首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
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   

2.
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   

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.
Summary. In this paper we show that: (1) The positions of the Cretaceous palaeomagnetic poles (PP) for South America and Africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole.
(2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time.
(3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data.
On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary.
Four 'hairpins' are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively.
The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption.  相似文献   

5.
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   

6.
Summary. Samples from the Nexø Sandstone of the Lower Cambrian- Precambrian boundary in South Bornholm reveal a stable NRM with a direction after magnetic cleaning of D = 226°, I = - 30° (α95= 11.5°). This NRM appears to originate in the detrital hematite grains rather than in the red cement of the sandstone. The stable NRM is likely to be of primary origin and reflects a Lower Cambrian pole at 104° W, 38° N (dp = 7°, dm = 11°). Apparent discrepancies between the Bornholm pole and the few other published Early Cambrian/Late Precambrian poles from the Baltic Shield are consistent with the suggestion of large polar movements in those times.  相似文献   

7.
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   

8.
In late 2005 and early 2006, the WTW Operating, LLC (W.T.W. Oil Co., Inc.) #1 Wilson well (T.D. = 5772 ft; 1759.3 m) was drilled for 1826 ft (556.6 m) into Precambrian basement underlying the Forest City Basin in northeastern Kansas. Approximately 4500 of the 380,000 wells drilled in Kansas penetrate Precambrian basement. Except for two previous wells drilled into the arkoses and basalts of the 1.1-Ga Midcontinent Rift and another well drilled in 1929 in basement on the Nemaha Uplift east of the Midcontinent Rift, this well represents the deepest penetration into basement rocks in the state to date. Granite is the typical lithology observed in wells that penetrate the Precambrian in the northern Midcontinent. Although no cores were taken to definitively identify lithologies, well cuttings and petrophysical logs indicate that this well encountered basement metamorphic rocks consisting of schist, gneiss, and amphibolitic gneiss, all cut by aplite dikes. The well was cased and perforated in the Precambrian, and then acidized. After several days of swabbing operations, the well produced shows of low-Btu gas, dominated by the non-flammable component gases of nitrogen (20%), carbon dioxide (43%), and helium (1%). Combustible components include methane (26%), hydrogen (10%), and higher molecular-weight hydrocarbons (1%). Although Coveney and others [Am. Assoc. Petroleum Geologists Bull., v. 71, no, 1, p. 39–48, 1987] identified H2-rich gas in two wells located close to the Midcontinent Rift in eastern Kansas, this study indicates that high levels of H2 may be a more widespread phenomenon than previously thought. Unlike previous results, the gases in this study have a significant component of hydrocarbon gas, as well as H2, N2, and CO2. Although redox reactions between iron-bearing minerals and groundwater are a possible source of H2 in the Precambrian basement rocks, the hydrocarbon gas does not exhibit the characteristics typically associated with proposed abiogenic hydrocarbon gases from Precambrian Shield sites in Canada, Finland, and South Africa. Compositional and isotopic signatures for gas from the #1 Wilson well are consistent with a predominantly thermogenic origin, with possible mixing with a component of microbial gas. Given the geologic history of uplift and rifting this region, and the major fracture systems present in the basement, this hydrocarbon gas likely migrated from source rocks and reservoirs in the overlying Paleozoic sediments and is not evidence for abiogenic hydrocarbons generated in situ in the Precambrian basement.  相似文献   

9.
Summary. In order to contribute to the resolution of the problem of the plate tectonic character of the Caribbean, a palaeomagnetic study has been carried out on some Jamaican igneous rocks. Sixteen Late Cretaceous intrusives and lavas and one Late Miocene lava sampled in five sites have been investigated. Because of widespread maghaemitization of the predominantly large-grained deuteric class 1 titanomagnetites, some difficulty was experienced in identifying stable directions of magnetization in the Cretaceous rock units. Using thermal demagnetization technique, two distinct directions of magnetization were obtained, significantly different from those observed in contemporaneous North American rocks. Nine units yield a palaeomagnetic pole at 143.8°W, 44.1°N, referred to as 'normal', while seven units yield'equatorial'poles situated both east and west of Jamaica. It is not possible to decide which of the two directions of magnetization reflects the Late Cretaceous geomagnetic field in Jamaica, but from other evidence they appear to merit palaeotectonic interpretation. They are consistent with the plate tectonic behaviour of the Caribbean since Late Cretaceous and its motion from the southwest relative to the present day frame of reference, with a simultaneous large anticlockwise horizontal rotation of Jamaica. Late Miocene lavas, containing high-coercivity magnetic material, yield closely grouped directions giving a palaeopole at 152.4°W, 73.3°N, supporting the deductions made from the Cretaceous data.  相似文献   

10.
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   

11.
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   

12.
summary . This study completes palaeomagnetic coverage of Lower Palaeozoic igneous rocks within the Welsh Borderlands, and includes data from Ordovician (Llanvirn and Caradoc) volcanics yielding positive fold tests and palaeomagnetic poles in agreement with other British Ordovician studies. A wide range of Late Ordovician-Lower Silurian (Llandovery) intrusions yield directions of magnetization unlike any previously identified from Lower Palaeozoic rocks of Britain, but highly stable to thermal and af demagnetization, and include a reversal of magnetization. The collective results define a large movement relative to the dipole axis over a period of less than 10 Ma between c .445 and 425 Ma; part of this movement can be explained in terms of complex rotations of the British region during late Ordovician and early Silurian tectonism within the paratectonic Caledonides.  相似文献   

13.
Summary. In addition to a component (A) of recent origin, two NRM components are distinguished in the Cambro-Ordovician redbeds of the Armorican Massif. In most sites other than those from northern Brittany the oldest (C) is probably Silurian or early Devonian, and is mainly carried by specularite with high blocking temperatures. This component was variably overprinted by a Devonian or early Carboniferous component (B3) which was probably acquired as a viscous PTRM on uplift after burial, and is carried by hematite pigment with intermediate to high blocking temperatures. In the red succession of Plourivo-Bréhec (northern Brittany) declination scatter of two intermediate to high blocking temperature components (B1 and B2) is consistent with clockwise rotation of the bulk of Europe during the late Carboniferous, implied independently by published European Carboniferous palaeomagnetic data.
Stable NRM in the Erquy Spilite Series yields a palaeomagnetic pole at 344° E, 35° N ( dp = 21°, dm = 22°), and was probably acquired during remagnetization following Late Precambrian or early Cambrian folding. This is consistent with a middle to late Cambrian age of remagnetization estimated by comparison with other poles of known age.
A palaeomagnetic pole position at 332° E, 34° S ( dp = 4°, dm = 7°) determined for the Hercynian Trégastel-Ploumanac'h complex is consistent with other middle to late Carboniferous poles from elsewhere in Europe.  相似文献   

14.
Summary. The remanent magnetization of intrusive/extrusive rocks of the 'basement' complex of East Maio constitutes four components that define two different axes of magnetization, at around dec. 328, inc. 12 and dec. 007, inc. 14 respectively. In general, two or more components co-exist in separate specimens or sites but both axes are present most frequently in the normal sense. The NNW-striking axis, the B-axis, fits very well with the Upper Cretaceous polar wander path for Africa. It is consequently inferred that the major phase of sheet intrusions in Maio dates from this time, probably from the interval 90–70 Myr bp. Comparisons of the directional dispersions in the folded and unfolded states suggest that this injection phase post-dates the uplift of the Central Igneous Complex of the island. The second axis of magnetization, the A -axis, agrees very well with late Teritary—Quaternary palaeomagnetic data for Africa and the Canary Islands. The A -axis is therefore regarded as of secondary origin, being the consequence of a thermal/ chemical overprint during the Miocene—Pliocene volcanism on the island. The occurrence of a 50–70 Myr long period of volcanic quiescence and erosion, between the termination of the early igneous activity (Upper Cretaceous) and the rejuvenated magmatism in Miocene/Pliocene time, is compatible with similar observations in the Canary Islands. In contrast to the palaeomagnetic conclusions, the K/Ar data only give ages around 10 Myr. The unusually young isotope dates are regarded as being due to an almost complete age resetting and are seen in conjuction with the overprinted magnetization. This explanation is further supported by the fact that K/Ar results of pillow lavas underlying Upper Jurassic limestones only give Tertiary ages.  相似文献   

15.
Summary. Stable natural remanent magnetization (NRM) in the Jersey Volcanics and in a single rhyolite dyke was probably acquired during the Cambrian before folding of the volcanics in the Cadomian Orogeny. After dip correction, the volcanics yield a palaeomagnetic pole at 323° E, 52° N ( dp = 33°, dm = 35°). In Jersey dolerite dykes three groups of stable NRM directions are recognized, with palaeomagnetic poles at 248° E, 26° N ( dp = 10°, dm = 20°), 339° E, 1° S ( dp = 9°, dm = 12°), and 336° E, 31° S ( dp = 5°, dm = 9°). Comparison with the European apparent polar wander path implies that stable NRM in these groups was acquired respectively during Late Precambrian or early Cambrian, Siluro-Devonian and middle Carboniferous time. The stable NRM of the Jersey lamprophyre dykes yields a palaeomagnetic pole at 322° E, 16° N ( dp = 31°, dm = 38°) and is probably of Silurian or Devonian age.
These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole.  相似文献   

16.
Measurements are described of the directions of remanent magnetization of 89 samples from nine lava flows and red beds. Stable remanent magnetization was isolated after AC demagnetizing. All the units have normal remanent magnetization, except one lava flow which yields a direction toward the north with positive inclination. From the mean direction of stable remanence, referred to the bedding, of each unit a virtual geomagnetic pole is computed; the mean of eight of these poles is 90·6 °E, 84·2° South, α95= 4·7° and represents the position of the palaeomagnetic pole for the exposures of the Sierra de Los Condores group from El Estrecho-Cerro Libertad. The position of this pole is reasonably close to the positions of the South American Lower Cretaceous palaeomagnetic poles for the Serra Geral and Vulcanitas Cerro Colorado formations and the trachybasaltic dykes from Rio Los Molinos. This supports the interpretations that the South Atlantic Ocean was formed in Lower Cretaceous times and that the Earth's magnetic field was on average similar to that of a geocentric dipole in South America in the Lower Cretaceous, and suggests that there has not been substantial relative movements between Central Argentina and Southern Brazil.  相似文献   

17.
Calcite and sedimentary fills in fractures cutting the Upper Devonian carbonates in the Holy Cross Mountains (HCM) were dated palaeomagnetically by comparison with the apparent polar wander path (APWP). Haematite-bearing calcite possessed well-defined components of natural remanent magnetization (NRM), which were preserved under thermal demagnetization to temperatures of approximately 500 °C, when specimens disintegrated. Although not completely demagnetized, some specimens revealed a stable NRM component before destruction, thus making a component analysis possible. Five components were determined using density point distribution and cluster analysis. One has a mean that is similar to the present-day local geomagnetic vector. The remaining four components yielded palaeomagnetic poles located at: A (70.3°S, 5.5°E), B (71.3°S, 31.2°E), C (48.7°S, 351.0°E, virtual geomagnetic pole), and D (11.6°S, 312.3°E). Antipodal polarities found in the fracture fills, together with dissimilarities in magnetization found in calcite and hosting carbonates, indicate the lack of simultaneous remagnetization, and different times of remanence acquisition for the rocks under comparison. Taking both palaeomagnetically inferred palaeolatitudes and regional tectonics into consideration, a Mesozoic (Cretaceous?) age is estimated for palaeopoles A and B, a Permian age for pole C, and a Carboniferous age for pole D. These age determinations are in line with the calcite ages estimated from isotopic studies. A comparative palaeomagnetic study performed on a well-dated Upper Devonian neptunian dyke of limestone and a Lower Triassic clastic vein yielded virtual geomagnetic poles (VGPs) close to the APWP for Baltica. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena and mineralization processes.  相似文献   

18.
Mineralogical maturity in dunefields of North America, Africa and Australia   总被引：1，自引：0，他引：1  
Daniel R. Muhs   《Geomorphology》2004,59(1-4):247
Studies of dunefields in central and western North America show that mineralogical maturity can provide new insights into the origin and evolution of aeolian sand bodies. Many of the world's great sand seas in Africa, Asia and Australia are quartz-dominated and thus can be considered to be mineralogically mature. The Algodones (California) and Parker (Arizona) dunes in the southwestern United States are also mature, but have inherited a high degree of mineralogical maturity from quartz-rich sedimentary rocks drained by the Colorado River. In Libya, sediments of the Zallaf sand sea, which are almost pure quartz, may have originated in a similar fashion. The Fort Morgan (Colorado) and Casper (Wyoming) dunefields in the central Great Plains of North America, and the Namib sand sea of southern Africa have an intermediate degree of mineralogical maturity because their sources are large rivers that drained both unweathered plutonic and metamorphic rocks and mature sedimentary rocks. Mojave Desert dunefields in the southwestern United States are quite immature because they are in basins adjacent to plutonic rocks that were their sources. Other dunefields in the Great Plains of North America (those in Nebraska and Texas) are more mature than any possible source sediments and therefore reflect mineralogical evolution over time. Such changes in composition can occur because of either of two opposing long-term states of the dunefield. In one state, dunes are stable for long periods of time and chemical weathering depletes feldspars and other weatherable minerals in the sediment body. In the other state, which is most likely for the Great Plains, abrasion and ballistic impacts deplete the carbonate minerals and feldspars because the dunes are active for longer periods than they are stable.  相似文献   

19.
Despite many years of study, the processes involved in the development of the continental margin of southern Africa and the distinctive topography of the hinterland remain poorly understood. Previous thermochronological studies carried out within a monotonic cooling framework have failed to take into account constraints provided by Mesozoic sedimentary basins along the southern margin. We report apatite fission track analysis and vitrinite reflectance data in outcrop samples from the Late Jurassic to Early Cretaceous sedimentary fill of the Oudtshoorn, Gamtoos and Algoa Basins (Uitenhage Group), as well as isolated sedimentary remnants further west, plus underlying Paleozoic rocks (Cape Supergroup) and Permian‐Triassic sandstones from the Karoo Supergroup around the Great Escarpment. Results define a series of major regional cooling episodes. Latest Triassic to Early Jurassic cooling which began between 205 and 180 Ma is seen dominantly in basement flanks to the Algoa and Gamtoos Basins. This episode may have affected a wider region but in most places any effects have been overprinted by later events. The effects of Early Cretaceous (beginning between 145 and 130 Ma) and Early to mid‐Cretaceous (120–100 Ma) cooling are both delimited by major structures, while Late Cretaceous (85–75 Ma) cooling appears to have affected the whole region. These cooling events are all interpreted as dominantly reflecting exhumation. Higher Late Cretaceous paleotemperatures in samples from the core of the Swartberg Range, coupled with evidence for localised Cenozoic cooling, are interpreted as representing Cenozoic differential exhumation of the mountain range. Late Cretaceous paleotemperatures between 60°C and 90°C in outcropping Uitenhage Group sediments from the Oudtshoorn, Gamtoos and Algoa Basins require burial by between 1.2 and 2.2 km prior to Late Cretaceous exhumation. Because these sediments lie in depositional contact with underlying Paleozoic rocks in many places, relatively uniform Late Cretaceous paleotemperatures across most of the region, in samples of both basin fill and underlying basement, suggest the whole region may have been buried prior to Late Cretaceous exhumation. Cenozoic cooling (beginning between 30 and 20 Ma) is focussed mainly in mountainous regions and is interpreted as representing denudation which produced the modern‐day relief. Features such as the Great Escarpment are not related to continental break up, as is often supposed, but are much younger (post‐30 Ma). This history of post‐breakup burial and subsequent episodic exhumation is very different from conventional ideas of passive margin evolution, and requires a radical re‐think of models for development of continental margins.  相似文献   

20.
出露在利文斯顿岛中部的白垩纪火山岩为玄武岩－玄武安山岩－安山岩－英安岩组合，基本属于钙碱性系列，但具有低钾拉斑玄武岩的某些特点。岩石富集Ｒｂ、Ｔｈ、Ｂａ，轻度亏损Ｚｒ、Ｔｉ、Ｈｆ、Ｎｂ，较强亏损Ｃｒ和Ｎｉ，而且随着岩浆从基性向酸性成分的演化，富集（或亏损）程度加大。岩石的轻稀土元素中度富集，岩石的（Ｃｅ／Ｙｂ）ＣＮ比值较低。白垩纪火山岩形成在一个造山环境，是南设得兰岩浆弧的一部分。然而，在利文斯顿岛不同地区出露的岩石其化学性质有所区别，因此认为它们可能源于不同的岩浆体系。在晚古生代至早中生代，利文斯顿岛中部处于一个独特的浊积扇的构造环境。主要产在利文斯顿岛中部和汉那角的鲍勒斯山组火山岩与其它地点的中生代火山岩相比，无论岩相学特征还是岩石化学性质均有所不同。因此认为它们的产生同时受到太平洋板块的俯冲作用和该地构造位置所控制。史莱夫角的橄榄玄武岩成分中碱高硅低，并且通常表现出高铝的特点，这些性质与百耳斯半岛的玄武岩极为相似。区域上火山活动不断发展，活动中心逐渐向北东迁移，导致粗玄岩岩颈和岩席生成，最后是英云闪长岩的侵入。更新世至现代的依诺特角组橄榄玄武岩以化学成分中的低硅高碱含量区别于中生代火山岩。岩石的高Ｍ值  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号