Topographic Effects on Magnetic Data and Their Influence on Semi-Automatic Interpretation Methods: a Cross-Correlation Approach
Most of the recent advances in magnetic surveying have focused on achieving higher levels of instrument sensitivity, and/or better definition of the morphology of the magnetic field through the use of measured magnetic field gradients. Semi-automatic interpretation routines are usually applied to the acquired magnetic data under the assumption that the observed magnetic dataset provides an unbiased representation of the magnetic mineral variations in the surface and subsurface geology. However, topographic effects on magnetic data are normally neglected. A common misconception is that magnetic data acquired (or transformed to) a surface that is parallel to the ground has no topographic effects on it. That is normally true when the observed magnetic anomalies are greater than 5000 nT and the topography is relatively flat; however topographic variations greater than 100 m can induce magnetic anomalies in the ±100 nT range. In this kind of situation any interpretation routine applied to the data will be biased by topography and therefore will fail in understanding the true nature of the subsurface geology. This work shows a cross correlation analysis between topography and the measured magnetic data, as a guideline to determine the areas where a magnetic terrain correction needs to be applied, prior to any subsequent modeling/interpretation routine. Calculations done on synthetic data show that in case of vertical total magnetization (induced plus remanent fields), there is a maximum correlation between topography and the observed magnetic field, on those areas of larger topographic changes. The topographic magnetic effect is then removed on the selected areas by means of computing the magnetic signature of a 3D body of uniform magnetic susceptibility, and limited by a digital elevation model of the area (top), and a flat surface at the bottom. Comparative results of using standard total magnetic intensity data and topography-corrected data for Euler Deconvolution and Tilt-depth are shown with synthetic data and two cases from the Bathurst Mining Camp (New Brunswick, Canada) and Baie Verte Peninsula (Newfoundland, Canada).
Connecting Crustal Faults and Tectonics from Puget Sound across the Cascade Range to the Yakima Fold and Thrust Belt, Washington: Evidence from New High-Resolution Aeromagnetic Data
A series of regional-scale faults in the Cascadia forearc, including the Tacoma, Seattle, and southern Whidbey Island faults, cuts the Puget Sound lowland in response to 6 mm/yr of north-south compression. The Cascadia backarc of Washington, on the other hand, currently migrates <3 mm/yr northeastward relative to stable North America (McCaffrey et al., 2007). The tectonic connection between these two regions has remained problematic in part because of the lack of clearly mapped faults across the Cascade Range. A new, high-resolution aeromagnetic survey of a portion of the Cascade Range and the Yakima fold and thrust belt offers a view of the tectonic connection between the forearc and backarc. The aeromagnetic survey was acquired in 2008 and includes the cities of Yakima and Ellensburg, Washington. Total-field intensity was measured at a nominal altitude of 250 m above terrain, along flight lines spaced 400 m apart, and along tie lines spaced 4 km apart. Upper crustal rocks in this region have diverse magnetic properties, ranging from highly magnetic rocks of the Miocene Columbia River Basalt Group (CRBG), with both normal and reverse polarities, to weakly magnetic sedimentary rocks of the Miocene Ellensburg Formation. These distinctive magnetic properties permit the mapping of important lithologies from their exposures to covered areas. High- gradient aeromagnetic lineaments correspond with mapped faults and folds of the CRBG and indicate where these structures extend beyond surface exposures. For example, a two-dimensional model of the northwest- striking Umtanum Ridge fault zone, based on the new aeromagnetic data and constrained by geologic mapping, consists of three thrust faults and associated folds that deform normal and reverse flows of the CRBG. In this model, Umtanum Ridge itself is viewed as a transpressional structure uplifted 1 to 2 km along northwest-striking thrust faults with opposing dip. Modeling indicates that these thrust faults penetrate through the entire Tertiary CRBG section and into underlying pre-Tertiary rocks, suggesting that thick-skinned tectonism is dominant here. Magnetic anomalies over Umtanum Ridge extend northwestward well beyond exposures of CRBG, thus allowing us to map this zone of deformation over large distances. Using the new aeromagnetic survey and older gravity and magnetic data, we speculate on possible tectonic connections between the Yakima fold belt in eastern Washington and active faults of the Puget Lowland. We suggest that the southern Whidbey Island fault truncates the Seattle fault about 35 km east of Seattle, then continues through the Cascade Range where it transfers strain southeastward to the Umtanum Ridge fault zone. The Tacoma fault may connect in the subsurface with the White River-Naches River fault zone in the Cascade Range and then may merge with the Umtanum Ridge fault zone farther east. In this view, active Puget Lowland faults converge near Snoqualmie Pass in the Cascade Range before connecting with the Yakima fold and thrust belt farther to the southeast. The distribution of earthquakes (MW ≤ 5.3) occurring during the past 35 years suggests that this confluence of faults 35 km east of Seattle may be seismically active.
Detailed Geological Features of The Arabian Peninsula Obtained From The Aeromagnetic Data
A new seamless high-resolution data set, is presented showing more detailed features of the Arabian Shield and the Cover Rocks than any of the existing compilations. This paper describe the procedures of editing and reprocessing 28 separate aeromagnetic surveys covering the Arabian Shield and the Cover Rocks areas and merging them into one regional aeromagnetic dataset. These surveys have different specification of time-span, flightline spacing, flightheight, flightline direction as well as data errors. Processing was not straight forward since some continuation lines did not overlap, resulting in the generation of significant noise in the derived grid. Also the archived digital aeromagnetic surveys over the Cover Rocks had locations of data points truncated to 3 decimal places of a degree; this resulted in zigzagging of location points along the flightlines. A further problem was the original data had more than one flightline assigned with the same line number. These shortcomings resulted in many automated processing errors which were impossible to simply correct by using the available commercial software. Therefore, specially written software was designed to solve these problems. Finally microlevelling each survey, bring all surveys to a common datum and merging the surveys into an integrated/unified survey at a grid cell size of 200 m was achieved. This new compilation was comprehensively used to map the tectonic fabric of the Arabian Shield and the extension of the fault systems beneath the Phanerozoic cover, enabling a better understanding of basement evolution.
Chronology of the Marsili Basin (Southern Tyrrhenian Sea) from new high resolution magnetic data
Inversion of new high-resolution magnetic data from the Marsili volcanic seamount and the surrounding basin in the Tyrrhenian Sea reveals NNE-SSE magnetization stripes ranging from the Matuyama chron to the Brunhes chron, including the short positive Jaramillo subchron. The detailed magnetic chronology shows that from the Matuyama chron the average half spreading rate was about 1.5 cm/yr, with a slight decrease between the Jaramillo and the Brunhes events, when the growth of the volcanic edifice overcame lateral spreading. Magnetic anomalies directly correlated with the seamount edifice indicate huge hydrothermal alteration which affects the top of the volcano. Statistical analysis of spreading rate and volume of erupted lava indicates that at the beginning of the Jaramillo subchron (1.07 Ma) the Marsili basin evolved from pure horizontal spreading to a super-inflation of basaltic volcanism forming the seamount as consequence of tearing of the Ionian slab. Our data gives us a snapshot of the geodynamic transition from an active backarc spreading phase to the vertical superinflation of the seafloor due to a Pleistocic detachment of the Ionian slab.
The Morin Anorthosite Complex, Canada: Example of a Remanence Dominated Magnetic Anomaly
The Morin Anorthosite Complex, in the Canadian Grenville Province, is delineated by a strongly negative aeromagnetic anomaly of 2000 nT. The 1.15 Ga anorthosite, jotunite and mangerite plutons were emplaced into metasedimentary and igneous rocks and later metamorphosed to 750°C. To investigate the negative anomaly, and magnetic properties of the associated rocks, we studied samples of anorthosite, jotunite and mangerite from the Morin Complex. Measurements of density, magnetic susceptibility, NRM, and hysteresis were collected on a suite of samples. Magnetic susceptibility ranges over three orders of magnitude from 2 x 10-4 to 3 x 10-1. Jotunites and mangerites are stronger, but the anorthosites were widely distributed over the entire range. NRM values showed considerable variability, from 0.03 A/m to 13 A/m, with anorthosite providing both the lowest and highest values. Anorthosite properties are strongly controlled by location; the ~1500 km2 western lobe (which preserves igneous textures) having high NRM and susceptibility values, while the ~1000 km2 eastern lobe (which is dominated by annealed mylonites) has low NRM and susceptibility. Calculations of the Koenigsberger ratio, Q, reveal all the anorthosites have ratios greater than 0.6, indicating that remanence dominates the anomaly. Jotunite and mangerite have Q values less than 0.5, indicating induced magnetization prevails. Hysteresis properties indicate multidomain magnetite is present, albeit in very small amounts; this masks the hemo-ilmenite observable in thin sections. As young basalt has NRM values of about 4 A/m, the anorthosites possess surprisingly large magnetizations for rocks possessing ~1% opaque minerals. As shown by Irving et al. (1978) the paleomagnetic signature of Morin samples is steeply negative; with the high Q values of the anorthosite this indicates the anomaly is remanent-dominated and related to strong magnetization of hemo-ilmenite.