Tilt-corrected Mean Direction Research Articles

New paleomagnetic results of the Upper Cretaceous to Lower Eocene sedimentary rocks from the Xigaze forearc basin and their tectonic implications

The India-Asia collision processes are under current debate, mainly due to the large variations of the pre-collision position of the southern margin of Asia, in the range of 10–30°N. To resolve this uncertainty, we report new paleomagnetic data of the Xigaze forearc basin sediments from the Zhongba Cuojiangding area. Characteristic remanent magnetizations (ChRMs) were obtained from the upper Cretaceous Qubeiya Formation (Fm) and the early Eocene Jialazi Fm. Both mean directions passed the fold test and also passed the reversal test. An elongation-inclination analysis for all the Jialazi Fm ChRM directions yielded an unflattened mean inclination of 41.6°, with 95% confidence limits between 36.0° and 49.0° (f = 0.76), consistent with the Qubeiya Fm results. The overall mean direction for the Qubeiya Fm is D = 334.8°, I = 42.7°, k = 11.8, α95 = 6.7°, n = 42 in stratigraphic coordinates, corresponding to a paleolatitude of 24.8°N and a paleopole at 67.1°N, 347.5°E with A95 = 7.1°. The tilt-corrected mean direction, combined with published data, for the Jialazi Fm is D = 340.5°, I = 41.6°, k = 8.7, α95 = 5.2°, n = 87, corresponding to a paleolatitude of 23.9°N and a paleopole at 71.6°N, 340.0°E with A95 = 5.0°. Our updated results indicate a paleolatitude of ~24°N for the Xigaze forearc basin during 70–50 Ma, suggesting that a significant Greater Asia is very unlikely, and strongly supporting a two-stage model for the India-Asia collision with a ~ 55 Ma initial collision of India with an equatorial intra-oceanic arc in the northern hemisphere low latitude followed by the collision between India-plus-arc and Asia in late Eocene.

Remagnetization of Permian Emeishan basalts: Constraints on the timing of native copper mineralization in northeast Yunnan Province, China

New paleomagnetic results from the Permian Emeishan basalts in the Zhaotong area, NE Yunnan province, China show four natural remanent magnetization components. Detailed stepwise thermal demagnetization of basaltic samples from 16 flows from the Dadi section, which represent basalt units III and IV, isolated two groups of characteristic remanent magnetizations. Samples in unit IV (five flows) record a southwest declination and a moderate downward inclination that is considered to be a partial remagnetized remanence. The bottom flows from unit III (11 flows) record a normal polarity direction, interpreted as a remagnetization, which yields a tilt-corrected mean direction of Ds/Is= 8.8°/31.6° (N = 9, ks= 39.7,α95= 8.3°), with a corresponding paleomagnetic pole at 77.1°N, 240.0°E (K = 49.2, A95= 7.4°). The secondary directions have steeper inclinations than primary ones that have been successfully recovered from other studies in this area of the Emeishan basalts. By comparison with the Phanerozoic paleomagnetic poles of the South China Block, the preferred timing of remagnetization is the Lower-Middle Jurassic. Field relationships suggest that the remagnetization of the Emeishan basalts is coeval with the spatially related, but localized, copper mineralization. Thus the timing of the main copper mineralization hosted in the Emeishan basalts is hypothesized to occur in the Early-Middle Jurassic.

Paleomagnetism and U-Pb Geochronology of Early Cretaceous Volcanic Rocks from the Qiangtang Block, Tibetan Plateau: Implications for the Qiangtang-Lhasa Collision

The collision between the Qiangtang and Lhasa blocks is a critical factor in understanding the geodynamics of the central Tibetan Plateau. However, the process of the Qiangtang-Lhasa collision remains contentious. A direct way to study the history of the Qiangtang-Lhasa collision would be to determine the evolution of the paleolatitude positions of the Qiangtang and Lhasa blocks during the Jurassic-Cretaceous. In this study, we present a combined paleomagnetic and geochronological study of the Early Cretaceous volcanic rocks dated at ~120–115 Ma in the western Qiangtang block. Stepwise thermal demagnetization succeeded in isolating the high-temperature characteristic directions of the samples. The tilt-corrected mean direction of the 16 sampling sites was Ds = 60.9°, Is = 45.9° with ɑ95 = 4.4°, which indicates that the Qiangtang block was situated at 27.6 ± 5.0°N during the Early Cretaceous (~120–115 Ma) (reference point: 32.9°N, 83.5°E). Our new data combined with previous reliable Cretaceous paleomagnetic results for the Lhasa block indicate that the collision between the western parts of the Qiangtang and Lhasa blocks may occur later than 115 Ma. The extent of the western segment of the Bangong-Nujiang Tethys Ocean was ~825 ± 600 km (7.5° ± 5.5° in latitude) at 120–115 Ma.

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is Ds = 1.7°, Is = 43.1°, ks = 403, α95 = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A95 = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.

New Early and Late Carboniferous paleomagnetic results from the Qaidam Block, NW China: Implications for the paleogeography of Central Asia

There is an ongoing debate regarding the paleogeographic evolution of the Qaidam Block (37.3°N/96.4°E) in northwest China during the late Paleozoic. To provide a reliable constraint on the paleogeographic reconstruction and to determine the relationship with the adjacent area, we performed a paleomagnetic investigation of Early and Late Carboniferous sediments from the Qaidam Block. Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from the samples. The tilt-corrected mean direction of the Early Carboniferous sediments is Ds/Is=298.6°/−46.4° with α95=4.7° and N=15 sites, corresponding to a paleopole at −3.2°N, 147.5°E with A95=4.8°. The Late Carboniferous paleomagnetic direction is Ds/Is=320.1°/−46.1° with α95=5.3° and N=8 sites, corresponding to a paleopole at −15.1°N, 132.2°E with A95=5.4°. A positive fold test for the Early Carboniferous paleomagnetic directions, and consistency with the Kiaman reversed superchron for the Late Carboniferous paleomagnetic directions, indicates the primary origin of the characteristic remanence of the studied section. The new paleomagnetic results suggest that the Qaidam Block was located at about 27°N and did not undergo significant N-S movement during the Carboniferous. However, the Qaidam Block experienced a counter-clockwise rotation of about 21° during this period. Comparisons of the paleomagnetic results from the major blocks of Central Asia suggest that the Qaidam Block may have experienced a relatively rapid northward movement and collided with the Tarim Block after the Late Permian. Based on the paleomagnetic poles of the Qaidam Block and adjacent blocks, we present a tentative paleogeographic reconstruction for central Asia during the Early and Late Carboniferous.

Paleomagnetic results from Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane, Tibet, China, and their tectonic implications

Results of a systematic paleomagnetic study are reported based on Late Carboniferous to Early Permian sedimentary rocks on the north slope of the Tanggula Mountains, in the northern Qiangtang terrane (NQT), Tibet, China. Data revealed that magnetic minerals in limestone samples from the Zarigen Formation (CP^ z )are primarily composed of magnetite, while those in sandstone samples from the Nuoribagaribao Formation (P nr ) are dominated by hematite alone, or hematite and magnetite in combination. Progressive thermal, or alternating field, demagnetization allowed us to isolate a stable high temperature component (HTC) in 127 specimens from 16 sites which successfully passed the conglomerate test, consistent with primary remnance. The tilt-corrected mean direction for Late Carboniferous to Early Permian rocks in the northern Qiangtang terrane is D s=30.2°, I s= - 40.9°, k s=269.0, a 95=2.3°, N =16, which yields a corresponding paleomagnetic pole at 25.7°N, 241.5°E (d p /d m =2.8°/1.7°), and a paleolatitude of 23.4°S. Our results, together with previously reported paleomagnetic data, indicate that: (1) the NQT in Tibet, China, was located at a low latitude in the southern hemisphere, and may have belonged to the northern margin of Gondwana during the Late Carboniferous to Early Permian; (2) the Paleo-Tethys Ocean was large during the Late Carboniferous to Early Permian, and (3) the NQT subsequently moved rapidly northwards, perhaps related to the fact that the Paleo-Tethys Ocean was rapidly contracting from the Late Permian to Late Triassic while the Bangong Lake-Nujiang Ocean, the northern branch of the Neo-Tethys Ocean, expanded rapidly during this time.

New Late Cretaceous paleomagnetic data from volcanic rocks and red beds from the Lhasa terrane and its implications for the paleolatitude of the southern margin of Asia prior to the collision with India

In order to improve estimation of the paleolatitude of the southern margin of Asia during the Late Cretaceous, as well as the constraints on the intracontinental convergence within Asia due to the India–Asia collision, we performed a combined geochronological and paleomagnetic investigation of Late Cretaceous red bed sequences and intercalated volcanic rocks from the Linzhou Basin in the Lhasa terrane (29.9°N/91.1°E). The results of potassium–argon (K–Ar) dating indicated that the basalt sequences are of Late Cretaceous age (68–75Ma). Stepwise thermal demagnetization successfully isolated high unblocking-temperature characteristic directions from 10 red bed sites and 11 lava flow sites. Positive fold and reversal tests indicate a primary origin for the characteristic remanence of the red beds and basalt sites. The consistent inclination recorded in the red beds and basalt lava flows indicates that no significant compaction-related inclination shallowing has occurred in the red beds from the studied section. The tilt-corrected mean direction is D/I=0.5°/20.2° with α95=6.4° and N=21 sites, corresponding to a paleopole at 70.5°N, 269.6°E with A95=4.9°. The paleomagnetic results yield a paleolatitude of 10.4±4.9°N for the southern margin of Asia in the Late Cretaceous. Compared with expected paleomagnetic directions from the Tethyan Himalaya and Asia, significant crustal shortening of 17.7±5.5° (1960±610km) and 9.4±4.7° (1040±520km), respectively, may have occurred between the southern margin of Asia and the Tethyan Himalaya, and within Asia, since the Late Cretaceous. Based on previous Cretaceous and Paleocene data from the Lhasa terrane, the latitude of the southern margin of Asia was located at a stable paleolatitudinal position (10°–15°N) during the Cretaceous and Paleocene.

Paleomagnetism of Late Jurassic to Early Cretaceous red beds from the Cardamom Mountains, southwestern Cambodia: Tectonic deformation of the Indochina Peninsula

Late Jurassic to Early Cretaceous red beds of the Phuquoc Formation were sampled at 33 sites from the Sihanoukville and Koah Kong areas of the Phuquoc–Kampot Som Basin, southwestern Cambodia. Two high-temperature remanent components with unblocking temperature ranging 650°–670 °C and 670–690 °C were identified. The magnetization direction for the former component (D=5.2°, I=18.5° with α95=3.1° in situ) reveals a negative fold test that indicates a post-folding secondary nature. However, the latter component, carried by specular hematite, is recognized as a primary remanent magnetization. A tilt-corrected mean direction of D=43.4°, I=31.9°(α95=3.6°) was calculated for the primary component at 11 sites, corresponding to a paleopole of 47.7°N, 178.9°E (A95=3.6°). When compared with the 130 Ma East Asian pole, a southward displacement of 6.0°±3.5° and a clockwise rotation of 33.1°±4.0° of the Phuquoc–Kampot Som Basin (as a part of the Indochina Block) with respect to East Asia were estimated. This estimate of the clockwise rotation is ∼15° larger than that of the Khorat Basin, which we attribute to dextral motion along the Wang Chao Fault since the mid-Oligocene. The comparison of the herein estimated clockwise rotation with the counter-clockwise rotation reported from the Da Lat area in Vietnam suggests the occurrence of a differential tectonic rotation in the southern tip of the Indochina Block. During the southward displacement of the Indochina Block, the non-rigid lithosphere under its southern tip moved heterogeneously, while the rigid lithosphere under the Khorat Basin moved homogeneously.

Combined paleomagnetic and geochronological study on Cretaceous strata of the Qiangtang terrane, central Tibet

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±0.46Ma (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 ~100Ma 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 ~100Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~550kmN–S convergence between the Qiangtang and Lhasa terranes happened after ~100Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~100Ma, which is most likely caused by the India–Asia collision.

Paleomagnetism of the Late Cretaceous ignimbrite from the Okhotsk-Chukotka Volcanic Belt, Kolyma-Omolon Composite Terrane: Tectonic implications

New Late Cretaceous paleomagnetic results from the Okhotsk-Chukotka Volcanic Belt in the Kolyma-Omolon Composite Terrane yield stable and consistent remanent directions. The Late Cretaceous (86–81Ma) ignimbrites from the Kholchan and Ola suites were sampled at 19 sites in the Magadan area (60.4°N, 151.0°E). We isolated the characteristic paleomagnetic directions from 16 sampled sites using an alternating field demagnetization procedure. The primary nature of these directions is ascertained by dual polarities and positive fold tests. A tilt-corrected mean direction (D=42.8°, I=84.7°, k=46.0, α95=10.0°) yields a paleomagnetic pole of 66.7°N, 168.5°E (A95=18.8°) which appears almost identical to the 90–67Ma pole reported from the Lake El’gygytgyn area of the Okhotsk-Chukotka Volcanic Belt (Chukotka Terrane). This consistency suggests that the Kolyma-Omolon Composite Terrane and Chukotka Terrane has acted as a single tectonic unit since 80Ma without any significant internal deformation. Accordingly, we calculate a combined 80Ma characteristic paleomagnetic pole (Long.=164.7°E, Lat.=68.0°, A95=10.9°, N=12) for the Kolyma-Omolon-Chukotka Block which falls 16.5–17.5° south of the same age poles from Europe and East Asia. We ascribe this discrepancy in pole positions to tectonic activity in the area and infer a southward displacement of 1640±1380km for the Kolyma-Omolon-Chukotka Block with respect to the North American and Eurasian blocks since 80Ma; more than 260km of it is attributed to tectonic displacement in the Arctic Ocean due to the opening of the Canadian Basin.

Tectonic deformation in the southern part of South China Block: Paleomagnetic study of the Early Cretaceous Xinlong Formation from Shangsi Foredeep Depozone in the Guangxi Province

To examine tectonic deformation in the southern part of the South China Block, red bed samples have been collected at 37 sites from the Early Cretaceous Xinlong Formation, south side of the Nanning town (108.3°E, 22.8°N). Stepwise thermal demagnetization and rock magnetic investigations revealed magnetite and hematite as the main carriers of remanent magnetizations in the studied samples. Pre-folding origin for hematite-related magnetic component is supported by positive fold test, whereas a syn-folding origin is assigned to a component carried by magnetite. A 100% tilt-corrected mean direction for hematite-related component (Dec/Inc=3.3°/41.7°, k=43.3, α95=4.1°, N=29) is considered to have been acquired at the time of deposition during the Early Cretaceous. Comparison of this direction with previously reported Cretaceous paleomagnetic directions from the Cathaysia Block (a part of the South China Block) indicates a gradual increase in counter-clockwise deflected declination toward southwest in this block. The amount of this counter-clockwise deflection increases up to 13.4°±4.7° in the southern part of the Cathaysia Block compared to the stable part of the South China Block. This deflection is ascribed to block rotation as a result of tectonic deformation in the South China Block during the indentation of India into Asia. Geological studies and seismic tomographic interpretations suggest that a large degree of counter-clockwise deflection remained restricted to the Nanpanjiang-Youjiang basin, where low-seismic velocities for both P and S waves prevail. The presence of mechanically weak crust is considered as a requisite condition for this rotation in the southern part of the South China Block.

Tectonic deformation of the southeastern tip of the Indochina Peninsula during its southward displacement in the Cenozoic time

The terminal collision of India with Asia brought about significant tectonic deformation into the Asian Continent. In order to depict these deformational features in the southeastern margin of the Indochina Peninsula, Late Cretaceous red beds of the Dak Rium Formation were sampled from Da Lat area (11.7°N, 108.3°E), the Kontum massif, Vietnam. Thermal demagnetization procedure revealed the presence of characteristic remanent magnetization (with unblocking temperatures around 680°C) in most samples. Primary nature of this magnetization is ascertained through positive fold test. A tilt-corrected mean direction calculated for the Dak Rium Formation is Dec/Inc=11.4°/35.4° (ks=331.7 α95=1.7°, N=21), with a corresponding paleopole at 76.5° N, 161.2° E (A95=1.7°). Compared with the European APWP, a negative inclination flattening (−15.2°±4.9°) is observed, indicating southward displacement of the Kontum massif by 9.2°±4.9° since Late Cretaceous. Since, comparable amount of southward displacement has been reported from 16 localities of the Indochina and Shan–Thai blocks, it seems a reasonable assumption to predict that the Indochina Peninsula underwent a southward displacement of 5.1°±2.4° (570km±270km) as a composite unit (including the Shan–Thai Block, Indochina Block and Kontum massif). On the contrary, 27.0°±9.8° counter-clockwise rotation of the Kontum massif with respect to the Indochina Block indicate a significant amount of internal deformation along the tip of south moving Indochina Peninsula. This rotation of the Kontum massif is attributed to 32–17Ma left lateral motion along the East Vietnam Boundary Fault, which makes up the 1000km long seaward continuation of the Ailao Shan–Red River shear zone. Tectonic deformation of the Indochina Peninsula was, thus, partly accommodated by local block rotation (e. g., counterclockwise rotation of the Kontum massif) and to some extent by internal tectonic deformation of the Shan–Thai Block.

Palaeomagnetism of late Cretaceous sediments from southern Tibet: Evidence for the consistent palaeolatitudes of the southern margin of Eurasia prior to the collision with India

Twenty sites were drilled in the late Cretaceous Shexing Formation for palaeomagnetic studies in the Lhasa terrane near the locality of Maxiang (29.9°N/90.7°E). The stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I = 350.8°/32.1° with α 95 = 8.1° and N = 20 sites, corresponding to a paleopole at 75.0°N, 306.7°E with A 95 = 6.8°. Positive fold tests indicate a primary origin for the characteristic remanence. Based on previous Cretaceous data mainly from the Takena Formation and Paleocene data from the Linzizong volcanic rocks near the city of Lhasa, the latitude of the southern margin of Asia is located at about 15°N, and yields a stable position of the Lhasa terrane during Cretaceous and Paleocene. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 ± 800 km (16.2 ± 7.6°) between southern Tibet and stable Eurasia and 1500 ± 830 km (14.4 ± 7.9°) between southern Tibet and stable India since the collision of India and Eurasia. A collision age between c. 54 and 47 Ma was determined using the results for the southern margin of Eurasia according to our new data and the extent of ‘Greater India’.

Magnetic overprinting of the Brachina Formation/Ulupa Siltstone, Southern Adelaide Foldbelt, prior to Delamerian deformation

The Proterozoic sediments of the Adelaide Geosyncline were deformed and metamorphosed during the Ordovician Delamerian Orogeny. In much of the Flinders Ranges, the relatively unmetamorphosed sediments, such as those of the Brachina and Bunyeroo formations in the Central Flinders Ranges, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic and cause prominent linear magnetic anomalies with amplitudes of 100 to 500 nT that can be traced for more than 50 km. The higher metamorphic grades are accompanied by the growth of metamorphic magnetite within beds that are thought to have originally contained siderite. These magnetic markers have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with magnetisations in the direction of the present magnetic field implying that remanence causes a significant proportion of the anomaly. The rocks display a strong thermochemical component, mostly acquired prior to deformation due to the Delamerian Orogeny. Remagnetisation prior to deformation is consistent with the high-temperature low-pressure metamorphism that characterises the eastern Mt Lofty Ranges. This secondary natural remanent magnetisation (NRM) component is similar to that of the nearby post-orogenic Black Hill Norite, which is believed to have intruded during the waning phases of the Delamerian Orogeny and is of the same magnetic polarity. We therefore have magnetic signatures of the same polarity that bracket the Delamerian Orogeny, both pre- and post-folding. The tilt corrected mean direction for the Ulupa Siltstone from 44 samples is D = 231.4°, I = 41.1° (α95 = 5.6°), and a pole position at latitude λp = 46.0°S, longitude φp = 49.6°E (A 95 = 6.4°).

New paleomagnetic results of Paleocene volcanic rocks from the Lhasa block: Tectonic implications for the collision of India and Asia

A combined geochronological and paleomagnetic investigation has been performed on Paleocene volcanic sequences in the Lhasa block near the localities of Mendui (30.1°N/90.9°E). A total of 15 sites have been sampled from rhyolitic tuffs. Stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I = 359.0°/26.1° with α 95 = 9.2° and N = 14 sites, corresponding to a paleopole at 73.6°N, 274.3°E with A 95 = 7.3°. Positive fold tests suggest a primary origin for the characteristic remanence. In order to provide a more accurate pole, we propose to combine site-mean directions from this study and Achache et al.'s (1984). The combined average palaeomagnetic direction from early Paleocene volcanic rocks is D = 356.6°, I = 25.9°, κ = 21.7, α 95 = 6.8° after tilt correction, N = 22 sites, corresponding to a pole at 73.2°N, 282.4°E with A 95 = 5.4°. The paleomagnetic results yield a paleolatitude of 13.6 ± 5.4°N for the southern margin of Eurasia at ∼ 55 Ma. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, significant crustal shortening of 1400 ± 600 km and 2000 ± 550 km respectively may have occurred between the southern margin of Eurasia and the stable India, and within Eurasia since the collision of India and Eurasia.

Magnetostratigraphy of the Lower Triassic beds from Chaohu (China) and its implications for the Induan–Olenekian stage boundary

A magnetostratigraphic study was performed on the lower 44 m of the West Pingdingshan section near Chaohu city, (Anhui province, China) in order to provide a magnetic polarity scale for the early Triassic. Data from 295 paleomagnetic samples are integrated with a detailed biostratigraphy and lithostratigraphy. The tilt-corrected mean direction from the West Pingdingshan section, passes the reversal and fold tests. The overall mean direction after tilt correction is D = 299.9°, I = 18.3° ( κ = 305.2, α 95 = 1.9, N = 19). The inferred paleolatitude of the sampling sites (31.6°N, 117.8°E) is about 9.4°, consistent with the stable South China block (SCB), though the declinations indicate some 101° counter-clockwise rotations with respect to the stable SCB since the Lower Triassic. Low-field anisotropy of magnetic susceptibility indicates evidence of weak strain. The lower part of the Yinkeng Formation is dominated by reversed polarity, with four normal polarity magnetozones (WP2n to WP5n), with evidence of some thinner (< 0.5 m thick) normal magnetozones. The continuous magnetostratigraphy from the Yinkeng Formation, provides additional high-resolution details of the polarity pattern through the later parts of the Induan into the lowest Olenekian. The magnetostratigraphic and biostratigraphic data show that the conodont marker for the base of the Olenekian (first presence of Neospathodus waageni) is shortly prior to the base of normal magnetozone WP5n. This provides a secondary marker for mapping the base of the Olenekian into successions without conodonts. This section provides the only well-integrated study from a Tethyan section across this boundary, but problems remain in definitively relating this boundary into Boreal sections with magnetostratigraphy.

Spatial gap between Lhasa and Qiangtang blocks inferred from Middle Jurassic to Cretaceous paleomagnetic data

We present paleomagnetic results from Middle Jurassic red sandstones of the Sangba Formation. Orientated samples were collected at 14 sites on north side of the Basu (30.1°N, 96.9°E), northeastern border of the Lhasa Block. After stepwise thermal demagnetization, high-temperature component with unblocking level of 680 °C is isolated from 7 sites. Primary nature of this magnetization is ascertained through positive fold and reversal tests at 95% confidence level. Combined with previously reported paleomagnetic results, tilt-corrected mean direction of D = 353.4°, I = 15.1°, α 95 = 14.1°, N = 8 is calculated, indicating that during Middle Jurassic the study area was located at 7.7°N ± 7.4°. Comparison with the Jurassic paleomagnetic data reported from eastern Tibet indicates that the Lhasa Block was located south of the Qiangtang Block at a latitudinal difference of 31 ± 11°. Compilation of paleomagnetic data further suggest that until the Cretaceous large spatial gap of more than 1200 km remained open between the Lhasa Block and the Qiangtang Block of eastern Himalayan syntaxis and eventually vanished during Tertiary. Cretaceous paleolatitude corresponding to this gap (13.0°N–31.0°N) is almost identical to that reported from Shan-Thai Block (25.4°N ± 5.2°). Furthermore, the Permo-Carboniferous sequences of Gondwanic affinity are distributed in western part of the Shan-Thai block as well as in the Lhasa and Qiangtang blocks. Keeping in view these similarities, we propose a new pre-collision (India vs. Asia) reconstruction model in which the Lhasa, Shan-Thai and Qiangtang blocks are shown in their respective order from south to north along southern margin of the Asian continent. The aftermath of gigantic India–Asia collision however introduced north–south compressional regime in the Asian Continent that eventually played a role in eastward extrusion of the Shan-Thai Block from an area between the Qiangtang and Lhasa blocks.

Paleomagnetic study of Cretaceous rocks from Pu'er, western Yunnan, China: Evidence of internal deformation of the Indochina block

Cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the Jingxing and Nanxin Formation around Pu'er (23.0°N, 101.0°E), western part of Yunnan province, China, in order to examine quantitative amount of tectonic deformation of the Simao Terrane due to collision of India with Asia. After stepwise thermal demagnetization, the high-temperature component magnetization with unblocking temperature of around 680 °C is isolated. Positive field tests show that the high-temperature component magnetization from 25 sites is primary for the Cretaceous formations. A tilt-corrected mean direction is D = 59.9°, I = 45.2° with α 95 = 5.1°, corresponding to a paleopole at 35.8°N, 173.1°E with A 95 = 5.6°. Comparison with Cretaceous paleomagnetic pole of the whole China block indicates that amount of clockwise rotation in the Pu'er area is evaluated to be 46 ± 9° with respect to the whole China block, which is consistent with large amount of rotation in the Mengla and Jinggu areas (47° and 66°) in the central part of the Simao Terrane. Clockwise rotation of the central part is larger by more than 20° compared to the Yunlong and Yongping areas of northern part of the Simao Terrane, suggesting that the Simao Terrane experienced internal deformation. The southward displacement of the Pu'er area expected from inclination data is 7.5 ± 6.6° with respect to the Chuan Dian fragment, which is a similar estimate to other areas in the Simao Terrane and the Indochina block. While the Simao Terrane displaced southward by 800 km together with clockwise rotation of 30° as a whole, further clockwise rotation occurred in the central part.

Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.

New Early Cretaceous paleomagnetic data from volcanic and red beds of the eastern Qaidam Block and its implications for tectonics of Central Asia

Abstract In order to better understand anomalously low paleomagnetic inclinations in the Cretaceous rocks of central Asia, a combined geochronologic and paleomagnetic investigation has been performed on Cretaceous basalt sequences intercalated with red beds from Zeku area in the eastern Qaidam Block, northeastern Tibetan Plateau. Potassium–argon (K–Ar) dating indicates that the basalt sequences are of Early Cretaceous age (89.9 ± 2.8 to 112.1 ± 2.4 Ma). Isothermal remanent magnetization (IRM), thermal demagnetization of a three-component IRM, and Curie point experiment suggest that magnetite and hematite dominate in red beds, magnetite in basalt flows. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 7 lava flows and 12 red beds sites. The ChRMs of red bed and basalt sites pass fold test and likely represent primary remanent magnetization. The tilt-corrected mean direction from red bed sites (basalts plus baked red bed site) is Ds = 18.4°, Is = 55.3°, κs = 69.1, α95 = 5.5°, N = 11 sites (Ds = 16.1°, Is = 52.0°, κs = 42.6, α95 = 8.6°, N = 8), corresponding to a pole at 75.2°N, 182.3°E with A95 = 7.07° (76.9°N, 194.9°E with A95 = 10.0°), respectively. The consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area. Compared with the Early Cretaceous poles from the North China Block (NCB) or Eurasia, insignificant post-Early Cretaceous northward motion may have occurred between the Qaidam Block and the NCB. On the basis of the existing regional paleomagnetic data, the Qaidam Block has not experienced wholescale vertical axis rotation since the Early Cretaceous with respect to NCB.