RECONSTRUCTING THE NORTHERN GULF OF CALIFORNIA-SALTON TROUGH OBLIQUE RIFT

We present preliminary results of a time-series reconstruction of the northern Gulf of California (GOC) and Salton Trough that highlights recent advances and unresolved controversies in understanding the strain history of this region. The final product will be a series of GIS-based paleotectonic maps that track oblique-divergent Pacific-North America plate boundary deformation in 2-million-year increments from 14 Ma to present. We focus on the regions of Sonora, Baja California Norte, southwest Arizona and southern California. Kinematic constraints include geologic, geodetic, and global plate-circuit data on the magnitude, timing, and direction of displacement and rotation of crustal blocks. Important constraints include the location of major dextral shear zones and cross-rift tie points of unique conglomerate deposits and correlative ignimbrites. Since ~6.1 Ma, ~90% of Pacific-North America relative plate motion has been located in the GOC, while ~10% has been accommodated by faults west of Baja California, a pattern corroborated by modern-day geodetic studies. Key to the reconstruction is the documented westward migration of transtensional strain as the GOC opened. Transtensional deformation initiated by 7 - 8 Ma on Isla Tiburón and adjacent coastal Sonora. Areas structurally inboard of these shear zones experienced older episodes of ENE-WSW extension with little to no evidence for dextral shear. By ca. 6 Ma strain migrated west and focused into marine pull-apart basins that now lie within the eastern GOC. Previous interpretations of seismic reflection data suggest that ca. 3.3–2.0 Ma strain migrated to the western GOC, forming new transtensional basins that host the modern-day plate boundary. In the Fish Creek-Vallecito basin of the Salton Trough region, rift-related non-marine sedimentation initiated ca. 8 Ma, followed by marine incursion at 6.3 Ma. The timing of marine incursion in the Salton Trough coincides well with marine incursion documented on southwest Isla Tiburón, constrained to 6.0 - 6.8 Ma. These areas of coincident localized subsidence and marine incursion suggest formation by this time of a focused plate boundary along the Gulf of California-Salton Trough axis. Overall, this reconstruction illustrates the 4-D evolution of lithospheric stretching that lead to the Gulf of California.