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1.
Measurement of samples from 154 sites in the continental sector of the Cameroon Volcanic Line yielded six palaeomagnetic poles, at 243.6°E, 84.6°N, α 95 = 6.8°; 224.3°E, 81.2°N, α 95 = 8.4°; 176.1°E, 82.0°N, α 95 = 8.5°; 164.3°E, 86.4°N, α 95 = 3.4°; 169.4°E, 82.6°N, α 95 = 4.6° and 174.7°E, 72.8°N, α 95 = 9.5°, belonging to rocks which have been dated by the K–Ar method at 0.4–0.9  Ma, 2.6  Ma, 6.5–11  Ma, 12–17  Ma, 20–24  Ma and 28–31  Ma, respectively. The results are in general agreement with other palaeomagnetic poles from Oligocene to Recent formations in Africa.
  The first three poles for rocks formed between 0.4 and 11  Ma are not significantly different from the present geographical pole. Together with other African poles for the same period, this suggests that the African continent has moved very little relative to the pole since 11  Ma. The other three poles for rocks dated between 12 and 31  Ma are significantly different from the present geographical pole, showing a 5° polar deviation from the present pole in the Miocene and 13° in the Middle Oligocene.  相似文献   

2.
Summary. From nine Upper Cretaceous—Lower Tertiary (85 ± 5–66 ± 5 Ma) volcanic hills in Central Argentina (33°S, 65°W), 26 hand samples were collected yielding a palaeomagnetic pole at 45°E 70°s ( A 95 = 12.1°; k = 13.6; N = 12) after AC cleaning. Three sites show normal and nine reversed polarity. This pole is close to the pole for the late Cretaceous (69 Ma) Andacolo Series.  相似文献   

3.
Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α95= 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α95= 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent 40Ar/39Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed.  相似文献   

4.
A palaeomagnetic study of 115 samples (328 specimens) from 22 sites of the Mid- to Upper Cretaceous Bagh Group underlying the Deccan Traps in the Man valley (22°  20'N, 75°  5'E) of the Narmada Basin is reported. A characteristic magnetization of dominantly reverse polarity has been isolated from the entire rock succession, whose depositional age is constrained within the Cretaceous Normal Superchron. Only a few samples in the uppermost strata have yielded either normal or mixed polarity directions. The overall mean of reverse magnetization is D m=144°, I m=47° ( α 95=2.8°, k =152, N =18 sites) with the corresponding S-pole position 28.7°S, 111.2°E ( A 95=3.1°) and a palaeolatitude of 28°S±3°. The characteristic remanence is carried dominantly by magnetite. Similar magnetizations of reverse polarity are also exhibited by Deccan basalt samples and a mafic dyke in the study area. This pole position falls near the Late Cretaceous segment of the Indian APWP and is concordant with poles reported from the Deccan basalt flows and dated DSDP cores (75–65  Ma) of the Indian Ocean. It is therefore concluded that the Bagh Group in the eastern part of the Narmada Basin has been pervasively remagnetized by the igneous activity of Deccan basalt effusion. This overprinted palaeomagnetic signature in the Bagh Group indicates a counter-clockwise rotation by 13°±3° and a latitudinal drift northwards by 3°±3° of the Indian subcontinent during Deccan volcanism.  相似文献   

5.
We evaluate the stress field in and around the southern Korean Peninsula with focal mechanism solutions, using the data collected from 71 earthquakes ( ML = 1.9–5.2) between 1999 and 2004. For this, the hypocentres were relocated and well-constrained fault plane solutions were obtained from the data set of 1270 clear P -wave polarities and 46 SH / P amplitude ratios. The focal mechanism solutions indicate that the prevailing faulting types in South Korea are strike-slip-dominant-oblique-slip faultings with minor reverse-slip component. The maximum principal stresses (σ1) estimated from fault-slip inversion analysis of the focal mechanism solutions show a similar orientation with E–W trend (269°–275°) and low-angle plunge (10°–25°) for all tectonic provinces in South Korea, consistent with the E–W trending maximum horizontal stress (σHmax) of the Amurian microplate reported from in situ stress measurements and earthquake focal mechanisms. The directions of the intermediate (σ2) and minimum (σ3) principal stresses of the Gyeongsang Basin are, however, about 90 deg off from those of the other tectonic provinces on a common σ2–σ3 plane, suggesting a permutation of σ2 and σ3. Our results incorporated with those from the kinematic studies of the Quaternary faults imply that NNW- to NE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea.  相似文献   

6.
207Pb/206Pb single-grain zircon, 40Ar/39Ar single-grain hornblende and biotite, and 40Ar/39Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4  Ma to about 3000  Ma. Whole-rock 40Ar/39Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900  Ma. There is no evidence of this or other, later events significantly affecting the argon systematics of the minerals from the pluton dated by the 40Ar/39Ar method.
  The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by 40Ar/39Ar dating of hornblende. This pole (18°N, 310°E, A 95=9°) yields a palaeolatitude of 0°, significantly different from other Archaean poles from the Kaapvaal Craton. The palaeolatitude difference implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150–1950  Ma) poles for southern Africa.  相似文献   

7.
Summary. Palaeomagnetic results are presented from the c . 160 km2 Caledonian synorogenic layered Fongen-Hyllingen gabbro complex (of probable late Silurian age) located about 75 km SE of Trondheim, Norway, in the allochthonous Seve-Kdli Nappe Complex. A total of 80 oriented samples from eight sites in the northern part of the gabbro were investigated. After detailed af demagnetization two stable high coercivity components emerge: one with a well defined NW direction with D =325°, I =−21° (α95=8°, N =8), and another, less well defined, probably younger, SW direction with D = 237°, I = 6° (α95= 9°, N = 8). Correction for dip of these two directions gives D = 329°, I =−7° (α95= 10°) and D = 238°, I =−11° (α95= 12°), respectively. The corresponding pole positions are P 1 : 19° N, 225° E and P 2: 19° S, 308° E, respectively. The reversed pole -P 2 of the SW direction lies close to other NW European palaeomagnetic poles of Caledonian, Upper Silurian-Lower Devonian age. However, the dominant pole PI is far away from these, and could be due to a late Caledonian geomagnetic excursion of considerable duration; or it could record a c . 90° rotation around a vertical axis of a crustal block within the Scandinavian Caledonides. Block rotation could have been related to nappe translation, although geological observations do not at present appear to support the occurrence of such an event.  相似文献   

8.
Further evidence for oceanic excitation of polar motion   总被引：3，自引：0，他引：3  
While the role of the atmosphere in driving variations in polar motion is well established, the importance of the oceans has been recognized only recently. Further evidence for the role of the oceans in the excitation of polar motion is presented. To estimate the equatorial excitation functions, χ 1 and χ 2 , for the ocean, we use velocity and mass fields from a constant-density ocean model, driven by observed surface wind stresses and atmospheric pressure, for the period 1993–1995; comparison with similar functions derived from a more complex density-stratified ocean model indicates the effectiveness of the simple constant-density modelling approach. Corresponding atmospheric excitation functions are computed from NCEP/NCAR re-analyses. Results indicate significant improvements in the agreement with the observed polar motion excitation when the simulated oceanic effects are added to atmospheric excitation. Correlations between the polar motion and the geophysical signals at periods of 15–150 days increase from 0.53 to 0.80 and from 0.75 to 0.88 for χ 1 and χ 2 , respectively. The oceanic signals are particularly important for seasonal variations in χ 1 (correlation increases from 0.28 to 0.85 when oceanic excitation is included). A positive impact of the oceans on more rapid polar motion is also observed, up to periods as short as 5 days. The sensitivity of the results to different forcing fields and different amounts of friction in the oceans is also discussed.  相似文献   

9.
Summary Nine basic dykes were sampled near Angmagssalik, east Greenland. Specimens have been treated by alternating field demagnetization in 11 steps up to 3000 (peak) oersted (300 ml). The 'cleaned' direction at all sites is recognized after treatment at 150 oersted. All specimens are reversely magnetized. The mean of the site mean directions has declination = 182°.0, inclination =−66°.9, it = 45, α95= 7°.7. This direction yields a palaeomagnetic pole (reversed) at 73°.4N, 139°.5E ( dp = 10°.7, dm = 12°.9) which is near, but significantly different from, that derived from lower Tertiary rocks in Greenland, namely 63°.2N, 184°.6E ( A 95= 4°.5). K-Ar ages of the nine dykes, based upon whole-rock and mineral separates, range from mid-Tertiary to Cambrian. It is impossible to reconcile these ages with the palaeomagnetic results. The palaeomagnetic evidence, supported by geological inference, suggests that all nine dykes are members of the east Greenland lower Tertiary dyke swarm, designated THOL1, of probable age c. 52 Ma.
The difference between the poles given above can be explained by supposing that the sampling area has tipped about a horizontal axis directed along 013°/193°, the angle of rotation being 13° (± 11°) anti-clockwise, when the axis is viewed along 013°. This local effect could have been due to block faulting when the north-east Atlantic started to open, or may be attributed to upwarping of the coast due to the weight of the ice-cap inland.  相似文献   

10.
Summary. Fifty-six orientated samples were collected from 13 sites on five dolerite dykes (between lat.14°23°N, long.77°43'E and lat.14°08'N, long. 77°49'E), which adjoin the south-western margin of the Cuddapah basin in Anantapur district, Andhra Pradesh. After af demagnetization, two dykes (five sites) striking ENE possess similar magnetic directions, (1) D = 57°, 1=-69° (K = 52, α95= 7°) and (2) D = 71°, I = -72° ( K = 260, α95= 5°). Again dykes (3) (three sites), and (4) (two sites) have similar strike (NE) and magnetic directions, D = 64°. I=-7 α( K = 142, α95 = 8°) and D = 53°, I =-8° (K = 142, α95= 6°) and dyke (5) (two sites) striking NW shows D = 320°, I = -34° ( K = 68, α95= 13°). Remanent directions estimated from total field magnetic anomaly data agree well with these results. Synthesis of these data with 10 other published palaeomagnetic studies of Precambrian dolerite dykes on the Indian peninsula, suggest that these three systems of dykes adjoining the Cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield. The radiometric data, however, are very sparse and these periods cannot be dated with confidence.  相似文献   

11.
Palaeomagnetic data for the Cretaceous Pirgua Subgroup from 14 different time units of basalts and red beds exposed in the north-western part of Argentina (25° 45' S 65° 50' W) are given.
After cleaning all the units show normally polarized magnetic remanence and yield a palaeomagnetic pole at 222° E 85° S ( d Φ= 7°, d χ= 10°).
The palaeomagnetic poles for the Pirgua Subgroup (Early to Late Cretaceous, 114–77 Myr), for the Vulcanitas Cerro Rumipalla Formation (Early Cretaceous,<118 Myr, Valencio & Vilas) and for the Poços de Caldas Alkaline Complex (Late Cretaceous, 75 Myr, Opdyke & McDonald) form a 'time-group' reflecting a quasi-static interval (mean pole position, 220° E 85° S, α95= 6°) and define a westward polar wander in Early Cretaceous time for South America.
Comparison of the positions of the Cretaceous palaeomagnetic poles for South America with those for Africa suggests that the separation of South America and Africa occurred in late Early Cretaceous time, after the effusion of the Serra Geral basalts.
The K-Ar ages of basalts of the Pirgua Subgroup (114 ± 5; 98 ± 1 and 77 ± 1 Myr) fix points of reference for three periods of normal polarity within the Cretaceous palaeomagnetic polarity column.  相似文献   

12.
Oxygen consumption by rotifers Macrotrachela musculosa and Trichotria truncata from Spitsbergen tundra (77°N) was measured using the method of Cartesian divers. The metabolic rate of M. musculosa was: 0.205 10−3mm3 02 per g 10−6 per hour at 2°C, 0.201 10−6mm3 at 6°C and 0.616 10−3mm3 02 per g 10−6 per hour at 10°C. The metabolic rate of Trichotria truncata at 6° was 0.103 10−3mm3 per g 10−6 per hour. The relation between body weight and oxygen consumption by M. musculosa at 2°C is expressed with the equation R = 0.18W0.67, with R – oxygen consumption in mm310−3 per individual per hour and W – wet weight of an animal in g 10−6.  相似文献   

13.
Summary. The Cordova gabbro of southern Ontario intrudes 1300 Myr old volcanic rocks of the Hastings Lowlands in the Grenville Structural Province. Three distinct vector magnetizations (A, B and C) have been isolated, using a combination of stable endpoints, subtracted vectors from orthogonal vector plots and converging remagnetization circles. The A magnetization, with mean direction D = 294° I =– 55.5° ( k = 42, α95= 5.5°, N = 18 sites), is a high coercivity, high blocking temperature remanence recorded by 49 samples. The B magnetization was isolated in 33 samples and has a mean direction D = 305.5° I =– 1.5° ( k = 24, α95, N = 11 sites). B has lower coercivities and blocking temperatures than A where the two are superimposed. The A and B palaeopoles, 151°E, 10.5°S ( dp = 6°, dm = 8°) and 165.5°E, 24°N ( dp = 5°, dm = 9.5°), fall on the Grenville Track around 900 and 820 Ma respectively. The A and B magnetizations thus date from uplift and cooling following the Grenvillian orogeny. The third magnetization, the C component, has been isolated in 23 samples. Its mean direction is D = 180° I = 27.5° ( k = 18, α95= 10.5°, N = 12 sites). The C is a low coercivity, low blocking temperature overprint of A and B. Its palaeopole, 102°E, 31°N ( dp = 6.5°, dm = 12°), is unlike post-1300 Precambrian poles for cratonic North America but matches Silurian and late Ordovician poles. 40Ar/39Ar plateau ages of 446 and 447 Ma determined by Lopez-Martinez and York for plagioclases from one of the Cordova samples confirm this age assignment. The C magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in Palaeozoic time, long after the Grenvillian orogeny.  相似文献   

14.
Summary. Stable components of magnetization have been isolated in 15 lava flows (mean K-Ar age 123 ± 4 Myr) from the alkaline sequence outcropping at El Salto-Almafuerte, Province of Cordoba, Argentina. Magnetic and geologic stratigraphy, as well as K-Ar ages indicate that this sequence was probably extruded in the Lower Cretaceous during the first volcanic cycle of the Sierra de los Cóndores Group (Vulcanitas Cerro Colorado Formation).
The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK7, is: 25° E, 72° S ( k = 35, α95= 6.5°); it is fairly close to other Lower Cretaceous palaeomagnetic poles for South America. The elongated distribution of Cretaceous palaeomagnetic poles suggest recurrent drift for South America in early Cretaceous time.
The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations.  相似文献   

15.
Summary. A method that enables the objective resolution of almost parallel multi-component magnetizations is described and demonstrated. A feature distinguishing this method from others is its simultaneous analysis of demagnetization data from a group of specimens, rather than the analysis of data from one specimen at a time. The only prerequisite is that the specimens are derived from a homogeneous source. Thus for a formation carrying a simple single component magnetization, all specimens from the formation may be simultaneously reduced. For a more complicated two component magnetization it is shown that only specimens from a particular site can be considered homogeneous, and for a complex three component system each sample often requires undivided attention. Thus the workload is proportionally increased to achieve analyses of comparable reliability from data of variable quality.
New pole positions from Mesozoic intrusions of the Sydney Basin, NSW are: from the Marsden Park Breccia pipe 48°S, 127°E ( A 95= 6°); the St Marys Breccia pipe 46°S, 150°E ( A 95= 8°); the Prospect Dolerite 60°S, 142°E ( A 95= 13°) and 53°S, 180°E ( A 95= 6°); and from the Dundas Breccia pipe 58°S, 162°E ( A 95= 36°) and 31°S, 195°E ( A 95= 16°). The last two formations possess multi-component magnetizations. These pole positions are consistent with previous results from south-eastern Australia.  相似文献   

16.
Upper Jurassic red sandstones and red siltstones were collected from 67 layers at 12 localities in the Penglaizhen formation. This formation is in the north of Bazhong county (31.8°N, 106.7°E) in the Sichuan basin, which is located in the northern part of the Yangtze craton. Thermal demagnetization isolated a high-temperature magnetic component with a maximum unblocking temperature of about 690 °C from 45 layers. The primary nature of the magnetization acquisition is ascertained through the presence of magnetostratigraphic sequences with normal and reversed polarities, as well as positive fold and reversal tests at the 95 per cent confidence level. The tilt-corrected mean direction of 36 layers is D = 20.0°, I = 28.8° with α 95 = 5.8°. A Late Jurassic palaeomagentic pole at 64.7°N, 236.0°E with A 95 = 7.0° is calculated from the palaeomagnetic directions of 11 localities. This pole position agrees with the two other Late Jurassic poles from the northern part of the Yangtze craton. A characteristic Late Jurassic pole is calculated from the three poles (68.6°N, 236.0°E with A 95 = 8.0°) for the northern part of the Yangtze craton. This pole position is significantly different from that for the southern part of the Yangtze craton. This suggests that the southern part of the Yangtze craton was subjected to southward extrusion by 1700 ± 1000  km with respect to the northern part. Intracraton deformation occurred within the Yangtze craton.  相似文献   

17.
By inversion analysis of the baseline changes and horizontal displacements observed with GPS (Global Positioning System) during 1990–1994, a high-angle reverse fault was detected in the Shikoku-Kinki region, southwest Japan. The active blind fault is characterized by reverse dip-slip (0.7±0.2  m yr−1 within a layer 17–26  km deep) with a length of 208±5  km, a (down-dip) width of 9±2  km, a dip-angle of 51°±2° and a strike direction of 40°±2° (NE). Evidence from the geological investigation of subfaults close to the southwestern portion of the fault, two historical earthquakes ( M L=7.0, 1789 and 6.4, 1955) near the centre of the fault, and an additional inversion analysis of the baseline changes recorded by the nationwide permanent GPS array from 18 January to 31 December 1995 partially demonstrates the existence of the fault, and suggests that it might be a reactivation of a pre-existing fault in this region. The fact that hardly any earthquakes ( M L>2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in this study, we evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar yr−1 at a depth of about 20  km around the epicentral area of the 1995 January 17  Kobe earthquake ( M L=7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar yr−1 during 1990–1994. The results suggest that the 1995  Kobe earthquake could have been induced or triggered by aseismic fault movement.  相似文献   

18.
The investigation of L g attenuation characteristics in the region bounding the western branch of the East African rift system using digital recordings from a seismic network located along the rift between Lake Rukwa and Lake Malawi is reported. A set of 24 recordings of L g waves from 12 regional earthquakes has been used for the determination of anelastic attenuation, Q Lg , and regional body-wave magnitude, m b Lg , scale. The events used have body-wave magnitudes, m b , between 4.6 and 5.5, which have been determined teleseismically and listed in ISC bulletins. The data were time-domain displacement amplitudes measured at 10 different frequencies (0.7–5.0  Hz). Q Lg and its frequency dependence, η , in the region can be represented in the form Q Lg = (186.2 ± 6.5)  f  (0.78±0.05). This model is in agreement with models established in other active tectonic regions. The L g -wave-based magnitude formula for the region is given by m b Lg = log   A + (3.76 ± 0.38)  log   D − (5.72 ± 1.06), where A is a half-peak-to-peak maximum amplitude of the 1  s L g wave amplitude in microns and D is the epicentral distance in kilometres. Magnitude results for the 12 regional earthquakes tested are in good agreement with the ISC body-wave magnitude scale.  相似文献   

19.
Summary. Palaeomagnetic data from 71 hand samples of igneous rocks of Late Ordovician age exposed in western Argentina (31.3°S, 69.4°W, Alcaparrosa Formation) are given. Stable remanent magnetization was isolated in the majority of samples; they yield a palaeomagnetic pole at 56°S 33°E ( N = 8, α95= 16°). Whole rock K-Ar age determinations yield an age of 416 ± 10 Myr for a pillow lava of the Alcaparrosa Formation.
Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent.  相似文献   

20.
We report on calculations of the on-shore run-up of waves that might be generated by the impact of subkilometre asteroids into the deep ocean. The calculations were done with the COULWAVE code, which models the propagation and shore-interaction of non-linear moderate- to long-wavelength waves  ( kh < π)  using the extended Boussinesq approximation. We carried out run-up calculations for several different situations: (1) laboratory-scale monochromatic wave trains onto simple slopes; (2) 10–100 m monochromatic wave trains onto simple slopes; (3) 10–100 m monochromatic wave trains onto a compound slope representing a typical bathymetric profile of the Pacific coast of North America; (4) time-variable scaled trains generated by the collapse of an impact cavity in deep water onto simple slopes and (5) full-amplitude trains onto the Pacific coast profile. For the last case, we also investigated the effects of bottom friction on the run-up. For all cases, we compare our results with the so-called 'Irribaren scaling': The relative run-up   R / H 0=ξ= s ( H 0/ L 0)−1/2  , where the run-up is   R , H 0  is the deep-water waveheight, L 0 is the deep-water wavelength, s is the slope and ξ is a dimensionless quantity known as the Irribaren number. Our results suggest that Irribaren scaling breaks down for shallow slopes   s ≤ 0.01  when  ξ < 0.1 − 0.2  , below which   R / H 0  is approximately constant. This regime corresponds to steep waves and very shallow slopes, which are the most relevant for impact tsunami, but also the most difficult to access experimentally.  相似文献   

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