Phanerozoic Evolution Research Articles

Imaging the basement architecture across the Cork Fault in Queensland using magnetic and gravity data

The basement rocks in central Queensland are largely obscured by the Phanerozoic sedimentary succession and direct information comes only from sparse geological data. 2.5D forward modelling of regional aeromagnetic and Bouguer gravity data has been undertaken to unveil the crustal architecture at the junction of the Proterozoic Mount Isa terrane and the Phanerozoic Thomson Orogen in Queensland. The most prominent geophysical character is represented by the abrupt termination of positive NNW-trending geophysical anomalies of the Mount Isa terrane against NE-trending, low amplitude gravity and magnetic anomalies of the Diamantina River Domain of the Thomson Orogen. Potential field forward modelling indicates that the lower basement crust of the Thomson Orogen is petrophysically indistinguishable from the crust of the Proterozoic Mount Isa terrane. We interpret that the prominent gradients associated with the Cork Fault represent the displacement and burial of the Mount Isa terrane crust under the Thomson Orogen. In this context, the Cork Fault is inferred to represent a fundamental crustal discontinuity but does not represent the eastern margin of Rodinia because the Thomson Orogen is floored by Precambrian continental crust.Initiation of the Cork Fault might have occurred as early as the Mesoproterozoic. At that time, the Cork Fault was a major structure that developed during the separation of the Mount Isa terrane and the Curnamona Province. It is inferred that this break-up occurred during initial N-S to NNW-SSE extension. The Cork Fault formed part of a network of major north- and south-dipping normal faults active at that time. The Cork Fault was repeatedly reactivated during the Neoproterozoic Rodinia break-up and during the Phanerozoic evolution of the Tasmanides. We suggest that the segment of the inferred ‘Tasman Line’ along the southern termination of the Mount Isa terrane should be regarded as a set of structures offsetting the Proterozoic crust rather than a simple lineament dividing the Proterozoic and the Phanerozoic Australian continent.

Transcurrent nature of the Teisseyre–Tornquist Zone in Central Europe: results of the POLCRUST-01 deep reflection seismic profile

Teisseyre–Tornquist Zone (TTZ) corresponds to a crustal boundary between the Precambrian East European Platform (EEP) and the Palaeozoic West European Platform. Although the zone has been controlling Phanerozoic evolution of large parts of Central Europe, its course, geometry and origin are still poorly constrained. Deep reflection seismic profile POLCRUST-01, recently acquired in SE Poland, for the first time allowed a precise comparison of the Ediacaran and later tectonic patterns to the deep crustal features of the TTZ and adjacent areas. The TTZ corresponds to the subvertical Tomaszow Fault separating the Radom–Kraśnik Elevation, composed of the typical EEP crust, from the Bilgoraj–Narol Block (BNB) in the SW, with a thinned crystalline basement showing affinities to the EEP crust. The BNB is a part of the larger Caledonian Łysogory Terrane as evidenced by its Lower Palaeozoic stratigraphy and gravity data. Thus, for the first time, the proximal Baltican affinity of this unit has been documented unambiguously. The Łysogory Terrane is delimited from the SW by the subvertical Cieszanow Fault Zone, corresponding to the Holy Cross Suture. The adjacent Malopolska Terrane is characterized by a distinct Early Palaeozoic stratigraphy, and lower-middle crust exhibiting SW-dipping reflective packages interpreted as NE-verging thrust and shear zones of a Neoproterozoic orogen. The observations from the POLCRUST-01 profile and regional comparisons indicate that the TTZ is a major Caledonian transcurrent zone between Poland and East Romania. In central Poland, the TTZ likely forms a narrow subvertical contact between the EEP and a proximal Kuiavia Terrane, as constrained by the deep refraction seismic data. To the NW, the zone extends towards the Pomeranian part of the Caledonide fold-and-thrust belt related to the Avalonia–Baltica collision zone (Thor Suture). South-east of Poland the TTZ corresponds to the Rava Ruska Fault Zone established as a Caledonian suture separating adjacent terrane, probably of a Baltican affinity. The East Romanian part of the TTZ conforms with the Sfântu Gheorghe Fault separating reworked EEP crust of the Pre-Dobrogean Depression from the North Dobrogea unit bearing a strong Variscan and Cimmerian overprint.

K–Ar illite dating to constrain multiple events in shallow crustal rocks: Implications for the Late Phanerozoic evolution of NE Asia

A recently developed illite-age-analysis (IAA) approach was applied to determine the multiple events for the Chugaryeong fault belt, Korea. Each event was determined by a combined approach of the optimized illite-polytype quantification and the K–Ar age-dating of clay fractions separated from the fault clays. The Late Cretaceous to Paleogene events (76.5±0.8, 69.1±0.6, 59.3±0.7, and 48.2±0.7Ma) were recognized by calculating the authigenic 1M/1Md illite ages on the IAA plots of the fault clays. The Early Cretaceous ages (121.7–124.7 and 112.4±1.5Ma) were also obtained from the convergent intercepts of 100% 2M1 illite on the IAA plots. The absence of the 2M1 illites in the host-rock indicates that the Early Cretaceous ages represent the timings of high-T hydrothermal events of >280°C. The 2M1 illites in the fault clays should be pre-formed by a fluid-rock interaction under a relatively high-T subsurface condition, and be mechanically reworked into the near surface along the fault by post-tectonic events. This is the first report determining the absolute age constraints of multi-activated tectonic events from a fault. These geochronological determinations of the multiple events recorded in the Chugaryeong fault belt are crucial to establish the tectonic evolution of the Korean Peninsula since the Late Cretaceous.

Tectonic control of continental weathering, atmospheric CO2, and climate over Phanerozoic times

We explore the role of continental drift on the Phanerozoic evolution of the carbon cycle and climate with the GEOCLIM numerical model. GEOCLIM couples a 3D-climate model, a global dynamic vegetation model and a mathematical description of the chemical weathering of the continental surfaces. Twenty-one timeslices covering the whole Phanerozoic are simulated, allowing the calculation of the climate and steady-state atmospheric CO2 for each of these timeslices. We show that continental drift is a first order control parameter of the geological carbon cycle and climate. The salient features are the following: (1) the Paleozoic environment is characterized by a long term CO2 decrease; (2) the Late Permian-Late Triassic period displays very high CO2 levels; (3) atmospheric CO2 is decreasing since the Middle Cretaceous. All these features are driven by the drift of the continents and the subsequent modulation of the water cycle and weathering rates. Furthermore, continental drift imprints the long term evolution of the tropical seawater surface temperatures with periodical oscillations characterized by a frequency of about 120 Myr, in agreement with available data. Finally, we emphasize a discrepancy existing between the climate reconstructions from the most recent apatite δ18O data and the CO2 inferred from proxies. Reconstructed CO2 levels from proxies are often unrealistically low for the distant geological past.

Destruction of the North China Craton: Delamination or thermal/chemical erosion? Mineral chemistry and oxygen isotope insights from websterite xenoliths

The destruction (or reactivation) of the North China Craton (NCC) is one of the important issues related to the Phanerozoic evolution of eastern China, although the processes of destruction remain debated. Two main mechanisms – delamination and thermal/chemical erosion – have been proposed based on the geochemistry of Mesozoic–Cenozoic basalts and entrained deep–seated xenoliths. A key criterion in distinguishing between these mechanisms is the nature of the melt, derived from delaminated crust or the asthenosphere, that modified the lithospheric mantle. Here we investigate the mechanism of destruction of the NCC based on mineral compositions and oxygen isotopic data from olivines, and strontium isotopic data for clinopyroxenes within websterite xenoliths from the Early Cretaceous Feixian basalts in the eastern NCC. Olivines in websterite xenoliths have higher Mg# (86–86.4), Ni content (2187–2468ppm), and lower Ca (983–1134ppm), Ti (58.1–76.1ppm), and Mn (1478–1639ppm) contents than olivine phenocrysts (Mg#=71.0–77.3, Ni=233–1038ppm, Ca=1286–2857ppm, Ti=120–300ppm, and Mn=2092–4106ppm) from Late Cretaceous basalts. Additionally, olivines in websterite xenolith have δ18O values of 7.10±0.21‰ to 8.40±0.21‰, evidently higher than those of typical mantle-derived olivines. Similarly, orthopyroxenes (Opx) and clinopyroxenes (Cpx) in the websterite xenoliths have much higher Mg# (86.3–89.2 and 87.5–90.3, respectively), and Ni contents (1097–1491ppm and 581–809ppm, respectively) than orthopyroxene- and clinopyroxene-phenocrysts (Opx: Mg#=82.2–83.9, Ni=730–798ppm; Cpx: Mg#=74.2–84.6, Ni=117–277ppm) from Late Cretaceous basalts. The 87Sr/86Sr ratios of clinopyroxenes in the websterite xenoliths range from 0.70862 to 0.70979, and are much higher than those of clinopyroxene grains from peridotite xenoliths and basalts. These data indicate that olivines are the residue of ancient lithospheric mantle that was modified intensively by a melt derived from recycled continental crust, and that the silicic and calcic metasomatic melt might have been derived from the partial melting of the subducted Yangtze slab and delaminated lower crust of the NCC. The existence of recycled continental crust in the Mesozoic lithospheric mantle implies that delamination was an important mechanism of destruction of the NCC.

Episodic widespread magma underplating beneath the North China Craton in the Phanerozoic: Implications for craton destruction

A comprehensive synthesis of U–Pb geochronology and Hf isotopes of zircons from granulite/pyroxenite xenoliths entrained in Phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the North China Craton (NCC) provides new insights into understanding the Phanerozoic evolution of the lower crust in this craton. Episodic widespread magma underplating into the ancient lower crust during Phanerozoic has been identified throughout the NCC from early Paleozoic to Cenozoic, broadly corresponding to the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies on the circum-craton mobile belts. The early Paleozoic (410–490Ma) ages come from xenoliths in the northern and southern margins as well as the central domain of the Eastern Block of the craton which mark the first phase of Phanerozoic magma underplating since the final cratonization of the NCC in the Paleoproterozoic. The magmatism coincided with the northward subduction of the Paleotethysian Ocean in the south and the southward subduction of the Paleoasian Ocean in the north. The subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton, marking the initiation of decratonization. The late Paleozoic event as represented by the 315Ma garnet pyroxenite and/or lherzolite xenoliths in Hannuoba was restricted to the northern and southern margins of the craton, correlating with the arc magmatism continuous associated with the subduction of the Paleotethysian and Paleoasian Oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle/lower crust. The early Mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the Paleotethysian and Paleoasian Oceans as well as the collisional orogeny between the NCC and the Yangtze Craton. The late Mesozoic (ca. 120Ma) was a major and widespread magmatic event which manifested throughout the NCC, associated with the geothermal overturn due to the giant south Pacific mantle plume. The Cenozoic magmatism, identified only in the dark clinopyroxenite xenoliths in the Hannuoba, was probably induced by the Himalayan movement in eastern Asia and might also have been influenced by the subduction of the Pacific Ocean to some extent. These widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the NCC revealed by U–Pb and Hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust, with a mixing of the old crust with melts. The process inevitably resulted in the compositional modification of the ancient lower crust, similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite — infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton.

Phanerozoic changes in the high-rank suprageneric diversity structure of brachiopods: Linear and non-linear effects

Phanerozoic evolution of brachiopods produced many linear (established by a comparison of successive geologic time units) and non-linear (established by a comparison of non-successive geologic time units) effects, which can be examined quantitatively by using the similarity coefficients (Czekanowski's Quantified Coefficient and Gower Index) and correlation tools. The high-rank suprageneric diversity structure accounts for a number of superfamilies in each of 26 orders for every epoch of geological time. The intensity of turnovers in this structure was generally low during the entire Phanerozoic. It was slightly stronger during the Early Paleozoic, but close to zero during the Cenozoic, when the high-rank suprageneric diversity structure of brachiopods stabilized finally. Significant turnovers took place at the Middle Cambrian–Early Ordovician, the Late Ordovician–Early Silurian, the Late Silurian–Early Devonian, the Middle Devonian–Mississippian, and the Permian–Triassic transitions. Influences of mass extinctions, both major like those End Ordovician or Permian/Triassic and minor like Early Jurassic or Jurassic/Cretaceous, on the high-rank suprageneric diversity structure of brachiopods is registered. The strongest was the consequences of the Permian/Triassic catastrophe, which perhaps even reset the brachiopod evolution. No evident direct relationships are established between intensity of turnovers and eustatic fluctuations. However, the changes in the diversity structure recorded with the Gower Index provide evidence that eustatic lowstands were more favorable for intensification in these changes.

Prelithified deformation of the Jurassic Bansong Group along the Gongsuwon Thrust, South Korea

The Gongsuwon Thrust in the Taebaeksan Basin is one of the important regional structures for the Phanerozoic tectonic evolution of the Korean Peninsula. The late Early Jurassic Bansong Group (Daedong Supergroup) is structurally overlain by the early Paleozoic Joseon Supergroup along the thrust in the Yeongwol area. We present four lines of evidence indicating that the siliciclastics of the Bansong Group suffered a prelithified deformation within the fault zone of the Gongsuwon Thrust; (1) strainfree quartz clasts both in the hinge and limb areas of isoclinal folds of the sandstone layer, indicating granular flow, (2) intact gravels in the conglomerate with pressure shadows filled by sandstone or shale matrix, (3) injection of mud into sandy layers, and (4) lensoidal marble blocks (Joseon Supergroup) with plastically deformed calcite grains within siliciclastics of the Bansong Group. We suggest that the deposition of the Bansong Group and the thrusting were coeval during the middle Mesozoic Daebo tectonic event.

Petrogenetic significance of high Fe/Mn ratios of the Cenozoic basalts from eastern China

2Chengdu Institute of Geology & Mineral Resources, Chengdu 610082, China The Cenozoic basalts from eastern China show commonly high Fe/Mn ratios (average = 68.6 ± 11.5) coupled with OIB-type trace element signature. The Cenozoic basalts form the northern margin and the southern margin of the North China Craton are studied in detail. Model calculations point out that the coupling feature of high Fe/Mn ratio with OIB-type trace element signature of these basalts cannot be produced by neither pyroxene/olivine crystallization nor remelting of previously melted mantle, but require partial melting of a garnet pyroxenite-rich mantle source. Combining these features of the Cenozoic basalts with the Phanerozoic lithospheric evolution of the eastern China, we suggest that the Cenozoic basalts were derived from a garnet pyroxenite-rich mantle source associated with continental crust delamination or oceanic crust subduction.

A multi-disciplinary study of Phanerozoic landscape development in West Greenland

The western margin of the Greenland craton has been much less stable in the Phanerozoic than previously thought. This new insight has come from close integration of independent datasets: geomorphological analysis of large-scale landscapes, apatite fission track analysis (AFTA), onshore and offshore stratigraphy and analysis of onshore fault and fracture systems. Each data set records specific and unique parts of the event chronology and is equally important to establish a consistent model. A key area for understanding the Mesozoic– Cenozoic landscape evolution and into the present is the uplifted part of the Nuussuaq Basin, where remnants of planation surfaces cut across the Cretaceous to Eocene sedimentary and volcanic rocks. Our integrated analysis concluded that the West Greenland mountains were formed by late Neogene tectonic uplift (Fig. 1) and also provided new insight into early Phanerozoic development. To understand our model, we present the different methods and the results that can be deduced from them.

Devonian events and correlations—a tribute to the lifetime achievements of Michael Robert House (1930–2002)

The Devonian was a peculiar time in the Phanerozoic evolution of the Earth. Most continents, including the large Gondwana and Laurussia cratons, formed a Pangea-type assembly around the tropical Prototethys and an increasingly hot, global, greenhouse climate prevailed, with a complete lack of major ice sheets, even in polar areas. There was gradual and increasing flooding of the continents, creating huge epicontinental seas that have no modern analogues. Under these conditions the plants finally conquered the land, with the innovation of deep roots in the Emsian, the appearance of seed precursors and trees in the Givetian, and the spread of vegetation into dry uplands in the late Famennian. In the marine realm, the largestknown Phanerozoic tropical reef belts surrounded craton margins and tropical islands. It was the time of the sudden radiation of early ammonoids, of the earliest episodic blooms of calcareousshelled, pelagic zooplankton (tentaculitoids), the rise to dominance of fishes, mostly of armoured forms and with giants reaching 10 m in length, but also including the first sharks, and the appearance of earliest tetrapods in marginal settings. However, the tropical and subtropical areas reaching up to 458 latitude were hardly a paradise. A combination of climatic, plate tectonic/magmatic and still poorly understood palaeoceanographic factors caused the recurrent sudden perturbation of stable ecological conditions by short-term global events of variable magnitude (e.g. House 1985), including two of the biggest mass extinctions that the Earth’s biosphere has experienced—the Upper Kellwasser Event at the Frasnian–Famennian boundary and the Hangenberg Events at the close of the Devonian. The global and regional correlation of Devonian rocks relies strongly on the available fossil record, on the facies distribution of marker fossils, on the recognition of eustatic sea-level change in sequence stratigraphy, and, as a relatively new development, on the application of stable isotope geochemistry and magnetic susceptibility. The Prototethys seaway, fortunately, allowed a free exchange of pelagic faunas through all oceans and seas. In the warm-water areas, there was only moderate endemism in conodonts, ammonoids, pelagic ostracodes and tentaculitoids, which are the main biostratigraphic marker groups. Low diversity benthic assemblages of the outer shelf also include many cosmopolitan groups but they differ strongly from fossil associations of the nearshore, neritic and photic zones. In the latter areas, including reef and biostrome complexes, the evolution and ecologically controlled distribution of brachiopods, stromatoporoids, rugose and tabulate corals, and trilobites provides refined regional time frameworks but the faunas are mostly characterized by endemism that prevents simple correlation across oceans and seaways. Palynomorphs that were washed into the seas enable the correlation of marine, fluvial, limnic and terrestrial deposits. The highly variable palaeoecology and taxonomic composition of the terrestrial, neritic and pelagic faunal assemblages are the fundamental problems of cross-facies correlations within the Devonian. Apart from overlapping facies and palaeobiogeographic ranges of some key taxa, event and physical stratigraphy are the major tools in reconstructing global changes in the Devonian world at high level of time resolution. The International Subcommission on Devonian Stratigraphy (SDS) was the first International Union of Geological Sciences (IUGS) Subcommission to complete the formal designation of all its series and stage subdivisions, but all ratified GSSPs (Global Stratotype Sections and Points), as the norm, were defined in the pelagic facies realm. Their recognition on all continents and in all other facies belts is still a major task to be resolved. As a first step to reach some progress in this wide, international and multidisciplinary scientific field, the Devonian Subcommission and the Institut Scientifique University Mohammed V, Rabat, especially with a major organizational input by its Titular Member and Vice-Chairman, Prof. Ahmed El Hassani, organized in March 2004 an international symposium (El Hassani 2004a) on ‘Devonian neritic–pelagic correlation and events’, followed by an excursion with the same topic to the continually inaccessible Dra Valley of SW Morocco (El Hassani 2004b).

Phanerozoic evolution of the Inner Mongolia–Daxinganling orogenic belt in North China: constraints from geochronology of ophiolites and associated formations

Abstract The Inner Mongolia–Daxinganling Orogenic Belt (IMDOB), located between the North China and South Mongolia Blocks, consists of several ENE–WSW- to NE–SW-trending zones including dismembered ophiolite blocks, metamorphic rocks and granitoids. Although numerous studies have been carried out on this belt, its tectonic evolution has been a subject of controversy, chiefly because of the lack of reliable geochronological data. Based on a synthesis of newly published geochronological data and our unpublished data for the IMDOB, we define two oceanic basins: Ondor Sum and Hegenshan. The former, probably the main one, was initiated during the Ordovician (>467 Ma) period, whereas the latter, representing a back-arc basin, opened on a pre-Permian basement at, or earlier than, Early Permian times ( c . 295 Ma). These two oceanic basins were separated by a magmatic arc (Sunid–Baolidao), and were probably closed simultaneously when the final orogenesis of the IMDOB occurred during the Triassic period (240–220 Ma). Importantly, the Triassic timing of the final orogenesis of the IMDOB due north of the North China Craton is essentially coeval with that of the Qinling–Dabie–Su–Lu orogenic belt on the southern margin of the North China Craton. It is inferred that this two-sided subduction–collision scenario in the Triassic may have contributed to the Mesozoic lithospheric thinning event of the North China Craton, although the details are unclear.

EVENTOS TECTÔNICOS RECORRENTES IMPRESSOS NO ARCABOUÇO ESTRATIGRÁFICO DO GRUPO ITARARÉ NA REGIÃO DE VILA VELHA, ESTADO DO PARANÁ

O presente trabalho avalia o controle tectônico na deposição e deformação do intervalo basal doGrupo Itararé no sudeste do Estado do Paraná e sua influência na anisotropia de reservatórios análogos. Apartir da análise estrutural do arenito Vila Velha, a observação das relações espaciais entre a conformaçãodos estratos e as estruturas descritas e o mapeamento geológico de detalhe, propõe-se um modelo deevolução tectono-sedimentar para a área. O Grupo Itararé foi depositado em contexto glacial, glácio-marinhoe costeiro, tendo sido afetado por vários eventos tectônicos que influenciaram a sedimentação e deformarama seção sedimentar. A descrição do arcabouço estrutural/estratigráfico sugere uma história tectônica complexa,do Carbonífero ao Eocretáceo, com estruturas tectonicamente ativas durante a deposição e a suareativação em eventos tectônicos posteriores. A natureza da sedimentação e a deformação tectônicasuperimposta em múltiplos eventos imprimiram no arenito intensa anisotropia, com diferentes escalas deheterogeneidades estruturais e estratigráficas que limitam unidades de fluxo e controlam a circulação defluidos no meio poroso.

Geotectonic evolution of the Great Basin

The crust of the Great Basin has occupied a range of tectonic settings through geologic time. Archean and Paleoproterozoic crustal genesis preceded residence of Laurentia within the Mesoproterozoic supercontinent Rodinia, which rifted in the late Neoproterozoic to delineate the Cordilleran flank of Laurentia. Successive stages of Phanerozoic evolution included (1) early to middle Paleozoic miogeoclinal sedimentation along a passive continental margin, (2) late Paleozoic to earliest Mesozoic thrusting of oceanic Antler and Sonoma allochthons over the continental margin in response to episodic slab rollback beneath an offshore Klamath-Sierran island-arc complex, (3) Mesozoic to mid-Cenozoic arc-rear and backarc thrusting, together with pulses of interior magmatism, associated with development of the Cordilleran magmatic arc to the west where subduction and arc accretion expanded the continental margin, and (4) middle to late Cenozoic crustal extension, which involved initial intra-arc to backarc deformation and later transtensional torsion of the continental block inland from the evolving San Andreas transform system. Potential metallogenic influences on Great Basin tectonic evolution included transfer of substance from mantle to crust by magmatism and associated metasomatism, and reworking of crustal materials by both magmatism and intracrustal fluid flow, the latter of which was induced both by thermal effects of magmatism and by reconfiguration of fluid-bearing rock masses during multiple episodes of Great Basin deformation.

Leaf architectural profiles of angiosperm floras across the Cretaceous/Tertiary boundary

The Cretaceous/Tertiary boundary extinction has long been considered one of the most important identifiable events in the course of Phanerozoic evolution. At times, the dramatic evidence for this has obscured the fact that any extinction event is selective and may not affect all groups of organisms in the same way. In this paper we examine a North American plant fossil database from the Mesozoic and Cenozoic eras in order to re-evaluate the evolutionary significance of the Cretaceous/Tertiary extinction on plants. When we compare the leaf architectural profiles of fossil floras in each stage of the Cretaceous and epoch of the Cenozoic, we find that the changes in leaf architecture at the Maastrichtian/Paleocene boundary cannot be statistically distinguished from the population of changes at other boundaries. To the extent that patterns in leaf architecture reflect ecosystem structure, we can therefore conclude that despite the local species or morphotype extinctions that are known to have taken place at the boundary, the effect of the extinction on the structure of plant ecosystems was either minor or short-lived. Certainly, the extinction seems insignificant compared with the dramatic changes in leaf architecture that accompanied the rise of angiosperms in the middle Cretaceous. This analysis also provides an example of the importance of time scales in the evaluation of macro-evolutionary pattern, and shows how the use of morphological categories instead of phylogenetic groups or simple diversity measures can produce rich and ecologically informative semi-quantitative proxy measurements of plant evolutionary patterns.

Terminal addition, the Cambrian radiation and the Phanerozoic evolution of bilaterian form

We examine terminal addition, the process of addition of serial elements in a posterior subterminal growth zone during animal development, across modern taxa and fossil material. We argue that terminal addition was the basal condition in Bilateria, and that modification of terminal addition was an important component of the rapid Cambrian evolution of novel bilaterian morphology. We categorize the often-convergent modifications of terminal addition from the presumed ancestral condition. Our focus on terminal addition and its modification highlights trends in the history of animal evolution evident in the fossil record. These trends appear to be the product of departure from the initial terminal addition state, as is evident in evolutionary patterns within-fossil groups such as trilobites, but is also more generally related to shifts in types of morphologic change through the early Phanerozoic. Our argument is contingent on dates of metazoan divergence that are roughly convergent with the first appearance of metazoan fossils in the latest Proterozoic and Cambrian, as well as on an inference of homology of terminal addition across bilaterian Metazoa.

Phanerozoic evolution of plants on the African plate

The Phanerozoic has witnessed major changes, with Africa being an integral part of supercontinental landmass agglomeration forming Gondwana, its amalgamation with smaller landmasses to form Pangaea, and later disintegrating to form the existing continents. At the same time climates, atmosphere, oceanic circulation and tectonic plates shifted. During this upheaval, life was evolving and organisms were adapting and with higher diversity came more ecological interactions, creating more habitats and thus influencing more biological radiation. Plants became more complex, developing from unicellular to multicellular organisms. Moving from water onto land, plants had to cope with desiccation, and features evolved enabling them to do this. Patterns of Phanerozoic plant evolution are strongly driven by major changes in the physical environment, most notably continental drift, climate change and bolide impacts, precipitating massive volcanism and other effects leading to mass global extinctions. The products of this evolution were early land plants during Silurian and Devonian times. Subjected to extinction events and environmental changes, these early plants gave rise to the pteridophytes reaching their diversity peak during the Carboniferous and Permian. After the demise of the pteridophytes, the gymnosperms dominated during the Triassic and Jurassic, followed by the emergence of the angiosperms in the Cretaceous. This melange of factors has produced the present plant diversity on earth, which we will examine in context of the African flora. Plant diversification in Gondwana and Laurasia will be covered focussing on Africa, while factors affecting the vegetation and species composition of the present flora will be discussed.

Detection of subtle basement faults with gravity and magnetic data in the Alberta Basin, Canada

Editor's note: This paper is based on a presentation at the 2003 Canadian Society of Petroleum Geologists/Canadian Society of Exploration Geophysicists Joint Convention in Calgary. Exploration for a wide range of mineral deposits is critically dependent on knowledge of the location and age of fractures and faults. Oil and gas fields in many sedimentary basins are distributed along fault-controlled linear trends, and fault identification is often used effectively for target-area selection in hydrocarbon exploration. Similarly, mineral deposits in various geologic settings are commonly associated with fluid-conducting faults. In the platformal, Phanerozoic Alberta sedimentary basin in western Canada (Figure 1), two fundamentally different types of crystalline-basement structure, formed in different tectonic conditions, are recognized (Lyatsky et al., 1999): Figure 1. Location of the northern Alberta study area in relation to main regional geologic features. 1. Archean and Early Proterozoic (Hudsonian and older) ductile orogenic structures, and 2. Middle Proterozoic to Recent cratonic ones. The influence of ancient ductile structures on the Alberta Basin seems largely confined to the control on Early Paleozoic depositional and drape patterns by the Precambrian erosional basement relief, which may to some extent be related to the basement lithology distribution controlled by ductile ancient structures. These structures were mostly healed and innactive during the Phanerozoic development and evolution of the Alberta Basin, and thus of limited significance to fluid migration. Their geophysical expression typically consists of large magnetic and gravity anomalies, which often obscure the potential-field signatures of the desirable brittle faults. The steep, brittle basement faults in the western Canadian platforms and Cordilleran foreland, although much more subtle than their spectacular counterparts in western United States, exerted considerable syn- and postdepositional influence on the Phanerozoic sedimentary cover. Occasionally, brittle faults and fractures follow the older orogenic basement structures, but commonly cut across them. Even when subresolution seismically, many …

Aplicação do índice "Relação Declividade-Extensão - RDE" na bacia do Rio do Peixe (SP) para detecção de deformações neotectônicas

This paper presents the identification of tectonic deformation through the application of the stream-gradient index (slope x length) method in the Rio do Peixe hydrographic basin, located in western State of Sao Paulo. The main principle used is that river valleys are the best tools for this type of analysis because of their fast adjustment to even the most gentle crustal deformations. This type of analysis for structural studies is extremely useful in the western region of the State of Sao Paulo, where deep chemical weathering results in a thick regolith and scarce outcrops. The stream length index is the ratio of the altimetric amplitude of each drainage and the natural logarithm of its length. The value obtained is plotted on maps at the median point of each drainage, allowing the drawings of lines with the same values (isodefs). Three anomalies (A, B and C) were identified along the Rio do Peixe valley representing uplift sectors indicated by the formation and distribution of Upper Quaternary deposits in terraces and modern fluvial plains and alluvial fans. In correlation with known tectonic data for this region it is possible to attribute the A and B anomalies to the Ribeirao Preto and Presidente Prudente crustal sutures, respectively. The smaller C anomaly needs further field investigation, but it could be associated with the Tres Lagoas crustal suture. It must be emphasised that these sutures are inherited from the Precambrian Basement which had a strong influence on the Phanerozoic evolution in the whole sedimentary and volcanic stratigraphic pile of the Parana Basin and on the dominant structural lineaments directions developed therein .The A, B and C anomalies are suggestive of renewed uplift as a result of neotectonic action in modern times.

Impact of atmospheric CO2 and galactic cosmic radiation on Phanerozoic climate change and the marine δ18O record

A new model is developed and applied to simulate the Phanerozoic evolution of seawater composition (dissolved Ca, Sr, dissolved inorganic carbon, alkalinity, pH, δ18O), marine carbonates (Sr/Ca, 87Sr/86Sr, δ13C, δ18O), atmospheric CO2 and surface temperature. The marine carbonate records (Sr/Ca, 87Sr/86Sr, δ13C) are used to reconstruct changes in volcanic/tectonic activity and organic carbon burial over the Phanerozoic. Seawater pH is calculated assuming saturation with respect to calcite and considering the changing concentration of dissolved Ca documented by brine inclusion data. The depth of calcite saturation is allowed to vary through time and the effects of changing temperature and pressure on the stability constants of the carbonate system are considered. Surface temperatures are calculated using the GEOCARB III approach considering also the changing flux of galactic cosmic radiation (GCR). It is assumed that GCR cools the surface of the Earth via enhanced cloud formation at low altitudes. The δ18O of marine carbonates is calculated considering the changing isotopic composition of seawater, the prevailing surface temperatures and seawater pH. Repeated model runs showed that the trends observed in the marine δ18O record can only be reproduced by the model if GCR is allowed to have a strong effect on surface temperature. The climate evolution predicted by the model is consistent with the geological record. Warm periods (Cambrian, Devonian, Triassic, Cretaceous) are characterized by low GCR levels. Cold periods during the late Carboniferous to early Permian and the late Cenozoic are marked by high GCR fluxes and low pCO2 values. The major glaciations occurring during these periods are the result of carbon cycling processes causing a draw‐down of atmospheric CO2 and a coevally prevailing dense cloud cover at low‐altitudes induced by strong GCR fluxes. The two moderately cool periods during the Ordovician ‐ Silurian and Jurassic ‐ early Cretaceous are characterized by both high pCO2 and GCR levels so that greenhouse warming compensated for the cooling effect of low‐altitude clouds. The very high Jurassic δ18O values observed in the geological record are caused by low pH values in surface waters rather than cold surface conditions.