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1.
《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   

2.

The Hastings Terrane comprises two or three major fragments of the arc‐related Tamworth Belt of the southern New England Orogen, eastern Australia, and is now located in an apparently allochthonous position outboard of the subduction complex. A palaeomagnetic investigation of many rock units has been undertaken to shed light on this anomalous location and orientation of this terrane. Although many of the units have been overprinted, pre‐deformational magnetizations have been isolated in red beds of the Late Carboniferous Kullatine Formation from the northern part of the terrane. After restoring these directions to their palaeohorizontal (pre‐plunging and pre‐folding) orientations they appear to have been rotated 130° clockwise (or 230° anti‐clockwise) when compared with coeval magnetizations from regions to the west of the Hastings Terrane. Although these data are insensitive to translational displacements, a clockwise rotation is incompatible with models previously proposed on geological grounds. While an anti‐clockwise rotation is in the same sense as these models the magnitude appears to be too great by about 100°. Nevertheless, the palaeomagnetically determined rotation brings the palaeoslopes of the Tamworth Belt, facing east, and the Northern Hastings Terrane, facing west before rotation and facing southeast after rotation, into better agreement. A pole position of 14.4°N, 155.6°E (A95 = 6.9°) has been determined for the Kullatine Formation (after plunge and bedding correction but not corrected for the hypothetical rotation). Reversed magnetizations interpreted to have formed during original cooling are present in the Werrikimbe Volcanics. The pole position from the Werrikimbe Volcanics is at 31.6° S, 185.3° E (A95 = 26.6°). These rocks are the volcanic expression of widespread igneous activity during the Late Triassic (～ 226 Ma). While this activity is an obvious potential cause of the magnetic overprinting found in the older units, the magnetic directions from the volcanics and the overprints are not coincident. However, because only a few units could be sampled, the error in the mean direction from the volcanics makes it difficult to make a fair comparison with the directions of overprinted units. The overprint poles determined from normal polarity magnetizations of the Kullatine Formation is at 61.0°S, 155.6°E (A95 = 6.9°) and a basalt from Ellenborough is at 50.7° S, 148.8° E (A95 = 15.4°), and from reversed polarity magnetizations, also from the basalt at Ellenborough is at 49.4° S, 146.2° E (A95 = 20.4°). These are closer to either an Early Permian or a mid‐Cretaceous position, rather than a Late Triassic position, on the Australian apparent polar wandering path. Therefore, despite their mixed polarity, and global observations that the Permian and mid‐Cretaceous geomagnetic fields were of constant polarities, the age of these overprint magnetizations appears to be either Early Permian or mid‐Cretaceous.  相似文献   

3.
ABSTRACT

We report geological and palaeomagnetic data from five discrete plutons in the southern part of the Peninsular Ranges batholith (PRB) and one pluton that is part of the Jurassic plutonic suite in the Vizcaíno peninsula. The PRB plutons are Cretaceous and belong to the Alisitos island arc. The Jurassic pluton intrudes a Triassic-Jurassic ophiolite.

Our study was designed to evaluate the palaeomagnetic homogeneity of the batholith from the Sierra San Pedro Mártir, at ~31°N, to about ~28.3°N. The Punta Prieta, Nuevo Rosarito, San Jerónimo, and La Rinconada plutons in the western zone of the PRB are characterized by magnetizations residing in magnetite. The Compostela pluton is emplaced in a transition zone and has a magnetization that resides in haematite. The five Cretaceous plutons yield a combined palaeopole at 80.3°N, 162.1°E, A95 = 9.8°, N = 5 that after correcting for the opening of the Gulf of California rotates to 77.6°N, 173.6°E, the rotated pole being in angular distance of only 4.4° from the North America reference pole. The Jurassic San Roque pluton yields a mean 0.6°N, 306.1°E, A95 = 9.2°, N = 10, which is discordant, showing a clockwise rotation of about 131° ± 16° and flattening of 9.5° ± 12.9° with respect to the 150 Ma cratonic reference palaeopole. The results suggest that the intrusion of the undeformed Cretaceous Punta Prieta to Compostela plutons (128.1 ± 1.4 and 100.5 ± 2.7 Ma, respectively) restricts tectonic accretion of the Jurassic-Early Cretaceous sequences to the North America margin to the time before mid-Cretaceous magmatism (~100 Ma) in the PRB near present latitude 28°N. Mesozoic and Cenozoic strike-slip faulting along the Vizcaíno margin can account for the 131° clockwise rotation of the San Roque pluton. Our results do not support significant latitudinal movement between Vizcaíno, the PRB, and mainland Mexico with the exception of the Neogene San Andreas Fault-related right lateral movement.  相似文献   

4.
《Tectonophysics》1999,301(1-2):133-144
We report the Cretaceous palaeomagnetic results from Hainan Island, south China. In Hainan island we collected the Early Cretaceous redbeds of the Lumuwan Formation at eleven sites. We also describe the tectonic kinematics for and around Hainan Island since the Cretaceous, deduced from our and previous palaeomagnetic results. The palaeolatitude of Hainan Island is 25.9°N (+3.4°/−3.2°), implying that Hainan island was situated about 7° north from the present position during the Cretaceous. The palaeopole of Hainan Island (latitude = 77.7°N, longitude = 162.1°E, k=65.6, and A95=4.4°) suggests 4.0±5.8° counterclockwise rotation and 14.1±5.5° southward translation relative to the suspected coherent part of the south China block (SCB) since the Cretaceous. The rotation and translation of similar sense (18.8±7.4° and 7.8±6.9°, respectively) are detected in the existing palaeomagnetic result from the Xinlong Formation in Guangxi, which is situated approximately 400 km north-northwest from Hainan Island. The southward translation of both areas seems to have been due to the southeastward extrusion of dissected zones within the southwestern part of the SCB in a similar pattern to the Indochina block, which had resulted from the indentation of India into Asia. This SW part seems to have slightly rotated counterclockwise, because its extrusion was probably smaller in scale than the Indochina block and therefore it was dragged out by the Indochina block. This hypothesis is supported by the existence of a northwest–southeast-trending fault system parallel to the Red River Fault.  相似文献   

5.
The apparent polar wander (APW) path from the Tarim block consists of palaeo-magnetic poles ofDevonian (λ=16°N, ψ= 165° E. A_(95)=4°). Late Carboniferous (λ=41° N, ψ=160° E, A_(95)=4°).Permian (λ=61°N, ψ=177° E. A_(95)=9°). Early Triassic (λ=69° N. ψ=183° E. A_(95)=11°) andJurassic/Cretaceous (λ=65° N, ψ=214° E. A_(95)=6°) times. On the basis of this APW path, it is con-cluded that the Tarim block was subducted beneath the Kazakstan plate between Devonian and Permiantimes. The Tarim, North China and South China blocks were sutured between the Early Triassic and EarlyCretaceous. Tarim had moved eastward some 2000 km relative to Siberia since the Cretaceous.  相似文献   

6.
Paleomagnetic investigations have been carried out on poorly determined radiometric age controls of Bhander sandstones within the vicinity of Bhopal Inlier of the Upper Vindhyan Supergroup. Available ages assigned to the Upper Vindhyan sequence range from Cambrian to the Mesoproterozoic and are derived from a variety of sources and methods. Paleomagnetic data generated from the Bhander Group of Bhopal Inlier yielded a mean declination of 357° and mean inclination of 58° (k=17.69, α95 = 16.38) with a Virtual Geomagnetic Pole (VGP) at 74° N, 69.0° E. This pole position is falling close to the Malani Igneous Suite (MIS) mean palaeomagnetic pole of 67.8° N and 72.5° E (A95=8.8°) by Gregory et al. (2009). The results obtained from this study and previous work on the 1073 Ma Majhgawan kimberlite, as well as detrital zircon geochronology of the Upper Bhander sandstone suggest that the Upper Vindhyan sequence may be older than is commonly thought earlier.  相似文献   

7.
Palaeomagnetic, rock magnetic and magnetic fabric results are presented for a Carboniferous (Visean to Westphalian) succession of felsic, mainly ignimbritic, volcanic and volcaniclastic rocks from the Rocky Creek Block of the northern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation of 734 samples from 64 sites show three groups of magnetic components with low (<300°C), intermediate (300–600°C) and high (500–680°C) unblocking temperature ranges. Well‐defined primary magnetisations have been determined for 28 sites with evidence of four overprint phases. The overprints arise from a mid‐Tertiary weathering event (or possibly recent viscous origin), and from fluid movements associated with the Late Cretaceous opening of the Tasman Sea, thrusting during the Middle Triassic main phase of the Hunter‐Bowen Orogeny, and latest Carboniferous — Early Permian formation of the Bowen‐Gunnedah‐Sydney Basin system. Rock magnetic tests establish that the primary magnetisation carriers in the volcanic rocks are mainly magnetite (predominantly single domain, or pseudo‐single domain, and little or no multidomain) and hematite. Optimal magnetic cleaning is achieved at high to very high temperatures, with subtle, but systematic, directional and statistical differences between primary components derived from the mainly hematite fraction and pseudo‐components derived from the mainly magnetite fraction. The 28 primary magnetisation results are presented as six mean‐site results, summarised below and representing 25 sites, and three single‐site results. Fold tests could be applied to five mean‐site results. These are all positive, but one of these results may represent a secondary magnetisation. The primary magnetisation results define a Visean to Westphalian pole path. This long pole path indi cates extensive latitudinal and rotational movement for the Rocky Creek Block, and potentially for the New England Orogen, as follows: (i) Yuendoo Rhyolite Member (Caroda Formation, Visean) pole 235.8°E, 27.7°S, ED95 = 9.0°, n = 3; (ii) Peri Rhyolite Member/Boomi Rhyolite Member (Clifden Formation, Namurian, 318.0 ± 3.4 Ma) pole 177.4°E, 63.4°S, ED95 = 5.2°, n = 3; (iii) tuffaceous beds above Boomi Rhyolite Member (Clifden Formation?, Namurian) pole 162.2°E, 59.1°S, ED95 = 10.2°, n = 3; ((iv) upper Clifden Formation/lower Rocky Creek Conglomerate (Namurian/Westphalian) pole 95.3°E, 49.6°S, ED95 = 8.1°, n = 3 (possible overprint)); (v) Rocky Creek Conglomerate (Westphalian) pole 136.5°E, 57.6°S, ED95 = 5.3°, n = 5; (vi) Lark Hill Formation (Westphalian) pole 127.0°E, 50.4°S, ED95 = 4.8°, n = 8.  相似文献   

8.
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   

9.
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   

10.
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   

11.
To constrain the age of Australian opal formation, we have undertaken a paleomagnetic study of oxidised ironstone ‘nuts’ from Yowah, Queensland. Following standard methods, we have calculated a mean direction of declination D = 191.4°, inclination I = 61.7° (α95 = 4.0°), indicating a paleomagnetic pole position at latitude λp = 71.3°S, longitude ?p = 119.4°E (A95 = 5.3°). The direction comprises both normal and reverse polarities that fail a reversal test most probably owing to contamination by small recent/present-day components. The mean direction should not be significantly affected. A chi-square comparison with paleomagnetic poles for dated Cenozoic rocks in eastern Australia, poles derived from the Global Moving Hotspot Reference Frame and the Cenozoic pole path for North America, appropriately transferred to Australian coordinates, yields a mean age estimate of 35 ± 7 Ma, i.e. late Eocene to early Oligocene. This is interpreted as the age of the ironstone formation, which places a maximum age for the formation of precious Yowah opal. This result confirms and tightens the age for the nearby Canaway weathered profile.  相似文献   

12.
《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   

13.
Three basic dyke swarms of post-Ellesmerian (post-Early Carboniferous) age in Nansen Land (83° N, 43° W) are still not dated numerically, but cross-cutting relationships show Group 1 to be older than Group 2, while Group 3 is the freshest and likely the youngest. Group 1 (the most northerly swarm) strikes N-S; Group 2 NW-SE, and Group 3 (the most southerly swarm) E-W. From more than 200 dykes 234 specimens from 28 sites were investigated palaeomagnetically. Group 1 dykes show unexpected shallow inclinations with a cleaned mean direction of (Dm, Im) = (151°, –5.8°), N = 7, k = 18.5, 95 = 13.9°. They show hydrothermal alterations, some remagnetization by lightning, and the low inclination indicates a low palaeo latitude. The palaeopole is (Plat, Plon) = (8.9° S, 14.0° W) with (dp, dm) = (7°, 14°), and is close to the North American Early Carboniferous mean pole, suggesting a syn- or early late-tectonic dyke injection. The polarity is reverse. Groups 2 and 3 of presumed Cretaceous or Tertiary age show dominantly normal and reverse polarities, respectively. Their mean directions per polarity are well grouped, with (Dm, Im) = (–30.6°, 76.7°), n = 13, k = 191.4, 95 = 3.9°, and (Dm, Im) = (133.4°, –76.7°), n = 10, k = 87.5, 95 = 5.9°, respectively. They are antipodal within 95% significance, and combining both swarms gives (Dm, Im) = (–37.5°, 76.8°), n = 23, k = 124.3, 95 = 2.7°, corresponding to a mean pole of (Plat, Plon) = (70.0° N, 185.1° E) with (dp, dm) = (4.7°, 5.0°), for which the spline of Late Cretaceous-Tertiary poles for all Greenland indicates a palaeomagnetic age of 57 ± 10 Ma. This pole (in present-day coordinates) is very close to the Late Cretaceous North American pole, in accordance with the fact that Greenland belongs to the North American craton, and that the two younger swarms are essentially postdating the opening of Baffin Bay.  相似文献   

14.
The 1766 ± 5 Ma Deschambault pegmatites are anorogenic intrusions emplaced into the Glennie domain at the end of the Trans-Hudson Orogeny (THO) in north-central Saskatchewan. They are composed mainly of orthoclase and quartz with minor biotite, muscovite, tourmaline and beryl. A coherent primary characteristic remanence is retained in all 18 sites (170 specimens) that resides in magnetite, hematite and minor pyrrhotite, giving a direction of Dec. = 28.3°, Inc. = 82.1°, α95 = 4.0°, and k = 77.5 based on alternating field and thermal step demagnetization and saturation remanence analyses. The pegmatites' pole position, along with recently published ∼1810 ± 10 Ma and ∼1795 ± 15 Ma poles for the THO, define a stillstand and hairpin in the apparent polar wander path for the THO that marks continent-continent collision of the Archean Superior, Sask and Hearne (?) cratons.  相似文献   

15.
《地学前缘(英文版)》2019,10(6):2313-2327
As one of the pivotal Gondwana–derived blocks, the kinematic history of the northern Qiangtang Block (in the Tibetan Plateau) remains unclear, mainly because quantitative paleomagnetic data to determine the paleoposition are sparse. Thus, for this study, we collected 226 samples (17 sites) from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block (NQB). Stepwise demagnetization isolated high temperature/field components from the samples. Both Early and Late Triassic datasets passed field tests at a 99% confidence level and were proved to be primary origins. Paleopoles were calculated to be at 24.9°N and 216.5°E with A95 = 8.2°(N = 8) for the Early Triassic dataset, and at 68.1°N, 179.9°E with A95 = 5.6° (N = 37) for the Late Triassic, the latter being combined with a coeval volcanic dataset published previously. These paleopoles correspond to paleolatitudes of 14.3°S±8.2° and 29.9°N±5.6°, respectively. Combining previously published results, we reconstructed a three-stage northward drift process for the NQB. (1) The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic; (2) thereafter, the block rapidly drifted northward from southern to northern hemispheres during the Triassic; and (3) the block converged with the Eurasian continent in the Late Triassic. The ∼4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of ∼11.85 cm/yr. The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.  相似文献   

16.
《International Geology Review》2012,54(11):1417-1442
ABSTRACT

The Ordos Basin, situated in the western part of the North China Craton, preserves the 150-million-year history of North China Craton disruption. Those sedimentary sources from Late Triassic to early Middle Jurassic are controlled by the southern Qinling orogenic belt and northern Yinshan orogenic belt. The Middle and Late Jurassic deposits are received from south, north, east, and west of the Ordos Basin. The Cretaceous deposits are composed of aeolian deposits, probably derived from the plateau to the east. The Ordos Basin records four stages of volcanism in the Mesozoic–Late Triassic (230–220 Ma), Early Jurassic (176 Ma), Middle Jurassic (161 Ma), and Early Cretaceous (132 Ma). Late Triassic and Early Jurassic tuff develop in the southern part of the Ordos Basin, Middle Jurassic in the northeastern part, while Early Cretaceous volcanic rocks have a banding distribution along the eastern part. Mesozoic tectonic evolution can be divided into five stages according to sedimentary and volcanic records: Late Triassic extension in a N–S direction (230–220 Ma), Late Triassic compression in a N–S direction (220–210 Ma), Late Triassic–Early Jurassic–Middle Jurassic extension in a N–S direction (210–168 Ma), Late Jurassic–Early Cretaceous compression in both N–S and E–W directions (168–136 Ma), and Early Cretaceous extension in a NE–SW direction (136–132 Ma).  相似文献   

17.
During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α95 = 3.6°), indicating a pole position at latitude λp = 77.6°S, longitude ?p = 113.2°E (A95 = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 105–106 years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.  相似文献   

18.
《Precambrian Research》2006,144(3-4):213-238
We report new palaeomagnetic results from a ca. 1300 to 800 Ma continental shelf succession on the southern margin of the North China Block. A total of 386 oriented core samples were subjected to stepwise demagnetisation. Two overprint components (‘A’ and ‘B’) were identified, with ‘A’ being a Recent geomagnetic field component and ‘B’ a likely Mesozoic remagnetisation related to collision of the North and South China Blocks. An interpreted primary remanence was isolated from six rock units. The most reliable results are as follow, in the order of stratigraphic ascendance. (1) Purple mudstone, muddy sandstone and andesite of the lower Yunmenshan Formation (Rb–Sr age ca. 1270 Ma) yields a high-temperature component that passes both reversal and fold tests and gives a palaeopole at (60.6°S, 87.0°E, A95 = 3.7°). (2) Mudstone in the overlying Baicaoping Formation yields a high-temperature component with a palaeopole at (43.0°S, 143.8°E, A95 = 11.1°). (3) Purple sandstone of the earliest Neoproterozoic Cuizhuang and Sanjiaotang Formations exhibits a high-temperature component that provides a palaeopole at (41.0°S, 44.8°E, A95 = 11.3°). Based on both our new results and a critical selection of available palaeomagnetic data, we construct a preliminary apparent polar wander path (APWP) for the North China Block between 1300 and 510 Ma. Regardless of alternative polarity options applicable to these poles, North China was located within equatorial latitudes for much of this interval. Comparing the North China poles with coeval poles from Laurentia suggests that the two continents were situated on the same plate between 1200 and 700 Ma. North China was thus likely part of the supercontinent Rodinia. Separation of North China and Laurentia occurred between 650 and 615 Ma.  相似文献   

19.
This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a95=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a95=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a95=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.  相似文献   

20.
The Nain and Ashin ophiolites consist of Mesozoic melange units that were emplaced in the Late Cretaceous onto the continental basement of the Central-East Iran microcontinent(CEIM).They largely consist of serpentinized peridotites slices;nonetheless,minor tectonic slices of sheeted dykes and pillow lavas-locally stratigraphically associated with radiolarian cherts-can be found in these ophiolitic melanges.Based on their whole rock geochemistry and mineral chemistry,these rocks can be divided into two geochemical groups.The sheeted dykes and most of the pillow lavas show island arc tholeiitic(IAT)affinity,whereas a few pillow lavas from the Nain ophiolites show calc-alkaline(CA)affinity.Petrogenetic modeling based on trace elements composition indicates that both IAT and CA rocks derived from partial melting of depleted mantle sources that underwent enrichment in subduction-derived components prior to melting.Petrogenetic modeling shows that these components were represented by pure aqueous fluids,or sediment melts,or a combination of both,suggesting that the studied rocks were formed in an arc-forearc tectonic setting.Our new biostratigraphic data indicate this arc-forearc setting was active in the Early Cretaceous.Previous tectonic interpretations suggested that the Nain ophiolites formed,in a Late Cretaceous backarc basin located in the south of the CEIM(the so-called Nain-Baft basin).However,recent studies showed that the CEIM underwent a counter-clockwise rotation in the Cenozoic,which displaced the Nain and Ashin ophiolites in their present day position from an original northeastward location.This evidence combined with our new data and a comparison of the chemical features of volcanic rocks from different ophiolites around the CEIM allow us to suggest that the Nain-Ashin volcanic rocks and dykes were formed in a volcanic arc that developed on the northern margin of the CEIM during the Early Cretaceous in association with the subduction,below the CEIM,of a Neo-Tethys oceanic branch that was existing between the CEIM and the southern margin of Eurasia.As a major conclusion of this paper,a new geodynamic model for the Cretaceous evolution of the CEIM and surrounding Neo-Tethyan oceanic basins is proposed.  相似文献   

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