PROVENANCE INVESTIGATION OF THE MIDDLE MIOCENE MAGDELENA FAN OF BAJA CALIFORNIA SUR, MEXICO

The Magdelena fan is a major submarine topographic feature at the base of the continental slope off western of Baja California Sur that records a distinct pulse of Middle Miocene quartzofeldspathic turbidite sedimentation from ~13-14 Ma, closely coincident with initiation of the modern Gulf of California-San Andreas plate boundary. The fan has no apparent relationship to the present topography and no obvious sediment source. A favored explanation is that it was transported NW away from basement source(s) near the present-day mouth of the Gulf of California by offset along the Tosco-Abreojos (proto-San Andreas) fault. In an attempt to constrain possible source regions for the Magdelena fan, we have measured U-Pb ages of detrital zircons from DSDP Site 471 core. Two samples (120 grains total) yield similar results; a broad normal distribution of U-Pb ages from 60-110 Ma (72% of total) with subordinate Tertiary (15-30 Ma; 9%), early Mesozoic (115-184 Ma; 15%) and early to Middle Proterozoic (1026-1790 Ma; 4%) ages. Nearby Mesozoic crystalline basement of the Los Cabos block at the southern tip of Baja California Peninsula is an obvious candidate to consider as a fan sediment source. Previous Los Cabos block data together with new zircon U/Pb ages and a large number (N >100) of new 40Ar/39Ar hornblende & biotite cooling ages indicate medial to Late Cretaceous emplacement ages, but with significant regional differences. In particular, ages for the southern Los Cabos block are restricted to 75-90 Ma. To further test this, we sampled detrital zircon U-Pb ages from modern drainages in the southern Los Cabos block. Virtually all detrital zircons (55 of 57) examined from a composite sample fall between 78-91 Ma. Hence we conclude that while basement rocks of the southern Los Cabos block may have contributed sediment to the Magdelena fan, other source regions are required to explain the zircon age distribution documented from Site 471.