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1.
Abstract

This study aims at unravel the geotectonic evolution of northern Greece prior to the already established Tertiary clockwise rotation. Therefore, Mesozoie sediments, Early Mesozoie ophiolites and Carboniferous granites were sampled. While the metamorphosed and/or too weakly magnetized limestones had to be rejected, the gabbros and serpentinites of the 80 km long Chalkidiki belt (40.4°N, 23.3”E), and the granites of the northern Pelagonian zone (40.8°N, 21.2°E) have yielded similar results interpretable in terms of geoleetonies. In both areas the demagnetizing process has revealed a poh phased magnetic evolution.

The oldest magnetizations, labelled M (D=311°, I=20°, a95, = 15°; VGP: 37°N, 272.5°, for the ophiolites; D=320.5°, I = 26°, a95 =11°; VGP : 46°N, 264.5”E, for the granites) are interpreted as overprints acquired in Late Jurassic-Cretaceous times. The younger magnetizations, called C2 (D = 66°, I = 28°, a95 = 9°; VGP : 28°N, 117°E, in the ophiolites ; D=64°, I = 2° a95, = 11°; VCP : 20°N, I28°E, in the granites) are Tertiary overprints. Northeasterly C’ directions with negative inclinations (and conversely) are considered as overprints empiaceli prior to the Ca magnetizations ; they are interpreted as due to a barkthrusting of the ophiolilic belt of Chalkidiki and of the N. Pelagonian granitic belt, during the Early - Middle Tertiary convergence phase. The large deviation from the M to the C2 directions, also observed by other authors in Mesozoic volcanics and sediments, results from a counterclockwise rotation of the Hellenides, probably in the Late Cretaceous as it is the case for the counterclockwise rotations of the western Mediterranean microplates. The deviation from the C2 to the present field direction is due to a clockwise rotation of all Hellenic zones, probably in several phases.  相似文献   

2.
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   

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.
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   

5.
《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   

6.
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   

7.
Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A95 = 2.6°), 63. 3°N/170.2°E (A95 = 6.9°), and 67.0°N/180.8°E (A95 = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.  相似文献   

8.
地磁场的缓慢变化(周期在一年以上)称为地磁场的长期变。由于长期变不仅与地磁场起源的理论关系极大,而且对于古地磁学中的基本假定(轴向地心偶极子假说)有着头等重要的意义,因此,近来颇受重视,讨论相当热烈。  相似文献   

9.
The stable magnetizations of the Tasmanian Dolerites are shown to fall into two distinct groups depending upon their directions and the geographical region of the dolerites. It has been suggested that this could be a result of significant age differences between the dolerites of these groups. A series of K‐Ar determinations indicates that there is no detectable systematic age differences and the average of the five bodies dated is 170.5 ± 8.0 m.y. (not significantly different from previous K‐Ar dates from a single body). A re‐appraisal of the palaeomagnetic data, in the light of the distinct groupings of the directions, yields two significantly different pole positions‐ Lat 50.7°S, Long. 174.5°E (A9r, = 5.2°) and Lat. 47.7 °S, Long. 123.5° (A95 = 9.5°)’. The former of these is in excellent agreement with pole positions from other Lower to Middle Jurassic rocks of Australia but the significance of the latter remains obscure.  相似文献   

10.
Detrital zircon U–Pb LAM-ICPMS age patterns for sandstones from the mid-Permian –Triassic part (Rakaia Terrane) of the accretionary wedge forming the Torlesse Composite Terrane in Otago, New Zealand, and from the early Permian Nambucca Block of the New England Orogen, eastern Australia, constrain the development of the early Gondwana margin. In Otago, the Triassic Torlesse samples have a major (64%), younger group of Permian–Early Triassic age components at ca 280, 255 and 240 Ma, and a minor (30%) older age group with a Precambrian–early Paleozoic range (ca 1000, 600 and 500 Ma). In Permian sandstones nearby, the younger, Late Permian age components are diminished (30%) with respect to the older Precambrian–early Paleozoic age group, which now also contains major (50%) and unusual Carboniferous age components at ca 350–330 Ma. Sandstones from the Nambucca Block, an early Permian extensional basin in the southern New England Orogen, follow the Torlesse pattern: the youngest. Early Permian age components are minor (<20%) and the overall age patterns are dominated (40%) by Carboniferous age components (ca 350–320 Ma). These latter zircons are inherited from either the adjacent Devonian–Carboniferous accretionary wedge (e.g. Texas-Woolomin and Coffs Harbour Blocks) or the forearc basin (Tamworth Belt) farther to the west, in which volcaniclastic-dominated sandstone units have very similar pre-Permian (principally Carboniferous) age components. This gradual variation in age patterns from Devonian–late Carboniferous time in Australia to Late Permian–mid-Cretaceous time in New Zealand suggests an evolutionary model for the Eastern Gondwanaland plate margin and the repositioning of its subduction zone. (1) A Devonian to Carboniferous accretionary wedge in the New England Orogen developing at a (present-day) Queensland position until late in the Carboniferous. (2) Early Permian outboard repositioning of the primary, magmatic arc allowing formation of extensional basins throughout the New England Orogen. (3) Early to mid-Permian translocation of the accretionary wedge and more inboard active-margin elements, southwards to their present position. This was accompanied by oroclinal bending which allowed the initiation of a new, late Permian to Early Triassic accretionary wedge (eventually the Torlesse Composite Terrane of New Zealand) in an offshore Queensland position. (4) Jurassic–Cretaceous development of this accretionary wedge offshore, in northern Zealandia, with southwards translation of the various constituent terranes of the Torlesse Composite Terrane to their present New Zealand position.  相似文献   

11.
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12.
New pole positions for Triassic and Cretaceous times have been obtained from volcanic and sedimentary sequences in Central Iran. These new results confirm the general trend of the Apparent Polar Wander Path (APWP) of the Central-East-Iran microplate (CEIM) from the Triassic through the Tertiary as published by Soffel and Förster (1983, 1984). Two new palaeopoles for the Triassic of the CEIM have been obtained; limestones and tuffs from the Nakhlak region yield a mean direction of 094.0°/25.0°, N=12, k=4.1,α 95=24.7°, after bedding correction, corresponding to a palaeopole position of 310.8°E; 3.9°S, and volcanic rocks from the Sirjan regions yield a mean direction of 114.5°/35.1°, N=44, k=45.9,α 95=3.2° after bedding correction and a palaeopole position of 295.8°E; 10.3°N. Combining these with the two previously published results yields a new palaeopole position of 317.5°E; 12.7°N, for the Triassic of the CEIM, thus confirming that large counterclockwise rotations of the CEIM have occurred since the Triassic time. New results have also been obtained from Cretaceous limestones from the Saghand region of the CEIM. The mean direction of 340.7°/26.3°, N=33, k=44.3,α 95=3.8°, and the corresponding palaeopole position of 283.1°E; 64.4°N, is in agreement with previously determined Cretaceous palaeopole positions of the CEIM. Furthermore, results have also been obtained from Triassic dolomite, limestone, sandstone and siltstone from the Natanz region, which is located to the west of the CEIM. A total of 161 specimens from 44 cores taken at five sites gave a mean direction of the five sites at 033.3°/25.1°, N=5, k=69.0,α 95=9.3° and a palaeopole position of 167.2°E; 53.7°N. They pass the positive fold test of McElhinny (1964) on the level of 99% confidence. This pole position is in fairly good agreement with the mean Triassic pole position of the Turan Plate (149°E; 49°N). It indicates that the area of Natanz has not undergone the large counterclockwise rotation relative to the Turan plate since the Triassic, which has been shown for the CEIM. A Triassic palaeogeographic reconstruction of Iran, Arabia (Gondwana) and the Turan Plate (Eurasia) is also presented.  相似文献   

13.
Paleomagnetic results from Upper Jurassic to Paleocene rocks in Peninsular Malaysia show counter clockwise (CCW) rotations, while clockwise rotations (CW) are predominantly found in older rocks. Continental redbeds of the Upper Jurassic to Lower Cretaceous Tembeling Group have a post folding remagnetization, giving a VGP at N54°E29°, corresponding to approximately 40° of CCW rotation relative to Eurasia and 60° CCW relative to the Indochina block (Khorat Plateau). Samples from Cretaceous to Paleocene mafic volcanics of the Kuantan dike swarm and the Segamat basalts give VGPs at N59°E47° and N34°E36°, respectively. These Malayasian data are indistinguishable from the Late Eocene and Oligocene VGPs reported for Borneo and the Celebes Sea and are similar to the Eocene VGPs reported for southwest Sulawesi and southwest Palawan. The occurrence of CCW deflected data over this large region suggests that much of Malaysia, Borneo, Sulawesi, and the Celebes Sea rotated approximately 30° to 40° CCW relative to the Geocentric Axial Dipole (GAD) between the Late Eocene and the Late Miocene, although not necessarily synchronously, nor as a single rigid plate. These regional CCW rotations are not consistent with simple extrusion based tectonic models. CW declinations have been measured in Late Triassic granites, Permian to Triassic volcanics, and remagnetized Paleozoic carbonates. The age of this magnetization is poorly understood and may be as old as Late Triassic, or as young as Middle or Late Cretaceous. The plate, or block rotations, giving rise to these directions are correspondingly weakly constrained.  相似文献   

14.
A systematic sedimentologic and paleomagnetic study was carried out in the Vaca Muerta Formation, cropping out in the northern Neuquén Basin, west-central Argentina. The studied section is c. 280 m-thick and represents a carbonate ramp system bearing ammonites that indicate Late Jurassic–Early Cretaceous ages. The Vaca Muerta Formation is one of the most important unconventional hydrocarbon reservoirs in the world and its thorough study has become a relevant target in Argentina. The J-K boundary is comprised within this unit, and although it is well-dated through biostratigraphy (mainly ammonites), the position of particularly the boundary is yet a matter of hot debate. Therefore, the systematic paleomagnetic and cyclostratigraphic study in the Vaca Muerta Formation was considered relevant in order to obtain the first Upper Jurassic–Lower Cretaceous magnetostratigraphy of the southern hemisphere on the first place and to precise the position of the J-K boundary in the Neuquén Basin, on the other. Biostratigraphy is well studied in the area, so that paleomagnetic sampling horizons were reliably tied, particularly through ammonites. Almost 450 standard specimens have been processed for this study distributed along 56 paleomagnetic sampling horizons that were dated using ammonites. Paleomagnetic behaviours showed to be very stable, and their quality and primary origin have been proved through several paleomagnetic field tests The resultant magnetostratigraphic scale is made up of 11 reverse and 10 normal polarity zones, spanning the Andean Virgatosphinctes mendozanus (lower Tithonian) to Spiticeras damesi Zones (upper Berriasian). These polarity zones were correlated with those of the International Geomagnetic Polarity Time Scale 2012 and 2016 through the correlation between Andean and Tethyan ammonite zones. Cyclostratigraphy on the other hand, proved to be quite consistent with the magnetostratigraphy. Through the correlation of the resultant paleomagnetic and cyclostratigraphic data, it was possible to date the section with unprecedented precision, and therefore, to establish the position of the Jurassic-Cretaceous boundary. The paleomagnetic pole calculated from the primary magnetization is located at: Lon = 191.6°E, Lat = 76.2°S, A95 = 3.5°, indicating a c. 24° clockwise rotation for the studied section, which is consistent with structural data of the region.  相似文献   

15.
The paleomagnetism of Upper Cretaceous magmatic rocks from 47 collecting sites (172 samples, 692 specimens) in the Apuseni Mountains was studied. After AF cleaning, characteristic magnetizations were identified for various collecting areas in the study zone, which defined a few spatial and temporal units for which paleomagnetic poles could be derived statistically. At 21 sampling sites the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of If = −38° and Df = −100°, with 95 = 6°. The paleomagnetic results show that to reach their present-day position the Apuseni Mountains moved to the north, around 14° with respect to Europe, or around 25° with respect to the geographic poles, between the Campanian and, probably, Late Miocene, while a clockwise rotation, of around 80°, was taking place.  相似文献   

16.
Northward indentation of the Indian Plate has brought about significant tectonic deformation into East Asia. A record of long-term tectonic deformation in this area for the past 50 M yr, particularly the vertical axis rotation, is available through paleomagnetic data. In order to depict rotational deformation in this area with respect to Eurasia, we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern Himalayan syntaxis in East Asia. This record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the Chuan Dian Fragment to as far as the northwestern part of the Indochina Peninsula. A limited zone that experienced a significant amount of clockwise rotation after an initial India–Asia collision is now located at 23.5°N, 101°E, far away from an area (27.5°N, 95.5°E) where an intense rotational motion has been viewed by a snapshot of GPS measurements. This discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within East Asia as a result of ongoing indentation of the Indian Plate. A quantitative comparison between the GPS and paleomagnetically determined clockwise rotation further suggests that following an initial India–Asia collision the crust at 30°N, 94°E paleoposition was subjected to southeastward displacement together with clockwise rotation, which eventually reached to present-day position of 23.5°N, 101°E, implying a crustal displacement of about 1000 km during the past 50 M yr.  相似文献   

17.
New gravity data along five profiles across the western side of the southern New England Fold Belt and the adjoining Gunnedah Basin show the Namoi Gravity High over the Tamworth Belt and the Meandarra Gravity Ridge over the Gunnedah Basin. Forward modelling of gravity anomalies, combined with previous geological mapping and a seismic-reflection transect acquired by Geoscience Australia, has led to iterative testing of models of the crustal structure of the southern New England Fold Belt, which indicates that the gravity anomalies can generally be explained using the densities of the presently exposed rock units. The Namoi Gravity High over the Tamworth Belt results from the high density of the rocks of this belt that reflects the mafic volcanic source of the older sedimentary rocks in the Tamworth Belt, the burial metamorphism of the pre-Permian units and the presence of some mafic volcanic units. Modelling shows that the Woolomin Association, present immediately east of the Peel Fault and constituting the most western part of the Tablelands Complex, also has a relatively high density of 2.72 – 2.75 t/m3, and this unit also contributes to the Namoi Gravity High. The Tamworth Belt can be modelled with a configuration where the Tablelands Complex has been thrust over the Tamworth Belt along the Peel Fault that dips steeply to the east. The Tamworth Belt is thrust westward over the Sydney – Gunnedah Basin for 15 – 30 km on the Mooki Fault, which has a shallow dip (～25°) to the east. The Meandarra Gravity Ridge in the Gunnedah Basin was modelled as a high-density volcanic rock unit with a density contrast of 0.25 t/m3 relative to the underlying rocks of the Lachlan Fold Belt. The modelled volcanic rock unit has a steep western margin, a gently tapering eastern margin and a thickness range of 4.5 – 6 km. These volcanic rocks are assumed to be Lower Permian and to be the western extension of the Permian Werrie Basalts that outcrop on the western edge of the Tamworth Belt and which have been argued to have formed in an extensional basin. Blind granitic plutons are inferred to occur near the Peel Fault along the central and the southern profiles.  相似文献   

18.
Jurassic to Cretaceous red sandstones were sampled at 33 sites from the Khlong Min and Lam Thap formations of the Trang Syncline (7.6°N, 99.6°E), the Peninsular Thailand. Rock magnetic experiments generally revealed hematite as a carrier of natural remanent magnetization. Stepwise thermal demagnetization isolates remanent components with unblocking temperatures of 620–690 °C. An easterly deflected declination (D = 31.1°, I = 12.2°, α95 = 13.9°, N = 9, in stratigraphic coordinates) is observed as pre-folding remanent magnetization from North Trang Syncline, whereas westerly deflected declination (D = 342.8°, I = 22.3°, α95 = 12.7°, N = 13 in geographic coordinates) appears in the post-folding remanent magnetization from West Trang Syncline. These observations suggest an occurrence of two opposite tectonic rotations in the Trang area, which as a part of Thai–Malay Peninsula received clockwise rotation after Jurassic together with Shan-Thai and Indochina blocks. Between the Late Cretaceous and Middle Miocene, this area as a part of southern Sundaland Block experienced up to 24.5° ± 11.5° counter-clockwise rotation with respect to South China Block. This post-Cretaceous tectonic rotation in Trang area is considered as a part of large scale counter-clockwise rotation experienced by the southern Sundaland Block (including the Peninsular Malaysia, Borneo and south Sulawesi areas) as a result of Australian Plate collision with southeast Asia. Within the framework of Sundaland Block, the northern boundary of counter-clockwise rotated zone lies between the Trang area and the Khorat Basin.  相似文献   

19.
This paper conducts systematic test research on the 2920 paleomagnetic directional samples taken from Ordovician-Paleogene sedimentary formation in the north slope of Qomolangma in south of Tibet and obtains the primary remanent magnetization component and counts the new data of paleomagnetism the times. Based on the characteristic remanent magnetization component, it calculates the geomagnetic pole position and latitude value of Himalaya block in Ordovician-Paleogene. According to the new data of paleomagnetism, it draws the palaeomagnetic polar wander curve and palaeolatitude change curve of the north slope of Qomolangma in Ordovician-Paleogene. It also makes a preliminary discussion to the structure evolution history and relative movement of Himalaya bloc. The research results show that many clockwise rotation movements had occurred to the Himalaya block in northern slope of Qomolangmain the process of northward drifting in the phanerozoic eon. In Ordovician-late Cretaceous, there the movement of about 20.0° clockwise rotation occurred in the process of northward drifting. However, 0.4° counterclockwise rotation occurred from the end of late Devonian epoch to the beginning of early carboniferous epoch; 6.0° and 8.0° counterclockwise rotation occurred in carboniferous period and early Triassic epoch respectively, which might be related with the tension crack of continental rift valley from late Devonian period to the beginning of early carboniferous epoch, carboniferous period and early Triassic epoch. From the Eocene epoch to Pliocene epoch, the Himalaya block generated about 28.0° clockwise while drifting northward with a relatively rapid speed. This was the result that since the Eocene epoch, due to the continuous expansion of mid-ocean ridge of the India Ocean, the neo-Tethys with the Yarlung Zangbo River as the main ocean basin closed to form orogenic movement and the strong continent-continent collision orogenic movement of the east and west Himalayas generated clockwise movement in the mid-Himalaya area. According to the calculation of palaeolatitude data, the Himalaya continent-continent collusion orogenic movement since the Eocene epoch caused the crustal structure in Indian Plate-Himalaya folded structural belt-Lhasa block to shorten by at least 1000 km. The systematic research on the paleomagnetism of Qomolangma area in the phanerozoic eon provides a scientific basis to further research the evolution of Gondwanaland, formation and extinction history of paleo-Tethys Ocean and uplift mechanism of the Qinghai-Tibet Plateau.  相似文献   

20.
The Upper Permian Emmaville Volcanics at Deepwater, northeastern New South Wales, consist of a diverse succession of calc‐alkaline silicic‐intermediate ignimbrites, volcaniclastics and minor lavas. This 2.5 km‐thick sequence underlies and outcrops extensively along the northern margin of the Dundee Rhyodacite Outlier at Dundee. Detailed mapping and facies analysis have revealed eight locally mappable units namely; Magistrate Volcanic Member (rhyolitic ignimbrites), Wollundi Mudstone Member, Dellwood Ignimbrite Member, Marrawarra Rhyolite Member, Top‐Crossing Sandstone Member, Arranmor Ignimbrite Member, Yarramundi Andesite Member (lava, breccia) and Welcome Volcanic Member (rhyolitic ignimbrites). All volcanic units are contained in two fault‐bounded blocks of different lithology and structure. The volcanic succession ranges in composition from andesite to high‐silica rhyolite (58.6–78% SiO2). Chemical characteristics include enrichment in K2O (>3.5%), Al2O3 and large‐ion lithophile elements (LILE: Rb, K and light rare‐earth elements (LREE)), and depletion in high field strength elements (HFSE: Ti, Nb and Zr). These geochemical attributes reflect a continental subduction‐related signature. The facies architecture indicates that the principal volcanic features of the Late Permian palaeogeography in northeastern New South Wales was a topographically subdued depression flanked by low‐angle ignimbrite sheets with rhyolitic‐intermediate volcanic centres rising gently from the sloping terrain. The succession demonstrates that during the Late Permian andesitic volcanism was present, although localised. A modern analogue for the setting of the Emmaville Volcanics is the Quaternary Taupo Volcanic Zone (New Zealand).  相似文献   

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