TRANSTENSIONAL RIFTING ASSOCIATED WITH VOLCANISM DURING ACCRETION OF LATE CRETACEOUS-EOCENE SEAMOUNT REMNANTS IN THE FRANCISCAN COMPLEX COASTAL BELT, NORTHERN CALIFORNIA

Small bodies of ocean floor entrained in the deformed Paleogene-Neogene Franciscan Complex Coastal Belt preserve a record of sea floor that otherwise was mostly subducted beneath North America. These basaltic rocks are expressed in aeromagnetic data indicating they are more widespread and structurally continuous than indicated by their mapped surface distribution (Langenheim et al., this meeting).

Foraminifer faunas of pelagic limestones intercalated with the basaltic rocks indicate mainly latest Cretaceous ages (Campanian-Maestrichtean) and a warm water, northern equatorial depositional setting. In the Wheatfield Fork terrane (WFT), however, Campanian-Maestrichtean basalts and limestones are overlain by a sequence of basalt and limestone containing a non-equatorial, late middle Eocene foraminifer fauna (about 45 ± 4 Ma).

We have compared the WFT geochemistry to other Coastal Belt basalts and to Paleogene (Siletz terrane) basalts in OR and WA, where pre-Paleogene basalts are not present. Trace and minor element scatter plots indicate a MORB to Ocean Island basalt origin for the Coastal Belt and most Siletz terrane basalts. The Eocene WFT basalt, however, has a chondrite-normalized rare-earth element (REE) pattern that is convex upward, with a maximum at Nd and Sm (Sm/Yb= 1.4; La/Sm=0.84). This pattern is comparable to basalts derived from a pyroxenitic melt source erupted during early Gulf of California rifting. Paleogene Siletz terrane basalts, in contrast, are strongly light REE- enriched, arguing against a compositional link with Paleogene Coastal Belt volcanism. We suggest that Late Cretaceous basalt entrained in the Coastal Belt was erupted on a seamount or plateau east of the Farallon-Kula and Farallon-Pacific ridges, which then collided with the California subduction margin north of 30° N in the Eocene (about 49-41 Ma). We speculate that deep extension or rifting occurred within the subducting seamount close to the Coastal Belt trench, producing Eocene (WFT) volcanism over the seamount.