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1.
Jurassic to Cretaceous red sandstones were sampled at 33 sites from the Khlong Min and Lam Thap formations of the Trang Syncline (7.6°N, 99.6°E), the Peninsular Thailand. Rock magnetic experiments generally revealed hematite as a carrier of natural remanent magnetization. Stepwise thermal demagnetization isolates remanent components with unblocking temperatures of 620–690 °C. An easterly deflected declination (D = 31.1°, I = 12.2°, α95 = 13.9°, N = 9, in stratigraphic coordinates) is observed as pre-folding remanent magnetization from North Trang Syncline, whereas westerly deflected declination (D = 342.8°, I = 22.3°, α95 = 12.7°, N = 13 in geographic coordinates) appears in the post-folding remanent magnetization from West Trang Syncline. These observations suggest an occurrence of two opposite tectonic rotations in the Trang area, which as a part of Thai–Malay Peninsula received clockwise rotation after Jurassic together with Shan-Thai and Indochina blocks. Between the Late Cretaceous and Middle Miocene, this area as a part of southern Sundaland Block experienced up to 24.5° ± 11.5° counter-clockwise rotation with respect to South China Block. This post-Cretaceous tectonic rotation in Trang area is considered as a part of large scale counter-clockwise rotation experienced by the southern Sundaland Block (including the Peninsular Malaysia, Borneo and south Sulawesi areas) as a result of Australian Plate collision with southeast Asia. Within the framework of Sundaland Block, the northern boundary of counter-clockwise rotated zone lies between the Trang area and the Khorat Basin.  相似文献   

2.
《Precambrian Research》2004,128(1-2):167-188
Thirty-nine oriented block samples of iron-formation were collected at 13 sites, including opposite limbs of major folds, from the 1.88-Ga Sokoman Formation (Knob Lake Group) in the Schefferville–Knob Lake area of the central New Québec Orogen, northern Québec. The samples assayed up to 80.24% Fe2O3T (54.08% Fe), implying Fe-enrichment of the iron-formation up to ore grade. Anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding-parallel fabric in the iron-formation, suggesting minimal alteration of the magnetic mineralogy since deposition and/or a mimetic secondary magnetic mineralogy. The iron-formation has not been internally deformed since the magnetic mineralogy was established. Analyses by variable-field translation balance and X-ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples. Following low-temperature pre-treatment as appropriate, stepwise thermal and alternating-field demagnetization of 218 specimens revealed a low-temperature, post-folding component (maximum Tub≈400 °C, D=27.1°, I=20.1°, α95=10.9°, from seven sites; pole position of 40.6°S, 257.0°E), and components carried by magnetite (maximum Tub≈580 °C, D=35.8°, I=3.9°, α95=9.1°, from 10 sites; pole position of 29.6°S, 250.9°E) and hematite (maximum Tub≈680 °C, D=40.0°, I=1.6°, α95=18.6°, from seven sites; pole position of 26.8°S, 247.0°E). The components carried by magnetite and hematite are pre-, syn- and post-folding depending on the sampling site, indicating that the magnetization was acquired continuously with deformation in the New Québec Orogen at 1.84–1.83 Ga. No evidence was found for acquisition of magnetization during the Mesozoic, when many of the iron oxide orebodies in the Schefferville–Knob Lake area are thought to have formed. Our findings imply that an episode of Fe-enrichment of iron-formation in the Sokoman Formation involved the circulation of hydrothermal fluids related to late Paleoproterozoic orogenesis. Such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central New Québec Orogen.  相似文献   

3.
《Precambrian Research》1999,93(2-3):201-213
New palaeomagnetic results are presented from the recently dated Palaeoproterozoic ultramafic Konchozero sill, and associated basalts (three sites, 38 oriented samples). Three stable components of remanence have been isolated during thermal and alternating field demagnetisation. The component I, with a mean direction of D=103°, I=40°, k=18, α95=11° (N=11 samples), pole position of 14°S, 282°E, has been obtained from the unaltered deeper part of the sill and from baked schists. The study of the baked contact confirms the conclusion that component I is supposed to be primary and corresponds to the Sm–Nd age of the sill of 1974±27 Ma. The palaeopole of component I is not consistent with the accepted Fennoscandian apparent polar wander path (APWP) for the period 2120–1880 Ma, and for that part the Fennoscandian APWP should be revised. Two other components (component II: D=349°, I=39°, k=35, α95=6°, N=19 samples, pole position 49°N, 231°E; and component III: D=17°, I=41°, k=44, α95=5°, N=19 samples, pole position 50°N, 190°E) fit the APWP well, with palaeomagnetically estimated ages of ca. 1860 and 1760 Ma respectively.  相似文献   

4.
We have conducted a paleomagnetic investigation on the Middle–Upper Jurassic marine strata exposed in the hanging wall of the Tanggula Thrust system near the Yanshiping area, northern Tibet. Progressive demagnetization experiments successfully isolated stable magnetization over a broad spectrum of demagnetization temperatures. The mean direction of the characteristic remanent magnetizations for the Middle–Late Jurassic Yanshiping Group in stratigraphic coordinates (D/I (Declination/Inclination) = 5.6°/60.3°, k = 22.9, α95 = 12.9°, N = 7 s) is much more clustered than the mean direction in geographic coordinates (D/I = 345.5°/37.2°, k = 2.5, α95 = 48.4°), indicating magnetization was not acquired after folding. Although the conventional fold test is positive, incremental untilting test on the characteristic remanent magnetization reveals that a maximum value of precision parameter k occurs at 82.1 ± 4.6% untilting (D/I = 3.3°/57.8°, k = 43.9, α95 = 9.2°), which indicates the ChRMs are probably acquired during Late Cretaceous folding. This synfolding magnetization component is therefore secondary. The corresponding pole position (84.4°N, 119.4°E with dp/dm = 13.5/9.9°) is inconsistent with Jurassic–Early Cretaceous paleopoles of the region, but the paleolatitude is consistent with the Late Cretaceous paleolatitude observed in the Qiangtang terrane and its periphery. The synfolding component is carried by both magnetite and hematite, which were identified by isothermal remnant magnetization acquisition experiments, unblocking temperatures of stable magnetic components, and Curie temperature determination and correlated with observed hydrothermal veins. Available geological evidences indicate that the synfolding magnetization is probably the result of chemical remagnetization caused by orogenic fluids or hydrothermal sources during the early uplift of the Tibetan Plateau.  相似文献   

5.
A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8 ± 0.46 Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D = 348.0°, I = 47.3°, k = 51.0, α95 = 5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A95 = 5.7° and yielding a paleolatitude of 29.3° ± 5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~ 100 Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D = 312.6°, I = 47.7°, k = 109.7, α95 = 3.0°, N = 22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A95 = 3.2° (yielding a paleolatitude of 28.7° ± 3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~ 100 Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~ 550 km N–S convergence between the Qiangtang and Lhasa terranes happened after ~ 100 Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~ 100 Ma, which is most likely caused by the India–Asia collision.  相似文献   

6.
The Linzizong Group (64–44 Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D = 183.6°, I = −12.4° (α95 = 8.1°) for the Dianzhong (64–60 Ma), D = 1.0°, I = 18.1° (α95 = 8.1°) for the Nianbo (60–50 Ma), and D = 12.4°, I = 23.2° (α95 = 7.3°) for the Pana (50–44 Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5 ± 1.5 Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.  相似文献   

7.
A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM (D = 131.8°, I = 15.7°, k = 196, α95 = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.  相似文献   

8.
We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α95=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.  相似文献   

9.
A combined magnetic fabric and paleomagnetic study has been carried out on the siliciclastic rocks gathered from a stratigraphic cross-section through the Nanpanjiang Basin, South China, in an attempt to extract the paleoflow information preserved in and, thus, constrain the possible origins of these clastic rocks. The sediments used for this study were formed by sediment-gravity flows along the southern margin of the South China block in the Middle Triassic time (ca. 245–228 Ma). The results show a normal distribution of both low field magnetic susceptibility values and natural remanent magnetization intensities, which along with the monotonic detrital framework mode, mainly comprising quartz and lithic particles, may suggest a single provenance involved in deposition of these clastic deposits. Anisotropy of magnetic susceptibility (AMS) analysis acquires primarily the sedimentary magnetic fabrics, which, in this study, reveal paleoflow directions ranging from NNW to ENE with an overall mean orientation of NE. Demagnetization on a part of samples isolates a characteristic remanent component averaged at D = 44.8°, I = 16.9°, κ = 9.7, α95 = 6. 5°, n = 55, corresponding to a paleolatitude N8.6° and a clockwise rotation of ca. 45° since the Middle Triassic for the studied cross-section. This mean direction passes fold tests and is consistent with the reference direction expected from the South China block at the 95% confidence level. Restoring this ∼45° declination renders an overall northward paleoflow, which, combined with other evidence, suggests a southern provenance for these sediments during deposition in the Middle Triassic time. In terms of the early Mesozoic plate framework of southeastern Asia, a tectonic scenario is proposed here, whereby the nearly N–S convergence of the Indochina and South China blocks and its related Indosinian orogeny in the Middle Triassic caused the formation of the Nanpanjiang foreland basin, which was filled by voluminous detritus shed from the uplifted orogenic belt on its southern side.  相似文献   

10.
We conducted paleomagnetic investigations on limestone from the Lower Carboniferous Huaitoutala Formation in the Qaidam Basin near Delingha City, Qinghai Province, China. The characteristic remanent magnetization (D = 5.8°, I = − 25.7°, k = 114.3, α95 = 4.8°) passes a fold test and indicates a paleopole position of − 39.2°N, 90.4°E and a paleolatitude of 13.5°N for the Qaidam Block for the early Carboniferous. Based on global tectonic reconstructions and paleontological evidence, we suggest that the Qaidam Block was adjacent to, but independent from, the North China, South China, Alashan–Hexi and Tarim blocks at this time. This result suggests that Pre-Carboniferous sutures reported around the Qaidam Basin represent collisional events within Gondwana, rather than the final sutures that gave rise to the present tectonic configuration.  相似文献   

11.
Paleomagnetic investigations of the folded Upper Namurian–Lower Moscovian “Hassi Bachir” Formation cropping out in the “Ahnet” basin (Central Sahara, Algeria) yield two magnetic components. A pre-folding primary magnetization (D = 136.1°, I = 22.0°, k = 217, α95 = 2.6°) enables us to define a paleomagnetic pole (32.8°S, 55.7°E, K = 328 and A95 = 2.0°) which better constrains a paleopole that was determined by Daly and Irving [Daly, L., Irving, E., 1983. Paléomagnétisme des roches carbonifères du Sahara central; analyse des aimantations juxtaposées; configurations de la Pangée. Ann. Geophys. 1, 207–216] for the same formation. A secondary component consists in a synfolding remagnetization and shows that post-Permian tectonics account for at least about half of the total folding in the studied area. This indicates that Mesozoic folding noted 150 km to the West in the Reggane basin [Smith, B., Derder, M.E.M., Henry, B., Bayou, B., Amenna, M., Djellit, H., Yelles, A.K., Garces, M., Beamud, E., Callot, J.P., Eschard, R., Chambers, A., Aifa, T., Ait Ouali, R., Gandriche, H., 2006. Relative importance of the Hercynian and post-Jurassic tectonic phases in the Saharan platform: a palaeomagnetic study of Jurassic sills in the Reggane basin (Algeria). Geophys. J. Int. 167, 380–396] is not local and affected at least the entire north-western part of the Hoggar area. This reconfirms that the folding of the Paleozoic cover in the Sahara platform should not be restricted to the Hercynian orogeny.  相似文献   

12.
The mid-late Eocene “Valley of Whales” in the Fayum province of Egypt contains hundreds of marine-mammals’ skeletons. Given its paleontological importance, we carried out a paleomagnetic study of the fossil-bearing formations. A sequence of basalts directly overlying the upper Eocene rocks in three distant clusters within a 25 km-long NW–SE graben in the southwestern part of the area was also studied. Thermal demagnetization of three-axis IRM was used to identify and eliminate sites dominated by hematite and/or goethite as potential remanence carriers. Progressive thermal demagnetization of the NRM isolated a characteristic NNE–SSW dual-polarity direction with a shallow inclination that passes both tilt and reversal tests. The mean tilt-corrected direction of the sedimentary formations is D/I = 16°/30° (k = 50, α95 = 3°) yielding a paleomagnetic pole at 70°N/159°E. The anisotropy of magnetic susceptibility (AMS) indicated that the observed inclinations were free from inclination shallowing, as did the nearly identical characteristic remanence of the overlying basalt flows (with a tilt-corrected reversed-polarity direction of D/I = 198°/−28° (k = 38, α95 = 7°) and a pole at 68°N/158°E). The new paleopoles place the Fayum province at a lower paleolatitude (15–17°N) than today (29.5°N), and point to the possible prevalence of tropical climate in northeast Africa during mid-late Eocene times. This tropical position is nearly identical to the paleolatitudes extrapolated from the mean of 36 coeval poles rotated from the other major cratons and from Africa itself. The declinations show a minor easterly deviation from those predicted by extrapolation from other continents. This is interpreted as due to a small clockwise rotation internal to NE Africa, possibly related to Red Sea/Gulf of Suez rifting after the late Eocene. The alternative explanation that the geomagnetic field had a non-zonal non-dipole field contribution is not favored.  相似文献   

13.
《Gondwana Research》2013,23(3-4):956-973
The configuration and the timing of assembly and break-up of Columbia are still matter of debate. In order to improve our knowledge about the Mesoproterozoic evolution of Columbia, a paleomagnetic study was carried out on the 1420 Ma Indiavaí mafic intrusive rocks that crosscut the polycyclic Proterozoic basement of the SW Amazonian Craton, in southwestern Mato Grosso State (Brazil). Alternating field and thermal demagnetization revealed south/southwest ChRM directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests, including thermomagnetic data, hysteresis data and the progressive acquisition of isothermal remanent magnetization. Different stable magnetization components isolated in host rocks from the basement 10 km NW away to the Indiavaí intrusion, further support the primary origin of the ChRM. A mean of the site mean directions was calculated at Dm = 209.8°, Im = 50.7° (α95 = 8.0°, K = 22.1), which yielded a paleomagnetic pole located at 249.7°E, 57.0°S (A95 = 8.6°). The similarity of this pole with the recently published 1420 Ma pole from the Nova Guarita dykes in northern Mato Grosso State suggests a similar tectonic framework for these two sites located 600 km apart, implying the bulk rigidity of the Rondonian-San Ignacio crust at that time. Furthermore these data provide new insights on the tectonic significance of the 1100–1000 Ma Nova Brasilândia belt—a major EW feature that cuts across the basement rocks of this province, which can now be interpreted as intracratonic, in contrast to previous interpretation. From a global perspective, a new Mesoproterozoic paleogeography of Columbia has been proposed based on comparison of these 1420 Ma poles and a 1780 Ma pole from Amazonia with other paleomagnetic poles of similar age from Baltica and Laurentia, a reconstruction in agreement with geological correlations.  相似文献   

14.
Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A95 = 2.6°), 63. 3°N/170.2°E (A95 = 6.9°), and 67.0°N/180.8°E (A95 = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.  相似文献   

15.
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16.
The Late Cretaceous location of the Lhasa Terrane is important for constraining the onset of India-Eurasia collision. However, the Late Cretaceous paleolatitude of the Lhasa Terrane is controversial. A primary magnetic component was isolated between 580 °C and 695 °C from Upper Cretaceous Jingzhushan Formation red-beds in the Dingqing area, in the northeastern edge of the Lhasa Terrane, Tibetan Plateau. The tilt-corrected site-mean direction is Ds/Is = 0.9°/24.3°, k = 46.8, α95 = 5.6°, corresponding to a pole of Plat./Plon. = 71.4°/273.1°, with A95 = 5.2°. The anisotropy-based inclination shallowing test of Hodych and Buchan (1994) demonstrates that inclination bias is not present in the Jingzhushan Formation. The Cretaceous and Paleogene poles of the Lhasa Terrane were filtered strictly based on the inclination shallowing test of red-beds and potential remagnetization of volcanic rocks. The summarized poles show that the Lhasa Terrane was situated at a paleolatitude of 13.2° ± 8.6°N in the Early Cretaceous, 10.8° ± 6.7°N in the Late Cretaceous and 15.2° ± 5.0°N in the Paleogene (reference point: 29.0°N, 87.5°E). The Late Cretaceous paleolatitude of the Lhasa Terrane (10.8° ± 6.7°N) represented the southern margin of Eurasia prior to the collision of India-Eurasia. Comparisons with the Late Cretaceous to Paleogene poles of the Tethyan Himalaya, and the 60 Ma reference pole of East Asia indicate that the initial collision of India-Eurasia occurred at the paleolatitude of 10.8° ± 6.7°N, since 60.5 ± 1.5 Ma (reference point: 29.0°N, 87.5°E), and subsequently ~ 1300 ± 910 km post-collision latitudinal crustal convergence occurred across the Tibet. The vast majority of post-collision crustal convergence was accommodated by the Cenozoic folding and thrust faulting across south Eurasia.  相似文献   

17.
Over 300 samples for paleomagnetic analysis and K–Ar dating were collected from 27 sites at NW–SE and NE–SW trending dike swarms (herein, NW dikes and NE dikes, respectively) in the Koshikijima Islands, northern Ryukyu Arc. The NW dikes are Middle Miocene in age and have directions (D = ? 37.7°, I = 51.8°, α95 = 9.6°, and κ = 40.8) that are deflected westward relative to the stable eastern Asian continent. Conversely, the NE dikes, of Late Miocene age, have directions (D = 16.1°, I = 57.7°, α95 = 7.1°, and κ = 41.9) that show no such deflection. These differences are interpreted as indicating that the Koshikijima Islands underwent approximately 40° of counter-clockwise rotation during the Middle to Late Miocene. A synthesis of the paleomagnetic and structural data suggests a three-stage history of extensional deformation: (1) displacement upon normal faults (F1 faults) without vertical-axis block rotation, (2) strike-slip reactivation of F1 faults and oblique-normal displacement on NE–SW-trending faults (F2 faults) with vertical-axis block rotation, and (3) oblique-normal displacement on F2 faults without vertical-axis block rotation. Regional differences in the timing and amount of counter-clockwise vertical-axis block rotations indicate that the northern Ryukyu Arc rotated as several distinct rigid blocks.  相似文献   

18.
We report 6 K–Ar ages and paleomagnetic data from 28 sites collected in Jurassic, Lower Cretaceous and Paleocene rocks of the Santa Marta massif, to test previous hypothesis of rotations and translations of this massif, whose rock assemblage differs from other basement-cored ranges adjacent to the Guyana margin. Three magnetic components were identified in this study. A first component has a direction parallel to the present magnetic field and was uncovered in all units (D = 352, I = 25.6, k = 57.35, a95 = 5.3, N = 12). A second component was isolated in Cretaceous limestone and Jurassic volcaniclastic rocks (D = 8.8, I = 8.3, k = 24.71, a95 = 13.7, N = 6), and it was interpreted as of Early Cretaceous age. In Jurassic sites with this component, Early Cretaceous K–Ar ages obtained from this and previous studies are interpreted as reset ages. The third component was uncovered in eight sites of Jurassic volcaniclastic rocks, and its direction indicates negative shallow to moderate inclinations and northeastward declinations. K–Ar ages in these sites are of Early (196.5 ± 4.9 Ma) to early Late Jurassic age (156.6 ± 8.9 Ma). Due to local structural complexity and too few Cretaceous outcrops to perform a reliable unconformity test, we only used two sites with (1) K–Ar ages, (2) less structural complexity, and (3) reliable structural data for Jurassic and Cretaceous rocks. The mean direction of the Jurassic component is (D = 20.4, I = −18.2, k = 46.9, a95 = 5.1, n = 18 specimens from two sites). These paleomagnetic data support previous models of northward along-margin translations of Grenvillian-cored massifs. Additionally, clockwise vertical-axis rotation of this massif, with respect to the stable craton, is also documented; the sense of rotation is similar to that proposed for the Perija Range and other ranges of the southern Caribbean margin. More data is needed to confirm the magnitudes of rotations and translations.  相似文献   

19.
Paleomagnetic studies of thick lava series are one of the most reliable sources of data on the ancient geomagnetic field. However, most of such data are younger than 5 Ma, with much fewer results on the rest of the Cenozoic and the Mesozoic. Two wholesome results are available for the Precambrian but none for the Paleozoic. Late Permian basalts and rhyolites from northeastern Kazakhstan were studied to obtain first estimates of the geomagnetic-field characteristics during that period. We present preliminary results on part of the collection (66 flows (sites)) from a section ~ 1600 m thick. The characteristic component of reversed polarity was isolated by stepwise demagnetization at all the sites with a slight error. This component is of prefolding age and, most likely, primary. No abnormal magnetization direction is observed in the data, and the average directions of the characteristic component at the sites are tightly clustered (D = 243.3°; I = − 57.0°; k = 79.1; α95 = 2.0°; 65 sites). As compared with the published data on Cenozoic and Mesozoic thick lava series, secular variation was much weaker in the Late Permian than in the Mesozoic or Cenozoic, and the geomagnetic field was less disturbed. Secular-variation models based on the Late Cenozoic data show even more dramatic differences.  相似文献   

20.
The Upper Jurassic basalts (150–160 Ma) described as the Ichetui Formation over the territory of the Tugnui, Margintui, and Maly Khamar-Daban volcanic structures have been studied paleomagnetically. It is shown that natural remanent magnetization still contains a component which may reflect the geomagnetic field direction at the beginning of the Late Jurassic. This is supported by reversal and conglomerate tests. Calculation of mean paleopole gives: Plat = 63.6°, Plong = 166.8°, α95 = 8.5°. These values well coincide with the data for the Badin Formation from Mogzon depression, which lies east of the study area and approximately dates from the Kimmeridgian-Oxfordian interval of the Late Jurassic. At the same time, those poles statistically differ from the European and Southeast Asian poles of the same age. The available paleomagnetic data suggest that at the beginning of the Late Jurassic the Mongol-Okhotsk Ocean was probably still open. Since the early Late Jurassic the continental blocks of Southeastern Asia and Siberian part of the Eurasian plate had been approaching, with the Siberian domain rotating clockwise. Analysis of the total of data shows that sinistral strike-slip deformations were present not only in southern Siberia but also between the Siberian and European Platforms. Thus, the deformations of the Central Asian crust in the early Late Jurassic reflect the intraplate strike-slip motions coeval with the closure of the Mongol-Okhotsk Ocean and are governed by the clockwise rotation of the Siberian part of the Eurasian plate relative to its European part.  相似文献   

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