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1.
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   

2.
Palaeomagnetic data from 182 hand samples collected in a rock sequence of about 620-m of red beds of Late Palaeozoic to Early Triassic age exposed in north-western Argentina (30.3° S 67.7° W), are given.
After cleaning, the majority of the Upper Palaeozoic samples (Middle Section of Paganzo Group) show reversed polarity and yield a palaeomagnetic pole at 78° S 249° E (α95= 3°). They also record a polarity transition which we have correlated with the Middle Permian Quebrada del Pimiento Normal Event. The position of the palaeomagnetic pole and the K-Ar age of a basalatic sill at the base of the sequence support this correlation.
Stable remanent magnetization has been isolated in the majority of samples from the Upper Section of the Paganzo Group; it is predominantly reversed and reveals three normal events and also three geomagnetic excursions suggesting an Illawarra Zone age (post Kiaman, Late Tatarian-Early Scythian). The palaeomagnetic pole of the reversely magnetized samples is located at 75° S 285° E(α95= 13°).
The red beds involved in this study are correlated with red beds from the Corumbataí Formation (State of Paraná, Brazil) and with igneous rocks from the Quebrada del Pimiento Formation (Province of Mendoza, Argentina).
The South American Middle and Upper Permian, Upper Permian—Lower Triassic, Lower, Middle and Upper Triassic and Middle Jurassic palaeomagnetic poles reflect a quasistatic period with mean pole at 82° S 244° E, (α95= 4°) which followed the South American Late Palaeozoic polar shift.  相似文献   

3.
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   

4.
New palaeomagnetic data from the Lower and Middle Cambrian sedimentary rocks of northern Siberia are presented. During stepwise thermal demagnetization the stable characteristic remanence (ChRM) directions have been isolated for three Cambrian formations. Both polarities have been observed, and mean ChRM directions (for normal polarity) are: Kessyusa Formation (Lower Cambrian) D = 145°, I = -40°, N = 12, α95= 12.8°; pole position: φ= 38°S, A = 165°E; Erkeket Formation (Lower Cambrian, stratigraphically highly) D = 152°, I = - 47°, N = 23, α95= 6.8°; pole position: φ= 45°S, A = 159°E; Yunkyulyabit-Yuryakh Formation (Middle Cambrian) D = 166°, I = - 33°, N = 38, α95= 4.6°; pole position: φ= 36°S, L = 140°E. These poles are in good agreement with the apparent polar wander path based on the bulk of existing Cambrian palaeomagnetic data from the Siberian platform. In Cambrian times, the Siberian platform probably occupied southerly latitudes stretching from about 35° to 0°, and was oriented 'reversely' with respect to its present position. Siberia moved northwards during the Cambrian by about 10° of latitude. This movement was accompanied by anticlockwise rotation of about 30°. The magnetostratigraphic results show the predominance of reversed polarity in the Early Cambrian and an approximately equal occurrence of both polarities in the part of the Middle Cambrian studied. These results are in good agreement with the palaeomagnetic polarity timescale for the Cambrian of the Siberian platform constructed previously by Khramov et al. (1987).  相似文献   

5.
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   

6.
Palaeomagnetic pole positions have been determined for a collection of igneous rocks, comprising nearly five hundred samples, from the Cape Verde Islands of Santa Antao, Sao Vicente, Sao Nicolao and Sao Tiago. Limited data from the islands of Sal, Maio and Fogo are also presented. Stratigraphic control suggusts that the lavas are overwhelmingly Miocene in age on Sao Tiago and Sao Nicolao. Similarity in the palaeomagnetic pole positions indicates that Miocene lavas are also dominant on Santa Antao and Sao Vicente.
Substantial areas within two of the islands are of reversed polarity only, suggesting either a rapid extrusion rate, or the existence of a long reversed polarity epoch during the Miocene period. The palaeomagnetic pole positions for each island are close to the present geographic pole, excluding the possibility of Post-Miocene differential crustal spreading (or rotation about a vertical axis) in this part of the Atlantic. The palaeomagnetic pole position for the entire survey is consistent with the Miocene geographic pole being removed from, but close to, the present geographic pole; and is in harmony with the European polar wandering curve.  相似文献   

7.
Recent interest has focused on whether South Korea may have undergone variable tectonic rotations since the Cretaceous. In an effort to contribute to the answer to this question, we have completed a palaeomagnetic reconnaissance study of Early Cretaceous sedimentary and igneous rocks from the Kyongsang basin in southeast Korea. Stepwise thermal demagnetization isolated well-defined characteristic magnetization in all samples. The palaeomagnetic directions reveal patterns of increasing amounts of clockwise (CW) rotation with increasing age for Aptian rock units. Palaeomagnetic declinations indicate clockwise vertical-axis rotations of R = 34.3° ± 6.9° for the early Aptian rock unit, R = 24.9° ± 10.6° for the middle Aptian, and R = −0.9° ± 11.8° for the late Aptian relative to eastern Asia. The new Cretaceous palaeomagnetic data from this study are consistent with the hypothesis that Korea and other major parts of eastern Asia occupied the same relative positions in terms of palaeolatitudes in the Cretaceous. An analysis of and comparison with previously reported palaeomagnetic data corroborates this hypothesis and suggests that much of Korea may have been connected to the North China Block since the early Palaeozoic. A plausible cause of the rotation is the westward subduction of the Kula plate underneath the Asian continent, which is inferred to have occurred during the Cretaceous according to several geological and tectonic analyses.  相似文献   

8.
Summary. Two late Mesozoic dolerite sills, situated near Agardhbukta on the east coast of Vestspitsbergen and dated radiométrically at 100 ± 4 Myr BP, have been sampled in five localities and subjected to detailed mineralogical and rock magnetic studies to determine the direction and origin of their magnetization. Although the sills lie outside the Tertiary orogenic belt, one locality (no. 4) has undergone strong hydrothermal alteration and a small part of another locality (no. 3) has also been affected. A conventional procedure based on examination of Zijderveld diagrams, applied to specimens demagnetized by alternating fields and thermally, yielded similar remanence directions at all five localities, except at the altered part of locality 3. Using a least squares computer méthod of analysis of step demagnetization data, comparable directions were isolated from all localities, including the altered part of locality 3. Except in this last case, all directions were reversed. The adjusted mean direction obtained from this analysis is D = 159.0°, I = 62.2°, α95= 9.0° yielding a palaeomagnetic pole situated at 225.0°, 54.3°N comparable with pole positions obtained from other late Mesozoic igneous rocks on Spitsbergen and distinct from palaeopoles derived from Mesozoic rocks in North America and Eurasia. This suggests that during the late Mesozoic Svalbard existed as a semi-independent microplate.  相似文献   

9.
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   

10.
Continental red sandstone and siltstone rocks of the Dewey Lake (Quartermaster) Formation at Maroon Cliffs, near Carlsbad, New Mexico, are characterized by two components of magnetization with partially overlapping laboratory unblocking temperature spectra. Both magnetizations display high coercivities (>100 mT), probably residing in haematite. A north-directed magnetization with steep positive inclination unblocks between 100 and 650 °C, isolating a predominantly northwest-directed magnetization, with shallow inclination, of near uniform normal polarity and maximum unblocking temperatures of 680 °C.
We collected samples from 24 palaeomagnetic sites (i.e. individual beds) from a ~60 m thick section of flat-lying strata disconformably overlying carbonate and evaporite rocks of the Rustler Formation. The upper member of the Rustler Formation contains a Late Permian (early Changxingian) marine invertebrate and conodont fauna. Of the sampled sites, four yield only steep magnetizations, interpreted to be recent overprints. Eight sites did not yield well-grouped site means and were excluded from the final calculations. The formation mean (dec = 337.7°, inc = 9.2°; k = 31.6, α 95 = 7.8°, N = 12 sites) defines a palaeomagnetic pole located at 55.2°N, 117.5°E, in good agreement with other Late Permian North American cratonic poles.
Correlation of the short polarity sequence of this section of Dewey Lake strata is unambiguous. Compared with the polarity stratigraphy of marine sections in Asia, and supported by isotopic age determinations on a widespread bentonite bed in Dewey Lake strata in west Texas (approximately 251 Ma) and fossil data for the underlying Rustler Formation, the magnetostratigraphy is consistent with deposition of the Dewey Lake Formation during the latest Changxingian (Late Permian) stage.  相似文献   

11.
40Ar/39Ar whole-rock and alkali feldspar ages demonstrate that dioritic to monzonitic dykes from Bøverbru and Lunner belong to the youngest recorded magmatic activity in the Oslo Rift region, southeast Norway. These dykes represent the terminal phase of rift and magmatic activity in the Oslo Graben, at the dawn of the Triassic (246–238 Ma).
  The Bøverbru and Lunner dyke ages are statistically concordant. However, the palaeomagnetic signature of the Bøverbru dyke is complex, and directions from the margins and the interior of the dyke differ in polarity. Therefore, the new Early Triassic palaeomagnetic pole for Baltica (Eurasia) is exclusively based on the less complex Lunner dykes and contacts (palaeomagnetic pole: latitude=52.9°N, longitude=164.4°E, dp / dm =4.5 ° /7.3°). The early Triassic palaeomagnetic pole [mean age: 243±5 Ma (2 σ )] is slightly different from the Upper Carboniferous–Permian (294–274 Ma) and Kiaman-aged poles from the Oslo Rift.  相似文献   

12.
Summary. Palaeomagnetic and K–Ar whole rock age studies are reported from samples of the Camaraca Formation exposed near Arica (18.6° S, 70.3° W), Chile. The Camaraca Formation is composed of andesites and interbedded marine shales which yield a fauna placing the formation in the Bajocian–Callovian stages of the Jurassic. Our sampling sites span a strati-graphic thickness of 2 km and yield a reversal stratigraphy of: N–R–N. The K–Ar isochron method, applied to least altered samples from the formation, gives a sharply defined age of 157 Myr which is in agreement with the palaeontologically assigned age of the formation. Normal and reversed directions of remanent magnetization, isolated by of and thermal de-magnetization methods, are statistically antiparallel. The pole position, computed from these directions of magnetization, is at 010° E, 70%0 S ( A 95= 6.0°). This pole position, when compared with the well-studied Chon Alice Formation of Argentina, suggests that the sampling region has under-gone a 28°± 28° counter-clockwise rotation about a local vertical axis. The large uncertainty (between 0° and 56°) in this estimate is due to the large scatter in the South American reference data. When compared with African Jurassic palaeopoles, with allowance made for the opening of the South Atlantic, a counter-clockwise rotation of 44°± 14° is indicated.  相似文献   

13.
About six separately orientated cores were collected at each of 14 sites distributed throughout the arcuate, west-dipping, 6  km thick, Freetown layered igneous complex. Alternating field and thermal demagnetization both isolate a stable component of remanent magnetism which corresponds to a palaeomagnetic south pole from 13 sites (nine reverse, four normal polarity) at 82.9°S, +32.7°E ( α 95 = 5.6°). This is indistinguishable from that reported in 1971 based on alternating field demagnetization of cores from 10 orientated hand samples.
  The difference between the Freetown pole (age: 193 ± 3  Ma) and other mid-Jurassic poles from West Africa could be due to its greater age. The difference between the whole West African Jurassic pole group and the Karoo pole from southern Africa, however, suggests moderate (∼10°) differential rotation of West Africa relative to the Kaapvaal craton.
  A prevalent magnetic foliation fabric coincides generally with the petrological layering, as might be expected, but a ubiquitous magnetic lineation is predominantly down-dip. This is compatible with a down-dip pyroxene lineation reported to be present in some field outcrops, and interpreted in terms of late-stage deformation during the slow crystallization and cooling of the large igneous body. However, a fold test shows that the igneous layering had already achieved its present attitude before the Complex cooled to ∼570 °C (the maximum blocking temperature of the characteristic remanence).  相似文献   

14.
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   

15.
Summary. Palaeomagnetic and isotopic results from the Kaoko lavas, Hoachanas basalts and dolerite sills of South-West Africa indicate that the Upper Triassic-Lower Jurassic Stormberg flows of South Africa may have extended into SW-Africa and that younger igneous events of Lower Cretaceous age were simultaneous with the Serra Geral volcanism in Brazil. Five analyses on three samples of the Keetmanshoop sills gave K-Ar ages between 178 ± 4 and 199 ± 4 Ma, four analyses of two samples of the Hoachanas basalts gave ages between 161 ± 3 and 173 ± 2 Ma and eight analyses of five samples of Kaoko basalt gave ages between 110±4 and 128 ± 2 Ma.
The components of remanent magnetization (RM) used to compute palaeomagnetic pole positions for the Kaoko lavas (48° N, 93° W, A95 = 3°) and for the Hoachanas basalts (61° N, 106° W, A95 = 7° are stable to alternating field (AF) and thermal demagnetization.
Correlation on a pre-drift map and on a map reconstructed for 112 Ma BP (before present) between the palaeomagnetic poles from the Kaoko and Serra Geral lavas suggests that the South Atlantic had not opened appreciably by 112 Ma BP. Cretaceous pole positions for S. America and Africa on a map reconstructed for 80 Ma BP are also discussed.  相似文献   

16.
A palaeomagnetic investigation has been carried out of rocks from the eastern part of the Voronezh Massif, which constitutes, together with the Ukrainian Shield, the Sarmatian segment in the southern part of the East European Craton. The samples were collected in a quarry close to the town of Pavlovsk (50.4°N, 40.1°E), where a syenitic-granitic body intrudes Archaean units. U–Pb (zircon) dating has yielded an age of 2080  Ma for the intrusion.
  Two characteristic magnetic components, A and B, were isolated by thermal and alternating-field demagnetization. Component A was obtained from granites and quartz syenites (11 samples) and has a mean direction of D = 229°, I = 28°, and a pole position at 12°N, 172°E. This pole is close to a contemporary mean pole (9°N, 187°E) for the Ukrainian Shield, which implies that the Voronezh Massif and the Shield constituted a single entity at 2.06  Ga. These poles differ from contemporaneous poles of the Fennoscandian Shield, indicating that the relative positions of the two shields were different from their present configuration about 2100  Myr ago.
  A component B, isolated only in quartz monzonites (five samples), has a mean direction D = 144°, I = 49°, and a pole position at 4°N, 251°E, which is close to late Sveconorwegian (approximately 900  Ma) poles for Baltica. This suggests that the East European Craton was consolidated some time between 2080 and 900  Ma. Comparison with other palaeomagnetic data permit us to narrow this time span to 1770–1340  Ma.  相似文献   

17.
Remanence directions, measured at 2  cm intervals along a composite 88  m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation ( r >0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.  相似文献   

18.
A palaeomagnetic study has been carried out on late Palaeozoic rocks exposed in the Sierras Australes thrust and fold belt of Buenos Aires province (Argentina), in the early Permian red sandstones and clay siltstones of the Tunas Formation. The sections sampled are exposed in the eastern parts of the belt, in Sierra de las Tunas (north) and Sierra de Pillahuincó (south). More than 300 specimens were collected from 25 sites, in three localities with different structural attitudes. Demagnetization at high temperatures isolated a characteristic remanence at 20 sites. All the localities have a reverse characteristic remanence, suggesting that the magnetization was acquired during the Kiaman interval. Stepwise tectonic tilt correction suggests that the Tunas Formation in these localities acquired its magnetization during folding in early Permian times. Palaeomagnetic poles were computed for each locality based on partial tilt-corrected remanence directions. Taking into account the fact that these localities are close to one another and that the rocks are all of reverse polarity, a group syntectonic palaeomagnetic pole called Tunas was calculated: longitude: 13.9°E, latitude: 63.0°S; A 95 = 5.4°, K = 39.7, N = 19. This pole is consistent with previously calculated poles from South America assigned to the early Permian. In age it corresponds to the early Permian San Rafaelic tectonic phase of the Sierras Australes. Independent geological evidence indicates that the Tunas Formation underwent syndepositional deformation. We conclude that the Tunas Formation was deposited, deformed and remagnetized, all during the early Permian.  相似文献   

19.
b
Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMs during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM—ARM1 and TRM—ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K—Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K—Ar ages determined are: I (13 |Mp 2 Myr), 0.504 Oe; II, 0.453 Oe; III, 0.439 Oe; VI (52 |Mp 10 Myr), 0.074 Oe; VII (14 |Mp 2 Myr), 0.187 Oe and VIII (12 |Mp 2 Myr), 0.251 Oe. These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary.  相似文献   

20.
Summary. Thirty-six palaeomagnetic sampling sites distributed within 6000 m of dominantly andesitic flows and tuffs of Cretaceous age from the La Serena area, Chile confirm the normal polarity bias of the Cretaceous period. Af, thermal and limited chemical demagnetization techniques have been used in testing the stability of the remanent magnetization isolated in samples from these sites. A positive fold test in the Quebrada Marquesa Formation, the second lowest in the stratigraphic pile, confirms that the magnetization isolated is pre-Tertiary in age. Ages calculated by the K–Ar whole rock method however, appear to have been variably up-dated probably due to argon loss caused by Cretaceous–Tertiary intrusives. Thermal and hydrothermal effects of these intrusions have probably reset the magnetization in the youngest formation of the volcanic pile. A composite palaeomagnetic pole calculated from the 30 site poles of the three lower formations (209° E, 81° S, A95= 4½°), is in good agreement with mid to Late Cretaceous poles derived from rock units of the stable platform of South America. The use of Andean–Caribbean palaeomagnetic data however, to resolve small time-dependent polar shifts within the Cretaceous and thus to estimate the time of opening of the south Atlantic is questioned. Many of the Andean–Caribbean Cretaceous poles appear to have been affected by local tectonic rotation.  相似文献   

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