Reconstructing the Strain History of 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 both recent advances as well as unresolved controversies in understanding the strain history of this region. A series of GIS-based paleotectonic maps are planned to track oblique-divergent Pacific-North America plate boundary deformation in 2-million-year increments from 14 Ma to 2 Ma and in 1-million-year increments from 2 Ma to present. Here we focus on the regions of Sonora, Baja California Norte, southwest Arizona and southern California. This reconstruction will complement those in the southern GOC (see abstract in this session). Kinematic constraints include geologic, geodetic, and global plate-circuit data on the magnitude, timing, and direction of displacement of crustal blocks. An important constraint includes the location of major shear zones, which are limited to the western part of the rift, in coastal Sonora, in GOC marine basins, and west of Baja California. Cross-rift geologic tie points include a fusulinid-bearing clast conglomerate exposed in coastal Sonora and northeast Baja California, outcrops of the Poway conglomerate exposed in northwest Sonora and near San Diego, and 12.5 Ma and 6.1-6.4 Ma correlative tuffs that both occur on western Isla Tiburón and in northeast Baja California. Since ~6.1 Ma the magnitude of extension across the northern GOC requires that ~90% of Pacific-North America relative plate motion has been located here 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 migration of transtensional strain westward as the GOC opened. In coastal Sonora, transtensional deformation initiated by ca. 7 Ma on northeast Isla Tiburón and along a major dextral fault system east of Isla Tiburón, herein named the Coastal Sonora Fault Zone. New mapping inland suggests that this fault zone may mark the eastern limit of significant dextral faulting. By ca. 6 Ma strain appears to have migrated west and focused into marine pull-apart basins that now lie within the eastern GOC. Previous interpretations of seismic reflection data suggest that these eastern basins were abandoned ca. 3.3-2.0 Ma as strain again migrated west, forming new transtensional basins that host the modern-day plate boundary in the western GOC. 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 (Dorsey et al., in press GSAB). The timing of marine incursion in the Salton Trough coincides well with marine incursion documented on southwest Isla Tiburón, constrained to between 6.7 and 4.1 Ma. These areas of coincident localized subsidence and associated marine incursion suggest formation by this time of a focused plate boundary along the Gulf of California-Salton Trough axis. Overall, this reconstruction illustrates a 4D example of the role that rift obliquity may play in the formation and architecture of passive margins.