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1.
The seismic, drilling and logging data reveal that a large-scale igneous intrusion with a width of 14 Km and a maximum thickness of 170 m intruded within the Paleogene Liushagang Formation in the Fushan Depression, Beibuwan Basin, South China Sea (SCS). In this study, we report the geochemistry and Sr-Nd-Pb-Hf isotopic compositions of the igneous rocks, and evaluate the thermal effect induced by this large-scale igneous intrusion on the host rocks. The analyzed igneous samples exhibit strong enrichment in light rare earth elements (LREE) and large-ion lithophile elements (LILE), having characteristics similar to intra-plate oceanic island basalts (OIB). The Sr-Nd-Pb-Hf isotopic data display narrow ranges (e.g. 87Sr/86Sri = 0.7042–0.7044, 143Nd/144Ndi = 0.5128–0.5129, 206Pb/204Pbi = 18.90–18.94, εHf(t) = +7.56∼+9.60). Geochemical and isotopic compositions suggest a mixed mantle source between depleted mid-ocean-ridge-basalt (MORB) mantle (DMM)-like mantle and enriched mantle type II (EMII, possibly the Hainan mantle plume). Vitrinite reflectance values, major mineralogical changes together with a maturity-related biomarker [Ts/(Ts + Tm)] all reveal significant thermal effect induced by the igneous intrusion. Vitrinite reflectance values of the host rock are up to 2.5% in the intrusion region, whereas lower reflectance values (0.62–0.73%) occur in the unaffected area of the same strata. Moreover, our results suggest that the host rocks above the igneous intrusion are characterised by higher maturity than below, which should be attributed to the different behavior of hydrothermal fluids. These observations suggest that the thermal effect of large-scale thick igneous intrusions is much more intense than that of thin igneous intrusions, and the behavior of hydrothermal fluids induced by magmatic intrusive events should be a critical impact factor during heat transfer process.  相似文献   

2.
The Late Cretaceous–Paleocene rifting in the NW Vøring Basin is characterized by four main fault complexes and pronounced upper-crustal structural segmentation. The fault complexes are linked by accommodation zones, which separate fault systems of different polarities and thick from thinner coeval sedimentary successions. Structural and stratigraphic analyses suggest that the early rift phase (∼81 to 65 Ma) was characterized by large-scale normal faulting, along-margin segmentation and varying structural styles; whereas the late rift phase (∼65 to 55 Ma) was associated with continued extension, regional uplift, intrusive igneous activity and subsequent erosion. The rifting ended with breakup at ∼55 Ma accompanied by massive, but gradually waning extrusive igneous activity over the next 3 Myr. The mode of rifting appears to have changed from brittle to more ductile extensional deformation from the early to late rift phase. The changing rift rheology is probably related to the arrival of the Iceland mantle plume and initiation of associated igneous activity. Hence, the NW Vøring Basin provides an example of complex interaction of structural and magmatic relationships during rifting and breakup.  相似文献   

3.
Rockall Island is composed of 52 ± 9 Ma (m.y.) aegirine granite and forms part of a nearly planated intrusive complex. The nearby Helen's Reef lies within the complex but is composed of Cretaceous (81 ± 3 Ma) microgabbros. The petrography and chemistry of the microgabbros differ from Mid-Ocean Ridge tholeiites but are similar to the Tertiary gabbroic rocks of northwestern Scotland. The relationship of the Cretaceous igneous activity to the evolution of the North Atlantic Ocean is discussed briefly.  相似文献   

4.
In 2013, the first discovery of gas pools in well LS 208 in intrusive rocks of the Songliao Basin (SB), NE China was made in the 2nd member of the Yingcheng Formation in the Yingtai rift depression, proving that intrusive rocks of the SB have the potential for gas exploration. However, the mechanisms behind the origin of reservoirs in intrusive rocks need to be identified for effective gas exploration. The gas pool in intrusive rocks can be characterized as a low-abundance, high-temperature, normal-pressure, methane-rich, and lithologic pool based on integrated coring, logging, seismic, and oil test methods. The intrusive rocks show primary and secondary porosities, such as shrinkage fractures (SF), spongy pores (SP), secondary sieve pores (SSP), and tectonic fractures (TF). The reservoir is of the fracture–pore type with low porosity and permeability. A capillary pressure curve for mercury intrusion indicates small pore-throat size, negative skewness, medium–high displacement pressure, and middle–low mercury saturation. The development of fractures was found to be related to the quenching effects of emplacement and tectonic inversion during the middle–late Campanian. SP and SSP formed during two phases. The first phase occurred during emplacement of the intrusive rock in the late Albian, when the intrusions underwent alteration by organic acids. The second phase occurred between the early Cenomanian and middle Campanian, when the intrusions underwent alteration by carbonic acid. The SF formed prior to oil charging, the SSP + SP formed during oil charging, and the TF formed during the middle–late Campanian and promoted the distribution of gas pools throughout the reservoir. The intrusive rocks in the SB and the adjacent basins were emplaced in the mudstone and coal units, and have great potential for gas exploration.  相似文献   

5.
The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to reconcile with the various syn-to post-rift mechanisms of permanent and/or transient movements proposed in the hypothetical context of a plume beneath Iceland. The epeirogenic movements can be explained as dynamic topographic responses to changing forms of small-scale convective flow in the upper mantle: tilting as coeval upwelling and downwelling above an edge-driven convection cell, sagging as a loss of dynamic support above a former upwelling. The inferred Cenozoic succession of epeirogenic tilting, sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening, a process that may also be the underlying cause of plate reorganisations. The postulated episodes of flow reorganisation in the NE Atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin, as well as on other Atlantic-type ‘passive’ margins.  相似文献   

6.
Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene–Mid Miocene) and after cessation of seafloor spreading (Mid Miocene–Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean–continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to suggest that the northern margin of the SCS is a magma-poor rifted margin.  相似文献   

7.
白垩纪以来太平洋上地幔组成和温度变化   总被引：1，自引：0，他引：1  
The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling(DSDP/ODP/IODP) sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge(the East Pacific Rise). The Ontong Java plateau(125–90 Ma) basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.  相似文献   

8.
This study presents new geochemical and isotope data on igneous rocks of the Vetlovaya marginal sea paleobasin (part of the Late Mesozoic–Cenozoic margin of the northwestern Pacific). The results show that the rock complexes of this marginal sea basin comprise igneous rocks with geochemical compositions similar to those of normal oceanic tholeiites, enriched transitional tholeiites, and ocean island and back-arc basin basalts. Island-arc tholeiitic basalts are present only rarely. The specific geochemical signatures of these rocks are interpreted as being related to mantle heterogeneity and the geodynamic conditions in the basin.  相似文献   

9.
Based on published estimates of areal extent, thickness and dating of igneous rocks related to formation of the North Atlantic Volcanic Province, we calculate the magmatic production since the first reported magmatism at 70 Ma until present. We relate the magmatism to the Icelandic Plume and estimate the total volumetric production to 22.1 × 106 km3. The magmatic production varied significantly with time, with a clear maximum of 55.5 m3/s around continental break-up at ca. 54 Ma. The lowest production is estimated at 4 m3/s, increasing to 7 m3/s at ca. 23 Ma. Two other pulses with increased activity are found around 40 Ma and in the late Miocene (10–5 Ma). The variations in productivity are consistent with a plume pulsing with a periodicity of ca. 15 million years. The same periodicity and relative timing is found for the Hawaiian Plume. If confirmed, these observations suggest that both plumes originate within the thin boundary layer near the Core-Mantle Boundary. This hypothesis may imply periodic heating of the earth’s core with subsequent heat-release to the mantle and increased global plume activity.  相似文献   

10.
Transverse ridges are elongate reliefs running parallel and adjacent to transform/fracture zones offsetting mid-ocean ridges. A major transverse ridge runs adjacent to the Vema transform (Central Atlantic), that offsets the Mid-Atlantic Ridge by 320 km. Multibeam morphobathymetric coverage of the entire Vema Transverse ridge shows it is an elongated (300 km), narrow (<30 km at the base) relief that constitutes a topographic anomaly rising up to 4 km above the predicted thermal contraction level. Morphology and lithology suggest that the Vema Transverse ridge is an uplifted sliver of oceanic lithosphere. Topographic and lithological asymmetry indicate that the transverse ridge was formed by flexure of a lithospheric sliver, uncoupled on its northern side by the transform fault. The transverse ridge can be subdivided in segments bound by topographic discontinuities that are probably fault-controlled, suggesting some differential uplift and/or tilting of the different segments. Two of the segments are capped by shallow water carbonate platforms, that formed about 3–4 m.y. ago, at which time the crust of the transverse ridge was close to sea level. Sampling by submersible and dredging indicates that a relatively undisturbed section of oceanic lithosphere is exposed on the northern slope of the transverse ridge. Preliminary studies of mantle-derived ultramafic rocks from this section suggest temporal variations in mantle composition. An inactive fracture zone scarp (Lema fracture zone) was mapped south of the Vema Transverse ridge. Based on morphology, a fossil RTI was identified about 80 km west of the presently active RTI, suggesting that a ridge jump might have occurred about 2.2 m.a. Most probable causes for the formation of the Vema Transverse ridge are vertical motions of lithospheric slivers due to small changes in the direction of spreading of the plates bordering the Vema Fracture Zone.  相似文献   

11.
Research on seamounts provides some of the best constraints for understanding intraplate volcanism, and samples from seamounts reveal crucial evidence about the geochemical makeup of the oceanic mantle. There are still many seamounts in the West Pacific Seamount Province(WPSP) that have not been studied, meaning their ages and geochemistry remain unknown. A better understanding of these seamount trails and their evolutionary history, investigated with age and geochemistry data, will enable better understanding of the geological processes operating underneath the Pacific Ocean Plate. Here, new ~(40)Ar/~(39) Ar ages and trace element and Sr-Nd-Pb isotopic data for seven basalt rocks from four seamounts in the WPSP are provided. Chemically, these rocks are all Oceanic Island Alkali basalt(OIA type); analysis of olivine phenocrysts shows that the magmas experienced strong olivine fractionation and changed from olivine + plagioclase to olivine + plagioclase + clinopyroxene cotectic during their evolution. Rare earth element(REE) patterns and a spider diagram of the samples in this study show OIB(Ocean Island Basalt) like behavior. The range of ~(87)Sr/~(86) Sr values is from 0.704 60 to 0.706 24, the range of ~(206)Pb/~(204) Pb values is from 18.241 to 18.599, and the range of ~(143)Nd/~(144) Nd values is from 0.512 646 to 0.512 826; together, these values indicate magma sources ranging from EMI to EMII. Finally, new ~(40)Ar/~(39) Ar age data show that these seamounts formed at ～97 and ～106 Ma, indicating that some may have undergone the same formation processes as seamounts in the eastern part of the Magellan Seamount Trail, but other seamounts likely have different origins.  相似文献   

12.
Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s-1, underlain by a refracting layer in which the average velocity is 4.5 km s-1. The velocity of 6.6 km s-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s-1. This is more than 1.0 km s-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.  相似文献   

13.
The Bauer microplate was an independent slab of oceanic lithosphere that from 17 Ma to 6 Ma grew from 1.4 × 105 km2 to 1.2 × 106 km2 between the rapidly diverging Pacific and Nazca plates. Growth was by accretion at the lengthening and overlapping axes of the (Bauer-Nazca) Galapagos Rise (GR) and the (Pacific-Bauer) East Pacific Rise (EPR). EPR and GR axial propagation to create and rapidly grow the counter-clockwise spinning microplate occurred in two phases: (1) 17–15Ma, when the EPR axis propagated north and the GR axis propagated south around a narrow (100- to 200-km-wide) core of older lithosphere; and (2) 8–6 Ma, when rapid northward propagation of the EPR axis resumed, overlapping ∼400 km of the fast-spreading Pacific-Nazca rise-crest and appending a large (200- to 400-km-wide) area of the west flank of that rise as a ‘northern annex’ to the microplate. Between 15 and 8 Ma the microplate grew principally by crustal accretion at the crest of its rises. The microplate was captured by the Nazca plate and the Galapagos Rise axis became extinct soon after 6 Ma, when the south end of the Pacific-Bauer EPR axis became aligned with the southern Pacific-Nazca EPR axis and its north end was linked by the Quebrada Transform to the northern Pacific-Nazca EPR axis. Incomplete multibeam bathymetry of the microplate margins, and of both flanks of the Pacific-Bauer and Bauer-Nazca Rises, together with archival magnetic and satellite altimetry data, clarifies the growth and (counter-clockwise) rotation of the microplate, and tests tectonic models derived from studies of the still active, much smaller, Easter and Juan Fernandez microplates. Our interpretations differ from model predictions in that Euler poles were not located on the microplate boundary, propagation in the 15–8 Ma phase of growth was not toward these poles, and microplate rotation rates were small (5°/m.y.) for much of its history, when long, bounding transform faults reduced coupling to Nazca plate motion. Some structures of the Bauer microplate boundary, such as deep rift valleys and a broad zone of thrust-faulted lithosphere, are, however, similar to those observed around the smaller, active microplates. Analysis of how the Bauer microplate was captured when coupling to the Pacific plate was reduced invites speculation on why risecrest microplates eventually lose their independence.  相似文献   

14.
The Orange Basin records the development of the Late Jurassic to present day volcanic-rifted passive margin of Namibia. Regional extension is recorded by a Late Jurassic to Lower Cretaceous Syn-rift Megasequence, which is separated from a Cretaceous to present day post-rift Megasequence by the Late Hauterivian (ca. 130 Ma) break-up unconformity. The Late Cretaceous Post-rift evolution of the basin is characterized by episodic gravitational collapse of the margin. Gravitational collapse is recorded as a series of shale-detached gravity slide systems, consisting of an up-dip extensional domain that is linked to a down-dip zone of contraction domain along a thin basal detachment of Turonian age. The extensional domain is characterized by basinward-dipping listric faults that sole into the basal detachment. The contractional domain consists of landward-dipping listric faults and strongly asymmetric basinward-verging thrust-related folds. Growth stratal patterns suggest that the gravitational collapse of the margin was short-lived, spanning from the Coniacian (ca. 90 Ma) to the Santonian (ca. 83 Ma). Structural restorations of the main gravity-driven system show a lack of balance between up-dip extension (24 km) and down-dip shortening (16 km). Gravity sliding in the Namibian margin is interpreted to have occurred as a series of episodic short-lived gravity sliding between the Cenomanian (ca. 100 Ma) and the Campanian (ca. 80 Ma). Gravity sliding and spreading are interpreted to be the result of episodic cratonic uplift combined with differential thermal subsidence. Sliding may have also been favoured by the presence of an efficient detachment layer in Turonian source rocks.  相似文献   

15.
Two-dimensional flexural backstripping and thermal modelling (assuming uniform stretching and cooling) is applied to four interpreted, depth-converted seismic profiles across the Rockall, Faroe–Shetland and Vøring basins, along 1600 km of the Atlantic continental margin of NW Europe. The results reveal a significant discrepancy between the modelled palaeo-depths for the base of the Cenozoic succession and those proven by geological evidence at control points (subaerial conditions or depositional depth ranges in wells). The discrepancy is of Rm-scale, much larger than the possible range of parameter error determined by sensitivity tests (up to 0.5 km). Assuming a Cretaceous rift episode (100 Ma), the discrepancy is at least 1.7 km in the Rockall Basin, up to 2.1 km in the Faroe–Shetland Basin and at least 1 km in the Vøring Basin (which also contains evidence of kilometre-scale uplift of the inner margin). Assuming (unproven) a second rift in the early Cenozoic (60 Ma), the discrepancy remains of kilometre-scale in the Rockall and Faroe–Shetland basins. The restorations also provide evidence of uplift, both above compressive structures and across the modelled profiles as seaward rotations of palaeo-bathymetric records. The palaeo-bathymetric discrepancy corresponds to an anomaly in subsidence that is the cumulative product of all the tectonic episodes that have affected the NW European margin, and may incorporate both permanent effects of the last episode of lithospheric extension and transient responses to the interaction of the margin with mantle convective flow. Any explanation must accommodate both the large magnitude of anomalous subsidence along the margin and evidence of its episodic character.  相似文献   

16.
New swath bathymetric, multichannel seismic and magnetic data reveal the complexity of the intersection between the extinct West Scotia Ridge (WSR) and the Shackleton Fracture Zone (SFZ), a first-order NW-SE trending high-relief ridge cutting across the Drake Passage. The SFZ is composed of shallow, ridge segments and depressions, largely parallel to the fracture zone with an `en echelon' pattern in plan view. These features are bounded by tectonic lineaments, interpreted as faults. The axial valley of the spreading center intersects the fracture zone in a complex area of deformation, where N120° E lineaments and E–W faults anastomose on both sides of the intersection. The fracture zone developed within an extensional regime, which facilitated the formation of oceanic transverse ridges parallel to the fracture zone and depressions attributed to pull-apart basins, bounded by normal and strike-slip faults.On the multichannel seismic (MCS) profiles, the igneous crust is well stratified, with numerous discontinuous high-amplitude reflectors and many irregular diffractions at the top, and a thicker layer below. The latter has sparse and weak reflectors, although it locally contains strong, dipping reflections. A bright, slightly undulating reflector observed below the spreading center axial valley at about 0.75 s (twt) depth in the igneous crust is interpreted as an indication of the relict axial magma chamber. Deep, high-amplitude subhorizontal and slightly dipping reflections are observed between 1.8 and 3.2 s (twt) below sea floor, but are preferentially located at about 2.8–3.0 s (twt) depth. Where these reflections are more continuous they may represent the Mohorovicic seismic discontinuity. More locally, short (2–3 km long), very high-amplitude reflections observed at 3.6 and 4.3 s (twt) depth below sea floor are attributed to an interlayered upper mantle transition zone. The MCS profiles also show a pattern of regularly spaced, steep-inclined reflectors, which cut across layers 2 and 3 of the oceanic crust. These reflectors are attributed to deformation under a transpressional regime that developed along the SFZ, shortly after spreading ceased at the WSR. Magnetic anomalies 5 to 5 E may be confidently identified on the flanks of the WSR. Our spreading model assumes slow rates (ca. 10–20 mm/yr), with slight asymmetries favoring the southeastern flank between 5C and 5, and the northwestern flank between 5 and extinction. The spreading rate asymmetry means that accretion was slower during formation of the steeper, shallower, southeastern flank than of the northwestern flank.  相似文献   

17.
Reconstructions of the Albian to Campanian foreland basin adjacent to the northern Canadian Cordillera are based on outcrop and well log correlations, seismic interpretation, and reconnaissance-level detrital zircon analysis. The succession is subdivided into two tectonostratigraphic units. First is an Albian tectonostratigraphic unit that was deposited on the flexural margin of a foreland basin. At the base is a shallow marine sandstone interval that was deposited during transgressive reworking of sediment from cratonic sources east of the basin that resulted in a dominant 2000–1800 Ma detrital zircon age fraction. Subsequent deposition in a west-facing muddy ramp setting was followed by east-to-west shoreface progradation into the basin.Near the Albian–Cenomanian boundary, regional uplift and exhumation resulted in an angular unconformity at the base of the Cenomanian–Campanian tectonostratigraphic unit. Renewed subsidence in the Cenomanian resulted in deposition of organic-rich, radioactive, black mudstone of the Slater River Formation in a foredeep setting. Cenomanian–Turonian time saw west-to-east progradation of a shoreface-shelf system from the orogenic margin of the foreland basin over the foredeep deposits. Detrital zircon age peaks of approximately 1300 Ma, 1000 Ma, and 400 Ma from a Turonian sample are consistent with recycling of Mississippian and older strata from the Cordillera west of the study area, and show that the orogen-attached depositional system delivered sediment from the orogen to the foreland basin. A near syndepositional detrital zircon age of ca. 93 Ma overlaps with known granitoid ages from the Cordillera. After the shelf system prograded across the study area, subsequent pulses of subsidence and uplift resulted in dramatic thickness variations across an older structural belt, the Keele Tectonic Zone, from the Turonian to the Campanian.The succession of depositional systems in the study area from flexural margin to foredeep to orogenic margin is attributed to coupled foreland propagation of the front of the Cordilleran orogen and the foreland basin. Propagation of crustal thickening and deformation toward the foreland is a typical feature of orogens and so the distal to proximal evolution of the foreland basin should also be considered as typical.  相似文献   

18.
The 1994 Tasmante swath-mapping and reflection seismic cruise covered 200 000 km2 of sea floor south and west of Tasmania. The survey provided a wealth of morphological, structural and sedimentological information, in an area of critical importance in reconstructing the break-up of East Gondwana.The west Tasmanian margin consists of a non-depositional continental shelf less than 50 km wide and a sedimented continental slope about 100 km wide. The adjacent 20 km of abyssal plain to the west is heavily sedimented, and beyond that is lightly sedimented Eocene oceanic crust formed as Australia and Antarctica separated. The swath data revealed systems of 100 m-deep downslope canyons and large lower-slope fault-blocks, striking 320° and dipping landward. These continental blocks lie adjacent to the continent ocean boundary (COB) and are up to 2500 m high and have 15°–20° scarps.The South Tasman Rise (STR) is bounded to the west by the Tasman Fracture Zone extending south to Antarctica. Adjacent to the STR, the fracture zone is represented by a scarp up to 2000 m high with slopes of 15–20°. The scarp consists of continental faultblocks dipping landward. Beyond the scarp to the west is a string of sheared parallel highs, and beyond that is lightly sedimented Oligocene oceanic crust 4200–4600 m deep with distinct E-W spreading fabric. The eastern margin of the bathymetric STR trends about 320° and is structurally controlled. The depression between it and the continental East Tasman Plateau (ETP) is heavily sedimented; its western part is underlain by thinned continental crust and its central part by oceanic crust of Late Cretaceous to Early Tertiary age. The southern margin of the STR is formed by N-S transform faults and south-dipping normal faults.The STR is cut into two major terrains by a N-S fracture zone at 146°15E. The western terrain is characterised by rotated basement blocks and intervening basins mostly trending 270°–290°. The eastern terrain is characterised by basement blocks and intervening strike-slip basins trending 300°–340°. Recent dredging of basement rocks suggests that the western terrain has Antarctic affinities, whereas the eastern terrain has Tasmanian affinities.Stretching and slow spreading between Australia and Antarctica was in a NW direction from 130–45 Ma, and fast spreading was in a N-S direction thereafter. The western STR terrain was attached to Antarctica during the early movement, and moved down the west coast of Tasmania along a 320° shear zone, forming the landward-dipping continental blocks along the present COB. The eastern terrain either moved with the western terrain, or was welded to it along the 146°15 E fracture zone in the Early Tertiary. At 45 Ma, fast spreading started in a N-S direction, and after some probable movement along the 146°15E fracture zone, the west and east STR terrains were welded together and became part of Australia.  相似文献   

19.
An improved gas chromatographic system was constructed to analyze oceanic dissolved N2, Ar and O2 with a higher accuracy and shorter analytical time. To obtain a higher accuracy of N2, Ar and O2 measurements, the following was added to the system: (I) an air trapping system; (II) a N2–CO2 trapping system after the operation of the air trapping system; (III) an active carbon column system for separating N2 and CO2 completely and (IV) the introduction of automatic valves controlling most of the system. Compared to previous studies, the precision of the measurements of N2, Ar and O2 concentrations was higher at 0.04%, 0.05% and 0.02%, respectively, and our analytical time was shorter at 600 s. Using the improved analytical technique, concentrations of N2 (CN2, 561.69–611.81 μmol/kg) and Ar (CAr, 15.126–16.238 μmol/kg), saturation states of N2 (ΔN2, − 5.1–0.9%) and Ar (ΔAr, − 7.0 to − 1.1%) from 0 m to 3000 m depth in the western North Pacific were observed during March 2005. Based on these data, we propose a new concept for estimating the amount of bubble injection (B). The total error in calculating B was estimated to be about 20%. We estimated B from 12 to 43 μmol/kg in this region using the observational values of N2 and Ar. As each water mass had a significantly different value of B even with an error of 20%, it is possible to use it as an index of sea surface state for when each water mass is produced in the sea surface mixed layer. Moreover, based on our values of B, we estimated preformed dissolved oxygen (DO) (CpreDO, 309–332 μmol/kg) and the saturation state of CpreDO (ΔpreDO, − 7.0 to − 1.2%) in this region. Thus, the difference between CpreDO and DO content in the ocean interior may be a more useful index for biogenic organic decomposition in the ocean field compared to Apparent Oxygen Utilization (AOU). Until now, the estimation of oceanic uptake of anthropogenic CO2 has used AOU as a major parameter. Therefore, it may be necessary to re-evaluate the oceanic uptake of anthropogenic CO2 based on our new concept of B.  相似文献   

20.
Polycyclic aromatic hydrocarbons (PAHs) were identified and measured in surface seawater and in the tissues (gills and mantle) of indigenous black mussels, Mytilus galloprovincialis, collected from three coastal sites of Saronikos Gulf (Greece), a gulf that exhibits high levels of pollution. The total PAHs measured by spectrofluorometry in the surface seawater were found in the range of 425–459 ng L−1 at the most polluted sites 1 and 2 (Elefsis Bay–Salamis Island) and in the range of 103–124 ng L−1 at site 3 (Aegina Island). PAHs' sources in seawater were identified by application of specific PAH ratios, such as phenanthrene/anthracene and fluoranthene/pyrene. Levels of PAHs in soft tissues (gills and mantle) of indigenous mussels were much higher than those reported for seawater. Total PAH concentrations in mantle tissues were in the range of 1300–1800 ng g−1 dry weight (dw) tissue at sites 1 and 2 and approximately 380 ng g−1 dw at site 3. In gill tissues total PAH concentrations were in the range of 1480–2400 ng g−1 dw at sites 1 and 2 and approximately 430 ng g−1 dw at site 3. PAHs composition was dominated by two-, three- and four-ring compounds in seawater, where 17 different PAH compounds were identified and measured in mussel tissues. Mussels can be used as sentinel organisms to monitoring PAHs' contamination, since they concentrate PAHs from the surrounding water media and therefore making the chemical analysis simpler and less prone to error than that for water. In surface seawater possible weathering and photodegradation due to hot climates contribute to reduced PAHs concentrations.  相似文献   

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