CONSTRAINING CRUSTAL EMPLACEMENT PROCESSES FROM THE VARIATION IN SEISMIC LAYER 2A THICKNESS AT THE EAST PACIFIC RISE

A stochastic model for the emplacement of dikes and lava flows at a fa st spreading ridge can generate an upper oceanic crustal structure sim ilar to that observed in seismic data from the East Pacific Rise (EPR) , in ocean drill holes, and in ophiolites. In this model the location of successive dike intrusion events relative to the ridge axis is dete rmined by a Gaussian probability function and the cumulative flow leng ths of the erupted lavas are chosen to build a Gaussian-shaped lava pi le, We interpret wide-angle seismic reflections from the steep velocit y gradient at the base of seismic layer 2A to be the extrusive/sheeted dike contact. Seismic data from the northern and southern EPR place c onstraints on the on-axis extrusive layer thickness (230 +/- 50 m), th e distance over which the thickening of the extrusive layer occurs (wi dth of the accretion zone = 1-3 km) and its off-axis thickness (300-80 0 m) Ophiolites and ocean drill holes (DSDP Hole 504B) provide additio nal estimates of the thickness of the extrusive layer and constrain th e thickness of the transition region from extrusives to sheeted dikes (similar to 100-200 m). A simple stochastic emplacement model, where t he lavas are described by one mean flow length, fits the thickening of the extrusive layer off-axis inferred from the deepening of seismic l ayer 2A, but the predicted transition from sheeted dikes to extrusives is too thick In order to match the dimensions and flat-topped shape o f the seismic layer 2A boundary as well as the thickness of the extrus ive-sheeted dike transition, a bimodal distribution of lava flows is u sed. Short flows, confined within the axial summit caldera (ASC), buil d up approximately half the extrusive volume. Occasional voluminous fl ows spill out of the ASC, or erupt outside of the ASC, and pond at a c onsiderable distance off-axis to build up the remainder of the extrusi ve section. The upper part of the final extrusive section will be domi nated by the off-axis flows, while the lower portions will be primaril y composed of short flows erupted within the ASC. Magnetic transition widths predicted from the overlap of lavas (similar to 2 km) in this m odel are similar to those measured in deep-tow studies. Assuming a smo othing function which acts over one seismic wavelength, the upper crus tal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the EPR, The ages of seafloor lavas in this model are younger than th e tectonic spreading model ages by similar to 30-70 kyr, in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the EPR.