End Of Early Miocene Research Articles

Age and conditions of formation of sedimentary cover of the Ulleung Plateau of the Sea of Japan according to micropaleontological data

The paper reports on the micropaleontologcal (diatoms, silicoflagellates, radiolarians, and pollen flora) data substantiating the age and conditions of sedimentary cover formation of the submarine Ulleung Plateau (Krishtofovich Rise) in the Sea of Japan. Five rock complexes with different age and origin were distinguished on the basis of micropaleontological and petrographic data. Complex 1 (tuffites, tuffogenous siltstones) with numerous freshwater diatoms and pollen flora that prove the lacustrine genesis and the Early Miocene age occurs at the base of the sedimentary cover. Complexes 2–5 are composed of marine tuffogenous sedimentary deposits of end of Early Miocene–Pleistocene age. Stratigraphic unconformity between continental and marine deposits involves a short-time interval in the end of the Early Miocene and points to rather fast tectonic submersion of the Ulleung Plateau. Marine sedimentation in bathyal conditions dominated from the end of the Early Miocene. In the Late Miocene, in the northern part of the plateau, the region of the large rise was characterized by shallow-water conditions, indicating supposed existence of an island territory in this place.

Late Eocene-early Miocene palaeogeographic evolution of central eastern Anatolian basins, the closure of the Neo-Tethys ocean and continental collision

In central eastern Anatolia which is located between Eurasia and Africa, the study of basin developments between late Eocene and early Miocene is of great importance for understanding the process of the closure of the Neo-Tethys Ocean and the formation of strike-slip faults and regional uplift. To study these, three basins were selected: the Sivas-Erzincan, Gurun-Akkisla-Divrigi (GAD), and Malatya basins. The study proposes that the opening of the GAD basin played a key role in the formation of the Ecemis fault, which started developing at the end of early Miocene, and in mountain uplift. All these basins are situated on continental blocks and oceanic crust, arranged from north to south as the Sakarya continent, the Izmir-Ankara-Erzincan ocean (Northern Neo-Tethys), the Kirsehir continent, the inner Tauride ocean, the Munzur-Binboga block, the Maden (=Berit) ocean, the Bitlis-Puturge block, the Cungus ocean and the Arabian continent.

KARABURUN YARIMADASI KUZEY KIYI KESİMİNİN NEOJEN STRATİGRAFİSİ

The terrigenous Neogene lithology in north of Karaburun peninsula is represented by LowerMiddle Miocene rock units which are cut and bounded by NW-SE trending synthetic faults.Dominant lacustrine sedimentation (Haseki and Hisarcik formations) of Early-Middle Mioceneperiod which was separated by upper and lower regional unconformities, and the Early Miocenemafic volcanism (Karaburun volcanites) were studied within the scope of Karaburun group.Haseki formation which reflects Early Miocene sedimentation begins with fills of alluvial fan ofthe Salman member and is basically formed by algal-biostromal Yeniliman limestone andlacustrine deposits of Aktepe member. With synsedimentary emplacement of the 1stphaseKaraburun volcanites over Yeniliman limestone platform, the dominant sequence which ismicritic limestone intercalating with diatomite of the Aktepe member was deposited inenvironmental conditions deepening towards shore-face. The effectiveness of NW trendingboundary faults which gave rise to 2ndphase of the Karaburun volcanism, shaping the westernedge of Foca depression towards the end of Early Miocene has not changed the dominantdepositional environment, and lacustrine sedimentation has continued with Hisarcik formation.Green and fine clastic shore-face sediments (Karaba¤lari member) of the Hisarcik formationwhich deposited the latest between Early Miocene-Middle Miocene overlie Aktepe sequencewith submarine parallel unconformity. Karaburun volcanism which is represented by highpotassium andesitic products with calc alkaline character (pyroclastics and olivine- phyriclavas) is synchronous with sedimentation of Haseki formation and has two phases. Pyroclasticrocks which are in facies of base-surge, air-fall and ash-fall in lesser amount were emplacedbefore lava eruptions. K/Ar aged, 1stphase products, which are 18.2±1.0 my according to K/Ardating, are in the position of stratigraphical reference level which separates Yenilimanlimestone from Aktepe member. Second phase products are emplaced in Aktepe sequence

Slowing extrusion tectonics: lowered estimate of post-Early Miocene slip rate for the Altyn Tagh fault

Determination of long-term slip rate for the Altyn Tagh fault is essential for testing whether Asian tectonics is dominated by lateral extrusion or distributed crustal shortening. Previous slip-history studies focused on either Quaternary slip-rate measurements or pre-Early Miocene total-offset estimates and do not allow a clear distinction between rates based on the two. The magmatic and metamorphic history revealed by SHRIMP zircon dating of clasts from Miocene conglomerate in the Xorkol basin north of the Altyn Tagh fault strikingly matches that of basement in the southern Qilian Shan and northern Qaidam regions south of the fault. This match requires that the post-Early Miocene long-term slip rate along the Altyn Tagh fault cannot exceed 10 mm/year, supporting the hypothesis of distributed crustal thickening for post-Early Miocene times. This low long-term slip rate and recently documented large pre-Early Miocene cumulative offset across the fault support a two-stage evolution, wherein Asian tectonics was dominated by lateral extrusion before the end of Early Miocene, and since then has been dominated by distributed crustal thickening and rapid plateau uplift.

Modeling of Neogene crustal block rotation: Case study of southeastern California

Numerical modeling using a two‐dimensional distinct element method (DEM) was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the Pacific‐North America transform plate boundary. Paleomagnetic and structural investigations have shown that large (30°–50°) horizontal rotations of crustal blocks occurred in the Eastern Transverse Zone and the Northeast Mojave Block (southeastern California) since the end of early Miocene. The main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not. The simulations were performed using different stress boundary conditions and different parameters of the fault strength. Applying an assumed ratio of 2 for the horizontal stress components as boundary condition, block rotations in the order of 40° only occur if (1) the major horizontal stress SH is oriented N20°E with a maximal deviation of 15° to the east or west, and (2) the fault strength described by the friction coefficient is smaller than 0.12, or 14% only of the value 0.85 for intact rocks (Byerlee's law). For a weakening model of the fault strength an initial friction coefficient of 0.85 allows also rotations of about 40° if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0.12 or smaller. For the same amount of rotation (40°) a decreasing stress ratio requires a lowering of the friction coefficient used, whereas an increase of the ratio allows a higher friction coefficient. The simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation, especially the cumulative slip values. Discrepancies in the observations and modeling results for the western parts of the Mojave Desert are the result of a neglect of the three‐dimensional deformation in our two‐dimensional modeling.

Cenozoic plate reconstruction of the South China Sea region

Reconstructions of the South China Sea region at 60 Ma, 40 Ma, 30 Ma, 20 Ma, 10 Ma and 5 Ma are presented. We have attempted to place the South China Sea Basin in a regional tectonic framework. The tectonic evolution of the major blocks surrounding the South China Sea were analyzed, as well as the relative motions of the Indian and Australian plates. We have tried to correct the tectonic models available in this region. A 3-D graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self-consistency. When the model conflicted with previous interpretations the input data were evaluated for alternative explanations. At least two, and possibly three, stages of extension can be recognized in this region. The earliest one, active in the Late Cretaceous to Eocene, involved NW-SE extension. The second one, active from the Late Eocene to Early Miocene involved north-south extension. The third stage of extension, which probably trended NW-SE, can be dated as post-Oligocene. The first extensional event produced the NE-SW trending proto-South China Sea and a series of sedimentary basins along the South China margin. Following the southeastward extrusion of Indochina, the proto-South China Sea was mostly consumed at the Palawan Trough. Renewed north-south extension in the South China continental margin started the present-day South China Sea spreading in the Oligocene. The southeastward extrusion of Indochina, blocked by Sundaland, resulted in the NW-SE trending opening of the South China Sea Basin in the Early Miocene. Collision of the North Palawan microcontinental block with the West Philippines block stopped the opening of the South China Sea at the end of Early Miocene. Spreading activity switched to the Sulu Sea Basin in the Middle Miocene but collision between the Sulu Ridge and the West Philippines at Mindanao halted the opening of the Sulu Sea at the end of the Middle Miocene. In the Late Miocene, Greater India continued its northward path and seems to have ripped open the Andaman Sea. In the Pliocene, subduction along the northern Manila Trench placed the North Luzon Arc on a collision path with the East Asia continental margin at Taiwan. Our reconstructions, along with detailed geological and geophysical information, may be used as a predictive tool for basin evolution models and block interactions in this region. The development of the South China Sea Basin, the Gulf of Thailand, the Malay Basin and the central Thailand basins are the result of collision-induced extensional forces. The Sulu, Celebes and Sumatra basins were formed as a consequence of prolonged subduction. The opening of the Pearl River Mouth, West Natuna, South China Sea, Sulu, and possibly Celebes, basins were terminated by various plate collisions. During the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution.

History of the development of the temperate forest flora in Kazakhstan, U.S.S.R. from the Oligocene to the Early Miocene

Exact dating of the floras existing in western Kazakhstan during the Oligocene and Early Miocene permits a detailed examination of the formation of a temperate flora during the Rupelian (Early Oligocene), Chattian (Late Oligocene), Aquitanian (the beginning of Early Miocene) and the Burdigalian (end of Early Miocene) Ages and at the same time an establishment of the sequence in the origin and the disappearance of a flora of Turgayan type (the “Turgayan flora” of Kryshtofovich) in Kazakhstan. The most important aspects (composition and terminology) of a previous subtropical flora of a Drevlyanian (possibly Volynian) ecological type (the “Poltavian” flora of Kryshtofovich), which had developed in Kazakhstan and the Ukraine during the Eocene, are also discussed.

Episodic post-rift subsidence of the United States Atlantic continental margin

Sediment thickness, paleobathymetry, and chronostratigraphy from COST wells offshore from Georgia and New Jersey indicate periods of rapid subsidence superimposed on the slower thermal subsidence of the continental margin. Rapid subsidence occurred during the Coniacian-Santonian, the Eocene and, in the COST wells off New Jersey, since the end of early Miocene. Once the maximum effects of water and sediment loading, compaction, and thermal cooling are removed, the residual vertical movements caused by tectonic and sea-level fluctuations can be analyzed. Because no global sea-level change can account for all residual movements, we propose that tectonism, variously amplified by loading, is responsible for the observed episodes of rapid subsidence. Synchroneity of subsidence with sea-floor–spreading changes in the North Atlantic suggests a unified cause for these events. Recognition of episodic subsidence may have implications for fault timing, petroleum potential, and global sea-level effects on passive margins.