Research Highlights - Jurassic Quiet Zone Deeptow Magnetics

Jurassic-aged ocean crust represents the oldest surviving ocean crust in the world today and consequently only resides in a few places, such as along the continental margins of the Atlantic Ocean and in the western Pacific Ocean. This Jurassic crust is marked by the disappearance of correlatable magnetic anomaly stripes in both the Atlantic and Pacific oceans, thus leading to name of the "Jurassic Quiet Zone" or JQZ [Larson and Chase, 1972; Larson and Hilde, 1975; Cande et al, 1978; Vogt and Einwich, 1979]. The precise origin of the JQZ is unclear. It could be period of constant polarity similar to the more well-known the Cretaceous Quiet Zone. It could be that the crust has suffered sufficient aging that the signal has been totally degraded. The equatorial latitude of the Pacific JQZ and deep water depths has meant that sea surface magnetic profiles are contaminated by noise from the strong diurnal and magnetic effects of the equatorial electrojet, so even measuring the magnetic field in these areas is difficult. A paper has been published in Journal of Geophysical Research, 1998 on the results of this study:

Sager, W.W., C.J. Weiss, M.A. Tivey, and H.P. Johnson, Geomagnetic polarity reversal model of deep-tow profiles from the Pacific Jurassic Quiet Zone, J. Geophys. Res., 103, 5269-5286, 1998.

Below we briefly describe the 1992 deeptow magnetic results from the Pacific Jurassic project. We revisited the area in 2002/2003 and those provisional results are now shown separately under jqz2003.html.

Map of Pacific Jurassic aged crust showing the seafloor depth based on satellite altimetry [Smith and Sandwell, 1997], where dark blue is deep and orange to white is shallow. The three sets of magnetic anomaly lineations are shown by bold black lines: the Japanese to the north, the Hawaiian to the east (right) and the Phoenix lineations to the south. Red line shows the deeptow magnetic profile lines collected in 1992.

To the right is an expanded scale map of the 1992 Deep-tow magnetic profiles collected in the Pigafetta Basin [Sager et al., 1998]. Again the seafloor depth is shown based on satellite altimetry as in the above figure. The observed deep-tow magnetic anomalies are corrected for depth variations of the tow fish and continued upward to the sea surface. Correlations between the profiles are shown with the red lines with anomaly identifications out to M41. See notes on geomagnetic polarity timescale for age in millions of years. Ocean Drilling Project (ODP) drill sites [Lancelot et al., 1990] are also shown for reference.

On the right is a plot showing the detailed correlation between the magnetic wiggles of the two 1992 deep-tow magnetic profiles from the Pigafetta basin [Sager et al., 1998]. The bottom panel shows the correlation (dashed lines) between the two 1992 Pigafetta Basin deep-tow magnetic profiles at 4.5 km depth. Middle panel shows the deep-tow profiles upward continued to sea surface level and the correlation with a redrawn version of the composite aeromagnetic profile of Handschumacher et al., [1988]. Top panel shows forward models of sea surface anomaly and near-bottom anomaly. These data have not been deskewed. Figure modified from Sager et al., [1998].

Upper panel of figure displays the near-bottom magnetic field measured along line 1 of the 1992 deep-tow survey and demonstrates the clear decrease in field intensity with age. The bottom panel is a composite sea surface profile (V3214 from Cande et al., 1978) and upward continued deep-tow profile and shows that the trend of decreasing anomaly amplitude with age has continued throughout the late Jurassic for at least 25 m.y.

Late-Jurassic to Early Cretaceous magnetic polarity reversal rate calculated for the GPTS of Harland et al., [1990] shown by solid line, and for Handschumacher et al. [1988] shown by white filled circles. These are compared with our deep-tow (black circles) and upward continued (black diamonds) reversal models. The deep-tow model implies an extremely high reversal rate while the more conservative sea surface shows an elevated rate that is consistent with the other time-scales. Reversal rate is calculated at 1 m.y. intervals with a 2 m.y. window and is based on a figure from Sager et al. [1998].

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Last revised: 2005/1/31