Oblique Seafloor Spreading, the Malpelo Plate Hypothesis, and Cocos-Nazca-Pacific Plate Motion Circuit
Gordon, Richard G
Doctor of Philosophy
We show that oblique seafloor spreading occurs in several regions where obliquity, α, was not recognized before. These include the slow spreading centers of the Red Sea (α≈20°), intermediate spreading centers of the Cocos-Nazca plate boundary between 91°W and 94°W (α≈9°), and superfast spreading centers of the East Pacific Rise at the Nazca-Pacific plate boundary between 29°S and 32°S (α≈10°) and perhaps between ≈16°S and ≈22°S (α≈4°). Thus, oblique spreading occurs across slow, intermediate, and superfast spreading centers, but not across fast spreading centers. Across slow and intermediate spreading centers, obliquity tends to decrease with increasing spreading rate, while across fast and superfast spreading centers it tends to increase with increasing spreading rate. Oblique spreading at intermediate and superfast spreading centers may be related to magma overpressure or to unusual directions of remote tectonic stress or to ongoing plate boundary reorganizations or to some combination of these. We show along a segment of the Cocos-Pacific plate boundary that inferred magma overpressure is only one-fourth as large as remote tectonic stress, consistent with a prior inference from other observations. The highest obliquity occurs along ridge segments lying 200 km to 1500 km from a mantle plume, but not all ridge segments near plumes spread obliquely. For one set of estimates of plume fluxes, the rate of plume flux delivered to ridges correlates positively and significantly with spreading rate. Using global multi-resolution topography, we estimate new transform-fault azimuths along the eastern Cocos-Nazca plate boundary and show that the direction of relative plate motion is 3.3° ±1.8° (95% confidence limits) clockwise of prior estimates. The new direction of Cocos-Nazca plate motion is, moreover, 4.9° ±2.7° (95% confidence limits) clockwise of the azimuth of the Panama transform fault. We infer that the plate east of the Panama transform fault is not the Nazca plate, but instead is a microplate that we term the Malpelo plate. With the improved transform fault data, the non-closure of the Nazca-Cocos-Pacific plate-motion circuit is reduced from 15.0 mm a–1 ±3.8 mm a–1 to 11.6 mm a–1 ±3.8 mm a–1 (95% confidence limits). Last, we examine the closure of the Cocos-Nazca-Pacific plate motion circuit, which were previously shown to fail closure by a linear velocity of 11.6 mm a–1 ±3.8 mm a–1. We tested eliminating the spreading rates along the Cocos-Pacific plate boundary north of the Orozco transform fault. The non-closure velocity is reduced to 9.9 mm a–1 ±3.8 mm a–1 (95% confidence limits). By further replacing the spreading rates with those from NUVEL plate motion model [DeMets et al., 1990] and keeping the new transform fault azimuth estimates, the non-closure velocity is reduced to 8.2 mm a–1 ±3.8 mm a–1 (95% confidence limits), which indicates significant non-closure. Finally, we tested further by eliminating both the spreading rates and transform faults along the traditionally defined Cocos-Nazca plate boundary east of the Galapagos transform fault, the non-closure velocity is reduced to 4.2 mm a–1 ±3.8 mm a–1 (95% confidence limits), which is insignificant. This result indicates that the plate boundary east of the Galapagos Transform Fault does not record motion between the Cocos and Nazca plate. Instead it records motion between a previously unrecognized plate and either the Cocos or Nazca plate.
Oblique seafloor spreading; The Malpelo plate; Non-closure of Cocos-Nazca-Pacific plate motion circuit