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1.
Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α95=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing40Ar/39Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.  相似文献   

2.
The Blake excursion was among the first recognized with directional and intensity behavior known mainly from marine sediment and Chinese loess. Age estimates for the directional shifts in sediments are poorly constrained to about 118−100 ka, i.e., at the marine isotope stage (MIS) 5e/5d boundary. Moreover, sediments at Lac du Bouchet maar, France and along the Portuguese margin reveal what may be a "post-Blake" excursion at about 105−95 ka. The excursional directions are associated with a prominent paleointensity minimum between about 125 and 95 ka in global stacked records. Lava flow recordings of the Blake excursion(s) have, however, been questionable because precise ages required for correlation with these sediment records are lacking. To establish new, independent records of the Blake excursion, and link these into a larger Quaternary GITS, we have undertaken 40Ar/39Ar incremental heating and unspiked K-Ar experiments on groundmass from the transitionally magnetized Inzolfato flow on Lipari Island. We also obtained 40Ar/39Ar incremental heating results for a lava flow on Amsterdam Island originally thought to record the Mono Lake excursion and from the transitionally magnetized El Calderon basalt flow, New Mexico that was K-Ar dated by Champion et al. (1988) at 128 ± 66 ka.Unspiked K-Ar ages of four samples from the Inzolfato flow are 102.5 ± 4.7, 101.3 ± 3.3, 97.1 ± 2.6, and 96.8 ± 3.1 ka and thus indistinguishable from one another. 40Ar/39Ar results are more complex, with three samples yielding discordant age spectra. Based on incremental heating data obtained in both the UW-Madison and Gif-sur-Yvette 40Ar/39Ar laboratories, a fourth sample yields six concordant age plateaus and a weighted mean age of 105.2 ± 1.4 ka that we take as the best estimate of time since the flow erupted. Five 40Ar/39Ar incremental heating experiments on the Amsterdam Island lava yield a plateau age of 120 ± 12 ka, whereas ages from two sites in the Calderon flow are 112 ± 23 and 101 ± 14 ka, together giving a weighted mean of 104 ± 12 ka. The age of 120 ± 12 ka from Amsterdam Island, though imprecise, correlates with the Blake excursion. In contrast, the 104–105 ka age obtained from both Lipari and New Mexico indicates that these lavas record a younger period of dynamo instability, most probably associated with the post-Blake excursion. These radioisotopic ages are consistent with the astronomical ages of two paleointensity minima in the PISO-1500 global stack. Our findings indicate that the Blake and post-Blake excursions are both global features of past geodynamo behavior and support the hypothesis that Brunhes chron excursions are temporally clustered into two groups of at least a half-dozen each spanning over 220 to 30 ka and 720 to 520 ka.  相似文献   

3.
A rock magnetic and paleomagnetic investigation was performed on some selected, radiometrically dated lava flows from the Mascota Volcanic Field (MVF), western Trans- Mexican Volcanic Belt. A set of rock-magnetic experiments and standard paleomagnetic analysis were carried out on 19 sites spanning the time interval from 2268 to 72 kyr. The paleomagnetic directions are anchored to absolute radiometric ages while no such information was available in previous studies. This makes possible to correctly evaluate the fluctuation of Earth’s magnetic field from Pliocene to Pleistocene and reveal the firm evidence of possible Levantine excursion. Both Ti-poor and Ti-rich titanomagnetites seem to carry the remanent magnetization with Curie temperatures ranging from 350°C to 537°C. Thirteen flows correspond to the Brunhes chron, one of them exhibits transitional directions, while the remaining six sites belong to the Matuyama chron. New and existing dataset for MVF were used to estimate the paleosecular variation parameters. The selected data include 35 Plio-Quaternary lava flows. After excluding the poor quality data, as well as the transitional directions, the mean paleodeclination is 356.1° and oaleoinclination 39.9°, which agree well with the geocentric axial dipole (GAD) and the expected paleodirections for the Plio-Pleistocene, as derived from the reference poles for the stable North America. The corresponding mean paleomagnetic poles are paleolongitude 226.7° and paleolatitude 86.0°. The virtual geomagnetic pole scatter for the MVF is 15.2°, which is consistent with the value expected from model G at latitude of 20° (this model provides an interpretation of the paleosecular variation at different latitudes for the time of interest). The combined paleomagnetic data, supported by positive reversal test, indicate no paleomagnetically detectable vertical-axis rotations in the study area. The evidence of one transitional directions was detected, which may correspond to the Levantine excursion (360-370 kyr) or unnamed event between 400-420 kyr.  相似文献   

4.
Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.  相似文献   

5.
Lacustrine sediments of the Wilson Creek Formation in the Mono Basin, California, record a paleomagnetic field excursion constrained by 14C and 40Ar/39Ar geochronology to have occurred within the last 50 ka. However, 14C and 40Ar/39Ar ages are discordant, making it difficult to distinguish which of two possible excursions during this period, the Mono Lake or Laschamp, is recorded in the Mono Basin. New 40Ar/39Ar age determinations from sanidine, as well as the first biotite and obsidian ages, for three of the nineteen rhyolitic ashes intercalated with these sediments are presented and compared to previous 14C and 40Ar/39Ar data sets. Although the sanidine ages of the three ashes are stratigraphically consistent with each other and previously determined 40Ar/39Ar ages for other ashes in the Wilson Creek Formation, each is significantly older than 14C ages obtained from stratigraphically equivalent beds, relative paleointensity field correlations, oxygen isotope records, and glacial histories. These data indicate an absence of juvenile, eruptive crystals and most likely reflect the incorporation of crystals from older volcanic centers or underlying sediment. We examine the strengths and weaknesses of all available geochronologic data for the section exposed at Wilson Creek to arrive at an internally consistent set of age constraints. Using these constraints we propose two new relative paleointensity correlations for the section, both of which indicate that the excursion recorded in the Mono Basin occurred at ～30–34 ka on the Greenland Ice Sheet Project 2 (GISP2) ice core time scale.  相似文献   

6.
We present rock-magnetic and paleomagnetic results obtained on samples belonging to a Neogene sequence of 11 successive lava flows and a dyke from La Gomera (Canary Islands, Spain). Analysis of thermomagnetic curves allows to distinguish three types of samples: (i) Type H samples with low-Ti titanomagnetite as the only carrier of remanence; (ii) type M samples with a main intermediate Curie-temperature phase (TC = 450°C) and low-Ti titanomagnetite; (iii) type L curves with a low Curie-temperature phase (TC = 120 to 200°C) and an intermediate Curie-temperature phase (TC = 400°C). Analysis of hysteresis parameters suggests that the grain size of most studied samples corresponds to pseudo single-domain particles, which can be also interpreted as a mixture of single-domain and multi-domain particles. Paleomagnetic experiments reveal only a single paleomagnetic component. Characteristic remanence of all studied lava flows and the dyke shows reverse polarity. The mean direction of the whole sequence is D = 188.2°, I =−35.4° (k = 46.9; α95 = 6.4°) and the calculated paleomagnetic pole yields a longitude λ= 150.7° and a latitude ϕ= 78.8° (k = 59.4; A95 = 5.7°). Secular variation is analysed through the scatter of virtual geomagnetic poles (VGP). A VGP angular scatter SB = 5.9 with an upper confidence limit Sup = 8.0 and a lower confidence limit Slow = 4.6 are obtained. This scatter is clearly smaller than the average for this latitude obtained for the last 5 Ma. The studied lava flows were probably emitted in a relatively short time interval.  相似文献   

7.
The palaeomagnetic and rock magnetic characteristics of some Cenozoic rocks from the Cairo–Fayum area have been investigated. A total number of 259 oriented core samples were collected at 32 sites located in rocks of Eocene (13 sites), Oligocene (11 sites) and Pliocene (9 sites) ages. Most of these rocks carry a weak but stable remanent magnetisation that is principally carried by hematite. Goethite and magnetite are also found in some samples as subordinate constituents. Careful thermal demagnetisation successfully enabled the isolation of the characteristic remanent magnetisation. Normal and reversed polarities that passed a reversal test have been recorded in the three age groups. This magnetisation is probably of primary origin and reflects the ages of the rocks. The resultant palaeomagnetic poles are considered reliable and represent a good contribution to the African palaeomagnetic database and should help in further refining of the Cenozoic APWP for Africa.  相似文献   

8.
The paleosecular variation (PSV) and polarity transitions are two major features of the Earth’s magnetic field. Both PSV and reversal studies are limited when age of studied units is poorly constrained. This is a case of Central and western Mexico volcanics. Although many studies have been devoted to these crucial problems and more than 200 paleomagnetic directions are available for the last 5 Ma, only few sites were dated directly. This paper presents new paleomagnetic results from seventeen independent cooling units in the Michoacán-Guanajuato Volcanic Field (MGVF) in western Mexico. Twelve sites are directly dated by 40Ar/39Ar or K-Ar methods and span from 2.78 to 0.56 Ma. The characteristic paleodirections are successfully isolated for 15 lava flows. The mean paleodirection (inclination I and declination D) obtained in this study is I = 28.8°, D = 354.9°, and Fisherian statistical parameters are k = 28, α95 = 7.3°, N=15, which corresponds to the mean paleomagnetic pole position Plat = 83.9°, Plong = 321.6°, K = 34, A95 = 6.6°. The paleodirections obtained in present study compiled with those, previously reported from the MGVF, are practically undistinguishable from the expected Plio-Quaternary paleodirections. The paleosecular variation is estimated through the study of the scatter of the virtual geomagnetic poles giving SF = 15.9 with SU =21.0 and SL = 12.7 (upper and lower limits respectively). These values agree reasonably well with the recent statistical Models. The oldest sites analyzed (the Santa Teresa and Cerro Alto) yield normal polarity magnetizations as expected for the cooling units belonging to the Gauss geomagnetic Chron. The interesting feature of the record comes from lava flows dated at about 2.35 Ma with clearly defined normal directions. This may point out the possible existence of a normal polarity magnetization in the Matuyama reversed Chron older than the Reunion and may be correlated to Halawa event interpreted as the Cryptochron C2r.2r-1. Another important feature of the geomagnetic record obtained from the MGVF is the evidence of fully reversed geomagnetic field within Bruhnes Chron, at about 0.56 Ma corresponding to the relative paleointensity minimum of global extent found in marine sediments at about 590 ka.  相似文献   

9.
Numerous records of the Matuyama-Brunhes geomagnetic transition have been obtained from paleomagnetic studies. Because few of the reversal records are of acceptable reliability, however, the exact behavior of the field during the transition has remained enigmatic. To provide confirmation of one of the more reliable records, we have re-examined the transition at two sites, 150 m apart, in lake sediments of Tecopa basin, southeastern California. The two sites are geographically very close to that of Valet et al. [10], who previously obtained a record from that site indicating that the transitional field was non-dipolar and axisymmetric.

The Matuyama-Brunhes reversal is recorded differently at each of our two sites and at that of Valet et al. [10]. Zones of mixed polarities and/or intermediate directions occur at all three sites but they differ greatly in polarity character, thickness and stratigraphic position. It appears that all three sites have provided mutually contradictory records of the transition. It is unlikely, therefore, that any of the records is acceptable for establishing the nature of the transition at this locality.

Obliteration of the transition is apparently the result of acquisition of a stable, normal-polarity overprint that appears to consist of two remanence components, one acquired during post-depositional compaction and dewatering, and one during later sediment diagenesis.  相似文献   


10.
Abstract We carried out paleomagnetic measurements and K–Ar dating on Neogene andesitic lavas and sills of the Shigarami Formation in North Fossa Magna, central Japan. The Shigarami Formation is distributed in the axial part of the Komiji Syncline in the folding zone of the southwestern North Fossa Magna. Results of the present study indicate that the Komiji Syncline was formed shortly after 4.42 ± 0.12 Ma during the Pliocene. The sedimentary rocks of the Shigarami Formation consist of shallow marine and fluvial deposits. Intrusions of andesitic sills are found in the shallow marine deposits and two andesitic lava flows are present in the fluvial deposits. Oriented samples were taken from the sills at four sites and from the lavas at three sites. The samples produced stable remanent magnetization through stepwise alternating field and thermal demagnetizations. Results of a positive fold test indicate that the stable remanent magnetizations concentrate around a mean reversed polarity of declination = 169.0°, inclination = ?58.5° and 95% confidence limit = 9.0° after corrections have been made according to the direction of the bedding of the sedimentary rocks. Four fresh samples were selected for K–Ar dating from the samples used for paleomagnetic measurements. The groundmass of three samples taken from the sills yield ages of 4.42 ± 0.12, 4.49 ± 0.22 and 4.69 ± 0.13 Ma, whereas the one taken from the lower lava has an age of 5.91 ± 0.26 Ma. We believe that the Komiji Syncline was formed after the emplacement of lavas and sills in the area, because the descending Miocene strata were folded concordantly with the Shigarami Formation. The Pliocene and Pleistocene strata rest unconformably on the folded strata. The deformation might have progressed during the Pliocene, then slowed down in the Early Pleistocene. Our results suggest that the northwestward motion of the Philippine Sea Plate and the collision of the Tanzawa Block affected not only the South Fossa Magna, but also the North Fossa Magna.  相似文献   

11.
In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250-330°C) and magnetite (demagnetising spectra: 15-50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by 40K/39Ar cooling and 40Ar/39Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N-S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38±8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3±11.7° (LH/Alaknanda valley) and 11.3±8.5° (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process.  相似文献   

12.
We report detailed rock-magnetic and paleomagnetic directional data from 35 lava flows (302 standard paleomagnetic cores) sampled in the Central-Northern region of Uruguay in order to contribute to the study of the paleosecular variation of the Earth’s magnetic field during early Cretaceous and to obtain precise Cretaceous paleomagnetic pole positions for stable South America. The average unit direction is rather precisely determined from 29 out of 35 sites. All A95 confidence angles are less than 8°, which points to small within-site dispersion and high directional stability. Normal polarity magnetizations are revealed for 19 sites and 10 are reversely magnetized. Two other sites yield well defined intermediate polarities. The mean direction, supported by a positive reversal test is in reasonably good agreement with the expected paleodirection for Early Cretaceous stable South America and in disagreement with a 10° clockwise rotation found in the previous studies. On the other hand, paleomagnetic poles are significantly different from the pole position suggested by hotspot reconstructions, which may be due to true polar wander or the hotspot motion. Our data suggest a different style of secular variation during (and just before) the Cretaceous Normal Superchron and the last 5 Ma, supporting a link between paleosecular variation and reversal frequency.  相似文献   

13.
We present new 40Ar/39Ar ages and paleomagnetic data for São Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. 40Ar/39Ar age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K–Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across São Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The 40Ar/39Ar ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0–0.1 Ma) and up to 0.78 Myr (0–0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78–0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction.  相似文献   

14.
 Outflow sheets of the Hiko tuff and the Racer Canyon tuff, which together extend over approximately 16 000 km2 around the Caliente caldera complex in southeastern Nevada, have long been considered to be products of simultaneous or near-simultaneous eruptions from inset calderas in the west and east ends, respectively, of the caldera complex. New high-precision 40Ar/39Ar geochronology and paleomagnetic data demonstrate that emplacement of the uppermost part of the Racer Canyon tuff at 18.33±0.03 Ma was nearly synchronous with emplacement of the single outflow cooling unit of the much larger overlying Hiko tuff at 18.32±0.04 Ma. Based on comparison with the geomagnetic polarity time scale derived from the sea-floor spreading record, we conclude that emplacement of the first of several outflow cooling units of the Racer Canyon tuff commenced approximately 0.5 m.y. earlier. Only one paleomagnetic polarity is found in the Hiko tuff, but at least two paleomagnetic reversals have been found in the Racer Canyon tuff. The two formations overlap in only one place, at and near Panaca Summit northeast of the center of the Caliente caldera complex; here the Hiko tuff is stratigraphically above the Racer Canyon tuff. This study demonstrates the power of combining 40Ar/39Ar and paleomagnetic data in conjunction with phenocryst compositional modes to resolve problematic stratigraphic correlations in complex ash-flow sequences where use of one method alone might not eliminate ambiguities. Received: 13 January 1997 / Accepted: 7 May 1997  相似文献   

15.
A joint palaeomagnetic and 40Ar/39Ar study has been performed on two olistolithic blocks from the Cabrières Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn-Early Caradoc age (ca 470-458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and 40Ar/39Ar whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence.The new data show that the Montagne Noire region was located at high southerly latitudes (68° +17/-15) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician.  相似文献   

16.
Paleomagnetic measurements have been made on a continuously sampled, 5-m section of a core from Clear Lake, California. The sediments studied span an 8000-year interval centered at 25,000 years B.P., the approximate date of the large-scale, counterclockwise loop of the magnetic vector recorded in sediments from Mono Lake, California. The data from Mono Lake have been interpreted as a geomagnetic excursion with a duration of 600–1000 years. Because Clear Lake is only 320 km from Mono Lake and since each sample from Clear Lake represents 26 years of sedimentation, the magnetic signature of the Mono Lake geomagnetic excursion should be recorded in detail in the Clear Lake samples. Aside from a generally uniform shallowing of inclinations due to compaction, the paleomagnetic record from Clear Lake contains no anomalous features which would correspond to the Mono Lake excursion. Thus it has yet to be shown that the Mono Lake excursion was recorded anywhere besides Mono Lake. Even if the existence of the excursion is ultimately confirmed, its usefulness as a magnetostratigraphic horizon is limited.  相似文献   

17.
Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.  相似文献   

18.
Cambro-Ordovician redbeds have been sampled in the Buçaco syncline, an Hercynian structure of northern Portugal. Secondary and characteristic components of magnetisation have been recovered for samples from seven sites on the same limb of the syncline using thermal and chemical demagnetisation techniques, suggesting both pre- and post-tectonic magnetisations. The secondary magnetisations include: (1) the present Earth field direction with normal and reverse polarity, interpreted as a stable Quaternary magnetisation; and (2) a group with only normal polarities, of constant inclination (50°) but variable declination (320–350°). The mean inclination of this second group suggests a Cretaceous origin, and acquisition of magnetisation during the opening of the Bay of Biscay could explain the data.There are two periods of sedimentation after deposition of the Cambro-Ordovician redbeds that correspond to the acquisition of the secondary remanent magnetisations. We propose a process involving the fossilisation of alteration products immediately preceding sedimentation in order to explain this correlation.  相似文献   

19.
Seventy sites of sills, flows and dikes from Northeastern Paraná Magmatic Province (PMP), were submitted to paleomagnetic, chemical and radiometric analyses. The rocks are high in TiO2 content, and similar in composition to the rocks from the northern region of PMP. The sills intrude mainly Paleozoic sediments, and can be subdivided into two domains; the northern being characterized by sills showing reversed polarities, and the southern essentially by sills of normal polarities. 40Ar/39Ar dating of three distinct sills gave plateau ages (129.9 ± 0.1, 130.3 ± 0.1 and 131.9 ± 0.4 Ma) that are similar to surface-outcropping flows of the Northern Paraná Basin, and the Ponta Grossa dikes. The new paleomagnetic data combined with existing data from the northern PMP allowed the calculation of a paleomagnetic pole at 71.4° E and 83.0° S (N = 92; α95=2.4°; k = 39). This pole is in good agreement with poles for central and southern PMP, which are slightly older than the northern PMP, as well as for the contemporaneous Central Alkaline Province (Paraguay) on the western side of PMP. In contrast, the coeval pole for the Ponta Grossa dikes (eastern border of PMP), however, is slightly displaced from that group of poles, suggesting that dikes in that area may have undergone some tectonic tilting.  相似文献   

20.
Paleomagnetic polarity data were obtained from nine sections of the Verde Formation, a late Tertiary carbonate-bearing lacustrine unit in central Arizona. This study tested the applicability of magnetostratigraphy as a geochronologic technique in a restricted terrestrial sedimentary basin, and its objective was to better define the age of the Verde Formation.Intensities of natural remanent magnetism (NRM) ranged from <10?7 to >10?4 gauss. Although secondary components of viscous magnetization commonly were observed, alternating field demagnetization was successful in revealing the polarity of the primary NRM at almost all sites. Thermomagnetic analysis, partial thermal demagnetization of NRM, and polished section analysis together indicate that the primary NRM is a depositional remanence carried by detrital magnetite. Intrabasin stratigraphic correlation of the sections, together with K-Ar ages on interbedded and underlying volcanic rocks has allowed construction of a composite magnetostratigraphic column for the Verde Formation that is correlated with the late Cenozoic polarity time scale. The correlation indicates nearly continuous sedimentation in the Verde basin from ～7.5 to ～2.5 m.y. ago.  相似文献   

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