1. Fault Mechanics and Strain Partitioning Session Axen, Umhoefer, Stock, Contreras, Tucholke, Grove, Janecke

2. What do we know?  Variability: strain is being accommodated by different mechanisms that display spatially distinct patterns that give rise to diverse basin architectures.  The north Gulf displays LANFS, deep sedimentary cover, and more distributed deformation.  The south part of the Gulf displays sea floor spreading and sedimentary cover is relatively thin.  Vertical axis rotation is an important 3D component of plate boundary strain, and understanding of these rotations came from observation in the northern Gulf. This could not have been possible in rifts where development of sedimentary systems is an important process.  The Baja peninsula behaves as a rigid body and the Gulf has accommodated ~250 km of displacement, i.e., the new “real estate” between Tiburon Island and San Felipe.

3. What do we need to know?  Better understanding of magmatic systems (pre and post rupturing), what controls them, and their influence on strain  Influence of preexisting geology (e.g., microplates) on the spatial distribution of strain.  Influence of previous subduction zone on strain distribution, e.g., basal shear along the interface between plates in driving extension along the Tosco-Abreojos fault (microplates stuck under Baja Peninsula ?)  Influence of sedimentation on distribution of strain in the northern part of the Gulf of California. Does it localize or promote distributed deformation?  Better understanding of 3D partitioning of strain and how discontinuous (brittle) and continuous (lower crustal flow) control the creation of accommodation space

4. How can we acquire this knowledge? More geophysical imaging beneath the Peninsular Ranges. Northern transect to characterize changes in deformation mechanisms from brittle to ductile and to asses the influence of sedimentary cover on deformation distribution - to really determine if the basement & mantle shear zones are broad or narrow. More earthquakes that are better located (e.g., with double difference relocations?) would help to define the strain field at depth We need an integration of strain record over multiple time scales (geological, paleoseismological, and geodetic) including records from sedimentary basins where exposed on land or where they can be imaged with hi-res seismics

5. Proyecto Cortes transect gravity seismic ray paths Crustal structure model Upper Delfin BasinUpper Tiburon Basin From Gonzalez-Fernandez et al., JGR 2005

6. Comparison with Salton Trough Note high density, 3.1 g/cm 3, in lower crust in middle of rift here

7. Comparison with Salton Trough, 2 P wave speeds are > 7 km/s in the mafic intrusive rocks in the lower crust of the Salton Trough From Fuis & Kohler, 1984, SEPM volume 40