Cenozoic Events Research Articles

Miocene sponge assemblages in the face of the Messinian Salinity Crisis-new data from the Atlanto-Mediterranean seaway.

The Messinian Salinity Crisis is considered as one of the most influential Cenozoic events that impacted negatively on the benthic fauna of the Mediterranean area. Changing environmental conditions, including a sharp reduction of water exchange between the Mediterranean Sea and the Atlantic Ocean, altered the geographical ranges of many organisms, including sponges (Porifera). Here, we report a unique assemblage of isolated sponge spicules from the upper Miocene of southwestern Spain. The newly recognized sponge fauna was inhabiting the Guadalquivir Basin-the corridor between the Mediterranean and the Atlantic Ocean at that time. It represents a taxonomically rich sponge community that consisted of members of "soft" and "lithistid" demosponges and hexactinellids. Demosponges are represented by at least thirty-four taxa, while hexactinellids are significantly rarer; only six taxa have been identified. From among eighteen taxa recognized to the species level, at least eight seem to be inhabiting this area to these days; six are recorded from adjacent areas, such as the Western Mediterranean, South European Atlantic Shelf, and the Azores, and three are present in the Red Sea and/or the Northern Atlantic. Intriguingly, some taxa seem to have their closest relatives in distant areas, such as the Indo-Pacific and Japanese waters which suggests that the range of some once widely-distributed populations shrunk after the isolation of the Mediterranean and the Messinian Salinity Crisis, surviving to the present day only in refugia.

Control of inherited structural fabric on the development and exhumation of passive margins – Insights from the Araçuaí Orogen (Brazil)

The Araçuaí Orogen, in eastern Brazil, was formed during the Neoproterozoic – Cambrian amalgamation of West Gondwana. During the Mesozoic – Cenozoic opening of the South Atlantic Ocean, and the associated divergent tectonics, the orogen developed as basement to the passive margin of South America and was progressively covered by thick offshore sedimentary basins, particularly the Espírito Santo, Mucuri, and Cumuruxatiba basins, in which hydrocarbon systems have been exploited. Our understanding of the Araçuaí Orogen's passive margin evolution, erosion, and sediment transfer to these basins ultimately depends on constraining the onshore exhumation in response to Mesozoic – Cenozoic events. Here, new and previously published data from apatite fission-track (AFT) analyses and inverse thermal history modelling of (Pre)Cambrian basement rocks from the Araçuaí Orogen resolve three discrete basement cooling and associated erosional exhumation episodes. In the Pre-Rift phase, Jurassic – Hauterivian erosion of the Araçuaí Orogen is most likely related to the adjoining intra-continental West Gondwana flexural subsidence, which increased hillslope and river erosion power. In the Rift and Transitional phases, Barremian – Albian accelerated phase of erosion is associated with the uplift of the Atlantic rift shoulders and the establishment of an oceanic base-level. In the Drift phase, reactivations in response to far-field stresses likely triggered a Late Cretaceous – Paleocene rapid erosion event. The rates at which these events unfolded vary spatially and are controlled by inherited structures. The Araçuaí Orogen experienced slower denudation rates in areas closer to the São Francisco Craton, which suggests that the tectonic reactivation and related surface uplift during the Mesozoic–Cenozoic is in first-order controlled by lithospheric rigidity. Furthermore, the structural framework of the Paramirim and Pirapora aulacogens and NE-oriented shear zones in the orogen’s southeast facilitated later reactivations. From the spatial pattern of denudation/exhumation of the Araçuaí Orogen during the Mesozoic – Cenozoic, we draw inferences on the tectonic development of the offshore basins regarding their hydrocarbon potentials.

Extensional tectonics, structural architecture modeling and geodynamic evolution in the Cretaceous Tinghir-Errachidia-Boudenib basin (Pre-African Trough, Morocco)

Structural evolution of the Cretaceous Tinghir-Errachidia-Boudenib foreland basin in the south front of the Moroccan High Atlas belt is documented here. Fieldwork, lithostratigraphic and new structural analysis coupled with borehole and geoelectrical data identify two ENE-to NE-trending sub-basins in the large Cretaceous Tinghir-Errachidia-Boudenib (TEB) basin. One named Errachidia sub-basin is situated in the west and the other Boudenib sub-basin is located in the east. They are separated by a major NE-SW trending anticlinal, inherited from a paleohigh structure, where the thickening of the Cretaceous series increases towards the center of the sub-basins and in their northern borders with the central-eastern High Atlas. The Cretaceous series clearly show a considerable thickness variation, angular unconformities and lateral facies change that was related to the syn-sedimentary normal faulting activity. The Albian-Turonian period is characterized by the development of ENE- and NW-trending fault systems. These latter are linked to an extensional tectonics event responsible for half-graben basin geometry associated with significant sedimentary rates. This considerable thickness depocenters geometry is related to normal fault coupled with the intra-basin faults. The intensity of subsidence increases from south to north and from west to east, with a high rate in the Boudenib sub-Basin. The compressive Cenozoic events caused the inversion of these extensional faulting and the folding of the High Atlas area with ENE-trending structures.

Phylogenetic relationships of a new catfish of the genus Trichomycterus (Siluriformes, Trichomycteridae) from the Brazilian Cerrado, and the role of Cenozoic events in the diversification of mountain catfishes

The Brazilian Cerrado highlands shelter the headwaters of the three largest South American hydrographic basins, where a great species diversity is concentrated, but some biological groups are still insufficiently known. The focal taxa of this study are trichomycterid catfishes of the subgenus Cryptocambeva, genusTrichomycterus, endemic to mountain areas of south-eastern Brazil. The primary objective of this study is to test through a molecular phylogeny if a new species collected in streams of the upper Rio Paraná basin draining the Serra da Canastra is sister toT. macrotrichopterus, endemic to the upper Rio São Francisco at another facet of the Serra da Canastra, as suggested by morphological data. The analysis corroborated sister group relationships between these two species, besides supporting four main clades inCryptocambeva, each of them endemic to distinct mountain regions. A time-calibrated analysis supported the divergence timing between the new species andT. macrotrichopterusat the Pliocene, which is chronologically compatible with the final period of intense fluvial configuration re-arrangement, when São Francisco headwater streams were captured by the Paraná basin. The new species herein described is similar toT. macrotrichopterusand distinguished from all other species ofCryptocambevaby having a long pectoral-fin filament. These two species are distinguished from each other by characteristics of the latero-sensory system, colour pattern and bone morphology.

Research Progress on Cenozoic Volcano Genesis and Fluid Action in Northeast China

The tectonic evolution of northeast China is closely related to the subduction of the Pacific plate. The dehydration of the slab subduction process produces metasomatic agents that have important effects on the physical and chemical properties of the mantle wedge, including the decrease of seismic wave velocity and the increase of Poisson’s ratio and electrical conductivity. In order to investigate the tectonic evolution and fluid action of northeast China, this paper compares the previous seismic and electromagnetic imaging results of northeast China and explores the relationship between the genesis of Cenozoic volcanoes and fluid action in northeast China through rheological analysis. The results show that the western Pacific plate subducted into the mantle transition zone beneath northeast China, and sustained dehydration occurred. The upward migration of these released water caused partial melting at the base of the upper mantle. Some of the upwelling streams pierced the weak tectonic boundary under the buoyancy effect, which finally formed the large-scale Cenozoic volcanic events in northeast China.

Tectonothermal events in the central North China Craton since the Mesozoic and their tectonic implications: Constraints from low-temperature thermochronology

The North China Craton (NCC) experienced important tectonic events during the Meso-Cenozoic as implied by many geological studies. Combined with existing low-temperature thermochronological data, new apatite fission track (AFT) and zircon (UTh)/He data from the Lüliangshan and Taihangshan mountains in this study reveal that the central NCC experienced tectonothermal events during the Meso-Cenozoic, i.e., ca. 150 Ma, 110–80 Ma, 50–40 Ma and 30–20 Ma. The Late Jurassic event caused the formation of the Lüliangshan basement-involved anticline. The main folding and thrusting in the central NCC developed in the Mesozoic instead of the Cenozoic and resulted from the low-angle subduction of the Paleo-Pacific ocean plate beneath the Eurasian Plate during the Late Jurassic; and the Late Cretaceous tectonic event stopped the Early Cretaceous extension across the eastern Asia, thrusting and sinistral strike-slip faulting occurred across the central NCC, which was possibly caused by a collision along the southeastern boundary of eastern Asia. The early Cenozoic event in central NCC was coeval with an event occurring throughout eastern Asia and may have been a regional response to changes in the movement direction and velocity of the Pacific Plate, and this event may have resulted in the formation of rift basins in the eastern NCC. The whole central NCC experienced uplift during this period in isostatic readjustment to this extension.

Unravelling the lithospheric-scale thermal field of the North Patagonian Massif plateau (Argentina) and its relations to the topographic evolution of the area

The North Patagonian Massif (NPM) area in Argentina includes a plateau of 1200 m a.s.l. (meters above sea level) average height, which is 500–700 m higher than its surrounding areas. The plateau shows no evidence of internal deformation, while the surrounding basins have been deformed during Cenozoic orogenic events. Previous works suggested that the plateau formation was caused by a lithospheric uplift event during the Paleogene. However, the causative processes responsible for the plateau origin and its current state remain speculative. To address some of these questions, we carried out 3D lithospheric-scale steady-state and transient thermal simulations of the NPM and its surroundings, as based on an existing 3D geological model of the area. Our results are indicative of a thicker and warmer lithosphere below the NPM plateau compared with its surroundings, suggesting that the plateau is still isostatically buoyant and thus explaining its present-day elevation. The transient thermal simulations agree with a heating event in the mantle during the Paleogene as the causative process leading to lithospheric uplift in the region and indicate that the thermo-mechanical effects of such an event would still be influencing the plateau evolution today. Although the elevation related to the heating would not be enough to reach the present plateau topography, we discuss other mechanisms, also connected with the mantle heating, that may have caused the observed relief. Lithosphere cooling in the plateau is ongoing, being delayed by the presence of a thick crust enriched in radiogenic minerals as compared to its sides, resulting in a thermal configuration that has yet to reach thermodynamic equilibrium.

The Palaeogene record of Himalayan erosion in the Andaman Basin

The Himalayan orogeny has been recognized as one of the most important Cenozoic events that shaped the geography, climate and ocean chemistry of our planet. The erosion in the Himalayas is believed to have played a critical role in crustal deformation and changes in the chemistry of the ocean water since the Eocene. In spite of the fact that the orogeny began after India–Asia collision at 59±1 Ma, the record of its earliest erosional history is meagre. In an attempt to fill this gap in the knowledge, we studied temporal changes in provenance of Paleogene–Neogene siliciclastic sediments of the Andaman Islands, deposited in a trench-forearc basin in the Bay of Bengal. Using Sr-isotope stratigraphy and tephrochronology we determined the timings of depositions of various lithologies. Sediment sources were identified using trace element and isotopic (Sr–Nd) fingerprinting. Results of our study suggest that the Myanmar Arc had remained a constant sediment source to the Andaman basin during 55–5 Ma, whereas the basin started receiving significant continental sands input after 35 Ma that increased with time until ~20 Ma. Geochemical provenance of these sands suggests their derivation from Precambrian crustal sources in the Himalaya, which probably is an outcome of higher erosional rates subsequent to a rapid exhumation of the orogen in the late Eocene and efficient sediment transport through the palaeo-channels of the rivers Brahmaputra and Ganga under optimal conditions of the Indian monsoon. Such a scenario is consistent with the idea that the Himalayan sediment input is the cause for the conspicuous rise in marine 87Sr/86Sr since ~40 Ma. Our data also suggest that since the Miocene, sediment sources in the Indo-Burman Ranges and the Myanmar arc have become the major contributors to the Andaman Basin through the Irrawaddy river system.

Extinction intensity during Ordovician and Cenozoic glaciations explained by cooling and palaeogeography

A striking feature of the marine fossil record is the variable intensity of extinction during superficially similar climate transitions. Here we combine climate models and species trait simulations to explore the degree to which differing palaeogeographic boundary conditions and differing magnitudes of cooling and glaciation can explain the relative intensity of marine extinction during greenhouse–icehouse transitions in the Late Ordovician and the Cenozoic. Simulations modelled the response of virtual species to cooling climate using a spatially explicit cellular automaton algorithm. We find that palaeogeography alone may be a contributing factor, as identical changes in meridional sea surface temperature gradients caused greater extinction in Late Ordovician simulations than in Cenozoic simulations. Differences in extinction from palaeogeography are significant, but by themselves are insufficient to explain observed differences in extinction intensity. However, when simulations included inferred changes in continental flooding and interval-specific models of sea surface temperature, predicted differences in relative extinction intensity were more consistent with observations from the fossil record. Our results support the hypothesis that intense extinction in the Late Ordovician is partially attributable to exceptionally rapid and severe cooling compared to Cenozoic events. High extinction intensity during Late Ordovician but not Cenozoic transitions to glacial conditions can be attributed to both temperature and palaeogeography, according to analysis combining climate models and simulations of virtual species.

The complete mitochondrial genome of Harpago chiragra and Lambis lambis (Gastropoda: Stromboidea): implications on the Littorinimorpha phylogeny

The complete mitochondrial genomes of Harpago chiragra and Lambis lambis (Strombidae) were determined with the size of 15,460 bp and 15,481 bp, respectively, and both sequences contained 13 protein-coding genes, 22 tRNAs, and two rRNAs. H. chiragra and L. lambis have similar mitochondrial features, corresponding to typical gastropod mitochondrial genomes, such as the conserved gene order, a high A + T content (66.22% for H. chiragra and 66.10% for L. lambis), and preference for A + T-rich codons. The start or termination codon of same protein-coding gene in H. chiragra was consistent with that in L. lambis, except for the termination codon of cox1 gene (TAG for H. chiragra and TAA for L. lambis) and the start codon of nad4 (GTG for H. chiragra and ATG for L. lambis). Pairwise sequence alignments detected different degrees of variations in H. chiragra and L. lambis mitochondrial genomes; and the two species had lower levels of genetic distance (0.202 for nucleotide sequence) and closest relationships as compared to Strombus gigas and Oncomelania hupensis. The 13 partitioned nucleotide sequences of protein coding genes of H. chiragra and L. lambis were aligned with representatives of the main lineages of gastropods and their phylogenetic relationships were inferred. H. chiragra and L. lambis share the same gene order as Littorinimorpha species, except Vermetoidea, which demonstrate a gene rearrangement in species. The reconstructed phylogeny supports three major clades within Littorinimorpha: 1) Stromboidea, Tonnoidea, Littorinoidea, and Naticoidea, 2) Rissooidea and Truncatelloidea, and 3) Vermetoidea. In addition, a relaxed molecular clock calibrated with fossils dated the diversification of Strombidae near 112 (44–206) Mya and a possible radiation is detected to occur between 45–75 Mya, providing implications to understand the Cenozoic replacement event (65–135 Mya) of Aporrhaidae by Strombidae.

Late Cenozoic structure and tectonics of the southern Sierra Nevada–San Joaquin Basin transition, California

AbstractThis paper presents a new synthesis for the late Cenozoic tectonic, paleogeographic, and geomorphologic evolution of the southern Sierra Nevada and adjacent eastern San Joaquin Basin. The southern Sierra Nevada and San Joaquin Basin contrast sharply, with the former constituting high-relief basement exposures and the latter constituting a Neogene marine basin with superposed low-relief uplifts actively forming along its margins. Nevertheless, we show that Neogene basinal conditions extended continuously eastward across much of the southern Sierra Nevada, and that during late Neogene–Quaternary time, the intra-Sierran basinal deposits were uplifted and fluvially reworked into the San Joaquin Basin. Early Neogene normal-sense growth faulting was widespread and instrumental in forming sediment accommodation spaces across the entire basinal system. Upon erosion of the intra-Sierran basinal deposits, structural relief that formed on the basement surface by the growth faults emerged as topographic relief. Such “weathered out” fossil fault scarps control much of the modern southern Sierra landscape. This Neogene high-angle fault system followed major Late Cretaceous basement structures that penetrated the crust and that formed in conjunction with partial loss of the region’s underlying mantle lithosphere. This left the region highly prone to surface faulting, volcanism, and surface uplift and/or subsidence transients during subsequent tectonic regimes. The effects of the early Neogene passage of the Mendocino Triple Junction were amplified as a result of the disrupted state of the region’s basement. This entailed widespread high-angle normal faulting, convecting mantle-sourced volcanism, and epeirogenic transients that were instrumental in sediment dispersal, deposition, and reworking patterns. Subsequent phases of epeirogenic deformation forced additional sediment reworking episodes across the southern Sierra Nevada–eastern San Joaquin Basin region during the late Miocene break-off and west tilt of the Sierra Nevada microplate and the Pliocene–Quaternary loss of the region’s residual mantle lithosphere that was left intact from the Late Cretaceous tectonic regime. These late Cenozoic events have left the high local-relief southern Sierra basement denuded of its Neogene basinal cover and emergent immediately adjacent to the eastern San Joaquin Basin and its eastern marginal uplift zone.

Jurassic extension and Cenozoic inversion tectonics in the Asturian Basin, NW Iberian Peninsula: 3D structural model and kinematic evolution

We constructed a geological map, a 3D model and cross-sections, carried out a structural analysis, determined the stress fields and tectonic transport vectors, restored a cross section and performed a subsidence analysis to unravel the kinematic evolution of the NE emerged portion of the Asturian Basin (NW Iberian Peninsula), where Jurassic rocks crop out. The major folds run NW-SE, normal faults exhibit three dominant orientations: NW-SE, NE-SW and E-W, and thrusts display E-W strikes. After Upper Triassic-Lower Jurassic thermal subsidence, Middle Jurassic doming occurred, accompanied by normal faulting, high heat flow and basin uplift, followed by Upper Jurassic high-rate basin subsidence. Another extensional event, possibly during Late Jurassic-Early Cretaceous, caused an increment in the normal faults displacement. A contractional event, probably of Cenozoic age, led to selective and irregularly distributed buttressing and fault reactivation as reverse or strike-slip faults, and folding and/or offset of some previous faults by new generation folds and thrusts. The Middle Jurassic event could be a precursor of the Bay of Biscay and North Atlantic opening that occurred from Late Jurassic to Early Cretaceous, whereas the Cenozoic event would be responsible for the Pyrenean and Cantabrian ranges and the partial closure of the Bay of Biscay.

Inferring Evolution of Habitat Usage and Body Size in Endangered, Seasonal Cynopoeciline Killifishes from the South American Atlantic Forest through an Integrative Approach (Cyprinodontiformes: Rivulidae).

Cynopoecilines comprise a diversified clade of small killifishes occurring in the Atlantic Forest, one of the most endangered biodiversity hotspots in the world. They are found in temporary pools of savannah-like and dense forest habitats, and most of them are highly threatened with extinction if not already extinct. The greatest gap in our knowledge of cynopoecilines stems from the absence of an integrative approach incorporating molecular phylogenetic data of species still found in their habitats with phylogenetic data taken from the rare and possibly extinct species without accessible molecular information. An integrative analysis combining 115 morphological characters with a multigene dataset of 2,108 bp comprising three nuclear loci (GLYT1, ENC1, Rho), provided a robust phylogeny of cynopoeciline killifishes, which was herein used to attain an accurate phylogenetic placement of nearly extinct species. The analysis indicates that the most recent common ancestor of the Cynopoecilini lived in open vegetation habitats of the Atlantic Forest of eastern Brazil and was a miniature species, reaching between 25 and 28 mm of standard length. The rare cases of cynopoecilines specialized in inhabiting pools within dense forests are interpreted as derived from four independent evolutionary events. Shifts in habitat usage and biogeographic patterns are tentatively associated to Cenozoic paleogeographic events, but the evolutionary history of cynopoecilines may be partially lost by a combination of poor past sampling and recent habitat decline. A sharp evolutionary shift directed to increased body size in a clade encompassing the genera Campellolebias and Cynopoecilus may be related to a parallel acquisition of an internally-fertilizing reproductive strategy, unique among aplocheiloid killifishes. This study reinforces the importance of adding morphological information to molecular databases as a tool to understand the biological complexity of organisms under intense pressure from loss of habitat.

Thermal history of the Jurassic marine sequences in the Qiangtang Basin, northern Tibetan Plateau: implication for the hydrocarbon preservation

The Jurassic marine sequences of the Qiangtang Basin in the northern part of the Tibet Plateau have the best petroleum exploration prospect. The prototype basin has undergone multi-stage and complicated tectonic events since the Cretaceous, especially the mid-Cretaceous tectonic event and the Cenozoic uplift of Tibetan Plateau. However, little information about the impact of these tectonic events on petroleum preservation has been done. In this paper, the burial and thermal histories modeling using vitrinite reflectance (Ro) are used to characterize the impact. The results showed that there is a regional falling of palaeotemperature in the Qiangtang Basin during the mid-Cretaceous indicating a regional tectonic event caused by the collision between Lhasa and Qiangtang blocks. However, the Cenozoic tectonic event caused by the uplift of Tibetan Plateau since 30 Ma was only recorded in the Southern Qiangtang Depression and not obvious in the Northern Qiangtang Depression, which indicates that uplift-denudation caused by the mid-Cretaceous tectonic event was extensive in whole Qiangtang Basin and more significant to the hydrocarbon accumulation than those in the Cenozoic. The event could damage the previously formed hydrocarbon traps and caused the leaking of cap-rocks for the dramatically denudation especially in the Southern Qiangtang Depression. We speculated that the Northern Qiangtang Depression has the lower thickness of denudation and barely influenced by the Cenozoic tectonic events and thus is primary exploration target for gas or gas hydrate.

The distribution of iodine and effects of phosphatization on it in the ferromanganese crusts from the Mid-Pacific Ocean

In the present paper, iodine (I), iron (Fe), manganese (Mn), cobalt (Co), phosphorus (P) and calcium (Ca) contents in three ferromanganese crusts from the Pacific Ocean are measured by spectrophotometric method and inductively coupled plasma atomic emission spectrometers (ICP-AES) to investigate the contents and distribution of iodine in ferromanganese crusts. The results show that iodine contents in three crusts vary between 27.1 and 836 mg/kg, with an average of 172 mg/kg, and the profile of iodine in the three crusts all exhibits a two-stage distribution zone: a young non-phosphatized zone and an old phosphatized zone that is rich in I, P and Ca. The iodine content ratios of old to young zone in MP5D44, CXD62–1 and CXD08–1 are 2.3, 3.4 and 13.7, respectively. The boundary depths of two-stage zone in MP5D44, CXD62–1 and CXD08–1 locate at 4.0 cm, 2.5 cm and 3.75 cm, respectively, and the time of iodine mutation in three crusts ranges from 17–37 Ma derived from 129I dating and Co empirical formula, which is consistent with the times of Cenozoic phosphatization events. The present study shows that the intensity of phosphatization is the main responsible for the distribution pattern of iodine in the crusts on the basis of the correlation analysis. Consequently, iodine is a sensitive indicator for phosphatization.

Exhuming the Meso–Cenozoic Kyrgyz Tianshan and Siberian Altai-Sayan: A review based on low-temperature thermochronology

Thermochronological datasets for the Kyrgyz Tianshan and Siberian Altai-Sayan within Central Asia reveal a punctuated exhumation history during the Meso–Cenozoic. In this paper, the datasets for both regions are collectively reviewed in order to speculate on the links between the Meso–Cenozoic exhumation of the continental Eurasian interior and the prevailing tectonic processes at the plate margins. Whereas most of the thermochronological data across both regions document late Jurassic–Cretaceous regional basement cooling, older landscape relics and dissecting fault zones throughout both regions preserve Triassic and Cenozoic events of rapid cooling, respectively. Triassic cooling is thought to reflect the Qiangtang–Eurasia collision and/or rifting/subsidence in the West Siberian basin. Alternatively, this cooling signal could be related with the terminal terrane-amalgamation of the Central Asian Orogenic Belt. For the Kygyz Tianshan, late Jurassic–Cretaceous regional exhumation and Cenozoic fault reactivations can be linked with specific tectonic events during the closure of the Palaeo-Tethys and Neo-Tethys Oceans, respectively. The effect of the progressive consumption of these oceans and the associated collisions of Cimmeria and India with Eurasia probably only had a minor effect on the exhumation of the Siberian Altai-Sayan. More likely, tectonic forces from the east (present-day coordinates) as a result of the building and collapse of the Mongol-Okhotsk orogen and rifting in the Baikal region shaped the current Siberian Altai-Sayan topography. Although many of these hypothesised links need to be tested further, they allow a first-order insight into the dynamic response and the stress propagation pathways from the Eurasian margin into the continental interior.

Nature and Evolution of the Lithospheric Mantle beneath the Hoggar Swell (Algeria): a Record from Mantle Xenoliths

Peridotite xenoliths exhumed by Quaternary alkaline magmatism in the Tahalgha district, southern Hoggar, represent fragments of the subcontinental lithospheric mantle beneath the boundary between the two major structural domains of the Tuareg Shield: the ‘Polycyclic Central Hoggar’ to the east and the ‘Western Hoggar’, or ‘Pharusian Belt’, to the west. Samples were collected from volcanic centres located on both sides of a major lithospheric shear zone at 4°35' separating these two domains. Although showing substantial variations in their deformation microstructures, equilibrium temperatures and modal and chemical compositions, the studied samples do not display any systematic changes of these features across the 4°35' fault. The observed variations rather record small-scale heterogeneities distributed throughout the study area and reflecting the widespread occurrence of vein conduits and metasomatized wall-rocks related to trans-lithospheric melt circulation during the Cenozoic. These features include partial annealing of pre-existing deformation microstructures, post-deformation metasomatic reactions, and trace-element enrichment, coupled with heating from 750–900°C (low-temperature lherzolites) to 900–1150°C (intermediate-T lherzolites and high-T harzburgites and wehrlites). Trace-element modelling confirms that the range of rare earth element (REE) variations observed in the Tahalgha clinopyroxenes may be accounted for by reactive porous flow involving a single stage of basaltic melt infiltration into a light REE (LREE)-depleted protolith. Whole-rock compositions record the final entrapment of disequilibrium metasomatic melts upon thermal relaxation of the veins–wall-rock system. The striking correlations between equilibrium temperatures and trace-element enrichment favor a scenario in which the high-temperature peridotites record advective heat transport along melt conduits, whereas the intermediate- and low-temperature lherzolites reflect conductive heating of the host Mechanical Boundary Layer. This indicates that the lithosphere did not reach thermal equilibrium, suggesting that the inferred heating event was transient and was rapidly erased by thermal relaxation down to the relatively low-temperature present-day geotherm. The low-T (<900°C) deformed lherzolites (porphyroclastic to equigranular) are characterized by only incipient annealing and LREE-depleted clinopyroxene compositions. They were only weakly affected by the Cenozoic events and could represent relatively well-preserved samples from rejuvenated Pan-African lithosphere. Extensive lithospheric rejuvenation occurred either regionally during the Pan-African orogeny, as a result of lithospheric delamination or thermomechanical erosion after thickening, or more locally along the meridional shear zones. The low-T Tahalgha lherzolites are comparable with lherzolites from Etang de Lherz, southern France, interpreted as lithospheric mantle rejuvenated by melt-induced refertilization during a late stage of the Variscan orogeny.

Assessment of the Cenozoic Erosion Amount Using Monte Carlo Type-Petroleum Systems Modeling of the Hammerfest Basin, Western Barents Sea

The Cenozoic uplift and erosion is often believed to be a major risk factor in hydrocarbon exploration in the Barents Sea causing petroleum redistribution and leakage from filled traps. Therefore, the estimation of erosion amount is an important but often underrepresented task in the basin modeling procedure. The assessment of erosion magnitudes and spatial distribution by geochemical and thermo chronological methods results in very different estimates and/or does not consider uncertainties of input data. In this study, this problem is approached by using Monte Carlo simulation techniques in secondary migration basin modeling. Thereby, amounts of early and late Cenozoic erosion episodes are described by probability distributions and the modeling results were evaluated considering their uncertainty ranges. In addition, overpressure and related leakage scenarios are considered in the petroleum basin models to study their effect on modeling results. It is shown that the early Cenozoic erosion event had a generally higher erosion magnitude than the late Cenozoic event (1.0-1.3 and 0.4-1.2 km respectively). Modeled erosion estimates are not very sensitive to overpressure modeling which is found to affect only the early Cenozoic erosion amount estimates at low degree.

Late Triassic thickening of the Songpan–Ganzi Triassic flysch at the edge of the northeastern Tibetan Plateau

Growing geologic evidence documents incremental Mesozoic and early Cenozoic shortening and thickening of the Tibetan crust prior to the onset of the main Cenozoic orogenic event. The Tibetan crust shows spatial and temporal variability in thickness, style, and timing of thickening, and in plateau-forming processes. The Songpan–Ganzi area of northeastern Tibet provides evidence for shortening and thickening of the crust in Late Triassic time. An oil exploratory well (HC-1) of 7012.4 m located in the area shows at least six tectonic repetitions, resulting in more than ∼46% thickening of the Triassic sequence. It indicates that the true thickness of the Songpan–Ganzi Triassic flysch is not 10–15 km as previously assumed, but not more than 3–5 km. Based on this evidence, combined with prior tectonostratigraphic studies, we propose that substantial crustal shortening and thickening, leading to initial plateau formation in the northeastern Tibetan Plateau, had already occurred during the Late Triassic.

Evidences of buried loads in the base of the crust of Borborema Plateau (NE Brazil) from Bouguer admittance estimates

Abstract In the Borborema Province (BP) – northeastern Brazil – two important Cenozoic events occurred at the surface: the Macau magmatism and the Borborema Plateau epeirogenesis. To obtain appropriated-scale geophysical data to explain the deep origins of these two events, different gravimetric/elevation databases were integrated with new surveys. Bouguer admittance estimates reveal that isostatic condition of the BP, especially in the Borborema Plateau, can be explained using elastic models to the lithosphere only if surface and buried loadings are combined. If the buried load is applied in the base of the crust, the ratio between buried and surface weights is circa 15 for a lithosphere with effective elastic thickness around 15 km and crust thickness around 33 km. From an interpretative viewpoint of the buried load, it is assumed that the lower crust under the Borborema Plateau might have an anomalous high value of density. Magmatic underplating might explain this fact as well as the observed surface magmatism and epeirogenesis. Crustal thickening of about 4 km under the Borborema Plateau and intracrustal seismic velocity discontinuity with high Vp/Vs ratio are geophysical facts consistent with magmatic underplating. However, the surface magmatism presents low volume and mainly alkaline composition – facts that are not entirely consistent with the hypothesis of magmatic underplating. Regardless the validity of this hypothesis, Cenozoic-to-present events in BP might be somewhat associated with imbalances in lithosphere-asthenospheric mantle and/or crust-lithospheric mantle systems. The existence of free-air anomalies showing no null integral over area and of an expressive positive geoid anomaly are geophysical evidences of these imbalances. Possibly, the Borborema Plateau is still suffering epeirogenesis. Post-depositional deformation found in Barreiras Formation strata, Late Quaternay fault reactivations, and AFT thermochronology analysis suggesting the existence of a cooling stage between 20 and 0 Ma might be geologic evidences of the continued action of epeirogenesis until the present. In addition, the relatively high level of the present intraplate seismicity recorded in several regions of the BP is another unequivocal geophysical evidence that the crust of the province is still submitted to accommodation processes.