Structural evolution of the East Pacific Rise axis from 13°10′N to 10°35′N: interpretations from SeaMARC I data

Abstract

In 1983 the SeaMARC I and the Sea Beam systems imaged the East Pacific Rise (EPR) and the Clipperton Transform Fault from 13°10′N to 9°50′N revealing both magmatic and tectonic segmentation along the rise axis. Tectonic segments are defined by an undulating axial zone of extension which widens and narrows at an average wavelength of 45 km. In addition, axial fissure-fault-density and -length curves reveal wavelengths of 30 and 56 km. The narrowest axial zones, < 0.5 km in width, correspond to regions of hydrothermal activity and are located atop regional morphologic highs quasi-harmonically spaced at 155 km. These highs define the major magma centers on the rise axis about which seamounts cluster. Superimposed on the intermediate-wavelength highs are smaller highs (from 10 to 50 km in length) which are truncated by overlapping spreading centers (OSC's) at their distal edges. The rise axis widens near OSC's while it narrows and disappears near the Clipperton Transform Fault suggesting that transforms and OSC's have different origins. Imagery on and adjacent to a few OSC's reveals that in some cases OSC's migrate along the axis forming relict off-axial structural traces at oblique angles (285°–330°) to the axis. Off-axial fault density and length curves have wavelengths similar to the axial curves. However, fault densities to the west of the axis are somewhat antisymmetric to fissure-fault densities on the axis. In contrast, faults to the east of the axis nearly mimic fissure-fault density and length curves on the axis suggesting that the fracture pattern is controlled by the state of deviatoric stress in the adjacent Cocos Plate to the east. The data suggest that large deep-seated magma diapirs may dominate the magmatic segmentation of the EPR generating dynamically supported 155 km wavelength highs. Smaller, shallower magma cupulas may rise as fingers to the seafloor above the large diapirs. Where the magma erupts may be a factor of the thin skin tectonic segmentation of the rise axis formed in response to the state of deviatoric stress in the upper crust. Small OCS's may form at crustal weak points and may migrate in response to magma body movement along the axis. The instability of both the magmatic and tectonic segmentation is apparent from the oblique alignment of both seamount chains and relict OSC's with their present-day counterparts.