Tertiary Time Research Articles

Exploring carbonate reservoirs potential, north Egypt

Carbonate reservoirs contain a significant portion of the world's oil reserves. The Middle East is home to many of these reservoirs. Carbonate reservoirs can be found in a few very large oil fields, including Egypt's Zohr field, the largest conventional gas field in Egypt. In this article, we present a couple of successful tertiary carbonate reservoirs that are bearing hydrocarbon in the Egyptian Western Desert and Gulf of Suez provinces, they are distributed across Egypt's subsurface, and they are characterized by heterogeneous porosity and permeability. These heterogeneities are caused by the wide spectrum of tectonostratigraphic environments in which carbonates are deposited and by subsequent diagenetic alteration of the original rock fabric. The structural analysis of the study areas based on the interpretation of both geophysical and geological data, in JD, Abu Sennan, and the North Amer areas in the Western Desert and offshore Gulf of Suez respectively showed that tectonostratigraphic history during tertiary time affected a great extent carbonate reservoir quality. Major fault trends formed secondary fracture porosity possibly allowing hydrothermal solutions to pass through the reservoirs and form secondary vuggy porosity. These characteristics are considered important factors of promising carbonate reservoirs. In this study, several carbonate reservoirs have been outlined. The study workflow helped us better identify carbonate prospects with high fractured density.

Geochemistry of Liquid Hydrocarbons and Natural Gases Combined with 1D Basin Modeling of the Oligocene Shale Source Rock System in the Offshore Nile Delta, Egypt.

The current study aims to integrate the geochemical characteristics of the Oligocene shale source rock system, oil, condensate, and natural gas samples in the Oligocene sandstone reservoirs from three exploration wells located in the offshore Nile Delta, East Mediterranean Sea, using organic geochemistry and a 1D basin modeling scheme. The Tineh shales exhibit total organic carbon values ranging between 0.90 and 1.89 wt %, along with hydrogen index values in the range of 54-240 mg hydrocarbon/g rock. The geochemical characterization suggests that the shale intervals of the Oligocene Tineh Formation contain type II-III and type III kerogens and, thereby, could be regarded as promising oil- and gas-prone source rocks with high contributions of gas generation potential. The study also reconstructs the 1D thermal and burial history models, showing that the Oligocene Tineh source rock system is in the main oil and wet gas generation phases from the late Miocene to the present time. The simulated basin models reveal the transformation (TR) of 10-50% kerogen to oil during the late Miocene-early Pliocene period and that the Oligocene Tineh source rock system has larger oil generation and expulsion competency, with a TR value of up to 65% during the early Pliocene-Pleistocene time period. The thermogenic gas was also formed during this time and continued to the present day. This study also investigated the presence of oil and condensate in the Oligocene sandstone reservoir samples and revealed that they were generated from mature source rock, ranging from moderately to highly mature stages. This source rock unit was deposited in fluvial to fluvial-deltaic environments under oxic mixed organic conditions and accumulated during the Tertiary time, as evidenced by the presence of the oleanane biomarker dating indicator. The molecular and isotope compositions of natural gases revealed that most of the natural gases in the Oligocene sandstone reservoir are mainly thermogenic methane gases that were generated from mainly mixed organic matter. The thermogenic methane gases were formed mainly from secondary cracking of oil and gas, with small contributions of primary kerogen cracking. The properties of natural gases together with oil and condensate in the Oligocene reservoir rocks suggest that most of the thermogenic methane gases and associated liquid hydrocarbons are derived primarily from the Oligocene shale source rock system and formed by primary kerogen cracking and secondary oil and oil/gas cracking in different thermal maturity stages. Therefore, the Oligocene Tineh Formation can be regarded as self-source generation and self-reservoir rock; hence, an intensive oil exploration and production program can be recommended whenever the Tineh source rock system is is well developed and deeply buried.

Evaluation and Interpretation of Legacy Major-Ion Chemistry for the Niobrara River Watershed, Northwestern and Northcentral Nebraska, USA

Evaluation and Interpretation of Legacy Major-Ion Chemistry for the Niobrara River Watershed, Northwestern and Northcentral Nebraska, USA

Experiences of working time intensification and extensification: examining the influence of logics of production in IT work

Abstract This article examines the intensification and extensification of working time among IT workers. As standardised hours have become increasingly eroded, understanding the nuanced experience of re-configured working time becomes vital. Informed by conceptual distinctions between clock time, process time and tertiary time, this study investigates how working time is constituted among, and experienced by, two distinct IT occupational categories: support workers and mobile apps developers. Theorising experiences of working time with recourse to distinct logics of production, which encompass and reflect labour markets, new technology product markets and occupational communities, illuminates how working time experiences are shaped. This facilitates an explanation of the underlying dynamics that surround the interaction between the three concepts of time, which advances our understanding of intensification and extensification.

Timeliness of Multimodal Care for At-Risk Breast Cancer Patients at a Safety Net Institution

IntroductionBecause limited data exist, we sought to evaluate timeliness of multimodal treatments in a safety net breast cancer population. MethodsBreast cancer patients treated at a safety net hospital from 2016 to 2020 were analyzed retrospectively. Time intervals were defined as primary time (PT) from diagnosis to initiation of primary intervention, secondary time (ST) from completion of primary to initiation of secondary intervention, and tertiary time (TT) from completion of secondary to initiation of tertiary intervention. Variables included primary language, insurance type, and race. ResultsOf 223 patients, 99 (44.4%) primarily spoke Spanish, 29 (13.0%) were of Black race, and 184 (82.5%) had Medicaid or uninsured status. Median (IQR) age at diagnosis was 55 (48-62) years. Primary intervention was surgical in 127/216 (58.8%); secondary intervention was systemic in 38/169 (22.5%); and tertiary intervention was radiation in 67/80 (83.8%). Overall, median days (IQR) for PT were 69 (53, 98), ST were 65 (42, 95), and TT were 69 (43, 88). PT was significantly longer in Black [105 (76, 142) days] patients compared to non-Hispanic White patients [68 (51, 107) days, P = 0.031)] and White Hispanic patients [65 (53,91) days, P = 0.014]. There were no significant differences in PT, ST, or TT by spoken language or insurance type. ConclusionsBlack patients remain at risk due to prolonged time to intervention. Spanish-speaking status was not associated with inferior timeliness or completion of multimodal care at a safety net hospital. Identifying safety net hospital barriers to achieving benchmarks for timely completion of all phases of multimodal care warrants further attention.

Wehrlite xenoliths and petrogenetic implications, Hosséré Do Guessa volcano, Adamawa plateau, Cameroon

Abstract Peridotite xenoliths of wehrlite composition, scarcely known in Adamawa plateau, Cameroon, were sampled by Mio-Pliocene basanites from Hosséré Do Guessa volcano. Their origins are discussed and elucidated trough petrography and mineral chemistry. Studied wehrlites exhibit poikilitic or protogranular textures and are composed of four main mantle phases (high Mg-olivine, augite, enstatite and Al-spinel). Petrographic and microprobe (Camebax SX100) chemical data (Fo90.8-91.4 olivine, Wo39.4-42.0 augite, En90.5-91.1 enstatite and Al-spinel) suggest a mantle origin for the Hosséré Do Guessa wehrlites. Hence, these rocks could not be considered cumulate. They have been equilibrated between 1,140 and 1,220°C, at pressures of 1.5–2.0 GPa, at 50–66 km deep, below the crust-mantle boundary. Wehrlites might result in reactions with carbonate/carbonatite melt, accompanying CO2 degassing and metasomatism by fluid phases. They suffered transpressional tectonics, during movement at Tertiary times of Pan-African strike-slip-faults, after solid-state tectonic relaxation.

Role of crustal fluids and thermo-mechanical structure for lower crustal seismicity: The Gargano Promontory (southern Italy)

Several regions around the globe are characterized by a seismically active lower crust, at depths where lithological and thermal conditions suggest stress release by ductile flow. The Gargano Promontory (GP, southern Italy) is an example where a recently installed seismic network has recorded an intense seismic activity at depths between 20 and 30 km, i.e. in the lower crust. The GP is located in proximity of the Gargano-Dubrovnik lineament, a seismogenic zone separating the central and southern Adriatic basins. These two basins constitute sites of sediments accumulation since Tertiary times. Another important basin in the region is represented by the Apennine foredeep, that includes the Candelaro area. We analyze the possible mechanisms controlling the distribution of seismicity in the GP to identify the factors that make the lower crust seismically active. To this aim, we construct a thermo-rheological model of a layered continental crust, calibrated on the basis of geometrical, lithological and thermal constraints. The model takes into account various crustal lithologies, the presence of fluids in the crystalline basement, lateral variations of geotherm and stress field. The numerical simulations show that the presence of fluids is a key factor controlling the cluster of seismicity in the lower crust. Moreover, the presence of water in the upper crystalline basement and sedimentary cover provides a plausible explanation for upper crustal seismicity in a zone of very high heat flow SW of the GP. The distribution of the seismicity is probably affected by the composition of the crystalline basement, with mafic bodies injected into the crust during the Paleocene magmatic phase that affected the Mediterranean region. In addition, fluid accumulation and overpressure may occur along detachment levels in the lower crust, leading to clustering of the earthquakes. Based on our findings, we hypothesize that the presence of hydrous diapiric upwelling(s) in the upper mantle can feed a deep fluid circulation system, inducing lower crustal seismicity. • A thermo-rheological model to explain lower crustal seismicity in the Gargano Promontory (Italy) is presented. • Fluid presence controls the Gargano seismic activity in the lower crust. • The Gargano crust is injected by mafic bodies at various depths.

The association between intracranial pressure and optic nerve sheath diameter on patients with head trauma.

Although intracranial pressure (ICP) monitoring is the gold standard method for measuring intracranial pressure after traumatic brain injury, optic nerve sheath diameter (ONSD) measurement with ultrasound (US) is also used in the evaluation of ICP. To investigate the association between a series of OSND measurements by US and changes in clinical presentation of the patient. Prospective study including 162 patients with traumatic brain injury. Age, sex, cerebral CT findings, ONSD levels by US at minutes 0, 60, and 120, Glasgow Coma Scale (GCS) within same period, change of consciousness, treatment, and mortality data were reviewed. The association of ONSD levels with GCS, change of consciousness, treatment, and mortality was evaluated. There was no difference in ONSD changes in the patients' sample within the period (p=0.326). ONSD significantly increased in patients who died (p<0.001), but not in those who survived (p=0.938). There was no significant change in ONSD of the patients who received anti-edema therapy (p=801), but significantly increased ONSD values were found in those who received anti-edema therapy (p=0.03). Patients without change of consciousness did not have any significant change in ONSD (p=0.672), but ONSD values increased in patients who consciousness became worse, and decreased in those who presented a recovery (respectively, p<0.001, p=0.002). A negative correlation was detected between ONSD values and GSC values measured at primary, secondary, and tertiary time periods (for all p<0.001). ONSD follow-up may be useful to monitor ICP increase in patients with acute traumatic brain injury.

How Two Different Cenozoic Geologic and Glacial History Paradigms Explain the Southcentral Montana Musselshell-Yellowstone River Drainage Divide Origin, USA

The accepted Cenozoic geologic and glacial history paradigm (accepted paradigm) considers the southcentral Montana Musselshell-Yellowstone River drainage divide to have originated during Tertiary (or preglacial) time while a new and different Cenozoic geologic and glacial history paradigm (new paradigm) describes how headward erosion of a northeast-oriented Musselshell River valley segment captured huge southeast-oriented meltwater floods to create the drainage divide late during a continental ice sheet&amp;rsquo;s melt history. Northwest to southeast oriented divide crossings (low points observed on detailed topographic maps where water once flowed across the drainage divide), southeast-oriented Yellowstone and Musselshell River segments immediately upstream from northeast-oriented Yellowstone and Musselshell River segments, and southeast- and northwest-oriented tributaries to northeast-oriented Yellowstone and Musselshell River segments indicate a major southeast-oriented drainage system predated the northeast-oriented Yellowstone and Musselshell River segments. Closeness of the divide crossings, divide crossing floor elevations, large escarpment-surrounded erosional amphitheater-shaped basins, and unusual flat-floored internally drained basin areas (straddling the drainage divide), all suggest the previous southeast-oriented drainage system moved large quantities of water which deeply eroded the region. In the mid-20th century geomorphologists working from the accepted paradigm perspective determined trying to explain such erosional landform evidence from the accepted paradigm perspective was a nonproductive research activity and now rarely investigate erosional landform origins. On the other hand, the new paradigm appears to explain most, if not all observed erosional landform features, although the two paradigms lead to significantly different regional Cenozoic geologic and glacial histories that cannot be easily compared.&amp;nbsp;&amp;nbsp;

PEDAGOGICAL STRATEGIES FOR EFFECTIVE ONLINE ENTREPRENEURIAL EDUCATION AS PERCEIVED BY BUSINESS EDUCATORS

This study was designed to determine the best pedagogical strategies for online entrepreneurial education. This was considered important as the rate of unemployment among graduates is on the increase, notwithstanding the efforts of tertiary institutions in Nigeria to teach all students entrepreneurship. There are also graduates who were not exposed to entrepreneurship in addition to people who had no privilege of studying in any tertiary institution full time. Given the favourable developments in science and technology, online education has become popular. In order to carry out the study, four research questions were asked relating to strategies for preparing students for online entrepreneurial education; creating conducive online learning environment; strategies for monitoring student’s progress and student assessment and data evaluation. Four hypotheses were formulated and tested at 0.05 level of significance. The population consisted of fifty business teacher educators drawn from selected universities and colleges of education in Enugu and Ebonyi States of Nigeria. Data which were collected using a 52-item structured questionnaire were analyzed using mean, standard deviation and t-test statistic. It was found, among others, that the 52 strategies were all accepted as effective strategies for online entrepreneurship education. There were no statistically significant differences in the mean responses of business teacher educators in the universities and those in the colleges of education on 47 of the strategies for online entrepreneurship education. It was concluded that the strategies are both relevant and appropriate for effective online entrepreneurship education. It was recommended that tertiary institutions and departments of business teacher education who offer online entrepreneurship education could apply the identified strategies. Article visualizations:

A new pipoid frog (Anura, Pipimorpha) from the Paleogene of Patagonia. Paleobiogeographical implications

The aim of the present contribution is to describe a new genus and species of Pipoidea from the Huitrera Formation (Eocene) from Patagonia, Argentina. The new genus shows a unique combination of characters indicating that it is a valid taxon different from other pipimorphs, including the coeval Llankibatrachus truebae. The phylogenetic analysis resulted in the nesting of the new taxon within a previously unrecognized endemic clade of South American aglossans. This new clade turns out to be the sister-group of crown-group Pipidae. This phylogenetic proposal reinforces the hypothesis sustaining the dispersal of pipids between Africa and South America through an island chain or a continental bridge across the Atlantic Ocean by Early Tertiary times.

Checklist of oribatid mites (Acari, Oribatida) of the Russian Far East and Northeast of China.

The present paper reviews the taxonomic studies of the mite suborder Oribatida in the Russian Far East South and Chinese North-East Territories. At present, 746 species of oribatid mites are registered in China, including 175 species in the soils of Northeast China. In the Russian Far East, there were 605 species of oribatids, including 344 species in the south of the Far East. The fauna of the oribatid mites of the Northeast of China and the south of the Russian Far East has 446 species and subspecies representing 190 genera and 80 families. 72 species of oribatid are common for both territories. The modern fauna of the oribatid mites of the Northeast of China and the south of the Far East was formed as a result of prolonged interaction between the boreal and palaearchaearctic faunas. The oribatid fauna of this region is distinguished by the presence of a large number of endemics, some of which are relics of the late Tertiary time and which can be considered as autochthonous. The checklist includes data from more than 100 locations of this enormous region. In addition, a short climatic and historical review of oribatid mites study is presented.

Disability as Inequality: Social Disparities, Health Disparities, and Participation in Daily Activities

Individuals with disabilities experience lower education levels, lower employment rates, fewer household resources, and poorer health than people without disabilities. Yet, despite comprising more than one-eighth of the US population, people with disabilities are seldom integrated into sociological studies of inequality. This study uses time use as a lens through which to understand one type of inequality between working-aged people with and without disabilities: participation in daily activities. It also tests whether social disparities (as suggested by the social model of disability) or health disparities (as suggested by the medical model of disability) explain a larger percentage of participation differences. I first consider if disability predicts daily time in market work, nonmarket work, tertiary (health-related) activities, and leisure—net of health and sociodemographic characteristics. Next, I utilize Oaxaca-Blinder decomposition to assess the relative contribution of these characteristics in explaining time differences. Results from the American Time Use Survey indicate that adults with disabilities spend less time than adults without disabilities in market work and more time in tertiary activities and leisure. There is no difference in nonmarket time. Health accounts for the largest percentage of the explained component of tertiary time differences, but depending on the choice of predictors, sociodemographic characteristics account for as much—or more—of the explained component of differences in market and leisure time. Results indicate the importance of disentangling disability from health in sociological studies of inequality. They also support a hybrid disability model in suggesting that both health and sociodemographic characteristics determine how disability shapes daily life.

Clay minerals of upper Cretaceous (Shiranish Formation) and lower Tertiary (Kolosh Formation) at selected sections from north Iraq

Clay minerals investigation is conducted using X-ray diffraction and scanning electron microscopy from the upper Cretaceous Shiranish Formation and lower Tertiary Kolosh Formation at two sections in Dohuk and Shaqlawa, northern Iraq. The study revealed the presence of smectite, palygorskite, illite, smectite-chlorite mixed layers, chlorite, and kaolinite. Smectite and palygorskite found to be formed authigenically in the marl of the Shiranish Formation, whereas, other minerals are detrital in origin especially in the Kolosh Formation. The mineral variation reflects the environmental changes from Cretaceous to Tertiary times. Variation in source rocks and change in drainage conditions may lead to increase in the effectiveness of leaching processes and hence transformation of smectite to palygorskite. These changes also effects on the prevalence of inherited or detrital types of clay minerals in the Kolosh sediments.&#x0D;

Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits

With very few exceptions, orogenic gold deposits formed in subduction-related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra-basinal rock sequences or de-volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post-gold orogenic overprint. Within the host volcano-sedimentary sequences at the province scale, world-class orogenic gold deposits are most commonly located in second-order structures adjacent to crustal scale faults and shear zones, representing the first-order ore-forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ∼10°–25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ∼70° cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre-existing structures, leading to deposit-scale focussing of ore-fluid and gold deposition. Anticlinal or antiformal fold hinges, particularly those of ‘locked-up’ folds with ∼30° apical angles and overturned back limbs, represent sites of brittle-ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits.In orogenic belts with abundant pre-gold granitic intrusions, particularly Precambrian granite-greenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano-sedimentary sequences are common sites of ore-fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore-forming fluids are commonly focused along strain gradients between more compressional zones where volcano-sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World-class orogenic gold deposits are commonly located in the deformed volcano-sedimentary sequences in such strain gradients adjacent to triple-point junctions defined by the granitic intrusions, or along the zones of assembly of micro-blocks on a regional scale. These repetitive province to district-scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology-based exploration targeting for orogenic gold deposits.

From obduction to continental collision: new data from Central Greece

AbstractThe aim of this paper is to contribute to deciphering the evolutionary history of the Hellenides by the study of a large sector of the chain located between the front of the ophiolitic units and the external zones classically attributed to the continental margin of Adria. In particular, the tectonic units located in Boeotia – a key area located in Central Greece at the boundary between the Internal and External Hellenides – were studied from structural, stratigraphic and biostratigraphic points of view. Addressing the main debated aspects concerning the origin of the ophiolite nappe(s), the tectonic evolution of the Hellenic orogen was revised with a particular emphasis on the period between obduction and continental collision. New findings were compared with consolidated data concerning the main metamorphic events recorded in the more Internal Hellenides, geochemistry and age of the ophiolites and main stratigraphic constraints obtained in other sectors of the belt. Finally, a new reconstruction of the tectonic evolution of this area was introduced and, in the context of the dispute concerning the origin of the ‘ophiolitic belts’ as a possible record of multiple oceanic basins, we put forward for consideration a ‘single ocean’ tectonic model spanning from Triassic up to Tertiary times, and valid for the whole Hellenic–Albanian sector.

Petrographic, mineral and pressure–temperature constraints on phyllites from the Variscan basement at Punta Bianca, Northern Apennines, Italy

We present a petrological study of phyllites from the Variscan basement at Punta Bianca (Northern Apennines). According to the literature the studied rocks are believed to be the result of Alpine and Variscan metamorphism of pre-Carboniferous pelites, which consist of Fe-chlorite, potassic white mica, quartz, minor paragonite, and accessory hematite, rutile, monazite, zircon, xenotime, and florencite. Microstructural observations indicate a main S2 foliation that had overprinted an earlier S1 foliation. The mineral assemblage can be related to both S2 and S1.Large flakes of relatively Mg-rich potassic white-mica along the two foliations are probably detrital grains that preserved their composition. The calculation of isochemical phase diagrams, their contouring by various mineral chemical parameters and the deformation microstructures of quartz indicate intermediate pressure-low temperature conditions (5-7 kbar and 300-400 °C). The anhedral habit and the very low Pb contents (< 50 ppm) in monazite point to dissolution of this mineral probably during Tertiary times.

Bauxite deposits in Suriname: Geological context and resource development

AbstractBauxite, the raw material of aluminum, has been one of the economically vital natural resources for Suriname. Mining operations started about a century ago, and subsequent development of a refinery industry and hydro-electric power made Suriname one of the foremost bauxite and alumina producers worldwide for a long period of time. This paper presents a concise survey of the main geological attributes of its bauxite deposits and examines significant aspects in the development of mining in the country where alumina dominated the export revenues until a decade ago. The lateritic bauxite deposits are spread across the northern part of the country and developed on various parent rocks during Late Cretaceous–Early Tertiary times. Bauxites in the coastal lowlands formed on Cenozoic sedimentary deposits, whereas plateau bauxites originated on various crystalline rocks in inland regions of the Precambrian Guiana Shield. The composition of parent rocks and timing of bauxitisation point to a genetic correspondence with West African bauxites and a strong control of paleoclimatic conditions on the distribution and properties of bauxite in both regions. The more accessible bauxite deposits in the coastal lowlands are almost mined out, whereas the plateau bauxites have been extensively explored but have not been brought into production to date. For economic and environmental reasons, the future of the bauxite industry in Suriname is currently uncertain.

Fluid‐induced breakdown of monazite in medium‐grade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy)

AbstractThe last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.

Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

AbstractThe stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non‐volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally &lt;10‐km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid‐crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth‐dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post‐rift. An Aptian–Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of post‐rift subsidence was delayed until post‐Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25–30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post‐rift period. This is consistent with edge‐driven, small‐scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian–Albian time.