Geometric Aspects of the Full Moment Tensors in the Gulf of California and the Mexican East Pacific Rise

Abstract

The East Pacific Rise (EPR) and the Gulf of California (GC) have different tectonic histories. While the EPR has been present for 75 Ma, the GC started only 12.5 Myr. The region that links both systems is the Tamayo Fracture Zone, where a diffuse triple junction is located. A key question to be solved is whether the source mechanisms in this region reflect important variations from the GC to the EPR. Therefore, we analyzed the seismic moment tensors of the GC and the EPR using a full moment tensor inversion. This source model is useful in extensional regimes where isotropic components or complex faults are present. The full moment tensor is the best representation of the fault and slip direction in a rifting process because it resolves for six free parameters, including complex sources of pure shear dislocations. The analysis is similar to the deviatoric case, but the interpretation is different, because physical characteristics in the model allow for choosing a realistic style of rupture. Our results show that there are similarities between focal mechanisms determined by full moment tensors computed for the southern part of the GC and the EPR. We suggest that the EPR is tectonically linked to the GC not only at the diffuse triple junction region but also along the entire province. The rupture patterns of the GC and the EPR are slightly different: whereas the GC is partitioned by means of NW–SE faults, the EPR ruptures through a faulting system NE–SW. The geometrical relations of the extensional province of the GC and the EPR were present since the crustal thinning of the rifting process. Strain partitioning of faults explains easily the nature of the oblique divergence of the GC and the EPR. In addition, in our analysis, we observe clockwise rotation in the structures of the southern part of the GC, suggesting that there is a change in the spatial partitioning of this region.