Flexural Basin Research Articles

Stratigraphic overview of the Neogene Transylvanian Basin, Romania

Although the Transylvanian Basin is often referred to as part of the Miocene Pannonian Basin System, we point out important tectonic and sedimentary differences reflected in the stratigraphy. The Neogene of the Transylvanian Basin can be split into two tectonic megasequences: Lower Miocene and Middle to Upper Miocene. The Early Miocene onset of the back-arc extension in the Pannonian Basin was not observed in the Transylvanian Basin, where shallow to deep-marine sediments were deposited in a flexural basin. Diverse types of facies developed in the continental (e.g. Sânmihai) and marine (e.g. Coruș, Chechiș, Hida) Lower Miocene formations. Although these are poorly constrained to the standard biostratigraphy, recent 87 Sr– 86 Sr data support their poor correlation with the Burdigalian. The Middle Miocene extension in the Transylvanian Basin is very weak compared with that in the Pannonian Basin. As the high-resolution Middle Miocene regional biostratigraphy of the Badenian and Sarmatian was calibrated by magnetostratigraphy and radiometric dating of the volcanic tuffs up to the Serravallian/Tortonian boundary, the influence of the regional tectonics on the basin's evolution and stratigraphy can be better understood. The regional tectonics produced a progressive basin isolation starting with the late part of the Middle Miocene. The uplift of the Carpathians isolated the basin at the beginning of the Pannonian and initiated the fan delta, lacustrine and fluvial sedimentation. The basin was rapidly uplifted and eroded while post-extensional subsidence was still ongoing in the Pannonian Basin. Endemic fossil assemblages allow only regional-scale correlations for this interval. The uppermost part of the stratigraphic record is represented by the volcano-sedimentary formations generated by the late evolution of the Eastern Carpathians. The sedimentation and subsidence pattern of the Transylvanian Basin reflect the effect of retreating subduction zone in the Eastern Carpathians.

Sedimentology and structure of the Paleogene Nummulitic series of Corsica: Reconstruction of the southern termination of the western Alpine foreland basin

AbstractNummulitic Limestones deposits are preserved along the tectonic contact between the Variscan basement and Alpine units of Corsica. These marine carbonates, dated from the Late Palaeocene to the Middle Eocene, were deposited within a foreland flexural basin that is considered to be the southern continuation of the Alpine foreland basin of southeast (SE) France. However, in contrast with the Nummulitic Limestones of SE France, those of Corsica are far less documented. This field‐based study constrains the sedimentology, stratigraphy and structure of the Nummulitic Limestones of Corsica in three localities (Balagne, Corte and Sari‐Solenzara) to identify factors that controlled foreland basin development and to clarify its significance within the early alpine orogen. The microfacies, microfaunal assemblages and siliciclastic fractions are characterised throughout the succession at each locality. The results indicate the existence of an important Variscan basement relief to the west of the basin (West Corsican Massif) that supplied early alluvial fans found at the base of the foreland succession in the northernmost Balagne area. Continuous high clastic input strongly reduced the development and diversity of the overlying Nummulitic Limestones facies and fauna. Further south, limestones in the Corte and Sari‐Solenzara areas are thicker and contain richer fauna. Three depositional models corresponding to the carbonate ramp system are proposed for the Nummulitic Limestones and used to construct paleogeographic maps illustrating the transgressive evolution of the Corsican foreland basin from the Early to the Late Eocene. Based on our results and available regional tectonic data and LT thermochronological data, we propose that the Nummulitic marine transgression took place within a continuous foreland basin encompassing southern Corsica and SE France during the early development of the western alpine arc.

Two-station Lg wave attenuation tomography in Eastern Asia

SUMMARY Utilizing over 31 000 Lg waveforms from 136 crustal earthquakes recorded at 346 regional stations, we conduct detailed tomographic mappings of the Lg Q structure across Eastern Asia in a frequency range from 0.5 to 4.0 Hz. By improving the standard two-station (TS) method, we effectively correct non-unity site response ratios using site responses estimated at individual stations. This innovative approach combines the flexible recording geometry of the TS method with the precision of reversed two-station (RTS) and reversed two-event (RTE) methods, producing a comprehensive data set devoid of source and site effects for Q tomography. To address unsolvable 3-D structural effects in the Lg spectral amplitude modelling, we justify these as modelling errors with a Gaussian distribution. This approach supports our SVD-based tomographic method, allowing for effective inversion of attenuation parameters and quantitative assessment of model resolution and errors. Our results reveal a complex relationship between Lg Q and the tectonic characteristics of Eastern Asia. In well-resolved regions, low Qo (1-Hz Q) values correspond to areas with high heat flow, partial melt, thick sediment and recent tectonic-thermal activities, in contrast to high Qo values in stable, ancient crusts lacking recent tectonic activity. Rift basins are characterized by low Lg Qo, whereas flexural basins generally have high Qo basements. We also note that post-formation factors, such as sedimentation and crustal flow intrusion, significantly impact Qo values. Furthermore, Lg Q shows a complex frequency relationship, though the power-law approximation with positive power η remains useful. The frequency dependence power η is inversely related to Qo: the regions with low Qo typically have high η and vice versa. This study provides reliable attenuation tomographic and relative site response models for Lg waves in Eastern Asia, pertinent for relative geophysical studies.

Organic geochemical characterization of the seeped oils and Paleogene source rocks in the Southwest Java Basin (SWJ), Indonesia; tectonostratigraphic consideration on oil to source rock correlation

This study's main aim is to establish the organic geochemical relationship between the seeped oils as well as between the oils with the exposed Paleogene rocks in the Southwest Java (SWJ) Basin. Based on organic source input, depositional setting, and age diagnostic-related biomarkers, three oil groups of SWJ are proposed. SWJ oil group 1 (Cipari oil) is characterized by a dominant terrestrial organic matter input with low marine influence and is considered to correlate with Bayah and Citatah Formations with probable contribution from the Karangsambung Formation. Maja oils represented the SWJ oil group 2 and were correlated with the Walat Formation, which was sourced from a moderate marine and terrestrial organic matter input. Moreover, SWJ oil group 3 (Kromong, Ciseti, and Bojongmangu oils) is inferred to be generated from a dominant marine organism input with low terrestrial influence. Their correlating rock unit is the Batuasih Formation. The rock lithofacies suggest an environment ranging from shallow marine/deltaic to deep marine setting. Their paleontological analysis yielded the age ranging from Middle Eocene to Early Oligocene. The SWJ Basin is recommended to be formed as a flexural basin initiated from the Middle Eocene, while their neighbouring producing basin (the Northwest Java/NWJ Basin) was formed later in the Early Oligocene as a back-arc extension basin. Besides differences in basin forming type, timing of initiation, and basin filling, the SWJ and NWJ Basins are also distinct in their oil characteristic. The SWJ oil is sourced dominantly by higher terrestrial plants and marine phytoplankton, while the NWJ oil is generated from mixed sources, mainly lacustrine organic input.

Cretaceous extensional and contractional stages in the Colombian Andes unraveled by a source-to-sink geochronological and thermochronological study in the Upper Magdalena Basin

Extensional and flexural basins can evolve through multiple stages under the same plate tectonic regime over tens of millions of years. The Cretaceous tectonic evolution of the Colombian Andes is characterized by shifts between contractional and extensional tectonics. The upper plate response to these changes is recorded in the Cretaceous sedimentary rocks along the Western, Central, and Eastern cordilleras, and their adjacent basins, including the Upper Magdalena hinterland basin. We integrated field observations, petrography, geochronology, and thermochronology in the Cretaceous sedimentary units in the southern Upper Magdalena Basin and in the adjacent crystalline basement to decipher the provenance of these units and the exhumation patterns of the source areas. These results suggest that the source areas were exhumed between the Late Jurassic to Early Cretaceous, which may have resulted from the combination of Jurassic contraction and the Early Cretaceous extension. Between ∼120 and 100 Ma, a major stage of crustal extension is recorded in the Caballos Formation. Subsequently, between ∼100 and 80 Ma, sedimentation in the Villeta Group marks the end of the extension and the onset of contraction. This contractional phase was characterized by minor rock uplift within the basin. Basin modification continued between 80 and 65 Ma, involving two successive contractional stages that changed the sedimentary patterns, leading to the burial of a previously exhumed horst block and the appearance of new source areas. These major changes in source areas and sedimentary systems are the result of deformation episodes during prolonged extensional and contractional phases. This geologic evolution highlights the stages that characterize extensional to contractional settings.

Onset of Iberian-European plate convergence: Late Cretaceous flexural response of a hot lithosphere (Aquitaine Basin, France)

The Aquitaine Basin is closely linked to the evolution of the Pyrenees, providing precious evidence of the early stages of the Pyrenean orogeny in the Late Cretaceous. Although the timing and geometry of the Early Cretaceous rifting stage is well constrained in the Pyrenees and surrounding north Pyrenean basins, the subsequent early inversion stage has resisted characterization. In this study, we used well log correlations and interpretation of seismic reflection profiles to determine the Late Cretaceous evolution of the sedimentary record in the Aquitaine Basin, spanning the transition from the postrift stage to the beginning of convergence. We identified two main deformation stages during the Late Cretaceous with radically different basin geometries in the western and eastern portions of the Aquitaine Basin. The western part appears to remain under a thermal subsidence regime, inherited from the postrift stage initiated in the Turonian, with little to no deformation. In contrast, the eastern part records early inversion, as indicated by the presence of a well-defined Campanian flexural basin. Between them, Coniacian and Santonian strata indicate a sharply increasing distinction between a very slowly subsiding North Aquitaine Domain (condensed sedimentation) and a strongly subsiding South Aquitaine Domain. We interpret this development as a consequence of lithospheric buckling resulting from far-field compressive strain between the Eurasia and Africa plates. During the Campanian, northward convergence of the Iberia plate led to the formation of a flexural basin with a narrower, steeper geometry than the subsequent Eocene basin that formed in nearly the same region during the Pyrenean orogeny. We attribute this difference to the development of the foreland over a lithosphere that was not thermally reequilibrated in Campanian time, roughly 10 Myr after rifting. This thermal inheritance resulted in lithosphere of very low rigidity. Sedimentary basins formed during both the Coniacian-Santonian and Campanian to early Maastrichtian were thus accommodated by short-wavelength flexural deformation of the lithosphere, a distinctive deformation pattern that represents an immediate response to the onset of compressive strain.

Geophysical signature of the transition zone between the sedimentary cover and the basement from an analogue of the Rhine Graben

Exploiting high temperatures geothermal resources in sedimentary and basement rocks, rifts or flexural basins to produce electricity is now possible because of the development of binary geothermal power plant technology. However it remains challenging because the presence of fluid and permeability at 4–5 km depth is necessary. A multi-scale and multi-disciplinary approach to increase our knowledge of the transition zone between the sedimentary cover and the basement have been undertaken to provide fundamental knowledge for the assessment of its geothermal potential. In this paper, we report out the results of a study performed on an exhumed transition zone in the Ringelbach area in the Vosges Mountain, on the flank of the Rhine graben. In this analogue of a deeply buried transition zone of the Rhine Graben, Triassic sandstones are still present on the top of the fractured and altered granitic basement providing the unique opportunity to study in-situ the physical properties of this transition zone. We focused here on electrical and seismic properties of the transition zone as they are the main physical parameters usually assessed with the help of geophysical methods during the exploration phase of a geothermal project.We show that altered porous and potentially permeable granite targeted in deep geothermal exploration has a clear signature on both electrical conductivity and seismic measurements that can be measured at core scale, borehole scale, and are still visible with surface geophysical methods such as refraction and reflection seismic and Controlled Source Electromagnetism at a few hundred meter depth. The results suggest that best discrimination between permeable and non permeable rocks may be provided by the joint interpretation of both resistivity and seismic velocities.

Flexural strike-slip basins

AbstractStrike-slip faults are classically associated with pull-apart basins where continental crust is thinned between two laterally offset fault segments. We propose a subsidence mechanism to explain the formation of a new type of basin where no substantial segment offset or syn-strike-slip thinning is observed. Such “flexural strike-slip basins” form due to a sediment load creating accommodation space by bending the lithosphere. We use a two-way coupling between the geodynamic code ASPECT and surface-processes code FastScape to show that flexural strike-slip basins emerge if sediment is deposited on thin lithosphere close to a strike-slip fault. These conditions were met at the Andaman Basin Central fault (Andaman Sea, Indian Ocean), where seismic reflection data provide evidence of a laterally extensive flexural basin with a depocenter located parallel to the strike-slip fault trace.

Classification and exploration potential of sedimentary basins based on the superposition and evolution process of prototype basins

Classification, superimposed evolution and sedimentary filling of prototype basins are analyzed based on the Wilson cycle principle of plate theory, by dissecting the evolution history of 483 sedimentary basins around the world since the Precambrian, combined with the three stress environments of tension, compression and shear. It is found that plate tectonic evolution controls the superimposed development process and petroleum-bearing conditions of the prototype basins in three aspects: first, more than 85% of the sedimentary basins in the world are developed from the superimposed development of two or more prototype basins; second, the superposition evolution process of the prototype basin takes Wilson cycle as the cycle and cycles in a fixed trajectory repeatedly. In each stage of a cycle, a specific type of prototype basin can be formed; third, each prototype basin can form a unique tectonic-sedimentary system, which determines its unique source, reservoir, cap conditions etc. For hydrocarbon accumulation, the later superimposed prototype basin can change the oil and gas accumulation conditions of the earlier prototype basin, and may form new petroleum systems. Based on this, by defining the type of a current basin as its prototype basin formed by the latest plate tectonic movement, 14 types of prototype basins can be classified in the world, namely, intracontinental growth rift, intracontinental aborted rift, intercontinental rift, passive continental margin, interior craton, trench, fore-arc rift, back-arc rift, back-arc depression, back-arc small ocean, peripheral foreland, back-arc foreland, strike-slip pull-apart, and strike-slip flexural basins. The classification scheme can ensure the uniqueness of the types of individual sedimentary basin, and make it possible to predict their oil and gas potential scientifically through analogy.

Significance of Northern Andes Terrane Extrusion and Genesis of the Interandean Valley: Paleomagnetic Evidence From the “Ecuadorian Orocline”

AbstractGPS data suggest that the NW South America corner forms a semi‐rigid block drifting NE‐ward along the regional dextral strike‐slip faults that bound an oceanic terrane accreted in Late Cretaceous times to W Ecuador and Colombia. However, the relevance of both strike‐slip versus thrust tectonics during Cenozoic times and their relation with oceanic terrane accretion are unclear. Here we report on the paleomagnetism of 31 mid‐upper Eocene to upper Miocene mainly volcanic sites from the Cordilleras Occidental and Real of southern Ecuador. Eleven sites show that the western Cordillera Occidental underwent a 24° ± 10° clockwise (CW) rotation with respect to South America after late Miocene times, while no rotation occurred further east. We relate the regional CW rotation to the emplacement of the Cordillera Occidental nappe onto the continental sediments of the Interandean Valley, blanketing the Cordillera's eastern margin. As rotation and continental sedimentation onset ages are similar, we interpret such tectonic depression as a narrow flexural basin formed ahead of the advancing nappe front. The 20°–30° CW Neogene rotation of the Cordillera Occidental is indistinguishable from the post‐Cretaceous rotation of the Coastal forearc oceanic blocks, implying that the whole W Ecuador Andean chain was detached and rotated over a mid‐crustal detachment during the last 10 Ma. Eocene‐Miocene paleomagnetic inclination values are systematically consistent with those expected for South America, thus excluding latitudinal terrane drift. We suggest that the Andes of Ecuador and Peru form the “Ecuadorian Orocline”, formed by opposing orogenic rotations around the Amazonian craton indenter.

Reconciling Spatial and Temporal Patterns of Cenozoic Shortening, Exhumation, and Subsidence in the Southern Bolivian Andes

The Bolivian Andes are an archetypal convergent margin orogen with a paired fold-thrust belt and foreland basin. Existing chronostratigraphic constraints highlight a discrepancy between unroofing of the Eastern Cordillera and Interandean Zone fold-thrust systems since 40 Ma and the onset of rapid sediment accumulation in the Subandean Chaco foreland after 11 Ma, previously attributed to Miocene climate shifts. New results from magnetostratigraphic, backstripping, erosional volumetric calculations, and flexural modeling efforts are integrated with existing structural and thermochronologic datasets to investigate the linkages between shortening, exhumation, and subsidence. Magnetostratigraphic and backstripping results determine tectonic subsidence in the Chaco foreland basin, which informs flexural models that evaluate topographic load and lithospheric parameters. These models show that Chaco foreland subsidence is consistent with a range of loading scenarios. Eroded volumes from the fold-thrust belt were sufficient to fill the Chaco foreland basin, further supporting the linkage between sediment source and sink. Erosional beveling of the Eastern Cordillera, local intermontane sediment accumulation after 30–25 Ma, and regional development of the high-elevation San Juan del Oro geomorphic surface from 25 to 10 Ma suggest that the western Eastern Cordillera did not store the large sediment volume expected from erosion of the fold-thrust belt, which arrived in the Subandean Zone after 11 Ma. Eocene to middle Miocene foreland basin accumulation was likely focused between the Eastern Cordillera and Interandean Zone, and has been almost completely recycled into the modern Subandean foreland basin. The delay between initial fold-thrust belt exhumation (early Cenozoic) and rapid Subandean subsidence (late Cenozoic) highlights the interplay between protracted shortening, underthrusting, and foreland basin recycling. Only with sufficient crustal shortening, accommodated by eastward advance of the fold-thrust belt and attendant underthrusting of Brazilian Shield lithosphere beneath the Subandes, did the Subandean zone enter proximal foreland basin deposystems after ca. 11 Ma. Prior to the late Miocene, the precursor flexural basin was situated westward and not wide enough to incorporate the distal Subandean Zone. These results highlight the interplay between a range of crustal and surface processes linked to tectonics and Miocene climate shifts on the evolution of the southern Bolivian Andes.

Mid-Ordovician subduction, obduction, and eduction in western Newfoundland; an eclogite problem

Abstract. The narrow, short-lived Taconic-Grampian Orogen occurs along the north-western margin of the Appalachian-Caledonian Belt from, at least, Alabama to Scotland, a result of the collision of a series of early Ordovician oceanic island arcs with the rifted margin of Laurentia. The present distribution of Taconian-Grampian ophiolites is unlikely to represent a single fore-arc from Alabama to Scotland colliding at the same time with the continental margin along its whole length; more likely is that there were several Ordovician arcs with separate ophiolites. The collision suture is at the thrust base of obducted fore-arc ophiolite complexes, and obduction distance was about two hundred kilometres. Footwalls to the ophiolites are, sequentially towards the continent, continental margin rift sediments and volcanics and overlying rise sediments, continental shelf slope carbonates, and sediments of foreland flexural basins. The regionally-flat obduction thrust complex between the ophiolite and the rifted Laurentian margin is the collision suture between arc and continent. A particular problem in drawing tectonic profiles across the Taconic-Grampian Zone is several orogen-parallel major strike-slip faults, both sinistral and dextral, of unknown displacements, which may juxtapose portions of different segments. In western Newfoundland, most of the Grenville basement beneath the Fleur-de-Lys metamorphic complex (Neoproterozoic to early Ordovician meta-sediments) was eclogitised during the Taconic Orogeny and separated by a massive shear zone from the overlying Fleur-de-Lys, which was metamorphosed at the same time but in the amphibolite facies. The shear zone continued either to a distal intracontinental “subduction zone” or to the main, sub-fore-arc, subduction zone beneath which the basement slipped down to depths of up to seventy kilometres at the same time as the ophiolite sheet and its previously-subcreted metamorphic sole were being obducted above. Subsequently, the eclogitised basement was returned to contact with the amphibolite-facies cover by extensional detachment eduction, possibly enhanced by subduction channel flow, which may have been caused by slab break-off and extension during subduction polarity flip. Although the basal ophiolite obduction thrust complex and the Fleur-de-Lys-basement subduction-eduction surfaces must have been initially gently-dipping to sub-horizontal, they were folded and broken by thrusts during late Taconian, late Ordovician Salinic-Mayoian, and Acadian shortening.

Late Paleozoic (Late Mississippian–Middle Permian) sediment provenance and dispersal in western equatorial Pangea

Late Mississippian to middle Permian sediment-dispersal networks of regional to continental scale in western equatorial Pangea, depicted here in a series of paleogeographic maps, developed in response to temporally and spatially changing influences of climate, eustasy, and a continent-wide late Paleozoic orogenic system. The orogenic system included linked Alleghanian, Ouachita-Marathon-Sonora collisional belts and associated foreland basin systems on Laurentia and magmatic arcs on Gondwana, intracratonic basement uplifts and basins of the Ancestral Rocky Mountains, flexural arches and intracratonic basins of the US midcontinent region, and basins and uplifts of the southwestern Laurentian transcurrent continental margin. Consideration of new and published U-Pb detrital-zircon datasets permits delineation of Laurentian sediment-dispersal networks of the developing supercontinent. The Transcontinental Arch deflected Late Mississippian transcontinental rivers with Alleghanian headwaters toward the southern midcontinent and nascent, deep-marine foreland basins along the Ouachita collision orogen. Pennsylvanian rivers likewise headed in the Alleghanian Orogen, transporting sediment southwest across the midcontinent and along the Alleghanian foreland basin to empty into the Arkoma Basin and Fort Worth Basin, which also received voluminous sediment from Gondwana and the Ouachita Orogen. Concomitantly, major growth of Ancestral Rocky Mountains uplifts yielded basement-derived sediment, much of which was retained in local flexural basins. Increased aridity drove ascendant eolian transport in early Permian (Artinskian) time, just as lowstand desiccation of a midcontinent seaway exposed unconsolidated silt and sand derived from eastern, western, and southern sources in an extensive interior desert sink. Eolian transport within the interior desert further mixed and deflated the already-cosmopolitan sediment, pushing it southwest toward the Permian Basin and westward beyond the Ancestral Rocky Mountains. Intercepted by newly developed monsoonal circulation, the deflated sediment came to reside in erg systems along the western marine margin of Pangea. Subtropical Pennsylvanian transcontinental fluvial networks were similar to those of modern big river systems of the eastern and midcontinent United States that drain toward the Gulf of Mexico. In contrast, Permian drainage networks yielding sediment to a continental desert more resembled Pleistocene sediment routes of the Arabian Plate; there, intermittent wadis draining western rift highlands and big rivers of the Mesopotamian foreland contribute sediment to Arabian eolian sands via zonal and monsoonal surface winds to create widespread sand seas of mixed Eurasian and Arabian provenance.

Net right-lateral slip of the Eastern Frontal Fault System, North Andes Sliver, northwestern South America

The NNE-directed tectonic escape of the North Andean block (NAB) or sliver (NAS) with respect to South America (SA) is generally accepted now. NAS is detached from continental South America (SA) by a 2000-km-long, NE-SW-trending dominant dextral fault system, known as the Eastern Frontal Front System (EFFS; Pennington's name and acronym kept for simplicity) that extends from the gulf of Guayaquil-Tumbes basin (GGTB) in offshore Ecuador to the Caribbean coasts of Venezuela in the Golfo Triste region. From SW to NE, EFFS comprises the following fault systems: In Ecuador, Puná-Pallatanga-Cosangá-Chingual (CCPP); In Colombia, Afiladores, Sibundoy, Algeciras and Guaicáramo; and in Venezuela, Boconó (BF).Several estimates of total dextral slip have been calculated along the EFFS. In Ecuador, the Pallatanga fault (part of the CCPP) displaces several large geologic and morphologic units in about 10 km. The net slip of EFFS at this western front of the Western cordillera can be as much as the double (≈20 km) if strain happens to be distributed between sub-parallel faults. Further south, at the very southern tip of the EFFS, from seismic data acquired from the GGTB, an estimate of 13.5–20 km of lengthening of this basin from the Plio-Quaternary boundary has been proposed.In Colombia, the Algeciras lazy Z-shaped pull-apart basin (Huila Dpt.) provides another estimate. The extent of the Quaternary basin fill measured along the regional fault trend on SGC geologic plate 345, is about 10 km. This is a minimum estimate because the fault shortcuts the basin in more recent times, increasing such slip in at least about 1–2 km. In Venezuela, although more reliable and frequent dextral offsets are of the order of 30 km, a larger 70–80 km offset in Mesozoic rocks was once proposed across BF. A supportive argument to the 30 km net slip along the Mérida Andes axis may be derived from a present mismatch of Bouguer anomaly minima between both adjacent flexural basins of no more than 40 km.From all the estimates of total slip here compiled along EFFS, we can conclude that the NE tectonic escape of NAS is most likely not larger than 30–35 km. This net slip may vary along EFFS strike, being smaller (close to 20 km) in the south. Therefore, despite the large size of NAS, its escape remains modest at plate scale. Remarkably, EFFS net slip is in the same order of magnitude as the ≈20 km for the Sumatra fault, which is as long as the EFFS (≈1900 km long).

U-Pb ZIRCON DATING OF MIDDLE EOCENE CLASTIC ROCKS FROM THE GERCUS MOLASSE, NE IRAQ: NEW CONSTRAINTS ON THEIR PROVENANCE, AND TECTONIC EVOLUTION

The provenance of Middle Eocene clastic rock from the Gercus Molasse, NE Iraq was determined by detrital zircon (DZ) U-Pb geochronology. The Gercus Molasse in the Iraqi segment of the north-eastern Zagros Thrust Zone provides an ideal example of foreland system evolution with respect to the transition from passive margin to the accretionary complex terrene-flexural foreland basins. The DZ U-Pb age spectra from the Gercus Molasse suggest that the foreland sediments either influx from multiple provenances or are the result of recycling from the accretionary complex terrane. During pre-accretion, however, the radiolarite basin (Qulqula Radiolarite, 221 Ma) located along Arabian passive margin likely acted as an intermediate sediment repository for most or all of the DZ. Representative DZ U-Pb measurements revealed that the Gercus clastic rocks fall into several separable age population ranges of 92-102 (Albian-Cenomanian), 221 (Upper Triassic), 395-511 (Cambrian), 570- 645 (Neoproterozoic), 1111 (Mesoproterozoic), and lesser numbers of Paleoproterozoic (1622-1991 Ma) ages. The source of Proterozoic detrital Zircons is enigmatic; the age peaks at 1.1, 1.5, 1.6, and 1.9 Ga (Proterozoic) does not correspond to any known outcrops of Precambrian rocks in Iraq, and it may be useful to continue to search for such basement. The detrital zircons with age populations at 0.63–0.86 Ga probably originated from the Arabian-Nubian Shield. The age peak at 0.55 Ga correlates with Cadomian Magmatism reported from north Gondwana. The age peaks at ~0.4 Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys. The youngest ages populations at 93 Ma indicate that fraction of DZ were transported directly from the contemporaneously active magmatic arc (Zagros Ophiolite segments). The paleogeography and tectonic evolution of the Neogene Zagros foreland basin were reconstructed and divided into two tectonic stages. The early stage is defined by the Campanian accreted terranes (i.e. orogenic wedge) form loads sufficient to produce flexural basin with a deepest part is situated next to the tip of the loads. This flexural basin is filled by the flysch clastics of the Maastrichtian– Early Eocene (i.e. referred to by the Tanjero-Kolosh flysch sequence). The late stage is marked by a synchronized modification of the clastics fill of the basin and changes in dip directions to compensate for the reduction of the load by both erosion and extension and the basin, therefore, was sealed by a shallowing upwards depositional sequence ending with the terrestrial Gercus Formation.

Provenance of Tanjero and Red Bed clastic sedimentary rocks revealed by detrital zircon SHRIMP dating, Kurdistan region, NE Iraq: Constraints on ocean closure and unroofing of Neo-Tethyan allochthons

Closure of Neo-Tethys between the Late Cretaceous and Miocene caused obduction of intra-Neo-Tethys volcanic arc allochthons onto the northern margin of the Arabian-Nubian Shield. This loading led to downwarping of the shield forming a flexural basin in which the Tanjero Formation and later Red Beds accumulated. Analysis of the detrital zircon populations in these sedimentary units provides additional evidence supporting the petrographic data for the presence and unroofing of the allochthons. Both the Tanjero Formation and Red Beds show evidence of initial derivation of detritus from the Nubian-Arabian Shield together with minor Paleozoic detritus from Egypt, Turkey and/or Iran. The youngest detrital zircon grains in the Tanjero Formation are of Paleocene age and probably represent northern derivation from the Sanandaj-Sirjan Zone. In contrast, the upper Miocene Red Beds contain detrital zircon of Eocene age that confirms the petrographic evidence for derivation from, and hence emplacement of, the lower allochthon of intra-Neo-Tethys Walash-Naopurdan rocks.

The bent prolongation of the 85°E Ridge south of 5°N – Fact or fiction?

The enigmatic 85°E Ridge crosses the Bay of Bengal from north to south. Its strong gravity anomaly low is associated with Cretaceous hotspot volcanism. South of 5°N, the gravity low bends into a SW-NE orientation and continues as far SW as the Afanasy Nikitin Seamounts. This change has been interpreted to represent a bent hotspot track. We report new constraints on the crustal structure and genesis beneath the SW-trending gravity low based on new refraction seismic, reflection seismic, and shipborne gravity data. Our findings show that the crustal structure across the gravity low does not differ significantly from the adjacent, 4.5 to 7 km thick oceanic crust. No basement ridge, significant crustal thickening, or magmatic underplating were identified. We found no evidence for a southern prolongation of the 85°E Ridge. Instead, our P-wave velocity and density models reveal the gravity low to express a flexural basin, which is a result of widespread mid-Miocene to recent lithospheric deformation in the Indian Ocean. The ~25 mGal negative gravity anomaly is therefore not related to the passage of the Indian plate over a mantle plume, casting doubt on the possibility that volcanism at the Afanasy Nikitin Seamounts might be related to the same plume as the northern 85°E Ridge. The northern 85°E Ridge may have been generated at the northern prolongation of the 86°E Fracture Zone.

Processes and causes of Phanerozoic tectonic evolution of the western Tarim Basin, northwest China

This paper addresses the Phanerozoic tectonic evolution of the western Tarim Basin based on an integrated stratigraphic, structural and tectonic analysis. P-wave velocity data show that the basin has a stable and rigid basement. The western Tarim Basin experienced a complex tectonic evolutionary history, and this evolution can be divided into six stages: Neoproterozoic to Early Ordovician, Middle Ordovician to Middle Devonian, Late Devonian to Permian, Triassic, Jurassic to Cretaceous and Paleogene to Quaternary. The western Tarim Basin was a rift basin in the Neoproterozoic to Early Ordovician. From the Middle Ordovician to Middle Devonian, the basin consisted of a flexural depression in the south and a depression that changed from a rift depression to a flexural depression in the north during each period, i.e., the Middle–Late Ordovician and the Silurian to Middle Devonian. During the Late Devonian to Permian, the basin was a depression basin early and then changed into a flexural basin late in each period, i.e., the Late Devonian to Carboniferous and the Permian. In the Triassic, the basin was a foreland basin, and from the Jurassic to Cretaceous, it was a downwarped basin. After the Paleogene, the basin became a rejuvenated foreland basin. Based on two cross sections, we conclude that the extension and shortening in the profile reflect the tectonic evolution of the Tarim Basin. The Tarim Basin has become a composite and superimposed sedimentary basin because of its long-term and complicated tectonic evolutionary history, highly rigid and stable basement and large size.

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: new constraints from Palaeogene sedimentary units and low-temperature thermochronometry

AbstractMountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa–Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian – early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U–Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification.

Age of synorogenic deposits and timing of folding in Dezful embayment, SW Zagros Fold Belt

Abstract Syntectonic deposits of the Mesopotamian flexural basin have been dated in the Lurestan and Fars salients in the Zagros foldbelt since the early 2000's but not in the Dezful embayment where most of the Zagros foreland oil is trapped. We use 237 samples collected for magnetostratigraphy to date shallow marine to fluvial-alluvial pre-growth and growth strata deposits along a >2700 m section in the NE flank of the Jarik anticline in north Dezful embayment. The Jarik anticline grew by a combination of compression and coeval diapirism like other structures in north Dezful embayment. The correlation of the local magnetostratigraphy with the Geomagnetic Polarity Time Scale indicates ages of 14.1 Ma and ~13.6 Ma for the bases of the Mishan and Aghajari formations. The bases of Lahbari Mb. and Bakhtyari Fm. are ~9.0 Ma and ~7.0 Ma. The onset of growth strata in the NE flank of the Jarik anticline is ~12.8 Ma with a duration of 5.8 Myr for this anticline. This study presents for the first time the age of syntectonic deposition and folding in the Dezful embayment that is essential to determine burial and generation of hydrocarbons in this oil-rich province.