Deep-tow magnetic anomaly study of the Pacific Jurassic Quiet Zone ...

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TOMINAGA ET AL.: JURASSIC QUIET ZONE MAGNETIC ANOMALIES
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Figure 5. Plot of seismic profile and deep-tow magnetic data. (a) Spatial relationship of seismic lines
from MESOPAC II [Lancelot, 1989] and survey lines from this study. Solid lines show magnetic survey
line in this study and dotted lines are lines 6-13 and 10 from MESOPAC II. (b) Line 10 seismic data from
MESOPAC II. Vertical line indicates the location of Hole 801 C. (c) Line 6-13 seismic data from
MESOPAC II. (d) Seafloor and oceanic basement interpreted from data shown in Figure 5c. (e) 5.5 km
level Line 3-9 magnetic line from this study shows that magnetic anomalies are not correlated to
basement topography.
Hole 801C. We used a paleofield inclination and declination
of 10° and 20°, respectively (from Larson and Sager
[1992]) and defined the strike of the magnetic anomalies as
135° clockwise from north (from Sager et al. [1998]).
Although Tivey et al. [2006] suggested that there may be
slight directional changes in anomaly strikes, we kept this
constant strike because the strike changes are uncertain
and this assumption is consistent with previous studies
[Handschumacher et al., 1988; Sager et al., 1998] (also
see sections 3.1, 4.1, and 4.4). Magnetic models require
appropriate values for the seafloor depth, sediment thickness,
and thickness of the magnetic source layer. While
seafloor depth is available from the DSL-120 side-scan
sonar data, the depth of igneous basement must be interpreted
from sparse, pre-existing seismic profiles [Abrams et
al., 1993] and the Hole 801C. We were unable to obtain
seismic data along the deep-tow profiles during the TN152
cruise because of time and logistical limitations. For simplicity
in modeling, we used a constant depth for the
seafloor and sediment thickness within each survey subarea:
5.6 and 6.1 km (seafloor and basement) for the M34
survey, 5.6 and 6.2 km for the NORTH survey, 5.5
and 5.9 km for the H801C survey, and 5.6 and 6.1 km
for the SOUTH survey. As shown in Figure 5, changes in
seafloor depth and sediment thickness are mostly gentle, so
these approximations should be reasonable. For reference,
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