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1.
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   

2.
The Pyrenees at the Iberia–Europe collision zone contain sediments showing Albian–Cenomanian high-temperature metamorphism, and coeval alkaline magmatic rocks. Stemming from different views on Jurassic–Cretaceous Iberian microplate kinematics, two schools of thought exist on the trigger of this thermal pulse: one invoking hyperextension of the Iberian and Eurasian margins, the other suggesting slab break-off. Competing scenarios for Mesozoic Iberian motion compatible with Pyrenean geology, comprise (1) transtensional eastward motion of Iberia versus Eurasia, or (2) strike-slip motion followed by orthogonal extension, both favoring hyperextension-related heating, and (3) scissor-style opening of the Bay of Biscay coupled with subduction in the Pyrenean realm, favoring the slab break-off hypothesis. We test these kinematic scenarios for Iberia against a newly compiled paleomagnetic dataset and conclude that the scissor-type scenario is the only one consistent with a well-defined ~ 35° counterclockwise rotation of Iberia during the Early Aptian. We proceed to show that when taking absolute plate motions into account, Aptian oceanic subduction in the Pyrenees followed by Late Aptian–Early Albian slab break-off should leave a slab remnant in the present-day mid-mantle below NW Africa. Mantle tomography shows the Reggane anomaly that matches the predicted position and dimension of such a slab remnant between 1900 and 1500 km depth below southern Algeria. Mantle tomography is therefore consistent with the scissor-type opening of the Bay of Biscay coupled with subduction in the Pyrenean realm. Slab break-off may thus explain high-temperature metamorphism and alkaline magmatism during the Albian–Cenomanian in the Pyrenees, whereas hyperextension that exhumed Pyrenean mantle bodies occurred much earlier, in the Jurassic.  相似文献   

3.
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   

4.
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5.
The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~ 70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve.  相似文献   

6.
Paleomagnetism has played an important role in quantifying the Mesozoic evolution of “Proto-Tibet”. In this paper, we present new paleomagnetic data from five Middle-Upper Jurassic sedimentary sequences (Quemo Co, Buqu, Xiali, Suowa and Xueshan Fms.) of the eastern North Qiangtang Terrane (QT) at Yanshiping (33.6°N, 92.1°E). The new paleomagnetic results form a large dataset (99 sites, 1702 samples) and reveal a paleopole at 79.1°N/306.9°E (dp = 3.9°, dm = 6.3°) for the Quemo Co Fm., at 68.9°N/313.8°E (dp = 2.1°, dm = 3.7°) for the Buqu Fm., at 66.1°N/332.1°E (dp = 2.7°, dm = 4.6°) for the Xiali Fm., at 72.4°N/318.6°E (dp = 3.9°, dm = 6.7°) for the Suowa Fm., and at 76.9°N/301.1°E (dp = 7.9°, dm = 13.2°) for the Xueshan Fm. These results indicate clockwise (CW) rotations of ~ 19.8 ± 9.4° between ~ 171.2 and 161.7 Ma and counterclockwise (CCW) rotations of ~ 15.4 ± 13.4° between ~ 161.7 and < 157.2 Ma for Yanshiping. We attribute the change in rotation sense at approximately ~ 161.7 Ma to the initial collision of the Lhasa and Qiangtang terranes. Using this and other paleomagnetic data from the Lhasa, Qiangtang and Tarim terranes, as well as other geological evidence (e.g., tectonism-related sedimentary sequences, volcanism, and HP metamorphism), we propose a new conceptual evolution model for the Mesozoic QT and Tethyan Oceans. The Longmo Co-Shuanghu oceanic slab was subducted before 248 Ma, followed by continental collision of the North-South Qiangtang subterranes between ~ 245 and 237 Ma. The Qiangtang Terrane experienced post-collisional exhumation between ~ 237 and 230 Ma during subduction of the Jinsha oceanic slab. The collision of the Qiangtang and Songpan-Ganzi terranes occurred between ~ 230 and 225 Ma. The QT experienced post-collisional relaxation from ~ 225 to ~ 200 Ma, followed by subsidence and extension-related exhumation between ~ 200 and 162 Ma in association with subduction of the Bangong-Nujiang oceanic slab. Finally, these events were followed by the scissor-like diachronous collisions of the Lhasa and Qiangtang terranes between ~ 162 Ma and the mid-Cretaceous.  相似文献   

7.
A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM (D = 131.8°, I = 15.7°, k = 196, α95 = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.  相似文献   

8.
《Gondwana Research》2013,23(3-4):974-991
It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~ 2000 km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north–central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle–Late Ordovician paleolatitudes and came to the conclusion that a nearly E–W trending active margin of the Kazakhstania landmass had existed at low (~ 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet–Akbastau, and with the Aktau–Junggar microcontinent by the Ordovician–Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW–SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt.  相似文献   

9.
The Marwar Supergroup refers to a 1000–2000 m thick marine and coastal sequence that covers a vast area of Rajasthan in NW–India. The Marwar Basin uncomformably overlies the ∼750–770 Ma rocks of the Malani Igneous Suite and is therefore considered Late Neoproterozoic to Early Cambrian in age. Upper Vindhyan basinal sediments (Bhander and Rewa Groups), exposed in the east and separated by the Aravalli–Delhi Fold Belt, have long been assumed to coeval with the Marwar Supergroup. Recent studies based on detrital zircon populations of the Marwar and Upper Vindhyan sequences show some similarity in the older populations, but the Vindhyan sequence shows no zircons younger than 1000 Ma whereas samples taken from the Marwar Basin show distinctly younger zircons. This observation led to speculation that the Upper Vindhyan and Marwar sequences did not develop coevally.While there are alternative explanations for why the two basins may differ in their detrital zircon populations, paleomagnetic studies may provide independent evidence for differences/similarities between the assumed coeval basins. We have collected samples in the Marwar Basin and present the paleomagnetic results. Previous paleomagnetic studies of Marwar basinal sediments were misinterpreted as being indistinguishable from the Upper Vindhyan sequence. The vast majority of our samples show directional characteristics similar to the previously published studies. We interpret these results to be a recent overprint. A small subset of hematite-bearing rocks from the Jodhpur Formation (basal Marwar) exhibit directional data (Dec = 89° Inc = −1° α95 = 9°) that are distinct from the Upper Vindhyan pole and may offer additional support for temporally distinct episodes of sedimentation in these proximal regions. A VGP based upon our directional data is reported at 1°S 344°E (dp = 5°, dm = 9°). We conclude that the Marwar Supergroup developed near the close of the Ediacaran Period and is part of a larger group of sedimentary basins that include the Huqf Supergroup (Oman), the Salt-Range (Pakistan), the Krol–Tal belt (Himalayas) and perhaps the Molo Supergroup (Madagascar).  相似文献   

10.
《Precambrian Research》2006,144(1-2):69-91
By using unusual combinations of demagnetization techniques, Proterozoic paleomagnetic vectors and paleopoles are provided for two recently discovered post-tectonic Proterozoic units near Armstrong, northern Ontario, and also for well-dated Gunflint Formation, which by previous techniques yielded problematical paleomagnetic data. The first paleomagnetic data are provided also for the Seagull Pluton and Inspiration Sills. Characteristic remanent magnetizations (ChRM) for the Pillar Lake Lavas indicate a Keweenawan age, more specifically ∼1000–1040 Ma by comparison with the well-established APWP for the Late Proterozoic Superior craton. Four combinations of demagnetization techniques yield declinations in the range 108–133° and inclinations in the range −65 to −70° (n = 100), which define paleopoles near 200 W/48 N corresponding to a location on the Keweenawan APWP near ∼1040 Ma. In the underlying basement a recently discovered Proterozoic igneous complex, the Waweig Troctolitic Complex, yields new paleomagnetic data with declination and inclination 42/−54 (n = 14) defining a paleopole at 238 W/09 N. Its ages may be 1400–1600 or ∼2000 Ma by comparison with the presently available, ambiguous and sparsely populated APWP. The first paleomagnetic results for the Seagull Pluton (U–Pb age 1113 Ma) yield a mean declination of 87.4/−75.7 (n = 32) corresponding to a Keweenawan paleopole near 233/42 N, consistent with other paleopoles near ∼1200 Ma. Tuffs of the oft studied but problematical Gunflint Formation (U–Pb age1878 Ma) yielded stable and presumably primary vectors using several different demagnetization techniques on the same specimens. Their mean primary declination and inclination ∼303/+48.8 (n = 17) yields a paleopole now located near 178 W/42 N, comparable with the published locations of paleopoles of ∼2000 Ma. Of broader interest, we recognized that low temperature demagnetization preceding conventional demagnetization techniques enhanced the isolation of characteristic vectors. Combining the conventional techniques (thermal and AF demagnetization) also improved the resolution of characteristic vectors not achieved by other means. Low grade metamorphism affected the non-tectonized Proterozoic cover to the Canadian shield, due to burial or hydrothermal effects, obfuscating or erasing primary vectors in some lithologies and especially at certain sites.  相似文献   

11.
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   

12.
R.L.M. Vissers  P.Th. Meijer 《Earth》2012,110(1-4):93-110
Following on paleomagnetic studies in the sixties showing ~ 35° counterclockwise rotation of Iberia during the Mesozoic, two classes of scenarios have been proposed for the motion history of Iberia which are currently competing. One class infers convergence in the Pyrenees in response to a scissor-type opening of the Bay of Biscay, described by a pole of rotation for Iberia with respect to Europe located within the Bay. The other class of scenarios assumes extensional or transtensional motions in the Pyrenees, compatible with opening of the Bay of Biscay described by a pole of rotation located in northern France. Although plate-kinematic studies over the last decade increasingly support the scissor-type model, geological studies in the Pyrenees have accumulated arguments in favour of an extensional or transtensional regime in the Pyrenean realm.We perform a detailed plate-kinematic analysis of the Late Jurassic and Cretaceous motion history of Iberia and surrounding plates with respect to Europe. A total of six sea-floor reconstructions in combination with paleomagnetic studies onland allow to recognize four distinct stages. (1) Early rifting and ultraslow spreading since the Kimmeridgean led to the development of an oceanic Neotethys domain north of Iberia. (2) This was followed by ~ 35° CCW rotation of Iberia during the Aptian, kinematically linked to progressive opening of the Bay of Biscay. (3) Motions in the Bay became stagnant during the Albian till Santonian, followed by the latest stages of spreading in the Bay, and (4) onset of largely Tertiary continental collision between Iberia and Europe eventually leading to the present day structure of the belt.Our analysis confirms the results of previous studies indicating that extensional or transtensional motions in the Pyrenean realm during opening of the Bay of Biscay and concurrent rotation of Iberia are incompatible with plate-kinematic reconstructions based on sea-floor anomalies. This invites a reappraisal of the geological data. Convergence in the Pyrenean realm during opening of the Bay and rotation of Iberia was accommodated by up to 300 km of subduction of mantle-dominated ocean floor exhumed during the late Jurassic and early Cretaceous. The stagnant stage in the progressive opening of the Bay indicates that convergence in the Pyrenean realm virtually came to a halt during the Albian. We hypothesize that the lithosphere previously subducted during Aptian convergence became gravitationally unstable, leading to asthenospheric upwelling and consequent magmatism and high temperature metamorphism in the overlying European margin now exposed in the North Pyrenean Zone. Aside from these magmatic and thermal effects, an enhanced gravitational potential energy of the remaining lithosphere column underlain by shallow asthenosphere may have led to a stress state allowing belt-parallel extensional deformation. Such a detachment scenario, inspired by plate-kinematic results, may provide an alternative to explain many of the geological data commonly quoted to infer a transtensional or extensional tectonic regime in the Pyrenees during the rotation of Iberia.  相似文献   

13.
In this paper we present new paleomagnetic and paleontological data from the Ordovician and Silurian carbonate rocks of Kotelny Island (the Anjou Archipelago), and from the Ordovician turbidities of Bennett Island (the De Long Archipelago). It is assumed that both archipelagos belong to the NSI (New Siberian Islands) terrane — a key tectonic element in the Arctic region. Ages of the studied rocks have been established by paleontological data and lithological correlations. Our new data on conodonts combined with those from previous studies of Ordovician and Silurian fauna indicate a biogeographic similarity between the shelves of the Siberian paleocontinent and the NSI in the Early Paleozoic. Three new paleomagnetic poles for the NSI (48.9°N, 13.8°E, A95 = 18.1° for 475 Ma; 45.5°N, 31.9°E, A95 = 11.0° for 465 Ma, and 33.7°N, 55.7°E, A95 = 11.0° for 435 Ma) fall between the south-eastern part of Central Europe and the Zagros Mountains. The similarity of paleomagnetic directions from Kotelny and Bennet islands confirms that both the Anjou and De Long archipelagos belong to the same terrane. Calculated paleolatitudes indicate that in Ordovician–Silurian times this terrane has been located between 30° and 45°, possibly in the northern hemisphere. Based on this observation, we suggest a linkage between the NSI and the Kolyma–Omolon superterrane. Comparison of apparent polar wander paths (APWPs) of the NSI, Siberia and other cratons/terranes suggests that the NSI drifted independently. We demonstrate that the structural line between Svyatoy Nos Peninsula and Great Lyakhovsky Island is the continuation of the Kolyma Loop suture on the Arctic shelf, and expect that the continuation of the South Anyui suture is to be found east of the NSI.  相似文献   

14.
Reconstruction of Mesozoic and Cenozoic sedimentary ‘cover’ on the Precambrian shield in the Lac de Gras diamond field, Northwest Territories, Canada, has been achieved using Cretaceous and early Tertiary sedimentary xenoliths and contemporaneous organic matter preserved in volcaniclastic sediments associated with late Cretaceous to early Tertiary kimberlite pipe intrusions, and in situ, Eocene crater lake, lacustrine and peat bog strata. Percent reflectance in oil (%Ro) of vitrinite within shale xenoliths for: (i) Albian to mid-Cenomanian to Turonian ranges from > 0.27 to 0.42 %Ro (mean = 0.38 %Ro), (ii) Maastrichtian to early Paleocene from 0.24 to < 0.30%; (iii) latest Paleocene to early middle Eocene 0.15 to < 0.23 %Ro (mean = 0.18 %Ro). These levels of thermal maturity are corroborated by Rock Eval pyrolysis Tmax (°C) and VIS region fluorescence of liptinites, with wavelengths of maximum emission for sporinite, prasinophyte alginite and dinoflagellates consistent with vitrinite reflectance of 0.20 to < 0.50 %Ro. Burial–thermal history modeling, constrained by measured vitrinite reflectance and porosity of shale xenoliths, predicts a maximum burial temperature for Mid to Late Albian strata (∼115 Ma) of 60 °C with ∼1.2 to 1.4 km of Cretaceous strata in the Lac de Gras kimberlite field region prior to major uplift and erosion, which began at 90 Ma. Late Paleocene to middle Eocene volcanic crater lake lacustrine to peat bog strata were only buried to a few hundreds of meters and are in a peat-brown coal stage of thermal maturation.  相似文献   

15.
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16.
A new paleomagnetic pole position is obtained from the well-dated (636.3 ± 4.9 Ma) Nantuo Formation in the Guzhang section, western Hunan Province, and the correlative Long’e section in eastern Guizhou Province, South China. Remagnetization of the recent geomagnetic field was identified and removed for both sections. The hard dual-polarity, interpreted as primary, component of the Nantuo Formation, directs east–westward with medium inclinations, yielding an average pole of 9.3°N, 165°E, A95 = 4.3° that, for the first time, passed a strata-bound reversals test. The new data are consistent with previously published paleomagnetic data of the Nantuo Formation from Malong county, central Yunnan Province, which passed a positive syn-sedimentary fold test. Together, these sites represent shallow- to deep-water sections across a shelf-to-basin transect centered at ∼33° paleolatitude. The sedimentary basin may have faced an expansive ocean toward the paleo-East. In the ∼750 Ma and ∼635 Ma global reconstructions, the South China Block (SCB) was best fitted in the northern hemisphere close to northwestern Australia. However, a direct SCB-northwestern Australia connection, inferred to have existed during the Early Cambrian–Early Devonian, had not formed by the time of ∼635 Ma.  相似文献   

17.
The processes of orocline formation are a topic of debate in geosciences. The Patagonian orocline has been a case in point for over a century. Large anomalous paleomagnetic pole rotations show that the orocline started to form at the same time as mid-Cretaceous closure of the Rocas Verdes Basin, today known from ophiolitic and basin fill remnants in the Patagonian and Fuegian Andes. Some studies therefore present bending of the Andes and closure of the basin as shared consequences of rotation of a small plate that was driven by subduction-related forces at the Pacific margin of Gondwana. An alternative view of the orocline is as a product of Cretaceous to Paleogene-aged sinistral oblique convergence at the plate-boundary scale. Geological data from Tierra del Fuego have been interpreted in support of both views. Here, I test these suggestions by comparing the Rocas Verdes Basin's tectonostratigraphy to predictions of a plate kinematic model for fragmentation of the western interior of Gondwana. The model is sufficient to explain the known history of basin opening to a width of ~ 100–300 km during the period 152–141 Ma and later closure in oblique plate convergence. As this convergence occurred by motion around a distant Euler pole, it could not have produced the Patagonian orocline by rotation of a lithospheric plate on its Pacific flank. The large anomalous paleomagnetic rotations of Tierra del Fuego, instead, are likely to have occurred within the crust by rotation and deformation of regional strike-slip faults and the intervening rocks to accommodate oblique convergence of the South American and Antarctic plates between Albian and Paleocene times.  相似文献   

18.
A new paleomagnetic study on well-dated (~ 155 Ma) volcanic rocks of the Tiaojishan Formation (Fm) in the northern margin of the North China Block (NCB) has been carried out. A total of 194 samples were collected from 26 sites in the Yanshan Belt areas of Luanping, Beipiao, and Shouwangfen. All samples were subjected to stepwise thermal demagnetization. After removal of a recent geomagnetic field viscous component, a stable high temperature component (HTC) was isolated. The inclinations of our new data are significantly steeper than those previously published from the Tiaojishan Fm in the Chengde area (Pei et al., 2011, Tectonophysics, 510, 370–380). Our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation (PSV), but the PSV can be averaged out by combining all the virtual geomagnetic poles (VGPs) from the Tiaojishan Fm in the region. The mean pole at 69.6°N/203.0°E (A95 = 5.6°) passes a reversal test and regional tilting test at 95% confidence and is thus considered as a primary paleomagnetic record. This newly determined pole of the Tiaojishan Fm is consistent with available Late Jurassic poles from red-beds in the southern part of the NCB, but they are incompatible with coeval poles of Siberia and the reference pole of Eurasia, indicating that convergence between Siberia and the NCB had not yet ended by ~ 155 Ma. Our calculation shows a ~ 1600-km latitudinal plate movement and crustal shortening between the Siberia and NCB after ~ 155 Ma. In addition, no significant vertical axis rotation was found either between our sampled areas or between the Yanshan Belt and the major part of the NCB after ~ 155 Ma.  相似文献   

19.
Elba Island, located midway between Corsica and mainland Italy, is a small but important fragment of the Adria Plate. It has a rich sedimentary record preserved in a stack of tectonic nappes of both continental margin and oceanic origin. Especially the detrital zircons in early Paleozoic to early Mesozoic metasedimentary rocks provide an archive of many important geological events in the island's history. Elba Island and Adria originated along the northern margin of Gondwana, but drifted north in Silurian times to become part of Europe. A large new dataset of LA-ICP-MS and SIMS U–Pb zircon ages allows us to trace this history. Three main stratigraphic units have been investigated. The oldest Porto Azzurro Unit was deposited in the early Cambrian and has zircon age distributions indicating a typical northern African provenance, most likely sourced from the Saharan Metacraton. The Ortano Unit has a simple, mostly unimodal Ordovician age distribution that is entirely dominated by metavolcanic rocks and their erosional products; a sample of the metavolcanic Ortano Porphyroids provided a SIMS U–Pb zircon age of 460 ± 3 Ma. This phase of intense volcanism is related to the subduction of the Rheic Ocean beneath Gondwana, terminating with initial rifting and subsequent opening of the Paleotethys. This also marks the onset of the separation of a range of European terranes, including Adria and future Elba Island, from Gondwana. The Permo-Triassic Monticiano–Roccastrada Unit is the first to show a European provenance with the appearance of large amounts of Variscan and late to post-Variscan detritus. The presence of Variscan detrital zircons in the Permo-Triassic sediments is unexpected, since a Variscan age signature is so far not well recorded in the Adria Plate. This dataset is the most comprehensive detrital zircon dataset so far available for the Adria Plate and documents Adria's close affinity to Africa in the Lower Paleozoic, as well as its initial rifting within an active continental margin setting during the Ordovician and its final separation and independent evolution since late Palaeozoic times.  相似文献   

20.
《Gondwana Research》2013,23(3-4):956-973
The configuration and the timing of assembly and break-up of Columbia are still matter of debate. In order to improve our knowledge about the Mesoproterozoic evolution of Columbia, a paleomagnetic study was carried out on the 1420 Ma Indiavaí mafic intrusive rocks that crosscut the polycyclic Proterozoic basement of the SW Amazonian Craton, in southwestern Mato Grosso State (Brazil). Alternating field and thermal demagnetization revealed south/southwest ChRM directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests, including thermomagnetic data, hysteresis data and the progressive acquisition of isothermal remanent magnetization. Different stable magnetization components isolated in host rocks from the basement 10 km NW away to the Indiavaí intrusion, further support the primary origin of the ChRM. A mean of the site mean directions was calculated at Dm = 209.8°, Im = 50.7° (α95 = 8.0°, K = 22.1), which yielded a paleomagnetic pole located at 249.7°E, 57.0°S (A95 = 8.6°). The similarity of this pole with the recently published 1420 Ma pole from the Nova Guarita dykes in northern Mato Grosso State suggests a similar tectonic framework for these two sites located 600 km apart, implying the bulk rigidity of the Rondonian-San Ignacio crust at that time. Furthermore these data provide new insights on the tectonic significance of the 1100–1000 Ma Nova Brasilândia belt—a major EW feature that cuts across the basement rocks of this province, which can now be interpreted as intracratonic, in contrast to previous interpretation. From a global perspective, a new Mesoproterozoic paleogeography of Columbia has been proposed based on comparison of these 1420 Ma poles and a 1780 Ma pole from Amazonia with other paleomagnetic poles of similar age from Baltica and Laurentia, a reconstruction in agreement with geological correlations.  相似文献   

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