Blocking Temperature Component Research Articles

Paleomagnetism and geochronological studies on a 450 km long 2216 Ma dyke from the Dharwar craton, southern India

Paleomagnetic and geochronological studies were carried out on a ∼ 450 km long (from 17 sites) N-S striking Paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km (increasing from North to South) in the Dharwar Craton, to study the characteristics of paleomagnetism and geochronology in vertical dimension. U-Pb/Pb-Pb dating on baddeleyite gives a crystallisation age of 2216.0 ± 0.9 Ma for long dyke AKLD. Paleomagnetic data from this well dated ∼ 2216 Ma dyke swarm in Dharwar Craton are of excellent quality. High coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions. Combined normal and reverse polarity remanences on AKLD and other N-S dykes define the most reliable paleomagnetic pole for ∼ 2216 Ma at latitude 36°S and longitude 312°E (A95 = 7°). Though paleomagnetic data is unavailable on other N-S dykes below the Cuddapah basin, high precision geochronology suggest that they are of similar age within errors. Though there is a variation in the crustal depth of Dharwar craton from north to south, consistent Pb-Pb/U-Pb baddeleyite geochronology and paleomagnetic studies along the AKLD established its continuity and preservation along its entire strike length. The virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test. Positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. Updated apparent polar wander path of Dharwar craton indicates relatively low drift rate during 2.21–2.08 Ga interval. Magnetogranulometry and SEM studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals.

Rock magnetic properties and paleomagnetic behavior of Neogene marine sediments from northern Chile

AbstractNeogene sediments from La Portada and Bahia Inglesa formations located at Mejillones Peninsula and Caldera Port, northern Chile carry multicomponent magnetizations, which can lead to misidentification of the characteristic paleomagnetic vector and consequently, errors in chronostratigraphic and structural interpretation of the strata. Here we present a complete rock magnetic and paleomagnetic behavior study of Neogene sediments from La Portada and Bahia Inglesa formations. Isothermal remanent magnetization, thermal‐magnetic susceptibility, magnetic hysteresis, and X‐ray diffractometer experiments are combined with paleomagnetic, alternating field, and thermal demagnetization data to determine magnetization history. Remanent magnetizations generally comprises three components: a low blocking temperature (Tb) component, below 150°C, close to the present‐day field in direction, interpreted as a thermoviscous component; an intermediate Tb component, between 150°C and 290°C, overprinted by the low and the high Tb components, considered as the survival of the characteristic detrital magnetization; and a high Tb component, above 290°C, thought to be of diagenetic origin. Detrital magnetite, titanomagnetite, and low titanium maghemite are identified as the main carrier of the magnetic remanence. Rock magnetic results of a minor group of samples at both locations detected the presence of a high‐coercivity mineral, possibly hematite, interpreted to carry the high Tb component and to represent oxidation of minerals in postdepositional processes.

Paleomagnetism of the middle Cretaceous Iritono granite in the Abukuma region, northeast Japan

We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with 40Ar– 39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar– 39Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity ( B c > 50–90 mT) or high blocking temperature ( T b > 350–560 °C) component and (2) L component, relatively low B c or low T b component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination ( I = 29.9°, D = 311.0°, α 95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude ( I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low- P/high- T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination ( I = 42.8°, D = 311.5°, α 95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point (∼ 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization.

Toward an understanding of the multicomponent magnetization of uplifted Neogene marine sediments in New Zealand

New Zealand contains many sequences of technically uplifted Miocene/Pliocene marine sediments, which have been central to the development of a biostratigraphic framework for temperate southern latitudes. Chronological control is almost invariably dependent on magnetostratigraphy. Interpretation of the natural remanent magnetization (NRM) of these sediments is, however, notoriously difficult because of the weak magnetization, secondary components that resist demagnetization, and thermal instability of the clay minerals. A new paleomagnetic study of Wanganui Basin sediments has yielded unusually well defined demagnetization data from which systematic characterization, explanation, and interpretation of the multicomponent nature of the NRM have been possible. In general, the magnetization consists of three components: (1) a low (<150°C) blocking temperature (Tb) component that is interpreted to be a recent thermoviscous component, usually close to the present day field in direction; (2) an intermediate (150–250°C) Tb component, which is thought to be all that remains of the primary detrital magnetization; and (3) a high (>250°C) Tb component that is considered to be diagenetic in origin. This interpretation of the intermediate and high Tb components is supported by rock magnetic data which show that specimens carrying strong high Tb components have a distinctly different spectrum of ferrimagnetic grains, specifically a population with much higher coercivities than is seen in specimens which lack a high Tb component. It is argued that these high‐coercivity, high Tb grains are secondary in origin, leading to the conclusion that the intermediate Tb component is the primary detrital magnetization. Consistency of the paleomagnetic data within and between sites also supports this conclusion. It is suggested that failure to recognize these three components has, in some cases in the past, led to misidentification of the primary magnetization. Using this hypothesis of the multi‐component magnetization, the Turakina River data of McGuire [1989] have been reinterpreted, yielding a high‐resolution magnetostratigraphy from the upper Gilbert, Chron 3n, (circa 4 Ma) to the lower Matuyama, Chron 2r. The correlation of this magnetostratigraphy with the geomagnetic polarity timescale, is supported by biostratigraphic constraints.

Low‐latitude and Southern Hemisphere origin of Anisian (Triassic) bedded chert in the Inuyama area, Mino terrane, central Japan

Paleomagnetic and paleontologic studies of Middle Triassic to earliest Jurassic radiolarian bedded chert in the Inuyama area, southern Mino terrane of central Japan, have shown that the chert was deposited in an equatorial region with a paleolatitude between 10°N and 10°S, and in particular, the lower to middle Anisian chert depositional basin was situated in the Southern Hemisphere. The chert has four characteristic components of remanent magnetization. The highest blocking temperature component, carried by hematite, shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition. The other three components are interpreted as a present field overprint and two postfolding remagnetizations. The absolute, tilt‐corrected inclinations of the primary component are less than 20° for all the samples of Middle Triassic to earliest Jurassic age, with the mean inclination of the lower to middle Anisian chert being 11.0°±4.3°. Paleomagnetic polarity reversals observed for the lower to middle Anisian chert are well correlated with those of the European Triassic sections, under paleontologic age constraints using radiolarians and conodonts, provided that the Anisian chert in Inuyama was deposited in the Southern Hemisphere. This conclusion illuminates the paleogeographic evolution of the chert‐bearing accretionary complexes not only in Japan but along the eastern margin of Asia as well.

A palaeomagnetic study of Jurassic intrusives from southern New South Wales: further evidence for a pre-Cenozoic dipole low

The results of rock magnetic, thermal demagnetization and Thellier palaeointensity studies are presented for two high-level intrusions from southern New South Wales, Australia. The Gingenbullen Dolerite (GB) (34.5°S, 150.3°E) and the Gibraltar Microsyenite (GS) (34.5°S, 150.4°E) were emplaced during the second of three major phases of igneous activity that affected the southern Sydney Basin and are K/Ar dated at 172 and 178 Ma, respectively. The magnetic mineralogy of the two intrusions is different: the GB is dominated by single-domain (SD)/pseudo-single-domain grains of magnetite, whereas the GS has both SD magnetite and haematite components, although the haematite component does not record a stable remanence. The GB records two opposed components of magnetization, above 100 °C, occupying distinct parts of the blocking temperature spectrum. The lower blocking temperature (LBT) component resides between 150 and 400 °C and is of normal polarity, having a mean direction of D/I = 021°/−79° (α95 = 8°, k = 63), with a corresponding VGP at Lat/Long = 54°S/137°E (dp = 12, dm = 13). This component is believed to be a TCRM or TVRM related to the initial opening of the Tasman Sea at ~90 Ma. The higher blocking temperature (HBT) ChRM component is recognized between 450 and 580 °C and is of reverse polarity, with a mean direction of D/I = 174°/+81° (α95 = 11°, k = 33) and a corresponding VGP at Lat/Long = 52°S, 153°E (dp = 17, dm = 18). The ChRM for the GS resides in the 100–450 °C region and, although the scatter is high, has a similar mean direction (D/I = 167°/+79°; α95 = 24°, k = 26) to the HBT component of the GB, with a pole position at Lat/Long = 54°S/158°E (dp = 30, dm = 31). There is no evidence of the GB LBT component in samples from the GS, suggesting that this intrusion was magnetically unaffected by the later event, which is also seen for other intrusions in the Sydney Basin. The VGPs calculated from the ChRM directions both plot on the Jurassic segment of the Australian APWP (Dunlop et al. 1997), providing further palaeomagnetic confirmation for the K/Ar ages. Thellier palaeointensity results were of variable quality, as expected for rocks of this age. The mean VDM values calculated for the ChRMs of the GB and GS were 1.3 × 1022 and 4.1 × 1022 Am2, respectively (approximately 16 and 51 per cent of the present value). On application of a cooling-rate correction estimate, the GB result falls to a minimum of 0.87 × 1022 Am2 (approximately 11 per cent of the present-day value), representing an overestimate of up to 56 per cent. These values, arising from a time window previously lacking in Thellier estimates, provide further confirmation of the existence of a pre-Cenozoic dipole low. The VDM values derived from the overprint in the GB samples yield an average of 4.8 × 1022 Am2 (from a range between 44 and 81 per cent of the present-day mean VDM) and are tentatively attributed to the Cretaceous Normal Superchron (CNS). These results may document evidence for a previously unrecognized low dipole moment during the bulk of the CNS, a possibility that requires further investigation by performing palaeointensity work on igneous rocks with radiometric ages between 110 and 90 Myr that carry unequivocal primary TRMs.

PALEOMAGNETISM OF THE DJEBEL REOUINA NAMURIAN FORMATION (TINDOUF BASIN, ALGERIA)

The paleomagnetic study of the Namurian of Reouina (28.9°N, 08.0°W) revealed the existence of two magnetization components, either juxtaposed or superimposed, besides a viscous component. The high blocking temperature component, carried by hematite, has a mean direction defined by D = 126.9° and I = 10.8°. It provides a Namurian paleomagnetic pole located at 28.4°S and 56.9°E (K = 642, A95=1.7°). The second component is carried at least in part, by grains with blocking temperatures lower than 550°C. Though well defined, it consists of two superimposed components, the high unblocking temperature component with a likely Permian overprint.

Palaeomagnetic controls on the emplacement of the Neapolitan Yellow Tuff (Campi Flegrei, Southern Italy)

Abstract Two outcrops of Neapolitan Yellow Tuff at Torregaveta and Posillipo have been investigated palaeomagnetically. Analyses of low blocking temperature components, in association with mineralogical, volcanological and magnetic anisotropy data, suggest the following sequence of events: (i) deposition of pyroclastics at temperatures not lower than 275°C, at least at Torregaveta; (ii) cooling associated with zeolitization and modification of the original positions of the clasts; (iii) further dislocation of the clasts after cooling; and (iv) lithification. Such a model reconciles otherwise conflicting volcanological and palaeomagnetic data.

Palaeomagnetism of the (early devonian) lower old red sandstones of South Wales: implications to variscan overprinting and differential regional rotations

Component structures of magnetic remanence in the Old Red Sandstones of South Wales are related to tectonic history by detailed study of three contrasting areas: (1) remote from, (2) bordering, and (3) within the Variscan orogenic belt. A uniform southerly shallow-directed component dominates remanence in each sector. In area 1, where it is considered most representative of the regional field, this magnetisation has a mean D I = 191.1/− 8.0° (151 samples in situ, α 95 = 2°) equivalent to a pole position at 341.1°E, 41.2°S. Fold tests show that it was acquired at an intermediate stage of orogenesis (and during the Carboniferous-Permian Reversed Superchron). It is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south. Remote from the orogenic front, a higher blocking temperature dual-polarity remanence of probable Lower Devonian age has sporadically survived ( D I = 246/38° , 27 samples tilt adjusted, α 95 = 7.2°) corresponding to a pole position at 297°E, 3°N. This remanence is entirely replaced close to the frontal zone of the orogen. Within the orogen, low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum. This region was deformed into upright folds before the magnetising event. Following remanence acquisition, continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs. Structures in the south were already more deformed than those in the north at the time of magnetisation, implying that the causative fluid front moved northwards with time. The palaeomagnetic vectors are rotated clockwise to the west in South Wales and, in part, record the effects of regional dextral transpression; the largest rotation however, is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain.

Palaeomagnetic results from the late Precambrian Chela Group of southwest Angola

Palaeomagnetic results from fourteen sites (56 samples) in the late Precambrian Chela Group of southwest Angola are reported. After alternating field demagnetization eight of the sites have mean directions closely grouped around D = 123°, I = −44 1 2 ° ; the dispersion of these directions increases substantially after tectonic corrections suggesting magnetic overprinting. Intensive thermal demagnetization proved more effective than alternating field demagnetization in resolving magnetic components. After removal of viscous components, samples from five sites (Group I) show a single component, similar to that observed after alternating field demagnetization. At four more sites (Group II) this same component is present together with another higher-temperature component. The mean direction for the component observed at all nine sites is D = 119.7°, I = −45.9° with k = 40.5. Tectonic corrections produced little change in mean direction but reduced k to 4.5; the data supports the magnetic overprint hypothesis. A high blocking temperature component was resolved by remagnetization circle analysis of three flat-lying sites (Group III) and this component is similar to the higher-temperature component observed at site DG (Group II) after tectonic correction has been made. The component observed in Group III samples ( D = 311°, I = 49°), confirmed by some samples from Group II, is similar to that observed in the Nosib Group just to the south in Namibia. It is suggested that the mean Group III result gives the primary direction of the Chela Group, that the data support the correlation of the Nosib and Chela Groups and that the Chela rocks were overprinted magnetically during an event subsequent to their tilting.

Paleomagnetism of mesozoic red chert from Cedros Island and the San Benito Islands, Baja California, Mexico revisited

Previous paleomagnetic study of Mesozoic subduction‐complex rocks on Cedros Island and the San Benito Islands, Mexico, shows that these rocks have been remagnetized; a single‐polarity magnetization was found in chert sections and underlying pillow basalt deposited during a mixed‐polarity interval of the geomagnetic field. Reanalysis of the Cedros and San Benito chert samples (101 total) shows that 13 samples also retain a high blocking‐temperature component of magnetization (600° to 680°C) indicating a polarity stratigraphy and deposition of the bedded chert near 2° ± 3° paleolatitude. These data are similar to those for red chert in California and Japan implying that many of the Mesozoic ophiolitic chert sequences now exposed around the Pacific rim were initially deposited within the equatorial zone of high biologic productivity. These new data also support accretion of the Cedros and San Benito chert to the American margin at tropical paleolatitudes requiring significant northward translation (and clockwise rotation) to bring them to their present position with respect to the continental interior.

Paleomagnetism in the Tajikistan: continental shortening of European margin in the Pamirs during Indian Eurasian collision

A stable and well defined magnetization with D = 350°, I = 55.5° and α 95 = 4° , carried by hematite, is isolated in the red beds of the Tajik basin. The fold test is positive (99% probability) and this magnetization predates the Late Eocene-Early Oligocene folding phase in the Tajik basin. The coordinates of the VGP are: 82°N, 323°E, dp = 4, dm = 6 with a paleolatitude of Dushanbe (38°38N, 68°51E) of 35.6 ± 5°. A high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of Ragoon, but it has not been isolated. This component predates the Long Cretaceous Normal Period and is compatible with the older stratigraphic age attributed to the formation (Valanginian); these arguments indicate that this component is likely of primary origin. The other component, of higher intensity and clearly isolated in all the sites, is a pre-folding remagnetization. The process of remagnetization is attributed to the tectonic events, such as fluids migration and slight heating during a few million years, which took place just before the first Late Eocene-Early Oligocene phase of folding An anticlockwise rotation of 26° encountered by the region of Dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation West of the Indian wedge and clockwise rotation to the east. A comparison of the paleolatitude of Dushanbe, computed from the direction of magnetization, and the paleolatitude expected from the apparent polar wander path for Eurasia, gives an estimate of the post-collision continental shortening north of the Tajik basin. This shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km. An estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of Eurasia at 45 Ma is proposed. The paleolatitude deduced from the prefolding remagnetization of the Red Beds of the Tajik basin is then compared with the results obtained by Soviet workers in Tertiary sediments of the same area, revealing strong contradictions in interpretation. This contradiction emphasizes the problem of the rigidity of European plate.

A palaeomagnetic study of the Basal Gneiss Region and Jotun Nappe Complex, central-southern Scandinavian Caledonides

SUMMARY In the southern sector of the Scandinavian Caledonides a metamorphic gneiss complex (the Basal Gneiss Region or BGR) underthrusts a nappe complex of Precambrian granulites (the Jotun Nappe) in an assemblage collectively recording orogenic events culminating in the Scandian (Upper Silurian and Lower Devonian) episode. Palaeomagnetic study of a large regional sample spanning the combined outcrop identifies properties strongly controlled by metamorphic retrogression to amphibolite and greenschist facies. Low blocking temperature (Ibt) components in the BGR have E+ directions with a mean of D = 69° 1 = 60° (N = 19 sites,α95= 7.0°); sporadic higher blocking temperature components define a dual polarity northeast negative/southwest positive (NE - /SW+) axis with a mean of D = 42°, 1 = -24° (20 sites, α95= 8.1°). Granulite facies rocks in the Jotun Nappe yield a high-quality palaeomagnetic record reflected in high $LQn values and dominated by medium and high blocking temperature (hbt) components. A dual polarity (‘A’) axis defines a palaeofield migration from NE+ to E+ during uplift and cooling of this terrane with a predominance of normal polarities. A smaller population of Ibt (‘B’) components show E+ directions distributed along the Mesozoic palaeofield migration path for Eurasia, and probably acquired during brittle tectonic events of this age. ‘A’ magnetizations from 86 sites define mean south pole positions between 301°E, 2°N and 325°E, 10°S linked to uplift-related cooling following the climactic Scandian orogenic episode and dated at 420–400 Ma from the collective geological and radiometric evidence. Both the outward and return segments of an APW loop may be represented. The polar swathe correlates with (i) a higher blocking temperature component from the BGR, (ii) the uplift magnetization record in the western orthotectonic Caledonides of Scotland, and (iii) primary magnetizations from the Siluro-Devonian molasse facies in Britain. This study shows that the westerly extension of the APW path identified by British Siluro-Devonian data is not peculiar to that crustal segment but applies to continental Europe, and presumably to Laurentia, as well. The pattern of remanence acquisition within the Caledonides suggests contrasting zones of thermal and thermochemical remanence, with the latter probably linked to fluid migration consequent upon orogenic loading.

Palaeomagnetism of (late Vendian-earliest Cambrian) minor alkaline intrusions, Fen Complex, southeast Norway

Emplacement of the Fen central complex (603-565 Ma) within the Fennoscandian Shield in southeast Norway was preceded by the emplacement of numerous minor alkaline intrusions into the surrounding gneisses. A palaeomagnetic sample of 28 of these bodies has identified a predominant SSE negative remanence carried by magnetite in some bodies and hematite in others. A sporadic high blocking temperature component appears to record localised effects associated with the development of the Oslo rift and igneous province to the east, but no major magnetic overprinting by post-emplacement events is recognised. The stable magnetisation vectors for twenty sites comprise a coherent population with those for two sites reversed with respect to the remainder; they yield a mean direction of D = 210°, I = 44° ( α 95 = 6.4° ) and a palaeomagnetic pole at 324°E, 50°S ( dpdm=4.9°7.9°). The difference between the pole position for this early phase of the Fen magmatism and that for the late metasomatic rødberg (322°E, 63°S) in the interior of the complex is interpreted in terms of continental movement during the late Vendian-earliest Cambrian interval of alkaline activity here. The defined direction of APW movement continues a motion recognised from other Vendian data but subsequent movements during Lower Cambrian times are unclear.

Palaeomagnetic results from the Lower Devonian Llandstadwell Formation, Dyfed, Wales—discussion

Component structures of magnetic remanence in the Old Red Sandstones of South Wales are related to tectonic history by detailed study of three contrasting areas: (1) remote from, (2) bordering, and (3) within the Variscan orogenic belt. A uniform southerly shallow-directed component dominates remanence in each sector. In area 1, where it is considered most representative of the regional field, this magnetisation has a mean DI = 191.1/− 8.0° (151 samples in situ, α95 = 2°) equivalent to a pole position at 341.1°E, 41.2°S. Fold tests show that it was acquired at an intermediate stage of orogenesis (and during the Carboniferous-Permian Reversed Superchron). It is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south. Remote from the orogenic front, a higher blocking temperature dual-polarity remanence of probable Lower Devonian age has sporadically survived (DI = 246/38°, 27 samples tilt adjusted, α95 = 7.2°) corresponding to a pole position at 297°E, 3°N. This remanence is entirely replaced close to the frontal zone of the orogen. Within the orogen, low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum. This region was deformed into upright folds before the magnetising event.Following remanence acquisition, continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs. Structures in the south were already more deformed than those in the north at the time of magnetisation, implying that the causative fluid front moved northwards with time. The palaeomagnetic vectors are rotated clockwise to the west in South Wales and, in part, record the effects of regional dextral transpression; the largest rotation however, is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain.

Ordovician docking of the Carolina Slate Belt: Paleomagnetic data

Paleomagnetic results from eight sites in argillaceous metasediments from Union County, North Carolina, in the central Carolina slate belt yield a consistent high blocking temperature magnetization component. The precision of the mean declination and inclination of all site means is maximized at 50% tilt correction, and the improvement in grouping relative to 0% and 100% tilt correction is significant at the 95% confidence level. This tends to indicate that the high blocking temperature component was acquired during folding. The metamorphism and deformation in this area are rather well constrained, by recent 40Ar/39Ar data, to the Late Ordovician (−450 Ma). We therefore interpret the computed pole position (29.6°N, 122.1°E, α95 = 5.11) as Late Ordovician in age and note that it lies close to some of the Late Ordovician poles for cratonic North America. The paleolatitude (22.2°S) of the sampling site is certainly unlike those derived from Armorica and Gondwana for this time interval and is rather similar to that proposed for the Iapetus coast of Laurentia during the Late Ordovician. Indeed, the magnetization (and deformation) may be contemporaneous with the docking of the Carolina slate belt onto the Laurentian continent.

Paleomagnetism of the westcoast complex and the geotectonics of the Vancouver island segment of the wrangellian subterrane

Paleomagnetic data from 46 sites (674 specimens) of the Westcoast Crystalline Gneiss Complex on the west coast of Vancouver Island using AF and thermal demagnetization methods yields a high blocking temperature WCB component (> 560°C) with a pole at 335°W, 66°N ( δ p = 4°, δ m = 6°) and a lower coercivity WCA component (∼ 25 mT, < 500°C) with a pole at 52°W, 79°N ( δ p = 7°, δ m = 8°). Further thermal demagnetization data from 24 sites in the Jurassic Island Intrusions also defines two high blocking temperature components. The IIA component pole is at 59°W, 79°N ( δ p = 7°, δ m = 8°) and IIB pole at 130°W, 73°N ( δ p = 12°, δ m = 13°). Combined with previous data from the Karmutsen Basalts and mid-Tertiary units on Vancouver Island and from the adjacent Coast Plutonic Complex, the geotectonic motions are examined for the Vancouver Island segment of the Wrangellian Subterrane of composite Terrane II of the Cordillera. The simplest hypothesis invokes relatively uniform translation for Terrane II from Upper Triassic to Eocene time producing ∼ 39° ± 6° of northward motion relative to the North American craton, combined with ∼40° of clockwise rotation during the Lower Tertiary.

Magnetic properties of marine limestones : Revs Geophys Space Phys, V20, N2, May 1982, P171–192

Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected. In this review, geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identification of the magnetic mineralogy are evaluated. The conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence (NRM), probably by a postdepositional alignment process. The effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation. Other remanence components can be acquired during diagenesis, in the soft sediment right after deposition or much later even in the indurated limestone. The magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration; magnetic techniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract. A particularly useful technique involves combined observations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequent destruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniques are magnetite, goethite, hematite, and maghemite. These magnetic minerals occur in different combinations in the marine limestones of central Europe and the Mediterranean realm. Pyrite is common in certain limestones, where it can be the precursor of an unwelcome late goethite. Pyrrhotite may be responsible for unstable magnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures (300°–400°C) due to the breakdown of a metastable mineral such as maghemite, goethite, or pyrrhotite. At high temperatures (above 500°C), pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespread component of NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibility is anisotropic and shows a classical ‘depositional’ pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbated sediments) implies that the anisotropy pattern results from postdepositional compaction of the sediment.

Remagnetization and tectonic rotation of Upper Precambrian and Lower Paleozoic strata from the Desert Range, southern Nevada

In the Desert Range of southern Nevada, miogeoclinal sedimentary rocks, mostly shallow‐water limestones of latest Precambrian through Early Ordovician age, yield three components of magnetization having different blocking temperature spectra: (1) a low blocking temperature component near the direction of the present axial‐dipole field; (2) an intermediate blocking temperature component with northerly declination and inclination ∼+60°; and (3) a characteristic component with southeasterly declination and inclination ∼−20°. Combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite. The intermediate magnetization probably reflects a viscous partial thermoremanent magnetization (VPTRM) imposed between the Late Cretaceous and mid‐Cenozoic; it was probably acquired when the strata were dipping slightly eastward. The characteristic magnetization is probably a VPTRM imposed during regional uplift in the Late Permian. The degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the Ordovician rocks; additionally, the characteristic magnetization in those rocks is younger than bedding disruption caused by major, late stylolitization. Red‐purple mudstones from the middle member of the Wood Canyon Formation, in which a directionally similar characteristic magnetization resides in fine‐grained hematite, also appear to have been remagnetized; in these rocks, the remagnetization probably reflects partial recrystallization, as the blocking temperatures are too high to have been reset by burial heating. The sampled sections have undergone relative tectonic rotation about a vertical axis, consistent with late Tertiary oroflexural bending that had been proposed on independent geologic evidence. The characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations, as the regional geology suggests the Desert Range strata were essentially horizontal throughout the Paleozoic. The total observed relative rotation is 44°±5°, representing absolute counterclockwise rotation (27°±4°) of the northern part of the Desert Range, and absolute clockwise rotation (17°±5°( of the southern part. The only unit in the Desert Range sequence that may retain a primary magnetization is the late Precambrian Rainstorm Member of the Johnnie Formation. The characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite; after correction for 18° of counterclockwise rotation, the resulting pole (5°N, 151°E) is near other late Hadrynian poles from North America.

Magnetic properties of marine limestones

Numerous paleomagnetic investigations have been carried out on limestones, but their rock magnetic properties have often been neglected. In this review, geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized, and laboratory techniques for the identification of the magnetic mineralogy are evaluated. The conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence (NRM), probably by a postdepositional alignment process. The effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation. Other remanence components can be acquired during diagenesis, in the soft sediment right after deposition or much later even in the indurated limestone. The magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration; magnetic techniques are more convenient. Direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract. A particularly useful technique involves combined observations of isothermal remanent magnetization (IRM) acquisition in strong fields up to at least 4 T and the subsequent destruction of this IRM during stepwise or continuous thermal demagnetization. The magnetic minerals most commonly identified by these techniques are magnetite, goethite, hematite, and maghemite. These magnetic minerals occur in different combinations in the marine limestones of central Europe and the Mediterranean realm. Pyrite is common in certain limestones, where it can be the precursor of an unwelcome late goethite. Pyrrhotite may be responsible for unstable magnetization in the Swiss Helvetic limestones but otherwise is rare. Thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures (300°–400°C) due to the breakdown of a metastable mineral such as maghemite, goethite, or pyrrhotite. At high temperatures (above 500°C), pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite. This necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent (VRM) behavior of the newly formed magnetite. VRM is a widespread component of NRM in many limestones, but it can usually be removed effectively by standard magnetic cleaning techniques. The susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles. The susceptibility is anisotropic and shows a classical ‘depositional’ pattern of the distribution of principal axes. However, the probable postdepositional mechanism of NRM acquisition (especially in bioturbated sediments) implies that the anisotropy pattern results from postdepositional compaction of the sediment.