Late Miocene Time Research Articles

Kinematics of the Southern Rhodope Core Complex (North Greece)

The Southern Rhodope Core Complex is a wide metamorphic dome exhumed in the northern Aegean as a result of large-scale extension from mid-Eocene to mid-Miocene times. Its roughly triangular shape is bordered on the SW by the Jurassic and Cretaceous metamorphic units of the Serbo-Macedonian in the Chalkidiki peninsula and on the N by the eclogite bearing gneisses of the Sideroneron massif. The main foliation of metamorphic rocks is flat lying up to 100 km core complex width. Most rocks display a stretching lineation trending NE–SW. The Kerdylion detachment zone located at the SW controlled the exhumation of the core complex from middle Eocene to mid-Oligocene. From late Oligocene to mid-Miocene exhumation is located inside the dome and is accompanied by the emplacement of the synkinematic plutons of Vrondou and Symvolon. Since late Miocene times, extensional basin sediments are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat-ramp-type structures. Evidence from Thassos Island is used to illustrate the sequence of deformation from stacking by thrusting of the metamorphic pile to ductile extension and finally to development of extensional Plio-Pleistocene sedimentary basin. Paleomagnetic data indicate that the core complex exhumation is controlled by a 30° dextral rotation of the Chalkidiki block. Extensional displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex scale. It is argued that the Rhodope Core Complex has recorded at least 120 km of extension in the North Aegean, since the last 40 My.

Occurrence of Epilucina californica (Conrad) (Bivalvia: Lucinidae) from the Neogene of Japan, with notes on the biogeographic history of Epilucina

ABSTRACT The California lucine, Epilucina californica (Conrad, 1837), is reported from lower Middle Miocene to Upper Pliocene strata in Japan. This extends the oldest record of this species and represents the first detailed record of the species and genus in the western North Pacific region. Based on the Paleogene fossil record of Epilucina and the general migration pattern of Neogene North Pacific mollusks, it is inferred that E. californica originated in the eastern North Pacific during Late Oligocene or Early Miocene time and migrated westward to the western North Pacific during the early Middle Miocene. The western North Pacific populations of E. californica became extinct by the end of the Pliocene, whereas the eastern North Pacific populations survive in California. This species is, therefore, regarded as a northeastern Pacific restriction taxon. Two Japanese fossil species, Lucina (Myrtea) nipponica Nomura and Hatai, 1936 and Lucina japonica Ozaki, 1958 are junior synonyms of E. californica.

Cenozoic landscape evolution of an East Antarctic oasis (Radok Lake area, northern Prince Charles Mountains), and its implications for the glacial and climatic history of Antarctica

Ice-free areas Antarctica reveal a multi-million year history of landscape evolution, but most attention up to now has focused on the Transantarctic Mountains. The Amery Oasis in the northern Prince Charles Mountains borders the Lambert Glacier—Amery Ice Shelf System that drains 1 million km 2 of the East Antarctic Ice Sheet, and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early Oligocene time. This glacial record has been deciphered by (i) geomorphological mapping from aerial photographs and on the ground, (ii) documenting the relationship between thick well-dated, uplifted glaciomarine strata and the underlying palaeolandscape, (iii) examining surficial sediment facies, and (iv) surface-exposure dating using 10Be and 26Al. The SE Amery Oasis records at least 10 million years of landscape evolution beginning with a pre-late Miocene phase of glacial erosion, followed by deposition of glaciomarine strata of the Battye Glacier Formation (Pagodroma Group) in late Miocene time. A wet-based ice sheet next expanded over the SE Amery Oasis, following which deposition of the glaciomarine Pliocene Bardin Bluffs Formation (Pagodroma Group) took place. Both formations were uplifted; by at least 500 and 200 m, respectively. Their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place. Higher-level bedrock areas were subjected to deep weathering and tor-formation. Early Pleistocene time was characterised by expansion of a cold-based ice sheet across the whole area, but it left little more than patches of sandy gravel and erratic blocks. Late Pleistocene expansion of local ice (the Battye Glacier) saw deposition of moraine-mound complexes on low ground around Radok Lake and ice-dammed lake phenomena. Subglacial drainage of the lake escaped to the east exhuming the sediment-filled gorges. Holocene landscape modification has been relatively superficial. Overall, the landscape of the Amery Oasis evolved primarily under the influence of wet-based (probably polythermal) glaciers in Miocene and Pliocene times, whereas the Quaternary Period was characterised mainly by cold-based glaciers that had comparatively little impact on the landscape.

Grain size of loess, palaeosol and Red Clay deposits on the Chinese Loess Plateau: Significance for understanding pedogenic alteration and palaeomonsoon evolution

To assess pedogenic modification to grain size distributions of loess, palaeosol and Red Clay deposits on the Chinese Loess Plateau and to understand long-term evolution of the East Asian palaeomonsoon since the late Miocene, we investigate a continuous loess–palaeosol–Red Clay sequence at Lingtai (south Chinese Loess Plateau) and another parallel Red Clay sequence at Zhaojiachuan (central Chinese Loess Plateau). By analyzing the grain size distributions of bulk samples and chemically isolated quartz samples, the relative intensity of pedogenic alteration of the loess, palaeosol and Red Clay deposits can be quantified. Comparisons of the grain size distributions and contents of different grain size fractions between the bulk and the quartz samples suggest that pedogenic alteration is apparently stronger in the Red Clay sequence than in the overlying loess–palaeosol deposits. Furthermore, grain size parameters derived from the bulk and the quartz samples, such as mean grain size, coarse fraction content and U-ratio, exhibit similar variations in the loess–palaeosol sequence (Quaternary, 0–2.6 Ma), whereas these grain size parameters of quartz samples show different variability from those of bulk samples in the Red Clay sequence (late Miocene to Pliocene, 2.6–7 Ma). Grain size of quartz from the loess–palaeosol and Red Clay deposits exhibits distinct and persistent oscillations from the late Miocene to the Pleistocene, implying that significant fluctuations of the palaeomonsoon climate in East Asia might have occurred at least since late Miocene time.

On-going orogeny in the outer-arc of the Timor–Tanimbar region, eastern Indonesia

The Timor–Tanimbar islands of eastern Indonesia form a non-volcanic arc in front of a 7 km deep fore-arc basin that separates it from a volcanic inner arc. The Timor–Tanimbar Islands expose one of the youngest high P/ T metamorphic belts in the world, providing us with an excellent opportunity to study the inception of orogenic processes, undisturbed by later tectonic events. Structural and petrological studies of the high P/ T metamorphic belt show that both deformation and metamorphic grade increase towards the centre of the 1 km thick crystalline belt. Kinematic indicators exhibit top-to-the-north sense of shear along the subhorizontal upper boundaries and top-to-the-south sense in the bottom boundaries of the high P/ T metamorphic belt. Overall configuration suggests that the high P/ T metamorphic rocks extruded as a thin sheet into a space between overlying ophiolites and underlying continental shelf sediments. Petrological study further illustrates that the central crystalline unit underwent a Barrovian-type overprint of the original high P/ T metamorphic assemblages during wedge extrusion, and the metamorphic grade ranged from pumpellyite-actinolite to upper amphibolite facies. Quaternary uplift, marked by elevation of recent reefs, was estimated to be about 1260 m in Timor in the west and decreases toward Tanimbar in the east. In contrast, radiometric ages for the high P/ T metamorphic rocks suggest that the exhumation of the high P/ T metamorphic belt started in western Timor in Late Miocene time and migrated toward the east. Thus, the tectonic evolution of this region is diachronous and youngs to the east. We conclude that the deep-seated high P/ T metamorphic belt extrudes into shallow crustal levels as a first step, followed by doming at a later stage. The so-called ‘mountain building’ process is restricted to the second stage. We attribute this Quaternary rapid uplift to rebound of the subducting Australian continental crust beneath Timor after it achieved positive buoyancy, due to break-off of the oceanic slab fringing the continental crust. In contrast, Tanimbar in the east has not yet been affected by later doming. A wide spectrum of processes, starting from extrusion of the high P/ T metamorphic rocks and ending with the later doming due to slab break-off, can be observed in the Timor–Tanimbar region.

Cenozoic orogenic growth in the Central Andes: Evidence from sedimentary rock provenance and apatite fission track thermochronology in the Fiambalá Basin, southernmost Puna Plateau margin (NW Argentina)

Intramontane sedimentary basins along the margin of continental plateaus often preserve strata that contain fundamental information regarding the pattern of orogenic growth. The sedimentary record of the clastic Miocene–Pliocene sequence deposited in the Fiambalá Basin, at the southern margin of the Puna Plateau (NW Argentina), documents the late Miocene paleodrainage evolution from headwaters to the west, towards headwaters in the ranges that constitute the border of the Puna Plateau to the north. Apatite Fission track (AFT) thermochronology of sedimentary and basement rocks show that the southern Puna Plateau was the source for the youngest, middle Miocene, detrital population detected in late Miocene rocks; and that the margin of the Puna Plateau expressed a high relief, possibly similar to or higher than at present, by late Miocene time. Cooling ages obtained from basement rocks at the southern Puna margin suggest that exhumation started in the Oligocene and continued until the middle Miocene. We interpret the basin reorganization and the creation of a high relief plateau margin to be the direct response of the source–basin system to a wholesale surface uplift event that may have occurred during the late Cenozoic in the Puna–Altiplano region. At this time coeval paleodrainage reorganization is observed not only in the Fiambalá Basin, but also in different basins along the southern and eastern Puna margin, suggesting a genetic link between the last stage of plateau formation and basin response. However, this event did not cause sufficient exhumation of basin bounding ranges to be recorded by AFT thermochronology. Our new data thus document a decoupling between late Cenozoic surface uplift and exhumation in the southern Puna Plateau. High relief achieved at the Puna margin by late Miocene time is linked to Oligocene–Miocene exhumation; no significant erosion (< 3 km) has occurred since in this arid highland.

Different stress states in the Burdur–Isparta region (SW Turkey) since Late Miocene times: a reflection of a transient stress regime

The Burdur–Isparta region is a transitional zone between two distinct neotectonic domains in Turkey: the Burdur graben system as the northeastern part of the Fethiye–Burdur fault zone and the Kovada graben at the apex of the Isparta angle. Each domain represents a particular stress regime. Fault data from unconsolidated Pliocene and Quaternary deposits were combined with stress states deduced from pre-Pliocene consolidated rocks in order to reconstruct the stress field evolution in the study area since Late Miocene. Different stress states were identified. They were put in a stratigraphic framework and U/Th dating of fault-related calcite precipitates proved to be an additional tool in constraining the history of the stress states. The reconstructed history reflects a transient stress regime. The evolving stress field indicates that the influence of the Kovada stress regime fades through time and that the influence of the Burdur stress regime is gaining importance, suggesting a northeastwards progression of the Fethiye–Burdur fault zone.

Geologie der Iberger Klippen und ihrer Flysch-Unterlage

Geologie der Iberger Klippen und ihrer Flysch-Unterlage

Petroleum systems of the Northern Malay Basin

The Northern Malay Basin petroleum province, offshore Peninsular Malaysia, comprises a central/basinal gas-rich area, flanked on both sides and to the south by mixed oil/gas zones. Non-associated gas fields in the central zone (Cakerawala to Bujang Trend) are found mainly in groups D and E reservoirs, in anticlinal traps formed by basin inversion during late Miocene times. This distribution may be biased by the depth of well penetrations in the basin centre due to the onset of overpressure. Oil occurs in faulted traps along the Western Hinge Fault Zone (Kapal to Beranang Trend), and is especially abundant on the NE ramp margin (Bunga Pakma-Raya Trend) where a separate kitchen may be present. Oil geochemistry reveals three main types of source rocks for the oils: lower coastal plain, fluvial marine and lacustrine source rocks. Most of the oils and condensates in the basin centre and on the Western Hinge Fault Zone are lower coastal plain oils, indicating charge from the basin centre. Lacustrine oils are restricted to the Bunga Pakma-Raya Trend on the NE flank, indicating charge from the basin centre as well as input from a small sub-basin to the northeast. Marine influence was found in oils from the most central position in the basin (Cakerawala-Bumi area). Vitrinite reflectance and basin modelling indicate that hydrocarbons were generated from source rocks within two main stratigraphic intervals: Group H and Group I, which are presently in the peak oil generation and gas generation stages, respectively. The basin-centre gas fields are charged from directly underneath, i.e. from the Group I kitchen, and from the post-mature shales in the older units (e.g. groups J and K). Vertical migration, assisted by deep-seated faults, is the dominant process in the basin centre. The enormous volume of thermogenic gas generated at the basin centre appears to have largely flushed out much of Group H oil that might have filled the D and E reservoirs initially. As a result, oil is more likely to have re-migrated and be trapped along the faulted basin margins, such as in the Western Hinge Fault Zone, away from the basin-centre gas kitchen. Limited oil accumulations may still exist in the basin centre where gas flushing is less effective. Oil could also be present below the regional overpressure seal (Group F) in the basin centre. The gases in the Northern Malay Basin contain varying amounts of CO 2 . High CO 2 concentrations (>50 mol%) are typical of reservoirs in groups I and older and are mainly derived from inorganic sources. Low CO 2 concentrations (<6 mol%) are more typical of the reservoirs in groups D and E, and are derived from organic sources (thermal degradation of kerogen). The inorganic CO 2 distribution appears to be governed by proximity to deep-seated faults that act as conduits for fluid migration.

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. Visegrad Castle Hill) and a hilltop lava dome (Szent Mihaly 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.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.

Crustal versus asthenospheric origin of relief of the Atlas Mountains of Morocco

We investigate the respective roles of crustal tectonic shortening and asthenospheric processes on the topography of the High Atlas and surrounding areas (Morocco). The lithospheric structure is modeled with a direct trial‐and‐error algorithm taking into account gravity (Bouguer and free air), geoid, heat flow, and topography. Three parallel cross sections, crossing the High Atlas and Anti‐Atlas ranges, show that the lithosphere is thinned to 60 km below these mountain ranges. An analysis of the effect of the lithospheric thinning allows us to conclude that the whole topography of the Anti‐Atlas, which belongs to the Sahara domain, is due to asthenospheric processes. In the High Atlas the lithospheric thinning explains a third of the relief of the western High Atlas, 500 m for a mean altitude of 1500 m, and half of the relief of the central High Atlas, 1000 m for a mean altitude of 2000 m. At the scale of Morocco the domain affected by lithospheric thinning forms an elongated NE‐SW strip crossing not only the main structural zones but also the Atlantic margin to the south and the Africa‐Eurasia plate boundary to the north. This major lithospheric thinning is associated with Miocene to recent alkaline volcanism and seismicity. We propose that this thermal anomaly is related to a shallow mantle plume, emplaced during middle to late Miocene time, during a period of relative tectonic quiescence.

Neogene vertebrates from Urumaco, Falcón State, Venezuela: Diversity and significance

Synopsis The first palaeontological discoveries in Urumaco were made more than half a century ago. Several research groups have worked in the region, but new vertebrate fossils are still being found in this area of northwestern Venezuela. The most conspicuous elements of the fauna are reptiles, because of their size and abundance, although fishes are by far the most diverse group. Among South American Neogene assemblages the whole fauna is most similar to those of Acre in Brazil and Entre Rios in Argentina. The crocodile fauna includes at least 12 species representing diverse ecomorphological types; most of them lived sympatrically, a unique case in worldwide crocodylomorph assemblages. At least four turtle species provide unique palaeobiogeographical or palaeobiological information. Several freshwater species, including Serrasalminae fish, thorny catfishes, silver croaker, redtail catfishes, matamata turtle, river dolphin and probable sirenians, are consistent with the hypothesis that a tributary and/or delta of the Orinoco existed in this area of north‐western Venezuela during late Miocene times.

Fault and fold interaction during the development of the Neogene-Quaternary Almería-Níjar basin (SE Betic Cordilleras)

Abstract The Neogene-Quaternary Almería-Níjar basin includes the Carboneras Fault, which constitutes a major left-lateral feature of the Betic Cordilleras. New gravity data help to determine the geometry of the sedimentary infill. The region underwent NE-SW extension during the Tortonian and local NW-SE compression during the first stages of Sierra Alhamilla uplift. During the Messinian, the sinistral strike-slip motion along the Carboneras Fault Zone, the dextral strike-slip motion along NW-SE-oriented faults, and the development of large folds such as the Sierra Alhamilla antiform, suggest clockwise rotation (towards the north) of the maximum stress axis (σ 1 ). During the Pliocene, a NNW-SSE-oriented compression also contributes to fold development. Finally, during the Quaternary, an ENE-WSW-directed extension controls the development of NW-SE-oriented normal oblique faults. The most recent local normal activity of the Carboneras Fault is related to this extension, whereas its behaviour as a left-lateral strike-slip fault may be a consequence of the accommodation of NW-SE normal fault displacements. Basic rock bodies, recognized by means of a detailed study of the magnetic anomalies, are related to the volcanic activity known to have occurred in the area in Late Miocene times.

Geometry and structural evolution of the Mesohellenic Trough (Greece): a new approach

Abstract The Mesohellenic Trough (MHT) is an elongate basin parallel to the Hellenide isopic zones that extends from southern Albania through northern Greece. The basin developed from Mid-Late Eocene to Mid-Late Miocene time related to Alpine orogenic processes. Structural and kinematic evidence shows that the MHT developed in response to successive tectonic events, involving isostatic crustal flexure, strike-slip and normal faulting, all related to inferred oblique convergence of the Apulian and Pelagonian microcontinents. The Mesohellenic Trough evolved as a piggyback basin above westward-emplacing ophiolites and higher Pelagonian units. This differs from previous interpretations that envisaged foreland flexure related to backthrusting, or subsidence associated with asymmetrical flexure, or normal faulting. The first stage of basin development during the Mid-Late Eocene was contemporaneous with the final emplacement of Pindos oceanic units and culminated in deformation and uplift of Eocene strata. The second phase was dominated by strikeslip faulting during Oligocene-Early Miocene time. The third stage was characterized by low-angle normal faulting at the eastern boundary of the MHT during the Early-Late Miocene. The evolution of the sedimentary basin ended around Late Miocene time, followed by rapid uplift and marine regression. A compressional event occurred during the latest Miocene. Finally, extensional tectonics affected the area from the Late Miocene to the present.

Growth, Demise, and Dolomitization of Miocene Carbonate Platforms on the Marion Plateau, Offshore NE Australia

Abstract Strontium-isotope stratigraphy has been used to examine the timing of depositional events and dolomitization in two Miocene carbonate platforms cored by Ocean Drilling Program (ODP) Leg 194, just seaward of the Great Barrier Reef. The results provide firm constraints for correlating surfaces and depositional stages between the two platforms and thereby relating seismic sequences previously defined in the off-platform sediments to the lithostratigraphic units described from cores in the seismically transparent platform-top sites. Oyster-bearing beds at the base of both platform successions yield early Oligocene ages (29–31 Ma), thus dating initial transgression of the Marion Plateau's volcanic basement. There followed a period of slow accumulation of shallow-water grainstones rich in quartz and phosphate grains in late Oligocene time (29–23 Ma; seismic Megasequence A). The main growth of the carbonate platforms took place in early to late Miocene time (23–7 Ma), comprising five depositional sequences. The first four of these (seismic Megasequence B) are common to both platforms and terminated with a possible karst surface at 10.7 Ma. Different sedimentologic expression of this megasequence in the two platforms reflects contrasting progradational versus aggradational geometries in the locations studied. The final growth stage (seismic Megasequence C) occurred only in the southern platform and terminated at 6.9 Ma. Both platform-demise events (10.7 and 6.9 Ma) approximately coincide with falls in global sea level combined with longer-term trends of decreasing water temperature. Sr-isotope ages of dolostones increase with depositional age, and older dolostones in the southern platform have more coarsely crystalline and fabric-destructive textures than overlying younger dolostones. These relationships are consistent with dolomitization by normal seawater shortly after deposition and overprinting of multiple times of dolomite recrystallization and cementation in the deeper strata.

Rapid late Miocene rise of the Bolivian Altiplano: Evidence for removal of mantle lithosphere

Abstract The oxygen isotopic compositions of carbonates deposited in the northern Altiplano demonstrate a rapid change in late Miocene time, which we attribute to an increase in elevation. Rainfall samples over an elevation transect adjacent to the Altiplano of northern Bolivia display a systematic decrease in mean annual δ 18 O values and define a local δ 18 O vs. altitude gradient [R. Gonfiantini, M.-A. Roche, J.-C. Olivry, J.-C. Fontes, G.M. Zuppi, The altitude effect on the isotopic composition of tropical rains, Chem. Geol. 181 (2001) 147-167.], which we apply to paleo-meteoric water values determined from carbonates. Age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of ∼2.5 to 3.5 km occurred between ∼10.3 Ma and 6.8 ± 0.4 Ma, when the Altiplano rose to its current elevation. Only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity. This change coincides with an ∼30% decrease in the rate of convergence between the South America and Nazca plates and the propagation of deformation into the eastern Subandean zone, which we infer to have resulted from the increase in the force per unit length applied by the elevated Andean plateau to the surrounding lowlands.

Geochronology of the Nabwal Hills: a record of earliest magmatism in the northern Kenyan Rift Valley

The Nabwal Hills, northeast of Lake Turkana, contain a record of magmatism associated with the initiation and early development of the East African Rift System in northernmost Kenya. The predominantly volcanic Asille Group, 1400 m thick, directly overlies metamorphic basement and comprises a sequence of basaltic lava flows with significant intervals of rhyolitic pyroclastic units, and minor intercalations of fluviatile sediments. The basement gneisses yield K–Ar cooling ages on biotite of 510 and 522 Ma, typically Pan-African. The 40Ar–39Ar ages on alkali feldspar crystals from the rhyolitic units are concordant and show that the Asille Group spans an interval from at least 34.3 to 15.8 Ma, continuing to at least as young as 13 Ma based on previous measurements. Vertebrate fossil sites, containing primate remains, at Irile and Nabwal are shown to be 17 ± 2 Ma old, Early Miocene, based upon K–Ar age measurements on immediately overlying basalts. Variably reliable whole rock K–Ar ages, determined on basalt samples from low in the sequence, indicate that volcanism commenced as early as 34.8 Ma ago. The overall geochronological results show that magmatism in the Nabwal Hills began about 35 Ma ago in Late Eocene times, interpreted as the time of initiation of crustal extension that led to the development of this segment of the East African Rift System. The Asille Group is tilted about 6° to the SSW. This tilting occurred later than 13 Ma ago, and prior to the eruption of the flat-lying Gombe Group basalts. These basalts may have begun erupting about 6 Ma ago in Late Miocene times, although much of this volcanism occurred between about 3.9 and 4.2 Ma ago in Pliocene times. It is suggested that the main rifting, which continues today, commenced in Late Miocene times, less than 13 Ma ago, and is partly reflected in the tilting of the Asille Group.

Alpine polyphase tectono-metamorphic evolution of the South Carpathians: A new overview

The main terrains involved in the Cretaceous–Tertiary tectonism in the South Carpathians segment of the European Alpine orogen are the Getic–Supragetic and Danubian continental crust fragments separated by the Severin oceanic crust-floored basin. During the Early–Middle Cretaceous times the Danubian microplate acted initially as a foreland unit strongly involved in the South Carpathians nappe stacking. Multistage folding/thrusting events, uplift/erosion and extensional stages and the development of associated sedimentary basins characterize the South Carpathians during Cretaceous to Tertiary convergence and collision events. The main Cretaceous tectogenetic events responsible for contraction and crustal thickening processes in the South Carpathians are Mid-Cretaceous (“Austrian phase”) and Latest Cretaceous (“Laramide” or “Getic phase”) in age. The architecture of the South Carpathians suggests polyphase tectonic evolution and mountain building and includes from top to bottom: the Getic–Supragetic basement/cover nappes, the Severin and Arjana cover nappes, and Danubian basement/cover nappes, all tectonically overriding the Moesian Platform. The Severin nappe complex (including Obarsia and Severin nappes) with Late Jurassic–Early Cretaceous ophiolites and turbidites is squeezed between the Danubian and Getic–Supragetic basement nappes as a result of successive thrusting of dismembered units during the inferred Mid- to Late Cretaceous subduction/collision followed by tectonic inversion processes. Early Cretaceous thick-skinned tectonics was replaced by thin-skinned tectonics in Late Cretaceous. Thus, the former Middle Cretaceous “Austrian” nappe stack and its Albian–Lower Senonian cover got incorporated in the intra-Senonian “Laramide/Getic” stacking of the Getic–Supragetic/Severin/Arjana nappes onto the Danubian nappe duplex. The two contraction events are separated by an extensional tectonic phase in the upper plate recorded by the intrusion of the “Banatitic” magmas (84–73 Ma). The overthrusting of the entire South Carpathian Cretaceous nappe stack onto the fold/thrust foredeep units and to the Moesian Platform took place in the Late Miocene (intra-Sarmatian) times and was followed by extensional events and sedimentary basin formation.

Late Miocene–Quaternary volcanism, tectonics and drainage system evolution in the East Carpathians, Romania

Middle Miocene (Sarmatian) convergence created the fold and thrust belt of the Eastern Carpathians of Romania, which subsequently experienced post-collisional crustal deformation combined with calc-alkaline and alkalic-basaltic volcanism in late Miocene–Quaternary time. This deformation led to the rise of the Cǎlimani–Gurghiu–Harghita volcanic mountains and to the subsidence of the N–S-oriented intramontane Borsec/Bilbor–Gheorgheni–Ciuc and Braşov pull-apart basins, and the E-oriented monocline-related Fǎgǎraş basin. The regional drainage network is the composite of: (1) Older E-, SE- and S-flowing rivers, which cross the Carpathians, radiate towards the foreland and were probably established during the Middle Miocene (Sarmatian) collision event. (2) A more recent drainage system related to the contemporaneous development of the volcanoes and intramontaneous basins, which generally drains westward into the Transylvanian Basin since late Miocene time and has been capturing the older river system. The older river drainage system has also been modified by Late Pliocene–Quaternary folding, thrusting and monoclinal tilting along the Pericarpathian orogenic front and by reactivated transverse high angle basement faults, which cross the Eastern Carpathian foreland.