Frequency-dependent shear wave splitting and heterogeneous anisotropic structure beneath the Gulf of California region

The Gulf of California region has undergone a major evolution over the past ∼15 Ma, since subduction of the now-extinct Farallon plate ceased and transform motion began, leading to the initiation of rifting and the opening of the Gulf of California ∼5 Ma ago. The character of dynamic processes in the upper mantle beneath the region remains poorly understood, but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes. In order to characterize more fully the anisotropic structure of the upper mantle beneath the region, I present measurements of SKS splitting at 14 broadband stations of the NARS-Baja array at periods between ∼8 and 50 s. The measured splitting parameters (ϕ, δt) at NARS-Baja stations exhibit a high degree of complexity, with dramatic lateral variations, significant backazimuthal variations at individual stations, and a large number of well-constrained null measurements over a range of backazimuths. The dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal-to-noise SKS arrivals. I find that while fast directions are generally insensitive to the SKS frequency content, delay times are frequency dependent at several stations. The measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the Gulf of California region is complex and exhibits a high degree of both lateral and vertical heterogeneity. The observed splitting is consistent with a scenario in which a large-scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with Farallon subduction. The pattern of mantle flow appears to be controlled by more localized processes, likely including small-scale buoyant upwelling associated with partial melting beneath the central part of the Gulf of California rift.