Negative Fold Test Research Articles

Where did rotational shortening occur in the Himalayas? – Inferences from palaeomagnetic remagnetisations

In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250–330°C) and magnetite (demagnetising spectra: 15–50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by 40K/39Ar cooling and 40Ar/39Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N–S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38±8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3±11.7° (LH/Alaknanda valley) and 11.3±8.5° (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process.

Coupling of late-orogenic tectonics and secondary pyrrhotite remanences: towards a separation of different rotation processes and quantification of rotational underthrusting in the western Himalaya (northern India)

The total amount of crustal shortening within the Indian plate includes straight forward and rotational shortening. The complex tectonic history of the western Himalaya impedes a direct observation of rotational shortening between India and its northern margin. A combination of palaeomagnetic results and tectonic structures along a ∼300km NW–SE section, from NW-Zanskar (33.9°N, 76.5°E) to Malari (30.0°N, 79.6°E) in northern India, enable a separation of rotations on different scales, correction of the observed rotations, and quantification of uniform rotational shortening. New palaeomagnetic results from metacarbonates of the Tethyan Himalaya in Sarchu, Losar, and Spiti and earlier studies from NW-Zanskar and Malari reveal a characteristic remanent magnetisation carried by pyrrhotite (unblocking spectrum: 250–330°C). Negative fold tests (99% level) indicated remanence acquisition after the main folding of the Himalaya. The formation of pyrrhotite during thermal events (T>200°C) suggests a remanence acquisition related to exhumation and cooling. Temperatures of about 200–300°C indicate a thermochemical remanence acquisition with an Eohimalayan age, dated in two areas by 40K/39Ar techniques on illites. The resulting rotation angles in respect to stable India show an ‘anomalous’ counterclockwise rotation in NW-Zanskar and a trend of decreasing clockwise rotations to the SE. These variations may be attributed to meso-scale (∼100km) tectonic features and oroclinal bending. For the first time block rotations with respect to stable India are separated into local, meso-scale and regional significant ones. After elimination of local and meso-scale effects, using structural analysis of late orogenic features, modelled rotations due to oroclinal bending were subtracted. Regionally significant rotations of 0–20° clockwise are interpreted to be caused only by uniform rotational underthrusting.

Juxtaposed and superimposed paleomagnetic primary and secondary components from the folded middle carboniferous sediments in the Reggane Basin (Saharan craton, Algeria)

A paleomagnetic study was carried out on the Middle Carboniferous sediments of the eastern margin of the Reggane Basin of Algeria. Seven sites (108 samples) in the Lower Serpukhovian and 11 sites (129 samples) in the Upper Serpukhovian, Bashkirian and Lower Moscovian levels were investigated.Besides a common, but generally limited, viscous remanent magnetization (component A) and a recent chemical remanent magnetization of reversed polarity (A′), two main components were identified: one of these (component B), is characterized by a negative fold test and has been identified as a Lower Jurassic remagnetization. The associated paleomagnetic pole obtained in the seven zones by combining characteristic remanent magnetization directions (ChRM) and great circles (λ=71.1°N, ϕ=251.4°E, A95=3.8°, K=254) lies in the vicinity of the NW African poles of similar ages. The second (component C) displays both normal and reversed polarities. Also determined by the combination of ChRM or stable end points and remagnetization circles, it yields a positive fold test which constrains the magnetization acquisition time and a positive reversal test which argues in favor of a “non-composite” nature of the component C. The normal polarities observed in the Lower Serpukhovian levels represent the latest normal event observed in Africa before the Kiaman superchron. The paleomagnetic South pole calculated from 10 sites (n=64 data) gathered in four large areas (λ=26.5°S, ϕ=44.7°E, A95=4.7°, K=383) is the first African Carboniferous pole founded on both positive reversal and fold tests. It lies only slightly apart from other Middle Carboniferous poles previously published for the northern part of Africa where no intraformational test were available to constrain the magnetization age.

The rotations of the Variscides during the Carboniferous collision: paleomagnetic constraints from the Vosges and the Massif Central (France)

This study aims at completing and refining results from previous paleomagnetic investigations carried out in various outcrops of the European Variscan belt. Sampling was performed in Visean effusive and intrusive rocks of the southern Vosges basin and the northern Massif Central, all emplaced in the time range 345–340Ma. Stepwise thermal demagnetization of 254 specimens from 13 sites of the Doller Valley and nine sites of the Massif Central has yielded a very similar paleomagnetic record for both areas. With the exception of post-Carboniferous overprints isolated in the southern Vosges, two groups of directions, with both normal and reversed polarity have been evidenced. The first group of directions, labelled C, occurs in both areas and is characterized by scarce pre-tectonic magnetizations (two sites) and by syn-tectonic overprints (five sites) acquired during the Late Visean convergence, i.e. in the time-range 340–328Ma. The mean pole Co is 28°S/45°E (k=139, α95=5°). The main group, labelled B, shows a negative fold test and a positive reversal test, indicating that the magnetizations are post-tectonic overprints, acquired after the main tightening phase, i.e. after 328Ma. At a few sites in both areas were evidenced syn-tectonic B magnetizations that are in favour of an overprinting in the context of post-Visean extension tectonics. The mean post-tectonic B direction of the Doller Valley (southern Vosges) 62°/−25° (k=52, α95=6°, N=11; VGP: 8°N/127°E) is consistent with the mean post-tectonic B direction 72°/−23° (k=126, α95=5°, N=6; VGP: 3°N/114°E) obtained from the tuffs, the grey microgranite and the dyke intrusive into the microgranite of the Massif Central. The occurrence of similar paleomagnetic records in the eastern and central Variscides implies that, from the Massif Central to the Sudetes, the belt has probably moved as one block, since Early Carboniferous.The change in inclination of the C components suggests a southward drift of the Moldanubian and Saxothuringien Variscides in the Late Visean (340–328Ma). According to geological features, in the same time, the Rhenohercynian crust was underplating the Saxothuringian arc and fore-arc zone. Subsequent to 328Ma, the drift turned into a rapid clockwise rotation by about 70° around a pole located on southeastern corner of Baltica, leading to the closure of the Rhenohercynian oceanic basin. In the Namurian, large-scale overprinting (B components), likely related to post-rotation extension, affected the internal belt. The convergence continuing, the Variscides underwent a second clockwise rotation by about 45° in the Late Namurian–Westphalian (320–305Ma). Stacking of the Rhenohercynian sedimentary cover and thrusting over the London–Brabant basement may be related with this motion.

First paleomagnetic data from the sedimentary cover of the French Penninic Alps: evidence for Tertiary counterclockwise rotations in the Western Alps

We present a paleomagnetic study performed in the Briançonnais sedimentary cover of the western Alpine Arc (France). Sampling was focused on Upper Jurassic rocks of the Briançonnais cover in the Briançon–Guillestre area. More than 100 samples from 11 sites were collected essentially from Ammonitico rosso limestones and sedimentary dykes of the Upper Jurassic. Thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite: (1) a low-temperature component with present-day magnetic field direction of viscous origin; (2) a well defined reverse-polarity component with a maximum unblocking temperature of 440°C (A component); and (3) a weak high-temperature component (B component) that experienced a viscous overprint during heating and was difficult to isolate. The A component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late Alpine extension. This component is interpreted as a secondary remagnetization acquired during the cooling path related to Late Eocene–Early Oligocene metamorphism. The mean A component direction, corrected for late Alpine extensional tilting, is D=142°, I=−57°, k=44, α95=8°. This direction significantly differs by 47° ± 13° from the expected direction for Eurasia. We interpret this difference to be related to a post-Eocene counterclockwise rotation about a vertical axis of the Penninic zone relative to stable Europe.

Paleomagnetism of the Paleoproterozoic Gowganda and Lorrain formations, Ontario: low paleolatitude for Huronian glaciation

To further our understanding of the paleolatitudes of Precambrian glaciations, a paleomagnetic study has been conducted on the glaciomarine Coleman Member of the Gowganda Formation and the conformably overlying deltaic Firstbrook Member and Lorrain Formation (Huronian Supergroup, ∼ 2.4-2.3 Ga) in Ontario, Canada. Many of the rocks, notably the gray and grayish red argillic facies, display unstable magnetization or overprints carried by magnetite and/or hematite that give negative fold tests. Some red sandstones, however, have stable high-temperature (∼ 690°C) components marked by very shallow northerly or southwesterly directions. Five sites in pale red sandstone from the Coleman Member near Elliot Lake give a dip-corrected A direction of D = 216.3°, I = 5.5° ( α 95 = 6.7 °, n = 30), and 17 sites in Lorrain red sandstone near Desbarats give a dip-corrected A direction of D = 5.4°, I = 5.5° ( α 95 = 5.9 °, n = 75). Unfavorable structural attitudes rendered fold tests inconclusive. These very shallow directions may be ascribed to early chemical remanent magnetization (CRM) because (a) some specimens display polarity reversals, (b) such directions are unknown, either as original or as overprint components, in post-Huronian rocks of the region, and (c) a prior site in Coleman red beds gave a high-temperature, shallow NNW direction that is not significantly different from Lorrain A and which a positive conglomerate test suggested is not an overprint. The very shallow directions are near primary directions for the slightly older (2.45 Ga) Matachewan dyke swarm in the region. Lorrain argillic rocks at five sites give a normal component B ( T = 670°C, D = 18.6°, I = 59.6°, α 95 = 6.1 °, n = 16, dip corrected) and a reverse component E ( T = 500°C, D = 190.9°, I = −60.9°, α 95 = 7.3 °, n = 14, in situ), which are close to a prefolding direction carried by magnetite previously identified in Coleman gray argillites (Morris A). However, a primary origin for Morris A is dubious because of the widespread occurrence of overprints carried by magnetite in Coleman gray argillites and because the fold test employed folds formed during the Penokean Orogeny at 1900-1800 Ma. Our results suggest that Morris A/Lorrain B and Lorrain E may data from immediately before and just after Penokean folding, respectively. Hence Coleman-sandstone A and Lorrain A may be better estimates of original directions. Those components and Matachewan paleomagnetic data constrain Huronian paleolatitudes and suggest that Huronian glaciation occurred within 11° and possibly within 4° of the paleoequator.

How far between Iran and Eurasia was the Turan plate during Triassic–Jurassic times?

Calc-alkaline volcanic deposits from the south-west of the Turan plate, near the city of Turkmenbasi (40°00'N, 52°58&E) in Turkmenistan, were studied paleomagnetically. These rocks have been affected by a greenschist-facies metamorphism, possibly of regional extent, that has been K/Ar-dated as 200 to 227 Ma old. A low-blocking-temperature component (D = 349°, I = 64°), close to the present field direction and probably of viscous or recent chemical origin, was isolated by a negative fold test at three sites. The mean direction of a high-blocking-temperature component isolated at 15 sites, mainly carried by magnetite, is scattered before and after tectonic correction and is therefore difficult to interpret. A group of seven sites with low inclinations before and after tectonic correction was isolated. The mean inclination of these sites (31 ± 8°), syn-folding or post-folding, corresponds to a paleolatitude of 17 ± 8° which is lower than the conventional Eurasian paleolatitudes for post-Permian times. The paleomagnetic data from the Turan and Iran plates constrain this low paleolatitude to the Late Triassic and Jurassic period. This requires a shortening of at least 7° between the Turan plate and Eurasia during this time.

New constraints on the bending of the Gibraltar Arc from palaeomagnetism of the Ronda peridotites (Betic Cordilleras, Spain)

Abstract The study of 210 cores (15 sites) from the Ronda peridotites (Sierra Bermeja and Sierra Alpujata, Alpujarride nappe complex, Betic zone), and from the granites intruding these peridotites and their country rocks shows the occurrence of two stable antipodal directions of magnetization ( D = 46°, I = 47°, α 95 = 6.6). The reverse polarity high-temperature component, only found in peridotites, is carried by hematite, while the normal polarity intermediate-temperature component is carried by magnetite in the peridotites, and by sulphides in the granites. Negative fold tests point to a late magnetization. The acquisition of remanence is attributed to the post-metamorphic cooling of the Alpujarrides, bracketed between 23 and 18 Ma by isotopic and stratigraphic data. Structural data and the homogeneity of the in situ mean palaeomagnetic directions preclude significant tilting of the massifs after their magnetization. The observed declination is interpreted as the result of a post-metamorphic, 46° ± 8° clockwise rotation of the Ronda massifs around a vertical axis. These results are compared with those from the Beni Bousera peridotites (southern branch of the Gibraltar Arc). In the latter massif, a c. 74° ± 11° anticlockwise rotation has been documented, and dated from the time of cooling of the peridotite unit. Therefore the opposite rotations of the Spanish and Moroccan massifs occurred rapidly during Early Miocene. A tectonic model involving extensional collapse with preferential displacement towards the Atlantic free-margin is favoured.

Paleomagnetism and 40Ar/39Ar geochronology of gabbro sills at Mariscal Mountain anticline, southern Big Bend National Park, Texas: Implications for the timing of Laramide tectonism and vertical axis rotations in the southern Cordilleran orogenic belt

Mariscal Mountain anticline is a major Laramide asymmetric fold located along the frontal margin of the Cordilleran orogenic belt in the Trans‐Pecos region of southwest Texas. Exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement, based largely on their arcuate outcrop geometry, has been interpreted to predate folding. Paleomagnetic fold tests on samples from large (≥ 30 m thick) and thin (≤ l m thick) gabbro sills are negative at the 99% confidence level. Baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro, whereas sample directions from a site well removed from thermal effects of the gabbro are scattered. Results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization. The negative fold test indicates that sill intrusion at Mariscal Mountain anticline postdated Laramide folding. Two 40Ar/39Ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37.0 ± 1.3 Ma (2σ). This date is similar to those from rocks of similar composition elsewhere in Trans‐Pecos Texas, but significantly younger than whole rock and pyroxene K‐Ar dates of Late Cretaceous and early Eocene age commonly cited for the Mariscal Mountain gabbro. The paleomagnetic and geochronologic data indicate that Laramide deformation had clearly ceased by late Eocene/early Oligocene time, consistent with absence of contractional deformation in middle Eocene to lower Oligocene volcanic and sedimentary rocks elsewhere in the region. Although paleomagnetic data from the gabbro sills may not adequately average secular variation, the in situ group mean direction (D = 348.6°, I = 50.9°, k = 65.1, α95 = 4.3°, n = 18 sites) is indistinguishable from expected mid‐Tertiary reference directions. This indicates no evidence of significant vertical axis rotation in the Mariscal Mountain area. Analysis of paleomagnetic data from studies elsewhere in the Big Bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations. Thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with Basin and Range extension is equivocal.

Miocene remagnetisation of carbonate platform and Antalya Complex units within the Isparta angle, SW Turkey

A palaeomagnetic study has been carried out within the Mesozoic and Tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep-sea volcanics and sediments of the Antalya Complex, exposed around the Isparta angle, SW Turkey. The Antalya Complex is interpreted as a mosaic of carbonate platforms, basinal sediments, volcanic and ophiolitic rocks which formed within a southerly strand of the Neotethyan ocean, adjacent to Gondwana. The results indicate a widespread remagnetisation event. Negative fold tests show that the remanence at most sites is of secondary origin (e.g., within the çirali lavas). The magnetisation is carried by magnetite of presumed authigenic origin. The remagnetisation event is believed to have occurred in the Early-Middle Miocene (Burdigalian-Langhian). It was possibly triggered by the migration of orogenic fluids ahead of the advancing Lycian nappes during their emplacement onto the carbonate platforms. Subsequent to remagnetisation, a large segment of the Isparta angle underwent an anticlockwise rotation of 30°. This rotation is attributed to the overall convergence and bending of the Hellenic arc and the final stages of emplacement of the Lycian Nappes during the Late Miocene, in agreement with previous studies. Previously, southerly palaeolatitudes were inferred from Late Triassic extrusives of the Gödene Zone (Čalbali Dag unit). The post-folding magnetisation identified here within the Çirali lavas of the Gödene Zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections. The available data cannot be used to substantiate an origin for the Antalya units south of the equator in the early Mesozoic. Instead, a position close to the northern margin of Gondwana is indicated.

Pervasive remagnetization of paleozoic rocks acquired at the time of Mesozoic folding in the South China Block

A total of 152 paleomagnetic samples was collected from 23 sites distributed over six Paleozoic formations, with ages ranging between Middle Ordovician and Early Permian, in the Tangshan type section in the Nanjing area of South China. Stepwise thermal demagnetization has successfully resolved two magnetization components from these samples. Component A with northerly shallow tilt‐corrected directions resides in approximately 95% of the total number of samples analyzed; this component passes fold tests at the 99% significance level in three formations. The overall mean direction of component A is 350/−3 (tilt‐corrected, a95=7.2). Component B, isolated from 19 samples in two formations (Cl and D3), yields negative fold tests at the 95% significance level and is, therefore, younger than the Mesozoic folding. The overall site mean direction of component B is 354/−4 (in situ, a95=8.4). The tilt‐corrected direction of component A clearly resembles the in situ direction of component B, indicating that both components are likely to be of approximately the same age. We infer that they are both remagnetizations, associated with the deformation. A review of the Paleozoic paleomagnetic data reported by other authors from adjacent areas shows that similar remagnetization situations may prevail. It is concluded that the northeast part of south China has experienced pervasive remagnetizations at the time of folding. The direction of this folding‐induced remagnetization yields a new well‐defined paleopole position (56°N, 314°E) for south China. Although this paleopole is significantly different from Mesozoic poles reported to date, we infer that a Triassic to Jurassic age is most likely for this remagnetization.

Paleomagnetism and paleogeography of the northern calcareous Alps (Austria)

Prefolding magnetization components have been resolved at 38 sites of Liassic “Adnet” limestones in the central Northern Calcareous Alps (NCA) and at one site in the Drauzug. The site mean directions are northeasterly and down, and are closer to coeval European directions than to Southern Alpine “African” directions. The western end of both the NCA and of the Drauzug yield different site mean directions and negative fold tests indicating remagnetization. The sampling localities have been rotated relative to one another during thrusting, however, comparison of paleomagnetic poles with the European apparent polar wander path implies an average clockwise rotation of about 30° for the NCA relative to Europe. Larger‐scale clockwise rotations in excess of 80°, relative to the Southern Alps, are implied by comparison with the Southern Alpine (African) polar wander path. Facies analysis of Jurassic and Lower Cretaceous basinal sediments in the central part of the NCA suggests that Liassic subsidence was related to presently E‐W sinistral strike‐slip faults, whereas Malm‐Berriasian subsidence was controlled by presently E‐W south‐dipping normal faults. Both paleomagnetic and facies data tend to indicate that the NCA did not undergo the “African” rotation seen in Southern Alpine paleomagnetic data. This implies the existence of a short‐lived tract of Triassic?‐Jurassic ocean floor between the NCA and the Southern Alps, which linked the Pindos Ocean to the Ligurian trough and allowed Adria to rotate independently from the NCA during the Late Jurassic. Eo‐Alpine deformation and metamorphism are associated with the Early Cretaceous closure of this “Hallstatt” trough.

Magnetic and tectonic history of the Late proterozoic upper little dal and coates lake groups of northwestern Canada

A pre- to syn-rift sequence (upper Little Dal Group, Coates Lake Group) in the Mackenzie Mountains of northwestern Canada reveals paleomagnetic directions in pigmental hematite (TH B, RR B) that are similar to hematite overprint directions (P2) of underlying Late Proterozoic strata. Paleopoles of these magnetic components from the Thundercloud and overlying Redstone River formations of the Coates Lake Group are respectively 202°W, 27°N ( N=6 sites, A 95=9°) and 206°W, 32°N ( N=4 sites, A 95=10°) The primary paleopole for the basal Thundercloud Formation (TH A) at 210°W, 01°N ( N=6 sites, A 95=21°) is close to a previously reported pole (222°W, 2°N) from diabases (dated by U-Pb at 778 ± 2 Ma) that intrude strata below Little Dal Group. The large error in TH AI caused by scatter along a small circle, is here attributed mainly to tectonic rotation of azimuths of section-mean directions. Based on this interpretation, the TH A pole position suggests a minimal unconformity below the Thundercloud Formation, contemporaneity of the sills and the basalts that cap the Little Dal Group, and uncomformity between Thundercloud and Redstone River Formations. Anomalous directions from the Little Dal basalts, and sequential directions in magnetic phases of the Redstone River Formation are also interpreted to record local block rotations during trans-tensional tectonism. The P2 disseminated pigmental hematite magnetization is interpreted to immediately postdate magnetizations from most other hematite phases, as well as magnetizations from (reduced) sulphide/magnetite beds at the base of the Coppercap Formation (upper Coates Lake Group). P2 is absent from, and likely predates deposition of, the upper Coppercap and overlying formations. Some poles (P3) that postdate P2 poles and indicate high paleonatitudes are here interpreted as Late Precambrian. Negative fold tests suggest that most other high-latitude poles from Late Proterozoic units of the region are of Cretaceous Laramide age.

Palaeomagnetic Results From the Tibetan Sedimentary Series of the Manang Area (North Central Nepal)

The remanent magnetization of Palaeozoic and Mesozoic carbonates from the Tibetan Sedimentary Series (TSS) of the Manang area north of the Annapurna Range (north central Nepal) has been investigated. the TSS forms part of the tectonic unit between the Indian-Eurasian plate boundary in the north and the Main Central Thrust (MCT) in the south. Apart from a present field direction, three remanence components have been separated: a normal polarity, post-folding remanence (negative fold test) is carried by pyrrhotite (the core corrected mean direction for five sites with 47 specimens from Silurian/Devonian rocks is decl. = 312.9°, incl. = 58.3°, α95 = 17.6°, k = 19.8). the origin of this component is not understood. A more precisely determined, reversed polarity, post-folding remanence (99 per cent negative fold test) is also carried by pyrrhotite (the core corrected mean direction for 12 sites with 229 specimens from Carboniferous/Triassic rocks is decl. = 196.4°, incl. = -65.9°, α95 = 3.2°, k = 183). One site of Lower Carinan age shows—in addition to the pyrrhotite component—a second, stable remanence direction carried by magnetite. the fold test is 99 per cent positive and there is a high probability that the remanence is of detrital origin (the bedding corrected direction for one site with 40 specimens is decl. = 334.3°, incl. = -54.1°, α95 = 4.2°, k = 30.2, palaeolatitude 34.6°S). The direction of the magnetite component coincides with other Triassic palaeodirection data from the literature and the inclination suggests a palaeoposition of the TSS at the northern margin of India at this time. the northern extent of India in the Triassic cannot be reconstructed from the data because of the strong anticlockwise rotation (48°) of stable India since this time. The inclination of the reverse polarity pyrrhotite component is 19° steeper than expected from the maximum northern position of the area. This suggests a regional northward tilt along the MCT which is supported by steep inclinations found in other areas. Declination data for the reverse polarity pyrrhotite component and for the magnetite component deviate clockwise from expected values for the area by 23°-34° (calculated from the Indian APWP). Utilizing Klootwijk, Conaghan & Powell's (1985) rotational underthrusting model, the minimum magnitude of continental underthrusting between the TSS and stable India at the longitude of central Nepal (84°E) has been calculated from the declination of our reverse polarity pyrrhotite component. A possible range of 520–1100 km results from consideration of different tectonic models.

Palaeomagnetic results from the southern Sierra Madre Oriental, Mexico: evidence for Early Cretaceous or Laramide remagnetization?

Abstract A large suite of samples from the Latest Triassic Huizachal and Early Jurassic Huayacocotla Groups and the Latest Jurassic Taman and Earliest Cretaceous Pimienta Formations was collected in the southern Sierra Madre Oriental for a palaeomagnetic study. Only the samples from three sites belonging to the Huizachal Group and the Las Juntas Formation possibly have retained their primary magnetization. If so, their pole position does not reveal any palaeomagnetically discernable motion relative to cratonic North America. All remaining sites were remagnetized as evidenced by a negative fold test at four sites and the fact that the pole positions cluster better at the 95% significance level if no structural corrections are applied. The tightness of the cluster ( α 95 = 4.6 °) and the same polarity, suggest that the samples were remagnetized at some common time. These results permit two interpretations. (1) If the sampling region has not suffered any significant tectonic rotation, then the remagnetization can be dated by comparison with the polar wander path for North America as Early Cretaceous (≈ 130 Ma). This Early Cretaceous phase of deformation is not recognized in the northern Sierra Madre Oriental and clearly pre-dates the Early Tertiary Laramide orogeny. (2) If the southern Sierra Madre Oriental did rotate counterclockwise by ≈ 20 °, then the remagnetization could have originated in any Cretaceous or Early Tertiary time, and may indeed be related to the Laramide deformation. In either case, the data imply that the southern Sierra Madre Oriental constitutes an independent tectonic domain.

On the origin and the tectonic implications of magnetic overprinting of the Old Red Sandstone, Shetland

SUMMARY The magnetic signature of the Old Red Sandstone (ORS) and post-orogenic plutonics on Shetland is influenced by three principal components, A (pole: S51,E003), B (pole: S24,E340) and C (pole: SlO,E003). As evidenced from negative fold tests, A is secondary, and most likely thermochemical in origin, presumably associated with hydrothermal fluids circulating in faults and crush zones subsequent to Permian-Early Triassic extensional reactivation of older Caledonian fault structures. From the ORS, the B component also can be proven to represent a magnetic overprint. The secondary nature of component B, and the fact that it reasonably can be correlated on both sides of the Walls Boundary Fault, commonly assumed to be the continuation of the Great Glen Fault, argues against recent suggestions of mega-shearing within the Great Glen Fault system. The precise time of acquisition of B is uncertain, but we consider a lower Carboniferous age synchronous with late-post orogenic plutonic activity (334-358 Ma) to be most likely. This implies that the B component carried by the plutonic rocks may represent a primary cooling event. C is exclusively carried out by Middle Devonian andesites and basalts from the Esha Ness Peninsula. It is evidently of post-fold origin, and we relate this earliest magnetic overprinting to Middle-Upper Devonian (Svalbardian) tectonism which affected the North Atlantic domain during this

Paleomagnetic results from the Upper Cretaceous Maudlow and Livingston formations, southwest Montana

A paleomagnetic investigation of welded tuff units from the Late Cretaceous Livingston and Maudlow Formations was undertaken to refine the Late Cretaceous to Paleocene APWP for cratonic North America. After progressive AF and thermal demagnetization, the Maudlow Formation (46.1°N, 248.9°E) yields a paleomagnetic pole at 70°N, 208°E (11 sites, A95 = 9°, K = 22). Tectonic correction improves the directional grouping, and the change is statistically significant. Two normal polarity intervals bounding a single reversed polarity interval are present. We interpret the reversed interval to be Chron 33r, on the basis of: [1] stratigraphic correlation of the Maudlow Formation with the Campanian Sedan Formation located 40 km to the east, and [2] K‐Ar dates of 83 ± 2 Ma at the middle of the sampled section and 74.9 ± 1 Ma near the top of the sampled section.The Livingston Formation (45.5°N, 250.1°E) yields a high Hc magnetization with a negative fold test, suggesting a post‐folding secondary magnetization. The Livingston mean direction yields a paleomagnetic pole at 72°N, 218°E (10 sites, A95 = 4°, K = 107) and is clearly distinguishable from a co‐existing lower Hc component reflecting a recent viscous overprint. We interpret the high Hc magnetization as Late Cretaceous to Paleocene in age.Neither the Maudlow Formation pole nor the pole derived from the high Hc overprint in the Livingston Formation agree with previously published poles for either the Late Cretaceous or Tertiary, but they do lie on a simple continuation of the APWP between Late Cretaceous and Paleocene mean poles. Secondary magnetizations found in the Franciscan Complex of California also yield poles which lie on a proposed extension of the APWP. If the Franciscan secondary poles represent an overprint event associated with early Tertiary accretion, and if the Livingston secondary pole represents a deformational event related to Laramide uplift of the nearby Beartooth Range, then together these three poles suggest that the North American APWP may continue farther to the east and south to define a Cretaceous‐Tertiary cusp.

Paleomagnetic and isotopic dating of thrust-belt deformation along the eastern edge of the Helena salient, northern Crazy Mountains Basin, Montana

Thrust-belt deformation along the eastern edge of the Disturbed Belt in the Helena salient of Montana has not been well dated owing to a lack of syn- or postorogenic strata. In situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern Crazy Mountains Basin, previously described as pre-, syn-, or posttectonic with respect to deformation, are well grouped (Dec. = 343°, Inc. = 61°, α95 = 5.0°, k = 46, n = 16 sites); a negative fold test is significant at minimally 95% confidence. Intrusion and magnetization acquisition thus postdate fold and thrust deformation. K-Ar age determinations of selected intrusions range from 52 to 48 Ma. Paleomagnetic and isotopic age data, combined with stratigraphic information, indicate that folding in the northern Crazy Mountains Basin is middle to late Paleocene in age. This age is in agreement with suggested dates for deformation in the northern Montana Disturbed Belt but is recognizably older than the youngest episodes of frontal deformation in the Utah-Idaho-Wyoming salient. Data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest Montana overlapped temporally and spatially, ranged from Late Cretaceous to earliest Eocene in age, and progressed from west to east through time.

Paleomagnetics of Ringerike Old Red Sandstone and related rocks, southern Norway: implications for pre-Carboniferous separation of Baltica and British terranes

Paleomagnetic study of two stratigraphic sections of the Upper Silurian-Lower Devonian Ringerike Sandstone, Oslo area, Norway, indicate a straightforward multicomponent NRM for these rocks. A lower temperature magnetization component gives a southwest and up (reversed polarity) in-situ direction, yields a negative fold test (i.e. post-folding) at the 99% confidence level, and is nearly identical to directions obtained from overlying Permo-Carboniferous lavas. A high temperature magnetization component exhibits both normal and reversed polarity directions, passes a fold test at the 95% confidence level, and is supported by a baked contact test which indicates a pre-intrusion age of magnetization. This characteristic high temperature component was successfully isolated in 79 samples, from 19 sites within the Ringerike succession. Results indicate a tilt corrected, characteristic magnetic direction of declination = 26°, inclination = −16°, which is consistent with earlier work. This direction corresponds to a time-integrated paleopole position of 19°N latitude, 164° longitude ( d p = 4.8°, d m = 9.3°). When compared to previous results from rocks of similar age from the British Isles, an approximately 20° latitudinal displacement between Britain and Baltica during Late Silurian-Early Devonian time is suggested.

Widespread late Mesozoic to Recent remagnetization of Paleozoic and lower Triassic sedimentary rocks from South China

Paleomagnetic results are described from eighteen formations sampled in the Nanjing (Jiangsu Province) and Yichang (Hubei Province) areas of South China. The marine platform carbonates and elastics range in age from early Cambrian to early Triassic. Progressive alternating field and/or thermal demagnetization analyses reveal predominantly single component magnetizations, sometimes of both normal and reverse polarity. These magnetizations, isolated in 457 of the 549 samples studied, typically have directions in geographic coordinates coincident with late Mesozoic to Recent fields. A secondary origin for the magnetizations is supported by negative fold tests in several units. A lower Carboniferous and possibly a lower Triassic limestone may have escaped the pervasive remagnetization; if the remanences isolated are indeed prefolding, a large counterclockwise tectonic rotation of the Nanjing area is implied by comparison with available Carboniferous and Triassic paleopoles reported from elsewhere in South China. This rotation may have been associated with Mesozoic left-lateral motion on the Tancheng-Lujiang fault zone.