Boulder, Colo., USA: The Geological Society of America regularly publishesarticles online ahead of print. GSA Bulletin topics includemultiple articles about the dynamics of China and Tibet; the end-Permianterrestrial extinction paradigm in South Africa; prehistoric lava flowsfrom the urban district of Catania (Etna volcano, Italy); the debatedorigins of granite, and "a tale of two Tweefonteins." You can find thesearticles athttps://bulletin.geoscienceworld.org/content/early/recent.
Authigenic berthierine and incipient chloritization in shallowly buriedsandstone reservoirs: Key role of the source-to-sink context
Maxime Virolle; Benjamin Brigaud; Daniel Beaufort; Patricia Patrier; EidAbdelrahman ...
Abstract:Chlorite is recognized worldwide as a key mineral that inhibits thedevelopment of quartz cement in deeply buried sandstone reservoirs.Iron-rich chlorite is mainly formed by the transformation of a precursorclay mineral; however, few studies have focused on the early stages beforethe crystallization of chlorite. This study analyzed shallowly buried(400−1000 m) coastal sandstones from within the Wealden Group of the ParisBasin. Berthierine, a 7 Å trioctahedral clay mineral belonging to theserpentine group, approximatively with same chemistry as chlorite but adifferent crystal structure, has been identified in a 900-m-deep well butnot in a 400−600-m-deep well. Berthierine has mainly been observed as claycoatings around detrital grains with a typical honeycomb texture.Nanopetrographic observations suggest that the honeycomb texturalorganization of the clay particles found in deeper buried sandstonereservoirs (>1500 m) is acquired from a berthierine precursor at shallowdepths. However, small amounts of quartz overgrowths are observed on thesurface of detrital grains at shallow depths and low temperature (below 40°C), and it is believed that precursor berthierine coatings are primarilyresponsible for the inhibition of quartz overgrowths before Fe-richchlorite is formed. This suggests that the key mineral primarilycontrolling the reservoir quality of deeply buried sandstone reservoirs isberthierine rather than iron-rich chlorite, which challenges the commonlyaccepted assertion that chlorite coating is the main process that inhibitsquartz overgrowths. The source-to-sink context of the Paris Basin duringthe Early Cretaceous was decisive with respect to the supply of sands andberthierine clay precursors (in particular kaolinite and iron-rich,hydroxy-interlayered clay minerals) to the center of the basin.
Subduction initiation-induced rapid emplacement of garnet-bearingperidotites at a nascent forearc: Petrological and Os-Li isotopicevidence from the Purang ophiolite, Tibet
Xiao-Han Gong; Ji-Feng Xu; Ren-Deng Shi; Ben-Xun Su; Qi-Shuai Huang ...
Abstract:Garnet-bearing peridotites commonly occur in the deeper parts of mature orthickened oceanic lithosphere, and are rarely exhumed and emplaced onto theseafloor. The Purang ophiolitic peridotites in south Tibet contain raresymplectite pseudomorphs after garnet, offering a unique window into thestill poorly understood evolution of the deep oceanic lithosphere. Here,integrated petrologic and Os-Li isotopic data are used to constrain theevolution and dynamics of emplacement for these garnet peridotiteprotoliths. The Purang peridotites show wide variations of chemicalcompositions (spinel Cr#: 0.2−0.8) and Os model ages (up to 2.0 Ga), thusrepresenting a piece of heterogeneous oceanic mantle lithosphere. Dunitechannels show two distinctive groups of Cr# of spinels and Os-isotopecompositions, with the low- to medium-Cr# (0.2−0.6) and high-Cr# (0.7−0.8)dunites reflecting the reaction of host lherzolites/harzburgites withpercolating mid-ocean ridge basalt−like and boninitic melts, respectively.This confirms recent subduction initiation-related melt percolation in thePurang peridotites. Coexisting olivines and pyroxenes in the peridotitesshow systematic Li elemental and isotopic disequilibrium, suggesting fastcooling of the peridotites to Li closure temperature shortly after the meltpercolations, likely during exhumation of the peridotites onto theseafloor. This supports a close link between subduction initiation andtectonic emplacement of the Purang peridotites. Combined with othergeological evidence, we suggest the Purang peridotites may originate fromthe deep part of old, thick oceanic lithosphere of the Neo-Tethys. Thisthick oceanic lithosphere was progressively weakened and thinned likelyduring widespread plume-lithosphere interaction, triggering thetransformation of garnet peridotite protoliths to spinel peridotites.Subsequently, initiation of a new subduction zone along the lithosphericweakness caused rapid ascent and emplacement of the Purang peridotites at anascent forearc.
A tale of two Tweefonteins: What physical correlation, geochronology,magnetic polarity stratigraphy, and palynology reveal about theend-Permian terrestrial extinction paradigm in South Africa
Robert A. Gastaldo; Johann Neveling; John W. Geissman; Sandra L. Kamo;Cindy V. Looy
Abstract:The contact between the Daptocephalus to Lystrosaurus declivis (previously Lystrosaurus)Assemblage Zones (AZs) described from continental deposits of the KarooBasin was commonly interpreted to represent an extinction crisis associatedwith the end-Permian mass-extinction event at ca. 251.901 ± 0.024 Ma. Thisterrestrial extinction model is based on several sections in the EasternCape and Free State Provinces of South Africa. Here, new stratigraphic andpaleontologic data are presented for the Eastern Cape Province, ingeochronologic and magnetostratigraphic context, wherein lithologic andbiologic changes are assessed over a physically correlated stratigraphyexceeding 4.5 km in distance. Spatial variation in lithofacies demonstratesthe gradational nature of lithostratigraphic boundaries and depositionaltrends. This pattern is mimicked by the distribution of vertebratesassigned to the Daptocephalus and L. declivis AZs wherediagnostic taxa of each co-occur as lateral equivalents in landscapesdominated by a Glossopteris flora. High-precision U-Pb zircon(chemical abrasion-isotope dilution-thermal ionization mass spectrometry)age results indicate maximum Changhsingian depositional dates that can beused as approximate tie points in our stratigraphic framework, which issupported by a magnetic polarity stratigraphy. The coeval nature ofdiagnostic pre- and post-extinction vertebrate taxa demonstrates that the L. declivis AZ did not replace the Daptocephalus AZstratigraphically, that a biotic crisis and turnover likely is absent, anda reevaluation is required for the utilization of these biozones here andglobally. Based on our data set, we propose a multidisciplinary approach tocorrelate the classic Upper Permian localities of the Eastern Cape Provincewith the Free State Province localities, which demonstrates theirtime-transgressive nature.
Paleomagnetic dating of prehistoric lava flows from the urban districtof Catania (Etna volcano, Italy)
Andrea Magli; Stefano Branca; Fabio Speranza; Gilda Risica; Gaia Siravo ...
Abstract:Determining the ages of past eruptions of active volcanoes whose slopeswere historically inhabited is vitally important for investigating therelationships between eruptive phenomena and human settlements. During itsalmost three-millennia-long history, Catania--the biggest city lying at thetoe of Etna volcano--was directly impacted only once by the huge lava flowemplaced during the A.D. 1669 Etna flank eruption. However, other lavaflows reached the present-day Catania urban district in prehistoric agesbefore the founding of the city in Greek times (729/728 B.C., i.e.,2679/2678 yr B.P.). In this work, the Holocene lava flows of Barriera delBosco, Larmisi, and San Giovanni Galermo, which are exposed in the Cataniaurban district, were paleomagnetically investigated at 12 sites (120oriented cores). Paleomagnetic dating was obtained by comparing flow-meanpaleomagnetic directions to updated geomagnetic reference models for theHolocene. The Barriera del Bosco flow turns out to represent the oldesteruptive event and is paleomagnetically dated to the 11,234−10,941 yr B.P.and 8395−8236 yr B.P. age intervals. The mean paleomagnetic directions fromthe San Giovanni Galermo and Larmisi flows overlap when statisticaluncertainties are considered. This datum, along with geologic, geochemical,and petrologic evidence, implies that the two lava flows can be consideredas parts of a single lava field that erupted in a narrow time windowbetween 5494 yr B.P. and 5387 yr B.P. The emplacement of such a huge lavaflow field may have buried several Neolithic settlements, which would thusexplain the scarce occurrence of archaeological sites of that age foundbelow the town of Catania.
Migmatite and leucogranite in a continental-scale exhumed strike-slipshear zone: Implications for tectonic evolution and initiation ofshearing
Junyu Li; Shuyun Cao; Xuemei Cheng; Franz Neubauer; Haobo Wang ...
Abstract:Plutons within continental strike-slip shear zones bear importantgeological processes on late-stage plate transpression andcontinent-continent collision and associated lateral block extrusion.Where, when, and how intrusions and shearing along transpressionalstrike-slip shear zones respond to plate interactions, however, are oftendebated. In this study, we investigated migmatite associated leucograniteand pegmatite from the exhumed >1000-km-long Ailao Shan-Red Riverleft-lateral strike-slip shear zone in Southeast Asia that was activeduring India-Eurasia plate convergence. Most zircons from the migmatitesand leucogranitic intrusions present inherited core-rim structure. Thedepletion of rare earth element patterns and positive Eu anomalies suggestthat leucosomes and leucogranites are the result of crustal anatexis.Zircon rims from the foliated migmatites and leucogranites record U-Pb agesof 41−28 Ma, revealing the timing of the Cenozoic crustal anatexis eventalong this strike-slip shear zone. Ages of the magmatic zircons from theunfoliated pegmatites provide the timing of the termination of ahigh-temperature tectono-thermal event and ductile left-lateral shearing at26−23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red Riverstrike-slip shear zone indicates that thickened crust underneath the shearzone involved previously subducted crust. We propose that the Cenozoicthermal state has an important effect on the crustal anatexis and thus onthe rheological behavior of the lithosphere by thermal weakening, whichplays an essential role in localizing the initiation of the deep-seatedlower-crustal shear zone.
The impact of a tear in the subducted Indian plate on the Miocenegeology of the Himalayan-Tibetan orogen
Rui Wang; Roberto F. Weinberg; Di-Cheng Zhu; Zeng-Qian Hou; Zhi-Ming Yang
Abstract:The Yadong-Gulu Rift, cutting across the Gangdese belt and Himalayanterranes, is currently associated with a thermal anomaly in the mantle andcrustal melting at 15−20 km depth. The rift follows the trace of a tear inthe underthrusted Indian continental lithospheric slab recognized by highresolution geophysical methods. The Miocene evolution of a 400-km-wide bandfollowing the trace of the tear and the rift, records differencesinterpreted as indicative of a higher heat flow than its surroundings. Inthe Gangdese belt, this band is characterized by high-Sr/Y graniticmagmatism that lasted 5 m.y. longer than elsewhere and by the highestvalues of εHf(i) and association with the largest porphyry Cu-Modeposits in the Gangdese belt. Anomalously young magmatic rocks continuesouth along the rift in the Tethyan and Higher Himalayas. Here, a300-km-wide belt includes some of the youngest Miocene Himalayanleucogranites; the only occurrence of mantle-derived mafic enclaves in aleucogranite; young mantle-derived lamprophyre dikes; and the youngest andhottest migmatites in the Higher Himalayas. These migmatites record ahistory of rapid exhumation contemporaneous with the exhumation of Miocenemafic eclogite blocks, which are unique to this region and which were bothheated to >800 °C at ca. 15−13 Ma, followed by isothermal decompression.We suggest that the prominent tear in the Indian lithosphere, sub-parallelto the rift, is the most likely source for these tectono-thermal anomaliessince the Miocene.
The timeline of prolonged accretionary processes in eastern CentralAsian Orogenic Belt: Insights from episodic Paleozoic intrusions incentral Inner Mongolia, North China
Lingling Yuan; Xiaohui Zhang; Zhili Yang
Abstract:Updating magmatic profile in crucial constituent terranes across theCentral Asian Orogenic Belt presents a key to chronicling the timeline ofprolonged accretionary processes and termination of the Paleo-Asian Oceanin the northern China−southern Mongolia tract. Here we performed asystematic geochronological and geochemical study on a spectrum ofPaleozoic intrusions from the Erenhot region in central Inner Mongolia,North China, within the hinterland of the tract, with four distinctmagmatic episodes unraveled. Combining these episodes with the previouslydocumented events from contiguous regions defines two majortectono-magmatic cycles. The early Paleozoic cycle (500−450 Ma) evolvedfrom initial fluid-fluxed tholeiitic and calc-alkaline granitoids tomelt-fertilized mafic-intermediate magmatism. It appears to experience theinitiation and maturation of a Western Pacific-type intra-oceanic arcsystem that culminated in ridge subduction. The late Paleozoic cycleexpanded in magmatic expression from sporadic Late Devonian (373−365 Ma)calc-alkaline intermediate-felsic pulses through Early-Middle Carboniferous(356−320 Ma) medium to high-K calc-alkaline flare-up to a LateCarboniferous−Early Permian (310−277 Ma) province of diverse lithologies.These magmatic episodes seem to encompass a complete active continentalarc−back-arc system that spanned from resuming oceanic plate subductionthrough slab rollback and backarc rifting to ridge-trench collision andbackarc basin closure. Featuring a Rodinia-aged terrane affinity and arepresentative Paleozoic magmatic profile, the Erenhot region provides anoptimal site for correlating the evolution of mosaic terranes in southernMongolia and northern China, and for evaluating the coupled evolution ofshifting tectonic regimes and plural crustal generation mechanisms within aretreating accretionary orogen.
Pure sediment-derived granites in a subduction zone
Jian Xu; Xiao-Ping Xia; Qiang Wang; Christopher J. Spencer; Chun-Kit Lai...
Abstract:The Earth is unique in the Solar System due to significant volumes ofgranite in the lithosphere. However, the origins of granites are stillhighly debated, especially sediment-derived granites, which are oftentreated as a geochemical end-member of the continental crust. In the Yunnanregion of South China, we identify the occurrence of pure sediment-derivedgranite in a subduction system. The suite of strongly peraluminous granitereported herein is interpreted to represent pure metasedimentary meltsbased on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopiccompositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granitesare characterized by radiogenically enriched Sr, Nd, and Hf isotopicsignatures. They show δ11B and δ18O signatures akinto those of continental shales. Geochemical modeling indicates nocontributions from the mantle that can be detected. Considering theregional tectonic evolution, these granites are suggested to be formed in asubduction zone by decompression melting of rapidly exhumed back-arcsediments. We posit that decompression melting was triggered by widespreadextension and thinning of the crust prompted by rollback of the subductingoceanic crust. These granites thus provide evidence that granite formationin subduction zones does not necessarily contribute to crustal growth.These subduction-related pure sediment-derived granites have differentelemental ratios and contents (e.g., Al2O3/TiO 2 and Yb) from the Himalayan leucogranites. Considering theirsource compositions (e.g., pelitic rocks), which are similar to those ofthe Himalayan leucogranites, these differences are likely due to theirhigher formation temperature and lower pressure despite a great similarityin isotopic compositions. Identification of pure sediment-derived, stronglyperaluminous granites (SPGs) in subduction systems provides an importantgeodynamic mechanism for crustal anatexis, which can both geochemically andtectonically complement their collisional counterparts identified in theHimalayas.
Mafic-ultramafic intrusion formed by multi-stage evolution of hydrousbasaltic melts
Qi-Wei Li; Jun-Hong Zhao; Mei-Fu Zhou; Jian-Feng Gao
Abstract:The magmatic processes beneath the active continental margins are verycomplicated and affect structures and compositions of the arc roots.Neoproterozoic igneous rocks are widely distributed around the margins ofthe Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is acomposite body, located at the northern margin of the Tarim Block, andconsists of gabbro, pyroxenite, and peridotite units. The gabbro unit has asecondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks fromthe Xingdier intrusion have a large range of MgO (12.9−32.8 wt%) and SiO 2 (43.0−57.9 wt%), and low K2O+Na2O(0.11−2.25 wt%) contents. They have right inclined chondrite-normalizedrare earth element patterns with (La/Yb)N ratios of 2.2−8.6.Their primitive mantle normalized trace element patterns show arc-affinitygeochemical features characterized by enrichment in Rb, Ba, Th, U, and Pband depletion in Nb, Ta, and Ti. They have variable initial 87Sr/86Sr ratios (0.7063−0.7093), εNd(t) values (−2.9 to −7.8),206Pb/204Pb (17.08−17.80), 207Pb/ 204Pb (15.42−15.49), and 208Pb/204Pbratios (37.48−38.05), forming an evolution trend from the peridotite unitto the gabbro and pyroxenite units. Clinopyroxene in the three units ischemically similar to those formed in hydrous magmas. The spinel inclusionsin olivine from the peridotite unit show unmixing texture and have high Alcontents and oxygen fugacity of ∼FMQ+1. Therefore, the parental magma wasprobably derived from a lithospheric mantle enriched by slab-derivedfluids. Rocks from the gabbro and peridotite units are proposed to havebeen derived from olivine-normative melts, whereas rocks from thepyroxenite unit are cumulates from the quartz-normative melts. Suchcontrasting parental magmas resulted from variable degrees of crustalcontamination and fractional crystallization in the arc root.
Origin of syn-collisional granitoids in the Gangdese orogen: Reworkingof the juvenile arc crust and the ancient continental crust
Yu-Wei Tang; Long Chen; Zi-Fu Zhao; Yong-Fei Zheng
Abstract:Granitoids at convergent plate boundaries can be produced either by partialmelting of crustal rocks (either continental or oceanic) or by fractionalcrystallization of mantle-derived mafic magmas. Whereas granitoid formationthrough partial melting of the continental crust results in reworking ofthe pre-existing continental crust, granitoid formation through eitherpartial melting of the oceanic crust or fractional crystallization of themafic magmas leads to growth of the continental crust. This category isprimarily based on the radiogenic Nd isotope compositions of crustal rocks;positive εNd(t) values indicate juvenile crust whereas negativeεNd(t) values indicate ancient crust. Positive εNd(t)values are common for syn-collisional granitoids in southern Tibet, whichleads to the hypothesis that continental collision zones are importantsites for the net growth of continental crust. This hypothesis is examinedthrough an integrated study of in situ zircon U-Pb ages and Hf isotopes,whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral Oisotopes for felsic igneous rocks of Eocene ages from the Gangdese orogenin southern Tibet. The results show that these rocks can be divided intotwo groups according to their emplacement ages and geochemical features.The first group is less granitic with lower SiO2 contents of59.82−64.41 wt%, and it was emplaced at 50−48 Ma in the early Eocene. Thesecond group is more granitic with higher SiO2 contents of63.93−68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The earlyEocene granitoids exhibit relatively depleted whole-rock Sr-Nd-Hf isotopecompositions with low (87Sr/86Sr)i ratiosof 0.7044−0.7048, positive εNd(t) values of 0.6−3.9, ε Hf(t) values of 6.5−10.5, zircon εHf(t) values of1.6−12.1, and zircon δ18O values of 5.28−6.26‰. These isotopiccharacteristics are quite similar to those of Late Cretaceous mafic arcigneous rocks in the Gangdese orogen, which indicates their derivation frompartial melting of the juvenile mafic arc crust. In comparison, the lateEocene granitoids have relatively lower MgO, Fe2O3,Al2O3, and heavy rare earth element (HREE) contentsbut higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb) N ratios. They also exhibit more enriched whole-rock Sr-Nd-Hfisotope compositions with high (87Sr/86Sr) i ratios of 0.7070−0.7085, negative εNd(t) values of−5.2 to −3.9 and neutral εHf(t) values of 0.9−2.3, andrelatively lower zircon εHf(t) values of −2.8−8.0 and slightlyhigher zircon δ18O values of 6.25−6.68‰. An integratedinterpretation of these geochemical features is that both the juvenile arccrust and the ancient continental crust partially melted to produce thelate Eocene granitoids. In this regard, the compositional evolution ofsyn-collisional granitoids from the early to late Eocene indicates atemporal change of their magma sources from the complete juvenile arc crustto a mixture of the juvenile and ancient crust. In either case, thesyn-collisional granitoids in the Gangdese orogen are the reworkingproducts of the pre-existing continental crust. Therefore, they do notcontribute to crustal growth in the continental collision zone.
Mantle source of tephritic porphyry in the Tarim Large Igneous Provinceconstrained from Mg, Zn, Sr, and Nd isotope systematics: Implicationsfor deep carbon cycling
Weiliang Kong; Zhaochong Zhang; Zhiguo Cheng; Bingxiang Liu; M. Santosh ...
Abstract:The nature and source of magmatism associated with large igneous provinces(LIPs) remain disputed. Here we investigate the role of recycled materialsthat contributed to mantle heterogeneity in the Tarim Large IgneousProvince (TLIP) in China through integrated Zn−Mg−Sr−Nd isotopes of a raretephritic rock suite. The Sr−Nd isotopes [(87Sr/86Sr)i = 0.70368−0.70629; εNd(t) = −0.25−4.64] and δ 26Mg values (−0.23‰ to −0.34‰) of the tephritic porphyries areconsistent with a normal mantle origin. In contrast, the whole rock andpyroxene phenocrysts yield δ66Zn values of +0.28‰ to +0.46‰ and+0.30‰ to +0.39‰, which are slightly heavier than those of the terrestrialmantle (+0.16 ± 0.06‰) and mid-oceanic-ridge basalts (MORBs) (+0.27 ±0.05‰). We exclude the possibility that the heavy Zn isotopes of theWajilitag tephritic porphyries are caused by magmatic processes such asfractional crystallization and partial melting and correlate the isotopicfeatures to the role of altered oceanic crust along with magnesite in themantle source. The Wajilitag tephritic porphyry displays trace-elementpatterns similar to those of the melts from natural hornblendite,especially those of hornblendite + peridotite. Additionally, thegeochemical characteristics such as enrichment in Nb and Ta, depletion inK, high TiO2, and constant Na2O/K2O ratiosalso suggest that the tephritic porphyries were derived from anamphibole-bearing source contributed by altered oceanic crust along withsedimentary carbonates. Our study provides insight into the contribution ofsubducted materials to the mantle heterogeneity beneath the TLIP andhighlights the role of subduction in the deep carbon cycle and subductedslab-lithosphere-plume interaction in the generation of LIPs.
Sedimentology, geochronology, and provenance of the late Cenozoic"Yangtze Gravel": Implications for Lower Yangtze River reorganizationand tectonic evolution in southeast China
Ping Wang; Hongbo Zheng; Yongdong Wang; Xiaochun Wei; Lingyu Tang ...
Abstract:The evolution of the Yangtze River, the longest river in Asia, provides aspectacular example for understanding the Cenozoic interaction betweentectonics, climate, and surficial processes. The oldest Lower Yangtzedeposits, characterized by ∼100-m-thick sequences of unconsolidatedconglomerate, sandstone, and siltstone, referred to as "Yangtze Gravel,"have been recently dated >23 Ma, indicating a pre-Miocene establishmentof a through-going river. However, the link between river integration andtectonic evolution has never been established due to the limited study ofthese sediments. Here, we report sedimentology, geochronology, andprovenance of the Yangtze Gravel based on 17 stratigraphic sections exposedalong the Lower Yangtze River. Our new chronostratigraphic results,including 40Ar/39Ar ages from the overlying basalt and fossil-basedstratigraphic correlation, suggest an early-middle Miocene age for thesesediments. Detailed analysis of lithofacies reveals several sequences ofcoarse-grained channel-belt deposits (channel fills and bars), indicatingbraided alluvial deposition across the Jianghan Basin, North Jiangsu-SouthYellow Sea Basin, and East China Sea Shelf Basin. This ancient LowerYangtze River is further characterized by petrography and detrital zirconU-Pb geochronology results which show similar provenance and erosionpattern as the present-day Yangtze River. However, the ancient river inearly-middle Miocene is an alluvial, bedload-dominated braided river withhigher stream power and a more prolonged course flowing into the East ChinaSea Shelf Basin. These differences between ancient and modern Lower YangtzeRiver reflect varied climate and paleogeography in southeast China duringthe late Cenozoic. Compared with the Paleogene red-colored, halite-bearing,Ephedripite pollen-dominated, lacustrine deposits in Jianghan Basin andNorth Jiangsu-South Yellow Sea Basin, the deposition of yellow togreen-colored, coarse-grained, arboreal pollen, and wood-dominated YangtzeGravel indicates a drainage reorganization from hydrologically closed lakesto a through-going river system during late Oligocene to early Miocene.During Paleogene, rift basins were filled by alluvial andfluvial-lacustrine deposition with restricted flow distance and localsources. From late Oligocene to early-middle Miocene, the post-riftsubsidence opens a path for the ancient Lower Yangtze River connecting theJianghan Basin, North Jiangsu-South Yellow Sea Basin, and East China SeaShelf Basin. We attribute the drainage reorganization of the Lower YangtzeRiver to be a surficial response to Cenozoic tectonics, particularly thewestern Pacific subduction, in southeast China. The deposition of thewidespread, coarse-grained Yangtze Gravel is probably due to the combinedeffects of catchment expansion and strong monsoonal climate in East Asia.
Thermotectonic events recorded by U-Pb geochronology and Zr-in-rutilethermometry of Ti oxides in basement rocks along the P2 fault, easternAthabasca Basin, Saskatchewan, Canada
E. Adlakha; K. Hattori
Abstract:Basement rocks below the Athabasca Basin, Saskatchewan, have been intenselyaltered through paleoweathering and multiple hydrothermal events, includingthe formation of world-class unconformity-type uranium deposits. Here, wedemonstrate the utility of Ti-oxide thermochronology for identifyingthermotectonic events in these altered rocks leading to uraniummineralization along basement structures. Rutile grains along the P2 fault,a major fault in the eastern Athabasca Basin, exhibit 207Pb/ 206Pb ages of ca. 1850−1700 Ma, with a weighted mean of 1757 ± 6Ma (mean square of weighted deviation [MSWD] = 1.4, n = 116). Theolder ages (~1770 Ma) record regional metamorphism reaching atemperature of 875 °C during the Trans-Hudson orogeny. Pb diffusionmodeling indicates that metamorphic rutile should exhibit cooling ages of1760−1750 Ma. Rutile grains showing young ages, ~1750 Ma, reflectisotopic resetting during regional asthenospheric upwelling between 1770and 1730 Ma related to the emplacement of the Kivalliq igneous suite to thenorth. This thermotectonic event (temperature ~550 °C) promotedhydrothermal activity to produce silicified rocks, i.e., "quartzite," alongthe P2 fault, which later focused mineralizing fluids for unconformity-typeuranium deposits. The young rutile ages also indicate that the basementrocks remained hot until 1700 Ma, providing the maximum age for thedeposition of the Athabasca sediments. Anatase yields a concordia age of1569 ± 31 Ma (MSWD = 0.30, n = 5), which is within uncertainty ofthe oldest ages for uraninite of the McArthur River deposit. This agecorresponds to the incursion of basinal fluids in the basement along the P2fault during uranium mineralization.
Superposition of Cretaceous and Cenozoic deformation in northern Tibet:A far-field response to the tectonic evolution of the Tethyan orogenicsystem
Ye Wang; Xuanhua Chen; Yaoyao Zhang; Zheng Yin; Andrew V. Zuza ...
Abstract:Although the Cenozoic Indo-Asian collision is largely responsible for theformation of the Tibetan plateau, the role of pre-Cenozoic structures incontrolling the timing and development of Cenozoic deformation remainspoorly understood. In this study we address this problem by conducting anintegrated investigation in the northern foreland of the Tibetan plateau,north of the Qilian Shan-Nan Shan thrust belt, NW China. The work involvesfield mapping, U-Pb detrital-zircon dating of Cretaceous strata in thenorthern foreland of the Tibetan plateau, examination of growth-stratarelationships, and construction and restoration of balanced cross sections.Our field mapping reveals multiple phases of deformation in the area sincethe Early Cretaceous, which was expressed by northwest-trending folding andnorthwest-striking thrusting that occurred in the early stages of the EarlyCretaceous. The compressional event was followed immediately by extensionand kinematically linked right-slip faulting in the later stage of theEarly Cretaceous. The area underwent gentle northwest-trending foldingsince the late Miocene. We estimate the magnitude of the Early Cretaceouscrustal shortening to be ∼35%, which we interpret to have resulted from afar-field response to the collision between the Lhasa and the Qiangtangterranes in the south. We suggest that the subsequent extension in theEarly Cretaceous was induced by orogenic collapse. U-Pb dating of detritalzircons, sourced from Lower Cretaceous sedimentary clasts from the northand the south, implies that the current foreland region of the Tibetanplateau was a topographic depression between two highland regions in theEarly Cretaceous. Our work also shows that the Miocene strata in theforeland region of the northern Tibetan plateau was dominantly sourced fromthe north, which implies that the rise of the Qilian Shan did not impactthe sediment dispersal in the current foreland region of the Tibetanplateau where this study was conducted.
