Berhorst, Arnim, Flueh, Ernst R., McIntosh, Kirk, Ranero, Cesar, Silver, E., Walther, C. H., Dole, J. and Strauch, Wilfried (2002) The crustal structure of the convergent Nicaraguan margin from a combined MCS and refraction study. [Talk] In: DGG - 62. Jahrestagung. , 03.- 08.03.2002, Hannover, Germany .

Abstract

We present seismic profiles from an extensive multi-channel-seismic and wide-angle survey acquired in June 2000
during R/V Maurice Ewing (cruise EW 00-05). The seismic measurements were carried out using a 6 km long
streamer, 14 ocean bottom hydrophones (OBH) and 9 landstations recording marine airgun shots from an 136 l
airgun-array. The objective of this study is to improve the knowledge about the structure of the convergent
Nicaraguan margin, which is located between the intensively studied margins of Guatemala to the north and Costa
Rica to the south.
The Cocos Plate, which is formed by the fast East Pacific Rise to the East and the Cocos Nazca Spreading Center
to the South subducts beneath Nicaragua nearly orthogonal to the trench. Here, the Cocos Plate was formed at the
East Pacific Rise about 24 Myr ago. The nearly 100 km wide continental shelf of Nicaragua includes the more than
10 km deep Sandino Basin. The seismic data yield detailed images of the subducting oceanic plate and the tectonic
structure of the continental plate.
A more than 250 km long transect from the outer rise to the volcanic arc and a 180 km long strike line along the
upper slope of the margin are presented. The wide-angle data were interpreted using forward modeling techniques.
The MCS data are processed up to a time migration and integrated into the refraction model. At the outer rise the
oceanic crust is only 5 km thick and some deep reflections cut trough the moho into the upper mantle. Towards the
trench the seafloor is strongly faulted in response to the plate flexure. The frontal sediments in the continental plate
are less than 1.5 km wide. The slope sediments are divided by a basement high into a shallower (< 3 km thick) part
to the southwest and the deep Sandino basin with 7 km of sediments close to the coastline. This basement high is
situated in the projection of the Santa Elena Peninsula in northern Costa Rica. The underlying basement shows a
high velocity and a high landward velocity gradient from 3.5 km/s at the tip of the margin wedge up to 6 km/s
below the Sandino basin. These velocities suggest that the margin wedge is composed of ophiolitic rock similar to
the Nicoya complex in Cost Rica. Beneath this basement we find an enigmatic high velocity material that trends
parallel to the subduction slab.

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