total field magnetic anomaly profiles can have complex forms which depend on: latitude of observation; geometry of the subsurface source of the anomalous magnetic field; and orientation of the profile. Examine Fig 9.11a in Lillie, which illustrates how a subsurface body magnetized parallel to the geomagnetic field at high latitude produces a symmetric magnetic anomaly which is positive over the body and negative over the flanks of the body. The mid-latitude location of Tucson yields magnetic anomaly profiles which are skewed. It is important that you understand the basic origin of these general features for the geometry of magnetic anomalies.
a. In Tucson, the geomagnetic field (a.k.a. regional magnetic field) has inclination ~60° and declination ~13°. For the purposes of this exercise, we will ignore the declination, but note that in published studies, declination is taken into account. Bold red arrows have been sketched in the figure on the next page that indicate the orientation of the geomagnetic field on the N-S oriented cross section of a buried, round body with anomalously strong induced magnetization. Also sketched on this figure are the induced magnetic field lines (red lines) of the buried, round body. Assuming paramagnetic behavior, draw arrows on the induced magnetic field lines indicating the directions of the magnetic field surrounding the buried, round body.
b. Sketch the profile of total field magnetic anomaly (resulting from the interaction of the anomalous field with the regional magnetic field in Tucson. Remember that where an anomalous field has a component parallel to the regional field at the surface, the resulting total field magnetic anomaly will be positive; where the anomalous field has a component antiparallel to the regional field at the surface, the resulting total field magnetic anomaly will be negative.
c. On the same graph used for b. sketch the profile of the total field magnetic anomaly (if the buried, round body exhibited diamagnetic behavior. Be sure to include in the sketch how this change in behavior would affect both the sign and amplitude of the total field magnetic anomaly.
If North America had the opposite sense of rotation about the same Euler pole during this time, where approximately would paleomagnetic poles from samples in North America be plotted on the map? Assume that the ages of these samples are the same as those in Table 1 and plot the estimated paleomagnetic pole locations using green diamonds.
2. Determining a Sea Floor Spreading Rate
The marine magnetic anomaly profile illustrated below was observed by towing a magnetometer behind an oceanographic vessel across the East Pacific Rise. The location of the profile was at ~55°S, 135°W where the East Pacific Rise forms the boundary between the Pacific and Antarctic plates. This location is at a sufficiently high latitude that the magnetic anomaly pattern is reasonably "user friendly", meaning that positive magnetic anomalies are above normal polarity oceanic crust and negative anomalies are above reversed polarity oceanic crust. Also, a marine magnetic anomaly profile at high latitude such as this should be roughly symmetric about the crest of a spreading oceanic ridge.
a. Find and label the point of symmetry for this marine magnetic anomaly profile. This location will indicate
a) Plot the approximate locations of the paleomagnetic poles from Table 1 on the map as white diamonds.
b) What is the relationship between these paleomagnetic poles and the small circle that passes through geographic north?
c) If we assume that the magnetic north pole is approximately located at the geographic north pole at any given time, explain why the relationship you described above exists.
d) If North America had the opposite sense of rotation about the same Euler pole during this time, where approximately would paleomagnetic poles from samples in North America be plotted on the map?