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.

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…Such a narrow dike intrusion width is supported by independent statistical observations of the proportion of dikes in ophiolites that exhibit one-sided cooling, due to having been split by a subsequent dike [1,17,18]. Studies of marine magnetic anomaly reversal widths and variability [4][5][6][7][8] suggest a relatively wide zone of lava emplacement, 1-3 km wide. However, these data can not resolve the relative contributions of the width of the intrusion zone and lava flow distribution to the lava emplacement process.…”

“…Work on the transition width of marine magnetic anomalies from deep-tow measurements on the Gorda Rise [4] suggest that the portion of the transition width due to constructional processes is 1.7 km. Near-bottom magnetic profiles from ridges in the Pacific covering intertnediate to fast spreading rates give an average half-width of crustal fortnation of 2-3 km [8].…”

“…[ 4,5]). At an active spreading center, steady state is only achieved statistically, since the accretion process is a series of discrete, successive events.…”

“…These two magmatic processes generate an upper crustal structure composed of lavas piled on top of one another, underlain by a sheeted dike complex. Constraints on the processes of upper crustal construction have come from ophiolite studies [1 ,2], observations in active rifts (e.g., Iceland [3D, modeling of marine magnetic anomaly shapes and transition thicknesses [4][5][6][7][8], mapping [9][10][11][12][13][14], dating [15], and drilling (e.g. DSDP Hole 504B [I6D of young ocean crust.…”

Comparison of the Chemical and Magnetic Stratigraphy of DSDP Sites 332 and 395

“…Macdonald 674 CHEMICAL AND MAGNETIC STRATIGRAPHY (1977), citing inversions of near-bottom east-west magnetic profiles near Sites 332 and 333, lent support to the concept that the Site 332 basalts erupted during a magnetic transition. Apparently, shortly after the Gauss-Gilbert reversal, a short-period reversal of the earth's field also occurred (Atwater and Mudie, 1973;Klitgord, 1974), and may have lasted 25,000 years. This may have extended the effective duration of the longer term Gauss-Gilbert reversal and the consequent erratic behavior of the earth's magnetic field.…”