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1.
J.D.A. Piper 《Physics of the Earth and Planetary Interiors》1981,27(3):164-186
Kimberlite and potassic lamprophyre dykes were intensively intruded into the early Proterozoic Nagssugtoqidian mobile belt of West Greenland during an important phase of brittle reactivation in Late Precambrian-Early Cambrian times (ca. 580-570 Ma) and during at least one other minor phase. Thermal and alternating field demagnetisation studies of 52 of these dykes identify primary components residing in the critical blocking temperature range distributed between shallow westerly and steep positive directions. Near the axis of the Proterozoic shear belt the dykes (predominantly lamprophyres) have closely grouped shallow directions with a reversal; near the margins of the shear belt dykes (predominantly kimberlites) have steeper and distributed directions. The cleaned components of magnetisation appear to be single, and the distribution of directions is interpreted to record a migration of the palaeofield axis which intersecting relationships show to have been from shallow to steep. The dyke directions are grouped to define representative mean palaeopoles of 215°E 3°N (LK1, A95 = 3.9°), 213°E 18°N (LK2, A95 = 6.1°), 203°E 46°N (LK3, A95 = 10.4°) and 259°E 54°N (LK4, A95 = 11.0°); a subsidiary direction recorded in five dykes near the southern margin of the shear belt (LK5, palaeopole at 297°E 16°S (A95 = 12.5°)) is derived entirely from lamprophyres and is possibly Silurian in age. An RbSr isochron on three lamprophyres of 1227 Ma and agreement of the remanence direction with ca. 1220 Ma rocks from elsewhere in Greenland suggests that the LK1 component is wholly or partly of that age.The remaining sequence of palaeopoles falls along the Hadrynian Polar Track and the age data relating to this track are re-evaluated. Evidence for a pre-800 Ma age is no longer valid and the new data from West Greenland confirm that the track is latest Precambrian to Early Cambrian in age. It is shown to connect poles of Late Precambrian and Lower Cambrian age and to embrace other data from the Laurentian shield. The rapid passage of the shield across the South Pole is consistent with the sedimentation sequences, and suggests a high-latitude origin for the tillite horizon of this age. The Hadrynian Track is also compared with the contemporaneous record from Gondwanaland and it is shown that the two shields were in juxtaposition in the identical reconstruction to the Proterozoic Supercontinent until earliest Cambrian times. This discovery links the Lower Cambrian marine transgression and the widespread ca. 580-560 Ma alkaline province in the Gondwanaland, Laurentian and Fennoscandian shields to major continental break up, and it conforms with evidence that the Iapetus Ocean did not open until Cambrian times. 相似文献
2.
J.D.A. Piper 《Physics of the Earth and Planetary Interiors》1980,23(3):165-187
The Svecofennian mobile zone occupies the bulk of the Fennoscandian shield and represents terrain subjected to profound tectonic activity and granite intrusion at ～1800 My. This study covers the palaeomagnetism of basic rocks within this belt in Sweden between 65.5 and 67.5°N (gabbros and diorites of Kallax, Niemisel, Sangis, Stora Lulevatten and the Gällivare, Jokkmokk and Voullerim regions) magnetised during uplift and cooling of this belt at ～1750-1550 My. AF and thermal demagnetisation define a consistent sequence of high to low blocking-temperature components identifying a migration of the geomagnetic field during part of this interval. Together with the Rådmansö gabbro-diorite of central-east Sweden (palaeomagnetic pole 201°E, 36°N) these components yield a comparable sequence of palaeopoles to those derived from uplift magnetisations of the contemporaneous Svecokarelian terrain of Finland. The post-tectonic, Uppsala metabasite suite possesses a magnetite-held (“A”) remanence comparable to Svecofennian uplift magnetisations from elsewhere; within the aureole of the Almunge alkaline complex this has been largely displaced by a low blocking temperature (“B”) remanence, possibly related to a late stage in the Svecofennian uplift cycle. The Hälleforsnäs giant dyke possesses a magnetite-held remanence attributed to initial cooling at 1518 My (palaeomagnetic pole 167°E, 27°N) and at least two high blocking-temperature components. One of these is correlated with the ～1000-800 My Sveconorwegian mobile activity of southwest Sweden; this latter component is represented as the univectorial remanence in dolerite dykes of this age, and sporadically as a secondary component in the adjacent Svecofennian terrain.The results are compiled with other palaeomagnetic poles from the Fennoscandian shield to derive a generalised apparent polar wandering path for the interval ～1750-1550 My. They define segments of a large loop which agrees closely with uplift magnetisations from the contemporaneous Hudsonian mobile terrain of the Laurentian shield on a single reconstruction derived from Upper Proterozoic (1450-1200 My) palaeomagnetic data. The two shields thus appear to have formed an integral continental unit during the interval 1750-1200 My. A geological reconstruction of the Siberian and Laurentian shields is also tested and found to yield general agreement with the palaeomagnetic evidence. The major geological implications of the collective reconstruction are an alignment of major tectonic trends and a gradual restriction of anorthosite-Rapakivi magmatism between the termination of the ～1800 and ～1100 My mobile episodes. 相似文献
3.
The Builth Volcanic Series of Llanvirnian age in Llanelwedd Quarries, mid-Wales, carries three components of natural remanent magnetisation. Component P, regarded as primary, is a thermochemical remanence directed at D = 181.7°, I = +54.5°, α95 = 4.4° relative to bedding. Component S is a secondary component with in situ D = 178.7°, I = ?6.7°, α95 = 5.4° and is believed to be a low-temperature chemical remanence (CRM) of Permo-Carboniferous age. Component R is directed close to the present geomagnetic field and is believed to be a recent viscous remanence (VRM).The results are of interest for three reasons. First, they are an unusually good example of multi-component NRM analysis, the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges. Second, they provide evidence of a Permo-Carboniferous event (possibly a mild thermal or hydrothermal pulse promoting CRM acquisition) some 40 km north of the Hercynian orogenic front. Third, they illustrate very clearly the importance of detailed demagnetisation: this work revises the pole position for these rocks by ～ 10° and removes an obstacle to the palaeomagnetic recognition of the ～ 1000 km wide Iapetus Ocean cutting Britain in Ordovician time. 相似文献
4.
J.C. Briden W.R. Fitches M.W. McElhinny 《Physics of the Earth and Planetary Interiors》1973,7(2):217-218
From a collection of 39 oriented hand-samples at 16 sites, total N.R.M. directions at 12 sites from the Elmina Sandstone (Devonian or possibly Carboniferous) of the Ghana coast fall in a group. Their in-situ mean (D = 334°, ) is significantly divergent from the local geomagnetic field, and does not correlate with expected Palaeozoic remanence directions. A bedding-tilt test suggests that the magnetisation is secondary, and comparison with other African data suggests a Mesozoic (possibly Cretaceous) age. The remanence is only partially stable against thermal demagnetisation. The observations are consistent with a remanence originating at the time of faulting, tilting and uplift which marked the beginning of rifting of South America from Africa. 相似文献
5.
Two contacts between Sudbury norite and northwest-trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement. A zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought. Subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement. Dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion (or sampling) surface at the time of magma emplacement. Remanence direction in one of the dikes and norite contact zones is not typical for the Sudbury dike swarm of 1250 Ma age, and this contact is not further considered. An earlier published result for a norite-dike contact was reconsidered because of complicated dike geometry and included in this study. In one of the four usable contacts the hybrid zone is represented by three samples, in another by one sample, and in the remaining two only the contact zone width could be used. The final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287°C (s.d. 13°) and 267°C (s.d. 11°) calculated without and with a correction for viscosity of the host remanence respectively. Using a gradient of 26°C/km for 1250 Ma ago indicates a burial depth of9.5 ± 2km at that time. The fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of Precambrian terranes, the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages. 相似文献
6.
Proterozoic supracrustal rocks of southwest Greenland and amphibolite dykes intruding the basement possess a thermal remanent magnetisation acquired during slow regional uplift and cooling between 1800 and 1600 m.y. following the Ketilidian mobile episode. Most samples from amphibolite dykes (mean palaeomagnetic pole 214°E, 31°N) possess a stable remanence associated with development of hematite during regional thermal metamorphism. Metavolcanics from the eastern part (eight sites, palaeomagnetic pole 230°E, 60°N, A95 = 15°) and western part (twelve sites, 279°E, 59°N, A95 = 17°) of Ars?k Island have magnetisations postdating folding and are related to KAr ages dating regional cooling (1700-1600 m.y.); magnetic properties are highly variable and partially stable remanence resides predominantly in pyrrhotite.These results agree in part with other palaeomagnetic results from the northern margin of the same craton, and currently available palaeomagnetic results assigned to the interval 1850-1600 m.y. are evaluated to define apparent polar wander movements. Two large polar movements are recognised during this interval with the possibility of a third at ca. 1800 m.y. It is concluded that apparent polar wander movements in Proterozoic times are most accurately described in terms of closed loops. 相似文献
7.
J.D.A. Piper 《Earth and Planetary Science Letters》1975,25(3):355-360
Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y. 相似文献
8.
C.J.D. Adams 《Earth and Planetary Science Letters》1975,28(1):98-104
Rb-Sr whole-rock analyses yield a Cambro-Ordovician (495 ± 11 m.y.) sedimentation age for the supposed Precambrian Greenland Group and a late Precambrian age, 680 ± 21 m.y., for parts of the Constant Gneiss, the first confirmation of Precambrian rocks in New Zealand. A Precambrian age for the Greenland Group is thus unlikely and the large area of Upper Cambrian-Lower Ordovician rocks now established can be considered as a lateral equivalent of the fossiliferous Lower Palaeozoic succession of northwest Nelson to the east. The Greenland Group, especially in the Paparoa Range has been affected subsequently by a thermal metamorphic overprint about 360 m.y. ago during the Tuhuan Orogeny. Although the Constant Gneiss must form the local basement to the Greenland Group in north Westland, the former does not appear to be the source of the sediments and the true provenance must lie elsewhere. 相似文献
9.
George E.J. Beckmann 《Physics of the Earth and Planetary Interiors》1983,32(1):85-99
A palaeomagnetic pole is established at 25.1°N 273.9°E (dp = 10.6°, dm = 14.3°) from the norite-charnockite complex at Angmagssalik, emplaced at 1800 Ma. A somewhat older palaeomagnetic pole at 4.2°S 246.7°E (dp = 4.2°, dm = 8.3°) is obtained from Archaean gneisses close to the northern boundary of the Nagssugtoqidian mobile belt; reversals of magnetization are present here. Both magnetizations were imposed during slow cooling following the (late) Nagssugtoqidian metamorphism.In general the gneisses, dyke amphibolites and granite of the Nagssugtoqidian mobile belt are unstably magnetized; their magnetization is attributable to the Earth's present field, and is often extremely weak.A pseudotachylyte within the Archaean gneisses has had a long cooling history. A fragment of the remanence reflects the magnetization characteristic of the Archaean gneisses, whereas most of the magnetization corresponds to a palaeomagnetic pole near that of the Angmagssalik complex. The pseudotachylyte is much older than its magnetizations.An apparent polar wander path is presented for Greenland at ca. 1750 Ma based on the above results and data from west Greenland. 相似文献
10.
L&#; Houyuan Wang Sumin Wu Naiqin Tong Guobang Yang Xiangdong Sheng Caiming Li Shijie Zhu Liping Wang Luo 《中国科学:地球科学(英文版)》2001,44(1):292-300
A new pollen record from the lake of Co Ngoin in the central Tibetan Plateau provides information on the vegetation and climate changes during the last 2.8 Ma. Seven major significant changes in pollen associations indicate the processes of vegetation change and possible tectonic uplifts. The seven changes in vegetation succession include a temperate montane conifer and broad-leaved mixed forest, cold temperate montane dark conifer forest, alpine shrub-meadow and alpine desert, montane dark coniferous forest and alpine shrub meadow, montane dark coniferous forest and alpine shrub meadow, montane dark coniferous forest and alpine meadow, and alpine desert and meadow. The pollen record provides the evidence of at least five times tectonic uplifts occurring at about 2.58 Ma, 1.87 Ma, 1.17 Ma, 0.83 Ma, and 0.3 Ma ago, respectively. Before 0.8 Ma, this region maintained the altitude below 4000 m a.s.l. Larger amplitude of uplift occurring at about 0.8 Ma ago enforced the plateau rising into cryosphere, shaping the basic topographic pattern of modern plateau. The major successions in vegetation of this area were largely controlled by stepwise uplift of the Tibetan Plateau.
相似文献11.
J.D.A. Piper 《Physics of the Earth and Planetary Interiors》1974,9(4):353-363
The Cunene Complex is the largest known anorthosite body and outcrops across the border between Angola and South West Africa. Palaeomagnetic results are reported from a traverse across the dark troctolitic facies of the anorthosite in Angola which yielded fifteen sites with two additional sites in gabbro bodies. Fourteen sites are stable to a.f. demagnetisation and a single site in the cumulative border zone of the anorthosite is reversed with respect to the remainder. Twelve sites combine to give a mean direction of D = 259°, I = ?46° (k = 7) with a virtual geomagnetic pole at 255°E and 3°S. The low overall precision is probably due to apparent polar movement during cooling of the Complex. Radiometric data are currently conflicting and imply that the anorthosite has an age between 1100 and 2600 m.y.; the only clear feature to emerge from age studies is a thermal overprinting at ca. 1100 m.y. The directions of magnetisation are shown to be most consistent with an age of ca. 2100 m.y. with cooling through the Curie point continuing to ca. 2000 m.y.A variety of magnetic tests demonstrate that magnetite is the principal remanence carrier in the dark troctolitic anorthosite where it occurs both as discrete grains and as fine rods in plagioclase. Lowrie-Fuller tests suggest that both these components include single domains but results from separated mineral fractions demonstrate that the bulk of the high coercivity remanence resides in magnetite rods within the feldspar. 相似文献
12.
A Precambrian metadolerite dyke has two distinct types of remanence carriers; those with medium/high coercivities (unblocking fields of 20–120 mT) and those with low coercivities (unblocking fields of <15 mT). Optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation-exsolution intergrowths of magnetite and ilmenite. All opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction. The submicron inclusions in the plagioclase are titanomagnetites(0 < x ≤ 0.14) with a size range between about 0.01 and 0.5 μm and axial ratios between 1 (equidimensional) and about 0.3. Many of these inclusions fall in the single-domain field but some are probably pseudo-single-domain. The large opaque grains contain almost pure magnetite and ilmenite and show no fine-scale exsolution; the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under Lorentz electron microscopy. There are also some primary ilmenites containing very fine exsolved haematite, and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite. Experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20–140 mT and blocking temperatures of 390–590°C and the large opaque grains acquire a remanence with unblocking fields of less than 15 mT but a wide range of blocking temperatures up to about 570°C. It is concluded that the medium/high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques. The contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape. 相似文献
13.
Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis
of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial
and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis
measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex
magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in
composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most
sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about
72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α95=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history
of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of
the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing40Ar/39Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed
Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of
the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr. 相似文献
14.
J.P. Hodych R.R. Pätzold K.L. Buchan 《Physics of the Earth and Planetary Interiors》1985,37(4):261-284
Oolitic hematite-bearing ironstones of the Silurian Red Mountain Formation of Alabama are shown to carry a single-component remanence stable enough to have survived major folding (of probable Permian age). Nevertheless, the remanence direction (ten sites yielding a paleopole at 38.0°N, 132.4°E with dm = 3.6°, dp = 1.9°), its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the Pennsylvanian—some 130 Ma after deposition. This remanence is likely a chemical remanent magnetization (CRM) acquired during diagenesis induced by heating due to deep burial under a Pennsylvanian clastic wedge. Two possible mechanisms for acquisition of CRM during deep-burial diagenesis are considered. In hypothesis I, the oolitic hematite transformed from original geothite when heated to about 80°C, acquiring CRM. In hypothesis II, the oolitic hematite originated from ferrihydrite and was too fine-grained to acquire stable CRM until heat raised the solubility of hematite allowing grain growth. Hypothesis I explains the timing of remanence acquisition better, but there is some evidence that oolitic goethites may be stable to considerably more than 80°C. Hypothesis II has some difficulty explaining preliminary paleomagnetic results from oolitic hematite-bearing ironstones of the Silurian Clinton Group, New York State. We prefer hypothesis I but both hypotheses remain plausible. Both hypotheses warn that continental red beds may also acquire CRM during diagenesis induced by deep-burial heating, long after deposition but before folding. 相似文献
15.
In the western part of the Gardar Igneous Province of southern Greenland, lamprophyre dykes intruded at ca. 1276-1254 m.y. RbSr biotite ages yield a palaeomagnetic pole at 206.5°E,3°N (nine sites, dψ = 5.1°, dχ = 10.1°) Slightly younger dolerite dykes with RbSr biotite ages in the range 1278-1263 m.y. give a pole at 201.5°E,8.5°N (24 sites, dψ = 4.7°, dχ = 9.4°), and the syeno-gabbro ring dyke of the Kûngnât complex (RbSr isochron age 1245 ± 17 m.y.) cutting both of these dykes swarms, gives a pole at 198.5°E, 3.5°N (four sites, dψ = 2.3°,dχ = 4.4°). All these rock units have the same polarity and the poles are identical to those from Mackenzie and related igneous rocks of North America (1280-1220 m.y.) after closure of the Davis Strait; they confirm that this part of the Gardar Province is a lateral extension of the Mackenzie igneous episode within the Laurentian craton.In the Tugtutôq region of the eastern part of the Gardar Province 47 NNE-trending dykes of various petrologic types, and intruded between 1175 ± 9 and 1168 ± 37 m.y. (RbSr isochron ages) yield a palaeomagnetic pole at 223.9° E, 36.4°N (dψ = 4.1°, dχ = 6.1°). Fifteen other dykes in this swarm were intruded during a transitional phase of the magnetic field which, however, does not appear to have achieved a complete reversal over a period of several millions of years. The majority of dykes studied are highly stable to AF and thermal demagnetisation and contain single high blocking temperature components with single Curie points in the range 380–560°C.Palaeomagnetic poles from the Gardar Province between ca. 1330 and 1160 m.y. in age define the earlier part of the Great Logan apparent polar-wander loop; they correlate closely with contemporaneous North American results and confirm the coherence of the Laurentian craton in Upper Proterozoic times. 相似文献
16.
J.D.A. Piper 《Earth and Planetary Science Letters》1980,46(3):443-461
The major Proterozoic igneous intrusions in the Swedish sector of the Baltic Shield are the Ragunda complex (1293 m.y., palaeomagnetic pole 165°E, 54°N) and the Nordingrågabbro-granite-anorthosite complex (1385 ± 30 m.y.). The latter body has been partially remagnetised by later post-Jotnian dolerites (1254 m.y.), and sites influenced by the dolerites have a stable magnetisation with a mean direction D = 45°, I = ?39°, (α95 = 4.3°). Elsewhere, the gabbro-anorthosite facies have a magnetisation of dual polarity predating the dolerite and recoverable at various stages of thermal and/or a.f. cleaning with a mean of D = 48°, I = 37° (α95 = 5.3°); medium and high coercivity remanence resides in large magnetite grains and fine, predominantly hematite, rods in feldspar megacrysts. The Nordingrårapakivi granite yields a mean, also including dual polarities, of D = 221°, I = ?25° (α95 = 13°), and the Gävle granite yields a mean of D = 26°, I = 17° (α95 = 13°).New data define the a.p.w. path for the Baltic Shield after final uplift and cooling of the ca. 1800 m.y. Svecofennian mobile belt and prior to intrusion of the post-Jotnian dolerites at 1250 m.y.; this (ca. 1500–1200 m.y.) path defines a double loop similar in size and shape to the contemporaneous path for the Laurentian Shield and the paths can be superimposed to define relative positions of the shields. They were in juxtaposition prior to 1200 m.y. with the optimum reconstruction obtained by rotation of approximately 64° about a Euler pole at 1°E, 36°N. Pre-1500 m.y. palaeomagnetic data are also shown to fit this same unique reconstruction. The main geological correlations are an alignment of the Lower/Middle Proterozoic major strike-slip zones, the structural trends within the pre-1700 m.y. mobile belts, and the Grenville and Sveconorwegian (ca. 1100 m.y.) mobile belts. The anorogenic magmatism characteristic of Proterozoic times became gradually more restricted to one active margin of the continental reconstruction as temperature gradients decreased and the crust consolidated. All of these Proterozoic tectonic/magmatic trends are parallel to the long axis of the continental reconstruction. 相似文献
17.
By observing, measuring the fluvial sediment grain size of mid-western segment of the Qilianshan Range and studying the correlation between the grain size and uplift of the plateau, we model the correlation. These models are applied to the Laojunmiao section and the process curve of the uplift of the northern Tibetan Plateau against age from 8.35 Ma is illustrated here. The process curve shows that the northern Tibetan Plateau surface has uplifted from the mean altitude of 900–3700 m since 8.35 MaBP. From 8.35 to 3.1 MaBP, the Tibetan Plateau uplifted slowly, uplifted amplitude is small, the total range is 420 m. From 3.1 MaBP up to now, the Tibetan Plateau uplifted tempestuously, showing that the uplift accelerated obviously later. It uplifted totally 2400 m. About 0.9 Ma ago, the northern Tibetan Plateau surface had uplifted to over 3000 m a.s.l., showing that the Tibetan Plateau surface had reached the cryosphere; and the mountain peaks had uplifted to more than 4000 m altitude, suggesting that there was a glacier developed on the mountains. 相似文献
18.
Bin Zhang Wen Chen JingBo Sun Shun Yu JiYuan Yin Jie Li Yan Zhang XinYu Liu Li Yang Xia Yuan 《中国科学:地球科学(英文版)》2016,59(2):349-361
The uplift and exhumation process in the Tianshan orogen since the late Paleozoic were likely related to the preservation of ore deposits. This study involved reconstructing the whole tectonic thermal history of the Ouxidaban pluton in central South Tianshan Mountains based on hornblende/plagioclase Ar-Ar and zircon/apatite(U-Th)/He methods. The thermal history and uplift process of central South Tianshan Mountains since the late Paleozoic were analyzed according to the results of previous works and cooling/exhumation rate features. The hornblende yields a plateau age of 382.6±3.6 Ma, and the plagioclase yields a weighted mean age of 265.8±4.9 Ma. The Ouxidaban pluton yields weighted mean zircon(U-Th)/He age of 185.8±4.3 Ma and apatite(U-Th)/He age of 31.1±2.9 Ma, respectively. Five stages of tectonic thermal history of South Tianshan Mountains since the late Paleozoic could be discriminated by the cooling curve and modeling simulation:(1) from the latest Silurian to Late Devonian, the average cooling rate of the Ouxidaban pluton was 7.84°C/Ma;(2) from the Late Devonian to the latest Middle Permian, the average cooling rate was about 2.07°C/Ma;(3) from the latest Middle Permian to the middle Eocene, the cooling rate decreased to about 0.68°C/Ma, suggesting that the tectonic activity was gentle at this time;(4) a sudden increase of the cooling rate(5.00°C/Ma) and the exhumation rate(0.17 mm/a), and crustal exhumation of ~1.83 km indicated that the Ouxidaban pluton would suffer a rapid uplift event during the Eocene(~46?35 Ma);(5) since the middle Eocene, the rapid uplift was sustained, and the average cooling rate since then has been 1.14°C/Ma with an exhumation rate of about 0.04 mm/a and an exhumation thickness of 1.33 km. The strong uplift since the Cenozoic would be related to a far-field effect from the Indian and Eurasian plates' collision. However, it was hysteretic that the remote effect was observed in the Tianshan orogenic belt. 相似文献
19.
We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16-35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process. 相似文献
20.
Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (40Ar/39Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60–100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01–0.03 mm/y) in relation with a northeastward tilting of 0.2–0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back. 相似文献