Cenozoic Examples Research Articles

Growth rate, extinction and survival amongst late Cenozoic bivalves of the North Atlantic

ABSTRACT Late Cenozoic bivalve extinction in the North Atlantic area has been attributed to environmental deterioration. Within scallops and oysters – groups with a high growth rate – certain taxa which grew exceptionally fast became extinct, while others which grew slower survived. Those which grew exceptionally fast would have obtained protection from predators thereby, so their extinction may have been due to the detrimental effect of environmental change on growth rate and ability to avoid predation, rather than environmental change per se. We investigated some glycymeridid and carditid bivalves – groups with a low growth rate – to see whether extinct forms grew faster than extant forms. Extinct Glycymeris subovata grew at about the same rate as the slowest-growing living glycymeridid and much slower than late Cenozoic examples of extant G. americana, which grew at about the same rate as the fastest-growing living glycymeridid. Extinct G. obovata and extinct Cardites squamulosa ampla also grew slower than G. americana. These findings indicate that within bivalve groups with a low growth rate, extinction or survival of taxa through the late Cenozoic was not influenced by whether they were relatively fast or slow growers. By implication, environmental change acted directly to cause extinctions in these groups.

Plastic Silica Conglomerate with an Extremophile Microbial Matrix in a Hot-Water Stream Paleoenvironment.

A new and unusual type of fossil, siliceous hot-spring deposit (sinter)-comprising monomictic, quartzose conglomerate encrusted with silicified microbial laminates-has been recognized in distal portions of Jurassic and Miocene paleo-geothermal fields of South and North America, respectively. The siliceous clasts are inferred to have originated as conduit-delivered hydrothermal silica gel, owing to their general plastic morphologies, which were then locally reworked and redistributed in geothermally influenced stream paleoenvironments. Today, hot-spring-fed streams and creeks, in places with silica-armored pavements, host microbial mats coating streambeds and/or growing over, and silicifying at, stream air-water interfaces, for example, in Yellowstone National Park (USA) and Waimangu Volcanic Valley (New Zealand). However, the modern deposits do not contain the plastically deformed silica cobbles evident in Mesozoic and Cenozoic examples described herein. Moreover, the fossil microbial laminates of this study are relatively dense and strongly coat the silica cobbles, suggesting the mats stabilized the clasts under fully submerged and hot, high-energy conditions. Thus, this new sinter facies, typically found a few kilometers from main spring-vent areas, is a perhaps unexpected extreme environment in which life took hold in hydrothermal-fluvial settings of the past, and may serve as an additional target in the search for fossil biosignatures of early Earth and possibly Mars.

Mantle Flow as a Trigger for Subduction Initiation: A Missing Element of the Wilson Cycle Concept

AbstractThe classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. This process, however, is impeded by the high strength of passive margins, and it has never occurred in Cenozoic times. Here using thermomechanical models, we show that additional forcing, provided by mantle flow, which is induced by neighboring subduction zones and midmantle slab remnants, can convert a passive margin into a subduction zone. Models suggest that this is a long‐term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the U.S. East Coast. Mantle suction force can similarly trigger subduction initiation along large oceanic fracture zones. We propose that new subduction zones will preferentially originate where subduction zones were active in the past, thus explaining the remarkable colocation of subduction zones during at least the last 400 Myr.

Geodynamics of divergent double subduction: 3‐D numerical modeling of a Cenozoic example in the Molucca Sea region, Indonesia

AbstractGeological observations reveal existence of a unique form of plate subduction featuring subduction on both sides of one single oceanic plate, which is termed divergent double subduction (DDS). DDS may play an important role in facilitating tectonic processes like closure of oceanic basins, accretion and amalgamation of magmatic arcs, and growth of continents. However, this type of subduction has been largely a conceptual model and the geodynamics behind DDS are still poorly constrained. The Molucca Sea subduction zone in SE Asia has been considered as a Cenozoic example of DDS based on geophysical and geological data and provides an opportunity for detailed assessment of how DDS occurs. Here we present 3‐D numerical modeling with aims to reproduce the geodynamic processes of DDS. Several factors that may have important influences on the evolution of DDS are evaluated, including the geometry of the subducting plate, the order of subduction initiation on both sides, the far‐field boundary conditions and thickness of the overriding plates, and the negative buoyancy of the subducting plate. Our results reproduce the observed asymmetrical shape of the subducting Molucca Sea plate and the bending of Halmahera and Sangihe arcs and suggest that DDS is possible if effective escape of the slab‐trapped upper mantle overcomes the space problem, otherwise the slab‐trapped mantle may hinder the sustainability of subduction. We therefore conclude that DDS is associated with closure of narrow and short oceanic plate, and large‐scale double subduction is rare in nature probably owing to space problem.

用化石生长线–日长推定地层年龄的方法：以元古界和新生界为例

地史期间地球的自转速度一直处于逐渐减慢的状态，导致地球1年的天数变少(化石生长线清楚地记录了这个事实)，日长(1天有多少小时)增加。构建起地球年龄、地层时代和自转速度(日长)这三者的函数关系后，即可用化石生长线记录的日长推定化石赋存地层的年龄。本文概要介绍了新提出的生长线–日长定年法的立论思路和计算方法。对前人研究过的元古界及新生界的若干实例用生长线–日长定年法进行计算并比较两者的结果，证明本方法可作为同位素定年法的验证和补充。 The rotational velocity of the earth had been becoming slower and slower during all the geohistorical process. It results in the day numbers in one year being fewer and fewer, which is recorded clearly by the growth lines on the fossils. Correspondingly, the day-long (hours in one day) had been becoming shorter and shorter during the geohistory. To make up a function relation among three variables, namely, the earth’s age, the stratigraphic chronology and the day-long, we can calculate the age of fossil-bearing strata by the data of day-long indicated by the fossil’s growth lines. The paper introduces, briefly, the theoretical thoughts and calculating formulas of the “growth line-day-long dating method” presented by the paper. Some Proterozoic and Cenozoic examples worked by the predecessors are re-dated by the new method, and the comparative study between the both demonstrates that the new method could act as a verification and supplement for the isotopic dating method.

Inversion tectonics under increasing rates of shortening and sedimentation: Cenozoic example from the Eastern Cordillera of Colombia

Abstract The Northern Andes of Colombia is a key locality for understanding tectonic inversion of symmetric rifts. A review of available data on structural geometry and deformation timing, and new thermochronology and provenance data from selected localities, enable the construction of balanced cross-sections and shortening budgets. During early deformation in the Palaeocene, most shortening was focused in the western sector of the orogen, in the Central Cordillera and the Magdallena Valley, although widely spaced and mild inversion occur in areas as far to the east as the Llanos Basin. After a period of tectonic quiescence in the Middle Eocene, deformation resumed across a former early Mesozoic graben in the Eastern Cordillera. Peak shortening rates and out-of-sequence reactivation of the main inversion faults were in place in latest Miocene time, during a phase of topographical growth. Our results indicate that coeval activation of basement highs and adjacent slower-slip shortcuts appear to be characteristic of inverted symmetric grabens. However, before reactivation and brittle faulting occur, strain hardening is required. Deformation rates in the Eastern Cordillera correlate with the westwards velocity of the South American Plate. A threshold convergence rate of approximately 2 cm year −1 seems to be necessary to activate shortening in the upper plate.

Stratigraphic record of subduction initiation in the Permian metasedimentary succession of the El Paso Mountains, California

Petrologic investigation of Permian metasedimentary rocks in the El Paso Mountains reveals a rock record interpreted to be consistent with the sedimentary pattern of the upper continental plate of a nascent subduction zone, based on geodynamic modeling and comparison with a Cenozoic example (Puysegur Ridge, New Zealand). Facies changes reveal a history of uplift (conglomerate), followed by subsidence (carbonate turbidite deposits) and deeper-water sedimentation (argillite, with portions deposited below the carbonate compensation depth [CCD]), and then gradual shallowing accompanied by the onset of nearby intermediate volcanism (volcaniclastic and bioclastic sediments) and construction of a volcanic edifice (andesitic lavas) in a shallow-marine environment. Comparison with Permian global sea-level curves indicates that initial uplift (relative sea-level fall) followed by deep subsidence (relative sea-level rise) are likely due to tectonic rather than eustatic effects. Shallowing during volcaniclastic sedimentation could have been due to both arc edifice building and global sea-level fall. Sandstone modal analysis suggests that the basin evolved from a tectonic setting involving compressive uplift to an arc basin setting. Geodynamic modeling implies the involvement of a transform/truncation fault in subduction initiation. Magmatic trends based on Permian paleogeography and timing suggest a limited nucleation of subduction in the El Paso Mountains followed by propagation southward. Furthermore, subduction initiation modeling suggests regional lithospheric flexure that may be reflected in coeval basins and uplift in the northern Mojave, Death Valley, and Inyo Mountains regions as well as in coeval facies changes on the western edge of the Colorado Plateau. Overall, the Permian section of the El Paso Mountains may be one of the few preserved Paleozoic sedimentary records of subduction inception along a continental margin.

Reservoir potential of sands formed in glaciomarine environments: an analogue study based on Cenozoic examples from McMurdo Sound, Antarctica

Abstract This paper provides documentation of unexpectedly high-reservoir-quality glaciomarine sands found in the Cenozoic succession beneath McMurdo Sound, Antarctica, as an analogue study for evaluations of hydrocarbon prospectivity in basins elsewhere. The Oligocene to Lower Miocene succession of the Victoria Land Basin, an extant portion of the West Antarctic Rift System, comprises diamictites, mudrocks and sandstones with minor conglomerates. These lithologies are arranged in repetitive stacking patterns (cycles), interpreted to record repeated advance and retreat of glaciers into and out of the basin, with attendant eustatic and isostatic effects. Phases of ice retreat within the cycles comprise an array of mudrocks, sandy mudrocks and sandstones, deposited mainly during relative sea-level highstands. Clean, well-sorted, unconsolidated and porous sands <25 m thick from such intervals, which are interpreted to be mainly deltaic in origin, were encountered. Some of these sands, which have visible porosity as high as 41%, flowed into the well bore together with significant volumes of cold formation water. Diagenetic modification of sands in these intervals is minimal, which can be attributed to the low-temperature nature of the subsurface environment. Accordingly, glaciomarine sands in near-field glaciogenic successions should be considered as potential reservoir facies in prospectivity assessments.

Numerical analysis of subduction initiation risk along the Atlantic American passive margins

As the oceanic lithosphere ages and cools, its density increases and at some stage exceeds the density of the underlying asthenosphere, so that the plate can sink spontaneously under its own weight. However, despite the fact that some seafloors are ∼170 m.y. old, an undeniable Cenozoic example of a passive continental margin transforming into an active margin by subduction initiation is not yet known. Several workers have indicated the sources of difficulty of such a transformation; however, no evaluations of existing passive margins from the viewpoint of their future stability have been provided. As suggested by recent numerical experiments with generalized passive margin structures, spontaneous subduction initiation may have a hidden initial phase that is not expressed in diagnostic features such as trench and magmatic arc. Here we analyze numerically the probability of subduction initiation along the Atlantic American passive margins based on their topography and lithospheric and crustal structure. According to our experimental results, proper subduction will likely start during the next 10–20 m.y. along the southern part of the Brazilian margin, while other Atlantic margins of North and South America are stable under the present geodynamic conditions.

Microcodium: An extensive review and a proposed non-rhizogenic biologically induced origin for its formation

Microcodium has been previously described as a mainly Cenozoic calcification pattern ascribed to various organisms. A review of the available literature and our data reveal two peaks in Microcodium abundance; the Moscovian–early Permian and the latest Cretaceous–Paleogene. A detailed analysis of late Paleozoic and Cenozoic examples leads to the following new conclusions. Typical Microcodium-forming unilayered ‘corn-cob’ aggregates of elongated grains and thick multilayered (palisade) replacing structures cannot be linked to smaller-grained intracellular root calcifications, as became widely accepted after the work of Klappa [Klappa, C.F., 1979. Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment. J. Sediment. Petrol. 49, 955–968.] Typical Microcodium is recognized from the early Carboniferous (with doubtful Devonian reports) to Quaternary as a biologically induced mineralization formed via dissolution/precipitation processes in various aerobic Ca-rich soil and subsoil terrestrial environments. Morphology and δ 13C signatures of Microcodium suggest that neither plants, algae, or roots and root-associated mycorrhiza regulate the formation of these fossil structures. Non-recrystallized Microcodium grains basically consist of slender (1.5–4 μm) curved radiating monocrystalline prisms with occasionally preserved hyphae-like morphology. Thin (0.5–3 μm) hypha-like canals can also be observed. These supposed hyphae may belong to actinobacteria. However, thin fungal mycelia cannot be excluded. We propose a model of Microcodium formation involving a mycelial saprotrophic organism responsible for substrate corrosion and associated bacteria capable of consuming acidic metabolites and CaCO 3 reprecipitation into the Microcodium structures.

Lipid biomarker patterns of methane-seep microbialites from the Mesozoic convergent margin of California

In order to reconstruct biogeochemical pathways at Mesozoic methane-seeps, a set of Late Jurassic (Tithonian) to Early Cretaceous (Aptian/Albian), 13C-depleted seep-limestones from forearc strata in western California were subjected to detailed molecular-isotopic biomarker analyses. Two of the microbial carbonate deposits are turbidite-hosted/fault-related, whereas one is hosted in serpentinite in a diapir-related setting. The limestones contain 13C-depleted archaeal lipid biomarkers such as crocetane ( δ 13C ∼ −80‰) and PMI (∼ −100‰), indicative of an involvement of anaerobic oxidation of methane (AOM) in carbonate precipitation. Isotopically depleted crocetane in the Tithonian sample represents the oldest reported occurrence of this compound at methane-seeps. In the set of samples, a series of strongly 13C-depleted, regular C 21 to C 24 isoprenoids possibly results from diagenetic alteration of archaeal sesterterpanylglycerol diethers as suggested by the presence of the putative intermediate 3,7,11,15,19-pentamethylicosanoic acid. 13C-depleted 17α(H),21β(H) and 17β(H),21α(H)-hopanes (C 30–C 34) with 22 S- and 22 R- isomer couplets (>C 31) are present in all samples in distributions indicative of a moderate thermal maturity. Low δ 13C values (−78‰ to −60‰) suggest that these are derived from anaerobic bacteria involved in AOM. Notably, 22 S-isomers are consistently enriched in 13C relative to their 22 R-counterparts. Our samples represent 70 myr of seepage activity and AOM along the Mesozoic margin of western California, filling the gap between the currently oldest methane-seep biomarker record from the Oxfordian (Late Jurassic) and the more widely recognised Cenozoic examples.

Trends in oolite dolomitization across the Neoproterozoic–Cambrian boundary: A case study from Death Valley, California

Changes in dolomite fabric within the Neoproterozoic–Cambrian stratigraphic succession in Death Valley, California preserve evidence for a first-order biogeochemical change in early diagenetic environments through the Neoproterozoic–Cambrian transition. Oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time. Two key generalizations can be made: (1) late Neoproterozoic dolomitization in Death Valley occurred very early in the paragenetic sequence, was volumetrically significant, and preserved fine detail of the original aragonite or calcite precursor; and (2) Early Cambrian dolomitization in Death Valley occurred late in the paragenetic sequence with burial, was commonly incomplete, and obliterated details of the calcite or aragonite precursor; early diagenetic dolomitization of any kind was absent. The dolomitization fabrics in Death Valley parallel the presumed pattern of seawater Mg/Ca with respect to mimetic dolomitization in the Paleozoic (low Mg/Ca, little/no mimetic dolomitization) and Cenozoic (higher Mg/Ca, more common mimetic dolomitization). However, the volume of Neoproterozoic mimetic dolomitization is immense compared to the Cenozoic examples. We speculate that a fundamental shift in the early diagenetic environment occurred in late Neoproterozoic time, perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment-capping microbial mats. An oscillatory trend in the fabric and volume of dolomite through the Phanerozoic has been suggested, but the termination of volumetrically significant mimetic dolomitization across the Neoproterozoic–Cambrian transition, a critical time in Earth history for many reasons, dwarfs the oscillatory trends noted in the Phanerozoic.

Subduction initiation: spontaneous and induced

The sinking of lithosphere at subduction zones couples Earth's exterior with its interior, spawns continental crust and powers a tectonic regime that is unique to our planet. In spite of its importance, it is unclear how subduction is initiated. Two general mechanisms are recognized: induced and spontaneous nucleation of subduction zones. Induced nucleation (INSZ) responds to continuing plate convergence following jamming of a subduction zone by buoyant crust. This results in regional compression, uplift and underthrusting that may yield a new subduction zone. Two subclasses of INSZ, transference and polarity reversal, are distinguished. Transference INSZ moves the new subduction zone outboard of the failed one. The Mussau Trench and the continuing development of a plate boundary SW of India in response to Indo–Asian collision are the best Cenozoic examples of transference INSZ processes. Polarity reversal INSZ also follows collision, but continued convergence in this case results in a new subduction zone forming behind the magmatic arc; the response of the Solomon convergent margin following collision with the Ontong Java Plateau is the best example of this mode. Spontaneous nucleation (SNSZ) results from gravitational instability of oceanic lithosphere and is required to begin the modern regime of plate tectonics. Lithospheric collapse initiates SNSZ, either at a passive margin or at a transform/fracture zone, in a fashion similar to lithospheric delamination. The theory of hypothesis predicts that seafloor spreading will occur in the location that becomes the forearc, as asthenosphere wells up to replace sunken lithosphere, and that seafloor spreading predates plate convergence. This is the origin of most boninites and ophiolites. Passive margin collapse is a corollary of the Wilson cycle but no Cenozoic examples are known; furthermore, the expected strength of the lithosphere makes this mode unlikely. Transform collapse SNSZ appears to have engendered new subduction zones along the western edge of the Pacific plate during the Eocene. Development of self-sustaining subduction in the case of SNSZ is signaled by the beginning of down-dip slab motion, causing chilling of the forearc mantle and retreat of the magmatic arc to a position that is 100–200 km from the trench. INSZ may affect only part of a plate margin, but SNSZ affects the entire margin in the new direction of convergence. INSZ and SNSZ can be distinguished by the record left on the upper plates: INSZ begins with strong compression and uplift, whereas SNSZ begins with rifting and seafloor spreading. Understanding conditions leading to SNSZ and how hinged subsidence of lithosphere changes to true subduction promise to be exciting and fruitful areas of future research.

Shallow Burial Dolomitization and Dedolomitization of Cenozoic Cool-Water Limestones, Southern Australia: Geochemistry and Origin

ABSTRACT The Gambier Limestone, a bryozoan-rich, cool-water carbonate of Late Eocene to Early Miocene age in the Otway basin of southeastern Australia, is about one-third variably dolomitized limestone and dolostone. Individual dolomite crystals range from 10 to 500 µm and are of three types: (1) silt-size crystals (4-64 µm diameters) having a brightly luminescent core (10-20 µm) of nearly stoichiometric dolomite with 1 mol% MnCO3 and SiO2 but no Fe, surrounded by a thin, dull or alternating dark and bright CL cortex that is nonstoichiometric with 44-46 mol % MgCO3 but sharp to diffuse zoning in Mn and Fe, (2) sand-size crystals, which also have a brightly luminescent and stoichiometric core and dark and bright non-stoichiometric thin inner cortex, but are mostly a thick outer cortex (at ca. > 50 µm diameter) that is nonluminescent and nonstoichiometric with some Fe but no Mn, and (3) identical sand-size crystals with a variably leached core and inner cortex that is either a void or a void filled with late calcite cement. These three crystal populations represent two distinct dolomites and their alteration product. The smallest of the silt-size crystals, which are rarely preserved, are also the cores of the larger crystals upon which grew an inner and outer cortex of somewhat different composition. The 18O values of dolomite rhombs (-1.6 to 2.9 ) correlate positively with 13C values (0.7 to 3.0), with the lowest values from silt-size or small sand-size crystals and the highest values from large sand-size crystals with leached, unfilled cores. Low and variable 18O and 13C values in conjunction with high and variable Mn and Fe contents in both the core and inner cortex of all crystals are consistent with formation from a mixture of brackish water and seawater. Higher values for the outer cortex in sand-size crystals are typical of those expected for precipitation from normal or near-normal seawater. Low Sr, Mn, and Fe contents in the largest crystals, in conjunction with fluid inclusions of seawater salinity in the outer cortex, are also consistent with the bulk of the large crystals having precipitated from seawater. Using the Sr-isotope evolution curve for Cenozoic seawater, Sr-isotope ratios of the dolomites can be divided into four distinct groups, which correspond to times of major transgressions. This Cenozoic example, in which dolomite petrography and crystal chemistry can be explained in terms of simple transgression-regression, offers a simple sedimentological explanation for many similar dolomites where processes of formation cannot be so easily constrained.

Development of a continental forearc: A Cenozoic example from the Central Andes, northern Chile

Development of a continental forearc: A Cenozoic example from the Central Andes, northern Chile

Development of a continental forearc: A Cenozoic example from the Central Andes, northern Chile

In order to understand the response of a continental forearc to changes in subduction-zone geodynamics, we constructed a high-resolution chronostratigraphic cross section across the Central Andean forearc of northern Chile (21°–24°S). The tectono-stratigraphic development of the forearc differs from established models. No relationship was found between changes in rate of relative plate convergence and amount and style of deformation. Forearc response to continual compression since the Oligocene has been uplift and segmentation into discrete tectono-stratigraphic zones. From west to east, these zones are the extensional Coastal Cordillera, the extensional and/or transtensional Central depression, and the transpressional and/or compressional Precordillera-Preandean depression. Each area has recorded almost continuous sedimentation from Oligocene (?Eocene) time to the present day. Accommodation space has been generated by basin-margin uplift rather than active subsidence. We propose a model in which uplift of the leading edge of the South American plate is driven by subcrustal accretion of material removed at the trench by subduction erosion. Uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge. The tectono-stratigraphic complexity exhibited within the Central Andean forearc is likely to be representative of Cordilleran-type margins and would be difficult to recognize in an ancient continental forearc.

Paleoecology of a Late Eocene Mobile Rockground Biota from North Otago, New Zealand

A distinctive mobile pebble and cobble rockground biota of Late Eocene age is described from volcaniclastic sediments between Oamaru and Kakanui, North Otago, New Zealand. Thousands of subangular to subrounded basaltic pebbles and cobbles are encrusted with a diverse range of very well-preserved epibionts including crustose coralline algae, serpulids, bivalves, foraminifera, brachiopods, and more than 70 species of cyclostome and cheilostome bryozoans. The preservation of thin sediment layers beneath and between encrusting bryozoan colonies indicates the probable occurrence, during life, of agglutinating microbial mats. The abundance of subspherical rhodoliths, the diversity of epibionts, and their occurrence on all faces of the volcanic clasts reflect intermediate levels of overturning and rolling in a moderately current-swept channel adjacent to small volcanic islands and seamounts. The occurrence of large foraminifera (Asteroclyna), bryozoans (including two extant species), and brachiopods with warm-water affinities indicates subtropical sea temperatures and water depths of 25-50 m for this community, which represents one of the few described Cenozoic examples of a mobile rockground biota.

Are Coronae Restricted to Venus?: Corona-Like Tectonovolcanic Structures on Earth

Coronae may not be tectonovolcanic features ‘unique to Venus’ because both the processes that lead to corona formation, and their final tectonovolcanic output (formation of domes, plateaus, extensional rings, etc.), are also found on Earth. Large-scale corona formation processes on Earth may be restricted (because of plate motion) but not absent. The same applies to resurfacing processes. We here suggest that at least, the early stages of corona formation can be recognized in intraplate tectonic settings on Earth. The African plate displays many Cenozoic examples of plume-related domal uplifts and volcanism (e.g., Hoggar, Tibesti, Darfur, Ethiopia). Furthermore, the east African rift system (EARS) around lake Victoria displays many striking features that resemble those of the Venus coronae associated with extensional belts. Among these are the following: (1) an overall elliptical shape; (2) the existence of a mantle plume (Kenya plume) centered beneath lake Victoria; (3) a central plateau (east African plateau); (4) an external extensional belt (the EARS east and west branches); (5) doming processes (Kenya dome); and last but not least (6) volcanism.

Evidence in North-East Greenland for Late Silurian-Early Devonian regional extension during the Caledonian orogeny

Crustal extension during and immediately following continental collision is recognized as a fundamental process within many orogenic belts. In the North Atlantic region, Caledonian (Ordovician-Devonian) orogenesis resulted from continental collision of Laurentia and Baltica. In North-East Greenland (Laurentia), regional extension occurred during the Late Silurian to Early Devonian, probably as a response to crustal thickening at an earlier stage in the orogeny. Extension resulted in a crustal geometry analogous to that of a metamorphic core complex. Recognition of this process in North-East Greenland substantially increases the area of the Caledonides known to have been affected by such extension, and provides additional evidence that this Paleozoic collisional orogen evolved in a manner similar to Mesozoic and Cenozoic examples.

Trench and slope basin deposits in an Archean metasedimentary belt, Superior Province, Canadian Shield

The identification of a late Archean arc–trench assemblage in northwestern Ontario provides the opportunity to compare depositional systems developed in a Precambrian convergent setting with Cenozoic examples. Two types of sedimentary associations exist in the accretionary complex. Medium- to thick-bedded Bouma A, AB, and ABC felsic turbidites dominate the belt. These are primarily organized into unstructured sequences and reflect deposition in a ramp-like environment with multiple feed points supplying sediment from a forearc basin. Mafic turbidites with possible shallow water reworked intervals form isolated pods within the metasedimentary belt. Erosion of upthrust blocks of sedimentary strata containing ultramafic masses supplied this sediment to elevated slope basins. These types of depositional systems are similar in many respects to those developed in Cenozoic and Holocene arc–trench settings.