The role of the Little salmon fault in accommodating deformation associated with the migration of the Mendocino triple junction

The boundary between the southern terminus of the Cascadia subduction zone (Csz) and the northern termination of the San Andreas transform system presents a unique opportunity to investigate crustal deformation at a migrating triple junction. This boundary is the trench-transform-transform Mendocino triple junction (MTJ), the northwesterly migrating intersection of the Pacific, Gorda/Juan De Fuca, and the North American plates. We use recent and published geologic, geomorphic, geodetic, seismic, and other available geophysical data to characterize the on-shore areas north and south of the MTJ and propose a deformation model for the boundary where e-w subduction gives way to NW translation. North of the MTJ and at the southern end of the Csz, the fold and thrust belt associated with the subducting plate interface is manifest on-land as several zones of northeasterly convergent thrust faults. Observations indicate that faults, focal mechanisms and other stress field indicators systematically change strike as you move from north to south within the on-land portion of the fold and thrust belt. Features in the north demonstrate a more north northwesterly strike than the features in the south which tend to strike more northwesterly as they approach the latitude of the MTJ. This is particularly true for the southernmost fault of the fold and thrust belt, the Little salmon fault (Lsf) which changes strike from roughly N30W at its northern exposure to approximately East-west at its southern exposure over a distance of 27 km. Geologic and paleoseismic data suggests the LSF accommodates significantly more horizontal contraction than the other faults of the southern Csz fold and thrust belt. South of the MTJ the northern San Andreas fault system consists of three primary fault splays that accommodate about 80% of the total Pacific-North American plate boundary motion. The primary faults from west to east are the San Andreas, Maacama, and Bartlett Springs fault zones. Fault slip rates diminish from west to east with the San Andreas accommodating roughly 20 mm/yr (51%), the Maacama accommodating 11 mm/yr (28%), and the Bartlett Springs accommodating 8 mm/yr (21%) of the fault slip distributed across the roughly 120 km wide shear zone. These varying slip rates along the primary fault zones result in the relatively coherent blocks between them to impinge into the southern Csz at accelerated rates from east to west with the block between the San Andreas and Maacama fault zones impinging the fastest. Our recent mapping and analysis of published data suggest this distribution of shear plays a key role in the migration and deformation of the MTJ and that the southern Lsf is accommodating the northwesterly impingement of the fault block between the San Andreas and the Maacama fault zones while the northern Lsf is accommodating contraction associated with the southern Csz. This suggests further characterization of the Lsf would be prudent to modeling both migration of the MTJ as well as deformation of the southern Csz.