GP33D-1598
Mesozoic Kinematic Evolution of the Central Atlantic Inferred From Regional Magnetic Anomalies
The magnetic anomaly signature of Central Atlantic margins is well defined by the gridded data published by Verhoef et al. (1996) on the North American region. However, a gridded dataset for the West African margin (South of the Canary Islands) was lacking. We have used magnetic data from the Geodas database, an Ifremer dataset and personnal communication from H.A. Roeser and W.J.M. Van der Linden in order to produce a gridded magnetic data of the Dakhla margin and to better constrain the kinematics of Central Atlantic early opening. Our model adopts the breakup timing of 195 Ma as proposed by Sahabi et al. (2004) - 20 myr earlier than what was generally proposed in previous models. According to our interpretation of the newly compiled magnetic data, the early opening of Central Atlantic was characterized by three distinct phases. In contrast to other models, we propose that for the first 30 myr (195-165 Ma, Lias-Dogger) the oceanic accretion was extremely slow (~0.8 cm/y). At the Blake Spur time, (around 165 Ma, Callovian basis), a drastic change occurred, both in the relative plate motions (initially NNW-SSE, it becomes NO-SE) and spreading rate (that increases up to ~ 4.8 cm/y). The BSMA (Blake Spur Magnetic Anomaly) is related to a great basement topographic change. From magnetic chron M22 (150 Ma, Tithonian basis) onwards, the spreading rate slowed down to about 2.6 cm/y and remained constant until magnetic chron M0 (125 Ma, Barremian-Aptian limit).
GP33D-1599
Sixth generation lithospheric magnetic field model, MF6, from CHAMP satellite magnetic measurements
The CHAMP satellite continues to provide highly accurate magnetic field measurements with decreasing orbital altitudes (<350km) at solar minimum conditions. A promising new CHAMP data product has become available, which provides the total field with one order of magnitude smaller noise amplitudes. The product is inferred from suitably merged Fluxgate and Overhauser magnetometer data. While the low-noise Fluxgate measurements are used in the short-period range (<900sec, or <6000km wavelength), we take advantage of the high stability provided by the Overhauser for the longer periods. The new data set is used for generating an improved lithospheric magnetic field model (MF6). Although MF6 is still in production at the time of writing this abstract, we anticipate significant benefits in terms of resolving small- scale low-amplitude crustal features from the new data. Further improvements include a new correction for steady ocean circulation and an expansion to higher spherical harmonic degrees of the model. http://geomag.org
GP33D-1600
SECULAR VARIATION OVER EUROPE:A NEW VIEW
The purpose of modern regional modelling is to describe the geomagnetic field over a restricted region of the Earth, providing a better spatial resolution of the local field for areas of high data density. One currently available method to obtain continuous regional descriptions of main field and secular variation is regularised Spherical Cap Harmonic Analyses, SCHA. The aim of the present work was to model the secular variation field over Europe, the region of highest spatial- temporal coverage with high-quality ground data, by means of SCHA for the past 40 years. In that attempt we mainly encountered two major difficulties: i) data are not representing the magnetic field aimed to be modeled and ii) known drawbacks of the modeling technique. Firstly, we applied an empirical procedure to minimise the external field contributions to the geomagnetic observatory annual means, as proposed by Verbanac et al., 2007. Further, we applied SCHA with physical regularization and adequate selection of model parametrisation to corrected annual means at 46 European observatory locations and additionally 11 'virtual observatories' chosen to improve the initial data distribution. The quality of the preferred model was verified by considering different criteria: comparisons of time series of model predictions and data and comparisons to a global model. The comparison of the overall rms misfit to all input data from our regional and global model suggests that our new model better describes the secular variation field and gives an improved fit to the features of the data that we assumed to be real internal secular variation. The present study is the first step in obtaining a detailed secular variation behaviour based on European observatory data.
GP33D-1601
Crustal Magnetization of Continental Shields and Surrounding Geological Provinces
WDMAM 2007 (World Digital Magnetic Anomaly Map) presents global distribution of magnetic lithosphere anomalies for the first time. Magnetization interpreted from these anomalies is compared between Archaean, Proterozoic and Phanerozoic continental provinces globally, and correlated with petrophysically-determined magnetizations of major regional data sets. http://projects.gtk.fi/WDMAM/
GP33D-1602
On the Comparison Between SCHA and R-SCHA Regional Modelling Techniques When Inverting Multilevel Magnetic Data Sets
Spherical Cap Harmonic Analysis (SCHA) has become a common tool for the regional modelling of potential fields since its introduction twenty years ago. The fact that the solutions provided by the technique satisfy Laplace equation, and the possibility of representing high-frequency fields with a small number of coefficients (compared to the global Spherical Harmonic Analysis) made of SCHA the preferred choice for the development, for example, of magnetic field models at national scale. However, Thebault et al. (2006) demonstrated that the traditional SCHA introduced by Haines (1985) presented some deficiencies, in particular related to the inversion of multilevel data sets. The authors presented the R-SCHA technique as an alternative method in which the introduction of a new set of basis functions and boundary conditions solved this issue. Perhaps due to the short lapse of time since the presentation of R-SCHA, the scientific community continues however to apply the classical SCHA approach when developing regional models using a blend of satellite and ground data. In this paper we present some comparisons between the SCHA and R-SCHA techniques applied to synthetic data computed using the shorter wavelength part of MF5 model at 0km, 400km, and 700km altitude, and also to a subset of the near-surface crustal field grid from the World Digital Magnetic Anomaly Map Project (WDMAM). The different tests show how R-SCHA is able to produce a consistent set of gauss coefficients avoiding the problems related to the traditional SCHA inverse problem, and how the R-SCHA model fits the radial variation of the field in a realistic way. These results should help the scientific community to evaluate the level of approximation accepted for the development of regional magnetic field models in the era of the Swarm multi- satellite mission.
GP33D-1603
Mapping dikes and faults in the Oregon forearc using high-resolution aeromagnetic data
The diversity of volcanic rocks in the Cascadia forearc is well displayed in high-resolution aeromagnetic data recently acquired over the Oregon Coast Range. The aeromagnetic survey extends from the Western Cascade Range to the continental shelf including the cities of Eugene and Florence, Oregon. Magnetic anomalies illuminate various geologic domains: (1) Numerous linear west-northwest-striking magnetic anomalies correspond in some places with mapped Oligocene and Eocene dikes and sills that intruded middle Eocene marine sedimentary rocks of the Tyee Formation. This pervasive linear pattern suggests that many similar dikes are concealed at shallow depth. (2) Arcuate, long-wavelength anomalies indicate folds within Eocene Siletz River Volcanics (SRV) lying beneath the Tyee Formation. (3) A pattern of short-wavelength anomalies is caused by Oligocene volcanic rocks of the Western Cascade Range. Superimposed on these anomaly patterns are linear, northeast-trending lineaments caused by various faults, including the Corvallis fault that juxtaposes SRV against Tyee Formation. To help differentiate these various magnetic signals, we have implemented a three-stage analysis. First, matched filters were designed to emphasize shallow magnetic sources. Second, a tilt-derivative (TDR) filter was applied to the shallow-source anomalies to distinguish noise from signal and map geologic contacts. Finally, an Euler analysis was applied using various structural indices to distinguish between dike and fault anomalies. This approach allows us to map both dikes and faults in detail, which is important since exposures are largely concealed in the heavily forested Coast Range. Modeling experiments indicate that we can detect dikes as thin as 25-75 m if they are separated by 1.5-2.5 km or more. In one case, a pair of WNW-striking magnetic anomalies overlies two separate mapped dikes, each about 25-75 m in thickness, 15 km long, and separated by 1.5 km. The magnetic doublet indicates that these two parallel dikes extend at least ~40-50 km and are offset approximately 1.5-2.0 km in two places by NNE-striking faults. We can resolve at least 40 sub- parallel, WNW-striking magnetic anomalies throughout the aeromagnetic survey. Assuming all of the WNW- striking linear anomalies are caused by dikes, and ignoring the possible presence of dikes thinner than our limits of resolution, we estimate that this episode of dike injection was accompanied by about 1 percent crustal extension. Taking into account 50° of clockwise rotation, the azimuth of maximum horizontal extension was 330°, approximately parallel to the Eocene convergent plate boundary.
GP33D-1604
Rock Magnetic and Geologic Characteristics of Faulted Sediments With Associated Aeromagnetic Anomalies in the Albuquerque Basin, Rio Grande Rift, New Mexico
The rock magnetic and geologic characteristics of basin sediments that generate aeromagnetic anomalies are little studied. Variations in rock magnetic properties are responsible for the many linear, short-wavelength, low- amplitude magnetic anomalies that are spatially associated with faults cutting Neogene basin sediments in the Rio Grande rift, including the San Ysidro normal fault that is well exposed in the northern part of the Albuquerque Basin. Magnetic susceptibility (MS) values from 310 sites distributed through a 1200-m-thick composite section of rift-filling sediments of Santa Fe Group and pre-rift sedimentary rocks juxtaposed by the San Ysidro fault have lognormal distributions with well-defined means that generally increase up section through eight map units: from 1.7 to 2.2E-4 in the pre-rift Cretaceous and Eocene rocks, from 9.9E-4 to 1.2E-3 in three members of the Miocene Zia Formation of the Santa Fe Group, and from 1.5E-3 to 3.5E-3 in three members of the Miocene-Pleistocene Arroyo Ojito Formation of the Santa Fe Group. Natural remanent magnetization measurements from oriented Santa Fe Group samples indicate Koenigsberger ratios are less than 0.3. Rock magnetic parameters (e.g., ARM/MS and S ratios) and petrography indicate that the amount of detrital magnetite and its variable oxidation to maghemite and hematite are the predominant controls of magnetic property variations within the Santa Fe Group sediments. Magnetite is present in rounded detrital grains that in reflected-light petrography include both homogeneous and subdivided types, indicating likely plutonic and volcanic provenances, respectively. Santa Fe Group sediments with highest magnetic susceptibility have greatest magnetic-grain size as indicated by lowest ARM/MS ratios. Magnetic susceptibility increases progressively with sediment grain size to pebbly sand within the Arroyo Ojito Formation (deposited in fluvial environments) but within the Zia Formation (deposited in mostly eolian environments) reaches highest values in fine to medium sands. Partial oxidation of detrital magnetite, decreasing MS, is spatially associated with calcite cementation in the Santa Fe Group; both oxidation and cementation probably reflect past flow of ground water through permeable horizons. Forward magnetic models of geologic cross sections that incorporate mean magnetic susceptibilities for the different stratigraphic units successfully mimic the aeromagnetic profiles across the San Ysidro fault. These models demonstrate that the stratigraphic juxtaposition of units having maximum magnetic contrast changes with different exposure levels into the fault. This study highlights several geologic factors such as sediment provenance, depositional facies, as well as post-depositional preservation and alteration of magnetic minerals as responsible for producing aeromagnetic anomalies in faulted basin sediments.