Detailed near-bottom geophysical study of the Gorda Rise

Abstract

A deeply towed instrument package was used to survey the fine details of topography, sediment distribution, and magnetization of the Gorda rise, an active spreading center off northern California. The gross form of the central rift valley is the result of large-scale normal faulting. The surface is broken into long, narrow, tilted steps parallel to the spreading center. Topographic features just outside the rift valley also have a tilted blocky aspect and are lineated and symmetrical with respect to features of the same age on the opposite flank. Flat terrigenous turbidite sediments are found in the valley floor and in deeps on the west flank, as might be expected. They also are found on the tops of the steps high in the rift valley walls, implying that the steps are uplifted to form the walls as they move out from the center. The magnetic field measured near the ocean bottom can be approximately simulated by using straightforward model calculations. The extremely complex short-wavelength anomalies (width less than 1 km) generally are caused by topographic effects, and from them the bulk magnetization of the topographic features was estimated to be about 0.009 emu/cm3 near anomaly 2 and 0.007 emu/cm3 near anomaly 3. Assuming these values of magnetization for the entire magnetic layer, the magnitude of long-wavelength anomalies shows that it must be about 0.5 km thick. In crust formed at a spreading rate of 37 mm/yr, magnetic field polarity reversals are recorded in the magnetic layer as a gradual zone of transition about 2 km wide. Of this width, 0.1 to 0.7 km can be attributed to the time it takes for a reversal to be completed, and the remaining width, about 1.7 km, can be attributed to the process of emplacement of magnetic material. Comparisons with emplacement models show that most magnetic material was emplaced within 2 km of the center (within 0.7 km if a pure dike injection model is assumed). The 2-km width of the emplacement effect acts as a smoothing function for events in the magnetic field, so that the record in the magnetic layer of events of duration less than about 10,000 years should be greatly attenuated. Two medium-wavelength anomalies are interpreted as possible normal polarity events centered at 3.4 and 3.55 m.y.