OCEANIC-CRUST THICKENS APPROACHING THE CLIPPERTON FRACTURE-ZONE

A multi-channel seismic reflection image shows the reflection Moho dip ping toward the Clipperton Fracture Zone in crust 1.4 my old. This sei smic line crosses the fracture zone at its eastern intersection with t he East Pacific Rise. The seismic observations are made in travel time , not depth. To establish constraints on crustal structure despite the absence of direct velocity determinations in this region, the possibl e effects of temperature, tectonism, and anomalous lithospheric struct ure have been considered. Conductive, advective, and frictional heatin g of the old crust proximal to the ridge-transform intersection can ex plain < 20% of the observed travel-time increase. Heating has a neglig ible effect on crustal seismic velocity beyond - 10 km from the ridge tip. The transform tectonized zone extends only 6 km from the ridge ti p. Serpentinization is unlikely to have thickened the seafloor-to-refl ection Moho section in this case. It is concluded that, contrary to co nventional wisdom, the 1.4 my old Cocos Plate crust thickens approachi ng the eastern Clipperton Ridge-Transform Intersection. Increase in th ickness must be at least 0.9 km between 22 and 3 km from the fracture zone.