Download Collaborative Research: Testing the Hypothesis of Ocean Core

Collaborative Research: Testing the Hypothesis of Ocean Core Complex Formation on Propagating Rifts
in the North East Pacific
H. Paul Johnson, Maurice Tivey & V. Dorsey Wanless
Project Summary – Oceanic core complexes (OCCs) are small oceanic plateaus with both smooth and
corrugated dome-like upper surfaces elevated above the surrounding topography and where lower crustal and
mantle rocks are directly exposed on the seafloor by slip on low-angle detachment faults. OCCs have recently
been shown to be a surprisingly common feature generated at slow-spreading mid-ocean ridges, with up to 50%
of oceanic crustal extension and accretion accounted for by OCC-style processes. The traditional paradigm
regarding the lithologic composition of ocean crust – a simple layered igneous oceanic crust, capped by
extrusive volcanic rock, is thus far from universal and will need to be revised, particularly for slow spreading
crust. A Chapman Conference in May, 2010 compiled recent data and models of OCCs, including the extensive
bibliography in the present reference section and concluded that OCCs form when magmatic crustal production,
as defined by the M parameter, is reduced to less than 0.5. Specifically, OCCs form when spreading is
dominated by amagmatic extension, and where mid-ocean ridge extension becomes asymmetric. To date the
majority of OCCs have been found in slow- to ultra-slow spreading environments where magmatic processes are
known to be episodic and variable. These conditions, however, are not unique to slow spreading ridges. A
reduction in magmatic accretion and asymmetric extension are conditions that are found associated with the
process of propagating rifts which are a common feature of medium-rate spreading centers. The dying rift
segment of a propagating rift experiences a dramatic reduction in magmatic supply as well as changes in
extension symmetry. We propose here that anomalous plateau-like features exposed on the dying pseudo-fault
segments of the southern Juan de Fuca and Gorda eastern ridge flanks associated with propagating rift traces
may be the result of OCC-like processes. To test this assertion, we request support for a pilot field program of
geophysical surveys (high resolution 30 kHz multibeam bathymetry, gravity, and magnetics), near-bottom
TowCam surveys and rock sampling at several sites on the southern Juan de Fuca and eastern Gorda plates,
where 3 decades of archive geophysical data strongly suggest the presence of OCC-like structures associated
with pseudo-fault traces. This new data would test the hypothesis that these features can be generated at the
dying segment of a propagating rift offset – at medium spreading-rate ridges in the NE Pacific.
Intellectual Merit – It is important to define the conditions under which OCCs form and to determine their
impact on oceanic crustal evolution. Recent studies of OCCs have shown that large deeply-penetrating
detachment faults can have substantial influence in the post-formation, ridge flank environment. Detachment
faults provide long-term access for seawater into the deeper and hotter crustal sections, and examples of both
high-temperature, low-flux and low-temperature, high-flux hydrothermal fluid circulation has been observed
occurring along these faults. Importantly, active fluid circulation is not limited to the near-axis environment, but
continues in older ridge flank crustal sections. Further, these low angle detachment faults appear to be weak
zones in ridge flank crustal strength, and are sites with continued tectonic activity that are located well outside
the crustal formation zone. The consequences of continued seismic activity and off-axis fluid circulation on
pseudo-faults at medium-spreading rate ridges are not yet clear, but the implications are potentially
transformative: for the sub-surface biosphere, the hydration of oceanic crustal rocks, and serving as possible
tectonic segmentation boundaries of the down-going plate within the active Cascadia subduction zone.
Broader Impacts – We will advance discovery and understanding of seafloor formation, while promoting
teaching, training and learning by including students at the graduate level for thesis work and at the
undergraduate level for student research projects as part of a multidisciplinary oceanographic cruise. We will
broadly distribute results through real-time web casts for the popular science media, use of outreach facilities at
University of Washington and Woods Hole Oceanographic Institute (WHOI) and by full/rapid archiving of the
data in publically accessible databases (NGDC, MGDS), as well through scientific publications and conferences.
The program supports a junior female post-doctoral researcher at WHOI and contributes to mentoring of an
early career scientist with her first Co-PI role in a major seagoing project. There is also a potentially important
benefit to society by investigating the nature of the oceanic crustal zone deformation within a propagating rift
wake that appears to contribute to the seismic segmentation of large megathrust earthquakes occurring on the
Cascadia Subduction Zone, adjacent to the well-populated western North American continent.