Early Mid Miocene Research Articles

The early-mid Miocene abyssal brown/green claystone from IODP Site U1503A in the northern South China Sea: Implications for paleoclimate and paleoceanography

Limited knowledge about the evolution of Pacific deepwater circulation during the early-mid Miocene hinders us from understanding the relationship between global climate and deepwater circulation during this interval. The marine pelagic brown/green sediments that formed in relation to deep ocean ventilation has crucial implications for the evolution of deepwater circulation. Here, we present sedimentological, mineralogical and geochemical records (reflectance a*, grain size, clay minerals, major and trace elements, and Sr-Nd isotopes) from brown and green claystone (∼19–12 Ma) from the South China Sea (SCS) International Ocean Discovery Program (IODP) Site U1503A to study the unique conditions that caused the sediment color transition and to reveal their implications for paleoclimate and paleoceanography. The results show that the sediment color transition was dictated by the variation in Fe (III) oxides (hematite). Clay minerals, rare earth elements and Sr-Nd isotope records reveal relatively stable provenances (mainly South China and Luzon). The poor correlation between sediment color and weathering proxies (endmember 3 (eolian dust), chemical index of alteration, Ti/Na, Al/K, and smectite/(illite + chlorite)) indicates that the influence of weathering intensity changes on sediment color may be negligible. The less enriched redox-sensitive elements (Ni, Co, V, Cr, and U) in brown claystone suggest a more oxidizing sedimentary environment than that of green claystone. We attribute this difference in redox conditions to the variations in the oxygen-rich bottom current in the northern SCS, which could reflect the changes in Pacific deepwater circulation. The synchronous variations in sediment color and global benthic isotope oxygen stack imply that Pacific deepwater circulation reorganized during the early-mid Miocene warmer-colder periods.

2D Seismic Data Analysis of Judaida Structure, Northern Iraq

This research aims to study the structural analysis of the 2D reflection seismic data for the Judaida subsurface structure located in Kirkuk province, northern Iraq. It is located 60 Km southwest of Kirkuk oil field, and 35 Km southwest of Jambur oil field, the Daquq River passes through the study area. The reflectors in the seismic section were picked and identified by using the synthetic seismograms generated from the logs data of the Jd-1 well. Three main seismic reflectors, Fatha, Jeribe, and the Euphrates were chosen. These mentioned sedimentary formations were deposited during the Middle Miocene, Lower Miocene, and Early-Mid Miocene respectively. Time and depth maps were drawn for these three reflectors by processing average data from Jd-1well. The structural interpretation results showed a structural closure on both time and depth domains in the middle of the study area. Judaida structure represented an asymmetrical longitudinal anticline fold. The Northeast limb has a dipping angle of 6-8 degrees more dip than the Southwest limb that has a dipping angle of 1-6 degrees, and the direction of the axis of the general structure is towards the Northwest-Southeast. The general dipping of the fold is toward the north east.

Reconstructing the southern Pelagonian domain in the Aegean Sea: Insights from U-Pb detrital zircon analysis, lithostratigraphic and structural study, and zircon (U-Th)/He thermochronology on Amorgos Island (SE Cyclades, Greece)

In the Attic-Cycladic Crystalline Complex (Aegean Sea, Greece), following early syn-convergence exhumation processes, the extensional tectonics resulted in the extreme denudation of the upper plate, preserved today in a few places. The Cycladic Blueschist Unit (CBU) and the underlying Cycladic Basement and the Basal Units have been exhumed in the footwall of detachment systems, juxtaposed against the Pelagonian-derived hanging wall fragments. Traditional views accept that Amorgos Unit occupies a structurally lower position, correlated to the Basal Unit. This study presents lithostratigraphic and structural data, integrated with detrital zircon MDA, provenance analyses, and (U-Th)/He ages, showing a new investigation of the paleogeographic and structural position of Amorgos in the Cycladic archipelago.The low-grade metamorphosed Amorgos Unit presents a tripartite lithostratigraphy: a basal metaconglomerate, a middle carbonate sequence with shallow-water to pelagic facies, and an Eocene (meta)flysch. An HP/LT metabasite huge block is incorporated within the metaconglomerate either by gravitational movements as an isolated olistolith or tectonic processes as a slice during the early deformation stage. Detrital zircon U-Pb analysis on the meta-siliciclastic deposits revealed that the basal metaconglomerate shows a dominant Ediacaran input with Mid-Permian and Precambrian MDAs for the matrix and the high-grade clasts, respectively. The metaflysch shows Paleozoic affinity (e.g., mainly Pelagonian-sourced) and Triassic-Jurassic MDAs. The deformation history of Amorgos includes an early-stage top-to-NW thrusting in the retro-wedge setting of the Late Eocene–Oligocene subduction zone and a late-stage top-to-SE extensional low- and high-angle normal faulting. Zircon (U-Th)/He ages reveal an Early–Mid Miocene (18–14 Ma) exhumation below ∼ 200 °C. Similarities on lithostratigraphy, structural inventory, exhumation history, and structural position of Amorgos Unit with the Santorini Detachment System Pelagonian hanging wall imply their close spatial relationship. From all the above, we propose that Amorgos belongs to the upper plate of the CBU and is paleogeographically located at the southern Pelagonian margin.

Shoreline migration paths and depositional architecture of early–mid Miocene deltaic clinoforms in response to sea‐level changes in the north‐eastern shelf margin, South China Sea

AbstractShoreline trajectory analysis improves the understanding of the correlation between clinoform architecture, shoreline and shelf‐edge development, and sea‐level changes. This study uses cores, well‐logs and seismic data to investigate the depositional architecture and shoreline migration paths of early−mid Miocene deltaic clinoforms in the northern Pearl River Mouth basin, South China Sea. Five clinoform types are observed, including sigmoidal, oblique, tangential oblique, parallel oblique and sigmoidal‐oblique clinoforms. Both the rollovers of deltaic clinoforms in seismic sections and the transitional points between delta plain and delta front deposits in well correlation cross‐sections can be used as the indicators for shoreline positions, provided that there are no significantly extended subaqueous deltas. This allows for the identification of three shoreline trajectory types from a single clinoform set and three major stages from the net shoreline trajectory for multiple clinoform sets. A landward‐directed net shoreline trajectory and ascending shoreline trajectories dominate in Stage 1 (23.8 to 17.5 Ma) with sigmoidal clinoforms transitioning into parallel oblique clinoforms, suggesting a rise in third and fourth‐order relative sea level. Stage 2 (17.5 to 13.8 Ma) displays flat or descending trajectories and an overall seaward‐directed net trajectory with a change from parallel to tangential oblique clinoform, indicating a relative sea‐level fall. The reoccurrence of sea‐level rise in Stage 3 (13.8 to 10.5 Ma) was inferred by a landward migration of parallel oblique clinoforms. This study shows that shoreline trajectory and net shoreline trajectory plotted from both seismic and well data is consistent with the regional sea‐level curves estimated from foraminiferal fossil records and the shelf‐edge development, which can be used to estimate the third and fourth‐order relative sea‐level changes in passive margins with wide, low‐gradient shelves. Furthermore, clinoform architecture and migration may have been controlled by the variations of subsidence/uplift and sediment supply produced by tectonic events.

Resolving the Cenozoic History of Rock Exhumation Along the Central Rocky Mountain Trench Using Apatite Low‐Temperature Thermochronology

AbstractThe Rocky Mountain Trench (RMT) extends through the Canadian Cordillera from the Yukon to northwest Montana. The RMT's impressive length and continuous nature have been interpreted to be the result of faulting, and localized erosion, along a continent‐scale zone of crustal weakness located beneath the RMT, possibly associated with the ancient continental margin. Despite the continuous nature of the RMT, its kinematics vary along strike from dextral strike‐slip faults in the north to normal faults in the south. The central RMT is thought to be the transition between these two structural zones. We use low‐temperature thermochronology to compare the cooling histories across the central RMT in eastern British Columbia. We report apatite fission track ages from 23 samples and apatite (U‐Th)/He ages from 25 samples along with thermal‐history models. Our results reveal three phases of rapid cooling that followed the Cretaceous‐Paleocene cordilleran thrusting in the Eocene, the early–mid Miocene, and from the late Miocene to recent. We find that the Malton Gneiss Complex exhumed as a horst structure 20–10 Ma bounded by the North Thompson Albreda Fault and the RMT. Normal faulting along the NTAF continued in the late Miocene as well as west side down normal faulting along the RMT. Our data suggest that extends along the southern RMT continued northward to at least this portion of the central RMT in multiple episodes during the Cenozoic. We suggest transtension occurred in our study area that was driven by orogenic collapse and lithospheric mantle delamination.

Lithofacies and eruptive conditions of the southernmost volcanoes in the world (87\xb0 S)

Neogene volcanic centres are uncommon in the Transantarctic Mountains but at least three basaltic examples occur within 300 km of South Pole, above 2200 m asl and inland of the margin of the West Antarctic Rift System. They are the southernmost volcanoes on Earth and have yielded Early—mid Miocene isotopic ages. Two of the centres, at Mt Early and Sheridan Bluff, have been examined. The centre at Mt Early is unequivocally glaciovolcanic. It formed a tall monogenetic volcanic edifice at least 1 km high and > 1.5 km in diameter. It erupted under significantly thicker-than-modern ice, which was probably a fast-moving ice stream at the eruptive site and resulted in a distinctive constructive architecture and lithofacies. It is the first described example of a glaciovolcano erupted beneath an ice stream. The characteristics of the second centre at Sheridan Bluff indicate that it was also a monogenetic volcano but with a shield-like profile, originally c. 6 km in basal diameter but just c. 400 m high. It probably erupted in a substantial pluvial lake in an ice-poor or ice-free environment. The strongly contrasting eruptive settings now identified by the volcanic sequences at both centres examined testify to a highly dynamic Antarctic Ice Sheet during the Early—mid Miocene.

Thermal history reconstruction from apatite fission-track analysis and vitrinite reflectance data of the Bongor Basin, the Republic of Chad

The Bongor Basin in southern Chad is one of the Cretaceous–Paleogene rift basins developed on the Precambrian crystalline basement and has been confirmed as a petroliferous basin in the last decade. Less than 400 m of Cenozoic unconsolidated sediments are separated by an unconformity from an underlying section of Lower Cretaceous units, in turn separated by another unconformity from underlying Precambrian basement. In addition, there is a locally low-angle unconformity within the Cenozoic section. A synthesis of apatite fission-track analysis data in four wells from the basin reveals two cooling episodes from Late Cretaceous to early Paleocene (beginning between 75 and 60 Ma) and mid-Miocene, respectively. The results suggest that regionally synchronous cooling is a likely scenario. The first exhumation between 75 and 60 Ma affected the whole basin, and the magnitude of uplift and erosion was approximately 1100–1250 m across the whole basin. In contrast, the second exhumation during the Miocene affected mainly the northern part of the basin while the magnitude was weak and could not be detected in the southeast of the basin. Potential trapping structures, for example, fault blocks and synsedimentary anticlines, formed prior to and inverted anticlines as a result of the first cooling phase of exhumation (strong compressional inversion) and were available for hydrocarbon migration and accumulation during the main phase of hydrocarbon generation. The Miocene exhumation was less pronounced and had weak or no impact on the hydrocarbon generation and accumulation.

Sedimentary evolution and controlling factors of Early-Mid Miocene Deltaic systems in the Northern Pearl River Mouth Basin, South China Sea

The Pearl River Mouth Basin is located at the northern continental shelf of the South China Sea. Since the early Miocene, the Paleo-pearl river transported a large amount of sediments to the northwest of the basin and resulted in the formation of a large-scale river-delta depositional system, which has become an important oil and gas reservoir in the study area. In the current paper, we investigate the characteristics and evolution of fluvial-deltaic depositional systems and their controlling factors, including sea level change, tectonic subsidence and sediment supply on the basis of 3D seismic, well logging and core data. Early-Mid Miocene succession can be divided into two second-order sequences (CS1–CS2) and eight third-order sequences (S1–S8). Deltaic systems developed in S1–S2 are relatively coarse in grain size, and the delta plain deposits are dominated by thickly stacked (100–180 m) distributary channel sand bodies and interpreted as braided delta depositional system. In the early stage (S1–S2), the braided delta systems mainly distributed in the west of the Baiyun Sag, which were proceeded by a retreat to the south of the Enping Sag along with sharply rising sea level. Following the transgression of S2, the provenance of the Paleo-Pearl River extended to the coastal region of South China, and the papleoclimate changed from warm and humid to dry and cold in the early Miocene, leading to the development of transition of braided river delta to meandering river delta, which was characterized by relatively fine grain deposits. During the deposition of S3–S6, well sorted and rounded fine sandstones of deltaic front deposits accumulated in the study area. The retrogradation to accretion and subsequent progradation of these meander delta systems are attributed to the sea level change in the study area. During the deposition of S7–S8, the delta front retreated to the south of the Enping depression as a result of minor sea level rise, reduction in sediment input, and subsidence rate. This resulted in the development of a wave-controlled deltaic depositional system.

Neogene to recent evolution of the Southern Gulf of Mexico basin: Tectonic controls on deep‐water sediment dispersal systems

AbstractNewly available two‐dimensional (2D) and limited three‐dimensional (3D) reflection seismic data coupled with publicly available well and core data were used to generate the first comprehensive regional basin evolution model for the deep‐water Neogene to recent southern Gulf of Mexico (GoM). This evolution is presented in the context of contemporaneous onshore tectonic drivers and predecessor basin tectonic history. Dynamic uplift to the west in North America, transpression to the south and tectonically influenced salt tectonics to the east served to collapse the margins of the southern Gulf of Mexico Basin throughout the late Neogene and into the modern. This collapse and salt inflation served to shut off efficient sediment transport into the axial deep basin. Newly developed nearshore extensional accommodation along both the western margin of the basin and salt‐rooted orogenic structures to the south and east appear to have baffled coarser‐caliber sediment input from southern (Mexican) sediment sources into the deep Gulf of Mexico Basin beginning in the late Miocene. This sequence is recorded in the southern GoM basin by a transition from large early–mid Miocene submarine channel belts to late Miocene–recent sediment wave fields and mass transport deposits. Improved resolution of new 2D and 3D subsurface seismic reflection data shows that sediment waves of the southern Gulf of Mexico are supercritical bedforms, long wavelength antidunes and cyclic steps, not contourites. These supercritical bedforms are among the most spatially expansive and morphologically largest documented globally. The location and evolution of these bedforms appears closely tied to the tectonically driven history of margin collapse, salt inflation and the starving of the deep basin of coarse‐grained sediment. Cumulatively, the tectonic drivers that intuitively should have increased sediment flux into the deep southern Gulf of Mexico acted to ‘pinch off’ the basin, starving it of sediment supplied from southern sediment sources.

Large-scale mass wasting in the western Indian Ocean constrains onset of East African rifting

Faulting and earthquakes occur extensively along the flanks of the East African Rift System, including an offshore branch in the western Indian Ocean, resulting in remobilization of sediment in the form of landslides. To date, constraints on the occurrence of submarine landslides at margin scale are lacking, leaving unanswered a link between rifting and slope instability. Here, we show the first overview of landslide deposits in the post-Eocene stratigraphy of the Tanzania margin and we present the discovery of one of the biggest landslides on Earth: the Mafia mega-slide. The emplacement of multiple landslides, including the Mafia mega-slide, during the early-mid Miocene is coeval with cratonic rifting in Tanzania, indicating that plateau uplift and rifting in East Africa triggered large and potentially tsunamigenic landslides likely through earthquake activity and enhanced sediment supply. This study is a first step to evaluate the risk associated with submarine landslides in the region.

A new pygmy armadillo (Cingulata, Euphractinae) from the late Miocene of Andean Argentina reveals an unexpected evolutionary history of the singular Prozaedyus lineage

The extinct euphractine Prozaedyus was recognized remarking the evident morphological resemblance of the dorsal carapace with the extant piche or pygmy armadillo Zaedyus pichiy. From a phylogenetic viewpoint, Prozaedyus has been interpreted as an early diverging Euphractinae restricted, at the moment, to the late Oligocene and early mid Miocene levels (Deseadan to Laventan SALMAs) of southern South America (Argentina, Chile, and Bolivia). We report here a specimen (MSJ-317) coming from the late Miocene (Loma de Las Tapias Formation; ~9 Ma, Chasicoan Stage/Age) of western Argentina (San Juan Province), which represents a new species of Prozaedyus. The specimen includes an almost complete skull with complete dental series, some articulated fragments of the dorsal carapace and several isolated fixed and mobile osteoderms. Absence of teeth in the premaxillae, ornamentation pattern of fixed osteoderms, and presence of small foramina in the posterior and lateral margins of both mobile and fixed osteoderms, are characters that allow its inclusion within the genus Prozaedyus. Differences with the other known species of this genus include smaller size (~35–40%), cranial characters (e.g. position of the infraorbital foramen, palatine suture, morphology of the occipital condyles), and ornamentation pattern of mobile osteoderms. The phylogenetic analysis carried out support its relation as a sister group of the other Prozaedyus species, revealing an ancient divergence for the new taxon, occurred before that of the late Oligocene-early mid Miocene forms, and a distant phylogenetic position from Zaedyus pichiy. This new finding represents the youngest record of the genus, extending its biochron several millions of years, and provides novel information on the poorly know post-Santacrucian forms. Additionally, the cranial anatomy of the new species suggests a carnivore/omnivore habit, similar to extant euphractine armadillos but contrary to the insectivore habit proposed for the other Prozaedyus species.

Taxonomic revision of the spider geckos of the genus gamura senso lato Blanford, 1874 (Sauria: Gekkonidae) in the Iranian Plateau

In this study, we present an integrative systematic revision of the spider gecko, Agamura senso lato, in Iran. We sampled 56 geckos of this complex from its distributional range in Iran and western Pakistan and sequenced these for two mitochondrial markers, cytochrome b and 12S ribosomal RNA, and one nuclear marker, melano-cortin 1 receptor. We combined our molecular data with species distribution modelling and morphological examinations to clarify Agamura persica systematics and biogeography. Due to a lack of published data, we used only our data to investigate the spatial and temporal origin of spider geckos within a complete geographic and phylogenetic context. The phylogenetic analyses confirm the monophyly of Agamura. Among spider geckos, Rhinogekko diverged around the early-mid Miocene (17 Mya) from the Lut Block, and then Cyrtopodion diverged from the Agamura clade about 15 Mya in the mid-Miocene as a result of the uplifting of the Zagros Mountains. Subsequent radiation across the Iranian Plateau took place during the mid-Pliocene. Agamura kermanensis exhibits deep divergence from two other species of Agamura (A. persica and A. cruralis), whereas no geographical substructure was observed on the Iranian Plateau for A. persica and A. cruralis. Our findings reveal that diversification is consistent with a biogeographical model explained by different dispersal waves and vicariant events on the Iranian Plateau during the last 18 Mya. The divergence times between clades are compatible with orogenic events in southern Iran that resulted from the collision with Arabia. According to the genetic differentiation of both mtDNA genes (12S and cytochrome b), the systematic status of A. cruralis is confirmed, the new clade was distinguished from the genus Agamura, monophyly of Rhinogekko was confirmed and the allocation of Cyrtopodion gastrophole to the Cyrtopodion clade was confirmed.

Mandibular Morphology of the Mid-Miocene Seal Devinophoca claytoni (Carnivora, Phocidae, Devinophocinae)

Abstract During several excavations in Slovakia at the base of the Malé Karpaty Mountains (near the junction of the Morava and Danube Rivers), two skulls and numerous mandibular, dental and postcranial bones of early mid-Miocene (16.3–12.8 Ma) seals were collected. Isolated mandibles and many individual teeth were found at this locality, with some teeth in situ corresponding morphologically with the mandible and skull of Devinophoca claytoni, and others perfectly associating to the mandible and skull of the recently described Devinophoca emryi. Based on this material, two species of the previously unknown phocid subfamily Devinophocinae Koretsky et Holec, 2002 have been described. However, the mandibular morphology of the type species Devinophoca claytoni has remained unknown. Here, we present a first description of the mandible of this species. Morphological assessments reveal that the D. claytoni mandible has posterior alveoli larger than anterior; flat mandibular body low in height; alveoli of p4 larger than m1; and a unique devinophocine combination of incisors (I3/1) that differs from those in the extant subfamilies Cystophorinae (I2/1), Monachinae (I2/2) and Phocinae (I3/2).

The Dynamics of the Developing Calcarenite Member of Pamutuan Formation In Cintaratu Area, Pangandaran, West Java

The investigated area is located in Cintaratu, Parigi District, Pangandaran Regency in the Southeastern part of Southern Mountain of West Java. The rock unit in the investigated area is described as the Calcarenite Member of the Pamutuan Formation. The present study applying detailed lithologic and paleontologic description reveals five lithologic units. Combining the field method with microscopic analysis leads to the verification of the age and environments of this carbonate rocks. The study concludes the development of reef complex in Early Mid Miocene, followed by the high energy of the dynamic environment producing the fragments of the lime clasts embedded in the packstone, indicates the deposition in the fore slope. The regression took place and moved the deposition environment to the low-energy condition at the back of the reef, producing bioclastic packstone containing larger foram. Rising seawater level amplified the dynamics of the seawater in mid-Middle Miocene. The formation of planktic packstone marked an initial transgression phase that was well-represented as regional transgressive in Southeast Asia. This phase continued until interbedded calcareous mudstone and sandstone were produced. Finally, in Late Mid Miocene the sea level regressive accumulated coarse materials of calcarenite grain size containing fragments of larger foram within shallower deposition environment. The NNW-SSE elongated distribution of five units of Calcerinite Member pointed out the configuration of Middle Miocene coastal line. In Early Middle Miocene, various carbonate rocks formation was primarily controlled by tectonic activities. Afterward, the rock deposition affected by the regional sea level rise and drop during Mid to Late Middle Miocene. The detailed description of the Calcarenite Member, therefore, contributed to the understanding of the paleogeography of Mid Miocene age and the dynamics of the environmental deposition.

Molecular phylogeny of Panicum s. str. (Poaceae, Panicoideae, Paniceae) and insights into its biogeography and evolution.

Panicum sensu stricto is a genus of grasses (Poaceae) with nearly, according to this study, 163 species distributed worldwide. This genus is included in the subtribe Panicinae together with Louisiella, the latter with 2 species. Panicum and subtribe Panicinae are characterized by including annual or perennial taxa with open and lax panicles, and spikelets with the lower glume reduced; all taxa also share a basic chromosome number of x = 9 and a Kranz leaf blade anatomy typical of the NAD-me subtype photosynthetic pathway. Nevertheless, the phylogenetic placements of many Panicum species, and the circumscription of the genus, remained untested. Therefore, phylogenetic analyses were conducted using sequence data from the ndhF plastid region, in an extensive worldwide sampling of Panicum and related genera, in order to infer evolutionary relationships and to provide a phylogenetic framework to review the classification of the genus. Diversification times, historical biogeography and evolutionary patterns of the life history (annual vs. perennial) in the subtribe and Panicum were also studied. Results obtained provide strong support for a monophyletic Panicum including 71 species and 7 sections, of which sections Arthragrostis and Yakirra are new in the genus; 7 new combinations are made here. Furthermore, 32 species traditionally assigned to Panicum were excluded from the genus, and discussed in other subtribes of Paniceae. Our study suggested that early diversification in subtribe Panicinae and Panicum occurred through the Early-Mid Miocene in the Neotropics, while the subsequent diversification of its sections mainly occurred in the Late Miocene-Pleistocene, involving multiple dispersals to all continents. Our analyses also showed that transition rates and changes between annual and perennial life history in Panicum were quite frequent, suggesting considerable lability of this trait. Changes of the life history, together with C4 photosynthesis, and the multiple dispersal events since the Mid Miocene, seem to have facilitated a widespread distribution of the genus. All these findings contribute to a better understanding of the systematics and evolution of Panicum.

Seismic analysis of early-mid Miocene carbonate platform in the southern Qiongdongnan Basin, South China Sea

The southern uplift of the Qiongdongnan Basin is a deepwater area in which no wells have beens drilled. The Miocene-Quaternary strata in the Xisha Islands, which are located 40–100 km to the south, are composed of carbonate reef formations. Paleotectonic and paleogeographic analyses of the basin suggest that the southern uplift experienced favorable geological conditions for the development of carbonate reefs during the Miocene. The high-impedance carbonates have high amplitudes and low frequencies on seismic profiles. The reefs are distributed on paleotectonic highs and are thicker than the contemporaneous formations. A forward model of the variation in carbonate thickness based on lithological and velocity information from wells in nearby regions can simulate the seismic response of carbonates with different thicknesses. We identified several important controlling points for determining the thickness of carbonates from seismic profiles, including the pinchout point, the λ/4 thickness point, and the λ/2 thickness point. We depict a carbonate thickness map in the deepwater area of the southern Qiongdongnan Basin based on this model. The carbonate thickness map, the paleotectonic and paleogeographic background, and the seismic response characteristics of reefs suggest that the carbonates that developed on the southern uplift of the Qiongdongnan Basin during the Miocene were mainly an isolated carbonate platform peninsula and ramp deposits. It consisted of gentle ramp platform, steep slope platform, platform depression, gravity flow, and reef bank facies.

Phylogenetic and paleobotanical evidence for late Miocene diversification of the Tertiary subtropical lineage of ivies (Hedera L., Araliaceae)

BackgroundHedera (ivies) is one of the few temperate genera of the primarily tropical Asian Palmate group of the Araliaceae, which extends its range out of Asia to Europe and the Mediterranean basin. Phylogenetic and phylogeographic results suggested Asia as the center of origin and the western Mediterranean region as one of the secondary centers of diversification. The bird-dispersed fleshy fruits of ivies suggest frequent dispersal over long distances (e.g. Macaronesian archipelagos), although reducing the impact of geographic barriers to gene flow in mainland species. Genetic isolation associated with geographic barriers and independent polyploidization events have been postulated as the main driving forces of diversification. In this study we aim to evaluate past and present diversification patterns in Hedera within a geographic and temporal framework to clarify the biogeographic history of the genus.ResultsPhylogenetic (biogeographic, time divergence and diversification) and phylogeographic (coalescence) analyses using four DNA regions (nrITS, trnH-psbA, trnT-trnL, rpl32) revealed a complex spatial pattern of lineage divergence. Scarce geographic limitation to gene flow and limited diversification are observed during the early-mid Miocene, followed by a diversification rate increase related to geographic divergence from the Tortonian/Messinian. Genetic and palaeobotanical evidence points the origin of the Hedera clade in Asia, followed by a gradual E-W Asian extinction and the progressive E-W Mediterranean colonization. The temporal framework for the E Asia - W Mediterranean westward colonization herein reported is congruent with the fossil record. Subsequent range expansion in Europe and back colonization to Asia is also inferred. Uneven diversification among geographic areas occurred from the Tortonian/Messinian onwards with limited diversification in the newly colonized European and Asian regions. Eastern and western Mediterranean regions acted as refugia for Miocene and post-Miocene lineages, with a similar role as consecutive centers of centrifugal dispersal (including islands) and speciation.ConclusionsThe Miocene Asian extinction and European survival of Hedera question the general pattern of Tertiary regional extinction of temperate angiosperms in Europe while they survived in Asia. The Tortonian/Messinian diversification increase of ivies in the Mediterranean challenges the idea that this aridity period was responsible for the extinction of the Mediterranean subtropical Tertiary flora. Differential responses of Hedera to geographic barriers throughout its evolutionary history, linked to spatial isolation related to historical geologic and climatic constraints may have shaped diversification of ivies in concert with recurrent polyploidy.

Seismic geomorphology and evolution of early–mid Miocene isolated carbonate build-ups in the Timor Sea, North West Shelf of Australia

Analysis of extensive 2D and 3D seismic data reveals the presence of about sixty isolated carbonate build-ups (ICBs) of early–mid Miocene age spreading over a wide area of the northwestern Bonaparte Basin. Individual build-ups are ∼100m thick with an average diameter of 3km. Integration of full-volume 3D seismic attribute analysis with extensive biostratigraphic and lithological data from exploration wells allows examining the evolution of ICBs and their associated morphologies at high-resolution. The development of ICBs took place above a seismically flat surface. The stratigraphic architecture of the build-ups typically consists of: (1) mid Burdigalian initiation (Tf1/CN2), (2) lateral expansion during the late Burdigalian (CN3), and (3) backstepping and drowning during the Langhian (Tf2/CN4). This is followed by sub-aerial exposure that probably corresponds to a major sea-level fall during the Serravallian. Only small patch reefs (pinnacles) developed afterwards during the late Miocene. The observed growth phases of the ICBs correlate with global sea-level fluctuations and major changes in global climate and oceanography. The role of local tectonics in the initiation and demise of these build-ups appears as minimal. Growth of the northwestern Bonaparte Basin ICBs is synchronous with the establishment of tropical carbonate factories elsewhere along the Australia's North West Shelf that suggest an acme of reef development during the early to middle Miocene.

Amblyomma parvum Aragão, 1908 (Acari: Ixodidae): Phylogeography and systematic considerations

The geographical distribution of Amblyomma parvum Aragão 1908 in the New World is disjunct, with two main clusters separated from each other by the Amazon basin. The main objectives of this study were to further investigate the systematic relationships within A. parvum, to determine whether or not populations from different geographical areas might represent cryptic species, and to reconstruct the phylogeographical evolutionary history of the species. The genetic diversity of A. parvum collected throughout its distributional range was analyzed by using 6 molecular markers: 5 mitochondrial [the small and the large ribosomal subunits 12rDNA and 16SrDNA, the cytochrome oxidase I (COI) and II (COII) and the control region or d-loop (DL)], and one nuclear (ITS2, Inter transcribed spacer 2). Phylogenetic trees were inferred by using maximum parsimony and Bayesian analyses. In addition, node dating was attempted for the main lineages identified phylogenetically. Although mitochondrial and nuclear topologies were not totally congruent, they all identified at least two main supported clusters, a Central American lineage, and a Brazilian-Argentinian lineage. Clade support and divergence values strongly suggest that the two lineages correspond to different taxonomic entities. Node dating placed the split between the Central American and the Brazilian-Argentinian lineages at approximately 5.8–4.9 Mya, just after the progressive replacement of the dry areas that occupied the northern part of South America by the Amazon Basin in the early-mid Miocene. This event might be the cause of fragmentation and putative speciation within the ancestral relatively xerophilic A. parvum population.

Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

The Doi Inthanon and Doi Suthep metamorphic core complexes in northern Thailand are comprised of amphibolite-grade migmatitic gneisses mantled by lower-grade mylonites and metasedimentary sequences, thought to represent Cordilleran-style core complexes exhumed through the mobilization of a low-angle detachment fault. Previous studies have interpreted two metamorphic events (Late Triassic and Late Cretaceous), followed by ductile extension between the late Eocene and late Oligocene, a model which infers movement on the detachment at ca. 40Ma, and which culminates in a rapid unroofing of the complexes in the early Miocene. The Chiang Mai Basin, the largest such Cenozoic Basin in the region, lies immediately to the east. Its development is related to the extension observed at Doi Inthanon and Doi Suthep, however it is not definitively dated, and models for its development have difficulty reconciling Miocene cooling ages with Eocene detachment movement. Here we present new in-situ LA-ICP-MS and SIMS U–Pb age data of zircon and monazite grains from gneiss and leucogranite samples taken from Doi Inthanon and Doi Suthep. Our new zircon data exhibit an older age range of 221–210Ma, with younger ages of ca. 72Ma, and 32–26Ma. Our monazite data imply an older age cluster at 83–67Ma, and a younger age cluster of 34–24Ma. While our data support the view of Indosinian basement being reworked in the Cretaceous, they also indicate a late Eocene–Oligocene tectonothermal event, resulting in prograde metamorphism and anatexis. We suggest that this later event is related to localized transpressional thickening associated with sinistral movement on the Mae Ping Fault, coupled with thickening at the restraining bend of the Mae Yuan Fault to the immediate west of Doi Inthanon. Further, this upper Oligocene age limit from our zircon and monazite data would imply a younger Miocene constraint on movement of the detachment, which, when combined with the previously recorded Miocene cooling ages, has implications for a model for the onset of extension and subsequent development of the Chiang Mai Basin in the early mid-Miocene.