Secondary Magnetization Research Articles

Chemical remanent magnetization in oceanic sheeted dikes

Marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non‐vertical boundaries between normally and reversely magnetized crust. The acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness. A petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust, ODP (Ocean Drilling Program) Hole 504B, demonstrates that these rocks carry a secondary chemical remanent magnetization (CRM) and not a thermoremanent magnetization (TRM). Primary titanomagnetites have mostly been altered to non‐magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its Curie temperature (∼600°C at ambient pressure) mainly by the alteration of olivine. The natural remanent magnetization (NRM) has been compared with laboratory anhysteretic remanent magnetization (ARM) and TRM. The intensities relate NRM < ARM < TRM, each by a factor of ≈ two. Because previous studies have shown that CRMs are much lower in intensity than TRMs and because of the common presence of secondary magnetite we conclude that the NRMs of the lower Hole 504B dikes are CRMs. The dikes exhibit the same reversed magnetization as the overlying extrusive basalts, and, thus, the timing of CRM acquisition is most likely confined to the time span between the crustal age of Hole 504B and the next younger field reversal, i.e. 0.25 m.y. after emplacement.

Far‐travelled Permian chert of the North Fork terrane, Klamath Mountains, California

Permian chert in the North Fork terrane and correlative rocks of the Klamath Mountains province has a remanent magnetization that is prefolding and presumably primary. Paleomagnetic results indicate that the chert formed at a paleolatitude of 8.6° ± 2.5° but in which hemisphere remains uncertain. This finding requires that these rocks have undergone at least 8.6° ± 4.4° of northward transport relative to Permian North America since their deposition. Paleontological evidence suggests that the Permian limestone of the Eastern Klamath terrane originated thousands of kilometers distant from North America. The limestone of the North Fork terrane may have formed at a similar or even greater distance as suggested by its faunal affinity to the Eastern Klamath terrane and more westerly position. Available evidence indicates that convergence of the North Fork and composite Central Metamorphic‐Eastern Klamath terranes occurred during Triassic or Early Jurassic time and that their joining together was a Middle Jurassic event. Primary and secondary magnetizations indicate that the new composite terrane containing these and other rocks of the Western Paleozoic and Triassic belt behaved as a single rigid block that has been latitudinally concordant with the North American craton since Middle Jurassic time.

Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5°N, 310.7°E ( K = 19.9, A 95 = 9.1°, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40 Ar/ 39 Ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9–1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40 Ar/ 39 Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated, even if 40 Ar/ 39 Ar dating is applied.

Timing of deformation and accretion of the Antimonio terrane, Sonora, from paleomagnetic data

At present, few paleomagnetic data exist to test tectonic models that link northern Mexico with the evolution of the southern Cordillera of North America. We present new paleomagnetic data for the Antimonio terrane in northwest Mexico. A moderate-inclination, dual-polarity yet secondary magnetization in Triassic-Jurassic strata of the Antimonio Formation and a primary remanence in Cretaceous igneous rocks (combined result: D = 338.9°, I = 57.4° α 95 = 5.6°, k = 34.8; N = 20 sites) are interpreted to indicate stability of the Antimonio terrane with respect to the craton since the Late Cretaceous. A more important result, however, is a shallow-inclination, dual-polarity magnetization in Antimonio Formation strata that fails a fold test ( D = 186.7°, I = −28.4°; α 95 = 7.6°, k = 28.2; N = 14 sites). We interpret the shallow-inclination magnetization as a secondary remanence acquired during accretion of the Antimonio terrane to the North American margin in the Middle or Late Jurassic, or during deformation within an essentially autochthonous Jurassic continental arc.

Paleomagnetism of Jurassic radiolarian chert above the Coast Range ophiolite at Stanley Mountain, California, and implications for its paleogeographic origins

Upper Jurassic red tuffaceous chert above the Coast Range ophiolite at Stanley Mountain, California (lat 35°N, long 240°E), contains three components of remanent magnetization. The first component (A; removed by ≈100–≈200°C) has a direction near the present-day field for southern California and is probably a recently acquired thermoviscous magnetization. A second component (B; removed between ≈100 and ≈600°C) is identical to that observed by previous workers in samples of underlying pillow basalt and overlying terrigenous sedimentary rocks. This component has constant normal polarity and direction throughout the entire section, although these rocks were deposited during a mixed polarity interval of the geomagnetic field. The B magnetization, therefore, is inferred to be a secondary magnetization acquired during accretion, uplift, or Miocene volcanism prior to regional clockwise rotation. The highest temperature component (C; removed between ≈480 and 680°C) is of dual polarity and is tentatively interpreted as a primary magnetization, although it fails a reversal test possibly due to contamination by B. Separation of the B and C components is best shown by samples with negative-inclination C directions, and a corrected mean direction using only these samples indicates an initial paleolatitude of 32°N ± 8°. Paleobiogeographic models relating radiolarian faunal distribution patterns to paleolatitude have apparently been incorrectly calibrated using the overprint B component. Few other paleomagnetic data have been incorporated in these models, and faunal distribution patterns are poorly known and mostly unquantified. The available data, therefore, do not support formation of the Coast Range ophiolite at Stanley Mountain near the paleoequator or accretion at ≈10°N paleolatitude, as has been previously suggested based on paleomagnetic data, but indicate deposition near expected paleolatitudes for North America (35°N ± 4°) during Late Jurassic time.

Use of renormalization group theory to explain the large variation of domain states observed in titanomagnetites and implications for paleomagnetism

Important experiments by Halgedahl (1991) demonstrate that a material can reach dramatically different domain states on cooling when the experiment is repeated many times. Renormalization group theory is applied to show that small thermal fluctuations close to the magnetic ordering temperature produce “predomain” structures. These predomain structures usually vary from one experiment to the next, even when the experimental conditions do not change. On cooling, these predomain structures are primarily responsible for determining the final domain states. Transdomain processes also occur during cooling, but these processes appear to be less important. Theory is presented to show that domain observations might be very useful to discriminate between primary thermal remanent magnetization and many forms of secondary magnetizations. In particular, a grain growth chemical remanent magnetization on the average will be characterized by a narrower distribution of magnetic domains in a given size grain than that associated with a thermal remanent magnetization.

Paleomagnetism of the Dockum Group (Upper Triassic), northwest Texas: Further evidence for the J‐1 cusp in the North America apparent polar wander path and implications for rate of Triassic apparent polar wander and Colorado plateau rotation

We report paleomagnetic data for 26 accepted sites collected in two sections of flat‐lying strata of the upper Carnian‐lower Norian (∼225 Ma) Dockum Group, northwest Texas. Six additional sites in coarse‐grained conglomeratic sandstones gave no usable results. The total assemblage of 26 VGPs is streaked along the Late Triassic ‐ earliest Jurassic track of the North America apparent polar wander path and their mean is inconsistent with the accepted upper Carnian‐lower Norian reference pole. In detail, 12 sites in grayish white (nonhematitic) sandstones have weak magnetizations (less than about 1 mA/m) carried by magnetite or maghemite that give a paleopole at 56.4°N–96.3°E (N=12 dual‐polarity VGPs; K=44.2; A95=6.6°) in close agreement with other results for upper Carnian‐lower Norian rocks in North America. The 14 remaining sites in tan and redcolored (hematitic) sandstones, siltstones, and claystones give high unblocking temperature characteristic magnetizations carried by hematite, with paleopoles at 59.0°N–53.8°E (normal; N=7 VGPs, K=62.4, A95=7.6°) and 59.3°N–77.8°E (reverse; N=7 VGPs, K=204.2, A95=4.2°). These poles fall along the younger track of poles and near the J‐I cusp of the North American APWP as defined by (unrotated) poles derived from Colorado plateau rocks. We suggest that the characteristic magnetization of the non‐hematitic sandstones is an “early” magnetization, acquired during or soon after deposition. However, the characteristic magnetization of the red bed sites is interpreted as a secondary magnetization, for which we infer an earliest Jurassic age. The secondary origin for this magnetization is supported by the observation of conflicting magnetostratigraphies. The extreme westward position of the poles derived from red bed sites, particularly those with normal polarity, confirms the general position of the J‐I cusp indicated by poles in the Piedmont province, the Newark basin, and the Colorado plateau; it also suggests that the magnitude of rotation of the Colorado plateau is no greater than about 5°. We compile an apparent polar wander path for North America, including Colorado plateau data, which suggests a fast rate of apparent polar wander throughout the Triassic period (about 0.8°/m.y.) with a gradual increase that preceded the opening of the Atlantic.

Low- to high-amplitude oscillations and secular variation in a 1.2 km late Miocene inclination record

A Miocene section of 2 km thickness was continuously cored near Szombathely, NW Hungary. A detailed magnetostratigraphic study has been integrated with results of lithologic, sedimentologic, and paleontologic studies. Progressive alternating field (a.f.) and thermal demagnetization, and rock magnetic and mineralogic studies indicate that the natural remanent magnetization (NRM) resides in magnetite and the strata contain only minor secondary magnetizations. Major polarity zones have been defined by inclinations and correlated with the geomagnetic polarity time scale for the interval 10-9 Ma. Inclinations for samples collected at 1 2 m intervals display fine-scale oscillations representing secular variation with period times of 400–700 years. Fluctuating amplitudes of oscillation range from low (less than 10° peak-to-peak) to high (more than 40°), reflecting apparently varying stabilities of the geomagnetic field. Additionally, amplitudes of oscillation progress from low to high, and return to low, forming oscillation cycles with a periodicity of 6.2 ± 1.8 kyr. The oscillations and boundaries of oscillation cycles are generally unrelated to lithology and stratigraphy. Many oscillation cycles appear to be incomplete and ‘interrupted’ by high-level oscillations. Some incomplete cycles appear to arise from brief interruptions to the depositional record, whereas other interruptions may irregularly arise from a separate component of the geomagnetic field.

Magnetic mineral assemblages and magnetic contrasts in diagenetic environments — with implications for studies of palaeomagnetism, hydrocarbon migration and exploration

Abstract Magnetic mineral assemblages in diagenetic environments are governed by microbial and inorganic/chemical processes. Empirical evidence and thermodynamic modelling of diagenetic environments ranging from near-surface settings to depths of about 6–7 km suggests that microbial processes are very important in surface and near-surface diagenetic settings, whereas inorganic diagenetic processes dominate in most deeper subsurface diagenetic settings. In all diagenetic environments, molecular oxygen and hydrocarbons are important because they control which types of bacteria can thrive and because they drive various redox reactions via changes in the redox potential. Iron oxides, such as hematite and goethite, are stable under relatively oxidizing/aerobic conditions. With decreasing redox potential, i.e. under relatively reducing/anaerobic conditions, magnetite and pyrrhotite are the most important stable magnetic minerals, whereas hematite is unstable. Other magnetic minerals, such as greigite, may be important in some instances. Pyrite and siderite are also important because they may form at the expense of magnetic minerals. A significant decrease in the redox potential in diagnetic settings, via the invasion of hydrocarbons during migration or seepage from traps, for example, almost invariability results in diagenetic remagnetization. As a result ‘positive’, ‘absent’ or ‘negative’ magnetic contrasts are generated relative to the total magnetization (or remanence, or susceptibility) before such a decrease, depending on the relative amounts of magnetic to non-magnetic minerals formed and destroyed. Thermodynamic modelling further suggests that magnetic contrasts are more probable and tend to become more ‘positive’ with depth and with proximity to hydrocarbon pools. Any significant change in the redox potential is likely to cause magnetic contrasts via the formation or destruction of magnetic minerals, thus generating secondary (chemical) magnetization and resetting original detrital or bacterial magnetization. The predicted magnetic contrasts are also likely to form as a result of hydrocarbon migration, and it appears possible to delineate migration timing and pathways by geomagnetic methods. Hence the predicted magnetic mineral assemblages can aid in the interpretation of palaeomagnetic studies, particularly via the recognition and proper interpretation of remagnetization. Magnetic contrasts, measured from the air, at the surface or on drill core, can be used for hydrocarbon exploration in association with other exploration methods.

Magnetostratigraphic correlations of the Jurassic carbonates from the Paris Basin: implications for petroleum exploration

Abstract Shallow water carbonates from the Middle Jurassic of the Paris Basin include oil reservoirs, mainly oolitic and bioclastic limestones. Biostratigraphic markers within these deposits are generally used for regional correlations. Unfortunately, these markers are not available in all cored sections. A pilot study has shown that palaeomagnetism could be used to better constrain the chronostratigraphic framework of the Jurassic shallow water carbonates. Because the magnetization of sediments is independent of facies and location, palaeomagnetic dating may allow higher resolution. Further magnetostratigraphic results are reported for three additional well dated core sections (53, 18.50 and 36.35 m in length) correlated throughout the Paris Basin. Specimens were subjected to alternating field and stepwise thermal demagnetization. The demagnetization paths show either one, two or three components. Based on the primary magnetization component, normal and reverse polarity intervals occur in the three sections. Rock magnetic experiments performed on representative specimens from both sections indicate that the bulk of the natural remanent magnetization is carried mainly by low coercivity minerals such as magnetite/maghemite and high coercivity minerals exclusively in altered layers of the cores. Single domain maghemite, probably biogenic in origin, has been observed by transmission electron microscopy. Larger detrital irregular titanomagnetite and secondary, framboidal magnetite spheres have been characterized with a scanning electron microprobe. The occurrence of the secondary magnetic spheres is probably related to the transformation of framboidal pyrite into magnetite during hydrocarbon migration. This has presumably resulted in the overprint of a secondary magnetization. The polarity sequences have been correlated with the magnetostratigraphic record defined for the Bathonian and Callovian. The correlation is fairly good for the Bathonian, but ambiguous for the Callovian. However, we used a characteristic reverse polarity interval occurring in all our cores, including that studied previously, to correlate strata throughout the Paris Basin. The positive correlation within these strata provides a strong potential for the analysis of reservoir geometry and quantitative modelling of the Middle Jurassic shallow water carbonates.

Application of magnetostratigraphy to Brent Group reservoir zonation in the Visund Field

Abstract Magnetostratigraphy has been applied to an increasing number of oil fields in the North Sea region with promising results for detailed chronostratigraphic reservoir correlation. However, the magnetization processes in hydrocarbon reservoir successions, and the potential use of magnetostratigraphy for reservoir zonation purposes, are not very well known. Therefore, aspects of these subjects are discussed on the basis of a magnetostratigraphic study of three different wells covering the Middle Jurassic Brent Group at the Visund Field, northern North Sea. Results from scanning electron microscopy and remanent coercivity analysis suggest that the primary magnetization defining the magnetozones in the Brent Group is a combination of detrital remanent magnetization and a chemical remanent magnetization, probably acquired contemporaneously shortly after deposition. Detailed thermal demagnetization has uncovered an important secondary magnetization component in most of the core samples. This magnetization is probably due to cooling of the cores from a reservoir temperature of about 110°C to surface temperature, and it causes the main disturbances in the palaeomagnetic results. Based on analyses of individual core samples, characteristic remanent magnetization components have been defined. The magnetopolarity stratigraphy for the individual wells is based on these components and the results are comparable from well to well. The well correlation covers a rock sequence with a thickness of about 200m, and both magnetostratigraphic and biostratigraphic results have been used in combination with the lithostratigraphy. The acme of Corollina represents the most important palyno-event, and the top and bottom of this palynozone is used as the main tie-level for two of the wells. Sixteen additional correlation lines have been derived from the magnetostratigraphic results, six of these considered as tentative. The correlation reveals that the magnetostratigraphic results are facies independent and some of the lithostratigraphic formation lines appear to be diachronous within this deltaic mega-sequence. Although the magnetostratigraphy only provides a relative time framework for the Brent Group, the results are encouraging and suggest the possibility of achieving high resolution chronostratigraphic well correlations for Middle Jurassic reservoir sequences in the northern North Sea area.

Results from a palaeomagnetic investigation of the Brent Group sediments in wells 34/10–16 and 34/10–17 showing evidence for complete remagnetization of the sediment

Abstract Cores from wells 34/10–16 and 34/10–17 in the Tampen Spur area, Norway were studied to establish a magnetic polarity stratigraphy for the Brent Group in the area. Approximately 100 samples from each well were treated by traditional thermal and alternating field demagnetization procedures and the apparent polarities determined. Thin sections were studied and scanning electron microscopy carried out with the purpose of verifying a primary origin of the remanent magnetization. Correlation of the two wells based on the polarity logs was in agreement with the lithoand biostratigraphic correlations. However, the mineralogical analysis did not determine any primary magnetic minerals, although potential remanence carrying diagenetic minerals were observed. This, combined with magnetic data indicating extensive remagnetization of the sediment, lead to the conclusion that the remanent magnetization of these sediments is late diagenetic, probably of Tertiary age. The results are not encouraging with respect to the use of palaeomagnetism as a chronostratigraphic tool for these sediments. It may, however, be useful for dating diagenetic events and, as a result of the strong secondary magnetization, magnetic orientation techniques can be successfully applied to the core material.

Miocene stable isotopic stratigraphy and magnetostratigraphy of Buff Bay, Jamaica

Previously reported biostratigraphic relationships from middle-upper Miocene sections exposed near Buff Bay, Jamaica (18°N, tropical bioprovince), differ from the subtropical North Atlantic (Sites 563 and 558). Time scales for this interval rely on correlations established at these subtropical sites, and the differences with the tropical section have implications to global correlations. Planktonic foraminiferal Zones N13 and N15 are thick at Buff Bay but are virtually absent at Sites 563 and 558; nannofossil Zone NN9 is associated with Zone N15 and uppermost Zone N14 at Buff Bay but is associated with Zone N16 at the other sites. Magnetostratigraphic data presented here further complicate the interpretation: Zone NN9 is associated with a thick normal magnetozone at Sites 563 and 558; at Buff Bay, it is associated with a thick reversed magnetozone. Although a secondary magnetization at Buff Bay makes it difficult to identify confidently Miocene normal magnetozones, the thick reversed magnetozone most likely represents the paleomagnetic field and correlates with Chron C5r. The magnetobiostratigraphic relationships require either diachrony of taxa or two mutually exclusive hiatuses in Jamaica and the North Atlantic. We address this problem by analyzing benthic foraminiferal δ 18 O and δ 13 C from the Buff Bay section. These isotopic data allow us to evaluate three hypotheses that reconcile the magneto-, bio-, and isotopic stratigraphic data and conclude that the first and last occurrences of five taxa were diachronous by ∼0.3-0.5 m.y. between tropical and subtropical locations. This requires revised age estimates for late middle to early late Miocene biostratigraphic datum levels. We suggest that the ranges of several taxa are useful for endemic tropical or subtropical zonations, but correlations between the low and midlatitudes were affected by an increase in latitudinal thermal gradients during the late middle Miocene. However, we admit that further studies are needed before this issue is resolved.

Paleomagnetism of the Carboniferous Tepuel Group, central Patagonia, Argentina

Samples of the Carboniferous glaciogenic sequences of the Tepuel Group, western central Patagonia, Argentina, were collected from 23 sites in two localities, Sierra de Tepuel (43.7°S, 70.5°W) and Las Salinas (43.2°S, 70.3°W). Most were fine sandstones to siltstones of late Visean to Westphalian age. A characteristic remanent magnetization was determined in most sites by alternating field and/or thermal cleaning. At Sierra de Tepuel all samples were partially overprinted by a secondary magnetization apparently related to Mesozoic magmatic activity in this area. However, site mean directions of the characteristic remanence from both localities passed a tilt test, indicating a pretectonic (pre‐Early Permian?) magnetization. Isothermal remanent magnetization experiments showed magnetite as the main and often the only magnetic carrier, while thin section analyses indicated a detrital origin for this mineral, suggesting a depositional or postdepositional magnetization. The paleomagnetic pole for the Tepuel Group (Namurian‐Westphalian) at 316.1°E, 31.7°S (number of sites N = 16, minor and major semi‐axes of confidence δp = 15.0°, δm = 16.0°), is considered a key pole for the Middle Carboniferous of South America and Gondwana, confirming the apparent polar wander path for the supercontinent for which the pole moved from central to southern Africa in the Early Carboniferous, followed by a rapid motion from southern Africa to Antarctica during the late Westphalian. The agreement of the Tepuel pole with this Gondwanan path confirms that Patagonia was already part of South America by the Middle Carboniferous.

Further palaeomagnetic data from Chitral (Eastern Hindukush): evidence for an early India-Asia contact

The Eastern Hindukush forms part of an elongate belt (“Central Domain”, collage of Cimmerian microcontinents) that encircles the northern part of the Indian subcontinent. A Gondwanan origin is commonly assumed for this belt, but a “Laurasian” origin for the Chitral region has been argued on palaeontological (Talent and Mawson, 1979) and palaeomagnetic (Klootwijk and Conaghan, 1979) grounds. The “Laurasian” view was based on a pilot study we undertook of Upper Devonian pisolitic ironstones from a thrust sheet at Kurāgh Spur in Chitral. Preliminary results showed a characteristic magnetization component [ D = 318°, I= −6.5°, N = 7 (block samples), k = 14, α 95 = 16.5°] indicating an equatorial palaeoposition. This component was thought to be of primary origin and was interpreted in terms of a Late Devonian “Laurasian” affinity of the Kurāgh Spur rocks. This controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the Reshūn-Kurāgh-Būni region of Chitral and the primary origin of the characteristic magnetization component refuted. Thermal demagnetization of 333 block samples from Middle to Upper Devonian variegated sediments, Permian quartz flysch, Permo-Triassic carbonates, and mid-Cretaceous redbeds showed two interpretable components. A softer component of recent origin (A); and a harder characteristic component (B) of both normal and reverse polarity whose mean direction [ D = 314.1°, I = 6.0°, N = 4 (thrust sheets), k = 198.2, α 95 = 6.5°] is comparable to the characteristic component observed in our preliminary study. However, the universal presence of this component throughout the thrust pile proves its overprint origin, which we attribute to initial India-Asia contact. Palaeomagnetic information pertinent to the controversy of a “Laurasian” versus a Gondwanan origin of the Chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt. Hence, we retract herewith our original conclusion of a Late Devonian “Laurasian” affinity of the Chitral region on the basis of the palaeomagnetic evidence. The secondary component (B) comprises a suite of secondary magnetizations, acquired at equatorial-to-low-northern palaeolatitudes, and is attributed to initial contact between Greater India and southern Asia. Component B has been observed previously in the Himalayan-Tibetan region, both north and south of the Indus-Tsangpo Suture zone. Identification is herein extended to the Hindukush region north of the Northern Kohistan (or Shyok) Suture zone, which is a western continuation of the Indus-Tsangpo Suture. Comparison of this suite of collision-at-tributed equatorial palaeolatitude data from the India-Asia convergence zone with new palaeolatitude constraints from the Ninetyeast Ridge on the northward movement of the Indian plate, constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of Greater India, indicates that initial contact between northwestern Greater India and southern Asia was established at, or before, the Cretaceous-Tertiary boundary. The overprint origin of component B at about this time is further supported by observations by Zeitler (1985) on rocks from the sampled area in Chitral of partially reset zircon fission-track ages around 68-55 Ma. The NW-SE declination axis of component B indicates a 60–70° counterclockwise rotation of the sampled thrust pile with respect to Eurasia and a counterclockwise rotation between 10 and 30° with respect to India. Some of the recent field components (A) show a comparable rotation and indicate that the tectonic activity that led to the formation of the Hindukush-Pamir-Karakorum syntaxial zone has continued into recent times.

Palaeomagnetic investigation of Middle Devonian limestones of Algeria and the Gondwana reconstruction

SUMMARY We have carried out a palaeomagnetic investigation of the upper middle Devonian marly limestone of the Hazzel Matti Formation outcropping at Meredoua in the northern Ahaggar in the Sahara Desert. the Givetian age of the formation is well constrained by palaeontological arguments. the formation has been deformed later than the Moscovian (upper middle Carboniferous). In the altered yellowish samples and in the heterogeneous samples made of mixed fresh and altered zones, several obviously secondary magnetization components are present. In the freshest bluegrey samples a SE down-dipping component can be isolated, particularly when a combined thermal and AF treatment is applied. This component, observed in 53 samples from three sites, passes the fold test of McFadden & Lowes (1981) and McFadden & Jones (1981) at the 95 per cent probability level. the mean direction for the three sites after tilt correction is: Ds= 118.9°, Is, = 34.2°, k = 378, α-95 = 6.3°, which corresponds to a south pole (MER) situated at Op=61.7°, λp= -16.2°, A95= 4.20 and places the site area under a palaeolatitude of -18.8°. However, the best grouping of the directions is achieved after 85 per cent of unfolding using a statistical method based on a bivariate extension of the Fisher's statistics (Legoff 1990; Legoff, Henry & Daly 1992). Such a tilt correction changes the pole position by 1.5°. As the α95 and K curves obtained from stepwise unfolding display a broad minimum and maximum value respectively, and because there was probably slight original syn-sedimentary dips, it is likely that 85 per cent of tilt correction is not significantly different from complete unfolding. The magnetic behaviour of the samples upon thermal and AF demagnetization, the evolution of the initial susceptibility upon thermal treatment, hysteresis cycles and investigation of the magnetic mineralogy through microscope examination and electron microprobe analyses lead to the following conclusion: greigite was probably a primary magnetic mineral formed during the diagenesis of the rock. However, its primary magnetization has been totally replaced by a VRM, except perhaps in rare samples in which the SE component can be found in the narrow range of 300-350°C. Magnetite is thought to carry the SE magnetization component and may have formed early during the diagenesis or soon after greigite. To consider the fold test negative would assign the MER pole a post-Moscovian age and place it in a situation inconsistent with the other poles of the West Gondwanian APWP, unless all the middle Carboniferous and probably lower Permian African poles are rejuvenated. We rather favour the interpretation of a positive fold test, which however does not constrain the magnetization to be of primary Givetian age; the SE component may as well have been acquired any time between the Givetian and at least the late Moscovian. Assuming that the MER pole has a Givetian age implies that the Rheic Ocean, which separated Laurussia from Gondwana in the early Palaeozoic, was almost closed in the middle Devonian, in agreement with palaeobiogeographic arguments; but it increases the discrepancy between the APWP of respectively West and East Gondwana. an intermediate lower Carboniferous age might be in better agreement with both African and Australian data. Clearly, however, more Gondwanian poles are needed to test the reliability of both West and East Gondwanian APWP, and check simultaneously the possibility of relative displacements between these two blocks during the Devono-Carboniferous time period.

Paleomagnetism of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite, southern Laramie Range, Wyoming and Colorado

We present the results of a combined paleomagnetic and 40Ar/39Ar geochronologic investigation of the Middle Proterozoic Laramie anorthosite complex and Sherman Granite in the southern Laramie Range of Wyoming and Colorado. Anorthosites and monzosyenites of the Laramie anorthosite complex yield a well‐defined characteristic magnetization of northeast declination (D) and moderate negative inclination (I), although antipodal normal and reverse polarity magnetizations are present at three sites. A grand mean direction from 29 of 35 sites in the complex is D = 44.6°, I = −48.7° (k = 77.4, α95 = 3.1°). Alternating field (AF) and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low‐Ti titanomagnetite of single or pseudo‐single domain character that occurs as elongate to rod‐shaped inclusions in plagioclase and potassium feldspar. The Sherman Granite contains a dual polarity magnetization that is less well defined than that of the Laramie anorthosite complex but similar in declination and inclination (D = 53.1°, I= −48.1°, k = 46.5, α95 = 7.6°, n = 8/14 sites); rock magnetic data indicate the primary carrier of remanence in Sherman Granite is magnetite. The 40Ar/39Ar geochronologic data from Sherman Granite hornblende, biotite, and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 Ma. Microcline data indicate that the Laramie anorthosite complex and Sherman Granite have probably not been thermally remagnetized. Paleomagnetic poles from the Laramie anorthosite complex and Sherman Granite are indistinguishable at the 95% confidence level, and individual virtual geomagnetic poles (VGPs) from both units are combined to provide a mean pole at 215.0°E, 6.7°S (K = 46.9, A95 = 3.5°, N = 37 VGPs). The location of this pole is similar to paleomagnetic poles derived from 1480 Ma to 1450 Ma intrusions elsewhere in North America, but it plots significantly north of those from Middle Proterozoic sedimentary strata of the Belt Supergroup and Sibley Group. In addition to the characteristic magnetization, samples from some sites in Sherman Granite contain a remanence of southeast declination and shallow negative inclination (D = 154.9°, I = −16.0°, k = 90.6, α95 = 9.7°, n = 4 sites). This secondary magnetization was probably acquired during late Paleozoic time.

Paleomagnetism of baked sedimentary rocks in the Newark and Culpeper basins: Evidence for the J1 cusp and significant Late Triassic apparent polar wander from the Mesozoic basins of North America

A paleomagnetic study of 14 sites in the baked sedimentary rocks of the Newark basin Passaic Formation in southeastern Pennsylvania reveals two types of magnetic behavior. Dark gray‐colored, baked sedimentary rocks have peak unblocking temperatures of 640°C, high magnetic intensities, and shallow, normal polarity, northeasterly directions. Light gray‐colored rocks have peak unblocking temperatures of less than 580°C, low magnetic intensities, and intermediate inclination, normal polarity, northwesterly directions. The low unblocking temperature magnetizations are secondary magnetizations which have declinations similar to but are shallower than the B remagnetization observed by Witte and Kent (1991) throughout the Newark basin. The discrepancy may be due to “underprinting” by an unresolved primary magnetization. The low unblocking temperature magnetization was probably acquired by growth of secondary magnetite during a hydrothermal event, as postulated by Sutter (1988), based on geochronologic data. The high unblocking temperature magnetization is significantly prefolding. Both the low‐peak unblocking temperature magnetization and the high‐peak unblocking temperature magnetization suggest a 15° counterclockwise block rotation of the Sassamansville syncline. If this rotation is removed from the high unblocking temperature sites collected around the fold, a stronger passage of the fold test results. Six sites were also collected from baked sediments and one site from diabase in northern Virginia's Culpeper basin, since Sutter's geochronological work indicated that the intrusives in the Culpeper basin are coeval to the Newark basin intrusives. Virtual geomagnetic poles (VGPs), based on the tilt‐corrected, high‐temperature Newark basin magnetizations, were compared with the VGPs calculated from the site means of a high‐temperature magnetization isolated from baked sedimentary rocks in the Culpeper basin and to the magnetizations reported by Raymond (1982) from dikes and sills. In this comparison the in situ Culpeper poles agreed with the prefolding Newark poles significantly better than the prefolding Culpeper poles. This result indicates that Culpeper intrusives erupted into already tilted sedimentary rocks. The paleomagnetic pole determined from the combined Culpeper baked sediments, dikes, and sills (in situ coordinates) and the Newark basin baked sediments (tilt‐corrected coordinates) lies at 60°N, 69°E and is of 201 Ma age. This latest Triassic/earliest Jurassic pole, when combined with the Newark basin Carnian results (Witte and Kent, 1989) and Norian results (Witte et al., 1991) corrected for a counterclockwise block rotation (Kodama et al., 1994), provides a record of significant polar wander from eastern North America's Mesozoic basins for the Late Triassic. This is consistent with observations made for a similar time period from rocks on the Colorado Plateau (Bazard and Butler, 1991). Comparison of the Newark/Culpeper pole to similar age poles from the Kayenta (Bazard and Butler, 1991) and Moenave Formations (Ekstrand and Butler, 1989) only requires small amounts (5°) of Colorado Plateau rotation. The pole also provides the first well‐dated evidence of the Jl cusp in North American apparent polar wander from rocks not located on the Colorado Plateau, thus giving strong support for the usefulness of paleomagnetic Euler pole analysis of apparent polar wander.

Geochemical constraints on the origin of secondary magnetizations in the Cambro-Ordovician Royer Dolomite, Arbuckle Mountains, southern Oklahoma

Geochemical and paleomagnetic results from Cambro-Ordovician Royer Dolomite in the Arbuckle Mountains, southern Oklahoma, suggest a connection between dolomite type and magnetizations. One dolomite type, which is most common in the vicinity of the Arbuckle Anticline, has a dull luminescence, low δ 18O values, coeval to slightly lower 87 Sr 86 Sr ratios and contains an easterly Cambro-Ordovician magnetization in magnetite. To the southeast, closer to the Ouachita Mountain front, most of the Royer dolomite contains a southeasterly and shallow magnetization in magnetite that is interpreted to be Late Paleozoic in age. In contrast to the other dolomites, these dolomites contain abundant authigenic K-feldspar and pyrite, radiogenic 87 Sr 86 Sr ratios, and have a bright (red) luminescence, all of which are consistent with alteration by basinal fluids. At one site, a stromatolitic bioherm, dolomites similar to those at the Arbuckle Anticline are surrounded by the altered dolomites. The distribution of specimen directions at this site is streaked between a Cambro-Ordovician and Late Paleozoic direction. The results of this study suggest a connection, either direct (by causing precipitation) or indirect (by increasing permeabilities for another fluid), between a radiogenic fluid and a chemical magnetization in magnetite. The relatively unaltered dolomites at the stromatolitic bioherm site escaped the almost pervasive alteration and remagnetization caused by the basinal fluids close to the Ouachita Mountains, a potential source for the fluids.

Late Permian age of NH4-bearing illite in anthracite from eastern Pennsylvania: Temporal limits on coalification in the central Appalachians

Authigenic NH 4 -bearing illites in coal fractures from the Anthracite region of eastern Pennsylvania yield individual K/Ar dates of 236-253 Ma (±8 m. y.) and a composite isochron of 253 Ma (±8 m.y.). Rb/Sr data for the same samples indicate an isochron of 260 Ma (±44 m.y.), consistent with the more precise K/Ar data. The age of NH 4 -bearing illite authigenesis should reflect the time at which coal attained anthracite rank, because mineralogical and geochemicai evidence indicates that NH 4 -bearing illite formed exclusively during anthracite formation (T = 200-275 °C) from organically derived nitrogen and a kaolinite precursor. Similar Late Permian K/Ar dates for both NH 4 -rich illite and K-illite in anthracite fractures and lack of correlation between percent NH 4 substitution in illite and the K/Ar date suggest that NH 4 does not influence significantly the closure temperature of illite and,thus, that NH 4 -rich illite can be used for dating the time of authigenesis. Given the uncertainties in closure temperatures for illite as well as the maximum temperatures for anthracite formation, we consider the oldest K/Ar date of 253 Ma (±8 m.y.) to be a minimum age for NH 4 -illite authigenesis and anthracite formation. Based on the retention of a detrital age signature in the Re-examination of syn- and post-folding secondary chemical remanent magnetizations in rocks from the central Appalachians indicates a late Early to Late Permian age (260-250 Ma) for Alleghanian deformation in the vicinity of the Anthracite region. The age of folding is similar to the 265-255 Ma age range for anthracite formation interpreted from illite dates and is consistent with previous studies of vitrinite reflectance fabrics in coal, which indicate that anthracite formation occurred prior to and during folding. Thus, age data in conjunction with paleomagnetic and vitrinite reflectance data allow a time period of 25-35 m.y. for anthracite formation and Alleghanian deformation, beginning after deposition of the coal in Late Pennsylvanian or Early Permian time (ca. 290-280 Ma) and concluding by the Late Permian time (ca. 260-250 Ma), significantly longer than previous estimates of less than 15 m.y. for this thermal/tectonic event.