DO MICROPLATES IN SUBDUCTION ZONES LEAVE A GEOLOGICAL RECORD

Active microplate boundaries in ocean-continent subduction zones may i nduce deformation of the overlying plate and spatial or geochemical va riations in the volcanic are. We discuss two modern cases. The first i s the South Gorda-Juan de Fuca plate boundary in the Cascadia subducti on zone, where there is little or no effect on the overriding plate an d the oceanic plate takes up much of the deformation. The second case is the Cocos-Rivera plate boundary in the Middle America trench, where the overlying Colima graben contains substantial deformation in a zon e extending from the trench to the volcanic are and the subduction-rel ated volcanism is spatially and geochemically complex. We apply these observations to boundaries of the Arguello, Monterey, Guadalupe, and M agdalena microplates, which existed in the subduction zone west of Baj a California at various times from 20 to 12.5 Ma. The past positions o f these boundaries relative to Baja California are constrained by glob al plate reconstructions, closure of the Gulf of California, and an es timate of extension in the Mexican Basin and Range province. Existing regional mapping and our additional reconnaissance mapping show that P aleocene to Eocene fluvial and marine sedimentary rocks south of Ensen ada along the western Baja California peninsula and eastward to the mi d-Miocene volcanic are are undeformed. Limited available data reveal n o major spatial or geochemical variations in the mid-Miocene volcanic are that might correlate with the past positions of the microplate bou ndaries. Thus these microplate boundaries had little to no effect on t he overriding continental plate. The nature of Guadalupe and Magdalena interactions with North America may have been closer to the South Gor da-Juan de Fuca example, with possible internal deformation of the mic roplates. The Monterey and Arguello microplates may have behaved like the modern Explorer plate, with largely strike-slip motion relative to North America during their last stages of existence. Tectonic pattern s similar to these examples may be expected from other plate boundarie s where a plate is fragmenting as it enters a subduction zone (e.g., t he Aluk plate in the trench beneath West Antarctica in early Tertiary time). Whether these microplates subsequently become attached to the o verriding continental plate or to a larger oceanic plate and whether t his causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries.