Pliocene Time Research Articles

Neotectonic analysis, active stress field and active faults seismic hazard assessment in Western Crete

Within the framework of this study the complicated fault system of Western Crete was napped in detail and its kinematic and dynamic setting was analysed in order to distinguish 13 major active and possible active fault zones, the seismic potential of which was assessed. Moreover, kinematic data and striations were used to estimate the corresponding stress field geometry. Two stress phases were recognized: 1st the N-S extension phase (D1) in Mid-Upper Miocene to Lower Pliocene times forming E-W normal faults that bound the Neogene basins; 2nd the E-W extension phase (D2) in Late Pliocene-recent times forming N-S trending active normal faults. Smaller, mainly NE-SW trending faults, with significant strike-slip component, indicate a kinematic compatibility to the D2 phase, acting as transfer faults between larger N-S fault zones. The faults were incorporated in a detailed seismic hazard analysis together with the available seismological data, involving both probabilistic and deterministic approaches, for seismic hazard assessment of several selected sites (municipalities).

Cenozoic North Atlantic deep circulation history recorded in contourite drifts, offshore Newfoundland, Canada

In the North Atlantic Ocean, contour-following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2km in thickness and extend for 100s of km. The stratigraphic records in the drifts have been used to reconstruct variations in North Atlantic deep-water circulation throughout the Cenozoic; however, uncertainties remain about certain aspects of the timing, intensity, depth distribution, and regional impact of these currents. Here, we use an integrated dataset of seismic-reflection profiles and IODP core data (lithology, biostratigraphy, and magnetostratigraphy) to document sedimentation history and the development of current effects in the Cretaceous to present sedimentary record on the J-Anomaly Ridge and Southeast Newfoundland Ridge, offshore Newfoundland, Canada. The Newfoundland ridges are in a key location, lying between well-studied areas in the northern and western North Atlantic and under the path of both the modern Deep Western Boundary Current and the Gulf Stream. Late Cretaceous through Early Eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation, but at ~47Ma, near the Early-Middle Eocene boundary, well developed contourite drifts began to accrete in paleo-water depths of ~4000–4500m, accompanied by an order-of-magnitude increase in terrigenous sediment mass accumulation rates. From this time forward, drift deposition, interrupted by brief episodes of erosion, continued unabated. This timing for the onset of persistent deep currents is coincident with reorganization of Atlantic circulation inferred from a change from biosiliceous to non-biosiliceous sedimentation in the western North Atlantic (Horizon AC) and with the current-eroded Intra-Eocene Unconformity (IEU) in the northern North Atlantic. A change in sedimentation style occurred within the Middle Eocene to upper Oligocene drift sequence, and it likely was related to a shift to deeper, more intense currents that eroded the widespread Horizon AU along the margin of eastern North America about Early Oligocene time. Beginning in the Late Oligocene (~25Ma) a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at ~3500–4500m paleodepth on the Southeast Newfoundland Ridge. This development correlates with widespread Late Oligocene through Miocene-Pliocene drift accumulation throughout the North Atlantic. The most recent phase of drift deposition, since Late Pliocene time (~3Ma), occurred after a shift to the ‘modern’ circulation system of deeper, swifter currents, and it includes mixed pelagic-hemipelagic sediments and ice-rafted debris that reflect glacial-interglacial influences on sedimentation.

Controls on Quaternary incision of the Northern Pyrenean foreland: Chronological and geomorphological constraints from the Lannemezan megafan, SW France

Alluvial fans and megafans hold a key position in the sediment routing system and are particularly sensitive to changes in geodynamic and climatic forcing, as well as autogenic fluvial processes. The Lannemezan megafan in the Northern Pyrenean foreland (SW France) was built during Miocene to Pliocene times and subsequently abandoned as the stream network deeply entrenched the foreland. We report new cosmogenic nuclide (10Be, 26Al) exposure dates for the abandoned fan surface and a series of alluvial terraces along the Neste and Garonne rivers. Our results show that abandonment of the fan occurred at or before ~300ka, and we suggest that this abandonment is the result of the autogenic dynamics of the river system leading to the capture of the feeding Neste River by the Garonne. However, the incision episodes that produced the terrace levels are concomitant to major climatic shifts. Detailed analysis of stream network morphometry does not reveal knickpoints or patterns in the steepness index and chi-values that could be linked to either active tectonic features, base-level change or network reorganization. We therefore suggest that fan abandonment and long-term incision occurred through autogenic processes and that climate shifts may have modulated the incision, allowing for the formation and persistence of alluvial terraces.

Pliocene–Pleistocene sedimentary–tectonic development of the Mesaoria (Mesarya) Basin in an incipient, diachronous collisional setting: facies evidence from the north of Cyprus

AbstractThe Mesaoria (Mesarya) Basin exemplifies multi-stage basin development within a regional setting of diachronous continental collision. The Plio-Pleistocene represented a period of major sediment accumulation between two topographic highs, the Kyrenia Range in the north and the Troodos Massif in the south. During Pliocene time, open-marine marls and chalks of the Nicosia (Lefkoşa) Formation accumulated in a shelf setting. The Early Pleistocene period was characterized by a relative fall in sea level and a change to shallower-water bioclastic deposition of the Athalassa (Gürpınar) Formation. The northern margin of the basin was approximately delineated by the E–W neotectonic Ovgos (Dar Dere) fault zone. A carbonate ramp system formed directly to the south of this structural feature. During Early Pleistocene time, the basin evolved from an open-marine shelf to semi-enclosed lagoons with deltaic deposits, and finally to a non-marine aeolian setting, flanked by the rising Kyrenia Range to the north. Synthesis of geological evidence from the Mesaoria (Mesarya) Basin as a whole, including outcrop and borehole evidence from the south, adjacent to the Troodos Massif, indicates that the Pliocene – Early Pleistocene represented a relatively quiescent period. This intervened between Late Miocene – earliest Pleistocene southward thrusting–folding of the Kyrenia Range and Pleistocene intense surface uplift of both the Kyrenia Range and the Troodos Massif. The basin development reflects flexurally controlled collapse during Late Miocene – earliest Pliocene time related to southward thrusting, followed by strike-slip during westward tectonic escape of Anatolia, and finally regional uplift controlled by under-thrusting of continental crust from the south, as collision progressed.

Geology of the Pergola–Melandro basin area, Southern Apennines, Italy

ABSTRACTThe Southern Apennines (SA) are part of the Apennine–Maghrebian chain, a segment of the circum-Mediterranean Alpine orogenic system. It is a NE-verging fold-and-thrust belt with an about N150°-striking axis developed since the late Oligocene-early Miocene. The Geological Map at 1:25,000 scale of the Pergola–Melandro basin area, presents a sector of the axial zone of the SA which represents a key area to reconstruct the tectonic evolution of this chain. The map describes the complex structural and stratigraphic relationships between the three main tectonic units forming this sector of the SA: (1) the carbonate slope succession of the Maddalena Mts Unit, interpreted as the eastern boundary of the Apennine carbonate Platform; (2) the Lagonegro Unit, resulting from the deformation of the homonym pelagic basin; (3) the strongly deformed Argille Variegate Group sandwiched between the two previous units. Three main contractional tectonic stages, occurring from middle Miocene to Pliocene, have been recognized. Since Pliocene times low-angle extensional tectonic contacts and tectono-gravitative detachments affected the tectonic pile.

Genesis of the Northern Adriatic Sea (Northern Italy) since early Pliocene

The Northern Adriatic Sea is a shallow and very flat shelf area located between the northern Apennines, the southern Alps and the Dinarides; its present day physiography is the result of the filling of a relatively deep Quaternary foredeep basin, developed due to the northeastward migration of the Apennine chain. Multichannel seismic profiles and well data have allowed documenting the stratigraphic architecture, the depositional systems and the physiographic evolution of the Northern Adriatic sea since early Pliocene time. In particular, three main depositional sequences bounded by regional unconformities were recognized. The Zanclean Sequence 1 documents first the drowning of late Messinian incised valleys and then the southward progradation of a shelf-slope system, which is inferred to be related to a tectonic phase of the Apenninic front. The Piacenzian-Gelasian Sequence 2 records a relatively rapid transgressive episode followed by minor southward progradation; the top of the sequence is associated with a major late Gelasian drowning event, linked to the NE-ward migration of the Apennine foredeep. The Calabrian to upper Pleistocene Sequence 3 testifies the infilling of accommodation previously created by the late Gelasian drowning event, and it initially accumulated in deep-water settings and then in shallow-water to continental settings. The upper part of Sequence 3, consisting of the paleo-Po deltaic system, is composed of seven high-frequency sequences inferred to record late Quaternary glacio-eustatic changes. These high-frequency sequences document the stepwise filling of the remaining accommodation, resulting in the development of the modern shelf.

New insights into the stratigraphic, paleogeographic and tectonic evolution and petroleum potential of Kerkennah Islands, Eastern Tunisia

This work presents general insights into the stratigraphic and paleogeographic evolution as well as the structural architecture and the petroleum potential of Kerkennah Islands, located in the Eastern Tunisia Foreland, from Cenomanian to Pliocene times. Available data from twenty wells mostly drilled in Cercina and Chergui fields are used to establish three lithostratigraphic correlations as well as isopach and isobath maps in order to point out thickness and depth variations of different geological formations present within our study area; in addition to a synthetic log and isoporosity map of the main carbonate reservoir (the nummulites enriched Reineche Member). The integrated geological study reveals relatively condensed but generally continuous sedimentation and a rugged substrate with horsts, grabens and tilted blocks due to the initiation and the individualization of Kerkennah arch throughout the studied geological times. Furthermore, a relationship was highlighted between the evolution of our study zone and those of Sirt basin, Western Mediterranean Sea and Pelagian troughs; this relationship is due to the outstanding location of Kerkennah Islands. The main Bou Dabbous source rock is thicker and more mature within the central-east of the Gulf of Gabes indicating therefore the southeast charge of Reineche reservoir which shows NW-SE trending tilted block system surrounded by normal faults representing the hydrocarbon migration pathways. Besides, the thick Oligo-Miocene formations deposited during the collapse of the Pelagian block caused the maturation of the Ypresian source rock, while the Pliocene unconformity allowed basin inversion and hydrocarbon migration.

Sicily in its Mediterranean geological frame

The Island of Sicily is generally considered to be the geological link between the North African Fold Belt and the Appennines, in Italy. This comes from a cylindristic meaning and is only partly exact. As a matter of fact, Sicily is essentially Greek; Ionian. Up to Middle Cretaceous time, the Sicilian area was a submerged shoal in the sea or the Panormide area, bordering the Ionian Ocean. This shoal lay between the future North African Fold Belt and the Appennines, forming an intermediate link between the Appenninic, Apulian, Panormian and Tunisian platforms. It was only during the Middle to Upper Cretaceous that the Atlantic and Ligure Oceans merged, making a continuous relationship between the Appenninic, Sicilian and North African sedimentary series. The key time periods are the Permian, Cretaceous and Oligo-Miocene periods leading to the formation of the actual Calabro-Sicilian arc. From the Permian to the present, the Sicilian geological history pertains to three oceanic domains: Ionian, Ligurian and Atlantic, of which the Ionian and Ligurian were under the influence of Tethys (Neo and Paleo-Tethys). The Tethysian identity of Sicily constitutes the major aspect of its geological history. However, the European and African plate tectonic movements complicated its structure. During the Middle Miocene subduction, southern Sicily became African, meanwhile its north-eastern part became, in Pliocene time, Maghrebian by accretion. Sicily is thus a truly geological patchwork, but its main section remains Ionian and now constitutes a link between North Africa and the Appennines. With older data, but also by means of recent results, we will replace Sicily in its Mediterranean frame, giving the mean stages of its paleogeographical and then its tectonic evolution. We will review the calabro-sicilian arc evolution from the Oligocene, developing the actual context and recalling the main fundamental play of the Numidian flysch.

Growth and demise of Cenozoic isolated carbonate platforms: New insights from the Mozambique Channel seamounts (SW Indian Ocean)

Although long-term evolutions of isolated shallow-water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades, their driving processes are still the subject of active debates. The Mozambique Channel (SW Indian Ocean) is characterized by several flat-topped seamounts ranging from 11°S to 21°S in latitudes. Based on a comprehensive geomorphologic study and on dredged samples analysis, we show that these features correspond to tropical isolated shallow-water carbonate platforms. Coupling strontium isotopy and foraminifera biostratigraphy, well-constrained chronostratigraphy results indicate that shallow-water carbonate production started in the Mozambique Channel during distinct Cenozoic periods ranging from Paleocene to Early Miocene. Our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism. While some of them kept developed until present days, forming modern carbonate systems, some others were drowned during Late Neogene and subsided to form guyots. Although different factors can be discussed, tectonic and volcanism appear as good potential triggers for demise episodes during Late Miocene-Early Pliocene times. Chronology and location of this geodynamical activity tend to emphasize influence of East African rift system until southern Mozambique Channel.

Reconstruction of Pliocene-Pleistocene sediment sources and weathering intensity in the paleo-life rich Olduvai and Laetoli basins of northern Tanzania using major and trace element geochemistry and Sr isotopic data

Major and trace elements and Sr isotopic compositions were analyzed for samples from the Pliocene-Pleistocene to Recent Olduvai and Laetoli basins of northern Tanzania with the aim of constraining their provenance and paleo-climatic conditions. The Olduvai samples are characterised by La/Cr ratios of 0.69–3.73, Th/Sc = 1.12–24.6 and 87Sr/86Sr = 0.70448–0.70563; features which suggest the volcanic facies of the nearby Olmoti as their possible source. The relatively lower values for the Laetoli basin (Th/Sc = 0.4–0.9 and 87Sr/86Sr = 0.70412–0.70508) coupled with Zr/Nb, Nb/Ta ans Zr/Hf ratios are suggestive of a mafic protolith inferred to be the Sadiman lavas. The calculated Chemical Index of Alteration (CIA) values for the Laetoli samples are 58–78% (mean = 65%) indicating higher weathering intensities of the source compared to 45% in the Olduvai samples. The calculated Mean Annual Precipitation (MAP) values are higher at Laetoli (mean = 950 ± 181 mm/yr) relative to Olduvai (mean = 694 ± 181 mm/yr) consistent with increased rainfall in the Pliocene during the deposition of Laetoli strata when compared to the Pleistocene times when the Olduvai sediments were deposited.The CIA and MAP patterns observed at Laetoli and Olduvai can be related to temporal changes in weather conditions during the Pliocene and Pleistocene, respectively. The Lower CIA and MAP values at Olduvai imply the prevalence of arid to semi-arid climatic conditions during the Pleistocene whereas the higher values at Laetoli correspond with wetter conditions during the Pliocene times. This could have the potential for improved eutrophication at Laetoli. The climatic inferences drawn from CIA and MAP data may point to a more favourable habitat for life proliferation in the wetter environment of Laetoli compared to the drier Olduvai. This inference is consistent with archaeological evidence which indicates a greater abundance of hominin fossils including the well preserved footprints in Laetoli.

Reactivation, inversion and basement faulting and thrusting in the Sierras Pampeanas of Córdoba (Argentina) during Andean flat-slab deformation

AbstractThe Sierras Pampeanas of Córdoba are the easternmost uplifted blocks caused by Andean foreland deformation, over 700 km from the Chile trench. This deformation started atc.340 Ma through basement faults, thrusts and reactivation of normal faults of the Cretaceous rift during the opening of the Atlantic Ocean. Other older faults, major oblique lineaments, were also reactivated. Thermochronological and geothermobarometric data indicate that some topographic relief could have been Palaeozoic–Mesozoic relicts and not only produced by the Andean orogeny. Faults are partially controlled by the early Cambrian S2metamorphic foliation, coincident with the curved fault traces at map scale. During Pliocene time, two deformation phases post-dating Miocene–Pliocene magmatism are recognized. Shallow seismicity data (c.25 km depth) indicate that the Sierras de Córdoba accommodate Quaternary displacement. Magnetotelluric studies detect the interface between the Pampia terrane and the Río de la Plata craton. The role of the oblique lineaments in the nucleation and development of the Tertiary faulting has been little considered; they could be correlated with an old pan-Gondwanan trend. During the Cretaceous period these lineaments worked in a transtensive way, producing the uplift of high-grade rocks and segmentation of the mountain chain favouring the diachronous uplift along the ranges. Recently, both the brittle–ductile transition atc.23 km depth and the crustal thickness have been determined by seismicity analysis. The oblique lineaments displace normally the Mohorovicic discontinuity. Main basement thrusts were probably rooted in the suture between the Pampia terrane and the Río de la Plata craton.

Hydrocarbon maturation modeling of Paleocene to Lower Miocene source rocks in the Niger Delta Basin: implications for hydrocarbon generation

This study evaluates the hydrocarbon generation potentials and time of generation for Paleocene to Lower Miocene source rock horizons from A-1, B-1, B-2, and C-1 wells in the Niger Delta Basin using 1D Petromod modeling software. Wells A-1, B-1 and B-2, and C-1 are located within the Central Swamp, the Coastal Swamp, and the Shallow Offshore depobelts, respectively. The thermal history was derived from the rifting–subsidence heat flow model. Maturity modeling were carried out by using Easy%Ro kinetic model and a heat flow history predicting present-day heat flow which were calibrated with measured temperature data. Results of the study suggest that these potential source rocks have attained maturity status to generate hydrocarbons, with vast differences existing in the timing of the onset of oil generation. Basin modeling suggests that Paleocene source rocks entered the oil generative window from the Oligocene to Miocene times with thermal maturity window that varies from gas generation to early-mature phase. The Eocene source rocks have also attained maturity from Miocene to Pliocene times, and their thermal maturity ranges from gas generation to early maturity stage. The Oligocene source rocks also began to generate oil during the Miocene and are currently within the early-mature to mid-mature stage. The thermal maturity window for the Lower Miocene source rocks ranges from immature to early-mature stage. The present modeling results reveals that higher levels of thermal maturity are attained in areas with high geothermal gradients and heat flow values while the cooler areas exhibits lower levels of maturation. The onset of the oil window lies at 2859 m at A-1 (Central Swamp), 3240 m at B-2 (Coastal Swamp), 4732 m at B-1 (Coastal Swamp), and 4344 m at C-1 (Shallow Offshore). The depth to the onset of oil window is found deeper in the Shallow Offshore and western parts of the study area than in the eastern and northwestern parts. The result of this study suggest that the Paleocene, Eocene, Oligocene, and Lower Miocene source rocks are the principal source rocks for oil and gas generation in the Niger Delta Basin.

Thermochronological constraints to late Cenozoic exhumation of the Barents Sea Shelf

The Cenozoic evolution of the Barents Sea has been widely debated for its implications on hydrocarbon exploration. In this paper, we provide the first, for the area, apatite (U–Th)/He thermochronology data on Early-Middle Jurassic and Early Cretaceous sandstone reservoir samples obtained from three wells located on the western part of the Barents Shelf. A large range of grain ages have been detected, ranging from 1.58 to 50.65 Ma. These thermochronological data, initially integrated with vitrinite reflectance measurements to properly constrain the burial history evolution, have been modelled on the basis of estimated maximum temperature in order to evaluate the cooling path to present-day temperatures. Outputs from modelling indicate that: 1) the amount of net uplift and denudation is in the order of 1000 m, and 2) the last important phase of exhumation occurred during late Miocene-early Pliocene time, therefore challenging the prevailing idea of substantial uplift linked to the observed shelf-progradation along the margin as a result of the Plio-Pleistocene glaciations. The timing of the distinct exhumation event documented here may be, instead, attributed to different mechanisms. These may include basin inversion widespread on the NW European margin that is probably related to local changes in the North Atlantic spreading vector, while other mechanisms may include thermal and lithospheric-scale anomalies observed at the northwestern corner of the Barents-Svalbard Shelf.

The northward tectonic transport in the southern Apennines: examples from the Capri Island and western Sorrento Peninsula (Italy)

We analyzed a thrust fault system located in the western Sorrento Peninsula and Capri Island (southern Italy) where several mesoscale structures related to the main thrusts, such as Riedel shear planes, overturned folds, minor thrust and back-thrust faults, suggest a dominant northward tectonic transport. Major and minor thrust faults, generally characterized by a ramp-flat geometry, involved the Mesozoic Apennine carbonates, the Middle Miocene foredeep, and the unconformable thrust-top basin deposits. The biostratigraphic analysis of calcareous nannoplankton assemblages on the thrust-top basin sediments indicates an age not older than late Tortonian. We propose that this out-of-sequence thrusting stage was related to a regional tectonic event widespread in the entire southern Apennines, probably occurred in the Pliocene time simultaneously with the activity of deep-seated thrust faults that involved the buried carbonates of the Apulian platform. These out-of-sequence thrust faults, here referred to as “envelopment thrusts,” were enucleated in a lower structural level with respect to the allochthonous wedge, representing the W–E segments of large regional arcuate structures.

Late Neogene oroclinal bending in the central Taurides: A record of terminal eastward subduction in southern Turkey?

The Tauride fold-thrusts belt formed during ∼S–N convergence between Africa and Eurasia since Cretaceous time. The western end of the central Taurides strike NW–SE, highly obliquely to the overall convergence direction, and connect to the NE–SW Beydağları–Lycian Nappe flank of the western Taurides, forming the so-called ‘Isparta Angle’. In Neogene time, the western and central Taurides and the inner part of the Isparta Angle became overlain by Neogene sedimentary basins including Manavgat, Köprüçay and Aksu, characterized by marine clastics and carbonates. The eastern limb of the Isparta Angle experienced multidirectional Miocene to Present extension, whereas E–W shortening affected the marine sedimentary basins in the heart of the Isparta Angle. To quantitatively reconstruct the Neogene kinematic evolution of the Taurides, towards restoring the subduction system accommodating Africa–Eurasia convergence, we paleomagnetically assess if and when vertical axis rotations affected the Manavgat, Köprüçay, and Aksu basins in Early Miocene to Pliocene times. We show that the northern Köprüçay Basin rotated ∼20–30° clockwise, the Manavgat Basin underwent ∼25–35° counterclockwise rotation, and the Aksu Basin underwent no rotation since the Early-Middle Miocene. It was previously shown that the Beydağları region underwent a post-Middle Miocene ∼20° counterclockwise rotation. These results show that the prominent oroclinal salient geometry of the western Taurides thus acquired, at least in part, since Miocene times, that the Köprüçay Basin rotated relative to the Aksu Basin along the Aksu thrust, and that the Beydağları platform rotated relative to the Aksu Basin along the Bucak thrust, which must have both been active until Late Neogene time. This synchronous E–W shortening in the heart of the Isparta Angle, and multidirectional extension in its eastern limb may be explained by relative westward retreat of an eastward dipping subducting Antalya slab that has previously been imaged by seismic tomography and a Benioff zone. The Neogene Bucak thrust west of the Aksu Basin may represent the most recent surface expression of the Antalya subduction zone.

Geophysical analysis of fault geometry and volcanic activity in the Erzincan Basin, Central Turkey: Complex evolution of a mature pull-apart basin

The Erzincan Basin is one of several Neogene sedimentary basins developed by prolonged right-lateral strike-slip along the North Anatolian Fault Zone (NAFZ), the intracontinental transform defining the present boundary between the Eurasian Plate to the north and accreted Anatolian terranes to the south. The basin has a strong asymmetry and young (<780ka) volcanic centers with widespread development of cross faults defining an advanced phase of pull-apart basin evolution. To isolate faults with no surface geomorphic or morphotectonic signatures in the young sedimentary cover, continuous magnetic profiles were conducted together with detailed interpretation of the regional Bouguer gravity map. This geophysical approach combined with surface mapping defines a fault geometry highlighting a series of buried structures including a fracture system 0.2–2.35km wide which conforms to the volcanic lineaments seen at the surface. A model is developed for the evolution of the Erzincan Basin with a history commencing as a simple pull-apart by right-lateral strike-slip on the developing NAFZ, probably in Early Pliocene times. Subsequent interaction with a major left-lateral (Ovacık) fault (OF) caused the focus of motion on the NAFZ to shift to the southwest and develop a complex fishbone fracture system. This became the focus of volcanic activity on three lineaments which migrated progressively southwards toward the axis of the basin. Continuing motion on the OF transformed the south east margin of the basin into an extensional zone and the tectonic history of the basin has been further complicated by its proximity to a major transform intersection between the NAFZ and OF. The signatures of recent volcanism and the development of cross faults on which much activity is now concentrated define a mature pull-apart advancing toward extinction.

Uplift of the Emei Shan, western Sichuan Basin: Implication for eastward propagation of the Tibetan Plateau in Early Miocene

The Emei Shan stands on the southeast side of the Longmen Shan. It marks the southeastern most edge of the eastern margin of the Tibetan Plateau (EMTP) and towers above the Sichuan Basin to its east, with the highest peak being 3099m. A large number of structural and thermo-chronological studies have been published focused on the Longmen Shan orogen, which is considered to be the northern part of the EMTP. However, for the southern part of the EMTP where the Emei Shan lies, its tectonics and uplift history are still poorly understood. This paper dates five granite samples from the Emei Shan batholith using the apatite fission track (AFT) method. The boundary faults of the Emei Shan and structural deformation in adjacent region are also investigated and mapped in detail. (1) In Miocene time, compressional stress from the eastward extrusion of the EMTP caused large-scale thrusting and mountain uplift along narrow Longmen Shan in northern EMTP, in contrast to southern EMTP where a broad and gentle Yaan−Emei Shan fold belt absorbed most of the stress. By Pliocene time, the southeastward extrusion of the Tibetan Plateau caused the transpressional stress and rotated the Chuandian fragment along the Xianshuihe−Xiaojiang fault belt. As a consequence, the Emei Shan region was intended northeastwards into the Sichuan Basin, bounded by two strike-slip faults on its north and southeast sides. (2) The AFT results show the Emei Shan uplifted since ∼25–20Ma ago. The threshold and main stage of uplift of the Emei Shan possibly lagged behind the Longmen Shan of approximately 5Myr. As the leading edge of fold belt, uplift of the Emei Shan indicated the eastward propagation of the EMTP was time-transgressive from Late Oligocene to Middle Miocene.

Solid bitumen in calcite veins from the Natih Formation in the Oman Mountains: Multiple phases of petroleum migration in a changing stress field

Solid bitumen in calcite veins in Natih limestone on the southern flank of the Jebel Akhdar Anticline provide evidence for at least two previously unrecognized petroleum migration events in the Oman Mountains. We present field observation of bituminous calcite veins combined with microscopy using highly polished thin sections. This allows imaging of solid bitumen in reflected light and study of its microstructural context in transmitted light.Straight and en échelon, black impregnated, bedding-normal Natih A calcite veins strike 110°. They formed as one of the earliest structures in an extensional stress regime at the end of Cretaceous (Turonian–Santonian) either by flexure of the Arabian plate (Wasia–Aruma break) or by subsidence during subduction. The veins always contain planar arrays of small (<10μm) intracrystal solid bitumen inclusions and may contain angular mosaic type solid bitumen associated with white, solid bitumen-free calcite and/or round, homogeneous type solid bitumen. The solid bitumen formed in multiple events and were most likely derived from the Natih B source rocks.During the first migration event, microfracturing during obduction of the Semail ophiolite and emplacement of the Hawasina thrust sheets in Santonian–Campanian times formed pathways for oil into the veins. Mosaic type solid bitumen formed during further vein growth. It has a higher solid bitumen reflectance (BRr=3.40–3.76%) combined with a high optical anisotropy compared to the underlaying thermally overmature Natih B source rock (BRr=3.10–3.14%). The higher reflectance of mosaic type solid bitumen is interpreted to result from deformation and does not reflect maximum burial temperatures which are inferred to have been about 225°C.The second migration event is indicated by low-reflective homogeneous type solid bitumen with palaeotemperatures (BRr=0.86–0.92%; about 145°C) lower than maximum burial. It occurs along pressure solution seams which cross-cut the veins showing that this is the latest event in the paragenetic sequence, interpreted to have formed after lateral intraformational oil migration from the active Natih oil kitchen located approximately 40km SW of the study area during doming and uplift of the Oman Mountains in Oligocene to Pliocene times.

Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Continental Borderland

Changing conditions along plate boundaries are thought to result in the reactivation of preexisting structures. The offshore southern California Borderland has undergone dramatic adjustments as conditions changed from subduction tectonics to transform tectonics, including major Miocene oblique extension, followed by transpressional fault reactivation. However, consensus is still lacking about stratigraphic age models, fault geometry, and slip history for the near-offshore area between southern Los Angeles and San Diego (California, USA). We interpret an extensive data set of seismic reflection, bathymetric, and stratigraphic data from that area to determine the three-dimensional geometry and kinematic evolution of the faults and folds and document how preexisting structures have changed their activity and type of slip through time. The resulting structural representation reveals a moderately landward-dipping San Mateo–Carlsbad fault that converges downward with the steeper, right-lateral Newport-Inglewood fault, forming a fault wedge affected by Quaternary contractional folding. This fault wedge deformed in transtension during late Miocene through Pliocene time. Subsequently, the San Mateo–Carlsbad fault underwent 0.6–1.0 km displacement, spatially varying between reverse right lateral and transtensional right lateral. In contrast, shallow parts of the previously identified gently dipping Oceanside detachment and the faults above it appear to have been inactive since the early Pliocene. These observations, together with new and revised geometric representations of additional steeper faults, and the evidence for a pervasive strike-slip component on these nearshore faults, suggest a need to revise the earthquake hazard estimates for the coastal region.

Constraints on plateau architecture and assembly from deep crustal xenoliths, northern Altiplano (SE Peru)

Newly discovered xenoliths within Pliocene and Quaternary intermediate volcanic rocks from southern Peru permit examination of lithospheric processes by which thick crust (60–70 km) and high average elevations (3–4 km) resulted within the Altiplano, the second most extensive orogenic plateau on Earth. The most common petrographic groups of xenoliths studied here are igneous or meta-igneous rocks with radiogenic isotopic ratios consistent with recent derivation from asthenospheric mantle ( 87 Sr/ 86 Sr = 0.704–0.709, 143 Nd/ 144 Nd = 0.5126–0.5129). A second group, consisting of felsic granulite xenoliths exhibiting more radiogenic compositions ( 87 Sr/ 86 Sr = 0.711–0.782, 143 Nd/ 144 Nd = 0.5121–0.5126), is interpreted as supracrustal rocks that underwent metamorphism at ~9 kbar (~30–35 km paleodepth, assuming a mean crustal density of 2.8 g/cm 3 ) and ~750 °C. These rocks are correlated with nonmetamorphosed rocks of the Mitu Group and assigned a Mesozoic (Upper Triassic or younger) age based on detrital zircon U-Pb ages. A felsic granulite Sm-Nd garnet whole-rock isochron of 42 ± 2 Ma demonstrates that garnet growth took place in Eocene time. Monazite grains associated with quenched anatectic melt networks in the same rocks yield ion microprobe U-Pb ages ranging from 3.2 ± 0.2 to 4.4 ± 0.3 Ma (2σ). These disparate geochronologic data sets are reconciled by a model wherein Mesozoic cover rocks were transferred to >30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time. Isothermal decompression and partial melting ensued as these rocks were entrained as xenoliths in volcanic host magmas and transported toward the surface. Mafic granulites and peridotites from the same xenolith suite comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of Central Andean crust. Calculated equilibrium pressures for these basement rocks are >11 kbar, suggesting that the basement-cover interface lies beneath the northernmost Altiplano at ~30–40 km below the surface. Together, these results indicate that crustal thickening under the northernmost Altiplano started earlier than major latest Oligocene and Miocene uplift episodes affecting the region and was coeval with a flat slab–related regional episode of deformation. Total shortening must have been at least 20% more than previous estimates in order to satisfy the basement to cover depth constraints provided by the xenolith data. Sedimentary rocks at >30 km paleodepth require that Andean basement thrusts decapitated earlier Triassic normal faults, trapping Paleozoic and Mesozoic rocks below the main decollement. Magma loading from intense Cenozoic plutonism within the plateau probably played an additional role in transporting Mesozoic cover rocks to >30 km and thickening the crust beneath the northern Altiplano.