Middle to late Miocene sequence stratigraphy of the Transylvanian Basin (Romania)

The history of Middle to Late Miocene evolution of the Transylvanian Basin was determined by the bordering Carpathian orogen evolution, the tectonic events being well recorded by the sedimentary history. The basin evolved in a back-arc setting, under a regional, compressional stress field. The major tectonic events produced during the Late Sarmatian and Post-Pannonian were related to the reactivation of the pre-Badenian fault systems. The Transylvanian Basin got uplifted after the Late Pannonian (? during the Pliocene), and at least 500 m of sedimentary cover was eroded.

Based on seismic and well-log interpretation, core and outcrop sedimentology, and microfauna, eight sequences were defined. The early Middle Miocene sequences are roughly synchronous to five 3rd order global sea-level cycles. Most of the recognized sequence boundaries are enhanced by regional tectonic events. The sedimentary evolution was also strongly influenced by salt-tectonics, active starting with the Late Sarmatian.

Two sequences were identified in the Lower Badenian deposits. The third sequence (late Early Badenian to early Mid Badenian) preserves information about deeper shelf settings. The lowstand of the following sequence was responsible for the deposition of the salt formation (late Mid Badenian), an important lithostratigraphic marker in the sedimentary record of the basin. In general, the Upper Badenian deposits (parts of the 4th and 5th sequences) belong to deep marine submarine fan systems. The Sarmatian (partially 5th, 6th and partially 7th sequences) was characterized by diverse salinity conditions, stretching from brackish to hypersaline, and by high tectonic instability, which induced several significant relative sea-level falls. During that time, deltaic (north) and fandeltaic (east) systems fed submarine fans, stacked between salt-related submarine heights (“channeled” deep-marine depocenters). Most of the Pannonian deposits (partially 7th and 8th sequences) belong to submarine fan systems, but shallower facies were also found in the western and eastern part of the basin.     

Incision of a river curvature due to exhumed Miocene volcanic landforms: Danube Bend, Hungary

A new model for the formation and relief evolution of the Danube Bend, northern Hungary, is discussed on geomorphological and volcanological grounds. We propose that the present-day U-shaped loop of the Danube Bend was partly inherited from the horseshoe caldera morphology of Keserűs Hill volcano, a mid-Miocene (ca 15 Ma) lava dome complex with an eroded central depression open to the north. According to combined palaeogeographical data and erosion rate calculations, the drainage pattern in the Danube Bend region was formed when Pleistocene tectonic movements resulted in river incision and sedimentary cover removal. Formation of the present curvature of the river was due to the exhumation of the horseshoe-shaped caldera as well as the surrounding resistant volcaniclastic successions (i.e. Visegrád Castle Hill) and a hilltop lava dome (Szent Mihály Hill). The process accelerated and the present narrow gorge of the Danube Bend was formed by very rapid, as young as late Quaternary differential tectonic uplift, also enhancing the original volcanic morphology. On the basis of comparative long-term erosion-rate calculations, we estimated successive elevation changes of the volcanic edifice, including partial burial in late Miocene time. In comparison with various order-of-magnitude changes, the mid-to-late Quaternary vertical movements show increased rates and/or base level drop in the Pannonian Basin.     

Closure of a seaway: stratigraphic record and facies (Guadix basin, Southern Spain)

During the late Tortonian (upper Miocene), the Guadix Basin in S Spain formed one of the Betic corridors that connected the Mediterranean Sea with the Atlantic Ocean. The closure of this connection occurred in a series of steps, documented by three sedimentary units. A lower unit, consisting of basinal marls, shallow-water calcarenites and sands records the formation of a wide seaway. During deposition of the following unit this narrowed to a strait no more than 2 km in wide, triggering an intensification of currents that caused migration of submarine dunes preserved as giant cross-beds in bioclastic sands and conglomerates. Current flowed from the Mediterranean to the Atlantic. The third unit constitutes the youngest marine episode of the filling of the Guadix Basin. At this stage, the connection between the Mediterranean Sea and the Atlantic Ocean was broken, and a system of coastal coral reefs was established in the northern part of the Basin.     

Thrusting and faulting in metamorphic and sedimentary units of Ligurian Alps: an example of integrated field work and geochemical analyses

In a sector placed in the SE part of the Alps–Apennine junction, a kilometre-scale shear zone has been identified as the Grognardo thrust zone (GTZ), which caused the NE-directed thrusting of metaophiolites (Voltri Group) and polymetamorphic continental crust slices (Valosio Unit) of Ligurian Alps onto Oligocene sediments of an episutural basin known as “Tertiary Piemonte Basin”. The structural setting of the GTZ is due to syn- to late-metamorphic deformation, followed by a brittle thrusting that occurred in the Late Aquitanian times and can thus be related to one of the main contractional tectonic events suffered by northern Apennines. The GTZ was then sealed by Lower Burdigalian carbonate platform sediments (Visone Formation). Transtensive faulting followed in post-Burdigalian times along NW–SE regional faults and displaced the previously coupled sedimentary and metamorphic units. The GTZ thus underwent a plastic-to-brittle evolution, during which carbonate-rich fluids largely sustained the deformation. In these stages, a complex vein network originated within both the metamorphic and sedimentary rocks. Field data and stable isotopic analyses (¹³C and ¹⁸O) of bulk rocks and veins show that fluid–rock interaction caused the carbonatisation of the rocks in the late-metamorphic stages and the cataclasis and recementation, by the action of isochemical cold carbonate groundwater during the thrusting events. Carbonate veins largely developed also during the transtensive faulting stages, with composition clearly different from that of the veins associated to thrust faults, as indicated by the strong depletion in ¹³C of carbonate fillings, suggesting the presence of exotic fluids, characterised by a high content of organic matter.     

宁夏中南部中新世构造活动的地质证据及其意义

通过区域性新生界的对比及不整合面分布的研究,发现宁夏地区中新世曾发生过两次构造运动,一次发生于早中新世,另一次发生于中中新世晚期,但以后者明显,分布也最广。其中前者集中在六盘山以西地区,后者分布在查汗布拉格—三关口—青铜峡—固原(查-固)断裂沿线。宁夏中北部大部分红柳沟组是在受中新世构造运动影响的环境下沉积的。发生于早中新世的构造运动集中体现在六盘山以西的地区和甘肃部分地区,该期运动使渐新统清水营组变形,由于这些现象多发生于海原断裂附近,因此推测在早中新世海原断裂就可能已活动了。而发生于中中新世晚期的运动体现在如下几个方面,在贺兰山中南段西侧发现了中中新世晚期的逆冲推覆活动,堆积了厚度较大的同构造沉积(红柳沟组);在查-固断裂沿线发现了由于中中新世晚期的构造活动而形成的红柳沟组与清水营组之间的角度不整合面,这些说明在该期青藏高原的变形前锋已到达查-固断裂,也就是鄂尔多斯西缘地区,早新生代盆地因此解体;与此同时盆地内部红柳沟组向上逐渐变粗,也说明中中新世晚期较强烈的构造运动已影响到了该区,香山地区开始隆起并分割了早先的盆地,使之成为背驮盆地。宁夏中南部并入到青藏高原东北缘逆冲构造楔之中,该逆冲构造楔中新世向北东扩展的原因可能是由于高原北部在同期显著的隆升运动使得逆冲楔顶角超过临界值,而向前扩展的,查-固断裂是早期类似于目前海原断裂的青藏高原变形前缘,整个高原东北部的扩展并不是前展式,而是一种没有特定顺序的变形。上述这些现象否定了前人认为宁夏地区第三系的变形发生在上新世末期—更新世以后的观点。     

藏南邛多江地区二长花岗岩^40Ar-^39Ar同位素年龄及其地质意义

本文以藏南邛多江变质核杂岩中花岗岩类侵入岩体为研究对象,分别对仲格耐和倾日二长花岗岩环(株)中的新鲜黑云母样品进行了~(40)Ar/~(39)Ar 同位素年龄测定。仲格耐二长花岗岩环黑云母样品的坪年龄和等时年龄分别为(13.30±0.32)Ma 和(12.90±2.90)Ma,而倾日二长花岗岩株黑云母样品的坪年龄和等时年龄则为(12.35±0.14)Ma 和(12.37±0.26)Ma。与倾日岩株相比,仲格耐岩环黑云母样品的坪年龄值相对较高些,暗示了其相对较早的形成时间。考虑到倾日二长花岗岩株形成之后基本未遭受到明显构造-岩浆活动影响,因此,12Ma 也许代表了黑云母的结晶时间,属中喜马拉雅时期构造-岩浆活动的产物。确切地讲,邛多江变质核杂岩及有关花岗岩类侵入岩的形成时间不是老第三纪的渐新世,而是新第三纪的中新世。本区花岗岩类侵入岩形成时代的确定,对于探讨本区中酸性岩浆岩的形成过程和揭示藏南拆离构造体系的动力学机制均具有重要意义。     

Miocene intra-arc bending at an arc-arc collision zone, central Japan

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra-arc deformation of central Japan and the collision tectonics of the Izu-Bonin Arc. The cusp structure of the pre-Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu-Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50-40°) continued until 6 Ma, resulting in the sharp bent structure of the pre-Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu-Bonin Arc on the northwestward migrating Philippine Sea Plate.     

Early exhumation of the collisional orogen and concurrent infill of foredeep basins in the Miocene Eurasian–Okhotsk Plate boundary, central Hokkaido, Japan: Inferences from K–Ar dating of granitoid clasts

Abstract   Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na₂O + K₂O/Al₂O₃ in molecule) versus A/CNK (Al₂O₃/CaO + Na₂O + K₂O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido.     

K–Ar ages of the Ohmine Granitic Rocks, south-west Japan

Abstract The Ohmine Granitic Rocks are a series of granitic rocks that are distributed in a chain stretching along the central axis of the Kii Peninsula. Their precise ages have not been determined, although precise ages have been reported for other geological units of the early to middle Miocene distributed over the peninsula. In this study, biotite K–Ar ages were obtained for the six major granitic plutons of the Ohmine Granitic Rocks: Dorogawa, Shirakura, Kose, Asahi, Tenguyama and Shiratani. Most are aged from 14.8 to 14.6 Ma. Although one pluton is older (15.4 ± 0.2 Ma) and two are younger (14.0 ± 0.2 Ma and 13.4 ± 0.1 Ma), these ages are excluded from the discussion of the mutual correlation among the plutons because some ambiguities exist in their ages. The age of the southernmost unit, the Katago–Mukuro Dykes, was not determined because of its intense alteration, but stratigraphic constraints suggest that it is younger than 16.1 Ma. The majority of the Ohmine Granitic Rocks concentrate within a narrow age window of approximately 14.8–14.6 Ma, although their geochemical/petrographical characteristics suggest that they were generated by multiple magma batches. The results of this study also reveal the simultaneous occurrence of the major activities of the Ohmine Granitic Rocks and the gigantic felsic igneous activities in the Kii Peninsula, such as the Kumano Acidic Rocks and the Muro Pyroclastic Flow Deposit.     

Ice-distal Upper Miocene marine strata from inland Antarctica

ABSTRACT Glacimarine strata of the Battye Glacier Formation (≈ 130 m thick), Pagodroma Group, exposed in the Amery Oasis of East Antarctica, provide a record of Late Miocene palaeoenvironmental conditions in an ice‐distal setting. The formation overlies the Amery Erosion Surface (≈ 300 m to ≈ 270 m above sea level) that formed during an advance of the Lambert Glacier into Prydz Bay (ODP Site 739), at least 750 km further north than today. Two lithological members: a grey and muddier Lower Member and a brown, sand‐rich Upper Member, reflect variation in proximity to the terminus of the Lambert Glacier. Ice‐distal, glacimarine, diatom‐bearing mud (up to 12% biogenic silica) and in situ articulated molluscs occur in the Lower Member. The Battye Glacier Formation is significant because of its inland location, which indicates that ice‐distal marine conditions existed 250 km inland from the current Amery Ice Shelf edge. Similar Neogene strata are known on land only from the Pliocene Sørsdal Formation in the Vestfold Hills, near the Antarctic coast. Three stratigraphic intervals of diatom‐bearing mud are recognized from glacially reworked clasts and from in situ strata informally referred to as the McLeod Beds and ‘Bed A’. The diatom‐bearing mud also contains sponge spicules and minor silicoflagellates and ebridians. Marine diatom biostratigraphy constrains the age of the beds to between 10·7 and 9·0 Ma (late Miocene). Abundant benthic diatoms suggest deposition within shallow euphotic waters. The high abundance of intercalary valves of Eucampia antarctica from an interval of the McLeod Beds suggests that there was less winter sea‐ice than in Prydz Bay today. It is unlikely that sea‐ice was perennial because the presence of Thalassionema spp. and Stellarima stellaris (Roper) Hasle et Sims suggests that summer sea‐surface temperatures were too warm (> 0°C and > 3°C respectively). The palaeoclimate at the time of deposition appears to have been analogous to that in modern fjords of East Greenland (e.g. Kangerdlugssuaq Fjord), which is consistent with the depositional model proposed previously for the Pagodroma Group. The three diatom‐bearing mud intervals were deposited in the Amery Oasis, ≈ 250 km inland of the current Amery Ice Shelf edge, when the East Antarctic Ice Sheet was reduced in size relative to today.