorphism associated with Miocene plutons. The meta-igneous rocks are considered to have formed from mid-Tertiary basaltic magmas with mixed within-plate/volcanic arc characteristics as seen in present-day volcanoes in the area, but enriched in immobile elements such as Ti, Zr, And Y. They were probably emplaced between 45 and 20 Ma, into a contemporaneous marine pull-apart or extensional basin with thin continental or oceanic floor. Many of the petrological and geochemical features of an ophiolite suite may be recognized in the immediate area.A chain of such basins developed immediately west of the LOFZ strike slip system, probably during an Eocene- Early Miocene period of oblique subduction-convergence in southern Chile, and closed when the approach angle became more orthogonal 25 Ma. Magmatic arc plutonism was resumed at about 20 Ma and continued into the latest Cenozoic.

Paleomagnetic studies combined with fission-track datings on the western arc of Sulawesi, east Indonesia

Paleomagnetic studies in conjunction with fission-track dating on the western arc of Sulawesi yield important evidence bearing on the tectonic history of the area. During the Paleogene to Early Miocene time interval the paleomagnetic pole for southwestern Sulawesi was situated at 36.5°E 44.8°N. This pole position is significantly different from that in the time interval Middle Miocene to Recent, which is consistent with the north pole of the axial geocentric dipole. This fact suggests that subsequent to the Paleogene to Early Miocene period, possibly 19–13 m.y. B.P., a major tectonic event occurred which caused about 40 degrees of anticlockwise rotation of the area. It is suggested by the present work that the postulated collision followed by welding of eastern Sulawesi with western Sulawesi during the Pliocene (Katili, 1978) may be the tectonic event mentioned above. In addition, our data does not support the hypothesis that western Sulawesi has been derived from the dispersal of Gondwanaland.

The Equatorial Mid-Ocean Canyon: A relict deep-sea channel on the Brazilian continental margin

The Equatorial Mid-Ocean Canyon is an erosional-depositional deep-sea channel which was formed by turbidity-current activity, was active during the Miocene, but is now a relict feature. The canyon parallels the Brazilian continental rise between 3°–5°S and 33°15′ to 28°W and descends eastward for at least 1,200 km with a gradient of 1:1,000. The canyon is 5 to 8 km wide and as much as 200 m deep. Although the bathymetric expression of the canyon on the sea floor begins at about 33°15′W, the canyon has been traced westward (up-slope) from this longitude in subsurface for at least 75 to 150 km. The trend of this buried portion of the canyon indicates that originally the head of the canyon was on the upper continental rise. The stratigraphic relationship between the canyon and a lower Miocene acoustic reflector of widespread regional extent suggests that the canyon formed during the late early Miocene period. Piston cores from the canyon floor, walls, and levees consist largely of pelagic foraminiferal marls and brown clays which indicate that the canyon has been inactive for at least the past million years. The exact time when the canyon became inactive is still uncertain, but it was probably middle to late Miocene. When active, the canyon channeled terrigenous sediments southeastward along the trend of the Fernando de Noronha Basin. The canyon apparently was abandoned and subsequently buried when the predominant direction of sediment dispersal shifted from southeastward along the Fernando de Noronha Basin to northeastward into the adjacent Guiana Basin, thus making the canyon path obsolete.

Hominoid tali from East Africa.

LEAKEY recently drew attention to the doubtful taxono-mic status of certain post-cranial bones known from two sites in Kenya, Songhor and Rusinga1. These sites have yielded material from the early Miocene period which belongs to primates of the genus Proconsul (= Dryopithecus (Proconsul)) and to a taxon previously known as “Siva-pithecus africanus”; the latter material has been transferred by Leakey to Kenyapithecus, a new genus created by him within the family Hominidae. This step has not gained universal acceptance and has been rejected by Pilbeam on the grounds that the material is inadequate for specific diagnosis, and he suggests that this material should be placed within the genus Dryopithecus2.

On the Geological and Physical Development of Guadeloupe

I. Introduction My visit to Guadeloupe in March, 1897, was for the purpose of extending my observations of the later geological features of the Antilles southward from Antigua; especially those bearing upon the changes of level of land and sea since the early Miocene Period, and their necessary variation in adjacent localities. The features of the Guadeloupe archipelago present some striking contrasts to those of the Antigua-Barbuda plateau. The main island is divided into Guadeloupe and Grande Terre. The former is traversed by high mountains, surmounted by recent volcanoes, which are unrepresented in Antigua; but all these features are reproduced in Dominica and other islands of the western belt of the Lesser Antilles. Grande Terre is a limestone-country comparable with Antigua, and, therefore, forms the principal subject of this paper. The islands of Désirade, Petite Terre, Marie Galante, and The Saints, belong to the same physical unit as Guadeloupe—the three former having characteristics similar to those of Grande Terre, and the last, a group of seven islands, are remnants of the older volcanic rocks. This archipelago is dissected to much greater depths than the Antigua-Barbuda plateau, and shows modifications in the erosion-features, helping us to further understand the history of the Antillean plateau, most of which is now submerged. The earliest geological contributions appear to have been those of William Maclure, shortly afterwards followed by the writings of Alexandre Moreau de Jonnès. Maclure's paper, along with the studies of Pierre Duchassaing, constitutes our principal knowledge of the Tertiary formations of