Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations

Abstract

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.

History of rift propagation and magnetization intensity for the Cocos‐Nazca sspreading Center

“…Slowly propagating offsets of the East Pacific Rise at 11ø45'N, 9øN, and 20ø40'S, with offsets of only 10-20 km, all show significant magnetization highs on the propagating tips of segments over 100 km long [Perram and Macdonald, 1990;Carbotte and Macdonald, 1992;Perram et al, 1993]. In particular, ridge offsets of 10-20 km may isolate the mantle upwelling beneath adjacent segments in the absence of a hotspot.…”

East Pacific Rise 18°–19°S: Asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities

“…Although the data coverage from 19øS to 19ø40'S is incomplete, we have tentatively included this area in our study in order to bridge the gap with the "dueling propagator" study area to the south [Perram et al, 1993]. The purpose of this survey was to image a 10-km-wide swath of the southern EPR with the SeaMARC II system.…”

“…The section of the EPR between the Garrett transform fault and the Easter microplate is spreading at ultrafast rates, ranging from 141 to 162 mm/a [Naar and DeMets et al, 1990;Perram et al, 1993]. Between 15ø30'S and 20ø00'S, the rise appears remarkably linear, with an overall strike of 009øE (Figure 1).…”

“…Interestingly, NUVEL-1 also predicts the mean fault azimuths accurately, but their variation with age (distance from the axis) is in a direction opposite of that predicted (Figure 12). Naar and Hey [1989,1991], Searle et al [1989], Rusby [1992], and Perram et al [1993] all report anomalous low spreading rates between the 20ø40'S OSC and the Easter microplate which they attribute to the nonrigid behavior of this portion of the Nazca plate. Evidence for a broad compression of the corner of the Nazca plate near 21ø-23øS also include diffuse seismicity, eastwest trending faults which overprint the seafloor fabric, and a progressive counterclockwise rotation of the ridge axis toward the Easter microplate, which is thought to be caused by the 53 ø-64 ø clockwise rotation of the Easter microplate relative to the Nazca plate since 3.4 Ma [Naar and Hey, 1991;Rusby, 1992;Rusby and Searle, 1993].…”

Tracking stress and hydrothermal activity along the Eastern Lau Spreading Center using seismic anisotropy

“…The area of disturbed seafloor morphology trailing away from the northernmost OSC, referred to as an OSC "wake", reveals a history of dueling ridge propagation by which the individual ridge limbs have moved both north and south over time. Within the wake, the trend of the seafloor fabric is variably misaligned with the general trend, a consequence of stress orientation changes as suggested by numerical models of OSC evolution (e.g., De Bremaecker and Swenson, 1990) and prior observations of rotated crustal blocks (e.g., Perram et al, 1993). The central OSC does not have a broad wake, but its southern limb has recently propagated north and its northern limb has receded, producing some disturbed topography.…”