The Lomonosov Ridge as a natural extension of the Eurasian continental margin into the Arctic Basin

The inregrated geological and geophysical studies carried out in recent years in the Lomonosov Ridge and at its junction with the Eurasian shelf revealed evidence for thinned (reduced) crust in the ridge (20–25 km) and its relationship with shelf structures. We compared the parameters of deep seismic cross-sections of the shelf and Lomonosov Ridge, thus proving the existence of continental crust in the latter. Also, we analyzed the deep structure of the junction between the Lomonosov Ridge and the shelf and established a genetic geologic relationship, with no evidence that the Lomonosov Ridge moved as a terrane with respect to the shelf. In addition, seismological studies independently confirm the relationship between the Lomonosov Ridge and the adjacent shelf.The Lomonosov Ridge is a continental-crust block of a craton. The craton was reworked during the Caledonian tectonomagmatic activity with the formation of a Precambrian–Caledonian seismically unsegmented basement (upper crust) and an epi-Caledonian platform cover. Afterward, the block subsided to bathyal depths in the Late Alpine. This block and the adjacent areas of the Eastern Arctic shelf developed in the platform regime till the Late Mesozoic.     

Typification of the Earth’s crust of Central Arctic Uplifts in the Arctic Ocean

The modern views on the structure of the oceanic and continental crust are discussed. The presented geological-geophysical information on the deep structure of the Earth’s crust of the Lomonosov Ridge, Mendeleev Rise, and Alpha Ridge, which make up the province of the Central Arctic Uplifts in the Arctic Ocean, is based on CMP, seismic-reflection, and seismic-refraction data obtained by Russian and Western researchers along geotraverses across the Amerasia Basin. It is established that the crust thickness beneath the Central Arctic Uplifts ranges from 22 to 40 km. Comparison of the obtained velocity sections with standard crust sections of different morphostructures in the World Ocean that are underlain by the typical oceanic crust demonstrates their difference with respect to the crustal structure and to the thickness of the entire crust and its individual layers. Within the continental crust, the supercritical waves reflected from the upper mantle surface play the dominant role. Their amplitude exceeds that of head and refracted waves by one to two orders of magnitude. In contrast, the refracted and, probably, interferential head waves are dominant within the oceanic crust. The Moho discontinuity is the only first-order boundary. In the consolidated oceanic crust, such boundaries are not known. The similarity in the velocity characteristics of the crust of the Alpha Ridge and Mendeleev Rise, on the one hand, and the continental crust beneath the Lomonosov Ridge, on the other, gives grounds to state that the crust of the Mendeleev Rise and Alpha Ridge belongs to the continental type. The interference mosaic pattern of the anomalous magnetic field of the Central Arctic Uplifts is an additional argument in favor of this statement. Such patterns are typical of the continental crust with intense intraplate volcanism. Interpretation of seismic crustal sections of the Central Arctic Uplifts and their comparison with allowance for characteristic features of the continental and oceanic crust indicate that the Earth’s crust of the uplifts has the continental structure.     

Grenvillian and Caledonian evolution of eastern Svalbard – a tale of two orogenies

Svalbard is located in the north-west corner of the Barents Sea shelf and the Eurasian Plate, in a key area for interpreting Caledonian and older orogens in the Arctic region. Recent U–Pb dating in the Nordaustlandet Terrane of eastern Svalbard shows this terrane to consist of a Grenville-age basement, overlain by Neoproterozoic to early Palaeozoic platformal sediments, and intruded by Caledonian anatectic granites. Deformation, metamorphism and crustal anatectic magmatism occurred both during the Grenvillian (960–940 Ma) and Caledonian (450–410 Ma) orogenies. This evolution shows great similarities with that of eastern Greenland. In the classical model, eastern Svalbard is placed outboard of central east Greenland in pre-Caledonian time. Alternatively, it may have been located north-east of Greenland and transferred west and rotated anticlockwise during Caledonian continent–continent collision. In the Neoproterozoic, easternmost Svalbard may have been part of a wider area of Grenville-age crust, now fragmented and dispersed around the Arctic.     

Geochemistry of organic matter of bottom sediments in the rises of the central Arctic Ocean

Based on geomorphological, lithological, and facial characteristics of the East Arctic continental margin, we studied the main factors controlling the Late Cenozoic supply of organic matter (OM) to the bottom sediments of the Central Arctic rises of the Arctic Ocean. Complex analysis of dispersed OM in the samples taken during the expeditions of the R/V “Akademik Fedorov” in 2000 and 2005 showed a significant difference between the sediments of the Lomonosov Ridge and Mendeleev Rise. The bottom sediments of the latter are strongly transformed and lack terrigenous components, as evidenced results from the main geochemical characteristics (contents of C_org, C_carb, N_org, bitumens, and humic acids) and the composition and distribution of hydrocarbon molecular markers (alkanes, saturated and aromatic cyclanes). The obtained data evidence that ancient sedimentary rocks containing genetically uniform deeply transformed (up to mesocatagenesis) OM played a significant role in the formation of the Pleistocene–Holocene sediments of the axial part of the Mendeleev Rise.     

Lomonosov ridge and the Eastern Arctic Shelf as elements of an integrated lithospheric plate: Comparative analysis of wrench faults

The notions of deformations in the juncture area of the Eastern Arctic Shelf and Lomonosov Ridge are highly contradictory. It has been suggested that these geostructures were divided by a large right-lateral wrench fault of the transform type, which is known as the Khatanga–Lomonosov Fault. Data obtained by interpretation of the A7 profile have been compared with seismic sections crossing large-sized wrench faults in other sedimentary basins. The investigations have shown that on the A7 profile there are no structures typical of large-sized wrench faults. The Eastern Arctic Shelf and Lomonosov Ridge, which are located on the same lithospheric plate, form an integrated structure where the ridge is a natural continuation of the shelf.     

探讨黔中古隆起形成机制及演化

黔中地区未出露中元古代褶皱基底。上覆新元古代沉积盖层厚度、沉积相及地层接触关系反馈出南华纪之前古隆起区已处于基底古地形的高部位。新元古代裂谷盆地演化及地球物理方面证据表明古隆起区与南北两侧边缘区基底存在一定差异。由此推测古隆起区沉积盖层之下可能有与四堡群相当的中元古代褶皱基底,并处于古地形的高部位。褶皱基底南北向的挤压...     

Oceanic basins in prehistory of the evolution of the Arctic Ocean

During geodynamic reconstruction of the Late Mezozoic-Cenozoic evolution of the Arctic Ocean, a problem arises: did this ocean originate as a legacy structure of ancient basins, or did it evolve independently? Solution of this problem requires finding indicators of older oceanic basins within the limits of the Arctic Region. The Arctic Region has structural-material complexes of several ancient oceans, namely, Mesoproterozoic, Late Neoproterozoic, Paleozoic (Caledonian and Hercynian), Middle Paleozoic-Late Jurassic, and those of the Arctic Ocean, including the Late Jurassic-Early Cretaceous Canadian, the Late Cretaceous-Paleocene Podvodnikov-Makarov, and the Cenozoic Eurasian basins. The appearances of all these oceans were determined by a complex of global geodynamical factors, which were principally changed in time, and, as a result of this, location and configuration of newly opened oceans, as well as ones of adjacent continents, which varied from stage to stage. By the end of the Paleozoic, fragments of the crust corresponding to Precambrian and Caledonian oceans were transported during plate-tectonic motions from southern and near equatorial latitudes to moderately high and arctic ones, and, finally, became parts of the Pangea II supercontinent. The Arctic Ocean that appeared after the Pangea II breakup (being a part of the Atlantic Ocean) has no direct either genetic or spatial relation with more ancient oceans.     

Interlinking the Hotspot Track in the Arctic and its Implications for Paleo-plate Reconstrution

The Siberian–Icelandic hotspot track is the only preserved continental hotspot track. Although the track and its associated age progression between 160 Ma and 60 Ma are not yet well understood, this section of the track is closely linked to the tectonic evolution of Amerasian Basin, the Alpha-Mendeleev Ridge and Baffin Bay. Using paleomagnetic data, volcanic structures and marine geophysical data, the paleogeography of Arctic plates (Eurasian plate, North American Plate, Greenland Plate and Alaska Microplate) was reconstructed and the Siberian–Icelandic hotspot track was interlinked between 160 Ma and 60 Ma. Our results suggested that the Alpha-Mendeleev Ridge could be a part of the hotspot track that formed between 160 Ma and 120 Ma. During this period, the hotspot controlled the tectonic evolution of Baffin Bay and the distribution of mafic rock in Greenland. Throughout the Mesozoic Era, the aforementioned Arctic plates experienced clockwise rotation and migrated northeast towards the North Pacific. The vertical influence from the ancient Icelandic mantle plume broke this balance, slowing down some plates and resulting in the opening of several ocean basins. This process controlled the tectonic evolution of the Arctic.     

The Arctida Craton and Neoproterozoic-Mesozoic orogenic belts of the Circum-Polar region

An attempt is made to characterize an assembly of Arctic tectonic units formed before the opening of the Arctic Ocean. This assembly comprises the epi-Grenville Arctida Craton (a fragment of Rodinia) and the marginal parts of the Precambrian Laurentia, Baltica, and Siberian cratons. The cratons are amalgamated by orogenic belts (trails of formerly closed oceans). These are the Late Neoproterozoic belts (Baikalides), Middle Paleozoic belts (Caledonides), Permo-Triassic belts (Hercynides), and Early Cretaceous belts (Late Kimmerides). Arctida encompasses an area from the Svalbard Archipelago in the west to North Alaska in the east. The Svalbard, Barents, Kara, and other cratons are often considered independent Precambrian minicratons, but actually they are constituents of Arctida subsequently broken down into several blocks. The Neoproterozoic orogenic belt extends as a discontinuous tract from the Barents-Ural-Novaya Zemlya region via the Taimyr Peninsula and shelf of the East Siberian Sea to North Alaska as an arcuate framework of Arctida, which separates it from the Baltica and Siberian cratons. The Caledonian orogenic belt consisting of the Scandian and Ellesmerian segments frames Arctida on the opposite side, separating it from the Laurentian Craton. The opposite position of the Baikalian and Caledonian orogenic belts in the Arctida framework makes it possible to judge about the time when the boundaries of this craton formed as a result of its detachment from Rodinia. The Hercynian orogenic belt in the Arctic Region comprises the Novozemel’sky (Novaya Zemlya) and Taimyr segments, which initially were an ending of the Ural Hercynides subsequenly separated by a strike-slip fault. The Mid-Cretaceous (Late Kimmerian) orogenic belt as an offset of Pacific is divergent. It was formed under the effect of the opened Canada Basin and accretion and collision at the Pacific margins. The undertaken typification of pre-Late Mesozoic tectonic units, for the time being debatable in some aspects, allows reconstruction of the oceanic basins that predated the formation of the Arctic Ocean.     

Mendeleev rise (arctic ocean) as a geological continuation of the continental margin of Eastern Siberia

The implemented deep seismic soundings have provided records of refracted and reflected P-waves up to offsets of 200–250 km, whereas the modern technique of ray-tracing and synthetic modelling enabled the wave fields to be decoded and the direct seismic problem about selection of velocity models of the crust to be solved correctly. The data acquired through the multichannel seismic reflection method have allowed us to identify the Late Paleozoic sedimentary unit on the shelf and trace it to the Mendeleev Rise as an intermediate complex. It has been shown that the principal structural elements of the consolidated crust and sedimentary cover of the East Siberian shelf are continued to the Mendeleev Rise, which has obvious tectonic features of an extended continental crust.     

Late Mesozoic and Cenozoic volcanosedimentary complexes from the submarine Vityaz Ridge,the Island Arc slope of the Kuril-Kamchatka trench,and its evolution

This paper describes the volcanosedimentary complexes of different ages (Late Cretaceous-Early Paleocene, Paleocene-Eocene (?), Oligocene-Early Miocene, and Pliocene-Pleistocene) that compose the basement and sedimentary cover of the submarine Vityaz Ridge. It was found that the Upper Cretaceous sedimentary rocks from the basement of the Vityaz Ridge (felsic) and the Lesser Kuril Ridge (mafic) have different compositions. Matrix mineral assemblages corresponding to the smectite and corrensite stages of epigenesis of Cenozoic rocks were distinguished, and a scheme of the Late Cretaceous-Pleistocene geological evolution of the region was proposed.     

Geophysical transects of the Labrador Sea: Labrador to southwest Greenland

In 1977 the Federal Institute for Geosciences and Natural Resources, Hannover, carried out a large scale multichannel reflection seismic survey in the Labrador Sea. This survey provided an opportunity for the direct comparison of the geologic structure of the Labrador and Greenland margins. The seismic records across the Labrador Shelf show a thick, prograding sedimentary wedge consisting of several seismic sequences onlapping an acoustic basement that dips steeply seaward. The surface of the acoustic basement is irregular below the continental slope, indicating Late Cretaceous—Early Tertiary faulting. The thick sedimentary section below the slope is divided by an unconformity, tentatively identified as Late Tertiary in age, into two seismic megasequencies which can be subdivided. The acoustic basement on the Greenland side is also strongly faulted but is overlain, in the south, by a thin sedimentary section. The sediment cover thickens on the Greenland Shelf to the north as the shelf becomes wider.As with more southerly parts of the western Atlantic margin, a positive free-air anomaly (30–50 mgal) lies landward of the shelf break off Labrador and a smaller negative anomaly follows the base of the slope. Similar, but generally narrower features are observed along the Greenland margin. West of the negative anomaly off the Greenland slope a narrow band of lower amplitude positive anomalies tends to be associated with an acoustic basement high observed in the reflection profiles. A landward negative gradient in the simple Airy isostatic anomaly across this margin suggests that the ocean—continent boundary is related to this high.Detailed magnetic measurements across the northern Labrador margin show that well-developed oceanic anomalies trending north-northwest lie east of the large Labrador Shelf gravity high, beyond the 2000 m isobath. Landward of these magnetic anomalies is a quiet magnetic zone within which the linear gravity high is parallel to the shelf break and correlates with a deep, sediment-filled basin. It is inferred that oceanic-type crust or greatly-attenuated continental crust underlies this basin and that continental crust thickens markedly westward of the gravity high over a distance of about 50 km.     

塔里木盆地早古生代构造古地理演化与烃源岩

塔里木陆块为一具前寒武系基底的克拉通盆地,早震旦世—寒武纪陆块内和边缘发生裂解,至中奥陶世转为被动大陆边缘,组建塔北和塔中两个遥相对应的碳酸盐台地和边缘斜坡,其间的阿瓦提—满加尔地区为克拉通内浅海—深水盆。满参1井以东至满加尔为欠补偿的深海槽盆,早期沉积了富生物营养链的烃源岩,晚奥陶世克拉通转为前陆碎屑岩沉积,满加尔坳陷反转为浊流盆地。碎屑岩由东向西、由南东向北西迁移,造成向塔北和塔中海侵上超,结束碳酸盐台地演化的同时,沉积了局限台地型和台缘斜坡灰泥丘相的烃源岩。奥陶纪时塔里木盆地演化和沉积相的配置,是加里东期盆山转换的重要反响,形成多个沉积-构造转换面。早加里东运动,造成下早奥陶统与寒武系的假整合;中加里东运动即晚奥陶世始,塔里木转为前陆盆地,塔北和塔中分别为前陆隆起,阿瓦提—满加尔为复合隆间盆地;晚加里东运动(始于早志留世)发生了大规模的海退。     

扬马延海脊构造-沉积演化及油气潜力

扬马延海脊位于北大西洋的北极圈附近,东格陵兰板块和挪威板块之间,冰岛东北方向。北极地区地域辽阔,油气资源丰富,但是恶劣的环境一直制约油气的勘探进展。在扬马延海脊的沉积演化过程中,扬马延海脊在第三纪前有着和东格陵兰陆架、挪威陆架相似的沉积序列,其构造演化经历了二叠纪陆内裂谷、三叠纪—侏罗纪同裂谷和微陆块漂移、白垩纪至今热沉降和被动陆缘等3个阶段。结合前人研究成果,对搜集的东格陵兰陆架、挪威陆架的油气地质资料分析,认为扬马延海脊可划分为扬马延盆地、扬马延西部构造带、扬马延中部凸起带、扬马延海槽、扬马延东部斜坡、扬马延南部复杂构造带6个构造单元,在其上发育着2套油气系统。同时扬马延海脊发育有伸展构造圈闭、地垒断块圈闭、构造圈闭和地层圈闭,这些圈闭为油气的赋存提供了良好的环境,也有利于划分有利油气勘探区带。研究结果可为进一步分析扬马延海脊构造特征等方面提供基础信息,同时对我国参与研究开发北极油气资源具有重大意义。     

Age of the Shirshov submarine ridge basement (Bering Sea) based on the results of investigation of zircons using the U-Pb SHRIMP method

The Shirshov Ridge holds an important position in the structure of the Bering Sea Basin. Stretching from north to south for over 500 km, it divides the Bering Sea into two deep water basins, the Aleutian and Komandorsky basins. The age of oceanic crust of the Aleutian basin based on linear magnetic anomalies is conventionally considered Early Cretaceous, of the Komandorsky basin — Miocene, according to the K-Ar dating of the basalts (9.8 Ma) exposed in 191 deep water drill borehole. Rocks belonging to the basement of the Shirshov Ridge were dredged during the 29th cruise of research vessel Dmitrii Mendeleev and are represented by amphibolitic gabbro whose composition is similar to that of gabbroids of mid-oceanic ridges. The age of metamorphism based on the results of K-Ar dating of amphibole is 47 ± 5 Ma. The U-Pb zircon dating method was used to determine the age of gabbro. Zircons were extracted from a ∼5 kg combined amphibolitic gabbroid sample, and the age of zircons was determined using a SHRIMP-II sensitive high resolution secondary ion microprobe (Center of Isotopic Studies, A.P. Karpinskii Russian Geological Research Institute, St. Petersburg). The average concordant age value for the 25 determinations performed based on 20 points for 18 grains is 72 ± 1.4 Ma (Late Campanian). For 5 grains, the measured age values are within the range of 88 ± 3.5 Ma to 126.5 ± 4.5 Ma. Given the western vergence of the thrust structure of the Shirshov Ridge, the acoustic basement of the Shirshov Ridge is most probably a complexly deformed oceanic crust of the Aleutian Trench, which most likely dates from the Early to Late Cretaceous.     

Tectonics and petroleum potential of the East Arctic province

Tectonics and petroleum potential of the underexplored East Arctic area have been investigated as part of an IPY (International Polar Year) project. The present-day scenery of the area began forming with opening of the Amerasia Ocean (Canada and Podvodnikov—Makarov Basins) in the Late Jurassic—Early Cretaceous and with Cretaceous—Cenozoic rifting related to spreading in the Eurasia Basin. The opening of oceans produced pull-apart and rift basins along continental slopes and shelves of the present-day Arctic fringing seas, which lie on a basement consisting of fragments of the Hyperborean craton and Early Paleozoic to Middle Cretaceous orogens. By analogy with basins of the Arctic and Atlantic passive margins, the Cretaceous—Cenozoic shelf and continental slope basins may be expected to have high petroleum potential, with oil and gas accumulations in their sediments and basement.     

Crustal growth stages in the Songino block of the Early Caledonian superterrane in Central Asia: I. Geological and geochronological data

The Early Caledonian folded area in Central Asia (Early Caledonian superterrane) hosts micro-continent fragments with an Early and Late Precambrian crystalline basement, the largest of them being the Dzabkhan and Tuva-Mongolian fragments. Their junction zone hosts exposures of crystalline rocks that were previously thought to be part of the Early Precambrian Dzabkhan microcontinent. The Bayannur zone in the southern part of the Songino block hosts the Baynnur gneiss-migmatite and Kholbonur metavolcanic-terrigenous metamorphic complexes. The former is believed to be the Early Proterozoic crystalline basement, and the latter is thought to unconformably overly the Late Riphean cover complex of the Songino block. Various rocks of the tectono-stratigraphic complexes in the Bayannur zone were studied geologically and geochronologically (by the U-Pb technique of zircon). Regional metamorphism and folding in the Bayannur Complex were dated at 802 ± 6 Ma. The Nd model ages lie within the range of 1.5–2.0 Ga and thus preclude the correlation of these rocks with those in the Archean and Early Proterozoic basement of the Dzabkhan microcontinent. The upper age limit for folding and metamorphism in the Bayannur zone is marked by postkinematic granites dated at 790 ± 3 Ma, and the lower limit of the volcano-sedimentary complex is determined by the Nd model age of the sandstone (1.3 Ga). The upper age limit of the volcano-plutonic rocks in this zone is set by the gabbroids and anorthosites: 783 ± 2 and 784 ± 3 Ma, respectively. The complex of island-arc granitoids in the Bayannur zone is dated at 859 ± 3 Ma. The age constraints make it possible to correlate crystalline rocks in the Bayannur Complex of the Sangino block and the Dzhargalant Complex in the Tarbagatai block. Currently available data testify that the Precambrian Khangai group of blocks in the Early Caledonian Central Asian superterrane includes continental crustal blocks related to the processes of Early Precambrian, Late Riphean, and Vendian tectonism.     

Continental ridges in the Arctic Ocean: Lorex constraints

Recent multidisciplinary geophysical measurements over the Lomonosov Ridge close to the North Pole support the widely held belief that it was formerly part of Eurasia. The known lithologies, ages, P-wave velocity structure and thickness of the crust along the outer Barents and Kara continental shelves are similar to permitted or measured values of these parameters newly acquired over the Lomonosov Ridge. Seismic, gravity and magnetic data in particular show that the ridge basement is most likely formed of early Mesozoic or older sedimentary or low-grade metasedimentary rocks over a crystalline core that is intermediate to basic in composition. Short-wavelength magnetic anomaly highs along the upper ridge flanks and crest may denote the presence of shallow igneous rocks. Because of the uncertain component of ice-rafted material, seafloor sediments recovered from the ridge by shallow sampling techniques cannot be clearly related to ridge basement lithology without further detailed analysis. The ridge is cut at the surface and at depth by normal faults that appear related to the development of the Makarov Basin. This and other data are consistent with the idea that the Makarov Basin was formed by continental stretching rather than simple seafloor spreading. Hence the flanking Alpha and Lomonosov ridges may originally have been part of the same continental block. It is suggested that in Late Cretaceous time this block was sheared from Eurasia along a trans-Arctic left-lateral offset that may have been associated with the opening of Baffin Bay. The continental block was later separated from Eurasia when the North Altantic rift extended into the Arctic region in the Early Tertiary. The data suggest that the Makarov Basin did not form before the onset of rifting in the Artic.     

Composition and characteristics of the ferromanganese crusts from the western Arctic Ocean

Layered ferromanganese crusts collected by dredge from a water depth range of 2770 to 2200 m on Mendeleev Ridge, Arctic Ocean, were analyzed for mineralogical and chemical compositions and dated using the excess ²³⁰Th technique. Comparison with crusts from other oceans reveals that Fe-Mn deposits of Mendeleev Ridge have the highest Fe/Mn ratios, are depleted in Mn, Co, and Ni, and enriched in Si and Al as well as some minor elements, Li, Th, Sc, As and V. However, the upper layer of the crusts shows Mn, Co, and Ni contents comparable to crusts from the Atlantic and Indian Oceans. Growth rates vary from 3.03 to 3.97 mm/Myr measured on the uppermost 2 mm. Mn and Fe oxyhydroxides (vernadite, ferroxyhyte, birnessite, todorokite and goethite) and nonmetalliferous detrital minerals characterize the Arctic crusts. Temporal changes in crust composition reflect changes in the depositional environment. Crust formation was dominated by three main processes: precipitation of Fe-Mn oxyhydroxides from ambient ocean water, sorption of metals by those Fe and Mn phases, and fluctuating but large inputs of terrigenous debris.     

U---Pb, Sr and Nd evidence for grenvillian and latest proterozoic tectonothermal activity in the spitsbergen caledonides, arctic ocean

Within the Caledonian complexes of northwestern Spitsbergen, high PT formations provide U---Pb zircon ages of 965±1 Ma of a metagranite and 955±1 Ma of a corona gabbro, indicating the influence of Grenvillian activity in the area. Various isotopic systems suggest that these rocks were partially derived by reworking of ancient crust (as old as Archaean). Eclogites and felsic agmatite indicate latest Proterozoic magmatic or metamorphic events (625₋₅⁺² and 661±2 Ma, respectively) by U---Pb zircon dating. The eclogitic metamorphism age is not fully constrained and ranges between 540 and 620 Ma; this occurred prior to the superimposed Caledonian metamorphism, indicated by a part of the K---Ar and Rb---Sr mineral cooling ages. The new data and other evidence of Precambrian tectonothermal activity on Svalbard suggest that the Early Palaeozoic and Late Proterozoic successions exposed elsewhere on Svalbard may also be underlain by Grenvillian or older basement rocks. Relationships to other Grenvillian and older terrains in the Arctic are reviewed.