Pliocene Time Research Articles

Ground Information and Selection of TBM for the Thessaloniki Metro, Greece

The Thessaloniki Metropolitan Railway is currently at its primary construction stage. The railway is composed of two separate ∼6-m-diameter parallel tunnels, each of which is ∼8 km long and has 13 stations. The span between the tunnel axes is approximately 12 m, and the depth of the red line (track level) varies from 15 m to 30 m. The geology of the urban area of Thessaloniki is characterized by the presence of Neogene and Quaternary deposits. The base formation for the study area is a very stiff to hard red clay, dating to Upper Miocene–Pliocene time. On top of this formation, Quaternary sediments have been deposited, most of which consist of sand and/or gravel in a clay-silt–dominated matrix, covered in places by anthropogenic fill. Ground investigation campaigns have incorporated a significant number of sampling boreholes and in situ and laboratory testing. In this study, data from borehole loggings, particle-size analyses, natural moisture content, Atterberg limits, permeability tests, pressure-meter tests, cone penetration tests (CPT), and standard penetration tests (SPT) were analyzed in order to obtain a better geological understanding, a geotechnical zonation, and a classification of the ground with respect to mechanized tunneling. The characteristics and parameters of the soils and the hydrogeological regime indicate that an EPBM (Earth Pressure Balance Machine) rather than a slurry TBM (Tunnel Boring Machine) should be used for the tunnel construction.

Depositional origin of tuffaceous units in the Pliocene Upper Siwalik Subgroup, Jammu (India), NW Himalaya

During late Pliocene times, extrabasinally derived acidic volcanic ashes were deposited as distal pyroclastic fallout in upland interfluvial–lacustrine settings in the northwestern part of the Siwalik basin. These ash beds occur as a bentonitized tuff band and tuffaceous mudstones in the Jammu region of India. We located and described 12 outcrops of this conspicuous, geographically widespread bentonitized tuff band and tuffaceous mudstone association at the same stratigraphic level, coincident with the Gauss–Matuyama boundary at 2.48 Ma. This bentonitized tuff band and tuffaceous mudstone association represents a stratigraphic marker horizon in the diachronous and sporadic vertebrate fauna-yielding Siwalik strata. The claystone, siltstone, and sandstone units and embedded bentonitized tuff band and tuffaceous mudstone beds represent a coarsening-upward sequence reflecting deposition in lacustrine proximal and distal bottomsets, pro-delta foresets, and mouth-bar facies capped by fluvial topsets. This study reports a chain of four contemporaneous, palaeo-lake basins at this stratigraphic level, which ranged in length from 2 to 7 km.

Strike-slip systems as the main tectonic features in the Plio-Quaternary kinematics of the Calabrian Arc

The oblique and diachronous collision of the Apennine-Maghrebian Chain with the Apulian (in the north-east) and Pelagian (in the south) continental forelands, has determined the characteristic arcuate structure of this orogen. The effects of Plio-Pleistocene deformation of the Calabrian Arc have been analysed on the basis of available reflection seismic profiles and using local time-structural maps reconstructed along the main structures. During this period, internal sectors of the Tertiary chain migrated forward on the oceanic Ionian foreland, and were cut by important strike-slip systems. These last have an orientation approximately coincident with that of the migration of the front, allowing differential movement of the different sectors of the arc, towards the weakly buoyant Ionian oceanic domain. The dataset suggests a clear connection between the development of the strike-slip systems cutting the chain and the direction of tectonic transport, towards the East during Late Messinian/Early Pliocene time, to the ESE during Late Pliocene/Early Pleistocene time, finally to the SSE during the Middle/Late Pleistocene to Present, showing a clockwise rotation in well defined stages during the kinematic evolution of the chain. The origin of the Strait of Messina during the different phases is also interpreted in the context of the analysed regional tectonic setting.

Atmospheric transport and concentration of diatoms in surficial and glacial sediments of the Allan Hills, Transantarctic Mountains

A global climate up to 3 °C warmer than present around 3 Ma is widely accepted, but the response of the Antarctic ice sheets at that time is still debated. The presence of rare Pliocene marine diatoms in the Sirius Group tillites of the Transantarctic Mountains has led some to infer that the warming caused a collapse of the ice sheet, exposing the Antarctic interior to inland seas. Here we document an alternative source for the Sirius Group diatoms through atmospheric transport onto the surface of the East Antarctic Ice Sheet to be carried downslope by katabatic winds flowing over the Allan Hills, one of many nunataks that border the East Antarctic Ice Sheet. Evidence comes from diatom remains recovered in windtraps, surficial sediments and Sirius Group deposits themselves. The abundance and preservation of the diatoms being actively transported and trapped in the Allan Hills far exceeds those observed in the Sirius Group. Melting of wind-blown snow during summer provides a mechanism to transport and concentrate these diatoms into “diatom-rich” layers. This mechanism, along with the extreme sparseness and sporadic occurrence of diatoms previously documented in the Sirius Group, suggests a more likely scenario in which diatoms from the atmosphere contaminated tillites that had been deposited beneath wet-based glaciers in much older and warmer times. In this scenario, Pliocene marine diatoms may have been injected into the troposphere by meteorite impact, or picked up from exposed land around the Ross Embayment following the disappearance of the Ross Ice Shelf in Pliocene times. We conclude that the Pliocene age assigned to the Sirius Group on the basis of rare diatoms should be regarded as unproven, and the inferred collapse of the East Antarctic Ice Sheet at that time unlikely.

Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran

Oligocene‐Miocene strata preserved in synclinal outcrop belts of the western Alborz Mountains record the onset of Arabia‐Eurasia collision‐related deformation in northern Iran. Two stratigraphic intervals, informally named the Gand Ab and Narijan units, represent a former basin system that existed in the Alborz. The Gand Ab unit is composed of marine lagoonal mudstones, fluvial and alluvial‐fan clastic rocks, fossiliferous Rupelian to Burdigalian marine carbonates, and basalt flows yielding 40Ar/39Ar ages of 32.7 ± 0.3 and 32.9 ± 0.2 Ma. The Gand Ab unit is correlated with the Oligocene–lower Miocene Qom Formation of central Iran and is considered a product of thermal subsidence following Eocene extension. The Narijan unit unconformably overlies the Gand Ab unit and is composed of fluvial‐lacustrine and alluvial fan sediments exhibiting contractional growth strata. We correlate the Narijan unit with the middle to upper Miocene Upper Red Formation of central Iran on the basis of lithofacies similarities, stratigraphic position, and an 8.74 ± 0.15 Ma microdiorite dike (40Ar/39Ar) that intruded the basal strata. Deformation timing is constrained by crosscutting relationships and independent thermochronological data. The Parachan thrust system along the eastern edge of the ancestral Taleghan‐Alamut basin is cut by dikes dated at 8.74 ± 0.15 Ma to 6.68 ± 0.07 Ma (40Ar/39Ar). Subhorizontal gravels that unconformably overlie tightly folded and faulted Narijan strata are capped by 2.86 ± 0.83 Ma (40Ar/39Ar) andesitic lava flows. These relationships suggest that Alborz deformation had migrated southward into the Taleghan‐Alamut basin by late Miocene time and shifted to its present location along the active range front by late Pliocene time. Data presented here demonstrate that shortening in the western Alborz Mountains had started by late middle Miocene time. This estimate is consistent with recent thermochronological results that place the onset of rapid exhumation in the western Alborz at ∼12 Ma. Moreover, nearly synchronous Miocene contraction in the Alborz, Zagros Mountains, Turkish‐Iranian plateau, and Anatolia suggests that the Arabia‐Eurasia collision affected a large region simultaneously, without a systematic outward progression of mountain building away from the collision zone.

Exhumation across the Indus Suture Zone: a record of back sliding of the hanging wall

AbstractThe Kohistan Arc Complex is an integral part of the NW Himalayan collision system and is bounded by two major suture zones, the Indus Suture Zone (ISZ) and the Northern Suture in the south and north respectively. Fission‐track analyses on samples collected along the Indus River across the arcuated ISZ in the Besham region are presented here. The footwall yields zircon and apatite fission‐track (FT) ages of ∼23 Ma and ∼3.7 Ma respectively; the hanging wall ages range from 24 to 42 Ma for zircon and ∼10 Ma for apatite. Thus, the change in ISZ kinematics from thrusting to normal faulting was not later than Oligocene and normal faulting on this ISZ segment was still active at least into early Pliocene times. At this time normal faulting had already ended at other ISZ segments, but it was still (or again) active across the ISZ in the Besham region most likely as a local phenomenon caused by the growth of the Indus Syntaxis, a transverse antiform parallel to the Nanga Parbat Syntaxis.

Polyphase deformation in a newly emerged accretionary prism: Folding, faulting and rotation in the southern Taiwan mountain range

In Taiwan, the oblique late Cenozoic collision of the Eurasian Plate with the Luzon arc provides an opportunity to observe the development of arc–continent collision. However, analyses of tectonic structures demonstrate that a simple ESE–WNW compression, as induced by the arc–continent collision, fails to explain all aspects of the Taiwan Mountain building processes. This is the case for the rapid uplift and exhumation of the metamorphic rocks, the east–west trending fold-and-thrust systems in southeastern Taiwan, the tectonic rotation of the Hengchun peninsula, the escape phenomenon in the Ilan and Pingtung Plains, which are difficult to interpret using a “simple collision mechanism”. In this paper we focus on the structural record of southern Taiwan mountain range, in order to clarify the process of deformation in the growing accretionary prism. In view of geological stratigraphy, the southern Taiwan is underlain by thick series of Miocene deposits, including dark gray argillites, flysch deposits with occasional interbeds of gray compact sandstone and disseminated marly nodules. Field works have been done to reconstruct the main tectonic paleostress and deformation events of this region. Polyphase brittle and fold structures are largely distributed in this area, which allows us to determine the tectonic history in terms of nature, direction, importance and relative chronology of the main deformation events of the Late Cenozoic. These results provide an important key to reconstruct the in situ geotectonic history of the southern Taiwan region, which can be discussed in terms of four main stages. The stage 0 corresponds to the Early to Middle Miocene time, with WNW–ESE extension. The stage I corresponds to the Late Miocene to Pliocene time, when the study area were incorporated into the accretionary prism by west-verging thrust faults. The stage II is Pliocene in age, with continuing compression between the volcanic arc and the continental margin producing series of east-verging backthrusts. During the Plio-Pleistocene time, arc–continent collision occurred in Taiwan, as stage III in our study area, which produced conspicuous transpressional structures in the southern Central Range. A new mountain building model, involving limited counterclockwise rotation in the transition zone between incipient collision and mature collision, is also proposed to explain the recent deformation mechanism of the southern Taiwan range.

Pliocene and Lower Pleistocene fluvial history of the Lower Rhine Embayment, Germany: Examples of the tectonic forcing of river courses

The Plio–Pleistocene succession has been studied in the northwestern and southern Lower Rhine Embayment by means of (a.o.) petrographical, geochemical and palaeomagnetic methods. Data obtained provide improved insight into the drainage pattern of the study area and give evidence to tectonically driven shifts of river courses. In the northwestern part of the region, the river Rhine changed its course gradually towards the north during Late Pliocene and Lower Pleistocene times. These shifts were forced by an increase of subsidence rates in the same direction. In the southern Lower Rhine Embayment, an infilling of the main channel of the Latest Pliocene Rhine river, the latter of which is represented by the newly defined “Öbel beds” throughout the Lower Rhine Embayment, was found in the central part of the tectonic unit of the Erft block (open-cast mine Hambach). Due to subsequent eastward tilting of this tectonic unit, the course of the Rhine shifted into the same direction. New palaeomagnetic results concerning the position of the Gauss–Matuyama boundary at the open-cast mine Hambach support this concept. A geochemical study of fine grained sediments has given evidence to a similar eastward shift of a local river entering the Lower Rhine Embayment from the south during Late Pliocene times. The Meuse river entered the Lower Rhine Embayment from the southwest during Pliocene times (“East Meuse river”). Probably, this course prevailed longer during the Early Pleistocene than previously thought. The advance of this river towards the east during Early Pleistocene times is probably due to the eastward tilting of the Erft block as well.

Late‐ to post‐orogenic exhumation of the Central Pyrenees revealed through combined thermochronological data and modelling

ABSTRACTApatite (U–Th)/He and fission track thermochronometry have been combined with 3D thermal modelling to constrain the late‐ to post‐orogenic exhumation history of the Central Pyrenees, Spain. Data from four massifs immediately north and south of the present drainage divide of the mountain belt reveal a diachroneity in the transition from syn‐ to post‐orogenic forcing of exhumation. Immediately south of the drainage divide, rapid exhumation of ∼1.5 mm year−1 decelerated after ∼30 Ma to ∼0.03 mm year−1. A similar transition occurred immediately north of the drainage divide at the same time. Further south, in the core of the Axial Zone antiformal stack of the Pyrenees, rapid (∼1 mm year−1), syn‐orogenic exhumation continued to ∼20 Ma, but slowed to ∼0.1–0.2 mm year−1 soon after that time. This order of magnitude decrease in exhumation rates across the orogen records the diachronous transition into a post‐orogenic state for the mountain belt. These data do not record rejuvenation of exhumation in Late Miocene or Pliocene times driven either by large‐scale base‐level change or an evolution to more erosive climatic conditions.

Large‐scale deformation in a locked collisional boundary: Interplay between subsidence and uplift, intraplate stress, and inherited lithospheric structure in the late stage of the SE Carpathians evolution

The interplay between slab dynamics and intraplate stresses in postcollisional times creates large near‐surface deformation, particularly in highly bent orogens with significant lateral variations in mechanical properties. This deformation is expressed through abnormal foredeep geometries and contrasting patterns of vertical movements. Intraplate folding is often the controlling mechanism, particularly when the orogenic belt is locked. The study of these tectonic processes in the SE Carpathians indicates a generalized subsidence period during latest Miocene–Pliocene times driven by the slab‐pull and an intraplate folding due to an overall Quaternary inversion. The latter accommodates ∼5 km ESE‐ward movement of this area with respect to the neighboring units, which creates complicated three‐dimensional deformation patterns potentially driven at a larger scale by the interaction between the Adriatic indentor and the entire Carpathians system. The lithospheric anisotropy inherited from the subduction times concentrates strain and induces large‐scale deformation far away from the active plate margins. This anisotropy is dynamic because of deep mantle processes related to the subducted slab during postcollisional times, such as thermal reequilibration or increase in slab dip.

Seismic expression and inferred depositional environments of Plio-Pleistocene sedimentary sequences in the southwestern Caspian Sea

Eight seismic profiles from the southwestern Caspian Sea were analyzed to establish the sedimentary environments and depositional history in the South Caspian Basin since Pliocene times. Based on reflection terminations, nine sequence boundaries (S1 to S9) were identified and traced across the study area. Consequently, nine depositional sequences (S1–S9) were defined on the basis of seismic facies analysis. The results suggest a transition from non-marine sedimentation in the Lower Pliocene to marine-dominated conditions in the Upper Pliocene (S1–S4). Marine conditions then continued to the present time; however, several sea-level changes led to the formation of sequences S5 to S9. Although no local sea-level curve is available due to a lack of well data, there is good agreement between the seismic stratigraphy of the study area and a published regional sea-level curve.

Sedimentary facies, depositional environments and palaeogeographic evolution of the Neogene Denizli Basin, SW Anatolia, Turkey

The Denizli Basin (southwestern Anatolia, Turkey) contains a record of environmental changes dating since the Early Miocene. Detailed facies analysis of the Neogene formations in this half-graben enables us to document successive depositional regimes and palaeogeographic settings. Sedimentation commenced in the Early Miocene with the deposition of alluvial-fan and fluvial facies (Kızılburun Formation). At this stage, alluvial fans sourced from elevated areas to the south prograded towards the basin centre. The Middle Miocene time saw the establishment of marginal lacustrine and wetland environments followed by the development of a shallow lake (Sazak Formation). The uppermost part of this unit consists of evaporitic saline lake and saline mudflat facies that grade upward into brackish lacustrine deposits of Late Miocene–Pliocene age (Kolankaya Formation). The lake became shallower at the end of the Pliocene time, as is indicated by expansion shoreface/foreshore facies. In the Early Quaternary, the Denizli Basin was transformed into a graben by the activation of ESE-trending normal faults. Alluvial fans were active at the basin margins, whereas a meandering river system occupied the basin central part. Oxygen isotope data from carbonates in the successive formations show an alternation of wetter climatic periods, when fresh water settings predominated, and very arid periods, when the basin hosted brackish to hypersaline lakes. The Neogene sedimentation was controlled by an active, ESE-trending major normal fault along the basin's southern margin and by climatically induced lake-level changes. The deposition was more or less continuous from the Early Miocene to Late Pliocene time, with local unconformities developed only in the uppermost part of the basin-fill succession. The unconformable base of the overlying Quaternary deposits reflects the basin's transformation from a half-graben into a graben system.

Spatial and temporal patterns of grain size and chemical weathering of the Chinese Red Clay Formation and implications for East Asian monsoon evolution

Palaeoclimatic interpretation of the Red Clay in the Chinese Loess Plateau is particularly important for understanding of the East Asian monsoon evolution from Pliocene to the Quaternary. In this study, a N–S transect involving five loess-Red Clay sections across the Loess Plateau was selected for detailed investigation with analyses of grain size and major element geochemistry, in order to address the spatial pattern. Results indicate that the Red Clay contains large amount of coarse (<6ϕ, i.e. >16 μm) fraction and shows a prominent N–S decreasing gradient much similar to that of the overlying loess, suggesting that the dust transport was dominantly by low-level northerly/northwesterly winds. Compared with overlying loess, the Red Clay is markedly fine and rather constant through all the sections, implying a distant and steady dust-source area about 200 km further north during most of the Pliocene time. Pedogenic leaching intensities inferred from geochemical data display a clear decreasing trend from south to north both for the Red Clay and the loess, suggesting that the modern spatial pattern of the East Asian monsoon regime has been maintained at least for the past 5 Ma. The N–S spatial gradient, however, might be smaller during the Pliocene than the Quaternary period. Temporally, two short periods of climatic deterioration implied by marked grain-size coarsening and low weathering intensity occurred at 5.3 and 3.5 Ma, respectively, which divide the Pliocene into two major stages. The 5.3–3.5 Ma stage is characterized by a steady state climate through the entire region, consistent with other records. In the 3.5–2.7 Ma stage, the Red Clay in the Jingbian section shows a noticeable coarsening in grain size than the previous stage, indicating some southward extension of the deserts, in agreement with mass accumulation rate record in the north Pacific. But the eolian dust deposition remained relatively stable in major part of the Loess Plateau until 2.7 Ma. The Red Clay deposition was then terminated by massive and coarse loess deposition in the entire region, suggesting dramatic intensification of the winter monsoon in response to the onset of extensive glaciation of Northern Hemisphere.

The Monte Carrubba Formation (Messinian, Sicily) and its correlatives: New light on basin-wide processes controlling sediment and biota distributions during the Palaeomediterranean–Mediterranean transition

A little known marine Miocene (Messinian) succession in the Hyblean region of south eastern Sicily preserves a carbonate ramp sequence which developed on the tectonically stable foreland margins of the African Plate (Pelagian Block). The ramp (Monte Carrubba Formation) was distally steepened and contained a shoalwater barrier and lagoonal inner ramp complex. The outer ramp is poorly preserved though distal deep water ramp facies (Tellaro Formation) in the south Hyblean region preserve a full Globorotalia mediterranea Subzone and major part of the Globigerina multiloba Subzone succession. This ramp sequence faithfully records the subtle interplay between the global eustatic curve and internal Palaeomediterranean base-level signatures for the Late Miocene. The earliest second order Palaeomediterranean lowstand occurred in the Late Tortonian at about 7.3 Ma and a second occurred at about 6.75 Ma within the Early Messinian G. mediterranea Subzone. Both were in the order of 40 m and global sea-level was probably restored to the Palaeomediterranean after the lowstands. Although the overriding control at the time was the global eustatic curve, harmonic discrepencies between it and the Palaeomediterranean curve resulted from constriction of the Atlantic connection during the latest Tortonian. From about 6.4 Ma until about 5.96 Ma increasingly restricted conditions developed across the Monte Carrubba inner ramp. These faithfully reflect many second order, constriction-driven Palaeomediterranean sea-level oscillations which were superimposed on to the falling global signal as the connecting Atlantic seaways became constricted. These oscillations resulted in major reductions in planktonic faunal diversity within the outer ramp, followed by total emergence of the inner ramp by about 6.05 Ma. Marine deposition continued until about 5.96 Ma in the southern part of the Hyblean region. However, between 6.05 Ma and about 5.6 Ma the amplitude of successive Palaeomediterranean sea-level oscillations increased to over 100 m, causing prolonged periods of total Palaeomediterranean isolation. The Hyblean carbonate suite also contains post-Messinian Salinity Crisis (post-MSC) ‘Lago Mare’ highstand deposits which are developed unconformably above the Monte Carrubba Formation. The widespread distribution of hyposaline to near-marine Congeria faunas within the Lago Mare episode argues for high post-MSC Mediterranean water levels in the Central Mediterranean Late Messinian. In this article we draw together disparate regional evidence from this stable foreland setting and assimilate it with published data from regions lying on the European Plate in order to demonstrate that the tectonically stable Monte Carrubba succession provides an unambiguous record of Central Palaeomediterranean base-level change during the Messinian. Furthermore, the post-MSC sequence demonstrates a prolonged Mediterranean base-level rise throughout the Late Messinian, leaving little readjustment necessary in order to restore Mediterranean base-level to global water levels in basal Pliocene times.

Invaders or endemics? Molecular phylogenetics, biogeography and systematics of Dreissena in the Balkans

Summary1. Zebra mussels and their relatives (Dreissena spp.) have been well studied in eastern, central and western Europe as well as in North America, because of their invasiveness and economic importance. Much less is known about the biology and biogeography of indigenous (endemic) taxa of Dreissena, in the Balkans. A better knowledge of these taxa could help us (i) understand the factors triggering invasiveness in some taxa and (ii) identify other potentially invasive species.2. Using a phylogenetic approach (2108 base pairs from three gene fragments), Dreissena spp. from natural lakes in the Balkans were studied to test whether invasive Dreissena populations occur in such lakes on the Balkan Peninsula, whether Dreissena stankovici really is endemic to the ancient Lakes Ohrid and Prespa, and to infer the phylogenetic and biogeographical relationships of Balkan dreissenids.3. No invasive species of Dreissena, such as Dreissena polymorpha, were recorded. The supposedly ‘endemic’D. stankovici is not restricted to the ancient Lakes Ohrid and Prespa, but is the most widespread and dominant species in the west‐central Balkans. Its southern sister taxon, Dreissena blanci, occurs sympatrically with D. stankovici in Lakes Prespa, Mikri Prespa and Pamvotis. Both species are classified into the subgenus Dreissena (Carinodreissena) of which the subgenus Dreissena (Dreissena) (which includes the invasive D. polymorpha) is the sister taxon. Dreissena blanci and D. stankovici are considered to represent distinct species.4. On a global scale, the two Balkan species have small ranges. An early Pliocene time frame for the divergence of the subgenera Carinodreissena and Dreissena is discussed, as well as potential colonization routes of the most recent common ancestor of Carinodreissena spp.5. The ambiguous taxonomy of dreissenids in the Balkans is addressed. As nominal D. blanci presbensis from Lake Prespa has nomenclatural priority over D. stankovici, the correct name for the latter taxon should be Dreissena presbensis.

Cenozoic provenance history of synorogenic conglomerates in western Argentina (Famatina belt): Implications for Central Andean foreland development

In Famatina, at the modern northern flat-slab segment of the Central Andean belt, synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting. Stratigraphically constrained provenance analyses record a multi-event history of the Andean foreland and a complex pattern of exhumed uplands, which, in turn, controlled the depocenter history. Western and eastern sources can be differentiated. The western domain was composed of equigranular granitoids and lower and upper Paleozoic volcano-sedimentary and sedimentary units, whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower Paleozoic clastic rocks. The surrounding regions expose medium- to high-grade metamorphic rocks, which are not recorded in any Cenozoic conglomerate of Famatina. Unroofing sequences can be deduced from the compositional trends. Conglomerate composition and paleocurrent data show input from intraba-sinal sources and important participation of basement since the beginning of the Neogene. We infer an early Miocene broken foreland during deposition of the Del Creston Formation.A significant source inversion, with clasts supplied from the eastern basement domain, is recorded ∼2000 m from the base, within the middle Miocene Del Abra Formation; reactivation of western granitic thrust sheets occurred by the late middle Miocene and late Miocene, as indicated by provenance studies of the Santo Domingo and El Durazno Formations. In contrast to simple asymmetric foreland models, provenance analysis documents early participation of crystalline rocks, suggesting early development of intrafore-land basement highs within the southern Central Andes. At a regional scale, basement thrusting and associated intermontane basin development were diachronous. This interpretation is supported by broad propagation of basement thrusting from the northern Sierras Pampeanas (and Famatina) toward the central and eastern Sierras Pampeanas between early-middle Miocene and late Miocene–Pliocene time. Early Miocene reconstructions of the Juan Fernandez Ridge position on the Central Andean margin suggest that during this interval, shallow subduction was likely unrelated to oceanic plateau subduction under the Sierras Pampeanas.

Fault‐related folds above the source fault of the 2004 mid‐Niigata Prefecture earthquake, in a fold‐and‐thrust belt caused by basin inversion along the eastern margin of the Japan Sea

The 2004 mid‐Niigata Prefecture earthquake occurred in a fold‐and‐thrust belt that has been growing since late Pliocene time in a Miocene rift basin along the eastern margin of the Japan Sea. We constructed the trajectory of the subsurface faults responsible for the growth of the folds from the geologic structure and stratigraphy in the source region, assuming that the folds have been growing as fault‐related folds because of inclined antithetic shear with a dip of 85° in the hanging wall above a single reverse fault. The fault trajectory constructed from the fold geometries nearly coincides with the geometries of the source fault of the main shock of the 2004 earthquake revealed by aftershocks, which supports that the rupture was along a geologic fault that has ruptured repeatedly during the last a few million years. A three‐dimensional fault model based on 12 fault trajectories constructed along parallel sections revealed that the main shock occurred on a convex bend in the fault surface and that the southern termination of the aftershock distribution nearly coincides with a concave bend in the fault. The close relation between the source fault and the geologic structure shows that it is possible to construct source fault geometry assuming inclined shear as deformation mechanism of a hanging wall and to infer the rupture areas from geologic data.

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.

Late Miocene transcurrent tectonics in NW Turkey: evidence from palaeomagnetism and 40Ar–39Ar dating of alkaline volcanic rocks

A number of intra-continental alkaline volcanic sequences in NW Turkey were emplaced along localized extensional gaps within dextral strike-slip fault zones prior to the initiation of the North Anatolian Fault Zone. This study presents new palaeomagnetic and 40Ar–39Ar geochronological results from the lava flows of NW Turkey as a contribution towards understanding the Neogene–Quaternary tectonic evolution of the region and possible roles of block rotations in the kinematic history of the region. 40Ar–39Ar analyses of basalt groundmass indicate that the major volume of alkaline lavas of NW Turkey spans about 4 million years of episodic volcanic activity. Palaeomagnetic results reveal clockwise rotations as high as 73° in Thrace and 33° anticlockwise rotations in the Biga Peninsula. Movement of some of the faults delimiting the areas of lava flows and the timing of volcanic eruptions are both older than the initiation age of the North Anatolian Fault Zone, implying that the region experienced transcurrent tectonics during Late Miocene to Pliocene times and that some of the presently active faults in the region are reactivated pre-existing structures.

A Pliocene ignimbrite flare-up along the Tepic-Zacoalco rift: Evidence for the initial stages of rifting between the Jalisco block (Mexico) and North America

The Tepic-Zacoalco rift, a NW-trending corridor ~50 × ~250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the TepicZacoalco rift are rhyolitic ash-fl ow tuffs and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New 40 Ar/ 39 Ar dates on 41 volcanic samples reveal a previously unrecognized, voluminous fl are-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-fl ow tuffs have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite fl are-up in the TepicZacoalco rift at 5–3 Ma may refl ect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12–6 Ma, prior to the transfer of Baja California from North America to the Pacifi c plate. Additionally, new 40 Ar/ 39 Ar dates show that the Sierra Madre Occidental volcanic province extends across the entire width of the Tepic-Zacoalco rift and terminates abruptly at the northern boundary of the Jalisco block near the Rio Ameca. In contrast, PaleoceneEocene basement from the Jalisco block extends northward into the Tepic-Zacoalco rift, where it is locally overlain by Sierra Madre Occidental rhyolites.