Late Tertiary Time Research Articles

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.

Slab bending, syn‐subduction normal faulting, and out‐of‐sequence thrusting in the Central Apennines

AbstractIn the Central Apennines, a Tertiary‐Present eastward‐vergent fold and thrust belt, thrusting mainly occurred in‐sequence, cutting across a Mesozoic to Early Neogene succession, deposited in rifted (Early Jurassic) to passive continental margin, to foredeep (late Miocene) conditions. However, the Simbruini Mts were characterized by the occurrence of regional out‐of‐sequence thrusts (the Vallepietra‐Filettino‐Monte Ortara thrust fault) and ofTertiary prethrusting normal faults, associated with the deposition of thick breccia deposits (Brecce della Renga, Tortonian‐early Messinian). A direct link between these two features is here proposed. Evidence for a prethrusting normal faulting origin for the Brecce della Renga is amply discussed. Although evidence for subduction‐related normal faulting is growing in the Apennines literature, nowhere in the region are known extensional systems so vast and their syn‐tectonic deposits so thick as in the Simbruini area. We propose that the Simbruini prethrusting normal faults were subduction related, i.e., they developed in the foreland due to anomalous bending of the lithosphere subducting in Late Tertiary time under the Apennines in the Simbruini Mts area. The associated breccia deposits were subsequently deformed during the development of the Simbruini Mts thrust front. We speculate that the prethrusting normal faults allowed the penetration of fluids within the subducting crust and possibly also in the mantle, weakening this part of the subducting plate. As a consequence, the weaker plate was anomalously down‐bent, leading to a subcritical state of the Apennines wedge. The regional scale out‐of‐sequence thrust faults developed in order to bring the Apennines wedge back to the critical state.

Cosmogenic 10Be-derived denudation rates of the Eastern and Southern European Alps

Denudation rates from cosmogenic 10Be measured in quartz from recent river sediment have previously been used in the Central Alps to argue that rock uplift occurs through isostatic response to erosion in the absence of ongoing convergence. We present new basin-averaged denudation rates from large rivers in the Eastern and Southern European Alps together with a detailed topographic analysis in order to infer the forces driving erosion. Denudation rates in the Eastern and Southern Alps of 170–1,400 mm ky−1 are within a similar range to those in the Central Alps for similar lithologies. However, these denudation rates vary considerably with lithology, and their variability generally increases with steeper landscapes, where correlations with topographic metrics also become poorer. Tertiary igneous rocks are associated with steep hillslopes and channels and low denudation rates, whereas pre-Alpine gneisses usually exhibit steep hillslopes and higher denudation rates. Molasse, flysch, and schists display lower mean basin slopes and channel gradients, and, despite their high erodibility, low erosion rates. Exceptionally low denudation rates are also measured in Permian rhyolite, which has high mean basin slopes. We invoke geomorphic inheritance as a major factor controlling erosion, such that large erosive glaciers in the late Quaternary cold periods were more effective in priming landscapes in the Central Alps for erosion than in the interior Eastern Alps. However, the difference in tectonic evolution of the Eastern and Central Alps potentially adds to differences in their geomorphic response; their deep structures differ significantly and, unlike the Central Alps, the Eastern Alps are affected by ongoing tectonic influx due to the slow motion and rotation of Adria. The result is a complex pattern of high mountain erosion in the Eastern Alps, which has evolved from one confined to the narrow belt of the Tauern Window in late Tertiary time to one affecting the entire underthrust basement, orogenic lid, and parts of the Southern Alps today.

Fossil leaf-impressions from the Late Tertiary sediments of Mahuadanr Valley, Latehar District, Jharkhand, India

Morphotaxonomical study of the leaf impressions collected from the Late Tertiary sediments of Mahuadanr Valley, Jharkhand, India has been done. The comparison of morphological features between the fossils and extant taxa revealed the occurrence of 13 species of 5 dicotyledonous families. On the basis of present assemblage, the palaeoclimate and phytogeography of Mahuadanr area during the Late Tertiary have been deduced. Present distribution of all the modern comparable species of the fossils indicates that these are presently found to grow in the tropical forests of the Himalayan foot hills, central India, south India and adjoining area of the Mahuadanr Valley, suggesting a mixed, mesophytic type of forest was flourishing in and around the fossil locality during the sedimentation. As most of the comparable species are found now-a-days in the vicinity of fossil locality of Mahuadanr it indicates that same flora has continued till now which suggests that there has been no marked climatic change in the area since the Late Tertiary time.

On the occurrence and formation of open fractures in the Jurassic reservoir sandstones of the Snøhvit Field, SW Barents Sea

Open fractures were described in core and Formation Micro Image (FMI) image logs in the Jurassic sandstones of the Tubåen, Nordmela and Stø formations in the Snøhvit Field, and 3D fracture network properties analysed using Computer Tomography (CT)scanning in selected core samples. The most frequent open fracture type is short stylolite-related fractures (F1), but longer open fractures are also present, with no obvious relationship to stylolites (F2). The F1 fracture densities are related generally to the clay content of the host rock, which controls the occurrence and spacing of the stylolites. The fractures are steep, with a N–S-dominant strike azimuth and significant spread. Although, generally, the F1 fractures are short, a percolating and 3D connected open fracture network across the core was found in most of the CT-scan samples. Open fractures were also found in the damage zone of a lately reactivated fault. The formation of the open fractures in the Snøhvit Field is related most likely to thermoelastic processes during removal of overburden in late Tertiary time. The presence of open fractures may influence reservoir flow, particularly in intervals containing a high frequency of stylolites and in the damage zones of reactivated faults.

Unroofing, incision, and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronometry

The source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics. The ~1.9 km uplift of the Colorado Plateau since the Late Cretaceous is a prime example of this problem. We used apatite (U-Th)/He thermochronometry (230 analyses; 36 samples) to provide the fi rst single-system, regional-scale proxy for the unroofi ng history of the southwestern quadrant of the plateau. The results confi rm overall southwest to northeast unroofi ng, from plateau margin to plateau interior. A single phase of unroofi ng along the plateau margin in Late Cretaceous to Early Tertiary (SevierLaramide) time contrasts with multiphase unroofi ng of the southwestern plateau interior in Early and mid- to Late Tertiary time. The Early Cretaceous was characterized by northeastward tilting and regional erosion, followed by aggradation of ≥1500 m of Upper Cretaceous sediments along the eroded plateau margin. Sevier-Laramide denudation affected the entire southwestern plateau, was concentrated along the plateau margin, and migrated from northwest to southeast. Following a period of relative stability of the landscape from ca. 50‐30 Ma, signifi cant unroofi ng of the southwestern plateau interior occurred between ca. 28 and 16 Ma. Additional denudation north of the Grand Canyon took place in latest Tertiary time. Mid-Tertiary dates from the Grand Canyon basement at the bottom of the Upper Granite Gorge limit signifi cant incision of the modern Grand Canyon below the Kaibab surface to 1500 m difference in vertical structural position. If these models are correct, they indicate that a “proto‐Grand Canyon” of kilometer-scale depth had incised post-Paleozoic strata by the Early Eocene. Evidence for kilometerscale mid-Tertiary relief in northeast-fl owing drainages along the plateau margin, as well as the mid-Tertiary episode of plateau interior unroofi ng, imply that the southwestern plateau interior had attained substantial elevation by at least 25‐20 Ma, if not much earlier. These observations are inconsistent with any model calling for exclusively Late Tertiary uplift of the southwestern plateau. Sevier-Laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere, expanding the Cordillera’s orogenic highlands into its lowstanding cratonic foreland. The onset of the Laramide slab’s demise at ca. 40 Ma and the major pulse of extension in the Basin and Range from ca. 16‐10 Ma appear to have had little infl uence on the denudation history of the southwestern plateau. In contrast, the post-Laramide unroofi ng episodes may be explained by drainage adjustments induced by rift-related lowering of regions adjacent to the plateau, without the need to otherwise modify the plateau lithosphere. Our data do not preclude a large component of post‐Early Eocene elevation gain (or the geodynamic mechanisms it may imply), but they do point toward Laramide-age buoyancy sources as the initial cause of signifi cant surface uplift, ending more than 500 m.y. of residence near sea level.

Uniform Permian40Ar/39Ar detrital mica ages in the eastern Qaidam Basin (NW China): where is the source?

AbstractNew single‐grain40Ar/39Ar detrital white‐mica ages from the Lulehe section at the eastern Qaidam Basin yield uniform Permian ages between 250 ± 3 and 279 ± 3 Ma throughout the whole Cenozoic sequence. This is inconsistent with the present hinterland, which is composed of early Palaeozoic metamorphic units with subordinate early Palaeozoic and few Permian granites. The new data indicate that Permian tectonic units are likely more widespread at the north‐eastern margin of the Tibetan plateau as known at present, particularly within the Qilian Mountains. The preferred explanation is that the Qaidam block represents a rigid indenter, which indented during late Tertiary times into early Palaeozoic orogenic units. This is consistent with recent findings of a NW‐trending sinistral Permian ductile shear zone and a dextral, NW‐trending Tertiary fault system close to the north‐eastern margin of the Qaidam Basin.

Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya

The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time. Primary components of the thrust sheet include Upper Proterozoic metasedimentary rocks of the Bhimphedi Group, Cambrian-Ordovician granite bodies, and Ordovician (through Devonian?) conglomerate, sandstone, shale, and limestone of the Phulchauki Group. Deformation, metamorphism, uplift, and erosion accompanied Tertiary emplacement of the thrust sheet, but there is also evidence of widespread early Paleozoic tectonism, such as: (1) Rocks of the Bhimphedi Group were metamorphosed at ca. 490 Ma, as indicated by Th-Pb ages of monazite inclusions in garnet crystals. (2) Bhimphedi rocks are interpreted to have been imbricated along a south-vergent(?) thrust system during early Paleozoic time, with U-Pb ages recording ductile deformation through ca. 484 Ma but ending (at least locally) by ca. 473 Ma. (3) Cambrian-Ordovician granite bodies may have been generated by crustal melting during thrust loading, and at least some are interpreted to have been emplaced as syntectonic sills along the early Paleozoic thrust faults. (4) Ordovician conglomeratic strata were shed from the uplifted Bhimphedi Group and Cambrian-Ordovician granite bodies and are interpreted to have accumulated in a foreland basin setting with respect to the early Paleozoic thrust system. Our findings, together with evidence for early Paleozoic ductile deformation and metamorphism in adjoining regions, indicate that early Paleozoic tectonism has played an important role in shaping the Himalayan orogen. This early Paleozoic tectonism has been overlooked in most current models for the formation of the Himalayan orogen.

Rejuvenation of The Pre-Cambrien Najd Fault System and Its Importance In The Oil Province of Saudi Arabia

Structural interpretation covering the Central Saudi Arabia was carried out through Landsat, Radar images, topographic maps and seismic data. Existing geology maps provide materialized evidence for this study. A strong northwest trending lineament system has been mapped in the region and is interpreted as being rela- ted to the reactivated Najd Fault System. Elongate hills, strait-going creeks, offset along valley and ridges, and pull-apart basins are evident, indicating rejuvenation of the Najd transtensional sinistral movement. Geological map shows evidence indicating a northwestward dragging on the sedimentary cover along a NW trending regio- nal lineament in the region which extend from South Ghawar to An Nafud Basin in Saudi Arabia. Northeast trend- ing structures in Saudi Arabia are related to the pre-Permian structuring and Oman-Masirah stress regime. The Ghawar (Saudi Arabia), Dukhan (Qatar) and other north-northwest trending anticlines were reshaped by the reju- venated Najd stress regime. The northwest trending sinistral Najd Fault was initiated in Precambrian and was reactivated at various times, probably during Paleozoic and late Jurassic times, and continuously, though of vari- able intensity, from late Cretaceous to the late Tertiary time. The Carboniferous, Late Cretaceous and Tertiary events are overprinted on one another in the region. The Oman stress regime and the stress regime that is rela- ted to oblique obduction of the Masirah Ophiolite are the principal controlling event for the anticline structures in Central Arabia. Dominant structural elements of the area are N-S anticline axis that suggests approximate E-W compressional stress direction, which is consistent with Oman and reactivated Najd tectonic regimes. The effect of the Tertiary orogenic overprint of the Oman-Masirah stress regime in eastern Arabia was to favor a slight renewed movement along the old basement faults such as the Najd Trend. Reactivated Najd system, Oman prin- cipal horizontal stress regime and obduction of the Masirah Ophiolite onto the Arabian continent have produced a combined effect on the structures in Eastern Arabian Plate. The Zagros stress regime may have produced Mineral Res. Exp., Bull., 132, 73-82, 2006 little effect in the region.

Left-lateral shear deformation found in southern part of the Gaoligong range in west Yunnan and its tectonic implications

It is generally understood that the Tertiary structural deformation in the Gaoligong range in west Yunnan was under the control of right lateral shear, resulting from the northward movement of Indian relative to Eurasian. In the southern part of Gaoligong range, however, we have recently found many pieces of evidence of left lateral shear deformation, featuring a 10 km wide shear zone cutting through all tectonic elements of the range in N S direction, which possibly occurred in late Tertiary time, and postdated the right lateral shear. This is a significant discovery for revealing characteristics of the Tertiary intracontinental deformation of the Gaoligong range and the interaction between the Indian and the Eurasian continents.

Generation, migration, accumulation, and dissipation of oil in Northern Iraq

ABSTRACT Most of the known oil accumulations of Northern Iraq probably originated by upward migration from earlier, deeper accumulations which were initially housed in stratigraphic or long-established structural traps, and which are now largely depleted. The earlier concentrations had their source in basinal sediments, into which the porous, primary-reservoir limestones pass at modest distances east of the present fields. Development of the region favored lateral migration from different basinal areas of Upper Jurassic and Lower-Middle Cretaceous time into different areas of primary accumulation. Important factors affecting primary accumulation included: (1) early emergence and porosity improvement of the reservoir limestones, followed by burial under seal-capable sediments; (2) the timely imposition of heavy and increasing depositional loads on the source sediments, and the progressive marginward advance of such loads; (3) progressive steepening of gradients trending upward from source to accumulation area; (4) limitation of the reservoir formations on the up-dip margin by truncation or by porosity trap conditions. In late Tertiary time, large-scale folding caused adjustments within the primary reservoirs, and associated fracturing permitted eventual escape to higher limestone reservoirs, or to dissipation at surface. The sulfurous, non-commercial crudes of Miocene and Upper Cretaceous reservoirs in the Qaiyarah area are thought to stem from basinal radiolarian Upper Jurassic sediments, which lie down dip, a few tens of miles east of these fields. Upper Cretaceous oils of Ain Zalah and Butmah drained upward from primary accumulations in Middle Cretaceous limestones, which were filled from basinal sediments of Lower Cretaceous age situated in a localized trough a few miles northeast of these structures. The huge Kirkuk accumulation, now housed in Eocene-Oligocene limestones, ascended from a precedent accumulation in porous Middle-Lower Cretaceous limestones, which drew its oil from globigerinal-radiolarian shales and limestones of the contemporaneous basin, a short distance east of the present field limits. Eocene-Oligocene globigerinal sediments, considered by some the obvious source material for Kirkuk oil, seemingly provided little or no part of the present accumulation. The reservoir formation may have been filled from these sources, to lose its oil by surface dissipation during the erosional episode preceding Lower Fars deposition. Upper Cretaceous basinal sediments probably contributed nothing to known oil field accumulations, though they may have subscribed to the spectacular impregnations of some exposed, Upper Cretaceous reef-type limestones. Neither Miocene nor pre-Upper Jurassic sediments have played any discernible role in providing oil to any producing field. Indigenous oils are thought to be negligible in the limestone-reservoir formations considered.

MIOCENE MAMMALIAN FAUNAS, OGALLALA GROUP, PAWNEE BUTTES AREA, WELD COUNTY, COLORADO

Abstract The Pawnee Buttes of northeastern Colorado lie within spectacular outcrops of Oligocene (White River Group) and Miocene (Ogallala Group) strata that form part of the Chalk Bluffs, an 150 mile long escarpment near the state line. These deposits were some of the earliest High Plains units to be explored by vertebrate paleontologists (Marsh, 1870; Cope, 1873). Consequently their fossil remains yielded the first information on North American faunas of mid- and late Tertiary time. Many parties subsequently exploited these deposits; their work, and his own investigations, were monographed by Galbreath in 1953. The present paper reviews the literature, a substantial part of the collections, as well as later stratigraphic work to present a revised litho- and biostratigraphy focused on the Pawnee Buttes area of eastern Weld County, Colorado. The oldest post-White River Group unit is the Martin Canyon Formation, the type locality of which is in adjacent Logan County. It contains an early Hemingfordian faun...

Peri-Tethyan neritic carbonate areas: distribution through time and driving factors

Neritic carbonates (NC) were one of the most prominent facies of the Peri-Tethyan domain. From the Moscovian to the end of Tertiary times, most parts of the area concerned were included in, or in the vicinity of, the intertropical domain, favourable to the deposition of NC. These optimal latitudinal conditions were strengthened by warm surface currents which globally ran from east westward in this part of the Tethyan realm. The settlement of NC was enhanced by the existence of huge shallow marine platforms on the southern and northern Peri-Tethyan margins and on isolated oceanic highs as well. Peri-Tethys atlas maps, explanatory notes and validation points have provided data on the main biological, sedimentological, geographical, and environmental features of NC. Areas covered by NC have been calculated from 20 maps encompassing the Moscovian to the Piacenzian/Gelasian, using an Image Analysis Optilab system. The latitudinal distribution and the total area covered by NC for each time slice have been plotted with reference to global factors driving the carbonate factory. Variations of areas through time have been analysed and discussed with respect to regional palaeogeographic conditions. Global factors driving the NC sedimentation in Peri-Tethyan areas have been successively examined. The dominant skeletal and non-skeletal mineralogy on carbonate deposition has been evaluated. Photozoan organisms have been dominant contributors; however, deposition of temperate Heterozoan carbonates in northern latitudes occurred during Artinskian, Wordian, Maastrichtian and Tertiary times. Adverse conditions of siliciclastic inputs on carbonate deposition settled at certain periods but many Peri-Tethyan NC areas faced without damage terrigenous belts. Sea level highstands generally allowed wide NC areas to form, but certain time intervals had about the same amount of area covered by NC, despite opposite – low vs high – eustatic stillstand. Latitudinal enlargement of NC areas was enhanced during Mesozoic and Early Cainozoic greenhouse regimes, while a latitudinal constriction occurred during Artinskian, Wordian and late Tertiary times, in relation to an icehouse mode. During early Jurassic, middle Cretaceous and early Tertiary times, ocean circulation seems to have controlled latitudinal shifts of the NC areas. Abrupt increases or reductions of areas covered by NC coincided with the onset of atmospheric-controlling magmatic events or of sudden oceanic events with climatic implications.

Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains.

Quantifying exhumation from apatite fission-track analysis and vitrinite reflectance data: precision, accuracy and latest results from the Atlantic margin of NW Europe

Abstract In areas where significant unconformities are present, palaeotemperatures derived from apatite fission-track analysis (AFTA) and vitrinite reflectance (VR) data through a vertical rock section can be used to estimate palaeogeothermal gradients and (by extrapolation to an assumed palaeo-surface temperature) amounts of exhumation (palaeoburial). AFTA also provides a direct estimate of the timing of exhumation. These parameters can be used to reconstruct more complete histories than those based purely on the preserved rock record. Precision and accuracy of these estimates are controlled by a range of theoretical and practical factors, perhaps the most important being the use of appropriate kinetic models. In extracting thermal history information from fission tracks in apatite, it is essential to use models that can describe variation in response between apatite grains within a sample. It is also important to recognize the limitations of the methods. AFTA and VR are dominated by maximum temperatures, preserving no information on events prior to a palaeo-thermal maximum. Recognition of this allows definition of key aspects of the history with greater precision. Results from NW Europe define a series of regionally synchronous palaeo-thermal episodes, with cooling beginning in Early Cretaceous, Early Tertiary and Late Tertiary times. Latest results show that Early Tertiary palaeo-thermal effects in NW England can be understood as being due to a combination of higher basal heat flow and deeper burial, and emphasize the importance of obtaining data from a vertical sequence of samples. Comparison with similar results from other parts of the world suggests that events at plate margins exert a key influence on the processes responsible for regional exhumation, as recognized through Mesozoic and Cenozoic times across NW Europe.

Fossil woods from Upper Tertiary sediments of Jammu region (Jammu & Kashmir) North-West India and their significance

Petrified dicotyledonous woods are reported for the first time from the Middle Siwalik sediments of Jammu region (Jammu & Kashmir) in the northwestern part of India. The fossil woods resembling modem woods of Dipterocarpus Gaertn.f. (Dipterocarpaceae), Bischofia Blume (Euphorbiaceae), Cassia Linn. and Kingiodendron (Roxb.) Harms (Leguminosae) have been described in detail. Based on the distribution of comparable extant genera and species, prevalence of humid climate and existence of a mixed lowland tropical forest, comprising moist deciduous to evergreen elements, in the area have been deduced. Evidently Bischofia, Dipterocarpus and Kingiodendron existed in the Jammu region during the Late Tertiary time and subsequently shrunk in their distribution due to climatic change. They no longer grow in the area as they require more humid conditions. The occurrence of Dipterocarpus is particularly noteworthy as it extends the limit of Dipterocarpaceae in the geological past as far west as Jammu region in the north-west India. The arboreal C3 forms show that the landscape was well forested.

Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating

Abstract Based on analyses of about 970 zircon grain fission-track (FT) ages from 44 clastic rock samples collected from six transects and two pooled ages of apatite concentrates separated from a diabase body, the thermo-kinematic evolution of the Taiwan mountain belt since the last orogenic phase (the Penglai Orogeny) has been delineated for the first time. As a consequence of the active Penglai Orogeny since late Tertiary times, pre-orogenic FTs in detrital zircons and apatites have been subjected to varying degrees of annealing by geothermal heating. The spatial boundary between the partial and complete resetting of zircons coincides well with that corresponding to the 260°C isotherm between the greenschist facies and prehnite–pumpellite facies defined previously by crystallinity of potassic micas. The age distribution along each cross-section exhibits younger ages from the mountain front toward the rear plate boundary between the Eurasian and the Philippine Sea plates, suggesting asymmetric cooling and exhumation. Along the strike direction (roughly north to south) of the regional structure, the age distribution reflects southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Spatially, the width of the zircon complete-reset zone gradually narrows down along the strike of the regional structure, reflecting the southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Temporally, zircon FT ages for the western margin of the completely reset zone are progressively younger from 5–6 Ma in the northern part to ca. 2 Ma in the south-central part, then resume to ca. 6 Ma for the southern end, where collision is only in its initial stage. The previously tectonized pre-Tertiary Peikang Basement High on the Asian continental margin plays an important role in defining the uplift-and-cooling history and shaping the major salient-and-recess structure and neotectonics of the mountain belt.

Fault distribution and timing in the Central Irish Sea Basin

Abstract Well data analysis and the interpretation of 2D and 3D seismic reflection data provide valuable insights into the distribution and timing of fault activity within the Central Irish Sea Basin (CISB). Structural and stratigraphic relationships have been used to constrain the timing of fault movements and to interpret the mapped fault patterns in terms of the tectonic evolution of the area. Four main fault trends are identified at the Top Lower Triassic Sherwood Sandstone Group level: I, a NE-SW fault trend that parallels the basin-bounding faults and is believed to be of Mesozoic age; II, a pervasive system of northsouth-trending faults that cross-cut the earlier NE-SW-trending faults, which manifests evidence of later Mesozoic extension followed by post-Oligocene transpressional fault reactivation; III, a NNE-SSW-trending, steeply dipping, fault set; IV, a WNW-ESE-trending conjugate extensional set that formed perpendicular to the NNE-SSW-trending transpressional faults during Late Tertiary dextral shearing. Early Tertiary axial centred basin inversion and regional exhumation have resulted in the elevation of the Sherwood Sandstone reservoir to shallow structural levels within the basin. Continued fault reactivation into Late Tertiary time has resulted in the compartmentalization of mapped structural closures and suggests that trap integrity is a major exploration risk factor in the CISB.

The influence of thermal history on hydrocarbon prospectivity in the Central Irish Sea Basin

Abstract Thermal history reconstruction studies of four hydrocarbon exploration wells located in the Central Irish Sea Basin (CISB) reveal three major regional episodes of heating and cooling. Units throughout the pre-Quaternary section intersected in wells 42/12-1, 42/16-1 and 42/17-1 began to cool from their maximum post-depositional palaeotemperatures in Early Cretaceous time, between 120 and 115 Ma. Cooling from subsequent palaeotemperature peaks began in Late Cretaceous-Early Tertiary (70–55 Ma) and Late Tertiary (25–0 Ma) time. Results from well 42/21-1 are dominated by the two more recent episodes, and show no evidence of the Early Cretaceous episode. This is thought to reflect a different structural setting of this well, within a North Celtic Sea-Cardigan Bay trend. Palaeotemperature profiles suggest that heating in each episode was due largely to deeper burial, with subsequent cooling caused mainly by uplift and erosion. A maximum of c. 3 km of additional Late Triassic to Early Cretaceous section is required to explain the observed Early Cretaceous palaeotemperatures. Appropriate values for the Late Cretaceous-Early Tertiary and Late Tertiary episoders are c. 2 km and c. 1 km, respectively. All of these cooling episodes correlate closely with similar episodes recognized from previous studies in surrounding regions, from onshore Ireland, Scotland, South Wales and northern, eastern, central and SW England, and each appears to be of truly regional extent. Exploration risk in the CISB generation can be significantly reduced through recognition of the major palaeothermal episodes that have affected the region, and the variation in the magnitude of their effects across the region. The challenge for future exploration in the region is to identify regions where the main phase of hydrocarbon generation post-dated structuring.

Late Tertiary faulting, footwall uplift and topography in western Ireland

Abstract The Erriff Fault in south Mayo, Ireland, is a rare example within the British Isles and Ireland of a sizeable onshore fault (normal displacement >500 m) at which a correlatable stratigraphic horizon is preserved in both the footwall and the hanging wall. That this displacement probably accumulated during Late Tertiary time makes the Erriff Fault (and other similar faults in the area) all the more interesting. Structural forward modelling indicates that footwall uplift and hanging-wall subsidence accompanying faulting, followed by further isostatic uplift following glacial erosion during the Pleistocene glaciations, exercised a fundamental control on the overall form of present-day topography in western Ireland.