In order to gain a better understanding of the behavior of microplates after their subduction, we studied two end-member scenarios for the post-subduction history of two offshore California microplates. In the first scenario, Monterey and Arguello microplate remnants are present today below the North America Plate, while in the second scenario subducted microplate remnants are absent. 3-D numerical modeling of the thermal evolution implied by these scenarios results in two different present-day thermal structures of the central and southern California upper mantle. By comparing the model-predicted surface heat flow values and seismic velocities to heat flow data and tomography, we find that we cannot discriminate between the two scenarios as they both are consistent with the data. This result means that the present-day upper mantle temperature field is relatively insensitive to the assumed microplate scenarios. A slabless window is not needed for the generation of partial melt either, which is consistent with earlier 2-D studies for this region.