Formation and evolution of upper oceanic crust from seismic data acquired over mature oceanic crust

DESCRIPTION: The upper oceanic crust and its physical properties are well known near the mid-ocean ridge axis where the crust is formed. However, little is known about how the properties of the upper oceanic crust change as the crust ages, primarily because of the lack of the appropriate datasets over older crust. An important question that remains unanswered is whether or not hydrothermal circulation continues as the crust spreads away from the ridge axis. Heat flow measurements suggest that hydrothermal circulation persists, but it has been impossible to clarify from existing datasets the effects and the controls on this late stage circulation. This project will use recently collected marine seismic data over mature oceanic crust (>50 Myr) offshore the Sumatra and Alaska subduction zones to address these problems. These two datasets are very high quality and unique in that they allow imaging of the upper part of the oceanic crust. Understanding hydrothermal circulation in older crust is important because we do not know the extent of the chemical exchanges that occur between the earth and oceans in these vast regions that represent about 70% of the surface of the earth. The study supports the training of a Ph.D. graduate student. The project also involves an international collaboration with French colleagues. This project will examine 2-D marine seismic data acquired over mature oceanic crust (>50 Myr) offshore the Sumatra and Alaska subduction zones. These data were acquired in 2011 on the R/V Marcus Langseth and in 2009 on the M/V Geowave Champion during the TIDES Program (IPG-Paris). The main objective of this study is to provide new insights on the seismic heterogeneity of the uppermost crust and the dominant processes that control the aging of upper oceanic crust. The long-streamer MCS data will provide seismic imaging of the upper crust including post-critical reflections seen at large offsets. The MCS imaging will also allow the characterization of variations in the sediment thickness, topography of the upper crust, and abundance of faulting within the sediments and upper crust. The downward continuation followed by travel time modeling of the streamer data will provide the thickness and P-wave velocity of the two layers that constitute the upper crust. Synthetic computation will generate further information on S-wave velocities and attenuation. Together the two datasets will allow the comparison between sections of oceanic crust formed at different spreading rates and of slightly different ages.