Late Cretaceous Paleogene Research Articles

Sources of clastic material of Cretaceous–Paleogene deposits in the southwestern part of the Amur–Zeya Basin: results of dating U–Pb detrital zircons

The results of our study indicate that at the Late Cretaceous–Paleogene boundary in the southeastern part of the Amur–Zeya Basin, the sedimentation conditions changed drastically, namely, the change of provenance areas of debris. In the Maastrichtian, the clastic material was mainly transported from the Bureya–Jiamusi Superterrane and the volcanic–plutonic belts of Khingan–Okhotsk and East Sikhote–Alin located to the east: sedimentation occurred simultaneously with magmatic activity. During the Danian Stage, the major source of debris to the southern part of the basin was located to the south of the young mountain system of the Lesser Khingan (the uplifted part of the basement of the Songliao Block).

History of anticlinal traps and oil and gas fields in Jurassic reservoirs in the northern West Siberian basin

The paper presents new paleotectonic data on the Bovanenkovo, Taz, and Gubkin dome-shaped uplifts and the Novyi Port, Urengoi, Kharampur, and Medvezh’e swells, with implications for the history of zone J2 and the Jurassic reservoir as a whole. Most of the large uplifts originated in the Early-Middle Jurassic and completed their evolution in the early Late Cretaceous (latest Cenomanian), generally (Bovanenkovo, Novyi Port, Kharampur, and Taz) or to a large extent (Urengoi and Gubkin), except for the much younger Medvezh’e swell, which formed in the Paleogene-Quaternary interval. The conditions for oil storage and retention in anticlinal traps of Jurassic zones J1 and J2 were the most favorable between the middle Late Cretaceous and the Eocene, inclusive. Gas accumulation lasted for a much longer time, from the Late Jurassic to the Present, and was especially rapid in the Late Cretaceous-Paleogene.

Prolonged high relief in the northern Cordilleran orogenic front during middle and late Eocene extension based on stable isotope paleoaltimetry

The paleoelevation and size of the North America Cordilleran orogen during the late Cretaceous–Paleogene contractional and subsequent extensional tectonics remain enigmatic. We present new estimates of paleorelief of the northern Cordilleran orogenic front during the middle and late Eocene using oxygen isotope compositions of unaltered molluscan fossils and paleosol carbonates in the Kishenehn basin. Bounded by several mountains ranges to the east, the Kishenehn basin was a half graben developed during middle Eocene to early Miocene collapse of the Cordilleran orogen. These mollusk taxa include three sympatric groups with affinities to wet tropical, semi-arid subtropical, and temperate environments. Our reconstructed surface water δ18O values vary between −19.8‰ and −6.3‰ (VSMOW) during the middle and late Eocene. The large differences in paleoenvironments and surface water δ18O values suggest that the catchment of the Kishenehn basin was at variable elevation. The estimated paleorelief between the basin and the surrounding mountains, based on both Rayleigh condensation model and predictions of Eocene precipitation isotope values using an isotope-enabled global climate model, is ∼4 km, and the basin floor was <1.5 km high. This high topography and high relief paleogeography suggest that the Cordilleran orogenic front reached an elevation of at least 4 km, and the crust thickness may have reached more than 55 km before Eocene gravitational collapse. We attribute the maintenance of high Eocene topography to the combination of an inherited thick crust, thermal uplift caused by mantle upwelling, and isostatic uplift caused by removing lower lithosphere or oceanic slab.

Tectono-magmatic evolution of the Chihuahua-Sinaloa border region in northern Mexico: Insights from zircon-apatite U–Pb geochronology, zircon Hf isotope composition and geochemistry of granodiorite intrusions

We present the whole-rock geochemistry, LA–ICP–MS zircon-apatite U–Pb ages and zircon Hf isotope composition of the granodioritic plutons at the southwestern boundary of Chihuahua with the states of Sinaloa and Sonora. These granodiorites are exposed in the north and south of the Rio El Fuerte in southwest Chihuahua and northern Sinaloa. The magmatism spans over a time period of 37Ma from 90 to 53Ma. Zircons are exclusively magmatic with strong oscillatory zoning. No inheritance of any age has been observed. Our new U–Pb dating (~250 analyses) does not support the involvement of older basement lithologies in the generation of the granitic magmas. The U–Pb apatite ages from granodiorites in southwest Chihuahua vary from 52 to 70Ma. These apatite ages are 1 to 20Ma younger than the corresponding zircon U–Pb crystallization ages, suggesting variable cooling rates from very fast to 15°C/Ma (~800°C to 500°C) and shallow to moderate emplacement depths. In contrast, U–Pb apatite ages from the Sinaloa batholith are restricted from 64 to 61Ma and are indistinguishable from the zircon U–Pb ages range from 67 to 60Ma within the error, indicating rapid cooling and very shallow emplacement. However, one sample from El Realito showed a larger difference of ~20Ma in zircon–apatite age pair: zircon 80±0.8Ma and apatite 60.6±4Ma, suggesting a slower cooling rate of ~15°C/Ma. The weighted mean initial εHf (t) isotope composition (2σ) of granodiorites varies from +1.8 to +5.2. The radiogenic Hf isotope composition coupled with previous Sr–Nd isotope data demonstrates a significant shift from multiple crustal sources in the Sonoran batholithic belt to the predominant contribution of the mantle-derived magmas in the southwest Chihuahua and northern Sinaloa. Based on U–Pb ages, the absence of inheritance, typical high Th/U ratio and radiogenic Hf isotope composition, we suggest that the Late Cretaceous–Paleogene magmatic rocks in this region are not derived from melting of a felsic older crust, neither evolved North American Proterozoic basement nor Jurassic metasedimentary rocks of the Guerrero terrane. Instead, the magma was primarily derived from partial melting of mantle related sources, possibly in the subcontinental mantle wedge above the Farallon plate. Our younger U–Pb zircon–apatite ages are well correlated with the late-stage cogenetic porphyry Cu–Au mineralization at 57 to 59Ma. If the origin of ore mineralization is related to the magma sources of host batholiths, then a minimal input from crustal melt is suggested in the genesis of Cu–Au porphyry system.

Stratigraphy of Reinga and Aotea basins, NW New Zealand: constraints from dredge samples on regional correlations and reservoir character

ABSTRACTSandstone, mudstone and limestone samples dredged in the Reinga and Aotea basins, NW New Zealand during voyage TAN1312 provide age and lithological constraints on the Cretaceous–Neogene succession. A total of 46 micropaleontology and 7 macropaleontology samples were examined along with 84 thin-sectioned petrographical samples. Some were examined by X-ray diffraction and porosity-permeability analyses. Late Cretaceous sandstones are dominated by feldspathic and lithofeldspathic compositions, with mixed granitic plutoniclastic and volcaniclastic provenance; a comparison with Pakawau Group of Taranaki Basin is appropriate. Late Cretaceous–Paleogene mudstones are widespread and display close petrographical and age similarities to the Whangai Formation facies of other parts of New Zealand and fine-grained carbonate facies of the Weber and Amuri formations of eastern North and South Islands, respectively. Cretaceous limestone and Paleogene sandstone were not recovered. Carbonates and mudstones dominate the Neogene succession of Reinga and Aotea basins; rare Neogene sandstones have feldspatholithic compositions and resemble Waitemata Group sandstones of the northern North Island. In terms of petroleum prospectivity, Cretaceous sandstones represent a potential reservoir facies but are lithic with low permeability.

Gondwanan Perspectives: Cretaceous—Paleogene Biota of West Antarctica

Antarctica has played a key role connecting all major land-masses of Gondwana, even during periods of their break up through the Cretaceous and Paleogene. The largely ice-covered rocks of Antarctica undoubtedly contain countless clues about the paleogeographic and biotic connections of southern Gondwana. The Antarctic Peninsula is the region that has provided and that likely will provide the most informative fossil remains from the Cretaceous–Paleogene. In particular, during the past two decades, geologic and paleontological explorations of the James Ross Basin, Weddell Sea, have revealed that this basin, located off the northeast tip of the Antarctic Peninsula (West Antarctica), contains one of the most important records of Late Cretaceous and early Paleogene life in the Southern Hemisphere.

An Upper Cretaceous paleo-aquifer system in the Eromanga Basin of the central Gawler Craton, South Australia: evidence from apatite fission track thermochronology

ABSTRACTAn apatite fission track (AFT) study of crystalline basement in the central Gawler Craton reveals apparent ages in the range of ca 430–58 Ma. The majority of samples underwent protracted monotonic cooling related to regional Paleozoic exhumation, consistent with long-term crustal stability as expected for cratonic interiors. However, multiple samples show evidence of Late Cretaceous–early Paleogene reheating, indicating a more dynamic low-temperature history. Inverse time–temperature modelling of AFT data indicates varying degrees of thermal overprinting between ∼60 and 110°C, with substantially overprinted and negligibly overprinted samples in close proximity (<1 km). Time–temperature histories for samples that experienced thermal overprinting reveal localised Late Cretaceous–early Paleogene (ca 100–50 Ma) heating that is significantly younger than the Paleozoic–early Mesozoic exhumation recorded regionally. The highly localised nature and non-systematic patterns of overprinting combined with the lack of major Mesozoic or Cenozoic fault structures are not consistent with a regional thermal event associated with substantial reburial and later exhumation. Rather, localised reheating was most likely caused by heated groundwater from the once-overlying Mesozoic Eromanga Basin aquifer system, whose modern discharge margin (∼400 km north of the study area) is marked by thermal mound springs that produce fluids with temperatures up to 100°C. Only basement rocks in close proximity to fluid pathways in the overlying aquifer would have recorded reheating, resulting in the observed sporadic distribution of partially overprinted samples. Thermal history modelling indicates rejuvenated apatite grains cooled to near-surface temperatures in the latest Cretaceous–Paleogene. This was likely in response to local removal of the overlying Eromanga Basin aquifer unit due to a relatively minor degree of exhumation (≤1 km) recorded regionally, which consequently disrupted the anomalous heating mechanism. These results show that the flow of heated groundwater is a feasible reheating mechanism for low-temperature thermochronometers, resulting in cooling patterns that may become decoupled from exhumation in cratonic interiors.

A new sediment-dwelling pholadid bivalve from Oligocene glaciomarine sediments of King George Island, West Antarctica

We present a re-description of the pholadid bivalve from the Oligocene Polonez Cove Formation, King George Island, West Antarctica, previously identified as Penitella sp. The study is based on a collection of 210 specimens, preserved exclusively in life position in flask-shaped Gastrochaenolites type borings which have been subsequently buried by glaciomarine diamictite. The systematic study showed that this pholadid is a new species belonging to the genus Pholadidea rather than to Penitella and we name it Pholadidea gradzinskii sp. nov. The species is one of very few Late Cretaceous—Paleogene pholadids that we could safely identify as Pholadidea. All of them are known exclusively from the southern Pacific and adjacent areas (New Zealand, Antarctica, and Patagonia). We demonstrate that the genus attained its Recent broad distribution before the middle Miocene, when the first species of Pholadidea appeared in the Northern Hemisphere. The mass occurrence of P. gradzinskii in the Oligocene of West Antarctica...

Fault-sealing capability and its impact on coalbed methane distribution in the Zhengzhuang field, southern Qinshui Basin, North China

Fault-sealing capability has critical influence on coalbed methane (CBM) accumulation and preservation in coal-bearing basins. Although southern Qinshui Basin is the first commercial CBM production area in China, detailed investigation on faults evolution and its impact on gas accumulation has never been performed. In this paper, the generation time and the evolution process of the sealing capability of seven major faults in the Zhengzhuang field were evaluated by using well-logging data and 3D seismic tectonic interpretation. Results indicate that most normal faults in the Zhengzhuang field were created from Late Cretaceous to Paleogene and after the key stage of hydrocarbon-generation. Fault-sealing capability was evaluated through the vertical pressure on the faults' surface (VPFS), the shale gouge ratio (SGR) and the clay-smear potential (CSP), the values of which for Zhengzhuang field are −3.2 MPa to 16 MPa, 0.36 to 0.91, and 2–40 m, respectively. These values demonstrate that the fault-sealing is complex and variable when the faults present in different scales or different positions in the faulting system. In the Zhengzhuang field, almost all faults exhibit good lateral sealing capability (LSC) with high values of SGR (>0.6) and CSP (>15 m) during fault evolution. Therefore, the current gas contents are primarily related to the vertical sealing capability (VSC) but not to the LSC of the faults. The VSC of the fault varies with the different stages during the fault evolution. In the Late Cretaceous-Paleogene, a key stage for gas dissipation, almost all faults possess low VSC, which caused generally low gas content (<22.4 m3/t) near the faults. In contrast, the gas content that was not affected by faults is commonly >25 m3/t. In the Neogene-Quaternary, the VSC was variable for different faults. For the faults with low VSC, they would gradually lead to low gas content. The results showed that the VSC positively correlated with the current gas contents near the faults in the Zhengzhuang field, southern Qinshui Basin, North China.

Orthopyroxene-enrichment in the lherzolite-websterite xenolith suite from Paleogene alkali basalts of the Poiana Ruscă Mountains (Romania)

Abstract In this paper we present the petrography and geochemistry of a recently collected lherzolite-websterite xenolith series and of clinopyroxene xenocrysts, hosted in Upper Cretaceous–Paleogene basanites of Poiana Ruscă (Romania), whose xenoliths show notable orthopyroxene-enrichment. In the series a slightly deformed porphyroclastic-equigranular textured series could represent the early mantle characteristics, and in many cases notable orthopyroxene growth and poikilitic texture formation was observed. The most abundant mantle lithology, Type A xenoliths have high Al and Na-contents but low mg# of the pyroxenes and low cr# of spinel suggesting a low degree (&lt; 10 %) of mafic melt removal. They are also generally poor in overall REE-s (rare earth elements) and have flat REY (rare earth elements+ Y) patterns with slight LREE-depletion. The geochemistry of the Type A xenoliths and calculated melt composition in equilibrium with the xenolith clinopyroxenes suggests that the percolating melt causing the poikilitization can be linked to a mafic, Al-Na-rich, volatile-poor melt and show similarity with the Late Cretaceous–Paleogene (66–72 Ma) subduction-related andesitic magmatism of Poiana Ruscă. Type B xenoliths, with their slightly different chemistry, suggest that, after the ancient depletion, the mantle went through a slight metasomatic event. A subsequent passage of mafic melts in the mantle, with similar compositions to the older andesitic magmatism of Poiana Ruscă, is recorded in the pyroxenites (Fe-rich xenoliths), whereas the megacrysts seem to be cogenetic with the host basanite. The Poiana Ruscă xenoliths differ from the orthopyroxene-enriched mantle xenoliths described previously from the Carpathian-Pannonian Region and from the Dacia block.

Late Cretaceous-Paleogene transform zone between the Eurasian and North American lithospheric plates

Within the limits of the Chukchi Sea and the Amerasian Basin of the Arctic Ocean, study have been carried out to calculate D function anomalies. The result was the discovery of elongated faults that cut, according to the positions of the upper and lower extents of the disturbing masses, both the upper crust and the upper mantle. It is shown that these faults are right-lateral strike-slips continuing the Late Cretaceous-Paleogene structures of the same type in the Bering Sea. This suggests that the en echelon strike-slip system of the Bering Sea, Chukchi Sea, and Amerasian Basin is a relic of the Late Cretaceous-Paleogene transform fault zone between the Eurasian and North American lithospheric plates.

Where was the Magura Ocean?

Abstract In the Late Jurassic to Early Cretaceous palaeogeography of the Alpine Tethys the term Ocean is used for different parts of these sedimentary areas: eg. Ligurian – Piedmont and Penninic, Magura, Pieniny, Valais and Ceahlau-Severins oceans. The Magura Ocean occupied the more northern position in the Alpine-Carpathian arc. During the Late Cretaceous–Paleogene tectono-sedimentary evolution the Magura Ocean was transformed into several (Magura, Dukla, Silesian, sub-Silesian and Skole) basins and intrabasinal source area ridges now incorporated into the Outer Western Carpathians.

ANCIENT VALLEYS IN THE WEST PRIBAIKALYE (HISTORY OF FORMATION)

The Late Cretaceous-Paleogene, Neogene, Late Pliocene-Eopleistocene, and Pleistocene-Holocene valleys have been distinguished in the West Pribaikalye. The ancient valleys are associated with the relic of the Late Cretaceous-Paleogene peneplain. They are confined to the various mineral occurrences and spatially and genetically are related to the Quaternary gold placers. The Late Pliocene is the time of the beginning of peneplain deformation and the Baikal dome growth, the drainage reorientation, gradual destruction of old low-graded relief, and formation of contemporary alpine one and goltsy. This epoch is also characterized by the development of river captures, retrograde erosion and formation of antecedent valleys.

MORPHOSTRUCTURE OF THE WEST SHOULDER OF THE BAIKAL RIFT

The southwest end of Baikal rift is a slit-like gap incised into plateau - the relic of the Late Cretaceous-Paleogene (initial) peneplain. This is the most ancient element of the rift system. The plateau is transferred along-strike into Late Pliocene-Quaternary Baikal dome, which comprises the most part of the Baikal rift. The morphostructural pattern of the dome exhibits relics of the initial peneplain deformed into local domes and young tilted horsts. Ancient landforms within the horsts were denuded. The vast system of overthrusts frames the Baikal rift from the west. Cenozoic rejuvenation of this system may probably be the result of the Baikal dome's rise. The evolution of the Baikal morphostructure may serve as additional argument for the passive rifting model.

Shell ornamentation as a likely exaptation: evidence from predatory drilling on Cenozoic bivalves

AbstractPredation is an important process in modern oceans and in the evolutionary history of marine ecosystems. Consequently, it has been hypothesized that shelled prey modified their ornamentation in response to predation. However, bivalve ornamentation has also been argued to be important in maintaining a stable life position in the sediment and in burrowing. To test whether concentric ribs were effective against drilling by carnivorous gastropods, we examined drill hole position and completeness for four Cenozoic bivalve species that differ in rib strength (Astarte radiata,A. goldfussi,Lirophora glyptocyma, andL. latilirata). The percentage of drill holes located between the ribs increases with increasing rib strength, whereas the percentage of drill holes on top of ribs decreases. This result suggests that gastropods select the drill hole site more effectively as rib strength increases, thereby saving time and energy, and that natural selection favors gastropods that select drill hole sites between ribs. Because of this greater stereotypy, the percentage of drill holes that are incomplete is generally lower in strongly ribbed species. The proportion of drill holes located on top of ribs is greater for incomplete than complete holes, implying that ribs can be effective against predators, but only when selected as the drilling location. We show that ribs are most effective against drilling predation for bivalves with moderately sized ribs, between which gastropods have difficulty siting drill holes. Concentric ribs are unlikely to have evolved as an adaptation against drilling predation because concentric ribs evolved in the Paleozoic and were already common in the Mesozoic, whereas drilling frequency increased later, in the Late Cretaceous–Paleogene. Moreover, rib strength of North AmericanAstartedid not change through this time interval. Thus, the ribs considered here are a likely exaptation to drilling given their effectiveness at deterring drilling predation on bivalves with moderate ribs.

Modelling petroleum generation of Late Cretaceous Dabut Formation in the Jiza-Qamar Basin, Eastern Yemen

The Jiza-Qamar Basin in eastern Yemen is still undergoing hydrocarbon exploration and hydrocarbon potential has not been assigned yet. In this study, subsurface samples from two onshore and offshore wells were collected to characterize the source rocks of Dabut Formation and to incorporate into basin modelling in order to know and determine the timing of petroleum generation and expulsion of the Dabut source rocks. Based on organic geochemical analysis, the Dabut sediments have variable TOC content in the range of 0.5–2.5 wt%, indicating fair to good source rock generative potential. The organic matter of Dabut source rock is dominated by Type III kerogen with minor mixed III–II kerogen contributions, and is thus considered to be mainly gas-prone and some oil-prone. This is supported by HI values in the range of 50–212 mg HC/g TOC. Vitrinite reflectance in the range of 0.63–0.88%Ro and pyrolysis Tmax in the range of 427–454 °C, generally indicate that the Dabut source rocks contain mature organic matter.One-dimensional basin modelling was performed to analyse the hydrocarbon generation and expulsion history of the Dabut source rocks in the Jiza-Qamar Basin based on the reconstruction of the burial and thermal maturity histories in order to improve our understanding of the hydrocarbon generation potential. Calibration of the model with measured vitrinite reflectance (%Ro) and borehole temperature (BHT) data indicates that the paleo-heat-flow was constant during the Late Cretaceous–Paleogene but increased during the Late Paleogene–Neogene and then decreased exponentially from Neogene to present-day.The modelled maturity history predicts that the oil generation (0.5% Ro) in the Dabut source rock began from about 56 Ma to 42 Ma and the peak hydrocarbon generation (0.86% Ro) occurred approximately from 14 Ma to 13 Ma. These models also indicate that the main phase of oil generation was at 56 Ma to 25 Ma, whereas the main phase of gas generation began from about 25 Ma to 14 Ma and still generated in the present-day.

Paleogene Grand Canyon incompatible with Tertiary paleogeography and stratigraphy

The Hualapai Plateau in northwest Arizona, the location of the western Grand Canyon, contains an unusually lengthy Tertiary stratigraphic record dominated by fluvial deposition and extending from at least late Paleocene through late Miocene time. The thickest and oldest Tertiary sections are best exposed in a system of partially re-exhumed Laramide paleocanyons. The Paleogene drainage system was locally disrupted and ponded by Laramide monoclines. In pre-Oligocene time, extensive alluvial fans spread southward from the Shivwits Plateau scarp across the current location of the modern Colorado River gorge to the northern margin of the Laramide drainage system at Hindu Canyon. Locally derived, fluvial Buck and Doe Conglomerate subsequently filled the disrupted Paleogene channels, spilled out over the local interfluves, and formed an extensive aggradational surface of low relief by late Oligocene time. Early Miocene volcanism filled in much of the relict Laramide relief. Erosional recession of the adjacent Shivwits Plateau escarpment shifted the northern Hualapai Plateau margin 8 km northeastward after the Laramide drainage episode and before the incision by the modern Colorado River. Partially exhumed tributaries to the Hindu Canyon paleochannel and associated sedimentary deposits bordering the southern edge of the Grand Canyon gorge demonstrate that local surface runoff flowed south, away from the modern Grand Canyon location, during early Paleogene time. Headwardly eroding Colorado River tributaries exhumed, captured, and reversed the flow of these tributaries to the Laramide canyon, beginning in late Miocene or Pliocene time. The geomorphic and stratigraphic records show no evidence of, and provide no space for, incision of a Late Cretaceous–Paleogene ancestral precursor to the modern Colorado River gorge. Instead, all the field evidence clearly supports a late Miocene–Pliocene origin for integration of the western Grand Canyon on the central Hualapai Plateau with the upper Colorado River.

Comparison of scaling relationships of extensional fault cores in tight carbonate and porous sandstone reservoirs

This paper presents a comparative analysis of the thicknesses–displacement relationships of fault cores in non-/low-porous carbonate and porous sandstone reservoirs. Fault thickness data were collected from extensional faults in mainly Cretaceous continental sandstones and Late Cretaceous–Paleogene marine carbonates exposed along the eastern flank of the Suez Rift, Egypt. The dataset consists of 730 thickness measurements, of which 313 are from 68 faults in carbonate rocks, and 417 are from 120 deformation band/microfaults and faults in sandstones. These data show that the increase in fault core thickness with displacement for the two lithologies is, overall, similar in log–log space, consistent with a power-law trend with an exponent of 0.5. However, for smaller faults, cores in carbonates are generally thicker than those in sandstones. The observation of nearly equal core thickness–displacement relationships for larger faults (&gt;10 m displacement) suggests that increased displacement reduces the geomechanical influence of lithology. In a statistical analysis of bins of fault displacement, small displacement faults seem to follow a skewed distribution towards smaller fault core thicknesses, moderate displacement faults have a normal distribution, while large displacement faults have a distribution skewed towards large core thicknesses. Each thickness distribution can be estimated with a mean and standard deviation. Fault core thickness variations are caused by interplaying factors such as fault geometric development, spatial and temporal positioning of shear and volumetric strains, and rheological changes. The variability in core thickness and intrinsic architecture of fault cores for any given location will cause significant variation in across-fault fluid flow, even for the same juxtaposition style.

Cenozoic contractional tectonics in the Fuegian Andes, southernmost South America: a model for the transference of orogenic shortening to the foreland

The Fuegian thrust-fold belt has been subjected to significant shortening during the Cenozoic. Although contemporaneous shortening and uplift was also recognized in the Fuegian Andes central belt (hinterland), previous studies stated that most of that deformation developed out-of-sequence with the thrust-fold belt and hence did not contribute to its shortening. Therefore, no suitable geometric and kinematic model has been proposed for the mechanism that links deformation in both domains. Here we address the style and timing of the younger (Late Cretaceous-Paleogene) structures of the central belt and the structural evolution of the thrust-fold belt, based on published and own data. We reinterpret the style of the central belt structures, proposing a new model in which basement thrusting in the central belt caused all the shortening in the foreland cover. We postulate that the basement was involved in a regional-scale duplex whose roof thrust was the decollement of the thin-skinned thrust-fold belt. Therefore, the total slip transferred through the roof thrust accounted for the shortening in the cover rocks. The basement duplex incorporated the underthrust crust from the footwall of the orogenic wedge through forward propagation, in sequence with the thrust-fold belt, from the Late Cretaceous to the Miocene.

Compositional variation of the late Cretaceous–Paleogene plutons from southwest Japan and its implication for ore genesis and continental growth

During the late Cretaceous–Paleogene period, the fluids liberated from the subducting slab along Japanese-arc subduction zone could have triggered the partial melting of the mantle wedge, whose composition was similar to the fertile primitive mantle. Underplating of the produced basaltic magmas beneath the continental crust could further facilitate the partial melting of the lower continental crusts. The primary magmas produced by the mixing of resultant partial melts underwent various degrees of fractional crystallization processes to produce the late Cretaceous–Paleogene plutons in southwest Japan, and have contributed to the growth of continental crust with new, non-recycled materials during the Phanerozoic.The factor analysis of chemical data from ore-barren Ryoke and W–Sn-ore-rich Sanyo plutonic rocks identifies three major groups of elements: the mafic group (Al, Fe, Mg, Mn, Ca, Sr, Zn, Co, V, P, Cu, Ni, and Cr); the felsic group (Si, K, Rb, Cs, Tl, Pb, Y, Nb, Ta, Hf, Ge, Sn, W, Th, U, and HREE); and the LREE+Zr group (Ba, La, Ce, Pr, Nd, Eu, Zr, and Hf). Mafic group and felsic group elements are inversely correlated. The concentrations of LREE+Zr group elements increase first, then, decrease with increasing SiO2 content. Therefore, the high silica samples of the Ryoke+Sanyo belts are low in LREE+Zr group elements. In the Ryoke samples Mo behaves like Zr, which results in the lowest Mo content in the Ryoke samples with high silica. The high Mo and Si contents in some Sanyo samples suggest that the zircon-compatible Mo4+ may be converted to incompatible Mo6+ under a relatively high oxygen fugacity condition at the late stage of magmatic differentiation. W and Sn behave as the felsic group elements in the Ryoke+Sanyo samples. Thus, their concentrations are high in the high silica granitoids, especially rock samples from the Sanyo belt, which are often associated with W–Sn ores. The factor analysis of the Sanin plutonic samples indicates that W still belongs to the felsic group, but Sn no longer correlates with W and behaves more like Zr due to the conversion of incompatible Sn2+ to zircon-compatible Sn4+ under a relatively high oxygen fugacity condition in the Sanin belt. Therefore, Sn content is low in high silica granitoids from Sanin belt. The high silica Sanin samples are often enriched in Mo, again suggesting the conversion of zircon-compatible Mo4+ into incompatible Mo6+ in the magmas of those samples. The high Mo granitoids from the Sanin belt are probable source rocks for the associated Mo ores in the area.