Final answer:
The western margin of Southern California and northern Mexico shifted to a transform (strike-slip) tectonic margin, specifically the San Andreas Fault, where the Pacific Plate is moving northwest against the North American Plate. Los Angeles is located on the Pacific Plate.
Step-by-step explanation:
The tectonic margin that defines the boundary between the Pacific and North American plates at the western margin of Southern California and northern Mexico is a transform fault margin, specifically a (transform) strike-slip boundary. This is most famously represented by the San Andreas Fault.
The Pacific Plate and the North American Plate form the San Andreas Fault, with the Pacific Plate moving northwest relative to the North American Plate. Cities such as Los Angeles and San Francisco are situated on the Pacific Plate. The ongoing lateral movement of these plates is significant for the future geography of California; according to scientific models, Los Angeles could end up closer to San Francisco in several million years.
Further north, the tectonic setting changes along the West Coast. For instance, near the coast of Washington and Oregon, the Juan de Fuca Plate interacts with the North American Plate forming a convergent boundary, leading to subduction.
This subductive movement has given rise to volcanic activity and the formation of the Cascade Range, which includes Mt. St. Helens. The ultimate fate of the relatively small Juan de Fuca Plate is expected to be complete subduction under the North American Plate.