Negative Fold Test Research Articles

New paleomagnetic and geochronologic results from late Paleozoic rocks in the Turfan-Hami block (NW China) and implications for the geodynamic evolution of the western Altaids

How and when the ocean-continent transition started in the western Altaids remain controversial. The paleomagnetic signals recorded by late Paleozoic rocks in the Turpan-Hami block can provide critical constraints on this issue. We conducted a new combined paleomagnetic and geochronologic study on the late Paleozoic rocks from the Turpan-Hami block. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating of volcanic beds from the Upper Carboniferous Qijiaojing and Julideneng Formations yielded ages of 313.1 ± 4.3 Ma and 309.6 ± 1.9 Ma to 308.1 ± 3.6 Ma, respectively. Meanwhile, the U-Pb age of the granite intruding the Julideneng Formation is 300.3 ± 2.4 Ma. Passing a series of fold tests, the characteristic remanent magnetization (ChRM) directions of the Qijiaojing Formation are likely primary and consistent with the Kiaman reversed superchron (ca. 319−267 Ma). However, the ChRM values of the Dananhu (Middle Devonian) and Julideneng Formations all represent reverse polarity with negative fold tests, which indicate remagnetizations related to the magmatic thermal events during the late Carboniferous. Thus, two high-quality paleomagnetic poles were obtained for the periods ca. 313−308 Ma at 44.4°N, 177.3°E (K = 22.1, A95 = 8.0°) and ca. 300 Ma at 47.8°N, 173.9°E (K = 116.0, A95 = 4.8°), respectively. Comparison with published coeval paleomagnetic poles of the blocks on both sides of the Tianshan sutures suggests that the central oceanic basin of the western Paleo-Asian Ocean (between the Siberian and Tarim blocks) had been closed since the late Carboniferous (ca. 310 Ma), apart from remnant seas. In addition, a sizeable clockwise rotation (∼58°) of the Turpan-Hami block had taken place during the early Permian, with a scissor-style shrinking of the remnant Bogda marine basin in the meantime. This study provides a new perspective for understanding the tectonic evolution of the western Altaids.

Interaction Between Mineralization and Rock Magnetization: New Constraints From a Silurian‐Lower Devonian Volcanogenic Massive Sulfide (VMS) Deposit

AbstractThe unclear relationship between mineralization, hydrothermal alteration, and rock magnetization in volcanogenic massive sulfide (VMS) deposits limits us from fully understanding the magnetic anomalies and remagnetization process in this type of deposit. We address the issue by conducting systemic paleomagnetic, rock magnetic, petrographic, and X‐Ray diffraction studies in the Dapingzhang VMS deposit on the northwestern Indochina Block. Magnetite is the dominant magnetic carrier of hydrothermally altered surrounding rocks and orebody. Magnetite consumption and secondary magnetite formation occurred at different stages and types of hydrothermal alteration. The overall decrease in magnetite concentration from chloritization via silicification to mineralization implies that the magnetite was mostly consumed during hydrothermal alterations. Secondary single domain magnetite, which can carry a remagnetization direction, is proposed to be formed during illitization via the smectite‐to‐illite transformation. Secondary superparamagnetic magnetite, which was most likely formed during the late stage of chloritization, is unable to carry the stable characteristic remanent magnetization. The site‐mean direction of high‐temperature and high‐coercivity components is Dg/Ig = 324.5°/43.1°, and kg = 35.1, with α95 = 6.8° before tilt correction, and Ds/Is = 316.2°/37.6°, and ks = 16.4, with α95 = 10.1° after tilt correction, with a negative fold test. However, plate reconstruction is limited by the uncertainty of the tilting process following mineralization and the possibility of remagnetization during burial alteration. Therefore, this study provides a mechanism for rock magnetic variation and remagnetization during VMS mineralization.

First Pre‐Miocene Paleomagnetic Data From the Calabrian Block Document a 160° Post‐Late Jurassic CCW Rotation as a Consequence of Left‐Lateral Shear Along Alpine Tethys

AbstractThe Calabrian block, along with Alboran, Kabylies, and Peloritani form isolated and enigmatic igneous/metamorphic terranes (AlKaPeCa) stacked over the Meso‐Cenozoic sedimentary successions of the Apennines and Maghrebides. They are commonly interpreted as fragments of the Hercynian chain rifted apart from Europe during Jurassic Alpine Tethys spreading, drifted southward during Neogene roll‐back of (Neo) Tethyan slab fragments for hundreds of kilometers on top of nappe piles. We report on the paleomagnetism of upper Triassic‐lower Miocene sedimentary rocks from the Longobucco succession that is transgressive over the crystalline Sila Massif (NE Calabria). Well‐defined magnetization directions carried by hematite were isolated in 10 sites (122 samples) in Jurassic rocks. Nine Toarcian and one Tithonian Ammonitico Rosso sites yielded a dual polarity “A” magnetization component whit a direction over 40° from the geocentric axial dipole (GAD) field direction, that supports a positive fold test. Five sites yielded a “B” normal polarity component NE (<40°) of the GAD direction characterized by a negative fold test. We interpret the B component as a Miocene magnetic overprint later clockwise rotated by ∼20° during the well‐known Pleistocene (1–2 Ma) rotation of Calabria. When corrected for such rotation, the A component defines a ∼160° counterclockwise (CCW) rotation of the Calabrian block with respect to Europe. Of these, ∼90° likely occurred along with Corsica‐Sardinia block during its Eocene‐Miocene rotation from the Provençal margin. Thus, the Calabrian block underwent an additional Cretaceous‐Eocene 70° CCW rotation that we relate to Early‐mid Cretaceous >500 km left‐lateral transcurrent motion between Africa and Europe.

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.

Palaeomagnetic time and space constraints of the Early Cretaceous Rhenodanubian Flysch zone (Eastern Alps)

The Rhenodanubian Flysch zone (RDF) is a Lower Cretaceous-lower Palaeocene turbidite succession extending for similar to 500 km from the Danube at Vienna to the Rhine Valley (Eastern Alps). It consists of calcareous and siliciclastic turbidite systems deposited in a trench abyssal plain. The age of deposition has been estimated through micropalaeontologic dating. However, palaeomagnetic studies constraining the age and the palaeolatitude of deposition of the RDF are still missing. Here, we present palaeomagnetic data from the Early Cretaceous Tristel and Rehbreingraben Formations of the RDF from two localities in the Bavarian Alps (Rehbrein Creek and Lainbach Valley, southern Germany), and from the stratigraphic equivalent of the Falknis Nappe (Liechtenstein). The quality of the palaeomagnetic signal has been assessed by either fold test (FT) or reversal test (RT). Sediments from the Falknis Nappe are characterized by a pervasive syntectonic magnetic overprint as tested by negative FT, and are thus excluded from the study. The sediments of the Rehbreingraben Formation at Rehbrein Creek, with positive RT, straddle magnetic polarity Chron M0r and the younger M'-1r' reverse event, with an age of similar to 127-123 Ma (late Barremian-early Aptian). At Lainbach Valley, no polarity reversals have been observed, but a positive FT gives confidence on the reliability of the data. The primary palaeomagnetic directions, after correction for inclination shallowing, allow to precisely constrain the depositional palaeolatitude of the Tristel and Rehbreingraben Formations around similar to 28 degrees N. In a palaeogeographic reconstruction of the Alpine Tethys at the Barremian/Aptian boundary, the RDF is located on the western margin of the Brianconnais terrain, which was separated from the European continent by the narrow Valais Ocean.

Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision

AbstractThe characteristic remanent magnetization (ChRM) isolated from Paleogene carbonate rocks of the Zongpu Formation in Gamba (28.3°N, 88.5°E) of southern Tibet has previously been interpreted to be primary. These data are pertinent for estimating the width of Greater India and dating the initiation of India‐Asia collision. We have reanalyzed the published ChRM directions and completed thorough rock magnetic tests and petrographic observations on specimens collected throughout the previously investigated sections. Negative nonparametric fold tests demonstrate that the ChRM has a synfolding or postfolding origin. Rock magnetic analyses reveal that the dominant magnetic carrier is magnetite. “Wasp‐waisted” hysteresis loops, suppressed Verwey transitions, high frequency‐dependent in‐phase magnetic susceptibility, and evidence that >70% of the ferrimagnetic material is superparamagnetic at room temperature are consistent with the rock‐magnetic fingerprint of remagnetized carbonate rocks. Scanning electron microscopy observations and energy‐dispersive X‐ray spectrometry analysis confirm that magnetite grains are authigenic. In summary, the carbonate rocks of the Zongpu Formation in Gamba have been chemically remagnetized. Thus, the early Paleogene latitude of the Tibetan Himalaya and size of Greater India have yet to be determined and the initiation of collision cannot yet be precisely dated by paleomagnetism. If collision began at 59 ± 1 Ma at ~19°N, as suggested by sedimentary records and paleomagnetic data from the Lhasa terrane, then a huge Greater India, as large as ~3500–3800 km, is required in the early Paleogene. This size, in sharp contrast to the few hundred kilometers estimated for the Early Cretaceous, implies an ever greater need for extension within Greater India during the Cretaceous.

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.

Genetic constraints from paleomagnetic dating for the Aliva zinc–lead deposit, Picos de Europa Unit, northern Spain

Aliva has been the most productive Zn–Pb mine in the Picos de Europa Mississippi Valley-type (MVT) district of Spain’s Cantabrian Zone. The mineralisation is hosted in a Variscan thrust sheet by strata of the Late Carboniferous (~309 ± 3 Ma) Picos de Europa limestone–shale formation. Paleomagnetic and rock magnetic analyses of 194 specimens from 23 sites show that the sphalerite–galena–dolomite ore carries a stable characteristic remanent magnetisation (ChRM) in magnetite–titanomagnetite with minor pyrrhotite. Uncorrected for bedding tilt, the ChRM’s direction is declination (D) = 347.2°, inclination (I) = 61.8° (N = 20 sites, k = 75.4, α95 = 4.3°). A negative paleomagnetic fold test shows that the MVT mineralisation is in one or more carbonate olistoliths of the Picos de Europa Formation in shales of the overlying Late Carboniferous Lebena Formation and that the ChRM entirely postdates Variscan orogenic deformation. No plausible tilt correction of the ChRM’s paleopole supports a previously proposed late Variscan (Permian–Triassic) age for the genesis of the MVT mineralisation at Aliva. The paleopole does support an age of 112 ± 8 Ma for the hydrothermal dolomitisation and MVT mineralisation event with a subsequent increase in bedding tilt by ~10° NNE during the Oligocene–Miocene Pyrenean (Alpine) Orogeny. The 112 ± 8 Ma Aptian–Albian age ties ore genesis to the 35° ± 2 counterclockwise rotation of the Iberian peninsula relative to stable Europe and strongly favours an origin in the rift’s flank associated with rift hydrothermal activity.

Can a primary remanence be retrieved from partially remagnetized Eocence volcanic rocks in the Nanmulin Basin (southern Tibet) to date the India‐Asia collision?

AbstractPaleomagnetic dating of the India‐Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5°N to 30°N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (~10°N). New zircon U‐Pb dates indicate an age of ~52 Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end‐member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low‐temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India‐Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1° (35.7°, 40.5°) (95% significance) and a secondary remanence with inclination of 42.9° (41.5°,44.4°) (95% significance). The paleolatitude defined by the modeled primary remanence—21°N (19.8°N, 23.1°N)—is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya‐Asia collision was situated at ~20°N and took place by ~50 Ma.

Paleomagnetism of the Neoproterozoic diamictites of the Qiaoenbrak formation in the Aksu area, NW China: Constraints on the paleogeographic position of the Tarim Block

The middle to late Neoproterozoic records the transition from the dispersal of supercontinent Rodinia to the subsequent assembly of supercontinent Gondwana. Determining paleogeographic positions of the major cratons during this transition is crucial to understand the displacement histories of these blocks. However, high-quality paleomagnetic results from well-dated rocks during this period remain sparse for the cratons. In this paper, we have carried out a paleomagnetic study of the succession of the Cryogenian Qiaoenbrak formation (Fm) in the Aksu area, NW China, which contains diamictites that are of probable glacial origin. The age of the studied section is constrained at around 730Ma based on the stratigraphy and recent zircon U–Pb chronologic data. Stepwise alternating field (AF) and thermal demagnetizations generally reveal two-component magnetizations. The low temperature (coercivity) component was removed by ∼300°C (∼30mT) and the high temperature (coercivity) component decayed toward the origin, defining the characteristic remanent magnetization (ChRM). Rock magnetic data indicate that magnetite is the major remanence carrier. A negative fold test for an isoclinal soft-sediment fold, the presence of both normal and reversed polarities, and positive conglomerate tests in conjunction with the presence of pristine sedimentary features by microscopic inspections of thin sections suggest that the remanence was acquired shortly after the deposition, i.e. post-depositional remanent magnetization (pDRM), which is very close to the magnetic field at the time of deposition. Fourteen out of 17 sites yield stable ChRMs with a mean direction of Ds=246.1°, Is=30.4°, ks=14.1, α95=11.0° in stratigraphic coordinates, corresponding to a paleopole (Q=6) at λ=6.3°S, φ=17.5°E, A95=9.1°. Together with geologic constraints, the new paleomagnetic results place the Tarim Block at 16.3±5.6°N adjacent to either the west or east of Australia (WOA or EOA) at around 730Ma, but the WOA position is preferred based on the overall relatively favorable evidence. In addition, since the studied diamictites of the Qiaoenbrak Fm that are of probable glaciogenic origin yield a low paleolatitude, this study may lend support to the “Snowball Earth” hypothesis during the Neoproterozoic times.

Remagnetization of Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania): tectonic implications

Abstract We present a pioneering paleomagnetic study on Upper Jurassic limestones from the Danubian Unit (Southern Carpathians, Romania). Thermal and alternating field demagnetizations were applied to define the characteristic remanent magnetization component in all six localities (81 samples). All samples have a normal polarity characteristic remanent magnetization. Negative regional and local fold tests suggest that this remanent magnetization is in fact a remagnetization produced by late diagenetic processes. The studied limestones were probably remagnetized during the collision of the Getic Unit and Danubian Unit which took place during the long normal polarity Chron C34 (82-118 Ma). The area mean direction (D = 75.5°, I = 50.0°, α95 = 10.2°, k = 44) implies about 75° clockwise rotation post remagnetization. Our paleomagnetic results further indicate the absence of significant relative rotation between the Getic Unit and the Danubian Unit during the Cenozoic.

E–W extension and block rotation of the southeastern Tibet: Unravelling late deformation stages in the eastern Himalayas (NW Bhutan) by means of pyrrhotite remanences

In the Himalayan chain the collision of India into Eurasia has produced some of the most complex crustal interactions along the Himalayan–Alpine Orogen. In NW Bhutan, middle to late Miocene deformation has been partitioned between conjugate strike-slip faulting, E–W extension along the Yadong-Gulu graben and kilometre-scale folding. To better understand the late deformation stages and their implications for the evolution of the eastern Himalayas, the palaeomagnetism in the erosional remnant of the Tethyan Himalayan rocks outcropping in NW Bhutan has been studied. Their position to the south of the trace of the inner South Tibetan Detachment, to the south of the Tibetan Plateau offers a unique possibility to study the Tertiary rotation of the Himalayas. Pyrrhotite is the carrier of the characteristic magnetisation based on 270–325 °C unblocking temperatures. The age of the remanence is ca. 13 Ma indicated by illite 40K/40Ar cooling ages and a negative fold test. Small circle intersection method applied to the pyrrhotite components shows a ca. 32° clockwise rotation with respect to stable India since 13 Ma. We suggest that this clockwise rotation is related to strain partitioning between NE-directed shortening, sinistral-slip along the Lingshi fault, and east–west extension. This represents a field-based explanation and a minimum onset age for present-day eastward motion of the upper-crust of SE-Tibet and NE-Himalayas.

Palaeomagnetism of the Howards Pass Zn-Pb deposits, Yukon, Canada

SUMMARY The Howards Pass district is located in the Selwyn Basin, Yukon. The district consists of >15 laminated clastic-dominated (CD) sphalerite–galena (Zn–Pb) deposits in the ‘zinc corridor’ that trends northwest–southeast and extends for ∼35 km. The stratiform mineralized zone, the Active Member, is hosted in carbonaceous cherts and black shales of the Early Silurian Road River Group. Using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) by the core-magnetization-angle method from 18 sites (339 specimens) in 18 variably oriented exploration drill hole cores from 6 Zn–Pb mineralized panels. Rock magnetic analyses show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite and titanomagnetite. The deposits’ mean ChRM direction yields a pole position of either ∼170 Ma on the North American apparent polar wander path or ∼162 Ma on a corrected path for the Intermontane Belt (IMB) terranes. A negative palaeomagnetic fold test indicates that the mineralization's ChRM is post-folding, setting a minimum age for regional metamorphic deformation of ∼170 ± 20 Ma and supporting an Early Jurassic arrival for the IMB's collision and accretion to North America. An autochthonous or para-autochthonous North American tectonic model is favoured for the ChRM of the metamorphosed Selwyn Basin strata rather than an allochthonous IMB model. Further, the palaeomagnetic age indicates that the coarse-grained Zn–Pb mineralization in fine fractures that cut the laminated fine-grained Zn–Pb mineralization at Howards Pass at least was formed by remobilization during Middle Jurassic metamorphism.

Paleomagnetic evidence for dextral strike-slip motion in the Pyrenees during alpine convergence (Mauléon basin, France)

The paleomagnetic study of the Aptian–Albian black marls from the inverted Early Cretaceous Mauléon basin in the northern Pyrenees reveals two components of magnetization that both yield negative fold tests. The carriers of each component show different unblocking temperatures indicating that ferrimagnetic iron sulphides are the main carriers of the intermediate temperature component (ITC unblocks around 360–400 °C) and magnetite is the main carrier of the high temperature component (HTC unblocks around 460–500 °C). Both components show a clockwise deviation in declination with respect to the reference field directions (ITC: D& I: 274, −60, α 95: 10°, k: 17; and HTC: D& I: 059, 52, α 95: 28°, k: 4; both in in situ coordinates). The reference direction for stable Europe for the Cenozoic is: D& I: 001, 50, ΔD: 5, Δ I: 3. However, the reversal test for both of the magnetization components in situ is positive. The structural evolution of the basin involves transtensional deformation during the main stage of basin formation, and inversion during compression with migration of hydrocarbons and development of thrusts and folds with axial planar cleavage. Secondary magnetizations were acquired at the latest stages of the evolution of the inverted basin (post-folding) as the result of a chemical mechanism, and they were blocked during two different, and likely close-spaced in time, polarity chrons. The blocking of the two different magnetizations, with different carriers, implies chemical changes in the environment from anoxic, sulphate-reduction to oxic. Inferred clockwise rotation of the paleomagnetic vectors can be interpreted in terms of dextral shear across the North-Pyrenean Zone during the Tertiary (probably Eocene, according to the plate tectonic scenario). This distributed dextral shear was probably favored by discrete shear along cleavage planes, as shown by geologic markers of consistent dextral strike-slip motion in non-competent units with strong cleavage development. There is no evidence of such dextral movement in the South-Pyrenean Zone, thus indicating a strong partitioning of deformation during the Pyrenean orogeny.

New 40Ar/39Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.

Detailed 40 Ar/ 39 Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by ∼69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at ∼69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at ∼79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. 40 Ar/ 39 Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including Eocene–Oligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 ± 0.18 Ma and 49.42 ± 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a ∼30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 ± 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.

A negative fold test on the Lorrain Formation of the Huronian Supergroup: Uncertainty on the paleolatitude of the Paleoproterozoic Gowganda glaciation and implications for the great oxygenation event

Previous paleomagnetic studies of the glaciogenic Gowganda and Lorrain formations have identified several low-inclination magnetic components of high thermal stability, which suggest low-latitude glaciation during deposition of the Huronian Supergroup, Canada. While extraordinary claims demand extraordinary proof, prior authors have been unable to support their interpretations of these components conclusively with any of the classic field stability tests (e.g., conglomerate, fold, and baked contact) capable of demonstrating that the magnetization was acquired at or soon enough after the time of deposition to be used to constrain the paleolatitude of the Gowganda or Lorrain formations. We report here the results of a fold test from the “purple siltstone” member of the Lorrain Formation near the town of Desbarats, Ontario, which indicate that none of the reported components dates to the time of deposition. Hence, the paleolatitude of the Gowganda glaciation is uncertain.Comparison of the lithostratigraphic, paleomagnetic, and radiometric constraints on the Huronian sequence and the Transvaal Supergroup of Southern Africa implies that the one verified low-latitude Paleoproterozoic glacial event (the Makganyene glaciation, Transvaal Supergroup, South Africa) is younger than the three glacial units of Canada. With this correlation, the physical rock record indicates that the ‘great oxygenation event’ began in the time interval between the Gowganda and Makganyene glaciations. These data are consistent with the sudden evolution of oxygenic photosynthesis destroying a methane greenhouse and thereby triggering the first Snowball Earth event in Earth history.

Palaeomagnetic investigation on Early-Middle Triassic sediments of the North China block: a new Early Triassic palaeopole and its tectonic implications

SUMMARY Palaeomagnetic and rock magnetic study is reported of Early‐Middle Triassic terrestrial sediments from the Jiaocheng and Yushe areas, Shanxi province. Thermomagnetic analysis indicates that magnetite and haematite are the main magnetic carriers. Thermal demagnetization shows that the majority of Early Triassic specimens from the Jiaocheng area contain an intermediate-temperature component (ITC) between 500 ‐250 ◦ C and a high-temperature component (HTC) between 500‐680 ◦ C. The ITC has a negative fold test and might be a Cretaceous or recent overprint. The HTC contains both normal and reversed polarities with positive fold and C-level reversal tests. We interpret the HTC as primary magnetization. Unfortunately, we failed to isolate a meaningful mean direction from Middle Triassic rocks of the Yushe area. New Early Triassic palaeopole from the Jiaocheng area confirms an ∼ 10 ◦ counter-clockwise displacement of the Taihang Early Triassic palaeopoles relative to their Ordos counterparts. This study thus suggests that the North China block (NCB) has generally behaved as rigid block since the Early Triassic, but there might be an ∼9 ◦ counter-clockwise rotation of the Taihang terrane with respect to stable NCB in the Late Triassic. The slight rotation of the Taihang terrane does not bear significantly on what has been previously concluded concerning the Mesozoic history of the NCB—South China block (SCB) collision. However, Triassic palaeomagnetic data from stable NCB indicate that the east end of the NCB and SCB did not undergo large convergence or significant opposite latitudinal displacement after the initial contact at the end of the Late Permian.

Paleomagnetic study on the Early Triassic red beds from Jiaocheng, Shanxi Province

Paleomagnetic and rock magnetic study has been conducted on the Early Triassic red beds of Liujiagou Formation from Jiaocheng, Shanxi Province. Hematite was shown as the main magnetic mineral. After eradicating an initial viscous component at room temperature to ~100℃－200℃, thermal demagnetization shows that most samples contain two remanence components, intermediate-temperature remanence component at 250℃－500℃ and high-temperature component at 500℃－680℃. The intermediate-temperature component has a negative fold test at the 95% confidence level. And the pole position of the intermediate-temperature component in geographic coordinates is correlated with the Middle Jurassic reference pole of the North China Block (NCB) within the 95% confidence, suggesting that it might be a remagnetization component acquired during the Yanshanian period. The high-temperature component contains both reversal and normal polarities with positive fold test and C-level positive reversal test at the 95% confidence level, which suggests that this high-temperature component can be regarded as primary magnetization. Comparison of this newly obtained Early Triassic paleopole with the coeval mean pole of the Ordos Basin suggests that a locally relative rotation may have happened between the Ordos and the Jiaocheng area of Shanxi Province. This rotation may be related with two faults: one is Lishi big fault separating Ordos from Shanxi and the other is Jiaocheng big fault, which is situated in the southeast of sampling locality and was still in motion during the Cenozoic.

Record of deformation by secondary magnetic remanences and magnetic anisotropy in the Nar/Phu valley (central Himalaya)

Secondary magnetic remanences residing in pyrrhotite and anisotropy of magnetic susceptibility (AMS) were studied in low-grade metamorphic carbonates of the Tethyan Himalaya in Nar/Phu valley (central Nepal) and used for interpretation of tectonic deformations. The characteristic remanence (ChRM) is likely of thermomagnetic origin related to post-peak metamorphic cooling occurring after the Eohimalayan phase (35–32 Ma). The ChRM postdates small-scale folding (main Himalayan folding F1 and F2) as shown by a negative fold test of site mean directions at 99% confidence level, and has been probably acquired between 32 and 25 Ma. Late-orogenic long-wavelength folding associated with the Chako antiform (CA) is recorded by the spatial dispersion of ChRM directions and the distribution of the main axes of the AMS tensor. The mean tilting of the ChRM direction since remanence acquisition (≈20–30°) approximately coincides with the tilting of the CA (31°) at the study area indicating that the pyrrhotite remanence predates the CA (CA formed at <18 Ma according to preliminary U/Pb dating). However, comparison of tilt angles of remanence directions and AMS tensor axes suggests that remanence acquisition was not completed before the onset of the CA formation. This could imply a younger age (Early Miocene or even younger) of the ChRM. Using the distribution of remanence directions along a small-circle as well as the distribution of AMS tensor axes, a clockwise mean rotation of 16° is obtained for a remanence age of ≈30 Ma. An Early Miocene remanence age would not change this result substantially. Compilation of rotations in the Tethyan Himalaya deduced from secondary pyrrhotite remanences reveals an increasing clockwise rotation from the Hidden valley in the W to the Shiar valley in the E (≈150 km distance), incompatible with an oroclinal bending model.

Cenozoic remagnetization of the Paleozoic rocks in the Kitakami massif of northeast Japan, and its tectonic implications

Secondary remanent magnetization is identified in the Paleozoic igneous and sedimentary rocks of the Kitakami massif. The secondary nature is shown by a negative fold test for the Permo–Carboniferous sedimentary rocks. The northwesterly paleomagnetic declination with moderate inclination (D=321.2°, I=56.5°, α95=5.2°, N=18) of this secondary remanent magnetization is almost parallel to the primary magnetization reported for the Cenozoic welded tuffs of northeast Japan, indicating that the Paleozoic rocks were subjected to remagnetization at any period between 62 and 16 Ma. The secondary magnetization of the serpentinized ultramafic rocks is carried by magnetite, which grew in veins and mesh rims of serpentine, whereas the carrier of the magnetization in limestones is fine-grained pyrrhotite. Combining this with the previously reported remagnetization of the Kitakami granitic rocks, it is suggested that rocks in the Kitakami massif were subjected to crystallization remanent magnetization at low-temperature conditions. Since serpentinization requires fluid migration, one of the most likely events is the eastward lateral migration of water into the Kitakami massif. We postulate a Cenozoic suturing of the Kitakami massif with the Asian continent as a plausible tectonic event for this fluid migration.