During the AMAR 1978 Expedition, over 200 separate basalt samples were collected by Alvin and dredge from the Narrowgate and AMAR segments of the Mid-Atlantic Ridge near 37°N to determine the origin of reversely magnetized rocks previously recognized in the FAMOUS rift area to the north and to study the variations in rock magnetic properties that occur along the strike of the Mid-Atlantic Ridge. Magnetic intensity determinations of rocks collected in the FAMOUS area indicate that intensity decays with time by a factor of 5 in less than 106 years. Rocks with low intensities have high Curie temperatures (Tc) indicating that low-temperature oxidation of the magnetic minerals takes place soon after the basalts are extruded and is pervasive in crust older than 106 years. Magnetic intensity (Jnrm) of the uppermost oceanic crust can be expressed as a function of age (t) by the expression J = 5.82t−1/3. By measuring Jnrm of the surface rocks from the three valley segments, the relative ages of the surface rocks from the FAMOUS, AMAR, and Narrowgate areas can be estimated to be 1×104 years, 3×104 years, and 4×104 years, respectively. One exception is Mt. Mars, located in the center of the Narrowgate area that has high Jnrm values (84 × 10−3 emu/cm3) and low Tc values (165°C), suggesting that this is an area where volcanic activity is becoming reactivated following a period of quiescence. Of the 55 oriented samples collected, eight are reversely magnetized. Although three of the reversed samples have Curie temperatures above 300°C and may have undergone self-reversal, the other five reversed samples have magnetic properties similar to normally magnetized rocks and may have been misoriented or extruded during a short reversed event within the Brunhes epoch. Because most of the surficial rock samples are normally magnetized, it is suggested that the previously recognized reversed zones, which appear in a deep-tow magnetometer profile that crossed the Narrowgate area, are located at depth and are the result of the spreading axis moving into older Matuyama crust that is now covered by more recent volcanism.
Van Wagoner, N. A., & Johnson, H. P. (1983). Magnetic properties of three segments of the Mid-Atlantic Ridge at 37°N: Famous, Narrowgate, and Amar: AMAR 2. Journal of Geophysical Research, 88(B6), 5065-5082. https://doi.org/10.1029/JB088iB06p05065