Structure of Charnockitic basement in a part of the Krishna-Godavari basin, Andhra Pradesh, India
Magnetic Properties of Deep Crustal Rocks From the Athabasca Granulite Terrane: Western Canadian Shield
Collecting magnetic data on deep crustal rocks is required for modeling and understanding the nature of long wavelength anomalies and aiding the interpretation of recently discovered large-scale Martian magnetic anomalies, in addition to preparing for upcoming international satellite missions. We present results from an Earth analog focused on the magnetic properties of deep crustal rocks from the Athabasca granulite terrane. The exposure represents >20,000 km2 of exhumed continental lower crust in the western Canadian Shield. A detailed aeromagnetic map (200 m resolution) publicly available from the Geological Survey of Canada guided the study of the relationship between aeromagnetic anomalies and the magnetic properties of 13 rock samples. Samples include granulite facies orthogneiss and schist, as well as granite and mafic dikes. Each sample had 1-3 cores drilled and oriented in a sandbox to within 10 degrees. Data were collected on the cores for susceptibility, density, and natural remanent magnetization (NRM), and Koenigsberger ratios (Q) were calculated for each sample. The Koenigsberger ratio (Q) is a measure of remanent magnetization versus induced magnetization and it provides information on the source of the observed anomalies. Susceptibilities ranged between 1.33x10-4 and 1.04 SI units, and NRM ranged between 1.11x10-2 and 38.8 A/m, while Q values were between 1.85x10-2 and 3.21 with 5 samples having Q values greater than one. A range of magnetic properties over several orders of magnitude is surprising for an area with large magnetic anomalies. Based on the Q values, some of the sampled units are dominated by induced anomalies (magnetic highs on the anomaly map) while others have a remanent component influencing the observed anomalies, particularly in the case of negative anomalies. Hysteresis properties along with microscope work indicate a range of oxide phases are present; on-going work is investigating which are the primary carriers of the induced and remanent magnetizations.
New geophysical views of Mt.Melbourne Volcano (East Antarctica)
Mt. Melbourne volcano is located along the transition between the Transantarctic Mountains and the West Antarctic Rift System. Recent volcanic activity is suggested by the occurrence of blankets of pyroclastic pumice and scoria fall around the eastern and southern flanks of Mt Melbourne and by pyroclastic layers interbedded with the summit snows. Geothermal activity in the crater area of Mount Melbourne may be linked to the intrusion of dykes within the last 200 years. Geophysical networks suggest that Mount Melbourne is a quiescent volcano, possibly characterised by slow internal dynamics. During the 2002-2003 Italian Antarctic campaign a high-resolution aeromagnetic survey was performed within the TIMM (Tectonics and Interior of Mt. Melbourne area) project. This helicopter-borne survey was flown at low-altitude and in drape-mode configuration (305 m above terrain) with a line separation less than 500 m. Our new high-resolution magnetic maps reveal the largely ice-covered magmatic and tectonic patters in the Mt. Melbourne volcano area. Additionally, in the frame of the UK-Italian ISODYN-WISE project (2005-06), an airborne ice-sounding radar survey was flown. We combine the sub-ice topography with images and models of the interior of Mt. Melbourne volcano, as derived from the high resolution aeromagnetic data and land gravity data. Our new geophysical maps and models also provide a new tool to study the regional setting of the volcano. In particular we re-assess whether there is geophysical evidence for coupling between strike-slip faulting, the Terror Rift, and Mount Melbourne volcano.
Magnetic Properties of Dropstones Along the East Antarctic Margin: Towards a Better Understanding of the Carriers of Antarctic Crustal Magnetic Anomalies
We present results of the magnetic properties of Antarctic dropstones collected during United States Antarctic Program cruise 01-01 of the Research Vessel Nathaniel B. Palmer. NBP01-01 sampled along the East Antarctic Margin and visited The George V Coast, Terre Adélie, Prydyz Bay, and Mac.Robertson Land. Samples were taken using jumbo piston coring and dredging techniques. Dropstones taken from cores and dredges were used as representative samples of Antarctic crust and studied for a better understanding of the carriers of Antarctic crustal magnetic anomalies. Thin sections were used to group dropstones according to chemical composition and metamorphic grade. Rocks recovered included; mafic gabbros, dolerite, and basalts, intermediate andesite, diorite, and granodiorites, a variety of felsic granitoids, low-grade metamorphics including slates and phyllites, and high-grade granitic gneisses. Mass susceptibility and natural remnant magnetization (NRM) were measured to understand each rock type's potential contributions to magnetic anomalies. Induced and remnant magnetizations were used to calculate the Koenigsberger ratio, Q = Total Remnant Magnetization / Total Induced Magnetization. The Q values of mafic igneous rocks were dominated by the NRM, with a mean Q value of 33.1. However, plutonic mafic rocks show a mean Q value of 0.089. Intermediate and felsic rocks display a wide range of Q values (0.09-3.79) with rocks showing both remnant and induced dominated components. Low-grade Metamorphic rocks showed high remanence components and a mean Q value of 7.62. High-grade metamorphic rocks have mean Q value of 8.08 and those with a granitic protolith display higher Q values up to 10.1. Work in progress includes hysteresis parameters to assess magnetic domain state and Curie temperature analysis.
Volcanic Structure of the Basaltic Shield Volcano of Socorro Island, Mathematician Ridge, Pacific Plate
Socorro Island is a pantellerite and perialkaline shield volcano built at the northern segment of the Mathematician ridge in the Pacific plate. Active spreading along the ridge terminated following major plate boundary re-organization and ridge jumping eastwards to the East Pacific Rise. Cessation of spreading and ridge jumping are recorded in the marine magnetic anomaly pattern that preserve a record of the plate re- organization and transform faulting. Socorro is the largest of the group of oceanic islands located at the intersection with the Clarion fracture zone built by intense activity which continues to present. The shallow structure and stratigraphy of Socorro Island are here investigated from magnetic surveys conducted in the southern sector between the coast and the Evermann shield volcano. Profiles cross the volcano summit and collapsed caldera rim and the active zone of Lomas Coloradas with cinder cones, domes and lava flows. Forward and inversion 2-D models show a shallow structure formed by trachite flows and the thick basaltic flows emplaced over the local andesitic basement which lies below present sea level. Residual anomalies mark the collapsed partial caldera rim and shallow structures in the Lomas Coloradas formation. Laboratory measurements of magnetic properties carried out on samples collected in the zone are used to infer contrasts in physical properties for the modeling. Magnetic susceptibility and induced magnetizations have been considered for the initial models. Data on remanent magnetization intensity and directions are available for the surface exposed lithologies, which show a relatively large range in declination and inclination for the remanent directions.
High-Resolution Magnetostratigraphic Analysis of the Denver Basin Kiowa Core; Testing Paleocene Astrochronologies
Marine records of orbitally driven climate cycles have been used to create an astronomically calibrated timescale for the Neogene Period (0-23 Ma). Recent studies have attempted to extend this approach to the Paleogene Period (23-65 Ma), despite various uncertainties related to astronomical solutions this far back, climate sensitivity in a greenhouse world, and stratigraphic completeness. We seek to test published Paleocene astrochronologies by comparing the durations of magnetic polarity chrons as determined by cyclostratigraphy to their durations as determined by U/Pb geochronology. To complete this test, the Late Cretaceous to Paleocene Kiowa Core of the Denver Basin was analyzed. This core was chosen as it is from a terrestrial basin with high sediment accumulation rates, contains abundant layers of volcanic ash, and records a reliable magnetostratigraphy. Twenty-three paleomagnetic samples were collected from the core to improve the stratigraphic resolution of magnetic reversals that were coarsely constrained by earlier research. Once the depths of the geomagnetic reversals were precisely determined, ash layers that best constrained these reversals were selected for U/Pb geochronology. Durations of periods of normal and reverse magnetization were estimated based on these radiometrically dated ashes. Initial results indicate that the interval between the Chron C28n/C28r reversal and the K/T Boundary is ca 1.27 million years in duration. This is within error of previously published astrochronologically determined durations for the same time interval. Although additional independent comparisons are needed for a more complete test, our initial results indicate a remarkable consistency between these different geochronometers.
High-Resolution Magnetostratigraphy of Core MD99-2236 (Cartwright Saddle, Offshore Labrador) Since the Last Deglaciation
The magnetic properties of a long and well-dated (18 AMS 14C dates) piston core (MD99-2236) sampled offshore Labrador (Cartwright Saddle) on board the R/V Marion Dufresne II in 1999 were analyzed at high- resolution in order to reconstruct the geomagnetic field behavior and to characterize several detrital carbonate events. The Natural, Anhysteretic, Isothermal and Saturated Isothemal Remanent Magnetizations were studied by progressives stepwise alternating field (AF) demagnetization at 1 cm intervals on u-channel samples using a cryogenic magnetometer at the Institut des sciences de la mer de Rimouski (ISMER). Hysteresis curves were also determined with an alternating gradient force magnetometer in order to characterize the magnetic mineralogy and grain size. The results indicate that the NRM is characterized by a strong, stable and single component magnetization most likely carried low coercivity minerals such as magnetite in the pseudo-single domain grain size range. The component inclination and declination were calculated by principal component analysis and exhibit maximum angular deviation values generally lower than 5°, highlighting well-defined directional data, except for some specific intervals characterized by higher carbonate content and multi-domain grains. Moreover, the component inclinations vary around the expected inclination (70.3°) for the latitude of the coring site based on a geocentric axial dipole (GAD) model. Component inclination and declination are presented and compared with other high-resolution paleomagnetic records from Eastern Canada, Iceland (MD99-2322 and -2269) and Europe. This comparison reveals that the sediments recorded coherent paleomagnetic secular variations (PSV), especially from 8 to 14 ka, where sedimentation rates are higher, the data of high quality and where previously published high-resolution RPI and PSV records for this period are rare. In addition, relative paleointensity (RPI) proxies were constructed by normalizing the NRM by the ARM. The quality of these proxies will be assessed and comparison with previously published RPI records will be presented. Finally, the ongoing rock-magnetic measurements performed in the detrital carbonate layers will be discussed.