January 25, 2007

Austin, Texas—Researchers report that an approach used for years to understand the structure of Earth's oceanic crust is flawed. This new finding has implications for understanding how new crust is formed at mid-ocean ridges, how heat and chemicals flow through oceanic crust and how life can exist in the hot, inhospitable environment deep below the seafloor.

Scientists Gail Christeson and Kirk McIntosh from The University of Texas at Austin’s Institute for Geophysics, and Jeffrey Karson from Syracuse University in New York, publish their findings in this week's edition of Nature. Their research reveals that seismic data, widely used by geoscientists to create a picture of the geology below the seafloor, cannot be used reliably to map the boundaries between lavas and dykes—streams of lava that have hardened—in young oceanic crust. Despite this limitation, seismic data may be key to understanding how fluids reside and circulate through the crust, as well as the extent of the biosphere, the zone in which life can survive in the subsurface.

The scientists’ work to understand the structure of young oceanic crust appears in an article entitled “Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust.” It is based on the results of two geophysical research cruises to the Hess Deep rift and the Blanco transform fault in the Pacific Ocean, rare locations where young oceanic crust is exposed along steep scarps revealing the layered geologic units.

Oceanic crust makes up two-thirds of Earth’s surface. It is made of igneous rock and formed at mid-ocean ridges, the largest volcanic system on our planet, from melting in the mantle. Eventually, oceanic crust is consumed at major earthquake-generating deep-sea trenches. The oceanic crust has been studied by geophysicists and geologists for many years—geophysicists using remote sensing techniques such as seismic exploration, and geologists with samples and direct observations. In part because of their different perspectives and techniques, geologists and geophysicists have been at odds over the basic definitions of oceanic crustal structure.

The simple picture of oceanic crust drawn by geologists is an upper layer of basaltic lavas, a middle layer of basaltic intrusive rock units, known as dykes, and a lower layer of gabbroic rocks—course-grained igneous rocks. The top layer of lavas formed when magma, or molten rock, erupted onto the seafloor. The middle layer of dykes were created as molten rock from the underlying magma chamber intruded into incrementally opening cracks at a spreading center, with younger dykes cross-cutting older dykes, eventually creating a massive collection of rock units referred to as a sheeted dyke complex. The incremental in-filling of cracks above a magma chamber is the essence of seafloor spreading.

Seismologists, on the other hand, divide oceanic crust (beneath any sedimentary material) into two basic layers: layer 2 and layer 3; layer 2 is typically subdivided further into layers 2A and 2B. Layer 2A, the subject of the research reported in Nature, is a commonly imaged horizon in the seismic data, known as the 2A reflector, which numerous studies have mapped over extensive regions of young oceanic crust.

The problem with trying to reconcile the two models of the structure of oceanic crust is that while seismic methods can be used to probe deeply into the crust and gather data over unlimited distances, the interpretation of results is usually by inference. Geologic methods, on the other hand, provide direct evidence and observation, but are limited by the few outcrops and drill holes where samples from the crust can be collected, and by the limitation of observing outcrop data with submersibles.

Co-investigator Kirk McIntosh, also of The University of Texas Institute for Geophysics, explains: “Our work addressed the extent to which seismic boundaries within the crust correlate with rock units at the Hess Deep rift and the Blanco transform fault, where nature offers a rare glimpse of what lies beneath the seafloor and the Earth’s crust-making processes.”

“Places like Hess Deep rift and the Blanco transform provide ‘windows’ into the internal structure of the oceanic crust in cliffs that are on the scale of the walls of the Grand Canyon- more than a mile high,” said Jeffrey Karson, co-investigator on the project and professor in the Department of Earth Sciences at Syracuse University.

“At the Hess Deep and the Blanco transform fault we were able to directly compare the seismic structure of upper oceanic crust with the known geology of the crust exposed and mapped by previous submersible dives,” said McIntosh.

“Prior to our study, there were no links between the geologic and seismological structure of oceanic crust except at a few deep drill holes,” said Christeson.

The scientists research show that while the layer 2A reflector is imaged near the top of the sheeted dyke complex at the Hess Deep rift, it is imaged far above the sheeted dykes section at the Blanco transform fault—a finding that does not support the hypothesis that seismic reflector 2A represents a geologic boundary between two different rock units.

“Many researchers interpret seismic reflector 2A as the geologic boundary between the upper layer of lavas and the underlying sheeted dykes,” said Gail Christeson, a research scientist at the Institute for Geophysics in the Jackson School of Geosciences at The University of Texas at Austin. “Our work shows that we can’t reliably use seismic methods to map the boundary between lavas and dykes in young oceanic crust,” said Christeson.

“However, the seismic data maps porosity,” said Karson. “Microbes live in this pore space—a very exciting frontier of geology/biology.”

The results also undermine an alternative hypothesis that the 2A reflector is associated with a chemical alteration boundary zone within the upper lava unit and raise the question: What are these seismic differences mapping?

“We propose that the 2A reflector corresponds to a chemical alteration front associated with a feature—possible a crack where minerals can precipitate as a result of increased temperature and decreased porosity,” said Christeson. “Such a hydrothermal alteration zone can occur either within the lava section or near the top of the sheeted dyke complex of oceanic crust.”

The work carried out at Hess Deep and the Blanco transform fault expands geologists’ and geophysicists’ understanding of the relationship between the seismic boundaries and the rock units of oceanic crust and provides a new avenue of research to learn more about the porosity structure of the upper oceanic crust.

This research was supported by the National Science Foundation.

The article, titled “Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust” is available online (by subscription) at:
http://www.nature.com/nature/journal/v445/n7126/full/nature05517.html

For more information, contact Katherine Ellins at kellins@ig.utexas.edu, 512-471-0347.

For more information about the Jackson School, contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.