ABSTRACT To understand the content and the causes of the changes in the Earth's magnetic ... more ABSTRACT To understand the content and the causes of the changes in the Earth's magnetic field beyond the observatory records one has to rely on archeomagnetic and lake sediment paleomagnetic data. The regional archeointensity curves are often of different quality and temporally variable which hampers the global analysis of the data in terms of dipole vs non-dipole field. We have developed a novel archeointensity database application utilizing MySQL, PHP (PHP Hypertext Preprocessor), and the Generic Mapping Tools (GMT) for ranking and displaying geomagnetic intensity data from the last 12000 years. Our application has the advantage that no specific software is required to query the database and view the results. Querying the database is performed using any Web browser; a fill-out form is used to enter the site location and a minimum ranking value to select the data points to be displayed. The form also features the possibility to select plotting of the data as an archeointensity curve with error bars, and a Virtual Axial Dipole Moment (VADM) or ancient field value (Ba) curve calculated using the CALS7K model (Continuous Archaeomagnetic and Lake Sediment geomagnetic model) of (Korte and Constable, 2005). The results of a query are displayed on a Web page containing a table summarizing the query parameters, a table showing the archeointensity values satisfying the query parameters, and a plot of VADM or Ba as a function of sample age. The database consists of eight related tables. The main one, INTENSITIES, stores the 3704 archeointensity measurements collected from 159 publications as VADM (and VDM when available) and Ba values, including their standard deviations and sampling locations. It also contains the number of samples and specimens measured from each site. The REFS table stores the references to a particular study. The names, latitudes, and longitudes of the regions where the samples were collected are stored in the SITES table. The MATERIALS, METHODS, SPECIMEN_TYPES and DATING_METHODS tables store information about the sample materials, intensity determination methods, specimen types and age determination methods. The SIGMA_COUNT table is used indirectly for ranking data according to the number of samples measured and their standard deviations. Each intensity measurement is assigned a score (0--2) depending on the number of specimens measured and their standard deviations, the intensity determination method, the type of specimens measured and materials. The ranking of each data point is calculated as the sum of the four scores and varies between 0 and 8. Additionally, users can select the parameters that will be included in the ranking.
ABSTRACT Oligocene flood volcanism in Yemen was erupted from 31-26 Ma producing ca. 2-3 km of bas... more ABSTRACT Oligocene flood volcanism in Yemen was erupted from 31-26 Ma producing ca. 2-3 km of basaltic lava flows, rhyolitic ignimbrites and airfall deposits, and minor andesitic lava flows. During the autumn of 1999 we sampled altogether 104 flows, drilling 7-10 cores per flow and using both magnetic and sun compass for orientation. In the present study we will compare the magnetostratigraphy and paleomagnetic pole of Oligocene Yemen flood volcanism with data from the Ethiopian Traps on the conjugate margin. Anisotropy of magnetic susceptibility data will be combined with isotopic fingerprinting of rhyolitic pyroclastic rocks and exhumed granite centres to identify the eruptive centres of the large volumes of rhyolitic magmas produced in this large igneous province.
ABSTRACT Paleomagnetic studies of Faeroe Islands flood basalts (85 lava flows, in average 7 drill... more ABSTRACT Paleomagnetic studies of Faeroe Islands flood basalts (85 lava flows, in average 7 drill cores per flow) agree with the previously suggested C26N-C25R-C25N-C24R magnetostratigraphy. Some parts of the volcanic sequence are characterized by several consecutive flows recording the same paleomagnetic field directions. After grouping such flows into directional groups we obtain 37 independent readings of the paleomagnetic field. The corresponding paleomagnetic pole lies at 72.2\deg N, 158.2\deg E (A95=6.3\deg, K=15, N=37). The estimated paleosecular variation, ASD (angular standard deviation) 21.2\deg +4.1\deg/-2.9\deg, is similar to previously published results for the given age and paleolatitude. Following paleodirectional and rock magnetic results, 108 samples from 27 flows were chosen for Thellier paleointensity experiments. Altogether 90 samples were rejected due to either chemical alterations during laboratory heatings or typical MD behaviour evidenced by pTRM-tails. Only 4 lava flows (18 samples) yielded acceptable paleointensity estimates, with flow mean VDM's ranging from 3.5 to 7.4x1022 Am2. The average VDM, 5.7 \pm 2.0x1022 Am2, is comparable to the suggested average dipole moment for the past 160 million years (Juarez et al., 1998). Reference: Juarez et al., Nature, 394, 878-881, 1998.
ABSTRACT In central West Greenland erosion has exposed large continuous geological sections throu... more ABSTRACT In central West Greenland erosion has exposed large continuous geological sections through volcanic rocks erupted during the early phases of the evolution of the Tertiary North Atlantic Igneous Province. A major finding in a 1994 reconnaissance sampling was the 200 m thick sequence of lava flows recording the C27n-C26r polarity transition (Riisager & Abrahamsen 1999, 2000). We present here paleomagnetic data from a new detailed sampling of two separate profiles offering a uniquely detailed record of a Paleocene geomagnetic transition in lavas. Based on multi-model photogrammetry as well as chemical and lithological analysis of one of the profiles, the 59 lavaflows in the transitional zone can be grouped into 17 magmatic events (eruptions). In general there is a good agreement with magmatic events also showing grouping of the paleomagnetic directions. The C27n-C26r transitional paleomagnetic directions will be presented and the VGP path will be compared to VGP paths of more recent records. References: Riisager, P. & N. Abrahamsen, Geophys. J. Int. 137 (1999) 774-782. Riisager, P. & N. Abrahamsen, Phys. Earth Planet. Inter. 118 (2000) 53-64.
ABSTRACT We present a case-study from the West Greenland flood volcanic province (part of the Pal... more ABSTRACT We present a case-study from the West Greenland flood volcanic province (part of the Paleogene North Atlantic igneous province) of combined paleomagnetic and multi-model stereo photogrammetry analyse. Our study is the first, in which photogrammetry profiles were photographed at the exact same location where paleomagnetic fieldwork was carried out, and a direct flow-to-flow comparison of the two datasets is possible. Photogrammetry has two major applications for correctly interpreting paleomagnetic datasets: (i) Field observations can be made in the laboratory with the possibility to zoom in and out on any part of the studied profile. This allows us to study both large-scale geological features (faults, unconformities etc.), which are not always easily recognized while working on the profiles, as well as finer details (soil horizons, variations in lava thickness, colour and morphology), which may have gone unnoticed during fieldwork. Most importantly photogrammetric field observations can be made in the laboratory at a point in time when paleomagnetic analyses have been completed. We demonstrate how photogrammetry aids us to correctly interpret the paleosecular variation record. (ii) Photogrammetry allows the exact position of any point on the profile to be determined in three-dimensional space with a precision better than ˜0.5 meter. By digitising ˜100 points along the best-defined and most easily traced flow boundaries over their entire length (typically ˜1 km), we can determine the attitude of strata with a precision better than ˜0.5o. Such highly precise dip/strike measurements are crucial for tectonic correction of sampled sequences. A comparison of photogrammetry and in-field dip/strike measurements suggests that although the in-field readings are taken at the most suitable and well-exposed locations, they are typically 5-6 o wrong.
The Ontong Java Plateau (OJP), a Large Igneous Province in the western Pacific, was thought to be... more The Ontong Java Plateau (OJP), a Large Igneous Province in the western Pacific, was thought to be formed by the starting plume head of the Louisville hotspot. The few existing preliminary paleomagnetic data from basement and basal sediments from previous OJP drilling Sites 289 and 807, however, suggests that the plateau was formed ~10° north of this hotspot's current location.
We present paleomagnetic data from basaltic pillow and lava flows drilled at four Ocean Drilling ... more We present paleomagnetic data from basaltic pillow and lava flows drilled at four Ocean Drilling Program (ODP) Leg 192 sites through the Early Cretaceous (~120 Ma) Ontong Java Plateau (OJP). Altogether 270 samples (out of 331) yielded well-defined characteristic remanent magnetization components all of which have negative inclinations, i.e. normal polarity. Dividing data into inclination groups we obtain 5, 7, 14 and 15 independent inclination estimates for the four sites. Statistical analysis suggests that paleosecular variation has been sufficiently sampled and site-mean inclinations therefore represent time-averaged fields. Of particular importance is the finding that all four site-mean inclinations are statistically indistinguishable, strongly supporting indirect seismic observation from the flat-lying sediments blanketing the OJP that the studied basalts have suffered little or no tectonic disturbance since their emplacement. Moreover, the corresponding paleomagnetic paleolatitudes agree excellently with paleomagnetic data from a previous ODP site (Site 807) drilled into the northern portion of the OJP. Two important conclusions can be drawn based on the presented dataset: (i) the Leg 192 combined mean inclination (Inc.=-41.4°, N=41, kappa= 66.0, alpha95 =2.6°) is inconsistent with the Early Cretaceous part of the Pacific apparent polar wander path, indicating that previous paleomagnetic poles derived mainly from seamount magnetic anomaly modeling must be used with care; (ii) the Leg 192 paleomagnetic paleolatitude for the central OJP is ~20° north of the paleogeographic location calculated from Pacific hotspot tracks assuming the hotspots have remained fixed. The difference between paleomagnetic and hotspot calculated paleolatitudes cannot be explained by true polar wander estimates derived from other lithospheric plates and our results are therefore consistent with and extend recent paleomagnetic studies of younger hotspot features in the northern Pacific Ocean that suggest Late Cretaceous to Eocene motion [...]
A combined paleomagnetic and 40Ar/39Ar study was carried out along eight stratigraphically overla... more A combined paleomagnetic and 40Ar/39Ar study was carried out along eight stratigraphically overlapping sections in the Oligocene Afro-Arabian flood volcanic province in Yemen (73 sites). The composite section covers the entire volcanic stratigraphy in the sampling region and represents five polarity zones that are correlated to the geomagnetic polarity time scale based on 40Ar/39Ar ages from this and previous studies.
ABSTRACT To understand the content and the causes of the changes in the Earth's magnetic ... more ABSTRACT To understand the content and the causes of the changes in the Earth's magnetic field beyond the observatory records one has to rely on archeomagnetic and lake sediment paleomagnetic data. The regional archeointensity curves are often of different quality and temporally variable which hampers the global analysis of the data in terms of dipole vs non-dipole field. We have developed a novel archeointensity database application utilizing MySQL, PHP (PHP Hypertext Preprocessor), and the Generic Mapping Tools (GMT) for ranking and displaying geomagnetic intensity data from the last 12000 years. Our application has the advantage that no specific software is required to query the database and view the results. Querying the database is performed using any Web browser; a fill-out form is used to enter the site location and a minimum ranking value to select the data points to be displayed. The form also features the possibility to select plotting of the data as an archeointensity curve with error bars, and a Virtual Axial Dipole Moment (VADM) or ancient field value (Ba) curve calculated using the CALS7K model (Continuous Archaeomagnetic and Lake Sediment geomagnetic model) of (Korte and Constable, 2005). The results of a query are displayed on a Web page containing a table summarizing the query parameters, a table showing the archeointensity values satisfying the query parameters, and a plot of VADM or Ba as a function of sample age. The database consists of eight related tables. The main one, INTENSITIES, stores the 3704 archeointensity measurements collected from 159 publications as VADM (and VDM when available) and Ba values, including their standard deviations and sampling locations. It also contains the number of samples and specimens measured from each site. The REFS table stores the references to a particular study. The names, latitudes, and longitudes of the regions where the samples were collected are stored in the SITES table. The MATERIALS, METHODS, SPECIMEN_TYPES and DATING_METHODS tables store information about the sample materials, intensity determination methods, specimen types and age determination methods. The SIGMA_COUNT table is used indirectly for ranking data according to the number of samples measured and their standard deviations. Each intensity measurement is assigned a score (0--2) depending on the number of specimens measured and their standard deviations, the intensity determination method, the type of specimens measured and materials. The ranking of each data point is calculated as the sum of the four scores and varies between 0 and 8. Additionally, users can select the parameters that will be included in the ranking.
ABSTRACT Oligocene flood volcanism in Yemen was erupted from 31-26 Ma producing ca. 2-3 km of bas... more ABSTRACT Oligocene flood volcanism in Yemen was erupted from 31-26 Ma producing ca. 2-3 km of basaltic lava flows, rhyolitic ignimbrites and airfall deposits, and minor andesitic lava flows. During the autumn of 1999 we sampled altogether 104 flows, drilling 7-10 cores per flow and using both magnetic and sun compass for orientation. In the present study we will compare the magnetostratigraphy and paleomagnetic pole of Oligocene Yemen flood volcanism with data from the Ethiopian Traps on the conjugate margin. Anisotropy of magnetic susceptibility data will be combined with isotopic fingerprinting of rhyolitic pyroclastic rocks and exhumed granite centres to identify the eruptive centres of the large volumes of rhyolitic magmas produced in this large igneous province.
ABSTRACT Paleomagnetic studies of Faeroe Islands flood basalts (85 lava flows, in average 7 drill... more ABSTRACT Paleomagnetic studies of Faeroe Islands flood basalts (85 lava flows, in average 7 drill cores per flow) agree with the previously suggested C26N-C25R-C25N-C24R magnetostratigraphy. Some parts of the volcanic sequence are characterized by several consecutive flows recording the same paleomagnetic field directions. After grouping such flows into directional groups we obtain 37 independent readings of the paleomagnetic field. The corresponding paleomagnetic pole lies at 72.2\deg N, 158.2\deg E (A95=6.3\deg, K=15, N=37). The estimated paleosecular variation, ASD (angular standard deviation) 21.2\deg +4.1\deg/-2.9\deg, is similar to previously published results for the given age and paleolatitude. Following paleodirectional and rock magnetic results, 108 samples from 27 flows were chosen for Thellier paleointensity experiments. Altogether 90 samples were rejected due to either chemical alterations during laboratory heatings or typical MD behaviour evidenced by pTRM-tails. Only 4 lava flows (18 samples) yielded acceptable paleointensity estimates, with flow mean VDM's ranging from 3.5 to 7.4x1022 Am2. The average VDM, 5.7 \pm 2.0x1022 Am2, is comparable to the suggested average dipole moment for the past 160 million years (Juarez et al., 1998). Reference: Juarez et al., Nature, 394, 878-881, 1998.
ABSTRACT In central West Greenland erosion has exposed large continuous geological sections throu... more ABSTRACT In central West Greenland erosion has exposed large continuous geological sections through volcanic rocks erupted during the early phases of the evolution of the Tertiary North Atlantic Igneous Province. A major finding in a 1994 reconnaissance sampling was the 200 m thick sequence of lava flows recording the C27n-C26r polarity transition (Riisager & Abrahamsen 1999, 2000). We present here paleomagnetic data from a new detailed sampling of two separate profiles offering a uniquely detailed record of a Paleocene geomagnetic transition in lavas. Based on multi-model photogrammetry as well as chemical and lithological analysis of one of the profiles, the 59 lavaflows in the transitional zone can be grouped into 17 magmatic events (eruptions). In general there is a good agreement with magmatic events also showing grouping of the paleomagnetic directions. The C27n-C26r transitional paleomagnetic directions will be presented and the VGP path will be compared to VGP paths of more recent records. References: Riisager, P. & N. Abrahamsen, Geophys. J. Int. 137 (1999) 774-782. Riisager, P. & N. Abrahamsen, Phys. Earth Planet. Inter. 118 (2000) 53-64.
ABSTRACT We present a case-study from the West Greenland flood volcanic province (part of the Pal... more ABSTRACT We present a case-study from the West Greenland flood volcanic province (part of the Paleogene North Atlantic igneous province) of combined paleomagnetic and multi-model stereo photogrammetry analyse. Our study is the first, in which photogrammetry profiles were photographed at the exact same location where paleomagnetic fieldwork was carried out, and a direct flow-to-flow comparison of the two datasets is possible. Photogrammetry has two major applications for correctly interpreting paleomagnetic datasets: (i) Field observations can be made in the laboratory with the possibility to zoom in and out on any part of the studied profile. This allows us to study both large-scale geological features (faults, unconformities etc.), which are not always easily recognized while working on the profiles, as well as finer details (soil horizons, variations in lava thickness, colour and morphology), which may have gone unnoticed during fieldwork. Most importantly photogrammetric field observations can be made in the laboratory at a point in time when paleomagnetic analyses have been completed. We demonstrate how photogrammetry aids us to correctly interpret the paleosecular variation record. (ii) Photogrammetry allows the exact position of any point on the profile to be determined in three-dimensional space with a precision better than ˜0.5 meter. By digitising ˜100 points along the best-defined and most easily traced flow boundaries over their entire length (typically ˜1 km), we can determine the attitude of strata with a precision better than ˜0.5o. Such highly precise dip/strike measurements are crucial for tectonic correction of sampled sequences. A comparison of photogrammetry and in-field dip/strike measurements suggests that although the in-field readings are taken at the most suitable and well-exposed locations, they are typically 5-6 o wrong.
The Ontong Java Plateau (OJP), a Large Igneous Province in the western Pacific, was thought to be... more The Ontong Java Plateau (OJP), a Large Igneous Province in the western Pacific, was thought to be formed by the starting plume head of the Louisville hotspot. The few existing preliminary paleomagnetic data from basement and basal sediments from previous OJP drilling Sites 289 and 807, however, suggests that the plateau was formed ~10° north of this hotspot's current location.
We present paleomagnetic data from basaltic pillow and lava flows drilled at four Ocean Drilling ... more We present paleomagnetic data from basaltic pillow and lava flows drilled at four Ocean Drilling Program (ODP) Leg 192 sites through the Early Cretaceous (~120 Ma) Ontong Java Plateau (OJP). Altogether 270 samples (out of 331) yielded well-defined characteristic remanent magnetization components all of which have negative inclinations, i.e. normal polarity. Dividing data into inclination groups we obtain 5, 7, 14 and 15 independent inclination estimates for the four sites. Statistical analysis suggests that paleosecular variation has been sufficiently sampled and site-mean inclinations therefore represent time-averaged fields. Of particular importance is the finding that all four site-mean inclinations are statistically indistinguishable, strongly supporting indirect seismic observation from the flat-lying sediments blanketing the OJP that the studied basalts have suffered little or no tectonic disturbance since their emplacement. Moreover, the corresponding paleomagnetic paleolatitudes agree excellently with paleomagnetic data from a previous ODP site (Site 807) drilled into the northern portion of the OJP. Two important conclusions can be drawn based on the presented dataset: (i) the Leg 192 combined mean inclination (Inc.=-41.4°, N=41, kappa= 66.0, alpha95 =2.6°) is inconsistent with the Early Cretaceous part of the Pacific apparent polar wander path, indicating that previous paleomagnetic poles derived mainly from seamount magnetic anomaly modeling must be used with care; (ii) the Leg 192 paleomagnetic paleolatitude for the central OJP is ~20° north of the paleogeographic location calculated from Pacific hotspot tracks assuming the hotspots have remained fixed. The difference between paleomagnetic and hotspot calculated paleolatitudes cannot be explained by true polar wander estimates derived from other lithospheric plates and our results are therefore consistent with and extend recent paleomagnetic studies of younger hotspot features in the northern Pacific Ocean that suggest Late Cretaceous to Eocene motion [...]
A combined paleomagnetic and 40Ar/39Ar study was carried out along eight stratigraphically overla... more A combined paleomagnetic and 40Ar/39Ar study was carried out along eight stratigraphically overlapping sections in the Oligocene Afro-Arabian flood volcanic province in Yemen (73 sites). The composite section covers the entire volcanic stratigraphy in the sampling region and represents five polarity zones that are correlated to the geomagnetic polarity time scale based on 40Ar/39Ar ages from this and previous studies.
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