Costa Rica Rift & 504B

Seismic layer 2a in the upper oceanic crust is variously interpreted to be the extrusive basaltic layer above sheeted dikes (2B) or a hydrothermal alteration front at which seismic velocities rapidly increase in the transition between high and low porosity rock. It is likely a combination between the two. A sharp velocity gradient between layers 2A and 2B produces a refracted seismic arrival that is recorded in the far offsets (> ~2.5 km) of seismic reflection and refraction data. This arrival is especially useful for determining the velocity of seismic layer 2A, which we can then use to constrain the thickness and degree of hydrothermal alteration as a function of such variables as time, sedimentation, and tectonic structure.

I’m currently studying variations in seismic layer 2A in crust formed at the Costa Rica Rift, which lies in the eastern equatorial Pacific east of the Galapagos hotspot. The study region extends 220 km from the rift axis to DSDP/ODP Hole 504B, which lies in 5.9 My old crust and is the deepest hole drilled into oceanic crust. The seismic dataset consists of 600 km of multichannel seismic reflection (MCS) data acquired aboard the R/V Maurice Ewing using a 3005 cu. in., 10-airgun source array and a 4 km, 160 channel digital hydrophone streamer. In addition, the survey acquired seismic refraction data with 25 sonobuoys, which recorded seismic arrivals at offsets of up to ~15-20 km, providing critical information at offsets greater than the 4 km offset available from the MCS data. The MCS and sonobuoy data, combined with drilling data from Hole 504B and nearby sites, will allow us to understand the factors affecting upper oceanic crustal structure at the Costa Rica Rift.

Floyd, J.S., Carbotte, S.M., and Mutter, J.C., 2011, Upper Crustal Seismic Structure and Velocity Variations from the Costa Rica Rift to Hole 504B from Multichannel Seismic Reflection and Sonobuoy Data, Abstract T34B-04, 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec.

Floyd, J.S., Mutter, J.C., and Carbotte, S.M., 2002, Seismic Reflection Imaging of the Evolution of Oceanic Crustal Structure at the Intermediate Rate Spreading Costa Rica Rift, Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract T12B-1318.